Two -dimensional superlattices of colloidal quantum dots...
Transcript of Two -dimensional superlattices of colloidal quantum dots...
Department of Inorganic and Physical Chemistry Research group Physics and Chemistry of Nanostructures
Two-dimensional superlattices of colloidal quantum dots - towards high performance
photodetectors
Thesis submitted to obtain the degree of Master of Science in Chemistry by
Willem WALRAVENS
Academic year 2013 - 2014
Promoter: prof. dr. ir. Zeger Hens
Copromoter: prof. dr. ir. Gunther Roelkens Supervisors: ir. Chen Hu and dr. Yolanda Justo
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The author and promoter give the permission to use this thesis for consultation and to
copy parts of it for personal use. Every other use is subject to the copyright laws, more
specifically the source must be extensively specified when using from this thesis.
De auteur en promotor geven de toelating deze scriptie voor consultatie beschikbaar te
stellen en delen ervan te kopieren voor persoonlijk gebruik. Elk ander gebruik valt onder
de beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting
uitdrukkelijk de bron te vermelden bij het aanhalen van resultaten uit deze scriptie.
Ghent, June 2014
The promotor The author
Prof. dr. ir. Zeger Hens Willem Walravens
Preface
Dear reader,
this thesis is the culmination of a year dedicated to experimental laboratory work. As a
last part of the master’s degree, this represents the transition from the classroom, where
the student is provided with clear-cut answers, to the laboratory, where questions and
possibilities reign. This rather different perspective is very exciting indeed, but it is often
a process of trial and error. Therefore I would like to thank my promoter, Zeger Hens, and
both my supervisors, Chen Hu and Yolanda Justo, for their guidance throughout the year
and for sharing their experience and insights. I would also like to thank Katrien Haustraete
for taking TEM images and Stijn Flamee for taking SEM images and introducing me to
the technique. Furthermore, all people who have helped me in one way or another this
year have my gratitude. I hope you enjoy this thesis as much as I did performing the
experiments and writing down the story of the results.
Willem Walravens
Ghent, June 2014
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Dutch summary
Deze thesis onderzoekt de mogelijkheid om PbSe quantum dot (QD) superstructuren te
gebruiken als actief sensormateriaal voor fotodetectie in het nabij-infrarood gedeelte van
het elektromagnetisch spectrum. De QDs werden gesynthetiseerd via de zogenaamde hot
injection methode. Vervolgens werden elektronisch gekoppelde QD structuren gevormd
via dropcasting op een vloeibare subfase en via Langmuir-Schaefer depositie. Verschillende
morfologieen werden verkregen door verandering van de samenstelling en temperatuur van
de subfase, en het tempo waarmee de QDs werden toegevoegd. De structuren werden
gekarateriseerd via TEM, SEM, AFM en FTIR. De fotogeleidende eigenschappen werden
onderzocht via I-V metingen van de lagen, afgezet op gouden elektroden. Alle gekoppelde
superstructuren vertoonden fotogeleiding en de resultaten wijzen verder op het belang van
de QD oppervlakte chemie en gecontroleerde trap state introductie voor het verbeteren
van de prestatie van de fotodetectoren.
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Quantum Dot Superstructures for Photodetection in the Near-Infrared
W. Walravensa,b, C. Hua,b, Y. Justoa, G. Roelkensb, Z. Hensa
a Physics and Chemistry of Nanostructures, Department of Inorganic and Physical Chemistry, Ghent University, 9000 Ghent, Belgium
b Photonics Research Group, INTEC Department, Ghent University, 9000 Ghent, Belgium
This paper reports the use of PbSe quantum dot (QD) superstructures as active sensing material for photodetection in the near-infrared. Electronically coupled QD superstructures were obtained via dropcasting on a liquid subphase and via Langmuir-Schaefer deposition. Different morphologies were obtained by changing the composition and temperature of the subphase, and the rate at which the QDs were added. The layers were characterized by TEM, SEM, AFM and FTIR. The photoconductive properties were determined via I-V measurements of the layers deposited on gold comb-like electrodes. All coupled superstructures show a photocurrent and the results point to the importance of the QD surface chemistry and directional trap state engineering as factors for enhancing the device performance.
Introduction
Colloidal quantum dots (QDs) are semiconductor nanocrystals which are dispersed as colloids in solution. Due to their dimensions below the exciton Bohr radius they exhibit a size dependent electronic structure via the quantum confinement effect. Quantum confinement translates in increasing band gap energies with decreasing crystal size, and thus enables spectrally tunable absorption spectra via careful control of the size. Their relatively cheap and easy synthesis makes them further very useful as building blocks for novel optoelectronic devices.
Colloidal QDs are synthesized via a bottom-up approach in a controlled crystallization of the semiconducting material. This is typically carried out in an organic solvent where the nanocrystals are stabilized with suitable organic ligands bound to the inorganic core. Colloidal dispersions make it possible to process QDs via solvent-based deposition techniques such as dropcasting, dipcoating, Langmuir-Blodgettry etc. These techniques have the advantage of being cheap, easy and fast compared to conventional vacuum- based techniques, which require complex and expensive equipment. However, production of high quality films via solvent-based techniques is an ongoing research and does not yet reach the standards of films produced by vacuum-based techniques.
Since QDs are light absorbing materials, they can be used as the active sensing material in photodetectors. More specifically, a photoconductor type of detector is more appropriate than a photodiode type of detector, as it makes better use of some of the advantages QDs offer. The operating principle of a photoconductor is schematically shown in Figure 1. A semiconducting sensing layer is placed between two biased electrodes. When a photon is
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absorbed, an electron is promoted to the conduction band, increasing the conductivity of the semiconducting layer. The rise in conductivity is detected as a photocurrent in the external circuit.
A first advantage of using QDs as sensing material is that they have a much larger surface area than bulk material. Since surface defects or chemical functionalization of the surface introduces trap states in the mid-gap region, electrons are trapped for a longer time and subsequently the holes can drift for a longer amount of time. In the case that a hole travels through the external circuit multiple times while the electron is trapped, a condition of photoconductive gain is created. This means that much larger photocurrents are generated per absorbed photon and thus the control over trap states provides a route to produce highly sensitive photodetectors.
A second advantage is that QDs exhibit multiple exciton generation (MEG) at lower photon energies than the bulk material (relative to the band gap energy Eg) [1]. MEG is a process in which a high energy photon is absorbed, at least twice the band gap energy. The excited electron can relax to the edge of the conduction band by releasing a photon with energy Eg. This photon is absorbed by a second electron, thereby creating a second exciton. Although this is an important advantage of QDs, it is less important for QD photodetectors in the infrared since it involves photon energies outside the region of interest.
Figure 1. Operating principle of a photoconductor. After absorption of a photon, an electron is excited to the conduction band (CB). The created hole in the valence band (VB) generates a photocurrent by circulating through the external circuit. Trapping of the electron in a trap state (TS) increases the lifetime of the exciton and thereby increases the photocurrent. After relaxation to the ground state, the photocurrent falls back to 0.
A functioning photoconductor however, requires charge transport between the electrodes. Since as-synthesized QDs have a relatively large insulating ligand shell, they do not allow charge carriers to be transported between the separate QDs. A procedure is thus required to remove the native ligand shell and induce an electronic coupling between QDs, thereby enabling a high mobility for photo-induced charge carriers. Several post-treatment strategies are found in literature, where interdot connections are formed after an initial layer formation. Such strategies include annealing [2, 3], ligand exchange [4-8] or ligand displacement [9, 10]. However, these strategies have the disadvantage of creating large cracks in the layer, which is detrimental for their conductivity. Pre-treatment includes ligand exchange prior to layer formation [11, 12]. This is a good strategy since it avoids crack formation, but a stable dispersion of inorganically capped PbSe QDs has not been reported yet. Other strategies influence the QD ligand shell at the moment the superlattice is formed. This is conveniently done by forming the superlattice on an immiscible, liquid subphase. On one hand this enables the addition of extra reagents and
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on the other hand it acts as a reservoir for the removed ligands. This method has successfully yielded connected QD superlattices by either adding formic acid to an acetonitrile subphase, or by forming the superlattice on ethylene glycol at elevated temperature [13, 14].
This paper intends to show the usability of connected QD superlattices as a photosensitive layer in a photoconductor type of device. To this end, superlattices were formed on a liquid subphase either by dropcasting or by using the Langmuir-Schaefer technique. Different morphologies were obtained by changing the composition and temperature of the subphase. Their photodetection capabilities were investigated via I-V measurements, and indicate that both the morphology and surface chemistry play an important role in the device performance.
Experimental PbSe quantum dot synthesis
PbSe semiconductor nanocrystals were synthesized by a method found in literature [14]. The crystals were grown at 180 °C for 60 s. After precipitation and purification of the obtained product, the QDs were dispersed in either toluene or hexane.
PbSe superlattice formation
Dropcasting on ethylene glycol. In this method, 10-50 µl of a PbSe QD solution with concentration varying from 0.29 µM to 2.9 µM, was cast onto 1 ml ethylene glycol, in a glass container with 10 mm diameter. Samples of the film were taken at the center of the vial after 1 h. After deposition, the substrate was immersed in deionized water to remove residual ethylene glycol and dried under a nitrogen flow. All the dropcasting experiments were carried out in a nitrogen purged glovebox.
Langmuir-Schaefer deposition. In this technique, 100 µl of a filtered 2.46 µM QD solution in hexane was added drop-wise to the Langmuir trough. After the solvent evaporated, the QDs were compressed to a desired pressure, by reducing the available surface area with 10 cm²/min.
Device fabrication
Photodetectors were prepared by patterning gold comb-like electrodes on a Si/SiO2 substrate via photolithography. The photosensitive layers were placed on top of the electrodes by direct stamping of the superlattices formed on the subphase. I-V measurements
I-V curves were obtained by sweeping a voltage from -5 V to +5 V across the electrodes and recording the current. The current was measured under different illumination levels, ranging from 0 mW to 800 mW, with a laser operating at a fixed wavelength of 1550 nm. From the resulting photocurrents, i.e. measured current minus dark current, responsivities (A/W, calculated as R=Iphoto/Pincident) and contrast ratios (Iphoto/Idark) were plotted.
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Results
PbSe QD synthesis The TEM image in Figure 2(a) shows that the synthesis yielded monodisperse, quasi-spherical PbSe nanocrystals. The size of the crystals was determined at 6.0 nm ± 4.25 % from the first exciton peak in the absorbance spectrum (Figure 2(b)).
Figure 2. (a) TEM image of the as-synthesized PbSe QDs. (b) Absorption spectrum corresponding to the QDs in (a). PbSe QD superlattices
Superlattices by dropcasting on ethylene glycol Addition of Na2S to the subphase. Figure 3(a-d) shows the microstructure of
monolayers formed by dropcasting 18 µl of a 0.29 µM solution at room temperature. From left to right, the amount of Na2S in the subphase increases from naught to a 100 fold excess, compared to the total amount of ligands in the QD solution.
The addition of Na2S to the subphase increased the amount of interparticle connections and eventually resulted in a dense, connected superlattice of QDs. The FTIR spectra in Figure 3(e) show a strong reduction of C-H stretch vibrations by increasing the amount of Na2S in the subphase. This indicates that Na2S successfully removed the native oleate ligands and that the QD surface becomes passivated with S2- ions.
The long range morphology of the obtained superlattices was studied with SEM (Figure 3(f-g)). The images show that the superlattices formed by the addition of a 10 fold (A) and a 50 fold (B) excess Na2S are very homogeneous with almost no cracks over an area of several tens of square micrometer.
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Figure 3. (a-d) TEM images of the superlattices formed by dropcasting on ethylene glycol. The amount of Na2S added to the subphase is no Na2S (a), a 10 fold excess (b), a 50 fold excess (c) and a 100 fold excess (d). Scale bars are 10 nm. (f-g) SEM images of superlattices formed with a 10 fold (f) and a 50 fold excess (g) Na2S. Scale bars are 1 µm. (e) FTIR spectra showing the decrease of C-H stretch vibrations by the addition of Na2S.
Temperature of the subphase. The effect of temperature on the formation of QD superlattices was studied by heating the subphase to 50 °C before adding the QD solution. Figure 4(a-b) shows the effect of adding 50 µl of a 1.4 µM QD solution at room temperature (a) and at 50 °C (b). At room temperature, the nanocrystals arranged in a hexagonal close packed structure and the distance between the crystals indicate that the ligand shell was not altered in a significant way. On the other hand, at elevated temperature, the individual nanocrystals merged together and formed a superlattice of connected QDs with a quasi-cubic symmetry. The FTIR spectra in Figure 4(d) confirm that the majority of oleate ligands are removed.
This procedure yielded a thicker, less homogeneous film. The surface profile in Figure 4(e) follows the line drawn in the AFM topography image in Figure 4(f), and shows that the superlattice mainly consists of a bilayered structure alternated with smaller monolayer regions. Figure 4(c) is an SEM image showing that the layer contains a number of small cracks with widths in the µm range.
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Figure 4. (a-b) TEM images of the superlattices formed by dropcasting a QD solution on ethylene glycol at room temperature (a) and at 50 °C (b). Scale bars are 50 nm. (c) SEM image showing crack formation at 50 °C. Scale bar is 1 µm. (d) FTIR spectra showing the decrease of C-H stretch vibrations in the superlattice shown in (b). (e) Surface profile following the line drawn in the topography image (f).
Rate of QD addition. The previously shown superlattices were obtained by adding the QD solution in one swift injection. A completely different morphology was obtained by adding the QDs in a drop-wise fashion to the subphase at 50 °C (Figure 5(a)). Instead of an assembly of individually connected QDs, the slow addition yields a structure where the QDs fuse together and form a molten-like network. Similar to the previous results, the native oleate ligands are successfully removed, as indicated by the removal of C-H stretch vibrations in the FTIR spectrum (Figure 5(b)). The AFM topography image shows that the layer has a thickness around 11 nm (Figure 5(c-d)).
Figure 5. (a) TEM image of the structure formed by drop-wise addition of the QDs at 50 °C. (b) FTIR spectra showing the decrease of C-H stretch vibrations in the superlattice shown in (a). (c) Surface profile following the line drawn in the topography image (d).
Superlattices by Langmuir-Schaefer deposition. In a first series of experiments with this technique, it was established that using water or ethylene glycol as subphase resulted in hexagonal close packed layers where the QDs retained their native ligand shell. On the other hand, by using diethylene glycol as subphase, the QDs instantaneously formed interparticle connections. Since the irreversible bonds reduced the mobility of the
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nanocrystals, it was impossible to compress the layer to yield a homogeneous, dense structure. To avoid this, two modifications were made to the subphase.
A first modification was using a subphase consisting of 30% diethylene glycol and 70% ethylene glycol. This drastically improved the quality of the layer, and homogeneous areas of several square centimeters were obtained. Figure 6(a) show that although the density is not maximal, the spread of the nanocrystals is relatively homogeneous. Figure 6(b) more clearly shows that connections are formed between QDs, although a significant amount is still separated by an oleate ligand shell.
The second modification was adding a 100 fold excess Na2S to ethylene glycol. A relatively dense layer was obtained where the nanocrystals have a rather random, non-symmetrical stacking (Figure 6(c)). A large fraction of the QDs show interparticle connections (Figure 6(d)), but also many nanocrystals are not connected, signaling an incomplete ligand removal. Although Na2S is a good reagent for oleate removal, the large volume and surface area of the subphase means that less S2- ions are available at the subphase-QD interface and thus less oleate ligands are exchanged.
The FTIR spectra in Figure 6(g) confirm that both modifications result in only a partial removal of the oleate ligands.
As for the long range morphology, SEM images indicate that reasonably homogeneous films were obtained (Figure 6(e-f)), considering the difficulty to compress layers where QDs form interparticle connections.
Figure 6. (a-b) TEM images of the structure formed by Langmuir-Schaefer deposition on an EG subphase containing 30% DEG. Scale bars are 100 nm (a) and 10 nm (b). (c-d) TEM images of the structure formed by Langmuir-Schaefer deposition on an EG subphase containing a 100 fold excess Na2S. Scale bars are 10 nm (c) and 5 nm (d). (e) SEM image corresponding to the layer shown in (a) and (b). (f) SEM image corresponding to the layer shown in (c) and (d). (g) FTIR spectra of the Langmuir-Schaefer films showing a partial reduction of the C-H stretch vibrations.
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PbSe QD photodetectors
Addition of Na2S to the subphase. The increase of interparticle connections directly enhanced the conducting behavior of the superlattice. This is illustrated by an increase of the dark current (at 5 V) from 0 µA to 10.5 µA going from no Na2S to a 50 fold excess, and an increase from 10.5 µA to 29.0 µA going from a 50 fold to a 100 fold excess. The increased conductivity allows the superlattice to produce more photocurrent per incident photon power. This resulted in a significant increase of both photocurrent and responsivity by the addition of more Na2S (Figure 7(a-b)). The comparable contrast ratio (Figure 7(c)) suggests that the observed increase in photocurrent and responsivity is mainly due to the increased conductivity and not due to an additional increase of the carrier lifetime.
Figure 7. Photocurrent (a), responsivity (b) and contrast ratio (c) at 5 V in function of illumination power, as obtained from the measured I-V curves. The curves correspond to the structures shown in Figure 3(c) (red curve) and Figure 3(d) (blue curve).
Temperature of the subphase. Fast addition of QDs. This structure (see Figure 4(b)) shows a remarkable increase of
conductivity, with a dark current (at 5 V) of 209.8 µA. On one hand, this is due to a structure with a high density of interconnected nanocrystals. On the other hand, the bilayered structure possibly serves as a bridge between cracks or isolated parts, thereby reducing the percolation path and activating 'dead' areas, which are otherwise effectively lost in a monolayer. The increased conductivity led to a significant increase in photocurrent and correspondingly, relatively high responsivities were obtained (Figure 8(a-b)). The contrast ratio is shown in Figure 8(c).
Figure 8. Photocurrent (a), responsivity (b) and contrast ratio (c) at 5 V in function of illumination power, as obtained from the measured I-V curves. The curves correspond to the structure shown in Figure 4(b).
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Drop-wise addition of QDs. The molten-like structure obtained by drop-wise addition of the QDs shows a dark current (at 5V) of 35.9 µA. Although the electrical connections in this structure are good, both the lower density and the more hindered percolation path contribute to a lower dark current. The photocurrents, responsivities and contrast ratios are shown in Figure 9(a-c).
Figure 9. Photocurrent (a), responsivity (b) and contrast ratio (c) at 5 V in function of illumination power, as obtained from the measured I-V curves. The curves correspond to the structure shown in Figure 5(a).
Langmuir-Schaefer films. The photodetectors produced by 4 Langmuir-Schaefer depositions have a very low dark conductivity, with dark currents (at 5 V) of 0.69 µA on the EG/DEG subphase and 0.79 µA on the Na2S containing subphase. Upon illumination, photocurrents of a few microamperes are generated, resulting in relatively low responsivities (Figure 10(a-b)). However, due to the low dark current, high contrast ratios were obtained in these structures (Figure 10(c)).
Figure 10. Photocurrent (a), responsivity (b) and contrast ratio (c) at 5 V in function of illumination power, as obtained from the measured I-V curves. The curves correspond to the structures shown in Figure 6(a) (red curve) and Figure 6(c) (blue curve).
Discussion The role of S2-
By taking into account the density and thickness, a responsivity per QD of 6 10-11 A/W is obtained for the S2- passivated structure (Figure 3(d)) while the non-S2- passivated structure (Figure 4(b)) yields 13 10-11 A/W per QD. It is seen that this quantity is about twice as high. At first sight this can be explained by the better conductivity of the latter, but since the dark current is about 7 times higher, another factor plays in favor of the first. If the same level of doping and quantum efficiency is assumed in the QDs, the responsivity scales with τlifetime/τtransit, meaning that the lifetime of the carrier is about 3.5
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times higher in the S2- passivated structure. This indicates that the S2-ions possibly introduce effective electron traps in the band structure. The role of surface area and morphology
Since the structures in Figure 4(b) and 5(a) are obtained by a similar procedure, it is assumed that they have a similar surface chemistry and that trap states are mainly introduced by surface defects. In this perspective, the higher contrast ratio in the molten structure can be rationalized by both a larger surface area, with possibly more surface defects, and a more complete removal of the organic ligands (see FTIR spectra in Figure 4(d) and 5(b)). Additionally, the complex morphology of the molten-like structure can possibly further increase the carrier lifetime by a principle of harder-to-find recombination centers. The role of surface passivation
Both Langmuir-Schaefer films show a very low dark current. However, upon illumination, photocurrents with relatively high contrast ratios are generated, meaning that a conductive path is available for the charge carriers. This indicates that few intrinsic mobile charge carriers are present in the structure. Since both Langmuir-Schaefer films still contain a large amount of native ligands, it is well possible that the oleate provides a better passivation and that in the other methods the many created surface states lead to an effective doping of the QDs, thereby increasing the dark current. This suggests that a superlattice with a well passivated surface, followed by controlled introduction of non-doping trap states could further increase the device performance.
Conclusion
In conclusion, this paper showed that several strategies successfully removed the native oleate ligand shell. This enabled the fabrication of nanometer sized, interconnected QD superlattices over a large area. The reported procedures include the addition of Na2S or diethylene glycol to the subphase, and increasing the temperature of the subphase.
The obtained structures furthermore showed promising features towards their implementation as sensitive layers in photoconductors. The results point to several important factors influencing the device performance, such as surface passivation, the nature of the trap states, carrier mobility, and morphology.
Acknowledgements
The research was supported by the Department of Inorganic and Physical Chemistry
and the INTEC Department of Ghent University. The authors would like to express their gratitude to Katrien Haustraete for taking TEM images and Stijn Flamee for taking SEM images.
References
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2. Wills, A.W., et al., Thermally Degradable Ligands for Nanocrystals. ACS Nano, 2010. 4(8): p. 4523-4530.
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Contents
Preface v
Dutch Summary vii
Article ix
1 Introduction 1
1.1 Colloidal Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 Quantum confinement . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Light absorption by quantum dots . . . . . . . . . . . . . . . . . . . 4
1.1.3 Colloidal synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Superlattices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 Thermodynamic considerations . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Superlattices in practice . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Application: photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.1 Photoconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3.2 Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4 Photoconductors based on quantum dot superlattices . . . . . . . . . . . . 14
2 Quantum Dot Synthesis 15
2.1 Synthesis of PbSe QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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Contents xxii
3 Quantum Dot Superlattices 20
3.1 Superlattices by dropcasting on ethylene glycol . . . . . . . . . . . . . . . 20
3.1.1 Addition of Na2S to the subphase . . . . . . . . . . . . . . . . . . . 21
3.1.2 Temperature of the subphase . . . . . . . . . . . . . . . . . . . . . 25
3.1.3 Rate of QD addition . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 Superlattices by Langmuir-Schaefer deposition . . . . . . . . . . . . . . . . 34
3.3 Light absorption by superlattices . . . . . . . . . . . . . . . . . . . . . . . 40
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.4.1 Oleate and the subphase . . . . . . . . . . . . . . . . . . . . . . . . 42
3.4.2 Oriented attachment . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.4.3 Langmuir films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.4.4 Absorption enhancement . . . . . . . . . . . . . . . . . . . . . . . . 46
3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4 Quantum Dot Photodetectors 49
4.1 Addition of Na2S to the subphase . . . . . . . . . . . . . . . . . . . . . . . 50
4.2 Temperature of the subphase . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3 Superlattices by Langmuir-Schaefer deposition . . . . . . . . . . . . . . . . 55
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4.1 Responsivity per QD . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4.2 Fast versus slow addition . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4.3 Langmuir-Schaefer films . . . . . . . . . . . . . . . . . . . . . . . . 58
4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
A QD Langmuir-Blodgett/Schaefer films 60
Bibliography 62
Chapter 1
Introduction
The research towards implementation of nanotechnology into integrated circuitry is a very
interesting and promising topic. The emerging field of photonics for example, aims at using
the peculiar interactions of nanocrystals with light to build devices which process photon
fluxes, much like today’s electronics process electrical signals. An important advantage is
that signals travel at the speed of light, making the devices possibly much faster than their
electronic counterpart. The combination with nanotechnology, resulting in faster, smaller
and greener devices, seems like a good fit for future technology. Crystals with sizes in the
nanometer range are easily prepared and scientists can use them as building blocks in a
multitude of devices. However, the fabrication of accurate, reliable and reproducible devices
in this direction demands a lot of research, not at least since matter behaves differently at
the nanoscale and formerly unknown effects can come into play.
This introduction first discusses nanocrystals of semiconducting material, i.e. quantum
dots (QDs), and how they behave differently from their bulk form, both electronically and
optically. In a second part, the organization of QDs in a structured manner, a so called
superlattice, is discussed. A thermodynamic point of view is given, and a review of the
state-of-the-art in superlattice formation is provided. The third part discusses photode-
tectors in general. Different types and their working principles are briefly summarized.
Finally, these three pillars are used to discuss the subject of this thesis; the fabrication of
1
Chapter 1. Introduction 2
a photodetector based on a QD superlattice.
1.1 Colloidal Quantum Dots
1.1.1 Quantum confinement
The electronic structure of a semiconducting material typically consists of two energy
bands: a filled or quasi-filled valence band, and an empty or quasi-empty conduction band.
Between these bands is a zone without energy levels, with an energy difference called the
band gap energy Eg. An exciton, i.e. a bound electron-hole pair, is created if an energy
equal to or larger than Eg is absorbed by an electron. This is the most applied property
of semiconductors.
The problem of an electron traveling a crystal can be studied by quantum mechanics as a
free particle in a box. In a first approximation, the boundaries of the box at x = 0 and
x = a can be seen as imposing an infinite potential barrier. However, for mathematical
convenience, the ensuing condition that the wavefunction should vanish at these boundaries
is often replaced by a periodic boundary condition. Here, it is required that the values
at x = 0 and x = a are identical. In 3 dimensions, the eigenfunctions and eigenenergies
obtained by solving the Schrodinger equation are represented by:
ψ(r) =
√8
a3/2sin(
nxπ
ax)sin(
nyπ
ay)sin(
nzπ
az) (1.1)
E =~2π2
2ma2(n2
x + n2y + n2
z) (1.2)
Important in this result is that the eigenenergies are proportional to 1/a2, meaning that
the eigenenergy (i.e., the kinetic energy) of an electron strongly increases with decreasing
size of the crystal. This increase can be seen as an extra energy cost in order to confine
an electron to a smaller space. In the end, the quantum confinement effect translates as
the presence of discrete energy levels near the edges of the valence and conduction band,
with the exact position of the energy levels, and thus the value of the band gap, being
determined by the size of the crystal.
Chapter 1. Introduction 3
In this context, a quantum dot is defined as a solid material where this confinement takes
place in three dimensions, and as a consequence, a QD behaves as something between an
atomic species and a bulk material. Their intermediate electronic structure is illustrated
in Figure 1.1.
One way to make the confinement effect observable in a semiconducting material is by
reducing the crystallite size to below the exciton Bohr radius. This is the most probable
distance between the hole and electron in an exciton and is given by:
aeh = εm
µeh
ab (1.3)
with ab the Bohr radius, m the mass, µeh the reduced mass of the exciton and ε the dielectric
constant. Depending on the material, the exciton Bohr radius in semiconducting materials
can be relatively large (up to 100 nm) due to a small reduced exciton mass or a large
dielectric constant. With the correct synthetic bottom-up approach, these crystallite sizes
can be easily obtained, and this has the advantage that quantum confinement is accessible
at room temperature.
Since the energy of an exciton is determined by the band gap energy, which in turn is
determined by the size of the crystal, quantum confinement can be exploited to tune the
absorption and emission spectra near the band gap to a desired value.
Figure 1.1: A schematic representation showing the electronic structure of a bulk semiconduc-
tor (left), a quantum dot (middle), and a single molecule (right). The electronic
structure of a QD is in between that of a bulk material and a single molecule.
Chapter 1. Introduction 4
1.1.2 Light absorption by quantum dots
When a photon flux is incident on a thin, homogeneous film of bulk semiconducting mate-
rial, the light beam is attenuated by absorption of photons. Equation 1.4 gives the intensity
of the beam after it has traveled a distance x through the film. In this equation, κ repre-
sents the extinction coefficient and λ the wavelength of the incident light. The attenuation
follows an exponential decay, and the characteristic decay length in a log10 scale is defined
as the absorption coefficient α of the bulk material (Equation 1.5).
I(x) = I0e− 4πκ
λx (1.4)
α =ln10A
L=
4πκ
λ(1.5)
For a colloidal solution of QDs, the intrinsic absorption coefficient of the QDs µi is related
to α by Equation 1.6 [15], where µi is obtained by dividing the absorption coefficient of the
composite (µ) with the volume fraction (f ) of QDs. On one hand, the intrinsic absorption
of QDs is enhanced by a factor n/ns, where n and ns are respectively the refractive indices
of the bulk material and the solvent. On the other hand, the intrinsic absorption is reduced
by the square of the local field factor fLF (0<fLF<1). This local field factor arises from a
partial screening of the electrical field of the incident photons, and can be rationalized as
follows. Since their diameter is much smaller than the wavelength of light, QDs subjected
to a photon flux can be described as small spheres in a homogeneous electrical field (E0).
This induces an opposing internal electrical field (Eint) in the QD, the size of which depends
on the depolarization factor of the material. This internal field effectively reduces the local
electrical field (Eloc) sensed by the QD, resulting in a smaller absorption coefficient. This
is schematically shown in Figure 1.2.
µi =µ
f=
n
ns
|fLF |2α (1.6)
Chapter 1. Introduction 5
Figure 1.2: A schematic representation showing that the electrical field sensed by the QD (Eloc)
is smaller than the incident field (E0) by induction of an internal opposing electrical
field in the QD (Eint).
1.1.3 Colloidal synthesis
Colloidal QDs can be synthesized at lab scale through a bottom-up approach. A well-
known method is the hot injection synthesis, where an appropriate precursor is heated to
its decomposition temperature, at which moment a solution containing the counter ion is
rapidly injected. The aim is to create very quickly a highly supersaturated mixture that
causes a sudden burst of nucleation. The nuclei start to grow and both nucleation and
growth relax the supersaturated situation. In a first instance, growth occurs via diffusion
of the ions in solution. However, as the concentration of ions decreases, growth by Ostwald
ripening becomes more important. This is a process in which large crystals grow larger at
the expense of ions and small crystals, which dissolve and redeposit onto larger crystals.The
timing of the growth process thus determines the average size and size distribution of the
crystals.
Since this reaction effectively leads to the formation of solid material, the formed crystals
have a tendency to precipitate. In order to prevent this, a surfactant with a favorable
solvent interaction is added to the reaction medium. This can be for example an amine
or carboxylic acid with a relatively long carbon chain. They are usually introduced as the
counter ion in the metal precursor solution. After quenching of the reaction and purification
of the reaction mixture, a colloidal solution of QDs, consisting of an inorganic core with
Chapter 1. Introduction 6
an organic capping shell is obtained.
1.2 Superlattices
1.2.1 Thermodynamic considerations
From a thermodynamic point of view, the formation of a superlattice can take place at con-
stant temperature and pressure when the total Gibbs free energy of the system decreases.
The driving force can be energetic, entropic or a combination of both, depending on the
type of interactions that play a role. The change of Gibbs free energy can be written in
terms of enthalpy and entropy as
∆Gsys(t) = ∆Hsys(t)− T∆Ssys(t) (1.7)
where t denotes the evolution of the superlattice formation. The Gibbs free energy can be
minimized by minimizing the enthalpy and/or by maximizing the entropy. Possible interac-
tions contributing to the enthalpy can be charge-charge, charge-dipole, dipole-dipole, and
Van der Waals interactions. The entropy is related to the number of possible microstates
within an ensemble of particles. Different systems can be distinguished, depending on the
thermodynamic driving forces that occur in the superlattice formation.
In a first system, the formation of a superlattice can be seen as a crystallization of non-
interacting hard spheres. This means that the contribution of the enthalpy is zero, and
the crystallization is entropy driven. At first sight, this seems counter intuitive, since crys-
tallization from solution introduces order to the system. However, it has been found that
in concentrated solutions (volume fractions above 49%), the free volume per nanocrystal
in an ordered lattice is larger than the free volume per nanocrystal in the disordered liq-
uid, and this compensates the loss in conformational freedom [8,9]. For a stable colloidal
dispersion, the hard sphere model can be applied for two reasons: the Van de Waals interac-
tions between the nanocrystals are effectively screened by the solvent, and the short-range
attraction between the inorganic cores is too weak to lead to aggregation (due to steric
hindrance of the ligands).
Chapter 1. Introduction 7
For systems where the enthalpy contributes to the free energy, different scenarios can take
place. For example, larger and anisotropic nanocrystals have stronger, directional Van der
Waals interactions and in binary, spherical nanocrystals with an inhomogeneous distri-
bution of elements, dipole moments can arise. When these forces extend over the ligand
molecules, the free energy during crystallization can be lowered by arranging certain facets
along the direction of these forces. This leads to a preferential orientation of nanocrystals
in the superlattice. These forces are electrostatic, so the nature of the solvent has a big in-
fluence, since a high dielectric constant more effectively screens Coulomb and dipole-dipole
interactions [30,29].
The formation of a superlattice can also be purely enthalpy driven. In this case chemical
bonds are formed between the nanocrystal building blocks and when the atomic bonding
happens via specific crystal facets, a process called oriented attachment takes place. The
big difference with self-assembled layers is the strength of the interactions: irreversible,
chemical bonds versus entropic and Van der Waals interactions. An important requirement
for oriented attachment is that the facets involved in attachment are available for bonding
and are not covered with bulky ligands, as is usually the case after synthesis.
A more practical way to look at superlattice formation is by considering the condition that
the chemical potentials (µ) of the different phases should be equal at equilibrium. Since the
chemical potential of nanocrystals in a stable dispersion is lower than that of nanocrystals
in a superlattice, a phase transition can be induced by increasing the chemical potential
of the colloidal dispersion. Equation 1.8 expresses the chemical potential in terms of the
standard chemical potential and concentration, and provides two possible ways to increase
µ. The first is to increase the concentration, for example by evaporation of the solvent.
The second is to increase the standard chemical potential. This can be done by addition of
a miscible non-solvent, which effectively increases the potential energy of the nanocrystals
in solution.
µ = µo + kBT lnc
c0(1.8)
Chapter 1. Introduction 8
1.2.2 Superlattices in practice
When using as-synthesized colloidal nanocrystals, i.e. nanocrystals with an inorganic core
and an organic shell, the easiest way to form a superlattice is by letting the crystals
self-assemble during evaporation of the solvent. The quasi-spherical nanocrystals arrange
themselves in a hexagonal close packed structure, which has the highest packing density.
Evaporation directly on a substrate has been extensively used to form self-assembled su-
perlattices consisting of metallic or semiconducting nanocrystals [22,3]. It is also possible to
form binary superlattices by introducing different sizes of nanocrystals, or by combining
metallic with semiconducting nanocrystals [26,19,28]. Two examples of self-assembled super-
lattices are shown in Figure 1.3. The figure illustrates that binary superlattices can form
many more conformations than the typical hexagonal packing, resembling more atomic
and ionic lattices.
Figure 1.3: (A) A self-assembled superlattice of 5 nm Au nanocrystals showing a hexagonal
close packed structure. (B) A binary superlattice with a AB13 unit cell, where A
are 11 nm γ − Fe2O3 nanocrystals and B 6 nm PbSe nanocrystals.
Another method to form monolayers, i.e. 2D superlattices, is by evaporating a dilute
solution on an immiscible liquid substrate and gently pushing the layer together. This is
known as the Langmuir technique and can yield very homogeneous monolayers over a large
area [18,20,32,1] (see Appendix A).
Although a wide variety of superlattices can be made with these techniques, a major draw-
back is that the layers are electrically insulating due to the organic capping molecules.
Since conducting layers are an important requirement for applications, a lot of research
Chapter 1. Introduction 9
focuses on ways to improve the conductivity. One solution is to heat the superlattice after
it has formed. At a high enough temperature, the ligands evaporate and the distance
between nanocrystals decreases. Unfortunately, the temperatures required to remove com-
monly used ligands also lead to sintering of the crystals and thereby losing the confinement
properties. This has been solved by exchanging oleate ligands with a ligand that degrades
at a temperature where sintering does not yet occur [33].
An alternative route, without heating, is exchanging the ligands with much smaller ones by
treating the superlattices with an appropriate reagent. These ligands can be, for example,
carboxylic acids with shorter carbon chains, or inorganic anions like S2-, OH-, NH2-, and
BF4- [31,16,17,24,27].
Instead of carrying out a ligand exchange reaction at the QD surface, it is also possible
to exploit the free energy difference between different crystal facets. Choi et al. used
DFT methods to calculate binding energies of the acetate molecule on different PbS QD
facets and found that it binds more strongly on the (111) facets than on the (100) facets,
with a difference of 0.346 ± 0.029 eV [5]. Baumgardner et al. used this to investigate
facet specific adsorption/desorption equilibria of oleate molecules on PbSe nanocrystals.
By treating self-assembled hexagonal superlattices with solvents that dissolve oleic acid,
preferential desorption at the (100) facets was observed, resulting in oriented attachment
along this direction. The conductivity of these layers increased by more than 3 orders of
magnitude [2,13].
All these post-treatments, however, lead to an inherent volume decrease in the superlattice,
and as a consequence unwanted crack formation occurs. Recently, Simon et al. showed that
low temperature annealing over a period of 24 h leads to an increased formation of small,
crystalline linkages between the nanocrystals [29]. This could prove an excellent method to
increase conductivity of the superlattices, while still preserving confinement. At the same
time, the volume decrease is minimal because the ligand shell remains mostly intact, and
this could strongly decrease crack formation.
Another way to prevent cracks is by exchanging the ligands before forming a superlattice.
Instead of treating the superlattice with a reagent, the nanocrystals are treated in solution.
Chapter 1. Introduction 10
Exchange with small inorganic ligands like S2- or OH- is typically carried out in a two-phase
system, where the apolar phase contains the nanocrystals capped with organic ligands and
the polar phase contains a salt of the exchanging ion. A successful exchange is signaled
by a phase transition of the crystals to the polar phase, usually formamide or dimethyl
sulfoxide (DMSO). This method has been successfully applied to a number of different
nanocrystals (e.g. CdSe, ZnS, InP, F2O3, Au) with a number of different inorganic ligands
(e.g. S2-, OH-, NH2-, BF4-) [23,6]. However, due to problematic charge interactions between
the crystals, a stable dispersion of inorganically capped lead chalcogenide nanocrystals has
not been reported yet.
New, promising methods to obtain conducting superlattices are a combination of pre- and
post-treatment, and influence the core-shell chemistry at the moment of superlattice for-
mation. For these methods to work, a solution of nanocrystals is cast upon a non-solvent,
where the crystals are allowed to form a superlattice. In this way, the nanocrystals still
preserve enough mobility to allow some movement. The relative higher mobility, compared
to nanocrystals on a solid substrate, enables the superlattice to contract uniformly with-
out forming large cracks. The choice of the liquid subphase and/or additives provides the
chemist with some useful parameters. Dong et al. assembled FePt, Au, and PbS superlat-
tices on an acetonitrile subphase, and carried out an in-situ ligand exchange by injecting
an appropriate reagent into the subphase after initial superlattice formation. The macro-
scopic contraction of the floating nanocrystals avoided crack formation and preserved the
nanocrystal ordering, significantly improving electrical transport in the superlattices [7].
Evers et al. reported PbSe superlattices formed by oriented attachment along specific
crystal facets, depending on the concentration of the nanocrystal solution. Ethylene gly-
col was used as subphase, and they showed that by keeping it at elevated temperatures,
oleate ligands absorb in the subphase, making the crystal facets available for interparticle
bonding [10].
Chapter 1. Introduction 11
1.3 Application: photodetectors
Photodetection is a broad field of research and it is almost impossible to underestimate
its technological importance. The electromagnetic spectrum, shown in Figure 1.4, encom-
passes a wide range of energy and each region is a source for a multitude of applications.
Photodetectors are typically used in the region from far infrared up to gamma rays and
applications can be grouped in two main types; communication and remote sensing. For
communication purposes, the radiation is used as a carrier for an encoded signal, while
for remote sensing applications, the radiation itself is the signal, and contains information
about an object or a substance. To give a faint idea about the range of applications, pho-
todetectors are used in motion detectors, all types of cameras, from normal video camera
to night vision, pollution detection in individual cells up to space-based environmental
monitoring, fiber-optic communication systems, detectors used in telescopes etc. The type
of application determines the specific technical requirements, such as the spectral range in
which the detector is sensitive, the speed at which successive signals can be distinguished,
the maximum optical power it can handle, the working temperature, size, robustness, and
last but not least, the production cost.
Figure 1.4: A presentation showing the different regions of the electromagnetic spectrum with
their corresponding wavelength and frequency.
Basically, a photodetector converts incident photons to electrical signals. An incident
photon can be absorbed by an electron, and depending on the energy of the photon, the
Chapter 1. Introduction 12
electron is either completely freed from its atomic or molecular environment, i.e. exter-
nal photoelectric effect, or gets excited to a higher atomic or molecular energy level, i.e.
internal photoelectric effect. External photoelectrons are typically observed in the X-ray
part of the spectrum, and are not considered here. Formation of internal photoelectrons
on the other hand, requires much lower energies and can be observed in the UV, visible,
and infrared part of the spectrum.
Photodetectors based on the internal photoelectric effect can be divided in two main classes:
photoconductors and photodiodes.
1.3.1 Photoconductors
A photoconductor typically consists of a semiconducting material between two biased metal
contacts. When an incident photon with sufficient energy is absorbed, an electron is excited
to the conduction band. The created electron-hole pair exists for a certain time until the
electron relaxes back to its ground state in the valence band. As long as the electron
remains in an excited state, a charge carrier circulates through the external electrical
circuit, creating a photocurrent in the semiconducting material. When the charge carrier
circulates the circuit multiple times while the exciton exists, a condition of photoconductive
gain is created. Trap states, which are intermediate energy levels originating from surface
defects or molecules bound to the QD surface, effectively increase the amount of time an
electron remains in an excited state, and thus the time a charge carrier exists. This leads
to an expression for the photoconductive gain G as
G =τlifetime
τtransit(1.9)
where τlifetime is the lifetime of the exciton and τtransit the time it takes for a free charge
carrier to pass from one metal contact to the other. A gain greater than 1 is thus obtained
when the exciton lifetime exceeds the time it takes for the charge carrier to travel between
electrodes. The mechanism of photoconductivity is schematically illustrated in Figure 1.5.
The internal signal amplification provided by photoconductive gain enables high sensitivi-
ties, since one created charge carrier creates a photocurrent that is many times higher. At
Chapter 1. Introduction 13
the same time, construction of electronic circuits is simplified because external amplifica-
tion is not needed if the gain is sufficiently high. However, the long exciton lifetime (∼ µs)
makes the detector respond relatively slowly to a time-changing photon flux, which limits
the range of frequencies at which these highly sensitive photoconductors can be used. This
trade-off between photoconductive gain and response time has an impact on the type of ap-
plications it can be used for. A photoconductor with high gain would, for example, not be
suited for a communication type of application, since these usually employ high frequency
signals in the GHz range. However, it is appropriate to use high gain photoconductors for
gas sensing, in order to detect e.g. toxic compounds at very low concentrations.
Figure 1.5: Schematic representation of the mechanism of a photoconductor. After absorption
of a photon, an electron is excited to the conduction band (CB). The created hole in
the valence band (VB) generates a photocurrent by circulating through the external
circuit. Trapping of the electron in a trap state (TS) increases the lifetime of the
exciton and thereby increases the photocurrent. After relaxation to the ground
state, the photocurrent falls back to 0.
1.3.2 Photodiodes
Photodiodes are very commonly used in today’s technology. They make use of the internal
electrical field of a pn-junction to propel photogenerated holes and electrons in opposite
directions. The separated charges are detected by a voltage difference across the junction.
Photodiodes typically have a very fast response time, making them suitable for high fre-
Chapter 1. Introduction 14
quency applications. On the downside, photodiodes can’t produce gain. This type will not
be discussed further, as it reaches outside the scope of this thesis.
1.4 Photoconductors based on quantum dot superlat-
tices
The subject of this thesis is to investigate the use of QDs as photosensitive material in
a photoconductor type of detector. Colloidal QDs have the advantage of being dispersed
in solution, thus enabling solution-based deposition techniques such as dropcasting, dip-
coating, Langmuir-Blodgett etc. Compared to conventional vacuum deposition techniques
such as chemical vapor deposition (CVD), sputtering, or molecular beam epitaxy (MBE),
solution-based techniques are low cost, easy to perform and fast. An extra benefit is that
the size of the QD allows spectral tuning of the band gap via the quantum confinement
effect. The downside of colloidal QDs is that their intrinsic absorption coefficient is lower
than the bulk material due to the local field factor. This means a less efficient photon-to-
current conversion.
The QD superlattice has to meet three important requirements in order to be useful for
photodetection. The first one is that the superlattice is highly conductive, since it allows
large photocurrents to travel through the layer. This is envisioned by a superlattice con-
sisting of electronically coupled QDs, providing high charge carrier mobilities. A second
requirement is that the superlattice has a low intrinsic carrier concentration. This reduces
the dark current and results in a higher contrast with the photocurrent, thereby enabling
higher sensitivities. The third requirement is that the superlattice is homogeneous and
crack-free over a large area, so that reliable and reproducible results can be obtained. An
possible extra benefit of a large area connected superlattice is that it increases the local
field factor and thus the intrinsic absorption coefficient of the material, since this structure
is somewhere between individual QDs and bulk material.
Chapter 2
Quantum Dot Synthesis
A first step towards the fabrication of photodetectors is the bottom-up synthesis of colloidal
QDs. This chapter first describes the synthesis of highly monodisperse PbSe nanocrystals
and their subsequent characterization by XRD, TEM, and NIR absorbance spectroscopy.
As an aid for the next chapter, this chapter also shows how high resolution TEM images
are used to obtain information on the crystallographic orientation of the QDs.
2.1 Synthesis of PbSe QDs
PbSe semiconductor nanocrystals were synthesized by the hot injection method. Two pre-
cursors were prepared separately before the synthesis was carried out. The first precursor,
lead oleate, was prepared by mixing 3.74 g PbO with 15.87 mL of oleic acid at 150 ◦C
for one hour. To the still hot, clear solution, 67.96 mL of diphenyl ether was added. The
Selenium precursor, TOPSe, was prepared by dissolving 3.52 g Se in 46.59 mL tri-octyl
phosphine (TOP) and 0.41 mL diphenyl phosphine at 150 ◦C. From the lead precursor,
20.5 mL was taken and heated to 180 ◦C. At this point, 15 mL of TOPSe was rapidly
injected. Successful nucleation was indicated by a blackening of the solution. The crystals
were further grown at 150 ◦C for 60 s, after which the reaction was quenched with 15 mL
butanol. The resulting nanocrystals were precipitated with 10 mL acetonitrile, a miscible
15
Chapter 2. Quantum Dot Synthesis 16
non-solvent, and centrifuged for 2 minutes at 10000 RPM. The liquid phase was decanted
and the quantum dots were again dispersed in 6 mL toluene. After a second wash with ace-
tonitrile and toluene the purified quantum dots were stored in a nitrogen purged glovebox.
All procedures were carried out under inert atmosphere.
2.2 Results and discussion
The TEM image in Figure 2.1A shows that the hot injection synthesis yielded monodis-
perse, quasi-spherical PbSe nanocrystals. The absorbance spectrum of the colloidal so-
lution is shown in Figure 2.1B. Three exciton peaks are seen at 1716 nm, 1350 nm, and
1034 nm, which are respectively transitions between the S-S, S-P, and P-P electronic states
of the PbSe QDs. The first exciton peak at 1716 nm was used to determine the average
QD diameter and size distribution via a commonly used method [21]. This resulted in an
average nanocrystal diameter of 6.0 nm ± 4.25 %. The method was also used to determine
the concentration of the stock solution (47.7 µM).
Figure 2.1: A: A TEM image of the as-synthesized PbSe QDs showing a quasi-spherical shape
and monodisperse size distribution. B: The absorption spectrum of a 200 times
diluted solution corresponding to the QDs in Figure A.
The XRD pattern in figure 2.2 confirms that the PbSe nanocrystals have a cubic crystal
structure, as is the case with bulk PbSe. The positions of the peaks were used to calculate
Chapter 2. Quantum Dot Synthesis 17
the interplanar distances by application of Bragg’s law (Equation 2.1), where n is the
diffraction order, λ the wavelength of the X-ray, dhkl the interplanar distance and θ the
angle between the incident ray and the scattering plane. The results are shown in Table
2.1. It should be noted that the sharp peaks in the XRD pattern originate from diffraction
of the Si substrate.
Figure 2.2: XRD pattern of the as-synthesized PbSe QDs showing diffraction peaks at the same
positions as cubic bulk PbSe.
nλ = 2dhklsinθ (2.1)
2θ (◦) dhkl (A) hkl
25.15 3.54 111
29.12 3.06 200
41.65 2.17 220
Table 2.1: Interplanar distances and indices of the planes, as determined from the peaks in the
XRD pattern.
Chapter 2. Quantum Dot Synthesis 18
High resolution images were used to measure the angles between crystal facets and the
distance between atomic planes. Table 2.2 summarizes the measured angles and distances
of some typical orientations of nanocrystals, shown in Figure 2.3. The angles are averaged
over the 8 angles around the nanocrystal, the distance is an average of 100 measure points.
The measured values, combined with the calculated interplanar distances from the XRD
pattern, allow a straightforward identification of the observed planes. Figure 2.3 uses a
crystal model consisting of 6 (100) planes and 12 (110) planes to visualize the orientation of
the nanocrystal on the TEM grid (A). When individual atoms are seen in a cubic structure,
as in Figure 2.3B, with an interatomic distance close to 3.06 A and angles between the
facets around 135◦, the particle is looked upon from the (100) direction and this plane is
parallel to the substrate. In the case that only atomic rows are seen, as in Figure 2.3C and
D, the particles are rotated around a specific axis, which is determined from the distance
between the atomic rows. A distance close to 3.06 A means that the particle is rotated
around a (100) axis and when a distance close to 2.17 A is found, the particle is rotated
around a (110) axis, as illustrated by the crystal models.
Chapter 2. Quantum Dot Synthesis 19
Figure 2.3: A: A PbSe unit cell and a crystal model of a 6 nm QD consisting of 6 (100) planes
and 12 (110) planes. B-D: TEM images and corresponding crystal models indicating
the lattice planes in several orientations of QDs. The particles are either not rotated
(B), rotated along a (100) axis (C), or rotated along a (110) axis (D).
Particle Distance atomic rows (A) Facet angles (◦)
Figure 2.3B 3.02 ± 0.05 135 ± 1.6
Figure 2.3C 3.05 ± 0.02 134 ± 3.6
Figure 2.3D 2.16 ± 0.10 134 ± 1.5
Table 2.2: The interatomic distances and angles between facets as measured from the corre-
sponding TEM images in Figure 2.3.
Chapter 3
Quantum Dot Superlattices
3.1 Superlattices by dropcasting on ethylene glycol
A first straightforward method to produce QD superlattices is by simply dropcasting a
QD solution. The use of a liquid subphase, onto which the QD solution is cast, not
only allows an easy transfer of the formed superlattice to other substrates, it also provides
opportunities to influence and control the formation of the superlattice. This section shows
how the composition of the subphase, the temperature of the subphase, and the rate at
which the QDs are added influences the microscopic morphology of the superlattice.
To form the superlattices shown in this section, 10-50 µL of a PbSe QD solution with
concentration varying from 0.29 µM to 2.9 µM, was cast onto 1 mL ethylene glycol
(HO− C2H4 −OH), in a glass container with 10 mm diameter. Upon addition of the
solution, the QDs spread on the liquid surface as the solvent evaporated. This left a thin
film on the subphase, and samples of the superlattice were taken at the center of the vial
after 1 h, either by fishing on a TEM grid or stamping on a Si or glass substrate. After de-
position, the substrate was immersed in deionized water to remove residual ethylene glycol.
The sample was then dried under a gentle nitrogen flow. The glass vial and substrates were
cleaned beforehand by rinsing with aceton, iso-propanol, and water. To prevent oxidation
of the QDs, all dropcasting experiments were carried out in a nitrogen purged glovebox.
20
Chapter 3. Quantum Dot Superlattices 21
3.1.1 Addition of Na2S to the subphase
Figure 3.1 shows the microstructure of monolayers formed by dropcasting 18 µL of a 0.29
µM QD solution at room temperature. This is the amount of nanocrystals needed to
form one monolayer of a cubic superlattice. From left to right, the amount of Na2S in the
subphase is increased from naught to a 100 fold excess, compared to the total amount of
ligands in the QD solution. This was calculated for 6 nm nanocrystals with an average
ligand density of 4 ligands/nm2 [14].
The superlattice formed on pure ethylene glycol (Figure 3.1A) shows a hexagonal distri-
bution. The average distance between the nanocrystals is 2.9 ± 0.8 nm, corresponding to
an intertwingled ligand shell stabilized by Van der Waals interactions (the length of oleic
acid is 1.97 nm). This indicates that the nanocrystals are surrounded by a complete ligand
shell, and that ethylene glycol at room temperature does not affect the ligand shell at the
QD surface. The surface density of nanocrystals in this layer is 15.56 · 103 QDs/µm2, as
determined from the TEM image.
The addition of a 10 fold excess Na2S to the subphase (Figure 3.1B) results in a fraction
of the nanocrystals connecting to each other, with an accompanying deviation from the
hexagonal structure. Although a significant fraction of nanocrystals remains separated
by the organic capping molecules, the increase in interparticle connections, compared to
no additional Na2S, is clear. Adding more Na2S subsequently increases the amount of
interparticle connections, as shown by the layer formed with a 50 fold excess in Figure
3.1C. At the same time, the surface density of nanocrystals increases due to a more closely
packed structure. Eventually, with even more Na2S in the subphase, the density of the
layer does not exhibit a dramatic increase, since at 50 fold excess most of the nanocrystals
are already connected. Figure 3.1D shows the layer formed on a subphase containing a 100
fold excess Na2S. The surface density, as determined from the TEM image, is 21.86 · 103
QDs/µm2.
Chapter 3. Quantum Dot Superlattices 22
Figure 3.1: TEM images showing the microstructure of superlattices formed by dropcasting a
QD solution on ethylene glycol. From left to right the amount of Na2S added to
the subphase is no Na2S (A), a 10 fold excess (B), a 50 fold excess (C) and a 100
fold excess (D). Increasing amounts of Na2S increases the amount of interparticle
connections and the surface density in the superlattice. Scale bars are 10 nm.
Figure 3.2 shows more detailed TEM images of the connections between nanocrystals. The
distance between the lattice planes in Figure 3.2A is 3.01 ± 0.16 A, which is approximately
the distance between Pb and Se along a (100) axis. The crystals are thus rotated along
this axis, and bind via (100) planes. Another example is shown in Figure 3.2B where the
interplanar distance is 2.18 ± 0.14 A. This is approximately the distance between (110)
planes, meaning that the nanocrystals are rotated along the (110) axis. This figure shows
that, although the orientation is different, the bonding still occurs via the (100) planes.
From inspection of all recorded TEM images, all connections between crystals were found
to occur via (100) planes, no bonding via (110) or (111) planes was observed.
Chapter 3. Quantum Dot Superlattices 23
Figure 3.2: High resolution TEM images showing interparticle connections via (100) planes
where the nanocrystals are slightly rotated along a (100) axis (A) or a (110) axis
(B).
The FTIR spectrum in Figure 3.3 shows how the addition of Na2S to the subphase affects
the ligand shell of the QDs. Without any Na2S, the spectrum shows relative strong C− H
stretch bands, originating from the oleate ligand shell. At a 50 fold excess, the bands
are strongly decreased in intensity, and at a 100 fold excess, no significant C− H stretch
bands are distinguishable. This shows that Na2S effectively removes oleate ions from the
QD surface, allowing the QDs to form interparticle connections. This is in agreement with
what is seen in the TEM images.
Chapter 3. Quantum Dot Superlattices 24
Figure 3.3: FTIR spectra showing the C−H stretch vibrations of superlattices formed on ethy-
lene glycol containing no Na2S (green), a 50 fold excess (blue) and a 100 fold excess
(red). The signals decrease with increasing amounts of Na2S, indicating removal of
the native oleate ligands.
Figure 3.4 shows SEM images of the layers formed on ethylene glycol containing a 10 fold
and a 50 fold excess Na2S (A and B respectively). A very homogeneous monolayer is
deposited on the Si wafer and the thickness is uniform over an area up to tens of µm2.
Figure 3.4: SEM images of superlattices formed on ethylene glycol containing a 10 fold excess
Na2S (A) and a 50 fold excess (B). Both superlattices consist of a homogeneous
monolayer with very few cracks over a large area (scale bars are 1 µm).
Chapter 3. Quantum Dot Superlattices 25
3.1.2 Temperature of the subphase
The effect of temperature on the formation of QD superlattices was studied by heating the
subphase to 50 ◦C before adding the QD solution. However, when monolayer amounts were
added, no layer was deposited on the substrate, most likely because convection currents in
the subphase drifted the nanocrystals away from the center of the vial. For this reason,
more material was added to the subphase.
Figure 3.5 shows the effect of adding 50 µL of a 1.4 µM QD solution to ethylene glycol at
room temperature (A) and at 50 ◦C (B). At room temperature, the nanocrystals arrange in
a hexagonal close packed structure and the distance between the crystals indicate that the
ligand shell has not been altered in a significant way. On the other hand, at elevated tem-
perature, the individual nanocrystals merge together and form a superlattice of connected
QDs with a quasi-cubic symmetry. The superlattice has a density of 22.13 · 103 QDs/µm2.
Figure 3.6 shows more detailed images of some connections found in the supperlattice.
The nanocrystals in Figure 3.6A are viewed along the (100) direction, as determined from
measurements of the angles (135 ± 2◦) and interatomic distances (3.04 ± 0.19 A) of crystal
1. This orientation means that the nanocrystals fuse via their (100) facets. In Figure 3.6B,
crystal 1 has an average interatomic distance of 3.05 ± 0.18 A and an average angle between
the facets of 135 ± 1◦, crystal 2 has an average distance of 3.05 ± 0.15 A between the
atomic rows. As indicated in the figure, this means that the bond between crystal 1 and
2 happens via the (100) facets. The bond between crystal 2 and 3 can either happens
via (100) or (110) facets, depending on the rotation of the nanocrystal. However, bonding
via the (110) facets means that the (100) facets between crystal 1 and 2 are rotated 45◦
relative to each other. Due to a large lattice mismatch between these orientations, this type
of bonding is very unlikely, which suggests that the bond between crystal 2 and 3 happens
via the (100) facets. For the same reason, bonding between different facets, between (100)
and (110) for example, is rather unlikely. For clarity, the arrangement of atoms in the
different possible orientations are shown in Figure 3.7.
From careful inspection of TEM images, it is found that the majority of connections happen
Chapter 3. Quantum Dot Superlattices 26
via (100) facets. Although fusion via (110) facets is also possible, this is not frequently
observed. An example of (110) bonding is shown in Figure 3.6C, where the average distance
between the atomic rows is 3.04 ± 0.16 A.
Figure 3.5: TEM images of the superlattices formed on pure ethylene glycol at room temper-
ature (A) and at 50 ◦C (B). The increased temperature leads to a more dense
structure of interconnected QDs, compared to a hexagonal close packed layer with
a larger interparticle distance at room temperature. Scale bars are 50 nm.
Figure 3.6: High resolution TEM images of interparticle connections found in the superlattice
formed at 50 ◦C. Most connection occur via (100) planes (A and B), although
bonding via (110) is also possible (C).
Chapter 3. Quantum Dot Superlattices 27
Figure 3.7: Crystal models showing the arrangement of Pb2+ and Se2- ions at the (100) and
(110) planes.
The FTIR spectrum in Figure 3.8 compares the layer formed at room temperature (green
curve) with the layer formed at 50 ◦C (blue curve). In the layer formed at room tem-
perature, strong absorption bands from C− H stretch vibrations are seen at 2925 cm-1
and 2850 cm-1. At 50 ◦C, a strong reduction of these signals is seen, indicating that the
majority of the ligand shell is removed from the QD surface. This shows that not only
the addition of a reagent to the subphase, but also moderately increasing the temperature
of the subphase is a successful strategy for removal of the ligand shell, and subsequently
leads to the formation of interparticle connections.
Chapter 3. Quantum Dot Superlattices 28
Figure 3.8: FTIR spectra showing the C−H stretch vibrations of superlattices formed on pure
ethylene glycol at room temperature (green) and at 50 ◦C (blue). The strong reduc-
tion of the signals indicate that the majority of native oleate ligands are removed.
The topography of the layer was measured with AFM. According to the surface profile in
Figure 3.9A, which follows the line drawn in Figure 3.9B, the superlattice mainly consists
of mono- and bilayers. Since more QDs were added to the subphase, this is a reasonable
result. Furthermore, the topography image shows that the layer contains small cracks with
widths around 1 µm (black lines), and that aggregations of up to 100 nm are spread across
the surface (yellow to white lines). This morphology is confirmed by the SEM images in
Figure 3.10. Figure 3.10A shows more in detail the cracks across the surface and Figure
3.10B is a higher magnification image showing the presence of mono- and bilayers. It
is clear that these layers are less homogeneous than the ones obtained in the previous
paragraph.
Chapter 3. Quantum Dot Superlattices 29
Figure 3.9: Surface profile (A) following the line drawn in the topography image (B) of the
superlattice formed on pure ethylene glycol at 50 ◦C. The superlattice primarily
consists of a bilayered structure with small portions consisting of a monolayer.
Figure 3.10: SEM images of the superlattice formed on pure ethylene glycol at 50 ◦C. The
images confirm the presence of cracks (A - scale bar 1 µm) and varying regions of
mono- and bilayers (B - scale bar 100 nm) as seen in the AFM image.
3.1.3 Rate of QD addition
Another parameter that influences the superlattice formation is the rate at which the
nanocrystals are added to the subphase. Figure 3.11 shows the different morphologies
obtained by adding 50 µL of a 0.9 µM QD solution either in one swift motion (A), or
Chapter 3. Quantum Dot Superlattices 30
in a drop-wise fashion at a rate of about 1 drop/s (B). In both cases, the solutions were
added at 50 ◦C. This section shows the results of adding a 0.9 µM QD solution in order
to properly compare with the structures formed at room temperature (only results with a
maximal concentration of 0.9 µM available). However, the molten structures were obtained
at different temperatures and with different concentrations, and addition of 1.4 µM resulted
in an identical morphology as the one shown here.
The fast addition results in superlattices where individual nanocrystals connect together,
similar to the superlattices obtained in the previous paragraph. On the other hand, when
the nanocrystals were added drop-wise, the morphology drastically changed. The crystals
fuse together and form a molten-like network where the individual nanocrystals cannot
be distinguished anymore. At the same time, the surface density of material decreases
considerably.
Figure 3.11: TEM images showing the microstructure of superlattices formed on pure ethylene
glycol at 50 ◦C, either by fast addition (A) or by drop-wise addition of the QD
solution (B). A drop-wise addition drastically changes the morphology from a
connected layer of QDs to a molten-like structure. Both scale bars are 50 nm.
The morphology of the layer at elevated temperature differs from that formed by drop-
wise addition at room temperature (Figure 3.12). Here, the nanocrystals arrange in a very
periodic structure over a large area. This is indicated by the Fourier transform, showing
well defined points in the reciprocal lattice (inset Figure 3.12A). From Figure 3.12B it
Chapter 3. Quantum Dot Superlattices 31
is clear that these structures are a stacking of hexagonal close packed layers where the
crystals are separated by their ligand shell. In general, the layers have a thicker stacking
of monolayers, compared to a fast addition at room temperature.
Figure 3.12: A: Superlattice formed by drop-wise addition of the QD solution at room tempera-
ture. The Fourier transform in the inset shows a long range hexagonal periodicity.
Scale bar is 50 nm. B: A higher magnification image of the same structure showing
that the superlattice consists of a stacking of hexagonal close packed layers. Scale
bar is 10 nm.
The FTIR spectrum in Figure 3.13 confirms that at room temperature the ligand shell
remains intact (green curve), as was the case in the previous section. Again, adding the
nanocrystals to the subphase at 50 ◦C removes the ligands from the QDs, as indicated by
the loss of C− H stretch vibrations in the spectrum (red curve). This means that removal
of the ligands is not significantly influenced by the rate at which the nanocrystals are added
but that it is, in this case, mainly determined by the temperature of the subphase.
Chapter 3. Quantum Dot Superlattices 32
Figure 3.13: FTIR spectra showing the C−H stretch vibrations of superlattices formed by
drop-wise addition of the QD solution at room temperature (green) and at 50
◦C (red). The strong reduction of the signals show that almost all native oleate
ligands are removed.
AFM was used to determine the thickness of the layer. Figure 3.14A shows the surface
profile following the line drawn in Figure 3.14B. The layer has a thickness of about 11 nm,
but since this layer is not comprised of individual nanocrystals, nothing can be said about
the thickness in terms of mono- or bilayers.
Chapter 3. Quantum Dot Superlattices 33
Figure 3.14: Surface profile (A) following the line drawn in the topography image (B) of the
superlattice formed by drop-wise addition of the QD solution at 50 ◦C. The profile
shows a thickness of about 11 nm.
In general, the long-range morphology of this layer is not as good as for the layers shown
previously. Rather than being uniform, the layer consists of a patchwork of smaller chunks
or flakes of material, with sizes of several µm2. This is illustrated by the SEM image in
Figure 3.15.
Figure 3.15: SEM image showing a small piece of the superlattice formed by drop-wise addition.
Scale bar is 1 µm.
Chapter 3. Quantum Dot Superlattices 34
3.2 Superlattices by Langmuir-Schaefer deposition
QD superlattices were also made by the formation of Langmuir films (see Appendix A for
a short explanation of the technique). For this, 100 µL of a filtered 2.46 µM QD solution
in hexane was added drop-wise to the Langmuir trough. After the solvent evaporated, the
nanocrystals were slowly compressed by two barriers, reducing the available surface area by
10 cm2/min. After compression to the desired pressure, samples were taken by stamping
the layer on a TEM grid, or a Si or glass substrate (i.e., Langmuir-Schaefer deposition).
The TEM images in Figure 3.16 show PbSe QD monolayers formed by Langmuir-Schaefer
deposition at room temperature. From left to right, the subphase was varied from water
to ethylene glycol to diethylene glycol. On water, the QDs arrange in a typical hexagonal
close packed structure, stabilized by Van der Waals interactions between neighboring ligand
shells. A similar ordering is seen with ethylene glycol as subphase. The interparticle
distance is comparable to the layer on water, indicating that at an elevated pressure of 25
mN/m, ethylene glycol does not significantly affect the ligand shell. However, the mobility
of the crystals on ethylene glycol is a bit lower due to stronger Van der Waals interactions
between ethylene glycol and oleate, and at 25 mN/m, the layer is a little less homogeneous
than on water (macroscopically).
Chapter 3. Quantum Dot Superlattices 35
Figure 3.16: TEM images of the superlattices formed by Langmuir-Schaefer deposition on wa-
ter (A), ethylene glycol (B) and diethylene glycol (C). (A) and (B) were taken at a
pressure of 25 mN/m and show a hexagonal close packed structure with no inter-
particle connections. (C) was taken at 4 mN/m and shows a disordered structure
of interconnected QDs. Scale bars are 10 nm.
On diethylene glycol, the morphology of the layer drastically changes from a dense hexag-
onal structure, to a less dense, somewhat disordered structure of interconnected nanocrys-
tals. This suggests that diethylene glycol, as opposed to ethylene glycol, effectively removes
the ligand shell at room temperature. Figure 3.17A shows a more detailed image of some
interconnected nanocrystals. The average distance between the atomic planes in Figure
3.17B is 3.02 ± 0.17 A, determined from 100 measure points. This shows that the crys-
tals are rotated around a (100) axis, and that connections in the direction perpendicular
to the lattice planes occur via (100) planes. Connections in the other direction, parallel
to the lattice planes, either occur via (100) or (110) planes, but as explained earlier, the
most probable bonding occurs via (100) planes due to lattice mismatches. A model of the
connected QDs with and without crystal shape is shown in Figure 3.17C.
The irreversible bonding between crystal facets introduces a rigidness to the structure, in
the sense that once bonds are formed in a small group of nanocrystals, they cannot easily
rearrange in a more close packed structure. Upon compression, this eventually leads to
aggregations of QDs and the layer gets an overall inhomogeneous thickness and morphology.
Chapter 3. Quantum Dot Superlattices 36
For this reason, the layer on diethylene glycol was sampled at 4 mN/m, instead of at 25
mN/m, as was the case with water and ethylene glycol.
Figure 3.17: A: High resolution TEM image of the structure formed on diethylene glycol. B:
High resolution TEM image showing that the connection between nanocrystals
occur via (100) planes and that they are slightly rotated along a (100) axis. C:
Crystal models of the bonds shown in (B) with and without crystal shape.
The formation of interparticle connections on diethylene glycol is in line with the desired
result, although the strong inhomogeneity is problematic for use in applications. In an
attempt to better control the formation of connections, monolayers were formed on a
subphase consisting of 70 % ethylene glycol and 30 % diethylene glycol. On this subphase,
the nanocrystals spread better than on pure diethylene glycol, and it was possible to
compress the layer to 12 mN/m without formation of problematic aggregations. After
compression, the layer was left to stabilize for 1 hour, during which the layer contracted,
and an accompanying drop in pressure was seen, from 12 mN/m to 9.4 mN/m. The
contraction was primarily in the direction parallel to the compressing bars of the apparatus,
and further compression only led to inhomogeneities in the layer.
The layer stamped on a Si wafer has a very homogeneous appearance across the surface.
This is confirmed by the SEM image in Figure 3.18A, which shows that the layer is homo-
geneous over an area of several tens of square micrometer. Some small ridges are visible,
Chapter 3. Quantum Dot Superlattices 37
originating from the applied compression after the layer had contracted. Figure 3.18B
shows that the surface is not completely covered with nanocrystals and thus the density of
the layer is not optimal, but overall, the QDs are evenly spread across the surface.
Figure 3.18: SEM images of the superlattice formed by Langmuir-Schaefer deposition on a sub-
phase consisting of 70 % ethylene glycol and 30 % diethylene glycol. A relatively
homogeneous film over a large area is obtained (A - scale bar 10 µm) although
the higher magnification image shows that the density of QDs is not maximal (B
- scale bar 100 nm).
The TEM image in Figure 3.19A confirms that the layer is not as dense as possible and that
many open gaps are present. A significant amount of QDs show interparticle connections
but at the same time, many nanocrystals retain their native ligand shell. The obtained su-
perlattice thus becomes a combination of connected and unconnected QDs (Figure 3.19B).
As expected from previous results, most connections occur via (100) planes. This result
confirms that DEG is a very good removing agent for the ligand shell, and that the amount
of DEG in the subphase determines the fraction of ligands that are lost.
Chapter 3. Quantum Dot Superlattices 38
Figure 3.19: TEM images showing the microstructure of the superlattice formed by Langmuir-
Schaefer deposition on a subphase consisting of 70 % ethylene glycol and 30 %
diethylene glycol. The scale bar in (A) is 100 nm, in (B) 10 nm. (C) is a high
resolution TEM image showing interparticle connections via (100) planes.
Inspired by the results in section 3.1.1, monolayers were formed on ethylene glycol con-
taining a 100 fold excess Na2S. The addition of Na2S changed the subphase in such a
way that the spread of the nanocrystals was retarded, leading to small spots of material
(approximately 5 cm in diameter) at the place where the drop of QD solution was added.
This decrease of QD mobility indicates an increased interaction with the subphase. After
compression to 12 mN/m, the layer was left to stabilize for 1 hour and again, a contraction
of the layer was seen. Due to the decreased mobility, aggregations were formed at the place
where the spots collided, although these were manageable and still homogeneous areas of
several square centimeters were obtained.
The TEM image in Figure 3.20A shows that a relatively dense layer is obtained where the
QDs have a rather random, non-symmetrical stacking. A large fraction of the QDs show
interparticle connections (Figure 3.20B), but also many QDs are not connected, signaling
an incomplete ligand removal.
Chapter 3. Quantum Dot Superlattices 39
Figure 3.20: TEM images of the superlattice formed by Langmuir-Schaefer deposition on an
ethylene glycol subphase containing a 100 fold excess Na2S.
Figure 3.21 shows the SEM image of a layer stamped on a Si wafer. The layer is somewhat
less homogeneous, with overall dense and less dense areas. However, no cracks or large
aggregations are seen across the surface.
Figure 3.21: SEM images of the superlattice formed by Langmuir-Schaefer deposition on an
ethylene glycol subphase containing a 100 fold excess Na2S.
Figure 3.22 shows the FTIR spectrum of the layers formed by Langmuir-Schaefer deposi-
tion. The green curve represents the spectrum of a monolayer formed on a H2O subphase.
In this layer, all organic ligands remain on the QD surface, and strong C− H stretch vi-
brations are seen. Both the addition of DEG (red curve) and the addition of Na2S (blue
Chapter 3. Quantum Dot Superlattices 40
curve) to the subphase result in a decrease of the C− H stretch bands. This indicates that
both additions have an effect on the ligand shell as partial removal of the organic ligands
is seen. Although a stoichiometric 100 fold excess of Na2S is added, ligand removal is not
complete, as was the case in section 3.1.1. This can be attributed to the larger volume
of the subphase, effectively decreasing the concentration of Na2S, and thus reducing the
available S2- ions at the subphase-QD interface.
Figure 3.22: FTIR spectra of the Langmuir-Schaefer films formed on different subphases: water
(green), ethylene glycol containing a 100 fold excess Na2S (blue) and ethylene
glycol containing 30 % diethylene glycol (red). Both additions to ethylene glycol
reduce the amount of oleate ligands, although a significant amount is still present.
3.3 Light absorption by superlattices
Figure 3.23 shows the absorption spectra of two superlattices. These structures were
selected because they illustrate the effect of interparticle connections and because they were
both very homogeneously deposited on a glass substrate. The red curve is the spectrum
of a hexagonal monolayer without interparticle connections, formed by Langmuir-Schaefer
deposition on water (see Figure 3.16A). The blue curve is the spectrum of the connected
superlattice formed by dropcasting on ethylene glycol containing a 100 fold excess Na2S
Chapter 3. Quantum Dot Superlattices 41
(see Figure 3.1D). The absorption onset of the connected superlattice is somewhat shifted
to longer wavelengths, followed by a steeper slope towards shorter wavelengths, resulting
in a higher absorbance at high photon energies.
The absorption at 400 nm and the density of the layer (Ns) were used to calculate a cross
section for the PbSe nanocrystals in both superlattices, by neglecting the reflection factor
in Equation 3.1. The result is summarized in Table 3.1. The density was determined from
the corresponding TEM images.
Figure 3.23: Absorbance spectra comparing the absorbance of an unconnected superlattice
(red) with a connected superlattice (blue).
σ = ln10A−RNs
(3.1)
A400nm Ns(10−3QDs/nm2) σ(10−14cm2)
Unconnected superlattice 0.11462 19.85 13.30
Connected superlattice 0.32683 21.86 34.42
Table 3.1: Cross sections of the connected and unconnected superlattice, calculated from Equa-
tion 3.1 without taking into account reflection of the superlattices.
Chapter 3. Quantum Dot Superlattices 42
3.4 Discussion
3.4.1 Oleate and the subphase
The organic ligand, oleate, is a surfactant molecule with a hydrophobic C18 carbon chain
and a hydrophilic carboxylate head group. The ligand is bound to Pb2+ ions at the QD
surface with the carboxylate group, forming Pb-oleate [14]. The high density of ligand
molecules, approximately 3-4 ligands/nm2, gives the faceted nanocrystal a quasi-spherical
shape. In other words, when the solvent is evaporated and the nanocrystals are allowed
to self-assemble, the ligand shell is responsible for the hexagonal close packed structure,
being the most dense and thus energetically favored stacking of an assembly of spheres. As
the ligands are removed from the QD surface, the non-spherical shape of the nanocrystal
becomes more expressed and the connected QD superlattice adopts a structure which
depends on the crystal structure and the crystal shape of the QD building blocks. In the
case of PbSe QDs, a tendency to form a cubic structure (although not perfect) is seen.
The experiments in this chapter show that several strategies can be used to remove the
organic capping molecules in order to form connected superlattices. A first route is the
addition of Na2S to the subphase. This removes the ligands by reacting with Pb-oleate
to form PbS and Na-oleate (Reaction 3.2). In this way, the excess Pb2+ ions at the QD
surface are compensated by S2- ions, which enables facets to fuse via the newly formed
stoichiometric surfaces.
Pb2+(C17H33COO−)2 + Na2S −→ PbS + 2Na+C17H33COO− (3.2)
The S2- ion is classified as an X-type ligand according to the Covalent Bond Classification
(C.B.C.) developed by M.L.H. Green [12]. This type of ligand donates an electron to the
metal cation, and forms normal covalent bonds.
Despite the long carbon chain, oleates formed with monovalent cations like Na+ are water
soluble (100 mg/ml) and thus dissolve in the ethylene glycol subphase. The reaction
is controlled by the amount of available Na2S at the surface of the subphase, in turn
Chapter 3. Quantum Dot Superlattices 43
determined by the concentration of the reagent.
This route has the advantage that the superlattice is formed at room temperature, which
excludes convection current and allows the amount of material at the surface of the sub-
phase to be better controlled. This results in the formation of very homogeneous mono-
layers of the superlattice, extending over a large area.
Removal of the organic ligand shell was also achieved by using as the subphase either
ethylene glycol at 50 ◦C or diethylene glycol at room temperature. According to the
C.B.C. system, both glycols are classified as an L-type ligand. This type of ligand donates
two electrons to the metal cation in the form of a lone pair. In this way, a dative covalent
bond is formed. The difference in ligating strength between EG and DEG follows from
their structure. EG contains two OH groups with a total of four lone pairs. DEG has an
additional OR2 functionality, contributing two extra lone pairs.
Owen et al. showed that Pb(OA)2 units are adsorbed relatively weakly to the nanocrystal
surface and that L-type ligands induce removal of the metal carboxylates by coordinating
with the metal cation [25]. The experiments here indicate that this occurs with EG and
DEG. After the metal carboxylates are removed, the facets obtain stoichiometric surfaces
which enable interparticle bonding.
3.4.2 Oriented attachment
Small PbSe nanocrystals consist of 6 (100) facets and 12 (110) facets. Both the (100)
and (110) facets are evenly occupied by Pb and Se atoms. The (111) planes, however, are
either completely occupied by Pb or Se atoms. The difference in electronegativity of both
elements sets up a charge interaction in the crystal, which leads to a dipole moment. Both
the magnitude and the direction of the dipole moment is determined by the distribution
of Pb- and Se-occupied (111) planes. Cho et al. calculated for PbSe nanocrystals that a
dipole moment along a (100) axis not only has the highest probability, but also the highest
magnitude [4]. This leads to a preferential orientation and attachment of the nanocrystals
along this axis.
Chapter 3. Quantum Dot Superlattices 44
This is confirmed by the analysis of high resolution TEM images in this chapter. It was
shown that the majority of interparticle connections occur via (100) facets, and that bond-
ing via (110) only rarely occurs. Bonding via (111) facets was not observed at all.
Figure 3.24 compares the superlattice obtained by Evers et al. (A) [10] with the one obtained
in this thesis (B). In both superlattices the nanocrystals bond via (100) facets, although
the symmetry of the superlattice is clearly different.
For nanocrystals fusing via (100) planes, the bonding occurs between a Pb atom of one
crystal and a Se atom of the other. This means that the crystals are shifted at least half
a lattice parameter relative to each other. As a consequence, the angle between the (100)
bonding directions is not exactly 90◦, as would be the case for a perfect cubic symmetry.
In the superlattice obtained by Evers, bonding directions around 87◦and 81◦are seen, re-
spectively corresponding to lattice jumps of 0.5 and 1.5 times the lattice parameter. In the
superlattice obtained in this thesis, the measured angles are around 75◦, corresponding to
a shift of 2.5 times the lattice parameter. A factor that possibly contributes to this relative
large shift is the crystal shape of the QDs. If the nanocrystals are slightly elongated along
one (110) axis, opposite (100) facets are already intrinsically shifted relative to each other.
This means that even if the bonding occurs via a minimal shift of half a lattice parameter,
the overall symmetry of the superlattice corresponds to a seemingly larger shift. More
specifically, opposite (100) facets are shifted by 2 whole lattice parameters if a crystal is 24
% elongated along one (110) axis, starting from a perfectly symmetrical 6 nm nanocrystal.
The drawing in Figure 3.25 more visually illustrates the effect of elongation on the lattice
symmetry.
Chapter 3. Quantum Dot Superlattices 45
Figure 3.24: A: Cubic PbSe superlattice as obtained by Evers et al. B: PbSe superlattice
obtained in this thesis. Scale bars are 50 nm.
Figure 3.25: Schematic representation of a superlattice formed with perfect symmetrical
nanocrystals (left) and a superlattice formed with nanocrystals elongated along
a (110) axis. An elongation of 24 % corresponds to a shift of opposite (100) planes
by 2 lattice parameters.
3.4.3 Langmuir films
The Langmuir technique allows well defined monolayers to be fabricated over a large area,
with the extra advantage that the results are easily reproduced. When there is no in-
teraction between subphase and QDs, the controlled application of pressure enables the
Chapter 3. Quantum Dot Superlattices 46
formation of monolayers with the highest possible density. However, when the subphase is
adjusted in such a way that it removes ligands from the QD surface, this removal is induced
from the moment the nanocrystals are added to the trough. This leads to QDs partially
stripped from their ligands spread over the surface of the subphase. Upon compression,
nanocrystals in close proximity to each other bind together, leading to many small, rigid
groups of connected QDs, which in turn connect together when more pressure is applied.
Since the bonds are irreversible, faults in the stacking order cannot be corrected by rear-
rangement of individual nanocrystals, and this mechanism subsequently does not yield the
most dense stacking of QDs. It can be seen that delaying the ligand removal to a moment
where all nanocrystals are already in close proximity to each other, would lead to a more
dense superlattice. This was partly achieved by adding a small amount of DEG to the
subphase and letting the layer contract after a first initial compression. This improved
the quality of the layer and resulted in a homogeneous superlattice over a much larger
area, although the density of QDs could still be higher. More experiments are needed to
see whether it is possible to further increase the surface density by controlling the rate of
ligand removal.
Another possible approach is that in a first step, a hexagonal close packed layer is formed,
and in a second step, a reagent or an impulse is given to initiate ligand removal. It is for
example possible to heat up the subphase by recirculating hot water around the trough.
Alternatively, a reagent could be added from beneath the layer by homogeneous injection
into the subphase, or from above by the flow of a reactive gas. A bit more far fetched could
be the design of ligands which, for example, dissociate under influence of UV light.
3.4.4 Absorption enhancement
Although the number of absorbance spectra taken was insufficient to make conclusive
statements, the spectra shown in Figure 3.23 illustrate that the formation of interparticle
connections over a long range possibly enhances the absorption cross section of the QDs.
This is indicated by the increased cross section of connected QDs compared to unconnected
Chapter 3. Quantum Dot Superlattices 47
QDs (Table 3.1). The cross section was estimated by neglecting the reflection factor in
Equation 3.1. Although the correction for reflection is relatively small (< 10 %) for un-
connected QDs [11], no reflection data is available for connected QDs. This could lead to an
overall smaller cross section then the one reported here, but the numbers are only given as
an indication.
The increase of intrinsic absorption can be qualitatively understood from Equation 1.6.
For separate QDs, the intrinsic absorption coefficient is decreased by a partial screening of
the incident electrical field, determined by the local field factor. For bulk material, this is
not the case, and the local field factor equals 1. It can be rationalized that a connected
QD superlattice behaves as something between separate QDs and bulk material, and that
the local field factor is significantly increased compared to individual QDs.
3.5 Conclusion
This chapter shows that connected QD superlattices can be obtained via different tech-
niques.
Formation of superlattices at room temperature provides a good control over layer thick-
ness, and homogeneous monolayers over a large area were possible. By adding Na2S to the
subphase, the native oleate ligands were removed via an ion exchange reaction, and this
subsequently resulted in relatively dense superlattices of connected QDs.
Connected superlattices were also obtained at elevated temperature. Control over layer
thickness and homogeneity was more difficult, resulting in overall bilayered structures with
more cracks. The rate at which the nanocrystals were added to the subphase proved to be
a determining factor for the microstructure of the superlattice. The postulated mechanism
for ligand removal involves the loss of Pb(OA)2 units, leaving the QD surface largely
unpassivated.
Superlattices formed via Langmuir-Schaefer deposition on different subphases resulted in
crack-free films of varying density. For large areas, this technique provides a better control
of the homogeneity of the superlattice, which becomes important for reproducible device
Chapter 3. Quantum Dot Superlattices 48
fabrication. The experiments further showed that adjustment of the subphase composition
provides a way to influence and control the surface chemistry of the QDs. Two examples
were shown, where the native ligands were removed by either adding Na2S or DEG to the
subphase.
Finally, the absorption measurements indicated that the formation of a connected QD
superlattice possibly enhances the intrinsic absorption coefficient by increasing the local
field factor. This could be an extra advantage of using electronically coupled QDs for
photodetection.
Chapter 4
Quantum Dot Photodetectors
Several QD superlattices described in the previous chapter were tested for their photocon-
ductive properties by recording I-V curves of the superlattices deposited on gold electrodes.
By sweeping a voltage across the electrodes, a certain current is induced, depending on the
microstructure of the superlattice. This current was measured under different illumination
levels, ranging from 0 to 17684 W/m2, with a laser operating at a fixed wavelength of
1550 nm. From the resulting photocurrents, i.e. measured current minus dark current,
responsivities (A/W, calculated as R = Iphoto/Pincident) and contrast ratios (Iphoto/Idark)
were plotted in order to get a view on the performance of the detectors. This chapter only
shows the results from I-V measurements in the -5V to +5V range. I-V curves were also
recorded in the -10V to +10V range, although no significant changes were observed, apart
from an approximate doubling of the dark- and photocurrents.
Furthermore, the superlattices were subjected to a laser with a wavelength varying from
2100 nm to 2400 nm to see if photoconductivity was observed at energies below the band
gap of the individual QDs.
As a reference to the results further shown in this chapter, a superlattice consisting of a
hexagonal close packed QD layer with an intact organic ligand shell, as in Figure 3.16A
and B, shows no dark- or photocurrent, pointing to the effective insulating properties of
the organic shell. All other manipulations carried out during the superlattice formation
49
Chapter 4. Quantum Dot Photodetectors 50
proved successful and resulted in an increase of the conductivity.
4.1 Addition of Na2S to the subphase
Figure 4.1 shows the measured I-V curves at different illumination levels of the superlattices
formed with a 50 fold (A) and a 100 fold (B) excess Na2S in the subphase. Figure 4.2 shows
the corresponding photocurrents (A), responsivities (B) and contrast ratios Iphoto/Idark (C)
at 5 V in function of the illumination level.
As explained earlier, additional Na2S led to the removal of native oleate ligands, and a
significant increase of interparticle connections, related to the excess amount that was
added. This directly enhances the conducting behavior of the superlattice by reducing the
time a charge carrier needs to travel between the electrodes (τtransit). This is illustrated
by an increase of the dark current (at 5 V) from 0 µA to 10.5 µA going from no Na2S to
a 50 fold excess, and an increase from 10.5 µA to 29.0 µA going from a 50 fold to a 100
fold excess. This relative high dark current most likely points to a considerable doping of
the nanocrystals by the S2- ions.
The reduced transit time allows the superlattice to produce more photocurrent per incident
photon power. This results in a significant increase of both photocurrent and responsivity
by the addition of more Na2S. The maximum photocurrent increases from 3.2 µA to 10.0
µA at 800 µW for a 50 fold and a 100 fold excess Na2S respectively. The maximum
responsivity is found at 50 µW, where an increase from 0.017 A/W to 0.062 A/W is seen.
With the surface of the laser beam (240 µm diameter) and the density of particles, it is
possible to express a responsivity per QD, resulting in 6 · 10-11 A/W per QD for the 100 fold
excess superlattice. Figure 4.2C shows that both superlattices have a comparable contrast
ratio, suggesting that the observed increase in photocurrent and responsivity is mainly due
to the reduced transit time and not by an additional increase of the carrier lifetime.
No photoconductivity was observed between 2100 nm to 2400 nm.
Chapter 4. Quantum Dot Photodetectors 51
Figure 4.1: Measured I-V curves of the superlattice formed with a 50 fold (A) and a 100 fold
(B) excess Na2S in the subphase.
Figure 4.2: Photocurrent (A), responsivity (B) and contrast ratio (C) of superlattices formed
with a 50 fold (red) and a 100 fold (blue) excess Na2S in the subphase.
4.2 Temperature of the subphase
Fast addition of QDs
Superlattice formation at elevated temperature, obtained by a fast addition of the QD
solution, resulted in a connected assembly of QDs (see Figure 3.5B). The measured I-V
curves are shown in Figure 4.3. Figure 4.4 shows the corresponding photocurrents (A),
responsivities (B) and contrast ratios Iphoto/Idark (C) at 5 V.
Chapter 4. Quantum Dot Photodetectors 52
The superlattice shows a remarkable increase of conductivity, with a dark current (at 5 V)
of 209.8 µA. On one hand, this is due to a structure with a high density of interconnected
particles, thus bringing in more freely moving charge carriers. On the other hand, the
bilayered structure can serve as a bridge between cracks or isolated parts, thereby reducing
the percolation path and activating ’dead’ areas, which are otherwise effectively lost in a
monolayer. Additionally, doping by surface defects could further increase the dark current
by increasing the carrier density. Of all experiments described in this thesis, illumination
of this QD superlattice leads to the highest photocurrents, with a maximal generated
photocurrent of 63.0 µA at an incident optical power of 800 µW. Correspondingly, the
responsivity of this superlattice is also the highest one obtained, and a maximal responsivity
of 0.26 is found at 50 µW. Expressed per QD this becomes 13 · 10-11 A/W per QD. Figure
4.4C shows a maximal contrast ratio Iphoto/Idark around 0.3.
Going to longer wavelengths, this superlattice still shows a small photocurrent between
2100 nm and 2400 nm. The responsivities in this spectral region are shown in Figure 4.5.
Even at 2400 nm, there is still a little photocurrent, indicating that the absorption onset
starts at even longer wavelengths.
Figure 4.3: Measured I-V curves of the superlattice formed by fast addition of the QDs to the
subphase at 50 ◦C.
Chapter 4. Quantum Dot Photodetectors 53
Figure 4.4: Photocurrent (A), responsivity (B) and contrast ratio (C) of superlattices formed
by fast addition of the QDs to the subphase at 50 ◦C.
Figure 4.5: Responsivity of the superlattice formed by fast addition of the QDs at 50 ◦C in the
2100 nm to 2400 nm spectral region, i.e. below the band gap energy of the colloidal
QDs.
Slow addition of QDs
Superlattice formation at elevated temperature, obtained by a drop-wise addition of the
QD solution, resulted in a molten-like structure (see Figure 3.11B). The measured I-V
curves are shown in Figure 4.6. Figure 4.7 shows the corresponding photocurrents (A),
responsivities (B) and contrast ratios Iphoto/Idark (C) at 5 V.
Chapter 4. Quantum Dot Photodetectors 54
The structure shows a dark current (at 5V) of 35.9 µA. Although the electrical connections
in this structure are good, both the lower density of material and the much more hindered
percolation path of charge carriers contribute to a lower dark current. The structure
generates a photocurrent of 35.6 µA at 800 µW, with a corresponding contrast ratio of
0.99. A maximal responsivity of 0.29 A/W is found at 25 µW.
This structure showed a photoconductive response up untill 2300 nm. At 2400 nm, no
photocurrent was observed. The responsivities in this region are shown in Figure 4.8.
Figure 4.6: Measured I-V curves of the superlattice formed by drop-wise addition of the QDs
to the subphase at 50 ◦C.
Figure 4.7: Photocurrent (A), responsivity (B) and contrast ratio (C) of superlattices formed
by drop-wise addition of the QDs to the subphase at 50 ◦C.
Chapter 4. Quantum Dot Photodetectors 55
Figure 4.8: Responsivity of the molten-like structure in the 2100 nm to 2400 nm spectral region,
i.e. below the band gap energy of the colloidal QDs.
4.3 Superlattices by Langmuir-Schaefer deposition
As stated earlier, the superlattices formed on pure water and ethylene glycol did not
show any conductivity. On the other hand, superlattices formed on a mixture of ethylene
glycol and diethylene glycol (Figure 3.18) and on a subphase containing Na2S (Figure
3.21) showed promising features and were homogeneous over a large area. Photodetectors
of these superlattices were made by depositing 4 layers via Langmuir-Schaefer deposition.
The measured I-V curves are shown in Figure 4.9. Figure 4.10 shows the corresponding
photocurrents (A), responsivities (B) and contrast ratios Iphoto/Idark (C) at 5 V.
Both superlattices have a very low dark conductivity, with dark currents (at 5 V) of 0.69
µA on the EG/DEG subphase and 0.79 µA on the Na2S containing subphase. Upon
illumination, photocurrents of several microamperes are generated, with maximal values
at 800 mW of 4.8 µA on the EG/DEG subphase and 7.5 µA on the Na2S containing
subphase. Correspondingly, the obtained responsivities in these superlattices are relatively
low, with the largest values found at 50 mW; 0.037 A/W for the EG/DEG subphase and
0.056 A/W for the Na2S containing subphase. Although the values of the photocurrents
Chapter 4. Quantum Dot Photodetectors 56
are not very high, the low dark current is responsible for fairly high contrast ratios, the
highest ones obtained. At an incident optical power of 800 mW, the ratios are 6.9 for the
EG/DEG subphase and 9.5 for the Na2S containing subphase.
The Langmuir-Schaefer films did not show photoconductivity between 2100 nm and 2400
nm.
Figure 4.9: Measured I-V curves of superlattices made by 4 Langmuir-Schaefer depositions,
formed on a subphase containing 70 % EG and 30 % DEG (A), and on an EG
subphase containing a 100 fold excess Na2S (B).
Figure 4.10: Photocurrent (A), responsivity (B) and contrast ratio (C) of superlattices made
by 4 Langmuir-Schaefer depositions, formed on a subphase containing 70 % EG
and 30 % DEG (red curves), and on an EG subphase containing a 100 fold excess
Na2S (blue curves).
Chapter 4. Quantum Dot Photodetectors 57
4.4 Discussion
4.4.1 Responsivity per QD
The responsivity per QD allows some comparison between different superlattices. Com-
paring the superlattice formed by fast addition at 50 ◦C to the superlattice formed by a
100 fold excess Na2S, it is seen that this quantity is about twice as high. At first sight this
can be explained by the better conductivity of the first, but since the dark current is about
7 times higher, another factor plays in favor of the latter. If the same level of doping and
quantum efficiency is assumed in the QDs, the responsivity scales with τlifetime/τtransit,
meaning that the lifetime of the carrier is about 3.5 times higher in the layer formed on
Na2S. This indicates that the S2- ions possibly introduce effective electron traps in the band
structure. Unless the structure consists of a significant amount of barriers or dead areas,
it is also possible that the S2- ions introduce some hole traps near the valence band, which
could partly explain the lower conductivity. However, these results are only indicative, and
more precise experiments aimed at determining the electronic structure of the superlattice
should provide more clarity.
4.4.2 Fast versus slow addition
The completely different morphology and their corresponding I-V characteristics of the
layers formed at 50 ◦C, i.e. the dense connected assembly of QDs and the less dense
molten-like structure, could provide some insights in properties influencing the photocon-
ductive behavior. Apart from a smaller amount of charge carriers present in the molten
structure, the transit time of the carriers is expected to be much higher due to the more
hindered, random percolation path. This can qualitatively explain the strongly reduced
dark current. Despite this, the relative generated photocurrent is significantly higher in the
molten structure, as illustrated by the increased Iphoto/Idark ratio. This is also shown by the
fact that both structures have a comparable responsivity, although the molten structure
has a much lower density.
Chapter 4. Quantum Dot Photodetectors 58
The mechanism of ligand removal is identical for both structures and occurs most likely by
removal or displacement of complete Pb(OA)2 units, as discussed earlier. This leaves the
surface of the QDs largely unpassivated, possibly creating many surface trap states and
thereby increasing the carrier lifetime and leading to additional doping. In this perspective,
the relative larger photocurrent in the molten structure can be rationalized by both a larger
surface area and a more complete removal of the organic ligands (see FTIR spectra in Figure
3.8 and 3.13). Additionally, the complex structure of the molten-like superlattice can
possibly further increase the carrier lifetime by a principle of harder-to-find recombination
centers.
Both these structures show photoconductivity from 2100 nm to 2300 nm, which indicates
that the absorption onset has red shifted by several hundreds of nanometers compared to
the colloidal QDs. This could result in a higher absorption coefficient at 1550 nm, possibly
explaining the higher responsivity at this wavelength. This is also a promising feature for
shifting the spectral response of QD based photodetectors towards longer wavelengths.
4.4.3 Langmuir-Schaefer films
Explaining the I-V curves of the Langmuir-Schaefer films is a bit more difficult. Both films
show a very low dark current. However, upon illumination, photocurrents with relatively
high Iphoto/Idark ratios are generated, meaning that a conductive path is available for the
charge carriers. This indicates that few intrinsic mobile charge carriers are present in
the structure. Since both Langmuir-Schaefer films still contain a large amount of native
ligands, it is well possible that the oleates provide a better passivation and that in the
other methods the many created surface states lead to an effective doping of the QDs,
thereby increasing the dark current.
Since both the dark currents and the amount of organic ligands are comparable for the
two Langmuir-Schaefer films, the difference in photocurrent can be seen as an effect of the
surface density and the nature of the introduced trap states. In the case of the EG/DEG
subphase, the ligand removal is postulated as the loss of complete Pb(OA)2 units, leaving
Chapter 4. Quantum Dot Photodetectors 59
part of the surface unpassivated. Since the Langmuir films were prepared under ambient
conditions, these sites are prone to oxidation, and partial passivation of the surface by
O2- ions or impurities of the subphase can take place. In the case of the EG subphase
containing Na2S, the surface is mainly passivated by oleate and S2- ions, which was shown
to increase the lifetime of the carrier. This allows more photocurrent to be generated,
increasing the responsivity and contrast ratio of the sulfur passivated QDs.
4.5 Conclusion
The I-V measurements in this chapter show that all manipulation made to remove the
native ligand shell improved the conductivity of the superlattices by providing connected
current pathways. The measurements further indicate that the precise nature of the surface
chemistry strongly influences the performance of the photodetector. More specifically,
passivation of the QD surface and controlled introduction of trap states are key factors for
enhancing the device performance.
An improved passivation of the QD surface ensures that the dark current remains low.
This has the advantage of enabling high contrast ratios, thereby increasing the sensitivity
of the device.
Introduction of trap states further increases the performance by increasing the exciton
lifetime. However, an important requirement is that the trap states do not act as a dopant.
This again increases the dark current and counteracts the advantage of passivation.
This thesis has shown that it is possible to make connected QD superlattices and that these
structures show promising features as photosensitive material. However, a more thorough
characterization and optimization of the chemical, and thus electronic, properties is needed.
Computational methods could prove a useful tool for screening and selecting possible trap
states or passivating agents. The use of a transistor type of setup would allow a more
precise study of the effect of carrier mobility and carrier density on the conductivity.
Appendix A
QD Langmuir-Blodgett/Schaefer
films
QD Langmuir films are QD monolayers formed by Langmuir-Blodgett/Schaefer deposition.
The films are produced on a Langmuir trough filled with a polar liquid, typically water.
The liquid in the trough is called the subphase. Due to the hydrophobic QD ligand shell, a
solution of QDs dispersed in a non-polar, volatile solvent can be dropcast on the subphase
without dissolving in it. As the solvent evaporates, the QDs evenly spread across the
surface of the subphase. At this stage, the distance between the QDs is relatively large.
By gently compressing the QDs with one or two moving barriers, the distance is gradually
decreased until eventually a dense, close packed monolayer is obtained.
The process of layer formation is monitored by careful measurement of the surface pressure.
This is achieved by partially immersing a Wilhelmy plate in the subphase and attaching
it to a microbalance. The surface pressure is determined from the variation of the force
exerted on the Wilhelmy plate via Equation A.1, where Π is the surface pressure and wp
and tp respectively the width and thickness of the Wilhelmy plate.
Π = − ∆F
2(wp + tp)(A.1)
The surface pressure is plotted in function of the available surface area and the resulting
pressure-area isotherm provides a way to monitor the formation of the close packed mono-
60
Appendix A. QD Langmuir-Blodgett/Schaefer films 61
layer. This is schematically shown in Figure A.1. At low pressure the QDs are regarded
as an ideal gas (G) due to the large interparticle distance. By gradually decreasing the
surface area, the interparticle distance decreases and the surface pressure increases. In this
process, the behavior of the QDs goes from gas-like (G) to liquid-like (L) to solid-like (S).
The formation of a close packed monolayer (S) is signaled by a very steep slope of the
isotherm.
Figure A.1: Pressure-area isotherm of the formation of a close packed monolayer. The behavior
of the QDs goes from gas-like (G) to liquid-like (L) to solid-like (S).
After compression, the film is transferred to a substrate of choice, either by vertically
pulling out a substrate, called Langmuir-Blodgett deposition, or by stamping the substrate
horizontally on the film, called Langmuir-Schaefer deposition (Figure A.2).
Figure A.2: Schematic representation of the Langmuir-Blodgett and Langmuir-Schaefer tech-
nique.
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