Photocurrent of CdSe nanocrystals on single-walled carbon nanotube-field effecttransistorSeung Yol Jeong, Seong Chu Lim, Dong Jae Bae, Young Hee Lee, Hyun Jin Shin, Seon-Mi Yoon, Jae YoungChoi, Ok Hwan Cha, Mun Seok Jeong, David Perello, and Minhee Yun Citation: Applied Physics Letters 92, 243103 (2008); doi: 10.1063/1.2944813 View online: http://dx.doi.org/10.1063/1.2944813 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of metal oxide morphology on electron injection from CdSe quantum dots to ZnO Appl. Phys. Lett. 102, 163119 (2013); 10.1063/1.4803173 Optical properties and effect of carrier tunnelling in CdSe colloidal quantum dots: A comparative study withdifferent ligands AIP Advances 2, 032132 (2012); 10.1063/1.4745080 Photo-induced charge transport in ZnS nanocrystals decorated single walled carbon nanotube field-effecttransistor Appl. Phys. Lett. 99, 173110 (2011); 10.1063/1.3656283 Metallic switching of semiconducting single-walled carbon nanotubes with ZnO thin film Appl. Phys. Lett. 92, 213501 (2008); 10.1063/1.2829800 Enhanced charge collection and reduced recombination of Cd S ∕ Ti O 2 quantum-dots sensitized solar cells inthe presence of single-walled carbon nanotubes Appl. Phys. Lett. 92, 153510 (2008); 10.1063/1.2911740

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Photocurrent of CdSe nanocrystals on single-walled carbon nanotube-fieldeffect transistor

Seung Yol Jeong,1 Seong Chu Lim,1 Dong Jae Bae,1 Young Hee Lee,1,a� Hyun Jin Shin,2

Seon-Mi Yoon,2 Jae Young Choi,2 Ok Hwan Cha,3 Mun Seok Jeong,3 David Perello,4

and Minhee Yun4

1BK21 Physics Division, Center Nanotubes and Nanostructured Composites, Sungkyunkwan AdvancedInstitute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea2Materials Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600,Republic of Korea3Advanced Photonics Research Institute, GIST, Gwangju 500-712, Republic of Korea4Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh,Pennsylvania 15261, USA

�Received 24 December 2007; accepted 22 May 2008; published online 16 June 2008�

CdSe nanocrystals �NCs� have been decorated on single-walled carbon nanotubes �SWCNTs� bycombining a method of chemically modified substrate along with gate-bias control. CdSe /ZnScore/shell quantum dots were negatively charged by adding mercaptoacetic acid. The silicon oxidesubstrate was decorated by octadecyltrichlorosilane and converted to hydrophobic surface. Thenegatively charged CdSe NCs were adsorbed on the SWCNT surface by applying a negative gatebias. The measured photocurrent clearly demonstrates that CdSe NCs decorated SWCNT can beused for photodetector and solar cell that are operable over a wide range of wavelengths. © 2008American Institute of Physics. �DOI: 10.1063/1.2944813�

CdSe nanocrystal �NC� is a promising candidate for usein future optoelectronic devices such as photodetectors, solarcells, and light emitting diodes �LEDs�.1–3 The semiconduc-tor NC shows quantum confinement effect similar to well-known quantum dots, with high quantum efficiency when theparticle size is less than the Bohr radius.4 The NC-embeddedconducting polymers have already been introduced forLEDs.5 However, the quantum efficiency was low �0.25%�resulting from inefficient charge transfer between the con-ducting polymer and NCs.6 This value is an order of magni-tude lower than that of conventional inorganic solar cells. Inan effort to increase the efficiency of NCs in the device, thecarbon nanotube �CNT� has been recently introduced as aballistic conducting channel.7,8 The CNTs and functionalizedNCs were simply combined using a chemical treatment.9,10

The idea was to use single-walled CNTs �SWCNTs� to pro-mote direct charge transport and also increase the efficiencyof charge transfer from the NCs, which is a major issue inconducting polymers due to the excitonic nature of carriers.11

While this chemical approach is simple and has a great po-tential for the application of optoelectronic devices, the fab-rication of highly sensitive devices is still challenging due tothe agglomeration of NCs on the bundled SWCNTs.12 An-other difficulty arises from an inevitable use of e-beam li-thography to fabricate the individual NC-SWCNT devices.

The purpose of this paper is twofold: �i� to demonstratethe use of gate modulation of a SWCNT-field effect transis-tor �FET� to locally decorate the SWCNT wall with NCs and�ii� to demonstrate the hybridized SWCNT/NCs as a sensi-tive photodetector that works in the visible wavelength atroom temperature. In this report, the negatively chargedCdSe NCs were attached to the individual SWCNTs grownby chemical vapor deposition �CVD�. The NC solution wasdropped by using a micropipette on a patterned substrate

with positively charged channel of SWCNT-FET that wascontrolled by applying a negative gate bias to the SWCNT-FET. This approach is a simple and straightforward methodto immobilize the NCs on individual SWCNTs and for directapplication in a highly sensitive photodetector.

Trioctyl phosphine oxide �TOPO�-coated CdSe /ZnSNCs were synthesized by a rapid injection of dimethyl-cadmium and TOP�trioxtyl phosphine�-selenide precursors ina hot TOPO using methods previously described.13,14 Thesurface ligands of CdSe NCs have been exchanged usingmercaptoacetic acid �MAA� to give a negative surface chargefollowing the previous report with minor modifications.15

Then, 10 mg of NCs dispersed in 1 ml of toluene was in-jected into 10 ml of MAA/toluene solution �typically20 mmol of MAA� drop by drop at a rate of 1 ml /h andheated at 100 °C for 3 h with stirring and, as a consequence,the MAA ligands were attached on the quantum dots thatbecame water soluble. The MAA-capped NCs were repeat-edly washed more than three times by chloroform and cen-trifugation to remove excess MAA. They were then redis-persed in � ·� ·�-tris �hydroxyl methyl� methyl amine buffersolution at pH=9, resulting in negatively charged CdSe /ZnSNCs in water. The SWCNT-FET was fabricated on a p-dopedsilicon substrate with an oxide thickness of 400 nm. Stan-dard optical photolithography has been employed to generatecircular patterns of catalysts �Fe� at the desired locations.16

Individual SWCNTs were synthesized by thermal CVD at900 °C for 10 min, using a gas mixture of hydrogen andmethane under atmospheric pressure. The second photoli-thography step produced large contact pads on the patternedcatalysts that served as a drain and source during subsequentcharacterization. For these electrodes, a Cr /Au �10 /20 nm�layer was deposited by the thermal evaporator at a vacuumpressure of �10−7 torr.

For a selective coating of hydrophilic NCs on theSWCNT-FET exclusively, the sample was first immerseda�Electronic mail: leeyoung@skku.edu.

APPLIED PHYSICS LETTERS 92, 243103 �2008�

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into a hexane solution �3 ml� containing 0.3 ml of1-dodecanethiol for 30 min and followed by rinsing withhexane, giving the 1-dodecanethiol coated gold electrodes.This substrate was then dipped into a mixture of hexane�3 ml� and OTS �0.3 ml� for 30 min and baked at 120 °C for30 min, followed by washing with hexane. This process re-sulted in the silicon dioxide substrate transforming into ahydrophobic surface. The NCs were then dropped on theSWCNT-FET, while a gate bias of −10 V was applied via aback gate for 10 min, and subsequently the substrate waswashed with de-ionized water �Fig. 1�a��. A confocal laserscanning microscope �Witec, alpha-SNOM�, Nd-YAG�neodymium-doped yttrium aluminum garnet� laser with532 nm wavelength� was used for identifying the presence ofNCs. The collected luminescence signal was analyzed by a30 cm single monochromator and charge-coupled device de-tector. The absorption spectroscopy was done by focusing anormal Xenon lamp �450 W� with a monochromator �gratingrange: 200–1500 nm� onto the active channel of NCs-SWCNT-FET. The photocurrent from the NCs-SWCNT-FETwas measured and normalized by the light intensity usingtwo lock-in amplifiers. All measurements were performed inambient condition at room temperature.

The fabricated CNT-FET electrode arrays were locatedon a 1�1 cm2 substrate, as shown in Fig. 1�b�. These arraysconsisted of ten pairs of electrodes connected by either iso-lated or a random SWCNT network between the electrodeswith a gap of 3 �m. Field-emission scanning electron micro-scope �FE-SEM� images were observed after decoration ofCdSe NCs �Figs. 1�c� and 1�d��. We visually confirmed theselective decoration of nanotubes with CdSe NCs using therandom network of CNTs �Fig. 1�d�� which is a black circlein Fig. 1�b�. The bright dotted CdSe NCs were decorated onthe nanotube surface. The Au electrode surface also indicatedthe presence of CdSe NCs �Fig. 1�d��. The negativelycharged NCs were selectively attached on a positivelycharged SWCNT that was induced by the negative gate bias.It is of note that without the gate bias, CdSe NCs were not

attached on the nanotube surface. This validates the effec-tiveness of our combined chemical modification and gate-bias method.

Because an isolated SWCNT is an effective conductingchannel that promotes the charge transfer from NCs, we se-lected an isolated individual SWCNT channel in Fig. 1�c�which is a white circled channel in Fig. 1�b�. The NC par-ticles are presumably attached on the SWCNT surface. How-ever, they were are not visible in SEM images due to thepoor resolution, which is also related to the different charg-ing state from random network SWCNTs. Figure 2 showsI-V curves of this individual CNT channel before decorationof the CdSe NCs. The resistance was relatively high, sug-gesting high contact resistance. As a function of gate voltage,the source-drain current clearly revealed gate modulation andhence the SWCNT is semiconducting and p type.

Figure 3�a� shows the photoluminescence from the chan-nel of the CdSe NC-decorated SWCNT from Fig. 1�c� with aclear peak position at 613 nm. The inset shows a confocallaser scanning microscopic image of the channel area. Theluminescence from the SWCNT and Au electrode is clearlyobserved, in good agreement with the physical presence ofCdSe NCs on the SWCNT and Au electrodes from SEMimages in Fig. 1. The absorption spectra are shown in Fig.3�b�. The peak appeared near 605 nm at a pH of 9. The pHwas chosen to maximize the negative charges on the CdSeNCs and this peak position was the same for NCs in toluene.Both factors indicate that the optical properties of CdSe NCswere not altered from the surface modification. The insetshows that the CdSe /ZnS nanoparticles had high crystallin-ity with a typical size of 7 nm. One may extract the diameterof CdSe from a band calculation using an effective massapproximation.17–19 The PL transition energy after a correc-tion of exciton binding energy is equivalent to our observed

FIG. 1. �Color online� The fabricated CdSe NC-SWCNT-FET: �a� a sche-matic of SiO2 substrate and Au electrode combined with gate-bias modula-tion, �b� the low magnification FE-SEM image indicating the center with tenarrays of electrodes on SiO2 substrate �inset: optical image of 1�1 cmarray�, �c� High magnification FE-SEM image of the individual NCs-SWCNT electrode corresponding to the white circle in �b� while a lowmagnification image is shown in the inset, which will be used for photocur-rent measurement, and �d� those corresponding to the black circle in �b�.

FIG. 2. I-V characteristics of the SWCNT-FET device: �a� Ids vs Vds char-acteristics and �b� Ids vs VgS characteristics �Vds range: −1–1 V, step:0.5 V�.

FIG. 3. �Color online� �a� Photoluminescence of the CdSe NC-SWCNT-FET �inset: confocal microscope image of the channel region, the whitearrow: NCs on an individual SWCNT, the white dotted arrow: silicon oxidesubstrate�. �b� Absorption spectra of CdSe NCs in toluene and in buffersolvent of pH 9 �inset: transmission electron microscope image of NCs witha typical size of 7 nm�.

243103-2 Jeong et al. Appl. Phys. Lett. 92, 243103 �2008�

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PL energy of 613 nm �2.02 eV�, which corresponds to theCdSe diameter of 3.2 nm. The calculated exciton bindingenergy was 78 meV. This yields the band edge energy of602 nm, which is close to the observed peak position of605 nm in the absorption spectroscopy.

Figure 4�a� shows the photocurrent as a function ofwavelength of the incident light. In this case, no source-drainbias was applied. No particular spectral response and a neg-ligible current were shown from the pure SWCNT-FET.Thus, the expected photocurrent that is contributed from vanHove singularity-related absorption peaks of SWCNTs wasnot observed in this case, unlike the previous report at lowtemperature.20,21 This may be ascribed to large thermal fluc-tuation at room temperature measurement and high noiselevel of our apparatus. On the other hand, a clear spectralresponse was observed from the CdSe NC-decoratedSWCNT-FET with peaks near 604.7 and 615.4 nm. Thesepeak positions are in excellent agreement with band edgeabsorption peak and exciton-related PL. The electrons thatare excited by the edge absorption and excitonic bands aretransferred to SWCNT channel to contribute as the photocur-rent. This interpretation was similar to the previous report onthe photoluminescence quenching in CdSe/SWCNTcomplex.22 Figure 4�b� shows the on/off response of incidentlight at a given laser energy of 514 nm as a function of time.A dark current of 0.08 pA in average at off state and light-oncurrent of 2 pA at on state were clearly repeatable for severalhundreds times without any appreciable change. As the ma-jority charge carriers are electrons donated from nanopar-ticles, the current flows from source to drain in the symmet-ric device, as described Fig. 1�a�. A more systematic study ofthe photocurrent on the dependence of different NC sizes andgate bias is required to utilize this concept for future photo-detectors and solar cell applications.

In summary, we have demonstrated a decoration of CdSeNCs on CNT surface. The decoration of CdSe NCs on CNTswas achieved by a chemical treatment of the NCs, silicon

oxide substrate, and Au electrodes combined with gate-biasmodulation. The CdSe NCs on SWCNTs demonstrated aclear spectral response and photocurrent response relatedspecifically to the CdSe NCs. This opens future opportunitiesfor applications in highly sensitive photodetectors and solarcells.

This work was supported by the MOE through the Star-Faculty project, KRF Grant funded by the Korea Govern-ment �MOEHRD, KRF-2005-005-J11902�, a grant from the21st Century Frontier Research Programs, in part KOSEFthrough the CNNC at SKKU, and by grants from AFOSR-MOST.

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FIG. 4. �Color online� �a� Photocurrent of CdSe NCs/SWCNT and pureSWCNT without NCs as a function of wavelength of the incident light in theSWCNT-FET device. The photocurrent was normalized by the spectral re-sponse of the light source. �b� The on/off photoresponse response as a func-tion of time for an incident light at a given laser energy of 514 nm.

243103-3 Jeong et al. Appl. Phys. Lett. 92, 243103 �2008�

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