Alexander A. Ezhov et al- Liquid-Crystalline Polymer Composites with CdS Nanorods: Structure and...
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8/3/2019 Alexander A. Ezhov et al- Liquid-Crystalline Polymer Composites with CdS Nanorods: Structure and Optical Properties
Published: September 13, 2011
r 2011 American Chemical Society 13353 dx.doi.org/10.1021/la203258r| Langmuir2011, 27, 1335313360
Liquid-Crystalline Polymer Composites with CdS Nanorods: Structureand Optical Properties
Alexander A. Ezhov,, Georgii A. Shandryuk, Galina N. Bondarenko, Alexey S. Merekalov,
Sergey S. Abramchuk,Alina M. Shatalova, Pramit Manna, Eugene R. Zubarev,*, and Raisa V. Talroze*,
A.V. Topchiev Institute of Petrochemical Synthesis, RAS, Moscow 119991, RussiaDepartment of Physics, M. V. Lomonosov Moscow State University, Moscow 119992, RussiaDepartment of Chemistry, Rice University, Houston, Texas 77005, United States
bS Supporting Information
INTRODUCTIONThe ability to combine inorganic nanoparticles (NPs) and
organic macromolecules into homogeneous composite materialsis an important goal, which is often difficult to achieve due to astrong tendency of such systems to undergo a macrophaseseparation.13 The incorporation of small spherical NPs intopolymer matrixes has been demonstrated via direct blending,4,5
in situsynthesis of NPs within polymer media,6,7 surface modifica-tion of NPs with monomers followed by polymerization from NPsurface, and grafting of preformed functionalized polymers toNPs.818 However, not only in-matrix dispersion, but also orga-nizationand packingof NPsare verydesirable for the fundamentalstudies of optoelectronic properties of such materials and their use
in novel photovoltaic devices. One of the approaches currentlydeveloped is based on liquid crystals as tunable matrixes providingthe alignment of NPs into larger organized structures in multipledimensions.1923 LC polymers, which combine basic properties ofconventional polymers and liquid crystals, also provide a way tocontrol the arrangement of anisotropic nanostructures within thematrix. These include oriented single wall carbon nanotubes in anematic LC polymer24 and gold nanorods in lyotropic LCs. Wehave previously used hydrogen-bonded LC polyacrylates as ma-trixes that govern the localization of CdSe quantum dots (QDs)due to the breaking of H-bonded dimers and the interaction of thepolymer carboxylic groups with the QDs.25,26 It resulted in theformation of two-dimensional QD nanolayers embedded into LC
polymer matrix. Unlike spherical (QDs) nanostructures, aniso-metric nanorods (NRs) are expected to be aligned by the LCpolymer matrix and control their orientation in a complexcomposite material.
The primary goal of this Article is to introduce a nanocompo-site system, in which a LC polymer serves as a matrix for CdSnanorods, and to describe their morphology, structure, andoptical properties. Side-chain LC polymers, poly[4-(n-acryloyl-oxyalkoxy)]benzoic acids (BA-nPA) with a varying length ofalkyl spacer, were used in this systematic study. These polymersare organized in smectic C phase amenable to macroscopicalignment when mechanical field is applied. For this study,CdS NRs with a diameter comparable to the thickness of thesmectic layers were chosen to preserve the mesophase structure.As a control system, we have also used an amorphous copolymerof norbornene and methylmethacrylate.
Synthesis of CdS NRs. In a 100 mL three-necked round-bottomflask was mixed 7 g of trioctylphosphine oxide (TOPO) with 830 mg oftetradecylphosphonic acid (TDPA) and 230 mg of CdO. The mixture
Received: August 20, 2011Revised: September 11, 2011
ABSTRACT: We report on the structure, uniaxial orientation, and photo-luminescent properties of CdS nanorods that form stable nanocompositeswith smectic C hydrogen-bonded polymers from the family of poly(4-(n-acryloyloxyalkoxy)benzoic acids. TEM analysis of microtomed films of nano-composites reveals that CdS nanorods form small domains that are homo-geneously distributed in the LC polymer matrix. They undergo long-rangeorientation with the formation of one-dimensional aggregates of rods whenthe composite films are uniaxially deformed. The Stokes photoluminescencewas observed from CdS NRs/LC polymer composites with emission peaklocated almost at the same wavelength as that of NRs solution in heptane. Ananti-Stokes photoluminescence (ASPL) in polymer nanocomposites was foundunder the excitation belowthe nanoparticlesgroundstate. The mechanism of ASPL was interpreted in terms of thermally populated states that are involved in the excitation process.These nanocomposites represent an unusual material in which the optical properties of anisotropic semiconductor nanostructurescan be controlled by mechanical deformation of liquid-crystalline matrix.
8/3/2019 Alexander A. Ezhov et al- Liquid-Crystalline Polymer Composites with CdS Nanorods: Structure and Optical Properties
13354 dx.doi.org/10.1021/la203258r |Langmuir2011, 27, 1335313360
was heated under argon atmosphere to 80 C for 1 h. During this heatingprocess, the mixture melted andturned into a dark redliquid.The liquidwasrapidly heated to 320 C and was kept at that temperature for 30 min. Thisprocess allowed CdO to form a complex with TDPA rendering the mixtureclear and colorless. The temperature was then decreased to 300 C, and6 mL of 0.281 M sulfursolutionin trioctylphosphine (S-TOP) wasinjectedrapidly uponvigorous stirring. Thereactionmixturewas keptat 300Cwithcontinuous stirring for another 1.5 h. After that, 14 mL of S-TOP solution
was introduced into thereaction mixture in a dropwisefashionat a rate ofca.0.2mL/min. When theaddition wascomplete, the heating wasstoppedandthe reaction mixture was allowed to cool. When the temperature reached80 C, the reaction mixture was transferred into several glass vials (30 mL),and10-fold excessof acetone/methanol mixture (50%vol) was added. The
vials were centrifuged at 3000 rpm for 5 min to obtain a yellow precipitate.
The precipitate was dissolved in chloroform. NRs werethenprecipitatedbyaddingexcess acetone, followed by centrifugationat 8000 rpmfor 4 minandsubsequent removal of the supernatant.
Polymer Synthesis. Monomeric 4-(n-acryloyloxyalkoxy)benzoicacids (BA-nA) were synthesized as described previously27 and polym-erized in benzene at 63 C for 75 h to form the polymer, BA-nPA. 2,20-
Azoisobutyronitrile (0.1% of the total weight of the monomer) was usedas an initiator. Polymers were precipitated from the benzene solution atroom temperature, then separated, dissolved in tetrahydrofuran (THF),reprecipitated with benzene, and dried overnight under vacuum. Theisolated yield of resulting polymers was 8090%.
Preparation of LC PolymerCdS NRs Composite Films. LCpolymer was dissolved in2 mL of THF at a concentration of 0.10.5wt%,andthe solution wasstirred at room temperature for2 h. CdSNRs solution
0.2wt %) wasadded dropwise to a polymer solutionunder vigorous stirring at room temperature. After 20 min of stirring, thecomposite product was precipitated from the solution by adding a 5-foldexcess of hexane. The precipitate was separated, filtered, and washed 23times with hexane and then dried under vacuum. To prepare film-likesamples, the resulting composite powder was heated between two poly-imide films at 200 C and pressed down to a designated thicknesscontrolled by a metal foil spacer (0.050.2 mm) under a constant load(23 kg).
Characterization of Composites. DSC curves were obtained ona differential scanning calorimeter DSC823e (Mettler Toledo) at aheating rate of 10 K/min under argon atmosphere. IR spectra wererecorded with IFS66 v/s(Bruker) (50 scans). Samples were prepared astablets from the powder pressed together with KBr. High temperature
FTIR measurements were carried out with in situ temperature cell(Bruker). TEM images and small area electron diffraction (SAED)patterns were obtained on transmission electron microscope LEO912
AB OMEGA (Carl Zeiss) operating at 100 kV voltage. Samples wereplaced on copper grids coated by Formvar film. Filmed samples (75 nmthick) were sliced by ultramicrotome Ultracut (Reichert-Jung) equipped
with an ultra diamond knife (DiATOME). SAED patterns were obtainedat accelerated voltage 100 kV and drawtube length 290 mm. To establishthecrystalstructureof CdSNRs, theSAED patterns of NRswere analyzedin terms of the equation R = L/dhkl that describes the relationship
between the radius Rof the peak in SAED pattern and dhkl spacing (L isthe hardware constant, and is the wavelength of the electrons). Thedetermination of the constant L in the equation was performed with thehelp of the SAED pattern of the well-known Au foil. These d-spacings
were 0.23553 nm (111), 0.20396 nm (200), 0.14422 nm (220),0.12299 nm (311), and 0.11776 nm (222). After the constant Ldetermination, the dhkl spacings for CdS were calculated from the SAEDpattern. The measured and calculated dhkl spacings were compared to dhklspacings of Greenockite and Hawleyite (cadmium minerals that consistofCdS in hexagonal and cubic forms, respectively). The spacings of Green-ockite were directly received from the published XRD data available fromthe PRUFF Project Database, whereas dhkl spacings of Hawleyite werecalculated with the use of Bragg equation from simulated XRD powderpattern created with the aid of free software Mercury 2.4 (Build RC5)from the Cambridge Crystallographic Data Centre (CCDC) and data fileobtained from Crystallography Open Database (COD) CIF collection.
Additionally, the location of the peaks in SAED patterns was compared tothose in simulated XRD powder patterns for Greenockite and Hawleyite,
which were calculated as described previously.X-ray diffraction measurements were performed using monochro-matic Cu K
radiation with a wavelength = 0.15148 nm generated by a
microfocus X-ray source (Rigaku Micromax M002+) and a 3-pinholecollimator (JJ-X-ray). The beam diameter was 0.2 mm. The scatteredintensity was detected by a two-dimensional gas detector (1024 1024pixels, Bruker HISTAR). As a calibration standard, silver behenate wasused with d001 = 5.838 nm. All measurements were performed with theincident beam normal to thefilm surface and in the center of the sample.Simultaneous measurements of the wide-angle and small-angle reflec-tions were done at a sample-to-detector distance of 232 mm. For aprecise determination of the smectic layer spacing, additional measure-ments were performed in a SAXS setup with a sample-to-detectordistance of 1759 mm.
Figure 1. TEM images of CdS NRs at low (A) and high magnification(B). Inset shows thediffraction pattern collected from thecentral area in panel A.
8/3/2019 Alexander A. Ezhov et al- Liquid-Crystalline Polymer Composites with CdS Nanorods: Structure and Optical Properties
13355 dx.doi.org/10.1021/la203258r |Langmuir2011, 27, 1335313360
UVvis absorption spectra of CdS NRs were measured in hexane
and heptane solutions by double-beam spectrophotometers SpecordUVvis, equipped with ADC for registration, and Specord M 82 (CarlZeiss Jena). Blank hexaneor heptane wasused as a reference. Theopticalpath was 1 cm.
The photoluminescence (PL) steady-state emission spectra of CdSNRs in solutions were collected on LS-55 (PerkinElmer) luminescencespectrometer and Hitachi F-4010 fluorescence spectrophotometer.Local laser beam PL spectra from hexane solution of CdS NRs, driedCdS NRs, and NRs composite films were obtained on a LabRAM HRRaman microscope (HORIBA Jobin Yvon). The argon ion laseroperating at 488.0 nm was used as a light source. The density of thelaser power on a sample surface was varied from 0.5 to 50 W cm2. PLspectra of solutions were measured in a quartz cell, and quartz coverslides were used for dried CdS NRs and NRs composite films. AllLabRAM HR Raman microscope measurements were carried out in theepi-illumination mode.
RESULTS AND DISCUSSION
Structure of LC PolymerCdS NRs Composites. TEMexamination shows that freshly prepared CdS nanostructures(Figure 1) have a rod-like shape (NRs) with an averagediameter ca. 5 nm and the length ranging from 30 to 100 nm.TEM image of a high concentration sample (Figure 1A) clearlydemonstrates that rod-like morphology is predominant underthese synthetic conditions. However, the imaging at highermagnification and lower concentration (Figure 1B) also revealsthe presence of branched structures and small fraction of
tetrapods. The crystal structure of CdS nanorods was analyzedon the basis of the small area electron diffraction (SAED)pattern (Figure 1, inset), which was collected from the centralarea shown in Figure 1A. The comparison ofdhkl spacings fromSAED patterns (see Supporting Information Figure S1 andTable S1) with XRD data of Greenockite CdS (wurtzitehexagonal phase) and Hawleyite CdS (zinc blende cubic phase)obtained with simulated XRD data shows that CdS NRs havea wurtzite crystal structure, which is consistent with the litera-ture reports.
The FTIR spectrum of isolated CdS NRs indicates thepresence of TOPO ligands on their surface, which generatesignals similar to those observed in pure TOPO (Figure 2A,
spectra 1 and 2, respectively). Strong spectral bands in700900 cm1 (PC bonds) and 10001200 cm1 (PdO bonds) regions are observed for a crystalline TOPO, whichbecome much broader and slightly shifted in its molten state
(Figure 2A, spectrum 3). The spectrum of CdS NRs (Figure 2A,spectrum 1) is consistent with the spectrum of molten TOPO,and one specific band appears at the same position in bothspectra (PdO at 1170 cm
1). Similarly, a characteristic sharpsignal is present at 1490 cm1 in these two spectra. In contrast,there is an intense band at 1102 cm1 in the CdS NRs (notpresent in pure TOPO), which may be assigned to the PdObond strongly coordinated to CdS. This shift in CdS/TOPOspectrum indicates that the PdO -bond is strongly delocalizeddue to its interaction with the CdS surface. On the other hand,spectral bands related to PC bond (700900 cm1 range) aresimilar in both systems (CdS NRs and pure TOPO). Theseresults prove that CdS nanostructures are coated with TOPO
Figure2. Comparison of FTIR spectra. (A)CdS NRscapped with TOPO (1), TOPO in thecrystalline(2) andmolten states(3), BA-7PApolymer (4),and its composite with 33 vol % CdS NRs (5). (B) BA-7PA polymer (1) and its composite containing 17 vol % CdS NRs (2).
Figure 3. SAXS curves of BA-10PA polymer (top) and its compositecontaining6.5 vol % of CdS NRs (A). X-raypatterns of uniaxially orientedfilms of the pure polymer (B) and its composite with CdS NRs (C).
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molecules, which strongly interact with the inorganic surface, butare not organized into a crystalline state. Figure 2A (spectra 4 and5) and B shows the spectra of CdS/TOPO composite with BA-7PAin comparison with the spectrum of thepure polymer. There
is no significant difference between the spectra of the polymerand its composite with CdS NRs. Moreover, there are no anyspectral signs for the presence of TOPO in the composite. Theremay be two reasons for the absence of TOPO related spectralbands. The first reason could be related to the low content ofCdS/TOPO within the composite (33%). The second reasoncould be the partial substitution of TOPO by a polymer on the
NRs surface followed by washing of free TOPO molecules fromthe composite in the process of its preparation. It is conceivablethat some TOPO ligands still remain on the surface of nanorods,but the sensitivity of FTIR is not sufficiently high to confirm it.Nevertheless, the absence of strong characteristic TOPO bandsin the spectrum of nanocomposite suggests that the majority ofthese ligands has been replaced by the polymer. We do not haveany specific spectral signature of the bonding between the matrixand CdS except for the change in the relative intensity of thespectral bands at 2552 and 2670 cm1 , which are slightlydecreased in the spectrum of CdS/BA-7PA (Figure 2B). Thesebands correspond to the vibration of OH bond in the cyclicdimers of carboxylic groups. The identical change in the spectraof a similar polymer BA-6PA was observed upon increasing the
temperature and was attributed to the decrease in the number of
Table 1. Thermal Characteristics of PB-nPA and TheirComposites with CdS NRs
polymer/CdS content, vol % Tg, C Tis, C
BA-7PA 95 181
6.5 97 174
9 97 178
17 97 178
33 96 176
38 97 181
BA-10PA 77 180
6.5 78 175
9 80 180
12 78 179
17 81 179
38 76 179
Figure 4. TEM images of BA-7PA composite containing 6.5 vol % of CdS NRs before (A,B) and after the uniaxial deformation (C). Double arrowindicates the direction offilm stretching, and the insets show the respective ED patterns.
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hydrogen-bonded dimers.28,29 We also can envision that theabove spectral changes in our system result from the conforma-tional changes in the side groups of LC polymers, when they arebrought into immediate vicinitywith the surfaceof CdSnanorods.
The presence of CdS NRs in the composites was confirmed byX-ray scattering data. As an example, the XRD patterns of BA-10PA and its composite are given in Figure 3. The tilted SmC
structure of the BA-nPA family of polymers30,31
is characterizedby an amorphous halo in the angular region 2 = 1330(Figure 3A, black curve) related to the distance between theside chains of the macromolecules and two small angle peaks (2 2.6 and 5.2) corresponding to the first- and second-orderreflections of the smectic H-bonded layers. Incorporation of CdSNRs into the polymer matrix leads to a slight shift (2 2.9 and5.8) in the small angle maxima (Figure 3A, red curve), whichindicates a 0.5 nm increase in the average value of the interlayerspacing. Most importantly, a well-defined maximum at 2 = 27(d= 0.325 nm) appears in the form of a spike on the amorphoushalo, which is related to the crystal structure of CdS NRs.Comparison of the dhkl spacings calculated from XRD patternof pure CdS NRs (see Supporting Information, Table S1) with
that of CdS NRs embedded into LC matrix (Figure 3B) allows usto assign this peak at 2 = 27 to the d002 spacing of the wurtzitephase (0.3315 nm).
To assess the interactions between the continuous LC matrixand the dispersed phase of CdS NRs, we investigated the phase behavior of various nanocomposites by differential scanningcalorimetry (DSC). Table 1 summarizes the glass transitiontemperatures (Tg) and LCisotropic phase transitions of com-posite samples. One can see that the observed transition tem-peratures do not change significantly with the insertion of asmuch as 38 vol % of CdS NRs. These data confirm that noappreciable distortion of the polymer matrix takes place and thatthe smectic mesogenic ordering is fully preserved. On the otherhand, it also suggests a fairly weak interaction between the NRs
and the polymer matrixes. This phase behavior is consistent withthe X-ray data (Figure 3A) showing only a slight increase in theinterlayer distance.
The internal structure and morphology of the compositeswere analyzed by TEM imaging of microtomed films measuringca. 75 nm in thickness (Figures 4). CdS NRs were found to formsmall domains (100200 nm) in the LC polymer matrix.Although the interaction between BA-nPA polymers and CdSNRs appears to be fairly weak, these domains are evenly distributedthroughout the LC matrix, and the composites remain stablewithout a macroscopic phase separation (Figure 4A,B). How-ever, the system is not entirely homogeneous, and some segrega-tion of NRs takes place. The packing density of NRs inside thedomains is relatively low and does not change appreciably in any
LC polymer used in our studies. We also performed a controlexperiment by dispersing CdS NRs in an amorphous matrixof norbornenemethyl methacrylate copolymer. As shown inFigure S2, there is a significant phase segregation manifested bya much higher packing density of CdS NRs (see SupportingInformation Figure S2). This comparative study supports theidea that the carboxylic acid groups of the substituted benzoicacids are capable of at least partial replacement of TOPOligands and favorable interaction with the surface of CdS NRs.In contrast, whenthepolymer lacks functional groups (norbornenemethyl methacrylate copolymer) that could strongly bind thesurface of NRs, only a limited dispersion of nanorods can beachieved.
The uniaxial deformation of the composite films above the
glass transition temperature results in a partial coalescence ofsmall domains and the formation of elongated wire-like struc-tures (Figure 4C). A combination of TEM images and EDpatterns clearly demonstrates that the CdS nanocrystals aremainly oriented along the stretching direction.32 The orientationof NRs proceeds together with the alignment of the polymermatrix as previously shown by the XRD pattern (Figure 3C).Four condensed wide angle maxima with an alternating azi-muthal distribution indicate the smectic C tilt within thelayers.30,31 The small angle maxima in XRD patterns of thecomposite system are localized at the equator (Figure 3C) andserve as a proof for the alignment of the smectic layers along thestretching axis.33As for 002 reflection of CdS NRs, it appears astwo well-defined arcs at the meridian of the XRD pattern
(Figure 3C). This meridian position proves that (002) planesof CdS NRs are oriented perpendicular to the stretching direc-tion. Because  is known to be the growth direction of CdSNRs (perpendicular to their long axis), one can conclude that thenanorods are mainly aligned parallel to the direction of externalmechanical field. This important information is not only inferredfrom the X-ray analysis, but is strongly corroborated by directTEM visualization (Figure 4C).
Optical Properties of Composites. Generally, the wave-length of the exciton absorption decreases with the particles sizedue to the quantum confinement of photogenerated electro-nhole pairs. Typical absorption and PL spectra of CdS NRs inhexane solution (Figure S2) exhibit a well-defined absorptionpeak at about 447 nm, which is considerably blue-shifted relative
to the peak of bulk CdS, indicating the quantum confinementeffect.10,11 If CdS NRs are excited at 380 nm, the emission peakappears at 466 nm, which implies that the Stokes shift is 19 nm(0.12 eV). This PL maximum is significantly blue-shifted fromthe macroscopic band gap of bulk CdS (500515 nm), which isconsistent with the strong confinement model34 and the nano-rods diameter of 5 nm.35,36 This well-defined maximum can beassigned to the optical transition of the first excitonic state.
The influence of the polymer matrix on PL properties wasstudied with the 2 wt % CdS NRs composites with amorphousand LC polymers. Figure 5 shows the absorption and PLspectrum of CdS NRs in heptane solution (excitation at400 nm) as well as PL spectra of NRs in amorphous and LC
Figure 5. UVvis absorption (1) and PL spectra (excitation wave-length 400 nm) of CdS NRs in heptane solution (2), in amorphousmatrix (3), and in LC polymer (BA-10PA) matrix (4) at 2 wt % content.
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polymer films. One can see that the emission peak of NRs doesnot significantly change when their dispersion in LC matrix(467 nm) is compared to a homogeneous heptane solution
(468 nm). In contrast, the peak maximum moves up by 7 nm ifthe amorphous polymer matrix is used. The intensity of thesecond broad PLpeak in 570700nm range is negligibly small inthe spectrum of CdS NRs in solution, but it strongly increases ifNRs are dispersed in polymer matrixes. This peak may beinterpreted as a PL of the defects (traps) usually localized atthe nanoparticles surface. The intensity ratio between the exitonand trap peaks becomes much lower in the amorphous matrix incomparison with the LC polymer. This fact together with the7 nm red shift of the exciton peak in the amorphous matrix maybe explained by much greater phase segregation of CdS NRs(Figure S3) followed by reabsorption of the high energy com-ponent of the exciton spectrum.
When the photoluminescence of CdS NRs in LC polymermatrix was induced at a wavelength longer than the PL emissionpeak, we observed an anti-Stokes photoluminescence (ASPL).
As an example, the PL spectrum of BA-7PA composite contain-ing 33 wt % of CdS NRs obtained under the excitation with theAr+-laser (488 nm) is given in Figure 6. The intensity of the anti-Stokes part of the spectrum is comparable to Stokes lumines-cence, whereas the PL maximum is located below the wavelengthof the excitation source. This phenomenon was previouslyobserved in various systems, which include bulk semiconductors,epitaxial heterostructures, and self-assembled quantum dotlayers.3743 However, there are no reports to date on theobservation of ASPL in CdS nanorods.
There are three mechanisms that may explain ASPL, a two-photon absorption recombination,4446Auger effect,47 and sur-face state processes.40 Whereas the first two mechanisms are
Figure 6. PL spectrum of BA-7PA composite with 33 vol % of Cd S NRs (A). The part of the same spectrum presented on the energy scale (B) thatshows the ASPL shift. Dotted lines indicate the wavelength and energy position of the exciting laser beam.
Figure 7. PL spectra of BA-7PA composite with 33 vol % CdS NRs recorded at different power density of the excitation light (A) at 488.5 nm and theintensity of the exciton (480481 nm) and trap (676677 nm) PL peaks as a function of the power density (B).
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nonlinear and require the initial photon to populate an inter-mediate state, the ASPL via surface state processes involvesthermally populated midgap states, which absorb a single photonleading to higher energy luminescence. In addition, surface stateprocesses show single photon power dependences.43 The corre-sponding ASPL spectra (A) and the intensity of ASPL peak (B)are shown in Figure 7 at different power density. One can see that
the intensity of ASPL depends linearly on the power density ofthe exciting radiation. The same linear curve corresponds to thedependence of PL intensity of surface traps on the power density.This linear relationship offers definitive proof that the ASPL ofCdS NRs is governed by the surface state processes.
In summary, we described the first example of CdS NRscomposites in LC polymers. The distribution of the smalldomains of NRs in the LC matrix is much more homogeneousthan in amorphous matrix, as confirmed by TEM analysis ofmicrotomed composite films. Importantly, the combination of X-ray and TEM data unambiguously proves that nanorodsundergo long-range orientation when the composite films areuniaxially deformed. In addition, an interesting formation ofone-dimensional aggregates of rods is observed within the LC
polymer matrix. We also demonstrated that the extent of CdSNRs microphase separation strongly influences their opticalproperties. These include the changes in the intensity, width,and position of the Stokes exiton emission peak as well as theratio of intensities of the Stokes emission over trap emissionpeaks. The position of the CdS NRs emission peak in an LCmatrix is nearly the same as it is in a homogeneous solution,whereas it is red-shifted in the amorphous matrix. The number ofdefects is much higher in the amorphous phase, and the trapemission prevails over the exciton emission. An anti-Stokesphotoluminescence (ASPL) in polymer nanocomposites isfound under the excitation below the nanoparticles ground state.Experimental evidence indicates that thermally populated statesare involved in the ASPL process.
bS Supporting Information. Experimental details describ-ing the XRD data and TEM images of nanocomposites, andUVvis spectra of CdS NRs. This material is available free ofcharge via the Internet at http://pubs.acs.org.
TEM and SAED measurements were performed in theResearch Center Transmission Electron Microscopy , andUV vis absorption and PL measurements were performed inpart in the Research Center Technology of the productionand complex investigation of new nanostructured materials ofM. V. LomonosovMSU. WearethankfultoProf. V. Yu.Timoshenkofor helpful discussions. This work was supported by the Minis-try of Education and Science of the Russian Federation(902.740.11.516, P914, P918) and the Presidium of the RussianAcademy of Sciences (no. 21). E.R.Z. acknowledges financialsupport provided by NSF (DMR-0547399, DMR-1105878).
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Liquid Crystalline Polymer Composites with CdS Nanorods:
Structure and Optical Properties
Alexander A. Ezhov,1,2
Georgii A. Shandryuk,1
Galina N. Bondarenko,1
Alexey S. Merekalov,1
Sergey S. Abramchuk,2
Alina M. Shatalova,1
Eugene R. Zubarev,3,*
Raisa V. Talroze1,*
1A.V. Topchiev Institute of Petrochemical Synthesis, RAS, Moscow 117912, Russia
2Moscow State University, Department of Physics, Moscow 119992, Russia
3Rice University, Department of Chemistry, Houston, TX 77005, USA
Figure S1. Small area electron diffraction (SAED) pattern with the identification of the
reflections. The pattern was collected from the area shown in Fig. 1A in the main text.
Table 1. Interplanar spacing dhkl of the CdS crystal lattice planes, plane indexes and the
relative scattering intensity.
CdS NRs CdS Greenockite CdS Hawleyite
dhkl,nm dhkl,nm hkl Intensity* dhkl,nm hkl Intensity*
0.36 0.35325 (100) 0.62 - - -
0.34 0.33150 (002) 0.48 - - -
- - - - 0.336 (111) 1.00
0.32 0.31176 (101) 1.00 - - -
- - - - 0.291 (200) 0.210.25 0.24173 (102) 0.29 - - -
- - - 0.206 (220) 0.420.21
0.20395 (110) 0.47 - - -
0.19 0.18736 (103) 0.44 - - -
- - - 0.175 (311) 0.27
0.17663 (200) 0.07 - - -0.18
0.17371 (112) 0.37 - - -
0.17067 (201) 0.15 - - -0.17
- - - 0.168 (222) 0.04
- 0.16575 (004) 0.03 - - -
- 0.15588 (202 0.06 - - -- 0.15005 (104) 0.03 - - -
- - - - 0.145 (400) 0.04
0.14 0.13797 (203) 0.14 - - -
- 0.13352 (210) 0.05 0.133 (331) 0.06
0.13 0.13089 (211) 0.11 0.130 (420) 0.03
- - - - 0.119 (422) 0.05
- - - - 0.112 (511) 0.03
Figure S2. UV-Vis absorption (1) and PL (2) spectra of CdS NRs in hexane solution (excitation
wave length = 380 nm). The small peak marked with at 430 nm corresponds to C-H vibration
band in the Raman spectrum of the solvent.
Figure S3. TEM images of amorphous (A) and LC polymer composites (B) containing 2 wt. %