Poly(N-Vinylcarbazole) (PVK) Photoconductivity Enhancement Induced by Doping with Cds Nanocrystals Through Chemical Hybridization

J. Phys. Chem. B 2000, 104, 11853-11858 11853

Poly(N-vinylcarbazole) (PVK) Photoconductivity Enhancement Induced by Doping with CdS Nanocrystals through Chemical Hybridization

Suhua Wang,† Shihe Yang,*,† Chunlei Yang,‡ Zongquan Li,‡ Jiannong Wang,‡ and Weikun Ge‡ Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, and Department of Physics, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ReceiVed: February 9, 2000; In Final Form: June 7, 2000

We have functionalized poly(N-vinyl carbazole) (PVK) by controlled sulfonation. CdS nanocystals of 3-20 nm across were synthesized in the sulfonated PVK matrix with the CdS molar fraction of ∼1-18%. The CdS nanoparticle size increased with the molar fraction of CdS. At high CdS molar fractions, the CdS nanocrystals exist in both cubic and hexagonal phases. Photoluminescence efficiency of PVK decreases when the molar fraction of CdS increases due to quenching through interfacial charge transfer. Photoluminescence attributable to the CdS nanocrystals can be observed only at low molar fractions of CdS. Significant enhancement in photoconductivity induced by the chemical doping of CdS in PVK has also been demonstrated.

Introduction formation of the CdS-PVK nanocomposite. The maximum molar fraction of CdS in the nanocomposite was ∼18% (∼5% Nanocomposites consisting of inorganic nanoparticles and v/v) based on the sulfonation degree of PVK, as determined by organic polymers often exhibit a host of mechanical, electrical, both X-ray photoelectron spectroscopy (XPS) and secondary optical and magnetic properties, which are far superior compared 7 1-4 ion mass spectrometry (SIMS). The average size of the with those of the individual components. These desirable particles, as determined using transmission electron microscopy properties are derived from a complex interplay between the (TEM), was 3-20 nm depending on the molar ratio CdS:PVK. building blocks and the interfaces separating the building The absorption edge is blue-shifted by 15-70 nm from that of blocks. bulk CdS. Decreased photoluminescence and enhanced photo- Poly(N-vinyl carbazole) (PVK) is a hole transport organic conductivity due to the dispersion of CdS nanoclusters in PVK semiconducting polymer. It has been widely used as an matrix have been observed. electronic and optical material.4 CdS is a well-known inorganic semiconductor. Hybrids of CdS nanoclusters and PVK promise Experimental Section both the excellent carrier generation efficiency and mobility of 6 the inorganic semiconductor and the processibility of the organic 1. Synthesis of Sulfonated PVK . A 0.76-mL (8.1 mmol) + polymer. Indeed, a photoconductivity enhancement of PVK has aliquot of acetic anhydride (99 %, Aldrich) was dissolved in been observed when CdS nanoclusters were finely dispersed in 4.0 mL of 1,2-dichloroethane in a 50-mL flask. Then, 0.28 mL the PVK matrix. In this case, the CdS nanoclusters act as a (5.0 mmol) of 95% sulfuric acid was added in a dropwise ° sensitizer for the photogeneration of charges and the PVK manner into the aforementioned solution at 10 C, and a polymer serves as the carrier transporting medium.5 transparent colorless solution was obtained. The concentration It should be pointed out that the nanocomposite CdS/PVK of acetyl sulfate in this solution was 1.0 M. The solution was reported previously was prepared simply by mixing PVK and stored for later use in the sulfonation of PVK. 4 PVK (secondary standard, Aldrich) was dried in a vacuum CdS nanoclusters or their precursors. This procedure introduced ° inevitably capping molecules or precursor molecules aside from oven at 50 C for 10 h before use. PVK (250 mg) was dissolved PVK and CdS. The effect of these molecules on the electrical in 5.0 mL of tetrahydrofuran (THF) at room temp. Seven and optical properties of the nanocomposite remains to be identical solutions were prepared in this way. A different amount investigated. We have recently taken a new approach for the of the sulfonating agent was added in a dropwise manner into preparation of a truly two-component nanocomposite of CdS- each of the seven PVK solutions under magnetic stirring, PVK by direct chemical hybridization. The thrust for pursuing resulting in seven solutions labeled as PVK-00, PVK-10, PVK- this synthetic approach is to better control surface properties of 15, PVK-30, PVK-40, PVK-50, and PVK-60 (Table 1). The the CdS nanoparticles in the nanocomposite for the enhancement two-digit numbers in the sample labels indicate rough molar ratios (x100) of the sulfonating agent to PVK. These solutions of photoconductivity. The electronic, structural, and composi- ∼ ° tional properties of the nanoparticle surface are the key to the containing the sulfonating agent were heated to 75 C and engineering of interfacial charge-separation characteristics in refluxed for 5 h. Then, 1.0 mL of ethanol was added to terminate 6 ∼ the nanocomposite. Our method consists of (1) sulfonation of the sulfonation reaction. At this stage, 20 mL of cyclohexane PVK,6 (2) preparation of the precursor PVK(SO ) Cd, and (3) was added to the solution to precipitate the sulfonation products. 3 2 The precipitate was vacuum filtrated, washed with ethanol (for samples with a ratio of sulfonation agent to carbazole of PVK * Corresponding author (E-mail: [email protected]). < † Department of Chemistry. 40%) or cyclohexane (for samples with a ratio of sulfonation ‡ Department of Physics. agent to carbazole of PVK >40%), and dried in a vacuum oven 10.1021/jp0005064 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000 11854 J. Phys. Chem. B, Vol. 104, No. 50, 2000 Wang et al.

TABLE 1: Characteristics of PVK Solutions molar fraction volume fraction fraction of of CdS in of CdS in product sulfonation, % PVK-CdS PVK-CdS (%) PVK-00 0 0 0 PVK-10 2.51 1:80 0.55 PVK-15 5.96 1:34 1.32 PVK-30 11.7 1:17 2.57 PVK-40 16.7 1:12 3.67 PVK-50 18.2 1:11 4.22 PVK-60 25.0 1:8 5.54 at 50 °C overnight. Finally, a light gray powder was obtained. Sulfonation of PVK was also carried out at different tempera- tures, but the optimum temperature for the sulfonation of PVK was 75 °C. The sulfonated PVKs reported in this work were all synthesized at this temperature. Figure 1. XRD pattern of the CdS nanoparticles in the chemically 2. Preparation of Cd-Exchanged Sulfonated PVK. First, derivatized nanocomposites (CdS:PVK ) 1:12). The vertical lines 100 mg of sulfonated PVK was dissolved in 40 mL of THF. indicate the standard XRD stick patterns for (b) hexagonal and (2) Then, 20 mL of 1 M CdCl2 aqueous solution (for PVK-10 and cubic phases of CdS. The peak intensities of the cubic phase are normalized to that of the (311) diffraction and the hexagonal phase to PVK-15, 1.0 g of CdCO3 powder was used) was slowly added to the THF solution. After the mixture was stirred for 24 h, the that of the (103) diffraction. solvent was removed by rotary evaporation at 75 °C for 30 min to precipitate the Cd-exchanged sulfonated PVK from the surface. The signal from the two electrodes of the sample was solution (for PVK-10 and PVK-15, the mixture was filtered to detected by a SR830 lock-in amplifier. All the measurements were taken for short circuits without external bias. The current- remove remnant CdCO3). The solid was washed six times with double deionized water, and then was dried in a vacuum oven. voltage measurement was carried out using a Hewlett-Packard In XPS spectra of the Cd-exchanged sulfonated PVK, peaks 4115A semiconductor analyzer. corresponding to the Cd 3d core levels were clearly observed The pristine PVK and doped PVK films were prepared by 5/2 - at 406.3 eV. The atomic ratio of cadmium to sulfur shows that spin coating on an indium tin-oxide (ITO) glass substrate. For roughly all the sulfonic acid groups are exchanged by the good film quality, a solvent mixture of THF and chloroform - cadmium ion. (volume fraction of chloroform: 1 10%) was used. After drying the fresh film in vaccuo for 1 h, Al was evaporated on the 3. Formation of the Chemically Derivatized Nanocom- surface of the nanocomposite film. posite CdS-PVK. Cd-exchanged sulfonated PVK (40 mg) was dissolved in 20 mL of THF, and 10 mL of hydrogen sulfide gas was then injected into the solution. Immediately, the Results and Discussion originally colorless solution turned yellow. Argon gas was 1. Nanocomposite Structures. A typical XRD pattern of the bubbled into the yellow solution at a rate of 3 mL/min to remove yellow CdS-PVK nanocomposite powder (PVK-30) is pre- the excessive hydrogen sulfide dissolved in the solution. The sented in Figure 1. Careful analysis of the XRD pattern shows appearance of the yellow color indicated the formation of the coexistence of cubic and hexagonal phases in the nanocom- the chemically derivatized nanocomposite CdS-PVK in the posite. Nearly all the diffraction peaks can be reasonably solution. assigned to the reflections from the lattice planes of the cubic 4. Sample Characterization. The size, shape, and crystal and hexagonal phases of CdS. For the cubic phase, the (200) structures of the CdS nanoparticles were determined using a diffraction peak at the angle of 31° is not obvious probably JEM100 CXII and JEM2010 high-resolution transmission because it is masked by the strong (101) reflection of the electron microscopes (HRTEM). The microscopes, operated at hexagonal phase. The (100) and (101) reflections of the 200 keV, have a spatial resolution of 0.17 nm. TEM samples hexagonal phase are both masked by the strong (111) reflection were prepared by placing a drop of the colloidal suspension on of the cubic phase at the angle of 26.5°, but they are still holey carbon-coated copper grids, the excess solvent was recognizable as two shoulders of the strong peak at the angle evaporated, and the sample was dried in a vacuum. X-ray of 26.5°. Furthermore, as can be seen from the XRD pattern, diffraction measurements were carried out on a PHILIP8338 the cubic phase is predominant over the hexagonal phase in model diffractometer with Cu KR incident radiation. The the CdS-PVK nanocomposite. samples for X-ray diffraction measurements were prepared by It is noteworthy that the XRD pattern of our CdS nanopar- rotary evaporation of the CdS nanoparticle suspension to a fine ticles synthesized at room temperature is similar to that of the powder. CdS nanoparticles that were prepared by annealing at 300 °C.8 Ultraviolet-visible (UV-vis) absorption spectra were ob- At 300 °C, transition occurs from cubic (â-CdS or Hwaleyite) tained using a Milton Roy Spectronic 300 spectrometer using to hexagonal (R-CdS or Greenockite) phase; thus, it is reasonable THF solution samples. Photoluminescence spectra were recorded for the coexistence of the two phases in the CdS nanoparticles using a He-Cd laser as the excitation light source at room annealed at this temperature.9 However, our synthesis was temperature for both solutions and films. A Spex500 spectrom- carried out at room temperature and the coexistence of cubic eter, a photomultiplier tube (PMT), and a photon counter were and hexagonal phases may be related to the method we used to used as the detection system. For photocurrent spectroscopy, a produce the CdS-PVK nanocomposite; that is, rapid nucleation 150 W Xe lamp served as a source of white light, which passed and growth from a H2S supersaturated solution. Moreover, the through a Spex1681 monochromator and was chopped at a cubic CdS is a metastable phase at room temperature.10 At the frequency of 400 Hz before being focused onto the sample initial instant of nucleation, the â-CdS phase may form CdS-Induced PVK Photoconductivity Enhancement J. Phys. Chem. B, Vol. 104, No. 50, 2000 11855

Figure 2. TEM images of the CdS nanoparticles in the chemically derivatized nanocomposites (CdS:PVK ) 1:12) at different magnifica- tion scales. preferably to the R-CdS because its isotropy is more compatible with a spherical nucleus than that of the anisotropic hexagonal structure.8 However, the small free energy difference between the cubic and hexagonal phases is likely to result in the formation of hexagonal phase nanocrystals.8 The fabrication of the two-phase CdS nanoparticles in this way could otherwise be attained at a much higher temperature (∼300 °C) by such methods as chemical bath deposition and aqueous solution precipitation at room temperature.8,11 The mean particle size is estimated to be ∼4 nm from the XRD line width using the Scherer equation.12 This value is smaller than that estimated from the TEM image (inset of Figure Figure 3. High-resolution TEM image of the CdS nanoparticles with 2b), which shows CdS particles with sizes in the range 4-20 the CdS:PVK molar fraction of 1:80. nm. This result is understandable because the CdS nanoparticles Figure 3 shows a HRTEM image of CdS nanoparticles prepared at this large molar ratio of CdS:PVK has a broad size prepared with a much lower Cd:PVK molar ratio (1:80). Most distribution, and the XRD peak width is primarily determined CdS nanoparticles are ∼3 nm in size, as can be seen in Figure by the small particles in the ensemble. 3a, in which four nanoparticles are apparent with fringe spacing TEM images reveal CdS nanoparticles in both spherical and of 3.4 Å. Again, this spacing can be assigned either to (111) of cubic shapes (Figure 2). The two CdS nanoparticles shown in cubic CdS (3.36 Å) or to (002) of hexagonal CdS (3.36 Å). Figure 2 are both single nanocrystallites. The CdS nanocube Occasionally, particles as large as 12 nm also can be found has definitely a cubic phase based on the two perpendicular (Figure 3b). The large CdS nanoparticle in Figure 3b has a sets of fringes at d spacings of 2.94 Å (200) and 2.84 Å (200), triangular shape, and its hexagonal structure can be appreciated and one diagonal set of fringes at a d spacing of 1.96 Å (220; from the observed fringe spacings of (100) and (010) (3.60 Å). see arrow in Figure 2). This result is also supported by the 2. Optical Properties. Figure 4 shows the UV-vis absorption selected electron diffraction pattern from the nanoparticle. On spectra of the PVK (curve a), sulfonated PVK (curve b), and the other hand, only one fringe spacing (3.41 Å) was observed the chemically derived nanocomposite CdS-PVK (PVK-30 and for the spherical nanoparticle. This spacing can be indexed either PVK-15; curves c and d, respectively). It is clear that there is to (111) of cubic CdS (3.36 Å) or to (002) of hexagonal CdS not much difference between the UV-vis spectra of the (3.36 Å). It is therefore unclear whether this spherical particle sulfonated PVK and the chemically derivatized nanocomposite has a cubic or hexagonal structure. CdS-PVK, except that a new broad absorption tail appears in We found that the CdS nanoparticle size is very much affected the latter above 370 nm. For the CdS-PVK nanocomposite with by the molar ratio CdS:PVK. The XRD and TEM patterns a molar ratio of 1:12, the absorption starts at ∼520 nm (curve already presented were obtained for a sample with a CdS:PVK d) and increases with decreasing wavelength. The absorption molar ratio of 1:12. For samples with CdS:PVK molar ratios < onset is determined to be 491 nm (2.53 eV) by plotting (Rhν)2 1:34, no X-ray diffraction patterns could be observed, presum- against hν and extrapolating (Rhν)2 to zero (R is the absorption ably due to the small size of the CdS nanoparticles. This result coefficient and hν is the photon energy), where the photon was confirmed by TEM measurements presented next. energy stands for the absorption onset (Figure 4, inset 1). This 11856 J. Phys. Chem. B, Vol. 104, No. 50, 2000 Wang et al.

Figure 4. UV-vis spectra of the (a) PVK, (b) sulfonated PVK, (c) chemically derivatized nanocomposite CdS-PVK (CdS:PVK ) 1:34), and (d) chemically derivatized nanocomposite CdS-PVK (CdS:PVK ) 1:12). Inset 1: plot of (Rhν)2 versus hν, obtained from the spectra labeled as c and d. The absorption edges are fitted to a direct transition.19 Inset 2: an enlargement of the spectra. tail is attributed to the band gap absorption of the CdS When photons are absorbed, electrons were excited into the nanoparticles and thus the band gap is calculated to be 2.53 lowest unoccupied molecular orbital (LUMO) of the carbazole eV. For these CdS nanoparticles in the chemically derivatized groups in PVK and holes were left in the highest occupied nanocomposites CdS-PVK, a particle size of 6.1 nm is molecular orbital (HOMO). The excited electrons, in the pure estimated from the band gap absorption onset using the Brus PVK solution, will return from the conduction band back to equation based on the effective mass approximation for semi- the valance band through a radiative process. In the system of conductor nanoparticles.13 For the CdS-PVK nanocomposite the as prepared PVK-CdS nanocomposites, however, the with a molar ratio of 1:34, a band gap of 2.79 eV is obtained excited electrons can also choose to migrate from PVK to the (curve c), and the average particle size is estimated to be CdS nanoparticles. This possibility is illustrated by the relative 3.8 nm. energy levels of PVK and CdS,11,14 as shown in Figure 6a. Such When the as-prepared chemically derivatized nanocomposite an interfacial charge-transfer brings down the transition prob- (CdS/PVK > 1:34) was excited with laser at a wavelength of ability from LUMO to HOMO and thus reduces the PVK 325 nm, the photoluminescence spectrum did not change, but photoluminescence. the intensity of the photoluminescence was reduced compared Electron-hole pairs in the CdS nanocrystals also form with that of the pure PVK solution with the same concentration through a direct absorption, as shown in Figure 6b. In this case, (Figure 5, curves 1 and 2). As can be seen from Figure 5, the electrons from PVK migrate favorably to CdS to fill the holes, higher the CdS:PVK molar ratio, the lower the intensity of the creating holes in PVK, or equivalently, the holes have been photoluminescence. The PL appears to be from the transition transferred to PVK. As a consequence, for the as-prepared large in the carbazole moieties of PVK, and the band gap emission CdS nanoparticles capped by PVK (Cd:PVK > 1:34), virtually from CdS was not observed. Both the reduced PL of PVK and no band gap luminescence could be observed.15 the absence of band-gap luminescence from CdS can be However, when the surface of the CdS nanoparticles was attributed to the quenching by interfacial change transfer. modified with dodecanthiol, which separates the nanoparticle Because amines (e.g., carbazole moiety in this system) are and PVK, the photoluminescence was significantly enhanced, effective complexing agents with the surfaces of CdS clusters, and in fact, the photoluminescence intensity is nearly comparable the quenching of the fluorescence of PVK in the chemically to that of the pure PVK solution (Figure 5, curve 3). Because derivatized nanocomposite CdS-PVK probably results from the the -SH group of dodecanthiol has a stronger interaction with close contact between the polymer and the CdS nanoparticles. the surface of CdS nanoparticles than the carbazole moiety of This close proximity makes possible the charge carrier transport PVK, it replaces PVK as a capping molecule on the nanoparticle through the interface between PVK and CdS nanoparticles. surface. Furthermore, the formation of the complex between CdS-Induced PVK Photoconductivity Enhancement J. Phys. Chem. B, Vol. 104, No. 50, 2000 11857

Figure 7. Photoluminescence spectra of (a) PVK and (b) CdS-PVK in THF with a CdS:PVK molar ratio of 1:80.

Figure 5. Photoluminescence spectra of pure PVK (curve 1), as- prepared CdS-PVK (curve 2), and thiol-activated CdS-PVK (curve 3) in THF: (a) CdS:PVK ) 1:34; and (b) CdS:PVK ) 1:12. Figure 8. Photoconductivity spectra of PVK and the CdS-PVK nanocomposite with CdS:PVK ) 1:80. Inset: CdS-PVK nanocomposite film configuration for photoconductivity measurements.

separating PVK and the nanoparticle, interfacial carrier transport was significantly hindered. As a result, the PVK in the nanocomposite is similar to that in the pure PVK solution, and thus the emission from the thiol-modified nanocomposite is enhanced compared with that of the as-prepared CdS-PVK nanocomposite. In general, the intensity of the photoluminescence from PVK (375-400 nm) decreases when the filling factor of CdS becomes larger in the CdS-PVK nanocomposite. Moreover, although the band gap photoluminescence was not observed from large CdS nanoparticles (corresponding to high filling factors of CdS), it was detected when the filling factor of CdS is <1:34. For example, with a CdS:PVK molar ratio of 1:80, photoluminescence was detected from the sample in THF solution peaking at ∼430-440 nm (Figure 7). The band gap is estimated to be 2.86 eV, which is apparently due to the presence of CdS. 3. Photoconductivity of the Nanocomposite. For electrical measurements, the CdS-PVK nanocomposite (CdS:PVK ) 1:34) was casted on an ITO glass by spin-coating to form a Figure 6. A schematic illustration of the energy band profiles of the transparent film. The film thickness was ∼150 nm, as deter- - chemically derivatized CdS PVK nanocomposites and the routes for mined with an Alpha-step 200 surface profiler. To form a excitation and charge transfer. The band positions of CdS nanoparticles were estimated based on the electron binding energies as measured by cathode, Al was evaporated on the surface of nanocomposite valence band photoemission.18,20 The curved arrows indicate the film, as illustrated in the inset of Figure 8. The size of the device direction of carrier transfer. is ∼0.5 cm2. To investigate the effect of the CdS nanoparticles doped into Cd2+ and -SH also removed the trapping sites on the CdS the PVK film, we examined the spectral response of the surfaces. Because of the space layer of the dodecanthiol photoconductive device with pure PVK and CdS-doped PVK. 11858 J. Phys. Chem. B, Vol. 104, No. 50, 2000 Wang et al.

Figure 8 shows the wavelength dependence of the short-circuit (PVK-10-CdS) and a blend nanocomposite (containing alky- photocurrent for a pure PVK film and a CdS-PVK nanocom- lthiol-capped CdS nanoparticles) under the same conditions. It posite film. For the pure PVK film, the photocurrent action is encouraging that the photoconductivity enhancement factor spectrum appears to be very similar to the absorption spectrum of the hybrid nanocomposite (2-3) is always larger than that (symbatic response) due to the small film thickness of our of the blend nanocomposite (1-2). Because this nanocomposite sample.16,17 Because only the electrons created by light close sample contains only ∼1 wt % CdS, further photoconductivity to the aluminum electrode will be effective in generating the enhancement is expected by increasing the CdS filling factor photocurrent, the polymer layer between the ITO and aluminum of the hybrid nanocomposite. electrodes should be sufficiently thin to observe the symbatic response. When the CdS nanoparticles are doped into the PVK Conclusion - film, the peak shifts to a longer wavelength by 6 7 nm with a In this work, we synthesized CdS nanocystals of a few long tail. This photocurrent spectrum, again, closely resembles nanometers across in sulfonated PVK matrixes with the CdS the corresponding absorption spectrum. The red-shift of the peak molar fraction of ∼1-18%. The sulfonated PVKs were prepared and the appearance of the long tail indicates the photoabsorption by controlled sulfonation of the PVK polymer. The CdS of the CdS nanoparticles. In addition, the doping of CdS nanoparticles size increased with the molar fraction of CdS. At nanoparticles into the PVK matrix aggrandizes the photocurrent high CdS molar fractions, the CdS nanoparticles exist predomi- efficiency of PVK by a factor of 1.7 at the wavelength indicated nantly in the cubic phase with a minor contribution from the by the vertical dashed line in Figure 8. Considering the hexagonal phase. Significant enhancement in photoconducty due integration of the short-circuit current over the wavelength, the to the chemical doping of CdS in PVK has been demonstrated. enhancement factor of the photocurrent efficiency of the hybrid Photoluminescence results indicate that interfacial electron nanocomposite is estimated to be nearly 3. These results show transfer occurs between the CdS nanoparticles and the PVK that the CdS nanoparticles have broadened the spectral range molecules. This transfer reduces the photoluminescence ef- of the photocurrent response and improved the photoconductivity ficiency, but enhances the photoconductivity of the CdS-PVK of PVK film. nanocomposite. Photoconductivity is a convolution of photoinduced charge generation and charge transport. Because a low density of Acknowledgment. This work was supported by an RGC nanoparticles has a small effect on the transport property of the grant administered by the UGC of Hong Kong. polymer matrix, their main function is the enhancement of the charge generation efficiency.17 The PVK polymer matrix is Supporting Information Available: Figure of conductivity mainly responsible for the charge transport. CdS can serve as data. This material is available free of charge via the Internet electron traps in PVK matrix judging from their band structures at http://pubs.acs.org. (Figure 6).18 After electron-hole pairs (or excitons) are gener- ated by photoabsorption in PVK molecules, which can only References and Notes absorb photons with energy >3.6 eV, the excited electrons can - (1) Colvin, V. L.; Schiamp, M. C.; Alivisatos, A. P. Nature 1994, 370, move to the conduction band of CdS in the CdS PVK 354. nanocomposites (Figure 6a). Such an interfacial charge transfer (2) Dabbousi, B. O.; Bawendi, M. G. Appl. Phys. Lett. 1995, 66, 1316. brings down the recombination probability of electrons and (3) Huynh, W. U.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. holes, and thus increases the lifetime of the holes in the HOMO (4) Wang, Y.; Herron, N. J. Lumin. 1996, 70, 48. of PVK (Figure 6c). As a result, the holes in the HOMO of (5) Wang, Y.; Herron, N. Chem. Phys. Lett. 1992, 200, 71. PVK have more chances to migrate and the photoconductivity (6) Wang, S. H.; Zeng, Z. H.; Yang, S. H.; Weng, L.-T.; Wong, P. C. of the nanocomposite increases. As illustrated in Figure 6b, the L.; Ho, K. Macromolecules 2000, 33, 3232. (7) Weng, L.-T.; Wong, P. C. L.; Ho, K.; Wang, S. H.; Zeng, Z. H.; CdS nanoparticles can absorb photons with energy 2.5 eV < E Yang, S. H. Anal. Chem. 2000, 72, 4908. < 3.6 eV, creating electron-hole pairs in the conduction and (8) Bandaranayake, R. J.; Wei, G. M.; Lin, J. Y. Appl. Phys. Lett. 1995, valence bands of CdS, respectively. The holes in the valence 67, 831. (9) Zelaya-Angel, O.; Alvarado-Gil, J. J.; Lozada-Marales, R.; Vargas, band of CdS have higher energy than that in the HOMO of H.; Ferreira da Silva, A. Appl. Phys. Lett. 1994, 64, 291. PVK, hence will intend to move to PVK, being accumulated in (10) Newman, K. E.; Lastas-Martines, A.; Kramer, B.; Barnett, S. A.; the HOMO of PVK (Figure 6c). It is interesting that the Ray, M. A.; Dow, J. D.; Greene, J. E. Phys. ReV. Lett. 1983, 50, 1466. (11) Vigil, O.; Riech, I.; Garcia-Rocha, M.; Zelaya-Angel, O. J. Vac. excitation of either PVK or CdS has the same effect of creating Sci. Tech. 1997, 15, 2282. a hole in PVK, and this results explains why the CdS (12) Taylor, A. X-ray Metallography; Wiley: New York, 1961; p 674. nanoparticles can enhance the photoconductivity and broaden (13) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 1832. the photocurrent response of PVK (Figure 8). (14) (a) Chao, C. I.; Chen, S. A. Appl. Phys. Lett. 1998, 73, 426. (b) Kido, J.; Shionoya, H.; Nagai, K. Appl. Phys. Lett. 1995, 67, 2281. Perhaps more convincing about the good performance of our (15) Spannel, L.; Hoase, M.; Weller, H. and Henglein, A. J. Am. Chem. hybrid nanocomposite is to make a direct comparison with that Soc. 1987, 109, 5649. of a blend nanocomposite with the same filling factor. Although (16) Harrison, M. G.; Gru¨ner, J.; Spencer, G. C. W. Phys. ReV.B1997, 55, 7831. there are a number of reports on the photoconductivity of blend (17) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV.B1996, nanocomposites of CdS-PVK, direct comparison of the data 54, 17628. with our results is difficult because different experimental (18) Luo, H.; Fundyna, J. K. Semiconductor Sci. Technol. 1995, 10, conditions involved (e.g., different filling factors, substrates, 1041. (19) Pankove, J. I. Optical Processes in Semiconductors; Dover Publica- measurement techniques among other things). Nevertheless, we tions: New York, 1990. examined the photoconductivity of a hybrid nanocomposite (20) Colvin, V. L.; Alivisatos, A. P. Phys. ReV. Lett. 1991, 66, 2786.