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

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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.
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