Preparation of Nanosized ZnS-Passivated CdS Particle Films via the ...

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Preparation of Nanosized ZnS-Passivated CdS Particle Films via the MOCVD Process with Co-fed Single Source Precursors Yung-Jung Hsu and Shih-Yuan Lu* Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan 30043, Republic of China Received May 1, 2003. In Final Form: November 7, 2003 A novel approach was developed to prepare thin films of nanosized ZnS-passivated CdS particles via a metal-organic chemical vapor deposition (MOCVD) process with co-fed single source precursors of CdS and ZnS. Single source precursors of CdS and ZnS with sufficiently different reactivity, as judged from thermogravimetry analysis, were prepared and paired up to form ZnS-passivated CdS, (CdS)ZnS, and CdS-modified ZnS, (ZnS)CdS, particle films in a one-step process. For comparison purposes, sequential layer growth of CdS/ZnS and ZnS/CdS particle films was also conducted, and single compound particle films were prepared. These films were characterized with absorption spectrometry, photoluminescence spectroscopy, scanning electron microscopy, and powder X-ray diffraction spectra. The photoluminescence efficiency of the resulting composite particle film of ZnS-passivated CdS was significantly enhanced as compared to that of the plain CdS film, due to the effective passivation of surface electronic states of CdS by ZnS, a material with a higher conduction band than that of CdS. As for particle films of CdS-modified ZnS, a decay in photoluminescence efficiency was observed. The enhancement or decay in photoluminescence efficiency was much more pronounced for the passivated and modified system than for the sequential layer system, proportional to the interfacial area between the CdS and ZnS phases.

Introduction Thin films of II-VI semiconductors such as CdS and ZnS are technologically important for their potential applications in optical coatings, solid-state solar cell windows, optoelectronic devices, and light-emitting diodes.1-7 In particular, ternary phase materials (e.g., CdxZn1-xS compounds and the core-shell (CdS)ZnS structure) have attracted much research attention in recent years because of the associated band gap tunability and enhanced optical properties.8-13 Deposition processes for CdS and ZnS films can be roughly divided into two categories: those carried out in gas phases and those in liquid phases. For the former, vacuum evaporation,14 sputtering,15 and chemical vapor * Corresponding author. E-mail: [email protected]. (1) Frigo, D. M.; Khan, O. F. Z.; O’Brien, P. J. Cryst. Growth 1989, 96, 989. (2) Pike, R. D.; Cui, H.; Kershaw, R.; Dwight, K.; Wold, A. Thin Solid Films 1993, 224, 221. (3) O’Brien, P.; Walsh, J. R.; Watson, I. M.; Hart, L.; Silva, S. R. P. J. Cryst. Growth 1996, 167, 133. (4) Motevalli, M.; O’Brien, P.; Walsh, J. R.; Watson, I. M. Polyhedron 1996, 15, 2801. (5) Berrigan, R. A.; Maung, N.; Irvine, S. J. C.; Cole-Hamilton, D. J.; Ellis, D. J. Cryst. Growth 1998, 195, 718. (6) Tsuji, M.; Aramoto, T.; Ohyama, H.; Hibino, T.; Omura, K. J. Cryst. Growth 2000, 214, 1142. (7) Uda, H.; Yonezawa, H.; Ohtsubo, Y.; Kosaka, M.; Sonomura, H. Sol. Energy Mater. Sol. Cells 2003, 75, 219. (8) Cheon, J.; Zink, J. I. J. Am. Chem. Soc. 1997, 119, 3838. (9) Nyman, M.; Jenkins, K.; Hampden-Smith, M. J.; Kodas, T. T.; Duesler, E. N. Chem. Mater. 1998, 10, 914. (10) Youn, H.-C.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1988, 92, 6320. (11) Sato, H.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1995, 34, 2493. (12) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. Colloids Surf., A 1996, 111, 195. (13) Huang, J.; Lianos, P. Langmuir 1998, 14, 4342. (14) Pal, U.; Silva-Gonza´lez, R.; Martı´nez-Montes, G.; GraciaJime´nez, M.; Vidal, M. A.; Torres, S. Thin Solid Films 1997, 305, 345. (15) Taneja, P.; Vasa, P.; Ayyub, P. Mater. Lett. 2002, 54, 343.

deposition3-7 are common practices. As for the latter, chemical bath deposition16 and electrodeposition17 are typical examples. Thin films of nanosized CdS-modified ZnS ((ZnS)CdS) particles and ZnS-passivated CdS ((CdS)ZnS) particles were prepared in this work by a hot-wall metal-organic chemical vapor deposition (MOCVD) process, using cofed single source precursors of CdS and ZnS. Here, single source precursors are chemical compounds containing both metal and chalcogen sources, decomposition of which gives the desired compound semiconductors.2-4 Conventionally, MOCVD processes for preparing II-VI compound semiconductors employ separate metal and chalcogen sources,1,4,7,18,19 which are independently introduced into the reactor. These reagents are normally highly toxic, and the possibility of contamination20 in the deposited film is also a concern, not to mention the complexity involved in handling two separate precursors. In recent years, dialkyldithiocarbamate-based single source precursors were developed to tackle the above-mentioned shortcomings.3,4 This single source precursor approach has been proved to be a simple and yet effective process for preparing highquality II-VI semiconductor films.21,22 We developed in this article a one-step MOCVD process to prepare thin films of nanosized particles of (ZnS)CdS and (CdS)ZnS, through a novel application of the single source precursor concept. The basic idea is to co-feed single (16) Boyle, D. S.; O’Brien, P.; Otway, D. J.; Robbe, O. J. Mater. Chem. 1999, 9, 725. (17) Boone, B. E.; Shannon, C. J. Phys. Chem. 1996, 100, 9480. (18) Zhenyi, F.; Yichao, C.; Yongliang, H.; Yaoyuan, Y.; Yanping, D.; Zewu, Y.; Hongchang, T.; Hongtao, X.; Heming, W. J. Cryst. Growth 2002, 237, 1707. (19) Hsu, C.-T. J. Cryst. Growth 2000, 208, 259. (20) Wright, P. J.; Cockayne, B.; Parbrook, P. J.; Oliver, P. E.; Jones, A. C. J. Cryst. Growth 1991, 108, 525. (21) Nomura, R.; Murai, T.; Toyosaki, T.; Matsuda, H. Thin Solid Films 1995, 271, 4. (22) Tran, N. H.; Lamb, R. N.; Mar, G. L. Colloids Surf., A 1999, 155, 93.

10.1021/la0347410 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/10/2003

Nanosized ZnS-Passivated CdS Particle Films

source precursors of CdS and ZnS of sufficient reactivity difference to the reactor and run the deposition at suitable temperatures according to the thermal decomposition temperatures of the two precursors. The precursor of higher reactivity would be thermally decomposed to form the native particles, and these first-formed particles then serve as a catalyst to promote the thermal decomposition of the precursor of lower reactivity.23 Note that in the presence of a catalyst, the precursor of lower reactivity can react at a lower temperature. As a result, the laterformed material is expected to grow on the surfaces of the first-formed particles. Consequently, an in situ surface passivation or modification is achieved. Cadmium sulfide as an optoelectronic material suffers from the existence of many surface electronic states and consequent degradation in luminescence quantum yield.12,15 Passivation of surface electronic states provided by a material with a conduction band located higher in the electronic energy level than that of CdS, such as ZnS, leads to a significant improvement in luminescence quantum yield.12,24 In this article, we developed a simple and convenient way to introduce this passivation in situ in a one-step process. Experimental Section Chemicals. Cadmium hydroxide (Aldrich, 99%), carbon disulfide (Aldrich, 99.9%), N-dipropylamine (Fluka, 98%), Ndibutylamine (Fluka, 98%), 1,1,1-trichloroethane, acetone, zinc nitrate (Showa, 99%), and sodium hydroxide were all used as received. Zinc hydroxide was prepared by precipitation from a stoichiometric mixture of aqueous zinc nitrate and sodium hydroxide. Synthesis of Single Source Precursors. Several metal dialkyldithiocarbamate, M(S2CNR1R2)2, precursors were prepared to meet the requirement of sufficiently different reactivity pairs. Here, M represents Cd or Zn and R1 and R2 are two alkyl groups adjusting which can vary the physical properties (melting point, volatility) and reactivity of the precursors to some extent. This type of precursor was typically synthesized through an acidbase reaction.3,4 Metal hydroxide M(OH)2, secondary amine HNR1R2, and carbon disulfide CS2 of stoichiometric proportions were mixed to react in boiling ethanol for 2 h. The reaction product was purified with the recrystallization process, and the desired single source precursors were obtained. For single source precursors of CdS, the following alkyl group combinations have been prepared and investigated: methyl and methyl, methyl and propyl, methyl and butyl, and propyl and propyl. As for single source precursors of ZnS, the following alkyl pairs were tried: methyl and methyl, ethyl and ethyl, methyl and propyl, methyl and butyl, propyl and propyl, and butyl and butyl. We judged the reactivity of these single source precursors by thermogravimetry analysis (TGA) and picked precursor pairs with an appropriate reactivity difference. For in situ formation of the CdS-modified ZnS particle film, (ZnS)CdS, we need the ZnS precursor to be of higher reactivity than the CdS precursor. A suitable precursor pair was found to be bis(dimethyldithiocarbamate) of zinc ([Zn(S2CNMe2)2]2, denoted as Zn11) and cadmium dipropyldithiocarbamate (Cd(S2CNPr2)2, denoted as Cd33). As for ZnS-passivated CdS particle films, (CdS)ZnS, cadmium dipropyldithiocarbamate (Cd33) as the higher reactivity precursor was paired up with bis(dibutyldithiocarbamate) of zinc ([Zn(S2CNBu2)2]2, denoted as Zn44). Precursor Zn11 was purchased from Fluka (purity > 97%). Its melting point is 252.8 °C. To prepare Cd33, cadmium hydroxide (7.3 g, 0.05 mol), N-dipropylamine (13.69 mL, 0.1 mol), and carbon disulfide (6.02 mL, 0.1 mol) were suspended in 150 mL of ethanol and stirred at 80 °C for reaction for 2 h. Suspended solids and almost all the solvent were removed under reduced pressure to precipitate the product. The solid product was purified with (23) Lu, S.-Y.; Chen, S.-W. J. Am. Ceram. Soc. 2000, 83, 709. (24) Lu, S.-Y.; Lin, I.-H. J. Phys. Chem. B 2003, 107, 6974.

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Figure 1. Schematic diagram of the hot-wall CVD system: 1, vacuum pump; 2, pressure controller; 3, cold trap; 4, quartz tube reactor; 5, three-zone furnace; 6, quartz plate substrate; 7, precursor heating tube; 8 and 9, precursor boats; 10, molecular sieve; 11, carrier gas. recrystallization in equal volumes of acetone and 1,1,1-trichloroethane. Finally, yellow crystalline Cd33 was obtained (16.53 g, yield of 79%). The preparation procedure for Zn44 was similar to that for Cd33, but the starting reactant mixture was composed of zinc hydroxide (4.95 g, 0.05 mol), N-dibutylamine (16.97 mL, 0.1 mol), and carbon disulfide (6.02 mL, 0.1 mol). After crystallization from acetone, the white product of Zn44 was obtained (17.64 g, yield of 82.9%). MOCVD Process for Preparation of (ZnS)CdS and (CdS)ZnS Particle Films. MOCVD was conducted in a simple reduced-pressure, hot-wall reactor as depicted in Figure 1. Amorphous quartz plates were used as the substrate for deposition of the particle films, and the two precursors were placed in two different precursor boats heated to appropriate temperatures for generation of precursor vapors. For deposition of (ZnS)CdS particle films (referred to as case 1 from here on), precursor temperatures were set at 200 °C for Zn11 and 160 °C for Cd33. As for deposition of (CdS)ZnS particle films (case 2), Cd33 and Zn44 were heated to 160 and 120 °C, respectively. The vapors of both precursors were introduced into the furnace by the carrier gas N2. The furnace temperatures were set at 330, 360, 380, and 400 °C, respectively, for different runs of case 1. As for case 2, furnace temperatures of 360, 380, 400, 450, and 500 °C were used. The deposition was run at a carrier gas flow rate of 200 sccm, a system pressure of 30 Torr, and a deposition time of 3 h. In addition, the sequential layer growth of CdS on ZnS (denoted as ZnS/CdS) and ZnS on CdS (denoted as CdS/ZnS) was carried out at deposition temperatures of 380 and 400 °C, respectively, with a 1.5 h deposition time for each layer. Plain CdS and ZnS particle films were also prepared at corresponding conditions for comparison purposes. Characterization. UV-visible absorption spectra were obtained using a Hitachi U-3300 spectrophotometer. For photoluminescence (PL) spectroscopy, a Hitachi F-4500 equipped with a xenon lamp (150 W) and a 700 V photomultiplier tube as the detector was used. Both absorption and photoluminescence spectra were obtained at room temperature under ambient atmosphere. Photoluminescence quantum yields (Qs) of the samples were determined from the following expression:25

Qs ) Qst(Ast/As)(Ds/Dst) Here, Qst is the quantum yield of the standard substance, tryptophan, known to be 0.16 at a concentration of 0.73 µg/mL in H2O; Ast and As are the absorbance values of the tryptophan solution and samples, respectively, at a wavelength of 320 nm; and Ds and Dst are the corresponding integrated wavenumber values for the sample and tryptophan, respectively, in the PL spectra. Scanning electron microscopy (SEM) graphs were taken with a Hitachi S-4700 operated at an accelerated voltage of 2 kV. All samples were sputtered with thin layers of Au before analysis. Energy dispersive spectrometry (EDS) analysis was performed at an accelerated voltage of 15 kV. (25) Hitachi Scientific Instrument Technical Data, FL No. 24.

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Figure 2. TGA curves of precursors Zn11, Cd33, and Zn44. Case 1 was to pair up Zn11 and Cd33 to prepare composite (ZnS)CdS particle films, and case 2 was for the preparation of (CdS)ZnS particle films using precursors Cd33 and Zn44. Powder X-ray diffraction (XRD) spectra were obtained with a MAC Sience MXP18 diffractometer using Cu KR radiation (λ ) 1.5418 Å). The acceleration voltage was 30 kV with a 20 mA current flux. Data were collected for 2θ in the range of 20-60° with a scan speed of 4°/min and a sample interval of 0.020°. TGA was performed with a Seiko SSC 5000. Measurements were carried out in an inert atmosphere provided by N2 flow of 100 mL/min at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) analysis was performed using a Setaram TGDSC 111. Measurements were carried out in an environment of N2 gas flowing at the rate of 100 mL/min. The heating rate was 5 °C/min. Nuclear magnetic resonance (NMR) measurements were performed using a Varian Uniytinova-500 (500 mHz). It was equipped with a 1H(13C/15N) triple resonance pfg probe on a Sun Ultra10 workstation.

Results and Discussion Reactivity of Single Source Precursors. Results of structural and physical characterizations of the prepared Cd33 and Zn44 are presented here. For Cd33, the main peaks of the 1H NMR spectra were as follows: δ ) 0.88 (3H, t, NCH2CH2CH3), δ ) 1.78 (2H, m, NCH2CH2CH3), and δ ) 3.77 (2H, t, NCH2CH2CH3). Major IR bands were observed at 240 cm-1 (Cd-S), 950 cm-1 (C-S), and 1480 cm-1 (CdN). Mass spectra showed an m/z value of 464, complying with the molecular weight of Cd(S2CNPr2)2. The melting point of Cd33 was determined by DSC to be 152.3 °C. As for Zn44, 1H NMR gave δ ) 0.92 (3H, t, NCH2CH2CH2CH3), δ ) 1.32 (2H, m, NCH2CH2CH2CH3), δ ) 1.70 (2H, m, NCH2CH2CH2CH3), and δ ) 3.74 (2H, t, NCH2CH2CH2CH3). Major IR bands were observed at 450 cm-1 (Zn-S), 966 cm-1 (C-S), and 1500 cm-1 (CdN). Mass spectra gave an m/z value of 946, complying with the molecular weight of [Zn(S2CNPr2)2]2. The melting point was determined by DSC to be 105.3 °C. The relative reactivity (thermal decomposition to form corresponding semiconductors) of the three chosen precursors was judged by thermogravimetry analyses as shown in Figure 2. The weight loss experienced by the sample with increasing temperature was caused mainly by thermal decomposition of the sample to form gaseous byproducts. Thus, TGA can serve as a convenient measure for precursor reactivity. However, one should take the concentration effect into account for judging the reaction rate of the precursor in the CVD reactor where the precursor concentration is much less than that encountered in the TGA. As is evident from Figure 2, Zn11 is the most reactive one among the three, with Cd33 coming

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Figure 3. UV-visible absorption spectra of (ZnS)CdS particle films compared with those of plain ZnS and CdS films at different deposition temperatures. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

next and Zn44 being the least reactive. The curves leveled off at high temperatures because of the formation of ZnS or CdS from the thermal decomposition of the corresponding precursors. The final remaining weight percentages (14, 29, and 12% for Zn11, Cd33, and Zn44, respectively) agreed quite well with the ratios of the molecular weights of CdS or ZnS versus those of the corresponding precursors (16, 27, and 11% for Zn11, Cd33, and Zn44, respectively), as they should. For case 1, Zn11 and Cd33 were paired up to prepare (ZnS)CdS particle films. We ran the deposition at 330 °C separately for Zn11 and Cd33 and found only ZnS produced, as determined by the EDS analysis. However, when Zn11 and Cd33 were co-fed into the system, the deposition run again at 330 °C gave deposits consisting of both ZnS and CdS. This phenomenon implied that Cd33 would react to form CdS in the presence of ZnS. In other words, based on the reactivity difference existing between Zn11 and Cd33, Zn11 decomposed first to form ZnS particles on the substrate surface through surface reaction, and these ZnS particles served as a catalyst for the subsequent formation of CdS to achieve the modification purpose of CdS on ZnS particles. The deposition proceeded in 3-D island growth mode because of the lattice mismatch between the amorphous quartz substrate and the resulting semiconductor, leading to formation of a particle film. A similar phenomenon occurred for case 2, too. At the deposition temperature of 360 °C, there were no detectable ZnS deposits formed from Zn44 alone, but ZnS was produced when Zn44 was co-fed with Cd33. Here, Cd33 was the more reactive precursor and formed CdS first, leading to ZnS-passivated CdS particle films. (ZnS)CdS Particle Film. The UV-visible absorption spectra of (ZnS)CdS at different deposition temperatures compared with those of plain ZnS and CdS films are shown in Figure 3. At the deposition temperature of 330 °C, for (ZnS)CdS film, there was a little red-shift in its absorption edge as compared to that for the plain ZnS film, indicating the formation of CdS promoted by the first-formed ZnS particles. Here, the absorption edges (appearing as a shoulder) of the plain ZnS and plain CdS films were at around 325 and 490 nm, respectively, a 10 nm blue-shift from their corresponding bulk values, 335 nm (3.7 eV) and 500 nm (2.5 eV), respectively, due to the nanosize of the composing particles. The absorption edge of the (ZnS)CdS film red-shifted even more to around 340 nm when the deposition temperature was increased to 380 °C,


Langmuir, Vol. 20, No. 1, 2004 197 Table 1. Relative Quantum Yields Qs of (ZnS)CdS and (CdS)ZnS As Compared to Those of Native ZnS and Native CdS, Respectively

Figure 4. PL spectra of (ZnS)CdS particle films compared with those of plain ZnS films at different deposition temperatures with the excitation wavelength set at 320 nm. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

indicating a more complete and thicker coverage of CdS on ZnS. When the deposition temperature was further increased to 400 °C, the absorption edge jumped to 475 nm, close to that of the plain CdS film deposited at 400 °C (around 490 nm). This phenomenon was later shown to be caused by the formation of CdS particles from the gas-phase reaction and subsequent deposition of these particles onto the substrate. At 400 °C, CdS was formed not only through the surface reaction route on the surface of ZnS particles, but also through the gas-phase reaction route in the main stream. These CdS particles deposited onto the substrate and gave rise to an apparent yellow color of the substrate. It was also possible for the gasphase-formed CdS particles to be embedded in the film. Plain ZnS films showed a light gray color with no surface shine, while plain CdS films showed an apparent yellow color. For (ZnS)CdS particle films deposited at 330 °C, the substrate color was similar to that for the plain ZnS film. When the deposition temperature was increased to 380 °C, the substrate surface was shiny and colorful, while at 400 °C, the substrate turned apparent yellow, indicating a dominant proportion of CdS particles on the substrate surface. The PL spectra of (ZnS)CdS and the corresponding plain ZnS films at the three deposition temperatures of 330, 360, and 380 °C are compared in Figure 4. The excitation wavelength was set at 320 nm. All films showed a near band edge emission of ZnS with the peaks centering around 390 nm. The (ZnS)CdS particle films showed basically optical properties of the native material, ZnS, indicating a thin coverage of CdS.26 The degradation in PL intensity caused by the surface electronic states of the later-formed modification material, CdS, was however rather pronounced. The variation in PL quantum yield (Qs) for (ZnS)CdS against corresponding plain ZnS films at the three deposition temperatures is tabulated in Table 1. A Qs degradation of at least 60% was observed due to the surface electronic states of the modifier, CdS. In Figure 5, we show the X-ray powder diffraction patterns of (ZnS)CdS particle films at the three deposition temperatures with those of plain ZnS and CdS films included for comparison. The diffraction patterns of the plain ZnS and CdS films exhibited peak positions corresponding to those of sphalerite and greenockite structures, respectively. For (ZnS)CdS films deposited at 330 and (26) Lu, S.-Y.; Wu, M.-L.; Chen, H.-L. J. Appl. Phys. 2003, 93, 5789.

(ZnS)CdS

quantum yield Qs

Qs change (%)

ZnS, 330 °C ZnS, 360 °C ZnS, 380 °C (ZnS)CdS, 330 °C (ZnS)CdS, 360 °C (ZnS)CdS, 380 °C ZnS-CdS, 380 °C

0.103 88 0.067 47 0.031 60 0.040 52 0.011 33 0.006 26 0.024 23

-61 -83 -80 -23

(CdS)ZnS

quantum yield Qs

Qs change (%)

CdS, 360 °C CdS, 380 °C CdS, 400 °C (CdS)ZnS, 360 °C (CdS)ZnS, 380 °C (CdS)ZnS, 400 °C CdS-ZnS, 400 °C

0.008 46 0.005 16 0.005 98 0.010 71 0.029 30 0.046 30 0.007 64

+27 +468 +675 +28

Figure 5. X-ray powder diffraction patterns of (ZnS)CdS particle films compared with those of plain ZnS and CdS films at different deposition temperatures. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

360 °C, the diffraction patterns resembled that of the plain ZnS film. As for (ZnS)CdS films deposited at 380 °C, the main diffraction peak, ZnS(111), was found to shift toward the peak position of CdS(101), implying the increasing influence exerted by the modifier, CdS. For (ZnS)CdS films deposited at 400 °C, the strong CdS(002) peak indicated the formation of plain CdS particles and the broadened peak lying between ZnS(111) and CdS(101) implied the continuing existence of (ZnS)CdS particles. These results are consistent with those of absorption and PL spectra and substrate color observation. The broadened peak lying between ZnS(111) and CdS(101) may also be attributable to the possible partial alloying of ZnS with CdS in the shell region. (CdS)ZnS Particle Films. The UV-visible absorption spectra of (CdS)ZnS particle films at different deposition temperatures compared with those of plain ZnS and CdS are shown in Figure 6. For (CdS)ZnS films deposited at 360 °C, the absorption edge experienced a slight blueshift as compared with that for the plain CdS film, indicating the influence of the passivation material, ZnS. This blue-shift became more pronounced as the deposition temperature was increased to 380 °C, indicating a more complete and thicker coverage of ZnS on CdS. A significant blue-shift occurred when the deposition temperature was further increased to 400 °C. This phenomenon was later shown to be caused by the even more successful passivation

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Figure 6. UV-vis absorption spectra of (CdS)ZnS particle films compared with those of plain CdS and ZnS films at different deposition temperatures. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

Figure 7. PL spectra of (CdS)ZnS particle films compared with those of plain CdS films at different deposition temperatures with the excitation wavelength set at 320 nm. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

of CdS by ZnS, instead of formation of plain ZnS particles, by closely examining the corresponding PL (Qs), XRD, and substrate color. However, at an even higher deposition temperature, 500 °C, plain CdS particles were again formed from the gas-phase reaction route in the main stream, which can be judged from the absorption edge’s returning to 470 nm (close to 490 nm of the plain CdS film) and the apparent yellow color of the substrate. Note that the absorption shoulder of curve e of Figure 6 was cut off because of scale limitation. The particle film of (CdS)ZnS deposited at 360 °C showed a yellow color similar to that of the plain CdS film. At the deposition temperatures of 380 and 400 °C, the substrates became shiny and colorful, but for deposition at 500 °C, the substrate turned apparent yellow again, indicating a dominant proportion of CdS particles on the substrate surface. The PL spectra of (CdS)ZnS and the corresponding plain CdS films at the deposition temperatures of 360, 380, and 400 °C are compared in Figure 7. The excitation wavelength was set at 320 nm. All films showed a near band edge emission of CdS with the peaks centering around 525 nm. The ZnS-passivated CdS particle film showed

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Figure 8. X-ray powder diffraction patterns of (CdS)ZnS particle films compared with those of plain CdS and ZnS films at different deposition temperatures. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

basically optical properties of the native material, CdS, the same as in the (ZnS)CdS case. The enhancement in PL intensity of the (CdS)ZnS film over the corresponding plain CdS film is evident from Figure 7. This enhancement came from the effective passivation of surface electronic states of CdS by ZnS.27-32 For the (CdS)ZnS structure, the location of the conduction band for native CdS was lower than that for the passivation material, ZnS, in the electronic energy level,24,33 and as a result the electrons from ZnS can transfer onto the CdS region to delocalize the surface traps of CdS, resulting in a positive contribution to PL efficiency.34 The variation in PL quantum yield for (CdS)ZnS against the corresponding plain CdS films at the deposition temperatures of 360, 380, and 400 °C is also tabulated in Table 1. The Qs enhancement increased with increasing deposition temperature, and a Qs enhancement as high as 675% was achieved through the effective surface passivation. In Figure 8, we show the X-ray powder diffraction patterns of (CdS)ZnS particle films prepared at the deposition temperatures of 360, 380, 400, and 500 °C with those of plain ZnS and CdS films included for comparison. For (CdS)ZnS films deposited at 360 and 380 °C, the diffraction patterns resembled that of the plain CdS film, but with a slight shift of the CdS(101) peak toward the ZnS(111) position. For films deposited at 400 °C, an apparent right-shift of the two main diffraction peaks was observed, implying the increasing influence exerted by the passivation material, ZnS. Note that between CdS(002) and CdS(101) there are no other crystalline phases that can be attributed to either CdS or ZnS. Therefore, the apparent shift of the strong peak was unlikely caused by generation of a new phase but rather is an indication of the strong influence of passivating ZnS on the CdS core. (27) Hao, E.; Sun, H.; Zhou, Z.; Liu, J.; Yang, B.; Shen, J. Chem. Mater. 1999, 11, 3096. (28) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407. (29) Xu, L.; Wang, L.; Huang, X.; Zhu, J.; Chen, H.; Chen, K. Physica E 2000, 8, 129. (30) Rajeshwar, K.; de Tacconi, N. R.; Chenthamarakshan, C. R. Chem. Mater. 2001, 13, 2765. (31) Revaprasadu, N.; Malik, M. A.; O’Brien, P.; Wakefield, G. Chem. Commun. 1999, 1573. (32) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576. (33) Wei, S.-H.; Zunger, A. Appl. Phys. Lett. 1998, 72, 2011. (34) Yoffe, A. D. Adv. Phys. 2001, 50, 1.


For nanosized particles, surface atoms account for a significant fraction of the constituent atoms of the particle. These surface atoms, when coordinated with atoms of other material to form an epitaxial core-shell structure, have to twist their crystalline structure to meet with that of the shell material. This twisting in crystalline structure causes a change in atomic d spacings of both core and shell materials and thus leads to shifted diffraction peaks.35 The peak lying between CdS(101) and ZnS(111) was in fact broadened due to the closeness of the CdS(101) and ZnS(111) peaks and a possible alloying between the two materials in the shell region. For films deposited at 500 °C, the two main peaks showed a left-shift, with the strong CdS(002) peak indicating the formation of CdS particles and the peak lying between ZnS(111) and CdS(101) implying the existence of (CdS)ZnS. These results are consistent with those of absorption and PL spectra and substrate color observation. Solid Solution Structure of Composite Particle Films. It may be argued that the composite particle films, (ZnS)CdS or (CdS)ZnS, obtained here may be in the form of a uniform mixture of individual particles or a solid solution film. If the composite particle film were a uniform mixture of individual particles, then one would not observe enhancement or degradation in PL quantum yield. In addition, to rule out the possibility of formation of solid solution films, we provided experimental evidence as follows. One of the composite deposits, (CdS)ZnS formed at 380 °C, was heated to 900 °C for 3 h in nitrogen to ensure a complete melting and mixing of CdS and ZnS to form a uniform solid solution (note here the melting points of nanoparticles are much lower than those of their bulk counterparts) with subsequent quenching to preserve the solid solution structure. We found that there was a blueshift (about 50 nm) in the PL spectrum for the heat-treated sample as compared to that of the as-deposited one. This shift came from the change of energy band gap because of the formation of the CdS-ZnS solid solution. The dominant energy band gap of CdS was enlarged by forming the solid solution with ZnS, a wider band gap material. As for the corresponding quantum yield, there was only a slight enhancement as compared to the plain CdS case, unlike the significant enhancement obtained for the (CdS)ZnS case. Consequently, the possibility of formation of solid solution films can be ruled out. Sequential Layer Growth of ZnS/CdS and CdS/ ZnS Particle Films. The sequentially grown ZnS/CdS and CdS/ZnS films were compared to the corresponding (ZnS)CdS and (CdS)ZnS films in their optical properties. Figure 9 shows the comparison in absorption spectra. Here, both sequentially grown films (ZnS/CdS and CdS/ZnS) showed absorption edges at around 490 nm, the same as that of the plain CdS film, while the edge positions of (ZnS)CdS and (CdS)ZnS films showed a red-shift and a blue-shift, relative to the absorption peaks of their corresponding native materials, ZnS and CdS, respectively. For sequentially grown films, there existed plain ZnS and CdS phases so that both films showed the same absorption peak and color regardless of the growth sequence. In addition, the substrate appearance was also different between the sequentially grown and composite particle films. The two sequentially grown films showed the same dark yellow color due to the existence of the plain CdS layer, while the composite particle films were shiny and colorful. The PL spectra of ZnS/CdS and CdS/ZnS films are compared with those of the corresponding composite films (35) Revaprasadu, N.; Malik, M. A.; O’Brien, P.; Wakefield, G. Chem. Commun. 1999, 1573.

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Figure 9. UV-visible absorption spectra of sequentially grown ZnS/CdS and CdS/ZnS particle films compared with those of the corresponding (ZnS)CdS and (CdS)ZnS films at the deposition temperatures of 380 and 400 °C, respectively. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and a total deposition time of 3 h.

Figure 10. PL spectra of sequentially grown ZnS/CdS and CdS/ZnS particle films compared with those of the corresponding (ZnS)CdS and (CdS)ZnS films at the deposition temperatures of 380 and 400 °C, respectively, with the excitation wavelength set at 320 nm. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and a total deposition time of 3 h.

in Figure 10. For ZnS/CdS film deposited at 380 °C, the spectrum showed a near band edge emission of the inner material, ZnS, at around 400 nm. If compared to that of the (ZnS)CdS film at the same deposition temperature, the PL intensity of the ZnS/CdS film was found to be stronger, although both were lower than that of the corresponding plain ZnS film. This is due to the much less interfacial area between ZnS and CdS phases in ZnS/CdS films than in (ZnS)CdS films, so that less PL degradation was caused by the surface states of laterformed CdS. From Table 1, the Qs degradation caused by the ZnS/CdS film was less than that by the (ZnS)CdS film. The CdS/ZnS film deposited at 400 °C showed a near band edge emission of the native material, CdS, at around 525 nm, with the intensity lower than that for the (CdS)ZnS film. Both films showed an enhancement over the plain CdS film because of the effective surface passivation. But the (CdS)ZnS film gained more enhancement than the CdS/ZnS film did, as is evident from Table 1, since this in situ passivation provided more CdS-ZnS interfacial area for passivating surface states of CdS.

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Figure 11. Schematic for the growth mechanism of particle films.

Growth Mechanism. A growth mechanism was proposed as shown in Figure 11 to illustrate the formation of films consisting of core-shell-like particles. During stage I, the precursor with higher reactivity decomposed to form islands on the substrate surface through surface reaction. In stage II, the first-formed islands served as a catalyst to promote the decomposition of the other precursor of lower reactivity on the surface of the firstformed islands. Stages III and IV repeated the island growth and subsequent induced formation of a shell-like structure to form a particle film. If the deposition temperature is high enough, CdS particles formed from the gas-phase reaction route would deposit onto the surface of the film to give an apparent yellow color of the substrate and a dramatic increase in surface roughness as depicted in stage V. Morphology Characterization. The morphology of the deposited substrate surface was observed with SEM. Typical SEM graphs for deposited plain and composite particle films that involved no gas phase formation and subsequent deposition of CdS particles are shown in Figure 12a,b. Basically, they were dense and smooth films consisting of particles of 10-25 nm in size. As is evident from Figure 12, there was no apparent morphological difference between plain ZnS and (ZnS)CdS particle films. We have also estimated the grain size of the particle films with XRD data by using Scherrer’s equation, and the results were reasonably slightly smaller than the particle size observed from the SEM graphs, indicating single crystal particles. If the deposition was run at a temperature that was too high, such as 400 °C for (ZnS)CdS films and 500 °C for (CdS)ZnS films, the gas-phase formation and subsequent deposition of CdS particles occurred, and the film became rougher and was composed of larger size particles of 50-150 nm, as shown in Figure 12c. The surface roughness data from AFM analyses of the composite films were also studied. For (ZnS)CdS films deposited at 330, 360, and 380 °C, they showed a roughness of around 3-4 nm. However, when the deposition temperature increased to 400 °C, the roughness increased dramatically to 17 nm. The variation of surface roughness with increasing deposition temperature showed a similar trend for (CdS)ZnS films. There was a dramatic increase in surface roughness as the deposition temperature was increased to 500 °C. Basically, the results of optical characterizations (absorption, photoluminescence, and apparent color), XRD data, and surface morphology of the composite films were consistent and supported the proposed growth mechanism. Conclusion We successfully prepared nanosized (ZnS)CdS and (CdS)ZnS particle films and achieved an in situ passivation

Figure 12. SEM graphs of (a) a plain ZnS particle film deposited at 380 °C, (b) a (ZnS)CdS particle film deposited at 380 °C, and (c) a (ZnS)CdS particle film deposited at 400 °C. Deposition conditions: system pressure of 30 Torr, carrier gas (N2) flow rate of 200 sccm, and deposition time of 3 h.

with a one-step MOCVD process, by using two co-fed single source precursors of sufficient reactivity difference. The reactivity of metal dialkyldithiocarbamate precursors was tuned by adjusting the two alkyl groups and was characterized by TGA. With an appropriate manipulation of the reactivity of the two co-fed precursors, composite (ZnS)CdS and (CdS)ZnS particle films were produced in situ. The optical and structural characteristics of these composite particle films were dominated by the native material with an increasing influence from the passivation/ modification material at an increasing deposition temperature. The PL quantum yield enhancement achieved by the effective passivation of CdS by ZnS and the PL quantum yield degradation caused by the modification of


ZnS by CdS were clearly observed. In addition, the high interfacial area between the CdS and ZnS phases in the composite cases gave rise to more pronounced degradation or enhancement in PL quantum yield as compared to the corresponding sequentially grown layer structure.

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Acknowledgment. The authors gratefully acknowledge the support of the National Science Council of the Republic of China under Grant NSC 90-2214-E-007-003. LA0347410