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Synthesis of ternary ZnxCd1 − xS nanowires by thermal evaporation and the study of their photoluminescence
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Materials Letters 62 (2008) 128 – 132 www.elsevier.com/locate/matlet
Synthesis of ternary ZnxCd1 − xS nanowires by thermal evaporation and the study of their photoluminescence Y.Y. Xi, Teresa L.Y. Cheung, Dickon H.L. Ng ⁎ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China Received 5 December 2006; accepted 26 April 2007 Available online 1 May 2007
Abstract Ternary ZnxCd1 − xS nanowires were synthesized on Au-coated Si (100) substrates by thermal evaporation method. The nanowires obtained under ambient and reduced pressure of ∼100 Torr were studied and compared. Both of them were single crystalline wurtzite structured with similar chemical composition. The tip of the nanowires obtained in ambient pressure contained an Au-rich particle, but that obtained under reduced pressure was impurity-free. Hence, both vapor–liquid–solid (VLS) and mixed vapor–solid (VS)–VLS growth mechanisms were proposed for these two types of nanowires. Furthermore, photoluminescence studies revealed that their intrinsic and extrinsic emission bands were within the visible region. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnxCd1 − xS; Thermal evaporation; Nanocomposites; Optical materials and properties
1. Introduction
2. Experimental procedures
One dimensional (1-D) nanostructured semi-conductors are of prime focuses in materials research due to their potential applications in lasers [1,2], waveguides [3], and optical switches [4]. Considerable studies have been devoted to the synthesis of binary II–VI nanostructures [5–8] and ternary systems [9–12]. By varying the stoichiometric ratio of a system, the optical properties can be modified. For example, ZnxCd1 − xS could be used in optoelectronic applications within the visible to UV spectral range, and also in solar energy driven devices [13]. Although many reports are available on ZnxCd1 − xS films [14– 19], only few 1-D micro- and nanostructures were documented. In view that the ternary ZnxCd1 − xS has a tremendous potential in optoelectronic applications [10,12], we have explored a simple and cost-effective way to fabricate single crystalline ZnxCd1 − xS nanowires by thermal evaporation. The effects of deposition pressure on the morphology, structure and optical properties of the nanowires have also been investigated.
Two grams of CdS powder (International Laboratory, USA) and 0.94 g of ZnCl2 powder (Fisher Scientific, UK) were mixed and put into an alumina boat and inserted into a horizontal tube furnace. The mixture was annealed at 450 °C for 12 h in argon atmosphere. The annealed powder was ground and placed into an alumina boat located at the center of the quartz tube. A number of Au-coated Si (100) substrates (1 × 1 cm2 each) were placed side-by-side on the downstream side covering a distance of 20 cm next to the source powders. The entire setup was further annealed at 900 °C for 100 min under different pressures in argon atmosphere: sample (A) under ambient pressure and sample (B) under reduced pressure ∼ 100 Torr. Microstructural characterization of the products was performed by using scanning electron microscopy (SEM, LEO 1450VP, Thronwood, NY) and transmission electron microscopy (TEM, CM120, Philips, Hillsboro, OR). Elemental and chemical analyses were carried out by the energy dispersive X-ray spectrometry (EDS, Oxford Link II, UK). The optical properties were studied by a photoluminescence spectrometer (PL Renishaw, Rm1000B, UK), in which the wavelength of the excitation laser was 325 nm.
⁎ Corresponding author. Tel.: +852 2609 6392; fax: +852 2603 5204. E-mail address: [email protected] (D.H.L. Ng). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.094
Y.Y. Xi et al. / Materials Letters 62 (2008) 128–132
3. Results and discussion 3.1. Morphologies, compositions and structures Thin white layers were observed on the substrates in both settings, and we had selected the ones with the greatest amount of products. The substrates located at ∼2 cm and ∼10 cm were selected for samples (A) and (B), respectively. SEM image of sample A (Fig. 1a) showed that the average length and diameter of the nanowires were ∼100 μm and ∼200 nm, and that in sample B (from Fig. 1b) were ∼100 μm and ∼350 nm. The nanowires found in sample A contained a droplet-tip (inset in Fig. 1a). EDS was used to determine the compositions of the droplet-tip and the body of the nanowire. The top EDS spectrum in Fig. 1b showed that the droplet contained Au, S, Cd and Zn (the Si peak was originated from the substrate) with atomic percentages of 44.9, 29.3, 10.7, and 15.1 at.%, respectively. It was evident that this Au-rich droplet was used as a catalyst in the growth of this wire during annealing. The bottom EDS spectrum in Fig. 1b was obtained from the region on the body, and 42.3 at.% Zn, 8.9 at.% Cd and 48.8 at.% S were detected. For the nanowire extracted from sample B (inset of Fig. 1c), no droplet-tip was observed. The EDS spectra obtained from the tip and the body were similar (Fig. 1d), which contained 45.1 at.% Zn, 8.0 at.% Cd, and 47.0 at. % S. The structures of the nanowires were further investigated by TEM and HRTEM. Fig. 2a and b showed the TEM and HRTEM images of a nanowire extracted from sample A, respectively, and the images of the nanowire from sample B were shown in Fig. 2c and d. The two selected area electron diffraction (SAED) patterns (insets in Fig. 2a and c) confirmed that the wires in both samples were single crystalline hexagonal wurtzite structured. The HRTEM images also revealed that the nanowires were free from dislocations and microcracks. We had measured the lattice parameter Δd between the atomic planes oriented normal to the growth axis of the nanowires, which were 0.630 nm (for sample A) and 0.334 nm (for sample B). The former spacing ¯0) plane. corresponded to the (0001) plane, and the latter to the (101
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Thus, different preferential growth directions were observed for the two ¯0N (for B). nanowire samples: b0001N (for A) and b101 Fig. 3 shows the XRD patterns of samples A (pattern a) and B (pattern b), both indexed to the wurtzite structure similar to that of ZnS. However, the relative intensities of the diffracted peaks, particularly the (100) and (002) peaks, were different. Such results were consistent with our claim that the products had different preferential growth directions. For our ZnxCd1 − xS system, the relationship between the composition value x and the lattice parameter cx along its c-axis can be described by the Vegard's law [20–22]: cx ¼ cCdS þ ðcZnS −cCdS Þx
ð1Þ
where cZnS and cCdS were the lattice constants along the c-axis of ZnS (when x = 1) and CdS (when x = 0) crystals, respectively. In calculating the x value, we had made use of the (002) and the (101) peaks in pattern a (Fig. 3) obtained from sample A, which lied at 2θ = 28.2° and 30.2°, respectively. From Eq. (1), the lattice constants a, c, and the x value of sample Awere determined to be 3.852 Å, 6.310 Å, and 0.89, respectively. Using similar approach for pattern b, the values were calculated to be 3.855 Å, 6.314 Å, and 0.88. Thus, the chemical formula for samples A and B were Zn0.89Cd0.11S and Zn0.88Cd0.12S, respectively. 3.2. Reactions and deposition zones The possible reactions during annealing of the raw materials were: CdS þ ZnCl2 →ZnS þ CdCl2
ð2Þ
xZnS þ ð1−xÞCdS→Znx Cd1−x S
ð3Þ
Eq. (2) described the reaction from CdS and ZnCl2 powders to produce ZnS and CdCl2, and ZnS further reacted with CdS via Eq. (3) to produce ZnxCd1 − xS. To prove the validity of these equations, the
Fig. 1. (a) and (c) are the SEM images of the ZnxCd1 − xS nanowires synthesized under ambient pressure, and ∼ 100 Torr, respectively. (b) EDS spectra taken at the tip (upper curve) and on the body (lower curve) of the nanowire shown in the inset of (a). (d) EDS spectrum taken from the nanowire shown in the inset of (c).
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Fig. 2. (a) and (b) are the TEM and HRTEM lattice images of a ZnxCd1 − xS nanowire synthesized under ambient pressure. (c) and (d) are those taken from the nanowire prepared under a pressure of ∼ 100 Torr. The insets in (a) and (c) are the corresponding SAED patterns. The large arrows in (b) and (d) indicate the growth directions of the two nanowires.
product mixture was characterized by XRD (pattern c in Fig. 3), which confirmed the existence of ZnxCd1 − xS, ZnCl2, CdS and CdCl2. Thus, both reactions were far from completion. We also found that the x value of the raw ZnxCd1 − xS compound was ∼ 0.43, which was different from those ZnxCd1 − xS after further annealing. Using the gas phase condensation technique, Sanchez-Lopez et al. [23] had demonstrated that CdS and ZnS would be decomposed into Cd and S vapors, and Zn and S vapors, respectively during annealing, and they traveled and deposited at different locations. These might be the cues to explain why the deposition zones in our cases were not the same in different situations. The melting point (420 °C) and boiling point (907 °C) of elemental Zn were relatively higher than those of Cd (321 °C and 765 °C). Hence, Cd evaporated earlier and traveled for longer distance before deposition. As a result, the atomic ratios of the nanowires near the source contained a slightly higher Zn content and those located further away contained higher Cd content. Moreover, under reduced pressure, the vaporization rate was enhanced, and the deposition could
occur at a lower temperature. In the deposition zone of sample B (∼10 cm from source), the temperature was determined to be about ∼50 °C lower than the target temperature (900 °C) at the powder source. Thus, the vapor tended to travel longer, and deposited on the region located ∼ 10 to 14 cm away from the source. Previous works on 1-D ZnO nanostructures, which were prepared either in ambient or in reduced pressure conditions, had shown that their growth directions were b0001N [24,25] and those of the CdS prepared in reduced pressure was b101¯0N [26,27]. Results of our current work showed that the zinc-rich ZnCdS sample (A) grown in the ambient condition had b0001N preferred growth which was the same as that of ZnO, while the Cd-rich ZnCdS sample (B) grown in reduced ¯0N preferred growth which was the same as that of pressure had b101 CdS. Although a direct relationship between the inert gas pressure in the annealing environment and the preferred growth direction of the products had not been established, it was evident that the orientation of the Zn-rich compound tended to follow that of the ZnO and the Cd-rich
Fig. 3. XRD patterns of the ZnxCd1 − xS nanowires synthesized under (a) ambient pressure, (b) ∼ 100 Torr; and (c) the reaction products via reaction (3).
Y.Y. Xi et al. / Materials Letters 62 (2008) 128–132
Fig. 4. Photoluminescence spectra of (a) Zn0.89Cd0.11S and (b) Zn0.88Cd0.12S nanowires. The gray lines are the Gaussian fittings.
compound tended to follow that of the CdS. As mentioned earlier, the pressure in the annealing system had a decisive effect on the location and composition of the products. We found that more Zn-rich ZnCdS product having b0001N preferred growth was obtained nearer to the source material when prepared in the ambient condition, and the Cd¯0N preferred growth was found further rich ZnCdS product having b101 away from the source material when prepared in the reduced pressure condition. Thus, it was clear that the pressure of the annealing environment would affect the location and composition of the product, which in turn governed the growth direction of the product. 3.3. Growth mechanisms Two distinct kinds of growth mechanisms were proposed for the two types of nanowires. The Au-rich particles found at the tips of the nanowires synthesized under ambient pressure suggested the VLS growth mechanism. During annealing, some parts of the Au film melted and converted into tiny liquid droplets. ZnxCd1 − xS vapor was carried away from the source and dissolved into the Au droplets. As the droplets became supersaturated, ZnxCd1 − xS precipitated at the liquid/ solid interface and eventually formed nanowires. On the other hand, no droplet-tip was found from the nanowires synthesized under reduced pressure, the conventional VLS route was not possible. In reduced pressure, large part of the Au film was molten. The vapor dissolved and precipitated to form ZnxCd1 − xS particles where localized supersaturation was reached. Since the density of ZnxCd1 − xS was much smaller than Au, some of them floated on the surface of molten Au and became the precursors for condensation of incoming ZnxCd1 − xS vapor as well as for the nucleation sites for the dissolved ZnxCd1 − xS. When more ZnxCd1 − xS was added, the nanowires were pushed and extended outward from the molten Au. In this case, the growth followed the combined VS mechanism at the tip and VLS mechanism at the base. 3.4. Photoluminescence Room temperature PL spectroscopy was performed on the ZnxCd1 − xS nanowire samples, the spectra are shown in Fig. 4a and b. Both spectra consisted of two luminescence bands: the weak one located at 363 nm (3.42 eV) corresponded to the intrinsic near band edge emission (NBE), and the stronger broad band corresponded to the extrinsic deep-level emission (DLE) in the lower energy region. Salem [19] had determined the band-gap energy (Eg) of ZnxCd1 − xS films by the following equation: Eg ¼ 2:337 þ 0:72x þ 0:563x2
ð4Þ
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Putting x = 0.88 into Eq. (4), we found that the Eg of Zn0.89Cd0.11S and Zn0.88Cd0.12S nanowires were ∼3.42 eV, which was rather consistent with the value obtained from our PL measurement. In addition, this Eg value was slightly smaller than that of the ZnS (3.62 eV), but larger than that of CdS (2.42 eV). Two Gaussian bands (in gray) had been fitted into the extrinsic broad band emissions of each sample as shown in Fig. 4. For the Zn0.89Cd0.11S nanowires obtained under ambient pressure, the three Gaussian peaks were located at 502, 564 and 682 nm (curve a of Fig. 4). For the Zn0.88Cd0.12S nanowires, two Gaussian peaks were found at 510 nm and 572 nm (curve b of Fig. 4). These broad PL signals were probably related to the defect-related emissions. Denzler et al. [28] proposed that lattice defects such as vacancies or interstitials of sulfur and zinc could have given rise to the luminescence in different spectral positions thus produce a broad spectrum. So far, the defect emissions in ZnS [28–30] and ZnxCd1 − xS ternary compound [10] remain unclear. Our results on DLE obtained at different pressures indicated that the deposition conditions might lead to different types of defects, surface states, or impurities in the ZnxCd1 − xS systems. Undoubtedly, more works must be performed before the origins and the nature related to the DLE of these ZnxCd1 − xS nanowires could be identified.
4. Conclusions Single crystalline wurtzite structured ZnxCd1 − xS nanowires with x = 0.88 and 0.89 were successfully synthesized by a simple thermal evaporation method under different pressure levels. The nanowires synthesized under ambient pressure grew along the b0001N directions via the tip-growth VLS mechanism; and the ones synthesized under ∼ 100 Torr grew along the b101¯0N direction via the combined base-growth VLS and tipgrowth VS mechanism. Eg of both samples was ∼ 3.42 eV and the broad DLE bands indicated defects, surface states or impurities in these nanowires. References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [2] K. Bando, T. Sawabe, K. Asaka, Y. Masumoto, J. Lumin. 108 (2004) 385. [3] X.F. Duan, Y. Hu, R. Agarwal, C.M. Lieber, Nature 421 (2003) 24. [4] H. Kind, H. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158. [5] S. Kar, S. Chaudhuri, J. Phys. Chem., B 110 (2006) 4542. [6] A. Varfolomeev, D. Zaretsky, V. Pokalyakin, S. Tereshin, S. Pramanik, S. Bandyopadhyay, Appl. Phys. Lett. 88 (2006) 113. [7] A.L. Pan, R.B. Liu, Q. Yang, Y.C. Zhu, G.Z. Yang, B.S. Zou, K.Q. Chen, J. Phys. Chem., B 109 (2005) 24268. [8] W.M. Kwok, A.B. Djurisic, Y.H. Leung, W.K. Chan, D.L. Phillips, Appl. Phys. Lett. 87 (2005) 093108. [9] A. Pan, H. Yang, R. Liu, R. Yu, B. Zou, Z.L. Wang, J. Am. Chem. Soc. 127 (2005) 15692. [10] Y.K. Liu, J.A. Zapien, Y.Y. Shan, C.Y. Geng, C.S. Lee, S.T. Lee, Adv. Mater. 17 (2005) 1372. [11] X.T. Zhang, Z. Liu, Q. Li, S.K. Hark, J. Phys. Chem., B 109 (2005) 17913. [12] Q. Nie, Q.L. Yuan, Q.S. Wang, Z. Xu, J. Mater. Sci. 39 (2004) 5611. [13] H.H.L. Kwok, M.Y. Leung, Y.M. Lam, J. Cryst. Growth 59 (1982) 421. [14] J.H. Lee, W.C. Song, J.S. Yi, K.J. Yang, W.D. Han, J. Hwang, Thin Solid Films 431 (2003) 349. [15] P. Kumar, A. Misra, D. Kumar, N. Dhama, T.P. Sharma, P.N. Dixit, Opt. Mater. 27 (2004) 261. [16] S.A. Kuhaimi, Z. Tulbah, J. Electrochem. Soc. 147 (2000) 214.
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[17] M.E. Rincon, M.W. Martinez, M. Miranda-Hernandez, Sol. Energy Mater. Sol. Cells 77 (2003) 25. [18] P.J. Sebastian, M. Ocampo, Sol. Energy Mater. Sol. Cells 44 (1996) 1. [19] A.M. Salem, Appl. Phys., A 74 (2002) 205. [20] P. Cherlin, E.L. Lind, A. Davis, J. Electrochem. Soc. 117 (1970) 233. [21] D.W.G. Ballentyne, B. Ray, Physica 27 (1961) 337. [22] G. Shimaoka, Y. Suzuki, Appl. Surf. Sci. 114 (1997) 528. [23] J.C. Sanchez-Lopez, E.P. Reddy, T.C. Rojas, Nanostruct. Mater. 12 (1999) 459. [24] N.C. Hung, G.Z. Wang, M.Y. Yau, D.H.L. Ng, J. Mater. Res. 19 (2004) 2226.
[25] G.Z. Wang, N.G. Ma, C.J. Deng, P. Yu, C.Y. To, N.C. Hung, M. Aravind, D.H.L. Ng, Mater. Lett. 58 (2004) 2195. [26] Y. Wang, G.Z. Wang, M.Y. Yau, C.Y. To, D.H.L. Ng, Chem. Phys. Lett. 407 (2005) 510. [27] Y. Wang, C.Y. To, D.H.L. Ng, Mater. Lett. 60 (2006) 1151. [28] D. Denzler, M. Olschewski, K. Sattler, J. Appl. Phys. 84 (1998) 2841. [29] Y.W. Wang, L.D. Zhang, C.H. Liang, G.Z. Wang, X.S. Deng, Chem. Phys. Lett. 357 (2002) 314. [30] Y.C. Zhu, Y. Bando, D.F. Xue, Appl. Phys. Lett. 82 (2003) 1769.
Materials Letters 62 (2008) 128 – 132 www.elsevier.com/locate/matlet
Synthesis of ternary ZnxCd1 − xS nanowires by thermal evaporation and the study of their photoluminescence Y.Y. Xi, Teresa L.Y. Cheung, Dickon H.L. Ng ⁎ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China Received 5 December 2006; accepted 26 April 2007 Available online 1 May 2007
Abstract Ternary ZnxCd1 − xS nanowires were synthesized on Au-coated Si (100) substrates by thermal evaporation method. The nanowires obtained under ambient and reduced pressure of ∼100 Torr were studied and compared. Both of them were single crystalline wurtzite structured with similar chemical composition. The tip of the nanowires obtained in ambient pressure contained an Au-rich particle, but that obtained under reduced pressure was impurity-free. Hence, both vapor–liquid–solid (VLS) and mixed vapor–solid (VS)–VLS growth mechanisms were proposed for these two types of nanowires. Furthermore, photoluminescence studies revealed that their intrinsic and extrinsic emission bands were within the visible region. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnxCd1 − xS; Thermal evaporation; Nanocomposites; Optical materials and properties
1. Introduction
2. Experimental procedures
One dimensional (1-D) nanostructured semi-conductors are of prime focuses in materials research due to their potential applications in lasers [1,2], waveguides [3], and optical switches [4]. Considerable studies have been devoted to the synthesis of binary II–VI nanostructures [5–8] and ternary systems [9–12]. By varying the stoichiometric ratio of a system, the optical properties can be modified. For example, ZnxCd1 − xS could be used in optoelectronic applications within the visible to UV spectral range, and also in solar energy driven devices [13]. Although many reports are available on ZnxCd1 − xS films [14– 19], only few 1-D micro- and nanostructures were documented. In view that the ternary ZnxCd1 − xS has a tremendous potential in optoelectronic applications [10,12], we have explored a simple and cost-effective way to fabricate single crystalline ZnxCd1 − xS nanowires by thermal evaporation. The effects of deposition pressure on the morphology, structure and optical properties of the nanowires have also been investigated.
Two grams of CdS powder (International Laboratory, USA) and 0.94 g of ZnCl2 powder (Fisher Scientific, UK) were mixed and put into an alumina boat and inserted into a horizontal tube furnace. The mixture was annealed at 450 °C for 12 h in argon atmosphere. The annealed powder was ground and placed into an alumina boat located at the center of the quartz tube. A number of Au-coated Si (100) substrates (1 × 1 cm2 each) were placed side-by-side on the downstream side covering a distance of 20 cm next to the source powders. The entire setup was further annealed at 900 °C for 100 min under different pressures in argon atmosphere: sample (A) under ambient pressure and sample (B) under reduced pressure ∼ 100 Torr. Microstructural characterization of the products was performed by using scanning electron microscopy (SEM, LEO 1450VP, Thronwood, NY) and transmission electron microscopy (TEM, CM120, Philips, Hillsboro, OR). Elemental and chemical analyses were carried out by the energy dispersive X-ray spectrometry (EDS, Oxford Link II, UK). The optical properties were studied by a photoluminescence spectrometer (PL Renishaw, Rm1000B, UK), in which the wavelength of the excitation laser was 325 nm.
⁎ Corresponding author. Tel.: +852 2609 6392; fax: +852 2603 5204. E-mail address: [email protected] (D.H.L. Ng). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.094
Y.Y. Xi et al. / Materials Letters 62 (2008) 128–132
3. Results and discussion 3.1. Morphologies, compositions and structures Thin white layers were observed on the substrates in both settings, and we had selected the ones with the greatest amount of products. The substrates located at ∼2 cm and ∼10 cm were selected for samples (A) and (B), respectively. SEM image of sample A (Fig. 1a) showed that the average length and diameter of the nanowires were ∼100 μm and ∼200 nm, and that in sample B (from Fig. 1b) were ∼100 μm and ∼350 nm. The nanowires found in sample A contained a droplet-tip (inset in Fig. 1a). EDS was used to determine the compositions of the droplet-tip and the body of the nanowire. The top EDS spectrum in Fig. 1b showed that the droplet contained Au, S, Cd and Zn (the Si peak was originated from the substrate) with atomic percentages of 44.9, 29.3, 10.7, and 15.1 at.%, respectively. It was evident that this Au-rich droplet was used as a catalyst in the growth of this wire during annealing. The bottom EDS spectrum in Fig. 1b was obtained from the region on the body, and 42.3 at.% Zn, 8.9 at.% Cd and 48.8 at.% S were detected. For the nanowire extracted from sample B (inset of Fig. 1c), no droplet-tip was observed. The EDS spectra obtained from the tip and the body were similar (Fig. 1d), which contained 45.1 at.% Zn, 8.0 at.% Cd, and 47.0 at. % S. The structures of the nanowires were further investigated by TEM and HRTEM. Fig. 2a and b showed the TEM and HRTEM images of a nanowire extracted from sample A, respectively, and the images of the nanowire from sample B were shown in Fig. 2c and d. The two selected area electron diffraction (SAED) patterns (insets in Fig. 2a and c) confirmed that the wires in both samples were single crystalline hexagonal wurtzite structured. The HRTEM images also revealed that the nanowires were free from dislocations and microcracks. We had measured the lattice parameter Δd between the atomic planes oriented normal to the growth axis of the nanowires, which were 0.630 nm (for sample A) and 0.334 nm (for sample B). The former spacing ¯0) plane. corresponded to the (0001) plane, and the latter to the (101
129
Thus, different preferential growth directions were observed for the two ¯0N (for B). nanowire samples: b0001N (for A) and b101 Fig. 3 shows the XRD patterns of samples A (pattern a) and B (pattern b), both indexed to the wurtzite structure similar to that of ZnS. However, the relative intensities of the diffracted peaks, particularly the (100) and (002) peaks, were different. Such results were consistent with our claim that the products had different preferential growth directions. For our ZnxCd1 − xS system, the relationship between the composition value x and the lattice parameter cx along its c-axis can be described by the Vegard's law [20–22]: cx ¼ cCdS þ ðcZnS −cCdS Þx
ð1Þ
where cZnS and cCdS were the lattice constants along the c-axis of ZnS (when x = 1) and CdS (when x = 0) crystals, respectively. In calculating the x value, we had made use of the (002) and the (101) peaks in pattern a (Fig. 3) obtained from sample A, which lied at 2θ = 28.2° and 30.2°, respectively. From Eq. (1), the lattice constants a, c, and the x value of sample Awere determined to be 3.852 Å, 6.310 Å, and 0.89, respectively. Using similar approach for pattern b, the values were calculated to be 3.855 Å, 6.314 Å, and 0.88. Thus, the chemical formula for samples A and B were Zn0.89Cd0.11S and Zn0.88Cd0.12S, respectively. 3.2. Reactions and deposition zones The possible reactions during annealing of the raw materials were: CdS þ ZnCl2 →ZnS þ CdCl2
ð2Þ
xZnS þ ð1−xÞCdS→Znx Cd1−x S
ð3Þ
Eq. (2) described the reaction from CdS and ZnCl2 powders to produce ZnS and CdCl2, and ZnS further reacted with CdS via Eq. (3) to produce ZnxCd1 − xS. To prove the validity of these equations, the
Fig. 1. (a) and (c) are the SEM images of the ZnxCd1 − xS nanowires synthesized under ambient pressure, and ∼ 100 Torr, respectively. (b) EDS spectra taken at the tip (upper curve) and on the body (lower curve) of the nanowire shown in the inset of (a). (d) EDS spectrum taken from the nanowire shown in the inset of (c).
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Fig. 2. (a) and (b) are the TEM and HRTEM lattice images of a ZnxCd1 − xS nanowire synthesized under ambient pressure. (c) and (d) are those taken from the nanowire prepared under a pressure of ∼ 100 Torr. The insets in (a) and (c) are the corresponding SAED patterns. The large arrows in (b) and (d) indicate the growth directions of the two nanowires.
product mixture was characterized by XRD (pattern c in Fig. 3), which confirmed the existence of ZnxCd1 − xS, ZnCl2, CdS and CdCl2. Thus, both reactions were far from completion. We also found that the x value of the raw ZnxCd1 − xS compound was ∼ 0.43, which was different from those ZnxCd1 − xS after further annealing. Using the gas phase condensation technique, Sanchez-Lopez et al. [23] had demonstrated that CdS and ZnS would be decomposed into Cd and S vapors, and Zn and S vapors, respectively during annealing, and they traveled and deposited at different locations. These might be the cues to explain why the deposition zones in our cases were not the same in different situations. The melting point (420 °C) and boiling point (907 °C) of elemental Zn were relatively higher than those of Cd (321 °C and 765 °C). Hence, Cd evaporated earlier and traveled for longer distance before deposition. As a result, the atomic ratios of the nanowires near the source contained a slightly higher Zn content and those located further away contained higher Cd content. Moreover, under reduced pressure, the vaporization rate was enhanced, and the deposition could
occur at a lower temperature. In the deposition zone of sample B (∼10 cm from source), the temperature was determined to be about ∼50 °C lower than the target temperature (900 °C) at the powder source. Thus, the vapor tended to travel longer, and deposited on the region located ∼ 10 to 14 cm away from the source. Previous works on 1-D ZnO nanostructures, which were prepared either in ambient or in reduced pressure conditions, had shown that their growth directions were b0001N [24,25] and those of the CdS prepared in reduced pressure was b101¯0N [26,27]. Results of our current work showed that the zinc-rich ZnCdS sample (A) grown in the ambient condition had b0001N preferred growth which was the same as that of ZnO, while the Cd-rich ZnCdS sample (B) grown in reduced ¯0N preferred growth which was the same as that of pressure had b101 CdS. Although a direct relationship between the inert gas pressure in the annealing environment and the preferred growth direction of the products had not been established, it was evident that the orientation of the Zn-rich compound tended to follow that of the ZnO and the Cd-rich
Fig. 3. XRD patterns of the ZnxCd1 − xS nanowires synthesized under (a) ambient pressure, (b) ∼ 100 Torr; and (c) the reaction products via reaction (3).
Y.Y. Xi et al. / Materials Letters 62 (2008) 128–132
Fig. 4. Photoluminescence spectra of (a) Zn0.89Cd0.11S and (b) Zn0.88Cd0.12S nanowires. The gray lines are the Gaussian fittings.
compound tended to follow that of the CdS. As mentioned earlier, the pressure in the annealing system had a decisive effect on the location and composition of the products. We found that more Zn-rich ZnCdS product having b0001N preferred growth was obtained nearer to the source material when prepared in the ambient condition, and the Cd¯0N preferred growth was found further rich ZnCdS product having b101 away from the source material when prepared in the reduced pressure condition. Thus, it was clear that the pressure of the annealing environment would affect the location and composition of the product, which in turn governed the growth direction of the product. 3.3. Growth mechanisms Two distinct kinds of growth mechanisms were proposed for the two types of nanowires. The Au-rich particles found at the tips of the nanowires synthesized under ambient pressure suggested the VLS growth mechanism. During annealing, some parts of the Au film melted and converted into tiny liquid droplets. ZnxCd1 − xS vapor was carried away from the source and dissolved into the Au droplets. As the droplets became supersaturated, ZnxCd1 − xS precipitated at the liquid/ solid interface and eventually formed nanowires. On the other hand, no droplet-tip was found from the nanowires synthesized under reduced pressure, the conventional VLS route was not possible. In reduced pressure, large part of the Au film was molten. The vapor dissolved and precipitated to form ZnxCd1 − xS particles where localized supersaturation was reached. Since the density of ZnxCd1 − xS was much smaller than Au, some of them floated on the surface of molten Au and became the precursors for condensation of incoming ZnxCd1 − xS vapor as well as for the nucleation sites for the dissolved ZnxCd1 − xS. When more ZnxCd1 − xS was added, the nanowires were pushed and extended outward from the molten Au. In this case, the growth followed the combined VS mechanism at the tip and VLS mechanism at the base. 3.4. Photoluminescence Room temperature PL spectroscopy was performed on the ZnxCd1 − xS nanowire samples, the spectra are shown in Fig. 4a and b. Both spectra consisted of two luminescence bands: the weak one located at 363 nm (3.42 eV) corresponded to the intrinsic near band edge emission (NBE), and the stronger broad band corresponded to the extrinsic deep-level emission (DLE) in the lower energy region. Salem [19] had determined the band-gap energy (Eg) of ZnxCd1 − xS films by the following equation: Eg ¼ 2:337 þ 0:72x þ 0:563x2
ð4Þ
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Putting x = 0.88 into Eq. (4), we found that the Eg of Zn0.89Cd0.11S and Zn0.88Cd0.12S nanowires were ∼3.42 eV, which was rather consistent with the value obtained from our PL measurement. In addition, this Eg value was slightly smaller than that of the ZnS (3.62 eV), but larger than that of CdS (2.42 eV). Two Gaussian bands (in gray) had been fitted into the extrinsic broad band emissions of each sample as shown in Fig. 4. For the Zn0.89Cd0.11S nanowires obtained under ambient pressure, the three Gaussian peaks were located at 502, 564 and 682 nm (curve a of Fig. 4). For the Zn0.88Cd0.12S nanowires, two Gaussian peaks were found at 510 nm and 572 nm (curve b of Fig. 4). These broad PL signals were probably related to the defect-related emissions. Denzler et al. [28] proposed that lattice defects such as vacancies or interstitials of sulfur and zinc could have given rise to the luminescence in different spectral positions thus produce a broad spectrum. So far, the defect emissions in ZnS [28–30] and ZnxCd1 − xS ternary compound [10] remain unclear. Our results on DLE obtained at different pressures indicated that the deposition conditions might lead to different types of defects, surface states, or impurities in the ZnxCd1 − xS systems. Undoubtedly, more works must be performed before the origins and the nature related to the DLE of these ZnxCd1 − xS nanowires could be identified.
4. Conclusions Single crystalline wurtzite structured ZnxCd1 − xS nanowires with x = 0.88 and 0.89 were successfully synthesized by a simple thermal evaporation method under different pressure levels. The nanowires synthesized under ambient pressure grew along the b0001N directions via the tip-growth VLS mechanism; and the ones synthesized under ∼ 100 Torr grew along the b101¯0N direction via the combined base-growth VLS and tipgrowth VS mechanism. Eg of both samples was ∼ 3.42 eV and the broad DLE bands indicated defects, surface states or impurities in these nanowires. References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [2] K. Bando, T. Sawabe, K. Asaka, Y. Masumoto, J. Lumin. 108 (2004) 385. [3] X.F. Duan, Y. Hu, R. Agarwal, C.M. Lieber, Nature 421 (2003) 24. [4] H. Kind, H. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158. [5] S. Kar, S. Chaudhuri, J. Phys. Chem., B 110 (2006) 4542. [6] A. Varfolomeev, D. Zaretsky, V. Pokalyakin, S. Tereshin, S. Pramanik, S. Bandyopadhyay, Appl. Phys. Lett. 88 (2006) 113. [7] A.L. Pan, R.B. Liu, Q. Yang, Y.C. Zhu, G.Z. Yang, B.S. Zou, K.Q. Chen, J. Phys. Chem., B 109 (2005) 24268. [8] W.M. Kwok, A.B. Djurisic, Y.H. Leung, W.K. Chan, D.L. Phillips, Appl. Phys. Lett. 87 (2005) 093108. [9] A. Pan, H. Yang, R. Liu, R. Yu, B. Zou, Z.L. Wang, J. Am. Chem. Soc. 127 (2005) 15692. [10] Y.K. Liu, J.A. Zapien, Y.Y. Shan, C.Y. Geng, C.S. Lee, S.T. Lee, Adv. Mater. 17 (2005) 1372. [11] X.T. Zhang, Z. Liu, Q. Li, S.K. Hark, J. Phys. Chem., B 109 (2005) 17913. [12] Q. Nie, Q.L. Yuan, Q.S. Wang, Z. Xu, J. Mater. Sci. 39 (2004) 5611. [13] H.H.L. Kwok, M.Y. Leung, Y.M. Lam, J. Cryst. Growth 59 (1982) 421. [14] J.H. Lee, W.C. Song, J.S. Yi, K.J. Yang, W.D. Han, J. Hwang, Thin Solid Films 431 (2003) 349. [15] P. Kumar, A. Misra, D. Kumar, N. Dhama, T.P. Sharma, P.N. Dixit, Opt. Mater. 27 (2004) 261. [16] S.A. Kuhaimi, Z. Tulbah, J. Electrochem. Soc. 147 (2000) 214.
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