Unambiguous evidence for wurtzite phase in capped CdS quantum dots

Unambiguous evidence for wurtzite phase in capped CdS quantum dots

Solid State Communications 146 (2008) 425–427 Contents lists available at ScienceDirect Solid State Communications journal homepage: www.elsevier.co...

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Solid State Communications 146 (2008) 425–427

Contents lists available at ScienceDirect

Solid State Communications journal homepage: www.elsevier.com/locate/ssc

Unambiguous evidence for wurtzite phase in capped CdS quantum dots Surendra K. Gautam a,1 , Dhananjai Pandey a,∗ , S.N. Upadhyay b , Shahid Anwar c , N.P. Lalla c a School of Materials Science & Technology, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India b Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India c UGC-DAE Consortium for Scientific Research, Indore-452001, India

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Article history: Received 20 December 2007 Accepted 16 March 2008 by D.D. Sarma Available online 22 March 2008 PACS: 61.82.Rx

a b s t r a c t The problem of identification of the correct crystalline structure of CdS nanoclusters below 2.5 nm in size is outlined. Structure of thiophenol capped CdS nanoclusters in the size range 1.2–4.3 nm, synthesized using cadmium acetate solution in methanol, is discussed using powder XRD and electron diffraction data. Unambiguous confirmation of the wurtzite phase in CdS nanoclusters below 2.5 nm size is reported. The observation of 102 wurtzite peak in the XRD patterns of such nanoclusters indicates low stacking fault concentration. © 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Quantum dots A. Semiconductor C. Wurtzite structure C. Stacking fault

CdS is one of the most studied II-VI compound semiconductors. The band gap of CdS in bulk crystalline form is ∼2.5 eV. Reduction of CdS particle size to below 5 nm enables tuning of the band gap from ∼2.5 eV to ∼4.5 eV due to quantum confinement effects [1– 3]. Such nanocrystalline CdS quantum dots are extremely useful for optoelectronic applications [1,4,5]. Bulk CdS is known to crystallize in the thermodynamically stable hexagonal wurtzite (W) form, although the cubic sphalerite (S) form has also been reported due to kinetic factors [6]. In the capped nanoparticles of CdS, both wurtzite [7,8] and sphalerite [9– 12] structures have been reported in the literature, based on visual inspection of the diffraction peaks. However, the large broadening of the various Bragg peaks due to size effects and stacking faults [3] masks the unambiguous determination of the correct crystal structure of such small nanoparticles. For example, an unambiguous identification of the W structure cannot be made on the basis of the cluster of three intense peaks (100, 002 and 101) near 2θ ∼ 26.5 degrees (for Cu Kα ) as they all merge into one peak for sizes less than 5 nm, mimicking the S structure. The most characteristic distinguishing peaks 102 and 103 of the W phase are the ones which are also affected by stacking faults [3]. The stacking faults broaden these reflections so much that the 102

∗ Corresponding author. Tel.: +91 542 2307047; fax: +91 542 2368707. E-mail address: [email protected] (D. Pandey). 1 On leave of absence from Department of Chemistry, Tribhuvan University, Nepal. 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.03.020

peak nearly merges with the background while 103 gets absorbed by the broadening of the neighbouring 110 and 112 reflections. For unambiguous identification of the W phase, it is therefore imperative to synthesize capped nanoparticles of CdS with low stacking fault probability so that the 102 peak is observable. Theoretical calculations of quantum confinement effects in CdS nanoparticles have revealed that the band gaps and exciton energies of small clusters are different for the W and S phases [13]. Further, the presence of stacking faults will introduce a large number of undesirable trap sites in the forbidden energy gap [14]. The synthesis of nearly stacking fault free and phase pure nanoclusters of CdS exhibiting blue shift is therefore of immense interest. We show here for the first time that, by using an appropriate Cd-source compound and by controlling the dilution of its solution, it is possible to prepare phase pure quantum dots of CdS with an unambiguous signature of W structure in the powder XRD pattern even in the 1.2 nm size range. We have synthesized thiophenol capped CdS nanocrystalline semiconductors using Cd(CH3 COO)2 .2H2 O as a cadmium source compound, Na2 S as a source of sulfur and methanol as the solvent by the chemical precipitation technique. Five different stock solutions of 0.05, 0.075, 0.10, 0.15 and 0.20 M concentrations of cadmium acetate and sodium sulfide in methanol were prepared. CdS nanoparticles were obtained by dropwise addition of Na2 S solution into the mixture of thiophenol and Cd-source compound solutions with continuous stirring and ultrasonication at room temperature [15]. In [10] also, capped CdS nanoclusters were synthesized using Cd(Ac)2 , Na2 S and thiophenol in which the molarity of Cd(Ac)2 solution was kept fixed while the molarity of

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Fig. 2. Variation of band gap and particle size of CdS nanoparticles with dilution of Cd(CH3 COO)2 solution. Inset shows (αhυ)2 vs band gap (eV) plot for 0.05M dilution showing an average band gap of 4.3 eV. Fig. 1. XRD patterns of thiophenol capped CdS nanoparticles prepared from solutions of Cd(CH3 COO)2 in methanol with varying dilutions (a) 0.20 M (b) 0.15 M (c) 0.10 M (d) 0.075 M and (e) 0.05 M. The fluctuations around the smoothened curves are due to the higher CdS to thiophenol ratio as compared to that used in [10] (see Fig. 1) where also similar fluctuations were observed.

the thiophenol solution was varied such that the ratio of the two molarities decreases. In our synthesis procedure, the molarity of the thiophenol solution used was always twice that of Cd(Ac)2 solution (i.e. the ratio of the two molarities was fixed at 0.5). For obtaining fine nanoparticles, the dilution of the Cd(Ac)2 solution was varied instead and the solution was sonicated [15] during precipitation. The as-synthesized samples were characterized by X-ray diffraction using Cu Kα radiation of wavelength λ = 0.15406 nm on a Rigaku 18 kW rotating anode based powder diffractometer working in the Bragg–Brentano geometry and fitted with a curved crystal graphite monochromator in the diffracted beam. The data were collected in continuous scan mode in the twotheta range of 19–120 degrees at a scan rate of two degrees per minute and at a step interval of 0.02 degree. The X-ray generator was operated at 40 kV and 150 mA, which was found to be adequate for characterizing the nanoparticles. Optical absorption spectra of the thiophenol capped CdS nanoparticels were recorded at room temperature using a Perkin Elmer UV-vis absorption spectrometer in the wavelength range of 600–250 nm. Fig. 1(a) to (e) depict the powder XRD patterns of thiophenol capped CdS nanoparticles synthesized using 0.20, 0.15, 0.10, 0.075 and 0.05 M solutions of cadmium acetate in methanol. The variation of the band gap, obtained from the measured absorption spectra as a function of the concentration of the cadmium acetate solution, is shown in Fig. 2. The average size (d) of the nanoparticles was calculated from the measured band gap (1Eg ) using the following phenomenological equation proposed by Sapra and Sarma [16]: −d/b1

1Eg = a1 e

−d/b2

+ a2 e

,

(1)

where a1 , b1 , a2 and b2 are 2.83, 8.22, 1.96 and 18.07 respectively. The variation of size, so obtained, with concentration of the cadmium acetate solution is also shown in Fig. 2. Figs. 3 and 4 depict the results of TEM studies on samples prepared using 0.20 and 0.05 M cadmium acetate solutions. The sizes of the CdS nanoparticles prepared using 0.20 and 0.05 M solutions of cadmium acetate are found to be 4.3 and 1.2 nm,

Fig. 3. (a) Electron diffraction pattern and (b) transmission electron microscope (TEM) image of capped CdS nanoclusters prepared using 0.20M cadmium acetate solution.

respectively, on the basis of the optical band gap data. These are broadly consistent with the TEM images shown in Fig. 3(b) and 4(b). The structure of the capped CdS nanoparticles synthesized using 0.20 M cadmium acetate solution could not be unambiguously confirmed using powder diffraction data since the characteristic 102 peak of W and 200 peak of S, which occur near 2θ ∼ 36.5 and 2θ ∼ 31.5 degrees, respectively, are not seen in Fig. 1(a). However, electron diffraction patterns confirm the W structure of this sample as all the powder diffraction rings could be indexed using

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Fig. 4. (a) Electron diffraction pattern and (b) transmission electron microscope (TEM) image of capped CdS nanoclusters prepared using 0.05 M cadmium acetate solution.

a hexagonal lattice. A typical electron diffraction pattern is shown in Fig. 3(a) along with the wurtzite indices of the diffraction rings. With increasing dilution of the cadmium acetate solution, the size of the nanoparticles decreases as can be inferred from Fig. 1(b) to (e) where the 110 and 112 peaks are no longer resolved due to large Scherrer broadening. In addition, new features appear in the XRD patterns which may be summarized as follows: (i) the 110 and 112 peaks of W phase in Fig. 1(a) merge into one asymmetric peak (centred around 2θ ∼ 50 degrees) with a subsidiary peak near the 004 position of the W phase with increasing dilution, (ii) the single peak, corresponding to the 100, 002 and 101 reflections (centred around 2θ ∼ 26.5 degrees) of the W phase in Fig. 1(a), splits at higher dilutions [see Fig. 1(d) and (e)] into two peaks corresponding to the 002 and 101 peaks of W phase, (iii) a small peak at the 102 position of the W phase appears, whose intensity increases with increasing dilution. The smallness of the nanocrystallites leads to broadening of all the Bragg reflections as per the Scherrer formula. However, the presence of stacking faults in the wurtzite phase leads to broadening of only those HK.L Bragg reflections for which H − K 6= 0 mod3 [17]. Thus the reflections like 002, 110, 112 and many others for which H − K = 0 mod3 will not be affected by faulting. But these reflections cannot be used for unambiguous confirmation of the W phase, as they are common to the S phase also. The prominent reflections, which are unique to the W phase and are not present in the S phase, are 100, 101, 102, 103, as the other reflections at angles higher than 2θ = 55 degrees are of very low intensity and are washed away into the background for fine nanocrystallites of size less than 4 nm. With decreasing nanocrystallite size and in the presence of stacking faults, the 100 and 101 peaks of W phase almost merge into a broad profile centred around the 002 position (2θ ∼ 26.5 degrees), as can be seen from Fig. 1(a) to (c). Further, the 103 peak nearly gets masked by the Scherrer broadening of the adjoining 110 and 112 peaks. This leaves only the 102 peak of W phase at 2θ ∼ 36.5 degrees in bulk CdS which, in principle, is not masked by

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the Scherrer broadening of the nearby Bragg peaks. However, the intensity of this peak is rather low and even a small concentration of stacking faults broadens it so much that it merges with the background (see for examples Fig. 12 of Ref. [3]). This is the reason why this peak is not seen in Fig. 1(a) for 4.3 nm particles. However, it is intriguing to note that the intensity of this reflection grows with decreasing particle size, as can be seen from Fig. 1(b)–(e), which has to be attributed to the decreasing concentration of stacking faults with size reduction. Further, the emergence of the 101 peak, from the broad peak centred around 002 position in Fig. 1(a), as the size decreases, further confirms the decreasing concentration of stacking faults. The 100 peak of the W phase, which occurs on the lower 2θ side of the 002 peak, and which is merged with 002 and 101 peaks in Fig. 1(a), is also a fault affected peak and should have emerged separately with decreasing stacking fault concentration in Fig. 1(d) and (e). The fact that this does not happen suggests that the size reduction is not only improving the crystalline perfection (less stacking fault concentration) but is also leading to a transformation of the shape. Wickham et al. [18] have simulated the powder diffraction patterns for clusters of II–VI compounds of different shapes and various stacking fault concentrations. A comparison of the simulated patterns shown in Ref. [18] with the patterns given in Fig. 1 suggests that the shape of the particles in Fig. 1(a) could be cylindrical with an aspect ratio slightly greater than one, whereas the shape corresponding to 1(e) may correspond to a slightly oblate sphere (see Fig. 2(e) and (a) of Ref. [18]). Our results thus clearly show that the stacking fault concentration decreases with decreasing particle size leading to unambiguous identification of W structure even in 1.2 nm (average spherical radius from optical band gap data) CdS clusters through the presence of the 102 peak. Our results also indicate that the shape of the nanoclusters may change from nearly cylindrical with low aspect ratio to oblate spherical leading to the emergence of the 101 peak of the W structure even in the 1.2 nm clusters. To the best of our knowledge, for CdS nanoclusters of 1.2 nm average size, such an unambiguous confirmation of the presence of bulk W structure has not been reported before. Acknowledgements One of the authors SKG acknowledges the partial financial support from University Grants Commission (UGC), Nepal. DP acknowledges helpful discussions with Professor D. D. Sarma. References [1] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [2] M.A. Marcus, W. Flood, M. Stiegerwald, L. Brus, M. Bawendi, J. Phys. Chem. 95 (1995) 1572. [3] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [4] Photoelectronic Materials and Devices edited by S. Laranch (Van Nostrand, Princeton, NJ, 1965); Handbook of Optics (McGrow – Hill, 1995), vol. 1: EG G VACTEC, Optoelectronics Data Book. [5] M. Euta, H. Kanzaki, K. Kobayashi, Y. Toyozawa, E. Hanamura (Eds.), Excitonic Processes in Solid, in: Springer Series in Solid State Sciences, vol. 60, Springer, Berlin, 1986. [6] M. Haase, A.P. Alivisatos, J. Phys. Chem. 96 (1992) 6756. [7] T. Vossmeyer, L. Katisikas, M. Giersig, I.G. Popovic, K. Diesner, A. Chemseddine, A. Eychmuller, H. Weller, J. Phys. Chem. 98 (1994) 7665. [8] A. Datta, A. Priyam, S. Chatterjee, A.K. Sinha, S.N. Bhattacharya, A. Saha, Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 239. [9] Y. Wang, N. Herron, Phys. Rev. B 42 (1990) 7253. [10] N. Herron, Y. Wang, H. Ecker, J. Am. Chem. Soc. 112 (1990) 1322. [11] C. Li, Y. Tang, K. Yao, F. Zhou, Q. Ma, H. Lin, M. Tao, J. Liang, Carbon 44 (2006) 2021. [12] B.A. Simmons, S. Li, V.T. John, G.L. McPherson, A. Bose, W. Zhou, J. He, Nano Letters 2 (2002) 263. [13] M.V. Ramakrishna, R.A. Friesner, J.Chem.Phys. 95 (1991) 8309. [14] M.G. Bawendi, A.R Kortan, M.L. Steigerwald, L.E. Brus, J. Chem. Phys. 11 (1989) 7281. [15] G. Wang, G. Li, C. Liang, L. Zhang, Chem. Lett. 30 (2001) 344. [16] S. Sapra, D.D. Sarma, Phys. Rev. B 69 (2004) 125304. [17] B.E. Warren, X-ray Diffraction, Addison Wesley, New York, 1969. [18] J.N. Wickham, A.B. Herhold, A.P. Alivisatos, Phys. Rev. Lett 84 (2000) 923.