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XANES studies of CdS nano-structures on porous silicon
Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229–233 www.elsevier.nl / locate / elspec
XANES studies of CdS nano-structures on porous silicon P. Zhang, P.S. Kim, T.K. Sham* Department of Chemistry, The University of Western Ontario, London, Ontario N6 A 5 B7, Canada Received 14 December 2000; received in revised form 15 March 2001; accepted 19 March 2001
Abstract CdS nano-structures were produced electrochemically using porous silicon (PS) as a template / substrate. AFM and SEM images show that the CdS deposit are grains of |100 nm, each of which is an aggregate of smaller particles of several nanometers. XANES studies of the CdS nano-structures together with bulk CdS were conducted. The electronic behavior and optical properties of the CdS / PS composite, particularly those relevant to nano-size effects, were discussed. 2001 Elsevier Science B.V. All rights reserved. Keywords: CdS; Nano-particles; XANES
1. Introduction The past decade has witnessed increased research interests in porous silicon (PS), especially its optical properties. Many other properties relevant to its nano-porous structure have also been exploited. For example, PS was recently reported to exhibit important advantages over a plain silicon wafer as a substrate on which ordered carbon nanotube arrays can be fabricated using a CVD method [1]. In order to examine whether or not PS is a suitable template to produce other semiconductor nano-structures, we have conducted a series of experiments using a model II–VI semiconductor compound CdS and an electrochemical method to deposit CdS on PS. The obtained CdS deposits were characterized with many techniques such as AFM, SEM, XRD, XPS, EXAFS, XANES and XEOL. In this report, we will focus on *Corresponding author. Tel.: 11-519-679-2111, extn. 6341; fax: 11-519-661-3022. E-mail address: [email protected] (T.K. Sham).
the synchrotron-based XANES (X-ray absorption near edge structure) and XEOL (X-ray excited optical luminescence) studies of its electronic and optical properties. Other results on chemical properties and characterization will be described elsewhere.
2. Experimental Porous silicon was prepared using a regular n-type N lightly-doped silicon wafer etched in 25% HF / ethanol solution at a constant current of 50 mA / cm 2 . The as-prepared PS was immediately rinsed with alcohol, acetone and dimethylsulfone (DMSO) and then employed as a cathode for CdS deposition. Preparation of CdS was conducted in an electroplating cell containing 10 g / l CdCl 2 and 6 g / l S solution (S dissolved in DMSO) at a constant current of 0.5 mA / cm 2 at a temperature of 110–1208C. After deposition, the sample was rinsed with hot DMSO, acetone and alcohol in sequence. XANES
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 01 )00297-3
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P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
was conducted at the DCM beam line of the Canadian Synchrotron Radiation Facility (CSRF) at Synchrotron Radiation Center (SRC), University of Wisconsin-Madison. The XEOL signal is detected using a JY H-100 monochromator coupled to a Hamamatsu photomultiplier (R943-02). The microscopic measurements were carried out at Surface Science Western (SSW) using a Topometrix TMX 2000 Explorer atomic force microscope (AFM) and a Hitachi S-4500 scanning electron microscope (SEM).
3. Results and discussion A three-dimensional AFM image of the CdS deposits is shown in Fig. 1a. The CdS deposits appear to be a dense grain-like structure, each grain being |100 nm. More detailed information about the morphology was provided by high-resolution field emission SEM (Fig. 1b). Aggregates of smaller particles of several nanometers can clearly be observed. The porous structure of a few nanometers on the PS surface can also be seen in the background. The formation of these smaller nano-particles is related to the nano-porous structure of the template since a similar procedure using a Si wafer does not yield such nano-CdS. It is of great interest to determine the chemical and physical properties of these CdS structures and the role of the nano-particles in the aggregate using absorption spectroscopy. Fig. 2 shows the S K-edge X-ray absorption near edge structures (XANES) of CdS / PS recorded in total electron yield (TEY) together with the TEY spectrum of a polycrystalline CdS powder (hexagonal crystalline phase, obtained commercially from Aldrich). From Fig. 2 we can clearly see a blue shift in both the whiteline maximum (|0.5 eV) and the absorption threshold (inflection point) in CdS / PS relative to bulk CdS. It is well established that when the size of a semiconductor particle is small enough (typically several nanometers), a band gap widening, or quantum size effect, will occur [2]. A blue shift in the absorption spectrum is strong evidence for quantum confinement. In CdS this occurs at about 7 nm diameter [2]. Close examination of Fig. 1b reveals that the dimension of the smaller CdS
particles on the surface of the aggregate is of the order of several nanometers. If these nano-particles are the main contributor to the aggregate, it would account for the blue shift (quantum size effect) in the S K-edge XANES of CdS / PS. Other noticeable differences were found when comparing the above-the-edge region of the XANES. The first two resonances, a 2 and b 2 , of CdS / PS are narrower relative to a 1 and b 1 of the bulk. The bulk peaks are characteristic of the 1s→conduction band transition in CdS [3]. Of particular interest is an additional feature found in the region 2480–2490 eV. Bulk CdS shows a peak at 2484.0 eV, c 1 , whereas CdS / PS has a peak, c 2 , at 2481.8 eV followed by a shoulder, c 29 , in the higher energy region. The EXAFS region (not shown) for both specimens exhibits similar oscillations, indicating the CdS on PS is crystalline. The sharp features b 2 / 2477.1 eV and c 2 / 2481.8 eV in CdS / PS are similar to the whitelines seen previously in SO 322 / 2477.4 eV (Na 2 SO 3 ) and SO 22 4 / 2481.3 eV (CaSO 4 ), as reported in the literature [4–6]. It is well known that when particles become smaller, more atoms will be located on the surface, resulting in increased surface sensitivity in electron yield measurements. Thus it is likely that, for CdS nanoparticles on PS, the surface is oxidized, forming a passivated layer of SO 22 x . The surface origin of the sulfate species is confirmed by its absence in the S K-edge XANES recorded in the X-ray fluorescence yield (FLY), a bulk sensitive mode (Fig. 3). It is evident from Fig. 3a that the oxide peaks b 2 and c 2 in the TEY of CdS / PS are suppressed significantly in FLY, indicating that SO 22 x are indeed surface components of the CdS particles. Such surface components are obviously not readily present in the bulk CdS powder sample, as deduced from the similarity of its TEY and FLY in Fig. 3b [7]. The optical properties of the PS-based composites have been widely investigated [8]. However, when both PS and nano-CdS contribute to the luminescence, identification of the origin of the luminescence becomes very challenging. X-ray excited optical luminescence (XEOL) using a tunable excitation photon source (synchrotron) is a powerful technique to facilitate this type of study by providing element-specific information about the luminescence
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
231
Fig. 1. (a) AFM image of CdS deposits on PS. (b) High resolution SEM. Smaller particles of several nanometers can be seen in each larger grain.
which in turn can be used to monitor the absorption across the edge (optical XAFS). Fig. 4 shows the XEOL with excitation energy below (A) and above the Si K-edge (B). The XEOL excited at the Si K-edge (B) shows a wide orange-red color lumines-
cence band together with a weak emission at |530 nm. This orange-red luminescence is a very typical band observed in many PS samples [9,10]. However, when the excitation photon energy was tuned to below the Si K-edge, the one absorption length
232
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
Fig. 2. S K-edge XANES (in TEY) of bulk CdS and deposits on PS (CdS / PS). The spectra have been shifted vertically for clarity. Inset: XANES at the edge jump area of the two specimens.
increases dramatically from 1.3 to 14 mm and now only a relatively small fraction of excitation photons are absorbed by Si atoms in the PS layer. Consequently, the orange-red color XEOL band becomes very weak. However, the green color luminescence band centered at |530 nm changes little and is now clearly visible. This XEOL band is similar to what we observed in polymer-stabilized CdS quantum dots
Fig. 4. XEOL spectra of CdS / PS excited at (A) Si pre-edge and (B) Si K-edge. Inset: Si K-edge XANES of CdS / PS.
[11] and is attributed to a nano-CdS origin. These observations indicate that both PS and CdS contribute to the luminescence, although the luminescence
Fig. 3. S K-edge XANES in TEY and FLY modes for (a) CdS / PS and (b) bulk CdS.
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
from CdS is interfered by that of PS because the latter has a more intense and broad luminescence band. This observation is consistent with the results of XEOL selectively excited near the S K-edge [12]. 4. Summary CdS grains of |100 nm were successfully electrodeposited on a PS template. These grains were found to be aggregates of smaller particles of a few nanometers. XANES and XEOL studies reveal that these smaller nanoparticles are passivated with oxides on the surface and are responsible for the green luminescence. Acknowledgements We thank CSRF staff K.H. Tan and Y.-F Hu for their assistance in the measurements at CSRF, and Surface Science Western (SSW) for the AFM and SEM measurements. Research at UWO is supported by NSERC and SRC is supported by US NSF grant [DMR-95- 31009.
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References [1] S. Fan, M. Chapline, N. Franklin, T. Tombler, A. Cassell, H. Dai, Science 283 (1999) 512. [2] M. Steigerwald, L. Brus, Acc. Chem. Res. 23 (1990) 183. [3] A. Bianconi, in: D. Koningsberger, R. Prins (Eds.), X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley, New York, 1988, p. 573. [4] D. Li, G. Bancroft, M. Kasrai, M. Fleet, X. Feng, K. Tan, Can. Mineralogist 33 (1995) 949. [5] D. Li, G. Bancroft, M. Kasrai, M. Fleet, X. Feng, K. Tan, B. Yang, J. Phys. Chem. Solids 55 (1995) 535. [6] D. Li, G. Bancroft, M. Kasrai, M. Fleet, B. Yang, X. Feng, K. Tan, M. Peng, Phys. Chem. Miner. 20 (1994) 489. [7] T.K. Sham, I. Coulthard, J. Synchrotron Rad. 6 (1999) 215. [8] A. Cullis, L. Canham, P. Calcott, J. Appl. Phys. 82 (1997) 909. [9] T.K. Sham, D. Jiang, I. Coulthard, J. Lorimer, X. Feng, K. Tan, F. Frigo, R. Rosenberg, D. Houghton, B. Brystkewicz, Nature 363 (1993) 331. [10] L. Canham, in: L. Canham (Ed.), Properties of Porous Silicon, INSPEC-IEE, London, 1997, p. 249. [11] P. Zhang, S. Naftel, T.K. Sham, J. Chem. Phys., submitted for publication. [12] The XEOLs below and just above the S K-edge are both dominated by a broad orange-red luminescence band with similar intensity. The details will be published elsewhere.
XANES studies of CdS nano-structures on porous silicon P. Zhang, P.S. Kim, T.K. Sham* Department of Chemistry, The University of Western Ontario, London, Ontario N6 A 5 B7, Canada Received 14 December 2000; received in revised form 15 March 2001; accepted 19 March 2001
Abstract CdS nano-structures were produced electrochemically using porous silicon (PS) as a template / substrate. AFM and SEM images show that the CdS deposit are grains of |100 nm, each of which is an aggregate of smaller particles of several nanometers. XANES studies of the CdS nano-structures together with bulk CdS were conducted. The electronic behavior and optical properties of the CdS / PS composite, particularly those relevant to nano-size effects, were discussed. 2001 Elsevier Science B.V. All rights reserved. Keywords: CdS; Nano-particles; XANES
1. Introduction The past decade has witnessed increased research interests in porous silicon (PS), especially its optical properties. Many other properties relevant to its nano-porous structure have also been exploited. For example, PS was recently reported to exhibit important advantages over a plain silicon wafer as a substrate on which ordered carbon nanotube arrays can be fabricated using a CVD method [1]. In order to examine whether or not PS is a suitable template to produce other semiconductor nano-structures, we have conducted a series of experiments using a model II–VI semiconductor compound CdS and an electrochemical method to deposit CdS on PS. The obtained CdS deposits were characterized with many techniques such as AFM, SEM, XRD, XPS, EXAFS, XANES and XEOL. In this report, we will focus on *Corresponding author. Tel.: 11-519-679-2111, extn. 6341; fax: 11-519-661-3022. E-mail address: [email protected] (T.K. Sham).
the synchrotron-based XANES (X-ray absorption near edge structure) and XEOL (X-ray excited optical luminescence) studies of its electronic and optical properties. Other results on chemical properties and characterization will be described elsewhere.
2. Experimental Porous silicon was prepared using a regular n-type N lightly-doped silicon wafer etched in 25% HF / ethanol solution at a constant current of 50 mA / cm 2 . The as-prepared PS was immediately rinsed with alcohol, acetone and dimethylsulfone (DMSO) and then employed as a cathode for CdS deposition. Preparation of CdS was conducted in an electroplating cell containing 10 g / l CdCl 2 and 6 g / l S solution (S dissolved in DMSO) at a constant current of 0.5 mA / cm 2 at a temperature of 110–1208C. After deposition, the sample was rinsed with hot DMSO, acetone and alcohol in sequence. XANES
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 01 )00297-3
230
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
was conducted at the DCM beam line of the Canadian Synchrotron Radiation Facility (CSRF) at Synchrotron Radiation Center (SRC), University of Wisconsin-Madison. The XEOL signal is detected using a JY H-100 monochromator coupled to a Hamamatsu photomultiplier (R943-02). The microscopic measurements were carried out at Surface Science Western (SSW) using a Topometrix TMX 2000 Explorer atomic force microscope (AFM) and a Hitachi S-4500 scanning electron microscope (SEM).
3. Results and discussion A three-dimensional AFM image of the CdS deposits is shown in Fig. 1a. The CdS deposits appear to be a dense grain-like structure, each grain being |100 nm. More detailed information about the morphology was provided by high-resolution field emission SEM (Fig. 1b). Aggregates of smaller particles of several nanometers can clearly be observed. The porous structure of a few nanometers on the PS surface can also be seen in the background. The formation of these smaller nano-particles is related to the nano-porous structure of the template since a similar procedure using a Si wafer does not yield such nano-CdS. It is of great interest to determine the chemical and physical properties of these CdS structures and the role of the nano-particles in the aggregate using absorption spectroscopy. Fig. 2 shows the S K-edge X-ray absorption near edge structures (XANES) of CdS / PS recorded in total electron yield (TEY) together with the TEY spectrum of a polycrystalline CdS powder (hexagonal crystalline phase, obtained commercially from Aldrich). From Fig. 2 we can clearly see a blue shift in both the whiteline maximum (|0.5 eV) and the absorption threshold (inflection point) in CdS / PS relative to bulk CdS. It is well established that when the size of a semiconductor particle is small enough (typically several nanometers), a band gap widening, or quantum size effect, will occur [2]. A blue shift in the absorption spectrum is strong evidence for quantum confinement. In CdS this occurs at about 7 nm diameter [2]. Close examination of Fig. 1b reveals that the dimension of the smaller CdS
particles on the surface of the aggregate is of the order of several nanometers. If these nano-particles are the main contributor to the aggregate, it would account for the blue shift (quantum size effect) in the S K-edge XANES of CdS / PS. Other noticeable differences were found when comparing the above-the-edge region of the XANES. The first two resonances, a 2 and b 2 , of CdS / PS are narrower relative to a 1 and b 1 of the bulk. The bulk peaks are characteristic of the 1s→conduction band transition in CdS [3]. Of particular interest is an additional feature found in the region 2480–2490 eV. Bulk CdS shows a peak at 2484.0 eV, c 1 , whereas CdS / PS has a peak, c 2 , at 2481.8 eV followed by a shoulder, c 29 , in the higher energy region. The EXAFS region (not shown) for both specimens exhibits similar oscillations, indicating the CdS on PS is crystalline. The sharp features b 2 / 2477.1 eV and c 2 / 2481.8 eV in CdS / PS are similar to the whitelines seen previously in SO 322 / 2477.4 eV (Na 2 SO 3 ) and SO 22 4 / 2481.3 eV (CaSO 4 ), as reported in the literature [4–6]. It is well known that when particles become smaller, more atoms will be located on the surface, resulting in increased surface sensitivity in electron yield measurements. Thus it is likely that, for CdS nanoparticles on PS, the surface is oxidized, forming a passivated layer of SO 22 x . The surface origin of the sulfate species is confirmed by its absence in the S K-edge XANES recorded in the X-ray fluorescence yield (FLY), a bulk sensitive mode (Fig. 3). It is evident from Fig. 3a that the oxide peaks b 2 and c 2 in the TEY of CdS / PS are suppressed significantly in FLY, indicating that SO 22 x are indeed surface components of the CdS particles. Such surface components are obviously not readily present in the bulk CdS powder sample, as deduced from the similarity of its TEY and FLY in Fig. 3b [7]. The optical properties of the PS-based composites have been widely investigated [8]. However, when both PS and nano-CdS contribute to the luminescence, identification of the origin of the luminescence becomes very challenging. X-ray excited optical luminescence (XEOL) using a tunable excitation photon source (synchrotron) is a powerful technique to facilitate this type of study by providing element-specific information about the luminescence
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
231
Fig. 1. (a) AFM image of CdS deposits on PS. (b) High resolution SEM. Smaller particles of several nanometers can be seen in each larger grain.
which in turn can be used to monitor the absorption across the edge (optical XAFS). Fig. 4 shows the XEOL with excitation energy below (A) and above the Si K-edge (B). The XEOL excited at the Si K-edge (B) shows a wide orange-red color lumines-
cence band together with a weak emission at |530 nm. This orange-red luminescence is a very typical band observed in many PS samples [9,10]. However, when the excitation photon energy was tuned to below the Si K-edge, the one absorption length
232
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
Fig. 2. S K-edge XANES (in TEY) of bulk CdS and deposits on PS (CdS / PS). The spectra have been shifted vertically for clarity. Inset: XANES at the edge jump area of the two specimens.
increases dramatically from 1.3 to 14 mm and now only a relatively small fraction of excitation photons are absorbed by Si atoms in the PS layer. Consequently, the orange-red color XEOL band becomes very weak. However, the green color luminescence band centered at |530 nm changes little and is now clearly visible. This XEOL band is similar to what we observed in polymer-stabilized CdS quantum dots
Fig. 4. XEOL spectra of CdS / PS excited at (A) Si pre-edge and (B) Si K-edge. Inset: Si K-edge XANES of CdS / PS.
[11] and is attributed to a nano-CdS origin. These observations indicate that both PS and CdS contribute to the luminescence, although the luminescence
Fig. 3. S K-edge XANES in TEY and FLY modes for (a) CdS / PS and (b) bulk CdS.
P. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 229 – 233
from CdS is interfered by that of PS because the latter has a more intense and broad luminescence band. This observation is consistent with the results of XEOL selectively excited near the S K-edge [12]. 4. Summary CdS grains of |100 nm were successfully electrodeposited on a PS template. These grains were found to be aggregates of smaller particles of a few nanometers. XANES and XEOL studies reveal that these smaller nanoparticles are passivated with oxides on the surface and are responsible for the green luminescence. Acknowledgements We thank CSRF staff K.H. Tan and Y.-F Hu for their assistance in the measurements at CSRF, and Surface Science Western (SSW) for the AFM and SEM measurements. Research at UWO is supported by NSERC and SRC is supported by US NSF grant [DMR-95- 31009.
233
References [1] S. Fan, M. Chapline, N. Franklin, T. Tombler, A. Cassell, H. Dai, Science 283 (1999) 512. [2] M. Steigerwald, L. Brus, Acc. Chem. Res. 23 (1990) 183. [3] A. Bianconi, in: D. Koningsberger, R. Prins (Eds.), X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley, New York, 1988, p. 573. [4] D. Li, G. Bancroft, M. Kasrai, M. Fleet, X. Feng, K. Tan, Can. Mineralogist 33 (1995) 949. [5] D. Li, G. Bancroft, M. Kasrai, M. Fleet, X. Feng, K. Tan, B. Yang, J. Phys. Chem. Solids 55 (1995) 535. [6] D. Li, G. Bancroft, M. Kasrai, M. Fleet, B. Yang, X. Feng, K. Tan, M. Peng, Phys. Chem. Miner. 20 (1994) 489. [7] T.K. Sham, I. Coulthard, J. Synchrotron Rad. 6 (1999) 215. [8] A. Cullis, L. Canham, P. Calcott, J. Appl. Phys. 82 (1997) 909. [9] T.K. Sham, D. Jiang, I. Coulthard, J. Lorimer, X. Feng, K. Tan, F. Frigo, R. Rosenberg, D. Houghton, B. Brystkewicz, Nature 363 (1993) 331. [10] L. Canham, in: L. Canham (Ed.), Properties of Porous Silicon, INSPEC-IEE, London, 1997, p. 249. [11] P. Zhang, S. Naftel, T.K. Sham, J. Chem. Phys., submitted for publication. [12] The XEOLs below and just above the S K-edge are both dominated by a broad orange-red luminescence band with similar intensity. The details will be published elsewhere.