Applied Surface Science

Applied Surface Science 257 (2011) 6779 6786 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Synthesis and characterization of Cu 2+ doped ZnS nanoparticles using TOPO and SHMP as capping agents M. Kuppayee a, G.K. Vanathi Nachiyar a, V. Ramasamy b, a Department of Physics, Sri Sarada College for Women, Salem, Tamilnadu, India b Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram, Tamilnadu 608002, India article info abstract Article history: Received 14 January 2011 Received in revised form 23 February 2011 Accepted 24 February 2011 Available online 3 March 2011 Keywords: ZnS:Cu 2+ nanoparticles Band gap Semiconductor Luminescence X-ray diffraction TEM Undoped and Cu 2+ doped (0.2 0.8%) ZnS nanoparticles have been synthesized through chemical precipitation method. Tri-n-octylphosphine oxide (TOPO) and sodium hexametaphosphate (SHMP) were used as capping agents. The synthesized nanoparticles have been analyzed using X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectrometer (FT-IR), UV vis spectrometer, photoluminescence (PL) and thermo gravimetric-differential scanning calorimetry (TG-DTA) analysis. The size of the particles is found to be 4 6 nm range. Photoluminescence spectra were recorded for ZnS:Cu 2+ under the excitation wavelength of 320 nm. The prepared Cu 2+ -doped sample shows efficient PL emission in 470 525 nm region. The capped ZnS:Cu emission intensity is enhanced than the uncapped particles. The doping ions were identified by electron spin resonance (ESR) spectrometer. The phase changes were observed in different temperatures. 2011 Elsevier B.V. All rights reserved. 1. Introduction Nanocrystalline materials have been studied extensively, especially II IV semiconductor nanoparticles, showing interesting size-dependent electrical and optical properties due to the quantum confinement effect of the carriers [1]. Especially, ZnS is a direct-transition semiconductor with the widest energy band gap among the groups II VI compound semiconductor materials, and it is an important material with an extensive range of applications from blue/green light-emitting diodes (LEDs) and electroluminescence devices (ELDs) to optoelectric modulators, data storage, data transfer and coatings which are sensitive to UV light. Then the most striking feature of ZnS nanocrystallites is that their chemical and physical properties differ dramatically from those of the bulk solids. This is particularly due to the high dispersity of nanocrystalline system, i.e. the number of atoms at the surfaces is comparable to the number of those located in the crystalline lattice [2]. In addition, their luminescence properties show to depend on the doped ions strongly because these ions can lead to the formation of novel luminescent centers in ZnS nanocrystallines. There have been a number of reports on the properties of transition metal ion doped ZnS nanocrystals prepared using different techniques [3 5]. Corresponding author. E-mail address: srsaranram@rediffmail.com (V. Ramasamy). ZnS nanoparticles have been mostly synthesized using various methods such as inverse micelle [6], zeolite [7] or vapor phase condensation method [8] and chemical synthesize method [9]. In this study, copper doped ZnS nanoparticles were prepared using chemical method. Generally doped ZnS semiconductor materials have a wide range of applications in luminescence devices. Accordingly, special attention has been paid to their luminescence properties. In ZnS:Mn nanoparticles, an orange luminescence has been well documented, which is attributed to the 4 T 1 6 A 1 transition of Mn 2+ ions excited via energy transfer from the host ZnS [1]. Similar intraionic emission from the characteristic 4f 7 4f 6 5d 1 transition of Eu 2+ was also observed in ZnS:Cu nanoparticles [10]. However, in the case of nanosized ZnS:Cu, the luminescence properties are still controversial. Two emission bands (blue and green) were observed together in the same sample of ZnS:Cu, such as 420 and 520 nm by Lee et al. [11] and 460 and 507 nm by Xu et al. [12]. On the other hand, a single emission peak was also observed at 480 nm by Kumar et al. [5] and at 415 nm by Huang et al. [13]. In this paper, we reported the synthesis and optical studies of ZnS:Cu nanoparticles. The grain size is calculated from the XRD line broadening and morphology of the same sample was studied from TEM analysis. Capping molecules were identified by FT-IR. The impurity ion (Cu) was identified by ESR. The optical properties of the samples by UV vis and photoluminescence and thermal properties by TG-DTA were studied. TOPO and SHMP were used as surfactants. 0169-4332/$ see front matter 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.124

6780 M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 2. Experimental All steps of the synthesis were carried out at room temperature and all chemicals used were AR grade. The water is used as a solvent in the experiments. First, a desired molar proportion of Zn(CH 3 COO) 2 2H 2 O in 50 ml de-ionized water and Cu II(CH 3 COO) 2 4H 2 O (wt% in Zn = 0.20%, 0.4%, 0.6%, and 0.8%) in 50 ml de-ionized water were dissolved and surfactant was added to control the growth of the nanocrystals during the reaction. Subsequently, the Na 2 S (50 ml) was added drop wise by an additional funnel to the above mixture. For an each experiment, the molar amounts of Zn(CH 3 COO) 2 and Na 2 S used were equal. Undoped ZnS nanoparticles were synthesized by following the same procedure without doping and capping agents. During the whole reaction process, the reactants were vigorously stirred under air atmosphere at 80 C. The formed nanocrystals were separated from the solution by centrifuging. After that, it is washed repeatedly using deionized water and then dried at 60 C under vacuum. Then, the powder samples of ZnS:Cu nanocrystals were obtained. The X-ray diffraction (XRD) patterns of the powdered samples were recorded using X pert PRO diffractometer with a Cu K radiation ( = 1.54060 Å) under the same conditions. The crystallite size was estimated using the Scherrer equation (0.9)/(ˇ cos ) at the full width at half maximum of the major XRD peak. The size and morphology of the nanoparticles were determined using TEM (PHILIPS-CM200; 20 200 kv). For sample preparation, dilute drops of suspension were allowed to dry slowly on carbon-coated copper grids for TEM measurement. The FT-IR spectra were obtained on an AVATOR 360 spectrometer. Electron spin resonance (ESR) spectra of the uncapped and capped powdered ZnS:Cu were measured on an EPR spectrometer (Bruker EMX Plus) at room temperature. The optical transmission/absorption spectra of the same particles in de-ionized water were recorded using UV-1650PC SHIMADZU spectrometer. Fluorescence measurements were performed on a RF-5301PC spectrophotometer. Emission (350 650 nm) spectra were recorded under 315, 302 and 294 nm for uncapped, TOPO and SHMP capped ZnS:Cu nanoparticles, respectively. The thermal analysis was carried out with SDT Q600 20 thermometer. 3. Results and discussion 3.1. Structural and morphology The XRD patterns of the ZnS and ZnS:Cu nanoparticles (Fig. 1) show very broad diffraction peaks, which is the characteristics of nanosized materials. For all the samples, three diffraction peak positions correspond to the lattice planes of (1 1 1), (2 2 0), and (3 1 1), are very well matched with the cubic zinc blended structure (JCPDS No. 05-0566). It also dictates that there is no diffraction peaks from copper impurities. Thus it is observed that the Cu 2+ ions are dispersed into the ZnS matrix. According to the Debye Scherrer formula [14], the mean crystalline sizes calculated from the fullwidth at half-maximum (FWHM) of these lines are about 5 and 5.3 nm for uncapped ZnS and ZnS:Cu nanoparticles. Further, relatively larger line broadening in surfactant (TOPO and SHMP) capped samples indicates their smaller particle size as compared to uncapped samples. From the XRD line width, the mean crystalline domain sizes have been estimated to be around 4.4 and 3.9 nm for TOPO and SHMP capped ZnS:Cu 2+ particles, respectively. This result indicates necessity of the capping agent for the reduction of particles size. By comparing the effect of two surfactants, the SHMP is used as an effective surfactant to reduce the particles size, which may be due to the presence of the sodium ions in this surfactant. Fig. 1. XRD spectra of ZnS, uncapped and surfactants capped ZnS:Cu 2+ nanoparticles. It is necessary to obtain the particle size and the information about the nanostructure by direct measurement, such as transmission electron microscope (TEM), which can reveal the size and morphology of the particles. Fig. 2 shows the TEM images of TOPOcapped ZnS:Cu 2+ nanoparticles with the corresponding diffraction pattern. Presence of fine ZnS:Cu 2+ nanoparticles with capped TOPO are clearly visible in the TEM picture (Fig. 2a). The diffraction pattern (Fig. 2c) of the sample consists of a central halo with concentric broad rings. The rings correspond to the reflections from (1 1 1), (2 2 0) and (3 1 1) planes confirmed the cubic structure of the bulk ZnS. Fig. 3a and b shows, TEM images of SHMP capped ZnS:Cu 2+ nanoparticles. As seen in Fig. 3, the particles are well dispersed with small size. The SAED pattern consists of broad diffuse rings due to small ZnS:Cu 2+ particle. The discrete bright spots in selected area electron diffraction pattern reveal well-crystallized cubic from in the ZnS:Cu 2+ nanoparticles. The average size of the nanocrystallites

M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 6781 Fig. 2. (a) and (b) TEM spectra of TOPO capped ZnS:Cu nanoparticles and (c) corresponding SAED pattern. Fig. 3. (a) and (b) TEM spectra of SHMP of ZnS:Cu nanoparticles and (c) corresponding SAED pattern.

6782 M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 Fig. 4. FT-IR spectra of pure (TOPO) and uncapped, TOPO and SHMP capped ZnS:Cu nanoparticles. determined from TEM is around 4.5 and 4 nm for TOPO and SHMP capped ZnS:Cu 2+ nanoparticles, respectively. The TEM images of the particles clearly show that the particles are in uniform sizes, which are in agreement with the XRD studies. 3.2. FT-IR study FT-IR spectra of the uncapped and capped with TOPO ZnS:Cu 2+ nanoparticles were recorded in the range 4000 400 cm 1 and are shown in Fig. 4a. The peaks appearing at 1110, 618 and 491 cm 1 are due to Zn S vibration and 2924, 2364 and 1635 cm 1 are due to microstructure formation of the sample. The obtained peak values are in good agreement with the reported values [15]. However, the presence of strong IR peak in capped particles at 1490 cm 1 along with a shoulder at 1467 cm 1 are due to P O stretching vibrational modes indicate the signatures of capping agent, i.e., TOPO bounded to ZnS:Cu nanocrystals. FT-IR spectra of the uncapped and SHMP capped ZnS:Cu 2+ nanoparticles were recorded and are shown in Fig. 4b. The larger number of IR bending modes (<1000 cm 1 ) observed for SHMP capped ZnS:Cu nanoparticles indicate the formation of smaller particles when compared with uncapped ones. The peaks observed at 1636 and 899 cm 1 indicate nitrogen oxygen interaction, whereas the peak appearing at 554 cm 1 may be due to sulfur oxygen interaction. The presence of the bands at 1261 and 1097 cm 1 ensures phosphorous oxygen interaction of covalent bonded phosphor to ZnS:Cu 2+. The presence of covalent bonded phosphate even after rigorous washing of colloidal nanoparticles ensures that SHMP inhibits the growth of particles by steric stabilization. The broad absorption peak in the range of 3410 3465 cm 1 corresponding to OH group indicates the existence of water absorbed in the surface of nanocrystals. The presence of this band can be clearly attributed to the adsorption of same atmospheric water during FTIR measurements. The bands at 1500 1650 and at 2370 cm 1 are due to the C O stretching modes arising from the absorption of atmospheric CO 2 on the surface of the nanoparticles [16].

M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 6783 Fig. 6. UV vis spectra of ZnS, uncapped, TOPO and SHMP capped ZnS:Cu nanoparticles. 3.4. Optical study Fig. 5. ESR spectra of pure (ZnS:Cu), TOPO and SHMP capped ZnS:Cu nanoparticles. 3.3. ESR study The ESR spectra of uncapped, TOPO and SHMP capped ZnS:Cu 2+ is shown in Fig. 5. The ESR spectrum of ZnS:Cu 2+ shows an intense broad signal, which could be attributed to Cu Cu interactions. The broadening suggests that exchange coupling may be occurred in the sample. Similarly, the same broad peak is observed in other two capped samples. However, the additional peaks observed in capped samples confirm the coating of capping agent such as TOPO and SHMP. It also confirms the presence of P in TOPO and Na in SHMP on the surface of ZnS:Cu nanoparticles. The formation of additional peaks and change in the intensity are due to the segregation of surfactant molecules at the surface of ZnS:Cu nanoparticles and chain length of the surfactants. 3.4.1. Optical absorption Fig. 6 shows the absorption behavior of ZnS:Cu 2+ nanoparticles dispersed in deionized water. The strong absorption peaks of SHMP, TOPO and uncapped particles at 296, 302 and 315 nm are assigned to the optical transition of the first excitonic state of the ZnS:Cu 2+ nanoparticles and the narrow shape is an evidence of very small size of the dispersed particles. The peak position of the absorption spectra has no change with respect to its doping and individual capping concentrations. Comparison of the capped with uncapped ZnS:Cu 2+, the capped particles are strongly blue shifted. This implies quantum confinement effect of small particles. Furthermore, the absorption peak of SHMP capped ZnS:Cu 2+ is highly blue shifted than TOPO capped particles. It may be due to the presence of the Na ions in the surfactant. The mean sizes of the ZnS:Cu 2+ nanoparticles in deionized water can be calculated from the onset absorption (or absorption edge) of the UV vis spectrum. In our case, the particles size were calculated using Brus equation [17] as 3.9 (4.24 ev), 4.3 (4.13 ev), 4.9 (4 ev) and 5.2 nm (3.94 ev) corresponding to SHMP, TOPO, uncapped ZnS and ZnS:Cu 2+ nanoparticles. These are in good agreement with the values from XRD and TEM. The above results indicate that the dimension of the produced ZnS:Cu 2+ nanoparticles and their corresponding optical properties could be controlled by the encapsulations. The passivation layers from the surfactants around the ZnS:Cu 2+ core could both prevent the growth of ZnS:Cu 2+ nanoparticles and act as a quantum well, which widens the band gap and therefore is responsible for the blue shift of UV vis absorption spectra. 3.4.2. Photoluminescence The room temperature PL spectra of the ZnS (inset) and different concentrations of Cu 2+ doped ZnS:Cu nanoparticles are shown in Fig. 7a and b. It can be seen that the peak intensity at 445 nm (inset) for undoped sample is much lesser than those for the doped samples. The peak is related with native defects (e.g. sulfur

6784 M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 Fig. 7. (a) PL spectra of different concentration (0.2 0.8%) of Cu doped ZnS nanoparticles and (b) concentration (Cu) versus intensity. vacancy). When Cu 2+ ions are doped into ZnS nanoparticles, more defect states will be introduced. Therefore, it is reasonable with some new peaks appeared in the longer wavelength side. Comparison of the two spectra, the impurity (Cu 2+ ) doped ZnS spectra is more red shifted from the undoped ZnS. The spectra show a broad emission spectrum around 400 600 nm. It is in agreement with earlier results in ZnS:Cu nanoparticles [18 21]. Three emission peaks were observed in the blue region, at 472 [22], 484 [23] and 496 nm [19 21], arises from the recombination between the shallow donor level (sulfur vacancy) and the t 2 level of Cu 2+ [23]. With an increase of the Cu 2+ concentration (from 0.4 to 0.6%), the green light position is systematically shifted to longer wavelength (from blue to green (524 nm)) [24]. Moreover, the intensity of the ZnS:Cu emission is decreased by higher concentration of the Cu 2+. Fig. 7b shows concentrations versus intensity. This illustrates an optimum concentration (0.4%) of doping Cu 2+ ions for enhanced PL emission. When Cu 2+ ions are incorporated in ZnS host lattice, the luminescence centers of Cu 2+ ions are formed. Hence from the PL emission spectra, it can be concluded that the Cu 2+ ions are incorporated successfully into the ZnS host lattice. Based on the PL results observed, the schematic energy-level diagram of ZnS:Cu nanoparticles is depicted and are shown in Fig. 8. Here V s stands for sulfur vacancy and Zn i stands for interstitial Zinc. This figure explained the emission mechanism of ZnS:Cu nanoparticles and illustrated the above assignments. It is worth mentioning that Xu [25] studied the PL properties of Cu 2+ -doped ZnS nanocrystallites and observed blue and green emissions at 460 and 507 nm, respectively. Also the research workers Sun et al. [26] and Huang et al. [13] observed a green emission at around 520 nm in ZnS:Cu 2+ colloidal solution. Cu 2+ in bulk ZnS has two well-known emission peaks. Chestnoy et al. [27] also reported that blue, green and yellow emission peaks were in appeared in ZnS bulk doped with Cu 2+ impurity. These results indicate a very complicated energy level structure of Cu 2+ impurity in ZnS. Fig. 9a and b shows enhanced photoluminescence spectra of different amounts of the TOPO capped ZnS:Cu (0.4%) nanoparticles. Fig. 9b indicates the PL intensity as a function of amount of the corresponding surfactants injection into colloidal solution. The PL intensity of TOPO increases up to 1.5 g, after which it starts decreasing. However, no observation in the shifting of peak position for different (TOPO) concentrations of the surfactants compared to ZnS:Cu. The peak position is shifted by adding the surfactant due to presence of the P and O in TOPO ligand on the ZnS:Cu matrix. Optimum concentration at 1.5 g of TOPO surfactant was selected by the observed enhanced PL emission. Fig. 10a and b Fig. 8. Energy level diagram.

M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 6785 Fig. 9. (a) PL spectra of different concentration of TOPO capped ZnS:Cu nanoparticles and (b) concentration versus intensity. shows PL emission of the SHMP capped ZnS:Cu (0.4%) nanoparticles and corresponding concentration versus PL intensity. It is observed (Fig. 10b) that the PL intensity is increased and maximum at 1 g of SHMP. After that the intensity is decreased. However, when compared with uncapped emission, it has higher in intensity which may be due to the Na ion in SHMP on the ZnS:Cu. It is also observed that the concentration of 1 g SHMP is the optimum level to obtain the maximum PL emission. 3.5. Thermal study The TG-DTA thermograms were recorded for ZnS:Cu 2+ nanoparticles in the temperature range from room temperature (RT) to 1000 C with an increment of 10 C/min in air atmosphere. Fig. 11 shows combined plots of TG and DTA. From, the TG data, it is noticed Fig. 10. (a) PL spectra of different concentration of SHMP capped ZnS:Cu nanoparticles and (b) concentration versus intensity. that the weight loss of the nanoparticles are found to take place up to 700 C. In DTA curve, the first endothermic peak is found at 65 C, which is attributed to the evaporation of the water. The endothermic peak around 230 C probably corresponds to the evaporation of organic and lattice deformation of ZnS:Cu 2+. The composition does not vary in the annealing range from 100 Cto200 C, whereas, as can be seen, beyond 230 C, most Cu 2+ ions are released from the ZnS matrix. The observed endothermic peak at 330 C is believed to be the beginning of phase transition. A broad exothermic peak at 400 C implies the improvement of the crystallinity of the sample. Additionally, above 500 C, there is a smooth downward trend in DTA curve with significant weight loss. This is may be due to release of residual sulfur ions from the sample. Fig. 11. TG-DTA curve of ZnS:Cu nanoparticles.

6786 M. Kuppayee et al. / Applied Surface Science 257 (2011) 6779 6786 4. Conclusions X-ray diffraction (XRD) patterns revealed that the particles exhibited which crystal structure. The estimated size of the uncapped ZnS, ZnS:Cu and surfactants capped ZnS:Cu is found to be in the range of 6 3 nm. For Cu 2+ -doped ZnS nanocrystallites, it is interpreted that the intensive emissions around 472, 484 and 496 nm may be due to Cu 2+ ions lying in the interstices of the ZnS lattice as it is possible that heterogeneous nucleation is facilitated in the presence of Cu 2+ ions in the ZnS structure. That is, Cu 2+ ions are substituted without affecting the coordination environment around the central metal. The TEM result revealed that the prepared particles are monodispersed by the addition of surfactants. ESR study shows the detection of impurity Cu ions. The shifting is observed by increasing the doping concentration. The absorption spectra of the all samples are more blue shifted due to the quantum confinement effect. The addition of surfactants yielded enhanced emissions. Moreover, the optimum levels of the capping agent are determined by increasing photoluminescence emission. Dictated the stability of the ZnS:Cu nanoparticles TG-DTA analysis. Acknowledgement The authors would like thank to SAIF, IIT, Bombay for providing TEM facility. References [1] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [2] L.E. Brus, Acc. Chem. Res. 23 (1990) 183. [3] F. Lacomi, Surf. Sci. 532 (2003) 816. [4] H. Yuan, S. Xie, D. Liu, X. Yan, Z. Zhou, L. Ci, J. Cryst. Growth 258 (2003) 225. [5] S.S. Kumar, et al., Nucl. Instrum. Meth. Phys. Res. B 251 (2006) 435 440. [6] H. Weller, Adv. Mater. 5 (1993) 88. [7] R. Maity, K.K. Chattopadhyay, Nanotechnology 15 (2004) 812. [8] R.N. Bharagava, D. Gallager, T. Welker, J. Lumin. 60 (1994) 275. [9] A.A. Khosravi, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagawat, M. Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702. [10] W. Chen, J.-O. Malm, V. Zwiller, Y. Huang, S. Liu, R. Wallenberg, J.-O. Bovin, L. Samuelson, Phys. Rev. B 61 (2000) 11021. [11] S. Lee, D. Song, d. Kim, J. Lee, S. Kim, I.Y. Park, Y.D. Choi, Mater. Lett. 58 (2004) 342. [12] S.J. Xu, S.J. Chua, B. Liu, L.M. Gan, C.H. Chew, G.Q. Xu, Appl. Phys. Lett. 73 (1994) 416. [13] J. Huang, Y. Yang, S. Xue, B. Yang, S. Liu, J. Shen, Appl. Phys. Lett. 70 (1997) 2335. [14] B.E. Warren, X-Ray Diffraction, Dover Publications, New York, 1990, p. 251. [15] B.S. Rema Devi, R. Raveendran, A.V. Vaidyan, Synthesis and characterization of Mn 2+ -doped ZnS nanoparticles, Pramana J. Phys. 68 (2007) 679. [16] S.B. Qadri, E.F. Skelton, D. Hsu, A.D. Dinsmore, J. Yang, H.F. Gray, B.R. Ratna, Size-induced transition-temperature reduction in nanoparticles of ZnS, Phys. Rev. B 60 (1999) 9191. [17] P. Vinotha Boorana Lakshmi, K. Sakthi Raj, K. Ramachandran, Cryst. Res. Technol. 44 (2009) 153. [18] S.J. Xu, S.J. Chua, B. Liu, L.M. Gan, C.H. Chew, G.Q. Xu, Appl. Phys. Lett. 72 (1994) 478. [19] P.H. Borse, N. Deshmukh, R.F. Shinde, S.K. Kulkarni, J. Mater. Sci. 34 (1999) 6087. [20] M. Wang, L. Sun, X. Fu, C. Liao, C. Yan, Solid State Commun. 115 (2000) 493. [21] H. Weller, U. Koch, M. Gutierrez, A. Henglien, Ber. Bunsenges Phys. Chem. 88 (1984) 649. [22] H. Wang, X. Lu, Y. Zhao, C. Wang, Mater. Lett. 60 (2006) 2480. [23] A.A. Khosravi, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagwat, M. Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702. [24] S. Lee, D. Song, D. Kim, J. Lee, S. Kim, I.Y. Park, Y.D. Choi, Mater. Lett. 58 (2004) 342. [25] X. Xu, Solid Luminescence, Chinese Academy of Science and University of Science and Technology, China, 1996. [26] L. Sun, C. Liu, C. Liao, C. Yan, Solid State Commun. 111 (1999) 483. [27] N. Chestnoy, T.D. Harris, R. Hull, L.E. Brus, J. Phys. Chem. 90 (1986) 3393.