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A solvothermal synthetic route to CdE (E = S, Se) semiconductor nanocrystalline
Materials Chemistry and Physics 58 (1999) 87±89
Materials Science Communication
A solvothermal synthetic route to CdE (E S, Se) semiconductor nanocrystalline Yadong Li*, Hongwei Liao, Yue Fan, Longquan Li, Yitai Qian Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China Received 1 June 1998; received in revised form 14 October 1998; accepted 27 October 1998
Abstract CdE (E S, Se) nanocrystallines have been successfully prepared through a solvothermal elemental combination reaction in diethylamine at 1808C. X-ray powder diffraction (XRD) pattern shows that CdS is the coexistence of two phases: cubic and hexagonal and CdSe is cubic. Transmission electron microscope (TEM) shows that the morphologies of CdS and CdSe are mostly spherical in shape. Raman spectrum of the obtained CdS indicates that there are two overtones of longitudinal optical (LO) phonon. X-ray photoelectron spectrum was used to analyze the composition of CdSe and the ratio of Cd to Se is 1.03 : 1. The in¯uence of solvent and time was initially investigated. We found that the coordinating ability of solvent strongly in¯uences the reaction process and structure, and morphology of the products. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Cadmium chalcogenide; II±VI semiconductor; Elemental reaction; Solvothermal
1. Introduction The preparation and characterization of II±VI nanoscale compound semiconductors have caused much attention in the past several years due to their great potential in many optoelectronic application [1±6]. So far, a number of reactions have been utilized to prepare II±VI family compounds for quite some time. They are summed up as follows: 1. The direct combination of the IIB metals and chalcogens at high temperature [7], 2. ionic reaction in liquid [8], 3. gas±liquid precipitation [9], 4. molecular precursor method [10] and 5. electrochemical method [11]. All of these methods are very useful and are of widespread importance, but there are some limitations to their utility, that some methods either require relatively elevated temperature, or use toxic agents such as H2E (E S, Se, Te), metalorganic compounds, or need significant energy input, or some products are easy to agglomerate, or it is difficult to control the particle size and size distribution. Now, there have been considerable efforts to simplify the synthetic route to II±VI semiconductors and avoid the use of complex reactions and toxic reagents. *Corresponding author. E-mail: [email protected]
CuS and ZnS have been successfully prepared employing elemental reaction at re¯ux in strongly coordinating solvents such as pyridine and N-methylimidazole [12,13]. Recently, Henshaw et al. [14] reported a room-temperature liquid ammonia route to metal chalcogenides, but most of the products are amorphous and need post heat treatment at high temperature. In this paper, we report a solvothermal elemental combination reaction to fabricate CdS, CdSe nanocrystallines in diethylamine at 1808C. The effects of solvent, temperature and time were investigated. It was found that the reaction and the structure of CdE were strongly in¯uenced by the solvent. 2. Experimental An appropriate amount of analytical pure Cd powder and E (E S, Se) elemental was stoichiometrically added into Te¯on-liner autoclave with 100 ml capacity. Then the autoclave was ®lled up to 70% of the total volume with diethylamine. The autoclave was maintained at 1808C for 8 h and then naturally cooled to room temperature. Products were collected by ®ltration and washed with distilled water several times to remove the residue of diethylamine. Finally, the products were washed with absolute alcohol and desiccated in vacuum at 808C for 3 h.
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0254-0584(98)00254-5
88
Y. Li et al. / Materials Chemistry and Physics 58 (1999) 87±89
The products were characterized by XRD, TEM, XPS and Raman spectroscopy. The XRD pattern was determined on a Rigaku D/max-rA X-ray diffractometer with graphite Ê ). The monochromatized Cu Ka radiation ( 1.5418 A TEM image was made on a Hitachi H-800 transmission electron microscope, using an accelerating voltage 200 kV. The XPS was performed with a VGESCALAB MKII X-ray photoelectron spectrometer, using non-monochromatized Mg Ka X-ray as the excitation source. Raman spectrum was recorded on a Spex 1403 spectrometer. 3. Results and discussion For the bulk materials, it is well known that CdS and CdSe have a highly stable hexagonal (wurtzite) phase from room temperature to melting point, while bulk CdTe has a cubic structure [15]. The cubic phase of CdS and CdSe exists at low temperature and it is the metastable phase. The cubic CdS and CdSe can transform into hexagonal phase at proper temperature. In the solvothermal process, the formation of CdE (E S, Se) is associated with the following reaction Cd E ! CdE E S; Se Fig. 1 shows the XRD patterns of the samples. Fig. 1(A) shows the XRD pattern of CdS. It is obvious that there exist two phases: hexagonal, wurtzite phase and cubic, zinc-blend phase. This unexpected cubic phase may be due to metastability [16] caused by the pressure produced by diethylamine at 1808C (about 8 atm). Fig. 1(B) shows the XRD pattern of CdSe. It is seen that CdSe was cubic in structure. When the time was less than 10 h, there existed a little unreacted cadmium powder. With the increase of the reaction time the little Cd impurity peaks become smaller and disappear. The average size of CdSe was 39 nm, estimating from the XRD pattern with Sherrer formula. Fig. 2 shows the TEM images of the samples. Fig. 2(A) shows the micrograph of CdS. From TEM image, one can see that morphology of the obtained CdS was spherical in
Fig. 2. TEM images of the samples (A) CdS and (B) CdSe.
average size about 10 nm. Fig. 2(B) shows the image of CdSe. It shows that CdSe was also spherical and had an average size of about 40 nm, which is consistent with the XRD result. Fig. 3 shows the UV±Vis absorption spectrum of the obtained CdS product, in which, there is an absorption peak with onset at 508 nm and maximum absorption at 482 nm. This is consistent with the characteristic absorption spectrum of the bulk CdS [17]. Raman spectroscopy was used to characterize the obtained CdS sample. From the results (Fig. 4), we found that the Raman spectrum of CdS sample was dominated by the overtones of the longitudinal optical phonon (LO). In the Raman spectrum of the CdS, there were two clearly discernible LO modes at 299 and 599 cmÿ1, respectively, in the range of 200±700 cmÿ1. The overtone of long wavelength
Fig. 3. UV±Vis absorption spectra of CdS.
Fig. 1. XRD patterns of the samples (A) CdS (~ hexagonal; * cubic) and (B) CdSe (~).
Fig. 4. Raman spectrum of CdS.
Y. Li et al. / Materials Chemistry and Physics 58 (1999) 87±89
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lamine at 1808C. XRD, TEM, XPS, Raman spectroscopy and optical spectrum were used to characterize the obtained samples. The results showed that CdS was the coexistence of cubic and hexagonal phases and CdSe was cubic phase. But there was no CdTe formed under the same conditions. The in¯uence of solvent, time and temperature on the products was investigated. Solvent plays a crucial role in the reaction during the solvothermal process. Fig. 5. XPS spectra of CdSe.
longitudinal optical vibration may be caused by a lattice deformation produced by electronic exciton in bulk CdS [18], which manifests in the laser Raman spectrum. XPS was carried out to derive composition information of the CdSe sample. In case of CdSe, Cd(3d) and Se(3d), core level regions were examined. Fig. 5 shows the XPS spectra of the CdSe sample. The two strong peaks at 54.20 and 405.65 eV correspond to Se(3d) and Cd(3d5/2) binding energy, respectively, for CdSe. No other obvious peak was observed. The quanti®cation of peaks gives the ratio of Cd to Se as 1.03 : 1. Elemental analysis shows that the ratio of Cd to Se as 1.05 : 1. These results were identical. In order to investigate the in¯uence of solvent on the products, we substitute diethylamine ®rst with benzene and then with ethylenediamine, keeping other conditions invariant. From the XRD results in benzene, there was no CdE and the product was mainly the unreacted cadmium. XRD patterns indicate that there are no sulphur and selenium, which may dissolve in benzene. However, in ethylenediamine, results showed that the obtained products were CdS, CdSe and CdTe. And all the products are nanoscale rod-like [19]. The above results indicated that only in coordinating amine solvents, CdE (E S, Se) could be prepared through the elemental reaction between cadmium and chalcogen at relatively low temperature (<2008C). This is the reason that in benzene no CdE was formed. In case of CdTe, XRD result shows that there is no CdTe and the product was the mixture of unreacted Cd and Te. Here, we could ®nd that the solvent plays a crucial role in the reaction during the solvothermal process. The in¯uence of time and temperature on the reaction process was investigated. When temperature was below 1408C, no CdE was formed. When temperature was 1408C, CdE was formed, but there existed unreacted cadmium. When time is less than 5 h, the reaction proceeds incompletely. In conclusion, CdE (E S, Se) nanocrystalline were prepared through solvothermal elemental reaction in diethy-
Acknowledgements We would like to acknowledge Prof. Zhou Guien and Ji Mingrong for valuable assistance with the XRD and XPS analysis. This work is supported by the National Natural Science Foundation of China and the National Nanometer Materials Climbing Project. References [1] L. Spanhel, M. Haase, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [2] M.G. Bawendi, D.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946. [3] M.C. Harris Liao, Y.H. Chang, Y.F. Chen, J.W. Hsu, J.M. Lin, W.C. Chou, Appl. Phys. Lett. 70 (1997) 2256. [4] B.P. Zhang, T. Yasuda, Y. Segawa, H. Yaguchi, K. Onabe, E. Edamatsu, T. Itoh, Appl. Phys. Lett. 70 (1997) 2413. [5] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [6] A. Ishibashi, Int. Symp. on Blue Laser and Light Emitting Diodes, Chiba, Japan, 1996. [7] H. Wiedemeir, A. Khanm, Trans. AIME 242 (1968) 1969. [8] K.J. Bandaranayake, G.W. Wen, J.Y. Lin, H.X. Jing, C.M. Sorensen, Appl. Phys. Lett. 67 (1995) 831. [9] R.B. Borade, Zeolites 7 (1987) 398. [10] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [11] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100 (1996) 14037. [12] E. Ramli, T.B. Rauchfuss, C.L. Stern, J. Am. Chem. Soc. 112 (1990) 4043. [13] S. Dev, E. Ramli, T.B. Rauchfuss, C.L. Stern, J. Am. Chem. Soc. 112 (1990) 6385. [14] G. Henshaw, I.P. Parkin, G.A. Shaw, J. Chem. Soc., Dalton Trans. (1997) 231. [15] B.J. Fitzpatrick, in: T.C. McGill, C.M.S. Torres, W. Gebhardt (Eds.), Growth and Optical Properties of Wide-gap II±VI Low-Dimensional Semiconductors, Plenum, New York, 1989, p. 67. [16] R.J. Bandaranayake, G.W. Wen, J.Y. Lin, H.X. Jiang, C.M. Sorensen, Appl. Phys. Lett. 67 (1995) 831. [17] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [18] J.F. Scott, R.C.C. Leite, T.C. Damen, Phys. Rev. 188 (1969) 1285. [19] Y.D. Li, H.W. Liao, Y. Ding, Y. Fan, Y. Zhang, Y.T. Qian, Angew. Chem., Int. Ed. Engl., submitted for publication.
Materials Science Communication
A solvothermal synthetic route to CdE (E S, Se) semiconductor nanocrystalline Yadong Li*, Hongwei Liao, Yue Fan, Longquan Li, Yitai Qian Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China Received 1 June 1998; received in revised form 14 October 1998; accepted 27 October 1998
Abstract CdE (E S, Se) nanocrystallines have been successfully prepared through a solvothermal elemental combination reaction in diethylamine at 1808C. X-ray powder diffraction (XRD) pattern shows that CdS is the coexistence of two phases: cubic and hexagonal and CdSe is cubic. Transmission electron microscope (TEM) shows that the morphologies of CdS and CdSe are mostly spherical in shape. Raman spectrum of the obtained CdS indicates that there are two overtones of longitudinal optical (LO) phonon. X-ray photoelectron spectrum was used to analyze the composition of CdSe and the ratio of Cd to Se is 1.03 : 1. The in¯uence of solvent and time was initially investigated. We found that the coordinating ability of solvent strongly in¯uences the reaction process and structure, and morphology of the products. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Cadmium chalcogenide; II±VI semiconductor; Elemental reaction; Solvothermal
1. Introduction The preparation and characterization of II±VI nanoscale compound semiconductors have caused much attention in the past several years due to their great potential in many optoelectronic application [1±6]. So far, a number of reactions have been utilized to prepare II±VI family compounds for quite some time. They are summed up as follows: 1. The direct combination of the IIB metals and chalcogens at high temperature [7], 2. ionic reaction in liquid [8], 3. gas±liquid precipitation [9], 4. molecular precursor method [10] and 5. electrochemical method [11]. All of these methods are very useful and are of widespread importance, but there are some limitations to their utility, that some methods either require relatively elevated temperature, or use toxic agents such as H2E (E S, Se, Te), metalorganic compounds, or need significant energy input, or some products are easy to agglomerate, or it is difficult to control the particle size and size distribution. Now, there have been considerable efforts to simplify the synthetic route to II±VI semiconductors and avoid the use of complex reactions and toxic reagents. *Corresponding author. E-mail: [email protected]
CuS and ZnS have been successfully prepared employing elemental reaction at re¯ux in strongly coordinating solvents such as pyridine and N-methylimidazole [12,13]. Recently, Henshaw et al. [14] reported a room-temperature liquid ammonia route to metal chalcogenides, but most of the products are amorphous and need post heat treatment at high temperature. In this paper, we report a solvothermal elemental combination reaction to fabricate CdS, CdSe nanocrystallines in diethylamine at 1808C. The effects of solvent, temperature and time were investigated. It was found that the reaction and the structure of CdE were strongly in¯uenced by the solvent. 2. Experimental An appropriate amount of analytical pure Cd powder and E (E S, Se) elemental was stoichiometrically added into Te¯on-liner autoclave with 100 ml capacity. Then the autoclave was ®lled up to 70% of the total volume with diethylamine. The autoclave was maintained at 1808C for 8 h and then naturally cooled to room temperature. Products were collected by ®ltration and washed with distilled water several times to remove the residue of diethylamine. Finally, the products were washed with absolute alcohol and desiccated in vacuum at 808C for 3 h.
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0254-0584(98)00254-5
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Y. Li et al. / Materials Chemistry and Physics 58 (1999) 87±89
The products were characterized by XRD, TEM, XPS and Raman spectroscopy. The XRD pattern was determined on a Rigaku D/max-rA X-ray diffractometer with graphite Ê ). The monochromatized Cu Ka radiation ( 1.5418 A TEM image was made on a Hitachi H-800 transmission electron microscope, using an accelerating voltage 200 kV. The XPS was performed with a VGESCALAB MKII X-ray photoelectron spectrometer, using non-monochromatized Mg Ka X-ray as the excitation source. Raman spectrum was recorded on a Spex 1403 spectrometer. 3. Results and discussion For the bulk materials, it is well known that CdS and CdSe have a highly stable hexagonal (wurtzite) phase from room temperature to melting point, while bulk CdTe has a cubic structure [15]. The cubic phase of CdS and CdSe exists at low temperature and it is the metastable phase. The cubic CdS and CdSe can transform into hexagonal phase at proper temperature. In the solvothermal process, the formation of CdE (E S, Se) is associated with the following reaction Cd E ! CdE E S; Se Fig. 1 shows the XRD patterns of the samples. Fig. 1(A) shows the XRD pattern of CdS. It is obvious that there exist two phases: hexagonal, wurtzite phase and cubic, zinc-blend phase. This unexpected cubic phase may be due to metastability [16] caused by the pressure produced by diethylamine at 1808C (about 8 atm). Fig. 1(B) shows the XRD pattern of CdSe. It is seen that CdSe was cubic in structure. When the time was less than 10 h, there existed a little unreacted cadmium powder. With the increase of the reaction time the little Cd impurity peaks become smaller and disappear. The average size of CdSe was 39 nm, estimating from the XRD pattern with Sherrer formula. Fig. 2 shows the TEM images of the samples. Fig. 2(A) shows the micrograph of CdS. From TEM image, one can see that morphology of the obtained CdS was spherical in
Fig. 2. TEM images of the samples (A) CdS and (B) CdSe.
average size about 10 nm. Fig. 2(B) shows the image of CdSe. It shows that CdSe was also spherical and had an average size of about 40 nm, which is consistent with the XRD result. Fig. 3 shows the UV±Vis absorption spectrum of the obtained CdS product, in which, there is an absorption peak with onset at 508 nm and maximum absorption at 482 nm. This is consistent with the characteristic absorption spectrum of the bulk CdS [17]. Raman spectroscopy was used to characterize the obtained CdS sample. From the results (Fig. 4), we found that the Raman spectrum of CdS sample was dominated by the overtones of the longitudinal optical phonon (LO). In the Raman spectrum of the CdS, there were two clearly discernible LO modes at 299 and 599 cmÿ1, respectively, in the range of 200±700 cmÿ1. The overtone of long wavelength
Fig. 3. UV±Vis absorption spectra of CdS.
Fig. 1. XRD patterns of the samples (A) CdS (~ hexagonal; * cubic) and (B) CdSe (~).
Fig. 4. Raman spectrum of CdS.
Y. Li et al. / Materials Chemistry and Physics 58 (1999) 87±89
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lamine at 1808C. XRD, TEM, XPS, Raman spectroscopy and optical spectrum were used to characterize the obtained samples. The results showed that CdS was the coexistence of cubic and hexagonal phases and CdSe was cubic phase. But there was no CdTe formed under the same conditions. The in¯uence of solvent, time and temperature on the products was investigated. Solvent plays a crucial role in the reaction during the solvothermal process. Fig. 5. XPS spectra of CdSe.
longitudinal optical vibration may be caused by a lattice deformation produced by electronic exciton in bulk CdS [18], which manifests in the laser Raman spectrum. XPS was carried out to derive composition information of the CdSe sample. In case of CdSe, Cd(3d) and Se(3d), core level regions were examined. Fig. 5 shows the XPS spectra of the CdSe sample. The two strong peaks at 54.20 and 405.65 eV correspond to Se(3d) and Cd(3d5/2) binding energy, respectively, for CdSe. No other obvious peak was observed. The quanti®cation of peaks gives the ratio of Cd to Se as 1.03 : 1. Elemental analysis shows that the ratio of Cd to Se as 1.05 : 1. These results were identical. In order to investigate the in¯uence of solvent on the products, we substitute diethylamine ®rst with benzene and then with ethylenediamine, keeping other conditions invariant. From the XRD results in benzene, there was no CdE and the product was mainly the unreacted cadmium. XRD patterns indicate that there are no sulphur and selenium, which may dissolve in benzene. However, in ethylenediamine, results showed that the obtained products were CdS, CdSe and CdTe. And all the products are nanoscale rod-like [19]. The above results indicated that only in coordinating amine solvents, CdE (E S, Se) could be prepared through the elemental reaction between cadmium and chalcogen at relatively low temperature (<2008C). This is the reason that in benzene no CdE was formed. In case of CdTe, XRD result shows that there is no CdTe and the product was the mixture of unreacted Cd and Te. Here, we could ®nd that the solvent plays a crucial role in the reaction during the solvothermal process. The in¯uence of time and temperature on the reaction process was investigated. When temperature was below 1408C, no CdE was formed. When temperature was 1408C, CdE was formed, but there existed unreacted cadmium. When time is less than 5 h, the reaction proceeds incompletely. In conclusion, CdE (E S, Se) nanocrystalline were prepared through solvothermal elemental reaction in diethy-
Acknowledgements We would like to acknowledge Prof. Zhou Guien and Ji Mingrong for valuable assistance with the XRD and XPS analysis. This work is supported by the National Natural Science Foundation of China and the National Nanometer Materials Climbing Project. References [1] L. Spanhel, M. Haase, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [2] M.G. Bawendi, D.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946. [3] M.C. Harris Liao, Y.H. Chang, Y.F. Chen, J.W. Hsu, J.M. Lin, W.C. Chou, Appl. Phys. Lett. 70 (1997) 2256. [4] B.P. Zhang, T. Yasuda, Y. Segawa, H. Yaguchi, K. Onabe, E. Edamatsu, T. Itoh, Appl. Phys. Lett. 70 (1997) 2413. [5] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [6] A. Ishibashi, Int. Symp. on Blue Laser and Light Emitting Diodes, Chiba, Japan, 1996. [7] H. Wiedemeir, A. Khanm, Trans. AIME 242 (1968) 1969. [8] K.J. Bandaranayake, G.W. Wen, J.Y. Lin, H.X. Jing, C.M. Sorensen, Appl. Phys. Lett. 67 (1995) 831. [9] R.B. Borade, Zeolites 7 (1987) 398. [10] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [11] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100 (1996) 14037. [12] E. Ramli, T.B. Rauchfuss, C.L. Stern, J. Am. Chem. Soc. 112 (1990) 4043. [13] S. Dev, E. Ramli, T.B. Rauchfuss, C.L. Stern, J. Am. Chem. Soc. 112 (1990) 6385. [14] G. Henshaw, I.P. Parkin, G.A. Shaw, J. Chem. Soc., Dalton Trans. (1997) 231. [15] B.J. Fitzpatrick, in: T.C. McGill, C.M.S. Torres, W. Gebhardt (Eds.), Growth and Optical Properties of Wide-gap II±VI Low-Dimensional Semiconductors, Plenum, New York, 1989, p. 67. [16] R.J. Bandaranayake, G.W. Wen, J.Y. Lin, H.X. Jiang, C.M. Sorensen, Appl. Phys. Lett. 67 (1995) 831. [17] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [18] J.F. Scott, R.C.C. Leite, T.C. Damen, Phys. Rev. 188 (1969) 1285. [19] Y.D. Li, H.W. Liao, Y. Ding, Y. Fan, Y. Zhang, Y.T. Qian, Angew. Chem., Int. Ed. Engl., submitted for publication.