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Preparation and thermal stability of silicon nanoparticles
Applied Surface Science 171 (2001) 44±48
Preparation and thermal stability of silicon nanoparticles Y. Zhu, H. Wang, P.P. Ong* Department of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 4 May 2000; accepted 11 July 2000
Abstract Silicon nanoparticles were prepared in a homemade apparatus by means of a dc sputtering method in which the condensates were collected directly from the cold surface of a liquid nitrogen trap. They were dispersed in 2-propanol under ultrasonic agitation, and dried in the atmosphere. The particles were found to compose of tiny silicon crystals and were only mildly oxidized. Various samples were prepared with different annealing times and temperatures in ultrahigh vacuum. XPS results show that, in the particles, the Si±O bonds of the Si4 state are the most stable, followed next by the unoxidised state Si0. The intermediate oxidation states are the least stable; they exist only at suf®ciently low temperatures (3008C or lower) and are converted to either Si0 or Si4 at higher temperatures. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal stability; Silicon nanoparticles; X-ray photoelectron spectroscopy (XPS)
Since the discovery of visible photoluminescence from nanometer-sized silicon such as porous silicon, silicon nanoparticles and silicon nanoparticles embedded in other materials at room temperature have attracted much attention in the study of siliconbased, light-emitting materials [1,2]. It is necessary to understand their luminescence processes in order to exploit their potential for applications in optoelectronics. However, owing to their low stability and reproducibility, porous silicon devices still seem very dif®cult to use in the industry. Therefore, it is important to clarify the stability of the nanometer-sized silicon and their surfaces believed to be responsible for the blue luminescence [3±5]. We have prepared nanometer-size silicon particles by an improvement in the dc sputtering preparation method. The as-prepared particles were con®rmed by transmission electron microscope (TEM) and energy dispersive X-ray spec* Corresponding author. Tel.: 65-8748002; fax: 65-7776126. E-mail address: [email protected] (P.P. Ong).
troscopy (EDS) to compose of tiny silicon crystals with diameters of less than 10 nm. Strong photoluminescence was observed in the wavelength region of 300±550 nm for the silicon particles in both its original and post-annealed forms [6]. In this paper, we present details of the preparation method and study of the thermal stability of the silicon nanoparticles by X-ray photoelectron spectroscopy (XPS). It is found that intermediate oxidation states of the nanoparticles are less stable than those in either the high oxidation states, or the unoxidized state of the silicon. Our silicon nanoparticles were prepared in a homemade dc sputtering apparatus and the harvested materials were dispersed in the form of colloids in 2-propanol liquid. Fig. 1 is a schematic of the apparatus. The preparation process consisted of the following steps: 1. The vacuum chamber was evacuated to better than 5 10ÿ6 Torr. 2. High purity (99.999%) dry Ar gas was introduced into the vacuum chamber to about 1 10ÿ2 Torr.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 5 4 5 - 6
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
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7. An adequate quantity of glass slices were cleaned by ultrasonic vibration in acetone and dried. 8. Some drops of the colloids were dropped on to the clean glass slices and dried, yielding the test samples. Each sample used for the TEM and EDS analyzes were prepared by placing a drop of the colloids on to the amorphous carbon ®lm covering the TEM copper grid. It was immediately dried and transferred to the TEM instrument for measurement in order to minimize oxidation in air. 9. Some of the samples obtained in (8) were variously annealed in ultrahigh vacuum of < 5 10ÿ8 Torr (i) at 3008C for 30 min, (ii) at 5008C for 30 min, and (iii) at 5008C for 6 h, respectively.
Fig. 1. Schematics of the preparation apparatus.
3. A negative DC high voltage (ÿ0.6 kV) was applied to the high purity (99.999%) silicon target so that its electric potential is negatively biased with respect to the cold surface of the liquid nitrogen stainless steel trap. 4. The sputtering process was started during which silicon nanoparticles were condensed on the cold liquid nitrogen trap; 5. After enough amounts of the particles were deposited on the trap, they were scraped off and placed in a sample collector, which was ®lled with frozen high purity (99.99%) 2-propanol. In order to preserve the tiny size of the particles, the sputtering time was limited to no more than about 10 min before each scraping. This process was then repeated if increased yield was required. Even in this cautious way, there were still residues of undesirable excessive clustering of the particles. 6. To reduce the clustering effect, the colloidal form of the samples suspended in the 2-propanol was vibrated by ultrasonic agitation so as to separate out the particles. In this way, it was possible to obtain silicon nanoparticles ®nely dispersed in the 2-propanol liquid while their total immersion in the liquid 2-propanol prevented their surfaces from oxidation by the air.
Fig. 2 is a TEM photograph of the typical particles obtained with a Philips CM300 HRTEM instrument. It shows that the silicon particles are in the form of crystals of less than 10 nm in diameter. This result should be compared to that of Baru et al. [7] who reported Si/SiO2 ®lms (Si wafer and pure silica target) prepared by the rf sputtering method with different Ar pressures but with the rest of the sputtering parameters held constant. The super¯uous silicon might be present either as nanocrystals or as amorphous clusters. In yet another modi®cation of the dc sputtering method wherein the Si sputtered materials were collected directly at the liquid-nitrogen-frozen solvent, the silicon nanocolloids so produced comprised silicon nanoparticles with silicon crystal cores covered by amorphous oxidized silicon shells [8]. Fig. 3 depicts the EDS spectrum of the as-prepared particles before annealing. The carbon and copper peaks arise from the TEM copper grid and the amorphous carbon ®lm covering it. The immeasurably small oxygen peak which should appear very near the C peak is overshadowed by this large peak in the EDS spectrum. This indicates that the original oxidation level of the as-prepared silicon particles remains very low throughout the dc discharge process, or even in the colloidal state, the last of which con®rmed that the particles suspended in the 2-propanol could keep the nanoparticles from direct atmospheric oxidization. XPS measurements were carried out on a VG ESCALAB MKII spectrometer, using a Mg Ka source (1253.6 eV photons) with the analyzer mode set at a constant analyzer energy of 20 eV. The X-ray source was run at 150 W (15 kV and 10 mA). Since the
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Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
Fig. 2. Typical TEM photograph of the particles. The circle indicates a typical nanocrystal.
Fig. 3. EDS spectrum of the as-prepared particles before annealing.
constituent elements of all our samples consisted of only silicon and oxygen (except for the absorption composition on the particle surfaces), and it was dif®cult to measure the XPS peak change of oxygen, we focused our attention on the changes of the silicon peaks in the XPS results. As XPS signals come from the surface and near surface layers of the sample, their analyses would help to distinguish between surface phenomena and those arising from the sub-surface or substrate. Fig. 4 shows the XPS Si 2p spectrum of the original unannealed sample. The XPS spectrum was ®tted with pure Gaussian functions of the four possible oxidized states and the unoxidized state in order to deconvolute the component peaks for each site type at its appropriate location. The full widths at half-maximum of each component were allowed to vary from 1.3 to 1.9 eV so as to yield the best ®t [9]. If we denote the Si and SiO2 oxidation bonding states as Si0 and Si4, and
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
47
Fig. 4. The deconvoluted XPS Si 2p spectrum of the original sample ®tted with pure Gaussian functions for each of the site types: Si0, Si1, Si2, Si3, Si4, showing the contributions from each site type.
the intermediate oxidation states as Si1, Si2 and Si3, then the lowest binding energy peak refers to Si0 and the highest binding energy peak to SiO2 [9,10]. All the peaks are present and their positions correspond, in the order of increasing binding energy, to Si0, Si1, Si2, Si3 and Si4, respectively. The comparative peak heights are in the order Si2 < Si4 < Si0 < Si1 < Si3 . If we take the intensity of Si2 as unit value, then the intensities of Si2, Si4, Si0, Si1, Si3 are respectively in the approximate ratio of 1:2.1:2.2:3.3:4.3. This ratio suggests that the silicon nanoparticles were not heavily oxidized after it was taken out of the liquid solvent. The result is consistent with the fact that the oxidation speed of the silicon micro particles is slow compared with that of the macroscopic silicon wafer under the same conditions [11]. Fig. 5 shows the Si 2p spectra of the different samples. All spectra were normalized to their peak values with respect to their base lines at about 97 eV. For clarity of display, they are arbitrarily spaced out vertically so as to prevent overlapping of the curves. Curve a refers to the original sample, and curves b, c, d are for the samples annealed in vacuum at 3008C, 5008C for 30 min, and at 5008C for 6 h, respectively. After the sample was annealed at 3008C for 30 min (curve b), the peaks of Si0, Si2, Si3 almost disappeared, while the peak of Si1 became more pronounced. The ratio of the peaks of Si4 and Si1 is
Fig. 5. XPS Si 2p spectra of the different samples. The four curves are arbitrarily displaced vertically in order to avoid overlapping of the curves. (a) original sample, (b)±(d) samples annealed at 3008C, 5008C for 30 min, and 5008C for 6 h in ultrahigh vacuum, respectively.
about 3:2. Because the annealing time was short (30 min) and was carried out in high vacuum, there was little opportunity for the silicon particles to encounter the residual oxygen atoms inside the vacuum chamber and combined with them. Instead, the change in oxygen composition is likely to occur via processes within the particles themselves. At the annealing temperature of 3008C, most of the Si±O bonds of the Si2, Si3 became unstable and a scrambling rearrangement took place. Some of the Si2, Si3 sites were converted to Si1 by losing oxygen atoms or ions which transmigrated to other sites, while some other Si2, Si3 sites received additional oxygen atoms or ions to form Si4. For the sample annealed at 5008C for 30 min (curve c), the Si±O bonds of Si1 also became unstable at this high temperature resulting in the disappearance of the Si1 peak, while the Si0 peak increased greatly. This suggests that the Si±O bonds in the Si1 sites tended to be reduced to Si0 with its oxygen atom or ion migrating to other Si particles to form new Si4 sites. For the sample annealed at 5008C for 6 h (curve d), the peak ratio of Si4:Si0 is about 6.3:1. The intensity of the Si0 peak was considerably reduced. As the annealing temperature was
48
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
high and the annealing time long, the residual oxygen molecules in the vacuum chamber had suf®cient chances to react with the silicon particles, even in the ultrahigh vacuum condition. However, this process could produce only the Si4 state but not the intermediate oxidation state. The corresponding intensity of the Si0 state was suppressed. In conclusion, our results show ®rstly that the freshly prepared silicon nanoparticles were only slightly oxidized because our samples were prepared in high vacuum condition and kept in high purity (99.99%) 2-propanol. Secondly, the oxidation states of the particles underwent different chemical stages when they were annealed. At ®rst the Si±O bonds of the Si2 and Si3 states became unstable at 3008C, and were depleted while the Si4 and Si1 states enhanced. Then, the Si1 state also became unstable at 5008C, and was reduced with a corresponding increase in the Si0 and Si4 states. With intense annealing, most of the remaining Si0 states were converted to Si4 by reaction with the residual oxygen in the vacuum chamber. However, no conversion to the less stable intermediate oxidation states took place. Of course, during all these processes, the oxygen atoms and ions could also diffuse in the particles at the same time. The full oxidation Si4 state of the silicon nanoparticles was the most stable, while the intermediate oxidation states were even less stable than the unoxidized state.
Acknowledgements We would like to thank Mr. H.K. Wong, Ms. W.C. Tjiu and Mr. H.H. Teo for their experimental assistance. This work was supported under NUS Research Grant No. 3979901. References [1] L.T. Caham, Appl. Phy. Lett. 57 (1990) 1046. [2] H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, T. Nakagiri, Appl. Phy. Lett. 56 (1990) 2379. [3] D.W. Cooke, B.L. Bennett, E.H. Farnum, W.L. Hults, K.E. Sickafus, J.F. Smith, J.L. Smith, T.N. Taylor, P. Tiwari, Appl. Phys. Lett. 68 (1996) 1663. [4] K. Kimura, S. Iwasaki, J. Appl. Phys. 83 (1998) 1345. [5] S. V. Gaponenko, Optial Properties of Semiconductor Nanocrystals, Cambridge University Press, Cambridge, 1998. [6] Y. Zhu, H. Wang, P. P. Ong, J. Phys. D: Appl. Phys. 33 (2000), 1965. [7] V.G. Baru, A.P. Chernushich, V.A. Luzanov, G.V. Stepanov, L.Yu. Zakharov, K.P. O'Donnell, I.V. Bradley, N.N. MelAnik, Appl. Phys. Lett. 69 (1996) 4148. [8] Y. Zhu, K. Kimura, L.D. Zhang, J. Vac. Sci. Technol. B15 (6) (1997) 2077. [9] W.K. Chio, F.W. Poon, F.C. Loh, K.L. Tan, J. Appl. Phys. 81 (1997) 7386. [10] F.J. Grunthaner, J. Maserjian, IEEE Trans. Nucl. Sci. NS-24 (1977) 2108. [11] Y. Zhu, K. Kimura, L. D. Zhang, Solid State Commun. 108 (1998) 545, and references therein.
Preparation and thermal stability of silicon nanoparticles Y. Zhu, H. Wang, P.P. Ong* Department of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 4 May 2000; accepted 11 July 2000
Abstract Silicon nanoparticles were prepared in a homemade apparatus by means of a dc sputtering method in which the condensates were collected directly from the cold surface of a liquid nitrogen trap. They were dispersed in 2-propanol under ultrasonic agitation, and dried in the atmosphere. The particles were found to compose of tiny silicon crystals and were only mildly oxidized. Various samples were prepared with different annealing times and temperatures in ultrahigh vacuum. XPS results show that, in the particles, the Si±O bonds of the Si4 state are the most stable, followed next by the unoxidised state Si0. The intermediate oxidation states are the least stable; they exist only at suf®ciently low temperatures (3008C or lower) and are converted to either Si0 or Si4 at higher temperatures. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal stability; Silicon nanoparticles; X-ray photoelectron spectroscopy (XPS)
Since the discovery of visible photoluminescence from nanometer-sized silicon such as porous silicon, silicon nanoparticles and silicon nanoparticles embedded in other materials at room temperature have attracted much attention in the study of siliconbased, light-emitting materials [1,2]. It is necessary to understand their luminescence processes in order to exploit their potential for applications in optoelectronics. However, owing to their low stability and reproducibility, porous silicon devices still seem very dif®cult to use in the industry. Therefore, it is important to clarify the stability of the nanometer-sized silicon and their surfaces believed to be responsible for the blue luminescence [3±5]. We have prepared nanometer-size silicon particles by an improvement in the dc sputtering preparation method. The as-prepared particles were con®rmed by transmission electron microscope (TEM) and energy dispersive X-ray spec* Corresponding author. Tel.: 65-8748002; fax: 65-7776126. E-mail address: [email protected] (P.P. Ong).
troscopy (EDS) to compose of tiny silicon crystals with diameters of less than 10 nm. Strong photoluminescence was observed in the wavelength region of 300±550 nm for the silicon particles in both its original and post-annealed forms [6]. In this paper, we present details of the preparation method and study of the thermal stability of the silicon nanoparticles by X-ray photoelectron spectroscopy (XPS). It is found that intermediate oxidation states of the nanoparticles are less stable than those in either the high oxidation states, or the unoxidized state of the silicon. Our silicon nanoparticles were prepared in a homemade dc sputtering apparatus and the harvested materials were dispersed in the form of colloids in 2-propanol liquid. Fig. 1 is a schematic of the apparatus. The preparation process consisted of the following steps: 1. The vacuum chamber was evacuated to better than 5 10ÿ6 Torr. 2. High purity (99.999%) dry Ar gas was introduced into the vacuum chamber to about 1 10ÿ2 Torr.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 5 4 5 - 6
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
45
7. An adequate quantity of glass slices were cleaned by ultrasonic vibration in acetone and dried. 8. Some drops of the colloids were dropped on to the clean glass slices and dried, yielding the test samples. Each sample used for the TEM and EDS analyzes were prepared by placing a drop of the colloids on to the amorphous carbon ®lm covering the TEM copper grid. It was immediately dried and transferred to the TEM instrument for measurement in order to minimize oxidation in air. 9. Some of the samples obtained in (8) were variously annealed in ultrahigh vacuum of < 5 10ÿ8 Torr (i) at 3008C for 30 min, (ii) at 5008C for 30 min, and (iii) at 5008C for 6 h, respectively.
Fig. 1. Schematics of the preparation apparatus.
3. A negative DC high voltage (ÿ0.6 kV) was applied to the high purity (99.999%) silicon target so that its electric potential is negatively biased with respect to the cold surface of the liquid nitrogen stainless steel trap. 4. The sputtering process was started during which silicon nanoparticles were condensed on the cold liquid nitrogen trap; 5. After enough amounts of the particles were deposited on the trap, they were scraped off and placed in a sample collector, which was ®lled with frozen high purity (99.99%) 2-propanol. In order to preserve the tiny size of the particles, the sputtering time was limited to no more than about 10 min before each scraping. This process was then repeated if increased yield was required. Even in this cautious way, there were still residues of undesirable excessive clustering of the particles. 6. To reduce the clustering effect, the colloidal form of the samples suspended in the 2-propanol was vibrated by ultrasonic agitation so as to separate out the particles. In this way, it was possible to obtain silicon nanoparticles ®nely dispersed in the 2-propanol liquid while their total immersion in the liquid 2-propanol prevented their surfaces from oxidation by the air.
Fig. 2 is a TEM photograph of the typical particles obtained with a Philips CM300 HRTEM instrument. It shows that the silicon particles are in the form of crystals of less than 10 nm in diameter. This result should be compared to that of Baru et al. [7] who reported Si/SiO2 ®lms (Si wafer and pure silica target) prepared by the rf sputtering method with different Ar pressures but with the rest of the sputtering parameters held constant. The super¯uous silicon might be present either as nanocrystals or as amorphous clusters. In yet another modi®cation of the dc sputtering method wherein the Si sputtered materials were collected directly at the liquid-nitrogen-frozen solvent, the silicon nanocolloids so produced comprised silicon nanoparticles with silicon crystal cores covered by amorphous oxidized silicon shells [8]. Fig. 3 depicts the EDS spectrum of the as-prepared particles before annealing. The carbon and copper peaks arise from the TEM copper grid and the amorphous carbon ®lm covering it. The immeasurably small oxygen peak which should appear very near the C peak is overshadowed by this large peak in the EDS spectrum. This indicates that the original oxidation level of the as-prepared silicon particles remains very low throughout the dc discharge process, or even in the colloidal state, the last of which con®rmed that the particles suspended in the 2-propanol could keep the nanoparticles from direct atmospheric oxidization. XPS measurements were carried out on a VG ESCALAB MKII spectrometer, using a Mg Ka source (1253.6 eV photons) with the analyzer mode set at a constant analyzer energy of 20 eV. The X-ray source was run at 150 W (15 kV and 10 mA). Since the
46
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
Fig. 2. Typical TEM photograph of the particles. The circle indicates a typical nanocrystal.
Fig. 3. EDS spectrum of the as-prepared particles before annealing.
constituent elements of all our samples consisted of only silicon and oxygen (except for the absorption composition on the particle surfaces), and it was dif®cult to measure the XPS peak change of oxygen, we focused our attention on the changes of the silicon peaks in the XPS results. As XPS signals come from the surface and near surface layers of the sample, their analyses would help to distinguish between surface phenomena and those arising from the sub-surface or substrate. Fig. 4 shows the XPS Si 2p spectrum of the original unannealed sample. The XPS spectrum was ®tted with pure Gaussian functions of the four possible oxidized states and the unoxidized state in order to deconvolute the component peaks for each site type at its appropriate location. The full widths at half-maximum of each component were allowed to vary from 1.3 to 1.9 eV so as to yield the best ®t [9]. If we denote the Si and SiO2 oxidation bonding states as Si0 and Si4, and
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
47
Fig. 4. The deconvoluted XPS Si 2p spectrum of the original sample ®tted with pure Gaussian functions for each of the site types: Si0, Si1, Si2, Si3, Si4, showing the contributions from each site type.
the intermediate oxidation states as Si1, Si2 and Si3, then the lowest binding energy peak refers to Si0 and the highest binding energy peak to SiO2 [9,10]. All the peaks are present and their positions correspond, in the order of increasing binding energy, to Si0, Si1, Si2, Si3 and Si4, respectively. The comparative peak heights are in the order Si2 < Si4 < Si0 < Si1 < Si3 . If we take the intensity of Si2 as unit value, then the intensities of Si2, Si4, Si0, Si1, Si3 are respectively in the approximate ratio of 1:2.1:2.2:3.3:4.3. This ratio suggests that the silicon nanoparticles were not heavily oxidized after it was taken out of the liquid solvent. The result is consistent with the fact that the oxidation speed of the silicon micro particles is slow compared with that of the macroscopic silicon wafer under the same conditions [11]. Fig. 5 shows the Si 2p spectra of the different samples. All spectra were normalized to their peak values with respect to their base lines at about 97 eV. For clarity of display, they are arbitrarily spaced out vertically so as to prevent overlapping of the curves. Curve a refers to the original sample, and curves b, c, d are for the samples annealed in vacuum at 3008C, 5008C for 30 min, and at 5008C for 6 h, respectively. After the sample was annealed at 3008C for 30 min (curve b), the peaks of Si0, Si2, Si3 almost disappeared, while the peak of Si1 became more pronounced. The ratio of the peaks of Si4 and Si1 is
Fig. 5. XPS Si 2p spectra of the different samples. The four curves are arbitrarily displaced vertically in order to avoid overlapping of the curves. (a) original sample, (b)±(d) samples annealed at 3008C, 5008C for 30 min, and 5008C for 6 h in ultrahigh vacuum, respectively.
about 3:2. Because the annealing time was short (30 min) and was carried out in high vacuum, there was little opportunity for the silicon particles to encounter the residual oxygen atoms inside the vacuum chamber and combined with them. Instead, the change in oxygen composition is likely to occur via processes within the particles themselves. At the annealing temperature of 3008C, most of the Si±O bonds of the Si2, Si3 became unstable and a scrambling rearrangement took place. Some of the Si2, Si3 sites were converted to Si1 by losing oxygen atoms or ions which transmigrated to other sites, while some other Si2, Si3 sites received additional oxygen atoms or ions to form Si4. For the sample annealed at 5008C for 30 min (curve c), the Si±O bonds of Si1 also became unstable at this high temperature resulting in the disappearance of the Si1 peak, while the Si0 peak increased greatly. This suggests that the Si±O bonds in the Si1 sites tended to be reduced to Si0 with its oxygen atom or ion migrating to other Si particles to form new Si4 sites. For the sample annealed at 5008C for 6 h (curve d), the peak ratio of Si4:Si0 is about 6.3:1. The intensity of the Si0 peak was considerably reduced. As the annealing temperature was
48
Y. Zhu et al. / Applied Surface Science 171 (2001) 44±48
high and the annealing time long, the residual oxygen molecules in the vacuum chamber had suf®cient chances to react with the silicon particles, even in the ultrahigh vacuum condition. However, this process could produce only the Si4 state but not the intermediate oxidation state. The corresponding intensity of the Si0 state was suppressed. In conclusion, our results show ®rstly that the freshly prepared silicon nanoparticles were only slightly oxidized because our samples were prepared in high vacuum condition and kept in high purity (99.99%) 2-propanol. Secondly, the oxidation states of the particles underwent different chemical stages when they were annealed. At ®rst the Si±O bonds of the Si2 and Si3 states became unstable at 3008C, and were depleted while the Si4 and Si1 states enhanced. Then, the Si1 state also became unstable at 5008C, and was reduced with a corresponding increase in the Si0 and Si4 states. With intense annealing, most of the remaining Si0 states were converted to Si4 by reaction with the residual oxygen in the vacuum chamber. However, no conversion to the less stable intermediate oxidation states took place. Of course, during all these processes, the oxygen atoms and ions could also diffuse in the particles at the same time. The full oxidation Si4 state of the silicon nanoparticles was the most stable, while the intermediate oxidation states were even less stable than the unoxidized state.
Acknowledgements We would like to thank Mr. H.K. Wong, Ms. W.C. Tjiu and Mr. H.H. Teo for their experimental assistance. This work was supported under NUS Research Grant No. 3979901. References [1] L.T. Caham, Appl. Phy. Lett. 57 (1990) 1046. [2] H. Takagi, H. Ogawa, Y. Yamazaki, A. Ishizaki, T. Nakagiri, Appl. Phy. Lett. 56 (1990) 2379. [3] D.W. Cooke, B.L. Bennett, E.H. Farnum, W.L. Hults, K.E. Sickafus, J.F. Smith, J.L. Smith, T.N. Taylor, P. Tiwari, Appl. Phys. Lett. 68 (1996) 1663. [4] K. Kimura, S. Iwasaki, J. Appl. Phys. 83 (1998) 1345. [5] S. V. Gaponenko, Optial Properties of Semiconductor Nanocrystals, Cambridge University Press, Cambridge, 1998. [6] Y. Zhu, H. Wang, P. P. Ong, J. Phys. D: Appl. Phys. 33 (2000), 1965. [7] V.G. Baru, A.P. Chernushich, V.A. Luzanov, G.V. Stepanov, L.Yu. Zakharov, K.P. O'Donnell, I.V. Bradley, N.N. MelAnik, Appl. Phys. Lett. 69 (1996) 4148. [8] Y. Zhu, K. Kimura, L.D. Zhang, J. Vac. Sci. Technol. B15 (6) (1997) 2077. [9] W.K. Chio, F.W. Poon, F.C. Loh, K.L. Tan, J. Appl. Phys. 81 (1997) 7386. [10] F.J. Grunthaner, J. Maserjian, IEEE Trans. Nucl. Sci. NS-24 (1977) 2108. [11] Y. Zhu, K. Kimura, L. D. Zhang, Solid State Commun. 108 (1998) 545, and references therein.