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Photoluminescence of CdS nanocrystals: effect of ageing
PERGAMON
Solid State Communications 111 (1999) 671–674
Photoluminescence of CdS nanocrystals: effect of ageing K.K. Nanda, S.N. Sahu* Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India Received 26 February 1999; accepted 1 June 1999 by S. Ushioda
Abstract We have studied the effect of ageing on the photoluminescence of uncapped CdS nanocrystals prepared using a chemical route. Two luminescence bands in the energy range 1.55–2.48 eV were observed whose intensities were found to increase with ageing. The luminescence properties obtained for different excitation are also presented. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; D. Optical properties; D. Luminescence
1. Introduction The study of optical properties of nanocrystals has become the topic of both theoretical and experimental interest. It is known that semiconductor nanocrystals exhibit the “quantum confinement effect” [1–5] when their sizes are comparable with the bulk exciton radius and as a result, the continuum of states are broken down into discrete states with an effective band gap blue shifted from that of the bulk. These nanocrystals are also characterised by large surface-to-volume ratios and influences the luminescence properties. Such quantum confinement effect as well as surface effects control the optical properties of semiconductor nanocrystals [6–9]. In general, II–VI semiconductor nanocrystals viz.: CdS and CdSe when exposed to atmosphere, the chalcogens (S or Se) at the surface is oxidized to sulfate or selenate [10]. In turn, this oxide will evaporate from the surface as a molecular species, leaving reduced Cd * Corresponding author. Fax: 1 91-674-581142. E-mail address: [email protected] (S.N. Sahu)
and a freshly exposed surface. Thus, it is interesting to study the photoluminescence of CdS nanocrystals with ageing which is the main aim of this work. The luminescence properties with different excitations are also discussed.
2. Experimental CdS crystals (dia. 12 nm) were grown by a precipitation technique using precursors as CdSO4, thiourea, and NH4OH. The preparation procedure and the characterization are published elsewhere [11,12]. XRD analysis of nanocrystalline CdS samples reveals a cubic phase along with CdO as a separate phase. The band gap of the 12 nm sized CdS sample is 3.14 eV. For the photoluminescence study, we used a Hg (Xe) lamp (Oriel Instruments) as a source of excitation and the PL signal is detected by a photomultiplier tube (PMT) coupled with a Stanford Research Systems lock-in amplifier (Model SR830 DSP) and the data were collected through a Personal Computer.
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00268-9
672
K.K. Nanda, S.N. Sahu / Solid State Communications 111 (1999) 671–674
Fig. 1. The photoluminescence spectrum of a fresh CdS nanocrystalline sample measured at 300 K. The excitation energy is 2.78 eV.
3. Results and discussion
Fig. 3. The effect photoluminescence spectra of CdS nanocrystalline samples with ageing recorded at 300 K. The excitation energy is 2.78 eV.
Fig. 1 shows the photoluminescence spectra of a freshly prepared nanocrystalline CdS sample taken at 300 K in the energy range 1.55–2.48 nm. The excitation energy is 2.78 eV. Two bands (A and B) are clearly observed in this energy range. As the energy of these two bands is less than the band gap energy, the transition involves donors, acceptors and surface traps. In the case of nanocrystals, there is a good
wave-function overlap between the host and impurity even at room temperature, and donor–acceptor transition is expected. It can be noted that the half width of band B (0.6 eV) is large compared with that of band A (0.17 eV). The energy band diagram of a nanoparticle and the bulk counterpart is shown in Fig. 2 (taken from Ref. [13]). It can be noted that the shallow
Fig. 2. The energy level diagram of a bulk and a nanocrystalline semiconductor. Surface states arising due to increased surface-to-volume ratio is shown in the figure.
K.K. Nanda, S.N. Sahu / Solid State Communications 111 (1999) 671–674
673
Table 1 Different parameters estimated from Fig. 3 Samples
Peak energy (A) (eV)
Peak energy (B) (eV)
B/A ratio
FWHM (A) (eV)
FWHM (B) (eV)
Fresh 15 days aged 30 days aged
1.74 1.74 1.74
2.185 2.11 1.98
2.0 1.81 1.22
0.17 0.17 0.17
0.6 0.49 0.45
traps move along with the intrinsic band as the crystalline size decreases. However, the shift of the deep traps lying close to the middle of the forbidden gap, is negligible. Thus, the transition involving the deep traps should not change in their energy with decreasing or increasing crystalline sizes. As the energy of peak B is higher than A, the assignment of B can be related to the shallow traps. Further, as the CdS sample is prepared from a chemical route, it is expected to have a size distribution which in turn results in a tailing in the optical absorption spectra and influence the luminescence properties. From the above discussion, it can be inferred that the shallow traps shift appreciably as the crystalline size decreases. Thus, a broad peak is observed for the band B. CdS samples of 12 nm sizes were aged between 15 and 30 days, and the PL was recorded using a 2.78 eV excitation (Fig. 3). Note a remarkable change of the luminescence bands in the PL
Fig. 4. The photoluminescence spectra of 15-day-old samples with different excitation energy (2.78 and 3.303 eV) recorded at 300 K.
spectra. The intensity ratio and the half width of both bands A and B are given in Table 1. It can be noted that the position of the B band shifts to lower energy for aged sample, whereas, there is no shift in the position of the band A. As the band B is associated with the transition from shallower traps, it should move to lower energy as the crystalline size increases with ageing. Also, the width of both bands decreases with ageing implies to an increased crystalline size and a decrease in the size distribution for the aged samples. Further, the increase of intensity of band A is faster compared to band B. We believe that both the transitions are associated with the transition from Cd–O complex [12]. As discussed earlier, the uncapped CdS nanocrystals are prone to oxidation that evaporates in ambient [10] and leaves a fresh surface with reduced Cd. This reduced Cd reacts with atmospheric oxygen to give Cd–O, thereby, increasing the density of Cd–O. As the transitions are associated with Cd–O, the intensity is expected to increase as observed in our case. However, our main conclusion does not depend on the exact identification of these peaks. The PL spectra studied with excitation energies of 3.303 and 2.788 eV are shown in Fig. 4. It can be noted that the position of the band B is different for different excitation, whereas, there is no shift for band A. The position of the band B shifts to higher energy as the excitation energy is increased in good agreement with earlier reported results [14,15]. Further, the intensity ratio (B/A) is greater for higher excitation energy (Table 2). As our CdS samples have size distribution, only the larger crystallites are excited with higher excitation wavelengths. However, when the sample is excited well above the luminescence band, the contribution comes from all the nanocrystals and enhancement of intensity is expected. In addition, a shift of the band B to higher energy is expected with increasing excitation energy.
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K.K. Nanda, S.N. Sahu / Solid State Communications 111 (1999) 671–674
Table 2 Different parameters estimated from Fig. 4 Excitation (eV)
Peak energy (A) (eV)
Peak energy (B) (eV)
B/A ratio
FWHM (A) (eV)
FWHM (B) (eV)
2.788 3.303
1.74 1.74
2.11 2.14
1.81 2.4
0.17 0.17
0.49 0.43
4. Conclusion The ageing of nanocrystalline CdS samples has been studied through PL measurements. It is observed that the luminescence intensity increases with ageing. The transition associated with the shallower trap changes in energy in the aged samples, whereas that associated with deep traps does not. The luminescence was also studied with different excitations. The results have been interpreted in terms of transitions from shallow traps and deep traps. References [1] L. Al, A.L. Efros, Efros, Sov. Phys. Semicond. 16 (1982) 722. [2] A.I. Akimov, A.A. Onushchenko, JETP Lett. 40 (1984) 1136.
[3] Y. Kayanuma, Phys. Rev. B 38 (1988) 7997. [4] Y. Kayanuma, Solid State Commun. 59 (1986) 405. [5] A.I. Ekimov, Al L. Efros, A.A. Onushchenko, Solid State Commun. 56 (1985) 921. [6] T. Arai, T. Yoshida, T. Ogawa, Jpn. J. Appl. Phys. 26 (1987) 396. [7] T. Arai, H. Fujumura, I. Umezu, T. Ogawa, A. Fujii, Jpn. J. Appl. Phys. 28 (1989) 484. [8] M. Agata, H. Kurase, S. Hayashi, K. Yamamoto, Solid State Commun. 76 (1990) 1061. [9] I. Umezu, T. Ogawa, T. Arai, Jpn. J. Appl. Phys. 28 (1989) 447. [10] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [11] S.N. Sahu, J. Mater. Sci. Mater. Electron. 6 (1995) 43. [12] K.K. Nanda, S.N. Sarangi, S.N. Sahu, Current Sci. 72 (1997) 110. [13] N. Chestony, T.D. Harris, R. Hull, L.E. Brus, J. Chem. Phys. 90 (1986) 3393. [14] P.A.M. Rodrigues et al., Solid State Commun. 94 (1995) 583. [15] S. Okamoto et al., Solid State Commun. 105 (1998) 7.
Solid State Communications 111 (1999) 671–674
Photoluminescence of CdS nanocrystals: effect of ageing K.K. Nanda, S.N. Sahu* Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India Received 26 February 1999; accepted 1 June 1999 by S. Ushioda
Abstract We have studied the effect of ageing on the photoluminescence of uncapped CdS nanocrystals prepared using a chemical route. Two luminescence bands in the energy range 1.55–2.48 eV were observed whose intensities were found to increase with ageing. The luminescence properties obtained for different excitation are also presented. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; D. Optical properties; D. Luminescence
1. Introduction The study of optical properties of nanocrystals has become the topic of both theoretical and experimental interest. It is known that semiconductor nanocrystals exhibit the “quantum confinement effect” [1–5] when their sizes are comparable with the bulk exciton radius and as a result, the continuum of states are broken down into discrete states with an effective band gap blue shifted from that of the bulk. These nanocrystals are also characterised by large surface-to-volume ratios and influences the luminescence properties. Such quantum confinement effect as well as surface effects control the optical properties of semiconductor nanocrystals [6–9]. In general, II–VI semiconductor nanocrystals viz.: CdS and CdSe when exposed to atmosphere, the chalcogens (S or Se) at the surface is oxidized to sulfate or selenate [10]. In turn, this oxide will evaporate from the surface as a molecular species, leaving reduced Cd * Corresponding author. Fax: 1 91-674-581142. E-mail address: [email protected] (S.N. Sahu)
and a freshly exposed surface. Thus, it is interesting to study the photoluminescence of CdS nanocrystals with ageing which is the main aim of this work. The luminescence properties with different excitations are also discussed.
2. Experimental CdS crystals (dia. 12 nm) were grown by a precipitation technique using precursors as CdSO4, thiourea, and NH4OH. The preparation procedure and the characterization are published elsewhere [11,12]. XRD analysis of nanocrystalline CdS samples reveals a cubic phase along with CdO as a separate phase. The band gap of the 12 nm sized CdS sample is 3.14 eV. For the photoluminescence study, we used a Hg (Xe) lamp (Oriel Instruments) as a source of excitation and the PL signal is detected by a photomultiplier tube (PMT) coupled with a Stanford Research Systems lock-in amplifier (Model SR830 DSP) and the data were collected through a Personal Computer.
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00268-9
672
K.K. Nanda, S.N. Sahu / Solid State Communications 111 (1999) 671–674
Fig. 1. The photoluminescence spectrum of a fresh CdS nanocrystalline sample measured at 300 K. The excitation energy is 2.78 eV.
3. Results and discussion
Fig. 3. The effect photoluminescence spectra of CdS nanocrystalline samples with ageing recorded at 300 K. The excitation energy is 2.78 eV.
Fig. 1 shows the photoluminescence spectra of a freshly prepared nanocrystalline CdS sample taken at 300 K in the energy range 1.55–2.48 nm. The excitation energy is 2.78 eV. Two bands (A and B) are clearly observed in this energy range. As the energy of these two bands is less than the band gap energy, the transition involves donors, acceptors and surface traps. In the case of nanocrystals, there is a good
wave-function overlap between the host and impurity even at room temperature, and donor–acceptor transition is expected. It can be noted that the half width of band B (0.6 eV) is large compared with that of band A (0.17 eV). The energy band diagram of a nanoparticle and the bulk counterpart is shown in Fig. 2 (taken from Ref. [13]). It can be noted that the shallow
Fig. 2. The energy level diagram of a bulk and a nanocrystalline semiconductor. Surface states arising due to increased surface-to-volume ratio is shown in the figure.
K.K. Nanda, S.N. Sahu / Solid State Communications 111 (1999) 671–674
673
Table 1 Different parameters estimated from Fig. 3 Samples
Peak energy (A) (eV)
Peak energy (B) (eV)
B/A ratio
FWHM (A) (eV)
FWHM (B) (eV)
Fresh 15 days aged 30 days aged
1.74 1.74 1.74
2.185 2.11 1.98
2.0 1.81 1.22
0.17 0.17 0.17
0.6 0.49 0.45
traps move along with the intrinsic band as the crystalline size decreases. However, the shift of the deep traps lying close to the middle of the forbidden gap, is negligible. Thus, the transition involving the deep traps should not change in their energy with decreasing or increasing crystalline sizes. As the energy of peak B is higher than A, the assignment of B can be related to the shallow traps. Further, as the CdS sample is prepared from a chemical route, it is expected to have a size distribution which in turn results in a tailing in the optical absorption spectra and influence the luminescence properties. From the above discussion, it can be inferred that the shallow traps shift appreciably as the crystalline size decreases. Thus, a broad peak is observed for the band B. CdS samples of 12 nm sizes were aged between 15 and 30 days, and the PL was recorded using a 2.78 eV excitation (Fig. 3). Note a remarkable change of the luminescence bands in the PL
Fig. 4. The photoluminescence spectra of 15-day-old samples with different excitation energy (2.78 and 3.303 eV) recorded at 300 K.
spectra. The intensity ratio and the half width of both bands A and B are given in Table 1. It can be noted that the position of the B band shifts to lower energy for aged sample, whereas, there is no shift in the position of the band A. As the band B is associated with the transition from shallower traps, it should move to lower energy as the crystalline size increases with ageing. Also, the width of both bands decreases with ageing implies to an increased crystalline size and a decrease in the size distribution for the aged samples. Further, the increase of intensity of band A is faster compared to band B. We believe that both the transitions are associated with the transition from Cd–O complex [12]. As discussed earlier, the uncapped CdS nanocrystals are prone to oxidation that evaporates in ambient [10] and leaves a fresh surface with reduced Cd. This reduced Cd reacts with atmospheric oxygen to give Cd–O, thereby, increasing the density of Cd–O. As the transitions are associated with Cd–O, the intensity is expected to increase as observed in our case. However, our main conclusion does not depend on the exact identification of these peaks. The PL spectra studied with excitation energies of 3.303 and 2.788 eV are shown in Fig. 4. It can be noted that the position of the band B is different for different excitation, whereas, there is no shift for band A. The position of the band B shifts to higher energy as the excitation energy is increased in good agreement with earlier reported results [14,15]. Further, the intensity ratio (B/A) is greater for higher excitation energy (Table 2). As our CdS samples have size distribution, only the larger crystallites are excited with higher excitation wavelengths. However, when the sample is excited well above the luminescence band, the contribution comes from all the nanocrystals and enhancement of intensity is expected. In addition, a shift of the band B to higher energy is expected with increasing excitation energy.
674
K.K. Nanda, S.N. Sahu / Solid State Communications 111 (1999) 671–674
Table 2 Different parameters estimated from Fig. 4 Excitation (eV)
Peak energy (A) (eV)
Peak energy (B) (eV)
B/A ratio
FWHM (A) (eV)
FWHM (B) (eV)
2.788 3.303
1.74 1.74
2.11 2.14
1.81 2.4
0.17 0.17
0.49 0.43
4. Conclusion The ageing of nanocrystalline CdS samples has been studied through PL measurements. It is observed that the luminescence intensity increases with ageing. The transition associated with the shallower trap changes in energy in the aged samples, whereas that associated with deep traps does not. The luminescence was also studied with different excitations. The results have been interpreted in terms of transitions from shallow traps and deep traps. References [1] L. Al, A.L. Efros, Efros, Sov. Phys. Semicond. 16 (1982) 722. [2] A.I. Akimov, A.A. Onushchenko, JETP Lett. 40 (1984) 1136.
[3] Y. Kayanuma, Phys. Rev. B 38 (1988) 7997. [4] Y. Kayanuma, Solid State Commun. 59 (1986) 405. [5] A.I. Ekimov, Al L. Efros, A.A. Onushchenko, Solid State Commun. 56 (1985) 921. [6] T. Arai, T. Yoshida, T. Ogawa, Jpn. J. Appl. Phys. 26 (1987) 396. [7] T. Arai, H. Fujumura, I. Umezu, T. Ogawa, A. Fujii, Jpn. J. Appl. Phys. 28 (1989) 484. [8] M. Agata, H. Kurase, S. Hayashi, K. Yamamoto, Solid State Commun. 76 (1990) 1061. [9] I. Umezu, T. Ogawa, T. Arai, Jpn. J. Appl. Phys. 28 (1989) 447. [10] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [11] S.N. Sahu, J. Mater. Sci. Mater. Electron. 6 (1995) 43. [12] K.K. Nanda, S.N. Sarangi, S.N. Sahu, Current Sci. 72 (1997) 110. [13] N. Chestony, T.D. Harris, R. Hull, L.E. Brus, J. Chem. Phys. 90 (1986) 3393. [14] P.A.M. Rodrigues et al., Solid State Commun. 94 (1995) 583. [15] S. Okamoto et al., Solid State Commun. 105 (1998) 7.