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Spectroscopy of single silicon nanoparticles
ARTICLE IN PRESS
Journal of Luminescence 108 (2004) 347–350
Spectroscopy of single silicon nanoparticles J. Martin, F. Cichos, C. von Borczyskowski* Institute of Physics, Optical Spectroscopy and Molecular Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany
Abstract Confocal microscopy has been performed on silicon nanoparticles prepared by gas-phase methods and electrochemical etching (single particles), respectively. Spectral line narrowing has been obtained for single particles. Spectra are in agreement with interstellar extended red emission (ERE) when properly choosing size distributions. Independent of preparation techniques, both types show similar behaviour with respect to (partly reversible in the dark) photobleaching accompanied by spectral red shifts on timescales of seconds upon 514 nm laser irradiation. r 2004 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Spectroscopy; Photophysics; Silicon; Extended red emission
1. Introduction Recently, several papers on optical properties of colloidal semiconductor nanoparticles such as CdSe have been published both for ensembles and single crystals [1–3]. II/VI semiconductor nanocrystals (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of the electron–hole pair leads to an increase in the effective band gap with decreasing crystallite size thus offering new ways for tuning optical and photonic properties. Similar relationships have been deduced for nanostructures prepared from the indirect-band gap semiconductor silicon such as porous silicon [4]. Silicon has an indirect optical band transition at 1.17 eV. Differ*Corresponding author. Tel.: +49-371-531-3035; fax: +49371-531-3060. E-mail address: [email protected] (C. von Borczyskowski).
ent techniques such as silicon implantation [4,5], etching of silicon resulting in porous nanocrystals [6], gas-phase preparation [7,8] and etching of silicon pillars [9] have been reported to prepare well-defined silicon nanocrystals (SiNC). Also preparation of colloidal types comparable to II/ VI semiconductor quantum dots have been reported [10]. Absorption and emission close to the band gap are in first-order forbidden for such an indirect band transition but become partly allowed by size dependent quantum confinement effects [11,12]. From experiments it is evident that SiNC without a protecting shell do not emit. Ample evidence experimentally provided that the quantum efficiency varies considerably [12,13] as a function of size and from particle to particle. This is also observed via optical detection of single (isolated) nanoparticles [9,15,16]. The reason for such a variation is not known with confidence, but suggestions have been made in favour of surface or (impurity) trap states in silicon oxide layers [17,18]. As is the case for colloidal semiconductor
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.073
ARTICLE IN PRESS J. Martin et al. / Journal of Luminescence 108 (2004) 347–350
nanoparticles and organic molecules, single SiNC show blinking effects [9,14,19]. The observation of the so-called extended red emission (ERE) in many interstellar clouds has been interpreted as being due to photoluminescence of SiNC excited by the interstellar radiation field. The related quantum efficiency is a key parameter for the assignment of SiNC to ERE [12,20,21]. Although Si atoms are not the most abundant material of interstellar grains, SiNC are of principal interest [22] since their optical properties can be investigated in great detail in the laboratory.
size selected particles non-size selelected particles
1.2
Normalized Emission
348
1.0
1.0
0.8
0.8
0.6
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0.4
0.4
0.2
0.2
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0.0 500
(a)
ERE from NGC 2327 non-size selected particles
1.2
600
700
500
800
Wavelength (nm)
(b)
600
700
800
Wavelength (nm)
Fig. 1. Fluorescence spectra of gas-phase produced silicon particles with and without size selection (a) and comparison with ERE from aninterstellar object NGC 2327 (b).
2. Experimental Experiments have been performed on bulk material of gas-phase produced SiNC [7,8]. Samples of particles produced by laser pyrolysis [8] could not be prepared in a way to obtain wellseparated single particles of sufficiently low background luminescence. Therefore, no studies with individual particles of this type have been performed. However, studies on particle ensembles with and without size selection were carried out. Using a preparation technique involving etching of porous silicon [6] and ultrasonic treatment we succeeded to separate individual particles and to observe their emission using confocal microscopy. The confocal microscope set up has been described elsewhere [23]. All experiments were carried out at room temperature.
3. Results and discussion 3.1. Gas-phase produced particles Fig. 1a shows the emission spectra of ensembles of size and non-size selected SiNC which have been prepared by pyrolysis [8]. The luminescence of the non-size selected ensemble in comparison with the ERE from the interstellar dust object NGC 2327 [20] can be seen in Fig. 1b. Laboratory experiments are in good agreement with the astrophysical observations, thus supporting the recently proposed analogy of SiNC luminescence
Fig. 2. Confocal microscope fluorescence image (23 23 mm2) of silicon nanocrystal islands (approx. 200,000 particles per dot) (a) and spectral evolution of bleaching (b).
with the ERE [12] of various interstellar objects. It is most likely, that interstellar dust clouds do not contain well-selected crystalline sizes. Far more likely is a combination of size selection and photoselection via interstellar radiation fields. Fig. 2a shows the image of ensembles of SiNC obtained by deposition using a mask. Some of the particle islands have been nearly bleached by illumination for 60 s with the 514.5 nm line of an argon-ion laser. A series of corresponding emission spectra reveals that the photobleaching of the ensemble luminescence goes along with a spectral shift towards longer wavelengths as shown in Fig. 2b. We conclude that the characteristic bleaching time varies with particle size and thus with the strength of the confinement. In addition, the bleaching dynamics are strongly non-exponential (see Fig. 4). The analysis of the involved time
ARTICLE IN PRESS J. Martin et al. / Journal of Luminescence 108 (2004) 347–350 14
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10 8 6 4 2 0 0
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80
Particle 1 Particle 2 Cluster
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trace of ensemble emission sum of individual traces
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0.8 0.6 0.4 0.2 0.0
500
600
700
800
900
0
5
Wavelength (nm)
10
15 20 Time (s)
25
30
Fig. 3. Emission spectra of 2 different single crystals and a cluster of silicon particles. The inset shows a time trace for a single particle.
Fig. 4. Comparison between bleaching behaviour of ensemble of gas-phase produced silicon crystals and the sum of 100 emission times traces of single etched particles.
constants points towards several bleaching mechanisms. This is also supported by the fact that bleaching is partly reversible.
silicon nanoparticles, which is non-exponential in character and incomplete (not decaying to zero) due to a saturation of the on/off time duration. This observation is consistent with the nonexponential and incomplete bleaching of the emission of the gas phase produced SiNC shown in Fig. 4.
3.2. Porous silicon particles Typical single-particle emission is between 550 and 650 nm with a spectral width of about 150 meV, which is much narrower than bands of the ensemble spectra as is shown in Fig. 3. Single SiNC show an emission intermittency (blinking) as can be seen in the inset of Fig. 3. Since this intermittency is connected to a dark state of the system, the study of the blinking of single particles gives insight into the population and depopulation rates of this state. The study of many individual time traces of SiNC emission reveals that the statistics of the duration of periods follow a power law. This is equivalent to the observation for other semiconductor particles and interpreted in terms of a nanoparticle ionization via Auger processes or charge tunneling. The charge ejected from the particle is accepted by a trap at the surface of the particle or its environment. A wide distribution of traps (potentials and locations) leads thus to the observed power law. Blinking of the studied SiNC shows a time dependence. Longer on periods and shorter off periods are observed at the beginning of the particle time traces. This leads to an apparent decay in the emission intensity of an ensemble of
4. Conclusions Comparing the results on SiNC obtained from two different preparation techniques shows clear evidence of quantum confinement and photoinduced bleaching in particle ensembles which is consistent with the blinking dynamics of single crystals. Most of the bleaching effects are reversible on a time scale of hours in the dark. The intrinsic nature of the physical and chemical origin might be similar to that observed for II/VI nanocrystals and is most likely due to photoinduced charge-transfer processes in combination with diffusion processes of charge-separated states within the shell or matrix.
Acknowledgements The work has been supported by the DFG Research Group 388 ‘‘Laboratory Astrophysics’’.
ARTICLE IN PRESS 350
J. Martin et al. / Journal of Luminescence 108 (2004) 347–350
Samples via gas phase techniques have been . provided by Prof. Huisken, MPI Gottingen. Illuminating discussions concerning the relationship with ERE with Prof. Huisken and Prof. Witt are gratefully acknowledged.
References [1] A.I. Ekimov, F. Hache, M.C. Schanne-Klein, D. Richard, C. Flytzanis, I.A. Kudriatsev, T.V. Yazeva, A.V. Rodina, Al.L. Efros, J. Opt. Soc. Am. B 10 (1993) 100. [2] A.P. Alvisatos, A.L. Harris, N.J. Steigerwald, M.L.E. Brus, J. Chem. Phys. 89 (1988) 3435. [3] D.J. Norris, M.G. Bawendi, Phys. Rev. B 53 (1996) 16338. [4] S. Cheylan, R.G. Elliman, K. Gaff, A. Durandet, Appl. Phys. Lett. 78 (2001) 1670. [5] S. Takeoka, M. Fujii, S. Hayashi, Physi. Rev. B 62 (2000) 16820. [6] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [7] S. Schuppler, S.I. Friedman, M.A. Marcus, D.L. Aaler, Y-H. Xie, F.M. Ross, Y.J. Chabol, T.D. Haris, L.E. Brus, W.L. Brown, E.E. Chabon, P.F. Szajowski, S.B. Christman, P.H. Citrin, Phys. Rev. B 52 (1995) 4910. [8] M. Ehbrecht, B. Kohn, F. Huisken, M.A. Laguna, V. Paillard, Phys. Rev. B 65 (1997) 6958.
[9] J. Valenta, R. Juhasz, J. Linnros, Appl. Phys. Lett. 80 (2002) 1070. [10] D.S. English, L.E. Pell, Z. Yu, P. Barbara, B.A. Kogel, Nanoletters 2 (2002) 681. [11] D.J. Lookwood, Solid State Commun. 92 (1994) 101. [12] G. Ledoux, J. Gong, F. Huisken, Appl. Phys. Lett. 79 (2001) 4028. [13] M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197. [14] M.D. Mason, G.M. Credo, K.D. Weston, S.K. Buratto, Phys. Rev. Lett. 80 (1998) 5405. [15] G.M. Credo, M.D. Manson, S.K. Buratto, Appl. Phys. Lett. 74 (1999) 1978. [16] D.S. English, L.E. Pell, Z. Yu, P.F. Barbara, B.A. Korgel, Nanoletters 2 (2002) 681. [17] J.L. Gole, F.P. Dudel, D. Grantier, D.A. Dixon, Phys. Rev. B 56 (1997) 2137. [18] Y. Mochizuki, M. Mizuta, Appl. Phys. Lett. 67 (1995) 1396. [19] L.A. Efros, M. Rosen, Phys. Rev. Lett. 78 (1997) 1110. [20] A.N. Witt, K.D. Gordon, D.G. Furton, Astro. Phys. J. 501 (1998) L111. [21] I.W.M. Smith, R.B. Rowe, Acc. Chem. Res. 33 (2000) 261. [22] D. Kovalev, H. Heckler, G. Polisski, F. Koch, Phys. Stat. Sol. (b) 215 (1999) 871. [23] F. Huisken, D. Amans, G. Ledoux, H. Hofmeister, F. Cichos, J. Martin, New J. Phys. 5 (2003) 1.1.
Journal of Luminescence 108 (2004) 347–350
Spectroscopy of single silicon nanoparticles J. Martin, F. Cichos, C. von Borczyskowski* Institute of Physics, Optical Spectroscopy and Molecular Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany
Abstract Confocal microscopy has been performed on silicon nanoparticles prepared by gas-phase methods and electrochemical etching (single particles), respectively. Spectral line narrowing has been obtained for single particles. Spectra are in agreement with interstellar extended red emission (ERE) when properly choosing size distributions. Independent of preparation techniques, both types show similar behaviour with respect to (partly reversible in the dark) photobleaching accompanied by spectral red shifts on timescales of seconds upon 514 nm laser irradiation. r 2004 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Spectroscopy; Photophysics; Silicon; Extended red emission
1. Introduction Recently, several papers on optical properties of colloidal semiconductor nanoparticles such as CdSe have been published both for ensembles and single crystals [1–3]. II/VI semiconductor nanocrystals (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of the electron–hole pair leads to an increase in the effective band gap with decreasing crystallite size thus offering new ways for tuning optical and photonic properties. Similar relationships have been deduced for nanostructures prepared from the indirect-band gap semiconductor silicon such as porous silicon [4]. Silicon has an indirect optical band transition at 1.17 eV. Differ*Corresponding author. Tel.: +49-371-531-3035; fax: +49371-531-3060. E-mail address: [email protected] (C. von Borczyskowski).
ent techniques such as silicon implantation [4,5], etching of silicon resulting in porous nanocrystals [6], gas-phase preparation [7,8] and etching of silicon pillars [9] have been reported to prepare well-defined silicon nanocrystals (SiNC). Also preparation of colloidal types comparable to II/ VI semiconductor quantum dots have been reported [10]. Absorption and emission close to the band gap are in first-order forbidden for such an indirect band transition but become partly allowed by size dependent quantum confinement effects [11,12]. From experiments it is evident that SiNC without a protecting shell do not emit. Ample evidence experimentally provided that the quantum efficiency varies considerably [12,13] as a function of size and from particle to particle. This is also observed via optical detection of single (isolated) nanoparticles [9,15,16]. The reason for such a variation is not known with confidence, but suggestions have been made in favour of surface or (impurity) trap states in silicon oxide layers [17,18]. As is the case for colloidal semiconductor
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.073
ARTICLE IN PRESS J. Martin et al. / Journal of Luminescence 108 (2004) 347–350
nanoparticles and organic molecules, single SiNC show blinking effects [9,14,19]. The observation of the so-called extended red emission (ERE) in many interstellar clouds has been interpreted as being due to photoluminescence of SiNC excited by the interstellar radiation field. The related quantum efficiency is a key parameter for the assignment of SiNC to ERE [12,20,21]. Although Si atoms are not the most abundant material of interstellar grains, SiNC are of principal interest [22] since their optical properties can be investigated in great detail in the laboratory.
size selected particles non-size selelected particles
1.2
Normalized Emission
348
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0 500
(a)
ERE from NGC 2327 non-size selected particles
1.2
600
700
500
800
Wavelength (nm)
(b)
600
700
800
Wavelength (nm)
Fig. 1. Fluorescence spectra of gas-phase produced silicon particles with and without size selection (a) and comparison with ERE from aninterstellar object NGC 2327 (b).
2. Experimental Experiments have been performed on bulk material of gas-phase produced SiNC [7,8]. Samples of particles produced by laser pyrolysis [8] could not be prepared in a way to obtain wellseparated single particles of sufficiently low background luminescence. Therefore, no studies with individual particles of this type have been performed. However, studies on particle ensembles with and without size selection were carried out. Using a preparation technique involving etching of porous silicon [6] and ultrasonic treatment we succeeded to separate individual particles and to observe their emission using confocal microscopy. The confocal microscope set up has been described elsewhere [23]. All experiments were carried out at room temperature.
3. Results and discussion 3.1. Gas-phase produced particles Fig. 1a shows the emission spectra of ensembles of size and non-size selected SiNC which have been prepared by pyrolysis [8]. The luminescence of the non-size selected ensemble in comparison with the ERE from the interstellar dust object NGC 2327 [20] can be seen in Fig. 1b. Laboratory experiments are in good agreement with the astrophysical observations, thus supporting the recently proposed analogy of SiNC luminescence
Fig. 2. Confocal microscope fluorescence image (23 23 mm2) of silicon nanocrystal islands (approx. 200,000 particles per dot) (a) and spectral evolution of bleaching (b).
with the ERE [12] of various interstellar objects. It is most likely, that interstellar dust clouds do not contain well-selected crystalline sizes. Far more likely is a combination of size selection and photoselection via interstellar radiation fields. Fig. 2a shows the image of ensembles of SiNC obtained by deposition using a mask. Some of the particle islands have been nearly bleached by illumination for 60 s with the 514.5 nm line of an argon-ion laser. A series of corresponding emission spectra reveals that the photobleaching of the ensemble luminescence goes along with a spectral shift towards longer wavelengths as shown in Fig. 2b. We conclude that the characteristic bleaching time varies with particle size and thus with the strength of the confinement. In addition, the bleaching dynamics are strongly non-exponential (see Fig. 4). The analysis of the involved time
ARTICLE IN PRESS J. Martin et al. / Journal of Luminescence 108 (2004) 347–350 14
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10 8 6 4 2 0 0
0.4
20 40 60 Observation Time (s)
80
Particle 1 Particle 2 Cluster
0.2 0.0
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trace of ensemble emission sum of individual traces
1.0
12 Counts (kHz)
Normalized Emission
1.0
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0.8 0.6 0.4 0.2 0.0
500
600
700
800
900
0
5
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10
15 20 Time (s)
25
30
Fig. 3. Emission spectra of 2 different single crystals and a cluster of silicon particles. The inset shows a time trace for a single particle.
Fig. 4. Comparison between bleaching behaviour of ensemble of gas-phase produced silicon crystals and the sum of 100 emission times traces of single etched particles.
constants points towards several bleaching mechanisms. This is also supported by the fact that bleaching is partly reversible.
silicon nanoparticles, which is non-exponential in character and incomplete (not decaying to zero) due to a saturation of the on/off time duration. This observation is consistent with the nonexponential and incomplete bleaching of the emission of the gas phase produced SiNC shown in Fig. 4.
3.2. Porous silicon particles Typical single-particle emission is between 550 and 650 nm with a spectral width of about 150 meV, which is much narrower than bands of the ensemble spectra as is shown in Fig. 3. Single SiNC show an emission intermittency (blinking) as can be seen in the inset of Fig. 3. Since this intermittency is connected to a dark state of the system, the study of the blinking of single particles gives insight into the population and depopulation rates of this state. The study of many individual time traces of SiNC emission reveals that the statistics of the duration of periods follow a power law. This is equivalent to the observation for other semiconductor particles and interpreted in terms of a nanoparticle ionization via Auger processes or charge tunneling. The charge ejected from the particle is accepted by a trap at the surface of the particle or its environment. A wide distribution of traps (potentials and locations) leads thus to the observed power law. Blinking of the studied SiNC shows a time dependence. Longer on periods and shorter off periods are observed at the beginning of the particle time traces. This leads to an apparent decay in the emission intensity of an ensemble of
4. Conclusions Comparing the results on SiNC obtained from two different preparation techniques shows clear evidence of quantum confinement and photoinduced bleaching in particle ensembles which is consistent with the blinking dynamics of single crystals. Most of the bleaching effects are reversible on a time scale of hours in the dark. The intrinsic nature of the physical and chemical origin might be similar to that observed for II/VI nanocrystals and is most likely due to photoinduced charge-transfer processes in combination with diffusion processes of charge-separated states within the shell or matrix.
Acknowledgements The work has been supported by the DFG Research Group 388 ‘‘Laboratory Astrophysics’’.
ARTICLE IN PRESS 350
J. Martin et al. / Journal of Luminescence 108 (2004) 347–350
Samples via gas phase techniques have been . provided by Prof. Huisken, MPI Gottingen. Illuminating discussions concerning the relationship with ERE with Prof. Huisken and Prof. Witt are gratefully acknowledged.
References [1] A.I. Ekimov, F. Hache, M.C. Schanne-Klein, D. Richard, C. Flytzanis, I.A. Kudriatsev, T.V. Yazeva, A.V. Rodina, Al.L. Efros, J. Opt. Soc. Am. B 10 (1993) 100. [2] A.P. Alvisatos, A.L. Harris, N.J. Steigerwald, M.L.E. Brus, J. Chem. Phys. 89 (1988) 3435. [3] D.J. Norris, M.G. Bawendi, Phys. Rev. B 53 (1996) 16338. [4] S. Cheylan, R.G. Elliman, K. Gaff, A. Durandet, Appl. Phys. Lett. 78 (2001) 1670. [5] S. Takeoka, M. Fujii, S. Hayashi, Physi. Rev. B 62 (2000) 16820. [6] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [7] S. Schuppler, S.I. Friedman, M.A. Marcus, D.L. Aaler, Y-H. Xie, F.M. Ross, Y.J. Chabol, T.D. Haris, L.E. Brus, W.L. Brown, E.E. Chabon, P.F. Szajowski, S.B. Christman, P.H. Citrin, Phys. Rev. B 52 (1995) 4910. [8] M. Ehbrecht, B. Kohn, F. Huisken, M.A. Laguna, V. Paillard, Phys. Rev. B 65 (1997) 6958.
[9] J. Valenta, R. Juhasz, J. Linnros, Appl. Phys. Lett. 80 (2002) 1070. [10] D.S. English, L.E. Pell, Z. Yu, P. Barbara, B.A. Kogel, Nanoletters 2 (2002) 681. [11] D.J. Lookwood, Solid State Commun. 92 (1994) 101. [12] G. Ledoux, J. Gong, F. Huisken, Appl. Phys. Lett. 79 (2001) 4028. [13] M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197. [14] M.D. Mason, G.M. Credo, K.D. Weston, S.K. Buratto, Phys. Rev. Lett. 80 (1998) 5405. [15] G.M. Credo, M.D. Manson, S.K. Buratto, Appl. Phys. Lett. 74 (1999) 1978. [16] D.S. English, L.E. Pell, Z. Yu, P.F. Barbara, B.A. Korgel, Nanoletters 2 (2002) 681. [17] J.L. Gole, F.P. Dudel, D. Grantier, D.A. Dixon, Phys. Rev. B 56 (1997) 2137. [18] Y. Mochizuki, M. Mizuta, Appl. Phys. Lett. 67 (1995) 1396. [19] L.A. Efros, M. Rosen, Phys. Rev. Lett. 78 (1997) 1110. [20] A.N. Witt, K.D. Gordon, D.G. Furton, Astro. Phys. J. 501 (1998) L111. [21] I.W.M. Smith, R.B. Rowe, Acc. Chem. Res. 33 (2000) 261. [22] D. Kovalev, H. Heckler, G. Polisski, F. Koch, Phys. Stat. Sol. (b) 215 (1999) 871. [23] F. Huisken, D. Amans, G. Ledoux, H. Hofmeister, F. Cichos, J. Martin, New J. Phys. 5 (2003) 1.1.