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Spectral diffusion of quasi localized excitons in single silicon nanocrystals
Journal of Luminescence 132 (2012) 2161–2165
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Spectral diffusion of quasi localized excitons in single silicon nanocrystals Joerg Martin 1, Frank Cichos 2, Christian von Borczyskowski n Centre for nanostructured Materials and Analytics, Institute of Physics, Chemnitz University of Technology, Reichenhainer Street 70, 09107 Chemnitz, Germany
a r t i c l e i n f o
abstract
Article history: Received 20 December 2011 Received in revised form 28 February 2012 Accepted 7 March 2012 Available online 17 March 2012
Evolution in time of photoluminescence spectra of SiOx capped single silicon nanocrystals has been investigated by means of confocal optical spectroscopy at room temperature. Large spectral jumps between subsequent spectra of up to 40 meV have been detected leading to noticeable line broadening and variation in the electron–phonon coupling. Further, a correlation between emission energy and emission intensity has been found and discussed in terms of an intrinsic Stark effect. Anti-correlated variations of the electron–phonon coupling to Si and SiO2 phonons as a function of photoluminescence energy indicate that the nearly localized excition is to some extent coupled to phonons in the shell covering the silicon nanocrystal. However, coupling is reduced upon increasing Stark effect, while at the same time coupling to phonons of the Si core increases. & 2012 Elsevier B.V. All rights reserved.
Keywords: Silicon nanocrystals Electron–phonon coupling Single particle detection Optical spectroscopy Spectral diffusion
1. Introduction Semiconductor nanocrystals are of scientific and practical interest since a few decades. Following the first observation of visible photoluminescence from silicon nanocrystals (Si-NC) [1] great effort has been made to understand the physical mechanisms beyond photoluminescence in detail both theoretically and experimentally [2–4]. Emission from quantum confined excitons exhibits properties, which make semiconductor quantum dots (QD) suitable for several applications in biology and optics. However, there are some aspects which remain to be solved. Due to quantum confinement, photoluminescence (PL) of these systems is directly coupled to the physical properties of the nanocrystals, such as size, surface or the dielectric environment. In most applications or investigations it is not possible to get control over all these properties at the same time. Therefore experiments on QD ensembles lack detailed results and sizeselected ensembles [5,6] or even single Si QD [7–9] have been investigated. Single particle methods revealed several new photophysical properties, such as size dependence of quantum efficiency [9], blinking (PL intermittency on long time scales) [10] and spontaneous spectral diffusion [11]. The interplay of electron– phonon coupling partly results in localization of the exciton (created in the core of the Si QD) in the covering SiOx shell [12]. n
Correspondence to: D09126 Chemnitz, Reichenhainer Street 70, Germany. E-mail address: [email protected] (C. von Borczyskowski). 1 Now at: Fraunhofer Institute for Electronic Nanosystems ENAS, TechnologieCampus 3, 09126 Chemnitz, Germany. 2 Now at: Molecular Nano-Photonics, Institute of Experimental Physics I, Leipzig University, Linne´straße 5, 09103 Leipzig, Germany. 0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.03.028
Very long excited state lifetimes in the order of microseconds [3] and strong PL intermittency [10] limit experimental investigations on spectral diffusion in single Si QD. Spectral shifts related to spectral diffusion result from changes in the local electrical field (Stark effect) [13]. Besides macroscopic external fields also randomly photogenerated charges [14] and mobile surface charges [15] can be the origin of local electric fields. An electrical field lowers the transition energy and therefore leads to red-shifted PL spectra compared to the nonfield configuration [13]. Additionally, the electrical field reduces the overlap of the electron and hole wavefunctions of excitons. That is why not only changes regarding the spectral position but also in the emission intensity are expected [13]. Additionally, this will considerably influence electron–phonon coupling [12]. In this paper we investigate for the first time systematically results related to spectral diffusion in silicon nanocrystals (Si-NC) and coupling of Si excitons to phonons in the oxide shell.
2. Experimental Silicon nanocrystals (Si-NC) have been produced by dehydration of silane in a gas-flow reactor. The setup and procedure including size selection is described in detail in Ref. [5].We obtain a layer of Si-NC on a filter paper which was mounted in front of the gas reactor. This paper was then immersed into toluene to disperse the particles into the solvent. The typical Si-NC diameter is about 2–3 nm capped with a SiOx shell [5,11,12,16]. PL can only be observed in the presence of a capping oxide layer. Following appropriate dilution the colloidal suspension was mixed with a solution of polymethyl methacrylate (PMMA) in toluene in order to embed the nanocrystals in a polymer. This mixture was then
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spin-casted on a quartz plate. After toluene evaporation we typically obtained 20–50 nm thick PMMA films with embedded well separated Si-NC. Single Si-NC were investigated with a home-built confocal microscope [10,12]. Si-NC photoluminescence (PL) was excited at 532 nm by a diode pumped solid state laser. The light was coupled into the set-up via an optical fibre and was focused onto the sample by a microscope lens (Carl Zeiss Jena) with 100x magnification and a numerical aperture of 0.9. Fixed excitation intensity was 6 103 W/cm2. This corresponds to 0.24 photons/10 6 s assuming an absorption cross section of 1.5 10–17 cm2 [28,29]. Taking an average lifetime of 2.5 10 6 s as determined in our experiments this corresponds to less than 1 exciton/Si-NC. We kept excitation intensity low to avoid fast photobleaching. Since PL signals are small we were not able to vary excitation intensity. However, spectral diffusion might be enhanced by photoexcitation. PL was collected by the same objective and imaged onto a pinhole by an achromatic lens. A second achromatic lens imaged the PL onto an avalanche photodiode (EG&G SPCMAQR 13) or a grating monochromator (Acton SP-300i) equipped with a liquid-nitrogen-cooled CCD (Princeton Instruments). Switching between the two detectors was done with a mirror. To separate the excitation from the PL a dichroic beam splitter and long pass filter of 540 nm cut-off wavelength (Omega Optics) was placed into the detection line. Sample scanning has been carried out by means of a closed-loop xyz-piezo translation stage (Physics Instruments) with maximum scan ranges of 80 mm in x, and y-direction and 20 mm in z-direction, respectively. With this setup we were able to record PL with a spatial resolution up to 250 nm and a time resolution up to some 10 ms/frame. All together we have investigated about 50 Si-NC which all showed spectral diffusion, but only 16 have been photostable enough to provide a long series of spectra. Four histograms of typical jump series are shown in Fig. 4. Detailed spectral deconvolution has been performed on two such series and which are shown in Fig. 1. Qualitatively all series behave the same but with a large scatter of fitting results (see also Appendix A). CCD camera and monochromator show in the respective energy range only a dispersion of 0.1%/nm. Since spectral jumps occur only over a small energy range of at most 90 meV we have not corrected for spectral response.
3. Results and discussion Two PL spectra are shown in Fig. 1 for two different single SiNC. The assignment to single Si-NC is based on the blinking phenomena (PL on–off intermittency), which is a very strong indication for the presence of a single quantum system [14]. Though the PL of the presently investigated Si-NCs is on for quite a long time, all NCs eventually blink or are photobleached in one single step. Both spectra shown in Fig. 1 exhibit the typical structure of small single Si-NC emission spectra grown from the gas phase or originating from porous silicon [11,12,16], namely consisting of a main transition and at least one side band. The main band is assigned to the recombination of electron and hole of a Si exciton, while the side bands are attributed to Si–O vibrational modes of the oxide shell of the NC [7,8,10–12,16]. The side bands suggest a strong coupling of core excitons and phonons. Recent experiments have shown that the side band structure of the PL spectra is very sensitive to the local dielectric environment of SiNC [11]. This indicates that the excitons are (quasi) localized close to the interface or even in the oxide shell [12,16]. Despite both spectra in Fig. 1 exhibit a very similar structure, there are differences with respect to the absolute PL energy and the width of the individual bands. Moreover, the spectral width depends critically on the averaging time as can be seen from the inset in Fig. 2. The full width at half maximum (FWHM) of the
Fig. 1. Typical PL spectrum of two single Si-NC with (A) a strong SiO2- and a weak Si-phonon band or with (B) a weak SiO2- and strong Si-Phonon band. The spectra have been fitted by 3 Gaussian lines. Averaging time is 0.5 s in each case. The Si no-phonon line is in both cases the band at highest PL energy, while the SiO2 phonon corresponds to band at lowest PL energy.
main band becomes 105 meV for 0.5 s averaging time and 150 meV for 50 s averaging time, respectively. This considerable line broadening results from spontaneous spectral diffusion as is illustrated in Fig. 2, in which a time series of PL spectra is shown for one single Si-NC. It has to be noted, that for clarity only 21 of total 100 spectra are shown. Nevertheless jumps in PL energy, spectral width and intensity are clearly visible. As shown in Fig. 3 we plot the time evolution of the PL energy of the main band and the (normalized) integrated PL intensity, which reveals strongly correlated statistical jumps (diffusion) for both observables. For this specific single Si-NC we determine a maximal diffusion range of nearly 70 meV. This range is similar to the one observed for CdSe quantum dots [13,17]. Similar large spectral shifts have been obtained for specific single molecules [18]. According to current models for spectral diffusion in semiconductor nanocrystals spectral diffusion may arise from time dependent local electric fields originating from charge fluctuations close to the nanocrystal [13,19,20]. These charges might be photogenerated from the QD itself by photoejecting charges from the exciton to the matrix or to the interface (e.g. oxide shell) [21] or they might be present right from the beginning [13]. This is in agreement with models to explain luminescence blinking of semiconductor nanocrystals or molecules [14]. Blinking has also been observed for Si-NC [7,8,10,11,22,23]. First, according to accepted models [24] one charge of the core exciton, most probably the electron, tunnels to the particle shell or interface. The charge (hole), which is left in the
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Fig. 2. Series of subsequent PL spectra of a single Si-NC with 0.5 s integration time over at time period of 50 s (Crystal 1 in Fig. 5). Excitation occurs at 532 nm (2.33 eV) with an excitation intensity of 6 103 W/cm2.The inset shows a comparison of PL spectra of a single spectrum with 1 s integration time (full line) and the sum of 100 subsequent spectra (broken line) of one typical Si-NC.
Fig. 3. Correlation of the PL energy of the main PL line of a single Si-NC ((1) in Fig. 4) at high PL energy (‘‘zero-phonon’’ line) and the total normalized intensity as a function of observation time. PL intensity has been set to 1 at the highest intensity during observation time of 50 s.
particle core, acts as a centre for non-radiative recombination for a subsequently generated exciton thus reducing the PL quantum efficiency. In case this charge (hole) is also trapped near the particle interface, subsequently generated excitons will recombine (partly) radiatively [14]. However, due to the presence of the electric dipole field generated by the (localized) charges, the PL energy is shifted towards lower energies due to an internal Stark effect, depending on the relative positions of the charges with respect to the exciton. This will be accompanied by a PL decrease resulting from increased nonradiative decay routes induced by an enhanced electron–phonon coupling. Even in case that only one charge is present, PL properties will be influenced in course of time as soon as the charge is diffusing. We have not detected dark periods in the present series of subsequent spectra. This means, that dark periods are either very short (shorter than our averaging time of 500 ms) or completely absent. This on-time is as long as several minutes before, finally, the PL is switched off for very long times [10,23]. Histograms of spectral jumps obtained for 4 different single Si-NC are shown in Fig. 4. It is clearly seen, that the jumps are on the same order of magnitude for each of the Si-NC, but show different types of distributions, some are symmetric and some are not. This is probably due to different local dielectric environments. Our
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investigations on PL blinking in semiconductor nanocrystals suggest, that the charge, which has been photoejected from the core is localized in spatially and/or energetically distributed (self-trapped) states [25]. It may diffuse among these states resulting in fluctuations of the quantum efficiency and spectral position of the subsequently generated exciton. According to the model outlined above distributions shown in Fig. 4 resemble an ‘image’ of spatial charge fluctuations in the nanocrystal shell or even in the PMMA matrix in case of very thin oxide layers [11]. The distributions of spectral jumps are therefore directly related to local dielectric properties. Beside this, another effect is quite obvious when looking at Fig. 5. Here we find a correlation between the total (integrated) PL intensity and the related PL energy (of the main peak). The PL intensity increases exponentially with increasing PL energy. Recently it has been found [6] that the radiative PL decay rate increases over a wide energy range exponentially with a characteristic energy En of about 250 meV crossing over to a smaller En at PL energies above 1.9 eV. It might be worthwhile to mention, that En is in the same energy range found for the energy difference (200 meV) between PL energies at which electron and hole localize separately [12]. The presently investigated Si-NCs have PL energies somewhat below the energy of 2.125 eV, at which electron localization is expected to take place. Since the radiative rate is directly related to the PL intensity we can compare the reported results with our findings. Fig. 5 reveals En to be close to 120 meV, which is a reasonable value as compared to results reported in the literature since we are at the high energy end of the PL of Si-NC. Commonly the broad PL range is related to different sizes of Si-NC. In our experiments on single NC, however, PL energy related intensities have to be assigned to other mechanisms than quantum size effects. The lowest intensity can be found for the spectrum with the lowest emission energy, related to a configuration with the strongest Stark effect in agreement with the model of quantum confined Stark effect [13,15]. An electric field interacts with the exciton by enlarging the separation of electron and hole wavefunctions thus diminishing the radiative decay rate because of the reduced overlap of the wavefunctions. Also the (phonon) side bands show a similar but quantitatively different behaviour. To investigate these effects in more detail we have deconvoluted each PL spectrum as shown in Fig. 1 into three bands, namely the main (zero phonon) band, a phonon sideband assigned to a (TO) Si phonon (kept fixed) at 56 meV [30] and a SiO2 phonon band with typical frequencies close to 150 meV [11,12]. Typical fitting results are also included in Fig. 1. The SiO2 phonon side bands are related to phonons of the SiOx-shell, meaning there is an electron–phonon coupling of excitons and shell phonons. The spectral deconvolution reveals a quantitative different PL intensity correlation of the main (nophonon) band with the PL energy of the main band as compared to the SiO2 phonon band intensity, respectively, as shown in Fig. 6. While the main band is decreasing by less than a factor of 2 over the respective PL energy range, the SiO2 band is (non-linearly) decreasing by almost a factor of 4 in the respective energy range. Obviously phonon activated and zero phonon transitions depend differently on the Stark effect. Qualitatively side band intensities are more strongly decreasing with decreasing PL energy. However, due to the strong scatter of the no-phonon band data (which is probably caused by the interfering deconvolution from the Si-phonons), it is not possible to confidently assign the kind of behaviour (linear or exponential). To get more insight we have calculated the Huang-Rhys factor S both for Si phonons S(Si) and SiO2 phonons S(SiO2) by relating the ratio of PL intensities of the respective phonon band In and the sum Sn of the main band and the remaining phonon band to the PL energy. Phonon side bands involving n phonons have intensities In ¼Sn exp(S)/n!, where S corresponds to the Huang-Rhys factor [27]. The results are shown in Fig. S1 of Appendix A. Though there is a large experimental scatter, the general trends are, that S(SiO2)
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Fig. 4. Distribution of spectral PL energy jumps of the main (zero-phonon) PL line during observation times of typically 60 s for 4 different Si-NC. Integration time for each spectrum is 0.5 s. Jump widths are related to spectral differences of subsequent spectra.
Fig. 5. Integrated PL intensity of a single Si-NC ((1) in Fig. 4) as a function of PL energy fluctuating during 60 s observation time. Note the log scale for the PL intensity.
increases with PL energy by about a factor of 1.2, while S(Si) decreases over the same range by about a factor of 2. This is in qualitative agreement with results obtained for electron–phonon coupling observed for Si-NC in various matrices with different dielectric constants [11]. Recent experiments have shown [11,16], that PL from single Si-NC is due to (nearly) localized excitons [12] in the SiO2 shell
showing strong SiO2 phonon side bands. We have reported PL intermittency (on–off blinking) for single Si-NC [10], which is assigned to photoinduced charge separation of the exciton [14] resulting in dark or at least dim states. In the present examples, however, PL is ‘‘on’’ (no blinking) for quite a long time of several minutes. This suggests that in these special cases photoinduced charge separation is absent. However, during this on-time spectral properties are nevertheless fluctuating. We speculate, that this might be resembling to an internal Stark effect [13,19,20] due to excess charges. In case that the Si-NC is charged, the (localized) exciton will experience shifts in PL energy and electron–phonon coupling [26] due to the created field gradient caused by the external charge as has already been observed for nanocrystals [15,21]. The corresponding magnitudes will depend on the position of the charge relative to the one of the (localized) exciton. Upon charge diffusion PL energy and intensity as well as electron–phonon coupling will fluctuate. Low PL energies and low integrated intensities are related to an increased Stark effect. Obviously the increase of the Stark effect results at the same time in a decrease of the coupling to SiO2 phonons but an increase of the coupling to Si phonons, which is recently not understood.
4. Conclusions Spontaneous spectral diffusion and strongly correlated PL intensity fluctuations have been detected for nearly localized excitons in single silicon nanocrystals capped with a SiOx shell. A considerably large
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discussions. Joerg Martin likes to thank Ruben Schmidt for support on experiments. Financial support of the DFG within the framework of FOR 388 ‘Laboratory Astrophysics’ is gratefully acknowledged.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2012. 03.028.
References
Fig. 6. (A) Integrated PL intensity of the main peak (‘‘zero-phonon’’ line) during observation time as a function of PL energy for single Si-NC 1. (B) Respective integrated PL intensity for the SiO2-phonon side band.
inhomogenous line broadening originates from this spectral diffusion. We have detected large spectral jumps between two subsequent spectra of up to 40 meV and a total spectral diffusion range of up to nearly 70 meV. The distribution of jumps and PL intensities reflects fluctuations in the local environment. Further, a correlation between PL energy and intensity could be found, which is in agreement with existing models for the influence of an internal Stark effect on optical properties. The electron–phonon coupling to Si and SiO2 phonons are obviously anti-correlated, which is not yet understood. The investigated Si-NC is most likely charged for the total time of observation of several minutes. We suggest that ‘‘permanent’’ charging prohibits the photoejection of additional charges.
Acknowledgements We thank Friedrich Huisken, University of Jena, for providing us with size selected silicon nanocrystals and for many stimulating
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] D.J. Lockwood (Ed.), Light Emission in Silicon: From Physics to Devices, Academic Press, New York, 1998. [3] G.J. Ledoux, J. Gong, F. Huisken, O. Guillois, C. Reynaud, Appl. Phys. Lett. 80 (2002) 4834. [4] M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197. [5] M. Ehbrecht, F. Huisken, Phys. Rev. B 59 (1999) 2975. [6] F. Huisken, G. Ledoux, O. Guillois, C. Reynaud, Adv. Mater. 14 (2002) 1861. [7] M.D. Mason, D.J. Sirbuly, P.J. Carson, S.K. Buratto, J. Chem. Phys. 114 (2001) 8119. [8] D.S. English, L.E. Pell, Z. Yu, P.F. Barbara, B.A. Korgel, Nano Lett. 2 (2002) 681. [9] J. Valenta, R. Juhasz, J. Linnros, Appl. Phys. Lett. 80 (2002) 1070. [10] F. Cichos, J. Martin, C. von Borczyskowski, Phys. Rev. B 70 (2004) 115314. [11] N. el-Kork, F. Huisken, C. von Borczyskowski, J. Appl. Phys. 110 (2011) 074312. [12] J. Martin, F. Cichos, F. Huisken, C. von Borczyskowski, Nano Lett. 8 (2008) 656. [13] J. Mueller, J.M. Lupton, A.L. Rogach, J. Feldmann, D.V. Talapin, H. Weller, Phys. Rev. Lett. 93 (2004) 167. [14] F. Cichos, C. von Borczyskowski, M. Orrit, Curr. Opinion Colloid Interface Sci. 12 (2007) 272. [15] T.D. Krauss, L.E. Brus, Phys. Rev. Lett. 83 (1999) 4840. [16] A.M. Chizhik, A.I. Chizhik, R. Gutbrod, A.J. Meixner, T. Schmidt, J. Sommerfeld, F. Huisken, Nano Lett. 9 (2009) 3239. [17] D.E. Go´mez, J. van Embden, P. Mulvaney, Appl. Phys. Lett. 88 (2006) 154106. ¨ ¨ [18] S. Krause, D. Kowerko, R. Borner, C.G. Hubner, C. von Borczyskowski, ChemPhysChem 12 (2011) 303. [19] S.A. Empedocles, M.G. Bawendi, Science 278 (1997) 2114; Y.-H. Kuo, Y. Kyu Lee, Y. Ge, S. Ren, J.E. Roth, T.I. Kamins, D.A.B. Miller, J.S. Harris, Nature 437 (2005) 1334. [20] R.G. Neuhauser, K.T. Shimizu, W.K. Woo, S.A. Empedocles, M.G. Bawendi, Phys. Rev. Lett. 85 (2000) 3301. [21] J. Martin, F. Cichos, C. von Borczyskowski, Opt. Spectrosc. 99 (2005) 281. [22] J. Martin, F. Cichos, C. von Borczyskowski, J. Lumin. 108 (2004) 347. [23] J. Martin, F. Cichos, I.Y. Chan, F. Huisken, C. von Borczyskowski, Isr. J. Chem. 44 (2004) 341. [24] R. Verberk, A.M. van Oijen, M. Orrit, Phys. Rev. B 66 (2002) 233202. [25] A. Issac, C. von Borczyskowski, F. Cichos, Phys. Rev. B 71 (2005) 161302. [26] N. Amecke, F. Cichos, J. Lumin. 131 (2011) 375. [27] I. de Maat-Gersdorf, T. Gregorkiewicz, C.A.J. Ammerlaan, N.A. Sobolev, Semicond. Sci. Technol. 10 (1995) 666. [28] G. Davies, Phys. Rep. 176 (1989) 83. ¨ [29] D. Kovalev, J. Diener, H. Heckler, G. Polisski, N. Kunzer, U.F. Koch, Phys. Rev. B 61 (2000) 4485. [30] D. Kovalev, H. Heckler, G. Polisski, U.F. Koch, Phys. Status Solidi (b) 215 (1999) 871.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Spectral diffusion of quasi localized excitons in single silicon nanocrystals Joerg Martin 1, Frank Cichos 2, Christian von Borczyskowski n Centre for nanostructured Materials and Analytics, Institute of Physics, Chemnitz University of Technology, Reichenhainer Street 70, 09107 Chemnitz, Germany
a r t i c l e i n f o
abstract
Article history: Received 20 December 2011 Received in revised form 28 February 2012 Accepted 7 March 2012 Available online 17 March 2012
Evolution in time of photoluminescence spectra of SiOx capped single silicon nanocrystals has been investigated by means of confocal optical spectroscopy at room temperature. Large spectral jumps between subsequent spectra of up to 40 meV have been detected leading to noticeable line broadening and variation in the electron–phonon coupling. Further, a correlation between emission energy and emission intensity has been found and discussed in terms of an intrinsic Stark effect. Anti-correlated variations of the electron–phonon coupling to Si and SiO2 phonons as a function of photoluminescence energy indicate that the nearly localized excition is to some extent coupled to phonons in the shell covering the silicon nanocrystal. However, coupling is reduced upon increasing Stark effect, while at the same time coupling to phonons of the Si core increases. & 2012 Elsevier B.V. All rights reserved.
Keywords: Silicon nanocrystals Electron–phonon coupling Single particle detection Optical spectroscopy Spectral diffusion
1. Introduction Semiconductor nanocrystals are of scientific and practical interest since a few decades. Following the first observation of visible photoluminescence from silicon nanocrystals (Si-NC) [1] great effort has been made to understand the physical mechanisms beyond photoluminescence in detail both theoretically and experimentally [2–4]. Emission from quantum confined excitons exhibits properties, which make semiconductor quantum dots (QD) suitable for several applications in biology and optics. However, there are some aspects which remain to be solved. Due to quantum confinement, photoluminescence (PL) of these systems is directly coupled to the physical properties of the nanocrystals, such as size, surface or the dielectric environment. In most applications or investigations it is not possible to get control over all these properties at the same time. Therefore experiments on QD ensembles lack detailed results and sizeselected ensembles [5,6] or even single Si QD [7–9] have been investigated. Single particle methods revealed several new photophysical properties, such as size dependence of quantum efficiency [9], blinking (PL intermittency on long time scales) [10] and spontaneous spectral diffusion [11]. The interplay of electron– phonon coupling partly results in localization of the exciton (created in the core of the Si QD) in the covering SiOx shell [12]. n
Correspondence to: D09126 Chemnitz, Reichenhainer Street 70, Germany. E-mail address: [email protected] (C. von Borczyskowski). 1 Now at: Fraunhofer Institute for Electronic Nanosystems ENAS, TechnologieCampus 3, 09126 Chemnitz, Germany. 2 Now at: Molecular Nano-Photonics, Institute of Experimental Physics I, Leipzig University, Linne´straße 5, 09103 Leipzig, Germany. 0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.03.028
Very long excited state lifetimes in the order of microseconds [3] and strong PL intermittency [10] limit experimental investigations on spectral diffusion in single Si QD. Spectral shifts related to spectral diffusion result from changes in the local electrical field (Stark effect) [13]. Besides macroscopic external fields also randomly photogenerated charges [14] and mobile surface charges [15] can be the origin of local electric fields. An electrical field lowers the transition energy and therefore leads to red-shifted PL spectra compared to the nonfield configuration [13]. Additionally, the electrical field reduces the overlap of the electron and hole wavefunctions of excitons. That is why not only changes regarding the spectral position but also in the emission intensity are expected [13]. Additionally, this will considerably influence electron–phonon coupling [12]. In this paper we investigate for the first time systematically results related to spectral diffusion in silicon nanocrystals (Si-NC) and coupling of Si excitons to phonons in the oxide shell.
2. Experimental Silicon nanocrystals (Si-NC) have been produced by dehydration of silane in a gas-flow reactor. The setup and procedure including size selection is described in detail in Ref. [5].We obtain a layer of Si-NC on a filter paper which was mounted in front of the gas reactor. This paper was then immersed into toluene to disperse the particles into the solvent. The typical Si-NC diameter is about 2–3 nm capped with a SiOx shell [5,11,12,16]. PL can only be observed in the presence of a capping oxide layer. Following appropriate dilution the colloidal suspension was mixed with a solution of polymethyl methacrylate (PMMA) in toluene in order to embed the nanocrystals in a polymer. This mixture was then
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spin-casted on a quartz plate. After toluene evaporation we typically obtained 20–50 nm thick PMMA films with embedded well separated Si-NC. Single Si-NC were investigated with a home-built confocal microscope [10,12]. Si-NC photoluminescence (PL) was excited at 532 nm by a diode pumped solid state laser. The light was coupled into the set-up via an optical fibre and was focused onto the sample by a microscope lens (Carl Zeiss Jena) with 100x magnification and a numerical aperture of 0.9. Fixed excitation intensity was 6 103 W/cm2. This corresponds to 0.24 photons/10 6 s assuming an absorption cross section of 1.5 10–17 cm2 [28,29]. Taking an average lifetime of 2.5 10 6 s as determined in our experiments this corresponds to less than 1 exciton/Si-NC. We kept excitation intensity low to avoid fast photobleaching. Since PL signals are small we were not able to vary excitation intensity. However, spectral diffusion might be enhanced by photoexcitation. PL was collected by the same objective and imaged onto a pinhole by an achromatic lens. A second achromatic lens imaged the PL onto an avalanche photodiode (EG&G SPCMAQR 13) or a grating monochromator (Acton SP-300i) equipped with a liquid-nitrogen-cooled CCD (Princeton Instruments). Switching between the two detectors was done with a mirror. To separate the excitation from the PL a dichroic beam splitter and long pass filter of 540 nm cut-off wavelength (Omega Optics) was placed into the detection line. Sample scanning has been carried out by means of a closed-loop xyz-piezo translation stage (Physics Instruments) with maximum scan ranges of 80 mm in x, and y-direction and 20 mm in z-direction, respectively. With this setup we were able to record PL with a spatial resolution up to 250 nm and a time resolution up to some 10 ms/frame. All together we have investigated about 50 Si-NC which all showed spectral diffusion, but only 16 have been photostable enough to provide a long series of spectra. Four histograms of typical jump series are shown in Fig. 4. Detailed spectral deconvolution has been performed on two such series and which are shown in Fig. 1. Qualitatively all series behave the same but with a large scatter of fitting results (see also Appendix A). CCD camera and monochromator show in the respective energy range only a dispersion of 0.1%/nm. Since spectral jumps occur only over a small energy range of at most 90 meV we have not corrected for spectral response.
3. Results and discussion Two PL spectra are shown in Fig. 1 for two different single SiNC. The assignment to single Si-NC is based on the blinking phenomena (PL on–off intermittency), which is a very strong indication for the presence of a single quantum system [14]. Though the PL of the presently investigated Si-NCs is on for quite a long time, all NCs eventually blink or are photobleached in one single step. Both spectra shown in Fig. 1 exhibit the typical structure of small single Si-NC emission spectra grown from the gas phase or originating from porous silicon [11,12,16], namely consisting of a main transition and at least one side band. The main band is assigned to the recombination of electron and hole of a Si exciton, while the side bands are attributed to Si–O vibrational modes of the oxide shell of the NC [7,8,10–12,16]. The side bands suggest a strong coupling of core excitons and phonons. Recent experiments have shown that the side band structure of the PL spectra is very sensitive to the local dielectric environment of SiNC [11]. This indicates that the excitons are (quasi) localized close to the interface or even in the oxide shell [12,16]. Despite both spectra in Fig. 1 exhibit a very similar structure, there are differences with respect to the absolute PL energy and the width of the individual bands. Moreover, the spectral width depends critically on the averaging time as can be seen from the inset in Fig. 2. The full width at half maximum (FWHM) of the
Fig. 1. Typical PL spectrum of two single Si-NC with (A) a strong SiO2- and a weak Si-phonon band or with (B) a weak SiO2- and strong Si-Phonon band. The spectra have been fitted by 3 Gaussian lines. Averaging time is 0.5 s in each case. The Si no-phonon line is in both cases the band at highest PL energy, while the SiO2 phonon corresponds to band at lowest PL energy.
main band becomes 105 meV for 0.5 s averaging time and 150 meV for 50 s averaging time, respectively. This considerable line broadening results from spontaneous spectral diffusion as is illustrated in Fig. 2, in which a time series of PL spectra is shown for one single Si-NC. It has to be noted, that for clarity only 21 of total 100 spectra are shown. Nevertheless jumps in PL energy, spectral width and intensity are clearly visible. As shown in Fig. 3 we plot the time evolution of the PL energy of the main band and the (normalized) integrated PL intensity, which reveals strongly correlated statistical jumps (diffusion) for both observables. For this specific single Si-NC we determine a maximal diffusion range of nearly 70 meV. This range is similar to the one observed for CdSe quantum dots [13,17]. Similar large spectral shifts have been obtained for specific single molecules [18]. According to current models for spectral diffusion in semiconductor nanocrystals spectral diffusion may arise from time dependent local electric fields originating from charge fluctuations close to the nanocrystal [13,19,20]. These charges might be photogenerated from the QD itself by photoejecting charges from the exciton to the matrix or to the interface (e.g. oxide shell) [21] or they might be present right from the beginning [13]. This is in agreement with models to explain luminescence blinking of semiconductor nanocrystals or molecules [14]. Blinking has also been observed for Si-NC [7,8,10,11,22,23]. First, according to accepted models [24] one charge of the core exciton, most probably the electron, tunnels to the particle shell or interface. The charge (hole), which is left in the
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Fig. 2. Series of subsequent PL spectra of a single Si-NC with 0.5 s integration time over at time period of 50 s (Crystal 1 in Fig. 5). Excitation occurs at 532 nm (2.33 eV) with an excitation intensity of 6 103 W/cm2.The inset shows a comparison of PL spectra of a single spectrum with 1 s integration time (full line) and the sum of 100 subsequent spectra (broken line) of one typical Si-NC.
Fig. 3. Correlation of the PL energy of the main PL line of a single Si-NC ((1) in Fig. 4) at high PL energy (‘‘zero-phonon’’ line) and the total normalized intensity as a function of observation time. PL intensity has been set to 1 at the highest intensity during observation time of 50 s.
particle core, acts as a centre for non-radiative recombination for a subsequently generated exciton thus reducing the PL quantum efficiency. In case this charge (hole) is also trapped near the particle interface, subsequently generated excitons will recombine (partly) radiatively [14]. However, due to the presence of the electric dipole field generated by the (localized) charges, the PL energy is shifted towards lower energies due to an internal Stark effect, depending on the relative positions of the charges with respect to the exciton. This will be accompanied by a PL decrease resulting from increased nonradiative decay routes induced by an enhanced electron–phonon coupling. Even in case that only one charge is present, PL properties will be influenced in course of time as soon as the charge is diffusing. We have not detected dark periods in the present series of subsequent spectra. This means, that dark periods are either very short (shorter than our averaging time of 500 ms) or completely absent. This on-time is as long as several minutes before, finally, the PL is switched off for very long times [10,23]. Histograms of spectral jumps obtained for 4 different single Si-NC are shown in Fig. 4. It is clearly seen, that the jumps are on the same order of magnitude for each of the Si-NC, but show different types of distributions, some are symmetric and some are not. This is probably due to different local dielectric environments. Our
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investigations on PL blinking in semiconductor nanocrystals suggest, that the charge, which has been photoejected from the core is localized in spatially and/or energetically distributed (self-trapped) states [25]. It may diffuse among these states resulting in fluctuations of the quantum efficiency and spectral position of the subsequently generated exciton. According to the model outlined above distributions shown in Fig. 4 resemble an ‘image’ of spatial charge fluctuations in the nanocrystal shell or even in the PMMA matrix in case of very thin oxide layers [11]. The distributions of spectral jumps are therefore directly related to local dielectric properties. Beside this, another effect is quite obvious when looking at Fig. 5. Here we find a correlation between the total (integrated) PL intensity and the related PL energy (of the main peak). The PL intensity increases exponentially with increasing PL energy. Recently it has been found [6] that the radiative PL decay rate increases over a wide energy range exponentially with a characteristic energy En of about 250 meV crossing over to a smaller En at PL energies above 1.9 eV. It might be worthwhile to mention, that En is in the same energy range found for the energy difference (200 meV) between PL energies at which electron and hole localize separately [12]. The presently investigated Si-NCs have PL energies somewhat below the energy of 2.125 eV, at which electron localization is expected to take place. Since the radiative rate is directly related to the PL intensity we can compare the reported results with our findings. Fig. 5 reveals En to be close to 120 meV, which is a reasonable value as compared to results reported in the literature since we are at the high energy end of the PL of Si-NC. Commonly the broad PL range is related to different sizes of Si-NC. In our experiments on single NC, however, PL energy related intensities have to be assigned to other mechanisms than quantum size effects. The lowest intensity can be found for the spectrum with the lowest emission energy, related to a configuration with the strongest Stark effect in agreement with the model of quantum confined Stark effect [13,15]. An electric field interacts with the exciton by enlarging the separation of electron and hole wavefunctions thus diminishing the radiative decay rate because of the reduced overlap of the wavefunctions. Also the (phonon) side bands show a similar but quantitatively different behaviour. To investigate these effects in more detail we have deconvoluted each PL spectrum as shown in Fig. 1 into three bands, namely the main (zero phonon) band, a phonon sideband assigned to a (TO) Si phonon (kept fixed) at 56 meV [30] and a SiO2 phonon band with typical frequencies close to 150 meV [11,12]. Typical fitting results are also included in Fig. 1. The SiO2 phonon side bands are related to phonons of the SiOx-shell, meaning there is an electron–phonon coupling of excitons and shell phonons. The spectral deconvolution reveals a quantitative different PL intensity correlation of the main (nophonon) band with the PL energy of the main band as compared to the SiO2 phonon band intensity, respectively, as shown in Fig. 6. While the main band is decreasing by less than a factor of 2 over the respective PL energy range, the SiO2 band is (non-linearly) decreasing by almost a factor of 4 in the respective energy range. Obviously phonon activated and zero phonon transitions depend differently on the Stark effect. Qualitatively side band intensities are more strongly decreasing with decreasing PL energy. However, due to the strong scatter of the no-phonon band data (which is probably caused by the interfering deconvolution from the Si-phonons), it is not possible to confidently assign the kind of behaviour (linear or exponential). To get more insight we have calculated the Huang-Rhys factor S both for Si phonons S(Si) and SiO2 phonons S(SiO2) by relating the ratio of PL intensities of the respective phonon band In and the sum Sn of the main band and the remaining phonon band to the PL energy. Phonon side bands involving n phonons have intensities In ¼Sn exp(S)/n!, where S corresponds to the Huang-Rhys factor [27]. The results are shown in Fig. S1 of Appendix A. Though there is a large experimental scatter, the general trends are, that S(SiO2)
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Fig. 4. Distribution of spectral PL energy jumps of the main (zero-phonon) PL line during observation times of typically 60 s for 4 different Si-NC. Integration time for each spectrum is 0.5 s. Jump widths are related to spectral differences of subsequent spectra.
Fig. 5. Integrated PL intensity of a single Si-NC ((1) in Fig. 4) as a function of PL energy fluctuating during 60 s observation time. Note the log scale for the PL intensity.
increases with PL energy by about a factor of 1.2, while S(Si) decreases over the same range by about a factor of 2. This is in qualitative agreement with results obtained for electron–phonon coupling observed for Si-NC in various matrices with different dielectric constants [11]. Recent experiments have shown [11,16], that PL from single Si-NC is due to (nearly) localized excitons [12] in the SiO2 shell
showing strong SiO2 phonon side bands. We have reported PL intermittency (on–off blinking) for single Si-NC [10], which is assigned to photoinduced charge separation of the exciton [14] resulting in dark or at least dim states. In the present examples, however, PL is ‘‘on’’ (no blinking) for quite a long time of several minutes. This suggests that in these special cases photoinduced charge separation is absent. However, during this on-time spectral properties are nevertheless fluctuating. We speculate, that this might be resembling to an internal Stark effect [13,19,20] due to excess charges. In case that the Si-NC is charged, the (localized) exciton will experience shifts in PL energy and electron–phonon coupling [26] due to the created field gradient caused by the external charge as has already been observed for nanocrystals [15,21]. The corresponding magnitudes will depend on the position of the charge relative to the one of the (localized) exciton. Upon charge diffusion PL energy and intensity as well as electron–phonon coupling will fluctuate. Low PL energies and low integrated intensities are related to an increased Stark effect. Obviously the increase of the Stark effect results at the same time in a decrease of the coupling to SiO2 phonons but an increase of the coupling to Si phonons, which is recently not understood.
4. Conclusions Spontaneous spectral diffusion and strongly correlated PL intensity fluctuations have been detected for nearly localized excitons in single silicon nanocrystals capped with a SiOx shell. A considerably large
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discussions. Joerg Martin likes to thank Ruben Schmidt for support on experiments. Financial support of the DFG within the framework of FOR 388 ‘Laboratory Astrophysics’ is gratefully acknowledged.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2012. 03.028.
References
Fig. 6. (A) Integrated PL intensity of the main peak (‘‘zero-phonon’’ line) during observation time as a function of PL energy for single Si-NC 1. (B) Respective integrated PL intensity for the SiO2-phonon side band.
inhomogenous line broadening originates from this spectral diffusion. We have detected large spectral jumps between two subsequent spectra of up to 40 meV and a total spectral diffusion range of up to nearly 70 meV. The distribution of jumps and PL intensities reflects fluctuations in the local environment. Further, a correlation between PL energy and intensity could be found, which is in agreement with existing models for the influence of an internal Stark effect on optical properties. The electron–phonon coupling to Si and SiO2 phonons are obviously anti-correlated, which is not yet understood. The investigated Si-NC is most likely charged for the total time of observation of several minutes. We suggest that ‘‘permanent’’ charging prohibits the photoejection of additional charges.
Acknowledgements We thank Friedrich Huisken, University of Jena, for providing us with size selected silicon nanocrystals and for many stimulating
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