- Email: [email protected]
Probing the excitonic emission of ZnO nanoparticles using UV–VUV excitations
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1798–1801
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Probing the excitonic emission of ZnO nanoparticles using UV–VUV excitations D. Tainoff a,, B. Masenelli a, P. Melinon a, A. Belsky b, G. Ledoux b, D. Amans b, C. Dujardin b, N. Fedorov c, P. Martin c a
LPMCN Universite´ de Lyon, Lyon, F-69003, France; Universite´ Lyon 1, CNRS, UMR5586, Villeurbanne F-69622, France LPCML Universite´ de Lyon, Lyon, F-69003, France; Universite´ Lyon 1, CNRS, UMR5620, Villeurbanne F-69622, France c CELIA, Universite´ Bordeaux 1, CNRS, CEA, 351, Cours de la Libe´ration 33405 Talence cedex, France b
a r t i c l e in f o
a b s t r a c t
Available online 30 May 2009
This study deals with the influence of the excitation (UV-lamp, UV-laser and VUV synchrotron radiation) on the 3.31 eV band of ZnO microcrystals and of variously treated nanoparticles. The nanoparticles are synthesized in ultra high vacuum condition and their stoichiometry and crystallinity can be controlled. This provides an efficient way to probe the influence of these factors on the excitonic emission. The energy and intensity of the excitation have a strong influence on the excitonic luminescence and particularly on the 3.31 eV emission band. The result of these experiments are used to probe the origins of this band which is found to be not linked to any surface phenomena. Indeed, the only way to fully explain our results is to consider that the 3.31 eV band involve the superposition of two emissions features: the first due to acceptor defects and the other originates form the LO phononic repliqua of the free exciton. & 2009 Elsevier B.V. All rights reserved.
Keywords: Exciton Defects Luminescence Nanoparticles Temperature VUV ZnO
1. Introduction Among wide-bandgap semiconductors for optoelectronic applications, ZnO has come to the forefront in the present decade because of the large value of its exciton binding energy (60 meV). However, in spite of many studies, the excitonic luminescence and particularly the 3.31 eV band (now called A band) often present in the low temperature emission spectra of various ZnO materials (microcrystal, nanorods, tetrapods, etc.) remains unclear. Depending on different studies this band was explained by three different hypotheses. Taking into account that the energy separating the A band from the free exciton is close to the 1LO energy, the first interpretation attributed the A band to a phonon repliqua of the free exciton [1]. Starting from ZnO p-doping studies, this band was then attributed to an acceptor localized state leading to a donor acceptor pair [2], a free to bound electronic transition [3,4], or the direct bounding of an exciton to an acceptor state [5]. Finally, the presence of the A band in many ZnO nanostructures has conducted this band to be interpreted as a surface excitonic contribution [6]. To be able to tell which mechanism is responsible for the 3.31 eV band in ZnO, it is first imperative to study different ZnO samples with different surface to volume ratio. Moreover these
Corresponding author.
E-mail address: [email protected] (D. Tainoff). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.099
samples must be well characterized and as perfect as possible, in terms of stoichiometry, crystallinity and impurity. In this work we use a simple and effective technique to synthesize uncapped ZnO nanoparticles (NP) that are deposited in ultra high vacuum (UHV). Depending on the synthesis parameters their crystallinity and stoichiometry can be controlled and adjusted in situ. Then we compare the optical properties of these NP to the properties of the microcrystalline powder used for the fabrication of the ablation target. This analysis is performed for the same samples at different temperatures and for three different excitations, Xe lamp at 300, 266 nm laser and VUV synchrotron radiation at 90 nm.
2. Experimental setup The nanoparticle assembled films are prepared by deposition of low energy neutral nanoparticles preformed in the gas phase with the LECBD technique [7]. A cluster generator based on a combined Nd-YAG laser vaporization-rare gas (He) condensation source is used to produce supersonic jet of NP with sizes ranging from a few tens to a few thousands of atoms (diameters from 2 to a few nm). The nucleation takes place in a supersonic nozzle where the atoms are quenched beyond the thermodynamic equilibrium. The stoichiometry of the NP is then controlled in situ by XPS/AES. Our previous in situ study of NP cathodoluminescence reveals that contrary to the microcrystal (MC) the NP
ARTICLE IN PRESS D. Tainoff et al. / Journal of Luminescence 129 (2009) 1798–1801
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does not presents any green luminescence band [8]. The size and the crystallinity are provided by TEM characterization that has shown that the NP have a mean diameter of 6 nm and are crystallized in the wurtzite phase [8]. The MC used for the fabrication of the ablation target is 99.99% pure and calibrated at 200 mesh [9]. Before optical measurement they are mashed on an indium substrate. The photoluminescence experiments have been performed with three different excitations: a Xe lamp emitting at 300 nm, a quadrupled YAG laser emitting at 266 and 90 nm VUV synchrotron radiation (SUPERLUMI line of DESY synchrotron, Hamburg). In all cases the luminescence is dispersed by a 1200 s/nm grating, the focal length of the spectrometer being one meter for VUV and UV-lamp excitation and 50 cm for laser experiment. In all cases the spectra are normalized to their maximum.
3. Results In Fig. 1a, one can observe the emission spectra of NP and MC samples excited with a Xe lamp. These two spectra are dominated by a band at 3.363 eV called D0X band and corresponding to an exciton bound to a neutral donor [10]. Close to this band there are other donor bound excitons [10] but our resolution does not permit to clearly identify them. The weak peak observed for the NP and MC spectra at 3.33 eV corresponds to the two electron satellite (TES) peak [11]. The 3.377 eV peak corresponding to the free exciton (FX) and the 3.31 eV peak (now called A band) are easily identifiable in the MC spectrum but are absent from the NP spectrum. This fact is not surprising for the FX peak that is often lacking in the 10 K spectra of nanostructures [11]. The lack of the 3.31 eV is more interesting because the surface is often invoked as the origin of this peak [6,12]. To better understand the mechanism of emission we have measured this experiment using VUV synchrotron radiation (Fig. 1b). For the NP the result of this experiment is equal to the previous one. This is not surprising because their emission spectrum is only composed by bound excitons. The MC spectrum is the same than for the Xe lamp excitation except for the FX contribution that is lacking in the case of VUV excitation. This fact shows that at 10 K the A band is not a FX repliqua. At VUV excitation the electronic excitations leading to the excitonic emission can strongly differ as compared to near gap excitation. Concerning near gap excitation the electron and the hole created do not have enough energy to go far from one another. Then they form free excitons, which relaxes through the creation of DX and other localized excitonic states. This is not the case for VUV excitation of 14 eV, capable to create 2–4 e–h pairs of relatively high energy. Here the hot electron and its associate hot hole can go far away one from the other leading to different mechanism of recombination that can override the FX state. In this case the localized excitonic states can be created by successful captures of holes and electrons. As the density of excitation changes with excitation energy, we have measured the influence of the intensity on the photoluminescence using a 266 nm laser more or less focalised with a lens (Fig. 1c). This experiment shows that the A band intensity compared to the DX one strongly depends on the laser fluence. Increasing the laser intensity increases the A band contribution until making it more important than the DX one in the MC (Fig. 1c). This phenomenon is not linked to a laser heating or defect creation because it is fully reversible and causes no shift on the peaks position. These results are in accordance with previous studies of intensity dependence of excitonic emission [12]. It is important to note that whatever the excitation intensity the A band is not present on the NP spectra.
Fig. 1. Emission spectra of ZnO microcrystal (MC) and nanoparticles (NP) at 10 K under different excitations; (a) Xe lamp excitation at 300 nm; (b) VUV excitation at 90 nm; and (c) laser excitation at 266 nm for two different intensities I0 and 100 I0. In all cases the spectra are normalized to their maximum of intensity.
We now present the emission spectra of MC and NP excited by a 266 nm laser and Xe lamp at 120 K (Fig. 2a and b). At this intermediate temperature the DX signal is divided by 100 but the signal/noise ratio is still good and we can well distinguish all the previously studied peaks. Unfortunately, in the case of VUV excitation, the signal becomes too noisy from 40 K. The modifications observed are qualitatively the same for a weak laser excitation (Fig. 2a) or a Xe lamp excitation (Fig. 2b). First, all the peaks are slightly shifted to the low energies. This shift is explained by the bandgap shift caused by the temperature
ARTICLE IN PRESS 1800
D. Tainoff et al. / Journal of Luminescence 129 (2009) 1798–1801
asymmetry of the A band depends reversibly on the excitation intensity. For the NP the effect of the intensity is less spectacular. We can see in Fig. 2b that the A band grows but never exceeds the intensity of the DX/FX band. Unfortunately we cannot deconvolute this DX/FX band taking into account the resolution of our system.
4. Discussion In this part we will discuss the previous results in regard of other results concerning the A band already published in the literature and exposed in introduction: 4.1. Surface contribution From our results, the A band is present in the emission spectra of MC but not in the NP spectra. As the NP have a huge surface to volume ratio compared to the MC, it is clear that the surface is not the origin of the A band. However, we can envisage another mechanism involving the surface and influencing the A band. Indeed, it is well known that the ZnO compounds are very sensitive to air and water [13,14]. This sensitivity would be increased in the case of NP due to there large surface volume ratio and could have an important influence on the emission spectra. We have tested this hypothesis using a UHV compatible electron gun to cape a ZnO NP sample with a 100-nm-thick layer of MgO. The gap of MgO is sufficiently larger compared to the ZnO one to permit a direct excitation of the ZnO NP by a 266 nm laser. The result is presented in Fig. 2c and shows that there is no difference between the capped and the uncapped sample. Finally, it is clear from our experiments that the surface is not the origin of the A band. 4.2. Acceptor localized state
Fig. 2. (a,b) Emission spectra of ZnO microcrystal (MC) and nanoparticles (NP) at 120 K under different excitation; (a) Xe lamp excitation at 300 nm; and (b) laser excitation at 266 nm for two different intensities I0 and 100 I0. (Fig. 2c) Emission spectra at 10 K of two different sample of ZnO NP excited by a laser at 266 nm. The black curve represents the emission of a capped sample. In all cases the spectra are normalized to their maximum of intensity.
increase. The MC spectra are dominated by the A band and FX for both excitation. Concerning the NP spectra we can remark that even if the DX still dominates the spectrum, the A band and the FX band are now clearly identifiable. However, the intensity of these two peaks is still much more important in the MC spectra. Finally, the more interesting feature of this temperature evolution is the asymmetric broadening of the more energetic component of the A band present in the MC. For the high-intensity spectrum of the MC, we can observe only one very broad symmetric A band that includes perhaps a little contribution from FX. This shows that at 120 K, the
This hypothesis could explain the lack of A band in NP at 10 K because the presence of extended defect in the NP is not energetically favourable [15]. To support this argument it is interesting to note the presence of a broad green band in the MC emission spectra at room temperature. This band is generally attributed to oxygen vacancies or other ‘‘volume related’’ defects [11] and is not present in our NP [8]. This fact supports the hypothesis that contrary to the MC, the NP does not contain welldefined defects. Moreover, at 10 K and under VUV excitation we can observe the A band without any contribution of the free exciton that reject the hypothesis of a phonon repliqua. Finally, the hypothesis of an acceptor localized state is compatible with the intensity dependence of the spectra profile and the Gaussian shape of the A band. But this hypothesis does not explain the asymmetric broadening of the A band at 120 K. 4.3. 1LO phonon repliqua When the temperature increases at 120 K (Fig. 2), the asymmetric shape of the A band suggests an interaction of the exciton with the 1LO phonon as studied by Seagall and Mahan [16]. Moreover, at this temperature, the energy between the A band and FX band is compatible with the 1LO energy reported in the literature for ZnO [17]. Finally, this assignment could explain why the asymmetric broadening of A band in the MC concords with the appearance of FX peak and A band in the nanoparticles. However, at 120 K and at high intensity excitation, the intensity of the A band increases but the FX one decreases. Moreover, for the high excitation intensity, the shape of the A band becomes again symmetric that is not in accordance with the hypothesis of the
ARTICLE IN PRESS D. Tainoff et al. / Journal of Luminescence 129 (2009) 1798–1801
1LO phonon repliqua. Finally, the only way to explain all our experimental results is to consider two temperature ranges. In a first range spanning from 10 to 100 K the A band could result from an acceptor defects. Then, from 120 to 300 K this band could be the superposition of the acceptor state previously described with a 1LO phonon repliqua of the free exciton.
1801
Acknowledgements This work was supported by the European Community Research Infrastructure Action within the FP6 Program through the Contract RII3-CT-2004-506008 (IA-SFS). The authors acknowledge G. Stryganyuk for his assistance in the SUPERLUMI experiments.
5. Conclusions References Among the different UV emission peaks the origin of the 3.31 eV (A band) remains unclear. Starting from industrial microcrystals (MC) we have synthesized ZnO nanoparticles (NP) in UHV and controlled in situ their quality. To explain the origin of the 3.31 eV band their emission spectrum has been studied for different excitation energies and/or intensities and different temperatures. Whatever the excitation energy or the temperature, the intensity of this band is weaker in the nanoparticles. This fact allows us to reject the hypothesis of a surface peak. At 10 K, the hypothesis of an acceptor localized state is the only one which is fully compatible with our results. At higher temperatures and at weak excitation intensity, the shift and the asymmetric broadening of the A band tends to prove the implication of an exciton–phonon interaction as studied by Seagall and Mahan [16]. However this explanation cannot explain the modification observed at high intensity excitation. Therefore, the only way to explain all the result is to consider that the A band originates from the superposition of a localized acceptor state and a 1LO phonon repliqua of the free exciton.
[1] C. Klingshirn, Phys. Status Solidi B 71 (1975) 547. [2] B.P. Zhang, N.T. Binh, Y. Segawa, K. Wakatsuki, N. Usami, Appl. Phys. Lett. 83 (2003) 1635. [3] Q.X. Zhao, M. Willander, R.E. Morjan, Q.-H. Hu, E.E.B. Campbell, Appl. Phys. Lett. 83 (2003) 165. [4] M. Schirra, R. Schneider, A. Reiser, G.M. Prinz, M. Feneberg, J. Biskupek, U. Kaiser, C.E. Krill, K. Thonke, R. Sauer, Phys. Rev. B 77 (2008) 125215. [5] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [6] J. Fallert, R. Hauschild, F. Stelzl, A. Urban, M. Wissinger, H. Zhou, C. Klingshirn, H. Kalt, J. Appl. Phys. 101 (2007) 073506. [7] A. Perez, et al., Mater. Trans. 8 (2001) 1460. [8] D. Tainoff, B. Masenelli, O. Boisron, G. Guiraud, P. Me´linon, J. Phys. Chem. C 112 (2008) 12623. [9] CERAC ref Z-10769. [10] A. Teke, U. Ozgur, S. Dogan, X. Gu, H. Morkoc, B. Nemeth, J. Nause, H.O. Everitt, Phys. Rev. B 70 (2004) 195207. [11] A.B. Djurisˇic´, Y.H. Leung, Small 8–9 (2006) 944. [12] Schneider, S.V. Zaitsev, G. Bacher, W. Jin, M. Winterer, J. Appl. Phys. 102 (2007) 023524. [13] O. Lupan, G. Chai, L. Chow, Microelectron. Eng. 85 (2008) 2220. [14] Shoou-Jinn Chang, et al., Nanotechnology 19 (2008) 095505. [15] Chia-Chun Chen, et al., Science 276 (1997) 398. [16] B. Segall, G.D. Mahan, Phys. Rev. 171 (1968) 935. [17] L. Wang, N.C. Giles, J. Appl. Phys. 94 (2003) 973.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Probing the excitonic emission of ZnO nanoparticles using UV–VUV excitations D. Tainoff a,, B. Masenelli a, P. Melinon a, A. Belsky b, G. Ledoux b, D. Amans b, C. Dujardin b, N. Fedorov c, P. Martin c a
LPMCN Universite´ de Lyon, Lyon, F-69003, France; Universite´ Lyon 1, CNRS, UMR5586, Villeurbanne F-69622, France LPCML Universite´ de Lyon, Lyon, F-69003, France; Universite´ Lyon 1, CNRS, UMR5620, Villeurbanne F-69622, France c CELIA, Universite´ Bordeaux 1, CNRS, CEA, 351, Cours de la Libe´ration 33405 Talence cedex, France b
a r t i c l e in f o
a b s t r a c t
Available online 30 May 2009
This study deals with the influence of the excitation (UV-lamp, UV-laser and VUV synchrotron radiation) on the 3.31 eV band of ZnO microcrystals and of variously treated nanoparticles. The nanoparticles are synthesized in ultra high vacuum condition and their stoichiometry and crystallinity can be controlled. This provides an efficient way to probe the influence of these factors on the excitonic emission. The energy and intensity of the excitation have a strong influence on the excitonic luminescence and particularly on the 3.31 eV emission band. The result of these experiments are used to probe the origins of this band which is found to be not linked to any surface phenomena. Indeed, the only way to fully explain our results is to consider that the 3.31 eV band involve the superposition of two emissions features: the first due to acceptor defects and the other originates form the LO phononic repliqua of the free exciton. & 2009 Elsevier B.V. All rights reserved.
Keywords: Exciton Defects Luminescence Nanoparticles Temperature VUV ZnO
1. Introduction Among wide-bandgap semiconductors for optoelectronic applications, ZnO has come to the forefront in the present decade because of the large value of its exciton binding energy (60 meV). However, in spite of many studies, the excitonic luminescence and particularly the 3.31 eV band (now called A band) often present in the low temperature emission spectra of various ZnO materials (microcrystal, nanorods, tetrapods, etc.) remains unclear. Depending on different studies this band was explained by three different hypotheses. Taking into account that the energy separating the A band from the free exciton is close to the 1LO energy, the first interpretation attributed the A band to a phonon repliqua of the free exciton [1]. Starting from ZnO p-doping studies, this band was then attributed to an acceptor localized state leading to a donor acceptor pair [2], a free to bound electronic transition [3,4], or the direct bounding of an exciton to an acceptor state [5]. Finally, the presence of the A band in many ZnO nanostructures has conducted this band to be interpreted as a surface excitonic contribution [6]. To be able to tell which mechanism is responsible for the 3.31 eV band in ZnO, it is first imperative to study different ZnO samples with different surface to volume ratio. Moreover these
Corresponding author.
E-mail address: [email protected] (D. Tainoff). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.099
samples must be well characterized and as perfect as possible, in terms of stoichiometry, crystallinity and impurity. In this work we use a simple and effective technique to synthesize uncapped ZnO nanoparticles (NP) that are deposited in ultra high vacuum (UHV). Depending on the synthesis parameters their crystallinity and stoichiometry can be controlled and adjusted in situ. Then we compare the optical properties of these NP to the properties of the microcrystalline powder used for the fabrication of the ablation target. This analysis is performed for the same samples at different temperatures and for three different excitations, Xe lamp at 300, 266 nm laser and VUV synchrotron radiation at 90 nm.
2. Experimental setup The nanoparticle assembled films are prepared by deposition of low energy neutral nanoparticles preformed in the gas phase with the LECBD technique [7]. A cluster generator based on a combined Nd-YAG laser vaporization-rare gas (He) condensation source is used to produce supersonic jet of NP with sizes ranging from a few tens to a few thousands of atoms (diameters from 2 to a few nm). The nucleation takes place in a supersonic nozzle where the atoms are quenched beyond the thermodynamic equilibrium. The stoichiometry of the NP is then controlled in situ by XPS/AES. Our previous in situ study of NP cathodoluminescence reveals that contrary to the microcrystal (MC) the NP
ARTICLE IN PRESS D. Tainoff et al. / Journal of Luminescence 129 (2009) 1798–1801
1799
does not presents any green luminescence band [8]. The size and the crystallinity are provided by TEM characterization that has shown that the NP have a mean diameter of 6 nm and are crystallized in the wurtzite phase [8]. The MC used for the fabrication of the ablation target is 99.99% pure and calibrated at 200 mesh [9]. Before optical measurement they are mashed on an indium substrate. The photoluminescence experiments have been performed with three different excitations: a Xe lamp emitting at 300 nm, a quadrupled YAG laser emitting at 266 and 90 nm VUV synchrotron radiation (SUPERLUMI line of DESY synchrotron, Hamburg). In all cases the luminescence is dispersed by a 1200 s/nm grating, the focal length of the spectrometer being one meter for VUV and UV-lamp excitation and 50 cm for laser experiment. In all cases the spectra are normalized to their maximum.
3. Results In Fig. 1a, one can observe the emission spectra of NP and MC samples excited with a Xe lamp. These two spectra are dominated by a band at 3.363 eV called D0X band and corresponding to an exciton bound to a neutral donor [10]. Close to this band there are other donor bound excitons [10] but our resolution does not permit to clearly identify them. The weak peak observed for the NP and MC spectra at 3.33 eV corresponds to the two electron satellite (TES) peak [11]. The 3.377 eV peak corresponding to the free exciton (FX) and the 3.31 eV peak (now called A band) are easily identifiable in the MC spectrum but are absent from the NP spectrum. This fact is not surprising for the FX peak that is often lacking in the 10 K spectra of nanostructures [11]. The lack of the 3.31 eV is more interesting because the surface is often invoked as the origin of this peak [6,12]. To better understand the mechanism of emission we have measured this experiment using VUV synchrotron radiation (Fig. 1b). For the NP the result of this experiment is equal to the previous one. This is not surprising because their emission spectrum is only composed by bound excitons. The MC spectrum is the same than for the Xe lamp excitation except for the FX contribution that is lacking in the case of VUV excitation. This fact shows that at 10 K the A band is not a FX repliqua. At VUV excitation the electronic excitations leading to the excitonic emission can strongly differ as compared to near gap excitation. Concerning near gap excitation the electron and the hole created do not have enough energy to go far from one another. Then they form free excitons, which relaxes through the creation of DX and other localized excitonic states. This is not the case for VUV excitation of 14 eV, capable to create 2–4 e–h pairs of relatively high energy. Here the hot electron and its associate hot hole can go far away one from the other leading to different mechanism of recombination that can override the FX state. In this case the localized excitonic states can be created by successful captures of holes and electrons. As the density of excitation changes with excitation energy, we have measured the influence of the intensity on the photoluminescence using a 266 nm laser more or less focalised with a lens (Fig. 1c). This experiment shows that the A band intensity compared to the DX one strongly depends on the laser fluence. Increasing the laser intensity increases the A band contribution until making it more important than the DX one in the MC (Fig. 1c). This phenomenon is not linked to a laser heating or defect creation because it is fully reversible and causes no shift on the peaks position. These results are in accordance with previous studies of intensity dependence of excitonic emission [12]. It is important to note that whatever the excitation intensity the A band is not present on the NP spectra.
Fig. 1. Emission spectra of ZnO microcrystal (MC) and nanoparticles (NP) at 10 K under different excitations; (a) Xe lamp excitation at 300 nm; (b) VUV excitation at 90 nm; and (c) laser excitation at 266 nm for two different intensities I0 and 100 I0. In all cases the spectra are normalized to their maximum of intensity.
We now present the emission spectra of MC and NP excited by a 266 nm laser and Xe lamp at 120 K (Fig. 2a and b). At this intermediate temperature the DX signal is divided by 100 but the signal/noise ratio is still good and we can well distinguish all the previously studied peaks. Unfortunately, in the case of VUV excitation, the signal becomes too noisy from 40 K. The modifications observed are qualitatively the same for a weak laser excitation (Fig. 2a) or a Xe lamp excitation (Fig. 2b). First, all the peaks are slightly shifted to the low energies. This shift is explained by the bandgap shift caused by the temperature
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D. Tainoff et al. / Journal of Luminescence 129 (2009) 1798–1801
asymmetry of the A band depends reversibly on the excitation intensity. For the NP the effect of the intensity is less spectacular. We can see in Fig. 2b that the A band grows but never exceeds the intensity of the DX/FX band. Unfortunately we cannot deconvolute this DX/FX band taking into account the resolution of our system.
4. Discussion In this part we will discuss the previous results in regard of other results concerning the A band already published in the literature and exposed in introduction: 4.1. Surface contribution From our results, the A band is present in the emission spectra of MC but not in the NP spectra. As the NP have a huge surface to volume ratio compared to the MC, it is clear that the surface is not the origin of the A band. However, we can envisage another mechanism involving the surface and influencing the A band. Indeed, it is well known that the ZnO compounds are very sensitive to air and water [13,14]. This sensitivity would be increased in the case of NP due to there large surface volume ratio and could have an important influence on the emission spectra. We have tested this hypothesis using a UHV compatible electron gun to cape a ZnO NP sample with a 100-nm-thick layer of MgO. The gap of MgO is sufficiently larger compared to the ZnO one to permit a direct excitation of the ZnO NP by a 266 nm laser. The result is presented in Fig. 2c and shows that there is no difference between the capped and the uncapped sample. Finally, it is clear from our experiments that the surface is not the origin of the A band. 4.2. Acceptor localized state
Fig. 2. (a,b) Emission spectra of ZnO microcrystal (MC) and nanoparticles (NP) at 120 K under different excitation; (a) Xe lamp excitation at 300 nm; and (b) laser excitation at 266 nm for two different intensities I0 and 100 I0. (Fig. 2c) Emission spectra at 10 K of two different sample of ZnO NP excited by a laser at 266 nm. The black curve represents the emission of a capped sample. In all cases the spectra are normalized to their maximum of intensity.
increase. The MC spectra are dominated by the A band and FX for both excitation. Concerning the NP spectra we can remark that even if the DX still dominates the spectrum, the A band and the FX band are now clearly identifiable. However, the intensity of these two peaks is still much more important in the MC spectra. Finally, the more interesting feature of this temperature evolution is the asymmetric broadening of the more energetic component of the A band present in the MC. For the high-intensity spectrum of the MC, we can observe only one very broad symmetric A band that includes perhaps a little contribution from FX. This shows that at 120 K, the
This hypothesis could explain the lack of A band in NP at 10 K because the presence of extended defect in the NP is not energetically favourable [15]. To support this argument it is interesting to note the presence of a broad green band in the MC emission spectra at room temperature. This band is generally attributed to oxygen vacancies or other ‘‘volume related’’ defects [11] and is not present in our NP [8]. This fact supports the hypothesis that contrary to the MC, the NP does not contain welldefined defects. Moreover, at 10 K and under VUV excitation we can observe the A band without any contribution of the free exciton that reject the hypothesis of a phonon repliqua. Finally, the hypothesis of an acceptor localized state is compatible with the intensity dependence of the spectra profile and the Gaussian shape of the A band. But this hypothesis does not explain the asymmetric broadening of the A band at 120 K. 4.3. 1LO phonon repliqua When the temperature increases at 120 K (Fig. 2), the asymmetric shape of the A band suggests an interaction of the exciton with the 1LO phonon as studied by Seagall and Mahan [16]. Moreover, at this temperature, the energy between the A band and FX band is compatible with the 1LO energy reported in the literature for ZnO [17]. Finally, this assignment could explain why the asymmetric broadening of A band in the MC concords with the appearance of FX peak and A band in the nanoparticles. However, at 120 K and at high intensity excitation, the intensity of the A band increases but the FX one decreases. Moreover, for the high excitation intensity, the shape of the A band becomes again symmetric that is not in accordance with the hypothesis of the
ARTICLE IN PRESS D. Tainoff et al. / Journal of Luminescence 129 (2009) 1798–1801
1LO phonon repliqua. Finally, the only way to explain all our experimental results is to consider two temperature ranges. In a first range spanning from 10 to 100 K the A band could result from an acceptor defects. Then, from 120 to 300 K this band could be the superposition of the acceptor state previously described with a 1LO phonon repliqua of the free exciton.
1801
Acknowledgements This work was supported by the European Community Research Infrastructure Action within the FP6 Program through the Contract RII3-CT-2004-506008 (IA-SFS). The authors acknowledge G. Stryganyuk for his assistance in the SUPERLUMI experiments.
5. Conclusions References Among the different UV emission peaks the origin of the 3.31 eV (A band) remains unclear. Starting from industrial microcrystals (MC) we have synthesized ZnO nanoparticles (NP) in UHV and controlled in situ their quality. To explain the origin of the 3.31 eV band their emission spectrum has been studied for different excitation energies and/or intensities and different temperatures. Whatever the excitation energy or the temperature, the intensity of this band is weaker in the nanoparticles. This fact allows us to reject the hypothesis of a surface peak. At 10 K, the hypothesis of an acceptor localized state is the only one which is fully compatible with our results. At higher temperatures and at weak excitation intensity, the shift and the asymmetric broadening of the A band tends to prove the implication of an exciton–phonon interaction as studied by Seagall and Mahan [16]. However this explanation cannot explain the modification observed at high intensity excitation. Therefore, the only way to explain all the result is to consider that the A band originates from the superposition of a localized acceptor state and a 1LO phonon repliqua of the free exciton.
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