The photoluminescence and photoluminescence excitation spectra for Zn 1-x Mn x O nanocrystals are presented. After annealing of powders in air, the intensity of the bands attributable to manganese decreases noticeably. This suggests that the oxygen vacancies affect the Zhang-Rice-like states appearing due to strong d-p-hybridization, which is confirmed by an increase in the band gap of Zn1-x Mn x O for low x. The origin of the 2.9-eV peak and the shape of its excitation spectrum are discussed qualitatively. For Zn1-x Mn x O nanocrystals, the shape of the excitation spectrum is as unusual as the intense absorption in the range (2.2-3.0) eV. © 2010 Pleiades Publishing, Ltd.We wish to thank T. Dietl, V.I. Anisimov, and A.V. Lukoyanov for the discussion of localized states in the Zn1–xMnxO system. This work was supported by the Russian Foundation for Basic Research (project nos. 07 02 00910_a and 08 02 99080r ofi), and Russian Federal Agency on Science and Innovations (grant no. 02.740.11.0217)

Ermakov, A. E.

Gruzdev, N. B.

Mysik, A. A.

Pustovarov, V. A.

Sokolov, V. I.

Uimin, M. A.

Journal of Experimental and Theoretical Physics

Institutional repository of Ural Federal University named after the first President of Russia B.N.Yeltsin

ISSN 10637761, Journal of Experimental and Theoretical Physics, 2010, Vol. 111, No. 2, pp. 231–235. © Pleiades Publishing, Inc., 2010.Original Russian Text © N.B. Gruzdev, V.I. Sokolov, A.E. Ermakov, M.A. Uimin, A.A. Mysik, V.A. Pustovarov, 2010, published in Zhurnal Éksperimental’noі i Teoreticheskoі Fiziki,2010, Vol. 138, No. 2, pp. 261–265.231Zn1 – xMnxO semiconductor crystals attract greatattention due to the potentiality for creating optoelectronic devices based on a semiconductor with ferromagnetic ordering with a Curie temperature TC abovethe room one. In recent years, interest in nanomaterials, including those based on Zn1 – xMnxO, hasincreased. As a rule, oxide compounds are produced inthe form of nanocrystals with a large number of defects(e.g., oxygen vacancies). Analysis of experimentalresults shows that the oxygen vacancies can play animportant role in the formation of magnetic orderingin oxide semiconductors with impurities of 3delements ZnO:Cr [1] and TiO2:Co [2] as well as inundoped ZnO [3]. Thus, it seems important to investigate ZnO and Zn1 – xMnxO nanopowders with sublattice oxygen vacancies by optical methods.Recently, a 2.9 eV photoluminescence (PL) peakwas detected for Zn0.99Mn0.01O nanopowders produced by the gasphase synthesis method [4]. Theexcitation spectrum of this emission exhibits threewide peaks in the region of interband transitions atenergies of 3.9, 4.5, and 5.3 eV. In this paper, we investigate the Zn0.99Mn0.01O system to understand why the2.9 eV peak appears in the PL spectrum and the wellstudied ZnMnS system for comparison. Our resultssuggest that a series of states the optical transitionsbetween which and the ground state of the crystal produce both the 2.9 eV peak in the PL spectrum and thefeatures in the excitation spectrum of this emission atenergies of 3.9, 4.5, and 5.3 eV in the region of interband transitions emerge in the Zn1 – xMnxO semiconductor compound.Zn0.99Mn0.01O nanopowders with a crystal size of30 nm were produced by the gasphase synthesismethod [5] and were annealed in air at a temperatureof 400°C for an hour. The samples for our measurements were produced by tablet pressing at a pressure of0.5 GPa. The Zn1 – xMnxO (x = 0.0016) single crystalswere annealed in air at a temperature of 700°C for20 hours. The manganese concentration in the nanopowders before and after annealing was measured byinductively coupled plasma mass spectroscopy. ThePL and PL excitation (PLE) spectra were measured inthe range (2⎯5.5) eV using two double DMP4 prismmonochromators (a reciprocal linear dispersion of10 Å mm–1 near 5 eV), an R635810 (Hamamatsu)photomultiplier tube, and a photoncounting system.A deuterium DDS400 lamp was used for excitation.The PLE spectra were normalized to the same numberof photons incident on the sample using yellow phosphor, which has an energyindependent PL quantumyield in the investigated energy range. The PL spectraare presented without any normalization to the spectral sensitivity of the optical section.Figure 1 presents the PL and PLE spectra forZn0.99Mn0.01O nanopowders before and after annealing. In both cases, the PL spectra exhibit lowenergypeaks with 2.10 and 2.34 eV maxima and a highenergy peak with a 2.9 eV maximum. The lowenergypeaks are attributable to deep defects in the crystalsthat passed thermal treatment in an oxygen atmosphere [6]. We believe that the number of oxygenvacancies decreased as a result of annealing in air. Theweakening of the 2.9 eV peak, which is pronounced inFig. 1a (curves 2 and 3), probably stems from the factExcitons and Photoluminescence in ZnOand Zn0.99Mn0.01O NanocrystalsN. B. Gruzdeva, V. I. Sokolova, A. E. Ermakova, M. A. Uimina, A. A. Mysika, and V. A. PustovarovbaInstitute of Metal Physics, Russian Academy of Sciences, Ural Division, ul. S. Kovalevskoi 18,Yekaterinburg, 620990 RussiabEltsin Ural State Technical University, Yekaterinburg, 620002 Russiaemail: visokolov@imp.uran.ruReceived October 30, 2009Abstract—The photoluminescence and photoluminescence excitation spectra for Zn1 – xMnxO nanocrystalsare presented. After annealing of powders in air, the intensity of the bands attributable to manganese decreasesnoticeably. This suggests that the oxygen vacancies affect the Zhang–Ricelike states appearing due to strongd–phybridization, which is confirmed by an increase in the band gap of Zn1 – xMnxO for low x. The originof the 2.9eV peak and the shape of its excitation spectrum are discussed qualitatively. For Zn1 – xMnxOnanocrystals, the shape of the excitation spectrum is as unusual as the intense absorption in the range (2.2–3.0) eV.DOI: 10.1134/S1063776110080121ELECTRONIC PROPERTIESOF SOLID232JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS  Vol. 111  No. 2  2010GRUZDEV et al.that this peak is produced by the Mn2+ + close environment complex, including the oxygen vacancy VO.An increase in the intensity with distinct peaks V1, V2,and V3 at energies of 3.9, 4.5, and 5.3 eV is observed inthe excitation spectrum of the 2.9 eV peak for anunannealed powder [4]. After annealing, the PL yielddecreases, but the increase in the intensity of the PLEspectrum with increasing photon energy is pronounced.Figure 2 presents the PL and PLE spectra for aZnO:Mn single crystal before and after annealing inair. The peak in the PL spectrum with an energy of2.9 eV after annealing decreases noticeably, while thePLE spectrum has a maximum near 4 eV and retains aclear tendency to increase near 5 eV.For comparison, we provide the PL and PLE spectra for a Zn0.995Mn0.005S single crystal (Fig. 3). Theemission with an energy of 2.12 eV is attributable tointracenter Mn2+ transitions (4T1–6A1), while the PLEspectrum contains the peaks due to the transitions tohighenergy Mn2+ states (4T2 and 4A2, 4E) and a maximum near Eg and a gradual decrease in the intensity atω > Eg are observed in the region of interband transitions. A similar decrease in the excitation intensity atω > Eg manifests itself for the 2.10eV emission peakin Zn0.99Mn0.01O nanopowders [4] attributable to the5003.53.010001.50400012000160002.52.0Photon energy, eV(a)150020002500Number of pulses2132000080000.150.2300.040.064Photon energy, eV(b)0.30.40.5Intensity, rel. units213 0.0240Fig. 1. (a) PL spectra for ZnO (1) and Zn0.99Mn0.01O (2) nanocrystals before annealing (excitation energy E = 3.64 eV, temperature T = 86 K) and an annealed Zn0.99Mn0.01O (3) nanocrystal (excitation energy E = 3.718 eV, temperature T = 90 K). (b) PLEspectra of the 2.10eV peak for ZnO (1) and Zn0.99Mn0.01O (2) nanocrystals and of the 2.9 eV peak for a Zn0.99Mn0.01O nanocrystal before annealing (3) (T = 86 K) and after annealing (4) (T = 90 K, excitation energy E = 3.89 eV).3.020003.504008002.52.0Photon energy, eV(a)40006000Number of pulses2112001.505.00.0125.50.0080.10.24.54.0Photon energy, eV(b)0.0160.020Intensity, rel. units2103.50.3Fig. 2. (a) PL spectra for a ZnO:Mn single crystal (0.16 at.%) before annealing (1) (excitation energy E = 3.84 eV, temperatureT = 80 K) and an annealed ZnO:Mn single crystal (2) (excitation energy E = 3.85 eV, temperature T = 300 K). (b) Excitationspectrum of the 2.9 eV peak for a ZnO:Mn single crystal before annealing (1) (T = 80 K) and after annealing of ZnO:Mn (2)(T = 300 K).JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS  Vol. 111  No. 2  2010EXCITONS AND PHOTOLUMINESCENCE IN ZnO AND Zn0.99Mn0.01O NANOCRYSTALS 233transitions via deep states (these spectra are also presented in Fig. 1b for comparison).We see that the excitation spectrum of the 2.9eVemission for Zn0.99Mn0.01O nanopowders differs significantly both from the excitation spectrum of the2.10 eV emission for the same Zn0.99Mn0.01O nanopowders via deep levels and from the excitation spectrumof the 2.12 eV emission for the Zn0.995Mn0.005S singlecrystal via intracenter Mn2+ transitions (4T1–6A1). Inthe latter cases, the PL level decreases at ω > Eg,because the absorption coefficient increases in theregion of fundamental absorption. As a result, thedepth of the layer that absorbs the exciting photonsdecreases and charge carriers emerge on the surface,where nonradiative annihilation exceeds radiativerecombination due to enhanced imperfection.The increase in the intensity of the excitation spectrum with increasing ω in the region of interbandtransitions is quite unusual for semiconductor materials. The cause of the increase in the intensity of the2.9 eV peak for Zn0.99Mn0.01O at ω > Eg is discussedbelow. However, on the other hand, there is a fundamental similarity in the behavior of the systems underconsideration. It manifests itself in the fact that PLattributable to manganese and its excitation inZn0.995Mn0.005S and Zn0.99Mn0.01O occur via a series oflocalized states coupled between themselves. ForZn0.995Mn0.005S at ω < Eg, this is the series of intracenter states of the d5 shell of Mn2+. For Zn0.99Mn0.01Oat ω > Eg, this is the series of localized states in thevalence band that appear due to strong d–phybridization (Fig. 4).Strong hybridization of the 3dstates of Mn2+ andthe 2pstates of the ions of the immediate environmentin Zn1 – xMnxO is attributable to a smaller cation–anion separation in ZnO than that in ZnSe and ZnS.Dietl [7] put forward the idea that gallium nitride andzinc oxide doped with 3dimpurities belong to thepoorly studied class of dissolved magnetic semiconductors with strong hybridization. An increase in thedegree of hybridization leads to an antiferromagneticp–dexchange interaction and hole localization on theions of the close environment. A Zhang–Rice stateintroduced for the oxide superconductor La2CuO4 [8],which is characterized by hole localization on theMn2+4O2– cluster [7], emerges in the band gap of suchcompounds.The existence of such a state was confirmed by calculations in the LSDA+U model [9]. The holes can beproduced through the electron transport from thebond closest to Mn2+ into the conduction band. As aresult, the hole is localized in the form of a Zhang–Ricelike state and a wide band of intense absorptionemerges. Thus, Dietl [7] explained the strong absorption in Zn1 – xMnxO in the range (2.2–3) eV that cannot be understood as the result of intracenter transitions commonly observed in Zn1 – xMnxSe andZn1 ⎯ xMnxS in this range or as transitions with chargetransfer, because Mn2+ in ZnO have no donor d5/d4 oracceptor d5/d6 levels in the band gap [10]. Thus, theabsorption bands can serve as a way of detectingZhang–Rice states. However, the Zhang–Ricelikestate in the PL spectrum does not manifest itself as aband shifted toward lower energies relative to the edgeof the absorption band, like the PL band inZn1 ⎯ xMnxSe and Zn1 – xMnxS due to the transition(4T1–6A1). This is explained by a nonradiative transition whose mechanism is not yet clear [11].In [7], the strong bonding condition is expressed asthe ratio of the impurity potential U to the criticalvalue of Uc at which a bound state is formed, U/Uc > 1.In this case, a bound Zhang–Ricelike state emergesand a positive addition to the change in band gap Eg(x)appears. If U/Uc < 1, then the addition is negative andEg(x) decreases at low x, as is observed for3.01.5 2.52.0Photon energy, eV(a)Number of pulses124000120001600080002000003.5 4.0 63 54Photon energy, eV(b)Intensity, rel. units00.40.60.20.8Fig. 3. (a) PL spectra for Zn1 – xMnxS (x = 0.55%) single crystals. The excitation energy E = 3.230 (1) and 4.782 (2) eV, T =300 K. (b) Excitation spectrum of the 2.12eV peak for a Zn1 – xMnxS (x = 0.55%) single crystal, T = 300 K.234JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS  Vol. 111  No. 2  2010GRUZDEV et al.Zn1 ⎯ xMnxSe [12]. In our experiment for nanopowders, the energy lines of the free excitons coupled tothe two upper inverted [6] valence subbands Γ7 and Γ9and the exciton coupled to the third, lower valencesubband Γ7 shift upward by 7 and 12 meV, respectively,which clearly indicates that Eg(x) increases. Therefore, we can confidently consider the condition forstrong bonding between the Mn2+ spin and the holeon the nearest oxygen ions in Zn1 – xMnxO to be satisfied [7].Given the above reasoning, the bands V1–V3 in thePLE spectrum of the 2.9 eV peak could be associatedwith the states that appear due to strong hybridizationdeep in the valence band. However, the decrease in theintensity of the 2.9 eV PL peak we revealed and theweakening of the excitation spectrum of this emissionin Zn1 – xMnxO after annealing in air probably stemfrom the fact that the vacancies play a significant rolein forming the 2.9 eV PL peak. It can be tentativelyassumed that PL is also produced by the Mn2+4O2–cluster whose close environment includes an oxygenvacancy, for example, in the second oxygen coordination sphere. In this case, strong hybridization of the dstates with the pstates of the nearest O2+ ions will alsotake place and this gives us grounds to believe that theenergy of the Zhang–Ricelike state will most likely bedifferent. The shallow state through which the emission with the 2.9 eV peak occurs is “pushed out” intothe band gap, i.e., the vacancy somehow activatesradiative recombination via the state that appears dueto strong d–phybridization. The wide bands of distorted states appear due to the same hybridization inthe valence band. It seems natural that there exists acommonality between the hybridizationdistortedstates of the valence band and the Zhang–Ricelikestate pushed out into the band gap, because both areformed in the region of the crystal around the Mn2+ions. This manifests itself in the PLE spectrum as thebands V1, V2, and V3 at energies of 3.9, 4.5, and 5.3 eV.It is these states that are shown in Fig. 4. The rise in theexcitation spectrum of the 2.9 eV peak can be understood as follows. The hole appearing at ω > Eg rapidlyloses its energy, moving over the energy states in spatially bounded regions around the Mn2+ ions and fallsinto a localized Zhang–Ricelike state, with theimmediate environment of Mn2+ being charged positively. Therefore, the electron falls to a hydrogenlikeorbit and recombines radiatively with a higher probability than that of nonradiative annihilation in the surface layers. The number of such radiative recombinations is determined by the density of the hybridizationdistorted states. Thus, we obtain a method for investigating these states deep in the valence band. Note thatthe bands V1, V2, and V3 manifest themselves in a veryweak form on the descent of the excitation spectrum ofthe 2.1eV peak in Zn1 – xMnxO nanopowders (Fig. 1b,curve 2). This is possible if the deep impurity centerthrough which the 2.1eV emission occurs is locatednear Mn2+.Thus, we found that the intensities of the 2.9 eVpeak in the PL spectrum and the 3.9, 4.5, and 5.3 eVpeaks in the excitation spectrum of the 2.9eV emission for Zn1 – xMnxO nanocrystals [4] decrease withreducing number of oxygen vacancies. This providesfurther evidence for an important role of the oxygenvacancies in forming the properties of oxide materials.We provided qualitative considerations that the 2.9eVpeak in the PL spectrum and the bands V1, V2, and V3in the excitation spectrum of the 2.9eV emissionresult from strong hybridization in the Zn1 – xMnxOsystem [7, 9]. Strong hybridization in Zn1 – xMnxO isconfirmed by an increase in the band gap for low values of x. On the whole, the origin of the 2.9eV peakand notably the shape of its excitation spectrum are asunexpected for Zn1 – xMnxO as the intense absorptionin the range (2.2–3) eV and require further studies.ACKNOWLEDGMENTSWe wish to thank T. Dietl, V.I. Anisimov, andA.V. Lukoyanov for the discussion of localized statesin the Zn1 – xMnxO system. This work was supportedby the Russian Foundation for Basic Research (projectnos. 070200910_a and 080299080rofi), and Russian Federal Agency on Science and Innovations(grant no. 02.740.11.0217).REFERENCES1. B. K. Roberts, A. B. Pakhomov, P. Voll, andK. M. Krishnan, Appl. Phys. Lett. 92, 162 511 (2008).6Energy, eV420ZnMnS ZnMnOGround state4T14T24E, 4A2EgV3V2V1ZRFig. 4. Left: the energy level diagram for Mn2+ in aZn1 ⎯ xMnxS single crystal; the 4T1–6A1 transitions forming the 2.12 eV luminescence band and the transitions tohigher intracenter states that form the peaks in the PLEspectrum are shown. Right: the diagram of localized statesfor a Zn1 – xMnxO compound; the transitions forming the2.9eV PL peak (ZR) and peaks V2 and V3 in the PLE spectrum are shown.JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS  Vol. 111  No. 2  2010EXCITONS AND PHOTOLUMINESCENCE IN ZnO AND Zn0.99Mn0.01O NANOCRYSTALS 2352. K. Kikoin and V. Fleurov, Phys. Rev. B: Condens. Matter 74, 174407 (2006).3. A. Sandaresan, R. Bhargavi, N. Rangarajan, U. Siddesh, and C. N. R. Rao, Phys. Rev. B: Condens. Matter74, 161306 (2006).4. V. I. Sokolov, A. Ye. Yermakov, M. A. Uimin,A. A. Mysik, V. A. Pustovarov, M. V. Chukichev, andN. B. Gruzdev, J. Lumin. 129, 1771 (2009).5. V. I. Sokolov, A. E. Yermakov, M. A. Uimin,A. A. Mysik, V. B. Vykhodets, T. E. Kurennykh,V. S. Gaviko, N. N. Shchegoleva, and N. B. Gruzdev,Zh. Éksp. Teor. Fiz. 132 (1), 77 (2007) [JETP 105 (1),65 (2007)].6. I. P. Kuz’mina and V. A. Nikitenko, Zinc Oxide: Preparation and Optical Properties (Nauka, Moscow, 1984),p. 111 [in Russian].7. T. Dietl, Phys. Rev. B: Condens. Matter 77, 085 208(2008).8. F. C. Zhang and T. M. Rice, Phys. Rev. B: Condens.Matter 37, 3759 (1988).9. T. Chanier, F. Virot, and R. Hayn, Phys. Rev. B: Condens. Matter 79, 205 204 (2009).10. K. A. Kikoin and V. N. Fleurov, Transition Metal Impurities in Semiconductors: Electronic Structure and Physical Properties (World Scientific, Singapore, 1994),p. 86.11. R. Beaulac, P. I. Archer, and D. R. Gamelin, J. SolidState Chem. 181, 1582 (2008).12. R. B. Bylsma, W. M. Becker, J. Kossut, U. Debska, andD. YoderShort, Phys. Rev. B: Condens. Matter 33,8207 (1986). Translated by V. Astakhov

Excitons and Photoluminescence in ZnO and Zn0.99Mn 0.01O Nanocrystals

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Excitons and Photoluminescence in ZnO and Zn0.99Mn 0.01O Nanocrystals

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