Local Atomic Structure and Chemical Bonds of Zinc Sulfide and Selenide Nanostructures in Porous Aluminum Oxide Matrices

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1 ISSN , Bulletin of the Russian Academy of Sciences. Physics, 2015, Vol. 79, No. 9, pp Allerton Press, Inc., Original Russian Text R.G. Valeev, A.N. Beltukov, V.V. Kriventsov, N.A. Mezentsev, A.I. Chukavin, 2015, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2015, Vol. 79, No. 9, pp Local Atomic Structure and Chemical Bonds of Zinc Sulfide and Selenide Nanostructures in Porous Aluminum Oxide Matrices R. G. Valeev a, b, A. N. Beltukov a, V. V. Kriventsov c, N. A. Mezentsev d, and A. I. Chukavin a a Physicotechnical Institute, Ural Branch, Russian Academy of Sciences, Izhevsk, Russia b Udmurt State University, Izhevsk, Russia c Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia d Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia rishatvaleev@mail.ru Abstract The local atomic structure and chemical bonds of ZnSe and ZnS nanocomposites are studied. Both materials are obtained via thermal evaporation of the materials powder onto porous matrices of anodic aluminum oxide and polycorr substrates. The mechanism responsible for the formation of the local atomic structure and the chemical bonds of zinc sulfide and selenide is studied. Relative to the films on polycorr surfaces, changes in the local atomic structure are observed in the materials obtained in porous matrices because of the difference between the mechanisms of condensation. DOI: /S INTRODUCTION Even though zinc sulfide and selenide have long studied thoroughly as materials used in semi-conducting engineering, nanostructures based on them (e.g., nanoparticles in oxide matrices) are still of great interest to researchers [1, 2]. In addition, methods have been developed in recent decades that enable us to obtain spatially correlated and ordered nanostructures whose properties are the coherent result of those of individual particles. The properties of materials are determined at the local atomic level: the presence or absence of longrange ordering considerably affects the electronic structure of semiconductors [3]. A dielectric matrix in which nanosize particles can form produces such nanoscale effects as dangling chemical bonds, resulting in the unique optical and electrical properties of semiconductor dielectric nanocomposites [4]. Gathering information on the parameters of local atomic environments (chemical bond lengths and coordination numbers) for specific atoms should thus help to explain those that are expanded relative to continuous (films and single crystals) media. In addition, the type of chemical bonds changes considerably (from covalent to partially ionic) upon moving from germanium to gallium arsenide and zinc selenide in the isoelectronic series. EXPERIMENTAL Aluminum oxide films with highly ordered pores were synthesized via twin-stage anodic oxidation [5]. The semi-conductors were sputtered onto porous surfaces via anodic abrasion and the resistive evaporation of polycrystalline powder in a high (10 5 Pa) vacuum [6]. Semiconductive films were simultaneously sputtered onto smooth polycorr surfaces to create the test samples. The local atomic structure of films was investigated via extended X-ray absorption fine structure (EXAFS) spectroscopy at the Siberian Center of Synchrotron and Terahertz Radiation (Novosibirsk) [7]. The VEPP-3 accelerating ring with its electron beam energy of 2 GeV and average current of 80 ma was used as our source of X-ray radiation. A (111)Si crystal was used for monochromatic radiation. EXAFS spectra were recorded using the fluorescence yield mode on the absorption К-edge E K specific to each of the studied materials and with a scanning pitch of 1.5 ev. For zinc selenide, E K was 9659 ev (for Zn) and ev (for Se); the energy was scanned over the ranges of ev and ev, respectively. For zinc sulfide, E K was 9659 ev and the energy was scanned over the range of ev. Spectra of the К-edge for sulfur were not obtained, since excitation in the soft X-ray region was needed for their registration, and this was not possible on the VEPP-3. Our EXAFS spectra were preliminarily treated using the standard technique in [8, 9]. The correlation functions and their parameters (chemical bond lengths and coordination numbers) were determined from the normalized oscillating parts of the χ(k) function by Fourier fitting over a k-space range of

2 1192 VALEEV et al Å 1 using the VIPER software [10]. Analysis of the resulting functions allowed us to determine the chemical bond lengths and coordination numbers for the first two coordination spheres. The chemical bonds were studied by X-ray photoelectron spectroscopy (XPS) using a SPECS spectrometer (Germany). The spectra were excited with Al K α radiation (E = ev). Before recording the spectrum, each sample s surface was cleaned for 5 min via Ar + ion etching at an energy of 4 kev and a current density of 30 µa. This was sufficient for the complete removal of surface impurities. We obtained both full spectra and spectra of the principal elements for all samples. RESULTS AND DISCUSSION 200 nm Fig. 1. Scanning electron microscopy image of our sample of zinc selenide nanostructure, obtained in the matrix of porous anodic aluminum oxide with a pore diameter of 40 nm. Zinc selenide The typical SEM pattern of nanostructured zinc selenide after removal of the porous anodic aluminum oxide matrix (pore diameter, 40 nm) is shown in Fig. 1. As can be seen, the nanostructures are threads and points formed by thread breakage and have diameters equal to those of the matrix pores. Analysis of the chemical composition and chemical bonds of ZnSe nanostructures in porous aluminum oxide revealed (Fig. 2) that the free surface contained adsorbed hydrocarbon impurities and adsorbed oxygen. The carbon concentration was as high as at % in the near-surface layers 1 3 nm thick. It did not exceed Intensity, arb. units Zn2p 3/2 Se3d O1s C1s Zn3s Zn3d Zn3p Se3d Zn LMV, LMM Fig. 2. XPS of nanostructured zinc selenide in the Al 2 O 3 matrix.

3 LOCAL ATOMIC STRUCTURE AND CHEMICAL BONDS 1193 Fourier transforms, arb. units K-edge Zn (c) (b) (a) 3 5% at depths of 8 10 nm, and there was no oxygen at these depths. The selenium : zinc ratio after removing the modified superthin surface layer corresponded almost exactly to the equiatomic ZnSe composition. The bonding energy for the Se3d line coincided with that of its metallic compounds. E bond of Se3d line exceeds the value typical of pure zinc, while the chemical shift is greater than the one characteristic of standard ZnO oxide. We may therefore conclude that our E bond of Zn2p and Se3d are is also that of ZnSe compound. Figure 3 shows the Fourier transforms of normalized oscillating parts of the absorption spectra at the Zn and Se К-edges for zinc selenide, obtained via thermal sputtering into matrix pores and onto smooth Al 2 O 3 surfaces. Fourier fitting allowed us to calculate the interatomic distances and coordination numbers for the first coordination spheres of the local atomic environments of Zn and Se atoms. The results are shown in Table 1. Under normal conditions, zinc selenide has a cubic (zincblende-type) crystalline structure in which each Zn and Se atom has four Se or Zn atoms, respectively, in its immediate environment. In our case, no changes in the interatomic distances were observed for ZnSe on either porous or smooth substrate surfaces, while the coordination number grew for zinc selenide implanted into the matrix. In addition, the Fourier transforms and table data show that there is an inverse redistribution of Zn and Se atoms between the first and the second coordination spheres for smooth films and zinc selenide, and for films inside the porous Al 2 O 3 matrix. In a smooth film, the first coordination sphere contains more zinc atoms and fewer atoms of selenium, while the second has more selenium atoms. The opposite is true for zinc selenide in porous aluminum oxide, due possibly to the formation of zinc selenide on the substrate occuring in parallel with atomic dissociation during reevaporation from the substrate. Zinc Sulfide K-edge Se R, Å R, Å Fig. 3. Fourier transforms of EXAFS spectra for the Zn and Se К-edges: (a) FEFF model; (b) ZnSe film on polycorr; (c) ZnSe in the porous Al 2 O 3 matrix. According to XPS results, the free surface of this sample contained adsorbed hydrocarbon impurities and adsorbed oxygen. The carbon impurities appeared to have been completely removed after 1 min of etching, (Fig. 4, plane spectrum). The energy bonding values for the S2p line agree with those of its metallic Table 1. Parameters of the ZnSe local atomic environment Sample R 1, Å N 1 R 2, Å N 2 Crystallography FEFF model ZnSe in the Al 2 O 3 matrix ZnSe film on polycorr 2.45 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.5 (1) Zn Se (top) and Se Zn (bottom) atomic pairs; (2) Zn Zn (top) and Se Se (bottom) atomic pairs.

4 1194 VALEEV et al. Intensity, arb. units S2p Zn2p 3/2 Zn2p Zn3s O1s Zn LVV Zn3p S2p Fig. 4. XPS of nanostructured zinc sulfide in the porous Al 2 O 3 matrix. compounds. E bond of the Zn2p 3/2 line is within the range typical of ZnS, and Zn : S ratio is almost equal to the equiatomic ratio. The Fourier transforms for ZnS in the porous aluminum oxide are close to the model, and we can see a bend in the range of the third coordination sphere while only two coordination spheres can be found for the film (Fig. 5). It follows from Table 2 that the parameters of the local atomic environment in ZnS Al 2 O 3 are also closer to being crystallographic than those of the film; this can be explained by the different mechanisms of growth for the films on the smooth aluminum oxide surface and the nanostructures in matrix. Table 2. Parameters of the ZnS local atomic environment Sample R 1, Å N 1 R 2, Å Crystallography FEFF model 2.34 ± ± ± 0.05 ZnS in the Al 2 O 3 matrix 2.33 ± ± ± 0.05 ZnS film on polycorr 2.33 ± ± ± 0.05 (1) Zn S; (2) Zn Zn atomic pairs.

5 LOCAL ATOMIC STRUCTURE AND CHEMICAL BONDS 1195 Fourier transforms, arb. units closed volume (Al 2 O 3 matrix pores), due to the higher probability of eventual collisions and the time one component is near another. Sulfur and selenium evaporate much more easily from a flat open surface than from a porous matrix. It is probably this factor that determines the established features of the formation of local structure and the nonstoichiometry of zinc sulfide and selenide nanostructures in the channels of porous matrices R, Å Fig. 5. Fourier transforms of normalized oscillating parts of the EXAFS spectra for the Zn К-edge. (a) FEFF model; (b) ZnS film on polycorr; (c) ZnS in the porous Al 2 O 3 matrix. CONCLUSIONS The deposition of semiconductor materials into the channels of a porous aluminum oxide matrix is promising in terms of their possible use in photonic and micro- and nanoelectronic devices. The mechanism behind the formation of local atomic structure can be quite complex, depending on the number of components in a material and the origin of its matrix. The likelihood of interaction between vapor components and the formation of a chemical compound is higher with a two-component vapor in a closed or partially (c) (b) (a) ACKNOWLEDGMENTS This work was performed on equipment of the Center for Collective use at the Siberian Center for Synchrotron and Terahertz Radiation. It was supported by the RF Ministry of Education and Science; by the Ural branch of the Russian Academy of Sciences, project nos. 12-S and 12-P ; by the Presidium of the Russian Academy of Sciences, project nos and 24.3; and by the Russian Foundation for Basic Research. REFERENCES 1. Taghavinia, N. and Yao, Y., Phys. E, 2004, vol. 21, p Shi, L., Xu, Y.M., and Li, Q., Nanotechnology, 2005, vol. 16, p Gorbachev, V.V. and Spitsyna, L.G., Fizika poluprovodnikov i metallov (Physics of Semiconductors and Metals), Moscow: Metallurgiya, Yuan, J.N., He, F.Y., Sun, D.C., et al., Chem. Mater., 2004, vol. 16, p Masuda, H. and Satoh, M., Jpn. J. Appl. Phys., Part 2, 1996, vol. 35, p. L Valeev, R.G., Krylov, P.N., and Romanov, E.A., J. Surf. Invest.: X-ray, Synchrotron Neutron Tech., 2007, vol. 1, p Kulipanov, G.N., Ancharov, A.I., Antokhin, E.I., et al., in Brilliant Light in Life and Material Sciences, Tsakanov, V. and Wiedemann, H., Eds., Springer, 2007, p Kochubey, D.I., EXAFS spektroskopiya v katalize (EXAFS Spectroscopy in Catalysis), Novosibirsk: Nauka, Zabinsky, S.I., Rehr, J.J., Ankudinov, A., et al., Phys. Rev. B, 1995, vol. 52, p viper.html Translated by O. Maslova