ISSN 1063-7745, Crystallography Reports, 2017, Vol. 62, No. 5, pp. 758–762. © Pleiades Publishing, Inc., 2017. Original Russian Text © A.E. Muslimov, A.V. Butashin, R.G. Valeev, S.N. Sulyanov, A.N. Beltiukov, A.B. Kolymagin, V.A. Babaev, V.M. Kanevsky, 2017, published in Kristallografiya, 2017, Vol. 62, No. 5, pp. 789–794.
SURFACE, THIN FILMS
Evolution of the Vanadium Pentoxide V2O5 Crystal Surface after Vacuum Annealing A. E. Muslimova,*, A. V. Butashina, R. G. Valeevb, S. N. Sulyanova, c, A. N. Beltiukovb, A. B. Kolymagina, V. A. Babaevd, and V. M. Kanevskya a Shubnikov
Institute of Crystallography, Federal Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia b Physical-Technical Institute, Russian Academy of Sciences, Ural Branch, Izhevsk, 426000 Russia c National Research Center “Kurchatov Institute,” Moscow, 123098 Russia d Dagestan State University, Makhachkala, 367000 Russia *e-mail:
[email protected] Received April 19, 2017
Abstract—A complex study of the (001) cleavage surface of a V2O5 single crystal annealed in vacuum at 450 and 550°C has been performed. Tunnel microscopy of the sample surface annealed in vacuum at 550°C showed the formation of a plane with a corundum structure, reconstructed according to the V2O3 (0001)(1/√3 × 1/√3)R30° type, on the surface. X-ray photoelectron spectroscopy revealed a significant modification of interatomic bonds in the surface layers of V2O5 single crystal after vacuum annealing at 550°C, which is related to the partial reduction of V5+ ions and formation of lower vanadium oxides. These modifications lead to a decrease in the electrical resistivity of V2O5 and the occurrence of bending at a temperature of 61.5°С in the temperature dependence of the sample resistance, which is indicative of phase transition. DOI: 10.1134/S1063774517050133
INTRODUCTION Vanadium belongs to the group of transition metals with unfilled d-electron shells. Due to this, it forms numerous phases of variable valence in compounds with oxygen, some of which undergo temperatureand pressure-induced phase transitions [1]. A phase transition changes not only the structure but also the electrical, optical, and other physical properties of materials, a circumstance making vanadium oxides an interesting class of materials, promising for both fundamental research and applications. The best-studied compounds are V2O3, VO2, and V2O5, where vanadium is in the V3+, V4+, and V5+ states. At the dielectric–metal phase transition in vanadium oxide V2O3 at Тt ~ –123°C, the trigonal lattice symmetry changes to the monoclinic one with an increase in the lattice volume by 1.6% [2, 3]. The oxide exhibits metallic properties at room temperature. V2O3 is a high-resistance conductor with a pronounced thermoresistive effect. V2O3-based thermistors are used, e.g., in cryogenic engineering as temperature sensors, self-regulating thermostat heaters, and contactless relays. A factor restricting the use of V2O3 is the ability of this compound to uncontrolled oxidation to stable V2O5 compound or a heterogeneous material
based on it, which indicates the trend of this oxide to aging and obvious modification of its properties [4]. The semiconductor–metal phase transition in vanadium dioxide VO2 occurs at Tt = 67°C: above Tt, this compound is a metal with a tetragonal lattice symmetry; below Tt, it is a semiconductor with a monoclinic lattice symmetry. Transparent VO2 films are considered to be promising coatings for adaptive glasses with an optical transmittance sharply changing at this transition. In addition, VO2 films are used in multilayer optical structures for nano- and picosecond interferometry due to the possibility of their ultrafast (approximately for 100 fs) transition from the semiconductor to metal phase under a short pulse with a sufficiently high energy [5]. The dielectric–metal phase transition was also observed in V2O5 crystals at a temperature of Tt = 280°C [6] without structural variations, but the mechanism of this transition has not been studied in detail. In particular, Pergament et al. [7, 8] attributed the transition in V2O5 to the presence of the VO2 phase, which arises in the V2O5 bulk in the form of separate filaments or inclusions during electroforming. In addition, we should note that the surface of fresh cleavages of V2O5 crystals, which are dielectrics at room temperature, can be analyzed by atomic-resolu-
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V2O5 annealed in vacuum in a VUP-2000 vacuum deposition system at residual pressures below 10–4 mmHg and temperatures of 450°C (type II) and 550°C (type III) for 30 min. XPS spectra of the samples were measured on a Specs X-ray electron spectrometer (Germany) at a constant transmission energy of 15 eV of the energy analyzer in MgKα radiation (1253.6 eV). The chemical states of the elements were identified using reference data. The experimental XPS data were processed applying the CasaXPS software package [14].
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Fig. 1. STM image of the atomic structure of the (001) plane of the V2O5 crystal (type I).
tion scanning tunneling microscopy not only in vacuum [9] but also in air [10]. V2O5 crystals belong to the orthorhombic system (sp. gr. Pmmn) with the unit-cell parameters a = 3.563 Å, b = 11.510 Å, c = 4.369 Å, and Z = 2 [11]. This compound congruently melts at a relatively low (690°С) temperature, and the crystals have a perfect cleavability along the (001) plane. The easiest way to obtain lower vanadium oxides (V2O3 and VO2) is the reducing annealing of V2O5 in vacuum. At the same time, there is a lack of unambiguous data on the phase formation in the V–O system when this method is used [9, 12, 13].
The STM study was carried out in an Integra Prima (NT MDT) scanning tunneling microscope, which allows one to determine the surface relief parameters, linear nanorelief sizes, and local tunnel conductivity of solid-state conducting structures. The maximum scanning area in the sample plane was 1.6 × 1.6 μm2, and the probe bias was 0.1 V. An important characteristic of the STM measurements is the presence of conducting tips (electrodes) with a small (up to one atom) radius, which are used to scan the sample surface. The conductive tips were prepared by beveled cutting and pulling a tungsten wire with a radius of about 0.2– 0.3 mm. The measurements were performed in two (dc and constant-height) STM modes. The electrical properties were studied using a Keithley 6487 picoamperemeter. RESULTS AND DISCUSSION
EXPERIMENTAL
Using the constant-height mode, we obtained images of the atomic structure of the (001) cleavage surface of V2O5 crystal (type I), which is formed mainly by oxygen atoms [13]. Such an image (Fig. 1) allows the geometric lattice parameters to be determined with a high accuracy. The spacing between oxygen rows is about 12.1 Å, which is fairly close to parameter b of the V2O5 crystal; the rows are oriented in the 〈100〉 direction. The observed periodicity of elements in these rows is about 3.6 Å, which corresponds to parameter a of the V2O5 crystal lattice. Thus, the STM state on the metric parameters of the cleavage surface structure for the V2O5 crystal are in satisfactory agreement with the structural data for the bulk crystal [13]. The surface roughness Ra was calculated (using the STM software) to be about 0.5 Å.
The samples for investigations were fabricated from V2O5 single crystals grown by slow cooling of vanadium pentoxide melt in air, whose X-ray diffraction patterns were indexed in the orthorhombic system with the above-mentioned unit-cell parameters. The X-ray diffraction patterns were recorded on a PANalytical X’PERT PRO diffractometer in the Bragg−Bretano geometry (CuKα radiation, λ = 1.5418 Å) with a nickel β filter. The range of diffraction angles was 2θ = 10°– 100°. Samples of three types were used . The type-I sample is a (001) cleavage of V2O5 single crystal. The samples of types II and III are the similar cleavages of
After vacuum annealing, the samples were divided in several groups for X-ray diffraction and STM studies. The samples for X-ray analysis were grinded into powder. A typical X-ray diffraction pattern for the type-II and type-III samples is presented in Fig. 2. A comparison with the model X-ray diffraction pattern calculated using the data of [15] for the V2O5 compound confirms that the initial V2O5 phase (type I) is preserved, while other phases in the V–O system are absent. At the same time, the STM study of the type-II sample surface revealed significant morphological changes: the surface roughness increased to 4 Å, as a
In this study, we carried out a complex investigation (scanning tunneling microscopy (STM), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS)) of the (001) cleavages of V2O5 crystals annealed in vacuum at different temperatures and analyzed the electrical properties of these crystals.
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Fig. 2. Typical X-ray diffraction pattern of the V2O5 compound annealed in vacuum (types II and III) (curve 1). Model X-ray diffraction pattern (curve 2) calculated using the data of [15] for the V2O5 compound (F is the line intensity).
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Fig. 3. STM image of the atomic structure of the type-III sample. The inset shows an enlarged image and a Fourier image of a separate area (indicated by an arrow).
result of which we failed to obtain an atomic-resolution image in the constant-height mode. The type-III sample surface was relatively smooth, with separate flaky islands. Scanning of the smooth areas yielded an atomic-resolution image (Fig. 3) with an interatomic spacing of a = 3.1 Å and sixfold symmetry. Taking into account that V2O3 among is the single representative of vanadium oxides that crystallizes into the trigonal system (corundum structural type, a = 4.9717 Å, and c = 14.005 Å), we may suggest the formation of a superstructure reconstructed according to the V2O3 (0001)-(1/√3 × 1/√3)R30° type on the type-III sample surface upon annealing. The found atomic-structure parameters (Fig. 3) are in good agreement with those reported in [16], where the above-mentioned superstructure was formed on the V2O3 (0001) surface at low oxygen partial pressures in the system. We may suggest that the reconstruction of V2O5 upon vacuum annealing occured in several stages: V2O5 → V6O13 → V2O3 [6]. The presence of the V2O3 phase in the surface layers was confirmed by the XPS data. The type-I and type-III samples were studied. Figure 4 presents the sample spectra in the binding energy range covering the oxygen 1s and vanadium 2p energy levels. The spectrum of the carbon 1s level (shown in the inset as a reference) demonstrates that the spectrometer calibration is not violated when recording spectra, because no line shifts are observed. The position of the peak with binding energy Еbind corresponds to carbon absorbed on the surface (Еbind = 284.5 eV), which is almost always present on the sample surface and usually vanishes after surface etching. Since we had to investigate specifically the surface
layer of the samples, they were not subjected to ion etching. At the same time, it can be seen that the carbon peak intensity decreases after the sample annealing, i.e., the annealing partially removes the adsorbed carbon from the surface. The position of the line with a binding energy of 530 eV for the type-I sample and 530.2 eV for the typeIII sample corresponds to the energy position of the oxygen 1s line in the chemical bond with the metal, but with a shift to lower binding energies (the correF, rel. units
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Fig. 4. XPS spectra of the type-I (curve 1) and type-III (curve 2) samples in the range of binding energies of O1s and V2p. The inset shows the spectrum of the carbon 1s line; F is the line intensity.
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Fig. 5. Decomposed XPS spectrum of the V2p3/2 line of the (a) type-I and (b) type-III samples (F is the line intensity): (1) experiment, (2) V5+, (3) V4+, (4) V3+, (5) V2+, (6) background, and (7) fitting.
sponding energy for a reference bulk sample is 531 eV, according to the table data of [17]). The increase in Еbind of the О1s peak for the type-I sample relative to that for the type-III sample can be explained by partial reduction of the V5+ oxide to intermediate lower oxides (Fig. 4), which have another crystal structure and, consequently, other chemical bond lengths. Figure 5 presents decomposed XPS spectra of the V2p3/2 line. Fitting was performed using a combination of Gaussian and Lorentzian functions [18]. It can be seen that the dominant phase in the type-I sample is V2O5 (Еbind = 517.2 eV) with a certain admixture of the I, μA 70 Annealed at 550˚C
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We investigated the temperature dependence of the resistance for all the samples (Fig. 6), using platinum contacts deposited onto the opposite sides of sample. It can be seen in Fig. 6 that the temperature dependences for the type-I and type-II samples are almost identical within the experimental error and typical of an insulator (bulk V2O5 at these temperatures).
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The type-III sample behaves quite differently. Its resistance decreases by a factor of about five upon annealing, which is related to the formation of layers or lower oxides in the surface region, which are a semiconductor (VO2) and a conductor (V2O3) at room temperature. A further increase in temperature to Т = 61.5°С gives rise to a phase transition. This temperature is close to Tt = 67°C—the temperature of the
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Fig. 6. Temperature dependences of current I for the type-I, type-II, and type-III samples at U = 0.5 V. CRYSTALLOGRAPHY REPORTS
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V2O3 phase (Еbind = 515.7 eV). After annealing, vanadium is reduced with the formation of the VO2 (Ebind = 516.7 eV) and V2O3 (Ebind = 515.6 eV) phases and a small admixture of VO (Ebind = 514.5 eV). In the ideal case, in situ annealing in the spectrometer chamber should lead to vanishing of V5+, but its repeated occurrence can be explained by the exposure of the sample under study to air after vacuum annealing. The energy positions of the decomposition peaks differ by ΔE = 1.1 eV, a value consistent with the reference data [17]. Thus, based on the results of our study, we may suggest that the vacuum annealing of V2O5 crystals leads to the formation of a layered V2O3/VO2/V2O5 structure (with the oxidation state rising towards the sample bulk) in the surface region up to several nanometers thick; this is no surprise, with allowance for the annealing conditions. The flakes (Fig. 3) are apparently due to the presence of the VO phase on the surface.
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phase transition in vanadium dioxide (VO2) [3]. We may suggest that, as in [7, 8], the transition in the type-III samples is explained by the presence of the VO2 phase, which arises in the bulk of V2O5 during electroforming as individual filaments or inclusions. Note that the drop of Tt to 61.5°С, observed in the type-III sample, is important for the vanadium dioxide applications and can be caused by the defects in the newly formed VO2 phase or the surface effects. Since the resistance jump is the vicinity of the transition point is fairly small, we may suggest that electroforming leads to the occurrence of the VO2 phase in the form of inclusions in the bulk of the V2O5 crystal. CONCLUSIONS We carried out a complex investigation of the (001) cleavage surface of V2O5 single crystal samples in the initial state (type I) and the samples subjected to additional vacuum annealing at temperatures of 450°C (type II) and 550°C (type III). The STM study of the type-I sample surface showed that it is not reconstructed, and the obtained image allows one to determine the geometric parameters of the V2O5 crystal lattice with a high accuracy. The type-II sample surface had increased roughness, and the STM study of the type-III sample revealed smooth surface areas with a regular symmetric structure, which can be considered as the (0001) reconstructed plane of the V2O3 crystal phase with a corundum structure. According to the XPS data, vacuum annealing significantly changes the interatomic bonds in the surface layers of the V2O5 single crystal, which was interpreted as a partial reduction of V5+ ions to the V4+, V3+, or even V2+ states. The changes observed after the vacuum annealing were only within the surface layers of the V2O5 single crystal, as was established by the STM, XSP, and X-ray diffraction analysis. The study of the temperature dependence of the resistance of the type-III sample revealed a bending at a temperature of 61.5°С, which is typical of the phase transition in the VO2 compound. ACKNOWLEDGMENTS This study was carried out on the equipment of the Center of Collective Use “Structural Diagnostics of Materials” of the Shubnikov Institute of Crystallography and the Center of Collective Use “Center for Surface Investigations and Physicochemical Analysis” of
the Physical-Technical Institute, Russian Academy of Sciences, Ural Branch, and supported by the Presidium of the Russian Academy of Sciences, Program of Fundamental Research no. 1 “Nanostructures: Physics, Chemistry, Biology, and Fundamentals of Technology.” REFERENCES 1. N. Bahlawane and D. Lenoble, Chem. Vap. Deposition 20, 299 (2014). 2. G. A. Berezovskii and I. E. Paukov, Izv. SO Akad. Nauk SSSR, No. 2(4), 19 (1981). 3. A. A. Bugaev, B. P. Zakharchena, and F. A. Chudnovskii, Metal–Semiconductor Phase Transition and Its Application (Nauka, Leningrad, 1978) [in Russian]. 4. O. V. Lyakh, Candidate’s Dissertation in Technical Sciences (Omsk, 2012). 5. V. N. Andreev and V. A. Klimov, Phys. Solid State 49, 2251 (2007). 6. M. Kang, I. Kim, S. W. Kim, et al., Appl. Phys. Lett. 98 (13), 131907 (2011). 7. A. L. Pergament, A. L. Stefanovich, and F. A. Chudnovskii, Fiz. Tverd. Tela 36 (10), 2988 (1994). 8. A. Pergament, G. Stefanovich, and V. Andreev, Appl. Phys. Lett. 98, 131907 (2011). 9. R.-P. Blum, H. Niehus, C. Hucho, et al., Phys. Rev. Lett. 99, 226103 (2007). 10. A. E. Muslimov, A. V. Butashin, and V. M. Kanevsky, Crystallogr. Rep. 62, 123 (2017). 11. W. H. Barnes, F. R. Ahmed, and H. G. Bachmann, Z. Kristallogr. 115, 110 (1961). 12. A. A. Baranov, M. S. Tret’yakova, S. A. Zhukova, and V. E. Turkov, Prikl. Fiz., No. 5, 55 (2016). 13. O. Ya. Berezina, V. P. Zlomanov, D. A. Kirienko, et al., Sovrem. Probl. Nauki Obraz., No. 4, 306 (2013). 14. N. Fairley, Casa XPS Manual (Casa Sofware, USA, 2010), p. 176. 15. Y. Wang, H. Shang, T. Chou, and G. Cao, J. Phys. Chem. B 109, 11361 (2005). 16. J. Schoiswohl, G. Tzvetkov, F. Pfuner, et al., Phys. Chem. Chem. Phys. 8, 1614 (2006). 17. V. I. Nefedov, X-Ray Photoelectron Spectroscopy of Chemical Compounds: A Handbook (Khimiya, Moscow, 1884) [in Russian], p. 150. 18. Surface Analysis by Auger- and X-Ray Photoelectron Spectroscopy, Ed. by D. Briggs and M. P. Seah (Wiley, Chichester, 1983).
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