Spectroscopic and structural characterisation of a VOx (x≈1) ultrathin epitaxial film on Pt (111)

Thin Solid Films 400 (2001) 154–159

Spectroscopic and structural characterisation of a VOx (xf1) ultrathin epitaxial film on Pt (111) Mikhail Petukhov1, G. Andrea Rizzi, Gaetano Granozzi* Dipartimento di Chimica Inorganica Metallorganica ed Analitica and Unita` di Ricerca INFM, Universita` di Padova, via Loredan 4, 35131 Padova, Italy

Abstract VOx ultrathin epitaxial films (0.8FxF1.3), grown on Pt(111) by evaporating vanadium in a controlled water background (1=10y7 Pa), have been chemically characterised by X-ray photoelectron spectroscopy (XPS) and X-ray-excited Auger electron spectroscopy (AES), which confirm the presence of V(II). The VO film shows a NaCl-type structure exposing the (111) plane, as proven by XPD. Multiple scattering calculations are compatible with an O-terminated surface and a surface relaxation of the outermost atomic layers, which leads to a V–O bond length contraction amounting to 7%. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Growth; Photoelectron diffraction; Epitaxy; Vanadium oxide; Platinum

1. Introduction Vanadium (II) oxide, VO, is a well-known example of a non-stoichiometric oxide. Like TiO, VO can be more correctly formulated as VOx, where 0.8FxF1.3, and therefore it may present both cation and oxygen vacancies w1,2x, which cause correlated electronic properties w1–3x. VO oxidises very easily to V2O3, and this is probably one of the reasons why only a few papers have been published so far on the electronic surface characterisation of VOx. A viable methodology to study such surfaces comes from the examination of ultrathin epitaxial VOx films. Actually, VOx epitaxial films were grown on Cu(100), Ni(110) w4,5x, Pd(111) w6x and on TiO2(110) w7,8x. In the case of the Cu(100) and Ni(110) substrates, VOx films were grown by exposure of deposited vanadium atoms to O2 (10y5 Pa range) at 310 K followed by annealing to 523 K. On Pd(111), the formation of VOx was obtained, only for V oxide coverage between 0.5 * Corresponding author. Tel.: q39-049-8275158; fax: q39-04988275161. E-mail address: [email protected] (G. Granozzi). 1 Permanent address: IGNP, RRC ‘Kurchatov Institute’, Kurchatov sq., Moscow 123182, Russia.

and 1 ML, as the product of V2O3 disproportion to VO and VO2 after annealing at 3508C. A completely different route was followed when TiO2(110) was used as substrate. A stepwise and controlled oxidation of metal deposits was performed by means of annealing cycles, carried out in UHV in order to promote oxygen diffusion from the bulk of the substrate to the surface w7x. This was also the only case in which the formation of a rocksalt-type structure was proven by the analysis of the Xray photoelectron diffraction pattern (XPD) w8x. A comparison between the XPS binding energy (BE) values and the full width at half-maximum (FWHM) data obtained in w5x and w7x shows some discrepancies, which may be associated either to the actual stoichiometry (as written above, VOx is a non-stoichiometric oxide) or to different structures. Pt(111) is a substrate commonly used to grow epitaxial oxides, mainly because of its chemical stability caused by a very compact surface structure. Its surface ˚ systems such as MnO and lattice parameter is 2.77 A; FeO with larger surface lattice parameters, 3.14 and ˚ respectively, were epitaxially grown on Pt(111) 3.09 A w9,10x, but maintaining their surface lattice parameter. It is well known that the variation in oxygen content in VO strongly affects the dimensions of the unit cell, and

0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 5 0 2 - 4

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referring to the (111) surface, its surface lattice param˚ to 2.92 A, ˚ where a lower eter changes from 2.85 A oxygen content corresponds to a shorter parameter. The small difference between the over-layer lattice parameter and the substrate parameter (2.8%) might allow a pseudomorphic growth of the VOx film. In this paper, we report the spectroscopic and structural characterisation of VOx (111) epitaxial ultrathin films grown on a Pt(111) surface by reactive evaporation of metallic vanadium in a controlled H2O background (1=10y7 Pa). The spectroscopic characterisation was carried out by means of X-ray photoelectron spectroscopy (XPS) and X-ray-excited Auger electron spectroscopy (AES). A structural characterisation was obtained by anglescanned X-ray photoelectron diffraction (XPD) coupled with multiple-scattering cluster–spherical wave (MSCSW) theory w11x. 2. Experimental The sample preparation was performed in a UHV preparation chamber operating at a base pressure of 2=10y8 Pa. The Pt(111) surface was cleaned by repeated cycles of Arq (Eps2 keV) ion bombardment at room temperature. The Pt sample was then annealed in 1=10y5 Pa of O2 at 6008C. This treatment was followed by an e-beam flash at 6008C. The sharp Pt(111) (1=1) low-energy electron diffraction (LEED) (VG Microtech Rear View LEED-RVL900) pattern and absence of XPS C1s and O1s lines at grazing angle (us708 from the sample normal) were taken as proof of a clean Pt(111) surface. The VOx film was obtained by reactive evaporation of V (99.95% pure vanadium wire, electron beam evaporator Caburn MDC, model EB90) in a H2O atmosphere (1=10y7 Pa), while the substrate was kept at room temperature. Distilled water was used for the exposure; cleaning and degassing procedures before the exposure were achieved by repeated freezing and warming cycles. During deposition, the composition of the chamber atmosphere was monitored by a quadrupole mass spectrometer (QMS). The XPS, X-ray-excited AES, and XPD experiments were performed in a modified VG Escalab MKII photoelectron spectrometer. AlKa radiation was used for XPS, AES spectra and XPD curves. Angle-scanned XPD measurements were obtained for V 2p3y2 and O 1s photoelectron signals. The line intensities were determined after a linear background subtraction. The sample was mounted on a two-axis goniometer, which allows sweeping of the electron emission direction with an angular resolution of "18 for both polar (u, with respect to the normal to ¯ ¯x the surface) and azimuthal {w, with respect to w121 direction of Pt(111) surface} angles. The chemical composition of the layers deposited was controlled by XPS and AES at us0 and 708. The BEs were calibrated with respect to Pt 4f7y2 (71.2 eV). The V 2p and O 1s

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Fig. 1. The ratios R(u)V 2pyPt 4f and R(u)O 1syPt 4f of integral peak intensity as a function of the polar angle u for a VOx film grown after 35 min of reactive deposition (ca. 7 ML).

XPD data were analysed by means of multiple-scattering cluster–spherical wave (MSC-SW) simulations calculated with the Multiple Scattering Calculation of Diffraction (MSCD) program, a package for the angle-scanned XPD simulation based on the Rehr–Albers (R–A) approximation w11x recently developed by Chen and Van Hove w12x. VO clusters containing approximately 290 atoms were used (see discussion). Convergence on multiple scattering and R-A orders was achieved by propagating scattering events to the eighth order and using (6=6) scattering matrices, respectively. Inelastic mean free paths for calculating inelastic attenuation of electron amplitudes were obtained by means of the Tanuma–Powell–Penn formula, known as TPP-2 w13x. Effects due to correlated vibrational damping, inner potential refraction at the surface and instrumental angular averaging have also been allowed. 3. Results and discussion The preparation procedure to obtain VOx ultrathin films has been reported in detail elsewhere w14x. The critical point is represented by a strict control of the residual water in the UHV chamber, which acts as oxidant with respect to the metallic vanadium deposit. The deposition rate and growth mode of the film were studied by an ARXPS experiment involving the peak intensity analysis for V 2p3y2, O 1s and Pt 4f lines (intensity of each peak was calculated after a linear background subtraction). The V 2p intensity was calculated by multiplying the values measured for the V 2p3y2 component by its branching ratio. Fig. 1 shows the ratios of V 2pyPt 4f wR(u)V 2pyPt 4fx and O 1syPt 4f wR(u)O 1syPt 4fx integral peak intensities as a function of the polar angle for a film obtained after 35 min of reactive deposition. In order to minimise diffraction effects, the ratios were calculated using the average of the polar curves taken along three different azimuthal

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Fig. 2. (a) XPS V 2p and O 1s core-level spectra of the VOx film obtained after 35 min of reactive deposition (ca. 7 ML). (b) X-Ray-excited vanadium L23M45M45 AES signals of the same VOx film

¯ x to directions in a 608 athimuthal interval (from w211 ¯ ¯ x directions of the substrate). This procedure w121 removes most of the original modulations and these curves can be fitted by the following equation w15x: RŽu.s

KØw1yexpŽydyŽLV(O)ØsinŽu...x

wŽ1yg.qgØexpŽydyŽLPtØsinŽu...x

where K is the ratio between the V 2p (O 1s) and Pt 4f peak intensities in a layer of semi-infinite thickness, d is the layer thickness, u is the polar emission angle, LPt and LV(O) are the inelastic attenuation lengths for photoelectrons emitted from Pt 4f and V 2p(O 1s), respectively, and g is the surface fractional coverage. Constant L values were used in the fitting, calculated by the TPP-2 formula w13x, assuming that the photoelectrons emitted from the Pt substrate originate only from the last interface layer. The L values adopted were ˚ for Pt 4f, O 1s and V 2p, respectively. 20, 17 and 17 A The cluster thickness d and the surface fractional coverage g were obtained from the fitting procedure. The best-fit values for the two curves were ds19 (V 2py ˚ (O 1syPt 4f); gs1.0 is identical for Pt4 f) and 21 A the two curves and indicates a layer-by-layer growth. From these data, a deposition growth rate of the order ˚ of 0.5 Aymin for VO(111) orientation (ca. 0.2 MLy min) can be estimated. The higher value estimated for the thickness of the film in the case of oxygen is probably due to the higher oxygen content on the surface of the film, and assuming a VOx(111) surface (see XPD data reported below), suggests an O-terminated surface. The XPS and X-ray-excited AES data for the 7-MLthick film are reported in Fig. 2. A complete discussion of these data has been reported in w14x. The BE for the V 2p3y2 peak (512.8 eV) (see Fig. 2a), is higher than the typical values (512.2–512.4 eV) for bulk metallic vanadium samples and lower than the values found for bulk V2O3 oxide (515.1–515.9 eV) w16,17x. This value

is in good agreement with the BE found for a bulk VOx sample w18x, obtained by the arc method from metal and V2O5 oxide, for which the V 2p3y2 peak was found at 513.5 eV. A VOx epitaxial film obtained using UHV annealing of metallic vanadium deposited on TiO2(110) w7x also showed V 2p3y2 at 513.5 eV. The BE of the O 1s peak (531.2 eV) is higher than that found for V2O3 (529.6–530.1 eV) w17x and closer to the values found for VO, 530.8 w8x and 531.1 eV w18x. The oxidation state of vanadium oxides can also be estimated from X-ray-excited AES spectra w16x. The Auger V L23M23M45 spectrum can be used as a finger print, because it has a significantly different structure for various oxidation states, due to changes in the 3d density of states in the valence region. Fig. 2b shows the X-ray-excited AES V L23M23M45 spectra for the 7ML vanadium oxide film obtained in the present experiment. Three components can be identified in the vanadium oxide spectrum. The low-intensity feature at Eks478 eV is due to an oxygen KL1L1 transition. Peak A of the V L23M23M45 Auger line at Eks472 eV originates from the d-band emission close to the Fermi level, which is not occupied in V2O5 w16x. The feature marked B at Eks468 eV originates from the valence band with V 3d–O 2p character, and of course is not present in the metallic vanadium. The position and relative intensity of the peaks reported are indicative of an oxidation state lower than V(III) w16x. Once demonstrated that the film is formed by V(II) oxide, we then focused our attention to the structural aspects. A diffuse, six-fold LEED pattern (Es76 eV) is apparent, even after 70 min of reactive deposition (ca. 14 ML). This evidence indicates that the film is growing epitaxially to the substrate, but it is characterised by a low degree of long-range order. The diffuse character of the LEED pattern did not allow us to determine the surface lattice constants.

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On the contrary, the XPD oscillations are rather strong, indicating that the film grows with a high degree of short-range order. A complete azimuthal scan (3608) (not reported) shows that the pattern has a three-fold symmetry axis. This evidence demonstrates that the film grows as a single domain structure. Fig. 3 shows the XPD 2p plots for (a) V 2p3y2 and (b) O 1s photoemission for the 7-ML-thick film. The plots were obtained by measurements of polar scans in the 608 sector ¯ ¯ x and w211 ¯ x directions of the Pt(111) delimited by w121 surface at steps of 38 in azimuthal angle and of 28 in polar angle, and by reflecting the data about the appropriate surface directions to achieve a full 2p projection. The 2p intensity maps in Fig. 3 are plotted as x functions, defined as xs(IyI0)yI0 , where I is the absolute intensity measured for the diffraction pattern and I0 is a smooth background obtained by a polynomial fitting of the azimuthal average of I. The 2p XPD plots reported in Fig. 3a,b show very similar diffraction patterns, and this is a clear indication that the V and O crystal sites are equivalent, as expected for a rock-salt structure. Before discussing the results of the MS simulations, it is useful to have a qualitative interpretation of the XPD pattern on the basis of the forward-scattering (FS) events. In Fig. 4, a view of the VO structure, exposing the cross-section of the rock-salt ¯ ¯ x and w211 ¯ x is report(111) surface along directions w211 ed. As indicated by the arrows, three FS directions are easily identified. The first one at us358 with respect to the surface normal (w011x rock-salt crystal direction) corresponds to the direction of V–V (or O–O) atomic rows contained in the f.c.c. V (or O) sub-lattice. The second corresponds to the V–O bond and is found at us558 (w100x rock-salt crystal direction) from the normal. The third direction is found at 708 from the ¯ x crystal direction) and also corresurface normal (w111 sponds to a V–O direction (the distance is longer than that corresponding to the w100x direction). The V–V (O–O) and V–O (O–V) FS peaks are easily identified in the experimental V 2p3y2 and O 1s 2p plots (Fig. 3a,b). The V–O feature is a very intense peak centred at 588 from the surface normal for V 2p3y2 and at 578 ¯ ¯ x (or for the O 1s signal, and aligned with the w121 equivalent) azimuth of the Pt(111) surface for both signals. The peak identifying the V–V (O–O) rows is found in at us378 for V 2p3y2 and at us368 for O1s, ¯ x (or equivalent) azimuth of and aligned with the w211 the Pt(111) surface. The less intense diffraction feature expected at us708, is actually found at us728 for both ¯ x (or O 1s and V 2p3y2, and aligned with the w211 equivalent) azimuth of the Pt surface. In order to extract structural information, MS simulations of the XPD 2p patterns were performed on a cluster formed by a sequence of six layers: three V

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layers and three oxygen layers. We first performed preliminary simulations assuming a VO structure with a ˚ and with an intersurface lattice parameter of 2.77 A ˚ layer spacing of 1.13 A (both bulk-like values). Two models were assumed: in the first one, the surface was O-terminated, while the second one assumed a Vtermination. As is evident in Fig. 3, where the 2p patterns calculated are compared with the experimental data for both V 2p and O 1s signals, the experimental data are satisfactorily reproduced only if an O-terminated surface is assumed (Fig. 3c–f). This is particularly evident in the V 2p 2p plots calculated (the vanadiumterminated simulation has rather different relative intensity value compared to experiments). A quantitative comparison with the experiment can be carried out by calculating the R factor, defined as: 2 2 .x Rs8iwŽxth,iyAxexp,i.2yŽxth,i qAxexp,i

where xth and xexp are the theoretical and experimental 2p plots, respectively, and A is a scaling factor that minimises the quantity R which takes into account the anisotropy differences between experimental and theoretical data w19x. The calculated R factors are 0.35 and 0.46 for an O-terminated surface for V 2p and O 1s emission, respectively, while for a V-terminated structure, the values are 0.40 and 0.48, respectively. We then attempted to obtain information on the actual lattice dimensions of the ultrathin film by systematically ˚ Howvarying the lattice constant from 2.92 to 2.77 A. ever, we only found a variation in the R factor of ca. 2%, which is not sufficient to draw any conclusion. We also attempted to test a (2=2) surface reconstruction, following the suggestion reported in w4,5x, but no improvement in the R factor was found. Thus, any further refinement of the structure was performed by ˚ or in other assuming a lattice parameter of 2.77 A, words, we assumed a pseudomorphic growth. Surface relaxation of the first few atomic layers is very often found in (111) surfaces of materials possessing a rock-salt structure. A shorter inter-layer spacing, where each layer is either a cation or anion layer, reduces a build-up of charge at the surface. Typical examples of this behaviour are found in similar systems, such as CoO, FeO and MnO w20–22x. XPD is rather sensitive to this parameter since, as discussed above, the polar angles corresponding to the V–O and V–V FS directions can be directly correlated with the interlayer distance. The FS peak corresponding to the V–O bond was found experimentally at us588 for V 2p3y2 and at us578 for the O 1s signal. The value expected for a bulk-terminated structure is 558. In the case of an Oterminated surface, the position of the V–O FS peak relative to the V 2p signal should be particularly sensitive to the inter-layer distance between a V and an oxygen layer. Thus, the distance found by a simple

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Fig. 3. The XPD 2p plots for (a) V 2p3y2 and (b) O 1s photoemission lines of the VOx film obtained after 35 min of the reactive deposition (ca. 7 ML). Reference directions are given with respect to the Pt(111) surface. Simulated XPD patterns are also reported for (c, e) V 2p and (d, f) O 1s lines.

trigonometric calculation, assuming the V–O bond to be ˚ (1.12 A ˚ is the value for a at 588 off normal, is 0.99 A non-relaxed structure, and according to the angular resolution of the goniometer, the experimental error of ˚ the inter-layer distance determination is 0.04 A). A quick look to the VO structure (side view) reported in Fig. 4 shows that the interlayer distance between two cation (or two anion) layers is directly correlated to the FS peak corresponding to the V–V direction. As written

above, this peak, expected at 358, is found at 378, so that the cation interlayer distance calculated would be ˚ We ran several calculations where the interlayer 2.12 A. distance between the first four atomic layers (this corresponds to two oxide layers) was systematically varied, and found a minimum in the R factor corresponding to an interlayer distance between the first four ˚ The R factors calculated were: atomic layers of 0.99 A. 0.35 for a pseudomorphic unrelaxed structure; 0.30 for

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gratefully acknowledged for their skilful technical assistance. References

Fig. 4. Side view of VO(111) surface. The main crystal directions are shown.

a structure where distance between the outer two layers ˚ 0.28 for a structure where the was relaxed to 0.99 A; ˚ and distance between the last three layers was 0.99 A; finally the minimum, Rs0.26, was found for three relaxed interlayer distances. In other words, the minimum R factor was found by assuming a relaxed surface where the interlayer spacing between the first four ˚ with an estimated atomic layers was the same: 0.99 A ˚ error of approximately 0.05 A. The corresponding V–O ˚ with a 7% contraction of the bond distance is 1.88 A, bond distance with respect to the distance of bulk VO. Acknowledgements This work has been partially funded by ‘Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II’ of the CNR, Rome, and by Ministero della Ricerca Scientifica e Tecnologica (MURST) through the National Program ‘Strati ultrasottili di ossidi e solfuri inorganici: crescita, caratterizzazione e reattivita` superficiale’. Mr Giuseppe Greggio and Mr Claudio Comaron are

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