An ARPEFS study of the structure of an epitaxial VO2 monolayer at the TiO2(110) surface

Sambi, Mauro; Negra, Michela Della; Granozzi, G.; Li, Z.S; Jørgensen, John; Møller, Preben J.

doi:10.1016/s0169-4332(98)00725-9

Cited by 32 publications

(22 citation statements)

References 12 publications

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“…To remove contaminations present on the surface of the powder particles, the supporting metal grid was heated to 700 K for 60 min under UHV conditions, as used in previous works112023. For the single crystal experiments a r-TiO 2 (110) specimen (MATECK GmbH, Jülich, Germany) with a size of 10 × 10 mm 2 was cleaned using previously published preparation procedures131 including Ar + ion sputtering and annealing steps until the sample was a clean (as judged from XPS data), well-ordered (as judged from the presence of well-defined spots in the LEED-pattern) and largely defect-free (as judged from the comparison of UPS-data with those reported in32). Exposure to H atoms was carried out by operating a hot tungsten filament in line of sight of the titania samples in an atmosphere of 1 × 10 −6 –5 × 10 −7 mbar of H 2 , as described previously33.…”

Section: Methodsmentioning

confidence: 99%

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

Sezen

Buchholz

Nefedov

et al. 2014

Sci Rep

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An important step in oxide photochemistry, the loading of electrons into shallow trap states, was studied using infrared (IR) spectroscopy on both, rutile TiO2 powders and single-crystal, r-TiO2(110) oriented samples. After UV-irradiation or n-doping by exposure to H-atoms broad IR absorption lines are observed for the powders at around 940 cm−1. For the single crystal substrates, the IR absorption bands arising from an excitation of the trapped electrons into higher-lying final states show additional features not observed in previous work. On the basis of our new, high-resolution data and theoretical studies on the polaron binding energy in rutile we propose that the trap states correspond to polarons and are thus intrinsic in nature. We assign the final states probed by the IR-experiments to hydrogenic states within the polaron potential. Implications of these observations for photochemistry on oxides will be briefly discussed.

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Section: Methodsmentioning

confidence: 99%

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

Sezen

Buchholz

Nefedov

et al. 2014

Sci Rep

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“…[17][18][19][20][21] This process is similar in nature to Al Cho's early GaAs films grown using "epitaxy by periodic annealing;" 22 epitaxy by periodic annealing has also been used to grow epitaxial SrTiO 3 on Si. 23,24 No electrical transport measurements on the VO 2 films made by this procedure were reported by Sambi et al [17][18][19][20][21] Our films grown on TiO 2 (001) substrates by MBE following this procedure [17][18][19][20][21] were epitaxial, but did not exhibit an MIT. Only after utilizing ~ 80% pure distilled ozone 25 in place of molecular oxygen was an MIT observed.…”

Section: -13mentioning

confidence: 98%

“…We began with the procedure described by Sambi et al for producing epitaxial VO 2 thin films on TiO 2 (110). [17][18][19][20][21] The key aspect of this procedure was the deposition of 0.2-0.5 monolayers (ML) of amorphous vanadium metal at room temperature followed by a 2 minute anneal at 423 K in 7.5×10 -7 -1.5×10 -6 Torr of oxygen during which it transforms into an epitaxial VO 2 layer. One ML corresponds to 5.2×10 14 vanadium atoms / cm 2 for growth on TiO 2 (110).…”

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confidence: 99%

Epitaxial growth of VO2 by periodic annealing

Tashman

Lee

Paik

et al. 2014

Applied Physics Letters

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We report the growth of ultrathin VO 2 films on rutile TiO 2 (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO 2 via solid-phase epitaxy.Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change ΔR/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm. Page 4 of 25The huge metal-insulator transition (MIT) exhibited by VO 2 in the vicinity of roomtemperature has made it a material of interest for uncooled microbolometer arrays, 1 gas sensing, 2 optical limiting, 3 and most recently MIT transistors. 4,5 In bulk single crystals this MIT occurs at a transition temperature (T c ) of 340 K and is accompanied by a change in structure from a hightemperature tetragonal form, to a low-temperature monoclinic form. 6 The change in resistivity through this transition in bulk VO 2 single crystals has been measured to be five orders of magnitude with a temperature hysteresis 0.5-1 K. 7 The change in resistivity in thick films (>100 nm) can be as high as four orders of magnitude, [8][9][10] but in thin films (<10 nm) is less than three orders of magnitude in all reports to date. 11-13While VO 2 presents an opportunity for emergent switching devices and sensors, its large carrier concentration (~10 In this paper we describe a process for the growth of ultrathin VO 2 films by reactive molecular-beam epitaxy (MBE) and show that they exhibit clear MITs in films as thin as 2.3 nm.We investigate the properties of these films with four-circle x-ray diffraction (XRD), low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and electronic transport measurements. We also describe a standardized method for calculating the magnitude, hysteresis, and T c of these transitions using Savitsky-Golay smoothing derivatives. 16 All films in this paper were grown by MBE in a Veeco Gen10. X-ray diffraction spectra were collected with a Rigaku Smartlab system utilizing Cu Κ α1 radiation with a 220 Ge twobounce incident-beam monochromator and a 220 Ge two-bounce diffraction side analyzer crystal. STEM images were taken with an FEI Tecnai G 2 F20. VO 2 film thicknesses were calculated with data from Rutherford backscattering spectrometry (RBS) assuming the calibration films had bulk VO 2 density. Electrical transport data was taken using the standard four-contact van der Pauw method in a Quantum Design Physics Property Measurement System (PPMS) with contacts made using gold wire and silver paint. All growth temperatures were measured using a thermocouple in the substrate cavity, but not in contact with the substrate.During growth the film was monitored using reflection ...

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“…As far as we know, the first study on VO 2 / TiO 2 film was reported by Z Zhang et al in 1992, which focused on the interfacial electronic interactions [27]. Later, Sambi et al successfully grew ultrathin epitaxial VO 2 on (110) TiO 2 surface and the room temperature phase of the VO 2 /TiO 2 films were regarded as 'pseudomorphic to substrate' with semiconductive character based on the data of XPD, LEED and angle resolved photoemission extended fine structure [28,29]. In 2001, Muraoka et al grew VO 2 thin films on the TiO 2 (001) and (110) substrates, and revealed that compressive strain along c-axis will induce reduction in the MIT transition temperature.…”

Section: Introductionmentioning

confidence: 99%

The tetragonal-like to rutile structural phase transition in epitaxial VO₂/TiO₂(001) thick films

et al. 2015

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A controllable metal-insulator transition (MIT) of VO 2 has been highly desired due to its huge potential applications in memory storage, smart windows or optical switching devices. Recently, interfacial strain engineering has been recognized as an effective approach to tuning the MIT of epitaxial VO 2 films. However, the strain-involved structural evolution during the MIT process is still not clear, which prevents comprehensively understanding and utilizing interfacial strain engineering in VO 2 films. In this work, we have systematically studied the epitaxial VO 2 thick films grown on TiO 2 (001) single crystal substrate and the structural transition at the boundary of MIT region. By using in situ temperature-dependent high-resolution x-ray diffractions, a tetragonal-like ('T-like') to 'rutile' structural phase transition is identified during the MIT process. The room-temperature crystal phase of epitaxial VO 2 /TiO 2 (001) thick film is clarified to be tetragonal-like, neither strained-rutile phase nor monoclinic phase. The calculated atomic structure of this T-like phase VO 2 resembles that of the M1 phase VO 2 , which has been verified by their similar Raman spectra. More, the crystal lattices of the coexisted phases in the MIT region were revealed in detail. The current findings will not only show some clues on the MIT mechanism study from the structural point of view, but also favor the interface engineering assisted VO 2 -based devices and applications in the future. OPEN ACCESS RECEIVED

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An ARPEFS study of the structure of an epitaxial VO2 monolayer at the TiO2(110) surface

Cited by 32 publications

References 12 publications

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

Epitaxial growth of VO2 by periodic annealing

The tetragonal-like to rutile structural phase transition in epitaxial VO₂/TiO₂(001) thick films

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An ARPEFS study of the structure of an epitaxial VO2 monolayer at the TiO2(110) surface

Cited by 32 publications

References 12 publications

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

Epitaxial growth of VO2 by periodic annealing

The tetragonal-like to rutile structural phase transition in epitaxial VO2/TiO2(001) thick films

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The tetragonal-like to rutile structural phase transition in epitaxial VO₂/TiO₂(001) thick films