In order to investigate band gap tunability in polar oxides, we measured the optical properties of a series of Bi͑Fe 1−x Mn x ͒O 3 thin films. The absorption response of the mixed metal solid solutions is approximately a linear combination of the characteristics of the two end members, a result that demonstrates straightforward band gap tunability in this system.

Phase-pure, stoichiometric, unstrained, epitaxial ͑001͒-oriented EuTiO 3 thin films have been grown on ͑001͒ SrTiO 3 substrates by reactive molecular-beam epitaxy. Magnetization measurements show antiferromagnetic behavior with T N = 5.5 K, similar to bulk EuTiO 3. Spectroscopic ellipsometry measurements reveal that EuTiO 3 films have a direct optical band gap of 0.93Ϯ 0.07 eV.

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 ...