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Abstract Straining the vanadium dimers along the rutilec‐axis can be used to tune the metal‐to‐insulator transition (MIT) of VO2but has thus far been limited to TiO2substrates. In this work VO2/MgF2epitaxial films are grown via molecular beam epitaxy (MBE) to strain engineer the transition temperature (TMIT). First, growth parameters are optimized by varying the synthesis temperature of the MgF2(001) substrate (TS) using a combination of X‐ray diffraction techniques, temperature dependent transport, and soft X‐ray photoelectron spectroscopy. It is determined thatTSvalues greater than 350 °C induce Mg and F interdiffusion and ultimately the relaxation of the VO2layer. Using the optimized growth temperature, VO2/MgF2(101) and (110) films are then synthesized. The three film orientations display MITs with transition temperatures in the range of 15–60 °C through precise strain engineering.
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Dynamics of the blue pump-induced ultrafast insulator-to-metal transition and relaxation in VO 2 /TiO 2 and VO 2 /TiO 2 :Nb thin films
We study the ultrafast time resolved response of 30 nm films of VO2on a TiO2substrate when 3.1 eV (400 nm wavelength) pump pulses were used to excite the insulator to metal transition (IMT). We found that the IMT threshold for these samples (≤30µJ/cm2) is more than 3 orders of magnitude lower than that generally reported for a more traditional 1.55 eV (800 nm wavelength) excitation. The samples also exhibited unusual reflectivity dynamics at near-threshold values of pump fluence where their fractional relative reflectivity ΔR/R initially increased before becoming negative after several hundreds of picoseconds, in stark contrast with uniformly negative ΔR/R observed for both higher 400 nm pump fluences and for 800 nm pump pulses. We explain the observed behavior by the interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2and use a simple diffusion model of the VO2phase transition to support qualitatively this hypothesis. We also compare the characteristics of the VO2films grown on undoped TiO2and on doped TiO2:Nb substrates and observe more pronounced reflectivity variation during IMT and faster relaxation to the insulating state for the VO2/TiO2:Nb sample.
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- Award ID(s):
- 1827536
- PAR ID:
- 10154704
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optical Materials Express
- Volume:
- 10
- Issue:
- 6
- ISSN:
- 2159-3930
- Format(s):
- Medium: X Size: Article No. 1393
- Size(s):
- Article No. 1393
- Sponsoring Org:
- National Science Foundation
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