Substrate-induced microstructure effects on the dynamics of the photo-induced metal–insulator transition in VO2 thin films (original) (raw)
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Physical Review B, 2013
We present a detailed study of the photoinduced insulator-metal transition in VO2 with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When exciting well above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the non-equilibrium transition, we observe that the electronic properties of the material evolve on a different, slower, timescale. This separation of timescales suggests that the early state of VO2, immediately after photoexcitation, is a non-equilibrium state that is not well defined by either the insulating or metallic phase.
Light-induced ultrafast phase transitions in VO 2 thin film
Applied Surface Science, 2006
Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68°C. As a kind of functional material, VO2 thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400nm on the
Substrate Effect on Optical Properties of Insulator-Metal Transition in VO2 Thin Films
2012
We have used Raman spectroscopy to investigate the optical properties of vanadium dioxide (VO 2 ) thin films deposited on different substrates during the thermally induced insulating to metallic phase transition. We observed a significant difference in transition temperature in VO 2 films similarly grown on quartz and sapphire substrates: the film grown on quartz displayed the phase transition at a lower temperature (T c =50 o C) compared a film grown on sapphire (T c =68 o C). We also investigated differences in the detected Raman signal for different wavelengths and polarizations of the excitation laser. We found that for either substrate, a longer wavelength (in our case 785 nm) yielded the clearest VO 2 Raman spectra, with no polarization dependence.
Applied Optics, 2015
Measurements of ultrafast light scattering within a hemisphere are performed for statistical analysis of nonequilibrium processes in VO 2 epitaxial film. A Gerchberg-Saxton error reduction algorithm is applied for accurate calculation of a surface autocorrelation function from light scattering data and for partial reconstruction of a power spectral density function. Upon ultrafast photoinduced phase transition of VO 2 , the elastic light scattering reveals anisotropic grain-size-dependent dynamics. It was found that the transition rate depends on the optical absorption and orientation of VO 2 grains with respect to polarization of the pump pulse. An observed stepwise evolution of surface autocorrelation length and transient anisotropy of the scattering field presumably originates from complex multistage transformation of VO 2 lattice on a subpicosecond time scale.
2005
We investigated optical absorption coefficient spectra of an epitaxial VO 2 film in wide photon energy (0.5-5.0 eV) and temperature (100-380 K) regions. In its insulating phase, we observed two d-d transition peaks around 1.3 eV and 2.7 eV and a charge transfer peak around 4.0 eV. As temperature goes above the metal-insulator transition temperature near 340 K, a large portion of the spectral weight of the peak around 4.0 eV becomes redistributed and a Drude-like peak appears. We initially applied the band picture to explain the details of the spectral weight changes, especially the temperature-dependent shift at 2.7 eV, but failed. To check whether the spectral changes are optical signatures of the electron-electron correlation effects, we applied the Hubbard model which takes into account orbital degeneracy. This orbitally degenerate Hubbard model could explain the details of the temperature-dependent peak shifts quite well. In addition, from the peak assignments based on the orbitally degenerate Hubbard model, we could obtain the values of U + ∆ (~ 3.4 eV) and J H (~ 0.7 eV), where U, ∆, and J H are the on-site Coulomb repulsion energy, the crystal field splitting between the t 2g bands, and the Hund's rule exchange energy, respectively. Our spectroscopic studies indicate that the electron-electron correlation could play an important role in the metal-insulator transition of VO 2 .
Abrupt metal–insulator transition observed in VO2 thin films induced by a switching voltage pulse
Physica B: Condensed Matter, 2005
An abrupt metal-insulator transition (MIT) was observed in VO2 thin films during the application of a switching voltage pulse to two-terminal devices. Any switching pulse over a threshold voltage for the MIT of 7.1 V enabled the device material to transform efficiently from an insulator to a metal. The characteristics of the transformation were analyzed by considering both the delay time and rise time of the measured current response. The extrapolated switching time of the MIT decreased down to 9 ns as the external load resistance decreased to zero. Observation of the intrinsic switching time of the MIT in the correlated oxide films is impossible because of the inhomogeneity of the material; both the metallic state and an insulating state co-exist in the measurement volume. This indicates that the intrinsic switching time is in the order of less than a nanosecond. The high switching speed might arise from a strong correlation effect (Coulomb repulsion) between the electrons in the material.
Optical limiting in pulsed laser deposited VO2 nanostructures
Optics Communications, 2012
Being a Mott type oxide, at a temperature of~68°C and ambient pressure, stoichiometric VO 2 undergoes a first order metal-insulator transition, which is accompanied by a reversible abrupt change in the band gap opening. From an optical point of view, this metal-insulator transition manifests itself by a significant and reversible variation of the refractive index under either a thermal stimuli or by photo-induction. This contribution reports on the ultrafast optical limiting in the IR regime of pulse laser deposited VO 2 nanostructures.
Journal of Materials Science: Materials in Electronics, 2017
of energy-related applications [1, 2]. It undergoes an abrupt reversible phase transition, known as metal-to-insulator transition (MIT) or semiconductor-to-metal (SMT) firstorder transition. At temperatures below the transition temperature (T MIT), VO 2 is in semiconducting state with monoclinic structure (space group P2 1 /c), in which the V atoms pair open an energy gap of 0.6 eV. While at temperatures above T MIT , VO 2 is in metallic state with rutile-tetragonal structure (space group P4 2 /mnm), in which overlap between the Fermi level and the V 3d band eliminates the band gap [3]. Most notably, this allotropic transition in crystal symmetry and electronic band structure, which can be triggered by some specific external stimuli such as temperature or voltage, was usually accompanied by an abrupt and dramatic change in physical properties. For example, the electrical resistance jumps up to four orders of magnitude and the optical transmission shows a distinct switching effect especially within the infrared wavelength region across the MIT boundary. With these unique and fascinating properties, VO 2 was considered to be one of the most promising energy-saving material for a wide range of energy-related applications, including smart windows for energy utilization, supercapacitors for energy storage, and thermoelectric generators for energy conversion. However, the intrinsic T MIT for bulk single crystals VO 2 has been well accepted to be ~341 K, the specific applications will be seriously limited by the relatively rigid T MIT [4]. Successfully modulating the T MIT to room temperature(RT) has been a longstanding research topic. For quite a long time, intentionally impurity doping had been commonly attempted to achieve controllable T MIT since both shifts towards lower and higher temperature can be realized by selecting appropriate dopant and ratios, i.e., doping with high-valent metal cations (such as W 6+ ,Nb 5+ ,Mo 6+) were reported to lower the T MIT , while doping with low-valent metal cations (such as Abstract VO 2 films were grown on TiO 2 (001) substrate by a radio frequency (RF)-plasma assisted oxide molecular beam epitaxy. An excellent reversible metal-to-insulator (MIT) transition accompanied with an abrupt change in both resistivity and infrared transmittance was observed at room temperature (RT), which was much lower than the 341 K for bulk single crystal VO 2. Remarkably, the MIT transition temperature (T MIT) deduced from resistivity-temperature curve was well consistent with that obtained from the temperature dependent IR transmittance. The lowed T MIT was supposed to be originated from the internal stress induced by the interface lattice mismatch between VO 2 film and TiO 2 substrate, this assumption was supported by both Raman measurement and X-ray diffraction (XRD) 2theta peak shift. This achievement will potentially open up new opportunities for advanced applications of VO 2-based devices where RT MIT is necessary.
Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide
Physical Review B, 2012
We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (VO 2) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in VO 2 is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales.