Vanadium Oxides: Synthesis, Properties, and Applications

Abstract

Correlated electrons in vanadium oxides are responsible for their extreme sensitivity to external stimuli such as pressure, temperature, or doping. As a result, several vanadium oxides undergo insulator-to-metal phase transition (IMT) accompanied by structural change. Unlike vanadium pentoxide (V2O5), vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) show IMT in their bulk phases. In this study, we have performed one-electron Kohn–Sham electronic band structure calculations of VO2, V2O3, and V2O5 in both metallic and insulating phases, implementing a full Ab initio simulation package based on Density Functional Theory (DFT), plane waves, and pseudopotentials (PPs). Electronic band structures are found to be influenced by crystal structure, crystal field splitting, and strong hybridization between O2p and V3d bands. “Intermediate bands” with narrow bandwidths, lying just below the higher conduction bands, are observed in V2O5 which play a critical role in optical and thermoelectric processes. Similar calculations are performed for both metallic and insulating phases of bulk VO2 and V2O3. Unlike in the metallic phase, bands corresponding to “valence electrons” considered in the PPs are found to be fully occupied in the insulating phases. Transport parameters such as Seebeck coefficient, electrical conductivity, and thermal (electronic) conductivity are studied as a function of temperature at a fixed value of the chemical potential close to the Fermi energy using Kohn–Sham band structure approach coupled with Boltzmann transport equations. Because of the layered structure and stability, only V2O5 shows significant thermoelectric properties. All the transport parameters have correctly depicted the highly anisotropic electrical conduction in V2O5. Maxima and crossovers are also seen in the temperature-dependent variation of Seebeck coefficient in V2O5, which can be consequences of “specific details” of the band structure and anisotropic electron–phonon interactions. For understanding the influence of phase transition on transport properties, we have also studied transport parameters of VO2 for both metallic and insulating phases. The Seebeck coefficient, at experimental critical temperature of 340 K, is found to change by 18.9 µV/K during IMT, which lies within 10% of the observed discontinuity of 17.3 µV/K. Numerical methods have been used to analyze the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates from infrared to vacuum ultraviolet range (up to 12 eV). The energies corresponding to the peaks in the reflectivity-energy (R-E) spectra are explained in terms of the Penn gap and the degree of anisotropy is found to be in the order of V2O3 < VO2 < V2O5. The effective number of electrons participating in the optical transitions is described using the “sum rule”. The optical absorption is found to occur followed by the transitions of d-electrons as well as the transitions from O2p to V3d states. The methods of preparation of vanadium oxides have been discussed. These include the sol–gel method, sputtering, chemical vapor deposition, and laser ablation. In the Honeywell microbolometer structure, the bolometer sensing element has been chosen to be VOx, with x equal to 1.8, along with other layers of Si3N4, air, Al and Si. The room temperature spectral emissivity of such a layered structure is analyzed using Multi-Rad, a simulation package that utilizes thin film optics in the form of matrix method of multilayers. Calculations show that the Si3N4 layer provides the much desired linear performance of the VOx-based bolometer. Applications of vanadium oxides have been summarized. Examples of application such as bolometers, coatings, thermistors, and batteries have been discussed.