Optical Properties Of VO2 Research Articles

Controllable preparation of VO2@m-SiO2 and its light transmittance and thermal insulation properties of PVB composite films

In this study, a hybrid approach using the Stöber and solvent extraction techniques was employed to synthesize mesoporous silicon-coated VO2 nanomaterials (VO2@m-SiO2, VmS) with controllable pore sizes and shell thicknesses. This synthesis was aimed at elucidating the influence of mesoporous silicon coatings on the thermochromic effect and optical properties of VO2. Experimental findings reveal that the mesoporous silica architecture markedly improve the phase-transition hysteresis of VO2 while mitigating the Mie effects attributable to particle coarsening, under the appropriate shell thickness. Furthermore, concomitant with the enlargement of the mesoporous aperture, the Tlum and ΔTsol of the VmS/PVB films also exhibit upward trends. Tlum reaches 73.33% and 62.24% in VmS3/PVB and VmS5/PVB, peak ΔTsol reaches 5.16% in VmS5/PVB. The facile synthesis procedure of VmS, in conjunction with its superior performance across phase transition, optics, and thermal insulation metrics, offers a viable pathway for industrial-scale deployment of VO2(M) in smart window applications.

Role of 4f electrons and 3d-4f hybridization in metal-insulator transition in RE (La, Nd, Sm, Eu, Dy and Er)-doped vanadium dioxide for thermochromic applications

Vanadium dioxide is a promising thermochromic material for novel applications to smart optoelectronic devices due to its reversible insulator-to-metal transition which is accompanied by rapid variation in its optical properties. In this work, rare-earth (La, Nd, Sm, Eu, Dy and Er) dopants substituting vanadium site were investigated in detail to control the phase transition temperature (TMIT) and optical properties of VO2. It is found that TMIT is mainly related to the nature of 3d-4f hybridization in RE-doped VO2(M1): the direct overlapping of 3d-4f orbitals contributes to V–V dimerization, but it is less pronounced to lower TMIT; the indirect 3d-4f interaction via O-2p makes 4f electrons itinerant to egπ states to assist MIT from M1 to R phase at lower temperatures. The dominating nature of 3d-4f hybridization varies with changing RE dopant's ionic radius and electronegativity. Er offered the lowest TMIT with a decreasing rate of 12.8 °C/at. % due to effective 4f-O2p-3d hybridization. RE dopants were also found effective in changing optical properties by scaling Eg1 and Eg2 bandgaps of VO2. Hence, the fashion of 3d-4f interaction in RE-doped VO2 is found to play important role in modulating thermochromic properties of VO2 for smart window applications and smart optoelectronics devices.

Phase Transition and Optical Properties of VO2 and Al: ZnO/VO2 Thin Films

Thermochromic Vanadium dioxide (VO2) has strong potential for smart window applications but its commercial scale usage is limited due to low visible light transmission. To address this issue, aluminum doped zinc oxide (AZO) anti-reflecting layer is integrated with VO2 layer in the present work. VO2 single layer and AZO/VO2 bilayer thin film samples were deposited by sputtering technique on quartz substrate. The single-phase growth of VO2 and AZO in single layer and bilayer thin film samples is confirmed by X-ray diffraction measurements. Monoclinic M1 phase of VO2 is detected in VO2 and AZO/VO2 thin film samples at room temperature. Monoclinic to rutile structural phase transition (SPT) in these samples is studied by performing temperature-dependent X-ray diffraction measurements. SPT in VO2 thin film samples is close to 68 °C and SPT temperature appears slightly lower in AZO/VO2 sample as compared to VO2 sample. Spectral transmittance measurement at room temperature showed significant improvement in the visible transmittance of AZO bilayer film than that of single layer VO2 thin film. These results demonstrate the possibility of integration of anti-reflecting layers such as AZO with VO2 layer for better visible transmittances suitable for large-scale smart window applications.

W-Nb Co-Doped VO<sub>2</sub> Films Realizing near Room-Temperature Transition and Satisfactory Thermochromic Performance for Smart Window

The W-Nb co-doped VO2 films are prepared through hydrothermal method. The effects of the Nb and W dopants are investigated respectively on the phase transition temperature (θc) and optical properties of VO2 by keeping the concentration of partner dopant at 1.0 at.%. The Nb doping induces a reduction of θc at a rate of ~ -13.0 °C per at.% Nb as Nb is less than ~1.5 at.%. For more than 1.5 at.% Nb, the θc shows a slight increase from ~23.0 °C. The W doping leads to a linear decrease of θc with a rate of ~ -17.2 °C per at.% W, more effective in reducing θc than the Nb dopant. However, the heavy W doping results in more serious deterioration of the solar energy modulation (ΔTsol) than the Nb doping. Therefore, taking use of the complementary advantages of W and Nb dopants, the 1.0 at.% W + 1.5 at.% Nb co-doped VO2 realizes the room-temperature transition at 23.0 °C with a satisfactory ΔTsol of ~9.6%, much better than the 1.5 at.% W + 1.0 at.% Nb co-doped which has a θc of ~22.1 °C and ΔTsol of ~5.3%. This work demonstrates the W-Nb co-doping is an effective doping formula in improving the performance of VO2 for smart window applications.

On/Off Ultra‐Short Spin Current for Single Pulse Magnetization Reversal in a Magnetic Memory Using VO2 Phase Transition

AbstractUltrafast magnetization reversal of magnetic thin films induced by one single femtosecond laser light pulse, without any applied magnetic nor electrical field, known as all‐optical switching has become one of the most attractive technologies for prospective ultrafast and low‐energy magnetic devices. A single femtosecond laser pulse switching of a ferromagnet in specific spin valves, driven by electron pulse originating from ultrafast demagnetization of a ferrimagnetic layer, has been recently demonstrated. This opens the way to ultrafast and energy‐efficient writing for magnetic memory. Here, a new functionality is presented through introducing a thin VO2 into the specific spin valve, where a switchable ultra‐short polarized electron pulse by thermally triggering the phase transition of VO2 to dynamically control the switching of the ferromagnet is experimentally achieved. It is demonstrated that the generated ultra‐short electron pulse can be free to pass through the VO2 in its metallic state while it is blocked in its insulating state. Additionally, the change on electrical and optical properties of VO2 during the phase transition leads to various possible magnetic states. On/off ultra‐short spin current controlled by phase transition paves the way to address specific ultra‐fast spintronic devices in large array devices.

Influence of proton irradiation on phase variation and optical properties of VO2(B) thin film

ABSTRACT The present work is focused on the fabrication of VO2(B) thin films by magnetron sputtering followed by proton irradiation using 5 MeV tandem accelerator. For the study of phase transitions, the proton ion beam applied was to the VO2(B) thin films at the rate of 1 × 1015 ions/cm2, 2 × 1015 ions/cm2, 7.5 × 1015 ions/cm2, 5 × 1016 ions/cm2 respectively. The VO2(B) thin-film shows prominent phase variation from VO2(B) to VO2(A) along with the amorphization phenomenon with the increasing doses of proton ions. The diffused reflectance spectroscopy shows increasing trends in reflectance from 20 to 39%. Energy bandgap was determined using a Tauc plot showing efficient decreasing trends w.r.t ion beam doses. Raman spectroscopy shows vibrations in the range from 144 to 883 cm−1 with different bands, confirming the phase transition towards VO2(A) by proton ion beam irradiation. The results are useful and promising for the smart use of various phases of VO2(B) thin film.

First-principle calculation of electronic and optical properties of VO2 by GGA-1/2 quasiparticle approximation

Correctly elucidating the strong electron correlations of vanadium dioxide (VO2) is of great significance to understand the physics of the metal–insulator transition (MIT) and develop potential applications of VO2. Standard density functional theory is believed to be inappropriate to describe the MIT of VO2. Herein, the recently developed GGA-1/2 quasiparticle approximation is employed to perform first-calculations on VO2. The electronic structures of the metallic and insulating phases of VO2 are well described. The GGA-1/2 calculations indicate that the preferential occupancy of the d// orbitals leads to strong Mott–Hubbard correlation, which induces the splitting of the d// orbitals and the MIT of VO2. The calculations on electron energy-loss function reveal that the satellite electronic energy-loss spectroscopy peak of metallic phase VO2 is resulting from the plasma resonance. This work demonstrates that the GGA-1/2 approach facilitates the electron correlation calculations of VO2 and suggests that the strong Coulomb correlation is necessary to trigger the MIT.

Alternating Coupling Regimes in a Plasmon–Molecule Hybrid Structure through a Phase-Change Material

Plasmon–molecule hybrid systems provide platforms for studying light–matter interactions. It is of great interest for systems where both strong and weak plasmon–molecule couplings exist and can transfer into each other freely. In this paper, we proposed a hybrid structure composed of a vanadium dioxide (VO2) layer sandwiched between a polymethyl methacrylate (PMMA) strip array and a gold substrate. The molecular vibration of PMMA interacts with the surface plasmon polaritons (SPPs) of the metal substrate, and the coupling regimes can be alternated through the phase transition of VO2. The electric fields at SPP resonances closely relate with the optical properties of VO2. When VO2 is an insulator, high electric fields in PMMA strips cause the strong plasmon–molecule coupling and new hybrid modes are formed. When VO2 is a metal, the electric fields in PMMA are moderate due to high loss of metallic VO2 and the plasmon–molecule coupling is in the weak coupling regime. The influence of the VO2 layer thickness on coupling properties was also discussed in detail for VO2 in different phases. Our study provides a new strategy to modulate the plasmon–molecule coupling and is inspiring for fundamental and practical research studies on light–matter interactions.

Thermally-induced optical modulation in a vanadium dioxide-on-silicon waveguide

In this paper, we report phase-pure vanadium dioxide (VO2) deposition on silicon-on-insulator and demonstrate switching/modulation exploiting the phase-change property. We present electrical and optical properties of VO2 during phase transition. Exploiting the phase change property, optical modulation is achieved by thermally tuning the VO2 phase using a lateral micro-heater beside the waveguide. We achieve an optical modulation extinction of 25 dB and a low insertion loss of 1.4 dB using a ring resonator with a VO2 patch. We also demonstrate the switching performance of a symmetric Mach-Zehnder interferometer and present a detailed discussion on the optimal operating point to achieve maximum modulation, higher speed, and lower insertion loss.

VO2-based switchable radiator for spacecraft thermal control

Direct calorimetric measurements of a solid state passive switchable radiator for spacecraft thermal control have been performed in a simulated space environment. Dynamic emissivity control is provided by the thermochromic phase change in a multilayer VO2 thin film based resonant absorber. The measured radiated power difference between 300 K and 373 K was 480 W/m2 corresponding to a 7× difference in radiative cooling power. We present theoretical and experimental radiator values for both normal and hemispherical as well the optical properties of VO2 as determined via infrared spectroscopic ellipsometry.

First-principle study of electronic structure and optical properties of Au-doped VO2

The electronic structure and optical properties of VO2 and Au-VO2 were studied using density functional theory. The calculation results show that the interaction between Au and O is stronger than that between V and O. There exists not only the covalent bonding but also ionic bonding in Au—O bond. The band gap of Au-VO2 is smaller than that of VO2, while the dielectric constant, conductivity, and intensity of optical absorption of Au-VO2 are larger than those of VO2.

Atomic and electronic structures of thermochromic VO2 with Sb-doping

Vanadium dioxide (VO2) is an attractive transition metal oxide owing to its intriguing metal to insulator phase transition and has great potential for applications in smart windows. In this paper, first-principles calculations were conducted and the results revealed that Sb is as an efficient dopant in improving the thermochromic properties of VO2 by lowering the phase transition temperature and enhancing the transmission of visible light, as well as increasing the near-IR absorption of VO2. These changes in the optical properties of VO2 are closely correlated with the variations in the atomic and electronic structures, where the VV chains along the [001] direction present dimerization features in VO2(R) with Sb-doping; in addition, the OV bonds appear less ionic due to the OSb ionic bonds. The calculation results are confirmed by the subsequent experiments, suggesting that our findings would be useful for further understanding the physics of atomic doping in VO2, which may provide useful guidance for the fabrication of high-quality VO2 based smart windows.

Au doping effects on electrical and optical properties of vanadium dioxides

Vanadium dioxides were fabricated on normal glass substrates using reactive radio frequency (RF) magnetron sputtering. The oxygen flow volume and annealed temperatures as growth parameters are systematically investigated. The electrical and optical properties of VO2 and Au:VO2 thin films with different growth conditions are discussed. The semiconductor-metal phase transition temperature decreased by ∼10°C for the sample with Au doping compared to the sample without Au doping. However, the optical transmittance of Au:VO2 thin films is much lower than that of bare VO2. These results show that Au doping has a marked effect on the electrical and optical properties.

New phase transition associated with configuration of oxygen vacancies in VO1.965 nanometer ceramics

Little attention was paid to the effect of oxygen vacancy configuration, although it is well known that oxygen vacancy plays a key role in determining the electrical and optical properties of VO2. By means of different heat treatments, the oxygen vacancy configuration in VO1.965 nanometer ceramic is controlled and its behavior is investigated in detail. It is found that VO1.965 ceramic aged at 60°C for a day displays a new first order phase transformation at about 30°C, which is associated with the oxygen vacancy distribution. An explanation based on the symmetry conforming principle is tentatively presented.

Electrically programmable photonic crystal slab based on the metal-insulator transition in VO2

The possibility of an electrically programmable photonic crystal is investigated theoretically based on the metal-insulator transition of vanadium dioxide (VO2). Recent experiments indicate that this phase transition of the first order can be controlled via external bias and results in significant changes in the electrical and optical properties of VO2. We propose a slab structure based on VO2 whose dielectric properties are modulated periodically in the in-plane direction by selectively inducing phase transition through a lithographically defined array of gate electrodes. A two-dimensional photonic band calculation predicts the presence of a band gap, clearly illustrating the feasibility of the proposed structure. The electrically controlled nature may enable the VO2- based structure to be programmable at a very high speed.

Optical Properties of the Vanadium Oxides VO2 and V2O5

The optical properties of VO2 and V2O5 were determined using spectroscopic ellipsometry. The unique physical properties of these materials will be presented and discussed in conjunction with the materials electronic band structure determined by calculations using the orthogonalized linear combination of atomic orbitalsmethod.