Strain engineering of optical properties in transparent VO2/muscovite heterostructures

Linear Interplay Between Raman Shift and Laser Irradiation in Photothermal-Strained Monoclinic Vanadium Dioxide

Vanadium dioxide as a strongly correlated electron material undergoes metal-insulator phase transitions and ferroelastic domain switching which highly couple to local strain distribution. Understanding the mechanisms and achieving the modulations require precise and high-resolution characterization of strain in vanadium dioxide. Micro-Raman spectroscopy is widely used to nondestructively characterize the strain on the surface of materials. However, vanadium dioxide is sensitive to multi-fields and with multiple physical properties correlated. It is vital and challenging to uncouple the multiple responses of vanadium dioxide to micro-Raman spectroscopy and achieve precise characterization of strain distribution. Herein, a linear relation between Raman shift and laser irradiation is revealed, which is originated from photothermal strain in monoclinic vanadium dioxide. By linear fitting and extrapolation, the strain-dependent coefficient is obtained for drifting of Raman shift and the intrinsic Raman shift without strain or laser irradiation, which enables to precisely characterize the strain distribution in vanadium dioxide.

Switchable 3D Photonic Crystals Based on the Insulator-to-Metal Transition in VO2

Photonic crystals (PhCs) are optical structures characterized by the spatial modulation of the dielectric function, which results in the formation of a photonic band gap (PBG) in the frequency spectrum. This PBG blocks the propagation of light, enabling filtering, confinement, and manipulation of light. Most of the research in this field has concentrated on static PhCs, which have fixed structural and material parameters, leading to a constant PBG. However, the growing demand for adaptive photonic devices has led to an increased interest in switchable PhCs, where the PBG can be reversibly activated or shifted. Vanadium dioxide (VO2) is particularly notable for its near-room-temperature insulator-to-metal transition (IMT), which is accompanied by significant changes in its optical properties. Here, we demonstrate a fabrication strategy for switchable three-dimensional (3D) PhCs, involving sacrificial templates and a VO2 atomic layer deposition (ALD) process in combination with an accurately controlled annealing procedure. The resulting VO2 inverse opal (IO) PhC achieves substantial control over PBG in the near-infrared (NIR) region. Specifically, the synthesized VO2 IO PhC exhibits PBGs near 1.49 and 1.03 μm in the dielectric and metallic states of the VO2 material, respectively, which can be reversibly switched by adjusting the external temperature. Furthermore, a temperature-dependent switch from a narrow-band NIR reflector to a broad-band absorber is revealed. This work highlights the potential of integrating VO2 into 3D templates in the development of switchable photonics with complex 3D structures, offering a promising avenue for the advancement of photonic devices with adaptable functionalities.

Impact of the uniaxial strain on terahertz modulation characteristics in flexible epitaxial VO2 film across the phase transition

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO₂) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO₂ film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.

Flexible strategy of epitaxial oxide thin films

Applying functional oxide thin films to flexible devices is of great interests within the rapid development of information technology. The challenges involve the contradiction between the high-temperature growth of high-quality oxide films and low melting point of the flexible supports. This review summarizes the developed methods to fabricate high-quality flexible oxide thin films with novel functionalities and applications. We start from the fabrication methods, e.g. direct growth on flexible buffered metal foils and layered mica, etching and transfer approach, as well as remote epitaxy technique. Then, various functionalities in flexible oxide films will be introduced, specifically, owing to the mechanical flexibility, some unique properties can be induced in flexible oxide films. Taking the advantages of the excellent physical properties, the flexible oxide films have been employed in various devices. Finally, future perspectives in this research field will be proposed to further develop this field from fabrication, functionality to device.

Facile Solution Process of VO2 Film with Mesh Morphology for Enhanced Thermochromic Performance

The fabrication and applications of VO₂ film continue to be of considerable interest due to their good thermochromic performance for smart windows. However, low visible transmittance (T_lum) and solar modulation efficiency (∆T_sol) impede the application of VO₂ film, and they are difficult to improve simultaneously. Here, a facile zinc solution process was employed to control the surface structure of dense VO₂ film and the processed VO₂ film showed enhanced visible transmittance and solar modulation efficiency, which were increased by 7.5% and 9.5%, respectively, compared with unprocessed VO₂ film. This process facilitated the growth of layered basic zinc acetate (LBZA) nanosheets to form mesh morphology on the surface of VO₂ film, where LBZA nanosheets enhance the visible transmittance as an anti-reflection film. The mesh morphology also strengthened the solar modulation efficiency with small caves between nanosheets by multiplying the times of reflection. By increasing the zinc concentration from 0.05 mol/L to 0.20 mol/L, there were more LBZA nanosheets on the surface of the VO₂ film, leading to an increase in the solar/near-infrared modulation efficiency. Therefore, this work revealed the relationship between the solution process, surface structure, and optical properties, and thus can provide a new method to prepare VO₂ composite film with desirable performance for applications in smart windows.

Infrared Transmission Characteristics of Phase Transitioning VO2 on Various Substrates

Infrared transmission characteristics of VO₂ thin films synthesized on multiple substrates, using a low-pressure direct oxidation technique, have been characterized. Material characterization of these films indicates high material quality, which resulted in large variation of electrical and optical properties at phase transition. A change in optical transmissivity greater than 80% was observed for these films utilizing infrared (IR) laser illumination at 1550 nm. Phase transition enabled by temperature change induced by a pulsed high-power laser beam resulted in modulated IR laser transmission with a low time constant in VO₂ on transparent quartz and muscovite substrates. Investigation of the effect of mechanical strain on phase transition in VO₂ grown on flexible muscovite substrate indicate shift in transition temperature to higher for tensile and lower for compressive strains.

Wafer-scale freestanding vanadium dioxide film

Vanadium dioxide (VO₂), with well-known metal-to-insulator phase transition, has been used to realize intriguing smart functions in photodetectors, modulators, and actuators. Wafer-scale freestanding VO₂ (f-VO₂) films are desirable for integrating VO₂ with other materials into multifunctional devices. Unfortunately, their preparation has yet to be achieved because the wafer-scale etching needs ultralong time and damages amphoteric VO₂ whether in acid or alkaline etchants. Here, we achieved wafer-scale f-VO₂ films by a nano-pinhole permeation-etching strategy in 6 min, far less than that by side etching (thousands of minutes). The f-VO₂ films retain their pristine metal-to-insulator transition and intrinsic mechanical properties and can be conformably transferred to arbitrary substrates. Integration of f-VO₂ films into diverse large-scale smart devices, including terahertz modulators, camouflageable photoactuators, and temperature-indicating strips, shows advantages in low insertion loss, fast response, and low triggering power. These f-VO₂ films find more intriguing applications by heterogeneous integration with other functional materials.

Acid Solution Processed VO2-Based Composite Films with Enhanced Thermochromic Properties for Smart Windows

As a typical thermochromic material, VO₂ coatings can be applied to smart windows by modulating the transmission of near infrared (NIR) light via phase transition. However, the inherent undesirable luminous transmittance (T_lum) and solar modulation efficiency (ΔT_sol) of pure VO₂ impede its practical application. In order to solve this problem, the porous VO₂ based composite film was prepared by magnetron sputtering and subsequent acid solution process with Zn₂V₂O₇ particles used as a sacrificial template to create pores, which showed excellent T_lum (72.1%) and enhanced ΔT_sol (10.7%) compared with pure VO₂ film. It was demonstrated that the porous structure of the film caused by acid solution process could improve the T_lum obviously and the isolated VO₂ nanoparticles presented strong localized surface plasmon resonance (LSPR) effects to enhance the ΔT_sol. Therefore, this method will provide a facile way to prepare VO₂ based films with excellent thermochromic performance and thus promote the application of the VO₂ based films in smart windows.