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Raman spectroscopy in transition metal dichalcogenides (TMDCs) helps determine their structural information and layer dependency. Because it is non‐destructive and fast, it is an archetypal spectroscopic technique to investigate the structure and defects in TMDCs. In our earlier study, we used a metal‐dielectric coating to enhance Raman signal of WS2 because the Raman Spectra measured from WS2 coated on the standard Si/SiO2 was significantly lower. This metal‐dielectric coating allowed access to the otherwise unavailable E12g and A1g modes of WS2. In this study, we compare the Raman spectra of WS2 on a Si/SiO2 to that of metal layers (Au [200 nm] and Al [200 nm]). A significant enhancement in the Raman signal of 2‐3L WS2 is observed for both the Au and Al coatings. Although 200 nm Au coating enhances the Raman Signal better than the 10 nm Au coating, it does not resolve the other signature vibrations other than E12g and A1g. Whereas, coating based on Al (200 nm) and Al (10 nm) both enhance the Raman signal of WS2. In the case of Al coating, we report well‐resolved vibrational modes such as the ones below <300 cm−1in a few layered WS2. This enhancement allows us to quantify the number of layers and also investigate the presence of defects. Photoluminescence measurements on 2–3L and 15L WS2 on Al (200 nm) confirm this. [ABSTRACT FROM AUTHOR]

Structural phase transition assisted micromechanical actuation of a vanadium dioxide (VO2) coated silicon microcantilever is presented. A 300 nm polycrystalline VO2 film was deposited over the silicon surface at 520 °C using metal organic chemical vapor deposition. The formation of the M1 monoclinic phase of the as-deposited VO2 film was confirmed by X-ray diffraction studies and further verified by temperature variable Raman spectroscopy. The heated VO2 film exhibits semiconductor-to-metal transition at 74 °C, which produces a change in the electrical resistance almost of three orders in magnitude. Consequently, the VO2 film undergoes structural phase transition from the monoclinic phase (011)M1 to a tetragonal phase (110)R. This generates a compressive stress within the VO2 film resulting in large, reversible cantilever deflection. This deflection was measured with a non-contact 3D optical profilometer, which does not require any vacuum conditions. Upon heating, the VO2 coated silicon cantilever produced a large reversible tip deflection of 14 μm at 50 °C. Several heating and cooling cycles indicate steep changes in the cantilever tip deflection with negligible hysteresis. In addition, the effect of thermal stress induced cantilever deflection was estimated to be as small as 6.4%, and hence can be ignored. These results were found to be repeatable within controlled experimental conditions. [ABSTRACT FROM AUTHOR]

Thin films of vanadium dioxide (VO2) are deposited on indium tin oxide (ITO), stainless steel (SS), and glass substrates using chemical vapour deposition. X-ray diffraction and Raman spectroscopy measurements confirmed the single phase nature of the VO2, which showed a phase transition from a low conducting state at low temperature(<68 °C) to a high conducting state at high temperature (>68 °C). This was confirmed by electrical conductance and infra-red reflectance measurements. X-ray photoelectron spectroscopy was used to measure the charge states of vanadium species. The optical constants of VO2 were determined using visible and near-infra red(NIR) reflectivity and show that the VO2 film on ITO has a lowered plasma frequency compared with VO2 on glass substrates. The thin films of VO2 enable a broadband of ultra-high absorption at mid-wave infra-red frequencies due to a Fabry-Pérot (F-P) like resonance due to the dielectric properties of ITO, SS, or glass. The tunability of this absorption band via VO2 thickness and the switchability by temperature makes the system attractive for absorptive coatings with controllable emissivity. [ABSTRACT FROM AUTHOR]

• Vanadium dioxide (VO 2) is deposited on glass slides by Chemical Vapor Deposition. • Structural characterization revealed the presence of phase pure monoclinic VO 2. • Electrical measurements of the as-deposited VO 2 did not show a sharp transition. • Surface analysis showed Vanadium in V3+, V4+ and V5+ states in as-deposited films. • Annealing the films enhanced the transition strength. In this work, VO 2 is deposited on borosilicate glass substrates via a single step Chemical Vapor Deposition (CVD). Though the structural characterization revealed the presence of phase pure polycrystalline VO 2 in simple monoclinic crystal structure (P2 1 /c), electrical measurements did not show a sharp transition. To understand the ambiguity, surface characterization was performed with X-ray Photoelectron Spectroscopy (XPS) on the as-deposited VO 2 thin films. Vanadium was present in V3+, V4+ and V5+ states. Following these measurements, annealing was performed in ambience at 200 °C for 10 minutes. Subsequent electrical measurements showed an improved phase transition, enhanced by an order of magnitude. We attribute the enhancement in transition to annealing of oxygen vacancies present in VO 2 thin films because of the deposition conditions. Decrease in oxygen vacancies was correlated with a decrease in the fraction of V3+ from XPS measurements. The sample annealed at 200°C for 10 minutes exhibited a higher resistance ratio (ΔA = 305) with an optimum hysteresis (4°C) and transition width(12°C), whereas the as-deposited sample exhibited a resistance ratio of ΔA = 32. The thermal treatment affected the characteristics of the transition. A narrow hysteresis of 4°C and a sharp transition width with a reasonable resistance ratio were obtained for the sample annealed at 200°C for 10 minutes. From this study, we understand that our synthesis of VO 2 thin films via CVD is prone to be off-stoichiometric and therefore, contains defects such as oxygen vacancies. We could mitigate the effect of these vacancies and engineer the stoichiometry by performing a simple annealing which in turn improved the strength of the phase transition. Drawbacks such as a poor-quality transition, resulting from synthesis parameters can be mitigated if we understand the effect of vanadium and oxygen vacancies. [ABSTRACT FROM AUTHOR]

• Comparison of two industrially scalable deposition techniques to develop large areas of VO 2. • UNSPACM is a cost-effective process that achieves electrically superior VO 2. • CVD is used to develop optically superior VO 2 over large areas. • Where optical applications are crucial, roughness is a key and CVD is preferred. • Different synthesis process alters morphology and stoichiometry. We compare the effect of two synthesis methods namely Chemical Vapor Deposition (CVD) and Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) on the Semiconductor-Metal Transition (SMT) characteristics of VO 2 thin films. The SMT characteristics of the films are measured electrically and optically. CVD films show a lower resistance ratio and SMT strength, whereas films synthesized via UNSPACM are rougher. We ascribe the reason for the observed behaviour to morphology and off-stoichiometry arising from the deposition process. [ABSTRACT FROM AUTHOR]

VO 2 displays a semiconducting to metallic (SMT) transition accessible near room temperature. This makes it one of the most sought after materials for electrical and optical switching. But this can be utilized only when the synthesis process yields phase pure VO 2 without other oxides of vanadium. Across the SMT, VO 2 exhibits difference crystal structures with a rich phase behavior of insulating monoclinic M1, M2 and T phases. The objective of this study is to synthesize phase pure VO 2 and to investigate its structural evolution and infrared switching during the transition. In this work, a rapid non-equilibrium process namely Solution Combustion Synthesis (SCS) was employed. The structural phase transition (SPT) of VO 2 nanostructures synthesized by SCS was investigated by in-situ temperature controlled XRD across the SMT. Gaussian curve fittings for measured XRD patterns revealed that competing phases of M1 and R significantly contribute to the observed pattern at every increase in temperature. The powders were further characterized by FTIR, DSC and DC electrical conductivity. These studies show that a sharp SMT was observed at 68–70 °C. Infrared transmittance experiments pinpointed the transition. Carrier density and mobility of VO 2 were calculated. This suggests that this VO 2 thus synthesized displays excellent phase transition behavior and can be utilized in optical and electrical switching. [ABSTRACT FROM AUTHOR]

Vanadium Oxide has been a frontrunner in the field of oxide electronics because of its metal-insulator transition (MIT). The interplay of different structures of VO2 has played a crucial role in deciding the magnitude of the first order MIT. Substitution doping has been found to introduce different polymorphs of VO2. Hence the role of substitution doping in stabilizing the competing phases of VO2 in the thin film form remains underexplored. Consequently there have been reports both discounting and approving such a stabilization of competing phases in VO2. It is reported in the literature that the bandwidth of the hysteresis and transition temperature of VO2 can be tuned by substitutional doping of VO2 with W. In this work, we have adopted a novel technique called, Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) to deposit VO2 and W- doped VO2 as thin films. XRD and Raman spectroscopy were used to investigate the role of tungsten on the structure of VO2 thin films. Morphology of the thin films was found to be consisting of globular and porous nanoparticles of size ~ 20nm. Transition temperature decreased with the addition of W. We found that for 2.0 at% Wdoping in VO2, the transition temperature has reduced from 68 °C to 25 °C. It is noted that W-doping in the process of reducing the transition temperature, alters the local structure and also increases room temperature carrier concentration. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO 2) has been a much sought after candidate for many electronics and photonics based applications. These applications demand CMOS compatibility and simple but scalable synthesis procedure. Herein we report the deposition of VO 2 on CMOS compatible Si substrates via easily scalable Metal Organic Chemical Vapor Deposition (MOCVD) at different substrate temperatures between 520 and 550 °C. The morphology of the films deposited at different growth temperatures varied drastically despite retaining VO 2 in the M1 phase. This was verified by XRD and Raman experiments. The thin films deposited at 535 °C showed sharp grain boundaries with a grain size of about 200 nm. Similarly, the electrical characteristics of the films deposited at 535 °C showed superior transition compared to the films deposited at other temperatures. However the films deposited at 550 °C retained the superiority in the transition strength, the transition width and the hysteresis of the transition increased at that temperatures. We understand that the reason for this observed behavior is the fraction of surface defects. It is seen from XPS measurements that vanadium was available in V3+, V4+ and V5+ oxidation states and the fractions of these individual components varied in all the thin films deposited at different temperatures. We believe that the defects fraction can give a handle to control the nature and quality of the transition in VO 2. • Single phase VO2 was grown at different temperatures using MOCVD on Si substrates. • Morphology of the thin films changed with the growth temperature. • Surface studies by XPS showed the presence of V in V3+, V4+ and V5+ states. • For a particular growth temperature, ratio of V3+, V4+ and V5+ states is optimal. • Varying growth temperature gives a handle to tweak defects and also the transition. [ABSTRACT FROM AUTHOR]

A tri-layer metamaterial absorber, composed of a metal structure/dielectric spacer/vanadium dioxide (VO 2 ) ground plane, is shown to switch reversibly between reflective and absorptive states as a function of temperature. The VO 2 film, which changes its conductivity by four orders of magnitude across a insulator–metal transition at about 68 °C, enables the switching by forming a resonant absorptive structure at high temperatures while being inactive at low temperatures. The fabricated metamaterial shows a modulation of the reflectivity levels of 58% at a frequency of 22.5 THz and 57% at a frequency of 34.5 THz. [ABSTRACT FROM AUTHOR]