Effect of W addition on the electrical switching of VO2 thin films

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 % W doping in VO2, the transition temperature has reduced from 68 o C to 25 o 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

Thermochromic VO2 thin films on ITO-coated glass substrates for broadband high absorption at infra-red frequencies

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 degrees C) to a high conducting state at high temperature (>68 degrees 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-Perot (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. Published by AIP Publishing

Thermally switchable metamaterial absorber with a VO2 ground plane

A tri-layer metamaterial absorber, composed of a metal structure/dielectric spacer/vanadium dioxide (VO2) ground plane, is shown to switch reversibly between reflective and absorptive states as a function of temperature. The VO2 film, which changes its conductivity by four orders of magnitude across a insulator-metal transition at about 68 degrees 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. (C) 2015 Elsevier B.V. All rights reserved

Phase transition induced micromechanical actuation in VO2 coated cantilever

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 degrees 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 degrees 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 mu m at 50 degrees 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. Published by AIP Publishing

Massive Open Online Courses (MOOCs) und ihre Anwendungsmöglichkeiten im Schulunterricht Informatik

Die vorliegende Arbeit versucht, die Frage zu beantworten, ob die Massive Open Online Courses (MOOCs) bereit für den schulischen Einsatz sind. Zunächst wird die breite Palette an E-Learning-Methoden beschrieben, und die Lernmanagementsysteme (LMS) werden erläutert, die für das E-Learning die Grundlage bilden. Anschließend werden auf Basis dieser Theorien die MOOCs im Detail erklärt. Für den empirischen Teil der Arbeit wurde ein MOOC zum Thema Datenschutz und Datensicherheit auf der Plattform Udemy.com erstellt und an ungefähr fünfzig Schülern getestet, die im Anschluss an den dreistündigen Kurs einen Fragebogen beantworteten. Die Auswertung dieser Studie zeigt, dass das Thema Datenschutz und Datensicherheit, wenn es in einem MOOC vermittelt wird, bei den Schülern sehr gut ankommt.This paper tries to find an answer to the question whether the Massive Open Online Courses (MOOCs) are ready for school use. First, there is a description of the wide range of e-learning methods and an explanation of Learning Management Systems (LMS), that are the basis of e- learning. Then the MOOCs are explained in detail based on these theories. For the empirical part of the work there was a MOOC on data protection and data security created on the platform Udemy.com and tested on about fifty students, who replied after the three hour course to a questionnaire. The analysis of this study shows that the issue of data protection and data security in the context of a MOOC is well received by the the pupils

Nanostructure-free Metal–Dielectric Stacks for Raman Scattering Enhancement and Defect Identification in CVD-Grown Tungsten Disulfide (2H-WS₂) Nanosheets

Low-wavenumber Raman (LWR) spectroscopy determines signatures in structural information and layer-to-layer dependency of transition metal dichalcogenides (TMDCs). It supports proper 2D TMDC analysis and subsequent layer verification. The nondestructive nature and ultrafast detection make LWR measurements imperative for layer variations and defect investigations. Interference-enhanced Raman scattering utilizes a metal–dielectric layer to enhance the Raman signal. This has been used to study graphene, C60, and Te. Here, we investigate using Al/Al2O3 coatings to enhance the LWR scattering of different 2H-WS2 layers and understand the structures of these large-area nanosheets. Phase-pure WS2 is synthesized by CVD, and the layers are exfoliated via ultrasonication at 80 kHz. Layers were drop-casted on Al/Al2O3 coatings of different thicknesses of Al2O3 to study differences in bilayers up to a few layers of 2H-WS2. We observe an enhancement of 30 times in the Raman signal (356 cm–1) corresponding to a quarter wave-thick 80 nm Al2O3 and characterize these 2H-WS2 films for their defects. Transfer-matrix calculations confirm the observed behavior to field enhancement. Our work shows how the structures and weaknesses of 2D materials such as WS2 can be better identified, provided an enhancing substrate is chosen

Visualization 1: High contrast switchability of VO₂ based metamaterial absorbers with ITO ground plane

Structural Uniformity Originally published in Optics Express on 17 April 2017 (oe-25-8-9116