No abstract

The ability of VO 2 to undergo semiconductor-to-metal phase transition (SMT) upon heating makes it a very attractive material for uncooled bolometers. The SMT of VO 2 represents a large temperature coefficient of resistance, which is an important parameter for the development of highly responsive microbolometers. However, other characteristics of the SMT of VO 2 such as its high transition temperature (341.2 K), the sharpness of the transition, its hysteresis, and the high room temperature resistivity limit the performance of this material in microbolometers. In this work, we grow a high-quality epitaxial ultrathin film VO 2 on c-plane Al 2 O 3 by pulsed laser deposition. The low deposition temperature and tuning the oxygen partial pressure during the growth process enable control over the grain size and oxygen vacancy concentration. This allowed controlling the SMT parameters of the samples. In particular, we show that the high density of grain boundaries associated with nanosized grains suppresses the thermal hysteresis of the SMT. Simultaneous control over the density of oxygen vacancies and the size of grains enables the adjustment of the temperature coefficient of resistance, room temperature resistivity, SMT temperature, sharpness, and thermal hysteresis toward suitable values for the fabrication of efficient VO 2 -based uncooled bolometers. Compared with other VO 2 fabrication methods, this approach can be viewed as a simpler alternative for VO 2 fabrication with favorable properties for practical bolometer applications.

Tungsten ditelluride (WTe 2) is a layered transition metal dichalcogenide (TMD) that has attracted increasing research interest in recent years. WTe 2 has demonstrated large non-saturating magnetoresistance, potential for spintronic applications and promise as a type-II Weyl semimetal. The majority of works on WTe 2 have relied on mechanically exfoliated flakes from chemical vapour transport (CVT)-grown crystals for their investigations. While producing high-quality samples, this method is hindered by several disadvantages including long synthesis time, high-temperature annealing and an inherent lack of scalability. In this work, a synthesis method is demonstrated that allows the production of large-area polycrystalline films of WTe 2. This is achieved by the reaction of pre-deposited films of W and Te at a relatively low temperature of 550 °C. Sputter X-ray photoelectron spectroscopy reveals the rapid but self-limiting nature of the oxidation of these WTe 2 films in ambient conditions. The WTe 2 films are composed of areas of micrometre-sized nanobelts that can be isolated and offer potential as an alternative to CVT-grown samples. These nanobelts are highly crystalline with low defect densities indicated by transmission electron microscopy and show promising initial electrical results.