Controlled oxygen plasma exposure of multilayer WSe ₂ can modulate the carrier type of WSe ₂ field effect transistors. XPS shows with increased exposure, the top WSe ₂ layer(s) oxidizes to amorphous WO _{( x −3)} layer-by-layer with a high degree of controllability as both single or 2 layers can be transformed. A systematic study of Raman and photoluminescence signatures clearly demonstrate the processing times necessary to selectively oxidized the top 1 or 2 layers of exfoliated WSe ₂ flakes. WSe ₂ devices exposed in the channel regions experience an increase in p-type conduction and large layer-dependent n-type suppression. Devices exposed in the contact region prior to metalization show slight increases of both p-type and n-type conduction. Devices made on flakes, which were completely exposed show suppression of p-type conduction and layer dependent suppression of n-type conduction. Based on these results we were able to fabricate self-rectifying lateral p–n homojunctions with rectification ratios exceeding 300.

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe${}_{0.98-z}$Cu${}_z$Te${}_{0.5}$Se${}_{0.5}$ with $z=0$, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy ($\le 12$ meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3$d$ transition metal dopants simply as effective electron donors.