Tailoring the electronic properties of transition-metal dichalcogenides via hydrogen-assisted phase, strain, and band gap engineering
In their two-dimensional (2D), monolayer form, transition metal dichalcogenides (TMDs) undergo an indirect-to-direct band-gap transition, which greatly enhances their radiative efficiency and is of high interest for optoelectronics. Furthermore, 2D-TMDs present clear evidence of polymorphism: in their most stable phase, the 2H structure, they are indeed direct-gap semiconductors, but their (semi)metallic 1T' phase can be easily stabilized by chemical treatments, mechanical deformations, or laser/electron-beam exposure. The possibility to locally induce an insulator-to-metal transition in 2D-TMDs has obvious technological implications, with tantalizing prospects for the realization of ultrathin devices. In this project, we propose to selectively trigger the 2H-1T' transition by exposing the sample surface to a controlled flux of hydrogen ions. Indeed, H chemisorption plays a key role in the chemical treatments that stabilize the 1T' phase. Moreover, H exposure leads to the formation of atomically thin micro- and nanobubbles on the surface of bulk TMDs. The formation of these bubbles, whose size and position can be controlled via electron-beam lithography, results in (1) a local exfoliation of the uppermost TMD layer, i.e., in a huge boost of the radiative efficiency and (2) in a significant lattice expansion, allowing for the introduction of large, site-controlled mechanical stresses in the sample. First of all, such stresses will be exploited to further manipulate the (strain-dependent) 2H-1T' transition. Also, the effects of the giant, strain-induced pseudo-magnetic fields associated with nanobubble formation should allow for the realization of valleytronic elements such as valley filters and beam splitters. This is particularly important in the context of 2H-1T' hybrid devices, where topological metallic states are expected to form at the edges of 1T' domains and would thus represent preferential channels for topologically protected valley transport.