凯发

Infraredand THz optics of atomically thin films of

transition-metaldichalcogenides

andpost-transition metal chalcogenides

Vladimir Fal’ko

National Graphene Institute, University of Manchester, Manchester, UK

Transition-metaldichalcogenides (TMD), MoS₂, MoSe₂, WS₂ andWSe₂ and post-transition metal chalcogenides (PTMC), InSe and GaSeare materials where a strong covalent bonding of atoms inside individual layerscoexists with a weak van der Waals coupling of the consecutive layer of thebulk crystal. Such peculiar bonding makes it possible to isolate mechanically,or to grow epitaxially atomically thin films of such compounds with a precise numberof atomic layers. At the same time, van der Waals nature of these crystalscoexists with strong hybridizations of – separately – conduction and valenceband orbitals in the consecutive monolayers in the film, making the bandstructure of few-layer atomically thin TMDs and PTMCs sensitive to the numberof layers in them. In particular, few-layer films acquire multiple subbands intheir electronic spectra, with a strong coupling of inter-subband transitionsof carriers (electrons in n-doped and holes in p-doped materials) without-of-plane polarised photons. Here, we show that, when n- or p-doped,few-layer films of TMDs and PTMCs become absorbers and emitters of infrared(IR) and THz light. Our density functional theory modelling and a speciallydesigned hybrid k·p theory for the monolayers of thesematerials, combined with the tight-binding model description of the interlayerhopping (HkpTB), predicts that optical activity of few-layer films of these twoclasses of compounds densely covers the range from IR (1.5 micron) for bilayerfilms to THz for the films with 3-10 layers. In a way, these thin films areanalogous to quantum wells in conventional semiconductors, and, by choosing thenumber of layers, and/or n- or p-doping in one of TMD and PTMC compounds, onecan tune such inter-subband transition energy to the desirable applicationrange, offering a new way how 2D materials can be harnessed for developing newtechnologies.