Charge Instability in Single-Layer TiTe₂ Mediated by van-der-Waals Bonding to Substrates

Single layers of transition metal dichalcogenides (TMDCs) are of interest for their emergent properties at the two-dimensional limit. However, the role of substrate effects is rarely discussed. For van-der-Waals bonding to an incommensurate substrate, the interaction can be expected to be weak, but is it negligible? Here, we report an experimental study of a model system to address this issue. The charge instability in a single-layer TiTe₂ grown on PtTe₂ films is examined as a function of the thickness of PtTe₂ films, which are themselves grown on a bilayer-graphene-terminated SiC substrate. The results show that the (2x2) charge density wave in single-layer TiTe₂ is strongly suppressed by increasing the film thickness of PtTe₂. Given the interfacial bonding remains of the weak incommensurate van-der-Waals type, the observed changes are correlated with a thickness-dependent metallicity transformation in the PtTe₂ substrate (from a wide-gap semiconductor to a semimetal). The results illustrate the crucial role of the substrate in single-layer physics.