posted on 2021-08-16, 16:41authored byJingfeng Li, Sadhu Kolekar, Mahdi Ghorbani-Asl, Tibor Lehnert, Johannes Biskupek, Ute Kaiser, Arkady V. Krasheninnikov, Matthias Batzill
Owing
to the relatively strong interlayer interaction, the platinum
dichalcogenides exhibit tunability of their electronic properties
by controlling the number of layers. Both PtSe2 and PtTe2 display a semimetal to semiconductor transition as they are
reduced to bi- or single layers. The value of the fundamental band
gap, however, has been inferred only from density functional theory
(DFT) calculations, which are notoriously challenging, as different
methods give different results, and currently, there is no experimental
data. Here, we determine the band gap as a function of the number
of layers by local scanning tunneling spectroscopy on molecular beam
epitaxy (MBE)- grown PtSe2 and PtTe2 islands.
We find band gaps of 1.8 and 0.6 eV for mono- and bilayer PtSe2, respectively, and 0.5 eV for monolayer PtTe2.
Trilayer PtSe2 and bilayer PtTe2 are semimetallic.
The experimental data are compared to DFT calculations carried out
at different levels of theory. The calculated band gaps may differ
significantly from the experimental values, emphasizing the importance
of the experimental work. We further show that the variations in the
calculated fundamental band gap in bilayer PtSe2 are related
to the computed separation of the layers, which depends on the choice
of the van der Waals functional. This sensitivity of the band gap
to interlayer separation also suggests that the gap can be tuned by
uniaxial stress, and our simulations indicate that only modest pressures
are required for a significant reduction of the gap, making Pt dichalcogenides
suitable materials for pressure sensing.