Self-organized electronic order in thin exfoliated layered transition metal dichalcogenides
Final Report Abstract
As our modern technology is driven by miniaturization it is very important to understand how materials behave when their size is reduced along one or more dimensions – up to the extreme limit of atomically thin layers, nanoribbons or quantum dots. In this project we wanted to develop experimental and theoretical methodology to explore the crossover from a three dimensional bulk material to a two-dimensional 2D material using the example of selected layered transition metal dichalcogenides (TMDCs) which show a strong tendency towards electronic order. To this end we intended to combine angle-resolved photoemission spectroscopy (ARPES), state-of-the-art x-ray scattering techniques and density functional theory in order to study the evolution of crystal structure, electronic structure and electronic order as a function of thickness, i. e. the number of layers in TMDCs. Despite several technical issues which are unavoidable in such a challenging project we reached the following important scientific achievements: i) We established a route to measure the electronic structure by means of angle resolved photoemission spectroscopy (ARPES) on exfoliated samples as a function of thickness using existing infrastructure at the MAESTRO beamline at the Advanced light source, Berkeley. First results demonstrate the feasibility and validity of the approach. However, a number of technical details still need to be refined in order to thoroughly investigate the evolution of electronic structure towards the 2D limit in exfoliated TMDCs. ii) We developed a realistic density functional theory (DFT) model to describe the peculiar charge density wave (CDW) stacking in 1T-TaS2 . It turned out that the CDW stacking in this system can induce a charge excitation gap at the Fermi level explaining the semiconducting properties of this material. This is of particular interest because the semiconducting transport properties of this material are commonly believed to stem from Mott-Hubbard type electron-electron correlations. This study not only sheds light on the peculiar low-temperature phase of 1T-TaS2 but also advances our understanding of interlayer interactions in TMDCs in general which might be also relevant for the much discussed twisted stack systems like twisted bilayer graphene. iii) We used non-resonant x-ray diffraction in order to study the structural properties of thin exfoliated TMDCs in particular in the direction perpendicular to the layers. It should be noted that such information are usually hard to obtain with complementary methods like scanning tunneling microscopy or electron diffraction. Specifically, we investigated the photo-induced so-called hidden state in 1T-TaS2 which has sparked large interest due to its metastable nature and the potentially technologically relevant drop in resistivity associated with the transition into this state. We found that the hidden state is characterized by a marked rearrangement of the CDW layer stacking in the direction perpendicular to the layers. In conclusion it turned out that the initially planned work plan was extremely challenging, in particular within the limited time frame of 18 months. However, many important milestones have been achieved and the acquired expertise and experience will be very important to continue this project and for the success of future projects. Most importantly, apart from the scientific achievements, a number of very promising international collaborations arose from this project which will in the future certainly advance the field of 2D materials and related systems.
Publications
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“Stacking-driven gap formation in layered 1T-TaS2 ”, Phys. Rev. B 98 (2018)
T. Ritschel, H. Berger, and J. Geck
