2D material heterostructures for electronic and optoelectronic applications
Wolfgang Mertin, Universität Duisburg-Essen, Duisburg, GermanyTilmar Kümmell, Universität Duisburg-Essen, Duisburg, GermanyEkaterina Nannen, Universität Duisburg-Essen, Duisburg, GermanyGerd Bacher, Universität Duisburg-Essen, Duisburg, Germany
Research on graphene and other two-dimensional crystals like transition-metal chalcogenides (TMCs) is one of the leading topics in current materials science. The whole range from electrically insulating systems like graphene oxide or BN, semiconducting TMCs such as MoS2, MoSe2, or WSe2 to highly conducting materials like graphene is available, making these materials ideal candidates as building blocks for atomically thin heterostructures. Challenges towards future electronic and optoelectronic applications are reproducible preparation techniques including the deposition of functional 2D layers on suitable substrates on the one hand and the development of a detailed understanding of their fundamental electrical and optical properties on the other hand.
We demonstrate CVD growth of monolayer graphene on copper foil where shape and size of the graphene grains can be controlled by the methane/hydrogen ratio and the chamber pressure, respectively. Embedding CVD graphene into electronic/ optoelectronic devices requires a transfer to the target substrate and the usage of advanced lithographic techniques for processing. We demonstrate the potential of graphene as transparent electrodes in large area quantum dot LEDs  and show the influence of different graphene/metal contact processing techniques (optical vs. electron beam lithography)  and architectures (2D vs. 1D contacts) for transistor applications.
In contrast to graphene with its zero bandgap, TMCs show a rich band structure with finite bandgap, making these materials in addition interesting for optoelectronic applications. We demonstrate gate control of the carrier distribution in the k space, monitored by photoluminescence in MoS2  and discuss the usage of TMC heterostructures for energy conversion in light emitters or solar cells.
 S. Wolff et al., Appl. Phys. A 120, 1197–1203, 2015
 C. A. Chavarin et al., Appl. Phys. A 122, 1-5, 2016
 T. Kümmell et al., Phys. Rev. B 91, 125305, 2015