Van der Waals heterostructures

Our work is based on studies of the morphological and transport properties of heterostructures based on graphene, hexagonal boron nitride (h-BN), and the organic semiconductor (OSC) N,N’–bis(n-octyl)-1,6-dicyanoperylene- 3,4:9,10-bis (dicarboximide) (PDI8-CN2). The first part of the work is devoted to studying morphology of PDI8-CN2 layers on single-layer graphene (SLG) and the effect of the presence of an OSC layer on the transport properties of a planar graphene field-effect transistor (GFET). Graphene flakes were obtained by mechanical exfoliation to conduct studies of layer morphology and fabricate GFETs. The PDI8-CN2 layers were deposited using the vacuum evaporation technique. It was shown that PDI8-CN2 molecules can have two orientations relative to the substrate surface. The atomic force microscopy (AFM) showed that for the first 2-3 layers, the molecules have a horizontal orientation, and subsequent layers are formed with the molecules in a vertical position. The dependence of the transport properties of a GFET on the number of molecular layers in an OSC layer with different molecular orientations is shown. Such dependence is caused by a different charge exchange at the graphene/organic semiconductor interface for different orientations of the molecules. We also show the possibility of obtaining large-area two-dimensional (2D) heterostructures based on graphene and hexagonal boron nitride (h-BN). The morphology of the resulting surface and issues that affect the performance of the heterostructure and were encountered during the manipulation of flakes with size of several tens of μm are described. The charge transport measurements of heterostructures were carried out at room temperature and at a temperature of 15 - 20 K to minimize the scattering of charge carriers by phonons. We show that the tunneling current between the graphene flakes is largely suppressed in a system with a potential barrier in the form of 30 h-BN layers. In the third part of our research, we present for the first time a vertical heterostructure based on two graphene layers separated by a layer of PDI8-CN2. The morphology of the OSC layer between the graphene flakes is described. Depending on the number of the OSC layers, the bottommost and topmost graphene flakes will be in contact with molecules which are oriented differently relative to the graphene layer. The horizontally ordered molecules are attached to the bottommost and vertically arranged molecules are in contact with the topmost graphene flake. The measurements of the charge transport properties of heterostructures were carried out at room temperature and at 14 - 30 K. The current-voltage (I-V ) characteristics measurements of the graphene/organic semiconductor/graphene (Gr/OSC/Gr) heterostructures showed similar behavior as in the currently reported heterostructures based on graphene and h-BN with the appearance of nonlinearity in the output curves after crossing a bias value of 0.5 V. The transfer chracteristics showed a typical V-shaped dependence for GFETs, except for the appearance of a second minor minimum. Such behavior of the heterostructures can be explained in terms of the alignment of the Fermi levels of both flakes with each other and with the Dirac points with charge transport through localized states in PDI8-CN2