Unraveling the Structural and Electronic Properties at the WSe₂–Graphene Interface for a Rational Design of van der Waals Heterostructures

Abstract

WSe2 thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe2 crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe2 islands nucleate from defective WSe x seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe2 islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe2 and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe2–graphene distance and the WSe2/WSe2 interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics.

A Perspective on the Application of Spatially Resolved ARPES for 2D Materials

“…One fascinating feature of this class of semiconducting TMDCs is their band structure around the K points which is composed by two spin-polarized branches due to the strong spin-orbit interaction originating from the transition metal ion’s d orbitals [ 79 ]. For example, for mono-layer WSe 2 the VBM is characterized by two bands with a spin-splitting of about 0.5 eV, making this material one of the most studied for spintronic devices [ 80 , 81 ]. The lack of inversion symmetry in mono-layer TMDCs creates unequal K and K′ valleys, the spin projection along their out-of-plane axis is well defined, and the two split bands are spin-up and spin-down.…”

“…An example of this is the micro-ARPES study on suspended MoS 2 compared to the more prevalent use of SiO x which confirms the extreme sensitivity of 2D layers to the underlying substrate [ 109 ]. Examples of spatially-localized ARPES on CVD grown TMDCs has been reported for WS 2 , WSe 2 and MoS 2 , where the band structure has been studied by micro-ARPES [ 78 , 81 , 110 , 111 , 112 , 113 ]. An interesting study of artificially built bi-layers of CVD-grown MoS 2 has been carried out by micro-ARPES; the band structure changes as a function of the angle between the layers and a remarkable dependence of the angle with the electron effective mass and of the position of the bands has been found [ 112 ].…”

“…Several epitaxial routes are based on the use of graphene because it is so well-studied that advanced growth on top of it are possible, for example, TMDCs are deposited onto graphene by CVD or PVD. Epitaxial graphene on SiC represents an excellent example of graphene used as a basis for the growth of other 2D materials such as TMDCs: WSe 2 [ 81 ], MoS 2 [ 99 ], MoSe 2 [ 114 ], and 2D chalcogenides such as single layer GaSe [ 115 , 116 ]. The latter material interacts greatly with the epitaxial graphene.…”

“…Interestingly this phenomenon has not always been detected. For WSe 2 /graphite [ 15 ], MoSe 2 and MoS 2 /graphene [ 13 , 114 , 131 ] it has been observed, however recently for WSe 2 /graphene it has not [ 81 ], so a more accurate investigation will be necessary to rule the effect of the heterostructures parameters, such as layer separation.…”

Spectroscopic photoemission and low-energy electron microscopy studies of the surface and electronic structure of two-dimensional materials

“…State-of-the-art spatially localized ARPES on synchrotron facilities can realize nanometer-sized beam spot [64][65][66]. The advantage of this method is that it enables high energy resolution to be achieved [67][68][69][70][71][72][73][74][75]. However, real-and momentum-space ARPES mapping are difficult, as they demands translation and rotation of either the sample stage or the electron analyzer in an ultrahigh vacuum environment [63].…”