Thickness characterization of atomically thin WSe2 on epitaxial graphene by low-energy electron reflectivity oscillations

Abstract

In this work, low-energy electron microscopy is employed to probe structural as well as electronic information in few-layer WSe 2 on epitaxial graphene on SiC. The emergence of unoccupied states in the WSe 2 -graphene heterostructures are studied using spectroscopic low-energy electron reflectivity. Reflectivity minima corresponding to specific WSe 2 states that are localized between the monolayers of each vertical heterostructure are shown to reveal the number of layers for each point on the surface. A theory for the origin of these states is developed and utilized to explain the experimentally observed featured in the WSe 2 electron reflectivity. This method allows for unambiguous counting of WSe 2 layers, and furthermore may be applied to other 2D transition metal dichalcogenide materials.

Epitaxial Growth of Two-Dimensional Layered Transition Metal Dichalcogenides

“…Fu et al, observed that a single orientation was favored in MBE when lower metal flux was used (91). Similar single oriented domains have been observed for WS2 (95) and WSe2 (Figure 6e) (56). Preference of a single orientation was explained based on metal atoms reacting with the BN defects and acting as nucleation or anchoring sites.…”

Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate

“…In addition to LEEM imaging, we obtain spectroscopic information of the sample under study by measuring the intensity of elastically reflected electrons as a function of the incidence electron kinetic energy, a technique called low-energy electron reflectivity (LEER). 56,59,60 In thin-film materials like graphene and transition metal dichalcogenides, the layer confinement of electrons leads to quantum-well states with energy spacing depending on the film thickness. 56,60 In the LEER experiment, incident electrons can couple resonantly to unoccupied quantum-well states, i.e., conduction bands with the wave vector perpendicular to the film, and experience a decreased reflection probability.…”

“…56,59,60 In thin-film materials like graphene and transition metal dichalcogenides, the layer confinement of electrons leads to quantum-well states with energy spacing depending on the film thickness. 56,60 In the LEER experiment, incident electrons can couple resonantly to unoccupied quantum-well states, i.e., conduction bands with the wave vector perpendicular to the film, and experience a decreased reflection probability. These distinct reflection minima result in a fingerprint for a given material and integer number of monolayers, which we can examine with high resolution in our setup.…”

“…vertical lines inFig. 6gindicate the energy positions of bulk WSe2 bands with strong in-plane character at the A-point of the Brillouin zone from Barrera et al56 and taking into account a work function offset of our electron emitter of 0.34 eV in this experiment. The coincidence with the minima in the LEER signal illustrates the spectroscopic features obtained for the WSe2 sample region.…”

Layered Two-Dimensional Heterostructures and Their Tunneling Characteristics

“…We replace the delta function of E L − E R with a version in terms of k, 5) where the zero of f (k R , k L ) is found using Eq. (B.4) to be region I: 6) with k = eV / v F . From this, it follows that |f (k 0 )| = v F .…”