| AVS 62nd International Symposium & Exhibition | |
| 2D Materials Focus Topic | Monday Sessions |
| Session 2D+EM+NS+PS+SP+SS+TF-MoM |
| Session: | 2D Materials: Growth and Fabrication |
| Presenter: | Sergio de la Barrera, Carnegie Mellon University |
| Authors: | S. de la Barrera, Carnegie Mellon University S. Satpathy, Carnegie Mellon University R. Feenstra, Carnegie Mellon University S. Wu, University of Washington X.D. Xu, University of Washington S. Vishwanath, University of Notre Dame X. Liu, University of Notre Dame J. Furdyna, University of Notre Dame D. Jena, University of Notre Dame H. Xing, University of Notre Dame Y.-C. Lin, Pennsylvania State University S.M. Eichfeld, Pennsylvania State University J.A. Robinson, Pennsylvania State University P. Mende, Carnegie Mellon University |
| Correspondent: | Click to Email |
Recent work on two-dimensional materials has focused on transition metal dichalcogenides (TMDs), owing to their semiconducting behavior. Characterizing as-grown TMDs is crucial in improving the understanding of the effects of growth conditions, and ultimately improving material quality. Low-energy electron microscopy (LEEM) is a powerful tool for this purpose, providing real-space images with ~10 nm spatial resolution as well as selected-area low-energy electron diffraction (µLEED) of local crystal orientation at length scales down to ~ 1 µm. Additionally, by varying the incident electron beam energy, low-energy electron reflectivity (LEER) spectra are extracted.
In this work, comparative LEEM results are presented from three TMD materials: MoS2 prepared by exfoliation (onto Si), MoSe2 grown by molecular beam epitaxy (MBE) (on epitaxial graphene), and WSe2 grown by chemical vapor deposition (CVD) (also on epitaxial graphene). It is found that for TMDs generally, the LEER spectra do not exhibit the oscillatory behavior (in the 0 – 6 eV range) that is seen for both graphene and hexagonal boron nitride (h-BN) for various numbers of monolayers (MLs). This lack of oscillatory behavior is interpreted as being due to the weak coupling of the interlayer states localized in between the MLs, which is itself a result of the relatively large out-of-plane lattice parameter. Nevertheless, additional “band structure” features in the LEER spectra permit clear identification of the TMD materials relative to the substrates. The exfoliated flakes are seen to extend over many 10’s of mm, the MBE-grown MoSe2 forms a nearly continuous film, and the CVD-grown WSe2 forms triangular islands several mm in extent. µLEED studies of the MBE-grown MoSe2 and CVD-grown WSe2 reveal preferential orientation with the underlying graphene substrates.
The reduced work functions of the TMD materials relative to the underlying substrate are clearly evident in the onset voltages for the LEER spectra (i.e. the onset shifts in accordance with the local work function of the surface). Most significantly, for the WSe2 islands, a predominant “tail” is observed in this onset, extending about 5V below the usual onset location. This tail is tentatively interpreted as arising from charging of the islands, perhaps due to polar termination at the edges of the TMD islands. Comparison of the data with simulated LEER spectra will be presented, as a test of this model for edge charge of the islands.
Work supported by the Center for Low Energy Systems Technology (LEAST), one of six SRC STARnet Centers sponsored by MARCO and DARPA, and by NSF-EFRI-1433496.