| AVS 64th International Symposium & Exhibition | |
| 2D Materials Focus Topic | Tuesday Sessions |
| Session 2D+AS+SA+SP-TuM |
| Session: | 2D Materials Characterization including Microscopy and Spectroscopy |
| Presenter: | Taisuke Ohta, Sandia National Laboratories, Center for Integrated Nanotechnologies |
| Authors: | T. Ohta, Sandia National Laboratories, Center for Integrated Nanotechnologies M. Berg, Sandia National Laboratories, Center for Integrated Nanotechnologies C. Chan, Sandia National Laboratories K. Keyshar, Rice University G. Gupta, University of Louisville P. Ajayan, Rice University A. Mohite, Los Alamos National Laboratory |
| Correspondent: | Click to Email |
The energy positions of the valence and conduction electronic states with respect to the vacuum level are essential parameters to evaluate how the band gaps of semiconductors or Fermi-levels of metals would line up with respect to each other. Defined as an energy separation between the vacuum level and the highest occupied electronic states, the ionization energy is of particular importance for atomically-thin two-dimensional (2D) crystals to predict the performance of their heterostructures useful in high performance electronics and opto-electronics. Ionization energies have been investigated based on theoretical calculations, but to the best of our knowledge, no systematic experimental confirmation is reported for the wide range of 2D crystals despite their importance.
Here, we present a new approach to study the electronic properties of prototypical 2D crystals, graphene, MoS2, WS2, and MoSe2 monolayer and multilayer flakes, supported on thick silicon oxide (SiO2) film using a photoemission electron microscopy combined with a deep ultraviolet (DUV) illumination. We determine the band alignments of monolayer to multilayer junctions in these four materials, and show that the ionization energy decreases from MoS2, WS2, to MoSe2 as predicted by density functional calculations. We postulate that the defects in SiO2 alleviate the charging of the 2D crystals thanks to the relatively low total photoemission current due to the low energy excitation by the DUV light. This study reveals a new metrology to uncover electronic properties intrinsic to 2D crystals supported on SiO2 substrates that interact minimally with the overlying 2D crystals.
The PEEM work was performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility (DE-AC04-94AL85000). T. O. is supported by the CINT user program and Sandia LDRD. The work performed by M. B. and C. C. are supported by a U.S. DOE, Office of Energy Efficiency and Renewable Energy SunShot Initiative award for BRIDGE (DE-FOA-0000654 CPS25859). K. K. was supported by the Army Research Office MURI grant W911NF-11-1-0362. A. D. M. is supported by LANL LDRD program. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.