K.A. Cochrane, B. Schuler, J-H. Lee, C. Kastl, C. Chen, E. Barnard, E. Wong, F. Ogletree, S. Aloni, A. Schwartzberg, J. Neaton, A. Weber-Bargioni
Lawrence Berkeley National Lab,
Keywords: transition metal dichalcogenides, scanning tunneling microscopy, scanning probe microscopy, defects
Summary:Two-dimensional transition metal dichalcogenides (2D-TMDs) are a novel class of materials with many potential applications including electronic and opto-electronic devices, quantum information systems, and catalysis.  Properties of these materials are significantly impacted by atomic-scale local morphology and defects within the lattice. Different geometric structures, defect densities, and single atom doping can influence transport, optical, and magnetic properties.  Particularly, the formation of electronic resonances that lie within the valence and conduction bands, referred to as in-gap states, result in significantly different photophysical properties than from a pristine lattice.  Few techniques have the capability of resolving the geometric, electronic, and opto-electronic properties at the atomic level. High resolution scanning probe microscopy (SPM) has the unique capability to examine these properties on relevant length scales. Scanning tunneling microscopy (STM) and spectroscopy (STS) are able to resolve the local density of states (LDOS) on the sub-angstrom scale providing a detailed picture of the electronic structure such as the presence and spatial distribution of in-gap states.  High-resolution non-contact atomic force microscopy (NC-AFM) with CO functionalized tips allows for the resolution of surface atoms and defects.  These atomic-scale characterization techniques allow for an extraordinarily detailed picture of the structure and functionality of point defects in 2D-TMDs. Previously [4,6], using LT-SPM, we identified and characterized point defects in monolayer tungsten disulfide (WS2) on graphene on silicon carbide. Commonly observed was a negatively charged defect, appearing is a depression approximately 3 nm in width, with a sharp in-gap resonance just above the valence band. High-resolution NC-AFM imaging indicated the defect sits at the chalcogen site and appears as a missing atom. Remarkably, after applying a local electric field (achieved by tunneling with a high current setpoint and bias), this defect can be reproducibly though not reversibly, converted to a new type of defect. This new defect shows trilobal symmetry in STM and in-gap resonances above and below the Fermi energy with striking vibrionic signatures. These features can be reproducibly fit with a Frank-Condon model resulting in Huang-Rhys (HR) factors of ~5 and ~2.5 and a lifetime broadening of 6 and 4 meV for monolayer and bilayer WS2 respectively. These HR values are on par with those for zero phonon lines previously observed with Raman spectroscopy in color centers in hBN . However, precious spectroscopic measurement techniques result in an average signal over a large area. Here, using STS we directly observe electron-phonon coupling of a single defect that we have created with an electric field. SPM techniques allow for an unprecedented view of the relationship between atomic structure therefore crucial optoelectronic properties.  Mak, K. and J. Shan. Nat. Photonics. 10, 216 (2016).  Lin, Z. et al. 2D Mat. 3, 22002 (2016).  Chow, P. et. al. ACS Nano. 9, 2, 1520 (2015).  Schuler, B. et al. arXiv:1810.02896v1 [cond-mat.mtrl-sci].  Gross, L. et al., Science. 325, 5944 (2009).  Schuler, B. et.al. in preparation.  Tawfik, S.A. et al. Nanoscale, 9, 13575 (2017).