Paper NM-MoM1
Identification of Point Defects in Transition Metal Dichalcogenides by Combining Atomic Resolution Force Microscopy, STM/STS and Density Functional Theory: Missing Vacancies in MoSe₂ and WS₂

Monday, December 3, 2018, 8:00 am, Room Naupaka Salon 5

Point defects can strongly influence material properties of 2D materials including Transition Metal Dichalcogenides (TMDs), where they can modify optical and transport properties, catalytic activity, and act as single photon emitters. It has been difficult to directly correlate specific defects with macroscopic measurements of TMD optical and transport properties. Scanning transmission electron microscopy (STEM) investigations have provided significant structural information, but STEM cannot directly probe electronic structure . In addition radiation damage is a significant problem for TMDs, making it difficult to determine intrinsic defect concentrations [1].

Here we report on the first applications of atomic resolution AFM to TMD point defects [2,3]. Cryogenic AFM/STM/STS studies using a CO molecule tip, when combined with advanced density functional excited-state theory, provide sufficient information for detailed point defect characterization. The experimental methods can resolve:

• defect location through AFM, including local strain with ~ 30 pm lateral and 3 pm vertical resolution. This is difficult to do with STM alone due to the strong convolution of geometric and electronic structure [4]
• electronic structure, through scanning tunneling spectroscopy (STS), which identifies localized resonances and in-gap states, and STM spatial maps of localized states and orbitals
• charge state, through Kelvin-probe AFM and STS charging peaks
• radial and reflection symmetry breaking, for example defects in the chalcogen plane appear very different in AFM if located on the upper or lower TMD surface, while the STS maps and spectra are similar. Defects localized in the metal plane have one AFM signature for a given STS structure.

The detailed information from scanning probe studies strongly constrains geometric structural models for theoretical simulations, and the results of these simulations can be directly compared to STS maps and local spectra, allowing detailed understanding of defects.

We will report on studies on MBE-grown MoSe₂ and CVD-grown WS₂. Based on STEM studies, chalcogen vacancies have been identified as the most common point defects, and have been predicted to have in-gap states. While our AFM/STM studies show chalcogen site defects whose AFM contrast is consistent with Se or S vacancies, they do not show any electronic in-gap states. In combination with theory, we identify these sites as substituted oxygen, which has very low STEM contrast.

[1] Wang, Robertson, Warner, Chem Soc Rev 2018.

[2] Barja, Refaely-Abramson, Schuler, Qiu et al, submitted.

[3] Schuler, Kastl, Chen et al, submitted.

[4] Barja et al, Nature Physics 2015.