Group-IV monochalcogenides are a family of semiconductors with orthorhombic lattices that resemble staggered black phosphorus. Their relatively low crystalline symmetry makes their physical properties highly tunable. Recently, the Low-dimensional Quantum Materials Group from the Beijing Academy of Quantum Information Science (BAQIS), in collaboration with the Hong Kong University of Science and Technology, conducted an in-depth study on defects in two-dimensional single-layer SnSe materials. This research involved a comprehensive analysis of both intrinsic and extrinsic point defects within a single van der Waals monolayer (comprising two atomic layers) of SnSe. The researchers demonstrated the capability to convert a substitution defect into a vacancy through the manipulation of the electric field between the scanning tunneling microscopy (STM) tip and the sample surface and achieved a success rate of nearly 100% in manipulating specific point defects. The related work was published in the journal ACS Nano on September 05, 2024, under the title “Identification and manipulation of atomic defects in monolayer SnSe.”

SnSe, with a moderate bandgap, is utilized in various applications including photodetectors, solar cells, photocatalysis, supercapacitors, gas sensors, memristors, thermoelectric materials, and anode materials for batteries. The extensive research on SnSe’s point defects is primarily driven by their significant impact on the material’s exceptional thermoelectric properties. However, there remains a lack of systematic research focusing on the local density of states (LDOSs) of all types of intrinsic defects in SnSe. Moreover, previous studies on SnSe defects were predominantly static and lacked exploration into the transition between different types of defects, particularly in regards to controlled conversion.

Researchers have extensively investigated the point defects in monolayer SnSe grown by molecular beam epitaxy (MBE), combining both STM studies and density functional theory (DFT) calculations. Eight types of intrinsic defects were identified, including 4 types of vacancies and 4 types of antisite substitutions. The vacancy defects consist of the loss of either a single Sn atom or a vertically oriented SnSe molecule. Most of the vacancy defects exhibit noncentrosymmetric appearances that are consistent with the in-plane polarization in monolayer SnSe. Surprisingly, the antisite substitution defects, involving a Se atom replacing a Sn atom, exhibit in two distinct atomistic configurations. Despite their similar topographic appearances, the energies of the extra electronic states they introduce within the band gap of monolayer SnSe show significant differences. Most interestingly, the researchers have achieved nearly 100% success in converting an antisite substitution defect into a Sn vacancy using STM tip manipulation. Furthermore, researchers identified 3 types of extrinsic point defects, including a Pb-substitution of a Sn atom and 2 types of adsorbates.

This study has unambiguously revealed all observable point defects as well as their atomic and electronic structures, establishing methods for their manipulation, hence clarifying the influence of the point defects on the electronic structure of SnSe. The results of this study can be applied in the rational band engineering of both ultrathin and bulk SnSe for applications in thermoelectric, photovoltaic and nonvolatile logical devices.

The first author of this study is Dr. Chengguang Yue, a senior engineer of BAQIS. The corresponding authors are Dr. Kai Chang from BAQIS, Associate Prof. Junwei Liu from Hong Kong University of Science and Technology, and Dr. Haicheng Lin from BAQIS. Other authors include Zhenqiao Huang at Hong Kong University of Science and Technology, Dr. Wen-Lin Wang and Dr. Zi’Ang Gao from BAQIS. This work is funded by the National Natural Science Foundation of China, National Key R&D Program of China, Beijing Municipal Science & Technology Commission, Innovation Program for Quantum Science and Technology, etc.

Link to the paper:

https://doi.org/10.1021/acsnano.4c04789