Scanning Tunneling Microscopy Study of Layered van der Waals Materials

Abstract

Understanding the structure, electronic properties, and defect characteristics of materials is essential for their application in devices. Scanning tunneling microscopy (STM) emerges as a crucial technique for investigating surface structures, electronic properties, and defects in materials. In this thesis, we employ STM to investigate the structure and electronic properties of several layered van der Waals materials, including FeTe, NiTe2-xSex, GeSe, and ReSe2. The thesis begins with an introduction about the layered van der Waals materials in Chapter 1, demonstrating their significant potential in applications. Chapter 2 provides information about STM, where elementary theories of tunneling current, the working principle of STM, and an introduction to home-built low-temperature STM are presented. Subsequent chapters focus on experimental studies using STM on specific layered van der Waals materials. The detailed information is as follows: Chapter 3: FeTe single crystal. Using STM, we investigate intrinsic defects present on the surface of the FeTe single crystal. Our STM images reveal the existence of Te vacancy located at both the top and bottom Te sites. Notably, at a temperature of 79 K, just above the phase transition temperature (TN = 60-70 K) of FeTe, we observe local charge ordering characterized by a 2a periodicity (a is the lattice constant) along both crystal axes. This charge order is attributed to the influence of the antiferromagnetic order. Therefore, our observations suggest the local presence of the antiferromagnetic phase at temperatures marginally over TN without establishing macroscopic ordering. Reference: Nguyen Huu Lam, et al. Journal of the Korean Physical Society 79, 552-556 (2021) Chapter 4: NiTe2-xSex single crystals. NiTe2 has recently attracted considerable attention due to its type-II Dirac band, which possesses intriguing topological properties and holds promise for various applications. The substitution of Te in NiTe2 with Se to form NiTe2-xSex alloys leads to interesting changes. Through scanning tunneling spectroscopy (STS) measurements, we have observed the evolutions of the type-II Dirac cones in the NiTe2-xSex alloys, which is evident in the changes of peaks near the Fermi level (EF) in the differential conductance (dI/dV) spectra. Supported by angle-resolved photoemission spectroscopy and density-functional theory calculations, we confirm the shifting of the Dirac point from +0.1 eV above EF in NiTe2 to -0.3 eV below EF in NiTeSe. This movement of the Dirac point correlates closely with the strength of spin-orbit coupling, which can be effectively controlled by Se substitution. Reference: Nguyen Huu Lam†, Phuong Lien Nguyen†, Byoung Ki Choi†, et al. ACS Nano 16, 11227−11233 (2022) Chapter 5: GeSe single crystal. In this section, we use STM to explore the cleaved GeSe surfaces. Depending on the STM tip states, the atomic features can display either a Se rectangular structure or a zigzag structure of both Ge and Se atoms. Additionally, we note the presence of various wrinkle structures on the GeSe surface, formed during the cleavage process. Due to the curved nature of the wrinkles, which causes local strains, we observe a reduction in the bandgap at the wrinkles (from ~1.2 eV in normal areas to ~1.0 eV at the wrinkle's center). Reference: Nguyen Huu Lam, et al. Surface Science 730, 122251 (2023) Chapter 6: Monolayer ReSe2 on bilayer graphene. Research into intrinsic defects within crystalline structures is a topic of interest spanning a variety of scientific fields. Our study employs STM to analyze the characteristics of Se vacancies in monolayer ReSe2 on a graphene substrate. We identified the common presence of Se4 vacancies near the Re layer among the potential Se vacancy locations. Additionally, a defect state associated with these vacancies is directly observed in our dI/dV spectroscopy, occurring at roughly -1.0 V near the valence band maximum. STM topography at the defect state level shows a bright protrusion near the Se4 vacancy, effectively spatially mapping its electronic state. These findings enhance our understanding of ReSe2/graphene heterojunctions with naturally existing Se vacancies. Reference: Nguyen Huu Lam†, Jae Hyeok Ko†, Byoung Ki Choi†, et al. Nanoscale Advances 5, 5513-5519 (2023)