There is no such thing as the perfect material; even single crystals possess structural defects. The role of atomic heterogeneity in the low dimensional electronic systems (1D/2D) is more decisive than in bulk materials (3D) due to dimensional confinement. Probing the electronic and magnetic structure of heterogeneities, such as structural defects, impurity, intercalation, step edge, hetero-interface, and disorder, can reveal the atomic-level understanding of low-dimensional electronic systems to enable new functionality.
Our research focuses on connecting atomistic details of the system's heterogeneities to collective behaviors to develop unprecedented functionality for devices. We explore heterogeneities in low-dimensional electronic systems by using scanning probe microscopies, such as scanning tunneling microscopy and spectroscopy (STM/S), atomic force microscopy (AFM), multi-probe STM (4P-STM), spin-polarized STM, scanning tunneling thermovoltage microscopy (STVthM), and so on. For example, STM/S revealed local electronic states of graphene-hBN in-plane heterostructure created by two-dimensional heteroepitaxy (Science 343 163 (2014), Nature communications 5 , 1 (2014)). Also, we characterized point defects in the few-layer 2H-MoTe2 as p-type and n-type dopants after photo-induced doping (Nature Electronics 1 512 (2018), Nature Electronics 4 38 (2021)). STVthM displayed thermoelectric power of atomic defects in SiC-graphene (Nano letters 13 3269 (2013)), and SP-STM presented chemical control of ferromagnetic surface states Co nanoislands (Nano Letters 17 292 (2017)). Recently, we developed spin-polarized (SP)-STVthM to measure spin-dependent thermoelectric power of Co nano-islands (Nano Letters 20 4910 (2020)).
Heterogeneity research on low dimensional electronic systems is based on expertise on atomistic controlling and understanding of materials with advanced probe microscopy. To do so, we are closely collaborating with the material research group for new low dimensional hetero structure, and theory group to combine measured data analysis with first principle calculations.
We have constructed ULT-HF-SPM, which can operate as both atomic force microscopy (AFM) and scanning tunneling microscopy (STM) under extreme conditions (2 K, 12 T). We can extend the atomic-resolution probe microscopy to the non-metallic samples by using the quartz tuning fork with conducting tip. It allows us to measure tunneling current and resonant frequency shift simultaneously as a function of the tip-sample distance. The ULT-HF-SPM system is also equipped with in-situ surface treatment techniques, such as low-temperature cleaving and thermal deposition at variable temperature, to investigate artificial low-dimensional electronic systems. We are also developing computer vision techniques and statistical analysis to process acquired multi-dimensional data set from the advanced microscopy. We exploit the ULT-HF-SPM to study localized electronic states of 1D/2D materials and their heterostructure under extreme environments regardless of local conductivity.
We utilize scanning tunneling thermovoltage microscopy (STVthM) to study defects and hetero interface in low-dimensional electronic systems. By using temperature gradient between tip and sample, we can explore the local thermoelectric power of the sample. Thermoelectric power of low-dimensional materials implies the opportunity to fabricate novel device platform for the energy conversion process. Beyond the bulk thermal property measurements using complex device lithography and fabrication, the STVthM provides a new route to study thermal properties of local heterogeneity in low dimensional materials. Recently, we developed spin-dependent STVthM for the spin caloritronics study by using spin-polarized probe. In addition, we utilized a mobile chamber to precisely control the number of defects and preserve the atomic-level surface cleanliness. We can move an as-grown sample from a chemical vapor deposition system to the STM chamber without air exposure.