Session 2

Tony Low Assistant Professor of Electrical and Computer Engineering, University of Minnesota

The infrared spectrum (~1mm – 1 µm) presents many opportunities for photonics applications, such as chemical and biosensing, and free space communication. In this talk, I will review the recent developments in the understanding of the infrared optoelectronics processes in graphene and black phosphorus, and their possible application space. The talk will be sectioned to answer the following questions: How does atomically thin materials like graphene and black phosphorus detects light (i.e. photodetector)? How does one controls light-matter interactions electrically (i.e. modulator)? How does one create metasurfaces from these materials to realize novel far-field phenomena such as anomalous reflection, focusing, cloaking and illusion optics, polarization rotation and conversion?

Seung Hyun Song CINAP, IBS, Sungkyunkwan University, Suwon, Korea

Monolayer molybdenum disulfide (MoS₂) has received intense interest as a strong candidate for next-generation electronics. However, the observed electrical properties of monolayer MoS₂ exhibit several anomalies: reported mobilities are unexpectedly low, < 150 cm²V^-1s^-1; samples are universally observed to have n-type characteristics; and contact resistances are large, regardless of contact metal work function. All these anomalies have been attributed to the presence of defects, but the mechanism behind this link has been elusive. Here we report the ionization dynamics of sulfur monovacancy defects in monolayer MoS₂ probed via noise nanospectroscopy, realized by combining noise-current analysis with atomic force microscopy (AFM). Due to the nanoscale dimension of the in situ channel defined by the tip size, we probe a few monovacancy defects at a time. Monovacancy defects exhibit switching between three distinct ionization configurations, corresponding to charge states 0, -1, and -2. The most probable charge configurations are 0 and -1, providing a plausible mechanism to explain the unexpectedly low mobility, large contact resistance, and universally observed n-type characteristics of MoS₂ monolayers.

Reference: Seung Hyun Song, Min-Kyu Joo, Michael Neumann, Hyun Kim, and Young Hee Lee, Probing defect dynamics in monolayer MoS₂ via noise nanospectroscopy. submitted.

Jong Seok Jeong Department of Chemical Engineering and Materials Science, U. of Minnesota

Despite progress in construction of novel devices and understanding their electronics, many important and fundamental electronic properties of these materials must be measured ex situ to prevent convoluting the response of the materials with details of transistor behavior and manufacturing difficulties. This problem has added great complexity to understand device performances. A major challenge for developing 2D devices is the control of atomic structures and the role of new electronic states at the resulting structures. However, the understanding of how these structures affect the local and overall electronic properties remains limited largely because this requires a systematic investigation at the atomic scale. Here, using high-quality low-noise scanning transmission electron microscopy energy dispersive x-ray spectroscopy (STEM EDX) maps of a model system, SrTiO₃, we demonstrate that analytical STEM can go beyond elemental profiling of whole atoms to quantitatively probe core-level electron orbitals. We also discuss the recent study of the local atomic and electronic structure of a new line defect in perovskite oxides.