Nanostructured interfaces have served as a crucial aspect to tailor both chemical and physical properties of today’s newly emerging materials, whose effects become increasingly important when the system dimension scales down. Therefore, a precise knowledge on the structural variation as well as the chemical inhomogeneity of nanostructured interfaces, ideally at atomic scale, is indispensable for the rational design of nanomaterials’ functionality. In this thesis, atomic-resolution transmission electron microscopy (TEM) study was conducted to investigate the critical roles played by two types of functional interfaces in nanomaterials, including the metal-semiconductor junction in 2D-material electronic devices and the metal-support heterostructure in atomically dispersed catalyst...[ Read more ]

Nanostructured interfaces have served as a crucial aspect to tailor both chemical and physical properties of today’s newly emerging materials, whose effects become increasingly important when the system dimension scales down. Therefore, a precise knowledge on the structural variation as well as the chemical inhomogeneity of nanostructured interfaces, ideally at atomic scale, is indispensable for the rational design of nanomaterials’ functionality. In this thesis, atomic-resolution transmission electron microscopy (TEM) study was conducted to investigate the critical roles played by two types of functional interfaces in nanomaterials, including the metal-semiconductor junction in 2D-material electronic devices and the metal-support heterostructure in atomically dispersed catalysts.

The dissertation starts with a brief history through the development of electron microscopy. The theoretical principles and instrumentation of electron-matter interactions and aberration-corrected electron optics utilized by the modern TEM will be discussed, followed by the introduction to two types of nanostructured interfaces as study subjects. The focus will then be placed on how the microscopic investigation of interfacial structure and composition can be correlated to the materials’ functionality.

As for the experimental techniques, different material-property measurements and TEM sample-preparation methods will be described in detail. Both the aberration-corrected scanning TEM and other complementary characterization tools, such as in-situ TEM and synchrotron X-ray absorption spectroscopy, are covered in this chapter.

By applying the atomic-scale TEM investigation to two distinct functional interfaces, results were obtained and analyzed in the following chapters:

Chapter 3 aims to fix the problem of bridging metal leads to two-dimensional transition metal dichalcogenide semiconductors by the oxygen-plasma-induced local structure distortion in the metal-semiconductor junction. Low-voltage atomic-resolution cross-section imaging of the contact region in practical 2H MoS₂ and WSe₂ field-effect devices reveals the interfacial distorted nanophase as the efficient carrier-injection path, which greatly enhances contact properties by achieving the record-low contact resistance of 90 Ωμm, towards the quantum limit, and the record-high field-effect mobility of 358,000 cm²V^-1s^-1, comparable to those of high-quality graphene devices. Such barrier-free electrical contact also enables the detection of prominent quantum transport at the low/fractional filling factors showing signals of intricate e-e interaction effects.

Chapter 4 concentrates on the effects of metal-support interactions on the metal dispersion behaviors of nanocarbon-supported catalysts. A wide range of single-atom catalysts have been synthesized due to the defect-engineered strong metal-support interactions and demonstrated to possess unparallel thermal stability on top of boosted catalytic performance. The unique metal-defect coordination was further extended to fabricate atomically dispersed bimetallic catalysts with composition-modulated morphologies and catalytic characteristics.

In summary, atomic-scale TEM can be widely applicable to other fundamental or industrial problems pertinent to the nanostructured interface, even more regarding the materials science. The unsolved points and prospects for future works will also be discussed in Chapter 5 at the end of this thesis.