Nanostructured interfaces have served as a crucial aspect to tailor both physical and chemical properties of today’s 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 two-dimensional-material devices and the metal-support interface in atomically dispersed catalysts.

Through engineering a local structure distortion in the metal-semiconductor junction by the soft oxygen-plasma treatment, barrier-free electrical contacts between metal leads and atomically thin transition metal dichalcogenide semiconductors (TMDCSCs) have been demonstrated. Low-voltage atomic-resolution cross-section TEM of the contact region from practical 2H MoS₂ and WSe₂ field-effect transistors (FETs) reveals the interfacial distorted nanophase as an efficient carrier-injection path, which greatly enhances contact properties by achieving the record-low contact resistance of 90 Ωµm and the record-high field-effect mobility of 358,000 cm²V^-1s^-1, comparable to those of high-quality graphene devices. Such high-quality electrical contact also enables the detection of prominent quantum transport at low filling factors, showing signals of intricate e-e interaction effects.

Moreover, by manipulating the strong metal-support interactions in the defective surface of nanodiamond-graphene hybrids (ND@G), a wide range of single-atom catalysts (SACs, e.g. Pd, Au, Cu) have been successfully synthesized, of which the unique metal-carbon interfaces were directly observed by atomic-scale TEM. Due to the full exposure of catalytically active sites anchored by carbon defects, the SACs have shown both boosted catalytic performance and unparallel thermal stability than conventional nanoparticle catalysts. The strategy was further extended to fabricate atomically dispersed bimetallic catalysts (e.g. Pt-Sn, Pt-Fe and Cu-Zn) with composition-modulated coordination and highly tunable catalytic characteristics.