As nanotechnology plays essential parts in developing of high performance or new concepts of electrical and optical devices, new class of nanomaterials has emerged to beat conventional optical and electronic devices. Mixed-dimensional hetero-interfaces, composed of low-dimensional material components, have been the focus of ongoing research efforts to surpass Si-based device and explore nano-optical phenomenon. The physical and electronic properties of these interfaces, such as transitions in density of states, intimacy of contact and localized strain, govern the charge/energy transfer between neighboring nanomaterials, endowing them entirely new optoelectronic functions. However, due to their deep-subwavelength nature challenged by diffraction-limit, a limited understanding of the fundamental physical properties of these interfaces exists. Deep-subwavelength imaging therefore takes important roles, as it enables collection of nano-optical and electrical signals within a tens of nanometer range. In this dissertation, I present multiple nanoscale interfaces where scanning probe techniques effectively and directly probe the mixed-dimensional interfaces with deep-subwavelength resolution. First, I optically investigate various nanoscale defects in as-grown MoSe2 using chemical vapor deposition, contributing to the rational synthesis of nanomaterials by providing direct evidence of defect distribution. Second, I introduce a universal strategy for characterizing buried semiconductor-metal contact interfaces involving 2D semiconductors and 3D metals and discuss critical factors that impact their optical and electrical properties. Third, I exhibit localized emissions at the 2D semiconductor -metal interfaces and reveal how charge transfer significantly influences the localized states. Finally, I provide experimental and computational evidence that strong light-matter interactions of quantum dot can be visualized using tip-enhanced nano-optical spectroscopy. The approaches presented in this dissertation offer valuable insights for studying various excitonic nanomaterial systems advancing the fields of nano-optical imaging as well as electronics based on these low-dimensional semiconductors.