Electrical contact resistance (ECR) is an important parameter to optimize thedesign and, evaluate the contact reliability and performance of small-scale electri- cal and electro-mechanical systems including those that are implemented in crucial radar and telecommunication technologies. State-of-the art electronic devices utiliz- ing metal-insulator-metal or molecular tunnel junctions are also affected to a great extent by ECR. Consequently, it is of utmost importance to understand the physical mechanisms pertinent to ECR and to precisely characterize ECR at different types of interfaces formed between mating surfaces. In this thesis, we primarily focused on measuring ECR at atomically flat, sub-micron interfaces formed between gold islands and highly oriented pyrolytic graphite (HOPG) via conductive atomic force microscopy (C-AFM). By means of proof-of-principle experiments, we were able to correlate the measured ECR in the diffusive conduction regime (where contact size is larger than the electron mean free path) to true, measurable contact areas. We also presented an improved approach that led to more reliable ECR measurements via C-AFM by suppressing the effects of tip deformation, humidity and temperature variations as well as substrate rough- ness on the experiments. The ECR values measured via this improved technique follow a Gaussian distribution with a much lower standard deviation compared to those obtained via conventional approaches. Finally, we demonstrated an improved method to synthesize gold islands of varying size on HOPG that can be utilized in future research to investigate the physical mechanisms of ECR in the intermediate conduction regime whereby the contact size is on the order of the electron mean free path. In summary, the findings reported in this thesis constitute the first mea- surements of ECR in the diffusive conduction regime as a function of true contact area at atomically flat interfaces. Based on crucial improvements in sample synthe- sis and measurement methodology reported in this thesis, future work will focus on elucidating the physical mechanisms of ECR across all electron conduction regimes.

A great number of chemical and mechanical phenomena, ranging from catalysis tofriction, are dictated by the atomic-scale structure and properties of material surfaces. Despite such enormous significance, the principal tools utilized to characterize surfaces at the atomic level rely heavily on strict environmental conditions such as ultrahigh vacuum and low temperature. Results obtained under such well-controlled, pristine conditions bear limited relevance to the great majority of processes and applications that often occur under ambient conditions. In this thesis, we report true atomic- resolution surface imaging via conductive atomic force microscopy (C-AFM) under ambient conditions, performed at high scanning speeds. We hypothesize that atomic resolution can be enabled by either (i) a confined, electrically conductive pathway at the tip–sample contact, or (ii) tunneling through a confined water layer accumulated on the sample surface under ambient conditions. Our approach delivers atomic- resolution maps on a variety of material surfaces that comprise defects including single atomic vacancies. Using our method, we also report the capability of in situ charge state manipulation of defects on MoS 2 . Finally, we employ the high-speed C- AFM methodology to study a thin transition metal carbide crystal (i.e., an MXene), α–Mo 2 C. Along with a variety of atomically-resolved defect structures, we observe an exotic electronic effect: room-temperature charge ordering. Our findings demonstrate that C-AFM can be utilized as a powerful tool for atomic-resolution imaging and manipulation of surface structure and electronics under ambient conditions, with wide-ranging applicability.