A great number of chemical and mechanical phenomena, ranging from catalysis to friction, are dictated by the atomic-scale structure and properties of material surfaces. Yet, the principal tools utilized to characterize surfaces at the atomic level rely on strict environmental conditions such as ultrahigh vacuum and low temperature. Results obtained under such well-controlled, pristine conditions bear little relevance to the great majority of processes and applications that often occur under ambient conditions. Here, we report true atomic-resolution surface imaging via conductive atomic force microscopy (C-AFM) under ambient conditions, performed at high scanning speeds. Our approach delivers atomic-resolution maps on a variety of material surfaces that comprise defects including single atomic vacancies. We hypothesize that atomic resolution can be enabled by either a confined, electrically conductive pathway or an individual, atomically sharp asperity at the tip–sample contact. Using our method, we report the capability of in situ charge state manipulation of defects on MoS2 and the observation of an exotic electronic effect: room-temperature charge ordering in a thin transitionmetal carbide (TMC) crystal (i.e., an MXene), α-Mo2C. 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.

Electrical contact resistance (ECR) measurements performed via conductive atomic force microscopy (C-AFM) suffer from poor reliability and reproducibility. These issues are due to a number of factors, including sample roughness, contamination via adsorbates, changes in environmental conditions such as humidity and temperature, as well as deformation of the tip apex caused by contact pressures and/or Joule heating. Consequently, ECR may vary dramatically from measurement to measurement even on a single sample tested with the same instrument. Here we present an approach aimed at improving the reliability of such measurements by addressing multiple sources of variability. In particular, we perform current-voltage spectroscopy on atomically flat terraces of highly oriented pyrolytic graphite (HOPG) under an inert nitrogen atmosphere and at controlled temperatures. The sample is annealed before the measurements to desorb adsorbates, and conductive diamond tips are used to limit tip apex deformation. These precautions lead to measured ECR values that follow a Gaussian distribution with significantly smaller standard deviation than those obtained under conventional measurement conditions. The key factor leading to this improvement is identified as the switch from ambient conditions to a dry nitrogen atmosphere. Despite these improvements, spontaneous changes in ECR are observed during measurements performed over several minutes. However, it is shown that such variations can be suppressed by applying a higher normal load.

An approach to measuring electrical contact resistance as a direct function of the true contact size at the nanoscale is presented. The approach involves conductive atomic force microscopy (C-AFM) measurements performed on a sample system comprising atomically flat interfaces (up to several hundreds of nanometers in lateral size) formed between gold islands and a highly oriented pyrolytic graphite (HOPG) substrate. The method overcomes issues associated with traditional C-AFM such that conduction can be correlated with a measurable true, conductive contact area. Proof-of-principle experiments performed on gold islands of varying size point toward an increasing contribution of the island-HOPG junction to the measured total resistance with decreasing island size. Atomistic simulations complement and elucidate experimental results, revealing the maximum island size below which the electrical contact resistance at the island-HOPG junction can be feasibly extracted from the measured total resistance.

Conductive-atomic force microscopy (C-AFM) and molecular dynamics (MD) simulations are used to investigate time-dependent electrical contact resistance (ECR) at the nanoscale. ECR is shown to decrease over time as measured using C-AFM and estimated using two approaches from MD simulations, although the experiments and simulations explore different time scales. The simulations show that time dependence of ECR is attributable to an increase in real contact area due to atoms diffusing into the contact. This diffusion-based aging is found to be a thermally activated process that depends on the local contact pressure. The results demonstrate that contact aging, previously identified as an important mechanism for friction, can significantly affect electrical conduction at the nanoscale. Graphical Abstract

Thin transition metal carbides (TMCs) garnered significant attention in recent years due to their attractive combination of mechanical and electrical properties with chemical and thermal stability. On the other hand, a complete picture of how defects affect the physical properties and application potential of this emerging class of materials is lacking. Here, we present an atomicresolution study of defects on thin crystals of molybdenum carbide (α-Mo 2 C) grown via chemical vapor deposition (CVD) by way of conductive atomic force microscopy (C-AFM) measurements under ambient conditions. Defects are characterized based on the type (enhancement/attenuation) and spatial extent (compact/extended) of the effect they have on the conductivity landscape of the crystal surfaces. Ab initio calculations performed by way of density functional theory (DFT) are employed to gather clues about the identity of the defects.