True Atomic-Resolution Surface Imaging and Manipulation under Ambient Conditions via Conductive Atomic Force Microscopy

Abstract

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.

Atomically Resolved Defects on Thin Molybdenum Carbide (α-Mo2C) Crystals

“…It should be noted that large-scale current images such as the one presented in Figure a allow to calculate a defect density on the order of 10 11 cm –2 for the α-Mo 2 C surface, which is comparable to what has been reported for transition metal dichalcogenides (TMDCs) in the literature . Of particular importance is the complete absence of these defects in the topographical images that are simultaneously recorded with the current data (see representative topographical map in Figure S5), which may be attributed to the loss of lateral resolution that AFM experiences in topographical imaging conducted under contact mode, due to blunt physical contacts formed between the tip and the sample …”

“…Motivated by the above, we present here a combined experimental and computational study aimed at investigating defects on the (100) surface of thin α-Mo 2 C crystals. We conducted atomic-resolution imaging experiments by employing high-speed C-AFM scanning under ambient conditions . This technique has recently proven its reliability on surfaces of a variety of materials (e.g., MoS 2 , WSe 2 , PtSe 2 , and Au) and was able to identify defects including but not limited to single atomic vacancies .…”

“…The recently introduced method of high-speed conductive atomic force microscopy (C-AFM) under ambient conditions is suited well to achieve this goal. 15 Motivated by the above, we present here a combined experimental and computational study aimed at investigating defects on the (100) surface of thin α-Mo 2 C crystals. We conducted atomic-resolution imaging experiments by employing high-speed C-AFM scanning under ambient conditions.…”

“…We conducted atomic-resolution imaging experiments by employing high-speed C-AFM scanning under ambient conditions. 15 This technique has recently proven its reliability on surfaces of a variety of materials (e.g., MoS 2 , WSe 2 , PtSe 2 , and Au) and was able to identify defects including but not limited to single atomic vacancies. 15 When it comes to the underlying mechanism enabling C-AFM to achieve atomic-resolution imaging under ambient conditions, it has been observed that the scanning speed plays a significant role such that the resolution reversibly improves or degrades with increasing or decreasing speed, respectively.…”

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite

“…Segregation of the structural defects at the domain walls [ 34 ], grain boundaries [ 36 ], and at the secondary phase locations [ 37 ] can modify the type of conductivity [ 38 ] and create a net of distributed channels for charge transport. Conductive atomic force microscopy (CAFM) is a useful approach to probe charge dynamics locally in the individual elements of the material microstructure: domain walls [ 30 , 31 , 32 ], the interior of the material or grain boundaries [ 36 ], or even around individual defects [ 1 , 39 , 40 ]. New technology of fast data acquisition and treatment motivates to further develop CAFM for the studying of disordered and distributed systems [ 41 , 42 ].…”