Interplay of the tip–sample junction stability and image contrast reversal on a Cu(111) surface revealed by the 3D force field

Abstract

Non-contact atomic force microscopy is used to measure the 3D force field on a dense-packed Cu(111) surface. An unexpected image contrast reversal is observed as the tip is moved towards the surface, with atoms appearing first as bright spots, whereas hollow and bridge sites turn bright at smaller tip-sample distances. Computer modeling is used to elucidate the nature of the image contrast. We find that the contrast reversal is essentially a geometrical effect, which, unlike in gold, is observable in Cu due to an unusually large stability of the tip-sample junction over large distances.

Site-Dependent Evolution of Electrical Conductance from Tunneling to Atomic Point Contact

“…The challenges of atomic point contacts have been addressed by various approaches, including the break-junction method [2][3][4][5], transmission electron microscopy [6], and scanning tunneling microscopy (STM) [7][8][9][10][11][12][13][14][15], as well as in theoretical calculations [16][17][18][19][20][21][22].…”

“…The histograms taken on metals, except noble ones, exhibit broad distributions with multiple peaks, which have been attributed to the stochastic nature of the contact formation. In fact, theoretical studies have indicated that the lateral configuration of the contact-forming atoms causes this significant variation in conductance [16][17][18][19][20][21][22].…”

3D Force Field Spectroscopy

“…(b) Utilizing an alternative approach, the volumetric f (x, y, z) data may be recorded in a layer-by-layer fashion, by combining a group of topographical NC-AFM images that contain f (x, y) data for set tip-sample distances z [26,29,31,32,36]. Volumetric maps of interaction force (F(x, y, z)) and energy (E(x, y, z)) are then obtained via the same procedures employed for the curveby-curve approach [22].…”

“…Additionally, contrast changes with respect to changing tip-sample distance (such as those in [26,32]) are directly observed during data acquisition, while such information only becomes observable after data processing in the case of curve-by-curve data acquisition.…”

“…While the DFS technique allows the acquisition of single interaction curves on defined lattice sites as indicated, converting the method into a comprehensive tool capable of collecting full three-dimensional (volumetric) maps of interaction forces and energies with atomic resolution took considerable effort due to limitations in terms of various experimental factors including tip and sample stability, as well as thermal drift. Eventually, thanks to advancements in instrumentation and experimental methodology such as low temperature operation [23] and atom-tracking/feedforward methods [24,25], the method of three-dimensional atomic force microscopy (3D-AFM) was established, implementation of which has now successfully been demonstrated by various research groups on a variety of sample surfaces [26][27][28][29][30][31][32][33][34][35][36][37]. Originally developed for vacuum conditions, the method has recently been extended to operation under liquid environments, thereby opening up tremendous possibilities for high-resolution investigation of biological material under close-to-natural conditions [38][39][40].…”