High-resolution imaging of C₆₀molecules using tuning-fork-based non-contact atomic force microscopy

Abstract

Recent advances in non-contact atomic force microscopy (nc-AFM) have led to the possibility of achieving unprecedented resolution within molecular structures, accomplished by probing short-range repulsive interaction forces. Here we investigate C(60) molecules adsorbed on KBr(111) and Cu(111) by tuning-fork-based nc-AFM. First, measurements of C(60) deposited on KBr(001) were conducted in cryogenic conditions revealing highly resolved nc-AFM images of the self-assembly. Using constant-frequency shift mode as well as three-dimensional spectroscopic measurements, we observe that the relatively weak molecule-substrate interaction generally leads to the disruption of molecular assembled structures when the tip is probing the short-range force regime. This particular issue hindered us in resolving the chemical structure of this molecule on the KBr surface. To obtain a better anchoring of C(60) molecules, nc-AFM measurements were performed on Cu(111). Sub-molecular resolutions within the molecules was achieved which allowed a direct and unambiguous visualization of their orientations on the supporting substrate. Furthermore, three-dimensional spectroscopic measurements of simultaneous force and current have been performed above the single molecules giving information of the C(60) molecular orientation as well as its local conductivity. We further discuss the different imaging modes in nc-AFM such as constant-frequency shift nc-AFM, constant-height nc-AFM and constant-current nc-AFM as well as three-dimensional spectroscopic measurement (3D-DFS) employed to achieve such resolution at the sub-molecular scale.

3D Force Field Spectroscopy

“…(a) The curve-by-curve method, involving the recording of individual curves of frequency shift as a function of tip-sample distance ( f (z)) at a number of lateral (x, y) locations on the sample surface [30,31,[33][34][35]37]. The curves are then [41] combined such that full three-dimensional maps of frequency shift ( f (x, y, z)) are formed that are subsequently converted to interaction force (F(x, y, z)) and energy (E(x, y, z)) maps [22].…”

“…In contrast, the feedforward procedure [25] is based on the real-time correction of drift during data acquisition by applying appropriate voltages to the scan piezo, which are predicted by relying on the assumption that the drift vector can be adequately determined in prior measurements. Both approaches have been successfully implemented in the past to perform 3D force field spectroscopy at room temperature and low temperatures on various sample surfaces [30,31,[33][34][35]37]. One potential shortcoming of the method is the fact that the drift vector may need to be frequently updated during actual measurements based on the unpredictability of thermal fluctuations and associated drift rates.…”

“…• Method A: Layer-by-layer acquisition of data, post-data-acquisition drift correction, operation at low temperatures [26,36] • Method B: Curve-by-curve acquisition of data, atom-tracking/feedforward procedures, operation mostly at room temperature [30,31,[33][34][35]37] • Method C: Curve-by-curve or line-by-line acquisition of data, use of a reference image for drift correction at fixed intervals during data acquisition, operation at low temperatures [27] It should be noted though, that the three methods listed above represent a generalization at best and different combinations of approaches can be utilized in different experiments to address specific issues. Using Table 2.1,…”

“…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].…”

Morphology Change of C60 Islands on Organic Crystals Observed by Atomic Force Microscopy

“…21 At low temperature, C 60 molecules were characterized with several SPM techniques on various surfaces and submolecular contrast have been obtained by scanning tunnelling microscopy (STM) [22][23][24] and non-contact atomic force microscopy (nc-AFM). [25][26][27] At room temperature (RT), two dimensional C 60 layers have been observed on metals [28][29][30] and semiconductors [31][32][33] while large three dimensional molecular islands or clusters have been revealed on ionic crystals. [34][35][36][37][38][39] Additionally, the controlled manipulation of C 60 molecules has also been demonstrated through the action of a SPM tip as single molecules [40][41][42][43] or as entire islands.…”