Quantitative Atomic Force Microscopy with Carbon Monoxide Terminated Tips

Abstract

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes. Scanning probe methods have been the workhorse of nanotechnology in obtaining atomic-scale structural and electronic information. While earlier experiments relied mostly on scanning tunneling microscopy (STM) [1][2][3][4], noncontact atomic force microscopy (AFM) has recently revolutionized the field of atomic-scale imaging by the impressive demonstrations of atomic resolution on bulk and ultrathin insulators and atomic identification and manipulation at room-temperature on semiconductor surfaces [5][6][7][8][9][10]. Improvements in AFM instrumentation have further extended these possibilities as demonstrated by the structural characterization of single crystal surfaces and isolated organic molecules with unprecedented spatial resolution [11][12][13][14][15]. These advances are based on the use of a quartz tuning fork force sensor operating with very small tip oscillation amplitudes, down to a fraction of an Å ngström [16,17]. As a consequence, the measured frequency shift Áf (proportional to the force gradient) is dominated by the short range (chemical) forces allowing the interaction energies to be measured down to the atomic scale [10,14,[16][17][18][19].Previous experiments have shown that using a carbon monoxide terminated tip (formed by controlled vertical manipulation of CO from the sample surface onto the tip) yields a well-defined tip apex and enhanced image resolution [12,20]. Here, we use the same approach and present precise measurement of intermolecular forces between two CO molecules: one adsorbed on the Cu(111) single crystal substrate and the other on the AFM tip as schematically illustrated in Fig. 1. The frequency shift, Áf, is measured as a function of tip-sample distance (z direction) and lateral distance between tip and the adsorbed CO molecule. By performing this experiment with a metalterminated and a CO-terminated tip, a three-dimensional map of the intermolecular interaction potential with subatomic resolution is obtained. At sufficiently large intermolecular separations this approach yields the ''true'' intermolecular interaction, i.e., a combination of chemical repulsion (due to the Pauli exclusion principle) and van der Waals (vdW) attract...

Origin of High-Resolution IETS-STM Images of Organic Molecules with Functionalized Tips

“…The sharp ridges observed in the intramolecular region often mimics the internal molecular structure [5], with only few exceptions [6,7]. The capability of AFM/STM to resolve internal atomic and chemical structure in real space opened new horizons for the characterization of molecules and surfaces [5,[8][9][10][11][12][13][14][15] at atomic scale.The origin of high resolution of molecular structures in AFM has been attributed to Pauli repulsion [2,16] and the bending of the functionalized tip apex [5]. Recently, we introduced a numerical model [7] which provides a unified insight into the detailed mechanism of the high resolution imaging with decorated tips in both AFM and STM.…”

“…The sharp ridges observed in the intramolecular region often mimics the internal molecular structure [5], with only few exceptions [6,7]. The capability of AFM/STM to resolve internal atomic and chemical structure in real space opened new horizons for the characterization of molecules and surfaces [5,[8][9][10][11][12][13][14][15] at atomic scale.…”

Image correction for atomic force microscopy images with functionalized tips

“…Whereas different tip functionalizations have been reported [2,3], CO was used most often, and is now being widely applied [2][3][4][5][6][7][8][9][10][11][12][13][14].…”

Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy

“…Since then, this technique has been widely applied [2][3][4][5][6][7][8][9][10]. AFM-based Kelvin probe force spectroscopy (KPFS) [11] provides the local contact potential difference (LCPD) between tip and sample [12][13][14].…”