Atomic force microscopy contact forces are shown to obey a Poisson distribution, so that the ratio of their variance to mean gives the force of a single chemical bond between the tip and sample. The derived single-bond force was able to distinguish nominal van der Waals interactions (60 ( 3 pN) from hydrogenbond interactions (181 ( 35 pN) between atomic force microscope tips and gold and mica surfaces, respectively. This technique greatly reduced sampling time and sample wear, allowed quantitative use of low-resolution force data from a commercially available instrument, and detected important chemical differences between surface functional groups on the samples. These experiments constitute an important step in obtaining chemically specific information in atomic force microscopy.

The gas-phase adsorption of 11-mercaptoundecanoic acid (HOOC(CH2)10SH) on Ni(111) in ultrahigh vacuum was studied using angle-dependent X-ray photoelectron spectroscopy (ADXPS) and temperature-programmed desorption (TPD). We present evidence which shows that 11-mercaptoundecanoic acid is adsorbed to Ni(111) via the sulfur atom with the carboxylic acid group disposed away from the surface. The monolayer thickness of this molecule, 10.4 ± 1.7 Å, estimated by photoelectron attenuation in ADXPS, was comparable to ellipsometric measurements of related n-alkanethiols on Au(111). The interaction of polar (CH3OH) and nonpolar (n-C6H14) probe molecules with this acid-terminated surface showed that the former interact more strongly than the latter as studied by TPD. A Redhead analysis was used to analyze the TPD spectra of CH3OH and n-C6H14 desorbing from this thiol-covered, acid-terminated Ni(111) surface. The calculated low-coverage desorption energy for CH3OH desorption from HOOC(CH2)10SH/Ni(111) was 41 kJ mol-1. The calculated low-coverage desorption energy for n-C6H14 desorption from HOOC(CH2)10SH/Ni(111) was 37 kJ mol-1.