Kohta Saitoh

Tip-enhanced Raman scattering (TERS) microscopy attracts increasing attention for visualization and characterization of strain distribution on crystalline samples at a nanoscale due to nano-sized fields localized at a metal tip. However, as the metal tip approaches close to a sample surface, a force acts between the tip and sample, which can induce unwanted perturbation on the local strain of the sample. Analysis and evaluation of intrinsic strains on samples with a high reliability demands a technique to correlate TERS spectra with the tip-sample force. Here, we present a TERS microscope based on a frequency-modulation atomic force microscope (FM-AFM) using a quartz tuning fork (QTF) as a force sensor. By continuously monitoring a shift in the resonance frequency of the QTF during TERS, the tip-sample force can be directly measured in both attractive and repulsive force regions, which is not possible by other AFMs. TERS spectra of single-walled carbon nanotubes (CNTs) were measured simultaneously while changing the tip-sample distance and hence the tip-sample force acting on the CNTs. We found that TERS occurs at the tip-sample distance where the repulsive force dominantly works and mostly decays out in the attractive force region.

We report measurements of noncontact friction between surfaces of NbSe2 and SrTiO3 and a sharp Pt-Ir tip that is oscillated laterally by a quartz tuning fork cantilever. At 4.2~K, the friction coefficients on both the metallic and insulating materials show a giant maximum at the tip-surface distance of several nanometers. The maximum is strongly correlated with an increase in the spring constant of the cantilever. These features can be understood phenomenologically by a distance-dependent relaxation mechanism with distributed time scales.