Abstract
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.
Highlights
In this paper, we aim to expand the previous work11 and to establish force-correlative spectroscopy in Tip-enhanced Raman scattering (TERS) microscopy, i.e., a technique to measure tip-sample force simultaneously with TERS spectra
Tip-enhanced spectra are discussed in conjunction with the tip-sample force in the repulsive force region and in the attractive force region that is not accessible with the conventional TERS microscopy based on contact- and tapping-mode atomic force microscope (AFM)
A high resolution TERS microscopy image of a single carbon nanotubes (CNTs) is shown to demonstrate the imaging capability of the developed TERS microscope based on frequency-modulation atomic force microscope (FM-AFM)
Summary
We aim to expand the previous work11 and to establish force-correlative spectroscopy in TERS microscopy, i.e., a technique to measure tip-sample force simultaneously with TERS spectra. From the amount of frequency shift ∆f, the tip-sample force can be determined, with positive and negative ∆f corresponding to the repulsive and attractive tip-sample force, respectively. We demonstrate simultaneous measurements of TERS spectra and tip-sample force using CNTs as a sample.
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