Anomalous Tip-Sample Distance Behavior on the Tip-Enhanced Raman Spectroscopy of Graphene in Ambient Conditions

Abstract

Tip-Enhanced Raman Spectroscopy (TERS) combines Raman spectroscopy with scanning probe microscopy to overcome the spatial resolution limitation imposed by light diffraction, offering a primary optical technique for the comprehensive study of two-dimensional (2D) materials. In this work, we investigate an anomalous decay profile of the TERS intensity of the graphene 2D band as the tip-sample separation changes, observations enabled by high TERS efficiency and accuracy in tip-approach and tip-retract procedures. The anomalous results can be properly described by the addition of an ad hoc deformation to the effective tip-sample distance, rationalized here as due to the presence of a liquid meniscus formed via capillary forces.

Figures (3)

• Figure 1: (a) Schematic diagram of the approach and retract experiments, where $z$ defines the tip-sample distance. (b) Raman spectrum of the 2D band of graphene at three different $z$ values (see legend), where the closest distance is assumed as $z_0=5$ nm novotny2012principles. (c) Normalized TERS near-field intensity $I^{NF}(z)$ of the 2D band (data points), overlaid with the fitting curves derived from the two TERS models discussed in this work, on a tip-retraction experiment.
• Figure 2: Normalized TERS near-field intensity decay $I^{NF}(z)$ alongside the corresponding tuning fork oscillation amplitude for the approach (a) and retraction (b) experiments. The arrows indicate the direction of the tip movement.
• Figure 3: (a) Normalized TERS intensity decay $I^{NF}(z)$ of the 2D band (black circles) and the fitted curve from both (deformed and undeformed) models as a function of the mechanical $z$ separation (same as Fig. \ref{['fig:diagram']}c, here in log scale). (b) Illustration of the deformation function $D(z)$, where $\Delta S$ (snap amplitude) represents the magnitude of the offset and $W_s$ (snap width) controls the transition range. (c) Normalized TERS intensity decay of the 2D band (black circles) plotted versus the effective separation $z_{\text{eff}}$. The fitted theoretical curve (gray dashed line) shown in (c) is the undeformed model cancado plotted against the undeformed $z$ values.