Spatially Coherent Tip-Enhanced Raman Spectroscopy Measurements of Electron–Phonon Interaction in a Graphene Device

Abstract

Coherence length (L c) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. L c decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity v g, reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.

Nano-Raman spectroscopy of 2D materials

“…Nevertheless, debate persists whether the measured coherence length (L C ) is governed by the phonon coherence length or the electron coherence length. As deliberated in [60], the phonon interpretation aligns with the theory of spatially coherent TERS [29][30][31]. However, it necessitates an exceptionally high optical phonon group velocity for graphene in the ballistic regime.…”

“…Similar to graphene, the depletion region at the interface between two differently doped regions of MoS 2 has been successfully imaged, as depicted in figures 5(a) and (c) [60]. Intriguingly, while charge oscillations observed in graphene extend over 50 nm across the junction region (figure 5(b)), in MoS 2 , this phenomenon appears to be twice as large (figure 5(c)).…”

“…Through analysis, it is observed that the coherence length (L C ) varies with carrier concentration, manipulated by adjusting the gate voltage (see figure 7(c)). Notably, high values of L C (∼40 nm) [30,31], indicative of doped graphene, are recorded, while L C reaches a minimum at the charge neutrality point [60]. This phenomenon is attributed to Kohn anomaly, wherein the lifetime G-band phonis due to electron-phonon interaction that excites electron-hole pair near the Fermi energy [61,62].…”

“…In graphene devices (figure 7(a)), altering the Fermi energy [59][60][61][62] leads to changes in the Raman intensity of both the G and 2D bands, correlating with the tip-sample distance from 5 nm to 50 nm (figure 7(b)). Through analysis, it is observed that the coherence length (L C ) varies with carrier concentration, manipulated by adjusting the gate voltage (see figure 7(c)).…”

Activity Origin and Catalytic Mechanism of the M–N–C Catalysts for the Oxygen Reduction Reaction

“…Combining SERS and scanning tunneling microscopy (STM) to reveal the origin of ORR activity is essential to address this issue. Ultrahigh vacuum tip enhanced Raman spectroscopy (UHVTERS) is a valuable method to combine the chemical information on SERS with the ultrahigh spatial resolution of STM. − UHVTERS can provide not only chemical information at subnanometer spatial resolution but adsorption-geometry information under controlled environments. , …”