Observation of well-defined Kohn-anomaly in high-quality graphene devices at room temperature

Abstract

Due to its ultra-thin nature, the study of graphene quantum optoelectronics, like gate-dependent graphene Raman properties, is obscured by interactions with substrates and surroundings. For instance, the use of doped silicon with a capping thermal oxide layer limited the observation to low temperatures of a well-defined Kohn-anomaly behavior, related to the breakdown of the adiabatic Born–Oppenheimer approximation. Here, we design an optoelectronic device consisting of single-layer graphene electrically contacted with thin graphite leads, seated on an atomically flat hexagonal boron nitride (hBN) substrate and gated with an ultra-thin gold (Au) layer. We show that this device is optically transparent, has no background optical peaks and photoluminescence from the device components, and no generation of laser-induced electrostatic doping (photodoping). This allows for room-temperature gate-dependent Raman spectroscopy effects that have only been observed at cryogenic temperatures so far, above all the Kohn-anomaly phonon energy normalization. The new device architecture by decoupling graphene optoelectronic properties from the substrate effects, allows for the observation of quantum phenomena at room temperature.

Nano-Raman spectroscopy of 2D materials

“…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)).…”

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

“…The device is purposely not encapsulated so that the TERS tip could interact with the graphene directly. The gate metal adopted is Au due to its high-quality performance for graphene Raman studies Figure a shows a schematic of the device used in this work, as well as the TERS tip interacting with the laser and the sample. , Once the tip was engaged (∼5 nm distant from the sample), a TERS spectrum was measured, and then the tip was pulled 0.5 nm up to measure the spectrum again.…”

“…To investigate the L c dependency on Fermi energy, we sweep a gate voltage with steps of 0.4 V, from −7 to +7 V in the Raman measurements with and without the presence of the tip. The conversion from the experimental gate voltage to n is estimated according to ref . Spectral fitting was performed using the FabNS’s PortoFlow Analysis Software (v1.12).…”