Abstract
Pristine graphene encapsulated in hexagonal boron nitride has transport properties rivalling suspended graphene, while being protected from contamination and mechanical damage. For high quality devices, it is important to avoid and monitor accidental doping and charge fluctuations. The 2D Raman double peak in intrinsic graphene can be used to optically determine charge density, with decreasing peak split corresponding to increasing charge density. We find strong correlations between the 2D1 and 2D2 split vs 2D line widths, intensities, and peak positions. Charge density fluctuations can be measured with orders of magnitude higher precision than previously accomplished using the G-band shift with charge. The two 2D intrinsic peaks can be associated with the “inner” and “outer” Raman scattering processes, with the counterintuitive assignment of the phonon closer to the K point in the KM direction (outer process) as the higher energy peak. Even low charge screening lifts the phonon Kohn anomaly near the K point for graphene encapsulated in hBN, and shifts the dominant intensity from the lower to the higher energy peak.
Highlights
MethodsThe encapsulated graphene sample was fabricated at Columbia University by Carlos Forsythe with their pick-and-place method that avoids direct contact between the pick-up polymer and graphene layer or the accompanying hexagonal boron nitride (hBN) surfaces that contact the graphene[21]
Extrapolating linearly, we find from this relationship that the charge neutrality point (CNP) corresponds to a 2D peak splitting of 20.4 cm−1
We have used Raman spectroscopy of graphene encapsulated in hexagonal boron nitride (hBN) to explore the intrinsic double peaked 2D phonon behaviour under low charge screening
Summary
The encapsulated graphene sample was fabricated at Columbia University by Carlos Forsythe with their pick-and-place method that avoids direct contact between the pick-up polymer and graphene layer or the accompanying hBN surfaces that contact the graphene[21]. This particular sample did not have functioning edge contacts, but similar samples have a room temperature mobility of ~3.2 × 104 cm2/Vs. The collected 2D Raman spectral map is fitted with two Voigt peak functions at all different locations on the hBN encapsulated graphene. The incident laser power was limited to 1.2 mW to avoid heating
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