Line shape of the Raman 2D peak of graphene in van der Waals heterostructures

Abstract

The Raman 2D line of graphene is widely used for device characterization and during device fabrication as it contains valuable information on, e.g., the direction and magnitude of mechanical strain and doping. Here, we present systematic asymmetries in the 2D line shape of exfoliated graphene and graphene grown by chemical vapor deposition. Both graphene crystals are fully encapsulated in van der Waals heterostructures, where hexagonal boron nitride and tungsten diselenide are used as substrate materials. In both material stacks, we find very low doping values and extremely homogeneous strain distributions in the graphene crystal, which is a hall mark of the outstanding electronic quality of these samples. By fitting double Lorentzian functions to the spectra to account for the contributions of inner and outer processes to the 2D peak, we find that the splitting of the sub‐peaks, 6.6±0.5cm−1 (hBN‐Gr‐WSe2) and 8.9±1.0cm−1 (hBN‐Gr‐hBN), is significantly lower than the values reported in previous studies on suspended graphene.

2D Raman band splitting in graphene: Charge screening and lifting of the K-point Kohn anomaly

“…Our results show the 2D 1 peak with the strongest intensity at the lowest doping level. A higher 2D 1 intensity has also been observed experimentally previously on both suspended 15 , 16 , 18 and hBN encapsulated 17 graphene. Based on the observed and predicted higher intensity for the inner process for pristine graphene, we tentatively assign the 2D 1 peak as originating from the inner process, and show below how this assignment is supported by the results.…”

“…It is counterintuitive that the 2D 2 phonons decrease with increased charge (Fig. 3a ), since increased screening lowers the Fermi velocity and increases the DR-selected q vector from q to q + Δq , which typically corresponds to a higher ω D 17 , 19 , 20 . The linear models above demonstrate that the decrease in 2D 2 with increased charge is due to a crossover of the unscreened and screened phonon dispersion, as seen in calculations 10 , 11 .…”

“…Indeed, early in the study of graphene, a symmetric 2D peak for graphene on SiO 2 /Si was identified as a signature of a single layer 14 . Later studies showed that the intrinsic 2D line shape of single layer suspended graphene is asymmetric 15 – 17 , but reverts to a symmetric line shape when charge is added by field doping to 2 × 10 11 cm −2 , indistinguishable in shape from graphene on SiO 2 18 . Hence, we can deduce that there is not a perfect compensation of the phonon trigonal warping at low doping condition, and that the two peaks can reveal the behaviour of the inner and outer scattering processes, respectively.…”

Nonvolatile Memories Based on Graphene and Related 2D Materials

“…In order to overcome such problem, dry transfer protocols, where 2D materials are protected against the contact with any liquid, have been developed to obtain cleaner interfaces . Cleaner surfaces will enable the achievement of the ultimate fundamental properties of 2D materials, namely extremely low interface trap density or dangling bonds . The transfer of graphene using pick‐and‐place techniques enabled the demonstration of extremely high charge‐carrier mobility (≈140 000 cm 2 V −1 s −1 at room temperature) in graphene transistors using h‐BN as the gate insulator.…”

Ab initio calculation of the G peak intensity of graphene: Laser-energy and Fermi-energy dependence and importance of quantum interference effects

“…Despite this apparent simplicity, the Raman spectrum yields a large amount of information on, amongst others, doping, strain, inter-layer interaction, and the underlying substrate [1][2][3][4] . To understand the influence of these quantities on the Raman spectrum of graphene, considerable effort has been made to understand the shape [5][6][7][8][9] , width [10][11][12][13] , height [14][15][16] , and position 11,[17][18][19][20][21] of the G and 2D peaks. While a clear picture for the 2D peak has been established 6,12,22,23 , a corresponding simple picture for the G peak is still missing.…”