Dielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride

Abstract

We investigate the influence of the substrate on the vibrational properties of graphene, comparing graphene on hexagonal boron nitride (hBN) with graphene on SiO2 by spatially resolved confocal Raman spectroscopy. By studying the G line we show that the average doping level and local doping domain fluctuations are significantly suppressed in graphene on hBN with respect to graphene on SiO2. In contrast to the G line, the 2D line of graphene on hBN shifts up in frequency compared to the one of graphene on SiO2. We show that this effect is due to a reduction of the Kohn anomaly at K through an enhanced screening by the dielectric substrate. We prove our theory to be consistent with Raman measurements on graphene surrounded by hBN (stronger screening) and recent findings on suspended graphene (no external screening).Graphene, a monoatomic carbon membrane with unique electronic properties [1, 2] is a promising candidate for flexible electronics, high frequency applications and spintronics [3]. However, graphene's ultimate surface-to-volume ratio makes the environment, in particular the substrate material, have a pronounced influence onto its intrinsic properties. For example, SiO 2 , the most common substrate material, exhibits surface roughness, dangling bonds and charge traps which introduce ripples, disorder [4], and doping domain fluctuations [5]. This limits carrier mobilities and the operation of graphene devices [6,7]. Therefore alternative substrates are required to overcome these limitations. Hexagonal boron nitride (hBN) has been identified as a very promising candidate [8][9][10][11]. A large (indirect) band gap, a lattice mismatch to graphite of less than 2%, and the absence of dangling bonds makes this atomically flat material a valuable and promising insulating counter part to graphene [12,13]. It has been shown that graphene can be successfully transferred to ultrathin hBN flakes leading to improved electronic transport properties compared to graphene on SiO 2 [8, 9]. Moreover, scanning tunneling microscopy experiments have shown that the sizes of individual electron-hole puddles are significantly increased while the disorder potential is reduced by roughly a factor 10 [10, 11]. Over the last years Raman spectroscopy has proven to be a powerful tool for characterizing graphene and studying its physical properties. For example, this technique has been successfully used (i) to distinguish single-layer graphene from few-layer graphene and graphite [16][17][18], (ii) to monitor doping levels [5,19], (iii) to study short range disorder and edge properties [20] and (iv) to investigate suspended [21] and nanostructured graphene [22]. Here we present spatially resolved confocal Raman spectroscopy measurements of graphene on hexagonal boron nitride substrates, which are compared with measurements of graphene on SiO 2 . We show that the average doping level and local doping domain fluctuations are significantly suppressed in graphene on hBN. This is visible in a redshift of the G-line which has also been observ...

Manifestation of Charged and Strained Graphene Layers in the Raman Response of Graphite Intercalation Compounds

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Insulating State in Low‐Disorder Graphene Nanoribbons

“…A representative Raman map of the FWHM of the Raman 2D line is shown in Figure b and a corresponding Raman spectrum is presented in Figure c, see highlighted point in Figure b. In comparison, the FWHM of the 2D line on SiO 2 was typically above 30 cm −1 . Electron beam lithography (EBL), deposition of an aluminum hard mask (20 nm), and reactive ion etching with a SF 6 /O 2 plasma were used to structure the heterostructure into the desired shape.…”

Vibrational and optical properties of MoS2: From monolayer to bulk

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