Abstract:We report on a scanning confocal Raman spectroscopy study investigating the strain-uniformity and the overall strain and doping of high-quality chemical vapour deposited (CVD) graphenebased heterostuctures on a large number of different substrate materials, including hexagonal boron nitride (hBN), transition metal dichalcogenides, silicon, different oxides and nitrides, as well as polymers. By applying a hBN-assisted, contamination free, dry transfer process for CVD graphene, high-quality heterostructures with low doping densities and low strain variations are assembled. The Raman spectra of these pristine heterostructures are sensitive to substrate-induced doping and strain variations and are thus used to probe the suitability of the substrate material for potential high-quality graphene devices. We find that the flatness of the substrate material is a key figure for gaining, or preserving high-quality graphene. IntroductionFor over a decade, graphene has been in the spotlight of solid state research. Its high charge carrier mobilities 1-4 and long spin diffusion lengths, 5,6 as well as its optical 7 and mechanical properties 8 promise a wide range of applications ranging from spintronics 9 to high frequency electronics, 10 ultra-sensitive sensors 11,12 and flexible optoelectronics. 13 In order to advance prototype devices to true applications, large effort has been put into growth 14-18 and contamination-free transfer 3,4,19,20 of high quality graphene based on chemical vapour deposition. However, as graphene and other two-dimensional (2d) materials consist only of surface atoms, the choice of substrate material has a large influence on their structural and electronic properties. 2,[21][22][23][24][25] In this work, we investigate strain, doping and the strain uniformity of high quality CVD graphene/hBN heterostructures placed on different substrate materials. Here, we follow a recently reported, contamination free, dry transfer process, where exfoliated hBN is used to pick up CVD graphene directly from the growth substrate. The obtained stack is subsequently placed on different target substrates. 4 This fabrication process yields high quality heterostructures with little intrinsic overall doping and low nanometre-scale strain variations. As the graphene crystal is covered, i.e. protected by hBN on the top side, modifications in doping and strain are purely due to the substrate at the bottom side of graphene, making our approach suitable for benchmarking the substrate suitability.

We present gate-controlled single-, double-, and triple-dot operation in electrostatically gapped bilayer graphene. Thanks to the recent advancements in sample fabrication, which include the encapsulation of bilayer graphene in hexagonal boron nitride and the use of graphite gates, it has become possible to electrostatically confine carriers in bilayer graphene and to completely pinch-off current through quantum dot devices. Here, we discuss the operation and characterization of electron-hole double dots. We show a remarkable degree of control of our device, which allows the implementation of two different gate-defined electron-hole double-dot systems with very similar energy scales. In the single-dot regime, we extract excited state energies and investigate their evolution in a parallel magnetic field, which is in agreement with a Zeeman-spin-splitting expected for a g-factor of 2.

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...