The specific rotational alignment of two-dimensional lattices results in a moiré superlattice with a larger period than the original lattices and allows one to engineer the electronic band structure of such materials. So far, transport signatures of such superlattices have been reported for graphene/hBN and graphene/graphene systems. Here we report moiré superlattices in fully hBN encapsulated graphene with both the top and the bottom hBN aligned to the graphene. In the graphene, two different moiré superlattices form with the top and the bottom hBN, respectively. The overlay of the two superlattices can result in a third superlattice with a period larger than the maximum period (14 nm) in the graphene/hBN system, which we explain in a simple model. This new type of band structure engineering allows one to artificially create an even wider spectrum of electronic properties in two-dimensional materials.

Using a simple setup to bend a flexible substrate, we demonstrate deterministic and reproducible in-situ strain tuning of graphene electronic devices. Central to this method is the full hBN encapsulation of graphene, which preserves the exceptional quality of pristine graphene for transport experiments. In addition, the on-substrate approach allows one to exploit strain effects in the full range of possible sample geometries and at the same time guarantees that changes in the gate capacitance remain negligible during the deformation process. We use Raman spectroscopy to spatially map the strain magnitude in devices with two different geometries and demonstrate the possibility to engineer a strain gradient, which is relevant for accessing the valley degree of freedom with pseudo-magnetic fields. Comparing the transport characteristics of a suspended device with those of an on-substrate device, we demonstrate that our new approach does not suffer from the ambiguities encountered in suspended devices.The large mechanical strength of two-dimensional (2D) crystals allows one to modify their optical and electronic properties by externally induced strain fields [1]. Graphene, one of the key examples of 2D materials, is of particular interest because of its peculiar electronic properties [2]. A series of intriguing effects were predicted for strained graphene, such as the appearance of a scalar potential [3], pseudomagnetic fields [3-5], valley filtering [6, 7] or superconductivity [8]. Different methods have been introduced to generate strain in graphene. One common approach is based on suspended graphene, where strain is induced by using different microactuators [9][10][11][12] or by simply bending a flexible substrate [13]. In other approaches, graphene is not suspended and strain can be generated by bending a flexible substrate [14], by using highly stressed metallic pads [15], or by placing graphene on periodic structures [16][17][18]. However, several challenges that need to be overcome simultaneously hampered the progress of these platforms for studying strain effects in transport experiments. First, complex fabrication usually significantly degrades the graphene quality and hinders the observation of the strain effects. In addition, the device is often limited to very basic structures, without the possibility of local gating or multiterminal devices. Second, mechanical deformations often result in changes in the gate capacitance that cannot be easily distinguished from the actual strain effects. The third challenge is that the strain should be in-situ tunable and non-hysteretic to disentangle strain effects from other effects.Here we report a straining method that meets all the above requirements. Instead of suspending the graphene, we encapsulate the graphene with hexagonal boron-( a ) ( b ) ( c ) pushing-wedge leads ∆z counter-supports substrate hBN/graphene/hBN 5 µm ( d ) device A device A device B x y device B Cr/Au polyimide FIG. 1. (a) Schematic cross section of our device and (b) of the three-point bendin...

Microscopic corrugations are ubiquitous in graphene even when placed on atomically flat substrates. These result in random local strain fluctuations limiting the carrier mobility of high quality hBN-supported graphene devices. We present transport measurements in hBN-encapsulated devices where such strain fluctuations can be in situ reduced by increasing the average uniaxial strain. When ∼ 0.2% of uniaxial strain is applied to the graphene, an enhancement of the carrier mobility by ∼ 35% is observed while the residual doping reduces by ∼ 39%. We demonstrate a strong correlation between the mobility and the residual doping, from which we conclude that random local strain fluctuations are the dominant source of disorder limiting the mobility in these devices. Our findings are also supported by Raman spectroscopy measurements. arXiv:1909.13484v1 [cond-mat.mes-hall]

By mechanically distorting a crystal lattice it is possible to engineer the electronic and optical properties of a material. In graphene, one of the major effects of such a distortion is an energy shift of the Dirac point, often described as a scalar potential. We demonstrate how such a scalar potential can be generated systematically over an entire electronic device and how the resulting changes in the graphene work function can be detected in transport experiments. Combined with Raman spectroscopy, we obtain a characteristic scalar potential consistent with recent theoretical estimates. This direct evidence for a scalar potential on a macroscopic scale due to deterministically generated strain in graphene paves the way for engineering the optical and electronic properties of graphene and similar materials by using external strain.

No abstract