Abstract
The observation of novel physical phenomena such as Hofstadter's butterfly, topological currents, and unconventional superconductivity in graphene has been enabled by the replacement of SiO2 with hexagonal boron nitride (hBN) as a substrate and by the ability to form superlattices in graphene/hBN heterostructures. These devices are commonly made by etching the graphene into a Hall-bar shape with metal contacts. The deposition of metal electrodes, the design, and specific configuration of contacts can have profound effects on the electronic properties of the devices possibly even affecting the alignment of graphene/hBN superlattices. In this work, we probe the strain configuration of graphene on hBN in contact with two types of metal contacts, two-dimensional (2D) top-contacts and one-dimensional edge-contacts. We show that top-contacts induce strain in the graphene layer along two opposing leads, leading to a complex strain pattern across the device channel. Edge-contacts, on the contrary, do not show such strain pattern. A finite-elements modeling simulation is used to confirm that the observed strain pattern is generated by the mechanical action of the metal contacts clamped to the graphene. Thermal annealing is shown to reduce the overall doping while increasing the overall strain, indicating an increased interaction between graphene and hBN. Surprisingly, we find that the two contact configurations lead to different twist-angles in graphene/hBN superlattices, which converge to the same value after thermal annealing. This observation confirms the self-locking mechanism of graphene/hBN superlattices also in the presence of strain gradients. Our experiments may have profound implications in the development of future electronic devices based on heterostructures and provide a new mechanism to induce complex strain patterns in 2D materials.
Highlights
The high charge carrier mobility attained at room temperature in graphene encapsulated in hexagonal boron nitride[1] has enabled the observation of ballistic transport over macroscopic distances[2−4] holding the promise for the development of room-temperature electrical equivalents of optical circuits
We have analyzed the strain induced in single-layer graphene deposited on hexagonal boron nitride (hBN) by 2D top-contacts and 1D edge-contacts
Using Raman spectroscopy mapping, we have shown that topcontacts induce strain in the graphene flake, pulling in opposite directions
Summary
Letter results in the formation of new Dirac points in the electronic band structure whose energy is determined by the Moiré wavelength.[11]. In the as-fabricated device, the data are distributed along the iso-strain axis with a vertical shift away from the pristine case This is thought to be due to Fermi velocity reduction, previously reported for graphene on hBN, which arises from van der Waals interlayer interaction.[36,37] Upon thermal annealing for 2 h in forming gas (H2/Ar, 10%/ 90%) at 200 °C the data set shifts vertically upward, suggesting a greater Fermi velocity reduction from increasing interlayer interaction (a strain/doping map of the device after thermal annealing is shown, Supporting Information). In the as-fabricated device the satellite peak (VSAT) is at more negative gate voltages for edge regions This can be clearly seen by taking the second derivative of the resistivity (d2ρxx/dVg2) indicating that different twist angles exist in the top and edge contact regions, a difference which disappears after annealing, Figure 5b.
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