Graphene is a honeycomb lattice entirely made of carbon only one atom thick; it is the first system discovered to be a perfect two-dimensional crystal. However, even graphene, with all of its ideal properties, is unavoidably deformed when isolated and placed on a supporting substrate. For instance, graphene may not lie perfectly flat but instead form “bumps.” Furthermore, its atoms may be randomly displaced from their ideal positions in the honeycomb lattice because of the force exerted by the atoms in the substrate. Scientists call these types of mechanical deformations “strain.” We show that when graphene exfoliated from graphite is placed on a supporting hexagonal boron nitride substrate, the random strain that appears in the carbon honeycomb lattice reduces the speed with which electrons can move inside the material.
Scientists have puzzled for a long time about the mechanism limiting the speed of electrons in real-world graphene devices. Many possibilities have been proposed, such as missing atoms or chemical impurities, structural defects in the honeycomb lattice, and unintentional “dirt.” All of these mechanisms can play a role in limiting the speed of electrons, which is why the situation has remained unclear for a long time. We use both statistical analyses and experimental verification and provide ample evidence that random strain fluctuations in the material set a limit on the electron speed for temperatures as low as 250 mK. We find that electron scattering is local (i.e., within the same valley) and is due to long-range potentials. The way in which strain affects electrons in graphene is very similar to the effect of a magnetic field. In other words, random strain generates a fluctuating pseudomagnetic field, which randomly deflects electrons and limits their speed. Since the amount of strain can strongly depend on the substrate or on the way in which the graphene layer is mounted on the substrate, the electron speed or mobility can vary significantly. We find that our results can also be applied to graphene mounted on and substrates.
Future electronic applications will necessitate a high electron speed. To optimize applications, therefore, we will need to focus our efforts on selecting substrates and assembly procedures that minimize strain.
