Nontechnical abstract: Graphene, a single atomic layer of carbon atoms, has a wealth of very extreme properties, e.g. being impermeable to gases even at only one atomic layer thickness, being extremely elastic, and having extremely high heat and electrical conductivity. The research group at Boston University is exploring how applying strain to graphene manipulates these properties for novel and interesting applications from mechanical resonators, and electronic and optical devices, to thermal management devices. In order to use strain engineering for these purposes, it is necessary to know how much friction is there to anchor the strained graphene. The researchers use miniature chambers covered by graphene to measure friction and how to control it by patterning the substrate. Graphene covered microchambers are strain tuned by applying a variable external pressure that deflects the suspended graphene membrane creating strain in both the suspended and supported regions. The strain response is measured using optical spectroscopy. Certain exotic strain distributions are predicted to affect the electrons in graphene in such a way that they get trapped and no longer can conduct electricity. The BU team is working on developing chamber shapes and friction patterning to achieve this state which can be turned on and off by varying the external pressure. The team is also studying how pressure can vary the heat conductivity in graphene.
Graphene is a good candidate for strain engineered devices since it can withstand a 20% extension without breaking. Hence huge strains can be induced and engineered for novel and interesting applications. Strain engineering affects many types of devices, from mechanical resonators to electronic and optical devices. Strain engineering also opens up new areas of exotic physics and applications, perhaps most spectacularly from creating magnetic pseudo fields with quantization of electrons and holes into Landau levels at room temperature. Therefore it is important to have a solid understanding of graphene-substrate interaction and friction under variable strain. The research team at Boston University has developed a method of applying variable strain by placing graphene to seal microchambers with variable external pressure. The graphene membrane deforms over the chamber and slides due to finite friction. With micro-Raman measurements the team is able to map out the strain profile and determine the friction coefficient which is pressure dependent. Knowledge of the friction dependence on substrate treatment allows strain patterning. Variable friction is achieved by patterning the surface treatment and hence local coefficient of friction. The varying friction is tailored to create strain distributions that will create high local pseudo magnetic field. The researchers are combining the strain-created high local pseudo magnetic fields with plasmonic patterning to overlap the plasmonic hotspots with the high pseudo field regions. The pseudo field response is then read out via Raman spectroscopy using phonon and Landau Level exciton interactions. Another application is graphene as high thermal conductivity conduits. Suspended graphene has been shown to have extremely high heat conductivity. Theory predicts that the out-of-plane phonons that carry the heat are much less efficient than the in-plane acoustic modes. Strain is predicted to remove the scattering of the in-plane modes into the out-of-plane modes as well as reducing the density of states so strain could drastically increase the already high thermal conductivity. The researchers are using their tunable strain on suspended graphene to experimentally measure the effect of strain on the thermal conductivity of graphene.
