Physical interaction of molecules with graphene : reflection, permeation and polarisability

There is a burgeoning use of graphene in the form of carbon nanotubes, nanoporous graphene, and graphene oxide laminates for detecting, identifying and filtering through molecules like water, air and DNA nucleotides. The exact molecular interactions with graphene that make these technologies possible are not entirely clear. While chemical interactions of molecules with graphene are being explored with great interest, fundamental explorations into the purely physical interactions have pointed to anomalies in the nature of molecular reflections on graphene, the molecular impermeability of graphene and the polarisability of interfacial layers of molecules on graphene. In this thesis, we employ novel nanoscale device structures using graphene layers and nanospaces sandwiched between them to study the physical interaction of relatively free molecules in fluid state at the surface of graphene via three physical responses they exhibit: reflection, permeation and change in polarisability. In the first study, we show that helium atoms completely specularly reflect off a pristine graphite surface. Using a graphite-nanospace heterostructure to construct ultraclean nanochannels, we demonstrate ballistic transport of helium atoms through them. Further by comparing with similarly constructed roughened graphite nanochannels we could attribute the specular reflections to the sub-angstrom smoothness of the graphene lattice. To study molecular permeability of graphene we utilize graphite microchambers with a suspended monolayer graphene top wall to create a very well-sealed chamber to detect permeation at previously untested resolution. We find that graphene is impermeable to helium atoms to an accuracy of 10^2 atoms/day which is 8-9 times more accurate than previous experiments. In order to study interfacial polarisability of molecules on graphene, we again utilize the graphite-nanospace heterostructures to construct capacitors with graphite plates and a nanometre thick water dielectric. A local dielectric constant probing AFM tip measures the dielectric constant of these thin water layers in the perpendicular direction to be anomalously suppressed to 2.1, owing to the structuring of interfacial water which completely suppresses its dipole polarisability. Overall, this thesis shows that graphene's physical interactions with molecules are unique in showing complete specular reflection of helium atoms, absolute impermeability to helium atoms, and anomalously low interfacial layer polarisability of water. The device structures used and the results of this thesis will help further graphene's use as a molecular separating, detecting and decoding membrane.