Title: "Graphene nanopore sensor for DNA sequencing"
Speaker: Chaitanya Sathe, Grad Student with Prof. Klaus Schulten
Inexpensive and fast methods to sequence the genome of individuals using nanopore technology revolutionizes modern medicine. The basis for using nanopores, drilled into a silicon membrane, to sequence DNA is to drive a DNA strand electrophoretically through the pore sandwiched between two electrolytic reservoirs. The flow of ions passing through the nanopore are altered depending on whether an A,C, G or T nucleotide is passing through the pore. Thus, in principle the sequence of the DNA strand can be discerned by recording the ionic current. The thickness of the membrane used to make the nanopore presents a fundamental limitation to the physical dimension that can be resolved. Typical solid-state membranes are too thick and usually fail to recognize single nucleotides on a DNA strand. Graphene is a sub-nanometer membrane, comprising of carbon atoms arranged in a honeycomb lattice, with remarkable electronic and mechanical properties. The thickness of a graphene membrane (3 ) is comparable to the vertical stacking distance between base pairs in the DNA (3.5 ) making graphene an ideal candidate for DNA sequencing. Resolving at the atomic level electric field-driven DNA translocation through graphene nanopores is crucial to guide the design of graphene-based sequencing devices. Molecular dynamics simulations, in principle, can achieve such resolution and are employed to investigate the effects of applied voltage, DNA conformation and sequence as well as pore charge on the translocation characteristics of DNA. We demonstrate that such simulations yield current characteristics consistent with recent measurements and suggest that under suitable bias conditions A-T and G-C base pairs can be discriminated using graphene nanopores.
Title: "Atomic-Scale Evidence for Strong Carrier Scattering at Graphene Grain Boundaries"
Speaker: Justin Koepke, Grad Student with Prof. Joe Lyding
Graphene is a two-dimensional of carbon atoms arranged in a honeycomb lattice, possessing phenomenal electronic, mechanical, and thermal properties. Chemical vapor deposition (CVD) growth on polycrystalline Cu foil as a method to grow wafer-scale graphene is interesting due to the ability to grow predominantly monolayer graphene and transfer it to other substrates. However, CVD graphene growth on Cu foil is non-epitaxial and results in many rotationally-misoriented grains. Simulations and transport measurements indicate that the grain boundaries (GBs) formed between these rotational domains impede carrier transport. Our scanning tunneling microscopy and spectroscopy (STM/S) results show atomic-scale evidence of carrier scattering from these graphene GBs. Our measurements corroborate the aperiodic GB topologies seen in recent TEM studies and the lack of epitaxy in graphene CVD on Cu. Our STS results clearly show a shift in doping due to the presence of the GBs forming barriers from ~ 20 meV to 0.20 eV. The length scale of the doping shift is ~ 1-2 nm. The superstructures observed adjacent to the GBs and confirmed with tight-binding calculations indicate strong intervalley carrier scattering from the GBs. The decay length of these electronic superstructures is on the same order as the doping shift, pinpointing the source of the scattering off of the potential barriers that the GBs induce. These results clearly identify the GBs as a source of strong carrier scattering in polycrystalline graphene, explaining why CVD graphene films have lower overall mobility and higher resistance than exfoliated graphene.