Technical. This project aims to develop understanding and materials techniques needed to control and exploit the effects of disorder and substrate interactions on the electronic properties of graphene. Graphene-substrate interactions will be studied to enable tuning of the electronic bandstructure, including opening a bandgap. High-mobility graphene enabled by this research will contribute to advanced device development, and allow the study of phenomena associated with Dirac fermions not accessible in particle physics, including Klein tunneling, Zitterbewegung, and the Schwinger mechanism. New low-temperature quantum phases may be observable in high-quality graphene, such as the fractional quantum Hall effect. Adsorbate interactions with graphene will be used to study electronic phenomena such as superconductivity in highly doped graphene and the Kondo effect in graphene with transition-metal impurities, as well as the study of ordering and phase transitions in the adsorbate layer. This research is expected to yield high-mobility graphene directly applicable to high-speed analog electronic devices operating at higher frequencies than presently possible, enabling high performance applications in communications and sensing. Producing a bandgap in graphene through substrate interaction may also enable high-speed, low-power logic applications of graphene transistors. Additionally, understanding interactions between graphene and the environment may lead to new types of chemical and biochemical sensors based on high-mobility graphene. Non-Technical. The project addresses fundamental research issues in a topical area of electronic/photonic materials science and condensed matter physics having technological relevance. Basic understanding gained is expected to lead to improved device performance, and to allow design of new components. The project integrates research and education providing students with hands-on laboratory experience and training while conducting forefront research. Two Ph.D. students will be trained in state-of-the-art interdisciplinary research in nanoelectronics and surface science. The investigators will work with the graduate students to design and implement nanotechnology demonstrations involving graphene and graphite, and used in outreach to K-12 students and teachers from under-represented groups.
Graphene is a single atom-thick sheet of carbon, and is interesting for its novel electronic properties. Electrons in graphene travel at a constant speed independent of their energy, much like photons (particles of light). In this project, we studied the effects of different kinds of disorder on how well graphene conducts electricity. Charged impurities (electrical charges near the surface of graphene; see figure "Charged impurities on graphene") had already been identified as one important source of disorder and electrical resistance in graphene. We found that if the positions of the charges are correlated, rather than random, then the resistance of graphene can be much lower (up to a factor of four times lower than for randomly placed charges). We also studied the effect of missing atoms (vacancies; see figure "Defects in graphene") on graphene's electrical resistance. Vacancies have a much stronger effect on the resistance than charged impurities. Moreover, vacancies have the ability to completely stop the electrons in graphene, which charged impurities cannot do: if enough vacancies are present in graphene it becomes insulating and no longer conducts electricity. In contrast if the number of charges nearby graphene is increased, the resistance always remains finite and graphene remains metallic (a conductor of electricity). We also studied how graphene deforms mechanically over a rough substrate. Graphene is the thinnest imaginable membrane (only one atom thick) and can easily deform by bending to accommodate features on a rough substrate. We studied graphene on substrates with randomly placed protrusions (silica nanoparticles of diameter 7 nanometers; see figure "Nanoparticles cause wrinkling in graphene"). We found that when the nanoparticles are sparse, graphene stretches to cover the nanoparticle. When the nanoparticles are closer, graphene can more easily accommodate the nanoparticles by wrinkling, and wrinkles form which connect the particles. Deformations are expected to affect the way electrons travel through graphene, and future research will study how the nanoparticle-induced bumps and wrinkles affect electron flow in graphene.
