The objective of this research is to understand the fundamental transport and scattering phenomena in graphene based electronic devices. The approach is to combine scanning probe microscopy and electrical transport measurements to both create and probe the local electronic properties of these devices. Intellectual Merit: Graphene has shown promise to augment silicon in future semiconductor technologies. However, as a gapless semiconductor it is not directly useable in digital electronics. This limitation can be overcome by opening a controlled bandgap by patterning the graphene into narrow strips. This research will explore ways of creating these nanoribbons with atomic control using scanning probe microscopy. Beyond creating these devices, the research will also examine the role of defects, edge roughness, surface morphology and charged impurities on the electronic properties of the nanoribbons. Furthermore, the scanning probe microscopy techniques developed in this proposal may be extended to study other low-dimensional systems. Broader Impacts: This research may lead to entirely new types of electronics devices made from graphene rather than silicon. These devices may significantly extend the capabilities of current semiconductor devices. A concurrent education and outreach program will involve the training of both undergraduate and graduate students and well as outreach to local K-12 schools through the creation of a set of demonstrations based on the research. The outreach to local schools will primarily reach under-represented groups and excite them about nanoscience at a young age.
This research project examined ways to improve the quality of graphene electronic devices. Graphene is a two-dimensional honeycomb lattice of carbon atoms and is therefore very sensitive to its environment. We have found that the standard silicon oxide substrate leads a large amount of charge fluctuations due to impurities in the oxide. During the course of the research we developed a new imaging technique to map these unwanted charged impurities with higher spatial and charge sensitivity. This development is important for future characterization of graphene devices. In order to improve the mobility of graphene devices a different choice of substrate was necessary. We have used hexagonal boron nitride as a substrate for graphene devices. It has the same crystal structure as graphene but with alternating boron and nitrogen atoms. This substrate leads to atomically flat graphene devices which allowed us to directly visualize the scattering of electron waves from edges. The decay of these standing waves with distance from the edge demonstrated the unique property that the electrons in graphene are resistant to backscattering. The third set of results from this project was the modification of graphene’s electronic properties by creating vertical heterostructures. The simplest case is trilayer graphene whose electronic properties depend on the stacking order of the three layers. We have found the ability to open a band gap in one configuration but not the other. When graphene is placed on hexagonal boron nitride, its electronic properties are modified by the presence of a periodic potential. The wavelength of this potential depends on the rotation angle between the two crystals. We have found that there is a new Dirac point created in the graphene by the periodic potential and its energy depends on the wavelength. This new Dirac point modifies the conductivity of graphene. The final area of results in this project was the response of graphene to optical excitations. We have determined the single shot damage threshold for graphene. This is the power that is required to burn a hole in graphene with a single laser pulse. We have also examined the time dependent response of the electrons and phonons in graphene as a function of energy. This information is critical to understand the fundamental limits of graphene devices. The broader impacts of the research include the development of new electronic materials and devices that may be used future applications. The grant also provided education and training for graduate and undergraduate students. The three undergraduate students working on the project have all decided to continue their education in graduate school. There was a post-doctoral research working on the early stages of the research. She has now started a faculty position. It is expected that all of the people who worked on the research will continue into careers in science related fields.
