Recent experiments have demonstrated that graphene, a monolayer of carbon atoms, exhibits surprisingly high room-temperature electron mobility. Such a remarkable electronic behavior may well enable ground-breaking advances in nanoelectronics when silicon-based technologies reach their natural limits imposed by fundamental physics. However, the control over the quality of graphene remains a major challenge because even a few atomic defects may markedly degrade the performance of a graphene device. The research team seeks to develop a novel thermo-mechanical method for perfecting graphene sheets by effectively removing defects through an integrated experimental and modeling effort. In particular, time-accelerated modeling methods will be developed to determine defect migration barriers and pathways and defect-defect reaction mechanisms under thermal and mechanical loadings. In parallel, experimental measurements of Raman topography, electrical transport, and low-temperature magnetotransport of suspended monolayer graphene devices will be performed before and after thermo-mechanical treatments, aiming at validating the modeling approaches and evaluating the effectiveness of the methods.
The novel thermo-mechanical methods developed for perfecting graphene sheets are expected to lead to major breakthroughs toward realization of the graphene-based next-generation electronics. The project will also help foster transformative progress for the analysis and manipulation of defects in graphene as well as other nano-materials in general. When combined with continuum mechanics theories, the research results will establish new constitutive equations relating thermo-mechanical loading with defect mobility, which will be valuable for improving existing graphene manufacturing processes and for designing future materials systems beyond graphene. On the educational front, the proposed research will generate many opportunities at both the college and K-12 levels. Graduate and undergraduate students at Penn State will benefit greatly from the multidisciplinary research experience in innovative, integrated experimental manipulation and computational nano-mechanics. The PIs will actively work with several organizations at Penn State to involve underrepresented groups including women and minority students in carrying out proposed research program.
Graphene is a monolayer of hexagonal lattice – a single-sheet of graphite – that possesses novel electronic and electromechanical properties including an exceedingly high mechanical strength. This project aimed at developing a novel thermo-mechanical means to perfect graphene sheets by pushing defects towards the edges of the graphene sheet and ultimately removing them from graphene, taking advantage of an enhanced defect mobility under thermal loading and enhanced defect migration directionality under tensile strains. Centering on the research theme, graphene-substrate interactions, graphene fracture, and small defect mobility in graphene under mechanical loading are evaluated in order to achieve a robust design. The project achieved several important technical findings: (I) Just as a piece of paper, external loading likely causes fracture of graphene in a tearing mode. When tearing graphene in vacuum, the produced nanoribbons always feature armchair edges. Whereas in the presence of oxygens, the produced nanoribbons always have zigzag edges. Given the strong correlations between the edge structures and the electrical properties of the nanoribbons, this findings demonstrated the controlled strain engineering of the nanoribbons with tailored bandgap. (II) Indentation, shear, and edge compression of a trenched graphene generate rippling morphologies that scale not only on the applied load and graphene size, but also on the graphene-substrate adhesion strength. (III) Both the mechanical strain and strain direction modulate the migration barrier of vacancy defects in graphene, which provided a fundamental guidance for strain engineering of defected graphene. (IV) While perfecting the structure of grapheme by removing defects will be important down the road, we found that the defects and charge traps from the dielectrics and graphene-dielectric interface, which are an integrated part of a graphene field effect transistor, actually represents the current challenge in improving the performance of a graphene field effect transistor. Therefore the development of graphene technology with graphene field effect transistor at its core must include the development of means of perfecting the graphene-dielectric interface and the dielectric itself to fulfill the full potential of graphene as a novel material for electronics. The proposed project represents an important step towards developing practical means to perfecting defects in graphene. The discipline will benefit from the experimental and numerical tools built in this project. For instance, the experimental manipulation and measurement of graphene deformation and electron mobility used in this project can be transplanted for other experiments. The multiscale modeling tools and the empirical force fields developed from the modeling group will also aid the research in the discipline to more accurately simulate graphene properties. The findings from this project will also offer a better understanding on graphene fracture, defect motion in graphene, and graphene-substrate adhesion, which provides fundamental guidance for experimental manipulations of graphene. The project provided opportunities for one undergraduate and three graduate students to conduct research in science and engineering areas. The students participated in this research acquired either experimental or computational skills that will enhance their success in research careers. The research results are incorporated into the courses (Multiscale modeling of Materials) in PSU taught by the PI. Research results have been disseminated by giving lectures within PSU and in different conferences
