Dynamical properties of nanocarbon

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Daniels, Colin Robert
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Electronic thesis
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In this thesis, we give insight into this problem in two ways. In the first half, we investigate the mechanisms behind two processes that can be used for the engineering of graphene-based devices using a Monte-Carlo approach to modeling processes that occur on timescales which are too long for traditional molecular dynamics (MD) approaches. In particular, we first discuss the extension of code that was previously developed in our research group and how this extension enables more accurate calculations by way of a custom-implemented interatomic potential, as well as increased performance by way of parallelization. The bulk of the chapter is then taken to discuss two case studies in which this code was used to give insights into the dynamics of irradiated GNRs under uniaxial strain as well as Joule-heated bilayer GNRs. In the first case, we see the successful use of our modified code and discover a potential avenue for spintronic device engineering in the case of AGNRs by way of the formation of zigzag-oriented domains that exhibit spin-dependent current flow. In the second case, we seek to understand the mechanisms behind the Joule heating of bilayer GNRs in collaboration with experiment. There, we see that we can not only understand the structural evolution of these bilayer GNRs but by performing transport calculations we gain insight into the time evolution of conductance as measured in experiment. We additionally illustrate the potential for band gap engineering via twist-angle modification in bonded bilayer GNRs.
Finally, we summarize the work described here and touch on future research directions, specifically that of using machine learning to solve the inverse problem of determining GNR structure from experimental Raman spectra.
In the second half, we first introduce and motivate the use of theoretical Raman spectra calculations in the context of assessing the structure and stability of new materials, and particularly we focus on GNRs. Discussing how to perform these calculations, we introduce multiple options for doing so and motivate the use of approximate methods over ab initio in some specific cases. We further demonstrate this by giving an overview of and discussing the implementation of, a publicly accessible web interface to calculate Raman spectra of carbon structures in essentially real-time. Next, we discuss graphene nanoribbons in the periodic model, and we see how, in concert with experiment, theoretical Raman calculations can give insight to the width of AGNRs and how ab initio methods in combination with a bond polarizability model can be used to asses the experimental stability and edge structure of newly-synthesized nanoribbons that have exotic properties. In the final section of this chapter, we see the limits of the periodic framework when considering GNRs synthesized for use in devices. In particular, we discuss the case of the experimental identification of a new Raman-active mode in finite-length GNRs where our theoretical calculations give insight to the physics behind it, its origin as a result of a zone-folding effect, and show how it can be used to asses the length of GNRs produced by synthesis.
Over the past few decades, graphene and graphene-based materials such as graphene nanoribbons (GNRs) have had a significant interest in them garnered due to their potential for use in devices. GNRs in particular due to their readily-tunable properties depending on their width or edge configuration. Unfortunately, until relatively recently, there was still the question of how to synthesize GNRs capable of being used in devices, since often one needs atomically-precise edges for GNRs to exhibit their desired properties. The development of the on-surface synthesis of GNRs has shown hope, but challenges remain for both synthesis alone as well as device development.
December 2019
School of Science
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Rensselaer Polytechnic Institute, Troy, NY
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