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Title:Structural and thermal properties of graphene - boron nitride superlattice
Author(s):Nandwana, Dinkar
Advisor(s):Ertekin, Elif
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
boron nitride
thermal conductivity
interface engineering
Abstract:Interfaces and their atomic structure play a key role in two–dimensional materials such as the graphene - boron nitride (C-BN) superlattices. C-BN superlattices with regular ordered subdomains of graphene and boron nitride have recently been synthesized and characterized as a potential candidate for optoelectronic and thermoelectric devices. However, detailed structure of the interface, and its effect on fundamental properties (such as the thermal conductivity) is unknown and largely complicated by the ∼ 2% lattice mismatch between the two materials. This work provides an in-depth view of the possible interfacial structures in C-BN superlattices, and quantifies their effect on the thermal conductivity. The equilibrium structures are predicted using atomistic total energy calculations and scaling arguments based on continuum theory. The lattice mismatch results in a competition, unique to two–dimensional systems, between mismatch strain and two strain–relieving mechanisms: nanoscale misfit dislocations, and ripples via out–of–plane deformation. For flat superlattices, a critical superlattice pitch (thickness) exists beyond which the interface is decorated by strain–relieving misfit dislocations. For superlattices that can deform out–of–plane, large–scale ripple formation serves as a highly efficient mechanism to relieve misfit strain. Using first–principles approach based on density functional perturbation theory (DFPT) and single–mode relaxation time (SMRT) approximation to the phonon Boltzmann transport equation, thermal transport property of one such stable structure is studied. Acoustic mismatch between graphene and boron nitride introduces wave interference effects (such as opening of phonon band gap) and phonon scattering mechanisms at the interface that severely affect the thermal conductivity. In the classical limit (large pitch superlattices), the thermal conductivity increases with the superlattice pitch due to the presence of interfacial thermal resistance at the interface. However, for ultra low pitch superlattices, influence of wave interference effects results in the modification of phonon group velocity and scattering time, and in turn results in a decrease in thermal conductivity with increasing pitch. The work can serve as a guide for interface engineering in C-BN superlattice and any two–dimensional superlattice in general, since the approach presented here provides a generalized framework.
Issue Date:2013-08-22
Rights Information:Copyright 2013 Dinkar Nandwana
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08

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