Flexoelectricity and piezoelectricity in BN-based hybrid layers

Abstract

In this thesis, molecular dynamics (MD) simulations were carried out with a Tersoff potential and Lennard-Jones (L-J) potential force field to predict the electromechanical response of single layer (SL) and multi-layer boron nitride nanosheets (BNSs). The electromechanical response of pristine and C-doped BNSs were studied using the charge dipole (C-D) potential model. The effects of various factors such as chirality, size of BNS, C doping concentration and different shape of pores were critically examined. The elastic and piezoelectric coefficients of BNS under tension and shear loading conditions were determined. The piezoelectric, flexoelectric and elastic coefficients of BNSs with graphene stripes were also examined. Comparisons of the (i) axial piezoelectric and flexoelectric coefficients of pristine BNS, and (ii) elastic coefficients of pristine and hybrid BNS with the existing results are found to be in good agreement. The flexoelectric coefficients of monolayer boron nitride-graphene heterostructures (BGHs) were also determined by imposing the bending deformation on the pristine BNS and BGHs. Three shapes of graphene domains were considered: triangular, trapezoidal, and circular. Overall polarization of BGHs was enhanced when the graphene domain surrounded by more N atoms than B atoms. The present thesis also deals with the electromechanical response of multi-layer BNSs under uniaxial tension test at ambient temperature (300 K), taking into account the effects of number of BN layers, chirality, fracture behaviour, and strain rate (SR) on the stress-strain response and deformation behaviour. Multi-layer BNSs with an even number of atomic layers (symmetry D6h) do not show piezoelectricity, whereas multi layer BNSs with an odd number of atomic layers (symmetry D3h) show piezoelectricity. As a result, the number of atomic layers show an inverse relationship with the piezo- and flexo-electric coefficients. This study highlights the possibility of developing light-weight and high-performance piezoelectric BN-based NEMS such as sensors, actuators and nanogenerators as the existing piezoelectric materials are heavy, brittle, and toxic. Keywords: Atomistic modeling; Boron nitride nanosheet; Boron-graphene heterostructures; Carbon-doping; Elastic properties; Multi-layer; Flexoelectric properties; Molecular dynamics simulation; Piezoelectric properties; Stone-Wales defects.