Defect-Mediated Piezoelectricity in Boron Nitride Nanotube
Authors: Yu Sheng 
Published: December 2020

Introduction

A zigzag boron nitride nanotube (ZBNNT) has a large intrinsic polarized dipole and a piezoelectric tunability with crystal chirality, nanotube diameter, and point defect. The piezoelectric coefficients (e33) of 8-ZBNNT with the VIA-group defects of SN and SeN are estimated to be 18.7×10-2 C/m2 and 14.9×10-2 C/m2, respectively. The coefficients of defect-mediated 8-ZBNNT are 97% and 57% bigger than that of pristine 8-ZBNNT. The elastic modulus of 8-ZBNNT is decreased due to the VIA-group defects, but is increased for the larger number of unit cells along zigzag rolling direction. The electronic band gap of 8-ZBNNT is reduced due to the VIA-group defects. The large piezoelectricity enhancement of 8-ZBNNT with VIA-group defect is attributable to the enhanced piezoelectricity from ionic contribution. Therefore, the unprecedented defect-mediated 8-ZBNNT provides a new material platform of mechanoelectronic devices for nanoscale sensors and energy conversions.

Modeling and methodology

Figure 1 shows the schematic of a single wall 8-ZBNNT with an atomic defect. The supercell is in a simple orthorhombic box with periodic boundary condition in 30 Å × 30 Å × Lc
where Lc
is the lattice parameter along the transport direction. The green atom “A” indicates the defect of a single substitution atom per a supercell. The average bond length is defined as the average length of atomic bond between the defect doping atom and the neighboring boron atoms. The first principle calculations were carried out by using the Virtual Nanolab Atomistix ToolKit (ATK) package with the density functional theory (DFT). The localized density approximation (LDA) exchange correlation with a double zeta polarized (DZP) basis was used with a mesh cut-off energy of 150 Ry. All atomic positions and lattice parameters were optimized by using the generalized gradient approximations (GGA) with the maximum Hellmann Feynman forces of 0.05 eV/Å, which was sufficient to obtain relaxed structures. The maximum number of fully self-consistent field (SCF) iteration steps was set to 100.

Defect-mediated piezoelectricity

The table I includes the piezoelectric coefficient (e33) and average bond length (a0) of 10- and 8-ZBNNT with different kinds of defects including SW deformation, IVA-group, and VIA-group defects within one supercell. Among the defects in ZBNNT, a single nitrogen vacancy or SW defect diminishes the piezoelectric coefficient (e33) of 10- and 8-ZBNNT. However, the atomic defects of IVA- and VIA-group elements in a nitrogen site enhance the piezoelectric coefficient. The piezoelectric coefficient of VIA-group defect is higher than that of IVA-group defect. The piezoelectric coefficient e33 of chalcogen defect SN and SeN in 8-ZBNNT is enhanced 97% and 57% from 9.49×10-2 C/m2 to 18.7×10-2 C/m2 and 14.9×10-2 C/m2, respectively, compared to that of pristine 8-ZBNNT. The table I includes the elastic modulus of pristine10-ZBNNT and 8-ZBNNT and the ZBNNT with defects. The elastic modulus of 10- and 8-ZBNNT with defects is smaller than that of pristine 10- and 8-ZBNNT. The elastic modulus of SW defects in ZBNNT is increased compared to that of pristine 10- and 8-ZBNNT. The structure with smaller elastic modulus has the larger strain response to the constant external stress. Therefore, the ZBNNT with IVA- and VIA-group atomic defects has an efficient response to the external stress for piezoelectric applications. 8-ZBNNT of SN and SeN with the smaller elastic modulus have the larger e33 compared to the structure with ON.

Defect-mediated electronic band structure

Figure 2 shows the electronic band structure of 8-ZBNNT with (a) pristine structure and (b) defects of ON, SN and SeN. The group VIA-group defects in 8-ZBNNT are n-type doping because the Fermi level is located in the conduction band while the VIA-group defects in 8-ZBNNT are p-type doping. The bandgap in 8-ZBNNT is also reduced from 3.37 eV to 3.16 eV, 3.12 eV and 3.20 eV for the VIA-group defect of ON, SN and SeN, respectively. However, the band gaps of 8-ZBNNT with the IVA-group defects of CN, SiN and GeN is slightly widened to 3.38 eV, 3.49 eV and 3.50 eV compared to the band gap 3.37 eV of pristine 8-ZBNNT, respectively.

Conclusions

The piezoelectricity of ZBNNT is significantly enhanced by atomic defects. The piezoelectric coefficients (e33) 8-ZBNNT with SN and SeN defects are increased up to 97% and 57%, respectively. The electronic band gap of 8-ZBNNT with defects is reduced. The large piezoelectricity enhancement of 8-ZBNNT with VIA-group defect is attributable to the enhanced piezoelectricity from ionic contribution. Therefore, the unprecedented defect-mediated 8-ZBNNT provides a new material platform of mechanoelectronic devices for nanoscale sensors and energy conversions.

Acknowledges

This work is supported by ARO W911NF-15-1-0535, NSF HRD-1137747, and NASA NNX15AQ03A.

Figure 1