# Effect of Net Charge on the Relative Stability of 2D Boron Allotropes

## TL;DR

This study investigates how electron doping influences the stability and bonding of 2D boron structures, revealing new stable allotropes and a doping-induced phase transition with potential piezoelectric properties.

## Contribution

It introduces previously unknown stable boron allotropes and demonstrates how electron doping alters their stability and structure, including phase transitions and piezoelectric response.

## Key findings

- Discovery of new stable 2D boron structures
- Electron doping causes phase transitions in borophene
- Doping levels can be doubled with layered electrides

## Abstract

We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our {\em ab initio} calculations for the neutral system reveal previously unknown stable 2D $\epsilon$-B and $\omega$-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of electron doping, the most stable allotrope changes from $\epsilon$-B to a buckled honeycomb structure. Additional electron doping, mimicking a transformation of boron to carbon, causes a gradual decrease in the degree of buckling of the honeycomb lattice that can be interpreted as piezoelectric response. Net electron doping can be achieved by placing borophene in direct contact with layered electrides such as Ca$_{2}$N. We find that electron doping can be doubled by changing from the B/Ca$_{2}$N bilayer to the Ca$_{2}$N/B/Ca$_{2}$N sandwich geometry.