Enhanced piezoelectricity and half-metallicity of fluorinated AlN nanosheets and nanoribbons: a first-principles study

Utilizing first-principles calculations, we have investigated the structural, mechanical, piezoelectric, and electronic properties of fluorinated AlN (F-AlN) nanosheets and nanoribbons. Unlike the fluorinated graphene and BN systems, the F-AlN sheet favours a stable boat-like buckled structure, which has a soft mechanical feature with an anisotropic Young modulus and Poisson ratio. Such structural flexibility and anisotropy result in an improved piezoelectric performance for the F-AlN system, whose piezoelectric coefficients, especially d(11) and e(11), are larger than those of hexagonal group III-V, group-III monochalcogenide, and transition metal dichalcogenide nanosheets. The F-AlN sheet and its armchair nanoribbons are semiconductors, while the zigzag ones become metallic. In particular, an intriguing half-metallic behaviour appears in these zigzag F-AlN nanoribbons, whose half-metal gaps are large for room-temperature operation. Moreover, armchair F-AlN nanoribbons can also be converted to half-metals by hole doping, which brings tunable half-metal gaps to the system. Our study demonstrates that fluorination is an efficient route to tailoring the properties of AlN nanomaterials, which have promising piezoelectric performance and half-metallic characteristics for potential applications in nano-sized energy harvesting and spintronic devices.