Strain-Induced Band-Gap Transitions in Hexagonal Boron Nitride

Project Details

• Student(s): Samir J. Daou (Chemical Enginering)
• Advisor(s): Dr. Serge Maalouf
• Department: Industrial & Mechanical
• Academic Year(s): 2025-2026

Abstract

Two-dimensional (2D) materials are atomically thin layers, exhibiting electronic properties that differ greatly from their bulk counterparts. One notable consequence of this is a highly tunable band gap, which is a property of particular interest in hexagonal boron nitride (h-BN), a wide band-gap 2D semiconductor known for its mechanical flexibility and chemical stability. Strain engineering has emerged as a practical way to tune h-BN’s electronic structure without chemical modification, and understanding how strain states drive band-gap changes and indirect-direct transitions is essential for the design of strain-engineered devices.

A series of density functional theory (DFT) simulations are performed on h-BN under three distinct deformation modes: strain along the zigzag direction, strain along the armchair direction, and in-plane shear strain. The strain magnitudes considered range from -5% to 5%. For each condition, the band structure is computed and plotted to track how energy levels shift and how the band gap evolves with deformation. The PBE functional yields a ground-state band gap of 4.57 eV, consistent with previously reported values. However, PBE is well known to systematically underestimate band gaps due to self-interaction errors. Hybrid functionals are an alternative to the PBE functional that can be used to accurately predict the band gaps of materials. HSE, an example of such hybrid functionals which incorporates a fraction of exact Hartree-Fock exchange, is also employed to provide a more accurate description of the band structure and a corrected band-gap magnitudes. In the case of h-BN, the band gap obtained using the HSE functional is of 5.58 eV, which compares well with experimentally reported values.

Results reveal a strong coupling between mechanical deformation and electronic structure, with both PBE and HSE predicting identical qualitative trends under uniaxial strain. Under uniaxial strain along the zigzag and armchair directions, the band gap initially increases under tensile loading before reaching a critical threshold beyond which an indirect-to-direct transition occurs, followed by a gradual reduction. Compressive strain produces a monotonic decrease in the band gap leading to a similar transition. Under shear strain, PBE predicts a weak and symmetric response, while HSE reveals a more pronounced and comparable modulation. These findings highlight the sensitivity of h-BN’s electronic structure to both the magnitude and direction of applied strain, underscoring h-BN’s strong potential for straintronics.