Functionalization-Driven Control of Flexural Acoustic Modes and Thermal Expansion in 2D BN Heterostructures

Abstract

Two-dimensional (2D) materials often display negative thermal expansion (NTE) at low temperatures due to flexural acoustic modes. Here we combine density functional perturbation theory (DFPT) with the quasi-harmonic approximation (QHA) to quantify how chemical functionalization of monolayer hexagonal boron nitride (h-BN) modifies its thermal expansion. Using ab initio phonons and QHA, we computed the linear thermal expansion coefficient (LTEC) for pristine h-BN, a graphitic boron–nitride–carbon alloy (h-CBN), and a carbon/oxygen functionalized BN (f-BN) identified by systematic small-cell evaluation of distinct substitutional configurations. We find that functionalization both preserves dynamical stability (no imaginary phonons) and engineers the flexural branch: f-BN exhibits a more linear out-of-plane acoustic (ZA) dispersion near Γ relative to h-BN. As a result, f-BN reduces the magnitude of NTE by ≈34% compared to pristine h-BN in 0–300 K (minima in this range: h-BN −6.5 × 10–6 K–1, f-BN −2.6 × 10–6 K–1, h-CBN −1.05 × 10–6 K–1, graphene −4.23 × 10–6 K–1), while retaining NTE up to 1000 K. We attribute this quantitative tuning to a partial linearization of the flexural (ZA) branch near Γ and a redistribution of negative mode Grüneisen parameters toward higher frequencies. These results clarify the microscopic origin of NTE control in functional BN and suggest a practical route to mitigate thermal-mismatch strain for BN-based heterostructures and device interfaces.