Layer-dependent thermal and thermoelectric properties of hydrogenated 2D diamond

Hydrogenated few layer graphene, also called thin film diamond, two-dimensional diamond or diamane are promising novel nano-materials with wide and direct band gap. A theoretical investigation is carried out to study the thermal and thermoelectric properties of hydrogenated graphene-based materials, namely graphane, bilayer (2LD), and trilayer diamane (3LD). Phonon dispersion relations, calculated within the harmonic approximation (HA), confirm their dynamical stability. The quasi-HA is then employed to assess the volume-dependent quantities, revealing positive thermal expansion coefficients for both diamane structures across the entire temperature range, in contrast to the negative expansion observed in graphane at low temperatures. The lattice thermal conductivity is obtained by solving the linearized Boltzmann transport equation using both the single-mode relaxation-time approximation and the exact iterative scheme which at 300 K, have values of 574, 1314, and 1282 W·m-1⋅K^-1for graphane, 2LD, and 3LD, respectively. It highlights the influence of hydrogenation and interlayer bonding on the conductivity. Mode-resolved analysis indicates that low-frequency acoustic phonons are the primary contributors to heat conduction, with increased lifetimes in multilayered structures. Thermoelectric properties are evaluated within the constant relaxation-time approximation, revealing a significant enhancement of the Seebeck coefficient upon hydrogenation. Among the studied systems, 2LD displays the best thermoelectric performance, reaching a maximum figure of merit (ZT) of approximately12×10-3at room temperature. These results demonstrate the tunability of thermal transport and thermoelectric efficiency in hydrogenated graphene structures, underscoring their potential for nanoscale thermal management and energy conversion applications.