On the nitrogen concentration and crystal size dependence of lattice thermal conductivity in diamond thin and nanofilms

This work comprehensively analyses lattice thermal conductivity (LTC) in diamond materials, focusing on the dual influence of nitrogen impurities and crystal size. Using a modified Debye-Callaway model, theoretical calculations are performed for two groups of diamond specimens. The first group comprises single-crystal diamonds with varying nitrogen concentrations, ranging from nearly pure to highly doped specimens. The second group comprises single crystals, microfilms, and nanofilms of varying sizes, allowing for an investigation of size-dependent effects. The study systematically examines the contributions of various phonon-scattering mechanisms, including phonon-phonon interactions (Normal and Umklapp processes), point defects, isotopes, boundary scattering, dislocations, phonon-electron interactions, and phonon resonance scattering for both longitudinal and transverse phonon modes. Notably, phonon resonance scattering plays a significant role in LTC reduction at low temperatures, where phonons interact with localized defect-induced vibrational modes. Nitrogen impurities act as strong scattering centers, significantly disrupting the lattice structure and reducing LTC. The degree of reduction correlates with nitrogen concentration, defect density, and platelet configurations. Crystal size is another critical factor, as smaller samples exhibit enhanced boundary scattering, resulting in a pronounced decline in LTC, particularly at nanoscale dimensions. The work further identifies systematic relationships between LTC, nitrogen concentration, and crystal size, providing key mathematical models to predict thermal transport behaviour. These findings offer valuable insights into tailoring diamond materials for advanced applications in thermal management, high-power electronics, and energy systems, where precise control of thermal properties is essential.