Spectral Analysis of the Heat Flow Across Crystalline and Amorphous Si-Water Interfaces

Nonequilibrium classical molecular dynamics simulations were employed to investigate thermal transport across crystalline and amorphous silicon (a-Si) surfaces in contact with water for different interfacial bonding strengths. A spectral analysis of heat transfer across the different interfaces revealed the characteristics of the phonon modes contributing to thermal transport. Low-frequency modes contributed the most in hydrophobic interfaces, while a shift toward contribution from higher frequency modes was found for hydrophilic surfaces. The shift to higher frequency modes was not significant for a-Si and crystalline Si(111) interfaces. In-plane phonon modes significantly contributed to heat transfer in Si(100), less significantly in a-Si, and had a minimum contribution in Si(111) hydrophilic interfaces. While the wettability and solid liquid bonding strength failed in explaining these observations, the interfacial liquid density depletion helped to understand the differences between Si(100) and a-Si interfaces with respect to the Si(111) interface. The interface liquid structure observed in the Si(100) but not in the a-Si system served as an explanation for the dominant contribution of in-plane modes in Si(100). These observations posed the density depletion and liquid structure at solid-liquid interfaces as useful parameters for explaining the underlying mechanisms of phonon transport at solid liquid interfaces.