Interfacial Energetics of C₂-C₁₈ Aliphatic Moieties on Hydrogenated Si(111) and Si(110) Surfaces: A DFT Study

We present a computational study based on density functional theory to systematically investigate how aliphatic moiety functionalization affects the interfacial electronic structure of H-terminated Si(111) and Si(110) surfaces. We explore the energetics, dipole formation, and charge transfer mechanisms for alkyl, alkenyl, and 1-alkynyl chains containing from 2 to 18 carbon atoms chemisorbed on both crystallographic orientations. Our analysis reveals that alkenyl moieties exhibit pronounced chain-length dependence of surface dipoles and tunneling barriers, whereas alkyl and 1-alkynyl chains show saturation effects for longer chains. We found that H-Si(111) exhibits surface dipoles up to 33% larger than H-Si(110), due to differences in atomic packing density and Si-H bond orientation. The resulting charge injection barriers for both thermionic and tunneling transport are quantified and discussed. The tilted geometry adopted by alkenyl moieties on Si(110) is rationalized through analysis of molecular orbital hybridization with surface states. These results provide quantitative guidelines for engineering interface energetics in silicon-based molecular electronic devices through rational choice of molecular termination and substrate orientation.