Effect of Surface Coverage and Composition on the Stability and Interfacial Dipole of Functionalized Silicon

A method for predicting the stability and interfacial dipole of mixed functionalized surfaces using first-principles density functional theory calculations is described, and calculated trends are consistent with previously published experimental data. Predicting the interfacial dipole is critical for photovoltaic and photoelectrochemical applications because the dipole can be tailored to enhance device performance by improving charge separation at the interface. To demonstrate the approach, the enthalpy of reaction and interfacial dipole as a function of coverage of 3,4,5-trifluorophenylacetylenyl (TFPA) moieties on Si(111) was analyzed for both mixed methyl/TFPA and mixed chlorine/TFPA-terminated surfaces. The enthalpy of reaction calculations show that the affinity for functionalization improves as a function of TFPA coverage for the mixed chlorine surface but instead remains constant for the mixed methyl surface across all coverages. The results indicate that the trend in enthalpy of reaction is a good predictor of the affinity for functionalization and the stability of the resulting surface. The interfacial dipole calculations show that the shift in dipole relative to the H-terminated Si(111) surface increases as a function of TFPA coverage with the mixed chlorine surface having a more positive shift than the mixed methyl across all coverages. We find that there are significant interactions between TFPA and neighboring -Cl or -CH₃ moieties that increase the magnitude of the interfacial dipole. This suggests that the magnitude of an interfacial dipole can be tuned by adjusting the chemical makeup of a mixed monolayer. All trends presented in this work were validated against experimental observations found in literature for both mixed methyl/TFPA and chlorine/TFPA surfaces.