Role of the head and/or tail groups of adsorbed -[X^{head group}]-Alkyl-[X^{tail group}] [X = O(H), S(H), NH₍₂₎] chains in controlling the work function of the functionalized h:si(111) surface

Using density functional theory (DFT) including van der Waals interactions, we examine work-function (WF) tuning of H:Si(111) over a range of 1.73 eV through the adsorption of alkyl monolayers with the general formula -[X^{head group}]-(C_nH_2n)-[X^{tail group}], X = O(H), S(H), NH₍₂₎. The WF is practically converged at four carbons (eight for oxygen) for head-group functionalization. For tail-group functionalization and with both head and tail groups, there is an odd-even effect in the behavior of the WF, with peak-to-peak amplitudes of up to 1.7 eV in the oscillations. This behavior can be explained through the orientation of the terminal-group dipole. The shift in the WF is largest for NH₂-linked chains and smallest for SH-linked chains, which can be rationalized in terms of interface dipoles. Our study reveals that the choice of the head and/or tail group effectively changes the impact of the alkyl chain length on the WF tuning using self-assembled monolayers. This is an important advance in utilizing hybrid functionalized Si surfaces.