American Chemical Society, Journal of Physical Chemistry C, 21(119), p. 11588-11597, 2015
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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 adsorption of alkyl monolayers with general formula -[Xhead-group]-(CnH2n)-[Xtail-group], X = O(H), S(H), NH(2). The WF is practically converged at 4 carbons (8 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 is explained through the orientation of the terminal-group's dipole. The shift in the WF is largest for NH2-linked and smallest for SH-linked chains and is rationalized in terms of interface dipoles. Our study reveals that the choice of the head- and/or tail-groups 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.