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Density Functional Theory with van der Waals Corrections Study of the Adsorption of Alkyl, Alkylthiol, Alkoxyl, and Amino-Alkyl Chains on the H:Si(111) Surface
journal contribution
posted on 2014-11-11, 00:00 authored by Hadi H. Arefi, Michael Nolan, Giorgos FagasSurface modification of silicon with
organic monolayers tethered
to the surface by different linkers is an important process in realizing
future miniaturized electronic and sensor devices. Understanding the
roles played by the nature of the linking group and the chain length
on the adsorption structures and stabilities of these assemblies is
vital to advance this technology. This paper presents a density functional
theory (DFT) study of the hydrogen passivated Si(111) surface modified
with alkyl chains of the general formula H:Si–(CH2)n–CH2 and H:Si–X–(CH2)n–CH3, where
X = NH, O, S and n = (0, 1, 3, 5, 7, 9, 11), at half
coverage. For (X)–hexane and (X)–dodecane functionalization,
we also examined various coverages up to full monolayer grafting in
order to validate the result of half covered surface and the linker
effect on the coverage. We find that it is necessary to take into
account the van der Waals interaction between the alkyl chains. The
strongest binding is for the oxygen linker, followed by S, N, and
C, irrespective of chain length. The result revealed that the sequence
of the stability is independent of coverage; however, linkers other
than carbon can shift the optimum coverage considerably and allow
further packing density. For all linkers apart from sulfur, structural
properties, in particular, surface-linker-chain angles, saturate to
a single value once n > 3. For sulfur, we identify
three regimes, namely, n = 0–3, n = 5–7, and n = 9–11, each with its
own characteristic adsorption structures. Where possible, our computational
results are shown to be consistent with the available experimental
data and show how the fundamental structural properties of modified
Si surfaces can be controlled by the choice of linking group and chain
length.