10.1021/la204770r.s001
Bart Rijksen
Bart
Rijksen
Sidharam P. Pujari
Sidharam P.
Pujari
Luc Scheres
Luc
Scheres
Cees J. M. van Rijn
Cees
J. M. van Rijn
J. E. Baio
J. E.
Baio
Tobias Weidner
Tobias
Weidner
Han Zuilhof
Han
Zuilhof
Hexadecadienyl Monolayers
on Hydrogen-Terminated Si(111):
Faster Monolayer Formation and Improved Surface Coverage Using the
Enyne Moiety
American Chemical Society
2012
IRRAS
NEXAFS
surface coverage
Si 2p region
contact angle measurements
C 16 dienyl layers
enyne
CH
DFT
monolayer formation
Alkenyl layers show
Quantitative XPS measurements
16 h
Molecular mechanics simulations
hexadec
2012-04-24 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Hexadecadienyl_Monolayers_on_Hydrogen_Terminated_Si_111_Faster_Monolayer_Formation_and_Improved_Surface_Coverage_Using_the_Enyne_Moiety/2528257
To further improve the coverage of organic monolayers
on hydrogen-terminated
silicon (H–Si) surfaces with respect to the hitherto best agents
(1-alkynes), it was hypothesized that enynes (H–CC–HCCH–R)
would be even better reagents for dense monolayer formation. To investigate
whether the increased delocalization of β-carbon radicals by
the enyne functionality indeed lowers the activation barrier, the
kinetics of monolayer formation by hexadec-3-en-1-yne and 1-hexadecyne
on H–Si(111) were followed by studying partially incomplete
monolayers. Ellipsometry and static contact angle measurements indeed
showed a faster increase of layer thickness and hydrophobicity for
the hexadec-3-en-1-yne-derived monolayers. This more rapid monolayer
formation was supported by IRRAS and XPS measurements that for the
enyne show a faster increase of the CH<sub>2</sub> stretching bands
and the amount of carbon at the surface (C/Si ratio), respectively.
Monolayer formation at room temperature yielded plateau values for
hexadec-3-en-1-yne and 1-hexadecyne after 8 and 16 h, respectively.
Additional experiments were performed for 16 h at 80° to ensure
full completion of the layers, which allows comparison of the quality
of both layers. Ellipsometry thicknesses (2.0 nm) and contact angles
(111–112°) indicated a high quality of both layers. XPS,
in combination with DFT calculations, revealed terminal attachment
of hexadec-3-en-1-yne to the H–Si surface, leading to dienyl
monolayers. Moreover, analysis of the Si<sub>2p</sub> region showed
no surface oxidation. Quantitative XPS measurements, obtained via
rotating Si samples, showed a higher surface coverage for C<sub>16</sub> dienyl layers than for C<sub>16</sub> alkenyl layers (63% vs 59%).
The dense packing of the layers was confirmed by IRRAS and NEXAFS
results. Molecular mechanics simulations were undertaken to understand
the differences in reactivity and surface coverage. Alkenyl layers
show more favorable packing energies for surface coverages up to 50–55%.
At higher coverages, this packing energy rises quickly, and there
the dienyl packing becomes more favorable. When the binding energies
are included the difference becomes more pronounced, and dense packing
of dienyl layers becomes more favorable by 2–3 kcal/mol. These
combined data show that enynes provide the highest-quality organic
monolayers reported on H–Si up to now.