1. Simultaneous Control of Surface Chemistry and Nanoscale Topography on the Si(100) Surface
Madeleine Beasley and K. T. Queeney
Department of Chemistry, Smith College, Northampton MA 01063
Introduction
Acknowledgments
Conclusions
Surface Hydride Species
Hydrosilylation
Multifunctionalization
Henry Dreyfus Teacher Scholar Award from the Camille and Henry
Dreyfus Foundation, Nancy Kershaw Tomlinson Memorial Fund
{110}
{110}
{110}
{110}
(100) (100)
2071
2082
2088
2109
2115
2134
2000
1 x 10-4
Frequency (cm-1)
Absorbance
Hydride
Species
Frequency
(cm-1)
Face Si-H Stretch
Vibration
Mono
Mono
Mono
Di
Di
Di
2071
2082
2088
2109
2115
2134
{110}
{110}
{110}
(100)
(100)
(100)
antisymmetric
strained
symmetric
Unstrained,
antisymmetric
strained
strained
1047
1166 2088 2250
3 hr.
2 hr.
1 hr.
4 x 10-4 2 x 10-4
Frequency (cm-1)
Absorbance
900
2070 2080 2090 2100 2110
2088
5 x 10-4
Absorbance
40:20 Min.
60:40 Min.
60:20 Min.
Frequency (cm-1)
Figure 1. Confocal laser scanning microsope images
showing micro-organism behavior on a variety of surface
chemistries and topographies.
1
Figure 2. a) Molecular model of a {110}-faceted hillock.
Unstrained {110} monohydrides are shown in pink,
strained {110} monohydrides are shown in orange,
and (100) dihydrides are shown in green.
2
b) AFM image (tapping mode) of an H-terminated
Si(100) surface after etching in deoxygenated water to
prduce the surface modeled above.
1 x 10-3
2800 2900 3000
Absorbance
Figure 3. FTIR spectrum showing all hydride species present on initial H-terminated surface.
Details for each are illustrated in the table above.
2
Figure 4. FTIR difference spectra showing the oxidation preference
for the {110} unstrained monohydride with a corresponding frequency
at 2088 cm
-1
.
1. J. Zhang, J. Huang, C. Say, M. Beasley, R. Gerdes, Rob Dorit and
K.T. Queeney (In Preparation).
2. Aldinger, B.S.; Hines, M.A. J. Phys. Chem. C. 2012, 116, 21499-21507.
3. Faggin, M.F.; Green, S.K.; Clark, I.T.; Queeney, K.T.; Hines, M.A.
J. Am. Chem. Soc. 2006, 128, 11455-11462.
4. Linford, M.R.; Chidsey, C.E.D. J. Am. Chem. Soc. 1993, 115, 12631-12632.
Figure 5. FTIR showing the replacement of SiH bonds with SiO
bonds through an O3
SiH intermediate.
Figure 9. Evidence for multifunctionalization of a surface with oxidation and
hydrosilylation.Figure 8. Comparison of CHx
stretch region for the multifunctionalized
surface to reference hydrosilylated surfaces.
Initial H-terminated Surface
Site-Selective Oxidation
Figure 6. CHx
stretch region of a completely hydrosilylated flat surface.
Figure 7. Comparison of hydrosilylation on a rough (water-etched), H-terminated surface with
the same reaction on a fully oxidized (SiO2
) surface.
Previous work3
has demonstrated that an
H-terminated Si(100) surface covered with
regular, nanoscale (~50-100 nm) hillocks can be generated by etching flat
H-terminated surfaces in deoxygenated water. While these features can
be seen directly with scanning probe techniques, the best insight into the
precise surface termination of the hillocks is provided by surface infrared
spectroscopy, which is exquisitely sensitive both to different silicon hydride
species and their local chemical environments.
2800 2850 2900 2950 3000
H Reference
Alkylated
CH3
stretch
asymmetric
CH2 stretch
symmetric
CH2 stretch
ν(CH)
Frequency (cm-1)
5 x 10-4
Absorbance
asymmetric
CH2 stretch
symmetric
CH3 stretch
symmetric
CH2 stretch
5 x 10-4
Absorbance
Alkylated
H Reference
ν(CHx
)
Frequency (cm-1)
2800 2850 2900 2950 3000
5 x 10-4
Absorbance
2750 2800 2850 2900 2950 3000 3050
ν(CH)
asymmetric
CH2 stretch
Frequency (cm-1)
Rough
SC-2
5 x 10-4
asymmetric
CH2 stretch
Absorbance
ν(CHx
)
Frequency (cm-1)
Rough
Fully oxidized
ν(SiOx
)
5 x 10-4
2800 2850 2900 2950 3000
Frequency (cm-1)
5 x 10-4
Absorbance
ν(CH)
Frequency (cm-1)
Absorbance
5 x 10-4
ν(CHx
)
2800
2800 2900 3000 900 1000 1100 1200 1300
Rough
Fully oxidized
Fully oxidized,
no alkylation
Frequency (cm-1)
900 1000 130012001100
Frequency (cm-1
)
5 x 10-4
ν(CHx
) ν(SiOx
)
partially oxidized
Multi-
functionalized
Alkylated
a)
b)
H H H H H
Initial Surface
H H HH H H H H H H
HH H
H
HH HH
Rough, H-Terminated Surface
24-hr Etch in
Ar-purged H2
O
An extensive body of work
has demonstrated that
cells and micro-organisms
behave differently on surfaces
with nanoscale (<100 nm)
topography than they do on
flat surfaces or on substrates
with conventional-scale
features. Recent work in
our lab has demonstrated
the combined effects of
surface chemistry and
nanoscale topography
on nucleation of biofilms of Pseudomonas aeruginosa.
The goal of the current work is to expand our ability to control the
combined effects of surface topography and chemistry by developing
a method to pattern both chemistry and topography simultaneously
on the nanoscale. To do this we will exploit naturally-occuring chemistry
that creates nanoscale topography on Si(100) and then explore whether
the resulting surface allows site-specific chemistry that will promote
nanoscale chemical patterning without the need for lithography.
Transmission FTIR reveals the characteristic
Si-H stretching modes of the hillock-covered
surface. Notably, this surface is reproducibly
created by a room-temperature etching
process. Work by Hines and coworkers2
has correlated the distinct Si-H peak
frequencies with surface species that are
attributed to the crystallographic planes that
form the sides and tops of the hillock features
that dominate this surface after long (~24-hr)
etch times.
Our goal was to determine whether we could selectively react (in this case, oxidize)
either the sides or tops of the hillocks preferentially, leaving the remaining face(s)
H-terminated for susbequent hydrosilylation. As shown in Figure 4, early stages of
controlled oxidation lead to preferential removal of monohydride species associated
with the sides of the hillocks, suggesting that the bulk of surface oxidation shown in
Figure 5 is confined to hillock sides as we had hoped.
Thermally-induced hydrosilylation
by reaction with 1-octadecene4
has
been widely used to form close-packed
alkyl layers on flat, hydrogen-
terminated Si surfaces. Figure 6
shows the resulting IR spectrum
of such a monolayer on flat
H:Si(100) in our lab. As shown in Figure 8,
this same hydrosilylation reaction has
been used successfully in our lab to
generate close-packed alkyl layers on the
surfaces with nanoscale hillocks.
For our goal of nanoscale chemical patterning to be achieved, this hydrosilylation reaction needs to be much less favorable on the pre-oxidized
hillock sides than on the unoxidized SiHx sites on the hillock tops. Figure 8 shows that, in fact, hydrosilylation will occur on fully oxidized sites, presumably
by reaction with surface silanols. However, this reaction is less extensive than on the unoxidized surface and results in a less well-packed monolayer,
as indicated by the asymmetric methylene stretch frequency. Analysis of the Si-O stretching region shows that hydrosilylation of oxidized sites is
characterized by growth of a ν(Si-O-C) band around 1100 cm-1
.
Multifunctionalized surfaces were attempted by partial (1 hour) oxidation to preferentially oxidize sites on the hillock sides, followed by short (15 minutes)
thermal hydrosilylation to minimize the less-favorable reaction with oxidized sites. Analysis of the CH stretching region of IR spectra of the resulting surfaces
reveals that the alkyl layers thus formed are close packed, which suggests they occur predominantly on regions of extended H termination and not on
oxidized areas. The Si-O region of the same IR spectra does not show evidence of extensive Si-O-C bond formation, which is also consistent with a lack
of hydrosilylation on oxidized regions.
We have strong evidence that we have successfully carried out spatially differentiated chemistry on the nanoscale on a surface with nanoscale topography.
Additional experiments (contact angle goniometry, atomic force microscopy) will be used to probe this reaction in more detail. We are also interested in
exploring the possibility of reacting the oxidized sites (e.g. via silanization) to introduce greater variety of chemical functionalization and perhaps to more
effectively hinder subsequent reaction at oxidized sites.
The ability to pattern surfaces with both topography and chemistry on length scales of 50-100 nm opens up new possibilities for surfaces that can be
tailored to enhance (or inhibit) cell and/or micro-organism attachment and growth.
Alkylated
Multifunctionalized
Multi-
functionalized
Fully oxidized,
hydrosilylated
Fully
hydrosilylated
2050 2100 2150 2200
1000 1100 1200 1300
2850 2900 2950 3000
Rough, H-Terminated Surface
H H H H H H
HH H
H
HH HH
Partially Oxidized Surface Multifunctionalized Surface
2000 2100 2200 2300