Mild methods to assemble and pattern organic monolayers on hydrogen-terminated Si(111)

Article abstract of DOI:10.1039/b503271g

Mild methods to assemble well-ordered organic monolayers of olefins on Si(111) using 2,2,6,6-tetramethyl-1-piperidinyloxy and to pattern these monolayers on the micrometer-size scale using soft lithography are reported. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503271g

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Mild methods to assemble and pattern organic monolayers on
hydrogen-terminated Si(111){
a
Samer N. Arafat, Samrat Dutta, Mathew Perring, Michael Mitchell, Paul J. A. Kenis and
a
a
b
b
a
Ned B. Bowden*
Received (in Columbia, MO, USA) 4th March 2005, Accepted 28th April 2005
First published as an Advance Article on the web 24th May 2005
DOI: 10.1039/b503271g
Mild methods to assemble well-ordered organic monolayers of
olefins on Si(111) using 2,2,6,6-tetramethyl-1-piperidinyloxy
and to pattern these monolayers on the micrometer-size scale
using soft lithography are reported.
glass, methods to assemble SAMs on silicon typically require harsh
1–4
reagents or conditions. Current methods to assemble SAMs on
silicon use ultraviolet light, Lewis acids, organic radicals from the
decomposition of diacyl peroxides at elevated temperatures, heat,
or halogenation of the surface followed by reaction with Grignard
reagents.
Combining patterned, organic self-assembled monolayers (SAMs,
Fig. 1) with semiconducting surfaces will have a wide impact on
the study of biological surfaces and molecular electronics.
Although mild methods exist to assemble and pattern SAMs on
electrically conducting and insulating substrates such as gold and
In this paper we report mild methods to assemble and pattern
organic SAMs on hydrogen-terminated Si(111). We assembled
SAMs using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in the
presence of 1-octadecene at room temperature without the need
5
for UV light (Fig. 1). Although TEMPO is a free radical, it is
stable at room temperature and unreactive with most functional
groups. In addition, we report a mild method to pattern SAMs on
Si(111) using soft lithography.
Our method to assemble ordered SAMs has three parts. First,
we cleaned silicon wafers in organic solvents, then we removed the
native silicon dioxide layer with 5:1 40% NH
a thin layer of silicon dioxide was grown by placing the wafer in
:3 30% H /concentrated sulfuric acid at 90 uC for 1 h. Finally,
hydrogen-terminated Si(111) was formed using established techni-
ques by immersion of a silicon wafer in 40% NH F under an
4
F/48% HF. Next,
1
2 2
O
4
6
atmosphere of Ar. The silicon shard was placed in a Schlenk flask
with TEMPO and 1-octadecene in a glove box under N for 24 h.
2
We studied the monolayers assembled from 1-octadecene and
TEMPO using ellipsometry, contact angle goniometry, and X-ray
photoelectron spectroscopy (XPS, Table 1). The ellipsometric data
for all of the SAMs were consistent with a monolayer of
1-octadecene rather than a multilayer.
We studied the contact angles of water for monolayers
assembled from various concentrations of TEMPO in neat
1
-octadecene (entries 1 to 5 in Table 1). The best contact angles
of 110u were measured on monolayers assembled from 1.0 and
.1 mole percent TEMPO. Others have reported that values for the
0
advancing contact angles of water on well-ordered SAMs exposing
1
methyl groups are 110–113u. Values for the advancing contact
angles of water on disordered SAMs exposing methylene groups
7
and on polyethylene are 102u. Thus, advancing contact angles of
Fig. 1 a) A Si(111)-H surface is generated by reacting a clean silicon
water on an alkyl SAM can provide evidence that the SAM is well-
ordered, yet they do not provide definitive proof. Our results
indicate that well-ordered monolayers are formed in the presence
of TEMPO, but disordered SAMs were obtained from 1-octade-
cene in the absence of TEMPO (entry 16) and from TEMPO in
the absence of 1-octadecene (entry 14).
4 2
wafer with 40% NH F under N . This surface is reacted with different
concentrations of TEMPO or derivatives of TEMPO in 1-octadecene to
form well-ordered monolayers. b) The different derivatives of TEMPO
used to assemble the monolayers.
{
Electronic supplementary information (ESI) available: details of method
of assembly of SAMs, synthesis of derivatives of TEMPO, and
representative examples of XPS spectra. See http://www.rsc.org/supp-
data/cc/b5/b503271g/
Interestingly, we saw some oxygen on the surface in the XPS
data for monolayers assembled from TEMPO and 1-octadecene
*ned-bowden@uiowa.edu
(entries 1–5 in Table 1). Oxygen could originate from SiO
2
or from
3
198 | Chem. Commun., 2005, 3198–3200
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Table 1 The contact angles and atomic compositions from XPS for various monolayers assembled on Si(111)
b
2
H O Contact
a
c
TEMPO-R (mole%)
angle (u)
XPS Atomic composition (%)
d
e
d
˚
Entry
Olefin
H
F
15
C
10
C
6
C
8
ellisp (A)
A
R
SiO
2
Si
F
C
O
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
C
C
C
C
C
C
C
C
C
C
C
C
C
18
18
18
18
18
18
18
18
18
18
18
18
18
H
H
H
H
H
H
H
H
H
H
H
H
H
36
36
36
36
36
36
36
36
36
36
36
36
36
10
1.0
0.1
0.01
0.001
—
19
21
15
—
19
17
15
14
17
19
18
20
—
—
—
106
110
110
108
106
114
112
111
110
112
111
111
111
85
104
107
107
107
103
113
111
110
108
107
105
107
106
71
0
0
0
0
0
0
0
0
0
0
0
0
0
5
2
43
39
36
32
34
31
31
33
35
35
33
34
33
29
36
37
0
0
0
0
0
18
6
2
0
0
0
0
0
0
8
0
52
54
59
62
59
46
58
60
55
60
60
61
61
18
24
51
6
7
5
6
7
4
4
7
10
5
7
6
6
48
29
12
1
0.1
0.01
0.001
1
1
1
1
1
1
1
1.0
0.1
1.0
0.1
f
f
None
None
C
0.1
0.1
89
102
78
91
g
H
18 36
trace
a
b
These values are the mole percent of TEMPO-R in the 1-octadecene. Values that are blank have a concentration of zero. The errors in the
c
advancing (A) and receding (R) contact angles were approximately ¡1u. We studied the Si(2p), F(1s), C(1s), and O(1s) peaks. The peak
corresponding to SiO appeared at approximately 102 eV in the Si(2p) high resolution scan. The olefin was 1-octadecene. The thickness was
2
f
measured by ellipsometry. The errors in the measurements were ¡5 A. The TEMPO and TEMPO-F15 in these entries were at the same
concentrations as TEMPO in entry 3. The monolayers were assembled in hexane. No TEMPO or derivatives of TEMPO were used to
assemble this monolayer.
d
e
˚
g
surface-bonded TEMPO. To determine whether TEMPO was
bonded to the surface, we assembled monolayers using TEMPO-
TEMPO-F15 in hexane and from neat 1-octadecene without
TEMPO had peaks in the XPS spectra that correspond to oxidized
silicon (entries 14 to 16 in Table 1).
F15 and 1-octadecene. The results in entries 6 to 9 in Table 1 show
that the fluorine is present and decreases in concentration as the
concentration used to assemble the monolayer decreased. These
results demonstrated that TEMPO-F15 is indeed bonded to the
surface. In summary, some of the oxygen on the surface is due to
TEMPO, but due to limitations of XPS, ellipsometry, and
structures of the monolayers we can not say whether all of the
oxygen on the surface is due to TEMPO.
To further study the quality of the monolayers, we investigated
the stability of monolayers from entry 11 in Table 1. Wafers left
exposed to ambient atmosphere for 48 days did not exhibit
changes in their contact angles and XPS spectra. Thus, the SAMs
are stable in air for weeks. We immersed the wafers in CHCl3,
toluene, or water for various durations of time. We removed the
wafers and characterized them by contact angle goniometry and
XPS. If the SAM is well-ordered and stable, we expect that
The presence of TEMPO on the surface raised an interesting
problem. TEMPO is expected to increase the disorder in the
monolayer of 1-octadecene as it occupies a large cross sectional
area near the silicon surface. TEMPO is shorter than 1-octadecene
so a gap above TEMPO should exist and disorder adjacent
molecules of 1-octadecene. To minimize the disorder caused by
TEMPO on the surface, we wished to fill in the ‘‘gap’’ above
TEMPO by assembling monolayers from TEMPO-C10 and
advancing contact angles of water will decrease by less than several
1
degrees and the XPS spectrum will not show the presence of SiO .
2
If the SAM is disordered and unstable, the contact angle will
significantly decrease and the XPS spectrum will show a peak for
1
. The data in Table 2 demonstrate that the SAMs are stable
SiO
2
in refluxing CHCl (boiling point 5 61 uC) for days and in water
3
up to 50 uC. They have limited stabilities in refluxing toluene
(boiling point 5 111 uC) and refluxing water. These results provide
further evidence that we assembled well-ordered monolayers and
that they are stable under a range of conditions, thus widening the
variety of potential applications.
6 8
TEMPO-C C . We expected that monolayers assembled from
these derivatives of TEMPO would be more ordered as the C10
and C C chains would partially fill the gap above TEMPO and
6
8
increase the order of adjacent 1-octadecene molecules. The results
in entries 10 to 13 in Table 1 support this hypothesis as the
advancing contact angles increased slightly from 110u for SAMs
assembled with TEMPO to 111–112u for SAMs assembled with
TEMPO-C10 and TEMPO-C C .
We developed a method to pattern these monolayers using soft
lithography. In our method, PDMS molds were brought into
contact with a silicon wafer to form a series of open channels with
three sides being PDMS and the fourth side hydrogen-terminated
silicon(111) (Fig. 2).
6
8
Others have shown that well-ordered monolayers on silicon
prevent the oxidation of silicon to silicon dioxide, and disordered
A monolayer terminated with carboxylic acid was assembled in
the microfluidic channels. The monolayer only assembled in the
channels; whereas, the silicon surface in contact with PDMS was
protected from the reagents. After the first monolayer was
assembled, we removed the PDMS and assembled a methyl-
terminated monolayer on the remainder of the surface. We
characterized the patterns using scanning electron microscopy to
1–4
monolayers do not prevent this oxidation. Thus, the absence of
oxidized silicon is further proof that our monolayers are well-
ordered. In Table 1 monolayers assembled from 1-octadecene with
TEMPO or derivatives of TEMPO showed no evidence of silicon
dioxide from the Si(2p) peak in the XPS spectra. In contrast,
disordered monolayers from the assembly of TEMPO or
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Table 2 Stabilities of SAMs (assembled from 0.1 mole % TEMPO-C10 and 1-octadecene) immersed in various solvents under ambient pressure
a
b
Contact angles (u)
XPS Atomic composition (%)
Solvent
CHCl
Temp.(uC)
Time (h)
2
A
R
SiO
2
Si
C
O
3
61
110
110
106
109
104
102
110
108
111
110
109
110
91
102
104
103
102
100
88
104
103
103
91
0
0
0
0
0
0
0
0
0
1
1
1
8
31
30
30
32
29
30
30
29
30
30
27
26
21
54
56
47
56
52
54
58
59
52
56
55
49
43
15
14
23
12
19
16
12
12
17
14
17
24
28
4
8
1
92
6
Toluene
Water
111
4
8
1
92
96
80
14
c
20
4
5
0
2
4
2
6
4
c
CHCl /H
3
2
O
Reflux
101
102
70
2
a
b
c
We measured the advancing (A) and receding (R) contact angles of water. We studied the Si(2p), F(1s), C(1s), and O(1s) peaks. The
wafers were refluxed for 2 h in CHCl then refluxed in H O for the indicated times.
3
2
We thank the University of Iowa, MPSF Program, Carver
Scientific Research Initiative Grants Program, and ACS-PRF for
funding. This work was partly carried out in the Center for
Microanalysis of Materials, University of Illinois, which is partially
supported by the U.S. Department of Energy under grant
DEFG02-91-ER45439. We gratefully thank Rick Haasch for his
help running the XPS.
a
Samer N. Arafat, Samrat Dutta, Mathew Perring, Michael Mitchell,
a
a
b
b
Paul J. A. Kenis and Ned B. Bowden*
a
a
Department of Chemistry, University of Iowa, Iowa City, IA 52242,
USA. E-mail: ned-bowden@uiowa.edu; Fax: +1 319 335-1270;
Tel: +1 319 335-1198
b
Department of Chemical and Biomolecular Engineering, University of
Illinois Urbana-Champaign, 600 S. Mathews Ave., Urbana, IL 61801,
USA
Notes and references
1
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2
terminated with methyl (Me) or carboxylic acid (CO H) groups. c) Water
117, 3145–3155.
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2
3
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5
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61) and slowly removed. Upon removal from NOA-61, the prepoly-
mer only coated the acid-terminated SAMs. The prepolymer was
cured under UV light and characterized as shown in Fig. 2d.
In conclusion, we report both a mild method to assemble well-
ordered SAMs on Si(111) and a mild method for their patterning.
These methods enable the assembly and patterning of monolayers
directly onto silicon without an intervening layer of silicon dioxide.
6
7
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3
200 | Chem. Commun., 2005, 3198–3200
This journal is ß The Royal Society of Chemistry 2005