Journal of Colloid and Interface Science 298 (2006) 267–273
www.elsevier.com/locate/jcis
Self-assembly of amino-functionalized monolayers on silicon surfaces and
preparation of superhydrophobic surfaces based on alkanoic acid dual layers
and surface roughening
Xiaoyan Song, Jin Zhai, Yilin Wang ∗ , Lei Jiang ∗
Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
Received 20 July 2005; accepted 22 November 2005
Available online 13 December 2005
Abstract
Reproducibly smooth amino-functionalized surfaces were obtained by deposition of aminopropyltrimethoxysilane (APTMS) at the vapor/solid
interface. Characteristics of these amino-functionalized surfaces were evaluated based on atomic force microscopy, water contact angle measurement and X-ray photoelectron spectroscopy. The results showed that APTMS modified surfaces are very homogeneous and the chemical reactivity
of modified surfaces can be ensured with high free amino content. Furthermore, for the purpose of tailoring the wettability of silicon surface, dual
self-assembled films were achieved by performing reaction between amino-functionalized surface and n-alkanoic acids with different chain length.
The wettability of the self-assembled films can be adjusted with altering the hydrocarbon chain length of alkanoic acids. Moreover, cooperation
of dual self-assembled films with surface roughening, superhydrophobic surfaces with CA larger than 153◦ were obtained. Thus, the wettability
of modified surfaces can be altered greatly with changing hydrocarbon chain length of self-assembled films.
© 2005 Elsevier Inc. All rights reserved.
Keywords: Amino-functionalized surface; Dual self-assembled films; Alkanoic acids; Hydrophobicity; Chain length; Superhydrophobic surface
1. Introduction
Reactive alkylsilanes anchored to hydroxyl-bearing surfaces, such as silica, glass and metal oxides, by a cross-linked
siloxane network result in the formation of self-assembled
monolayers (SAMs), which opened the door to the creation
of new surfaces with relatively robust chemical and physical
properties [1–5]. Among the silanating agents, aminopropylalkoxysilanes are of special interest owing to their bifunctional
nature. The aminopropylalkoxysilane behaves as a “primer”
molecule providing an anchor to the surface and a linkage point
for the attachment of other molecules to the surface. There is
a growing interest in amino-functionalized surfaces for various applications to immobilize different kinds of molecules,
such as enzymes or antibodies [6,7], liquid crystal molecules
[8,9], fluorescence molecules [10,11], organic dyes [12], inor* Corresponding authors.
E-mail addresses: yilinwang@iccas.ac.cn (Y. Wang), jianglei@iccas.ac.cn
(L. Jiang).
0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2005.11.048
ganic nanoparticles [13,14], ordering metal nanoparticles [15,
16] and so on. A major challenge in all these applications of
amino-functionalized surface is to have a sufficiently high content of primary amines available on the surface. In addition,
homogeneity in surface morphology is also desired in most applications. However, there is a fundamental problem associated
with the self-assembly of aminopropylalkoxysilane on surfaces.
It is the amino group that makes the self-assembly reaction
more complex. The morphology of aminopropylalkoxysilane
modified surfaces depends strongly on the silylation conditions,
such as concentration, solvent quality, temperature and reaction
time [9,17]. Much attention has been concentrated toward controlling the amino surface morphology through optimization of
deposition conditions [14,18–20].
Most of the self-assembly processes of aminopropylalkoxysilane were performed at solution/solid interface. The deposition of silane from liquid phase has been found to suffer from
the deposition of aggregated alkylsilane molecules, which frequently degrades the quality of SAMs. However, in practice,
it is impossible to stop the competitive self-polymerization re-
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X. Song et al. / Journal of Colloid and Interface Science 298 (2006) 267–273
action taking place in solution. While, the vapor-phase method
is considered to be more convenient and the undesired adsorption of aminopropylalkoxysilane aggregates can be prevented,
because the oligomeric precursors in solution have lower vapor
pressure and are rarely vaporized [21,22]. Nevertheless, there
have been only a few of studies [22–24] about deposition of
aminopropylalkoxysilane at vapor/solid interface. Most of these
studies require high vacuum, high temperature, and longer reaction time, which appear to be too rigorous to be handled.
Compared with these methods, the silanization method used
by Sugimura et al. [21] is simpler. The substrate was sealed in
dry N2 atmosphere under atmospheric pressure conditions and
contacted with the vapor of aminosilane vapored at a moderate
temperature.
In the application in biochips or other microelectromechanical systems, controlling the interaction of substrates with environmental surroundings, such as adhesion, lubrication, or
inertness, is also critical. One efficient resolvent is to control the wettability of the substrates, which can be successfully tailored by terminally attaching various organic modifiers. Deposition of SAMs offers one of the highest quality
routes. The focus of SAMs is mainly on thiols immobilized on
gold surfaces, silanes on hydroxylated surfaces, and Langmuir–
Blodgett films [25]. The SAMs of n-alkanoic acid formed on
metal oxide surface are also promising, due to interests in
areas of lubrication [26], corrosion resistance [27], and promoting biocompatibility [28]. This kind of self-assembly reactions are often performed through electrochemical method
[28], Langmuir–Blodgett technique [29] and chemical reaction
directly between alkanoic acids and hydroxyl surface [26]. It
is known that primary amino groups can react with carboxylic
acids in the presence of diimide coupling reagents. Thus, dual
self-assembled films can be achieved by performing the reaction between amino-functionalized surface and alkanoic acids.
By this means, the wettability of silicon surfaces will be tailored from hydrophilicity to hydrophobicity. Moreover, as is
well known, modificating rough surfaces by lowering the surface energy is one of the approaches for rendering surfaces with
superhydrophobicity. Thus, superhydrophobic surfaces can also
be fabricated with a combination of dual self-assembled films
with surface roughening.
In the present paper, we have prepared reproducibly smooth
aminopropyltrimethoxysilane (APTMS) surface through chemical vapor deposition (CVD) on a silicon surface. We have also
prepared dual self-assembled films by performing the reaction
between amino-functionalized surface and alkanoic acids with
different hydrocarbon chain length in the existence of diimide
coupling reagents. The properties of the self-assembled films
were characterized by atomic force microscopy (AFM), water contact angle measurement (CA), and X-ray photoelectron
spectroscopy (XPS). The dependence of water CA on hydrocarbon chain length was also investigated. In addition, superhydrophobic surfaces were fabricated through combination of
self-assembly dual films with laser etching. The purpose of the
present study is to investigate the preparation of reproducibly
smooth amino-functionalized surface and adjust and control
the wettability of surface from hydrophilicity to hydrophobicity
and then superhydrophobicity.
2. Experimental
2.1. Materials
3-Aminopropyltrimethoxysilane (APTMS, H2 N(CH2 )3 Si
(OCH3 )3 ) was obtained from TCI and stored in desiccator. Octanoic acid (C8), decanoic acid (C10), dodecanoic acid (C12),
myristic acid (C14), palmitic acid (C16), and stearic acid (C18)
were obtained from ACROS. n-Hexane and toluene of analytical purity were obtained from Sinopharm Chemical Reagent
Co., Ltd. (SCRC) and distilled over sodium before use. N, N Dicyclohexylcarbodiimide (DCCD) and dehydrated ethanol
were also obtained from SCRC and used without further purification. The single crystal silicon wafers (100) polished on one
side were obtained from General Research Institute for Nonferrous Metals.
2.2. Preparation of the self-assembled films
The polished silicon wafers were ultrasonicated in detergent
solution and acetone for 30 min. After rinsed with water, the
samples were submerged in freshly prepared mixture of H2 SO4
(98%) and H2 O2 (30%) at a volume ratio of 7:3. The solution
was heated to 80 ◦ C for 1 h to remove organic contaminants.
Then the samples were rinsed thoroughly with copious Milli-Q
water and dried with a N2 gas stream. After such treatments,
a thin oxide layer formed on the silicon surface and the substrates were placed in a 50-mL sealed vessel with a container
filled with 0.7 mL toluene and 0.1 mL APTMS. There was no
direct contact between the liquid and the substrates. The vessel was put in an oven maintained at 100 ◦ C for 1 h. APTMS
vaporized and reacted with the OH groups of surface, resulting
in the formation of APTMS monolayer. Then, the substrates
were ultrasonicated for 5 min in toluene and ethanol, respectively, and dried under a flow of N2 . The APTMS-coated silicon
wafers were immersed into a mixture solution of alkanoic acid
and DCCD in n-hexane for 24 h (the concentrations of acid
and DCCD are all 3 mM). A monolayer of alkanoic acid was
produced on the top of APTMS film. After withdrawn from the
solution, the dual-layer films were ultrasonicated with n-hexane
for 5 min and blow dried with N2 .
2.3. Surface characterization
Water contact angle (CA) was measured with a 5-µl water
droplet at ambient temperature with an optical contact angle
meter (Dataphysics Inc., OCA20). The CA values reported are
averages of five measurements made on different areas of surfaces. All measurements for all surfaces were within ±2.0◦ of
the averages. Morphology of the sample surfaces was observed
with AFM (Digital Instruments, Nanoscope III A, tapping
mode) and field-emission scanning electron microscopy (SEM,
JEOL JSM-6700F, Japan). The composition of the treated surfaces was studied by X-ray photoelectron spectroscopy (VG
X. Song et al. / Journal of Colloid and Interface Science 298 (2006) 267–273
ESCALAB MKII spectrometer) with an AlKα monochromatic
X-ray source. The binding energy scales were referenced to
284.6 eV as determined by the location of the maximum peaks
in the C 1s spectra of hydrocarbon (CHx), associated with adventitious contamination.
2.4. Roughening of silicon surface
The roughening of silicon surface was realized with QuikLazeII Laser etching machine (New Wave Research, USA). The
laser pulse was UV light and the repetition rate was 20 Hz. The
laser pulse dredged grooves in micrometer scale on silicon surface to form different regular patterns. By adjusting the size of
the laser spot and the intensity of the laser pulse, the width and
depth of the microgrooves could be altered.
3. Results and discussion
3.1. Preparation of smooth APTMS surface
Haller and Bein published some systematic work [22,24,30]
concerning the formation of amino-terminated self-assembled
monolayers (SAMs) on SiO2 surfaces. The surface modification was performed in an apparatus where silicon wafers were
in contact with the vapor phase of a 5 wt% solution of aminosilane in toluene refluxed for 16 h at 120 ◦ C. Although this
silanization method leads to homogeneous monolayers, it appears to be quite cumbersome. To simplify the self-assembly
process, we take advantage of another CVD method [4,21].
Without direct contact, the hydroxylic substrates and aminosilane solution were both placed in a sealed vessel filled with N2 .
The vessel was heated to 100 ◦ C. Thus, the amino-terminated
layers are prepared via self-assembly of APTMS. The properties of the deposited films were characterized further by AFM,
static CA measurement and XPS spectra.
269
As is well known, the sensitivity to water is enhanced
for aminopropylalkoxysilanes because the amino in the threeposition group self-catalyzes the hydrolysis [31]. Besides, the
amino group in the aminosilane molecule may bind via hydrogen bond or form covalent linkage with the alkoxy or hydroxy groups in another aminosilane molecule or silanol that
are not engaged in surface modification reaction [21,30]. Consequently, the morphology of the silicon surfaces modified with
APTMS tends to be complex and depends strongly on the reaction conditions. For the sake of optimization of the deposition
time, herein, we have investigated the effect of elongation deposition time on the APTMS self-assembly performed at vapor/solid interface. The morphologies of the APTMS modified
silicon surfaces prepared by reaction time of 1 and 3 h in this
paper are shown in Figs. 1a and 1b, respectively. Clearly, the
modified silicon surface for 1 h appears to be very smooth and
shows no existence of aggregation particles. The root-meansquare (RMS) roughness value for the surface modified for 1 h
is 0.45 nm. Fig. 1 also shows the height profile along the white
line in the AFM image. The height of little protuberance in
Fig. 1a is about 0.7 nm, indicating a uniform monolayer assembly. The RMS roughness value for the modified surface for
the condition of 3 h is 0.78 nm and the formation of aggregates
with the height of about 3.5 nm can be observed on the APTMS
self-assembled surface.
It has been demonstrated that the wetting behavior of
APTMS thin films is the result of several contributions, including amino, alkoxy and silanol groups in the film, and a
high CA is attributed to a high degree of cross-linking [30].
Accordingly, the water CA is also of great importance in evaluating the self-assembled film. The water CA of APTMS thin
films made by 1 h reaches 54.5◦ . As reported, the CA of various aminoalkoxysilane SAMs ranging from 40◦ to 63◦ [1,8,9,
21,32]. It has been reported that a value of approximately 60◦
is expected for the 100% amino-terminated monolayer [33].
Fig. 1. AFM images of APTMS self-assembled film on smooth silicon surface through chemical vapor deposition after (a) 1 h, (b) 3 h, and the height profile along
the white line part in the images.
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X. Song et al. / Journal of Colloid and Interface Science 298 (2006) 267–273
Fig. 2. XPS survey spectra for (a) bare silicon surface, (b) silicon surface
self-assembled with APTMS, and (c) silicon surface sequentially modified with
APTMS and STA.
The wetting ability of APTMS surface prepared here is in
good agreement with those of the entirely amino-terminated
SAMs reported in previous studies. With prolongation of the
self-assembly time to 3 h, the wetting ability of the APTMS
self-assembled film tends to decrease, reaching 72.9◦ . This may
originate from the fact that if the reaction time is long, APTMS
molecules will polymerize to form large aggregates and the film
obtained gets less ordered, even if the reaction is preformed at
vapor/solid interface, while within short reaction time, APTMS
molecules might be assembled mainly in the form of monomers
or small aggregates. Consequently, for the long reaction time,
amine moieties will not orient outward completely and the hydrophobicity of the film is improved. A short reaction time is
advantageous for more uniform films.
As mentioned above, by optimization of deposition conditions, a smooth APTMS surface can be prepared through CVD.
However, little information is available in literatures concerning
the chemical state of the film surface, for instance, the extent to
which surface amino groups are reactive. XPS has been proven
to be a powerful tool to investigate the composition and structure of the SAMs. Survey scans of bare silicon surface and
APTMS surface presented in Figs. 2a and 2b show the presence
of Si 2s, Si 2p, C 1s, N 1s, and O 1s. Upon functionalization
with APTMS, increase in the C 1s intensity and appearance of
the N 1s signals are observed. Since the survey scans can only
(a)
provide a qualitative understanding of the surface chemistry,
high-resolution data for C and N elements are also collected,
which is shown in Fig. 3.
There are two peaks arising from C 1s of the APTMS surface presented in Fig. 3a, first peak at 284.6 eV is due to
the CH2 group in APTMS, while the second peak centered
at 286.1 eV might originate from the C atoms bonded to the
N atoms (C–NH2 ) [6]. In Fig. 3b, N spectrum can be divided
into two peaks, centered at 398.7 and 400.7 eV. APTMS molecules are hydrolyzed at the methoxy end, and then condensed
with the substrate hydroxyl groups to produce siloxanes. While
the methoxy groups attach to the substrate, the amino moieties
orient themselves outward and are available for reaction with
other molecules. In all the applications of amino-functionalized
surfaces, the reactive primary-amine moieties exposed on the
substrate surface serve as a platform to interact with other molecules. These free primary amines correspond to the peak component at 398.7 eV in the N 1s spectrum. However, some amino
groups may also undergo H-bonding with each other or with
substrate hydroxyls, as revealed by the presence of a shoulder
at 400.7 eV [14]. In the present report, the free primary-amine
content can reach 71.6%, which is established as the percentage
of free –NH2 peak component in the XPS N 1s spectra.
Above all, a reproducibly smooth amino functioned surface
with high free primary-amine content can be prepared through
CVD process for further reaction.
3.2. Reactions of amino surface with alkanoic acids
The APTMS surface exhibits reactivity expected from primary amino groups. It has been reported that primary amino
groups can react with carboxylic acid in the existence of diimide coupling reagents [22]. For the purpose of adjusting the
hydrophobicity of the modified surface, we also perform the
deposition of alkanoic acids with different chain length on the
APTMS surface. Petri et al. [17] demonstrated that the APTMS
monolayers should be used directly after the silanization reaction or stored in desiccator in order to keep the surface reactivity, which will be degraded because of the adsorption of impurities from the atmosphere. Accordingly, fresh prepared APTMS
surfaces were immersed into a mixture solution of alkanoic acid
(b)
Fig. 3. XPS spectra for silicon surface modified with APTMS. (a) C 1s spectrum; (b) N 1s spectrum.
X. Song et al. / Journal of Colloid and Interface Science 298 (2006) 267–273
Fig. 4. Static contact angles of silicon surfaces sequentially modified with
APTMS and different alkanoic acids as a function of alkanoic acid chain length.
and DCCD in n-hexane for 24 h. With DCCD as dehydration
regent, the alkanoic acid can react with surface amino groups to
form covalent amide bond, instead of being adsorbed onto the
surface through hydrogen bonding or electrostatic interaction.
The reaction conditions are chosen according to the previous
reports [13,34].
Evidence for the reaction is provided by the difference in
the water CA values in Fig. 4 and XPS spectra in Figs. 2b
and 2c. The static CA of self-assembled film can be adjusted
in the range from 77.6◦ to 96.3◦ . The wettability of dual film of
APTMS and octanoic acid (APTMS-C8) is weakly hydrophobic (CA = 77.6◦ ). It is seen that the contact angle value exhibits
a monotonic dependence on the chain length of alkanoic acid,
i.e., the contact angle is enhanced gradually with increasing the
length of hydrocarbon chain. That is to say that the wettability of dual film turns to be more hydrophobic, such as the CA
of surface modified with APTMS and stearic acid (APTMSC18) is 96.3◦ . In the case of surface modified with APTMS
and alkanoic acids having short hydrocarbon chain, for example APTMS-C8, dual self-assembled film will reconstruct
when the test liquid contacts with self-assembled surface. Hydrophilic amide group will contact with test liquid, which will
raise surface energies and lower water CA. Whereas, the surface
reconstruction may be absent or very much lower for longer hydrocarbon chain, for example APTMS-C18, which lowers the
surface energies and improves CA.
The dependence of wettability on chain length of different
chemicals has been investigated, such as alkylsilane [35,36],
alkanoic acid [26,28,37] and thiol [25]. Some results showed
that the wettability of the surface is independent of alkyl chain
length. Others suggested that the chain length affects the packing density and the conformation of the molecular chain. The
dependence of wettability on chain length of alkanoic acid may
be attributed to the final structure of the dual self-assembled
film. Shustak et al. [28] prepared the alkanoic acid SAMs on
stainless steel by applying a potential to the stainless steel in
an organic electrolyte. A clear trend of increase in the water CA can be seen with elongation of hydrocarbon chain
length of alkanoic acids. Thünemann investigated the interaction between polyethyleneimine and perfluorinated carboxylic
acids [38]. He deemed that the wettability and the surface en-
271
ergy of complex surfaces decrease with increasing carboxylic
acids chain length. As a result of the attractive van der Waals
force between the alkane chains of the alkanoic acids, long
hydrocarbon chain tends to form densely packed, highly ordered, monomolecular assemblies with solid-state-like properties. While, alkanoic acids with short hydrocarbon chain may
constitute films that are not crystalline and may have some preferred orientation.
We also investigate the reaction of alkanoic acids and
APTMS surface through XPS spectra. Taking stearic acid
(STA) as an example, after the APTMS surface has been immersed in STA and DCCD mixed solution for 24 h, the C 1s
peak of XPS survey scan spectrum in Fig. 2c becomes more
intense, compared with that of APTMS surface (Fig. 2b). It implies that a much thicker hydrocarbon layer forms on the silicon
surface, which will improve the hydrophobicity of the selfassembled film. The high-resolution data of the self-assembled
film before and after treatment with STA are shown in Fig. 5. As
mentioned above, before immersed in STA solution, the N 1s
spectrum can be deconvoluted into two peaks, 398.7 eV due
to free aliphatic amines and 400.7 eV assigned to H-bonded
amino groups. After treated with STA, a new peak centered at
400.1 eV emerged, which can be attributed to the amide-N and
implies the formation of amide [7]. The new C 1s peak at binding energy 288.2 eV can be assigned to the amide-C [7], which
again indicates the newly introduced amide functionality. With
the appearance of the amide-N signal, the decrease in the free
primary amino groups centered at 398.7 eV is observed. The
content of the primary amino groups decreases from 71.6 to
36.1%. These results demonstrate that amino groups mostly react with STA, with some still unreacted. Compared with the
CH2 –CH2 component centered at 284.6 eV of APTMS surface
shown in Fig. 3a, it is also observed that the STA modification
results in an increase in the CH2 –CH2 component from 65.5
to 84.5%, indicating that more hydrocarbon chains have been
introduced in the dual self-assembled film.
3.3. Fabrication of superhydrophobic surfaces
Generally, water CA on smooth surfaces cannot exceed 120◦
through tailoring surface chemistry [39]. One important approach for rendering superhydrophobicity of surfaces is increasing surface roughness [15]. According to our previous
work [40], laser etching was taken advantage to introduce surface roughness. Fig. 6a shows a SEM image of silicon surface
roughened with laser etching machine. The rough surface exhibits vertically intersecting micrometer-scale grooves, which
are generated with laser etching with ca. 5 µm in depth and 4 µm
in width. Between every two microgrooves, micrometer-scale
convexes are generated. The convexes are ca. 7 µm in width.
The magnified SEM image of the microconvexes in Fig. 6b
shows that the microconvexes consist of many nanometer-scale
particles and pores, which are generated during laser etching.
The laser etching machine dredges grooves on silicon surface;
meanwhile, the sputtered chippings are oxidized into SiO2 and
stack on the adjacent convexes. By this means, nanometer-scale
protuberances and pores are formed.
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X. Song et al. / Journal of Colloid and Interface Science 298 (2006) 267–273
(a)
(b)
Fig. 5. XPS spectra for silicon surfaces sequentially modified with APTMS and STA. (a) N 1s spectrum; (b) C 1s spectrum.
Fig. 6. SEM images of etched silicon surfaces with laser etching machine without modification. (a) Rough surface; (b) magnified image of the microconvex.
rough surfaces is not as significant as that on the smooth surfaces. The great improvement of water CA may be explained
with Cassie’s theory [41]. The nanometer- and micrometerscale binary roughness fabricated in this paper intensifies the
proportion of air trapped in pores and contributes to the formation of superhydrophobic surfaces.
4. Conclusions
Fig. 7. Comparison of static contact angles of smooth and rough silicon surfaces sequentially modified with APTMS and alkanoic acids with different
chain length.
Here, the self-assembly of APTMS and alkanoic acid is
also preformed on rough silicon surfaces. Compared with those
modified on smooth silicon surfaces, the hydrophobicity of
modified rough surfaces is improved significantly, as shown in
Fig. 7. No matter how many carbon atoms involved in alkanoic
acid chains, the static CAs are all raised above 150◦ . Similar
to the wettability of modified smooth surfaces, with increasing
hydrocarbon length, the static CA also exhibits a monotonic
dependence to some extent, increasing from 153.8◦ (C8) to
159.2◦ (C18). Nevertheless, the increase of CA on modified
In summary, self-assembly of APTMS by chemical vapor
deposition was explored aiming at obtaining reproducibly homogeneous and reactive amino-functionalized surface. Compared with the liquid-phase self-assembly process reported in
previous works, a smooth SAM was successfully formed on silicon surface. Water CA measurements and XPS spectra showed
that the chemical reactivity of the amino-functionalized surface
was ensured with high content of free primary amino. Furthermore, APTMS self-assembled films are modified with alkanoic
acids with different chain length. The hydrophobicity of the
modified surface exhibits a monotonic dependence on the chain
length of alkanoic acids. Finally, we also fabricate superhydrophobic surfaces through coordinating dual self-assembled
film and surface roughness. Thus the wettability of modified
surfaces can be controlled and adjusted greatly with different elements of self-assembly, such as functional group, chain length
and surface roughness.
X. Song et al. / Journal of Colloid and Interface Science 298 (2006) 267–273
Acknowledgment
We are grateful for the financial support from the National Natural Science Foundation of China (Grants 20233010,
20473101).
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