Ultralow adhesion and friction of fluoro-hydro alkyne-derived self-assembled monolayers on H-terminated Si(111)

Article abstract of DOI:10.1021/la303893u

New fluorine-containing terminal alkynes were synthesized and self-assembled onto Si(111) substrates to obtain fluorine-containing organic monolayers. The monolayers were analyzed in detail by ellipsometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS), static water contact angle measurements (CA), and atomic force microscopy (AFM). The SAMs exhibit excellent hydrophobicity, with static water contact angles of up to 119 and low critical surface tensions of 5-20 mN/m depending on the number of F atoms per molecule. IRRAS confirmed the formation of highly ordered monolayers, as indicated by the antisymmetric and symmetric stretching vibrations of the CH₂ moieties at 2918-2920 and 2850-2851 cm^-1, respectively. Upon increasing the number of fluorine atoms in the alkyne chains from 0 to 17, the adhesion of bare silica probes to the SAMs in air decreases from 11.6 ± 0.20 mJ/m ² for fluorine-free (F0) alkyne monolayers to as low as 3.2 ± 0.03 mJ/m² for a heptadecafluoro-hexadecyne (F17)-based monolayer. Likewise, the friction coefficient decreases from 5.7 × 10^-2 to 1.2 × 10^-2. The combination of high ordering, excellent hydrophobicity, low adhesion, and low friction makes these fluoro-hydro alkyne-derived monolayers highly promising candidates for use in high-performance microelectronic devices.

Full text of DOI:10.1021/la303893u

Article
pubs.acs.org/Langmuir
Ultralow Adhesion and Friction of Fluoro-Hydro Alkyne-Derived Self-
Assembled Monolayers on H‑Terminated Si(111)
Sidharam P. Pujari,† Evan Spruijt,‡ Martien A. Cohen Stuart,‡ Cees J. M. van Rijn,† Jos M. J. Paulusse,†
and Han Zuilhof*,†,§
†Laboratory of Organic Chemistry and ‡Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Wageningen,
The Netherlands
§Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah, Saudi Arabia.
S
* Supporting Information
ABSTRACT: New fluorine-containing terminal alkynes were synthesized and self-
assembled onto Si(111) substrates to obtain fluorine-containing organic monolayers.
The monolayers were analyzed in detail by ellipsometry, X-ray photoelectron
spectroscopy (XPS), Fourier transform infrared reflection absorption spectroscopy
(FT-IRRAS), static water contact angle measurements (CA), and atomic force
microscopy (AFM). The SAMs exhibit excellent hydrophobicity, with static water
contact angles of up to 119° and low critical surface tensions of 5−20 mN/m depending
on the number of F atoms per molecule. IRRAS confirmed the formation of highly
ordered monolayers, as indicated by the antisymmetric and symmetric stretching
vibrations of the CH2 moieties at 2918−2920 and 2850−2851 cm−1, respectively. Upon
increasing the number of fluorine atoms in the alkyne chains from 0 to 17, the adhesion
of bare silica probes to the SAMs in air decreases from 11.6 0.20 mJ/m2 for fluorine-
free (F0) alkyne monolayers to as low as 3.2
0.03 mJ/m2 for a heptadecafluoro-
hexadecyne (F17)-based monolayer. Likewise, the friction coefficient decreases from 5.7 × 10−2 to 1.2 × 10−2. The combination
of high ordering, excellent hydrophobicity, low adhesion, and low friction makes these fluoro-hydro alkyne-derived monolayers
highly promising candidates for use in high-performance microelectronic devices.
adhesion, and low-friction surface coatings on silicon for use in
microcomponent devices.
1. INTRODUCTION
Studies on low-adhesion and low-friction surfaces are
particularly important for high-performance technological
devices. One of the great challenges in many microcomponent
devices, such as micro- and nanoelectro-mechanical systems
(MEMS and NEMS), is to reduce adhesion to and friction on
surfaces as much as possible while keeping the surface highly
robust and resistant to wear. The typical dimensions of MEMS
and NEMS are a few to several hundreds of micrometers, and
they are primarily made from silicon. On these small scales,
surface properties such as van der Waals and capillary forces
greatly influence the performance of these mechanical
systems.1−3 Without suitable surface modification, Si shows
high friction, adhesion, and wear.4−6 Therefore, several types of
thin films have been investigated as coatings for Si with
enhanced tribological properties of silicon.7 In particular, thin
organic fluorocarbon−hydrocarbon films have been found to
produce low-adhesion and low-friction lubricants.8−12 In
practice, poly(tetrafluoroethylene) (PTFE) is now the primary
coating material in many microelectro-mechanical systems.5
However, these films are not covalently bound to silicon and
are therefore prone to wear. Moreover, PTFE films are
relatively thick, which may still lead to high adhesion and
friction forces when high loads are applied and the films are
compressed. Here, we report on ultrathin covalently bound
fluoro-hydro alkyne-derived monolayers as high-quality, low-
Densely packed organic monolayers, which are covalently
bound via Si−CHC linkages to crystalline silicon surfaces
without an interfacial silicon oxide (SiO2) layer, are receiving
increasing interest13−15 mainly because of the potential of Si−C
bound monolayers for applications in micro- and nano-
electronics as well as in biochemical sensors.16−22 This field
has recently been reviewed.23 With the advent of nano-
technology, lubricating monolayers are highly desired because
they are covalently attached to the substrate and therefore are
not easily damaged and even if they break they do not yield
particulates that may cause damage. Among the different
organic monolayers grafted onto the surface, fluorinated
organic thin films have drawn much attention for their
outstanding chemical stability, thermal stability, unique
wettability, and nonadhesive properties.24,25 On the nanometer
scale, the physical state of thin-film systems may be influenced
by a number of forces, including intermolecular interactions
between molecules making up the film,26 surface interactions in
adsorbed films, and solvent interactions in detached film
systems.27
Received: September 28, 2012
Revised: November 27, 2012
© XXXX American Chemical Society
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The formation of a dipole between the last fluorinated
carbon (R−CH2−CF2−R1) and the first methylene group (R−
CH2−CF2−R1) in fluorinated organic monolayers causes the
electronegative groups to be oriented normal to the surface.
This gives rise to an important decrease in adhesion to and
friction of the fluorinated monolayers. In addition, fluorination
leads to an enhancement of the hydrophobicity and
oleophobicity.28,29 The diverse effects that varying degrees of
fluorination cause and the ratio between fluorocarbons and
hydrocarbons are under extensive study with the aim of
improving the tribological properties and enhancing the
hydrophobicity of these monolayers.30−32
A range of synthetic methods has been successfully used to
prepare covalently bound monolayers on solid surfaces. For
silicon, extensive reviews are provided by Gooding et al.33,34
and Zuilhof et al.23,35 The formation of a silicon−carbon bond
via hydrosilylation is a key step in the coating of a silicon
surface with a monolayer because it greatly improves both the
packing and stability of SAMs.34 Grafting of alkynes onto the Si
surface can be achieved in various ways.36−41 The mildest
approach already allows for the preparation of high-quality
covalently bound organic monolayers at room temperature.42
As the assembly on the silicon surface progresses, the interchain
steric hindrance of CH2 and CF2 becomes dominant and
prevents the bonding of alkyl chains to every silicon atom of the
substrate. Because the resistance toward oxidation depends on
the density of monolayer packing,15 a technique to develop
tightly packed monolayers is desired in order to minimize water
penetration, oxide formation, and, consequently, the degrada-
tion of the silicon substrate.
In this article, we prepare alkyne-based SAMs, which are
known to be stable and to form densely packed mono-
layers.23,43 By contrast, alkene-based SAMs, in which a Si−
CH2−CH2 single bond allows for a higher degree of free
rotation around the chain axis, give rise to a lower packing
density. Apart from the enhanced rotation, the saturation of the
carbon chain is another important feature: when an alkyne is
attached to silicon, an alkene (Si−CHC linkage) remains,
which takes up a smaller volume and thus causes less interchain
repulsion than caused by the Si−CH2−CH2 linkage that is
formed upon attaching an alkene to silicon. The enhanced
packing density of alkyne-derived monolayers compared to
alkene is also due to the smaller tilt angle with respect to the
surface normal and the higher ordering of the remaining alkene
moieties, facilitated by attractive π−π interactions.15 Finally, it
is remarked that the packing density increases with longer
carbon chains for alkyne-derived monolayers as well as alkene-
derived monolayers.13,44−46
In studying the characteristics of our S−CHC linked
monolayers together with the requirements for ultralow surface
tension and adhesion properties, we expect that partially
fluorinated monolayers on a hydrogenated, oxide-free Si (H−
Si) surface will combine several of the highly desirable
characteristics strived for in this field. This article presents
the synthesis of novel fluoro-hydro alkynes with a varying
number of fluorine atoms (no. of F atoms = 0−17) at a
constant chain length (C16) (Figure 1) and their application to
monolayer formation on oxide-free H−Si(111) surfaces. The
resulting monolayers are characterized by ellipsometry, X-ray
photoelectron spectroscopy (XPS), Fourier transform infrared
reflection absorption spectroscopy (FT-IRRAS), advancing and
static water contact angle measurements (CA), and critical
surface tension measurements. Subsequently, the adhesion and
Figure 1. Fluoro-hydro alkynes employed in monolayer formation.
friction of these monolayers were studied as a function of
fluorine content by colloidal probe atomic force microscopy
(CP-AFM) to reveal unprecedentedly low surface tensions and
adhesion properties.
2. EXPERIMENTAL SECTION
2.1. Materials. The synthesis procedures and spectroscopic
characterization of hexadec-1-yne (F0), 16-fluorohexadec-1-yne (F1),
16,16,16-trifluorohexadec-1-yne (F3), 13,13,14,14,15,15,16,16,16-non-
afluoro-hexadec-1-yne (F9), and 9,9,10,10,11,11,12,12,13,13,14,14,-
15,15,16,16,16-heptadecafluoro-hexadec-1-yne (F17) are described in
Supporting Information Schemes S1−S4. For rinsing and contact
angle measurements, Milli-Q water (resistivity 18.3 MΩ cm) was used.
Hexadecane (C16, 99%), tetradecane (C14, 99%), dodecane (C12,
99%), decane (C10, 99%), heptane (C7, 99%), and hexane (C6, 99%)
were used to determine the critical surface tension and were obtained
from Aldrich and used as received. Sulfuric acid (Sigma-Aldrich, 95−
97%), hydrogen peroxide (Acros Organics, 35%), ammonium fluoride
(Riedel-de Haen, 40%, semiconductor grade VLSI Puranal Honeywell
̈
17600), and acetone (Aldrich, semiconductor grade VLSI Puranal
Honeywell 17617) were used as received. Silicon wafers were (111)-
oriented single-side and doubly polished 500−550 μm thick,
phosphorus-doped n-type, and have a resistivity of 2.0−8.0 Ω cm
with a 0.2° miscut angle along the ⟨112⟩ plane (Siltronix).
2.2. Hydrogen-Terminated Si(111) Surfaces. Hydrogen-termi-
nated Si(111) surfaces (H−Si) were prepared by chemical etching as
previously reported.45,47 All liquid reagents were continuously purged
with a flow of argon. N-type Si(111) was cut into an appropriately
sized substrate (10 × 10 mm2) and subsequently cleaned in a
sonication bath with acetone and then with Milli-Q water. The Si
wafer was oxidized in freshly prepared piranha solution (H2SO4/H2O2
3:1) for at least 20 min. After piranha treatment, the substrates were
immersed immediately in water and rinsed thoroughly, followed by
drying with a stream of argon. Subsequently, the substrates were
etched in an argon-saturated 40% aqueous NH4F solution for 15 min,
rinsed with Milli-Q water, and finally dried with a stream of argon.
2.3. Preparation of Fluoro-Hydro Alkyne-Derived Mono-
layers on Si(111). A three-necked flat-bottomed flask connected to a
thin capillary as the argon inlet and to a reflux condenser connected to
a vacuum pump was charged with individual neat fluoro-hydro alkyne
(Scheme 1), flushed with argon, and heated to 80 °C in order to
remove traces of oxygen and moisture. The freshly etched Si(111)
substrate was placed in fluoro-hydro alkyne. The reaction was carried
out at 80 °C under an argon atmosphere at an argon pressure of 2−5
mbar for 16 h. After the reaction had been stopped, the modified
surfaces were rinsed and sonicated with CH2Cl2 for 5 min to remove
any physisorbed fluoro-hydro alkynes.
2.4. Monolayer Characterization. 2.4.1. Contact Angle
Measurements. Contact angle measurements were performed on a
Kruss DSA 100 contact angle goniometer with an automated drop
̈
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Scheme 1. Synthesis Procedure Used to Create Fluoro-Hydro Alkynes, with F3 Given as an Example
dispenser and image video capture system. The static contact angles of
six small droplets (3.0 μL volume of liquid) dispensed on modified
silicon surfaces with a microliter syringe with a stainless steel needle
(diameter = 0.51 mm) were determined using a Tangent 2 fitting
model. The digital drop images were processed by the image analysis
system, which calculated both the left and right contact angles from
the drop shape with an accuracy of 1.0°. For advancing contact angle
determinations on the same polymer samples, droplets of 1 μL total
volume were applied at 10 μL/min and monitored by video recording.
Reported angles are averaged over at least five droplets
2.4.2. Ellipsometry. The thickness of the modified silicon surfaces
(in the dry state) was measured using a rotating analyzer ellipsometer
(Sentech Instruments type SE-400) operating at 632.8 nm (He−Ne
laser) with an angle of incidence of 70°. The optical constants of the
substrate were determined with a piece of freshly etched H−Si(111)
(n = 3.819 and k = 0.057). The thicknesses of the monolayers were
determined with a planar three-layer (ambient, monolayer, and
substrate) isotropic model with a refractive index for the organic
monolayers of 1.46 (F0), 1.44 (F1), 1.40 (F3), 1.38 (F9), and 1.35
(F17).48 Each reported value of the layer thickness is the average of
eight measurements taken at different locations on the substrate with
an error of <1 Å.
converted directly into friction force, following the method of Liu et
al.53 Cantilevers were calibrated using the reversible bending of an 8.9
-μm-thick glass fiber, leading to a lateral conversion factor of
approximately 17.7 nN/V. The overall error in the conversion factor
determined in this way was estimated to be 15%.53
2.4.4. X-ray Photoelectron Spectroscopy (XPS). XPS measure-
ments were performed using a JPS-9200 photoelectron spectrometer
(JEOL, Japan). A monochromatic Al Kα X-ray source (hν = 1486.7
eV, 12 kV, 20 mA) using an analyzer pass energy of 10 eV was used.
The base pressure in the chamber during measurements was 3 × 10−7
Torr, and spectra were collected at room temperature. The intensity of
the XPS core-level electron was measured as the peak area after
standard background subtraction according to the linear procedure.
The takeoff angle φ (angle between sample and detector) of 80° is
defined to a precision of 1°. The typical sample size was 1 × 1 cm2. All
XPS spectra were evaluated using the Casa XPS software (version
2.3.15). The symmetrical GL(30) line shape was employed, which
consists of a Gaussian (70%) and a Lorentzian (30%) component. The
fwhm of each component was constrained to ∼1.0 eV. The relative
areas of each component peak were fixed by the stoichiometry of the
main hydrocarbon (CH2), which was assigned as aliphatic carbon with
a binding energy of 285.00 eV.
2.4.3. Atomic Force Microscopy. Force measurements are
performed on a Nanoscope IIIA AFM (Digital Instruments) equipped
with a PicoForce scanner. Spherical silica particles (R = 3.0 μm, rms
2.4.5. Fourier Transform Infrared Reflection Absorption Spec-
troscopy (IRRAS). IRRA spectra were recorded on a Bruker Tensor 27
FT-IR spectrometer using a variable-angle reflection unit (Auto
Seagull, Harrick Scientific). A Harrick grid polarizer was installed in
front of the detector and was used to record spectra with p-polarized
(parallel) radiation with respect to the plane of incidence at the sample
surface. All spectra were obtained at an incident angle of 68° (2048
scans). The resolution was set at 2 cm−1 per modulation center. The
final spectra were obtained using a piranha-oxidized reference surface
as the background. Data were collected as differential reflectance
versus wavenumber. All spectra were recorded at room temperature
under a dry atmosphere.
2.4.6. Molecular Modeling. For the molecular modeling study, the
same method is followed as reported by Scheres et al.43,54 In short,
Materials Studio software (version 5.0) was used to construct and
optimize the monolayers. All monolayers were formed from five
standard cells containing a decenyl chain (representing one of the
alkynes) attached in an all-trans conformation to four Si atoms. The Si
atoms represent the first four layers of the Si substrate obtained by
cleaving a Si crystal along the (111) plane. The structures were placed
in a box to obtain the standard cells. The standard cells were copied in
the directions of the Si substrate to form larger unit cells. By replacing
some of the attached chains with hydrogen atoms, different
substitution patterns and substitution percentages were obtained. All
unit cells were finally copied to form the final large simulation cells
roughness value of 6.83
2.06 nm)49 were attached to triangular
standard silicon nitride cantilevers (Bruker probes, NP-B, spring
constant between 0.10 and 0.16 N/m) using Norland optical adhesive
61 and cured with UV light (365 nm). Before use, the colloidal probes
were cleaned with excess ethanol followed by 5 min of air plasma
cleaning. Both adhesion and friction measurements were carried out in
air at a relative humidity of 44 2%.
Adhesion forces were measured using a scan range of 1.0 μm for
modified surfaces and 10 μm for oxide surfaces at a scan rate of 0.5 Hz.
At least 200 separate force curves were recorded for every surface. For
each cantilever, the normal spring constant was determined using the
thermal tuning method introduced by Hutter and Bechhoefer,50
correcting for the nonideality of the spring and the fact that the
deflection sensitivity was measured for a supported cantilever.51 The
overall error in the measured adhesion forces is the sum of
uncertainties in the voltage measurement, deflection sensitivity, and
spring constant and was estimated to be 10%.32,52
Friction forces were obtained from the trace and retrace of 5 × 5
μm2 lateral force images under varying normal loads (FN = 0−80 nN).
The lateral force images are measured at a constant speed of 5 μm/s
under a 90° angle with respect to the cantilever’s long axis. The
average lateral force difference signal ([μtrace − μretrace]/2, in V) was
C
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□
●
○
△
Figure 2. (a) Zisman plot showing the linear regression of cos θ vs different n-alkanes (from C6 to C16) on F17 ( ), F9 ( ), F3 ( ), F1 ( ), and
2
■
F0 ( ) monolayers on H-terminated Si(111). The line coefficient r = 0.98 0.01 indicates a high degree of confidence. (b) Critical surface tension
( ) and work of adhesion ( ) on SAMs derived from the hexadecyne series as a function of the total number of fluorine atoms per chain. Critical
■
○
surface tensions (γc) are obtained from extrapolation to cos θ = 1 at 24 2 °C.
van der Waals interaction or hydrogen bonding in C−F...H−
representing the whole modified Si surface. The unit cells and the large
C.55 Hence, the grafting of the fluorinated alkynes required
simulation cells were optimized using a polymer consistent force field
(PCFF) with the Smart Minimizer routine and high-convergence
criteria. Periodic boundary conditions were applied.
reaction conditions of at least 80 °C for 16 h.
Advancing and static water contact angle measurements were
determined for all monolayers (Supporting Information Table
S1). The hexadecyne (F0)-based monolayer displayed a static
3. RESULTS AND DISCUSSION
water contact angle of 110°, in agreement with earlier observed
values.42 The hydrophobicity of the SAMs based on F1−F17
are typical of the presence of fluorinated moieties.28 The water
contact angle increases with the increasing number of fluorines
in the monolayer (static water contact angle up to 119° for
F17) and compares favorably with that of PTFE (115°). For F9
and F17, higher contact angles were observed because of the
weak dipole−dipole interaction between the CF3 terminus and
water molecules. In comparison, a PTFE surface displays a
lower static water contact angle because the CF2 groups are less
capable of reducing the surface energy than CF3 groups.56 The
lower polarizability of fluorine compared to that of hydrogen
leads to very weak van der Waals interactions among the CF3
terminus and polar water molecules.57 There have been a
number of studies on the influence of terminal groups.58,59
Colorado et al. obtained for the Au···S−(CH2)15 − x(CF2)xCF3
series of monolayers a smaller contact angle (108°) for x = 0
(comparable to that for our F3 monolayer, θ = 113°).28 This
smaller contact angle is likely due to a more horizontal
organization of the CH2−CF3 terminus, thus exposing the CH2
moiety. In our case, the monolayer is likely more organized,
presenting only the CF3 group head up, yielding a higher
hydrophobicity.
From the advancing water contact angle, the apparent work
of adhesion can be calculated. Figure 2 shows how the work of
adhesion decreases with an increasing number of fluorine
atoms. The strongest decrease occurs between F3 and F9, as
discussed above. Between F9 and F17, little change is observed.
A similar trend is found in the critical surface tension of the
monolayers. Critical surface tensions are obtained from Zisman
plots of static contact angles of a series of linear alkanes on the
monolayers. A Zisman plot can be used to investigate the
surface energy of a low-energy solid surface.60 When the surface
energy of the solid is comparable to that of the liquid, a
transition from partial to complete wetting is observed. Surfaces
with very low surface energies are difficult to wet and may never
achieve complete wetting.
3.1. Synthesis of Fluoro-Hydro Alkynes. The synthe-
sized fluoro-hydro alkynes are depicted in Figure 1. Fluorine-
free 1-hexadecyne (F0) was obtained by the tosylation of 1-
tetradecanol, followed by nucleophilic substitution with lithium
acetylide. F3 was synthesized in six steps (Scheme 1; full
experimental details in Supporting Information), consisting of
the coupling of THP-protected propargyl alcohol to 1-
bromododecane, followed by deprotection of the THP-group
affording pentadec-2-yn-1-ol. The isomerization of pentadec-2-
yn-1-ol afforded the terminal alkyne, which was again tosylated
and subsequently converted to the corresponding iodide to
increase the reactivity for the final reaction with Me3SiCF3 to
yield F3. A similar procedure was followed in preparing F1
from commercially available hexadec-7-yn-1-ol. After isomer-
ization and tosylation, treatment with KF afforded F1.
Heptadecafluoro alkyne F9 was obtained via a four-step
reaction (Supporting Information Scheme S1). The radical
chain addition of 1,1,1,2,2,3,3,4,4-nonafluoro-4-iodobutane to
9-decene-1-ol afforded 11,11,12,12,13,13,14,14,14-nonafluoro-
9-iodotetradecan-1-ol, and was followed by the reduction of the
iodide to give reduced 11,11,12,12,13,13,14,14,14-nonafluor-
otetradecan-1-ol. The tosylation of 11,11,12,12,13,13,14,14,14-
nonafluorotetradecan-1-ol was followed by nucleophilic sub-
stitution with lithium acetylide to obtain fluoro-hydro alkyne
F9. F17 was synthesized following the same procedure but
starting with 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluoro-8-
iodooctane and 5-hexen-1-ol.
3.2. Monolayer Formation. In our search for a mild and
generally applicable method for grafting the newly synthesized
fluoro-hydro-containing alkynes onto Si substrates, we
investigated different reaction times and temperatures (data
not shown). Typically, the Si substrate was immersed in neat
alkyne under oxygen-free and water-free conditions. Although
high-quality monolayers were formed on Si(111) with F0 even
at room temperature under ambient light,42 the modification
with fluorinated alkynes was not completed under these
conditions and physisorption dominated on the basis of weak
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Table 1. XPS Data Atomic C/Si Ratio and XPS and Ellipsometry Thickness
monolayer
C/Si
C/F ratio theory
C/F ratio exp
XPS thickness (nm)
ellipsometry thickness (nm)
refractive index
F0
F1
F3
F9
F17
40.9/59.1
38.7/61.3
38.4/61.6
37.2/62.8
37.8/62.2
2.07
2.01
1.91
1.84
1.89
2.09
2.02
1.95
1.91
1.93
1.46
1.44
1.40
1.39
1.35
16.0
5.3
15.0
5.2
1.8
1.9
0.9
1.0
Figure 3. High-resolution XPS data of an F9-based monolayer on an oxide-free H−Si(111) surface. (a) Si 2p and (b) C 1s. (c) Schematic
representation of the F9 monolayer on Si(111).
In that case, measuring the wetting contact angle (θ) for
different values of the liquid surface tension (γL) and
extrapolating to cos θ = 1 (i.e., complete wetting) will yield a
critical surface tension (γc) that is a measure for the solid
surface energy and an essential parameter in many practical
applications.61 In general, the critical surface tension and the
details of the extrapolation depend on the molecular character-
istics of the liquid.61 However, in a homologous series of simple
molecular liquids such as n-alkanes, the van der Waals forces
dominate the liquid surface energy, and Zisman found that
cos θ depends linearly on γL.60 Using this approach, Zisman
determined critical surface tensions for a variety of solids with a
low-energy surface. The lowest critical surface tension found
was 6 mN/m for condensed monolayers of perfluorolauric acid
on platinum, whereas Teflon has a critical surface tension of
18.5 mN/m.
The monolayers in this study stand out by displaying
extraordinarily low surface tensions. Figure 2a shows Zisman
plots for fluoro-hydro alkyne-derived monolayers F0−F17
using a homologous series of n-alkanes. In all cases, a linear
relationship is found between cos θ and γL, in agreement with
the original findings of Zisman.60 The corresponding critical
surface tensions are shown in Figure 2b. Surfaces with a low
critical surface tension are expected to suffer least from
adhesion to the surface, which is desirable in many micro-
electronic devices. First, the critical surface tension for F0 is
found to be 20.1 mN/m, which is lower than characteristic
values reported for CH3 termination in the literature (22−24
mN/m),62 revealing the high organization of the monolayer.
Increasing the number of F atoms in the monolayer further
decreases the critical surface tension to 5.6 0.21 mN/m for
F17, which is, as far as we know, currently the lowest for any
solid surface. Consequently, only condensed inert gases would
show complete wetting on this surface.60,63
Table 1 summarizes monolayer thicknesses measured by
ellipsometry as well as XPS.13 The observed refractive index
(RI) of the F17 monolayer on Si(111) is close to that of the
analogous F17 silane reported by Geer et al.48 Monolayers
prepared from compounds F9 and F17 display slightly lower
thicknesses than those obtained from F0, F1, and F3. The
relaxed fluoro-hydro alkyne molecules in the SAMs are most
likely not straight but helical and bent, similar to fluorinated
alkyl thiols on Au. Such monolayers were reported to display
tilt angles of 30−35° with respect to the surface normal.31,64,65
The lower thicknesses may also relate to differences in the
effective cross-sectional area of the −CF2−CF3 and −CH2−
CH3 moieties (28 and 18 Å2, respectively) and fit it less
optimally with the intersite distances on the Si(111) surface.66
The fluoro-hydro alkyne-derived monolayers were also
analyzed by XPS, and the resulting C 1s and Si 2p high-
resolution spectra of an F9-based monolayer are depicted in
Figure 3. The peak labels in the spectra correspond to the
carbon atoms having different environments in the modified
monolayer, which was deconvoluted into six distinct
components. Hydrocarbon CH2 C 1s (signal 2) calibrated at
a binding energy of 285.0 eV corresponds to carbon atoms
involved in CH2 moieties of F9, whereas signal 1 at 283.6 eV is
assigned to the carbon bound to the less electronegative Si.42
The relative intensity of this Si−C signal is ∼1/16 of the total C
signal, in line with expectations. The C 1s signal (3) at 286.1 eV
corresponds to the methylene carbon atom involved in the
−CF2−CH2− group. The shoulder peak observed at 290.9 eV
corresponds to the −CF2−CH2− group (4). The C 1s signal
(5) at 291.6 eV corresponds to the other CF2 groups, whereas
the highest binding energy (293.8 eV) is observed for the
terminal CF3 group (Figure 3b).63 This assignment is in
excellent agreement with computational C 1s and F 1s XPS
E
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data obtained from B3LYP/6-311G(d,p) calculations (Support-
ing Information Figure S1).67
peak frequencies are typically correlated with diminished
ordering, it may be thought that the higher values observed
for F17 display a reduced ordering. However, another
electronic factor comes into play here. The increased
electronegativity of the chain yields an upward shift of the
CH2 stretching vibrations, as exemplified by B3LYP/6-311G-
(d,p) data obtained for frequencies of the C−H stretching
vibrations of model compounds CH3−(CH2)7−(CH2)7CH3
(i.e., n-hexadecane) and CH3−(CH2)7−(CF2)7CF3 using scaled
(0.9679)70 optimized B3LYP/6-311G(d,p) data. As can be seen
(Figure 4B), fluorination by itself decreases the C−H stretching
vibrational frequencies by approximately 3 cm−1. If one thus
compensates for this electronic effect on the CH2 stretching
vibrations, the degree of ordering in our fluorinated monolayers
seems to equal that observed for F0, pointing to highly ordered
monolayers throughout the entire fluoro-hydro alkyne series.
This high ordering, of course, does not imply identical
conformations because the F9 and F17 chains may well display
helical conformations.71
The Si 2p high-resolution spectrum (Figure 3a) shows
binding energies of the Si 2p1/2 and Si 2p3/2 doublet at 100.1
and 99.5 eV, respectively. Importantly, no contribution related
to oxide or suboxide species in the energy range of 101−104 eV
was observed, indicating the oxide-free nature of the silicon
substrate underneath this partially fluorinated monolayer.
Whereas excellent surface passivation has been shown before
for alkyne-derived F0 monolayers,13 apparently fluorination
does not hamper the formation of a fully surface-covering
monolayer.
3.3. Infrared Reflection−Absorption Spectroscopy.
Qualitative differences in the conformational order of the
alkyl chains in the monolayers were evaluated using infrared
reflection−adsorption spectroscopy (IRRAS) by monitoring
the antisymmetric and symmetric CH2 stretching vibrations
(Figure 4A).13,68,69 For increases in fluorine content from F0 to
3.4. Adhesion and Friction. To assess the adhesion and
friction characteristics of the modified Si(111) surfaces, atomic
force microscopy was used. Both adhesion to and friction on
surfaces greatly influence the performance of microelectronic
devices. Colloidal probe AFM allows us to measure both
properties with high accuracy. The colloidal probe acts as a
model solid object that can interact with the modified surfaces.
Previously, covalently bound alkene SAMs and adsorbed
fluorine-containing surface coatings have been shown to reduce
adhesion and increase lubrication significantly in solution.52,72
However, most devices operate in air or vacuum, where
adhesion is expected to be much larger. Moreover, capillary
condensation may occur between surfaces that are hydrophilic
enough, leading to even stronger adhesive interactions. Under
these conditions, a candidate surface coating should still
effectively reduce adhesion and provide sufficient lubrication.
Therefore, fluoro-hydro alkyne-derived monolayers were
subjected to the most stringent adhesion and friction test by
carrying out the AFM measurements in air with a smooth silica
probe as a hydrophilic solid particle.
The fluorinated monolayers in this study display very low
adhesion forces. Figure 5 shows the adhesion of a silica probe
particle to the monolayers when the surfaces are compressed at
a load of 10 nN. Adhesion forces decrease with increasing
fluorine content in the monolayers from 35 nN (11.6 0.20
mJ/m2) for the F0-based monolayer to as low as 9.8 nN (3.2
0.03 mJ/m2) for the F17-based monolayer. As far as we know,
this is the lowest adhesion force observed for any flat surface.
This low adhesion is attributed to the high degree of ordering
in the monolayer, which allows little reorganization within the
monolayer to increase attractive interactions between the
monolayer and the probe. Only in the case of F0 monolayers
was a small nonzero attraction measured upon approaching the
surfaces.
The measured adhesion originates from van der Waals
interactions between the probe particle and the fluoro-hydro
alkyne-coated Si(111) surface. Stable water capillary bridges
cannot be formed because of the high water contact angle of
the SAMs and the low relative humidity.73 The low adhesion
forces and thus low surface energies of the F9- and F17-based
monolayers confirm the low polarizability of the terminal CF2−
CF3 groups. The F9- and F17-based monolayers show lower
adhesion than do −(CH2)n−CF2−CF3 SAMs on silica surfaces
and are even comparable to rough surfaces of PEG-grafted and
Figure 4. (A) IRRAS spectra (between 2800 and 3000 cm−1) for
fluorinated monolayers on H−Si(111). (B) DFT calculations of
relevant vibrational frequencies of CH3−(CH2)7−(CX2)7CX3 (X = H
and F) optimized by B3LYP/6-311G(d,p) using a scaling factor of
0.9679.
F17, the SAMs exhibited slightly increasing values for the
antisymmetric CH2 stretching vibrations from 2918 to 2920
cm−1 as well as symmetric stretching vibrations ranging from
2850 to 2851 cm−1. The low values observed for F0 are, as
reported before,13 attributed to the high degree of short-range
ordering in these monolayers. Because higher values of these
F
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Figure 5. (A) Representative force−distance curves for SAMs with different fluorinated monolayers. In all cases, a load of 10 nN is applied. (B) The
adhesion force, defined as the minimum in the force−distance plot, for different monolayers.
fluorinated nanoparticles in air.72 Clearly, the high quality of
these monolayers further reduces the surface energy and leads
to minimal adhesion. Combining these monolayers with
structured surfaces would thus be very interesting for the
construction of robust superhydrophobic surfaces.
The low thickness of the monolayers compared to that of
other PTFE coatings74 leads to limited compressibility, and this
suggests that the adhesion will not increase significantly when
the applied load is increased. Figure 6 shows the increase in
fixed layer thickness of 2.0 nm in all cases. The elastic modulus
of the monolayer decreases with increasing fluorine content by
a total factor of 2 (from F0 to F17). This decrease may be
explained by the higher order and crystallinity of F0-, F1-, and
F3-based monolayers compared to those of F9- and F17-based
monolayers. In addition, the model gives a prediction for the
compression of the SAM and the area of contact between the
silica probe and the SAM as a function of the normal load.
Using these values, we can calculate the work of adhesion
between the silica probe and the fluoro-hydro alkyne-coated
Si(111) and an effective Hamaker constant using the Derjaguin
approximation. A typical indentation of the SAM at a normal
load of 10 nN is 1.3 nm, leading to a contact area of 0.030 μm2.
Combined with the adhesion forces in Figure 5, the effective
Hamaker constant is found to decrease from 15.4kBT for the
F0-derived monolayer to 5.1kBT for the monolayer obtained
from F17. There is good agreement between the Hamaker
constant for the F0 monolayer with the theoretical prediction
for the interaction between a silica probe and Si(111) covered
with a crystalline hydrocarbon layer.77
Besides adhesion, the friction of laterally moving or rolling
objects on the surfaces of microelectronic devices is an
important aspect that governs their performance. Minimal
friction coefficients or high lubrication are desirable. The lateral
friction force on the fluorinated monolayers was measured
using the same colloidal probe setup as for the adhesion
measurements. The load was varied from 0 to 75 nN, and the
lateral friction was recorded on several 5 × 5 μm2 areas
(Supporting Information Figure S2). The mean friction forces
are depicted in Figure 7. The friction forces follow an apparent
Gaussian distribution around the mean values, and the typical
relative standard deviation is shown in Figure 7A. Variations
between the mean friction forces on three independently
modified surfaces were found to be smaller than the variations
in the friction force on one surface. The friction coefficients are
calculated from the slopes of the data in Figure 7B. At zero
load, a small but nonzero friction value results from the
adhesion between the probe and monolayer. The magnitude of
this apparent adhesion, obtained by extrapolating to zero
friction force, is lower than the adhesion found in normal force
measurements (Figure 6). This is probably due to the weak
stick−slip nature of the frictional motion, for which the mean
frictional force can be lower than for smooth sliding.78,79
This friction at zero load decreases with increasing fluorine
content, in agreement with the adhesion measurements. For
Figure 6. Adhesion forces of a colloidal silica probe as a function of
the normal load for different monolayers. Solid lines are fits of the data
with a JKR model of a soft layer with 1.8 nm thickness on top of H−
Si(111).
adhesion with increasing load for all monolayers in air. Indeed,
adhesion increases only slightly (30%), with the increase being
largest for the F0-based monolayer. The increasing adhesion
originates from the elastic compression of the surfaces, which
yields a larger contact area between the solid probe and the
monolayers. The JKR model can be used to predict the load
deformation of compressed surfaces when their interaction is
governed by short-range adhesion only.75 In this case, the total
compression is determined by three materials with very
different elastic moduli: the silica probe, the Si(111) substrate,
and the fluoro-hydro alkyne-derived monolayer. Therefore, a
two-layer substrate JKR model was used in which a transition
function describes the change in the elastic modulus of the
substrate from the top of the SAM (150−300 MPa) to deep
inside the Si(111) (160 GPa).76 The solid lines in Figure 6
represent fits of the data to this modified JKR model with a
G
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result of the dipole orientation of the last fluorinated carbon
and the first methylene group (R−CH2−CF2−R1), are densely
packed, and display a high short-range organization, leading to
an overall low friction coefficient.
3.5. Molecular Modeling. To substantiate the optimum
substitution percentages of the fluoro-hydro alkyne-derived
monolayers on H−Si(111) and to investigate the structural
differences between different fluorinated monolayers, models of
the monolayers were studied by molecular mechanics
(Supporting Information Figure S3 and S4). Packing energies
of SAMs on H−Si(111) for different surface coverages are
depicted in Figure 8. The average packing energy per chain was
Figure 8. Surface coverage for fluoro-hydro alkyne-derived monolayers
F0−F17 on H−Si(111).
then calculated according to the literature,43,54 and the optimal
surface coverage was obtained after fitting to a parabolic
function. The optimum surface coverage decreases from 56%
for F0 to 42% for F17 because of the larger cross section of the
CF2/CF3 moieties. These values are confirmed by the
experimentally determined monolayer thicknesses (displayed
in Table 1), with a clear trend in the direction of greater surface
coverage for F0 as compared to that for F17.
Figure 7. (A) Lateral friction force of fluorinated SAMs on Si(111) as
a function of the applied normal load. The error bars represent the
standard deviation in mean friction force between three independently
modified surfaces. The highlighted error bar at a normal load of 75 nN
shows the relative variation in the friction force on a single surface
(Supporting Information Figure S2). (B) The friction coefficients,
obtained from the slope of the friction force versus normal load, for
different monolayers.
4. CONCLUSIONS
Partially fluorinated Si−CHC bound monolayers on oxide-
free Si(111) surfaces display an extremely low surface tension
and adhesion. Such self-assembled organic monolayers are
uniform, densely packed, and characterized by a high degree of
short-range ordering. This set of properties yields a highly
uniform upward presentation of CF3 moieties, yielding a low
interaction with the outside world as evidenced by a critical
surface tension of 5.6 0.21 mN/m, an adhesion force of 3.2
0.03 mJ/m2, and a friction coefficient of 1.2 × 10−2 for the F17
monolayer (Si−(HCCH)−(CH2)6−C8F17). Each of these
values is either the lowest currently measured for any flat
surface or close to it. The present study suggests that fluoro-
hydro alkyne-derived monolayers on silicon surfaces signifi-
cantly expand the scope of strong covalently bound nanometer
monolayers and display significant potential in MEMS/NEMS
applications.
F0-based monolayers, a friction coefficient of 5.7 × 10−2 is
found, in good agreement with the results for SAMs on gold.52
With increasing fluorine content in the monolayers, the friction
coefficient decreases to the rather low value of 1.2 × 10−2 for
F17-based monolayers. These values are comparable to the data
reported for C16H33 thiol monolayers on Au (3.0 × 10−3) and
C16H33 alcohol monolayers on Si (1.3 × 10−2).80 Interestingly,
for SAMs on Au the friction coefficient increases with an
increase in fluorination81 or shows no change,52 in marked
contrast to our data. Kim et al. have summarized the factors
that can influence friction: (i) chemical structure and binding of
an organic molecule to a substrate, (ii) packing density and
monolayer order, (iii) gauche conformation and or surface
coverage, (iv) mechanical properties such as elastic constant
and rigidity of the monolayer, (v) terminal polar or nonpolar
functional groups, and (vi) surface dipole orientations.66,82 In
this case, the fluorine-rich monolayers (especially F9 and F17)
are expected to have a lower polarizability on their surface as a
ASSOCIATED CONTENT
* Supporting Information
Synthesis and characterization of fluorinated alkynes, contact
angle, molecular modeling, DFT calculations of the chemical
shifts in XPS of the carbon atoms, full citation for Gaussian 09,
■
S
H
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(17) Zigah, D.; Herrier, C.; Scheres, L.; Giesbers, M.; Fabre, B.;
Hapiot, P.; Zuilhof, H. Tuning the Electronic Communication
between Redox Centers Bound to Insulating Surfaces. Angew. Chem.,
Int. Ed. 2010, 49, 3157−3160.
(18) Hunger, R.; Fritsche, R.; Jaeckel, B.; Jaegermann, W.; Webb, L.
J.; Lewis, N. S. Chemical and Electronic Characterization of Methyl-
Terminated Si(111) Surfaces by High-Resolution Synchrotron Photo-
electron Spectroscopy. Phys. Rev. B 2005, 72, 045317.
and AFM friction image. This material is available free of charge
via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
*Phone: +31-317-482361. E-mail: han.zuilhof@wur.nl.
■
Notes
(19) Faucheux, A.; Gouget-Laemmel, A. C.; de Villeneuve, C. H.;
Boukherroub, R.; Ozanam, F.; Allongue, P.; Chazalviel, J. N. Well-
defined Carboxyl-terminated Alkyl Monolayers Grafted onto H-
Si(111): Packing Density from a Combined AFM and Quantitative
IR Study. Langmuir 2006, 22, 153−162.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Mabel Caipa Campos, Radostina Manova, and
Dr. Luc Scheres for discussions and help with the surface
modifications. Dr. Marcel Giesbers, Barend van Lagen, Frank
Claassen, and Elbert van der Klift are acknowledged for
instrumental assistance. This project was partially funded by the
Marie Curie European Union FP7 project “Surface Physics for
Advanced Materials” research program and by ASML, The
Netherlands.
(20) Maboudian, R.; Ashurst, W. R.; Carraro, C. Tribological
Challenges in Micromechanical Systems. Tribol. Lett. 2002, 12, 95−
100.
(21) Seitz, O.; Vilan, A.; Cohen, H.; Hwang, J.; Haeming, M.;
Schoell, A.; Umbach, E.; Kahn, A.; Cahen, D. Doping Molecular
Monolayers: Effects on Electrical Transport Through Alkyl Chains on
Silicon. Adv. Funct. Mater. 2008, 18, 2102−2113.
(22) Bocking, T.; Kilian, K. A.; Gaus, K.; Gooding, J. J. Modifying
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Porous Silicon with Self-Assembled Monolayers for Biomedical
Applications: The Influence of Surface Coverage on Stability and
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(23) Li, Y.; Calder, S.; Yaffe, O.; Cahen, D.; Haick, H.; Kronik, L.;
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