Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by adsorption of an active surfactant on a solid surface.81,82 Alkanethiols are widely known to have a
c)
7 mm
41
high affinity for gold, and the resulting SAMs of thiols on gold have been extensively studied.83-
86 SAMs have received considerable attention in the development of materials for molecular-
based electronic devices,87 biocompatibility,88 wetting,89 adhesion90 and corrosion prevention.91 In contrast to ultrathin films made by molecular beam epitaxy(MBE) and chemical vapor deposition (CVD), SAMs are highly ordered and oriented86,92 and can incorporate a wide range of functional groups both in the alkyl chains and at the chain termina.82 One of the fortuitous properties of gold as the metal substrate is it is relatively inert and does not form stable oxide surfaces;93 therefore gold surfaces can be cleaned simply by removing physically and chemically adsorbed contaminants via chemical or physical methods. Porter demonstrated that long-chain thiols form densely packed, crystalline films featuring alkyl chains tilted from the surface normal by 20-30° in which the monolayer becomes increasingly disordered with lower packing density and coverage as the length of the chains decreases.94 Chemisorption of alkanethiols on gold is a kinetic phenomenon82,84,95 forming the Au(I) thiolate (RS-) species as shown below.
RS-H + Aun0 RS-Au+Aun0+ ½ H2
That reaction may be considered as an oxidative addition of S-H bond to gold surface, followed by reductive elimination of the hydrogen.82 On the basis of the bond energies of RS-H, H2 and RS-Au (87 kcal/mol, 104 kcal/mol and 40 kcal/mol, respectively) the enthalpy for
adsorption of alkanethiolates on gold -is about -5 kcal/mol (exothermic), and thus a spontaneous process.82,96 The bonding of thiolate to gold is very strong with a homolytic bond strength of ~44 kcal/mol82,97 that results in pinning of the thiolate to a specific site on gold through a covalent, slightly polar Au-S bond.97 Studies have shown that functional groups at the chain termina exposed at the surface adopt specific orientations but have relatively little effect on the structure
42
of the film in the interior region of hydrocarbon chains.98 These properties provide a basis for understanding the behavior and reactivity of organic surfaces derived from SAMs of thiols on gold.
Characterization of SAMs Contact angle goniometry
The technique of contact angle goniometry provides a convenient means to quantify the wettability (i.e., surface energy) of surfaces by measuring the wetting angle, or contact angle, when a 1 μL sessile drop of water (or other liquid) is placed in contact with the surface. The angle formed between the solid/liquid interface and the liquid/vapor interface and the vertex where the three interfaces meet is referred to as the contact angle. The angle is measured between the plane tangent to the surface of the drop of water where it meets the surface and the plane tangent to the surface of the solid. As shown in Figure 2.19a for a drop of liquid on a surface, Young’s equation (Eqn 2.3) is used to describe the interactions between the forces of cohesion and adhesion and to determine the surface free energy.99,100
Eqn 2.3
θ is the contact angle
γsl is the solid/liquid interfacial energy γsv is the solid surface free energy γlv is the liquid surface free energy
If the interaction between the surface and the water is strong, the measurement gives a low contact angle that indicates a hydrophilic surface with a high degree of wettability and low
43
surface free energy as shown in Figure 2.19b. If the interaction between the surface and water is weak, the measurement gives a high contact angle that indicates a hydrophobic surface with a low degree of wettability and high surface energy as shown in Figure 2.19b and 2.19c. For our purpose, the contact angle method is an ideal method to characterize the relative hydrophobic or hydrophilic character of SAMs with head groups featuring varying polarity.
Figure 2.19. Illustration of contact angle a) for a drop of liquid on a surface, b) drops of liquid
on hydrophobic(left) and hydrophilic(right) surfaces, and c) a 1µL drop of water on SAM I.
Ellipsometry
Ellipsometry is an optical technique for determining the thickness of molecular films that measures the change in polarization of coherent light upon reflection from a surface or a film on surface.101,102 Ellipsometry is used predominantly to characterize the properties (e.g., crystallinity, chemical composition, roughness of surfaces and thin films) without damaging or destroying the film or substrate. Measurements are made to determine the optical constants of the surface, dielectric constants, and thickness of the film. The reflection of polarized light from surfaces and thin films is expressed in terms of the reflection coefficients Rp for light polarized
parallel to the plane of incidence and Rs for light polarized perpendicular to the plane of
incidence.101 Those coefficients represent the change of amplitude and phase of the light on
1 mm
c)
b)
a)
γ
lvγ
sv vapor solid liquidγ
sl θ44
reflection. By equation 2.4, the reflection of light from a surface is characterized by the complex reflection coefficient tan Ψ exp(i∆), and hence by the two quantities ∆ and Ψ. Figure 2.20 shows the basic components of an ellipsometer.
Eqn 2.4
ρ = ratio of the parallel Rp and normal reflection Rs coefficient
Ψ = ratio of the change in amplitude after reflecting off the surface Δ = change in phase between the light which is polarized parallel to
the light beam, and light which is polarized perpendicular to the light beam.
A collimated unpolarized or circularly polarized beam of light from a suitable light source (L) is passed through a polarizer (P) to convert unpolarized light to linearly polarized light. The light then passes through a wave retarder (or compensator, Q) to convert linearly polarized light into elliptically polarized light. The light is next incident upon a flat sample (S) and reflected, causing its polarization to be modified due to change in refractive index of the sample. The change in polarization of the light is then measured by an analyzer (A) and a photoelectric detector placed behind it (D). The instrument is operated with the azimuth of the compensator fixed at 45° from the plane of incidence. The polarizer and the analyzer are made to rotate alternatively around the optical beam until the light leaving the analyzer is totally extinguished, or minimized. The extinction or the null condition is then determined by the output current or voltage of the photometer readout. From the readings P, Q and A at which the light
45
intensity is extinguished, ∆ and Ψ of equation are calculated, then the optical constants describing the refractive index (k) and extinction coefficient (n) and the thickness of the films on the surface are calculated.
Figure 2.20. Components of an ellipsometer. An unpolarized beam of light from source L passes
through a polarizer P. The elliptically polarized light from the compensator Q is next incident on the sample S and reflected. The change in the polarization of the light is then measured by an analyzer A and a photoelectric detector D.
Grazing-angle FT-IR
Confirmation of the functional groups present in the organic components of SAMs was carried out using grazing incidence infrared spectroscopy. This technique has been used widely to characterize monolayers formed on metal substrates.94,103-105 The incident IR beam is reflected off the surface at an angle into the detector as shown in Figure 2.21. Due to the small amount of material actually present on the surface in a SAM, the optical path must be purged with nitrogen prior the experiment to ensure that absorption by water vapor and other gases in air do not interfere with absorption by molecules in the SAM
S A D Q P Φ
46
Figure 2.21. Grazing-angle IR measurement of absorption by a SAM.
Although grazing angle IR operates similarly to traditional transmission IR used on bulk samples, not all absorption bands that appear in a bulk sample of the thiol are observed for monolayers using the grazing-angle technique. Some absorption bands may be absent due to the orientation of the bonds relative to the gold surface. Only those vibrations with transition dipoles perpendicular to the surface will absorb strongly enough to be observed by the grazing-angle technique, while vibrations with transition dipoles parallel to the surface generally do not absorb strongly enough to be observed.105,106 That effect, called the “metal-surface selection rule”, determines which components of the molecule are IR active when observed by grazing angle.107 A molecule adsorbed on a metal surface induces a local, opposite charge in the substrate that enhances the transition dipoles oriented perpendicular to the substrate and cancels out the dipoles for parallel orientationas illustrated in Figure 2.22.
SAM IR detector θ θ IR source
47
Figure 2.22. Molecular dipoles oriented perpendicular (left) and parallel (right) to the surfaces of
a metal substrate and the corresponding charges induced in the substrate; dipoles perpendicular to the surface are enhanced and IR active, whereas dipoles parallel to the surface are negated and IR inactive.
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