Polar and azimuthal alignment of a nematic liquid crystal by alkylsilane self-assembled monolayers: effects of chain-length and mechanical rubbing

Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.     

Spreading kinetics of water drops on self-assembled monolayers of thiols: significance of inertial effects

Spreading of 5-15 microL water drops on self-assembled monolayers of 1-hexadecanethiol and 11-mercapto-1-undecanol, both homogeneous and mixed compositions, formed on gold-coated silicon wafers or glass slides was recorded with a high-speed video camera. The time (t) evolution of the drop base diameter (D) during spreading was analyzed by a power law-correlation: D approximately t(n). The n value was found to increase from n = 0.3-0.5 for water drops on hydrophobic surfaces characterized by the advancing water contact angle of thetaA = 94-104 degrees to n = 0.5-0.8 on less hydrophobic surfaces (thetaA = 45-66 degrees ). These experimental values were found to be of similar magnitude as the literature values reported for small drops and bubbles, which spread over a variety of different substrates including water and water-ethanol drops on self-assembled monolayers of alkylsilanes, air bubbles in water on glass, molten metals on solid metals and ceramics, hydrocarbon drops on water, and others. Inertial effects, which are often not accounted for in the analysis of spreading results, appear to have an impact on the spreading kinetics of small drops in at least the first few milliseconds of the spreading phenomenon.     

Odd and even model self-assembled monolayers: links between friction and structure

The friction between an amorphous carbon tip and two n-alkane monolayers has been examined using classical molecular dynamics simulations. The two monolayers have the same packing density, but the chains comprising each monolayer differ in length by one -CH2- unit. The simulations show that the monolayers composed of C13 chains have higher friction than those composed of C14 chains when sliding in the direction of chain cant; the difference in friction becomes more pronounced as the load is increased. Examination of the contact forces between the chains and the tip, along with conformational differences between the two chain types, lends insight into the friction differences.     

Interfacial friction of surfaces grafted with one- and two-component self-assembled monolayers

We present a quantitative study of the nanoscale frictional properties of one-component (pure) and two-component (mixed) alkylsilane self-assembled monolayers (SAMs). The load and velocity dependence of the friction force was measured in air and ethanol using lateral force microscopy (LFM). It was observed that for SAMs with well-ordered structure (pure SAMs and mixed SAMs composed of two long chain molecules) friction depends nonlinearly on load, at low loads, both in air and in ethanol. These observations are consistent with the low-load contact area predictions of the Johnson-Kendall-Roberts (JKR) theory, indicating that for well-ordered SAMs friction force is proportional to contact area and that the true contact area is determined by elastic deformation of the SAM by the LFM probe. In ambient air, the magnitude of the friction force measured using mixed SAMs is found to be similar to that obtained using pure SAMs at the same external load. Changing the medium to ethanol, however, leads to dramatically lower friction in the mixed SAMs. An analysis of the friction data using a thermally activated Eyring model that takes into account the monolayer viscoelasticity suggests that the better friction properties of the mixed SAMs are a consequence of greater disorder and higher molecular mobility in the outer layer/canopy. These findings indicate that multi-tiered SAM coatings comprising a highly ordered underlayer and a disordered, mobile canopy can provide the basis for low-friction coatings for small mechanical systems.     

Imaging of micropatterned self-assembled monolayers with adsorbed liquid crystals

We report a novel method of imaging micropatterned self-assembled monolayers (SAMs) using adsorbed films of thermotropic (smectic or nematic) mesophase which can then be studied by optical microscopy. Three alkylthiols, functionalized with CH3, OH and COOH groups, were used in various combinations to form patterned SAMs. Two alkylcyanobiphenyls (7CB and 9CB) were used as the liquid crystal imaging reagents. The images are formed by the contrast generated by the different alignments of adsorbed smectic or nematic films induced by different regions of the pattern. The spatial resolution is at least to 4 μm.     

Imidazolium ion-terminated self-assembled monolayers on Au: effects of counteranions on surface wettability

Self-assembled monolayers presenting imidazolium ions at the tail ends (SAMIMs) having different counteranions have been prepared on Au, and the measurement of water contact angles of the surfaces proved to be an extremely valuable simple technique for quantifying the effects of counteranions on hydrophilicity and hydrophobicity of SAMIMs, which will be extrapolated to the water miscibility of the related ionic liquids.     

Measurement of the kinetics of photo-oxidation of self-assembled monolayers using friction force microscopy

The advancement of molecular nanotechnology requires new tools for the characterization of surface chemical reactivity with nanometer spatial resolution. While spectroscopy on sub-100 nm length scales remains challenging, friction force microscopy (FFM) is a promising tool for the characterization of molecular materials, although to date it has been little used in studies of surface reactivity. Here we report the use of FFM to measure the kinetics of photo-oxidation of self-assembled monolayers (SAMs) of alkanethiols adsorbed on gold surfaces. Two alternative approaches (analysis of friction-load plots and the use of line sections through images of patterned materials) are compared and found to yield data in very good agreement, with rate constants being found to be in good agreement despite being carried out on different microscopes. The use of line-section analysis provides a convenient method for the quantification of the extent of reaction in nanometer-scale patterns created in SAMs by the novel approach of scanning near-field photolithography.     

Alkanethiols on platinum: multicomponent self-assembled monolayers

We have studied the formation of self-assembled monolayers (SAMs) of n-alkanethiols on platinum thin films using X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and contact angle (CA) measurements. Specifically, SAMs of 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were grown on polycrystalline Pt films, and the effects of Pt surface preparation, deposition conditions, and solvent treatments on the initial quality and stability of the monolayer in air were investigated. The SAMs prepared under ambient conditions on piranha-cleaned and UV/ozone-cleaned substrates were compared to monolayers formed on template-stripped Pt in an inert atmosphere. We found that alkanethiols deposited from 1 mM ethanolic solutions on piranha-cleaned Pt formed densely packed monolayers in which alkyl chains were oriented close to the surface normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely oxidized in about 1 month. No evidence was found of molecules being weakly bound within the monolayer or having undergone C-S bond scission; however, three distinct sulfur states were observed for all samples in the XPS of the S 2p region. The lowest- and highest-binding-energy components are assigned to alkylthiolate and partially oxidized alkylthiolate species, respectively. The remaining S 2p component (approximately one-third of the sulfur layer), intermediate in binding energy between the other two components, is attributed to a chemisorbed species with a S binding configuration distinct from the majority alkylthiolate: for example, S bound to Pt bound to O, S with a different Pt coordination number, or S in an adsorbed disulfide.     

Odd-even effects in charge transport across self-assembled monolayers

This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (Ag(TS)) surfaces support the SAMs, while top electrodes of eutectic gallium-indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (～1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga(2)O(3)/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes--the "odd-even effect"--is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J(0). This paper includes a discussion of the possible origins of the odd-even effect but does not endorse a single explanation.     

Smectic liquid crystal alignment using mechanically rubbed n-octadecylsiloxane self-assembled monolayers

In recent years a variety of techniques has appeared for the fabrication and manipulation of self-assembled monolayers (SAMs). This development now offers new tools for the study and control at the molecular level of the interaction of liquid crystals (LCs) with solid surfaces, a research area of great importance for liquid crystal applications. In this paper we show that mechanically rubbed octadecylsiloxane SAMs generate a novel surface alignment of LCs in which the in-plane surface anisotropy usually accompanying rubbing is operative, but only for smectics in which the mean molecular long axis, [ncirc], is tilted from the layer normal. On our SAMs smectic phases align with the layers parallel to the SAM surface, and in tilted smectics the surface component of [ncirc] is along the rubbing direction. This anisotropy is absent in the nematic phases which align with [ncirc] strictly normal to the surface. This behaviour can be understood in terms of a rubbed SAM monolayer surface, which is low energy, molecularly smooth, and rendered anisotropic by the rubbing. UV irradiation of rubbed SAMs gave excellent planar alignment ([ncirc] parallel to the surface). This type of control over LC alignment has not been previously reported.     

Conglomerate crystallization on self-assembled monolayers

In this communication, we demonstrate that chiral self-assembled monolayers can be used for polymorphism control of chiral crystals. We studied the crystallization of DL-glutamic acid on chiral self-assembled monolayers and showed that crystallization of DL-glutamic acid on the chiral SAMs resulted in stabilization of the metastable conglomerate form.     

Combined effect of chain length and phase state on adhesion/friction behavior of self-assembled monolayers

The combined effects of phase state and chain length on the adhesion and friction behavior of self-assembled monolayers (SAMs) are demonstrated using atomic force microscopy (AFM). The phase state of n-alkyltrichlorosilane monolayers of varying chain length (C-8, C-12, and C-18) was controlled by adjusting the preparation temperature. The adhesion forces and friction coefficients were observed to increase dramatically around the phase-transition temperature of each monolayer. The phase state effect was more prominent for the longer chain SAMs, which is attributed to the larger deformation volume associated with disordered monolayers. The adhesion/friction diagram for chain length effect with a wide range of phase states is successfully presented. This study reveals that the chain length effect on adhesion/friction can be correctly evaluated by examining the phase-state dependence of adhesion/friction behind the chain length effect.     

Odd-even effects in photoemission from terphenyl-substituted alkanethiolate self-assembled monolayers

Self-assembled monolayers (SAMs) formed from 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2-(CH2)nSH (TPn, n = 1-6) on polycrystalline (111) gold and silver substrates have been characterized by synchrotron-based high-resolution X-ray photoelectron spectroscopy. The intensities, binding energy positions, and width of most photoemission lines exhibited pronounced odd-even effects, i.e., systematic and periodic variation, depending on either odd or even number of the methylene units in the aliphatic linker of the TPn molecules. The detailed analysis of these effects provides important information on the bonding and arrangement of the chemisorbed sulfur headgroups in the TPn films and balance of the structural forces in alkanethiolate SAMs.     

Hydroxide ion adsorption on self-assembled monolayers

It is argued, on the basis of density functional calculations, that a self-assembled monolayer of oligo(ethylene glycol) or n-alkanes in contact with water will preferentially adsorb hydroxyl ions (either from autoionization of water or added to the solution) on both methoxy- and hydroxide-terminated endgroups, thus charging the surface region of the SAM negatively with an estimated charge density of about 1 microC/cm(2) in agreement with recent experiments. The negative charging can explain long-ranged forces between opposing SAM surfaces. On dense SAMs, hydroxyl ions are highly mobile. Hydronium ions can absorb by penetration into the SAM provided there is enough lateral space for their encapsulation. The important role of hydration is demonstrated by calculating the excess binding energy of adsorption using a Born-Haber cycle.     

Phosphonate self-assembled monolayers on aluminum surfaces

Substrates of aluminum (Al) deposited by physical vapor deposition onto Si substrates and then chemically reacted with perfluorodecylphosphonic acid (PFDPAlSi), decylphosphonic acid (DPAlSi), and octadecylphosphonic acid (ODPAlSi) were studied by x-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and friction force microscopy, a derivative of AFM, to characterize their surface chemical composition, roughness, and micro-/nanotribological properties. XPS analysis confirmed the presence of perfluorinated and nonperfluorinated alkylphosphonate molecules on the PFDPAlSi, DPAlSi, and ODPAlSi. The sessile drop static contact angle of pure water on PFDPAlSi was typically more than 130 degrees and on DPAlSi and ODPAlSi typically more than 125 degrees indicating that all phosphonic acid reacted AlSi samples were very hydrophobic. The surface roughness for PFDPAlSi, DPAlSi, ODPAlSi, and bare AlSi was approximately 35 nm as determined by AFM. The surface energy for PFDPAlSi was determined to be approximately 11 mNm by the Zisman plot method compared to 21 and 20 mNm for DPAlSi and ODPAlSi, respectively. Tribology involves the measure of lateral forces due to friction and adhesion between two surfaces. Friction, adhesion, and wear play important roles in the performance of micro-/nanoelectromechanical systems. PFDPAlSi gave the lowest adhesion and coefficient of friction values while bare AlSi gave the highest. The adhesion and coefficient of friction values for DPAlSi and ODPAlSi were comparable.     

Plasma reactions of self-assembled monolayers to model oxygen atom effects on polymers

The plasma treatment of self-assembled monolayers of octadecyl mercaptan on gold substrates has been investigated as a model for oxygen atom effects on polymers. Both O₂ and H₂O low pressure gas plasmas have been used. X-ray photoelectron spectroscopy has revealed that the two plasma treatments differ from each other in the extent of oxidation and etch rate with O₂ being the more aggressive plasma. The results have confirmed that the plasma modification of organic surfaces involves a balance between surface oxidation and surface etching. The well-defined structure of the monolayer enables quantitation of these atom-substrate reactions. © 1998 John Wiley & Sons, Ltd.     

1-Dodecanethiol self-assembled monolayers on cobalt

Cobalt and its alloys are used in a broad range of application fields. However, the use of this metal is especially limited by its strongly oxidizable nature. The use of alkanethiol self-assembled monolayers (SAMs) is a very efficient way to protect against such oxidation and/or to inhibit corrosion. This surface modification method has been particularly applied to oxidizable metals such as copper or nickel, yet the modification of cobalt surfaces by alkanethiol SAMs received limited attention up to now. In this work, we study the influence of parameters by which to control the self-assembly process of 1-dodecanethiol monolayers on cobalt: nature of the surface pretreatment, solvent, immersion time, and concentration. Each of these parameters has been optimized to obtain a densely packed and stable monolayer able to efficiently prevent the reoxidation of the modified cobalt substrates. The obtained monolayers were characterized by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-absorption spectroscopy, and contact angle measurements. The stability of the optimized 1-dodecanethiol monolayer upon air exposure for 28 days has been confirmed by XPS.     

Observations of focal conic domains in smectic liquid crystals aligned on patterned self-assembled monolayers

Patterned Self-Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of nCBs in their smectic-A phase were created using microcontact printing of functionalized organothiols on gold films. By patterning the surface with homeotropic and planar aligning SAMs, the location and formation of the focal conic domains (FCDs) can be controlled. Polarizing microscopy was used to study the formation of FCDs in circle, stripe and checkerboard pattern geometries. Fluorescent confocal microscopy (FCM) was used for the first time to measure the eccentricity of FCDs that form along a stripe pattern.     

Electroreduction of oxygen on octadecylmercaptan self-assembled monolayers

The reduction of oxygen has been studied on octadecylmercaptan self-assembled monolayers adsorbed on gold substrates in borate buffer solutions with a rotating disc electrode. A great inhibition of the oxygen reduction and other electrochemical reactions by these monolayers has been found. However, after polarisation at –0.80 V_SHE the protecting properties of the film against electron transfer reactions are lost, and a behaviour similar to bare gold is observed. Ex situ XPS indicates that the thiol monolayer has not been desorbed to a large extent during oxygen reduction. Disorders of the monolayer structure and desorption of thiol molecules are proposed as the main reasons for the accessibility of electrochemical reactions to the surface.     

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C₆₀). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.