Phase state effect on adhesion behavior of self-assembled monolayers

The effect of phase state of self-assembled monolayers (SAMs) on adhesion behavior was studied using a combination of atomic force microscopy (AFM) and Johnson-Kendall-Roberts (JKR) methods. The phase state of SAMs was controlled by adjusting the reaction temperature. Order-to-disorder structural transitions in monolayers of n-alkyltrichlorosilanes resulted in dramatic increases in adhesion force and adhesion hysteresis, which represents the first report of alterations in adhesion properties due to phase changes of monolayers without any effect of chain length and surface heterogeneity. This increase in mechanical deformation of the disordered monolayer is understood to be caused by increases in (1) molecular contact between the AFM tip and a disordered monolayer due to the more deformable state of the latter and (2) monolayer deformation during unloading by the JKR probe lens. Adhesion hysteresis was found to have greater sensitivity toward the unloading rate for disordered monolayers. The occurrence of maximum hysteresis at faster rates proves that monolayer chain mobility increases with structural disorder, resulting in increased mechanical deformation.     

Effect of pendent chains on the interfacial properties of thin polydimethylsiloxane (PDMS) networks

The interfacial properties of end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin cross-linked PDMS films (～10 μm thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically varying the concentration of monofunctional PDMS in a mixture with telechelic precursor molecules, structures ranging from near-ideal elastic networks to poorly cross-linked networks composed of a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing bead probes was used to quantify the effect of network structure on the interfacial friction coefficient and residual force. Indentation measurements employing an AFM in force mode were used to characterize the elastic modulus and the pull-off force for the films as a function of pendent chain content. These measurements were complemented with conventional mechanical rheometry measurements on similar thick network films to determine their bulk rheological properties. All networks studied manifested interfacial friction coefficients substantially lower than that of bare silicon. PDMS networks with the lowest pendent chain content displayed friction coefficients close to 1 order of magnitude lower than that of bare silicon, whereas networks with the highest pendent chain content manifested friction coefficients about 3 times lower than that of bare silicon. At intermediate sliding velocities, a crossover in the interfacial friction coefficient was observed, wherein cross-linked PDMS films with the least amount of pendent chains exhibit the highest friction coefficient. These observations are discussed in terms of the structure of the films and relaxation dynamics of elastic strands and dangling chains in tethered network films.     

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.     

Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.     

Nanografting: modeling and simulation

We present a simple phenomenological model of the nanografting process with an emphasis on the formation of binary self-assembled monolayers. This model includes dynamical processes that are involved in natural growth experiments, including molecular deposition, surface diffusion, and the phase transition from physisorption to chemisorption, and we show that it predicts domain formation in ungrafted deposition that matches experiment. The one-order-of-magnitude faster kinetics that is found in the nanografting experiments compared to natural self-assembly (or unconstrained self-assembly) is described with a key assumption that the deposition rate is greatly enhanced in the small region confined between the back side of the AFM tip and the edge of the previously deposited self-assembled monolayer. Monte Carlo simulations based on this model reproduce experimental observations concerning the variation of SAM heterogeneity with AFM tip speed. Our simulations demonstrate that the faster the AFM tip displaces adsorbed molecules in a monolayer, the more heterogeneous are the monolayers formed behind the tip, as this allows space and time for the formation of phase-segregated domains.     

Nanoscale aggregate structures of trisiloxane surfactants at the solid-liquid interface

The self-associating structures at the solid-liquid interface of three nonionic trisiloxane surfactants ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)n OH (n = 6, 8, and 12), or BEn, are studied as a function of substrate properties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactants are known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energy surfaces. This study also attempts to relate the BEn surface aggregate structures at the solid-liquid interface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highly oriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregation concentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more on oxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-covered oxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparing the force curves obtained by tips with and without thiol modification. The surface aggregate structures of the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorption behavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus, our results confirm previous work showing that superwetting shares its main features with other classes of surfactants.     

Nanopatterning of alkynes on hydrogen-terminated silicon surfaces by scanning probe-induced cathodic electrografting

The electrochemical cathodic electrografting reaction, previously demonstrated on bulk silicon surfaces, can be patterned on the nanoscale utilizing conducting probe atomic force microscopy (CP-AFM). Alkyne electrografting is a particularly useful chemical technique since it leads to direct covalent attachment of conjugated alkynes to silicon. In addition, application of a forward bias during the reaction renders the surface less sensitive to oxidation and the resulting monolayers are very stable in air and basic aqueous solution. Alkyne monolayer lines can be drawn down to 40 nm resolution using a Pt-coated AFM tip, and the heights of the monolayers scale with the molecular length of the alkyne. The tip is biased (+) and the surface is biased (-) to drive the cathodic electrografting reaction under ambient conditions. The resistance of the monolayers to fluoride, as well as friction force microscopy, indicate that the alkynes are covalently bonded to the surface, not oxide-based, and hydrophobic. The reaction does not work with alkenes, and therefore hydrosilylation is not the primary mode of reaction. Wider lines (300 nm) can be produced using broadened Pt-coated AFM tips. This reaction could be important for the interfacing of conjugated molecules directly to silicon in a spatially controlled fashion.     

Nanoscopic Pd line arrays using nanocontact printed dendrimers

High-density Pd line arrays with 55 nm line-width were obtained using nanocontact-printed dendrimer monolayers. Elastomeric PDMS stamps for nanocontact printing were replicated from silicon master molds which were fabricated by UV nanoimprinting in combination with reactive ion etching. The fabrication method effectively controlled the aspect ratios of high-density lines for resolving the problems encountered in both replicating silicon masters to PDMS stamps and printing with the replicated PDMS stamps. Using the PDMS nanostamp with an optimized aspect ratio, a self-assembled monolayer of dendrimer was patterned on a Pd film via nanocontact printing, which was facilitated by the strong interaction between Pd and amine groups of the dendrimer. The patterned self-assembled monolayer was used as an etch-resist mask against the wet etchant of Pd, leaving behind a high-density Pd line array over large areas. The resulting functional Pd nanopattern is of practical significance in microelectronics and bio- or gas-sensing devices.     

Alternative method for fabricating chemically functionalized AFM tips: silane modification of HF-treated Si3N4 probes

An alternative method for fabricating functionalized, atomic force microscopy (AFM) tips is presented. This technique is simple and requires only minimal preparation and tip modification to generate chemically sensitive probes that have a robust organic monolayer of flexible terminal chemistry attached to the surface. Specifically, commercially microfabricated Si3N4 AFM tips were modified with self-assembled monolayers (SAMs) of octadecyltrichlorosilane and (11-bromoundecyl)trichlorosilane after removing the native silicon oxide surface layer with concentrated hydrofluoric acid. The structure of these SAM films on solid silicon nitride surfaces was studied using contact angle goniometry and Fourier transform infrared spectroscopy. Pull-off force measurements on various bare (mica, graphite, and silicon) and SAM-functionalized substrates confirm that mechanically robust, long-chain organic silane SAMs can be formed on HF-treated Si3N4 tips without the presence of an intervening oxide layer. Adhesion experiments show that the integrity of the organic film on the chemically modified tips is maintained over repeated measurements and that the functionalized tips can be used for chemical sensing experiments since strong discrimination between different surface chemistries is possible.     

Tribological properties of self-assembled monolayers of catecholic imidazolium and the spin-coated films of ionic liquids

A novel compound of an imidazolium type of ionic liquid (IL) containing a biomimetic catecholic functional group normally seen in mussel adhesive proteins was synthesized. The IL can be immobilized on a silicon surface and a variety of other engineering material surfaces via the catecholic anchor, allowing the tribological protection of these substrates for engineering applications. The surface wetting and adhesive properties and the tribological property of the synthesized self-assembled monolayers (SAMs) are successfully modulated by altering the counteranions. The chemical composition and wettability of the IL SAMs were characterized by means of X-ray photoelectron spectroscopy (XPS) and contact angle (CA) measurements. The adhesive and friction forces were measured with an atomic force microscope (AFM) on the nanometer scale. IL composite films were prepared by spin coating thin IL films on top of the SAMs. The macrotribological properties of these IL composite films were investigated with a pin-on-disk tribometer. The results indicate that the presence of IL SAMs on a surface can improve the wettability of spin-coated ionic liquids and thus the film quality and the tribological properties. These films registered a reduced friction coefficient and a significantly enhanced durability and load-carrying capacity. The tribological properties of the composite films are better than those of pure IL films because the presence of the monolayers improves the adhesion and compatibility of spin-coated IL films with substrates.     

Simulations of nanotribology with realistic probe tip models

We present the results of massively parallel molecular dynamics simulations aimed at understanding the nanotribological properties of alkylsilane self-assembled monolayers (SAMs) on amorphous silica. In contrast to studies with opposing flat plates, as found in the bulk of the simulation literature, we use a model system with a realistic AFM tip (radius of curvature ranging from 3 to 30 nm) in contact with a SAM-coated silica substrate. We compare the differences in response between systems in which chains are fully physisorbed, fully chemisorbed, and systems with a mixture of the two. Our results demonstrate that the ubiquitous JKR and DMT models do not accurately describe the contact mechanics of these systems. In shear simulations, we find that the chain length has minimal effects on both the friction force and coefficient. The tip radius affects the friction force only (i.e., the coefficient is unchanged) by a constant shift in magnitude due to the increase in pull-off force with increasing radius. We also find that at extremely low loads, on the order of 10 nN, shearing from the tip causes damage to the physisorbed monolayers by removal of molecules.     

Oriented binding of the His6-tagged carboxyl-tail of the L-type Ca2+ channel alpha1-subunit to a new NTA-functionalized self-assembled monolayer

Oriented stable binding of functional proteins on surfaces is of fundamental interest for receptor/ligand studies in atomic force microscopy (AFM) and surface plasmon resonance (SPR) experiments. Here we have chosen the His6-tagged carboxyl-tail (C-tail) of the alpha1c-subunit of the L-type Ca2+ channel and calmodulin (CaM) as its cognitive partner as a model system to develop a new functional surface. Covalently attached self-assembled monolayers on ultraflat gold containing NTA-thiols to which the His6-tagged C-tail was bound and thiols with triethylene-glycol groups as matrix-thiols represented the system of choice. The topography of this surface was characterized using AFM; its ability to bind C-tail proteins oriented and stable was confirmed by SPR measurements and by complementary force spectroscopy experiments with a CaM4-construct covalently attached to the tip. The developed anchoring strategy can now be used to study receptor/ligand interactions in general applying force spectroscopy and SPR on His6-tagged proteins oriented immobilized onto this new NTA-functionalized self-assembled monolayer.     

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.     

Comparative study of the adhesion, friction, and mechanical properties of CF3- and CH3-terminated alkanethiol monolayers

We report the results of a direct comparison of the adhesion, friction, and mechanical properties between alkanethiol self-assembled monolayer films terminated by either CH(3) or CF(3) end groups using both interfacial force (IFM) and atomic force (AFM) microscopies. The purpose of this work is to gain insight into the detailed origins of the differing frictional behavior previously observed with AFM. The IFM results reveal an increased adhesive interaction for the CF(3)-terminated film due to the highly polar nature of the end groups. In agreement with earlier studies, the AFM results show two linear regions with differing frictional slopes for the CH(3)-terminated film but only a single slope for the CF(3)-terminated film. We contrast the differences between these techniques, approximately 100 times smaller tips for the AFM, and discuss the role of the mechanical properties, the increased adhesive interaction, and the amount of disorder present in the film in creating differences in frictional behavior between the two systems. We conclude that increased adhesion for the CF(3)-terminated film plays an important role in the observed differences in frictional behavior, while the differences between the two techniques can be traced to the different tip sizes and the consequent responses to the presence of disorder in the films.     

Characterization and tribological investigation of self-assembled trimethoxysilyl-functionalized poly(phthalazinone ether ketone)thin films on glass substrates

<正>Trimethoxysilyl-functionalized PPEK(PKGS) films had been designed to serve as wear resistant coatings for silicon surfaces. These surface films were formed by a dip-coating technique applied to self-assembled monolayers(SAMs).The formation and wetting behavior of PKGS films were characterized by means of contact angle measurement.The friction coefficient of the film prepared is very low(about 0.1),and the anti-wear behavior is good,with a lack of failure after sliding over 1800 s.     

硅表面复合自组装膜的制备及其摩擦性能

采用自组装技术在单晶硅表面制备了3-氨基丙基三乙氧基硅烷(APTES)-SiO2-APTES复合膜,并对其表面的组成、结构及摩擦性能进行了表征.结果表明:复合膜表面对水的接触角约为63°,且表面平整、致密,其平均粗糙度(Ra)约为0.963nm.通过原子力显微镜(AFM)和透射电子显微镜(TEM)观察到夹层中SiO2颗粒的粒径约为20-50nm,较均匀地分布在第一层APTES膜的表面.与APTES自组装单层膜(SAMs)相比,APTES-SiO2-APTES复合膜由于纳米SiO2颗粒的引入而表现出更低的摩擦系数和更长的耐磨寿命.     

Engineering silicon oxide surfaces using self-assembled monolayers

Although a molecular monolayer is only a few nanometers thick it can completely change the properties of a surface. Molecular monolayers can be readily prepared using the Langmuir-Blodgett methodology or by chemisorption on metal and oxide surfaces. This Review focuses on the use of chemisorbed self-assembled monolayers (SAMs) as a platform for the functionalization of silicon oxide surfaces. The controlled organization of molecules and molecular assemblies on silicon oxide will have a prominent place in "bottom-up" nanofabrication, which could revolutionize fields such as nanoelectronics and biotechnology in the near future. In recent years, self-assembled monolayers on silicon oxide have reached a high level of sophistication and have been combined with various lithographic patterning methods to develop new nanofabrication protocols and biological arrays. Nanoscale control over surface properties is of paramount importance to advance from 2D patterning to 3D fabrication.     

Chemical force titrations of functionalized Si(111) surfaces

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.     

Nanotribology of octadecyltrichlorosilane monolayers and silicon: self-mated versus unmated interfaces and local packing density effects

We use atomic force microscopy (AFM) to determine the frictional properties of nanoscale single-asperity contacts involving octadecyltrichlorosilane (OTS) monolayers and silicon. Quantitative AFM measurements in the wearless regime are performed using both uncoated and OTS-coated silicon AFM tips in contact with both uncoated and OTS-coated silicon surfaces, providing four pairs of either self-mated or unmated interfaces. Striking differences in the frictional responses of the four pairs of interfaces are found. First, lower friction occurs with OTS present on either the tip or substrate, and friction is yet lower when OTS is present on both. Second, the shape of the friction versus load plot strongly depends on whether the substrate is coated with OTS, regardless of whether the tip is coated. Uncoated substrates exhibit the common sublinear dependence, consistent with friction being directly proportional to the area of contact. However, coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution to the frictional behavior of OTS. Direct in situ comparison of two intrinsic OTS structural phases on the substrate is also performed. We observe frictional contrast for different local molecular packing densities of the otherwise identical molecules. The phase with lower packing density exhibits higher friction, in agreement with related previous work, but decisively observed here in single, continuous images involving the same molecules. Lateral stiffness measurements show no distinction between the two OTS structural phases, demonstrating that the difference in friction is not due to divergent stiffnesses of the two phases. Therefore, the packing density directly affects the interface's intrinsic resistance to friction, that is, the interfacial shear strength.     

Nanoscale patterning of alkyl monolayers on silicon using the atomic force microscope

Self-assembled monolayers (SAMs) of 1-alkenes on hydrogen-passivated silicon substrates were successfully patterned on the nanometer scale using an atomic force microscope (AFM) probe tip. Nanoshaving experiments on alkyl monolayers formed on H-Si(111) not only demonstrate the flexibility of this technique but also show that patterning with an AFM probe is a viable method for creating well-defined, nanoscale features in a monolayer matrix in a reproducible and controlled manner. Features of varying depths (2-15 nm) were created in the alkyl monolayers by controlling the applied load and the number of etching scans made at high applied loads. The patterning on these SAM films is compared with the patterning of alkyl siloxane monolayers on silicon and mica.