Investigation of nanotribological properties of self-assembled monolayers with alkyl and biphenyl spacer chains (invited)

Understanding the relationships between molecular structure and nanotribological performance of self-assembled monolayers (SAMs) are quite important for molecular tailoring for efficient lubrication. For this purpose, SAMs, having alkyl and biphenyl spacer chains with different surface terminal groups (-CH3, -COOH, and -OH), and head groups (-SH and -OH), were prepared. The influence of spacer chains, surface terminal groups, and head groups on adhesion, friction and wear properties were investigated by contact mode atomic force microscopy (AFM). The relative stiffness of SAMs was determined by force modulation mode AFM and indentation experiments using load-displacement curves. The friction properties of SAMs are explained using a molecular spring model in which local stiffness governs the friction properties. Micropatterned SAMs specimen were fabricated and studied to verify the molecular spring model. The influence of relative humidity, temperature and velocity on adhesion and friction was studied. The failure mechanisms of SAMs and substrates were investigated by wear and continuous microscratch AFM technique. Based on these studies, the adhesion, friction and wear mechanisms of SAMs at molecular scale are discussed.     

Effect of self-assembled monolayers modified slider on head-disk tribology under volatile organic contamination

Self-assembled monolayers (SAMs) with different endgroups were established on slider surface. The effect of the SAMs coated slider on head-disk tribology under volatile organic contamination (VOC) of octamethylcyclotetrasiloxane (D₄) was investigated using a contact start/stop (CSS) tester. The slider surfaces before and after the CSS tests were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). The contact angle measurement and TOF-SIMS analysis proved that the SAMs were successfully formed on the slider surface. All the SAMs reduced the friction under the pollutant vapor. The transfer of lubricating oil onto the slider surface was detected after the CSS tests. It was found that the slider with a low surface free energy associated with small amount of lubricating oil transfer. The little the lubricating oil transfer was, the low the frictions were. These results indicate that a slider with low surface free energy can reduce the loss of lubricating oil from the disk surface, and hence improve the tribological properties of hard-disk interface (HDI) under VOC.     

Thermal decomposition of perfluorodecylsiloxane self-assembled monolayers in air

The thermal decomposition of perfluorodecylsiloxane self-assembled monolayers (SAMs) in air has been studied using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle analysis. It is found that the monolayers are stable in air up to about 300 degrees C. Above 350 degrees C the monolayers primarily decompose through CC bond cleavage, with a gradual reduction in chain length.     

Friction and fracture properties of polysilicon coated with self-assembled monolayers

The increasing use of small micromechanical devices and advanced sensors has led to concern about the failure modes and reliability of these structures. The enormous promise will not materialize without substantial progress in overcoming the stiction, friction and wear associated with such devices and understanding the mechanical behavior of MEMS materials and structures. Self-assembled monolayers (SAMs) are release and anti-stiction coatings for MEMS. In this paper, the anti-stiction properties of octadecyltrichlorosilane (OTS) SAM were calculated. The microtribological properties of OTS SAM were investigated with a ball-on-flat microtribometer. The influence of OTS SAM on the mechanical properties of micromachined polysilicon films for MEMS was investigated with an accurate evaluation using the microtensile test device. It was concluded that the OTS SAM has good anti-stiction properties and low friction coefficients. The hydrophobic property of OTS is the main factor leading to an increase in the average fracture strength of micromachined polysilicon up to 32.46%. Thus, the operational stability and lifetime of MEMS can be raised when coated with self-assembled monolayers.     

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm²) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.     

Orientation effects of micro-grooves on sliding surfaces

Based on a brief review of the researches related to the patterns of micro-grooves for tribological purpose, experiments were designed and carried out to study the orientation effects of grooves on the friction performance. The experimental data indicated that the grooves perpendicular or parallel to the sliding direction have a strong impact on the friction performance of sliding surfaces, and the merits of perpendicular or parallel orientation may swap under different contact conditions. The results were then discussed from the aspects of hydrodynamic effect, lubricant supply effect and contact stress effect.     

Molecular contributions to the frictional properties of fluorinated self-assembled monolayers

The frictional properties of self-assembled monolayers (SAMs)formed from four different species (n-octyltrichlorosilane,1H,1H,2H,2H-perfluorooctyltrichlorosilane, tridecanethiol, and 13,13,13-trifluorotridecanethiol) were measured on the molecular scale using atomic force microscopy (AFM). On this scale, monolayers containing partially fluorinated alkyl chains exhibited higher frictional properties than monolayers containing analogous fully hydrogenated alkyl chains. Systematic comparison of the frictional properties of these SAMs provided insight into the molecular contributions to the frictional response. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tribological properties of self-assembled monolayers on Au,SiOx and Si surfaces

Using interfacial force microscopy (IFM), the tribological properties of self-assembled monolayers (SAM) on Si surfaces produced by a new chemical strategy are investigated and compared to those of “classical” SAM systems, which include alkanethiols on Au and alkylsilanes on SiO_x. The new SAM films are prepared by depositing n-alkyl chains with OH-terminations onto Cl-terminated Si substrates. The chemical nature of the actual lubricating molecules, n-dodecyl, is kept constant in all three thin film systems for direct comparison and similarities and differences in tribological properties are observed. The adhesion strength is virtually identical for all three systems; however, frictional properties differ due to differences in film packing. Differences in the chemical bonds that attach the lubricant molecules to the substrate are also discussed as they influence variations in film wear and durability. It is demonstrated that the new SAM films are capable of controlling the friction and adhesion of Si surfaces equally well as the classical SAMs and are potentially more reproducible and more durable.     

Friction behaviour of chemical vapor deposited self-assembled monolayers on silicon wafer

Friction characteristics of self-assembled monolayers (SAMs) coated on Si-wafer (1 0 0) by chemical vapor deposition technique were studied experimentally at nano and micro-scales. Four self-assembled monolayers, such as dimethyldichlorosilane (DMDC), diphenyldichlorosilane (DPDC), perfluorooctyltrichlorosilane (PFOTS) and perfluorodecanoicacid (PFDA) coated on Si-wafer (1 0 0) were used as test materials. Nano-scale friction was measured using atomic force microscopy (AFM) in the range of 0–40 nN normal loads, in LFM (lateral force microscopy) mode, using a contact mode type Si₃N₄ tip. Results showed that the friction of SAMs at this scale was influenced by their physical/chemical properties, while that of Si-wafer by its inherent adhesion. Further, micro-scale friction tests were also performed with a ball-on-flat type micro-tribotester under reciprocating motion. Friction was measured in the range of 1500–4800 μN applied normal loads using glass balls of varying radii, viz., 0.25, 0.5 and 1 mm. It was observed that the performance of SAMs was more superior to Si-wafer even at micro-scale, except for PFDA. Evidences obtained using scanning electron microscope showed that Si-wafer and PFDA exhibited wear at this scale. Wear in the case of Si-wafer was due to solid–solid adhesion and that in the case of PFDA due to the influence of humidity (moisture). The micro-scale friction in both these materials was severely influenced by their wear.     

Periodicities in the properties associated with the friction of model self-assembled monolayers

The classical molecular dynamics simulations presented here examine the periodicities associated with the sliding of a diamond counterface across a monolayer of hydrocarbon chains that are covalently bound to a diamond substrate. Periodicities observed in a number of system quantities are a result of the tight packing of the monolayer and the commensurate structure of the diamond counterface. The packing and commensurability of the system force synchronized motion of the chains during sliding contact. This implies that the size of the simulations for this special case can be reduced so that the simulations can be conducted with sliding speeds and time durations that may bridge the gap between theory and experiment.     

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions prepared by microcontact printing (microCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.     

Achieving precision and reproducibility for writing patterns of n-alkanethiol self-assembled monolayers with automated nanografting

Nanografting is a high-precision approach for scanning probe lithography, which provides unique advantages and capabilities for rapidly writing arrays of nanopatterns of thiol self-assembled monolayers (SAMs). Nanografting is accomplished by force- induced displacement of molecules of a matrix SAM, followed immediately by the self-assembly of n-alkanethiol ink molecules from solution. The feedback loop used to control the atomic force microscope tip position and displacement enables exquisite control of forces applied to the surface, ranging from pico to nanonewtons. To achieve high-resolution writing at the nanoscale, the writing speed, direction, and applied force need to be optimized. There are strategies for programing the tip translation, which will improve the uniformity, alignment, and geometries of nanopatterns written using open-loop feedback control. This article addresses the mechanics of automated nanografting and demonstrates results for various writing strategies when nanografting patterns of n-alkanethiol SAMs.     

Inflammatory cell recruitment and adhesion to methyl-terminated self-assembled monolayers: effect of implantation time

The contribution of methyl groups in implant-triggered inflammation was investigated in vivo using self-assembled monolayers (SAMs) of alkanethiols on gold. The CH(3)-coated implants were inserted in an air-pouch cavity induced in BALB/c mice. The in situ inflammatory response was monitored 24, 48, and 72 hours later. Inflammatory cells recovered from the air pouches were counted and observed by light microscopy. The cellularity of the implant surfaces was defined by scanning electron microscopy (SEM). In comparison with gold implants, the CH(3)-coated SAMs recruited a significantly higher number of inflammatory cells. Polymorphonuclear leukocytes (PMN) were more numerous than mononuclear cells (Mo) in the exudates recovered from the air pouches with CH(3)-coated SAMs. The opposite PMN/Mo proportion was observed in air pouches of the two control groups (mice receiving gold implants or sham-operated animals). A low density of adherent cells was seen on CH(3)-coated implants, with no significant quantitative differences during the time course of the study. In contrast, the gold-coated surfaces were covered with numerous cells during all of the 3 days of the inflammation. In conclusion, implants with CH(3) surfaces are likely to induce PMN-dominated local acute inflammation but these surfaces are not associated with a significant adherence of leukocytes to the implant.     

Characterization of mixed self-assembled monolayers for immobilization of streptavidin using chemical force microscopy

Mixed self-assembled monolayers (SAMs) to immobilize streptavidin on a gold surface were investigated by measuring the pull-off force between an AFM tip and a biotin-modified surface using CFM. Biotin-LC-NHS was modified on SAMs prepared from a mixed solution of cystamine and MEOH. Increased pull-off forces between the AFM tip and the surface were observed with an increased cystamine mole fraction in the solution. Streptavidin was immobilized onto biotin-LC-NHS modified mixed SAMs and analyzed by tapping AFM. Also, the formation of mixed SAMs containing MUOH and MBDA was confirmed using CFM. The measured pull-off forces on the only MBDA surface were larger than those on the surface with MUOH. These results can be applied to determine an optimal mixing ratio of MUOH and MBDA SAMs that reduces non-specific streptavidin binding onto a surface.     

Direct deformation study of AFM probe tips modified by hydrophobic alkylsilane self-assembled monolayers

The in-use wear of atomic force microscopy (AFM) probe tips is crucial for the reliability of AFM measurements. Increase of tip size for several nanometers is difficult to monitor but it can already taint subsequent AFM data. We have developed a method to study the shape evolution of AFM probe tips in nanometer scale. This approach provides direct comparison of probe shape profiles, and thus can help in evaluation of the level of tip damage and quality of acquired AFM data. Consequently, the shape degradation of probes modified by hydrophobic alkylsilane self-assembled monolayers (SAMs) was studied. The tip wear length and wear volume were adopted to quantitatively verify the effectiveness of hydrophobic coatings. When compared with their silicon counterparts, probes modified by SAM materials exhibit superior wear-resistant behavior in tapping mode scans.     

Effect of nanometer surface morphology on surface stress and adsorption kinetics of alkanethiol self-assembled monolayers

Microcantilevers undergo quasi-static bending due to adsorption-induced stress when adsorption is confined to a single surface. Understanding the origin of surface stress is crucial for optimizing sensor performance. We have investigated the effect of surface morphology of gold-coated cantilevers on the adsorption characteristics of self-assembled monolayers of alkanethiols. Self-assembly of alkanethiols from liquid phase closely follows Langmuir-type kinetics up to a single monolayer assembly. Our results indicate that alkanethiol adsorption-induced surface stress is largely unaffected by surface roughness of the cantilever. Unlike prior reports that suggest surface roughness enhances adsorption-induced stress, we observe that nanometer-size roughness slightly decreases surface stress and adsorption kinetics.     

Immobilization of iron storage protein on a gold electrode based on self-assembled monolayers

Ferritin is a globular protein consisting of 24 subunits to form a hollow shell and is capable of storing iron in the cavity. Findings that the naturally existing iron core of ferritin can be readily extracted and replaced with a variety of electroactive materials make ferritin suitable for biosensor and biofuel cell applications. The immobilization of ferritin on the electrode surface is essential for various bioelectronic applications. In this work, based on self-assembled monolayers, ferritin was immobilized on a gold electrode through two different methods: chemisorption of thiolated ferritin onto bare gold electrodes and covalent binding of ferritin to succinimidyl alkanedisulfide-modified Au electrodes. Effects of experimental conditions on the ferritin immobilization were investigated. The ferritin immobilized on the gold electrode was characterized by atomic force microscopy and cyclic voltammetry.     

Unique domain structure of pi-conjugated tolanethioacetate self-assembled monolayers on Au(111)

Surface structures of tolanethioacetate (TTA) self-assembled monolayers (SAMs) on Au(111), formed in a 0.5mM N,N'-dimethylformamide (DMF) solution, were investigated as a function of solution temperature by scanning tunneling microscopy (STM). The STM study revealed that thioacetyl-terminated TTA molecules on Au(111) did not form ordered SAMs at room temperature and 80 degrees C, whereas the adsorption of TTA molecules on Au(111) at a solution temperature of 50 degrees C led to the formation of two mixed phases: a disordered phase and a unique ordered phase, consisting rod-like domains, which can be described as a ( radical6x radical13)15 degrees superstructure. From this study, it was demonstrated that solution temperature is a key factor controlling the two-dimensional SAM structure of TTA molecules.     

硫醇自组装膜在金表面吸附行为的AFM力曲线研究

本文用原子力显微镜（AFM）对硫醇自组装金电极表面随机取点测得力曲线，并对力曲线粘附力数据进行统计分析，结果表明随自组装液中硫醇浓度改变，粘附力特征呈现出关联性的变化。力曲线所揭示的自组装膜吸附行为的变化规律和传统电化学方法交流阻抗（EIS）测试所得结论一致，表明AFM力曲线技术可应用于自组膜吸附行为的研究。     

Dynamic variations in adhesion of self-assembled monolayers on nanoasperities probed by atomic force microscopy

Octadecyltriethoxysilane (OTE) self- assembled monolayers (SAMs) and their effects on friction and adhesion have been investigated on various combinations of functionalized and unfunctionalized silicon oxide surfaces including the oxidized surface of crystalline Si(100), silica nanoparticle films, and oxidized Si atomic force microscopy (AFM) tips. Force-distance spectroscopy was utilized to probe and compare the properties of the OTE SAMs on silica asperities with nanoscale curvature against these same monolayers on surfaces with sub-1 nm roughness (flat surfaces). It was found that adhesion between SAMs and silicon oxide surfaces can vary significantly when assembly takes place on surfaces with nanoscopic curvature as compared to flat surfaces. Observations indicate that the SAM structure present during force measurements is dynamic in nature, which yields different adhesion values when measured with variations of both tip-sample contact time and tip-approach/retract rates. These results point the need in reporting a number of measurement parameters when probing adhesion by SAM functionalized tips.