Influence of chain ordering on frictional properties of self-assembled monolayers (SAMs) in nano-lubrication

Identifying the Role of Interfacial Long-Range Order in Regulating the Solid Electrolyte Interphase in Lithium Metal Batteries

The self-assembled monolayer (SAM) technique, known for its customizable molecular segments and active end groups, is widely recognized as a powerful tool for regulating the interfacial properties of high-energy-density lithium metal batteries. However, it remains unclear how the degree of long-range order in SAMs affects the solid electrolyte interphase (SEI). In this study, we precisely controlled the hydrolysis of silanes to construct monolayers with varying degrees of long-range order and investigated their effects on the SEI nanostructure and lithium anode performance. The results indicate that the degree of long-range order in SAMs significantly influences the decomposition kinetics of the carbon-fluorine bond in lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), promoting the formation of a LiF-rich SEI and profoundly affecting the long-term stability of the highly sensitive anode during electrochemical processes. These findings provide new insights and directions for the molecular design of SAMs tailored for long-lasting lithium metal interfaces.

Friction Force Mapping of Molecular Ordering and Mesoscopic Phase Transformations in Layered-Crystalline Organic Semiconductor Films

It is critical to understand molecular ordering processes in small-molecule organic semiconductor (OSC) films in optimizing electronic device applications, although it is difficult to observe and investigate the ordering characteristics at a mesoscopic or device scale. Here, we report that friction force microscopy (FFM) allows visualizing the ordering transformation process from a thermodynamically metastable phase to a stable phase at a mesoscopic scale. We utilized 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) as a typical highly layered-crystalline OSC. We found that the friction force between an AFM tip and spin-coated OSC films significantly depends on whether local film states are in metastable monolayer phase or stable bilayer-type herringbone (b-LHB) phase that exhibits high carrier mobility. The formation of the stable b-LHB phase leads to lower friction than the metastable monolayer phase, clearly visualizing the molecular order. Force map (Fmap) analysis indicates that the lower friction in the b-LHB phase should be associated with the reduction of interfacial adhesion force. Notably, the observed results demonstrate that the spin-coated thin film changes from continuous film with the monolayer phase to rugged microcrystal grains with the b-LHB phase when left at ambient conditions. By contrast, an appropriate post-thermal annealing process facilitates the phase transformation without inducing such morphological changes. The technique provides a unique and effective tool for revealing the relationship between processing conditions and device performance in polycrystalline OSC films.

Stimuli-Responsive Oligourea Molecular Films

We have designed and synthesized a helical cysteamine-terminated oligourea foldamer composed of ten urea residues featuring side carboxyl and amine groups. The carboxyl group is located in proximity to the C-terminus of the oligourea and hence at the negative pole of the helix dipole. The amine group is located close to the N-terminus and hence at the positive pole of the helix dipole. Beyond the already remarkable dipole moment inherent in oligourea 2.5 helices, the incorporation of additional charges originating from the carboxylic and amine groups is supposed to impact the overall charge distribution along the molecule. These molecules were self-assembled into monolayers on a gold substrate, allowing us to investigate the influence of an electric field on these polar helices. By applying surface-enhanced infrared reflection-absorption spectroscopy, we proved that molecules within the monolayers tend to reorient themselves more vertically when a negative bias is applied to the surface. It was also found that surface-confined oligourea molecules affected by the external electric field tend to rearrange the electron density at urea groups, leading to the stabilization of the resonance structure with charge transfer character. The presence of the external electric field also affected the nanomechanical properties of the oligourea films, suggesting that molecules also tend to reorient in the ambient environment without an electrolyte solution. Under the same conditions, the helical oligourea displayed a robust piezoresponse, particularly noteworthy given the slender thickness of the monolayer, which measured approximately 1.2 nm. This observation demonstrates that thin molecular films composed of oligoureas may exhibit stimulus-responsive properties. This, in turn, may be used in nanotechnology systems as actuators or functional films, enabling precise control of their thickness in the range of even fractions of nanometers.

Synthesis and investigation of phosphorus-free ionic liquids as multifunctional lubricating additives

Two phosphorus-free multifunctional protic ionic liquids were devolpoed for lubricating additives. The low corrosion, excellent tribological behavior and antioxidation property make them suitable as multifunctional additives in various lubricants.

Controlled Growth of Organosilane Micropatterns on Hydrophilic and Hydrophobic Surfaces Templated by Vacuum Ultraviolet Photolithography

In this report, micropatterns of (3-aminopropyl)trimethoxysilane (APTMS) were developed on hydrophilic and hydrophobic surfaces after patterning using 172 nm vacuum ultraviolet (VUV) photolithography. Self-assembled monolayers (SAMs) formed on Si substrates through UV hydrosilylation of 1-hexadecene (HD) and 10-undecenoic acid (UDA) were used as hydrophilic and hydrophobic surfaces, respectively. For templating the HD- and UDA-SAMs, the VUV light was exposed to HD- and UDA-SAMs from the slits of photomasks in atmospheric and evacuated environments, respectively. Various oxygenated groups were generated at the exposed domains of HD-SAM, while the COOH groups were trimmed from the irradiated domains of UDA-SAM. The APTMS molecules were immobilized on the domains that were terminated by oxygenated groups after chemical vapor deposition (CVD). The thicknesses of the developed APTMS micropatterns increased significantly by raising the CVD temperature and in the presence of ambient air in the CVD Teflon container as well. The increase in thicknesses was ascribed to the formation of APTMS multilayers, which were mediated by H₃N⁺ ions. Also, the developed APTMS micropatterns on the UDA-SAM patterned by VUV light irradiation in a high-vacuum environment (HV-VUV) were thicker than those on the VUV/(O) patterned HD-SAM due to the presence of inactive oxygenated groups at the surface of VUV/(O)-terminated domains of HD-SAM such as COO-C and C-O-C groups. The presence of water or ambient air facilitated the silane coupling between the silyl groups with the oxygenated and amino groups The combination of VUV photolithography and the CVD method with control of the conditions would enable us to control the thicknesses and shapes of the developed APTMS micropatterns. These findings illustrate the applicability of VUV photolithography for templating hydrophobic and hydrophobic surfaces toward the development of organosilane architectures, which can be feasible for several applications.

Scale-Dependent Friction-Coverage Relations and Nonlocal Dissipation in Surfactant Monolayers

Surfactant molecules, known as organic friction modifiers (OFMs), are routinely added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases; however, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we investigate various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. We show that the differences between the friction-coverage relations from macroscale and nanoscale experiments are due to molecular plowing in the latter. For our small tip radii, the friction coefficient and indentation depth both have a nonmonotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. We rationalize the nonmonotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, we find that friction predominately originates from plowing of the monolayers by the leading edge of the tip, where gauche defects are created, while thermal dissipation is mostly localized in molecules toward the trailing edge of the tip, where the chains return to a more extended conformation.

UV/Ozone-Assisted Rapid Formation of High-Quality Tribological Self-Assembled Monolayer

UV/ozone (UVO)-assisted formation of self-assembled monolayer (SAM) of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS) was prepared on a glass surface. The effect of UVO exposure time on surface roughness and hydrophilicity was investigated through goniometer and atomic force microscope (AFM), and deposition time-dependent SAM quality was detected by AFM and X-ray photoelectron spectroscopy (XPS). The glass surface became smooth with UVO radiation after 10 min, and the hydrophilicity was also improved after the treatment. Confirmed by surface topography detection and chemical composition analysis, a high-quality SAM can be formed rapidly on glass with 10 min UVO treatment followed by 2 h deposition in PFDS solution. Excellent tribological performances of SAM coated with UVO treatment glass were demonstrated by friction and wear tests on AFM compared to film-deposited glass without UVO treatment and original glass. The study sheds a light on preparing high-quality lubrication and antiwear self-assembled films on the surface of engineering materials.

Vacuum Ultraviolet Treatment of Acid- and Ester-Terminated Self-Assembled Monolayers: Chemical Conversions and Friction Reduction

We have prepared COOH- and COOCH₃-terminated self-assembled monolayers (SAMs) from undec-10-enoic acid (UDA) and methyl undec-10-enoate (MUDO) molecules on hydrogen-terminated silicon (H-Si) substrates through ultraviolet (UV) irradiation. The as-prepared UDA- and MUDO-SAMs were exposed to 172 nm vacuum-UV (VUV) light in a high vacuum environment (HV, <10^-3 Pa) for different periods. The presence of COO components at the surfaces of these SAMs without prior oxidation would simplify the understanding of the origin of the chemical conversions and the changes of surface properties, as the prior oxidation would change the surface properties and generate different oxygenated groups. After the HV-VUV treatment, the abundance of COOH and COOCH₃ components of these SAMs decreased without significant dissociation of their C-C backbones. Degradation of these components occurred through dissociating their C-O bonds, resulting in different C═O components. Also, the occurrence of Norrish type pathways resulted in a slight decrease of carbon content and produced CH₃ components. We have applied the HV-VUV lithography to control the abundance of COOH and COOCH₃ components in well-defined areas and to investigate the friction differences between the irradiated and masked areas. The irradiated areas exhibited lower friction than the masked areas without observing significant height contrasts between these areas. The reduction in friction was attributed to the conversion of the COOH and COOCH₃ components to less adhesive components such as C═O and CH₃. These experiments suggest the HV-VUV treatments as an approach for low damage dry surface modifications and reductive lithographic techniques at surfaces terminated by acid and ester groups.

Low Damage Reductive Patterning of Oxidized Alkyl Self-Assembled Monolayers through Vacuum Ultraviolet Light Irradiation in an Evacuated Environment

Through 172 nm vacuum ultraviolet light irradiation in a high vacuum condition (HV-VUV), well-defined micropatterns with a varied periodic friction were fabricated at the surface of self-assembled monolayers (SAMs) terminated with oxygenated groups. No apparent height contrast between the HV-VUV-irradiated and -masked areas was observed, which indicated the stability of the C-C skeleton of the assembled molecules. The trimming of oxygenated groups occurred through dissociating the C-O bonds and promoting the occurrence of α- and β-cleavages in the C═O-containing components. Hence, the HV-VUV treatment trimmed the oxygenated groups without degrading the C-C skeleton. The HV-VUV treatment influenced the order of the assembled molecules, and the step-terrace structure was distorted. The decrease in friction at the HV-VUV-irradiated domains was attributed to the dissociation of oxygenated groups. (3-Aminopropyl)trimethoxysilane (APTMS) aggregated at the masked areas of the HV-VUV-patterned SAM, where the oxygenated groups worked as anchors. APTMS aggregations did not exist at the irradiated areas, indicating the trimming of the oxygenated groups at these areas. The direct assembling of APTMS on the Si substrate at the irradiated areas was prevented by the remaining C-C skeleton.

Role of oxygen functional groups in reduced graphene oxide for lubrication

Functionalized and fully characterized graphene-based lubricant additives are potential 2D materials for energy-efficient tribological applications in machine elements, especially at macroscopic contacts. Two different reduced graphene oxide (rGO) derivatives, terminated by hydroxyl and epoxy-hydroxyl groups, were prepared and blended with two different molecular weights of polyethylene glycol (PEG) for tribological investigation. Epoxy-hydroxyl-terminated rGO dispersed in PEG showed significantly smaller values of the friction coefficient. In this condition, PEG chains intercalate between the functionalized graphene sheets, and shear can take place between the PEG and rGO sheets. However, the friction coefficient was unaffected when hydroxyl-terminated rGO was coupled with PEG. This can be explained by the strong coupling between graphene sheets through hydroxyl units, causing the interaction of PEG with the rGO to be non- effective for lubrication. On the other hand, antiwear properties of hydroxyl-terminated rGO were significantly enhanced compared to epoxy-hydroxyl functionalized rGO due to the integrity of graphene sheet clusters.

Load-Induced Frictional Transition at a Well-Defined Alkane Loop Surface

Self-assembled monolayers (SAMs) have attracted considerable attention as a tool to confer desirable properties on material surfaces. So far, molecules used for the SAM formation are generally limited to linear ones and thus chain ends dominate the surface properties. In this study, we have successfully demonstrated unique frictional properties of a SAM composed of alkane loops from cyclic alkanedisulfide on a gold substrate, where both sulfurs are bound to gold. The frictional response was proportional to the load. However, once the load went beyond a threshold value, the frictional response became more dominant. Such a frictional transition was reversible and repeatable and was not discerned for a corresponding SAM composed of n-alkyl chains. The load-induced change in the frictional response from the alkane loops could be associated with the conformational change of the alkane loops. The present results differ from most studies, in which the surface properties are designed on the basis of functional chain end groups.

Lubrication of Individual Microcontacts by a Self-Assembled Alkyl Phosphonic Acid Monolayer on α-Al2O3(0001)

We report on the tribological behavior of a self-assembled alkyl phosphonic acid monolayer on the microscale using the colloidal probe technique. Friction-load data and adhesion forces were measured with borosilicate glass particles on uncoated and octadecylphosphonic acid (ODPA) coated α-Al2O3(0001) surfaces. A significant decrease in friction force was observed after surface coating, while the adhesion force was only moderately reduced. We assume the lubrication effect of the ODPA self-assembled monolayer (SAM) to be close to the maximum obtainable of alkyl phosphonic acids in the studied system due to the high molecular order which was confirmed by vibrational sum-frequency generation. At small loads, a nonlinear dependence of friction force to load was maintained after surface coating. However, a shift from a contact behavior well described by the DMT model toward the JKR model occurred that is possibly related to the altered elastic properties of the coated surface. With increasing load, a linear friction-load behavior was observed on the coated samples. Molecular plowing and adhesive interactions were identified as responsible mechanisms. In all friction experiments, we could not detect any wear neither of the colloidal probes nor at the surfaces of uncoated and coated samples. This proves the high wear resistivity of the studied ODPA SAM.

Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM Parameters in Order to Achieve Sub-100-nm Spatially Resolved Epoxidation of Alkenes Grafted onto a Surface

Scanning probe lithography (SPL) appears to be a reliable alternative to the use of masks in traditional lithography techniques as it offers the possibility of directly producing specific chemical functionalities with nanoscale spatial control. We have recently extend the range of applications of catalytic SPL (cSPL) by introducing a homogeneous catalyst immobilized on the apex of a scanning probe. Here we investigate the importance of atomic force microscopy (AFM) physical parameters (applied force, writing speed, and interline distance) on the resultant chemical activity in this cSPL methodology through the direct topographic observation of nanostructured surfaces. Indeed, an alkene-terminated self-assembled monolayer (alkene-SAM) on a silicon wafer was locally epoxidized using a scanning probe tip with a covalently grafted manganese complex bearing the 1,4,7-triazacyclononane macrocycle as the ligand. In a post-transformation process, N-octylpiperazine was covalently grafted to the surface via a selective nucleophilic ring-opening reaction. With this procedure, we could write various patterns on the surface with high spatial control. The catalytic AFM probe thus appears to be very robust because a total area close to 500 μm(2) was patterned without any noticeable loss of catalytic activity. Finally, this methodology allowed us to reach a lower lateral line resolution down to 40 nm, thus being competitive and complementary to the other nanolithographical techniques for the nanostructuration of surfaces.

Dielectric relaxation properties of carboxylic acid-terminated n-alkyl monolayers tethered to Si(1 1 1): dynamics of dipoles and gauche defects

Molecular-level insights into the organization and dynamics of n-alkyl monolayers covalently bonded to Si(1 1 1) were gained from admittance measurements of dipolar relaxation in rectifying Hg|| HOOC-C10H(25-n) Si junctions performed as a function of applied voltage and temperature. A collective behavior of dipole dynamics is inferred from the non-Debye asymmetric relaxation peak shape and strong coupling of the dipole relaxation path with some bending vibrations of the n-alkyl OML (multi-excitation entropy model). A variety of relaxation mechanisms is observed in the frequency range (0.1 Hz-10 MHz) with different dependence of relaxation frequency and dipolar strength on measurement temperature and applied voltage. Their microscopic origin is discussed by comparing the activation energy of relaxation frequency with previous molecular mechanics calculations of saddle point energy barriers for structural defects such as gauche conformations or chain kinks in n-alkanes assemblies. Gauche conformations organized in pairs (kinks) have vanishing relaxation strength below an order-disorder transition temperature T(D) =  175 K and their probability strongly increases with applied reverse voltage, above T(D). The presence of hydrogen bonds between terminal carboxylic acid functionalities is inferred from a comparison with a similar junction bearing a low density of carboxylic acid end groups. This temperature-dependent hydrogen-bond network provides some additional stiffness against external electrostatic stress, as deduced from the rather weak sensitivity of relaxation frequencies to applied bias voltage.

Microtribological properties of a spin-coated thin film of 1-butyl-3-(propyltrimethoxysilane)imidazolium bis(mandelato)borate ionic liquid

A thin film of 1-butyl-3-(propyltrimethoxysilane)imidazolium bis(mandelato)borate ionic liquid prepared by spin coating on silicon surface exhibited significantly low friction and excellent wear-resistivity.

Challenges and developments of self-assembled monolayers and polymer brushes as a green lubrication solution for tribological applications

Surface initiated polymer brushes, grafted from self-assembled initiating monolayers on a contact surface, provide opportunities to develop innovative solutions for friction reduction in tribological systems.