Molecular Dynamics Simulations of Alkylsilane Monolayers on Silica Nanoasperities: Impact of Surface Curvature on Monolayer Structure and Pathways for Energy Dissipation in Tribological Contacts

Self-assembled monolayers (SAMs) of alkylsilanes have been considered as wear reducing layers in tribological applications, particularly to reduce stiction and wear in microelectromechanical systems (MEMS) devices. Though these films successfully reduce interfacial forces, they are easily damaged during impact and shear. Surface roughness at the nanoscale is believed to play an important role in the failure of these films because it effects both the formation and quality of SAMs, and it focuses interfacial contact forces to very small areas, magnifying the locally applied pressure and shear on the lubricant film. To complement our prior studies employing Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) experiments in which silica nanoparticles are used to simulate nanoasperities and to refine our analysis of these films to a molecular level, classical molecular dynamics simulations have been employed to understand the impact of nanoscopic surface curvature on the properties of alkylsilane SAMs. Amorphous silica nanoparticles of various radii were prepared to simulate single asperities on a rough MEMS device surface, or AFM tips, which were then functionalized with alkylsilane SAMs of varying chain lengths. Factors related to the tribological performance of the film, including <i>gauche</i> defect density and exposed silica surface area, were examined to understand the impact of surface curvature on the film. Additionally, because the packing density of the films has been found to be relatively low for alkylsilane SAMs on surfaces with nanoscopic curvature, packing density studies were performed on simulated silica surfaces lacking curvature to understand the relative impact of these two important factors. It was found that both curvature and packing density affect the film quality; however, packing density was found to have the strongest correlation to film quality, demonstrating that greater priority should be given to the reduction of free volume within the films to improve their structural rigidity, to better passivate the underlying surfaces of the devices, and to improve the extent and accessibility of nondestructive dissipation pathways, all of which will lead to improved friction and wear resistance. While focused on silica nanoasperities, these MD simulations afford general approaches for studies of ligand effects on a range of surfaces with nanoscopic curvature such as metal oxide nanoparticles and quantum dots

Molecular Dynamics Simulations of Alkylsilane Monolayers on Silica Nanoasperities: Impact of Surface Curvature on Monolayer Structure and Pathways for Energy Dissipation in Tribological Contacts

Self-assembled monolayers (SAMs) of alkylsilanes have been considered as wear reducing layers in tribological applications, particularly to reduce stiction and wear in microelectromechanical systems (MEMS) devices. Though these films successfully reduce interfacial forces, they are easily damaged during impact and shear. Surface roughness at the nanoscale is believed to play an important role in the failure of these films because it effects both the formation and quality of SAMs, and it focuses interfacial contact forces to very small areas, magnifying the locally applied pressure and shear on the lubricant film. To complement our prior studies employing Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) experiments in which silica nanoparticles are used to simulate nanoasperities and to refine our analysis of these films to a molecular level, classical molecular dynamics simulations have been employed to understand the impact of nanoscopic surface curvature on the properties of alkylsilane SAMs. Amorphous silica nanoparticles of various radii were prepared to simulate single asperities on a rough MEMS device surface, or AFM tips, which were then functionalized with alkylsilane SAMs of varying chain lengths. Factors related to the tribological performance of the film, including <i>gauche</i> defect density and exposed silica surface area, were examined to understand the impact of surface curvature on the film. Additionally, because the packing density of the films has been found to be relatively low for alkylsilane SAMs on surfaces with nanoscopic curvature, packing density studies were performed on simulated silica surfaces lacking curvature to understand the relative impact of these two important factors. It was found that both curvature and packing density affect the film quality; however, packing density was found to have the strongest correlation to film quality, demonstrating that greater priority should be given to the reduction of free volume within the films to improve their structural rigidity, to better passivate the underlying surfaces of the devices, and to improve the extent and accessibility of nondestructive dissipation pathways, all of which will lead to improved friction and wear resistance. While focused on silica nanoasperities, these MD simulations afford general approaches for studies of ligand effects on a range of surfaces with nanoscopic curvature such as metal oxide nanoparticles and quantum dots

Driving Surface Chemistry at the Nanometer Scale Using Localized Heat and Stress

Driving and measuring chemical reactions at the nanoscale is crucial for developing safer, more efficient, and environment-friendly reactors and for surface engineering. Quantitative understanding of surface chemical reactions in real operating environments is challenging due to resolution and environmental limitations of existing techniques. Here we report an atomic force microscope technique that can measure reaction kinetics driven at the nanoscale by multiphysical stimuli in an ambient environment. We demonstrate the technique by measuring local reduction of graphene oxide as a function of both temperature and force at the sliding contact. Kinetic parameters measured with this technique reveal alternative reaction pathways of graphene oxide reduction previously unexplored with bulk processing techniques. This technique can be extended to understand and precisely tailor the nanoscale surface chemistry of any two-dimensional material in response to a wide range of external, multiphysical stimuli

Surface Curvature Enhances the Electrotunability of Ionic Liquid Lubrication

Ionic liquids (ILs) are a promising class of lubricants that allow dynamic friction control at electrified interfaces. In the real world, surfaces inevitably exhibit some degree of roughness, which can influence lubrication. In this work, we deposited single-layer graphene onto 20 nm silica nanoparticle films to investigate the effect of surface curvature and electrostatic potential on both the lubricious behavior and interfacial layering structure of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide on graphene. Normal force and friction force measurements were conducted by atomic force microscopy using a sharp silicon tip. Our results reveal that the friction coefficient at the lubricated tip–graphene contacts significantly depends on surface curvature. Two friction coefficients are measured on graphene peaks and valleys with a higher coefficient measured at lower loads (pressures), whereas only one friction coefficient is measured on smooth graphene. Moreover, the electrotunability of the friction coefficient at low loads is observed to be significantly enhanced in peaks and valleys compared with smooth graphene. This is associated with the promoted overscreening of surface charge on convex interfaces and the steric hindrance at concave interfaces, which leads to more layers of ions (electrostatically) bound to the surface, i.e., thicker boundary films (electrical double layers). This work opens new avenues to control IL lubrication on the nanoscale by combining topographic features and an electric field

Robust and Flexible Aramid Nanofiber/Graphene Layer-by-Layer Electrodes

Aramid nanofibers (ANFs), or nanoscale Kevlar fibers, are of interest for their high mechanical performance and functional nanostructure. The dispersible nature of ANFs opens up processing opportunities for creating mechanically robust and flexible nanocomposites, particularly for energy and power applications. The challenge is to manipulate ANFs into an electrode structure that balances mechanical and electrochemical performance to yield a robust and flexible electrode. Here, ANFs and graphene oxide (GO) sheets are blended using layer-by-layer (LbL) assembly to achieve mechanically flexible supercapacitor electrodes. After reduction, the resulting electrodes exhibit an ANF-rich structure where ANFs act as a polymer matrix that interfacially interacts with reduced graphene oxide sheets. It is shown that ANF/GO deposition proceeds by hydrogen bonding and π–π interactions, leading to linear growth (1.2 nm/layer pairs) and a composition of 75 wt % ANFs and 25 wt % GO sheets. Chemical reduction leads to a high areal capacitance of 221 μF/cm², corresponding to 78 F/cm³. Nanomechanical testing shows that the electrodes have a modulus intermediate between those of the two native materials. No cracks or defects are observed upon flexing ANF/GO films 1000 times at a radius of 5 mm, whereas a GO control shows extensive cracking. These results demonstrate that electrodes containing ANFs and reduced GO sheets are promising for flexible, mechanically robust energy and power

Effects of Direct Solvent-Quantum Dot Interaction on the Optical Properties of Colloidal Monolayer WS₂ Quantum Dots

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS₂ QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS₂ QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS₂ QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments

Using Particle Lithography to Tailor the Architecture of Au Nanoparticle Plasmonic Nanoring Arrays

The facile assembly of metal nanostructured arrays is a fundamental step in the design of plasmon enhanced chemical sensing and solar cell architectures. Here we have investigated methods of creating controlled formations of two-dimensional periodic arrays comprised of 20 nm Au nanoparticles (NPs) on a hydrophilic polymer surface using particle lithography. To direct the assembly process, capillary force and NP concentration both play critical roles on the resulting nanostructured arrays. As such, tuning these experimental parameters can directly be used to modify the nature of the nanostructures formed. To explore this, two different concentrations of Au NP solutions (∼7 × 10¹¹ or 4 × 10¹² NPs/mL) were used in conjunction with a fixed concentration of polystyrene microspheres (PS MS, ∼6 × 10⁹ PS MS/mL). Assembly at a relative humidity (RH) of 45% with the higher concentration resulted in the formation of well-defined Au nanorings of ca. 23 nm in height and 881 nm in diameter with a pitch of 2.5 μm. Assembly at 65% RH with the lower concentration of NPs resulted in Au nanodonut arrays comprised of isolated single Au NPs. To explore the extent of coupling in the well-defined structures, dark field scattering spectra were collected and showed a broad localized surface plasmon resonance (LSPR) peak with a shoulder, which full-wave electrodynamics modeling (finite-difference time domain (FDTD) method) attributed to be a result of pronounced particle–particle coupling along the circumference of the nanoring array

Reversible Changes in Solution pH Resulting from Changes in Thermoresponsive Polymer Solubility

Pendant groups on polymers that have lower-critical solution temperature (LCST) properties experience a water-like environment below the LCST where the polymer is soluble but are less hydrated above the LCST when the polymer phase separates from solution. When these pendant groups are amphoteric groups like carboxylate salts or ammonium salts, the change in solvation that accompanies the polymer precipitation event significantly changes these groups’ acidity or basicity. These changes in acidity or basicity can lead to carboxylate salts forming carboxylic acid groups by capturing protons from the bulk solvent or ammonium salts reverting to the neutral amine by release of protons to the bulk solvent, respectively. When polymers like poly­(<i>N</i>-isopropylacrylamide) that contain a sufficient loading of such comonomers are dissolved in solutions whose pH is near the p<i>K</i>_a of the pendant acid or basic group and undergo an LCST event, the LCST event can change the bulk solution pH. These changes are reversible. These effects were visually followed using common indicators with soluble polymers and or by monitoring solution pH as a function of temperature. LCST events triggered by the addition of a kosmotropic salt lead to similar reversible solution pH changes

Fabrication and Electrochemical Performance of Structured Mesoscale Open Shell V₂O₅ Networks

Crystalline vanadium pentoxide (V₂O₅) has attracted significant interest as a potential cathode material for energy storage applications due to its high theoretical capacity. Unfortunately, the material suffers from low conductivity as well as slow lithium ion diffusion, both of which affect how fast the electrode can be charged/discharged and how many times it can be cycled. Colloidal crystal templating (CCT) provides a simple approach to create well-organized 3-D nanostructures of materials, resulting in a significant increase in surface area that can lead to marked improvements in electrochemical performance. Here, a single layer of open shell V₂O₅ architectures ca. 1 μm in height with ca. 100 nm wall thickness was fabricated using CCT, and the electrochemical properties of these assemblies were evaluated. A decrease in polarization effects, resulting from the higher surface area mesostructured features, was found to produce significantly enhanced electrochemical performance. The discharge capacity of an unpatterned thin film of V₂O₅ (∼8.1 μAh/cm²) was found to increase to ∼10.2 μAh/cm² when the material was patterned by CCT, affording enhanced charge storage capabilities as well as a decrease in the irreversible degradation during charge–discharge cycling. This work demonstrates the importance of creating mesoscale electrode surfaces for improving the performance of energy storage devices and provides fundamental understanding of the means to improve device performance

Reversible Changes in Solution pH Resulting from Changes in Thermoresponsive Polymer Solubility

Pendant groups on polymers that have lower-critical solution temperature (LCST) properties experience a water-like environment below the LCST where the polymer is soluble but are less hydrated above the LCST when the polymer phase separates from solution. When these pendant groups are amphoteric groups like carboxylate salts or ammonium salts, the change in solvation that accompanies the polymer precipitation event significantly changes these groups’ acidity or basicity. These changes in acidity or basicity can lead to carboxylate salts forming carboxylic acid groups by capturing protons from the bulk solvent or ammonium salts reverting to the neutral amine by release of protons to the bulk solvent, respectively. When polymers like poly­(<i>N</i>-isopropylacrylamide) that contain a sufficient loading of such comonomers are dissolved in solutions whose pH is near the p<i>K</i>_a of the pendant acid or basic group and undergo an LCST event, the LCST event can change the bulk solution pH. These changes are reversible. These effects were visually followed using common indicators with soluble polymers and or by monitoring solution pH as a function of temperature. LCST events triggered by the addition of a kosmotropic salt lead to similar reversible solution pH changes