The functional implementation of graphene as a solid boundary lubricant requires the ability to control its frictional response across a variety of interfaces. This is challenging, as being a single atomic layer thick, the nanotribological properties of graphene depend highly on the competing interaction strengths with the converse sides of the top and bottom contacts of the interfaces it is placed in between. One method to modulate these interactions is to tune the surface chemistry (of one or both counter-faces) with self-assembled monolayers (SAMs). To fully understand the effects on the graphene/SAM (G-SAM) composite interfaces formed, however, first necessitates a basic understanding of graphene–SAM interactions. To explore graphene–SAM adhesive and frictional interactions over a range of chemical functionalities, SAMs were used to functionalize atomic force microscopy (AFM) tips with varying terminal end-groups (−NH2, −CH3, and –phenyl, compared to unfunctionalized −OH terminated reference tips). AFM pull-off force measurements and thermal gravimetric analysis (TGA) were used to evaluate the work of adhesion (mJ/m2) and interaction energy (kcal/mol) of the functionalized tips with graphene. Friction force microscopy (FFM) measurements were performed with the same functionalized AFM tips to examine the graphene-molecule frictional response. Tip–graphene interaction strength was increased for hydrophobic and aromatic functional groups. The frictional response was found to depend on a balance of graphene-molecule adhesion and shear strain.

Here we report on the effect of local molecular organization or “tertiary structure” on the charge transport properties of thiol-tethered tetraphenylporphyrin (ZnTPPF4-SC5SH) nanoscale clusters of ca. 5 nm in lateral dimension embedded within a dodecanethiol (C12) monolayer on Au(111). The structure of the clusters in the mixed monolayers and their resulting transport properties were monitored by Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM) and Spectroscopy (STS). The mixed films were deposited on Au(111) for a period of one to five days, during which the lateral dimensions of the ZnTPPF4-SC5SH islands that were formed after one day reduced by nearly 35% on average by five days, accompanied by a noticeable depletion of the surrounding C12 monolayer. These subtle changes in mixed monolayer morphology were accompanied by drastic differences in conductance. The ZnTPPF4-SC5SH clusters assembled for one day exhibited highly reproducible I–V spectra with simple tunneling behavior. By three days, this evolved into bias-induced switching of conductance, with a ∼100–1000 fold increase. Furthermore, current fluctuations started to become significant, and then dominated transport across the ZnTPPF4-SC5SH clusters assembled over five days. Our data suggests that this evolution can be understood by slow surface diffusion, enabling the ZnTPPF4-SC5SH molecules to overcome initial steric hindrance in the early stages of island formation in the C12 monolayer (at day one), to reach a more energetically-favored, close-packed organization, as noted by the decrease in island size (by day three). However, when desorption of the supporting matrix of C12 became pronounced (by day five), the ZnTPPF4-SC5SH clusters began to lose stabilization, and stochastic switching was then observed to dominate transport in the clusters, illustrating the critical nature of the local organization on these transport properties.

Nanoscale carbon lubricants such as graphene, have garnered increased interest as protective surface coatings for devices, but its tribological properties have been shown to depend on its interactions with the underlying substrate surface and its degree of surface conformity. This conformity is especially of interest as real interfaces exhibit roughness on the order of ∼10 nm that can dramatically impact the contact area between the graphene film and the substrate. To examine the combined effects of surface interaction strength and roughness on the frictional properties of graphene, a combination of Atomic Force Microscopy (AFM) and Raman microspectroscopy has been used to explore substrate interactions and the frictional properties of single and few-layer graphene as a coating on silica nanoparticle films, which yield surfaces that mimic the nanoscaled asperities found in realistic devices. The interactions between the graphene and the substrate have been controlled by comparing their binding to hydrophilic (silanol terminated) and hydrophobic (octadecyltrichlorosilane modified) silica surfaces. AFM measurements revealed that graphene only partially conforms to the rough surfaces, with decreasing conformity, as the number of layers increase. Under higher mechanical loading the graphene conformity could be reversibly increased, allowing for a local estimation of the out-of-plane bending modulus of the film. The frictional properties were also found to depend on the number of layers, with the largest friction observed on single layers, ultimately decreasing to that of bulk graphite. This trend however, was found to disappear, depending on the tip-sample contact area and interfacial shear strain of the graphene associated with its adhesion to the substrate.

•Compilation of computed interlayer sliding interaction parameters.•Nanoscale and macroscale tribological studies on 2D nanomaterials.•Single atomic layered lubricants.•2D nanomaterials as lubricant additives.This review focuses on recent developments in the use of 2D nanomaterials for controlling the frictional properties of surfaces and interfaces. While materials such as MoS2 and graphite have been investigated for some time, a host of other layered nanomaterials have emerged as alternatives for friction modification. These advanced lubrication schemes provide an opportunity to address growing needs in energy and materials efficiency and device compatibility, offering improved boundary and solid lubrication of contacting and sliding interfaces. Here, we describe both computational and experimental investigations of the mechanisms and implementations of 2D nanomaterials in the lubrication of interfaces.

Two different nickel precursors (NiCl2 or Ni(CH3COCH2COCH3)2) in the presence of 1-dodecanethiol and a mixture of oleylamine and oleic acid were used for a one-pot colloidal synthesis to produce high purity Ni3S4 nanoparticles with controlled shapes in high yields and narrow size distributions. By simply changing the nickel precursors, the shape of Ni3S4 nanocrystals can be readily tuned from triangular nanoprisms to tetrahedra (nanopyramids). The produced nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction, selected area electron diffraction, ultraviolet-visible spectroscopy and superconducting quantum interference device (SQUID). TEM tomography (3D-TEM) was employed to determine details of the particle shapes. SQUID measurements confirmed that particle shape and domain size could dramatically impact their magnetic properties, where tetrahedral nanopyramids of Ni3S4 nanoparticles showed ferromagnetic properties while the Ni3S4 nanoprisms showed random antiferromagnetic interactions between magnetic centers.

Self-assembled monolayers have been used extensively as surface modifications and model systems for friction and adhesion mitigation on surfaces. From experiment, it is unclear to what extent and under what conditions the substrate plays a role in the modification of these surface forces, but because SAMs are relatively compliant and thin, it is reasonable to assume that the unique frictional characteristics of these monolayers is driven in part by substrate effects. Molecular dynamics simulation and methods developed for analysis of total interaction area, and direct substrate interaction, have been employed to investigate the structure of surface asperity contacts coated with SAMs, examining these interactions and determining what role substrate interactions and other possible dissipation mechanisms are involved in the friction response of SAMs. It was observed that for sparse OTS films, typical of films formed on rough or asperity surfaces, substrate interactions are extensive, leading to increased tribochemistry and strain at sliding interfaces. For densely packed films, it was found that even pressures on the order of a few GPa do not lead to direct substrate interaction, but there is a distinct and localized increase in the compressive strain on the film, indicating the development of new dissipative modes during sliding at high pressures including conformational changes and wear of the films.

A series of experiments employing scanning tunneling microscopy (STM) have been developed for the physical chemistry laboratory. These experiments are designed to engage students in cutting edge research techniques while introducing and reinforcing topics in physical chemistry, quantum mechanics, solid-state chemistry, and the electronic structure of molecules and materials. In the first of three experiments, students are introduced to the basics of STM operation while imaging and conducting spectroscopy on the highly oriented pyrolytic graphite (HOPG) surface. Images of the surface are used to determine the crystal structure of the material, and scanning tunneling spectroscopy is used to determine the electronic properties of the material and study the tunneling phenomenon. In the second experiment, the students image the Au(111) surface as well as a series of alkanethiol self-assembled monolayers (SAMs) of different chains lengths on the Au(111) surface. They examine the structural and electronic properties of the metal surface and the adlattice structure of the film. Finally, in the third experiment, the students examine the conductance of molecules adsorbed onto the Au(111) surface, including the alkanethiol SAMs and a thiol-tethered porphyrin molecule or a dimercaptostilbene embedded into the SAM matrix. By measuring the tunneling efficiency and spectroscopic characteristics of these molecules, the students can explore the relationship between chemical structure and charge transport efficiency. The experiments provide advanced chemistry students an opportunity to view and study materials at the atomic and molecular length scales and provide an opportunity to apply their understanding of quantum mechanical concepts to real systems.Keywords: Hands-On Learning/Manipulatives; Laboratory Instruction; Nanotechnology; Physical Chemistry; Physical Properties; Spectroscopy; Surface Science; Upper-Division Undergraduate;

To achieve a better understanding of the mechanical effects of adsorbed films at surface contacts, methods were developed to map and examine the pressure distribution of nanoasperity contacts, modeled by molecular dynamics simulation. The methods employ smoothing functions to project the atomic forces obtained in contact simulation onto the contact plane for fitting to standard continuum contact models and subsequent analysis. Importantly, these methods allow for contact evolution between nanoscopic asperity–asperity contacts to be examined because these are the central load-bearing junctions at interfaces. To demonstrate the application and features of this approach, it was employed to examine the evolution of contact between silica nanoasperities, with an increasing density of octadecyltrichlorosilane (OTS) films employed as a model adsorbate film. Linearly increasing contact radius and linearly decreasing maximal pressure were observed as a function of the film packing density. Because contact between the underlying, high-energy silica surfaces is undesirable, the evolution of silica contact was also examined using these same methods. As more molecules were introduced into the contact, a sharp transition was observed from the narrow, high-pressure interaction between the underlying substrates, to a broad, substantially lower pressure interaction, indicating a sharp transition from the dry to lubricated condition. To study the dependence of these behaviors on contact morphology, silica nanoasperities in contact with a flat silica surface were also examined. Similar behavior, including the broadening of the contact area and the minimization of direct surface contact, were observed. The method developed herein is applicable to a variety of systems and can be employed to optimize surface protection and pressure redistribution by boundary lubricants. This method can also be extended to AFM adhesion measurements where a detailed understanding of the true contact area is critical for the quantitative determinations of molecular forces and local surface mechanics.

We show the suppression of luminescence quenching by metal nanoparticles (MNPs) in the plasmon enhancement of luminescence via fast sensitized energy transfer in Mn-doped quantum dots (QDs). The rapid intraparticle energy transfer between exciton and Mn, occurring on a few picoseconds time scale, separates the absorber (exciton) from the emitter (Mn), whose emission is detuned far from the plasmon of the MNP. The rapid temporal separation of the absorber and emitter combined with the reduced spectral overlap between Mn and plasmonic MNP suppresses the quenching of the luminescence while taking advantage of the plasmon-enhanced excitation. We compared the plasmon enhancement of exciton and Mn luminescence intensities in undoped and doped QDs simultaneously as a function of the distance between MNP and QD layers in a multilayer structure to examine the expected advantage of the reduced quenching in the sensitized luminescence. At the optimum MNP–QD layer distance, Mn luminescence exhibits stronger net enhancement than that of the exciton, which can be explained with a model incorporating fast sensitization along with reduced emitter–MNP spectral overlap. This study demonstrates that materials exhibiting fast sensitized luminescence that is sufficiently red-shifted from that of the sensitizer can be superior to usual luminophores in harvesting plasmon enhancement of luminescence by suppressing quenching.Keywords: doped semiconducting nanocrystals; plasmonics

Polymers like poly(N-isopropylacrylamide) (PNIPAM) exhibit lower critical solution temperature (LCST) behavior. A variety of reports have shown that brush grafts of PNIPAM on surfaces exhibit similar temperature responsiveness. We recently described an alternative synthetic approach to such surfaces that affords surfaces with similar LCST-like behavior. We also noted how such surfaces’ wettability can change in response to the identity and concentration of solutes. Here we show that this synthetic procedure can be extended to glass surfaces and to more complex surfaces present in porous glass frits. Functionalized glass surfaces exhibit solute-dependent wetting behavior analogous to that previously reported. We further show that the resulting responsive nanocomposite grafts on such frits exhibit “smart” responsive permeability with a greater than 1000-fold difference in permeability to water versus aqueous solutions of sodium sulfate. This “smart” permeability is ascribed to the solute-dependent wettability behavior of the responsive PNIPAM component of the nanocomposite graft, which is sensitive both to the identity and concentration of the solute anion and to temperature.

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 gauche 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.

In order to prepare visible-light responsive iodine-doped TiO2, a new facile synthetic approach was proposed, which started with the cost-efficient and environmentally friendly precursor of undoped anatase TiO2 to form nanotube structures as templates that collapsed and recrystallized into I-TiO2 nanopowders in HIO3 solution, followed by annealing at different temperatures. The modification of TiO2 to incorporate iodine and form titanium dioxide with significantly enhanced absorption in the visible range of the spectrum was investigated. The extent of iodine dopant incorporation was determined by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX) and was found to be homogenously distributed on each nanostructure as determined by electron energy-loss spectroscopy (EELS) elemental mapping and EDX spectroscopy. The modified TiO2 exhibits a dramatically extended absorption edge beyond 800 nm as compared to the original and unmodified TiO2.Graphical abstractAs-synthesized I-TiO2 nanoparticles show significantly enhanced visible-light absorption, with the dopant iodine homogenously dispersed on each I-TiO2 nanostructure based on EELS elemental mapping.Highlights► Iodine-TiO2 nanoparticles by a new facile two-step hydrothermal method. ► Significantly enhanced light absorption in the visible range of the spectrum. ► Homogenous dopant distribution within each nanostructure.

“Smart” acrylamide copolymers containing N-isopropyl and 4-N-amino-2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl (TEMPO) groups undergo reversible redox behavior leading to LCST changes. Oxidation (NaOCl/H2O) or reduction (ascorbic acid) changes the copolymer's LCST from 18 °C to 35–40 °C.

Nanoparticles, each consisting of one of the three molecular corrolazine (Cz) compounds, H3(TBP8Cz), MnIII(TBP8Cz), and FeIII(TBP8Cz) (TBP8Cz = octakis(4-tert-butylphenyl)corrolazinato), were prepared via a facile mixed-solvent technique. The corrolazine nanoparticles (MCz-NPs) were formed in H2O/THF (10:1) in the presence of a small amount of a polyethylene glycol derivative (TEG-ME) added as a stabilizer. This technique allows highly hydrophobic Czs to be “dissolved” in an aqueous environment as nanoparticles, which remain in solution for several months without visible precipitation. The MCz-NPs were characterized by UV−visible spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) imaging, and shown to be spherical particles from 100−600 nm in diameter with low polydispersity indices (PDI = 0.003−0.261). Particle size is strongly dependent on Cz concentration. The H3Cz-NPs were adsorbed on to a modified self-assembled monolayer (SAM) surface and imaged by atomic force microscopy (AFM). Adsorption resulted in disassembly of the larger H3Cz-NPs to smaller H3Cz-NPs, whereby the resulting particle size can be controlled by the surface energy of the monolayer. The FeIIICz-NPs were shown to be competent catalysts for the oxidation of cyclohexene with either PFIB or H2O2 as external oxidant. The reactivity and product selectivity seen for FeIIICz-NPs differs dramatically from that seen for the molecular species in organic solvents, suggesting that both the nanoparticle structure and the aqueous conditions may contribute to significant changes in the mechanism of action of the FeIIICz catalyst.

Self-assembled monolayers (SAMs) have been widely studied as potential lubricants for microelectromechanical system (MEMS) devices. However, these single-layer films have nominally been found to be insufficient for mitigating wear in sliding contacts because of their rapid breakdown under the high pressures found within the nanoasperity junctions at such interfaces. As such, there is a critical need to explore approaches beyond simple, single-component SAMs toward films that introduce additional lubricant molecules into the system. Because alcohol vapors have previously been shown to reduce wear in MEMS devices, here we have investigated a mixed monolayer consisting of an octadecyltrichlorosilane (OTS) SAM infused with 3-phenyl-1-propanol (3P1P), assembled on silica nanoparticle films. A combination of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), and FTIR spectroscopy was employed to investigate the structural and frictional properties of the mixed monolayers and to evaluate surface wear as a function of time. The nanoparticle film/AFM tip junction provides a ready mimic for the asperity−asperity contacts found in MEMS devices. Here it was found that for a mixed monolayer of OTS with ca. 15% 3P1P, the surfaces showed dramatically reduced friction and no wear under the same load conditions as surfaces with an OTS SAM alone. Moreover, the multicomponent film also displayed no increase in friction and exhibited no wear even after 14 h of shearing contact in an AFM at loads that would break down the OTS layer. The ability of the OTS SAM to trap short-chain alcohols, such as 3P1P, and to release them under load suggests a simple MEMS lubrication scheme that could be readily integrated into MEMS device architectures.