Nanotribology and Nanofabrication of MoO3 Structures by Atomic Force Microscopy

Bifunctional electrocatalytic hybrid heterostructures for polysulfide anchoring/conversion for a stable lithium-sulfur battery

In situ phase engineering of transition metal dichalcogenides (TMDs) with controlled sulfur vacancies offers a promising strategy for superior-performance lithium-sulfur (Li-S) batteries. Herein, we demonstrate a bifunctional approach by designing a sulfur host material using 1T-MoS2/MoO3 heterostructures grown directly on carbon nanopot-resembling designer structures (CMS). The metallic phase (1T-MoS2) with MoO3 synergistically contributes to exceptional electronic transport, increased interlayer spacing, and more electrochemically active sites across its basal plane. Carbon nanopot structures and sulfur vacancies within the TMDs act as anchoring sites for lithium polysulfides (LiPSs). Additionally, the specifically phase-engineered 2D heterostructure promotes their efficient conversion into the electrochemically favorable Li2S phase. This dual functionality is expected to significantly improve the rate capability and cycle life stability of Li-S batteries. This translates to a high reversible rate capacity of 1205 mA h g-1 at a current density of 0.2 A g-1. The sulfur-loaded CMS nanostructure shows an excellent cycling life with a decay rate of only 0.078% over 1100 cycles at 1 A g-1, underscoring the effectiveness of the in situ phase engineering approach for creating a stable Li-S battery.

Defect-Free Nanowelding of Bilayer SnSe Nanoplates

Nanowelding is a bottom-up technique to create custom-designed nanostructures and devices beyond the precision of lithographic methods. Here, a new technique is reported based on anisotropic lubricity at the van der Waals interface between monolayer and bilayer SnSe nanoplates and a graphene substrate to achieve precise control of the crystal orientation and the interface during the welding process. As-grown SnSe monolayer and bilayer nanoplates are commensurate with graphene's armchair direction but lack commensuration along graphene's zigzag direction, resulting in a reduced friction along that direction and a rail-like, 1D movement that permits joining nanoplates with high precision. This way, molecular beam epitaxially grown SnSe nanoplates of lateral sizes 30-100 nm are manipulated by the tip of a scanning tunneling microscope at room temperature. In situ annealing is applied afterward to weld contacting nanoplates without atomic defects at the interface. This technique can be generalized to any van der Waals interfaces with anisotropic lubricity and is highly promising for the construction of complex quantum devices, such as field effect transistors, quantum interference devices, lateral tunneling junctions, and solid-state qubits.

van der Waals Epitaxy, Superlubricity, and Polarization of the 2D Ferroelectric SnS

Tin monosulfide (SnS) is a two-dimensional layered semiconductor that exhibits in-plane ferroelectric order at very small thicknesses and is of interest in highly scaled devices. Here we report the epitaxial growth of SnS on hexagonal boron nitride (hBN) using a pulsed metal-organic chemical vapor deposition process. Lattice matching is observed between the SnS(100) and hBN{11̅0} planes, with no evidence of strain. Atomic force microscopy reveals superlubricity along the commensurate direction of the SnS/hBN interface, and first-principles calculations suggest that friction is controlled by the edges of the SnS islands, rather than interface interactions. Differential phase contrast imaging detects remnant polarization in SnS islands with domains that are not dictated by step-edges in the SnS. The growth of ferroelectric SnS on high quality hBN substrates is a promising step toward electrically switchable ferroelectric semiconducting devices.

Registry-Dependent Peeling of Layered Material Interfaces: The Case of Graphene Nanoribbons on Hexagonal Boron Nitride

Peeling of layered materials from supporting substrates, which is central for exfoliation and transfer processes, is found to be dominated by lattice commensurability effects in both low and high velocity limits. For a graphene nanoribbon atop a hexagonal boron nitride surface, the microscopic peeling behavior ranges from stick-slip, through smooth-sliding, to pure peeling regimes, depending on the relative orientation of the contacting surfaces and the peeling angle. The underlying mechanisms stem from the intimate relation between interfacial registry, interlayer interactions, and friction. This, in turn, allows for devising simple models for extracting the interfacial adhesion energy from the peeling force traces.

Computational Prediction of Superlubric Layered Heterojunctions

Structural superlubricity has attracted increasing interest in modern tribology. However, experimental identification of superlubric interfaces among the vast number of heterojunctions is a trial-and-error and time-consuming approach. In this work, based on the requirements on the in-plane stiffnesses of layered materials and the interfacial interactions at the sliding incommensurate interfaces of heterojunctions for structural superlubricity, we propose criteria for predicting structural superlubricity between heterojunctions. Based on these criteria, we identify 61 heterojunctions with potential superlubricity features from 208 candidates by screening the data of first-principles calculations. This work provides a universal route for accelerating the discovery of new superlubric heterojunctions.

Pervasive orientational and directional locking at geometrically heterogeneous sliding interfaces

Understanding the drift motion and dynamical locking of crystalline clusters on patterned substrates is important for the diffusion and manipulation of nano- and microscale objects on surfaces. In a previous work, we studied the orientational and directional locking of colloidal two-dimensional clusters with triangular structure driven across a triangular substrate lattice. Here we show with experiments and simulations that such locking features arise for clusters with arbitrary lattice structure sliding across arbitrary regular substrates. Similar to triangular-triangular contacts, orientational and directional locking are strongly correlated via the real- and reciprocal-space Moiré patterns of the contacting surfaces. Due to the different symmetries of the surfaces in contact, however, the relation between the locking orientation and the locking direction becomes more complicated compared to interfaces composed of identical lattice symmetries. We provide a generalized formalism which describes the relation between the locking orientation and locking direction with arbitrary lattice symmetries.

Nanoscale Friction Behavior of Transition-Metal Dichalcogenides: Role of the Chalcogenide

Despite extensive research on the tribological properties of MoS₂, the frictional characteristics of other members of the transition-metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into the factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS₂, MoSe₂, and MoTe₂ in both bulk and monolayer forms through a combination of atomic force microscopy experiments and molecular dynamics simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS₂ has the highest friction among these materials and MoTe₂ has the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between the TMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.

Structural lubricity in soft and hard matter systems

Over the recent decades there has been tremendous progress in understanding and controlling friction between surfaces in relative motion. However the complex nature of the involved processes has forced most of this work to be of rather empirical nature. Two very distinctive physical systems, hard two-dimensional layered materials and soft microscopic systems, such as optically or topographically trapped colloids, have recently opened novel rationally designed lines of research in the field of tribology, leading to a number of new discoveries. Here, we provide an overview of these emerging directions of research, and discuss how the interplay between hard and soft matter promotes our understanding of frictional phenomena.

Structural lubricity is one of the most interesting concepts in modern tribology, which promises to achieve ultra-low friction over a wide range of length-scales. Here the authors highlight novel research lines in this area achievable by combining theoretical and experimental efforts on hard two-dimensional materials and soft colloidal and cold ion systems.

Friction Anisotropy of MoS2: Effect of Tip-Sample Contact Quality

Atomic-scale friction measured for a single asperity sliding on 2D materials depend on the direction of scanning relative to the material's crystal lattice. Here, nanoscale friction anisotropy of wrinkle-free bulk and monolayer MoS₂ is characterized using atomic force microscopy and molecular dynamics simulations. Both techniques show 180° periodicity (2-fold symmetry) of atomic-lattice stick-slip friction vs. the tip's scanning direction with respect to the MoS₂ surface. The 60° periodicity (6-fold symmetry) expected from the MoS₂ surface's symmetry is only recovered in simulations where the sample is rotated, as opposed to the scanning direction changed. All observations are explained by the potential energy landscape of the tip-sample contact, in contrast with nanoscale topographic wrinkles that have been proposed previously as the source of anisotropy. These results demonstrate the importance of the tip-sample contact quality in determining the potential energy landscape and, in turn, friction at the nanoscale.

Structural Superlubricity Based on Crystalline Materials

Herein, structural superlubricity, a fascinating phenomenon where the friction is ultralow due to the lateral interaction cancellation resulted from incommensurate contact crystalline surfaces, is reviewed. Various kinds of nano- and microscale materials such as 2D materials, metals, and compounds are used for the fabrication. For homogeneous frictional pairs, superlow friction forces exist in most relative orientations with incommensurate configuration. Heterojunctions bear no resemblance to homogeneous contact, since the lattice constants are naturally mismatched which leads to a robust structural superlubricity with any orientation of the two different surfaces. A discussion on the perspectives of this field is also provided to meet the existing challenges and chart the future.

Origin of Nanoscale Friction Contrast between Supported Graphene, MoS2, and a Graphene/MoS2 Heterostructure

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS₂. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS₂, and a graphene/MoS₂ heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS₂ monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS₂ monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

Structural superlubricity and ultralow friction across the length scales

Structural superlubricity, a state of ultralow friction and wear between crystalline surfaces, is a fundamental phenomenon in modern tribology that defines a new approach to lubrication. Early measurements involved nanometre-scale contacts between layered materials, but recent experimental advances have extended its applicability to the micrometre scale. This is an important step towards practical utilization of structural superlubricity in future technological applications, such as durable nano- and micro-electromechanical devices, hard drives, mobile frictionless connectors, and mechanical bearings operating under extreme conditions. Here we provide an overview of the field, including its birth and main achievements, the current state of the art and the challenges to fulfilling its potential.

Molecules on rails: friction anisotropy and preferential sliding directions of organic nanocrystallites on two-dimensional materials

Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants for nanomechanical systems. So far, their frictional properties at the nanoscale have been studied by standard friction force microscopy. However, lateral manipulation of nanoparticles is a more suitable method to study the dependence of friction on the crystallography of two contacting surfaces. Still, such experiments are lacking. In this study, we combine atomic force microscopy (AFM) based lateral manipulation and molecular dynamics simulations in order to investigate the movements of organic needle-like nanocrystallites grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We observe that nanoneedle fragments - when pushed by an AFM tip - do not move along the original pushing directions. Instead, they slide on the 2D materials preferentially along the needles' growth directions, which act as invisible rails along commensurate directions. Further, when the nanocrystallites were rotated by applying a torque with the AFM tip across the preferential sliding directions, we find an increase of the torsional signal of the AFM cantilever. We demonstrate in conjunction with simulations that both, the significant friction anisotropy and preferential sliding directions are determined by the complex epitaxial relation and arise from the commensurate and incommensurate states between the organic nanocrystallites and the 2D materials.

Nanoserpents: Graphene Nanoribbon Motion on Two-Dimensional Hexagonal Materials

We demonstrate snake-like motion of graphene nanoribbons atop graphene and hexagonal boron nitride ( h-BN) substrates using fully atomistic nonequilibrium molecular dynamics simulations. The sliding dynamics of the edge-pulled nanoribbons is found to be determined by the interplay between in-plane ribbon elasticity and interfacial lattice mismatch. This results in an unusual dependence of the friction-force on the ribbon's length, exhibiting an initial linear rise that levels-off above a junction-dependent threshold value dictated by the pre-slip stress distribution within the slider. As part of this letter, we present the LAMMPS implementation of the registry-dependent interlayer potentials for graphene, h-BN, and their heterojunctions that were used herein, which provides enhanced performance and accuracy.

Structural superlubricity in graphite flakes assembled under ambient conditions

To date, structural superlubricity in microscale contacts is mostly observed in intrinsic graphite flakes that are cleaved by shearing from HOPG mesas in situ or friction pairs assembled in vacuum due to the high requirement of ultra clean interface for superlubricity, which severely limits their practical applications. Herein, we report observations on microscale structural superlubricity in graphite flake pairs assembled under ambient conditions, where contaminants are inevitably present at the interfaces. For such friction pairs, we find a novel running-in phenomenon, where the friction decreases with reciprocating motions, but no morphological or chemical changes can be observed. The underlying mechanism for the new running-in process is revealed to be the removal of third bodies confined between the surfaces. Our results improve the understanding of microscale superlubricity and may help extend the practical applications of superlubricity.

Friction between van der Waals Solids during Lattice Directed Sliding

Nanometer-scale crystals of the two-dimensional oxide molybdenum trioxide (MoO₃) were formed atop the transition metal dichalcogenides MoS₂ and MoSe₂. The MoO₃ nanocrystals are partially commensurate with the dichalcogenide substrates, being aligned only along one of the substrate's crystallographic axes. These nanocrystals can be slid only along the aligned direction and maintain their alignment with the substrate during motion. Using an AFM probe to oscillate the nanocrystals, it was found that the lateral force required to move them increased linearly with nanocrystal area. The slope of this curve, the interfacial shear strength, was significantly lower than for macroscale systems. It also depended strongly on the duration and the velocity of sliding of the crystal, suggesting a thermal activation model for the system. Finally, it was found that lower commensuration between the nanocrystal and the substrate increased the interfacial shear, a trend opposite that predicted theoretically.

Tribological Performance of Green Lubricant Enhanced by Sulfidation IF-MoS₂

Biopolymers reinforced with nanoparticle (NP) additives are widely used in tribological applications. In this study, the effect of NP additives on the tribological properties of a green lubricant hydroxypropyl methylcellulose (HPMC) composite was investigated. The IF-MoS₂ NPs were prepared using the newly developed gas phase sulfidation method to form a multilayered, polyhedral structure. The number of layers and crystallinity of IF-MoS₂ increased with sulfidation time and temperature. The dispersity of NPs in the HPMC was investigated using Raman and EDS mapping and showed great uniformity. The use of NPs with HPMC enhanced the tribological performance of the composites as expected. The analysis of the worn surface shows that the friction behavior of the HPMC composite with added NPs is very sensitive to the NP structure. The wear mechanisms vary with NP structure and depend on their lubricating behaviors.

Single-Molecule Tribology: Force Microscopy Manipulation of a Porphyrin Derivative on a Copper Surface

The low-temperature mechanical response of a single porphyrin molecule attached to the apex of an atomic force microscope (AFM) tip during vertical and lateral manipulations is studied. We find that approach-retraction cycles as well as surface scanning with the terminated tip result in atomic-scale friction patterns induced by the internal reorientations of the molecule. With a joint experimental and computational effort, we identify the dicyanophenyl side groups of the molecule interacting with the surface as the dominant factor determining the observed frictional behavior. To this end, we developed a generalized Prandtl-Tomlinson model parametrized using density functional theory calculations that includes the internal degrees of freedom of the side group with respect to the core and its interactions with the underlying surface. We demonstrate that the friction pattern results from the variations of the bond length and bond angles between the dicyanophenyl side group and the porphyrin backbone as well as those of the CN group facing the surface during the lateral and vertical motion of the AFM tip.

High photocatalytic performance of a type-II α-MoO3@MoS2heterojunction: from theory to experiment

A remarkably enhanced photocatalytic ability of a α-MoO₃@MoS₂hybrid rod@sphere structure was obtained.

A simple criterion for determining the static friction force between nanowires and flat substrates using the most-bent-state method

A simple criterion was developed to assess the appropriateness of the currently available models that estimate the static friction force between nanowires and substrates using the 'most-bent-state' method. Our experimental testing of the static friction force between Al2O3 nanowires and Si substrate verified our theoretical analysis, as well as the establishment of the criterion. It was found that the models are valid only for the bent nanowires with the ratio of wire length over the minimum curvature radius [Formula: see text] no greater than 1. For the cases with [Formula: see text] greater than 1, the static friction force was overestimated as it neglected the effect of its tangential component.

New growth modes of molybdenum oxide layered 1D structures using alternative catalysts: transverse mode vs. axial mode

Both transverse and axial growth modes were discovered in the CVD synthesis of molybdenum oxide (MoO₃) 1D structures using alkali metal based catalysts. A modified vapor–solid–solid (VSS) mechanism was proposed.

Striped nanoscale friction and edge rigidity of MoS2layers

Lateral force microscopy (LFM) is used to probe the nanoscale elastic and frictional characteristics of molybdenum disulfide (MoS₂).

Channeling motion of gold nanospheres on a rippled glassed surface

Gold nanospheres have been manipulated by atomic force microscopy on a rippled glass surface produced by ion beam sputtering and coated with an ultrathin (10 nm thick) graphitic layer. This substrate is characterized by irregular wavy grooves running parallel to a preferential direction. Measurements in ambient conditions show that the motion of the nanoparticles is confined to single grooves ('channels'), along which the particles move till they are trapped by local bottlenecks. At this point, the particles cross the ripple pattern in a series of consecutive jumps and continue their longitudinal motion along a different channel. Moreover, due to the asymmetric shape of the ripple profiles, the jumps occur in the direction of minimum slope, resembling a ratchet mechanism. Our results are discussed, extending a collisional model, which was recently developed for the manipulation of nanospheres on flat surfaces, to the specific geometry of this problem.

Intercalation anode material for lithium ion battery based on molybdenum dioxide

MoO2 is one of the most studied anode systems in lithium ion batteries. Previously, the reaction of MoO2 with lithium via conversion reaction has been widely studied. The present study highlights the possible application of MoO2 as an intercalation-based anode material to improve the safety of lithium ion batteries. Nanobelts of MoO2 are prepared by reduction of MoO3 nanobelts under hydrogen atmosphere. The intercalation behavior of MoO2 is specially focused upon by limiting the charge-discharge cycling to narrow potential window of 1.0 to 2.2 V vs Li/Li(+) to avoid conversion reaction. An excellent electrochemical stability over 200 cycles is achieved at a current rate of 100 mAh g(-1). A phase transformation from monoclinic to orthorhombic to monoclinic is observed during the lithiation process, which is reversible during delithiation process and is confirmed by ex-situ XRD and electrochemical impedance spectroscopy. To further demonstrate the viability of MoO2 as a commercial anode material, MoO2 is tested in a full-cell configuration against LiFePO4. The full-cell assembly is cycled for 100 cycles and stable performance is observed. The combination showed an energy density of 70 Wh kg(-1) after 100 cycles.

Scaling laws of structural lubricity

"Structural lubricity" refers to a unique friction state in which two flat surfaces are sliding past each other with ultralow resistance due to incommensurate atomic lattice structures. In this case, theory anticipates sublinear scaling for the area dependence of friction. Here, we experimentally confirm these predictions by measuring the sliding resistance of amorphous antimony and crystalline gold nanoparticles on crystalline graphite. For the amorphous particles a square root relation between friction and contact area is observed. For crystalline gold particles we find a more complex scaling behavior related to variations in particle shape and orientation. These results allow us to link mesoscopic friction to atomic principles.

Spinning and translational motion of Sb nanoislands manipulated on MoS2

Antimony nanoislands grown on a MoS2 surface in ultra-high vacuum have been manipulated by atomic force microscopy (AFM) in ambient conditions. The island profiles have been digitized and provided as an input to a collisional algorithm based on classical mechanics. Assuming that the islands are rigid and static friction is high enough to prevent further motion after the passage of the probing tip, the direction of motion and the angle of rotation of the islands have been reproduced numerically. For a given spacing between the scan lines, the angle of deflection with respect to the fast scan direction and the angular speed of the islands are expected to vary with the friction between islands and substrate. From a comparison between model and experiment a shear strength in the order of 0.2 MPa is estimated.

Observation of high-speed microscale superlubricity in graphite

A sheared microscopic graphite mesa retracts spontaneously to minimize interfacial energy. Using an optical knife-edge technique, we report first measurements of the speeds of such self-retracting motion (SRM) from the mm/s range at room temperature to 25 m/s at 235°C [corrected]. This remarkably high speed is comparable with the upper theoretical limit found for sliding interfaces exhibiting structural superlubricity. We observe a strong temperature dependence of SRM speed which is consistent with a thermally activated mechanism of translational motion that involves successive pinning and depinning events at interfacial defects. The activation energy for depinning is estimated to be 0.1-1 eV.

Measurement of the friction between single polystyrene nanospheres and silicon surface using atomic force microscopy

In the present work, the individual nanoparticles have been manipulated on a silicon surface, using atomic force microscope (AFM) techniques. As a model system, near-spherical polystyrene nanoparticles with radii from 28.85 nm to 228.2 nm were deposited on a nanosmooth silicon wafer. Experiments demonstrated that when the normal force is above a threshold load, nanoparticles could steadily be pushed by the tip of the AFM along the defined pathway. The tests allow us to quantitatively study the interfacial friction between the nanoparticle and the surface. It was found that the friction could be affected by various factors such as the load, the particle size, and the surface treatment. The results showed that the friction between particles and substrate is proportional to the two-third power of the radius, which is in agreement with the Hertzian theory. It can also be seen that the ratio between the kinetic and the static friction was slightly changed from 0.3 to 0.6, depending on the size of the particles. However, the value of the ratio was little affected by other factors such as the particles' location, the tip normal force and the surface modification. The results provided new insights into the intriguing friction phenomenon on the nanoscale.

Robust Superlubricity in Graphene/h-BN Heterojunctions

The sliding energy landscape of the heterogeneous graphene/h-BN interface is studied by means of the registry index. For a graphene flake sliding on top of h-BN, the anisotropy of the sliding energy corrugation with respect to the misfit angle between the two naturally mismatched lattices is found to reduce with the flake size. For sufficiently large flakes, the sliding energy corrugation is expected to be at least an order of magnitude lower than that obtained for matching lattices regardless of the relative interlayer orientation. Therefore, in contrast to the case of the homogeneous graphene interface where flake reorientations are known to eliminate superlubricty, here, a stable low-friction state is expected to occur. Our results mark heterogeneous layered interfaces as promising candidates for dry lubrication purposes.

Stability, dynamics, and lubrication of MoS2 platelets and nanotubes

A model is introduced to investigate structure, stability, dynamics, and properties of MoS(2). The tribological behavior of the material is obtained from the autocorrelation function, ACF, of the forces, using the Green-Kubo equation, and by the classical Amontons' laws. In the idealized system, i.e. without defects, junctions, vacancies, asperities, and impurities, both models find a superlubrication regime, in agreement with some experiments. In nanotubes, NTs, friction is an order of magnitude lower than in the layered systems. The calculations also show that there is a substantial stabilization, per atom, for the formation of multiwall NTs with at least four walls.

Static friction between silicon nanowires and elastomeric substrates

This paper reports the first direct measurements of static friction force and interfacial shear strength between silicon (Si) nanowires (NWs) and poly(dimethylsiloxane) (PDMS). A micromanipulator is used to manipulate and deform the NWs under a high-magnification optical microscope in real time. The static friction force is measured based on "the most-bent state" of the NWs. The static friction and interface shear strength are found to depend on the ultraviolet/ozone (UVO) treatment of PDMS. The shear strength starts at 0.30 MPa without UVO treatment, increases rapidly up to 10.57 MPa at 60 min of treatment and decreases for longer treatment. Water contact angle measurements suggest that the UVO-induced hydrophobic-to-hydrophilic conversion of PDMS surface is responsible for the increase in the static friction, while the hydrophobic recovery effect contributes to the decrease. The static friction between NWs and PDMS is of critical relevance to many device applications of NWs including NW-based flexible/stretchable electronics, NW assembly and nanocomposites (e.g., supercapacitors). Our results will enable quantitative interface design and control for such applications.

Strain-release assembly of nanowires on stretchable substrates

A simple yet effective method for assembly of highly aligned nanowires (NWs) on stretchable substrates is reported. In this method, NWs were first transferred to a strained stretchable substrate. After the strain was released, the NWs aligned in the transverse direction and the area coverage of the NWs on the substrate increased. This method can be applied to any NWs deposited on a stretchable film and can be repeated multiple times to increase the alignment and density of the NWs. For silver (Ag) and silicon (Si) NWs on poly(dimethylsiloxane) (PDMS) substrates, the probability of NW alignment increased from 29% to 90% for Ag NWs, and from 25% to 88% for Si NWs after two assembly steps; the density increased by 60% and 75% for the Ag and Si NWs, respectively. The large-strain elasticity of the substrate and the static friction between the NWs and the substrate play key roles in this assembly method. We find that a model that takes into account the volume incompressibility of PDMS reliably predicts the degree of NW alignment and NW density. The utility of this assembly method was demonstrated by fabricating a strain sensor array composed of aligned Si NWs on a PDMS substrate, with a device yield of 95%.

The interaction of an atomic force microscope tip with a nano-object: a model for determining the lateral force

A calculation of the lateral force interaction between an atomic force microscope (AFM) tip and a nano-object on a substrate is presented. In particular, the case where the AFM tip is used to manipulate the nano-object is considered; i.e., the tip is displaced across the nano-object with the feedback off. The Hamaker equations are used to calculate the force when the tip and sample are not in contact and the Johnson, Kendall and Roberts (JKR) or Derjaguin, Muller and Toporov (DMT) formalisms are used for the contact force. The effect of the material parameters, the choice of contact theory and the shape of the nano-object on the resulting lateral forces are explored. The calculation is applied to an experimental system consisting of a cadmium selenide nanorod on graphite.

Tribo-electrochemical surface modification of tantalum using in situ AFM techniques

A scanning-probe-based technique to observe tribo-electrochemically stimulated surface was demonstrated. The configuration consists of an electrochemical cell attached to an atomic force microscope (AFM) scanner. Under an applied electrical potential and in various chemical environments, the surface morphology, roughness, skew, bearing ratio, as well as surface adhesive forces between probes were measured, and the effects of mechano-electrochemical stimuli were evaluated. The effects of mechanical, electrochemical, and mechano-electrochemical stimuli were found to compete during AFM sliding process. Their effects do not follow a linear relationship, implying that the mechanical stimulus promotes electrochemical reactions. Similarly, electrochemically enhanced mechanical removal of surface materials is possible.

Data acquisition and analysis procedures for high-resolution atomic force microscopy in three dimensions

Data acquisition and analysis procedures for noncontact atomic force microscopy that allow the recording of dense three-dimensional (3D) surface force and energy fields with atomic resolution are presented. The main obstacles for producing high-quality 3D force maps are long acquisition times that lead to data sets being distorted by drift, and tip changes. Both problems are reduced but not eliminated by low-temperature operation. The procedures presented here employ an image-by-image data acquisition scheme that cuts measurement times by avoiding repeated recording of redundant information, while allowing post-acquisition drift correction. All steps are detailed with the example of measurements performed on highly oriented pyrolytic graphite in ultrahigh vacuum at a temperature of 6 K. The area covered spans several unit cells laterally and vertically from the attractive region to where no force could be measured. The resulting fine data mesh maps piconewton forces with <7 pm lateral and<2 pm vertical resolution. From this 3D data set, two-dimensional cuts along any plane can be plotted. Cuts in a plane parallel to the sample surface show atomic resolution, while cuts along the surface normal visualize how the attractive atomic force fields extend into vacuum. At the same time, maps of the tip-sample potential energy, the lateral tip-sample forces, and the energy dissipated during cantilever oscillation can be produced with identical resolution.

Manipulation of cadmium selenide nanorods with an atomic force microscope

We have used an atomic force microscope (AFM) to manipulate and study ligand-capped cadmium selenide nanorods deposited on highly oriented pyrolitic graphite (HOPG). The AFM tip was used to manipulate (i.e., translate and rotate) the nanorods by applying a force perpendicular to the nanorod axis. The manipulation result was shown to depend on the point of impact of the AFM tip with the nanorod and whether the nanorod had been manipulated previously. Forces applied parallel to the nanorod axis, however, did not give rise to manipulation. These results are interpreted by considering the atomic-scale interactions of the HOPG substrate with the organic ligands surrounding the nanorods. The vertical deflection of the cantilever was recorded during manipulation and was combined with a model in order to estimate the value of the horizontal force between the tip and nanorod during manipulation. This horizontal force is estimated to be on the order of a few tens of nN.

Gluing nanoparticles with a polymer bonding layer: the strength of an adhesive bond

The adhesive joint between silica nanoparticles and ultrathin poly(vinylpyridine) (PVP) layers (thickness between 3 and 100 nm) was tested using the cantilever of an atomic force microscope. Specifically, the strength of the adhesive bond (or practical adhesion) was probed in a tearing contact mode, when the particle was removed by applying a tangential force parallel to the substrate surface. The effect of the polymer molecular weight and layer thickness on the particle (practical) adhesion was investigated. It was found that the particles were removed by destroying the cohesive contact zone and that the PVP layer thickness had a pronounced effect on the force needed to destroy the adhesive joint. In particular, the greater the layer thickness, the larger was the required break force. However, the strength of the adhesive joint was estimated to be higher for a thinner layer. It is suggested that mechanical properties of the system as well as molecular characteristics of the PVP layer are responsible for the trend observed. The molecular weight of the polymer did not significantly affect the strength of the adhesive bond.

From MoO3 nanobelts to MoO2 nanorods: structure transformation and electrical transport

The MoO(2) nanorods (NRs) were synthesized by simple hydrogen reduction using the MoO(3) nanobelts (NBs) as the templates. The growth mechanism of one-dimensional (1D) MoO(2) nanostructure can be explained by the cleavage process due to the defects in the MoO(3) NBs. Different I/V characteristics of individual MoO(2) NRs were obtained at different bias voltages, which can be explained by Ohmic and Schottky conduction mechanisms, and the resistivity increased at high bias voltage probably because of the oxidation of MoO(2) NRs with large specific surface area.

Frictional duality observed during nanoparticle sliding

One of the most fundamental questions in tribology concerns the area dependence of friction at the nanoscale. Here, experiments are presented where the frictional resistance of nanoparticles is measured by pushing them with the tip of an atomic force microscope. We find two coexisting frictional states: While some particles show finite friction increasing linearly with the interface areas of up to 310 000 nm(2), other particles assume a state of frictionless sliding. The results further suggest a link between the degree of surface contamination and the occurrence of this duality.