Lateral Force Microscopy Research Articles

Few-layer tin sulfide (SnS): Controlled synthesis, thickness dependent vibrational properties, and ferroelectricity

• Consistently ultrathin SnS was synthesized on SnS 2 van der Waals substrates. • Real-time microscopy establishes underlying growth processes. • Experiment and calculations identify thickness-dependent vibrational properties. • First high-quality PFM maps of SnS ferroelectric polarization and domains. • Non-equilibrium stacking provides net polarization independent of thickness. Group IV monochalcogenides, anisotropic van der Waals crystals (and black phosphorus analogues), are attracting increasing interest due to a number of exceptional properties including anisotropic optoelectronics and multiferroicity predicted and in part realized in the ultrathin limit. Due to their enhanced chemical reactivity, both exfoliation and synthesis of monolayer and few-layer crystals pose challenges not found in other 2D/layered materials, such as the transition metal dichalcogenides. Here, we show that SnS synthesis on SnS 2 van der Waals substrates can address these challenges and consistently produces few-layer flakes, a capability that is explained via analysis of real-time microscopy of the growth process. Raman spectroscopy combined with efficient computations of the Raman-active modes across an extended thickness range enables a comprehensive understanding of the evolution of the vibrational properties of SnS with number of layers. Lateral piezoresponse force microscopy provides unprecedented insight into the stacking-dependent polarization and ferroelectric domain structures in large few-layer SnS flakes. The combined results establish a basis for further fundamental studies and applications of SnS and other group IV monochalcogenides in the few-layer regime.

Direct curvature measurement of the compartments in bamboo-shaped multi-walled carbon nanotubes via scanning probe microscopy

Bamboo-shaped multi-walled carbon nanotubes (BS-MWCNTs) have compartmented structures inherently obtained during their catalytic growth, and the curvature of the compartmented structure is known to be determined by the morphology of the metal catalysts. In this study, the inside curvature of the BS-MWCNTs was directly measured through scanning probe microscopy (SPM). The surface of the compartment structures of BS-MWCNTs has discontinuous graphene layers and different frictional force levels depending on the curvature direction. That of the inside curvature can be directly observed through tribological analysis by adding and subtracting the lateral force microscopy images obtained on opposite sides along the axial direction of the BS-MWCNT (diameter of 500 nm). This tells us the direction of the inside curvature of the BS-MWCNT, which was also confirmed by identifying the growth direction of the BS-MWCNTs via scanning electron microscopy. Our demonstration implies that SPM can give the same insight into the structural characterization of nanomaterials that is relatively inexpensive and more user-friendly than currently used methods.

The effect of electrochemical oxidation time on the surface morphology of oxidized indium phosphide

The surface features of oxide layers (< 100nm) grown on single-crystal n-InP (100) samples in acid electrolyte on initial stage were investigated by atomic force and lateral force microscopy. The time of electrochemical oxidation of samples was the variable parameter. There is difference found in the oxide layer phase structure grown in the electrolyte during different time. The initially formed (17 s) islands (a nucleated oxide phase) were divided, then coalescence appeared (26 s), and the occurrence of a second phase was detected. With an increase in the time (40 s) of oxidation, the surface morphology becomes more uniform.

Synergistic nano-tribological interaction between zinc dialkyldithiophosphate (ZDDP) and methyl oleate for biodiesel-fueled engines

In biodiesel-fueled compression-ignition (CI) engines, dilution by unburned biodiesel has been found to have adverse effects on the boundary lubrication properties of additives in fully formulated engine lubricants. Such dilution of engine lubricants could be even more pronounced for CI engines running on higher blend concentrations of biodiesel. Given the nanoscopic nature of the interaction, this study seeks to determine the nano-tribological properties of an engine lubricant additive (e.g., zinc dialkyldithiophosphate (ZDDP)) when diluted with a fatty acid methyl ester (e.g., methyl oleate). Using lateral force microscopy (LFM) together with a fluid imaging technique, the lowest nanoscopic friction forces and coefficient of friction values (0.068–0.085) were measured for ZDDP when diluted with 70 vol% of methyl oleate. These values are also observed to be lower than those measured for neat ZDDP and neat methyl oleate, respectively, under similar conditions. Subsequently, interpreting the data with the Eyring thermal activation energy approach, it could then be elucidated that the lower frictional losses observed for the contact lubricated with this volumetric mixture are a result of the lower potential energy barrier and activation energy required to initiate sliding. These energy values are approximated to be 2.6% and 28.9% (respectively) lower than that of the contact lubricated with neat ZDDP. It was also found that the mixture, at this volumetric concentration, possesses the highest possible pressure activation energy (load-carrying capacity) along with the lowest possible shear activation energy (shearing), potentially indicating optimum tribological conditions for boundary lubrication. Thus, the findings of this study suggest that an optimum concentration threshold exists in which a synergistic nano-tribological interaction between additives and fatty acid methyl esters can be attained, potentially reducing boundary frictional losses of lubricated conjunctions. Such findings could prove to be essential in effectively formulating synergistic additive concentrations for engine lubricants used in biodiesel-fueled CI engines.

Friction force reduction for electrical terminals using graphene coating

Multi-layer graphene, serving as a conductive solid lubricant, is coated on the metal surface of electrical terminals. This graphene layer reduces the wear and the friction between two sliding metal surfaces while maintaining the same level of electrical conduction when a pair of terminals engage. The friction between the metal surfaces was tested under dry sliding in a cyclical insertion process with and without the graphene coating. Comprehensive characterizations were performed on the terminals to examine the insertion effects on graphene using scanning electron microscopy, four-probe resistance characterization, lateral force microscopy, and Raman spectroscopy. With the thin graphene layers grown by plasma enhanced chemical vapor deposition on gold (Au) and silver (Ag) terminals, the insertional forces can be reduced by 74 % and 34 % after the first cycle and 79 % and 32 % after the 10th cycle of terminal engagement compared with pristine Au and Ag terminals. The resistance of engaged terminals remains almost unchanged with the graphene coating. Graphene stays on the terminals to prevent wear-out during the cyclic insertional process and survives the industrial standardized reliability test through high humidity and thermal cycling with almost no change.

Fibril size-dependent control of polar ordering in type I collagen membranes

The most abundant protein in the human body, collagen, is widely used in tissue culture and engineering applications, spanning from substrate functionalization to fibrillar architectures and three-dimensional constructs. Collagen piezoelectricity provides an opportunity to exploit electromechanical coupling in these applications, wherein an applied mechanical stress generates charge, which might influence ion screening, protein absorption, and cell response. In type I collagen, the polarization direction follows the fibril orientation. Thus, control of fibril orientation and size in a collagen film or membrane may provide control of the polarization, enabling the creation of regions of uniform polarization direction. Here, aligned substrate-supported type I collagen membranes having fibril sizes from ∼100–500 nm are deposited using different osmotic concentrations (90, 190, and 290 mOsm/kg, from low to high ionic strength) to investigate the correlation between fibril size and piezoelectric properties. Lateral piezoresponse force microscopy is used to show that regions of uniform polarization orientation, as determined through 2D correlation analysis, decrease with increasing fibril size.

High-speed residue-free transfer of two-dimensional materials using PDMS stamp and water infiltration

A high-speed residue-free transfer method using PDMS (polydimethylsiloxane) stamp and water infiltration between graphene and a hydrophilic surface is reported. Monolayer graphene was transferred from an enhanced fluorinated Al2O3 surface using PDMS. Water infiltration dramatically reduced the time required to separate the graphene from the Al2O3 substrate to a few minutes. The graphene was then successfully transferred to a target substrate (SiO2) using the PDMS stamp. Atomic force microscopy and lateral force microscopy was used to confirm the absence of residue on the transferred graphene surface.

Effect of focused nanosecond laser pulse irradiation on microtribological properties of diamond-like films

We have examined the effect of focused nanosecond laser pulses on the formation and tribological properties of microstructures in the form of a matrix of craters produced in a – C : H : Si : O diamond-like films (DLFs), both undoped and doped with tungsten. The studies are performed by scanning probe microscopy (SPM) using SPM probes of standard design with a tip and probes without a tip (tipless-type probes). A special procedure is developed for assessing the wettability (contact angle) of individual sections of the laser-induced microstructure by the local adhesion force. According to estimates, the material inside the crater becomes softer and more hydrophobic as a result of laser irradiation. This is manifested to the greatest extent in tungsten-doped DLFs, where the crater walls are arranged like terraces formed by ‘nanoflakes’, i.e. grains of the substance. Scanning of microstructures with a tipless probe in the regime of lateral force microscopy demonstrates a decrease in friction in the crater area.

Crystallographic polarity measurements in two-terminal GaN nanowire devices by lateral piezoresponse force microscopy∗∗Contribution of an agency of the U.S. government; not subject to copyright.

Lateral piezoresponse force microscopy (L-PFM) is demonstrated as a reliable method for determining the crystallographic polarity of individual, dispersed GaN nanowires that were functional components in electrical test structures. In contrast to PFM measurements of vertically oriented (as-grown) nanowires, where a biased probe tip couples to out-of-plane deformations through the d33 piezoelectic coefficient, the L-PFM measurements in this study were implemented on horizontally oriented nanowires that coupled to shear deformations through the d15 coefficient. L-PFM phase-polarity relationships were determined experimentally using a bulk m-plane GaN sample with a known [0001] direction and further indicated that the sign of the d15 piezoelectric coefficient was negative. L-PFM phase images successfully revealed the in-plane [0001] orientation of self-assembed GaN nanowires as part of a growth polarity study and results were validated against scanning transmission electron microscopy lattice images. Combined characterization of electrical properties and crystallographic polarity was also implemented for two-terminal GaN/Al0.1Ga0.9N/GaN nanowires devices, demonstrating L-PFM measurements as a viable tool for assessing correlations between device rectification and polarization-induced band bending.

Thermal activation Eyring energy approach to characterise the dependence of nanoscale friction on the surface roughness

Atomic Force Microscope (AFM) is used to characterise the frictional response of surfaces with varying roughness parameters in dry and in the presence of fully formulated lubricants. The surface roughness has shown to affect nanoscale friction. The characteristics involved the investigation of roughness, small-scale adhesive forces and nanoscale friction using AFM in lateral force mode. The fluid-cell Lateral Force Microscopy (LFM) results were used to model thermal activation Eyring energy components in conjunction with the relevant continuum contact mechanics model. The paper shows that a combination of LFM, for dry and fluid-cell LFM and thermal activation Eyring energy barrier approach is a useful tool to explain the effect of surface roughness on nanoscale friction.

Direct tuning of graphene work function via chemical vapor deposition control

Besides its unprecedented physical and chemical characteristics, graphene is also well known for its formidable potential of being a next-generation device material. Work function (WF) of graphene is a crucial factor in the fabrication of graphene-based electronic devices because it determines the energy band alignment and whether the contact in the interface is Ohmic or Schottky. Tuning of graphene WF, therefore, is strongly demanded in many types of electronic and optoelectronic devices. Whereas study on work function tuning induced by doping or chemical functionalization has been widely conducted, attempt to tune the WF of graphene by controlling chemical vapor deposition (CVD) condition is not sufficient in spite of its simplicity. Here we report the successful WF tuning method for graphene grown on a Cu foil with a novel CVD growth recipe, in which the CH4/H2 gas ratio is changed. Kelvin probe force microscopy (KPFM) verifies that the WF-tuned regions, where the WF increases by the order of ~250 meV, coexist with the regions of intrinsic WF within a single graphene flake. By combining KPFM with lateral force microscopy (LFM), it is demonstrated that the WF-tuned area can be manipulated by pressing it with an atomic force microscopy (AFM) tip and the tuned WF returns to the intrinsic WF of graphene. A highly plausible mechanism for the WF tuning is suggested, in which the increased graphene-substrate distance by excess H2 gases may cause the WF increase within a single graphene flake. This novel WF tuning method via a simple CVD growth control provides a new direction to manipulate the WF of various 2-dimensional nanosheets as well as graphene.

Strumming a Single Chemical Bond.

Atomic force microscopy and scanning tunneling microscopy can image the internal structure of molecules adsorbed on surfaces. One reliable method is to terminate the tip with a nonreactive adsorbate, often a single CO molecule, and to collect data at a close distance where Pauli repulsion plays a strong role. Lateral force microscopy, in which the tip oscillates laterally, probes similar interactions but has the unique ability to pull the CO over a chemical bond, load it as a torsional spring, and release it as it snaps over with each oscillation cycle. This produces measurable energy dissipation. The dissipation has a characteristic decay length in the vertical direction of 4 pm, which is 13 times smaller than the decay length in typical STM or AFM experiments.

Roles of structural and chemical defects in graphene on quenching of nearby fluorophores

Oxidation scanning probe lithography was used to systematically fabricate graphene/defective graphene ribbon arrays on a graphene sheet. The tribological, structural, and chemical natures of the defects created by o-SPL were characterized using lateral force microscopy (LFM), micro-Raman (μ-RS) and micro-X-ray spectroscopy (μ-XPS), respectively. While LFM revealed all fabricated patterns exhibited increase in friction, μ-RS and μ-XPS analysis showed that a transition of defect type from structural to chemical defects occurred in the defective graphene ribbons at a threshold voltage (8.5 V). Subsequently, micro-photoluminescence (μ-PL) quenching spectroscopy was measured for a thin layer of polystyrene containing isolated fluorophores (MEH-PPV) spin cast on the ribbon patterns. Whilst the quenching efficiency was suppressed more significantly as defect concentration increased, for pure structural defects, the μ-PL lifetime varied linearly with defect concentration and then saturated to a constant as chemical defects dominated. Among all oxygen related functional groups, the presence of CO bonds led to greatly reduced quenching efficiency while simultaneously reducing the strong blue spectral shift seen for pure graphene with the spectrum at higher CO bond concentration approaching to pristine MEH-PPV. This work demonstrated that local chemical and structural composition in 2D quencher strongly affects the quenching efficiency and dynamics.

Calibration of the in-plane PFM response by the lateral force curves

Calibration technique for lateral piezoresponse force microscopy (PFM) based on lateral force curves measurements is introduced and discussed in terms of relative error connected with experimental realization. Dependences of lateral calibration coefficient on the tip motion velocity, normal loading force, humidity environment, and position of laser spot on the cantilever beam are considered. The maximal error of the measurements does not exceed 10%, which is close to the error of typical vertical PFM signal calibration based on force-distance curves. Proposed calibration technique is easy and accurate, which is important for the future development of quantitative PFM method.

Thermal-Controlled Frictional Behaviour of Nanopatterned Self-assembled Monolayers as Triboactive Surfaces

Friction is an important limitation of energy efficiency performances of MEMS/NEMS but is, in the same time, a great opportunity for harvesting energy by designing optimized tribo-electric nano-Generators (TENG). Thus, frictional behaviour can be accurately controlled in real time by using thermally sensitive periodic patterned self-assembled monolayers of n-octadecyltrichlorosilane (OTS) grafted on MEMS surfaces. Nanopatterns are currently used in order to limit the wear rate without modifying the frictional behaviour. In this work, patterns have been created by micro-contact printing ($$\upmu \hbox {CP}$$) using a polydimethylsiloxane (PDMS) stamp displaying a trapezoidal profile. Hence, pattern periodicity can be continuously changed—and then optimized from discontinuous to pseudo-continuous—by applying a controlled normal load on the soft PDMS stamp. A multiscale tribological study has been carried out on these nanopatterns by using both single-asperity and multi-asperity nanotribometers. Lateral force microscopy (LFM) provides the individual frictional behaviour of each pattern’s component, whereas the multi-asperity nanotribometer rather gives the emerging frictional behaviour induced by the patterning according to temperature. As a macroscopic crucial parameter while designing TENG’s devices, this macroscopic behaviour has to be carefully optimized for each practical applications at the molecular scale. Thus, the microscale frictional behaviour can be precisely optimized by the pattern’s periodicity, whereas the macroscopic one can be accurately controlled with values of friction coefficient ranging from 0.12 to 0.04 by varying the contact temperature. In addition, any inertial effects observed in the thermal-controlled frictional behaviour of nanopatterns can be drastically reduced using infra-red emission as thermal source.

Competitive Molecular Interaction between Zinc Dialkydithiophosphate (ZDDP) and Methyl Oleate along Boundary Lubrication Regime

Fatty acid methyl ester (FAME) possesses good lubricity properties, prompting numerous research work to explore their potential as lubricant additives. However, fully formulated lubricants consist of additive packages, which includes essential anti-wear agents, such as zinc dialkyldithiophosphate (ZDDP). Adding FAME into a lubricant could lead to adverse tribological effects if the competitive molecular interaction between different additives are not well understood. Therefore, the present study determines the tribological impact of molecular interaction between ZDDP and methyl oleate along boundary lubrication regime. Lateral force microscopy, coupled with fluid imaging approach, is adopted, allowing for the analysis to focus on asperity level interaction. A silicon nitride tip is used to slide against a mirror-polished steel substrate while being fully submerged in the tested fluids. The tested fluids are mixtures of ZDDP and methyl oleate, blended at different volumetric percentages. It is observed that the lowest friction force is measured for ZDDP-methyl oleate mixture containing 70-vol% of methyl oleate. Interestingly, the friction force measured for such mixture is found to be lower than that of neat methyl oleate. This finding indicates that an optimum blending ratio between FAME and ZDDP is essential in achieving better boundary lubrication performance for tribological conjunctions.

Friction titration measurements of electrochemically generated mixed alkylthiol monolayers on polycrystalline gold

An electrochemical procedure was used to generate a heterogeneous mixed monolayer composed of 1-undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUA) on polycrystalline gold and the interface was characterized over a range of solution pH using Lateral Force Microscopy (LFM). To generate the heterogeneous mixture, a single component layer of UDT was formed on the gold electrode and portions were removed by reductively desorbing molecules selectively from the Au(111) facets. The voids created by this method were backfilled with MUA using either short-term passive incubation or electrochemically assisted deposition and LFM was used to gather friction maps of the surface in pH adjusted KClO4. From pH 3 to 8, the friction of both UDT and MUA zones in the mixed layer were relatively constant but above this range, the friction of the MUA regions increased significantly. The elevated friction was attributed to the ionization of the MUA groups since pH induced frictional differences were not observed when backfilling with the non-ionizable thiol 11-mercapto-1-undecanol (MUD). The relative difference in friction between the MUA and UDT zones were fit to a modified Henderson-Hasselbach equation to extract the apparent surface pKa (or pK1/2) of MUA bound to the Au(111) facets. The results were similar for both passive and electrochemically backfilled MUA and rendered a pK1/2 of 10.3 ± 0.8. The elevated pK1/2 was attributed to hydrogen bonding of the COOH groups when tightly packed on Au(111) as well as the presence of UDT, which created a local hydrophobic environment near the MUA molecules.

Asperity level frictional interactions of cylinder bore materials and lubricant composition

Parasitic frictional losses in internal combustion engines of race vehicles adversely affect their performance. A significant proportion of these losses occur within the piston-cylinder system. This paper presents a study of the compatibility of cylinder bore surface materials with typical lubricant base constituent stock (poly alpha olefin and polyolester) as well as a fully formulated lubricant. Nanoscale boundary friction is measured using lateral force microscopy. The effect of material properties, nanoscale roughness and lubricant species upon underlying mechanisms of generated friction is presented. Advanced cylinder materials and coatings and lubricant molecular species used for high performance engines are investigated, and an integrated approach not hitherto reported in literature.

Nanoscale frictional characterisation of base and fully formulated lubricants based on activation energy components

Abstract Tribofilms are activated using precision sliding strip microscale tribometry with a base and a fully formulated lubricant with a ZDDP anti-wear additive. The employed tribometry uses combined pressure, shear and temperature activation. The chemical compositions of the formed tribofilms are ascertained through use of Photoelectron X–ray Spectroscopy (XPS). Nanoscale frictional measurements of the tribofilms are reported using fluid cell lateral force microscopy (LFM). The measured coefficient of interfacial boundary shear strength is used with analytical contact mechanics to relate the in-situ conditions to the activation energy components of the Eyring potential cage model. The paper shows that combined LFM and the Eyring model can explain the variations in the frictional characteristics of formed tribofilms.

Surface Effect on Frictional Properties for Thin Hydrogel Films of Poly(vinyl ether)

Frictional properties play a key role in the performance of hydrogels in applications such as soft contact lenses, cell sheets, artificial articular cartilage, etc. Here we characterize the swollen state and frictional properties of thin hydrogel films composed of poly(2-methoxyethyl vinyl ether) on a nanometer scale using neutron reflectivity (NR) in conjunction with lateral force microscopy, leading to the discussion about the extent to which surface chains impact the frictional properties of gels. NR measurement revealed that the density profile for the hydrogel films in the interfacial region with water along the direction normal to the interface was well described by a parabolic function, which was generally used for swollen polymer brushes in a liquid. Lateral, or frictional, force (FL) first increased with increasing normal force (FN) and then reached a region where the dependence of FL on FN was extremely subtle. That is, there exist two regimes, I and II, for the relationship between FN and FL. T...