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Friction properties in the electric field are important for the application of graphene as a solid lubricant in graphene-based micro/nanoelectromechanical systems. The studies based on conductive atomic force microscopy show that interfacial water between graphene and the SiO

Graphene with excellent mechanical, tribological and electrical properties has enormous potential as solid lubricant at sliding electrical contact interfaces. Here, ultra-low friction (the coefficient of friction is about 0.028), ultra-high current stability (the coefficient of variation is as low as 4.74%) and effective wear-resistance properties (keep stable under contact pressure of 5.93 GPa for 10,000 cycles) of graphene as solid lubricant are found at electrical contact interface under the combined effect of the load and current. The high current stability and ultra-low friction could attribute to stable electric contact and conductive quality at the interface due to the high flexibility and carrier mobility of graphene based on the molecular dynamic simulations and atomic stick-slip behavior calculations. The high wear resistance at sliding electrical contact interface derives from the ultra-strong mechanical strength of graphene for combined effect of electricity and load. Our studies found that graphene with ultra-low friction, ultra-high conductivity, flexibility, and mechanical strength is unique as solid lubricant in various devices with sliding electrical contact interfaces.

Molybdenum disulfide (MoS2) with a hydrophobic property and layered structure possesses an excellent lubricative property and has been widely used as a lubricant in various areas, including satellites, aircraft, and new energy vehicles. Aging is a ubiquitous phenomenon in MoS2 and plays a key role in its tribological application for shortening its service life. The effect of the surface and microstructure on the lubricative properties of MoS2 aging under different environments, including deionized water (DI water), ultraviolet/ozone (UV/Ozone), and high-temperature, was investigated. First, the lubrication of MoS2 transiently degrades because of physical adsorption and recovers after mechanical removal. The lubrication of MoS2 also degrades slightly when its surface becomes hydrophilic, thereby enhancing the adhesion energy due to atomic oxygen interaction under UV/Ozone exposure. Second, the lubrication of MoS2 degrades irreversibly because of the formation of stripes with the destroyed structures under accelerated aging. The lubrication of MoS2 further degrades with the formation of small triangular pits under high-temperature annealing. Finally, the lubrication of MoS2 deteriorates due to the destroyed structure and complete oxidation. The severe aging of MoS2 is accompanied with large triangular pits due to anisotropic oxidation etching of MoS2. The lubrication failure of MoS2 was determined on the basis of structural defect formation and surface property degradation induced by the extent of oxygen diffusion. The enhanced out-of-plane deformation due to the reduced out-of-plane stiffness and the increased energy barriers of defects are fundamentally responsible for the lubrication degradation of MoS2 at the atomic scale. These findings can provide new insights into the atomic-scale mechanism underlying the lubrication failure of MoS2 and pave the way for the realization of MoS2-based lubrication application under various environments.

Atomically thin two-dimensional molybdenum disulfide (MoS2) is well known for its excellent lubrication characteristics and is usually used as a solid lubricant in diverse micro/nanoelectromechanical systems (MEMS/NEMS). The friction on atomically thin MoS2 deposited on a SiO2/Si substrate is reduced almost five times to achieve an ultra-low friction state (coefficient of friction nearly 0.0045) by rubbing the surface with an AFM tip under the electric field. The electric field leads to a shift and accumulation of charges at the interface between MoS2 and the SiO2/Si substrate. Then, electronic tight-binding with high interfacial bonding strength is experimentally found by the charges transferring during the rubbing process. The ultra-low friction state of atomically thin MoS2 could attribute to the electronic tight-binding between MoS2 and the SiO2/Si substrate, which suppresses the atomic-scale deformation and limits the local pinning capability of MoS2. The ultra-low friction state on atomically thin MoS2 is patterned further by controllably regulating position, time, and electric field during the rubbing process. This approach can provide an additional channel to achieve ultra-low friction on MoS2 related two-dimensional materials with semiconductor properties. The nanopatterning of ultra-low friction could promote and expand engineering applications of MoS2 as lubricants in various MEMS/NEMS with nano-scale components.

The nanofriction properties of hexagonal boron nitride (h-BN) are vital for its application as a substrate for graphene devices and solid lubricants in micro- and nano-electromechanical devices. In this work, the nanofriction characteristics of h-BN on Si/SiO2 substrates with a bias voltage are explored using a conductive atomic force microscopy (AFM) tip sliding on the h-BN surface under different substrate bias voltages. The results show that the nanofriction on h-BN increases with an increase in the applied bias difference (Vt−s) between the conductive tip and the substrate. The nanofriction under negative Vt−s is larger than that under positive Vt−s. The variation in nanofriction is relevant to the electrostatic interaction caused by the charging effect. The electrostatic force between opposite charges localized on the conductive tip and at the SiO2/Si interface increases with an increase in Vt−s. Owing to the characteristics of p-type silicon, a positive Vt−s will first cause depletion of majority carriers, which results in a difference of nanofriction under positive and negative Vt−s. Our findings provide an approach for manipulating the nanofriction of 2D insulating material surfaces through an applied electric field, and are helpful for designing a substrate for graphene devices.

The deformation induced atomic-scale stick-slip behaviors are unique and prominent friction characteristics of atomically thin two-dimensional (2D) materials. Here, the deformation induced atomic-scale frictional characteristics of atomically thin 2D materials under different load were explored at different velocity using atomic force microscopy. The evolution of contact quality and puckering forming play important role in atomic-scale frictional characteristics of 2D materials. The critical load was proposed for the strengthening effect at transient period of atomic-scale friction. The velocity-dependent evolution of contact quality affects the strengthening effect when the normal load is less than the critical load. Strengthening degree and distance increase with the increase of the normal load because of high puckering and strong interfacial bonding when the load exceeds the critical load. The suspended 2D materials on asperities of rough substrate possess stronger strengthening effect than smooth substrate because of large deformation space and flexibility. The stick time in stick-slip behavior elongated with the increase of load as the lattice of 2D materials on rough substrate is locally distorted by large deformation under high load. The correlation between the strengthening effect and friction of 2D materials could potentially provide deep insights into the tribological behavior of atomically thin 2D materials.

The current-carrying nanofriction characteristics play an important role in the performance, reliability, and lifetime of graphene-based micro/nanoelectromechanical systems and nanoelectronic devices. The atomic-scale friction characteristics of graphene were investigated using conductive atomic force microscopy by applying positive-bias and negative-bias voltages. The atomic-scale friction increased with applied voltages. Also, the friction under positive-bias voltages was lower than under negative-bias voltages, and the friction difference increased with the voltages. The different frictional behaviors resulted from the inherent work function difference and the water molecules between the tip and graphene. The applied voltages amplified the effect of the work function difference on the friction, and the water molecules played different roles under negative-bias and positive-bias voltages. The friction increased rapidly with the continuous increase of negative-bias voltages due to the electrochemical oxidation of graphene. Nevertheless, the friction under positive-bias voltages remained low and the structure of graphene was unchanged. These experimental observations were further explained by modeling the atomic-scale friction with a modified Prandtl-Tomlinson model. The model allowed the determination of the basic potential barrier and the voltage-induced potential barrier between the tip and graphene. The calculation based on the model indicated that the negative-bias voltages induced a larger potential barrier than the positive-bias voltages. The studies suggest that graphene can show a better lubricant performance by working as a lubricant coating for the cathodes of the sliding electrical contact interfaces.

The 60-degree rotational symmetry of graphene layers leads to remarkable physical and mechanical phenomena. One corresponds to ultralow sliding friction and friction coefficient between incommensurate graphene layers. A pick-up method was proposed to pick graphite flake to microsphere probe and achieved graphite and graphite tribo-pair. Ultralow friction was attained for various rotation angles, and the friction coefficient was kept at the range of 10− 3. Density functional theory (DFT) calculation was employed to reveal the angular dependence of friction, and the calculations are partially consistent with the experimental results. The difference of angle-dependence ultralow friction between experimental results and DFT calculations was observed, which was greatly influenced by the stiffness of the system. Friction hysteresis, abnormal shape of pull-off curves and long jumps in stick-slip were further demonstrated the stiffness of the system had changed during the friction measurements. The contact geometry in our experiment was a finite, symmetric, nanometer-sized (less than 1nm), flexible graphite flake and a rigid graphite surface. The tribological system offers an experimental platform to study interlayer friction and contact geometry.

Robust superlubrication across nano- and microscales is highly desirable at the interface with asperities of different sizes in durable micro/nanoelectromechanical systems under a harsh environment. A novel method to fabricate superlubric interfaces across nano- and microscales is developed by combining a batch of surface modification with atomically thin graphene. The robust superlubric interface across nano- and microscales between hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembly monolayers (SAMs) and graphene was achieved under high relative humidity, sliding speed, and contact pressure. The superlubric mechanisms at the interface of FDTS/graphene could be attributed to the following at different scales: the hydrophobicity of FDTS SAMs and graphene preventing the capillary interaction of the interfacial friction under high relative humidity; the high elastic modulus of graphene leading to small interfacial contact area; the compressing and orientating of FDTS SAMs decreasing interfacial shear strength under high contact pressure; the surface modification of FDTS molecules reducing the interfacial potential barriers when sliding on the atomically thin graphene. The robust superlubric interface across nano- and microscales reducing the friction at the complicated interfaces with asperities at different scales and improving the performance and durability have great potentials in the field of micro/nano mechanical systems.

The nanofriction on molybdenum disulfide (MoS2) plays a crucial role in the long-term stability and reliability of micro- and nano-electromechanical systems (MEMS/NEMS). The anisotropic nanofriction force on MoS2 as a function of thickness was studied by a calibrated atomic force microscopy (AFM) in ambient air. The nanofriction force on MoS2 with a thickness of 45.23 nm presents a periodicity of 180° correlating to the same periodicity of the sawtooth-type lattice orientation. The two-dimensional Prandtl-Tomlinson model that the longer moving length along the armchair lattice orientation dissipated more energy with respect to zigzag lattice orientation was adopted in interpreting the anisotropic nanofriction on MoS2 with a thickness of 45.23 nm. The observed anisotropic nanofriction on MoS2 with a thickness of 4.18 nm was attributed to the combination of the lattice orientation and puckering effect. The anisotropic nanofriction on MoS2 with a thickness of 1.49 nm was mainly due to the puckering effect dominating the nanofriction. As the thickness of MoS2 increased, the magnitude of nanofriction decreased, and the anisotropy ratio of nanofriction increased. The structural anisotropy coupling with thickness play an important role in the anisotropic nanofriction on MoS2. This study could provide the clue for the design of MoS2 and other two-dimensional materials in lubrication applications.

Molybdenum disulfide (MoS

The electric-carrying nanofriction properties of graphene are vitally important in the reliability and lifetime of graphene-based micro-/nanoelectromechanical systems. However, ubiquitous atomic-scale steps on graphene have an impact on their friction properties. In this work, electric-carrying nanofriction properties of atomic-scale steps on graphene were studied using the conductive atomic force microscope. Lateral force at uncovered step increased noticeably with the increasing voltages. Electrostatic interaction between the tip and dangling bonds along the step edges significantly increases the lateral force. In contrast, the lateral force of covered steps has only a slight increase as the voltage increases. In addition, oxidation of graphene is more likely to occur at uncovered step due to water molecules adsorbed by dangling bonds. The observations provide a deeper insight into the frictional behavior of atomic-scale steps on graphene.

We report a novel methodology for fabricating a sub-micron spherical atomic force microscope (AFM) tip controllably-a silicon sub-micron sphere atop microcantilevers, which is desired for precise nanoscale tribology measurements, biological studies, and colloid science. Silicon sub-micron spheres are fabricated through swelling of single-crystal silicon with proper high-energy helium ion dosing, a traditionally undesired phenomenon known in helium ion microscopy. Silicon sub-micron spheres with diameters from 100 nm to 1 μm are demonstrated, and the placement of silicon sub-micron spheres can be as accurate as 10 nm or even below. This AFM tip demonstrates robust measurements during friction tests on graphene/silicon oxide substrates for more than 10 000 cycles. This AFM tip overcomes a critical challenge of reducing the size of spherical AFM tips from the micrometer scale to the sub-micron scale and is promising in cross-scale mechanics studies, nanotribology, colloid science, and biology.

The tunable nanofriction of graphene as a lubricant material is important for its application in micro/nano-electromechanical devices. A tunable nanofriction array of graphene on a silicon dioxide/silicon (SiO2/Si) substrate was achieved via functionalization using negative-bias conductive atomic force microscopy by introducing an electron releasing process to avoid electron saturation. Based on the electron releasing process, the functionalization degree of graphene and thus the nanofriction were controllably tuned by the tip voltage. Moreover, the nanofriction of the functionalized array was further enhanced by applying biased tip voltage. The nanofriction on the functionalized array increased with an increase in bias voltage due to the polarization-induced electrostatic force, and the voltage increased the meniscus force. The tuning of the nanofriction of functionalized graphene on an insulating substrate can provide insight into the electric-carrying friction behavior of graphene as a solid lubricant in micro/nano-electromechanical systems (MEMS/NEMS) and the fabrication of nanodevices with a patterned surface.

The atomic-scale steps are ubiquitous on graphene. The frictional behavior of graphene at atomic-scale steps under different relative humidity (RH) is very important for application of graphene as solid lubricant in complicated environment. Frictional properties at atomic-scale graphene steps were investigated using calibrated atomic force microscopy (AFM) in various RH. Lateral force at uncovered atomic step decreased with the increase of RH, while lateral force at covered step were independent of RH. Uncovered steps possessed higher work function than graphene plane from the scanning Kelvin probe microscopy (SKPM) measurement due to the existence of dangling bonds and oxygen-containing functional groups. Then the adsorption of water molecule on hydrophilic oxygen containing functional groups at uncovered step decreased the work function and reduced the lateral force under high RH. Also, lateral force at steps increased even fast than plane after plasma treatment, because dangling bonds is easier to be oxide functionalization. The studies provide a further understanding of frictional properties at atomic scale steps and could be helpful in the applications of graphene as solid lubricant in humid environment.

Graphene and molybdenum disulfide (MoS2) are promising solid lubricants to deal with the interfacial friction and adhesion concerns in silver nanowires (Ag NWs)-based nanodevices, but their difference in friction performance is seldom considered. Here, the difference in friction performance between graphene and MoS2 has been comparatively studied by manipulating the Ag NWs on graphene and MoS2 surfaces through atomic force microscopy (AFM) tip-on-side and tip-on-top manipulations. The tip-on-side and tip-on-top manipulations demonstrate the atomically thin MoS2 has better friction performance than graphene. The tip-on-top manipulation further shows the shear strength in NW–MoS2 interface is smaller than in NW–graphene interface. The underlying mechanism for the friction difference between the two interfaces is attributed to the different interfacial adhesion interactions. The relatively small adhesion interaction in NW–MoS2 interface leads to a small interfacial shear strength, resulting in a small friction when the NW slides on MoS2 surface. The measured water contact angles, calculated work of adhesion and the adhesion force directly measured by AFM tip confirm the relatively small adhesion interaction in NW–MoS2 interface. These findings suggest that MoS2 may be more appropriate as lubricants for application in the NW-based nanodevices.

Nanotribological characteristics of silver nanowires (Ag NWs) are of great importance for the reliability of their applications where involving mechanical interactions. The frictional behaviors of the Ag NWs with different lengths on SiO2/Si substrate have been investigated directly by atomic force microscopy (AFM) nanomanipulation. The relatively short and long Ag NWs behave like the rigid rods and flexible beams, respectively, and the critical aspect ratio of NWs for the two cases is found to be about 20. The relatively short NWs demonstrates three forms of motion with different frictional behaviors. The friction of the relatively long NWs increases with the bend of the NWs. The long Ag NWs display extraordinary flexibility that can be folded to different shapes, and the folded NWs show a similar frictional behavior with the rigid rods. Different simplified mechanical models are established to match the frictional behaviors of the corresponding Ag NWs. The adhesion between the Ag NWs and substrate is calculated by an indirect method based on the van der Waals force equation to assess their adhesive attraction. These findings may provide insight into the frictional characteristics of Ag NWs and contribute to the quantitative interface design and control for their applications.

In this work, a green and sustainable strategy for the combined design containing both lubrication and photocatalytic degradation at different stages of base oils has been suggested. More concretely, Fe3O4@MoS2 nanohybrid was prepared and used as an additive for liquid paraffin (LP), resulting in enhanced lubricating properties together with significant photocatalytic degradation performance. The nanohybrid was prepared via a hydrothermal method, which was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectrometer (EDS), and ultraviolet–visible (UV–vis) spectroscopy. The frictional properties together with photocatalytic degradation behaviors of LP mixtures containing MoS2, Fe3O4, and Fe3O4@MoS2 nanohybrid were under a comparative investigation. The results show that the LP mixture containing 0.08 wt % of Fe3O4@MoS2 nanohybrid can achieve the maximum reduction of friction coefficient; while that containing 0.10 wt % ...

Nanofriction and nanoscale triboelectrification is a universally existing phenomenon and important in many branches of nanotechnology, but how the nanoscale triboelectrification affects the nanofriction during the relative sliding at the contact interface is not fully investigated. Here, nanoscale triboelectrification and nanofriction on the insulating SiO2 surface were investigated with atomic force microscopy (AFM) and scanning Kelvin probe microscopy. By the AFM tip sliding on the SiO2 surface, both triboelectric charges and nanofriction were investigated with the applied load, sliding cycle, and external electric field. Whether the tribocharges on the SiO2 surface were negative or positive, the nanofriction increases with the enhanced tribocharge density by nanoscale triboelectrification, while decreases with the reduced tribocharges by charge diffusion. The change of nanofriction correlates strongly with the variation in tribocharge density due to the tribocharge induced excess electrostatic interactions at the sliding interface. These findings not only add to our understanding of the role of nanoscale triboelectrification on nanofriction, but also help to control energy dissipation of sliding nanofriction, which is also important in many branches of nanotechnology, such as data storage and nanoelectromechanical system.

Flexible pressure sensor becomes increasingly important with the development of ultra-sensitive touch technology, electronic skin, and human health monitoring, which requires higher sensitivity. Here, a novel strategy is proposed by designing conductive polymer composites consisting of four components with different dimensions. Through a simple one-step hydrothermal method, the ternary composites of silver-carbon nanotubes-reduced graphene oxide (Ag-CNTs-rGO) were synthesized. The ternary composites-wrapped polydimethylsiloxane (PDMS) sponges, Ag-CNTs-rGO/PDMS, were fabricated by a dip-drying method. The conductive polymer sponges as pressure sensors showed improved sensitivity of 33 kPa−1 in the range of 0–0.05 kPa, which is twelve times higher than that of the conductive sponge without Ag nanoparticles. The uniform coating of Ag nanoparticles on CNTs and rGO not only enhanced the conductivity of the Ag-CNTs-rGO composites but also reduced the contact resistance between carbon materials. The CNTs-rGO matrix accelerated electron transport throughout the conductive sponge. The good flexible stability of the CNTs-rGO matrix and porous PDMS structures enabled the Ag-CNTs-rGO/PDMS sponge to assemble flexible circuits and act as elastic electric contacts. The combination of zero-dimensional metal nanoparticles, one-dimensional CNTs, and two-dimensional graphene as conductive fillers in three-dimensional porous structures provides new insight into acquiring the ultrahigh-performance flexible pressure sensors.

Interfacial friction is of crucial importance to ensure the friction-reducing and anti-wear properties of mechanical microstructures in micro/nanoelectromechanical systems (MEMS/NEMS). An ultra-low frictional interface combining hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembled monolayers (SAMs) coated on an AFM tip with mechanically exfoliated molybdenum disulfide (MoS2) nanosheets deposited on a planar Si/SiO2 substrate was achieved. The FDTS SAMs/MoS2 interface between the FDTS SAMs and the MoS2 nanosheets exhibits an ultra-low friction force that is independent of the relative humidity. The incommensurate contact with ultra-low energy dissipation between FDTS and MoS2 nanosheets and hydrophobic surface properties lead to this ultra-low frictional FDTS SAMs/MoS2 interface. Also, the MoS2 nanosheets have a high elastic modulus, which gives them a smaller contact area than the FDTS SAMs and contributes to the low friction. The excellent hydrophobic properties of both the FDTS SAMs and MoS2 enable them to be unaffected by the relative humidity by preventing the capillary interaction. This study paves the way for extensive applications in reducing the friction of nanoscale contact interfaces.

High-performance composite films based on ionic liquids (ILs) and two-dimensional MoS2 have attracted considerable attention because of their unique structure and properties, especially in the field of lubrication. Ultrathin MoS2 nanosheets obtained via a liquid exfoliation process exhibited good dispersion in the organic solution of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) without the addition of any other additives. A series of uniform composite films with different amounts of MoS2 were spin-coated onto the hydroxylated Si/SiO2 wafers, and the tribological performance of the composite films was studied. The load-carrying capacity and anti-wear properties of the IL films were greatly improved with the addition of MoS2 nanosheets, and the enhanced tribological properties of the composite films were attributed to the synergistic effect of the IL and MoS2. The low-friction composite films may provide a kind of reference and be useful for the design of ultralow friction films for practical lubrication applications.

Graphene is one of the key building blocks for a broad range of flexible/stretchable electronics and has great potential as a solid lubricant in flexible micro/nanoelectromechanical systems, soft robot and microfluidic devices due to their excellent electrical, mechanical and tribological properties. The nanotribological properties of graphene on soft elastic substrate were studied using the calibrated atomic force microscopy (AFM). The nanotribological properties of graphene on soft elastic substrate were enhanced by the elastic deformation compared with the hard SiO2/Si substrate. Also the friction force of graphene on soft elastic substrate decreases with the increase of the thickness and shows a sub-linear dependence on the indentation depth. A novel model of the elastic deformation enhancing puckering effect was proposed to explain the nanotribological properties of graphene on soft elastic substrate. The atomic-scale frictional behaviors of graphene on soft elastic substrate confirmed the mechanism of the elastic deformation enhanced puckering effect. These studies can provide a fundamental understanding of graphene as a solid lubricant on soft elastic substrate for graphene-based flexible MEMS/NEMS devices.

Graphene as a well-known solid lubricant is widely used in micro- and nano-scale mechanical devices, decreasing the friction of graphene as far as possible is a perpetual task to improve the performance of these devices. A novel approach was proposed to decrease the friction of graphene against atomic force microscopy (AFM) tip by plasma treatment of the substrate. The plasma treatment of the substrate enhances the adhesive attraction between graphene and SiO 2 substrate by generating stronger van der Waals attraction. Enhancing the adhesive attraction can indeed decrease the friction of graphene, regardless of the thickness of graphene and the kinds of AFM tip. Longer time of plasma treatment results in stronger adhesive attraction, leads to smaller friction. The decreased friction is mainly due to the combined action of the suppressed puckering of graphene and the reduced ability of graphene to adjust its atomic configuration. This novel approach will promote the engineering application of graphene and other related 2D materials as lubricants in MEMS/NEMS.

Atomic-scale steps generally presented in 2-dimensional materials have important influence on the overall nanotribological properties of surface. Frictional properties at atomic-scale steps of two types of 2-dimensional materials are studied using calibrated atomic force microscopy (AFM) tip sliding against the steps. The lateral force at uncovered step is larger than covered step due to the bending of step edge. The lateral force at monolayer uncovered step edge of h-BN is lower than graphene because h-BN possesses higher interlayer bonding strength than graphene and the bending of h-BN step edge is suppressed to some extent. The high uncovered step exhibits much larger lateral force than low uncovered step, which could be mainly induced by increased bending stiffness of step edge rather than increased step height. The results revealed that interlayer bonding strength and bending stiffness have great influence on the lateral force at atomic-scale steps. The studies can provide a further understanding of frictional properties at atomic scale steps and could be helpful for the applications of 2-dimensional materials as lubricant coating.

The design of pH-sensitive supramolecular drug delivery systems for efficient antineoplastic drug delivery remains a huge challenge. Herein, we describe the development of pH-responsive core–shell tecto dendrimers (CSTDs) formed using benzimidazole (BM)-modified generation 3 (G3) poly(amidoamine) (PAMAM) dendrimers (G3.NHAc-BM) as a shell and β-cyclodextrin (CD)-modified G5 PAMAM dendrimers (G5.NHAc-CD) as a core. By virtue of the host–guest recognition and pH-responsiveness of BM/β-CD assembly, the pH-sensitive supramolecular CSTDs of G5.NHAc-CD/BM-G3.NHAc were formed and adopted to encapsulate the anticancer drug doxorubicin (DOX) via hydrophobic interactions for pH-responsive drug delivery applications. The synthesis of dendrimer derivatives and the loading of the DOX were well characterized via different methods. We show that the encapsulated DOX can be released in a sustained manner with a rapid release speed under a slightly acidic pH condition (pH < 6), which is similar to acidic tumor microenvironment. The enhanced intracellular release of DOX and improved anticancer activity of the drug-loaded pH-responsive CSTDs were demonstrated and compared with the control CSTDs formed without pH-responsiveness through flow cytometry and viability assays of cancer cells. Furthermore, the pH-sensitive CSTDs also showed efficient drug penetration and growth inhibition of three-dimensional tumor spheroids owing to the faster DOX release in an acidic pH environment. The pH-sensitive G5.NHAc-CD/BM-G3.NHAc CSTDs may be employed as a valuable intelligent delivery system for various anticancer drugs.

Friction and wear are common physical phenomena in nature, while lubrication is an effective way to decrease unnecessary friction and wear. With the rapid development of environmental protection consciousness, designing novel lubricants or lubricating additives with excellent lubricating performance and little pollution is the increasing demand of the green tribology. In this work, nanoscale coral-like molybdenum disulfide (MoS2) was prepared via a hydrothermal method. It was first used as a lubricating additive for liquid paraffin (LP), and then, it acted as a catalyst for the photoinduced degradation of waste LP after testing. Many influencing factors (modification, concentration, morphology and temperature) that can influence the tribological properties and photocatalytic degradation level were investigated. Experimental results and related analyses indicate that modification by cetyltrimethylammonium bromide (CTAB) and a proper synthesis temperature are helpful to decrease the fiction coefficient, and organic modification, bigger surface area, narrower bandgap and more rim sites with oxidation activity are advantageous to achieve better LP degradation levels. Therefore, coral-like MoS2 can serve as both lubricating additive and photocatalyst at different working stages in the full life cycle of LP, which exhibits great potential in developing environment-friendly lubricating systems.

Nanofriction properties of graphene at electrified interfaces are important for the application of graphene as a solid lubricant in graphene-based electric-carrying interfaces. The nanofriction experiments based on conductive atomic force microscopy show that dynamic electron transfer determines nanofriction of graphene at the electrified interfaces. Nanofriction at highly conductive interfaces remain constant because the fast electron transfer decreases the electrical potential difference. However, nanofriction at low conductive interfaces increases because electron blocking increases the electrical potential difference. The nanofriction of graphene correlated negatively with loads at low conductive interfaces because high loads decrease the contact resistance to release blocked electrons. Furthermore, large graphene possesses higher nanofriction than small graphene. Scanning Kelvin force microscopy measurements show that the larger electron capacity of large graphene can maintain the electrical potential difference at interfaces. These studies provide beneficial guidelines for the applications of graphene as a solid lubricant at electrified interfaces.

Chromium (Cr)-based coatings have been widely used to strengthen the friction reduction and wear resistance on various kinds of surface. Here, the stable aqueous dispersion of oxidized multi-walled carbon nanotube (MWCNT) and graphene oxide nanosheets (GOS) was obtained by ultrasonic oxidation treatment. Then, MWCNT-Cr and GOS-Cr composite coatings were prepared using the direct current electrochemical co-deposition process on 420 stainless steel in the electrolyte with the addition of MWCNT and GOS under different current density and temperature. The morphology, structure, hardness and tribological properties of MWCNT-Cr and GOS-Cr composite coating are comparatively studied using pure Cr coating as a baseline. The friction reduction performance of MWCNT-Cr and GOS-Cr composite coatings was improved at optimum current density and temperature. The anti-wear properties of MWCNT-Cr and GOS-Cr composite coatings were enhanced by uniform embedment of MWCNT and GOS in coatings increasing the hardness and lubricity. This study suggests that the introduction of oxidized MWCNT and GOS with good dispersion could enhance the wear resistance and friction reduction of pure Cr coating due to their excellent dispersion, mechanical, and lubricant properties.

Graphene is a two-dimensional nanomaterial with high specific surface area, excellent physical and chemical properties with great potential in sensor applications. Uniform nanocrystalline Pt–Pd nanoparticle-decorated reduced graphene oxide (Pt–Pd/RGO) has been synthesized via a one-step chemical reduction. Dielectrophoresis applied to Pt–Pd/RGO fabricated the sensor. The hydrogen sensing properties of Pt–Pd/RGO were studied by recording the resistance changes following exposure to hydrogen at different concentrations, temperatures, flow rate and with different carrier gases. The Pt–Pd/RGO sensor has a stable and repeatable response due to carrier donation and crystal lattice expansion over hydrogenation and dehydrogenation. The maximum response tends to increase with the increase of hydrogen concentration or decrease of operating temperature, but remained almost unaffected by the flow rate. The nitrogen as carrier gas possesses higher response and longer recovery time than air because oxygen participates in the reaction. These studies have potential for the development of novel H2 sensors.

Novel Sn–Bi/reduced graphene oxide nanosheets (RGOS) nanocomposites were fabricated using electro-chemical deposition process at room temperature. The RGOS dispersed and distributed in Sn–Bi alloy. Nano-sized Sn–Bi grain clusters grew on the surface of RGOS and the Sn–Bi grains became finer after the incorporation of RGOS. The mechanical properties including micro-hardness and shear strength were enhanced by the incorporation of RGOS. The enhance mechanism of RGOS in Sn–Bi/RGOS nanocomposites was proposed by finite element method simulation. The graphene with extra-thin thickness and huge aspect ratio lead to an effective stress transfer and stress concentration occurs near the edge of graphene. The Sn–Bi/RGOS nanocomposites are promising in application as TIM of high power electronics, integrated circuits.

Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.

Graphene shows great potential applications as a solid lubricant in micro- and nanoelectromechanical systems (MEMS/NEMS). An atomic-scale friction strengthening effect in a few initial atomic friction periods usually occurred on few-layer graphene. Here, velocity dependent friction strengthening was observed in atomic-scale frictional behavior of graphene by atomic force microscopy (AFM). The degree of the friction strengthening decreases with the increase of velocity first and then reaches a plateau. This could be attributed to the interaction potential between the tip and graphene at high velocity which is weaker than that at low velocity, because the strong tip-graphene contact interface needs a longer time to evolve. The subatomic-scale stick-slip behavior in the conventional stick-slip motion supports the weak interaction between the tip and graphene at high velocity. These findings can provide a deeper understanding of the atomic-scale friction mechanism of graphene and other two-dimensional materials.

A type of novel Sn–Bi/Graphene nanocomposite was proposed as thermal interface material. The mechanical properties of Sn–Bi/Graphene nanocomposite were investigated by theoretical calculation as a function of the weight fraction of graphene. The stress distribution of single lap shear test of the Sn–Bi/Graphene nanocomposite solder was simulated by finite element method. The enhancement mechanism of Sn–Bi/Graphene composite was studied further by a 3-dimensional Sn–Bi/Graphene nanocomposite micro model. The strength of lap joint was mainly influenced by shear stress and initial crack occurs at the edge of lap joint. The shear modulus and strength of Sn–Bi/Graphene nanocomposite lap joint increased with the weight fraction of graphene. The shear modulus of Sn–Bi/Graphene nanocomposite was ∼192% greater than pure Sn–Bi alloy when the content of graphene increased to 1.0 wt.%. The graphene reinforcement improved stress strength as graphene leading an effective stress transfer. Stress concentration was existed near the edge of graphene, where the initial failure may occur. The novel Sn–Bi/Graphene nanocomposites are promising application as a thermal interface material with improved mechanical properties in the near future.

The nanotribological characteristics of silver nanowires (Ag NWs) are of great importance for the reliability of their applications in flexible nanodevices involving mechanical interactions. The frictional behaviors of Ag NWs on graphite substrate were directly investigated by atomic force microscopy (AFM) nanomanipulation. The relatively short NWs demonstrate three forms of motion-rotation, translation and a combination of the two-whose frictional behaviors behave like rigid rods. The relatively long Ag NW shows characteristics of a flexible beam, whose friction increases with an increase in the bending angle of the NW. The friction between the NW and substrate increases linearly with an increase in the length of the NW. The long Ag NW displays extraordinary flexibility that can be folded to different shapes, and the friction of the folded NW becomes smaller due to the decreased bending deformation. The critical aspect ratio of the Ag NW on graphite substrate for two different frictional behaviors between the relatively long and short NWs is found to be 12-15. These findings can deepen the understanding of the frictional characteristics of Ag NWs and contribute to their quantitative interface design.

Nowadays, with the rapid development of environmental protection awareness, the demand for the emergence of a green counterpart of lubricant additive plays a more and more important role in reducing friction and wear as the times require. In this paper, full-hollow and semihollow molybdenum disulfide (MoS2) microspheres were prepared via a hydrothermal method and were characterized and confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). According to our results, both fully hollow and semihollow MoS2 microspheres possessed excellent lubrication-enhancing effects for liquid paraffin (LP), while full-hollow samples after friction provided better photocatalytic degradation properties than semihollow samples after friction. Related analysis indicated that curved layer opened structures with more rim and edge sites, bigger surface area, and narrower band gap made full-hollow MoS2 samples achieve a better photocatalytic level. Thus, it was a sustainable solution for both lubrication-enhancing and photocatalytic degradation functions during different stages of the usage of lubricating oils, which suggests a potential strategy for achieving environmentally friendly developments.

Palladium/tin oxide/reduced graphene oxide (Pd/SnO