Observing and Modeling the Wear Process of Heterogeneous Interface.

Understanding and controlling the wear process of heterogeneous interfaces between soft and hard phases is crucial for designing and fabricating materials, such as improving the wear resistance of particle reinforced metal matrix composites and the accuracy and efficiency of chemical mechanical polishing. However, the wear process can be hardly observed, as interfaces are buried under the surface. Here, we proposed a nanowear test method by combining focused ion beam cutting to expose interfaces, atomic force microscopy to rub against interfaces, and scanning electron microscope to characterize the interface damage. Using this method, three typical wear forms had been observed in Al/SiC composite, i.e., merely matrix wear, particle fracture, and particle pullout. A theoretical model was proposed that revealed that the increasing interfacial friction would induce particle fracture or pullout, depending on the particle edge angle and tip edge angle. This work sheds light on wear control in composites and nanofabrication.

Ultralow-Friction at Cryogenic Temperature Induced by Hydrogen Correlated Quantum Effect.

Temperature is one of the governing factors affecting friction of solids. Undesired high friction state has been generally reported at cryogenic temperatures due to the prohibition of thermally activated processes, following conventional Arrhenius equation. This has brought huge difficulties to lubrication at extremely low temperatures in industry. Here, the study uncovers a hydrogen-correlated sub-Arrhenius friction behavior in hydrogenated amorphous carbon (a-C:H) film at cryogenic temperatures, and a stable ultralow-friction over a wide temperature range (103-348 K) is achieved. This is attributed to hydrogen-transfer-induced mild structural ordering transformation, confirmed by machine-learning-based molecular dynamics simulations. The anomalous sub-Arrhenius temperature dependence of structural ordering transformation rate is well-described by a quantum mechanical tunneling (QMT) modified Arrhenius model, which is correlated with quantum delocalization of hydrogen in tribochemical reactions. This work reveals a hydrogen-correlated friction mechanism overcoming the Arrhenius temperature dependence and provides a new pathway for achieving ultralow friction under cryogenic conditions.

Autophagy inhibition improves the targeted radionuclide therapy efficacy of 131I-FAP-2286 in pancreatic cancer xenografts.

PURPOSES: Radiotherapy can induce tumor cell autophagy, which might impair the antitumoral effect. This study aims to investigate the effect of autophagy inhibition on the targeted radionuclide therapy (TRT) efficacy of 131I-FAP-2286 in pancreatic cancer. METHODS: Human pancreatic cancer PANC-1 cells were exposed to 131I-FAP-2286 radiotherapy alone or with the autophagy inhibitor 3-MA. The autophagy level and proliferative activity of PANC-1 cells were analyzed. The pancreatic cancer xenograft-bearing nude mice were established by the co-injection of PANC-1 cells and pancreatic cancer-associated fibroblasts (CAFs), and then were randomly divided into four groups and treated with saline (control group), 3-MA, 131I-FAP-2286 and 131I-FAP-2286 + 3-MA, respectively. SPECT/CT imaging was performed to evaluate the bio-distribution of 131I-FAP-2286 in pancreatic cancer-bearing mice. The therapeutic effect of tumor was evaluated by 18F-FDG PET/CT imaging, tumor volume measurements, and the hematoxylin and eosin (H&E) staining, and immunohistochemical staining assay of tumor tissues. RESULTS: 131I-FAP-2286 inhibited proliferation and increased the autophagy level of PANC-1 cells in a dose-dependent manner. 3-MA promoted 131I-FAP-2286-induced apoptosis of PANC-1 cells via suppressing autophagy. SPECT/CT imaging of pancreatic cancer xenograft-bearing nude mice showed that 131I-FAP-2286 can target the tumor effectively. According to 18F-FDG PET/CT imaging, the tumor growth curves and immunohistochemical analysis, 131I-FAP-2286 TRT was capable of suppressing the growth of pancreatic tumor accompanying with autophagy induction, but the addition of 3-MA enabled 131I-FAP-2286 to achieve a better therapeutic effect along with the autophagy inhibition. In addition, 3-MA alone did not inhibit tumor growth. CONCLUSIONS: 131I-FAP-2286 exposure induces the protective autophagy of pancreatic cancer cells, and the application of autophagy inhibitor is capable of enhancing the TRT therapeutic effect.

Domino-like stacking order switching in twisted monolayer-multilayer graphene.

Atomic reconstruction has been widely observed in two-dimensional van der Waals structures with small twist angles1-7. This unusual behaviour leads to many novel phenomena, including strong electronic correlation, spontaneous ferromagnetism and topologically protected states1,5,8-14. Nevertheless, atomic reconstruction typically occurs spontaneously, exhibiting only one single stable state. Using conductive atomic force microscopy, here we show that, for small-angle twisted monolayer-multilayer graphene, there exist two metastable reconstruction states with distinct stacking orders and strain soliton structures. More importantly, we demonstrate that these two reconstruction states can be reversibly switched, and the switching can propagate spontaneously in an unusual domino-like fashion. Assisted by lattice-resolved conductive atomic force microscopy imaging and atomistic simulations, the detailed structure of the strain soliton networks has been identified and the associated propagation mechanism is attributed to the strong mechanical coupling among solitons. The fine structure of the bistable states is critical for understanding the unique properties of van der Waals structures with tiny twists, and the switching mechanism offers a viable means for manipulating their stacking states.

Fluctuation of Interfacial Electronic Properties Induces Friction Tuning under an Electric Field.

Mysteries about the origin of friction have remained for centuries. Especially, how friction is tuned by an electric field is still unclear. Present tuning mechanisms mainly focus on the atomic configurations and electrostatic force, yet the role of interfacial electronic properties is not fully understood. Here, we investigate a unique friction tuning effect induced by an electric current in a conductive atomic force microscopy experiment and uncover two main tuning mechanisms of friction by the fluctuation of electronic properties during sliding: (1) electric-field-induced electron density redistribution and (2) current-induced electron transfer. We put forward an electronic level friction model unraveling the relationship between the friction tuning and the electronic property fluctuation (EPF) under electric field/current, which is applicable to tribosystems ranging from conductors to semiconductors and insulators, including two-dimensional material interfaces. This model provides theoretical guidance for tribosystem design and friction control, proposing a new perspective in understanding the origin of friction.

Dual-Scale Stick-Slip Friction on Graphene/h-BN Moiré Superlattice Structure.

Using atomic force microscopy, we have shown that friction on graphene/h-BN superlattice structures may exhibit unusual moiré-scale stick slip in addition to the regular ones observed at the atomic scale. Such dual-scale slip instability will lead to unique length-scale dependent energy dissipation when the different slip mechanisms are sequentially activated. Assisted by an improved theoretical model and comparative experiments, we find that accumulation and unstable release of the in-plane strain of the graphene layer is the key mechanism underlying the moiré-scale behavior. This work highlights the distinct role of the internal state of the van der Waals interfaces in determining the rich dynamics and energy dissipation of layer-structured materials.

A New Pathway for Superlubricity in a Multilayered MoS2-Ag Film under Cryogenic Environment.

A fundamental cryogenic study in tribology from 20 to 300 K revealed that a kind of disulfide film could exhibit a superlubricity state. Inspired by this, we designed a more delicate experiment and reported an extremely low friction coefficient for a multilayered MoS2-Ag film in a cryogenic environment against a bare steel ball under a high load. The results showed that the multilayered MoS2-Ag film could undergo a pressure exceeding 2 GPa to maintain a superlow friction coefficient of below 0.001 at 170 K. The film material was transferred to the sliding contacts to form an antifriction tribofilm. The superlubricity mechanism was attributed to the formation of MoS2-wrapped Ag nanoparticles accumulated at the sliding interface through nanoparticle movement and layered-structure sliding. This new kind of multilayered MoS2-Ag film provides a novel design for a solid lubricant and broadens the application of solid lubrication films under harsh working conditions for mechanical engineering.

Unraveling the Friction Evolution Mechanism of Diamond-Like Carbon Film during Nanoscale Running-In Process toward Superlubricity.

Diamond-like carbon (DLC) films are capable of achieving superlubricity at sliding interfaces by a rapid running-in process. However, fundamental mechanisms governing the friction evolution during this running-in processes remain elusive especially at the nanoscale, which hinders strategic tailoring of tribosystems for minimizing friction and wear. Here, it is revealed that the running-in governing superlubricity of DLC demonstrates two sub-stages in single-asperity nanocontacts. The first stage, mechanical removal of a thin oxide layer, is described quantitatively by a stress-activated Arrhenius model. In the second stage, a large friction decrease occurs due to a structural ordering transformation, with the kinetics well described by the Johnson-Mehl-Avrami-Kolmogorov model with a modified load dependence of the activation energy. The direct observation of a graphitic-layered transfer film formation together with the measured Avrami exponent reveal the primary mechanism of the ordering transformation. The findings provide fundamental insights into friction evolution mechanisms, and design criteria for superlubricity.

Umbilical cord blood CD34+ cells administration improved neurobehavioral status and alleviated brain injury in a mouse model of cerebral palsy.

PURPOSE: Cerebral palsy (CP) is the most common neuromuscular disease in children, and currently, there is no cure. Several studies have reported the benefits of umbilical cord blood (UCB) cell treatment for CP. However, these studies either examined the effects of UCB cell fraction with a short experimental period or used neonatal rat models for a long-term study which displayed an insufficient immunological reaction and clearance of human stem cells. Here, we developed a CP model by hypoxia-ischemic injury (HI) using immunodeficient mice and examined the effects of human UCB CD34+ hematopoietic stem cells (HSCs) on CP therapy over a period of 8 weeks. METHODS: Sixty postnatal day-9 (P9) mouse pups were randomly divided into 4 groups (n = 15/group) as follows: (1) sham operation (control group), (2) HI-induced CP model, (3) CP model with CD34+ HSC transplantation, and (4) CP model with CD34- cell transplantation. Eight weeks after insult, the sensorimotor performance was analyzed by rotarod treadmill, gait dynamic, and open field assays. The pathological changes in brain tissue of mice were determined by HE staining, Nissl staining, and MBP immunohistochemistry of the hippocampus in the mice. RESULTS: HI brain injury in mice pups resulted in significant behavioral deficits and loss of neurons. Both CD34+ HSCs and CD34- cells improved the neurobehavioral statuses and alleviated the pathological brain injury. In comparison with CD34- cells, the CD34+ HSC compartments were more effective. CONCLUSION: These findings indicate that CD34+ HSC transplantation was neuroprotective in neonatal mice and could be an effective therapy for CP.

Understanding Interlayer Contact Conductance in Twisted Bilayer Graphene.

Bilayer or few-layer 2D materials showing novel electrical properties in electronic device applications have aroused increasing interest in recent years. Obtaining a comprehensive understanding of interlayer contact conductance still remains a challenge, but is significant for improving the performance of bilayer or few-layer 2D electronic devices. Here, conductive atomic force microscope (C-AFM) experiments are reported to explore the interlayer contact conductance between bilayer graphene (BLG) with various twisted stacking structures fabricated by the chemical vapor deposition (CVD) method. The current maps show that the interlayer contact conductance between BLG strongly depends on the twist angle. The interlayer contact conductance of 0° AB-stacking bilayer graphene (AB-BLG) is ≈4 times as large as that of 30° twisted bilayer graphene (t-BLG), which indicates that the twist angle-dependent interlayer contact conductance originates from the coupling-decoupling transitions. Moreover, the moiré superlattice-level current images of t-BLG show modulations of local interlayer contact conductance. Density functional theory calculations together with a theoretical model reproduce the C-AFM current map and show that the modulation is mainly attributed to the overall contribution of local interfacial carrier density and tunneling barrier.

Structure and tensile properties of the forewing costal vein of the honeybee Apis mellifera.

In this study, we investigated the morphological features and tensile properties of the forewing costal vein of the honeybee (Apis mellifera) under fresh, dry and in vitro-time varied conditions. The costal vein is composed of an outer sub-vein and an inner vein starting from the wing base to nearly 50% of the wing span and then they are fused into one vein extending to the wing tip. Confocal laser scanning microscopy revealed that the outer sub-vein with red autofluorescence is stiffer than the inner one with green autofluorescence, and the membrane in the gap between the sub-veins exhibited a long blue-autofluorescence resilin stripe. Considering the irregular cross-sectional shape of the costal vein, cross-sections of the tested specimens after tensile failure were analysed using scanning electron microscopy, to precisely calculate their cross-sectional areas by a customized MATLAB program. The Young's modulus and tensile strength of fresh specimens were ∼4.78 GPa and ∼119.84 MPa, which are lower than those of dry specimens (∼9.08 GPa and ∼154.45 MPa). However, the tensile strain had the opposite relationship (fresh: ∼0.031, dry: ∼0.018). Thus, specimen desiccation results in increasing stiffness and brittleness. The morphological features and material properties of the costal vein taken together represent a tradeoff between both deformability and stiffness. Our study provides guidance for material selection and bionic design of the technical wings of flapping micro aerial vehicles.

Modeling Atomic-Scale Electrical Contact Quality Across Two-Dimensional Interfaces.

Contacting interfaces with physical isolation and weak interactions usually act as barriers for electrical conduction. The electrical contact conductance across interfaces has long been correlated with the true contact area or the "contact quantity". Much of the physical understanding of the interfacial electrical contact quality was primarily based on Landauer's theory or Richardson formulation. However, a quantitative model directly connecting contact conductance to interfacial atomistic structures still remains absent. Here, we measure the atomic-scale local electrical contact conductance instead of local electronic surface states in graphene/Ru(0001) superstructure, via atomically resolved conductive atomic force microscopy. By defining the "quality" of individual atom-atom contact as the carrier tunneling probability along the interatomic electron transport pathways, we establish a relationship between the atomic-scale contact quality and local interfacial atomistic structure. This real-space model unravels the atomic-level spatial modulation of contact conductance, and the twist angle-dependent interlayer conductance between misoriented graphene layers.

Layer-dependent anisotropic frictional behavior in two-dimensional monolayer hybrid perovskite/ITO layered heterojunctions.

Two-dimensional (2D) organic-inorganic hybrid perovskites, which possess outstanding optical and electrical properties, are promising semiconductor materials that have attracted significant interest in widespread applications. The frictional behavior of 2D perovskite materials with other transparent conductive materials, such as indium tin oxide (ITO), offers promising developments in optoelectronic devices. Therefore, the understanding of this frictional behavior is essential. Atomic force microscopy (AFM) is employed here to measure the frictional behavior between the (001) plane of the 2D organic-inorganic hybrid (C4H9NH3)2PbBr4 perovskite and the (111) plane of the ITO. The experimental analyses characterizing the nature of the friction in a single-crystalline heterojunction are reported. Based on the results of the analyses of interfaces between 2D monolayer perovskites and ITO, a strong anisotropy of friction is clearly demonstrated. The anisotropy of friction is observed as a four-fold symmetry with low a frictional coefficient, 0.035, in misaligned contacts, and, 0.015, in aligned contacts in the heterojunction configuration. In addition, atomistic simulations reveal underlying frictional mechanisms in the dynamical regimes. A new phenomenon discovered in the studies establishes that the measured frictional anisotropy surprisingly depends on the number of atomic layers in the 2D perovskite. The frictional anisotropy decreases significantly with the increase in the number of layers up to 16 layers, and then it becomes independent of the thickness. Our results are predicted to be of a general nature and should be applicable to other 2D hybrid perovskite heterojunction configurations, and thus, furthers the development of adaptive and stretchable optoelectronic nanodevices.

Tuning Local Electrical Conductivity via Fine Atomic Scale Structures of Two-Dimensional Interfaces.

Two-dimensional (2D) materials have seen a broad range of applications in electronic and optoelectronic applications; however, full realization of this potential hitherto largely hinges on the quality and performance of the electrical contacts formed between 2D materials and their surrounding metals/semiconductors. Despite the progress in revealing the charge injecting mechanisms and enhancing electrical conductance using various interfacial treatments, how the microstructure of contact interfaces affects local electrical conductivity is still very limited. Here, using conductive atomic force microscopy (c-AFM), for the first time, we directly confirm the conjecture that the electrical conductivity of physisorbed 2D material-metal/semiconductor interfaces is determined by the local electronic charge transfer. Using lattice-resolved conductivity mapping and first-principles calculations, we demonstrate that the electronic charge transfer, thereby electrical conductivity, can be fine-tuned by the topological defects of 2D materials and the atomic stacking with respect to the substrate. Our finding provides a novel route to engineer the electrical contact properties by exploiting fine atomic interactions; in the meantime, it also suggests a convenient and nondestructive means of probing subtle interactions along 2D heterogeneous interfaces.

Three-Dimensional Graphene Networks with Abundant Sharp Edge Sites for Efficient Electrocatalytic Hydrogen Evolution.

To achieve sustainable production of hydrogen (H2 ) through water splitting, establishing efficient and earth-abundant electrocatalysts is of great necessity. Morphology engineering of graphene is now shown to modulate the electronic structure of carbon skeleton and in turn endow it with excellent ability of proton reduction. Three-dimensional (3D) graphene networks with a high density of sharp edge sites are synthesized. Electrocatalytic measurements indicate that the obtained 3D graphene networks can electrocatalyze H2 evolution with an extremely low onset potential of about 18 mV in 0.5 m H2 SO4 solution, together with good stability. A combination of control experiments and density functional theory (DFT) investigations indicates that the exceptional H2 evolution performance is attributed to the abundant sharp edge sites of the advanced frameworks, which are responsible for promoting the adsorption and reduction of protons.

Surface Orientation and Temperature Effects on the Interaction of Silicon with Water: Molecular Dynamics Simulations Using ReaxFF Reactive Force Field.

In this work, we use ReaxFF molecular dynamics simulations to investigate the interaction between water molecules and silicon surfaces with different orientations under ambient temperatures of 300 and 500 K. We studied the water adsorption and dissociation processes as well as the silicon oxidation process on the Si (100), (110), and (111) surfaces. The simulation results indicate that water can adsorb on the Si surfaces in the forms of molecular adsorption and dissociative adsorption, making the surfaces terminated by H2O, OH, and H species. The molecular adsorption of H2O dominates the (100) and (110) surfaces, whereas the dissociative adsorption dominates the (111) surface. Besides, the adsorbed hydroxyl oxygen can insert into the Si-Si bond of the substrate to make the surface oxidized, forming the Si-O-Si bonds. Our simulation results also indicate that the (100) surface is mostly terminated by H whereas (111) is mostly terminated by OH. The higher temperature causes more H2O to dissociate and also make all these surfaces more oxidized. Our results are consistent with most experiments. This study sheds lights on the wet oxidation process of Si and Si surface structure evolution in microelectromechanical systems as well as the Si chemical mechanical polishing process.

[Biological characteristics of mesenchymal stem cell and hematopoietic stem cell in the co-culture system].

The aim of the present study was to obtain the qualified hematopoietic stem/progenitor cells (HSC/HPC) and human umbilical cord-mesenchymal stem cells (MSC) in vitro in the co-culture system. Cord blood mononuclear cells were separated from umbilical cord blood by Ficoll lymphocyte separation medium, and then CD34+ HSC was collected by MACS immunomagnetic beads. The selected CD34+ HSC/HPC and MSC were transferred into culture flask. IMDM culture medium with 15% AB-type cord plasma supplemented with interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), stem cell factor (SCF) and FMS-like tyrosine kinase 3 ligand (Flt-3L) factors were used as the co-culture system for the amplification of HSC/HPC and MSC. The cellular growth status and proliferation on day 6 and 10 after co-culture were observed by using inverted microscope. The percentage of positive expression of CD34 in HSC/HPC, as well as the percentages of positive expressions of CD105, CD90, CD73, CD45, CD34 and HLA-DR in the 4th generation MSC, was tested by flow cytometry. Semisolid colony culture was used to test the HSC/HPC colony forming ability. The osteogenic, chondrogenesis and adipogenic ability of the 4th generation MSC were assessed. The karyotype analysis of MSC was conducted by colchicines. The results demonstrated that the HSC/HPC of co-culture group showed higher ability of amplification, CFU-GM and higher CD34+ percentage compared with the control group. The co-cultured MSC maintained the ability to differentiate into bone cells, fat cells and chondrocytes. And the karyotype stability of MSC remained normal. These results reveal that the appropriate co-culture system for MSC and HSC is developed, and via this co-culture system we could gain both two kinds of these cells. The MSCs under the co-culture system maintain the biological characteristics. The CFU-GM ability, cell counting and the flow cytometry results of HSC/HPC under the co-culture system are conform to the criterion, showing that the biological functions of HSC/HPC are maintained.

Tribochemical mechanism of amorphous silica asperities in aqueous environment: a reactive molecular dynamics study.

Reactive molecular dynamics (ReaxFF) simulations are used to explore the atomic-level tribochemical mechanism of amorphous silica (a-SiO2) in a nanoscale, single-asperity contact in an aqueous environment. These sliding simulations are performed in both a phosphoric acid solution and in pure water under different normal pressures. The results show that tribochemical processes have profound consequences on tribological performance. Water molecules could help avoid direct adhesive interaction between a-SiO2 surfaces in pure water under low normal load. However, formation and rupture of interfacial siloxane bonds are obviously observed under higher normal load. In phosphoric acid solution, polymerization of phosphoric acid molecules occurs, yielding oligomers under lower load, and tribochemical reactions between the molecules and the sliding surfaces could enhance wear under higher load. The bridging oxygen atoms in silica play an important role in the formation of interfacial covalent bonds, and hydrogen is found to have a weakening effect on these bonds, resulting in the rupture during shear-related loading. This work sheds light on tribochemical reactions as a mechanism for lubrication and wear in water-based or other tribological systems.

Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study.

The atomic-scale friction of the fluorographene (FG)/MoS2 heterostructure is investigated using first-principles calculations. Due to the intrinsic lattice mismatch and formation of periodic Moiré patterns, the potential energy surface of the FG/MoS2 heterostructure is ultrasmooth and the interlayer shear strength is reduced by nearly two orders of magnitude, compared with both FG/FG and MoS2/MoS2 bilayers, entering the superlubricity regime. The size dependency of superlubricity is revealed as being based on the relationship between the emergence of Moiré patterns and the lattice mismatch ratio for heterostructures.

Molecular dynamics simulations of wetting behavior of water droplets on polytetrafluorethylene surfaces.

Molecular dynamics simulations are performed to simulate the wetting behavior of nanosized water droplets on flat and pillar polytetrafluorethylene surfaces. The results show that the cutoff of the Lennard-Jones (LJ) potential has a large effect on the simulated value of the contact angle and some suggestions are given on how to choose an appropriate cutoff. On flat surfaces, the contact angle is independent of the size of the water droplet, which was determined by the energy parameters of the LJ potential. Furthermore, on pillar surfaces, two different equilibrium states are present: wetted contact and cross contact. For the wetted contact state, the contact angle increases with increasing droplet size and pillar size within a certain range. However, for the cross contact state, the contact angle and droplet size are uncorrelated, which results from the layering and structuring of molecules after their penetration into the hollows between pillars. However, additional simulations show that the final state depends on the initial geometry and the cross contact state is a metastable wetting state.