Versatile technique for assessing thickness of 2D layered materials by XPS.

X-ray photoelectron spectroscopy (XPS) has been utilized as a versatile method for thickness characterization of various two-dimensional (2D) films. Accurate thickness can be measured simultaneously while acquiring XPS data for chemical characterization of 2D films having thickness up to approximately 10 nm. For validating the developed technique, thicknesses of few-layer graphene (FLG), MoS2 and amorphous boron nitride (a-BN) layer, produced by microwave plasma chemical vapor deposition (MPCVD), plasma enhanced chemical vapor deposition (PECVD), and pulsed laser deposition (PLD) respectively, were accurately measured. The intensity ratio between photoemission peaks recorded for the films (C 1s, Mo 3d, B 1s) and the substrates (Cu 2p, Al 2p, Si 2p) is the primary input parameter for thickness calculation, in addition to the atomic densities of the substrate and the film, and the corresponding electron attenuation length (EAL). The XPS data was used with a proposed model for thickness calculations, which was verified by cross-sectional transmission electron microscope (TEM) measurement of thickness for all the films. The XPS method determines thickness values averaged over an analysis area which is orders of magnitude larger than the typical area in cross-sectional TEM imaging, hence provides an advanced approach for thickness measurement over large areas of 2D materials. The study confirms that the versatile XPS method allows rapid and reliable assessment of the 2D material thickness and this method can facilitate in tailoring growth conditions for producing very thin 2D materials effectively over a large area. Furthermore, the XPS measurement for a typical 2D material is non-destructive and does not require special sample preparation. Therefore, after XPS analysis, exactly the same sample can undergo further processing or utilization.

Thermal anisotropy in nano-crystalline MoS2 thin films.

In this work, we grow thin MoS2 films (50-150 nm) uniformly over large areas (>1 cm(2)) with strong basal plane (002) or edge plane (100) orientations to characterize thermal anisotropy. Measurement results are correlated with molecular dynamics simulations of thermal transport for perfect and defective MoS2 crystals. The correlation between predicted (simulations) and measured (experimental) thermal conductivity are attributed to factors such as crystalline domain orientation and size, thereby demonstrating the importance of thermal boundary scattering in limiting thermal conductivity in nano-crystalline MoS2 thin films. Furthermore, we demonstrate that the cross-plane thermal conductivity of the films is strongly impacted by exposure to ambient humidity.

MoVN-Cu Coatings for In Situ Tribocatalytic Formation of Carbon-Rich Tribofilms in Low-Viscosity Fuels.

Inhibiting the tribological failure of mechanical assemblies which rely on fuels for lubrication is an obstacle to maintaining the lifetime of these systems with low-viscosity and low-lubricity fuels. In the present study, a MoVN-Cu nanocomposite coating was tribologically evaluated for durability in high- and low-viscosity fuels as a function of temperature, load, and sliding velocity conditions. The results indicate that the MoVN-Cu coating is effective in decreasing wear and friction relative to an uncoated steel surface. Raman spectroscopy, transmission electron microscopy, and electron-dispersive spectroscopy analysis of the MoVN-Cu worn surfaces confirmed the presence of an amorphous carbon-rich tribofilm which provides easy shearing and low friction during sliding. Further, the characterization of the formed tribofilm revealed the presence of nanoscale copper clusters overlapping with the carbon peak intensities supporting the tribocatalytic origin of the surface protection. The tribological assessment of the MoVN-Cu coating reveals that the coefficient of friction decreased with increasing material wear and initial contact pressure. These findings suggest that MoVN-Cu is a promising protective coating for fuel-lubricated assemblies due to its adaptive ability to replenish lubricious tribofilms from hydrocarbon environments.

Temperature driven evolution of thermal, electrical, and optical properties of Ti-Al-N coatings.

Monolithic single phase cubic (c) Ti1-x Al x N thin films are used in various industrial applications due to their high thermal stability, which beneficially effects lifetime and performance of cutting and milling tools, but also find increasing utilization in electronic and optical devices. The present study elucidates the temperature-driven evolution of heat conductivity, electrical resistivity and optical reflectance from room temperature up to 1400 °C and links them to structural and chemical changes in Ti1-x Al x N coatings. It is shown that various decomposition phenomena, involving recovery and spinodal decomposition (known to account for the age hardening phenomenon in c-Ti1-x Al x N), as well as the cubic to wurtzite phase transformation of spinodally formed AlN-enriched domains, effectively increase the thermal conductivity of the coatings from ∼3.8 W m-1 K-1 by a factor of three, while the electrical resistivity is reduced by one order of magnitude. A change in the coating color from metallic grey after deposition to reddish-golden after annealing to 1400 °C is related to the film structure and discussed in terms of film reflectivity.

Vacuum phonon tunneling.

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

Processing, structure, and properties of nanostructured multifunctional tribological coatings.

Nanostructured, nanocomposite binary (TiC-a:C), ternary (Cr-Al-N), quaternary (Ti-B-C-N) and quinternary (Ti-Si-B-C-N) multicomponent films have been deposited using unbalanced magnetron sputtering (UBMS) and closed field unbalanced magnetron sputtering (CFUBMS) from both elemental and composite targets. Approaches to control the film chemistry, volume fraction and size of the multicomponent species, and pulsed ion energy (ion flux) bombardment to tailor the structure and properties of the films for specific tribological applications, e.g., low friction coefficient and low wear rate, are emphasized. The synthesized films are characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), nanoindentation, and microtribometry. The relationships between processing parameters (pulsed ion energy and ion flux), thin film microstructure, mechanical and tribological properties are being investigated in terms of the nanocrystalline-nanocrystalline and nanocrystalline-amorphous composite thin film systems that are generated. In the Ti-Si-B-C-N films, nanocomposites of solid solutions, e.g., nanosized (Ti,C,N)B2 and Ti(C,N) crystallites are embedded in an amorphous TiSi2 and SiC matrix including some carbon, SiB4, BN, CN(x), TiO2 and B2O3 components. The Ti-Si-B-C-N coating with up to 150 W Si target power exhibited a hardness of about 35 GPa, a high H/E ratio of 0.095, and a low wear rate of from approximately 3 to approximately 10 x 10(-6) mm3/(Nm). In another aspect, using increased ion energy and ion flux, which are generated by pulsing the power of the target(s) in a closed field arrangement, to provide improved ion bombardment on tailoring the structure and properties of TiC-a:C and Cr-Al-N coatings are demonstrated. It was found that highly energetic species (up to hundreds eV) were found in the plasma by pulsing the power of the target(s) during magnetron sputtering. Applying higher pulse frequency and longer reverse time (lower duty cycle) will result in higher ion energy and ion flux in the plasma, which can be utilized to improve the film structure and properties. For example, optimum properties of the TiC-a:C coating were a hardness of 35 to 40 GPa and a COF of 0.2 to 0.22 for moderate maximum ion energies of 70 to 100 eV, and a super high hardness of 41 GPa and low wear rate of 3.41 x 10(-6) mm3N(-1) m(-1) was obtained for Cr-Al-N coatings deposited with a maximum ion energy of 122 eV. These conditions can be achieved by adjusting the pulsing parameters and target voltages. However, the pulsed ion energy together with the applied substrate bias are need to be carefully controlled in order to avoid excessive ion bombardment (e.g., the maximum ion energy is larger than 180 eV in the current study), which will responsible for an increase in point and line defects, and high residual stress in the crystalline structure.

TiCaPCON-Supported Pt- and Fe-Based Nanoparticles and Related Antibacterial Activity.

A rapid increase in the number of antibiotic-resistant bacteria urgently requires the development of new more effective yet safe materials to fight infection. Herein, we uncovered the contribution of different metal nanoparticles (NPs) (Pt, Fe, and their combination) homogeneously distributed over the surface of nanostructured TiCaPCON films in the total antibacterial activity toward eight types of clinically isolated bacterial strains (Escherichia coli K261, Klebsiella pneumoniae B1079k/17-3, Acinetobacter baumannii B1280A/17, Staphylococcus aureus no. 839, Staphylococcus epidermidis i5189-1, Enterococcus faecium Ya-235: VanA, E. faecium I-237: VanA, and E. coli U20) taking into account various factors that can affect bacterial mechanisms: surface chemistry and phase composition, wettability, ion release, generation of reactive oxygen species (ROS), potential difference and polarity change between NPs and the surrounding matrix, formation of microgalvanic couples on the sample surfaces, and contribution of a passive oxide layer, formed on the surface of films, to general kinetics of the NP dissolution. The results indicated that metal ion implantation and subsequent annealing significantly changed the chemistry of the TiCaPCON film surface. This, in turn, greatly affected the shedding of ions, ROS formation, potential difference between film components, and antibacterial activity. The presence of NPs was critical for ROS generation under UV or daylight irradiation. By eliminating the potential contribution of ions and ROS, we have shown that bacteria can be killed using direct microgalvanic interactions. The possibility of charge redistribution at the interfaces between Pt NPs and TiO2 (anatase and rutile), TiC, TiN, and TiCN components was demonstrated using density functional theory calculations. The TiCaPCON-supported Pt and Fe NPs were not toxic for lymphocytes and had no effect on the ability of lymphocytes to activate in response to a mitogen. This study provides new insights into understanding and designing of antibacterial yet biologically safe surfaces.

Hexagonal MoTe2 with Amorphous BN Passivation Layer for Improved Oxidation Resistance and Endurance of 2D Field Effect Transistors.

Environmental and thermal stability of two-dimensional (2D) transition metal dichalcogenides (TMDs) remains a fundamental challenge towards enabling robust electronic devices. Few-layer 2H-MoTe2 with an amorphous boron nitride (a-BN) covering layer was synthesized as a channel for back-gated field effect transistors (FET) and compared to uncovered MoTe2. A systematic approach was taken to understand the effects of heat treatment in air on the performance of FET devices. Atmospheric oxygen was shown to negatively affect uncoated MoTe2 devices while BN-covered FETs showed considerably enhanced chemical and electronic characteristic stability. Uncapped MoTe2 FET devices, which were heated in air for one minute, showed a polarity switch from n- to p-type at 150 °C, while BN-MoTe2 devices switched only after 200 °C of heat treatment. Time-dependent experiments at 100 °C showed that uncapped MoTe2 samples exhibited the polarity switch after 15 min of heat treatment while the BN-capped device maintained its n-type conductivity for the maximum 60 min duration of the experiment. X-ray photoelectron spectroscopy (XPS) analysis suggests that oxygen incorporation into MoTe2 was the primary doping mechanism for the polarity switch. This work demonstrates the effectiveness of an a-BN capping layer in preserving few-layer MoTe2 material quality and controlling its conductivity type at elevated temperatures in an atmospheric environment.

Effect of Substrate Support on Dynamic Graphene/Metal Electrical Contacts.

Recent advances in graphene and other two-dimensional (2D) material synthesis and characterization have led to their use in emerging technologies, including flexible electronics. However, a major challenge is electrical contact stability, especially under mechanical straining or dynamic loading, which can be important for 2D material use in microelectromechanical systems. In this letter, we investigate the stability of dynamic electrical contacts at a graphene/metal interface using atomic force microscopy (AFM), under static conditions with variable normal loads and under sliding conditions with variable speeds. Our results demonstrate that contact resistance depends on the nature of the graphene support, specifically whether the graphene is free-standing or supported by a substrate, as well as on the contact load and sliding velocity. The results of the dynamic AFM experiments are corroborated by simulations, which show that the presence of a stiff substrate, increased load, and reduced sliding velocity lead to a more stable low-resistance contact.

Nanoparticle-wetted surfaces for relays and energy transmission contacts.

Submonolayer coatings of noble-metal nanoparticle liquids (NPLs) are shown to provide replenishable surfaces with robust asperities and metallic conductivity that extends the durability of electrical relays by 10 to 100 times (depending on the current driven through the contact) as compared to alternative approaches. NPLs are single-component materials consisting of a metal nanoparticle core (5-20 nm Au or Pt nanoparticles) surrounded by a covalently tethered ionic-liquid corona of 1.5 to 2 nm. Common relay failure modes, such as stiction, surface distortion, and contact shorting, are suppressed with the addition of a submonolayer of NPLs to the contact surfaces. This distribution of NPLs results in a force profile for a contact-retraction cycle that is distinct from bare Au contacts and thicker, multilayer coatings of NPLs. Postmortem examination reveals a substantial decrease in topological change of the electrode surface relative to bare contacts, as well as an indication of lateral migration of the nanoparticles from the periphery towards the contact. A general extension of this concept to dynamic physical interfaces experiencing impact, sliding, or rolling affords alternatives to increase reliability and reduced losses for transmittance of electrical and mechanical energy.

Scanning Tunneling Microscopy Observation of Phonon Condensate.

Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.

Pulsed laser syntheses of layer-structured WS2 nanomaterials in water.

We used water as an environmental friendly medium for the synthesis of hexagonal WS2 nanoparticles by the pulsed laser method. The materials collected on substrates were oriented with the 2H-WS2 basal planes parallel to the surface. The use of water, UV lasers, and large WS2 targets prevented the nanoparticles from restructuring into inorganic fullerenes, which were observed in research using hydrocarbon solvents, longer wavelength lasers, and dispersed powder targets. Fairly good dispersion of nanoparticles suggests that large surface areas are available for chemical reactivity.

Ab initio study of basal slip in Nb(2)AlC.

Using ab initio calculations, we have studied shearing in Nb(2)AlC, where NbC and Al layers are interleaved. The stress-strain analysis of this deformation mode reveals Nb-Al bond breaking, while the Nb-C bond length decreases by 4.1%. Furthermore, there is no evidence for phase transformation during deformation. This is consistent with basal slip and may be understood on the basis of the electronic structure: bands below the Fermi level are responsible for the dd bonding between NbC basal planes and only a single band with a weak dd interaction is not resistant to shearing, while all other bands are unaffected. The Al-Nb bonding character can be described as mainly metallic with weak covalent-ionic contributions. Our study demonstrates that Al layers move with relative ease under shear strain. Phase conservation upon shearing is unusual for carbides and may be due to the layered nature of the phase studied. Here, we describe the electronic origin of basal slip in Nb(2)AlC, the atomic mechanism which enables reversible plasticity in this class of materials.

Activating "Invisible" Glue: Using Electron Beam for Enhancement of Interfacial Properties of Graphene-Metal Contact.

Interfacial contact of two-dimensional graphene with three-dimensional metal electrodes is crucial to engineering high-performance graphene-based nanodevices with superior performance. Here, we report on the development of a rapid "nanowelding" method for enhancing properties of interface to graphene buried under metal electrodes using a focused electron beam induced deposition (FEBID). High energy electron irradiation activates two-dimensional graphene structure by generation of structural defects at the interface to metal contacts with subsequent strong bonding via FEBID of an atomically thin graphitic interlayer formed by low energy secondary electron-assisted dissociation of entrapped hydrocarbon contaminants. Comprehensive investigation is conducted to demonstrate formation of the FEBID graphitic interlayer and its impact on contact properties of graphene devices achieved via strong electromechanical coupling at graphene-metal interfaces. Reduction of the device electrical resistance by ∼50% at a Dirac point and by ∼30% at the gate voltage far from the Dirac point is obtained with concurrent improvement in thermomechanical reliability of the contact interface. Importantly, the process is rapid and has an excellent insertion potential into a conventional fabrication workflow of graphene-based nanodevices through single-step postprocessing modification of interfacial properties at the buried heterogeneous contact.

Approach to multifunctional device platform with epitaxial graphene on transition metal oxide.

Heterostructures consisting of two-dimensional materials have shown new physical phenomena, novel electronic and optical properties, and new device concepts not observed in bulk material systems or purely three dimensional heterostructures. These new effects originated mostly from the van der Waals interaction between the different layers. Here we report that a new optical and electronic device platform can be provided by heterostructures of 2D graphene with a metal oxide (TiO2). Our novel direct synthesis of graphene/TiO2 heterostructure is achieved by C60 deposition on transition Ti metal surface using a molecular beam epitaxy approach and O2 intercalation method, which is compatible with wafer scale growth of heterostructures. As-grown heterostructures exhibit inherent photosensitivity in the visible light spectrum with high photo responsivity. The photo sensitivity is 25 times higher than that of reported graphene photo detectors. The improved responsivity is attributed to optical transitions between O 2p orbitals in the valence band of TiO2 and C 2p orbitals in the conduction band of graphene enabled by Coulomb interactions at the interface. In addition, this heterostructure provides a platform for realization of bottom gated graphene field effect devices with graphene and TiO2 playing the roles of channel and gate dielectric layers, respectively.

Photo-sensitivity of large area physical vapor deposited mono and bilayer MoS2.

We present photosensitivity in large area physical vapour deposited mono and bi-layer MoS2 films. Photo-voltaic effect was observed in single layer MoS2 without any apparent rectifying junctions, making device fabrication straightforward. For bi-layers, no such effect was present, suggesting strong size effect in light-matter interaction. The photo-voltaic effect was observed to highly direction dependent in the film plane, which suggests that the oblique deposition configuration plays a key role in developing the rectifying potential gradient. To the best of our knowledge, this is the first report of any large area and transfer free MoS2 photo device with performance comparable to their exfoliated counterparts.

Controlling the physicochemical state of carbon on graphene using focused electron-beam-induced deposition.

Focused electron-beam-induced deposition (FEBID) is a promising nanolithography technique using "direct-write" patterning by carbon line and dot deposits on graphene. Understanding interactions between deposited carbon molecules and graphene enables highly localized modification of graphene properties, which is foundational to the FEBID utility as a nanopatterning tool. In this study, we demonstrate a unique possibility to induce dramatically different adsorption states of FEBID-produced carbon deposits on graphene, through density functional theory calculations and complementary Raman experiments. Specifically, an amorphous carbon deposit formed by direct irradiation of high energy primary electrons exhibits unusually strong interactions with graphene via covalent bonding, whereas the FEBID carbon formed due to low-energy secondary electrons is only weakly interacting with graphene via physisorption. These observations not only are of fundamental importance to basic physical chemistry of FEBID carbon-graphene interactions but also enable the use of selective laser-assisted postdeposition ablation to effectively remove the parasitically deposited, physisorbed carbon films for improving FEBID patterning resolution.

Using amphiphilic nanostructures to enable long-range ensemble coalescence and surface rejuvenation in dropwise condensation.

Controlling coalescence events in a heterogeneous ensemble of condensing droplets on a surface is an outstanding fundamental challenge in surface and interfacial sciences, with a broad practical importance in applications ranging from thermal management of high-performance electronic devices to moisture management in high-humidity environments. Nature-inspired superhydrophobic surfaces have been actively explored to enhance heat and mass transfer rates by achieving favorable dynamics during dropwise condensation; however, the effectiveness of such chemically homogeneous surfaces has been limited because condensing droplets tend to form as pinned Wenzel drops rather than mobile Cassie ones. Here, we introduce an amphiphilic nanostructured surface, consisting of a hydrophilic base with hydrophobic tips, which promotes the periodic regeneration of nucleation sites for small droplets, thus rendering the surface self-rejuvenating. This unique amphiphilic nanointerface generates an arrangement of condensed Wenzel droplets that are fluidically linked by a wetted sublayer, promoting previously unobserved coalescence events where numerous droplets simultaneously merge, without direct contact. Such ensemble coalescences rapidly create fresh nucleation sites, thereby shifting the overall population toward smaller droplets and enhancing the rates of mass and heat transfer during condensation.

In situ transmission electron microscopy of solid-liquid phase transition of silica encapsulated bismuth nanoparticles.

The solid-liquid phase transition of silica encapsulated bismuth nanoparticles was studied by in situ transmission electron microscopy (TEM). The nanoparticles were prepared by a two-step chemical synthesis process involving thermal decomposition of organometallic precursors for nucleating bismuth and a sol-gel process for growing silica. The microstructural and chemical analyses of the nanoparticles were performed using high-resolution TEM, Z-contrast imaging, focused ion beam milling, and X-ray energy dispersive spectroscopy. Solid-liquid-solid phase transitions of the nanoparticles were directly recorded by electron diffractions and TEM images. The silica encapsulation of the nanoparticles prevented agglomeration and allowed particles to preserve their original volume upon melting, which is desirable for applications of phase change nanoparticles with consistently repeatable thermal properties.

Synthesis of continuous TiC nanofibers and/or nanoribbons through electrospinning followed by carbothermal reduction.

Continuous titanium carbide (TiC) nanofibers that possess an intriguing nanoribbon morphology with a width and thickness of approximately 300 nm and approximately 40 nm, respectively, and containing TiC crystallites with sizes ranging from 5 nm to 30 nm were synthesized through electrospinning followed by carbothermal reduction.