John E. Bultman

Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition

Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350 °C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters.

Investigation into three-dimensional laser processing of tribological coatings

Abstract Tribological coating design has evolved from single layer/single phase to multilayer and composite architectures. The early single-layer coatings were not efficient for arresting cracks, distributing loads, relaxing stress, and preventing adhesive failures. Second-generation coatings with multilayer, gradient, and composite architectures added another dimension to the coating design and allowed much better accommodation of stresses and crack arresting. This paper considers the further evolution of coating design to three dimensions, where the lateral property variation was added to the cross-thickness property variation. The three-dimensional design considerably improved the tribological characteristics of hard coatings, by permitting solid lubricant replenishment inside the friction contacts. A functionally gradient Ti–TiC–TiC/diamond-like carbon coating with an upper layer of a tough nanocrystalline/amorphous composite was used for load support, crack prevention, and stress equalization. This coating was processed with laser irradiation to form grooved tracks along wear paths, which were then filled with MoS2. This provided a solid lubricant reservoir in the lateral dimension of the coating. The three-dimensional coating was tested in long-duration sliding tests at fixed and cycling humidity. The coating exhibited environmental adaptation with friction coefficients of 0.15 in humid air and 0.02 in dry nitrogen. The wear life was increased by at least one order of magnitude in comparison to that for a hard gradient coating with a top layer of MoS2 without any three-dimensional laser processing. Discussions of three-dimensional coating tribological properties, friction and wear mechanisms are provided. The environmental adaptation achieved with three-dimensional coating processing can be beneficial for aerospace applications.

Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition

Abstract The tribological properties of diamond-like carbon (DLC) coatings produced by pulsed laser deposition (PLD) are investigated. Films are grown onto steel substrates to 0.5 μm using a 248 nm laser to ablate graphite and polycarbonate targets in high vacuum. Chemical bonding is studied with Raman, XPS and EELS techniques; mechanical and tribological properties are evaluated using microindentation and ball-on-disk friction tests. Coatings grown from graphite targets are amorphous DLC (a-C), while those grown from polycarbonate targets are amorphous hydrogenated carbon (a-C:H). The hardness of the a-C coatings is 55–70 GPa and the hardness of the a-C:H coatings is 12–20 GPa depending on the substrate bias. Friction coefficients of the coatings against steel and sapphire balls are determined in several environments: in air as a function of relative humidity, in dry nitrogen, and in 10 Pa vacuum. For a-C coatings, the friction coefficients are typically below 0.1 and are as low as 0.03 in dry nitrogen. In wear tests, a critical contact pressure of 1.4 GPa led to catastrophic adhesive failure of a-C coatings, whereas failure of a-C:H coatings is by wear-through after 5 x 10 3 cycles. Extremely low wear rates of 10 −9 mm 3 N −1 m −1 are found for a-C coatings at the contact pressure of 0.8 GPa.

Characterization of air-annealed, pulsed laser deposited ZnO-WS2 solid film lubricants by transmission electron microscopy

Abstract ZnO-WS 2 is a candidate high temperature solid film lubricant for aerospace applications that exhibits adaptive lubricant behavior. In the as-deposited state, room temperature (RT) pulsed laser deposited (PLD) ZnO-WS 2 films are amorphous, but when wear-tested, the crystalline phases WS 2 , WO 3 and ZnWO 4 are produced. Of these, WS 2 is a lubricant phase at low temperatures (≤ ∼ 450°C) while ZnWO 4 , which is formed by reaction of the film with air, becomes lubricious above 600°C. If this material is to be used at elevated temperatures, the characterization of the microstructural and chemical changes that occur when these films are heated in air is extremely important to the understanding of the dynamics of this system. As-deposited films and films heated in air at increasing temperatures to 800°C were examined by transmission electron microscopy (TEM), Raman spectroscopy, and scanning electron microscopy (SEM). Cross-sectional TEM (XTEM) of the as-deposited RT-PLD ZnO-WS 2 films showed that they were fully dense and amorphous. A periodic structure was seen that was due to density variations and was attributed to a smaller angular distribution of W in the plume compared to the other elements. At 500°C, an approximately 37 nm-thick film of WO 3 and ZnWO 4 formed at the surface. At 600°C, a 150–200 nm-thick mixed oxide layer of ZnWO 4 and WO 3 formed at the surface with the WS 2 phase forming below it. The volume fraction of WS 2 decreased with increasing depth from the surface. Above 600°C, surface roughening of the film was seen as well as significant grain growth of the WO 3 and ZnWO 4 phases. The ZnO phase was not detected in any of the films heated in air. The dynamics of the nucleation of these lubricant phases are advantageous with respect to applications: the high temperature lubricant phase, ZnWO 4 , is available at the surface while the low temperature phase, WS 2 , remains intact to provide lubrication when the temperature is lowered.

Cross-plane thermal properties of transition metal dichalcogenides

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a >10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5–10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals.

TiC coatings prepared by pulsed laser deposition and magnetron sputtering

Abstract Tribological properties of TiC coatings grown by pulsed laser deposition (PLD) and magnetron sputtering were investigated. The PLD TiC coatings grown at room temperature were found to be much harder than the TiC coatings grown by magnetron sputtering under the given experimental conditions. The hardness of PLD coatings deposited at room temperature was as high as 44 GPa, in contrast to ~20 GPa of the magnetron sputtered ones. The coefficient of friction of the PLD films measured with a pin-on-disk type tribometer had a typical value of about 0.2 when using a 440C stainless steel pin. Scratch tests indicated that magnetron-sputtered TiC coatings adhered well to the stainless steel substrates. The relatively poor adhesion of the PLD coatings obtained from this scratch test was probably partly due to their high brittleness and the relatively weak adhesive bond. However, the adhesion of PLD coatings could be improved by raising the substrate temperature slightly to 300 °C or by in situ laser annealing. The former was also able to maintain their hardness at a relatively high level. The adhesion of magnetron-sputtered TiC coatings could be modified by inserting a metallic interlayer between the coating and stainless steel substrate. Mo interlayer had a detrimental effect on the adhesion caused probably by the poor stress bearing capability of the porous Mo film deposited at low temperature. However, the insertion of both Ti and Cr interlayers enhanced the adhesion of TiC by as much as 25%.

Pulsed Laser Deposition and Properties of Mn+1AX n Phase Formulated Ti3SiC2 Thin Films

The ternary phase ceramic, Ti3SiC2, has often been synthesized through reactive hot pressing, providing bulk samples for studying its mechanical and physical properties. Chemical vapor deposition has been the most popular route to make Ti3SiC2 films. Recently, magnetron sputtering (MS) and pulsed laser deposition (PLD) have been used to produce good quality films. In this paper, we present the results on the synthesis and tribological characterization of Ti3SiC2 thin films prepared by PLD. The films had a surface roughness of 0.46 nm, a friction coefficient of 0.2 in humid air, and hardness of between 30 and 40 GPa. The transfer films were identified on the surface of counterparts using scanning electron microscopy. Anisotropic layer structure of Ti3SiC2 and nano crystallites in the coatings observed by X-ray diffraction and transmission electron microscopy are related to the low friction and high hardness. A specially designed sample coated with half Ti3SiC2 and half TiC was fabricated for comparing the properties between the two materials using a lateral force microscope. Lateral force images of the coatings indicated that the lateral force against Ti3SiC2 was lower than against TiC. PLD Ti3SiC2 coatings may be produced at near room temperature to 300 °C, which is acceptable for many commercial applications.

Limited thermal conductance of metal-carbon interfaces

The thermal conductance for a series of metal-graphite interfaces has been experimentally measured with time-domain thermoreflectance (TDTR). For metals with Debye temperatures up to ∼400 K, a linear relationship exists with the thermal conductance values. For metals with Debye temperatures in excess of ∼400 K, the measured metal-graphite thermal conductance values remain constant near 60 MW m−2 K−1. Titanium showed slightly higher conductance than aluminum, despite the closeness of atomic mass and Debye temperature for the two metals. Surface analysis was used to identify the presence of titanium carbide at the interface in contrast to the aluminum and gold-carbon interfaces (with no detectable carbide phases). It was also observed that air-cleaved graphite surfaces in contact with metals yielded slightly higher thermal conductance than graphite surfaces cleaved in vacuo. Examination of samples with scanning electron microscopy revealed that the lack of absorbed molecules on the graphite surface resulted...

Evolution of surface topography in pulsed-laser-deposited thin films of MoS2

Abstract Films of MoS 2 were grown at various thickness onto silicon substrates at room temperature using pulsed laser deposition. Planar and cleaved cross-sectional samples examined in a scanning electron microscope showed that very thick films (thicker than about 450 nm) transitioned from being fully dense to having extensive porosity. The porosity is directly correlated with the accumulated incorporation of spherical particles within the film. Transmission electron microscopy (TEM) results from samples of differential thicknesses, acquired from early and later stages of deposition, showed that the volume fraction of the particles increased with the thickness. Decreasing the energy density of the laser beam by underfocusing relative to the target was shown to decrease dramatically the number of particles. Simulation of the deposition process by adding digitized, binary TEM images of the differential thickness samples, to an equivalent thickness of 450 nm, indicated that the porosity begins at a total particle projected area fractio of about 35%.

Redox Exfoliation of Layered Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have attracted considerable attention in a diverse array of applications due to the breadth of possible property suites relative to other low-dimensional nanomaterials (e.g., graphene, aluminosilicates). Here, we demonstrate an alternative methodology for the exfoliation of bulk crystallites of group V-VII layered TMDs under quiescent, benchtop conditions using mild redox chemistry. Anionic polyoxometalate species generated from edge sites adsorb to the TMD surface and create Coulombic repulsion that drives layer separation without the use of shear forces. This method is generalizable (MS2, MSe2, and MTe2) and effective in preparing high-concentration (>1 mg/mL) dispersions with narrow layer thickness distributions more rapidly and with safer reagents than alternative solution-based approaches. Finally, exfoliation of these TMDs is demonstrated in a range of solvent systems that were previously inaccessible due to large surface energy differences. These characteristics could be beneficial in the preparation of high-quality films and monoliths.