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Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. Here we present optical dispersion engineering in a superlattice structure comprising alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate greater than 90% narrow band absorption in less than 4 nm of active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in square-centimetre samples. These superlattices show evidence of strong light–matter coupling and exciton–polariton formation with geometry-tuneable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically thin layers. Square-centimetre scale, multilayer superlattice structures based on atomically thin two-dimensional chalcogenide monolayers enable the realization of excitonic metamaterials.

Porous polymeric covalent organic frameworks (COFs) have been under intense synthetic investigation with over 100 unique structural motifs known. In order to realize the true potential of these materials, converting the powders into thin films with strict control of thickness and morphology is necessary and accomplished through techniques including interfacial synthesis, chemical exfoliation and mechanical delamination. Recent progress in the construction and tailored properties of thin film COFs are highlighted in this review, addressing mechanical properties as well as application-focused properties in filtration, electronics, sensors, electrochemistry, magnetics, optoelectronics and beyond. Additionally, heterogeneous integration of these thin films with other inorganic and organic materials is discussed, revealing exciting opportunities to integrate COF thin films with other state of the art material and device systems.

High-power, flexible electronic devices require substrates that effectively dissipate heat while maintaining mechanical compliance. Nanocomposite substrates with ∼20 μm graphite nanoplatelets were ...

Mechanical transfer of high-performing thin-film devices onto arbitrary substrates represents an exciting opportunity to improve device performance, explore nontraditional manufacturing approaches, and paves the way for soft, conformal, and flexible electronics. Using a two-dimensional boron nitride release layer, we demonstrate the transfer of AlGaN/GaN high-electron mobility transistors (HEMTs) to arbitrary substrates through both direct van der Waals bonding and with a polymer adhesive interlayer. No device degradation was observed because of the transfer process, and a significant reduction in device temperature (327-132 °C at 600 mW) was observed when directly bonded to a silicon carbide (SiC) wafer relative to the starting wafer. With the use of a benzocyclobutene (BCB) adhesion interlayer, devices were easily transferred and characterized on Kapton and ceramic films, representing an exciting opportunity for integration onto arbitrary substrates. Upon reduction of this polymer adhesive layer thickness, the AlGaN/GaN HEMTs transferred onto a BCB/SiC substrate resulted in comparable peak temperatures during operation at powers as high as 600 mW to the as-grown wafer, revealing that by optimizing interlayer characteristics such as thickness and thermal conductivity, transferrable devices on polymer layers can still improve performance outputs.

Since the isolation of graphene in 2004, there has been an exponentially growing number of reports on layered two-dimensional (2D) materials for applications ranging from protective coatings to biochemical sensing. Due to the exceptional, and often tunable, electrical, optical, electrochemical, and physical properties of these materials, they can serve as the active sensing element or a supporting substrate for diverse healthcare applications. In this review, we provide a survey of the recent reports on the applications of 2D materials in biosensing and other emerging healthcare areas, ranging from wearable technologies to optogenetics to neural interfacing. Specifically, this review provides (i) a holistic evaluation of relevant material properties across a wide range of 2D systems, (ii) a comparison of 2D material-based biosensors to the state-of-the-art, (iii) relevant material synthesis approaches specifically reported for healthcare applications, and (iv) the technological considerations to facilitate mass production and commercialization.

Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A−1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

Transition metal tellurides (TMTs) are an exciting group of two-dimensional materials with a wide variety of polytypes and properties. Here, we demonstrate a simple and versatile two-step method for producing MoTe2, WTe2, and PtTe2 films via tellurization of thin metals at temperatures between 400 and 700 °C. Across this temperature range, monoclinic 1T′ phase of MoTe2, orthorhombic Td phase of WTe2, and hexagonal 2H phase of PtTe2 were formed. Based on x-ray diffraction and Raman analysis, temperatures greater than 600 °C were found to produce the best quality MoTe2 and WTe2. In contrast, lower temperatures (400 °C) were preferred for PtTe2, which becomes discontinuous and eventually decomposes above 650 °C. The presence of H2 in the tellurization process was critical to facilitate the formation of H2Te, which is known to be more reactive than Te vapor. In the absence of H2, neither MoTe2 nor WTe2 formed, and although PtTe2 was formed under pure N2, the crystal quality was significantly reduced. Temperature-dependent resistivity (ρ) measurements were performed on the best quality TMT films revealing all films to be highly conductive. MoTe2 showed metallic behavior up to 205 K where it underwent a phase transition from the semimetallic Td to semiconducting 1T′ phase. WTe2 exhibited a consistent semiconducting behavior with a small positive increase in ρ with decreasing temperature, and PtTe2 showed a metallic dependence from 10 K up to room temperature. Spectroscopic ellipsometry for TMT films provides complex optical constants n and k from ultraviolet to infrared.

Effective chemical sensor devices must facilitate both the detection of analytes at ultralow concentrations and the ability to distinguish one analyte from another. Sensors built using two-dimensional nanomaterials have demonstrated record-level sensitivity toward certain chemical vapor species, but the specificity of chemical analyte detection remains lacking. To address this deficiency, this work pioneers the use of a broadband fiber-optic sensor coated with thin-film MoS2 where selectivity is achieved through observing changes in the visible spectrum transmission during exposure to different aliphatic and aromatic vapors. A significant loss in transmission across the fiber was observed near peaks in the refractive index associated with the C, B, and A excitons as well as at peaks associated with defect states. Several mechanisms for achieving selectivity are investigated, including deciphering donor/acceptor molecules, aromatic compounds, analytes with high refractive index, and intercalants such as aniline-based compounds. Moreover, the sensor device is entirely reusable and demonstrates reversible, empirical, and selective detection of aniline down to 6 ppm.

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers., 4 figures + supporting

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. Here we present optical dispersion engineering in a superlattice structure comprising alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate greater than 90% narrow band absorption in less than 4 nm of active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in square-centimetre samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tuneable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically thin layers.

The semiconductor-metal junction is one of the most critical factors for high performance electronic devices. In two-dimensional (2D) semiconductor devices, minimizing the voltage drop at this junction is particularly challenging and important. Despite numerous studies concerning contact resistance in 2D semiconductors, the exact nature of the buried interface under a three-dimensional (3D) metal remains unclear. Herein, we report the direct measurement of electrical and optical responses of 2D semiconductor-metal buried interfaces using a recently developed metal-assisted transfer technique to expose the buried interface which is then directly investigated using scanning probe techniques. We characterize the spatially varying electronic and optical properties of this buried interface with < 20 nm resolution. To be specific, potential, conductance and photoluminescence at the buried metal/MoS$_2$ interface are correlated as a function of a variety of metal deposition conditions as well as the type of metal contacts. We observe that direct evaporation of Au on MoS$_2$ induces a large strain of ~5% in the MoS$_2$ which, coupled with charge transfer, leads to degenerate doping of the MoS$_2$ underneath the contact. These factors lead to improvement of contact resistance to record values of 138 kohm-um, as measured using local conductance probes. This approach was adopted to characterize MoS$_2$-In/Au alloy interfaces, demonstrating contact resistance as low as 63 kohm-um. Our results highlight that the MoS$_2$/Metal interface is sensitive to device fabrication methods, and provides a universal strategy to characterize buried contact interfaces involving 2D semiconductors., 6 figures + supplement

Beginning in the mid-20th century with the invention of a coherent beam of light with precisely controlled emission known as the laser, engineers and scientists have been pushing the boundaries as to the applications of these incredible devices. Utilizing lasers as a tool to manipulate nanomaterials opens the possibility of creating unique chemistries, structures, devices, and architectures by controlling the light–matter interactions within the material. In this chapter, we provide an introduction into these light–matter interactions at the nanoscale to include laser heating, ablation, and plasmonics, provide examples of nanochemistries achievable through this technique, and discuss applications for future implementation to include film transfer, nanostructuring, and device implementation.

Developing characterization strategies to better understand nanoscale features in two-dimensional nanomaterials is of crucial importance, as the properties of these materials are many times driven by nanoscale and microscale chemical and structural modifications within the material. For the case of large area monolayer MoSe2 flakes, kelvin probe force microscopy coupled with tip-enhanced photoluminescence was utilized to evaluate such features including internal grain boundaries, edge effects, bilayer contributions, and effects of oxidation/aging, many of which are invisible to topographical mapping. A reduction in surface potential due to n-type behavior was observed at the edge of the flakes as well as near grain boundaries. Potential phase mapping, which corresponds to the local dielectric constant, depicted local biexciton and trion states in optically-active regions of interest such as grain boundaries. Finally, nanoscale surface potential and photoluminescence mapping was performed at several stages of oxidation, revealing that various oxidative states can be evaluated during the aging process. Importantly, all of the characterization performed in this study was non-destructive and rapid, crucial for quality evaluation of an exciting class of two-dimensional nanomaterials.

Electronic circuits, the backbone of modern electronic devices, require precise integration of conducting, insulating, and semiconducting materials in two- and three-dimensional space to control the flow of electric current. Alternative strategies to pattern these materials outside of a cleanroom environment, such as additive manufacturing, have enabled rapid prototyping and eliminated design constraints imposed by traditional fabrication. In this work, a transformative manufacturing approach using laser processing is implemented to directly realize conducting, insulating, and semiconducting phases within an amorphous molybdenum disulfide thin film precursor. This is achieved by varying the incident visible (514 nm) laser intensity and raster-scanning the thin film a-MoS2 sample (900 nm thick) at different speeds for micro-scale control of the crystallization and reaction kinetics. The overall result is the transformation of select regions of the a-MoS2 film into MoO2, MoO3, and 2H-MoS2 phases, exhibiting conducting, insulating, and semiconducting properties, respectively. A mechanism for this precursor transformation based on crystallization and oxidation is developed using a thermal model paired with a description of the reaction kinetics. Finally, by engineering the architecture of the three crystalline phases, electrical devices such as a resistor, capacitor, and chemical sensor were laser-written directly within the precursor film, representing an entirely transformative manufacturing approach for the fabrication of electronic circuitry.

Multiplex electronic antigen sensors for detection of SARS-Cov-2 spike glycoproteins and hemagglutinin from influenza A are fabricated using scalable processes for straightforward transition to economical mass-production. The sensors utilize the sensitivity and surface chemistry of a 2D MoS

Two-dimensional nanomaterials exhibit exceptional multifunctional properties including high-electron mobilities/saturation velocities, high surface to volume ratios, unique layered structures and mechanical compliance, positioning the class of materials to be influential in next-generation flexible electronics for applications in wearables and the Internet of things. In this perspective, three key areas of interest are identified that take advantage of the multifunctional nature of these materials including molecular sensing, van der Waals transfer and compliant radio frequency electronics. Significantly more progress needs to be made to realize commercialization of these materials, but the revolutionary accessible properties may reveal themselves in these three key areas of future flexible electronic systems.

The control of ultra-thin MoS2 film growth via annealing of thin film MoO3 precursor films was investigated. Through analysis by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, it was demonstrated that there is significant competition between sublimation and surface diffusion, resulting in little to no grain growth at annealing temperatures of 250 °C–550 °C. At annealing temperatures above the MoO3 melting point (801 °C) and subsequent cooling, grain growth does occur. MoO3 decomposition products (Mo4O11 and MoO2) and a reaction product between the MoO3 film and Al2O3 substrate were also detected at annealing temperatures of 500 °C and above. The formation of the Al2(MoO4)3 phase at the film-substrate interface and the subsequent decomposition of the Al2(MoO4)3 producing MoO3 precursor at 850 °C yielded the largest grain 2D MoS2 films after sulfidation. This three step (physical vapor deposition, precursor annealing, and sulfidation) process yielded fully coalesced films of 1.5 layers thick, demonstrating the viability of high temperature annealing to obtain an interface layer that decomposes to produce MoO3 precursors resulting in very thin 2D MoS2 films.

Ultra-thin amorphous boron oxynitride films were deposited at room temperature on silicon and polymer substrates using radio frequency magnetron sputtering of a hexagonal boron nitride target in varied argon-nitrogen background gas ratios. The effects of nitrogen content in the sputtering gas, changes in target-to-substrate distance, and magnetron power density on the film composition were investigated with X-ray photoelectron spectroscopy and correlated to the dielectric breakdown strength and bandgap obtained from current-voltage measurements and Tauc plots. Films grown in pure Ar atmosphere at a large target-to-substrate distance displayed compositions containing metallic boron, which led to a decrease in the dielectric breakdown strength. The use of Ar:N2 mixtures and pure N2 eliminated metallic boron bonding. Decreasing the target-to-substrate distance and target power density during reactive sputtering resulted in B:N stoichiometry ratio closer to one and about 25–30 at.% oxygen. Smooth surface morphology films grown in pure N2 at a small target-to-substrate distance exhibited dielectric breakdown strength of 2.1–2.4 MV cm−1at 30 nm film thickness and bandgap of 5.4 eV. The results show the suitability of amorphous boron oxynitride for application as an insulator or gate dielectric in thin-films flexible transistors and other electronics when a low processing temperature is required.

Since the isolation of graphene in 2004, there has been an exponentially growing number of reports on layered two-dimensional (2D) materials for applications ranging from protective coatings to biochemical sensing. Due to the exceptional, and often tunable, electrical, optical, electrochemical, and physical properties of these materials, they can serve as the active sensing element or a supporting substrate for diverse healthcare applications. In this review, we provide a survey of the recent reports on the applications of 2D materials in biosensing and other emerging healthcare areas, ranging from wearable technologies to optogenetics to neural interfacing. Specifically, this review provides (i) a holistic evaluation of relevant material properties across a wide range of 2D systems, (ii) a comparison of 2D material-based biosensors to the state-of-the-art, (iii) relevant material synthesis approaches specifically reported for healthcare applications, and (iv) the technological considerations to facilitate mass production and commercialization.

In this paper we demonstrate a metal organic chemical vapor deposition (MOCVD) process for growth of few layer hBN films on Ni(111) on sapphire substrates using triethylborane (TEB) and ammonia (NH3). Ni(111) was selected as a substrate due to its symmetry and close lattice matching to hBN. Using atomic force microscopy (AFM) we find hBN is well aligned to the Ni below with in plane alignment between the hBN zig zag edge and the &lt, 110&gt, of Ni. We further investigate the growth process exploring interaction between precursors and the Ni(111) substrate. Under TEB pre-exposure Ni-B and graphitic compounds form which disrupts the formation of layered phase pure hBN, while NH3 pre-exposure results in high quality films. Tunnel transport of films was investigated by conductive-probe AFM demonstrating films to be highly resistive. These findings improve our understanding of the chemistry and mechanisms involved in hBN growth on metal surfaces by MOCVD.

Next-generation nanoelectronics based on 2D materials ideally will require reliable, flexible, transparent, and versatile dielectrics for transistor gate barriers, environmental passivation layers, capacitor spacers, and other device elements. Ultrathin amorphous boron nitride of thicknesses from 2 to 17 nm, described in this work, may offer these attributes, as the material is demonstrated to be universal in structure and stoichiometric chemistry on numerous substrates including flexible polydimethylsiloxane, amorphous silicon dioxide, crystalline Al2O3, other 2D materials including graphene, 2D MoS2, and conducting metals and metal foils. The versatile, large area pulsed laser deposition growth technique is performed at temperatures less than 200 °C and without modifying processing conditions, allowing for seamless integration into 2D device architectures. A device-scale dielectric constant of 5.9 ± 0.65 at 1 kHz, breakdown voltage of 9.8 ± 1.0 MV cm−1, and bandgap of 4.5 eV were measured for various thicknesses of the ultrathin a-BN material, representing values higher than previously reported chemical vapor deposited h-BN and nearing single crystal h-BN.

Influenza results in tens of thousands of deaths annually in the USA and hundreds of thousands worldwide. COVID-19, caused by the SARS-Cov-2 virus, is even more devastating in terms of patient mortality. At the time of this writing, the nanoscopic SARS-Cov-2 virus has paralyzed the world economy and resulted in what are likely permanent changes in our expectations of society and daily life. New technology is needed to reduce the economic and social impacts of diseases such as COVID-19 and prevent additional negative consequences resulting from subsequent pandemics. As viruses such as Influenza A and SARS-Cov-2 are transmitted from person to person by exposure to infected secretions, inexpensive at-home or workplace tests for the analysis of the virus content within those secretions, such as saliva or mucus from the nasopharynx (as in a swab-based test) or oropharynx (as in a saliva-based test), will be critical for a safe return to work, school, and cultural activities. The most reliable approaches for viral sensing are polymerase chain reaction and protein detection via enzyme-linked immunosorbent assay; however, these approaches require extensive sample handling, laboratory infrastructure, and long sample-to-result time. Advances are leading to increased point-of-care capability for these testing methods, but even this effort is insufficient for curbing the impact of the current pandemic. There are many options for alternative virus (or antigen) detection currently in development. These novel approaches are more amenable for testing in home or workplace without specialized equipment and training and include measurements of mass changes, heat of adsorption, electrochemical changes, changes in optical properties, and changes in electronic properties. Of these transduction mechanisms, electronic property measurements of materials as they interact with virus-containing secretions offer the greatest potential for simplicity, selectivity, and sensitivity needed to revolutionize traditional laboratory assays for at-home pathogen detection. We have, therefore, focused this review on the operation and architecture of electronic antigen sensors, specifically those demonstrating a change in electrical conductivity when interacting with a specific antigen, with hopes that a brief summary of over five decades of research in this area will be beneficial to those developing alternative, user-friendly routes for detection of viruses at this or any time. A key element in electronic virus sensing with useful sensitivity is the use of nanomaterials with ultrahigh surface-to-volume ratios, maximizing the change in charge carrier density upon adsorption events. So-called “low-dimensional materials” are materials characterized by nanoscopic length scales in at least one dimension. One-dimensional nanomaterials such as nanowires and nanotubes are well-established as effective sensing materials with potential for high sensitivity; however, their realization on a large scale has been challenging. Two-dimensional materials are planar materials with thicknesses of one or a few molecular layers and represent the ultimate limit of the surface-to-volume ratio with promising demonstrations of large-scale production and sensitive, selective virus sensing with many options for functionalization. All aspects of 2D sensor fabrication, functionalization, and use are addressed.

To combat the ever-increasing challenge of thermal management in nanoelectronic devices and reduce the risk of overheating during operation, material interfaces near the active region of a device must be designed for efficient thermal transport. In this study, metal contacts on amorphous boron nitride (a-BN) thin films, a promising dielectric material in two-dimensional systems, are evaluated for the relative thermal transport efficiency. Metals with small atomic masses and high Debye temperatures, aluminum and titanium, reveal as much as a six times improvement in interface thermal conductance, as measured by time domain thermoreflectance, compared to metals of larger atomic masses and low Debye temperatures such as gold and tantalum. The interface transport between a-BN and higher Debye temperature metals prove to depend on the growth conditions, and good initial film wettability was crucial for ensuring a quality metal contact. Interfacial bonding was evaluated by x-ray photoelectronic spectroscopy and the impact on a-BN chemistry and oxidation potential contributed to the understanding of phonon transport across the interface. The study described herein reinforces the crucial requirement to design effective thermal contacts in high-power electronic devices where Joule heating is a concern.

Large-area, flexible, two-dimensional transition metal dichalcogenide semiconductor materials (MoS2 and WSe2) were synthesized via magnetron sputtering of amorphous stoichiometric precursor materials on polydimethylsiloxane polymer substrates. Purely amorphous precursor materials and amorphous materials with pre-existing nanocrystalline regions observed via transmission electron microscopy were grown for the studies presented here. The MoS2 and WSe2 material precursors were then illuminated with a pulsed 532 nm laser to induce crystallization to their semiconducting hexagonal phases. The laser optics included an axicon lens to shape the Gaussian pulsed laser into a “Bessel beam” characterized by annular ring geometry. The pattern of the beam, with its rings of high-intensity laser light around a higher-intensity core, produced crystalline rings of the material around an ablation zone on the polymer substrate for MoS2 and WSe2 materials. The crystalline structure and density of atomic defects over the crystalline regions decreased as the same sample area was illuminated with additional pulses. The lateral coherence of the crystal lattice increased with the first 4 pulses but decreased with each subsequent pulse. The impact of preexisting nanocrystalline nanoinclusions in an amorphous film on the crystallization rate for WSe2 precursor materials was examined. The presence of nanocrystalline regions in the amorphous materials increased the crystallization rate under the photonic annealing conditions examined here. This approach of direct synthesis and patterning of materials is a route toward the fabrication of inexpensive flexible electronic devices.

Plasma assisted physical vapor deposition processes can provide alternative and scalable approaches for synthesis of two-dimensional (2D) materials. While plasma species with high kinetic energies and chemical activities are beneficial for reduced temperature growth of crystalline 2D layers, precise control of these characteristics for reduced defect generation under ion bombardment is one of the main challenges for plasma assisted growth processes. In this study a variable intensity magnetic field was used to control chemistry, energy, and spatial density distribution of plasma produced by pulsed magnetron sputtering of MoS 2 in 2.0 Pa argon. A magnetic coil was used to deflect plasma flux and control electron and ion densities at the substrate surface. Plasma emission and mass-spectrometry studies showed the abundance of excited neutrals and positive ions of Ar, Mo and S with no evidence for MoS radicals and negative S ions. Ar 1 + abundance was several orders of magnitude above that of Mo 1 + and S 1 + and argon ions were therefore the primary source of the substrate bombardment. Wavelength specific plasma imaging and mass/energy analysis demonstrated that the tunable magnetic field can effectively steer ionized Ar species away from the substrate with about a factor of three reduction of their abundance at the condensation surface. Spatial distributions of sputtered Mo and S species were also influenced but to a different degree, which was dependent on their ionization state. Tunable magnetic filtering helped to reduce unnecessary Ar bombardment by maintaining incident ion energies 2 films. The argon ion filtering approach was verified by producing three monolayer thick polycrystalline 2D MoS 2 films over wafer-scale areas allowing for a scalable direct synthesis needed for device manufacturing.

High power impulse magnetron sputtering was used to deposit thin (~ 100 nm) zinc oxide (ZnO) films from a ceramic ZnO target onto substrates heated to 150 °C. The resulting films had strong crystallinity, highly aligned (002) texture and low surface roughness (root mean square roughness less than 10 nm), as determined by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and atomic force spectroscopy measurements. Deposition pressure and target–substrate distance had the greatest effect on film microstructure. The degree of alignment in the films was strongly dependent on the gas pressure. Deposition at pressures less than 0.93 Pa resulted in a bimodal distribution of grain sizes. An initial growth layer with preferred orientations (101) and (002) parallel to the interface was observed at the film–substrate interface under all conditions examined here; the extent of that competitive region was dependent on growth conditions. Time-resolved current measurements of the target and ion energy distributions, determined using energy resolved mass spectrometry, were correlated to film microstructure in order to investigate the effect of plasma conditions on film nucleation and growth.

Recent computational studies predict mechanical strain–induced changes in thermal transport, which is yet to be validated by experimental data. In this article, we present experimental evidence of an increase in thermal conductivity of nominally 200-nm-thick freestanding amorphous silicon thin films under externally applied tensile loading. Using a combination of nanomechanical testing and infrared microscopy, we show that 2.5% tensile strain can increase thermal conductivity from 1 to 2.4 W/m-K. We propose that such an increase in thermal conductivity might be due to strain-induced changes in microstructure and/or carrier density. Microstructural and optical reflectivity characterization through Raman and infrared spectroscopy are presented to investigate this hypothesis.

Progress in the design and exploration of hard coatings with high temperature adaptive behavior in tribological contacts is reviewed. When coupled with most recent surface engineering strategies for high temperature contact thermal management, this progress opens a huge opportunity for adaptive coating applications on machine parts, where oils and coolants are commonly used. The adaptive mechanisms discussed here include metal diffusion and formation of lubricant phases at worn surfaces, thermally- and mechanically-induced phase transitions in hexagonal solids, contact surface tribo-chemical evolutions to form phases with low melting point, formation of easy to shear solid oxides, and others. All of these adaptive mechanisms are combined in nanocomposite coatings with synergistic self-adaptation of surface structure and chemistry to lubricate from ambient temperatures to 1000 °C and provide surface chemical and structural reversibility during temperature cycling to maintain low friction coefficients. The review also highlights emerging surface adaptive concepts, where advances with ab initio modeling of intrinsically layered solids point to new compositions for thermally stable, easy to shear ceramic coatings, load- and temperature-adaptive surfaces with arrays of compliant carbon and boron nitride nanotubes as well as low friction two-dimensional structures. Approaches for self-regulation of coating thermal conductivity, heat flow, and thermal spike mitigations are discussed in the context of surface structure evolution and phase transitions. Future progress is linked to the development of in situ exploration techniques, capable of identifying adaptive surface chemistry and structural evolutions in broad temperature regimes. When combined with predictive modeling, such approaches drastically accelerate adaptive coating developments. The review identifies opportunities, strategies, and challenges for designs and applications of hard coatings with high temperature adaptive lubrication and contact thermal management.

This article provides an overview of the latest research developments on binary and ternary oxide coatings that have the potential to be used as solid lubricants at elevated temperatures. The review focuses on understanding the major mechanisms that lead to a reduction in friction and/or wear in high temperature lubricious oxides. Changes in the structural, chemical, and electronic properties of these oxides as a function of temperature will be correlated to their mechanical and tribological performance using a range of experimental tools in addition to modeling based on ab initio calculations and molecular dynamics simulation methods. This review also includes a discussion of the industrial applications of these coatings as well as of potential improvements to the coating design and other anticipated future developments.

A strong resurgence of interest in vdW solid processing has accompanied two-dimensional (2D) materials research in the past decade. `Two dimensional` is terminology generally reserved for solids comprised of 5 molecular layers or less. As materials approach this ultimate thickness limit, unique properties emerge. vdW solids are well-suited for 2D materials research and applications as the anisotropic weak inter-layer bonding facilitates separation of bulk crystals into molecular constituents. Despite the great effort fueling the 2D materials revolution, electronics and other devices that truly capitalize on the benefits afforded by 2D van der Waals materials are not yet commercially available. The widespread use of 2D vdW solids is primarily inhibited by the lack of reliable, large-area synthesis approaches. Chemical vapor deposition is the primary means by which large area 2D TMD films are grown. This approach, however, has serious shortcomings, such as a lack of repeatable growth rates and kinetics, as well as the requirement for high processing temperatures (typically >650 °C). Physical vapor deposition (PVD) constitutes a family of synthesis processes with inherent qualities enabling large-scale 2D vdW materials processing. With no fundamental limitations on size, uniformity over large areas (>1 m2) for diverse materials, PVD has been demonstrated on thicker vdW films for decades. PVD processes are often characterized by the presence of energetic particles with kinetic energies that serve as an additional knob, along with process temperature and pressure, for control of growth kinetics. Ideally, a process would enable utilization of functional and mechanical properties of 2D vdW solids via synthesis directly on flexible polymer substrates at suitable temperatures. This objective is straightforward with PVD-based approaches. Here we review PVD techniques applied to the best-known vdW solids including graphene, transition metal dichalcogenides (such as MoS2), and boron nitride. Key points from decades of prior studies are summarized with the objective of applying this knowledge to hasten process development for future applications of large-area 2D vdW solids in advanced optical and electronic device applications meeting the needs of humankind beyond our current capabilities.

Accurate prediction of heat transfer from micro and nanoscale devices is very important for their operation and reliability. While the general consensus is that the heat transfer coefficient increases at the smaller scales, the literature also indicates contradiction in the scaling across length scales. In this paper, we present an experimental technique to accurately characterize heat transfer from micro to nano scales using infrared microscopy on nanofabricated specimens that are integrated with micro heaters. The heat transfer coefficient for quiescent air is found to be about two orders of magnitude higher than its bulk counterpart. It also showed pronounced inversely proportional relationship with temperature and suggested that solid to air conduction is the dominant mode of heat transfer. It was found that pressure only indirectly influences heat transfer. Based on the experimental data, we present an empirical relationship for the heat transfer coefficient that depends on the ratio of the surface area to cross-sectional area, temperature of the heated solid and its proximity to other solid bodies.

Vertically aligned arrays of multiwall carbon nanotubes (MWCNT) were decorated with gold (Au) nanoparticles of different diameter and areal densities and spun into yarns. The melting point of Au nanoparticles determined by differential scanning calorimetry was approximately 260 °C, well below the oxidation temperature of carbon. A continuous yarn was formed while pulling out a bundle of CNTs from the metalized CNT array. Relatively low temperature (300 °C) thermal processing of the metalized yarn resulted in a 30% improvement in thermal conductivity, 40% increase in electrical conductivity and a 4× increase in elastic modulus. Cross-sections of the yarn were examined with transmission electron microscopy to characterize the physical nature of the metal–nanotube interface. The deposition procedure described to decorate the nanotube yarns is easily scalable to larger CNT arrays or other configurations for commercial applications, such as medical implants, lightweight conductors, smart uniforms for the soldiers, and conformal electronics in aerospace industry.

The effect of V additions on reactively magnetron sputtered ZrO2 coatings was investigated with respect to their structural and mechanical properties. Coatings with different V-contents were co-sputtered from Zr and V targets using an Ar + O2 discharge. The X-ray diffraction patterns show a change from monoclinic (0–2.2 at.% V) to cubic/tetragonal ZrO2 based phase (5.8 at.% V) and finally X-ray amorphous structure at even higher V-content (17.4 at.% V). Hardness and Young's modulus decrease beyond 5.8 at.% V from 17.4 to 7.5 GPa and from 230 to 150 GPa, respectively. At 25 °C, the coefficient of friction (COF) is ~ 0.2 for low V-contents and above 0.5 for high V-contents, respectively. Differential scanning calorimetry of powder samples showed a characteristic exothermic peak at ~ 600 °C, which is due to the formation of the ZrV2O7 phase. This phase decomposes above 740 °C in ZrO2 and liquid V2O5. The latter phase provides a self-lubricious layer in the sliding contact, decreasing the COF below 0.2 at 800 °C for V-contents of 17.4 at.%. The endothermic nature of melting of the V2O5 phase is suggested as the basis for thermal management abilities of tribological coatings, where the frictional heat generated in severely loaded sliding contacts and thus local flash temperatures are efficiently reduced.

Molecular dynamics is used to study the interfacial thermal conductance between graphitic structures and metals. It is shown that with different metals, the conductance can vary by ∼4-fold, allowing the control of thermal transport in nanocomposites and nanoelectronic devices. The experimental values of conductance are higher by 10–20 MW m−2 K−1 compared to simulations. We suggest that in addition to lattice vibrations, an electromagnetic coupling between Johnson–Nyquist electric currents in the metal and graphite may contribute to the interfacial thermal conductance.

Tungsten is an important material for energy harvesting applications due to its high chemical and thermal stability; however, the difficulty in obtaining single-crystal films leads to a large variation in the optical properties. The present work focuses on the radiative and optical properties of thin tungsten films at wavelengths from 1 to 20 μm (wavenumbers from 10,000 to 500 cm −1 ), considering microstructural variations. Four films of a nominal thickness of 70 nm were deposited on silicon substrates using DC magnetron sputtering, and the effect of pre- and post-deposition treatments was investigated. Several analytical instruments were used to characterize the crystalline phases and microstructures, including X-ray diffraction, Rutherford backscattering, X-ray photoelectron spectroscopy, and scanning electron microscopy. The transmittance and reflectance of the film–substrate composites were measured at room temperature using a Fourier-transform infrared spectrometer. The dielectric function of each sample was obtained by fitting the measured radiative properties using the Drude–Lorentz dispersion model. The difference in the radiative properties between samples was analyzed and related to the crystalline phases and density.

Vapor phase growth of gold, nickel and titanium metal nanoparticles on multiwall carbon nanotube (MWCNT) buckypaper by sputtering was investigated. The size and distribution of nanoparticles was dependent on the intrinsic binding energy of the metal elements, but could be altered to mimic that of metals with different binding energies by in situ modification of the MWCNT surfaces by energetic metal ions or annealing of the buckypaper. A range of average gold particle diameters from approximately 5–30 nm could be produced depending on the intrinsic sputter process parameters (especially metal ion flux and kinetic energy) and defect density of the MWCNT surfaces, which could also be controlled by annealing prior to sputtering. The diameter of the MWCNTs had a significant influence on the geometry of the nanoparticles. Particles were elongated along the nanotube axis for tube diameters

Hafnium oxide (HfO 2 or hafnia) holds promise as a high-index dielectric in optical devices and thermal barrier coatings, because of its transparency over a broad spectrum (from the ultraviolet to the mid-infrared) and chemical and thermal stability at high temperatures. In the present work, thin hafnia films of thicknesses from about 180 to 500 nm are deposited on Si substrates using reactive magnetron sputtering. The crystalline structure and surface topography are characterized by X-ray diffraction and atomic force microscopy, respectively. The optical and radiative properties of the film–substrate composites are measured at room temperature using spectroellipsometry and Fourier-transform infrared spectrometry. The optical constants are obtained from about 0.37 to 500 μm by fitting suitable models to the experimental results. Optical properties and dielectric function modeling are discussed with correlation to both film thickness and surface roughness. It is found that a single-oscillator dielectric-function model can describe radiative properties from about 1 to 20 μm. By combining Cauchy's formula (for the visible and near-infrared regions) with a multiple-oscillator Lorentz model (for the far-infrared region), a dielectric function is obtained for the HfO 2 films that is applicable from the visible to the far-infrared.

The performance and lifetime of sensors, microelectronic devices, mini air vehicle components and other systems can benefit from maintenance of a constant temperature profile or a local temperature that does not exceed a pre-selected, critical value during short-term transient loading events. In this work, the utility of surfaces featuring phase change materials (PCMs) encapsulated within micro-reservoirs was evaluated as a passive thermal management system for mitigation of transient temperature spikes. Patterned silicon substrates with 40 μm diameter, 10 μm deep trenches were prepared by reactive ion etching in an Ar/CF 4 plasma. The features were filled with an organic phase change material possessing a high latent heat and known to undergo melting at a target operating temperature of approximately 60 °C. An infrared microscope was used to produce temporally and spatially resolved temperature maps of the surface during heating. When a constant heat flux of approximately 200 W m − 2 was used for uniform heating of the sample area from 50 to 75 °C the PCM encapsulated materials demonstrated an isothermal plateau period lasting 5–8 s near the PCM melting point. A similar plateau was also observed during cooling below the PCM melting point. From the isothermal time during heating the areal thermal energy storage density was estimated to be approximately 800 J m − 2 , close to that predicted by the calculated volume of PCM contained within the sample. The effects of PCM inclusions on surface temperature gradients during pulsed laser heating were also investigated. An infrared (980 nm) laser at a fixed power and repetition rate was focused into a 1 mm diameter beam on surfaces with and without PCM encapsulation for rapid heating (analogous to heating from resistive contacts in microelectronic circuits) of different duration from 0.5 to 5.0 s. The surface temperature stayed below the desired temperature limit for 100 mW laser pulses lasting up to 2 s in duration. Transient temperature profiles of Si/PCM surfaces showed that micro-scale volumes of embedded phase change material stored sufficient quantities of heat to stabilize and otherwise control surface temperatures for potentially useful periods of time in selected applications.

High power impulse magnetron sputtering (HiPIMS) was studied during the growth of hafnium films at argon pressures ranging from 0.80 to 5.33 Pa with a fixed pulse length (50 μs) and frequency (200 Hz). The effect of inert gas pressure on the plasma conditions and film structure was investigated. The peak target current increased with pressure, but its sensitivity decreased above 2.00 Pa, which corresponded to an increased ratio of ions to neutrals in the plasma. A comparison of plasma characteristics between Hf and Ti HiPIMS growth was made. In addition to pressure, the target currents were affected by the physical properties of the target material, particularly the secondary ionization energy and atomic mass. Sputtering gas rarefaction phenomena were found to be more pronounced for Hf, and as a result, the process characteristics and film properties had a strong interdependence on argon pressure discussed in this study. The microstructure of the hafnium films was analyzed with scanning electron microscopy and X-ray diffraction. When compared to Hf films deposited by dc magnetron sputtering, the HiPIMS process resulted in a decreased grain size and promoted the growth of the (100) orientation in the Hf films. These results demonstrate that Hf HiPIMS sputtering regimes have much stronger dependence on the working gas pressure compared to titanium, and these need to be taken into account to ensure that films are dense and have the desired morphology and crystallographic orientation.