Thin Metal Films Research Articles

Underlying Substrate Effect on Electrochemical Activity for Hydrogen Evolution Reaction with Low‐Platinum‐Loaded Catalysts

Platinum is known as the best catalyst for the hydrogen evolution reaction (HER) but the scarcity and high cost of Pt limit its widespread applicability. Herein, the role of the underlying substrate on the HER activity of dispersed Pt atoms is uncovered. A direct current magnetron sputtering technique is utilized to deposit transition metal (TM) thin films of W, Ti, and Ta as underlying substrates for extremely low loading of Pt (<1.5 at%). The electrocatalytic performance of as‐synthesized samples for the HER is examined in both alkali and acidic media. The results show that despite the low loading of Pt, the Pt/TM catalysts produce hydrogen at a rate comparable to that of pristine bulk Pt. Pt/TM catalysts also display good stability with less than 5% decay in performance after 10 h of continuous HER operation. Based on the computational study, the excellent performance is attributed to the modified electronic properties of the Pt atoms, offering ideal binding energy for HER due to interaction with the underlying substrates. This work provides a robust and industry‐friendly route toward designing efficient catalytic systems for important electrochemical reactions such as HER and others.

Emergent superconductivity in topological-kagome-magnet/metal heterostructures

Itinerant kagome lattice magnets exhibit many novel correlated and topological quantum electronic states with broken time-reversal symmetry. Superconductivity, however, has not been observed in this class of materials, presenting a roadblock in a promising path toward topological superconductivity. Here, we report that novel superconductivity can emerge at the interface of kagome Chern magnet TbMn6Sn6 and metal heterostructures when elemental metallic thin films are deposited on either the top (001) surface or the side surfaces. Superconductivity is also successfully induced and systematically studied by using various types of metallic tips on different TbMn6Sn6 surfaces in point-contact measurements. The anisotropy of the superconducting upper critical field suggests that the emergent superconductivity is quasi-two-dimensional. Remarkably, the interface superconductor couples to the magnetic order of the kagome metal and exhibits a hysteretic magnetoresistance in the superconducting states. Taking into account the spin-orbit coupling, the observed interface superconductivity can be a surprising and more realistic realization of the p-wave topological superconductors theoretically proposed for two-dimensional semiconductors proximity-coupled to s-wave superconductors and insulating ferromagnets. Our findings of robust superconductivity in topological-Chern-magnet/metal heterostructures offer a new direction for investigating spin-triplet pairing and topological superconductivity.

A CFD study of the gas reaction path in growth of InN films in metal–organic chemical vapor deposition

During simulation of the growth of indium nitride (InN) thin films in metal–organic chemical vapor deposition (MOCVD), different choices of kinetic parameters (activation energy Ea and pre-exponential factor A) for gas-phase reactions give rise to many uncertainties in the gas reaction path and growth rate. This study summarized and compared the kinetic parameters of gas-phase reactions during InN MOCVD growth. The InN MOCVD growth in a close-coupled showerhead reactor was simulated with varying kinetic parameters of gas-phase reactions. The effects of kinetic parameters on the gas reaction path were determined by comparing molar concentrations and growth rates of primary In-containing substances. The results show that with varying kinetic parameters, the gas reaction path is likely to be dominated by the addition or trimethylindium pyrolysis path, or to proceed through both reaction paths. Although the reaction path changes with varying kinetic parameters, the predicted growth rates do not vary substantially, because the total rate of transport of In-containing substances to the substrate changes little with the reaction path. The results also indicate that the contribution of pyrolytic products to the growth rate is greater than that of amine, whereas H radicals have minimal effect on the gas reaction path in InN MOCVD growth.

Advances in the Fabrication of Nanoporous Anodic Aluminum Oxide and Its Applications to Sensors: A Review.

Nanoporous anodic aluminum oxide (AAO) is an important template for 1D nanomaterial synthesis. It is used as an etching template for nanopattern transfer in a variety of contexts, including nanostructured material synthesis, electrical sensors, optical sensors, photonic and electronic devices, photocatalysis, and hardness and anticorrosion improvement. In this review, we focus on various fabrication methods, pore geometry modification, and recent advances of AAO, as well as sensor applications linked to our environment, daily life, and safety. Pore geometry is concerned with the material composition, applied voltage mold, electrolyte type, temperature, and anodizing time during the fabrication of AAOs and for adjusting their pore size and profile. The applied voltage can be divided into four types: direct current anodization (DCA), reverse pulse anodization, pulse anodization (PA), and hybrid pulse anodization (HPA). Conventional AAOs are fabricated using DCA and mild anodization (MA) at a relatively low temperature (-5~15 °C) to reduce the Joule heating effect. Moreover, the issues of costly high-purity aluminum and a long processing time can be improved using HPA to diminish the Joule heating effect at relatively high temperatures of 20-30 °C with cheap low-purity (≤99%) aluminum. The AAO-based sensors discussed here are primarily divided into electrical sensors and optical sensors; the performance of both sensors is affected by the sensing material and pore geometry. The electrical sensor is usually used for humidity or gas measurement applications and has a thin metal film on the surface as an electrode. On the contrary, the AAO optical sensor is a well-known sensor for detecting various substances with four kinds of mechanisms: interference, photoluminescence, surface plasma resonance, and surface-enhanced Raman scattering (SERS). Especially for SERS mechanisms, AAO can be used either as a solid support for coating metal nanoparticles or a template for depositing the metal content through the nanopores to form the nanodots or nanowires for detecting substances. High-performance sensors will play a crucial role in our living environments and promote our quality of life in the future.

Breakup-Free and Colorful Liquid Metal Thin Films via Electrochemical Oxidation.

Thin-film metal conductors featuring high conductivity and stretchability are basic building blocks for high-performance conformable electronics. Gallium-based liquid metals are attractive candidates for thin-film conductors due to their intrinsic stretchability and ease of processing. Moreover, the phase change nature of liquid metal provides an opportunity to create conformal electronics in a substrate-free manner. However, thin liquid metal films tend to break during the solid-to-liquid transition due to the high surface tension of liquid metal. Here, we created breakup-free liquid metal thin films by the electrochemical oxidation of solid gallium films. We show that electrochemical oxidation can enhance the mechanical strength of the gallium oxide layer and its interfacial adhesion to the gallium core. When heated to the liquid state, the oxidized gallium films can maintain their structural integrity on various solid substrates, hydrogels, and even the water surface. The solid-liquid phase change-induced stiffness decrease allowed the gallium films to be conformably attached to various nonplanar surfaces upon heating or water transfer printing. Moreover, we also found that enhanced electrochemical oxidation can result in the formation of structure color due to nanoporous structures on the film surface. We also demonstrate the applications of oxidized liquid metal films in functional electronics, electrophysiological monitoring, and tattoo art.

Armor for Steel: Facile Synthesis of Hexagonal Boron Nitride Films on Various Substrates

AbstractWhile hexagonal boron nitride (hBN) has been widely used as a buffer or encapsulation layer for emerging electronic devices, interest in utilizing it for large‐area chemical barrier coating has somewhat faded. A chemical vapor deposition process is reported here for the conformal growth of hBN on large surfaces of various alloys and steels, regardless of their complex shapes. In contrast to the previously reported very limited protection by hBN against corrosion and oxidation, protection of steels against 10% HCl and oxidation resistance at 850 °C in air is demonstrated. Furthermore, an order of magnitude reduction in the friction coefficient of the hBN coated steels is shown. The growth mechanism is revealed in experiments on thin metal films, where the tunable growth of single‐crystal hBN with a selected number of layers is demonstrated. The key distinction of the process is the use of N2 gas, which gets activated exclusively on the catalyst's surface and eliminates adverse gas‐phase reactions. This rate‐limiting step allowed independent control of activated nitrogen along with boron coming from a solid source (like elemental boron). Using abundant and benign precursors, this approach can be readily adopted for large‐scale hBN synthesis in applications where cost, production volume, and process safety are essential.

Thermal Lattice Field during Ultra-Short Laser Pulse Irradiation of Metal Targets: A Fokker–Planck Analytical Model

The ultrafast fs laser pulse heating of thin metal films is studied for the first time using the two-temperature model on the basis of the Fokker–Planck formalism. The incident laser radiation is multi-modal, while the electron temperature is described during the first 2 fs. The predictions are intended for use by experimentalists in optoelectronics, photonics, laser processing, electronics, and bio- and nanomedicine. The crucial role of the nano-sized spatial dimensions of the metal sample is highlighted. A significant result of this study is the interdependence between the target’s size, the phonon/lattice characteristics, and the coefficient β (the quotient of non-diffusive phenomena), which varies between zero (pure diffusive case) and one (pure non-diffusive case).

Interfacing neural cells with typical microelectronics materials for future manufacturing

The biocompatibility of materials used in electronic devices is critical for the development of implantable devices like pacemakers and neuroprosthetics, as well as in future biomanufacturing. Biocompatibility refers to the ability of these materials to interact with living cells and tissues without causing an adverse response. Therefore, it is essential to evaluate the biocompatibility of metals and semiconductor materials used in electronic devices to ensure their safe use in medical applications. Here, we evaluated the biocompatibility of a collection of diced silicon chips coated with a variety of metal thin films, interfacing them with different cell types, including murine mastocytoma cells in suspension culture, adherent NIH 3T3 fibroblasts, and human induced pluripotent stem cell (iPSC)-derived neural progenitor cells (NPCs). All materials tested were biocompatible and showed the potential to support neural differentiation of iPSC-NPCs, creating an opportunity to use these materials in a scalable production of a range of biohybrid devices such as electronic devices to study neural behaviors and neuropathies.

Flexible metasurfaces as sub-6 GHz frequency selective surfaces for 5G applications

The deployment of fifth-generation mobile network (5G), beyond 5G and sixth-generation mobile network platforms encounters challenges of blockage, interference, and path loss in radio mobile environments. Metasurfaces provide a promising solution to address these limitations. In this paper, we present a methodology for developing ultrathin flexible metasurface-based frequency selective surfaces (FSSs). Our approach combines thermal evaporation for metallic thin films with a macroscopic metasurface mask (something analogous to screen-printing but using thermal evaporation instead of inks). As a proof of concept, we fabricate a sub-6 GHz metasurface-based FSS using gold deposition on a flexible polyethylene terephthalate substrate. Experimental results are validated through numerical full-wave simulations using COMSOL Multiphysics and equivalent-circuit model simulations. The metasurface operates within the primary frequency band utilized in 5G networks (3–5 GHz), indicating its potential applicability across a wide range of flexible, conformal, and wearable devices. The fabricated FSS can be installed on surfaces of any shape, such as flat or curved windows, as well as on walls or other external surfaces. This methodology offers practical solutions for wireless communications and enhancing signal transmission in diverse environments.

Precise quantification of the adhesion between metallic thin films and silicon wafer

The performance and reliability of integrated circuits depend on the mechanical and electrical properties of the interfaces between thin films and the silicon wafer. One of the essential properties of the interface is its adhesion energy. However, conventional adhesion characterization methods could be further improved, especially in providing repeatable and reliable measurements. In this paper, we measured the adhesive strength of the interface between tungsten thin films and silicon wafers in micro-sized specimens using a direct push-to-pull device built into a nano-indenter. A scratch method with a transparent theoretical model was applied to quantify the same interfaces' adhesive strength, adhesion energy, and fracture resistance. Microscopic and mechanical analysis clarified the physical origin of thin film delamination. The current method precisely quantified the interfaces' adhesive strength and fracture resistance. This method is promising for sub-micron thin film designs in the semiconductor industry with its superior repeatability, reliability, and ease of operation.

Thermophysical properties of pressure-sensitive paint

The paper deals with the problem of the thermophysical properties of pressure-sensitive paint (PSP). The paint was used to determine the pressure distribution on the surface of models examined in wind tunnel aerodynamic tests. The aim of the study was to provide reliable data on the values of thermophysical parameters and their dependence on temperature. PSP is a thin-film structure that is applied layer by layer a specific number of times on a metallic substrate, and determining the thermal properties requires non-standard test procedures to obtain reliable results. The measurements were carried out on paint fragments separated from the flat substrate of a metallic thin film (molybdenum), from which samples were obtained for complementary studies of the thermophysical properties. The study includes weight measurements, thermogravimetric analysis, micro-calorimetric analysis, and measurement of effective thermal diffusivity using the temperature oscillation technique. The findings are complemented with structural test data collected by means of scanning electron microscopy, surface characterisation, and density determination by the displacement method. The analysis of the structural test data indicates the micro-structural isotropy of the examined material with geometric anisotropy of the layer as a whole. In this way, the results of thermal diffusivity tests can be properly evaluated, and reliable data can be obtained on the complete thermophysical properties of the PSP layer. The indirectly determined thermal conductivity, i.e., by recalculating the specific heat, density, and thermal diffusivity data, allows the test material to be classified as an insulator. Apart from the presentation of the test results and calculations, this paper also contains a brief discussion of the test procedures.

Sputter‐Deposited α‐MoO3 Interlayers for van der Waals Epitaxy and Film Transfer

AbstractIntegration of functional thin films onto flexible substrates is driven by the need to improve the performance and durability of flexible electronic devices. A van der Waals epitaxy technology that accomplishes the transfer of oxide or metal thin films via exfoliation or dissolution of sacrificial α‐MoO3 layers produced by sputtering is presented. The α‐MoO3 thin films, consisting of weakly bonded 2D layers, grow epitaxially on SrTiO3 (001) substrates, exhibiting mosaic domains rotated by 90°. Metallic Au films grown on the α‐MoO3 are transferred by mechanical exfoliation or by dissolving the α‐MoO3 in water at 45 °C. Spinel‐structured CoFe2O4 thin films grown on α‐MoO3 layers are easily transferred to flexible substrates via mechanical exfoliation, and the magnetic anisotropy of the transferred CoFe2O4 films is modulated by bending.

Suppression of spin pumping at metal interfaces

An electrically conductive metal typically transmits or absorbs a spin current. Here, we report on evidence that interfacing two metal thin films can suppress spin transmission and absorption. We examine spin pumping in spin-source/spacer/spin-sink heterostructures, where the spacer consists of metallic Cu and Cr thin films. The Cu/Cr spacer largely suppresses spin pumping—i.e., neither transmitting nor absorbing a significant amount of spin current—even though Cu or Cr alone transmits a sizable spin current. The antiferromagnetism of Cr is not essential for the suppression of spin pumping, as we observe similar suppression with Cu/V spacers with V as a nonmagnetic analog of Cr. We speculate that diverse combinations of spin-transparent metals may form interfaces that suppress spin pumping, although the underlying mechanism remains unclear. Our work may stimulate a new perspective on spin transport in metallic multilayers.

Annealing-induced changes in wear resistance and nanomechanical properties of CuZr metallic glass thin films

Over recent years, metallic glass thin films (MGTFs) have found extensive applications in advanced micro-engineering systems. Consequently, there is a need to thoroughly assess the nanomechanical and tribological behaviors of MGTFs to optimize the design of efficient components. In this study, we employed the nanoindentation technique in various modes to investigate the elastic heterogeneity, tribological response, and mechanical properties of CuZr amorphous films. Before conducting the mechanical tests, annealing treatments at 500 K and 600 K were performed to create samples with different stored energies. The thermal history analysis revealed that the annealing process reduced the stored energy in the microstructure. Furthermore, the pre-annealing treatment resulted in increased hardness and Young’s modulus of the thin films. Additionally, higher annealing temperatures significantly improved the wear resistance of the MGTFs. Observing the serration dynamics in the scratching test, we noticed that the annealing treatment induced larger shear bands on the wear track side. Moreover, the increase in annealing temperature led to a reduction in elastic heterogeneity, which was consistent with the enthalpy relaxation values in the samples. This suggests that the annealing temperature enhanced the densely packed atomic structure, leading to the stabilization of the thin films.

Hollow-core fiber sensor based on the long-range Tamm plasmon polariton with enhanced figure of merit

The potential of long-rang Tamm plasmon polariton (LRTPP) for sensing applications is theoretically investigated in a planar distributed Bragg reflector (DBR) structure containing a thin metal film. Results show that both sensitivity and figure of merit (FOM) increase with increasing incidence angle under transverse electric polarization. Due to the difficulty of achieving very large incidence angles in planar structures in practice, a LRTPP based hollow-core fiber (HF) sensor is proposed, where all transmitted lights in the fiber core have large incidence angle close to 90° on the inner surface of the HF. The ray transmission model is adopted to calculate the transmission spectra of the proposed HF LRTPP sensor. A comprehensive theoretical analysis about the influence of the structural parameters on the sensor performance is carried out, including the thickness of the metal layer and the number of periods of the DBRs. The HF LRTPP sensor achieves a sensitivity of 774 nm/RIU and FOM of 227.6 RIU−1 at the refractive index (RI) around 1.00 for gas sensing. In the RI range of 1.33–1.53, a sensitivity of 705 nm/RIU and FOM of 201.4 RIU−1 are achieved. The FOM of the HF LRTPP sensor is 2–7 times higher than that of the HF Tamm plasmon polariton (TPP) sensor, while the sensitivities are comparable. Benefiting from the lower attenuation and narrower resonance dip, the idea of utilizing LRTPP mode in DBR-metal-DBR structure to improve the sensor performance will provide a new way for the design of TPP sensors and promote the related researches.

Nanoscale thinning of metal-coated polypropylene films by Helium-ion irradiation

Polypropylene (PP) films have a wide range of applications, e.g. as dielectric materials for metallized film capacitors. In this article, we present a method for thickness reduction of PP films by ion irradiation, which has a direct effect on device capacitance. We show that the thickness of PP layers can be reduced by irradiation with He+ ions and controlled on the nanometer scale by the irradiation dose. The effect of different thin metal film coatings on PP surface was also investigated. The metal coatings were used for two reasons: they function as one of the metal electrodes in the capacitor structure, and they minimize sample charging during ion irradiation. Three different metallization materials were investigated: 5 nm Pt60Pd40, 5 nm Au, and 15 nm Al. We studied two technologically relevant PP films: the thinnest commercially available biaxially oriented polypropylene (BOPP) and spin-coated polypropylene (SC-PP) thin films. The irradiation was done with a focused Helium-ion beam (He-FIB) in a Zeiss Orion NanoFab Microscope at a landing energy of 30 keV with doses in a range of 5.4 × 10–5 nC/μm2 to 8.1 × 10–3 nC/μm2. An atomic force microscope (AFM) was used to analyze the details of surface modification: the surface height of the irradiated regions and surface morphology changes caused by the irradiation. For all applied doses, the Al-coated samples demonstrated smaller surface-height reduction compared to the Pt60Pd40 and Au-coated samples. We speculate that the possible factors responsible for this effect include differences in the thickness and the crystalline-grain orientation (texture) of the metallization films. Both BOPP and spin-coated PP presented surface ridges at the borders between the irradiated and non-irradiated regions. It can be attributed to the mechanical strain induced by the material modification.

Real-time topography inspection of DPPC monolayers using a surface-plasmon resonance sensor

This paper presents an alternative optical characterization of biosensors based on supported lipid monolayers (SLMs). Developing these biosensors requires precise thickness characterization of the films to understand their structure and dynamics. This paper proposes an optical technique to measure the thickness, optical properties, and location of a dipalmitoyl-phosphatidylcholine (DPPC) SLM on top of a metallic thin film. DPPC SLMs are of interest for biosensing applications, such as detecting pulmonary-related infections like SARS-CoV-2, Avian Influenza, and the H1N1 influenza virus. The monolayer was fabricated using the Langmuir–Blodgett technique, and the experimental characterization consisted of measuring the surface-plasmon resonance angle in the Kretschmann configuration. This technique provides an alternative option for real-time visual inspection and determination of the location and shape of DPPC monolayers in large areas. Therefore, it offers a useful tool for further developing SLM-based biosensors.

Creep-dominated fatigue of freestanding gold thin films studied by bulge testing

Although metallic thin films are widely used in microelectronics, their high-cycle fatigue properties are not well-understood. Cyclic testing was conducted on 150-nm thick freestanding gold thin films using nominal stress amplitudes between 50 and 150 MPa at 23–100 °C. As expected, lifetime decreases with increasing stress amplitude and temperature. The strain at failure hardly exceeds 1.5%, which is in agreement with previous studies on metallic thin films. The hysteresis stress-strain curves suggest that strain accumulates mostly due to cyclic creep. Based on this observation, we calculated the stress exponent n and activation energy Q – which are typical creep parameters – from the apparent steady-state strain rate observed during cyclic testing. Comparison to preexisting creep bulge tests show that the higher the temperature, the more pronounced the influence of creep on the fatigue behavior. Especially at 100 °C, creep mechanisms clearly dominate. At lower temperatures (23 °C) the data indicate that creep still plays a role, but intrinsic cycling effects become more pronounced. That leads us to postulate that the fatigue lifetime of gold thin films is closely connected to their creep properties.

Solid-source metal-organic MBE for elemental Ir and Ru films

Thin films of elemental metals play a very important role in modern electronic nano-devices as conduction pathways, spacer layers, spin-current generators/detectors, and many other important functionalities. In this work, by exploiting the chemistry of solid metal-organic source precursors, we demonstrate the molecular beam epitaxy synthesis of elemental Ir and Ru metal thin films. The synthesis of these metals is enabled by thermodynamic and kinetic selection of the metal phase as the metal-organic precursor decomposes on the substrate surface. Film growth under different conditions was studied using a combination of in situ and ex situ structural and compositional characterization techniques. The critical role of substrate temperature, oxygen reactivity, and precursor flux in tuning film composition and quality is discussed in the context of precursor adsorption, decomposition, and crystal growth. Computed thermodynamics quantifies the driving force for metal or oxide formation as a function of synthesis conditions and changes in chemical potential. These results indicate that bulk thermodynamics are a plausible origin for the formation of Ir metal at low temperatures, while Ru metal formation is likely mediated by kinetics.

Effects of RF Magnetron Sputtering Power on the Mechanical Behavior of Zr-Cu-Based Metallic Glass Thin Films.

Zirconium-based metallic glass films are promising materials for nanoelectronic and biomedical applications, but their mechanical behavior under different conditions is not well understood. This study investigates the effects of radio frequency (RF) power and test temperature on the nanostructure, morphology, and creep behavior of Zr55Cu30Al10Ni5 metallic glass films prepared by RF magnetron sputtering. The films were characterized by X-ray diffraction and microscopy, and their mechanical properties were measured by a bulge test system. The results show that the films were amorphous and exhibited a transition from noncolumnar to columnar morphology as the RF power increased from 75 W to 125 W. The columnar morphology reduced the creep resistance, Young's modulus, residual stress, and hardness of the films. The creep behavior of the films was also influenced by the test temperature, with higher temperature leading to higher creep strain and lower creep stress. The findings of this study provide insights into the optimization of the sputtering parameters and the design of zirconium-based metallic glass films for various applications.