Ultrathin Metal Films Research Articles

Bimorph electrothermal micro-gripper with large deformation, precise and rapid response, and low operating voltage

Micro-grippers are highly desired in engineering, robotics, and biomedicine. However, on the basis of satisfying the requirements of miniaturization, precise manipulation, and low power consumption, the existing micro-grippers are difficult to achieve rapid response simultaneously. In this paper, we present a bimorph electrothermal micro-gripper that composed of several metal ultrathin films with high surface-to-volume ratios, allowing rapid heating and cooling processes. Patterns of these films are exquisitely designed so that the micro-gripper naturally forms an embedded circuit to optimize the current distribution. The micro-gripper can be precisely actuated under voltages below 2 V, while dramatically responding to pulse voltages up to 100 Hz. By interacting with a silica particle 96 times heavier than its weight, potential applications of the micro-gripper in robotics, organic tissue engineering, and interventional surgery can be shown. The advantage to be compatible with other semiconductor components ensures that the functions of the micro-gripper can be further expanded.

High resolution crystal orientation mapping of ultrathin films in SEM and TEM

Ultrathin metallic films are important functional materials for optical and microelectronic devices. Dedicated characterization with high spatial resolution and sufficient field of view is key to the understanding of the relation between microstructure and optical and electrical properties of such thin films. Here, we have applied on-axis transmission Kikuchi diffraction (TKD) and scanning precession electron diffraction (SPED) to study the microstructure of 10 nm thick polycrystalline gold films. The study compares the results obtained from the same specimen region by the two techniques and provides insights on the limits of each diffraction technique. We compare the physical spatial resolution of on-axis TKD and SPED and discuss challenges due to the larger probe size in scanning electron microscopy (SEM). Moreover, we present an improvement for the physical spatial resolution (PSR) of on-axis TKD through acquisition in immersion mode. We show how this method extends the capabilities of SEM-based microstructure characterization of ultrathin films and achieve PSR comparable to semi-automated SPED. • State of the art on-axis TKD SEM system comparison with SPED TEM system. • High resolution orientation mapping of ultrathin nano-crystalline metal films. • Improving TKD physical spatial resolution maps by immersion mode.

Transparent absorber composed of two stacked ultrathin metal films perforated with small holes.

A transparent absorber refers to the device which can absorb light strongly within a narrow frequency range but transmit light efficiently outside that range. Because of the contradiction between absorption and transmission, however, the performances of the transparent absorbers are usually compromised. In this work, we propose a transparent absorber based on a sandwiched metal-insulator-metal (MIM) structure, i.e., two perforated ultrathin metal films separated by a central dielectric layer. This structure has the advantage that the narrow-band absorption can be greatly enhanced because of the cooperation of surface-plasmon polariton (SPP) mode and multiple reflections in the dielectric cavity. Moreover, the ultrathin thickness of the stacked metal films enables high transmission when the wavelength of incident light deviates from the SPP resonance. A semi-analytical Fabry-Perot model has been employed to describe the optical properties, which agrees well with the simulation. The dependence of optical properties on the structural parameters has also been studied systematically. In addition, by covering the transparent absorber with an antireflection layer, highly efficient absorption of red (∼87% @ 629 nm), green (∼89% @ 524 nm), or blue (∼68% @ 472 nm) light and high transmission (∼80%) in the transparent region have been suggested. With its excellent visible-wavelength selective absorption, polarization independence, high angle-tolerance, and structural simplicity, the proposed MIM transparent absorber may have potential applications in the display technology and other smart scenarios.

Designing independent water transport channels to improve water flooding in ultra-thin nanoporous film cathodes for PEMFCs

Utilization of 3D nanostructured Pt cathodes could obviously improve performances of proton exchange membrane fuel cells (PEMFCs) owing to the reduced tortuosity and the bi-continuous nanoporous structure. However, these cathodes usually suffer from the flooding problem ascribed to the ionomer-free and nanoscale pores which are more susceptible to water condensation. In this paper, ultra-thin nanoporous metal films (100 nm) were utilized to construct PEMFC cathodes and independent transport channels were designed separately for water and gas aiming at the flooding problem. Nanoporous gold (NPG) film was used as the model support for loading Pt nanoparticles owing to its controllable and stable structure. After optimizing the polytetrafluoroethylene (PTFE) content and carbon loading in the gas diffusion layer (GDL), plasma treatment under O2 atmosphere was used to pattern the GDL with independent water transport channels. The obtained liquid permeation coefficients and oxygen gains demonstrated the obviously improved water and O2 transport. By using a home-made optimized GDL and a nanoporous film cathode with pore size ∼60 nm, the flooding problem could be facilely solved. With a Pt loading of ∼16 μg cm−2, this 3D nanostructured cathode exhibits a PEMFC performance of ∼957 mW cm−2 at 80 °C. The Pt power efficiency is about 4 times higher than that of the commercial Pt/C cathode (50 μg cm−2, 756 mW cm−2). Obviously, this study provides a simple but effective methodology to solve the water flooding problem in the ultra-thin nanoporous film cathodes which is applicable for other types of 3D nanostructured PEMFC cathodes.

Deep Etching of Silicon Based on Metal-Assisted Chemical Etching.

A deep etching method for silicon “micro”structures was successfully developed. This wet etching process is based on metal-assisted chemical etching (MACE), which was previously mainly utilized to etch the features that have lateral dimensions of “nanometers.” In this novel MACE, the critical improvement was to promote the “out-of-plane” mass transfer at the metal/Si interface with an ultrathin metal film. This enabled us to etch micrometer-wide holes, which was previously challenging due to the mass transport limitation. In addition, it was found that when ethanol was used as a solvent instead of water, the formation of porous defects was suppressed. Under the optimized etch conditions, deep (>200 μm) and vertical (>88°) holes could be carved out at a fast etch rate (>0.4 μm/min). This novel deep MACE will find utility in applications such as microelectromechanical systems (MEMS) devices or biosensors.

Advances in Flexible Metallic Transparent Electrodes (Small 19/2022)

Metal-Based Transparent Electrodes In article number 2106006, Viet Huong Nguyen and co-workers review recent progress and remaining challenges in metallic nanostructure-based transparent electrodes (TEs), including oxide/metal/oxide multilayers, metal grids, metal nanowire/nanofiber/nanotrough percolating networks, metal meshes based on cracked templated, dealloyed ultrathin metallic films, and some important nanocomposites. The various fields in which metal-based TEs have been successfully applied as well as emerging and potential applications are summarized.

Dual functional anode for organic light-emitting devices by directly imprinted nanostructured ultrathin Au film

The ultrathin metal film with excellent electrical conductivity and high visible-band transmittance has attracted considerable attention as a transparent electrode for the organic light-emitting devices (OLEDs). However, the deficient surface morphology and poor continuity of low thickness evaporated metal films and the surface plasmon-polaritons (SPPs) mode induced energy loss still seriously limit the actual efficiency of OLEDs. In this work, the thermal nanoimprint lithography has been further modified and directly applied to fabricate nanostructures onto the ultrathin Au film. During the imprinting process, the surface smoothness and conductivity of the Au film are obviously optimized with the formation of nanostructures. After integrating proper nanostructures into OLEDs, the photons that trapped by SPPs mode within the device were effectively out-coupled. The current efficiency and luminance have been enhanced 45.3% and 18.1%, respectively. Furthermore, the emitting properties of the OLEDs were also modified by the nanostructures, and the directional transmission capability of output light was significantly improved.

Fully 3D‐Printed, Ultrathin Capacitors via Multi‐Material Microsputtering

AbstractThis study reports the first fully additively manufactured capacitors as a proof‐of‐concept demonstration of direct‐write, ultrathin‐film electronic components made via multi‐material microplasma sputtering. This is also the first demonstration of a cleanroom‐quality, multi‐material electrical device produced entirely through additive manufacturing. Ultrathin metal and dielectric films are deposited at <80 °C and atmospheric pressure conditions on a substrate using a novel, continuously fed, dual target microsputtering printhead. The conductive films are created by sputtering gold in an air atmosphere and shown to attain near‐bulk electrical resistivity. The dielectric films are created by sputtering aluminum in a gas blend of argon and air; the aluminum oxidizes in the high‐energy, high‐collisionality plasma, forming alumina nanoparticles that are deposited on the substrate. Ultra‐thin (35 nm) alumina films showed extremely high resistivity (100 GΩ‐m) and dielectric strength (6.2 GV m−1). Also, the frequency response of the capacitor is satisfactorily described by the universal dielectric response typically found in heterogenous dielectrics. It is hypothesized that the dielectric response is the result of the presence of condensed water in the pores of the alumina film.

Simultaneous measurement of the exchange parameter and saturation magnetization using propagating spin waves

The exchange interaction in ferromagnetic ultra thin films is a critical parameter in magnetization-based storage and logic devices, yet the accurate measurement of it remains a challenge. While a variety of approaches are currently used to determine the exchange parameter, each has its limitations, and good agreement among them has not been achieved. To date, neutron scattering, magnetometry, Brillouin light scattering, spin-torque ferromagnetic resonance spectroscopy, and Kerr microscopy have all been used to determine the exchange parameter. Here, we present a method that exploits the wavevector selectivity of Brillouin light scattering to measure the spin wave dispersion in both the backward volume and Damon–Eshbach orientations. The exchange, saturation magnetization, and magnetic thickness are then determined by a simultaneous fit of both dispersion branches with general spin wave theory without any prior knowledge of the thickness of a magnetic “dead layer.” In this work, we demonstrate the strength of this technique for ultrathin metallic films, typical of those commonly used in industrial applications for magnetic random-access memory.

Synchronously controlled optical modes in the transmittance and reflectance spectra of multilayer photonic structure with dual-frequency nematic liquid crystal.

A method for electrically controlled synchronous mode tuning in the transmittance and reflectance spectra of a photonic structure consisting of an asymmetric dielectric Fabry-Pérot microcavity and an ultrathin metallic film has been proposed. The excitation of a broadband Tamm plasmon polariton at a metal/Bragg mirror interface is accompanied by the two phenomena opposing one another. Due to the strong absorption induced by the metal, a sufficient rejection level in the reflection of the off-resonant radiation has been obtained in a wide spectral range, which almost coincides with the photonic band gap, while at the cavity mode frequencies the structure becomes transparent for the reflected radiation. This leads to the appearance of a series of narrow resonance peaks in the reflectance spectra that coincide in frequency with the transmittance peaks. Based on the structural transformations in the dual-frequency nematic liquid crystal used as a photonic structure defect, the tuning of the modes corresponding to the extraordinary waves to both the short- and long-wavelength spectral ranges has been implemented.

Formation of inhomogeneous oxide and suboxide layers on an ultra-thin metal film by multiple oxidation and ion sputtering

A technique of controlled formation of multilayer oxide and suboxide layers on a thin metal film. An opportunity of controlled generation of films containing an ultra-thin suboxide layer inside a higher oxide layer and films being a periodic layered structure with alternating oxide layers of various oxidation levels. A structure containing alternating ultra-thin layers of higher oxide and suboxide layers on a niobium film. Keywords: metal-oxide films, controlled film formation, layer chemical and phase profiling, X-ray photoelectron spectroscopy.

Resistivity Scaling Transition in Ultrathin Metal Film at Critical Thickness and Its Implication for the Transparent Conductor Applications

AbstractUnderstanding of ultrathin metal film's electrical and optical properties at sub‐10 nm thickness may provide important engineering insight on its application as a transparent conductor. Here, a rapid change is observed in the ultrathin metal film's electrical and optical scaling properties as the thickness shrinks to below a certain critical thickness dc. Below this thickness, the metal film's electrical properties are shown to be strongly influenced by the inhomogeneity of the film which can be modeled via general effective media theory by incorporating size‐effect contribution. As a result, below dc, carrier's scattering time rapidly decreases with a reduced mean free path leading to a rapid rise in resistivity. Also, the film's optical loss increases while the optical transmission plateaus below dc. As one promising application of thin metal film is transparent conductor where the film's electrical and optical properties are equally important, its maximum theoretical figure‐of‐merit is shown, which is determined at this dc serving as an important engineering metric.

Formation of Surface-Wrinkled Metal Nanosheets via Thermally Assisted Nanotransfer Printing

Nanopatterning methods for pattern formation of high-resolution nanostructures are essential for the fabrication of various electronic devices, including wearable displays, high-performance semiconductor devices, and smart biosensor systems. Among advanced nanopatterning methods, nanotransfer printing (nTP) has attracted considerable attention due to its process simplicity, low cost, and great pattern resolution. However, to diversify the pattern geometries for wide device applications, more effective and useful nTP based patterning methods must be developed. Here, we introduce a facile and practical nanofabrication method to obtain various three-dimensional (3D) ultra-thin metallic films via thermally assisted nTP (T-nTP). We show how to generate surface-wrinkled 3D nanostructures, such as angular line, concave-valley, and convex-hill structures. We also demonstrate the principle for effectively forming 3D nanosheets by T-nTP, using Si master molds with a low aspect ratio (A/R ≤ 1). In addition, we explain how to obtain a 3D wavy structure when using a mold with high A/R (≥ 3), based on the isotropic deposition process. We also produced a highly ordered 3D Au nanosheet on flexible PET over a large area (> 15 µm). We expect that this T-nTP approach using various Si mold shapes will be applied for the useful fabrication of various metal/oxide nanostructured devices with high surface area.

Liquid lithium metal processing into ultrathin metal anodes for solid state batteries

Lithium metal anodes are among the most promising candidates for further increasing the energy density of lithium ion batteries and all-solid-state batteries. A reduction of the anode thickness by using ultrathin lithium metal films is a crucial requirement to achieve a significant overall reduction of thickness on cell level. However, besides anode stabilization, realizing scalable technologies for an efficient production of thin lithium metal anodes is one of the most challenging obstacles for the success of various next-generation battery chemistries. In this publication we introduce a disruptive lithium melt deposition process for thin lithium metal coating on thin copper current collector foils. The wetting of molten lithium on the substrate can only be achieved through a lithiophilic interlayer. As a result fast and homogeneous lithium spreading on the substrate is enabled allowing roll-to-roll coating with liquid-deposition technologies as demonstrated in this contribution with a speed of several meters per minute and reaching 100 mm width. With this new process the anode thickness can be tuned in a wide range (1–30 µm). Evaluation in a prototype solid battery system shows high electrochemical lithium utilization and no detrimental effects compared to commercially available lithium reference foils.

Influence of roughness, porosity and grain morphology on the optical properties of ultrathin Ag films

The dielectric function ε(ω) determines the optical behavior of thin and ultrathin metal films, crucial for many research areas and devices. Surprisingly, even for the paradigmatic case of evaporated Ag films, a large scattering of ε(ω) exists, mainly because the film is modelled by neglecting grain structure and surface roughness. Here, we quantitatively describe the optical - morphological relationships for Ag films in the 5.4 to 63 nm range thermally evaporated on fused silica. The experimental reflectance and transmission spectra are reproduced by a multilayer model employing an effective medium approximation for each layer. For the 9 nm thick film we describe how the film grain size and shape, porosity and surface roughness determine the reflectance and transmission spectra, and the total film ε(ω). Moreover, we provide the thickness dependent trend of the morphology-optical response correlation thus obtaining a complete quantitative description of the Ag films behavior up to the transition to the bulk regime. These results make this work a milestone in the morphology-optical understanding for ultrathin metal films and open new perspectives in the interpretation of thin film properties and in the design of the optical response of a tailored system.

Nonlinear plasmonic response in atomically thin metal films.

Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light-matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.

Study of Pt growth on Si, Al2O3, Au, and Ni surfaces by plasma enhanced atomic layer deposition

Atomic layer deposition is a powerful technique for achieving atomic-level control in the deposition of thin films and nanoparticles. The ultrathin noble metal films can be applied in many functional devices, but it is a challenge to obtain such films since the island growth mode generally happens instead of the layer growth mode. In this work, the nucleation and growth of platinum on Si, Al2O3, Au, and Ni substrates were studied using (MeCp)PtMe3 and O2 plasma as a precursor and a co-reactant, respectively. The evolution of Pt coverage on different surfaces was observed and discussed based on the experimental results by x-ray photoelectron spectra. The chemical adsorption of the precursor and the following processes like metal atom diffusion on substrate surfaces and up-stepping onto the existing metal islands were considered to dominate the growth before continuous films formed. The chemisorption determined the metal coverage on bare substrate surfaces, and the total coverage was influenced by metal atom diffusion and up-stepping behaviors that are determined by surface energy relationships between the deposited metal and substrate surfaces. Pt films were easier to form on Al2O3 and Ni surfaces compared with on Si and Au surfaces, respectively. A model was proposed to help to understand the mechanisms in the nucleation and growth processes, involving the chemisorption, diffusion, and up-stepping, which contributed to prepare ultrathin continuous Pt films on different substrates.

Ultrathin Metals on a Transparent Seed and Application to Infrared Reflectors.

Ultrathin metal films (UTMFs) are widely used in optoelectronic applications, from transparent conductors to photovoltaic cells, low emissivity windows, and plasmonic metasurfaces. During the initial deposition phase, many metals tend to form islands on the receiving substrate rather than a physically connected (percolated) network, which eventually evolves into continuous films as the thickness increases. For example, in the case of Ag and Au on dielectric surfaces, percolation begins when the thickness of the metal film is at least about 5 nm. It is known that the type of growth can be changed when a proper seed layer is used. Here, we show that a CuO layer directly deposited on a substrate can dramatically influence surface wetting and promote early percolation of polycrystalline Ag and Au UTMFs. We demonstrate that the proposed seed is effective even when its thickness is sub-nanometric, in this way maintaining the full transparency of the receiving substrate. The Ag and Au films seeded with CuO showed a percolation thickness close to 1 nm and were morphologically and optically characterized from an ultraviolet (λ = 300 nm) to a midinfrared (λ = 15 μm) wavelength. Infrared reflectors, a mirror and a resonant plasmonic structure, were also demonstrated and uniquely tuned by electrical gating, this being possible owing to the small thickness of the constituting Au UTMF.

Brightening single-photon emitters by combining an ultrathin metallic antenna and a silicon quasi-BIC antenna

Bright single-photon emitters (SPEs) are fundamental components in many quantum applications. However, it is difficult to simultaneously get large Purcell enhancements and quantum yields in metallic nanostructures because of the huge losses in the metallic nanostructures. Herein, we propose to combine an ultrathin metallic bowtie antenna with a silicon antenna above a metallic substrate to simultaneously get large Purcell enhancements, quantum yields, and collection efficiencies. As a result, the brightness of SPEs in the hybrid nanostructure is greatly increased. Due to the deep subwavelength field confinement (mode size < 10 nm) of surface plasmons in the ultrathin metallic film (thickness < 4 nm), the Purcell enhancement of the metallic bowtie antenna improves by more than 25 times when the metal thickness decreases from 20 nm to 2 nm. In the hybrid nanostructures by combining an ultrathin metallic bowtie antenna with a silicon antenna, the Purcell enhancement (Fp≈2.6 × 106) in the hybrid nanostructures is 63 times greater than those (≤ 4.1 × 104) in the previous metallic and hybrid nanostructures. Because of the reduced ratio of electromagnetic fields in the ultrathin metallic bowtie antenna when the high-index silicon antenna is under the quasi-BIC state, a high quantum yield (QY ≈ 0.70) is obtained. Moreover, the good radiation directivity of the quasi-BIC (bound state in the continuum) mode of the silicon antenna and the reflection of the metallic substrate result in a high collection efficiency (CE ≈ 0.71). Consequently, the overall enhancement factor of brightness of a SPE in the hybrid nanostructure is EF* ≈ Fp × QY × CE ≈ 1.3 × 106, which is 5.6 × 102 times greater than those (EF* ≤ 2.2 × 103) in the previous metallic and hybrid nanostructures.

Development of latent fingermarks by unbalanced magnetron sputtering deposited ultra-thin metal film

Thin metal films with thickness above 50 nm have been successful used for developing level 1 features of latent fingermarks. In order to easily show the level 2 and level 3 features of the fingermarks, thinner films are needed to be fabricated. Three kinds of metal films including cooper, chromium and aluminum with film thickness ranging from 5 nm to 30 nm were deposited by unbalanced magnetron sputtering. The efficacy of film thickness and metal type on developing latent fingermark was investigated. The most clearly developed fingermark was obtained by films with thickness of 10–20 nm. Cooper films exhibit better effects on showing Level 2 features than chromium and aluminum films with the same thickness. The optical contrast of fingermark ridges and valleys was influenced by the morphology difference of film deposited on ridge and valley areas, which were related to shadowing effect affected by film thickness and metal type determined re-emission effect.