Ultrathin Metal Layers Research Articles

A two-dimensional thin film structure with spectral selective emission capability suitable for high-temperature environments

Spectral selective emission materials, characterized by their high emission efficiency within specific wavelength ranges, play a crucial role in various fields such as infrared stealth and radiative cooling. In this study, leveraging the tunneling effect of ultrathin metal layers and the impedance matching principle, we designed a one-dimensional multilayer thin film structure composed of Mo/Ge/Al. This design successfully achieved spectral selective emission within the 3-14 μm infrared spectrum range, showcasing superior high-temperature infrared stealth capabilities. Research findings indicate that the infrared atmospheric window (ε3-5 μm=0.40, ε8-14 μm=0.30) maintains a low emissivity within the temperature range from room temperature to 400°C, while high emissivity in non-infrared atmospheric windows (ε5-8 μm=0.81) enables radiative cooling. The use of high emissivity in the non-infrared atmospheric window reduced the surface temperature of this structure by 15.3°C compared to untreated Si substrate under similar heating conditions. The temperature reduction was 34.1°C when compared to a Si substrate coated with a 200 nm Al film. This straightforward and easily scalable structure not only presents novel solutions for applications in infrared stealth and radiative cooling but also holds vast potential for diverse fields including military, aerospace, and industrial manufacturing, injecting fresh impetus and possibilities into technological advancement.

Ultrathin metal-dielectric planar interface for high-performance photonic spin Hall effect

Abstract This study explores the photonic spin Hall effect (PSHE) with a focus on its sensitivity to incident wave polarization and physical parameter variations. Traditionally surface plasmonic resonance (SPR) systems mainly enhance H-polarized PSHE. However, our proposed method involves an ultrathin metal layer, capped with a glass dielectric layer on a glass substrate, to achieve high-performance PSHE for both H- and V-polarization waves. This high-performance PSHE in terms of enhancement in PSHE magnitudes arises from the simultaneous presence of SPR and waveguiding effects, resulting in the emergence of hybrid TE and TM modes. Investigating Au and Ag ultrathin metal layers, we find that the highest H-polarized PSHE occurs at ~1.52 × 10^6 nm and ~2.05 × 10^5 nm for Au and Ag, respectively. Similarly, for Vpolarized incident light, maximum enhanced PSHE is observed at ~1.58 × 10^5 nm and ~9.48 × 10^4 nm for Au and Ag, respectively. By adjusting the thickness of the metal and/or glass cap layer, precise control over PSHE amplitude and active polarization response is achievable. This research unveils unique capabilities for generating hybrid modes that enhance PSHE for both polarizations, offering a potential platform for developing tunable polarizationfavorable spin optics optoelectronic devices.

Engineering of Co/MgO interface with combination of ultrathin heavy metal insertion and post-oxidation for voltage-controlled magnetic anisotropy effect

The voltage-controlled magnetic anisotropy (VCMA) effect in ferromagnet/insulator junctions provides an effective way to manipulate electron spins, which can form the basis of future magnetic memory technologies. Recent studies have revealed that the VCMA effect can be strongly tuned by a process of “interface engineering” exploiting ultrathin heavy metal layers and an electron depletion effect. To further decrease the numbers of electrons, chemical reactions, such as surface oxidation of ferromagnets, may also be an effective way to achieve this depletion. However, the knowledge of combined effect of heavy metal layers and oxidation is still lacking. Here, we demonstrate that dual interfacial engineering using an insertion of heavy metals (Pt or Re) and a post-oxidation process can have a remarkable effect on the perpendicular magnetic anisotropy and the VCMA effect. Interestingly, a strong enhancement of the perpendicular magnetic anisotropy is observed by dual interfacial engineering with Pt insertion, although it does not occur with Pt insertion or surface oxidation alone. Furthermore, even a sign reversal of the additional VCMA effect due to the ultrathin heavy metal layers is observed by utilizing dual interfacial engineering. These findings provide another degree of freedom for designing voltage-controlled spintronic devices and pave the way to interfacial spin–orbit engineering for the VCMA effect.

ZnO/Ag/ZnO multilayers as an n-type transparent conducting electrode for transparent organic light-emitting diodes

Alternative transparent conducting electrodes (TCEs) have gained significant attention in recent decades in an effort to replace costly indium-tin oxide (ITO) in organic optoelectronic devices. For example, state-of-the-art top-emitting and transparent organic light-emitting diodes (OLEDs) in the display industry require a transparent or semitransparent cathode for electron injection. Current OLED display technology employs ultrathin metallic (typically Mg:Ag) layers as semitransparent cathodes. However, recent studies have shown that metal oxide/metal/metal oxide multilayers can perform better optically and offer improved stability compared to semitransparent metal cathodes. Deposition of these multilayers with minimal damage to the underlying organic emissive layer is imperative. Here, we investigate a ZnO/Ag/ZnO TCE multilayer TCE deposited under mild sputtering conditions that are safe for the emissive layer. We demonstrate ∼ 85 % transparent electrodes with a sheet resistance of 1.47 Ω/sq, which is more conductive than the industry standard, ITO. We also integrate the multilayer TCE as the cathode in a transparent OLED device built on ITO, demonstrating the compatibility of this TCE with the existing OLED technology.

Comprehensive effect on the development of multifunctional low emissive and transparent conductive amorphous-SIZO/Ag/amorphous-SIZO multilayer structure

Wide band gap oxide semiconductors based on transparent conducting electrodes (TCEs) are promising for the fabrication of a wide range of electronic and optoelectronic devices, which include flat panel displays, photovoltaics, transparent heaters, and smart windows. High optical transmittance in the visible spectral range and low electrical resistivity as obtained by intrinsic and/or extrinsic doping, lead to a high figure of merit (FoM) for oxide based TCEs. Recently, we have used wide band gap amorphous Si–In–Zn–O (SIZO) to realize a relatively new class of TCEs using SIZO/Ag/SIZO multilayer structures, which exhibit high FoM in addition to infrared reflection capability due to the embedded ultrathin Ag metal layer. These multifunctional TCEs are found to be highly useful for low emissive smart window applications, which minimize the outside infrared to coming inside and efficiently reflect inside heat to emit outside. In addition, amorphous OMO based TCEs show excellent mechanical endurance, unlike crystalline ITO based coatings. The optical and low emission characteristics of amorphous SIZO OMO were optimized to transmittance of 93.4 % at 550 nm and 58.6 % at 950 nm.

Realizing Ultrathin LiSn Alloy Anode by Tuning the Wettability of Molten Li for High-Energy-Density Batteries.

Despite being heralded as the "holy grail" of anodes for their high theoretical specific capacity, lithium (Li) metal anodes still face practical challenges due to difficulties in fabricating ultrathin Li with controllable thickness and suppressing Li dendrites growth. Herein, we introduce a simple and cost-effective dip-coating method to fabricate ultrathin lithium-tin (LiSn) anode with adjustable thicknesses ranging from 4.5 to 45 μm. The in situ formation of Li22Sn5 alloy improves the wettability of the molten Li, enabling the casting of ultrathin Li metal layers on different substrates. More importantly, the abundant Li22Sn5 lithiophilic sites significantly lower the nucleation overpotential, inducing uniform Li deposition and accelerating the electrochemical reaction at the interface. As a result, the symmetric cell assembled with LiSn-Cu electrodes can cycle stably for more than 120 h with a charge/discharge depth of 50%, which is 1.5 times longer than the lifespan of the pure Li anode. In the full cells paired with NCM cathode, the discharge specific capacity is improved from 13.84 to 70.31 mA h g-1 with the LiSn-Cu anode at 8 C. The LiSn-Cu||NCM full cell realized a high energy density of 724.9 Wh kg-1 at the active material level with an N/P ratio of 1.4.

Atomic-scale characterization of defects in oxygen plasma-treated graphene by scanning tunneling microscopy

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

Sensitivity of a Process for Heating Thin Metal Film Described by the Dual-Phase Lag Equation with Temperature-Dependent Thermophysical Parameters to Perturbations of Lag Times

In the paper, an equation with two delay times (dual-phase lag Equation (DPLE)) in a version that takes into account the dependence of thermophysical parameters (volumetric specific heat and thermal conductivity) on temperature is considered. In particular, an analysis of the sensitivity of transient temperature field in relation to disturbances in delay times (the relaxation and thermalization times) is performed. The sensitivity model concerns the process of heating an ultrathin metal layer with a laser beam. First, the equation with two delay times in the case of temperature-dependent thermophysical parameters is presented. Next, the sensitivity equations with respect to delay times are derived using the direct method. The algorithms for solving the basic and sensitivity tasks are also briefly presented. At the stage of computations, an authorial program based on the implicit scheme of a finite-difference method is developed. In the final part of the paper, examples of numerical solutions (for layers made from gold and nickel) are presented. The research conducted here shows that disturbances in the temperature field are clearly visible and depend, on the one hand, on the thermophysical parameters of the material, and on the other hand, on the intensity of heating with an external heat source.

Overcoming Copper Substrate Thermodynamic Limitations in Anode-Free Lithium Pouch Cells via In Situ Seed Implantation.

Anode-free lithium metal batteries (AFLMBs) exhibit enhanced energy density and cost-effective manufacturing, albeit constrained by short lifespans due to inhomogeneous lithium nucleation and growth on the inherently lithiophobic Cu current collector. Although numerous attempts at Cu surface modifications aim to mitigate this thermodynamic limitation, they often result in substantial irreversible capacity loss and/or lack the stability required for practical applications. Here, we present an in situ seed implantation (ISI) strategy to address the aforementioned challenge. A 36 s ISI treatment created an ultrathin lithium metal layer, composed of uniform lithium nuclei with ∼100 nm in diameter, equating to 0.05 mAh cm-2, on the Cu substrate. This approach facilitates dense lithium deposition during cycles, effectively doubling the lifespan of an Ah-level 437 Wh kg-1 AFLMB. Our ISI strategy offers a straightforward and efficient solution that maintains battery energy density and manufacturing cost effectiveness, and its application extends beyond lithium metal.

All-Solid-State Batteries with Pre-plated Ultrathin Lithium Metal Enabled by No Pressure Holding and Characterized by In Situ Electron Paramagnetic Resonance Imaging

All-solid-state batteries with ultrathin lithium metal, close to the anode-free solid-state batteries, could achieve high energy density. However, it is not trivial to plate an ultrathin lithium metal layer on a Cu current collector (Cu-CC). In situ electron paramagnetic resonance (EPR) imaging showed that the pre-plating on Cu-CC in a Li-In|Li6PS5Cl|Cu cell without pressure holding produced a lithium metal layer with a small area. However, when a Li-In|Li6PS5Cl|Cu cell was held under a considerable pressure, this cell would produce a lithium metal layer with a large area. The Cu-CC with the ultrathin lithium metal layer of a small surface could be extracted and then assembled with LiNiO2 and Li6PS5Cl to achieve better electrochemical performance compared to that of a large surface, because a small area of the lithium metal layer would lead to fewer side reactions and less consumption of lithium ions. This work suggests the importance of the control of the lithium plating and the observation of lithium plating by in situ EPR imaging.

A Mo/Si multilayer film based selective thermal emitter for high-temperature infrared stealth application

Thermal radiation regulation technology plays a vital role in the fields of military camouflage and energy utilization. Here, a Mo/Si multilayer film based selective thermal emitter for high-temperature infrared stealth application is demonstrated by virtue of the tunneling effect of ultrathin metal layers as well as impedance matching to modulate the radiation properties. The fabricated selective emitter exhibits low emissivity of ε3-5μm≈0.26 and ε8-14μm≈0.36 at the atmospheric windows for infrared “invisibility” and high emissivity of ε5-8μm≈0.75 at the non-atmospheric window for radiative cooling. As observed from infrared thermal images, the prepared sample shows reduced surface temperature compared to the ambient temperature. The radiation temperature on sample surface is only 425.7 °C when the heating temperature increase to 600 °C, indicating the good dissipation ability of surface heat at high temperature. Moreover, the multilayer film device exhibits good infrared camouflage capability and thermal stability as the heating temperature reaches up to 650 °C. The fabricated selective thermal emitter has promising prospects in high-temperature infrared stealth technology.

Polyethylene terephthalate-based cathode current collectors coated by ultrathin aluminum metal layers for commercial lithium-ion batteries with high security and long-term cycling stability

With the rapidly increasing market demand of lithium-ion batteries (LIBs), safety has become the main focus and challenge in realizing high-energy and high-safety LIBs. In this paper, a light-weight composite current collector, which is prepared by sandwiching a polyethylene terephthalate (PET) film between two ultrathin aluminum (Al) layers, is designed to improve the safety performances of the LIBs. The PET film is selected as the supporting film due to its low density, superior mechanical properties, excellent thermal stability, and remarkable flame resistance. The thermal and mechanical stabilities of the PET-Al based electrode systems, as well as the electrochemical performances of the corresponding LIBs, are evaluated. Nail penetration test further is carried out to investigate the internal short-circuit behaviors of the batteries, which is the most extreme and hazardous condition and may cause thermal runaway and catastrophic accidents. The results well demonstrate that the proposed PET-Al current collector can slow down the temperature rise of the entire battery during the nail penetration, and effectively achieve the aim of improving the reliability performances of LIBs.

Melting lithium alloying to improve the affinity of Cu foil for ultra-thin lithium metal anode.

Before lithium (Li) metal can be formally used as the anode material of Li-ion battery, the key technical defects of Li metal electrode, such as low active Li metal proportion and low mechanical strength, must be solved. Herein, the surface affinity of molten lithium-copper (LiCu) alloy with Cu foil is improved by alloying Cu and Li in a molten state. The surface of Cu foil naturally adsorbs an ultra-thin (∼30μm) composite Li metal layer. The ultra-thin composite Li metal layer can greatly reduce the amount of inactive Li, and the Cu foil improves the mechanical strength and engineering workability of Li metal anode. In addition, the enhanced Young's modulus facilitates the uniform Li plating/stripping process. As a result, the stable cycle stability of up to 600h and the average overpotential of 13mV (area specific capacity is 1 mAh cm-2 and current density is 1mAcm-2) are achieved. The cycle life is higher than 150h even though the maximum utilization rate of Li is greater than 50%. The Li metal full battery assembled with the commercial NCM811 cathode shows more stable cycle performance and Coulombic efficiency. Such strategy can effectively pave the way for the practical application of Li metal anode.

Stable Ultrathin Ag Electrodes by Tailoring the Surface of Plastic Substrates for Flexible Organic Light-Emitting Devices.

Flexible transparent metal electrodes (FTMEs) have significant application potentials in the fields of flexible optoelectronic devices due to their outstanding optical transmittance and electrical conductivity. However, obtaining excellent optoelectrical properties and mechanical flexibility of FTMEs is challenging because ultrathin metal layers usually follow an island growth mode. In this paper, flexible transparent ultrathin Ag electrodes with high mechanical stability and good optoelectrical properties were exploited by tailoring the surface properties of plastic substrates with ultraviolet-ozone (UVO) treatment for regulating the nucleation and growth kinetics of Ag films. The composite transparent electrodes of Ag (9 nm)/MoO3 (20 nm) fabricated on the UVO-treated polyethylene terephthalate (PET) substrates possess a low sheet resistance of ∼7.9 Ω/sq, a high optical transmittance of ∼87.2% at 550 nm, a long-period environmental stability of 30 days (∼65 °C, ∼80% humidity), and excellent mechanical flexibility of 100,000 bending cycles at a bending radius of 1.5 mm. These properties are derived from the surface treatment of PET substrates by UVO, which increases substrate surface energy and produces chemical nucleation sites of the phenolic hydroxyl groups. The phenolic hydroxyl groups generated on the PET surface not only provided efficient nucleation sites for subsequent Ag film growth but also formed C-O-Ag bonds between the substrate surface and the Ag layer, which act as "anchor chains" to fix firmly the Ag atoms on the substrate surface. As a universal applicability strategy, the composite electrodes on the UVO-treated polyethylene naphthalate (PEN) and norland optical adhesive 63 (NOA63) substrates also possess excellent optoelectrical properties and mechanical flexibility. Based on the ultrathin Ag composite electrodes, the flexible white organic light-emitting devices with PET, PEN, and NOA63 as substrates present the maximum current efficiencies of 53.0, 77.0, and 65.2 cd/A, respectively.

Dependency of Ag wetting on the oxygen nonstoichiometry of oxide surfaces

Oxide substrates are frequently modified to have either an oxygen deficiency or excess to synthesize a completely continuous, ultrathin metal layer on them by strengthening the metal–oxide adhesion. However, conventional surface treatments using ionic irradiation inevitably induce detrimental structural irregularities, along with chemical modification, on the surface of oxide substrates. This study explores the dependency of Ag wetting on the oxygen deficiency and excess on the surface of a ZnO substrate. The favorable contribution of the increased oxygen deficiency and excess of ZnO surfaces in improving Ag wetting during the inceptive growth stages is evidenced by both experimental results and numerical interpretations. This improved Ag wetting is attributed to a significant reduction in free energy at the Ag–ZnO interfaces, whereas pinholes remain in thicker Ag layers on the ZnO surfaces. This finding extends earlier studies, reporting a favorable role of an increased oxygen deficiency on Ag wetting, with the point that oxygen excess beyond a threshold value is also helpful in improving Ag wetting. Our findings redefine the underlying Ag wetting mechanisms of the nonstoichiometric surface of oxide substrates. Moreover, the study provides guidelines for implementing a continuous ultrathin Ag layer while avoiding ion-induced damage to the oxide substrates.

Tunable Polarized Microcavity Characterized by Magnetic Circular Dichroism Spectrum.

Tunable resonator is a powerful building block in fields like color filtering and optical sensing. The control of its polarization characteristics can significantly expand the applications. Nevertheless, the methods for resonator dynamic tuning are limited. Here, a magnetically regulated circular polarized resonant microcavity is demonstrated with an ultrathin ferrimagnetic composite metal layer Ta/CoTb. We successfully tuned the cavity resonant frequency and polarization performance. A huge magnetic circular dichroism (MCD) signal (∼3.41%) is observed, and the microcavity valley position shifts 5.41 nm when a small magnetic field is applied. This resonant cavity has two-stable states at 0 T due to the magnetic remanence of CoTb film and can be switched using a tiny magnetic field (∼0.01 T). Our result shows that the ferrimagnetic film-based tunable microcavity can be a highly promising candidate for on-chip magneto-optical (MO) devices.

Ultrathin TiN Epitaxial Films as Transparent Conductive Electrodes.

Titanium nitride (TiN), a transition-metal compound with tight covalent Ti-N bonding, has a high melting temperature and superior mechanical and chemical stabilities compared to noble metals. With a reduction in thickness, the optical transmittance of TiN films can be drastically increased, and in combination with its excellent electrical conductivity, the ultrathin and continuous TiN film can be considered as an ideal alternative of the metal oxide electrodes. However, the deposition of ultrathin and continuous metallic layer with a smooth surface morphology is a major challenge for typical deposition methods such as thermal evaporation or reactive sputtering. In particular, defects mainly related with oxygen contents and surface scattering can significantly limit the performance of ultrathin TiN films. In this work, ultrathin TiN films with 2-10 nm in thickness are grown by using the nitrogen plasma-assisted molecular-beam epitaxy (MBE) method in an ultrahigh vacuum environment. Excellent surface morphology with a root-mean-square roughness of ≤0.12 nm and a high optical transparency of 75% over the whole visible regime are achieved for ultrathin TiN epitaxial films. The dielectric properties determined by the spectroscopic ellipsometry and the electrical properties measured by the terahertz spectroscopy and the Hall effect method reveal that the percolation thickness of the TiN epitaxial film is less than 2.4 nm and its electrical conductivity is higher than 1.1 × 104 Ω-1 cm-1. These features make MBE-grown ultrathin TiN epitaxial films a good candidate for robust, low cost, and large-area transparent conductive electrodes.

Numerical Simulation of the Optical Coatings based on Ag Infrared Reflector

Ultra-thin metal layer (thickness below 15 nm) is of great interest as optical coatings in heat reflectors systems. However, producing these thin layers has been a continuing challenge because their thicknesses of individual layers depart from the conventional values. In this work, Infrared Reflective Optical Coatings are designed using thin layers to the visible transmittance at wavelengths 400~800 nm while reflecting infrared wavelengths 800-3000 nm. Three different design approaches have been used, namely single layers of metal (Ag), bi-layers design metal/ dielectric structure, and multilayer systems were designed on glass substrates for reflectors applications. The dielectrics materials (SiO2, SiN, Nb2O5, CdTe and AlAs) are chosen in this study. A detailed mathematical analysis based on characteristic matrix theory is presented to predict heat Multilayers reflectors. The optical performance of three alternating layers Nb2O5(25nm)/Ag(15nm)/Nb2O5(25nm) on substrate in range of (400-3000 nm) show the maximum visible transmittance about 89% and maximum IR reflectance 83. % at 1900 nm with stack SiO2(25nm)/Ag(15nm)/SiO2(25nm). Also the designs of stacks SiO2 (25nm) /Ag (15nm)/ SiO2(25nm)/ Ag(15nm)/ SiO2(25nm) and SiN (25nm) / Ag (15nm) /SiN (25nm)/Ag(15nm) / SiN(25nm) show the maximum reflection of IR wavelengths compared to the three alternating layers structure.

Universal Design of Ultra-Broadband and Ultra-Compact High Order Mode Pass Filters Based on Enhanced Absorption in Ultrathin Metal Layers

In hybrid plasmonic Si waveguides integrated with ultrathin Au stripes, mode hybridization effects between quasi-TE modes of different orders and various bound modes are first presented. Enabled by nearly complete mode-selective absorption attributed to the enlarged mode propagation attenuation (MPA) coefficient for lower-order quasi-TE modes under circumstances of mode hybridizations, multi-segment tapered structures are theoretically proposed to constitute high order mode (HOM) pass filters. It is believed that these innovative observations would have potential applications in the ultra-compact on-chip mode division multiplexing (MDM) systems, and provide inspirations for the development of multimode photonics.

Polyethylene Terephthalate-Based Cathode Current Collectors Coated by Ultrathin Aluminum Metal Layers for Commercial Lithium-Ion Batteries with High Security and Long-Term Cycling Stability

Polyethylene Terephthalate-Based Cathode Current Collectors Coated by Ultrathin Aluminum Metal Layers for Commercial Lithium-Ion Batteries with High Security and Long-Term Cycling Stability