Ultrathin Metal Films Research Articles

Transparent Electrodes for Efficient Optoelectronics

With the development of new generations of optoelectronic devices that combine high performance and novel functionalities (e.g., flexibility/bendability, adaptability, semi or full transparency), several classes of transparent electrodes have been developed in recent years. These range from optimized transparent conductive oxides (TCOs), which are historically the most commonly used transparent electrodes, to new electrodes made from nano‐ and 2D materials (e.g., metal nanowire networks and graphene), and to hybrid electrodes that integrate TCOs or dielectrics with nanowires, metal grids, or ultrathin metal films. Here, the most relevant transparent electrodes developed to date are introduced, their fundamental properties are described, and their materials are classified according to specific application requirements in high efficiency solar cells and flexible organic light‐emitting diodes (OLEDs). This information serves as a guideline for selecting and developing appropriate transparent electrodes according to intended application requirements and functionality.

Ultrathin Metal films for Transparent Electrodes of Flexible Optoelectronic Devices

The need for the development of transparent conductive electrodes (TCEs) supported on flexible polymer substrates has explosively increased in response to flexible polymer‐based photovoltaic and display technologies; these TCEs replace conventional indium tin oxide (ITO) that exhibits poor performance on heat‐sensitive polymers. An efficient, flexible TCE is required to exhibit high electrical conductance and high optical transmittance, as well as excellent mechanical flexibility and long‐term stability, simultaneously. Recent advances in technologies utilizing an ultrathin noble‐metal film in a dielectric/metal/dielectric structure, or its derivatives, have attracted attention as promising alternatives that can satisfy the requirements of flexible TCEs. This review will survey the background knowledge and recent updates of synthetic strategies and design rules toward highly efficient, flexible TCEs based on ultrathin metal films, with a special focus on the principal features and available methodologies involved in the fabrication of highly transparent, conductive, ultrathin noble‐metal films. This survey will also cover the practical applications of TCEs to flexible organic solar cells and light‐emitting diodes.

Self‐Encapsulating Thermostable and Air‐Resilient Semitransparent Perovskite Solar Cells

Semitransparent perovskite solar cells (PSCs) are of interest for application in tandem solar cells and building‐integrated photovoltaics. Unfortunately, several perovskites decompose when exposed to moisture or elevated temperatures. Concomitantly, metal electrodes can be degraded by the corrosive decomposition products of the perovskite. This is even the more problematic for semitransparent PSCs, in which the semitransparent top electrode is based on ultrathin metal films. Here, we demonstrate outstandingly robust PSCs with semitransparent top electrodes, where an ultrathin Ag layer is sandwiched between SnOx grown by low‐temperature atomic layer deposition. The SnOx forms an electrically conductive permeation barrier, which protects both the perovskite and the ultrathin silver electrode against the detrimental impact of moisture. At the same time, the SnOx cladding layer underneath the ultra‐thin Ag layer shields the metal against corrosive halide compounds leaking out of the perovskite. Our semitransparent PSCs show an efficiency higher than 11% along with about 70% average transmittance in the near‐infrared region (λ > 800 nm) and an average transmittance of 29% for λ = 400–900 nm. The devices reveal an astonishing stability over more than 4500 hours regardless if they are exposed to ambient atmosphere or to elevated temperatures.

Recent progress of multilayer composite transparent conductive film

Transparent conductive films (TCFs), which transmit light and conduct electrical current simultaneously, are widely used as transparent electrodes across technical fields such as flat-panel displays, touch screens, solar cells and light-emitting devices. In present, TCO films such as tin-doped indium oxide (ITO), aluminum-doped zinc oxide (AZO) and fluorine doped SnO2 (FTO) are the most widely used transparent electrodes in these devices. However, the manufacturing of ITO film requires precious raw materials indium and other TCO films still have to improve their conductivity and transmittance in visible region. Moreover, due to the inherent brittleness of oxide materials, the flexibilities of TCO films are poor, which could not meet the application requirements of flexible electronics. More and more researchers are focusing on finding new transparent conductive materials as substitutes. Multilayer composite TCFs based on ultrathin metal film exhibit high conductivity, good transparency and excellent flexibility. In this feature article, we review the recent progress of multilayer composite TCFs with dielectric/metal/dielectric (DMD) structure by describing the basic principles, the materials and thickness selection of each layer, the structural types, the methods of photoelectric performance improvement, and other important properties of multilayer composite TCFs with the DMD structure. Due to the good conductivity and ductility of metal layers, multilayer composite TCFs with the DMD structure show low sheet resistance and excellent mechanical flexibility. Some metals such as Ag, Au, Cu, Al, Pd, Pt, Mo, Ni, In and metal alloys have been used as the metal interlayer of DMD electrodes. Among them, the most frequently used metals are Ag, Cu and Au. The two dielectric layers in DMD electrodes may be utilized to improve the overall transmittance in the visible spectral range by optical interference within the multilayer structure, while their thickness can be chosen as a function of the device properties requested. As the metal provides a high lateral conductivity, the dielectric layers in the DMD electrodes do not require being highly conductive, therefore they can be selected in a wide range of materials such as ITO, AZO, FTO, MoO3, WO3, TiO2, ZnS, etc. Experimental data have shown that the same transmittance and sheet resistance values could be obtained with DMD structures composed of different dielectric and metal combinations. In addition, high temperature deposition and annealing are not required to achieve good electrical conductivity, in spite of the higher resistivity achieved by dielectric films prepared without heating. Therefore, DMD electrodes are suitable to deposit onto unheated plastic substrates by continuous roll-to-roll techniques. The efficiencies of DMD electrodes are seriously constrained by a deleterious trade-off between the optical transmittances and electrical conductivities of the metal layers. An improvement in the electrical conductivity requires an increase in the thickness of the metal layer, but the increase of thickness seriously reduces the transmittance. Moreover, due to the three dimensional island growth mode of metal layers deposited using vacuum coating techniques, the metal film usually has a threshold thickness that is the minimum possible thickness for forming a continuous layer to provide sufficient electrical paths. Below the threshold thickness, both the electrical resistivity and the optical absorption rapidly increase. The conductivity and transmittance of a thin metal layer are normally optimized at or near the percolation threshold thickness, since a high optical transmission is required in most cases. Therefore, reducing the percolation threshold thickness of metal layers is a key to improve their conductivity and transmittance simultaneously. In the latest years, some impressive improvements have been achieved by controlling the underlay material, seed layer, dopants and deposition rate at the deposition of metal layer. So far, multilayer composite TCFs with the DMD structure have primarily been applied in solar cell and light-emitting devices, where multilayer composite TCFs may be used as cathodes or anodes, even intermediate electrodes due to the electrode work function easily adjusted by the selection of dielectric layer material. When the multilayer electrodes are applied in solar cell, higher power conversion efficiencies have been achieved compared with devices fabricated on single-layer TCO electrodes. Although there are some challenges yet to overcome to optimize the processing and performance of multilayer composite TCFs, multilayer composite TCFs with the DMD structure remain a highly suitable candidate for various flexible electronic applications in the near future.

Influence of the surface morphology on the magnetoresistance of ultrathin films of ferromagnetic metals and their alloys

The microstructure, morphology and magnetoresistive properties of as-deposited and annealed at 700K discontinuous films of ferromagnetic metals and their alloys are investigated. It is established that the isotropic field dependences stemming from the spin-dependent electron tunneling between ferromagnetic islands are observed for as-deposited Co films with an effective thickness (d) of dCo = 5-25 nm as well as for Fe films (dFe = 10-30 nm). It is shown that the maximum values of the isotropic magnetoresistance take place in the case when the size of the islands is equal to 3-5nm and the width of the dielectric barrier between them is 1-3nm. The maximum TMR values for the films annealed at 700 K are obtained when the dielectric barrier is 2-5 nm wide.

Ultrahigh-capacity dynamic holographic displays via anisotropic nanoholes.

For the miniaturization of optical holographic and data recording devices, large information capacity or data density is indispensable but difficult to obtain using traditional technologies. In this paper, an ultrahigh-capacity metasurface hologram is proposed by encoding information in deep-subwavelength scale nanohole arrays, which can be reconstructed via a light beam with proper designed incident angles. The imaging information capacity of the two-dimensional (2D) hologram, defined by the distortion-free region, can be increased 11.5 times, which is experimentally demonstrated by focused ion beam (FIB) milling of an ultrathin metallic film. We also prove the feasibility of a three-dimensional (3D) hologram of spiral lines designed by using the point source algorithm. Benefitting from the ultrahigh capacity of the deep-subwavelength metasurface, dynamic holographic displays can be realized by controlling the incident angle. The method proposed here can also be leveraged to achieve large capacity optical storage, colorful holographic displays, lithography technology etc.

Simulating the opto-thermal processes involved in laser induced self-assembly of surface and sub-surface plasmonic nano-structuring

Nano-structuring of metals is one of the greatest challenges for the future of plasmonic and photonic devices. Such a technology calls for the development of ultra-fast, high-throughput and low cost fabrication techniques. Laser processing accounts for the aforementioned properties, representing an unrivalled tool towards the anticipated arrival of modules based in metallic nano-structures, with an extra advantage: the ease of scalability. Specifically, laser nano-structuring of an ultra-thin metal film or an alternating metal film on a substrate/metal film on a substrate results respectively on surface (metallic nanoparticles on the surface of the substrate) or subsurface (metallic nanoparticles embedded in a dielectric matrix) plasmonic patterns with many applications. In this work we investigate theoretically the photo-thermal processes involved in surface and sub-surface plasmonic nano-structuring and compare to experiments. To this end, we present a design process and develop functional plasmonic nano-structures with pre-determined morphology by tuning the annealing parameters like the laser fluence and wavelength and/or the structure parameters like the thickness of the metallic film and the volume ratio of the metal film on a substrate-metal composite. For the surface plasmonic nano-structuring we utilize the ability to tune the laser's wavelength to either match the absorption spectral profile of the metal or to be resonant with the plasma oscillation frequency, i.e. we utilize different optical absorption mechanisms that are size-selective. Thus, we overcome a great challenge of laser induced self assembly by combining simultaneously large-scale character with nanometer scale precision. For subsurface plasmonic nano-structuring, on the other hand, we utilize the temperature gradients that are developed spatially across the metal/dielectric nano-composite structure during the laser treatment. We find that the developed temperature gradients are strongly depended on the nanocrystalline character of the dielectric host which determines its thermal conductivity, the composition of the ceramic/metal and the total thickness of the nano-composite film. The aforementioned material parameters combined with the laser annealing parameters can be used to pre-design the final morphology of the sub-surface plasmonic structure. The proposed processes can serve as a platform that will stimulate further progress towards the engineering of plasmonic devices.

Cherenkov terahertz surface plasmon excitation by an electron beam over an ultrathin metal film

The mechanism of Cherenkov excitation of terahertz (THz) surface plasma wave (SPW), by a relativistic electron beam propagating over an ultrathin metal film deposited on glass, is investigated. The SPW field falls off exponentially in vacuum as well as glass, while the surface plasmon resonant frequency is lowered by the reduction of film thickness. The SPW field causes density bunching of the beam leading to current modulation and generation of THz radiation via the Cherenkov interaction. The frequency of the THz decreases with the energy of the beam, whereas the growth rate increases.

Band-blocking optical filter including ultrathin metallic film

A new multilayered optical filter is designed using high- and low-index materials and ultrathin metallic film providing band-blocking characteristic. Power absorption, reflection and transmission characteristics of the structure depending on the high/low index values of the layers and stack numbers of layers are investigated. Overall results indicate that band-blocking filter in visible region is realized with quasi-periodically settled layers in the form of Fibonacci Series and proper adjustment of the structural parameters. Designed structure is useful for the absorbtion of special wavelengths in the visible region in 670–730 nm band by eliminating reflection and transmission.

Interplay of quantum size effect, anisotropy and surface stress shapes the instability of thin metal films

Morphological instability of a planar surface ([111], [011], or [001]) of an ultra-thin metal film is studied in a parameter space formed by three major effects (the quantum size effect, the surface energy anisotropy and the surface stress) that influence a film dewetting. The analysis is based on the extended Mullins equation, where the effects are cast as functions of the film thickness. The formulation of the quantum size effect (Z. Zhang et al., PRL 80, 5381 (1998)) includes the oscillation of the surface energy with thickness caused by electrons confinement. By systematically comparing the effects, their contributions into the overall stability (or instability) is highlighted.

Imaging and controlling plasmonic interference fields at buried interfaces.

Capturing and controlling plasmons at buried interfaces with nanometre and femtosecond resolution has yet to be achieved and is critical for next generation plasmonic devices. Here we use light to excite plasmonic interference patterns at a buried metal–dielectric interface in a nanostructured thin film. Plasmons are launched from a photoexcited array of nanocavities and their propagation is followed via photon-induced near-field electron microscopy (PINEM). The resulting movie directly captures the plasmon dynamics, allowing quantification of their group velocity at ∼0.3 times the speed of light, consistent with our theoretical predictions. Furthermore, we show that the light polarization and nanocavity design can be tailored to shape transient plasmonic gratings at the nanoscale. This work, demonstrating dynamical imaging with PINEM, paves the way for the femtosecond and nanometre visualization and control of plasmonic fields in advanced heterostructures based on novel two-dimensional materials such as graphene, MoS2, and ultrathin metal films.

Tunable high-efficiency light absorption of monolayer graphene via Tamm plasmon polaritons.

Highly efficient absorption of light at telecommunication wavelengths in monolayer graphene is numerically and theoretically investigated in a multi-layer photonic configuration, which consists of an ultrathin metal film coated on a dielectric Bragg mirror and a graphene sheet in a silica spacer. Our results show that the absolute absorption of light in the single-atomic-layer graphene can significantly reach 80% (34.8-fold enhancement compared to the intrinsic value of 2.3%), due to the strong field confinement of Tamm plasmon polaritons in the silica spacer. Moreover, we find that the absorption efficiency of graphene is critically dependent on the position of the graphene in the spacer. The operating wavelength is flexibly tunable by adjusting the incident angle of light and the thicknesses of metal and Bragg mirror layers. The meaningful improvement and tunability of graphene absorption may find favorable applications for the realization of high-performance graphene optoelectronic devices, such as detectors and modulators.

Highly Efficient Photoacoustic Conversion by Facilitated Heat Transfer in Ultrathin Metal Film Sandwiched by Polymer Layers

Photoacoustic (PA) conversion of metal film absorbers is known to be inefficient because of their low thermal expansion and high optical reflection, as compared to polymeric materials containing light absorbing fillers. Here, highly efficient PA conversion is demonstrated in metal films. By using a metal film absorber sandwiched by transparent polymer layers, PA conversion is significantly enhanced, which is even comparable to in the highest reported in the CNT‐polymer composites. Such enhancement is accomplished by ultrathin metal film (10 nm) capable of facilitating heat transfer to the adjacent polymers having high thermal expansion coefficient. This thin metal layer also allows integration of a photonic resonance cavity, effectively compensating the potential absorption loss of the thin metal. This strategy allows for easy spatial PA signal patterns and high conversion efficiency, which not only can be implemented for deep tissue PA imaging of implants or tools, but also provides a guideline for designing photoacoustic transmitters and contrast agents.

Вплив неоднорідностей поверхні на умови перенесення заряду в ультратонких плівках металів

Вплив неоднорідностей поверхні на умови перенесення заряду в ультратонких плівках металів

Self-Terminated Electrodeposition Reactions for Electrocatalysis

Development of a sustainable hydrogen economy based on water electrolysis and fuel cells is constrained by the performance, longevity and high cost of Pt group electrocatalysts. Important strategies for improving catalytic reactivity include alloying, forming bimetallic or alloyed surfaces, and engineering substrate–catalyst interactions. At the same time, significant effort is underway to minimize the loading of Pt group metals by examining new synthesis methods and electrode geometries. Of particular interest is atomic layer deposition (ALD) for growing ultrathin films that maximize utilization of rare, expensive metals and facilitate the exploration of bimetallic effects in electrocatalysis. Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and Fe-group metals and alloys thereof. The mechanism of self-terminated deposition reactions will be discussed and the utility of these films as bimetallic electrocatalysts will be demonstrated. For example, monolayer equivalent Pt and Ir films were found to match or exceed the best known specific HER/HOR activity for bulk polycrystalline Pt or Ir electrodes and thereby offer the promise of increasing the scalability of these important precious metals in energy conversion devices.

Electrochemical Atomic Layer Deposition of Copper Mediated By Underpotentially Deposited Zinc

Electrochemical Atomic Layer Deposition (e-ALD) is attractive for materials fabrication in a variety of applications including nano-electronics, catalysis and energy conversion. This technique enables deposition of ultrathin metal and metal alloy films, while providing unprecedented atomic-level control over surface topography through layer-by-layer assembly of metal monolayers [1, 2]. Conventional e-ALD of copper (Cu) consists of two key steps repeated sequentially [3]: (i) Underpotential deposition (UPD) of a sacrificial metal monolayer such as lead (Pb) followed by (ii) Surface-Limited Redox Replacement (SLRR) of the sacrificial Pb-UPD layer by nobler Cu. In this study, we present a novel lead-free process for e-ALD of Cu mediated by zinc (Zn) UPD. This process eliminates lead-containing chemistries and thus it is advantageous in terms of its practical applicability. In this process, a sacrificial monolayer of Zn is underpotentially deposited onto the substrate (e.g., ruthenium Ru). Under open-circuit conditions, the sacrificial Zn monolayer undergoes redox replacement by nobler Cu. This provides a monolayer of Cu on the substrate surface. UPD-SLRR cycles are repeated to build multi-layers of Cu with controlled thickness while minimizing surface roughness evolution. These are characteristics of true e-ALD. Cyclic voltammetry (CV) was performed to confirm the Zn-UPD behavior on a PVD Cu substrate from an alkaline electrolyte containing complexed Zn2+. Quartz crystal microgravimetry (QCM) was employed to further study the Zn-UPD process on a Cu coated quartz crystal. The frequency of the quartz crystal decreased (indicative of mass addition) and reached a time-invariant value of -7 Hz after complete formation of a Zn-UPD monolayer. Based on results of CV and microgravimetry, the following Cu e-ALD protocol was established: Underpotential deposition of Zn onto the Cu substrate at -1.1 V vs. Ag/AgCl for 20 sec followed by SLRR of Zn-UPD monolayer by Cu under open-circuit conditions. In the present talk, we will demonstrate the basic feasibility of the proposed novel Cu e-ALD process. Fig. 1 shows the potential and QCM frequency response during the Cu e-ALD process. The important features of the process are as follows: (i) The steady-state QCM frequency shift of ~7 Hz during the Zn-UPD step, indicative of a mass gain of about 124 ng/cm2 (i.e., ~1 monolayer of Zn). (ii) During the SLRR period, the potential drifts anodic indicating replacement of the Zn monolayer by nobler Cu. (iii) The frequency shift is constant during the SLRR period. (iv) After each SLRR cycle, consistent frequency decrease of ~ 6–7 Hz is observed during the Zn-UPD step, indicating the layer-by-layer growth of smooth Cu layers. In this talk, we will discuss the Cu e-ALD deposit characteristics such as the thickness, roughness and composition. Possible extensions of the proposed e-ALD approach to fabrication of other metallic nano-films will be discussed.

The Spontaneous Deposition of Au on Pt (111) and Polycrystalline Pt

The development of durable fuel cell catalysts with high activity and low Pt loading is of paramount importance for the fuel cell’s industrial applicability. Conventionally Pt and Pt group metals have been shown to have the highest catalytic activity, however their durability has remained a concern due to CO poisoning and Pt dissolution [1, 2]. This has thusly necessitated the need for mixed metal catalysts, with improved durability, and conserved, or higher activity when compared to bulk Pt. It has been shown that the decoration of catalytically active metals with sub-monolayers of foreign metals can improve catalysts’ durability as well as its activity due to synergistic effects [3-7]. In addition it has been demonstrated that the creation of continuous ultrathin noble metal films on foreign substrates exhibit enhanced electronic and chemisorption properties when compared to the bulk material. This is explained through d-band theory and orbital mixing demonstrated by Norskov et al. [ 8-10 ] . These enhancements have created the need for simple low cost fabrication techniques of these materials, such as spontaneous deposition. Conventional spontaneous deposition involves the adsorption of an ordered ad-layer of a metal complex on a substrate. This adsorption is strong enough to withstand rinsing of the crystal, and then reduction in a solution of the supporting electrolyte. The limited amount of adsorbed complex results in excellent control of the amount of deposited metal, and the spontaneous deposition sequence can then be cycled multiple times in order to generate deposits of desired thicknesses. The spontaneous deposition of Au was previously discovered by this group when demonstrating the surface limited redox replacement of Au on Pt [11], and in this report it will be fundamentally demonstrated. Electrochemically, it will be shown that the spontaneous deposition of Au on Pt operates via a hybrid deposition mode, one that consists of conventional spontaneous deposition as well as an electroless step involving Pt oxidation and [AuCl4]- complex reduction. In addition, via STM, the ordered ad-layer of the [AuCl4]- complex will be demonstrated and it will be shown that deposits generated via spontaneous deposition proceed via a nearly perfect 2-D layer by layer growth mode on both Pt and Au. Finally, it will be shown that the spontaneous deposition of Au on Pt is a suitable strategy for the decoration of Pt with sub-monolayer to nearly perfect multilayer Au films.

Ultrathin and Nanostructured Au Films with Gradient of Effective Thickness. Optical and Plasmonic Properties

In present work the results of investigation of optical (transmission spectra) and plasmonic (surface plasmon-polariton resonance) properties of ultrathin and nanostructured Au films are presents. Methods and techniques for the syntheses of samples of ultrathin and nanostructured metallic films, and for the experimental studies of optical and plasmonic properties are representative. Au films on SiO2 (optic glass) substrates were investigated.

Ultrathin continuous lossy metal film for broadband light absorption and hot electron collection

A broadband perfect light absorption scheme relying on planar layers of lossy metallic and semiconductor films without structure pattering for hot electron photodetection is proposed and demonstrated. The nontrivial reflection phase shift occurring in the ultrathin lossy metal film is responsible for the strong resonance effects of the metal–semiconductor–metal (M-S-M) structures. Broadband and perfect light absorption is demonstrated experimentally for the platinum (Pt)—titanium dioxide (TiO2)—aluminum (Al) absorber, in which the ultrathin (10 nm) Pt layer electrically serves as the hot electron emitter. Under the monochromatic light illumination, the proposed M-S-M diode shows a clear hot-electron-based photoresponse.

Ultrathin Metal Films with Defined Topographical Structures as In Vitro Cell Culture Platforms for Unveiling Vascular Cell Behaviors.

Implanted material surfaces make direct contact with body tissues to work on its own purpose. Therefore, studies of the surface properties of implantable materials that determine cell fate are very important for successful implantation. Although numerous studies have addressed the relationship between cells and material surfaces, nonmetallic surfaces and metallic surfaces likely produce different cellular responses because of their intrinsic differences in surface energy, roughness, and chemical composition. Moreover, given the nontransparent property of metal materials, which hampers the real-time imaging of cellular behavior, a detailed cellular-level analysis at the metal-tissue interface has not been performed. In this study, metal-based cell culture platforms (MCPs) with defined microscale topographical patterns are developed using a combination of photolithography and direct current magnetron sputtering techniques. The MCPs allow to observe vascular cells on metals in real time and identify the selective regulation of human aortic smooth muscle cells and Human umbilical vein endothelial cells (HUVECs) by metallic surface topography. Additionally, atomic force microscopy, contact angles, and energy-dispersive X-ray spectroscopy analyses show that the MCPs exhibit nearly identical chemical properties with their bulk counterparts, demonstrating that MCPs can be utilized as an in vitro cell culture platform system for understanding the cellular behavior on metal substrates.