Ultrathin Metal Layers Research Articles

Effect of Mg insertion on time‐dependent dielectric breakdown in MgO‐based magnetic tunnel junctions

The stress-induced breakdown characteristics in magnesium oxide (MgO)-based magnetic tunnel junctions (MTJs) including an inserted Mg layer 0.5 nm thick is investigated, above or below an MgO tunnel barrier. Regardless of the insertion position, the inserted layer suppressed the time-dependent dielectric breakdown (TDDB) observed in the case of electron tunnelling into the Mg-inserted interface. This indicates that the Mg insertion significantly suppressed trap site formation at the anode-side barrier/electrode interface. The improvement of TDDB by the Mg insertion was confirmed through constant voltage stress experiments. Therefore, interface modification by means of inserting an ultra-thin metallic layer is highly effective in improving the reliability of an MTJ tunnel barrier for practical applications.

Large Voltage-Induced Changes in the Perpendicular Magnetic Anisotropy of an MgO-Based Tunnel Junction with an Ultrathin Fe Layer

Voltage control of magnetic anisotropy (VCMA) in an ultrathin ferromagnet metal layer is a promising approach for ultralow-power spin manipulation, but interface effects spoil its efficiency for applications such as memory devices. The authors study VCMA in the fully epitaxial ultrathin Fe layer of a magnetic tunnel junction, and achieve an efficiency of almost 300 fJ V${}^{-1}$ m${}^{-1}$ with high perpendicular anisotropy, which satisfies the requirements for gigabit-class spintronic memory.

Omnidirectional, polarization-independent, ultra-broadband metamaterial perfect absorber using field-penetration and reflected-wave-cancellation.

In this work, we present the result of nickel (Ni)-based metamaterial perfect absorbers (MPA) with ultra-broadband close-to-one absorbance. The experimental broadband characteristic is significantly improved over the past effort on metamaterial perfect absorbers. An in-depth physical picture and quantitative analysis is presented to reveal the physical origin of its ultrabroadband nature. The key constituent is the cancellation of the reflected wave using ultra-thin, moderate-extinction metallic films. The ultra-thin metal thickness can reduce the reflection as the optical field penetrates through the metallic films. This leads to minimal reflection at each ultra-thin metal layer, and light is penetrating into the Ni/SiO2 stacking. More intuitively, when the layer thickness is much smaller than the photon wavelength, the layer is essentially invisible to the photons. This results in absorption in the metal thin-film through penetration while there is minimal reflection by the metal film. More importantly, the experimental evidence for omni-directionality and polarization-insensitivity are established for the proposed design. Detailed measurement is conducted. Due to the ultrathin metal layers and the satisfactory tolerance in dielectric thickness, the broadband absorption has minimal degradation at oblique incidence. Such a wide angle, polarization-insensitive, ultra-broadband MPA can be very promising in the future, and the optical physics using sub-skin-depth metal film can also facilitate miniaturized high-performance nano-photonic devices.

Resonances of hybridized bound plasmon modes in optically-thin metallic nanoslit arrays for narrow-band transmissive filtering

Abstract We present extremely narrow-band transmission of light through optically-thin metallic nanoslit arrays on substrates coated with high/low-index dielectric layers for high-finesse wavelength filtering. The transmission peak is shown resulting from resonances of various hybrid plasmon modes, formed by symmetric or antisymmetric coupling of the symmetric and anti-symmetric bound surface plasmon modes at the ultrathin metal layer with the optical waveguide mode in the dielectric layers. Band-width of the peak can be down to a few nanometers, with no side peaks comparable in transmittance. For the purpose, all structure components have critical roles, clarified with numerical simulations.

Prediction of transmittance spectra for transparent composite electrodes with ultra-thin metal layers

Recent interest in indium-free transparent composite-electrodes (TCEs) has motivated theoretical and experimental efforts to better understand and enhance their electrical and optical properties. Various tools have been developed to calculate the optical transmittance of multilayer thin-film structures based on the transfer-matrix method. However, the factors that affect the accuracy of these calculations have not been investigated very much. In this study, two sets of TCEs, TiO2/Au/TiO2 and TiO2/Ag/TiO2, were fabricated to study the factors that affect the accuracy of transmittance predictions. We found that the predicted transmittance can deviate significantly from measured transmittance for TCEs that have ultra-thin plasmonic metal layers. The ultrathin metal layer in the TCE is typically discontinuous. When light interacts with the metallic islands in this discontinuous layer, localized surface plasmons are generated. This causes extra light absorption, which then leads to the actual transmittance being lower than the predicted transmittance.

Observation of quantum oscillation of work function in ultrathin-metal/semiconductor junctions

Quantization in energy level due to confinement is generally observed for semiconductors. This property is used for various quantum devices, and it helps to improve the characteristics of conventional devices. Here, the authors have demonstrated the quantum size effects in ultrathin metal (Ni) layers sandwiched between two large band-gap materials. The metal work function is found to oscillate as a function of its thickness. The thermionic emission current bears the signature of the oscillating work function, which has a linear relationship with barrier heights. This methodology allows direct observation of quantum oscillations in metals at room temperature using a Schottky diode and electrical measurements using source-measure-units. The observed phenomena can provide additional mechanism to tune the barrier height of metal/semiconductor junctions, which are used for various electronic devices.

Broadband perfect absorber based on one ultrathin layer of refractory metal.

Broadband perfect absorber based on one ultrathin layer of the refractory metal chromium without structure patterning is proposed and demonstrated. The ideal permittivity of the metal layer for achieving broadband perfect absorption is derived based on the impedance transformation method. Since the permittivity of the refractory metal chromium matches this ideal permittivity well in the visible and near-infrared range, a silica-chromium-silica three-layer absorber is fabricated to demonstrate the broadband perfect absorption. The experimental results under normal incidence show that the absorption is above 90% over the wavelength range of 0.4-1.4 μm, and the measurements under angled incidence within 400-800 nm prove that the absorber is angle-insensitive and polarization-independent.

Design of near-unity transmittance dielectric/Ag/ITO electrodes for GaN-based light-emitting diodes

We designed a near-unity transmittance dielectric/Ag/ITO electrode for high-efficiency GaN-based light-emitting diodes by using the scattering matrix method. The transmittance of an ultrathin metal layer, sandwiched between a dielectric layer and an ITO layer, was investigated as a function of the thickness and the optical constant of each constituent layer. Three different metals (Ag, Au, and Al) were examined as the metal layer. The analytical simulation indicated that the transmittance of a dielectric/metal/ITO multilayer film is maximized with an approximately 10-nm-thick Ag layer. Additionally, the transmittance also tends to increase as the refractive index of the upper dielectric layer increases. By tailoring the thickness of the dielectric layer and the ITO layer, the dielectric/Ag/ITO structure yielded a transmittance of 0.97, which surpasses the maximum transmittance (0.91) of a single ITO film. Furthermore, this extraordinary transmittance was present for other visible wavelengths of light, including violet and green colors. A complex phasor diagram model confirmed that the transmittance of the dielectric/metal/ITO multilayer film is influenced by the interference of reflected partial waves. These numerical findings underpin a rational design principle for metal-based multilayer films that are utilized as transparent electrodes for the development of efficient light-emitting diodes and solar cell devices.

Sub-surface laser nanostructuring in stratified metal/dielectric media: a versatile platform towards flexible, durable and large-scale plasmonic writing

Laser nanostructuring of pure ultrathin metal layers or ceramic/metal composite thin films has emerged as a promising route for the fabrication of plasmonic patterns with applications in information storage, cryptography, and security tagging. However, the environmental sensitivity of pure Ag layers and the complexity of ceramic/metal composite film growth hinder the implementation of this technology to large-scale production, as well as its combination with flexible substrates. In the present work we investigate an alternative pathway, namely, starting from non-plasmonic multilayer metal/dielectric layers, whose growth is compatible with large scale production such as in-line sputtering and roll-to-roll deposition, which are then transformed into plasmonic templates by single-shot UV-laser annealing (LA). This entirely cold, large-scale process leads to a subsurface nanoconstruction involving plasmonic Ag nanoparticles (NPs) embedded in a hard and inert dielectric matrix on top of both rigid and flexible substrates. The subsurface encapsulation of Ag NPs provides durability and long-term stability, while the cold character of LA suits the use of sensitive flexible substrates. The morphology of the final composite film depends primarily on the nanocrystalline character of the dielectric host and its thermal conductivity. We demonstrate the emergence of a localized surface plasmon resonance, and its tunability depending on the applied fluence and environmental pressure. The results are well explained by theoretical photothermal modeling. Overall, our findings qualify the proposed process as an excellent candidate for versatile, large-scale optical encoding applications.

Electrochemical Catalytic Activities of Pseudomorphic Pt Monolayers Electrodeposited on Au(111) for Various Reactions

Introduction. Ultra-thin metal layers on the foreign metal substrates have been attracting interests because of their unique physical and chemical properties, particularly their high electro-catalytic activities. Such special catalytic activity is due to their unique surface atomic arrangements and their induced surface electronic energy. Because Pt is one of the best catalysts for various chemical reactions, Pt is one of the most precious and expensive metals, and Pt resources are limited, Pt ultrathin layer formed on the foreign metal substrates is greatly expected that catalytic activity of Pt becomes higher and that loading amount of Pt becomes less. Recently, we succeeded in electrochemically construction of a pseudomorphic Pt monolayer on a single crystal Au(111) surface and its atomic arrangement was precisely confirmed to be pseudomorphic by resonance surface X-ray scattering (RSXS) measurements [1,2]. Moreover, this pseudomorphic Pt monolayer on the Au(111) surface has higher electro-catalytic activity than Pt(111) [3,4]. In this report, electro-catalytic activities of the electrochemically prepared pseudomorphic Pt monolayers on the Au(111) surface for hydrogen oxidation reaction (HOR) and methanol oxidation reaction (MOR) were investigated. Experimentals. Pseudomorphic Pt monolayer on Au(111) (Pt/Au(111)) was electrochemically deposited by the same procedures as previously reported [1-4]. HOR measurements were carried out in a hydrogen saturated 0.1 M HClO4 with a scan rate of 50 mV s-1 at several rotation rates by using rotation disk electrode (RDE) system. MOR measurements were performed in a nitrogen saturated 1.0 M CH3OH + 0.1 M HClO4 with a scan rate of 50 mV s-1 from 0.20 V to 1.15 V (vs. RHE). Same measurements of Au(111) and Pt(111) single crystal electrodes were carried out and the results were compared and examined. Results and Discussion. As a limitation of the space, the results of MOR were shown here. Cyclic volutammograms (CVs) for MOR of the Pt/Au(111), Pt(111) and Au(111) electrode show as follows. Oxidation current at Au(111) did not flow at all. On the other hand, oxidation current at Pt/Au(111) and Pt(111) started to flow from ca. 0.5 V (vs. RHE). As compared the result of Pt/Au(111) with that of Pt(111), the starting potential to flow oxidation current and the peak current density at Pt/Au(111) were more negative and higher than those at Pt (111), respectively. Thus, we can conclude that the electro-catalytic activity of the pseudomorphic Pt monolayer formed on Au(111) for MOR is higher than those of Pt(111) and Au(111). MOR can be simply indicated as the equation (1). CH3OH + H2O →CO2 + 6H+ + 6e- (1)However, as soon as a methanol molecule is adsorbed on the Pt surface at relatively negative potential, the reaction (2) takes place. CH3OH → COad + 4H+ + 4e- (2)This phenomenon is so-called CO poisoning [5], which causes the loss of electro-catalytic activity of Pt. When the potential is positively made, adsorbed CO is oxidized and then desorbed as equation (3). COad + H2O → CO2 + 2H+ + 2e- (3)Thus, the above results of higher electro-catalytic activity of Pt/Au(111) for MOR were caused by the decrease of adsorption energy of CO, because of the changes of atomic arrangement and surface energy [6].Dependence of the thickness of the pseudomorphic Pt layers on the electro-catalytic activity is now under investigation. Acknowledgement. This work was supported by New Energy and Industrial Technology Development Organization (NEDO). References. [1] T. Kondo, M. Shibata, N. Hayashi, H. Fukumitsu, T. Masuda, S. Takakusagi, and K. Uosaki, Electrochim. Acta, 55, 8302 (2010).[2] M. Shibata, N. Hayashi, T. Sakurai, A. Kurokawa, H. Fukumitsu, T. Masuda, K. Uosaki, and T. Kondo, J. Phys. Chem. C, 116, 26464 (2012).[3] T. Kondo, C. Song, N. Hayashi, T. Sakurai, M. Shibata, H. Notsu, and I. Yagi, Chem. Lett., 40, 1235 (2011).[4] M. Ishizaki, M. Kawabuchi, I. Yagi, and T. Kondo, 224th ECS Meeting, San Francisco, #43 (2013).[5] B. Beden, C. Lamy, A. Bewick, K. Kunimatsu, J. Electroanal. Chem. 121 (1981) 343.[6] T. E. Shubina, M. T. M. Koper, Electrochem. Acta 47 (2002) 3621.

Ultrathin Sheets of Metal or Metal Sulfide from Molecularly Thin Sheets of Metal Thiolates in Solution

Materials that exist as single molecule thick two-dimensional sheets are in great demand because they hold promise as precursors for synthesis of layered functional materials. We demonstrate that metal thiolates, that exist as lamellar assemblies in the neat state, can be disassembled into individual molecular sheets simply by dilution in apolar organic solvents and that these can form ultrathin metallic layers on substrates upon heat treatment. We establish the pathway to the disassembly of metal thiolates in solution using a combination of techniques, including X-ray diffraction, light scattering, FTIR, and TEM. Our results indicate that the lamellar structure of Pd-thiolates is preserved in toluene up to a concentration of 300% w/v and the average intersheet distance is unchanged. Interestingly, the dynamics of the Pd-thiolate sheets remain correlated even on diluting them up to 30% w/v, though the disorder within the lamellar stacks increases with a decrease in their coherence length. Finally, at dilutions less than about 5% w/v, individual sheets of these structures can be accessed that are isolated and directly observed using TEM. Heat treatment of the ultrathin films of metal thiolates deposited on appropriate substrates resulted in the formation of metal or metal sulfides with retention of sheetlike morphologies.

Ultrafast lasers and solids in highly excited states: results of hydrodynamics and molecular dynamics simulations

Action of ultrafast optical and X-ray lasers on metals is considered. It is known that under certain conditions surface structures appear as result of irradiation. Generation of nano-structures is usually associated with excitation of surface plasmons. But often structures do not have forms of ripples, and their spacial scales are order of magnitude less than optical wavelength. In the paper full description of surface nano-structures is given for the case of single shot laser action onto well polished boundaries. Plasmon effects are insignificant for this case and also for X-ray pulses. It is shown that structures are formed after laser illumination in a process of mechanical spallation of ultrathin surface layer of molten metal. Spallation is accompanied by a strong foaming of melt, breaking of foam, and freezing of foam remnants. Those remnants form chaotic nano-structures observed in experiments.

Sensitivity improvement of the surface acoustic wave ultraviolet sensor based on zinc oxide nanoparticle layer with an ultrathin gold layer

In this study, we demonstrate the improvement in the sensitivity of surface acoustic wave (SAW) ultraviolet (UV) sensor with a UV-sensitive zinc oxide nanoparticles (ZnO NPs) layer and an ultrathin gold layer. The additional gold layer plays an important role in the enhancement of photoresponse by increasing the overall sheet conductivity of the ZnO/Au layer, thereby improving the sensitivity of SAW UV sensor. The SAW sensor fabricated in this study consists of UV-sensitive ZnO NPs layer deposited onto a 41°Y–X LiNbO3 substrate that can propagate shear-horizontal (S-H) waves. The sheet conductivity of the film was calculated by transmission line method, using which the sensitivity of the sensor was estimated. The morphological and structural properties of ZnO NPs were evaluated by using scanning electron microscope (SEM) and X-ray diffraction (XRD). In addition, the effect of Au/ZnO bilayer on the UV absorption of the device was analyzed by using spectrophotometer. The device was irradiated with a commercial UV lamp, the intensity of which was assessed by a UV meter. In addition, the current–voltage characteristics and time responses of the device were determined by using a semiconductor parameter analyzer, in order to investigate the effects of annealing conditions and the additional deposition of an ultrathin gold metal layer. The improvement in the sensitivity of the SAW UV sensor with the gold layer was verified by the phase shift of S21 from 55° to 198°.

Wetting Layer: The Key Player in Plasma-Assisted Silicon Nanowire Growth Mediated by Tin

Unidirectional growth of silicon nanowires (SiNWs) can be achieved via a vapor–liquid–solid (VLS) mechanism. Interestingly, when adopting low surface tension metals such as tin or indium to mediate the growth of SiNWs in a plasma-assisted VLS process, the standard scenario is challenged by the fact that low surface tension metals tend to coat the high energy sidewalls of SiNWs. Moreover, we show that tin-assisted SiNW growth can continue without any apparent metal droplet on top. To address these issues, we investigate the time evolution of tin-assisted SiNW growth. We suggest that, during the initial growth phase, an ultrathin metal layer wetting the SiNW sidewalls allows silicon adatoms to diffuse toward the top of the nanowires, and thus enhances the axial growth while suppressing the radial one (in a way similar to molecular beam epitaxy growth of SiNWs). Later on, the wetting layer can assist the growth even when the tin droplets are exhausted from the top of SiNWs. We propose that this phenomenon ma...

Noble Metal Nanostructures Influence of Structure and Environment on Their Optical Properties

Optical properties of nanostructured materials, isolated nanoparticles, and structures composed of both metals and semiconductors are broadly discussed. Fundamentals of the origin of surface plasmons as well as the surface plasmon resonance sensing are described and documented on a number of examples. Localized plasmon sensing and surface‐enhanced Raman spectroscopy are subjected to special interest since those techniques are inherently associated with the direct application of plasmonic structures. The possibility of tailoring the optical properties of ultra‐thin metal layers via controlling their shape and morphology by postdeposition annealing is documented. Special attention is paid to the contribution of bimetallic particles and layers as well as metal structures encapsulated in semiconductors and dielectrics to the optical response. The opportunity to tune the properties of materials over a large scale of values opens up entirely new application possibilities of optical active structures. The nature of surface plasmons predetermines noble metal nanostructures to be promising great materials for development of modern label‐free sensing methods based on plasmon resonance—SPR and LSPR sensing.

Nanofabrication of TaS2 conducting layers nanopatterned with Ta2O5 insulating regions via AFM

It is demonstrated how local oxidation nanolithography performed with an atomic force microscope (AFM-LON) may be successfully employed for the nanopatterning of insulating regions of Ta2O5 on TaS2 ultrathin metallic layers. This provides a simple approach for the fabrication of electronic devices, such as single-electron transistors, at the nanoscale.

Controlled, Low-Coverage Metal Oxide Activation of Silicon for Organic Functionalization: Unraveling the Phosphonate Bond

Deposition of thin films and grafting of organic molecules on semiconductor surfaces, particularly oxide surfaces, are widely studied as means of passivation and functionalization for a variety of applications. However, organic functionalization of silicon oxide is challenging, as the currently used molecules (silanes and phosphonates) do not form layers that are stable in aqueous environments and present challenges during the grafting process. For instance, the chemical grafting of phosphonates requires high temperature (140 °C) to perform. Modification of SiO(2) surfaces with metal oxides is an attractive alternative since strong bonds can be established between metal oxides and relevant molecules (silanes, phosphonates). While such modification is possible using vapor-phase methods, such as atomic layer deposition and physical vapor-phase deposition, wet chemical processing is inexpensive and technologically very attractive. We describe here a simple wet chemical method to deposit an ultrathin layer of metal oxide/hydroxide groups. Further, using a model surface with exactly one-third monolayer OH groups on oxide-free Si surfaces, the precise adsorption geometry on single Al(OH)(3) groups is shown to be bidentate, and the distance between the Al and P atoms is determined to be the main influencing parameter for a thermodynamically stable formation of the Al-O-P bond.

Graphene-like metal-on-silicon field-effect transistor

This paper presents a field-effect transistor with a channel consisting of a two-dimensional electron gas located at the interface between an ultrathin metallic film of Ni and a p-type Si(111) substrate. The gate length is L = 2 μm, its width is W = 180 μm, and the source–drain separation is 188 μm, the role of the gate dielectric being played by the surface states of the ultrathin metal layer. We have demonstrated that the two-dimensional electron gas channel is modulated by the gate voltage. The dependence of the drain current on the drain voltage has no saturation region, similar to a field-effect transistor based on graphene. The drain current is 2 mA at a drain voltage of 3 V and a gate voltage of 1.07 V, while the transconductance is 0.6 mS for a drain voltage of 6 V and a gate voltage of 1 V. However, the transport in this transistor is not ambipolar, as in graphene, but unipolar.

An effective bilayer cathode buffer for highly efficient small molecule organic solar cells

Novel organic/ultrathin low work function metal bilayer cathode buffers for small molecule organic solar cells are proposed. Ultrathin low work function metal layers possess a high built-in electric field for effective carrier extraction and a high cathode reflectivity for maximum absorption in the photoactive layers. This leads to a significant increase of short circuit current density and fill factor of cells. By integrating this bilayer cathode buffer with DTDCTB:C60 small molecular heterojunction, the device exhibits a high power conversion efficiency of up to 5.28%, which is an improvement of 22% compared to a device with a traditional single organic layer buffer.

Single-step formation of a graphene–metal hybrid transparent and electrically conductive film

We report here a rapid (10 s of heating) graphene growth method that can be carried out on any desired substrate, including an insulator, thus negating the need for the transfer from the metal substrate. This technique is based on metal-induced crystallization of amorphous carbon (a-C) to graphene, and involves an ultra-thin metal layer that is less than 10 nm in thickness. Rapid annealing of a bilayer of a-C and metal deposited on the surface leads to the formation of graphene film, and to subsequent breaking-up of the thin metal layer underneath the film, thus resulting in the formation of a graphene–metal hybrid film which is both transparent and electrically conducting. Based on Raman studies, we have also systematically compared ultra-thin metal-induced crystallization behavior with a case of conventional thick metal. Based on the present investigation, it was observed that the dominant growth mechanism in ultra-thin metal-induced crystallization is nucleation controlled.