Metallic Islands Research Articles

Modelled kinetics of the rate of hydrogen evolution as a function of metal catalyst loading in the photocatalysed reforming of methanol by Pt (or Pd)/TiO2

The rate of hydrogen evolution, r(H2), due to the photocatalysed reforming of methanol, MeOH, is studied as a function of Pt loading on P25 TiO2 both in the aqueous (at room temperature) and gas phase (at 100 °C). A similar study, using Pd on P25, has been reported earlier [M. Bowker et al. J. Catalysis, 2003, 217, 427–433] and a perimeter-based Metal-Support Interface (MSI) kinetic model used to interpret the results. Here a new kinetic model is introduced which appears to provide a better fit to all the r(H2) vs wt% metal data sets, in which the rate of reaction is proportional to an extended area around each metal island, rather than its perimeter. A simple theoretical rationale is provided for this expanding photocatalytic area and overlap (EPAO) kinetic model in which each metal island forms an electric field with the surrounding TiO2 so as to act as a sink for electrons photogenerated in the surrounding TiO2 film, thereby allowing it to effect the reduction of water, leaving the remaining photogenerated holes to oxidise the methanol adsorbed on the TiO2. Some support for this very simple kinetic and theoretical model is provided by the results of a brief study of the oxidation of soot deposited on and around a Pt 'dot' on a sol-gel TiO2 film, in that, upon irradiation of this system a zone of activity is visibly revealed by the gradual disappearance of the soot around the Pt ‘dot’, the radius of which appears to be proportional to the radius of the metal ‘dot’.

Atomic layer deposition of cobalt oxide on oxide substrates and low temperature reduction to form ultrathin cobalt metal films

The authors report the deposition of 4.5-nm-thick cobalt (II) oxide on SiO2/Si(001) and MgO(001) substrates at 180–270 °C by atomic layer deposition using bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt (II) and water as coreactants. The resulting CoO film is smooth and carbon-free. CoO can be reduced to Co metal using hydrogen or deuterium gas at 400–500 °C in a vacuum furnace, but the high temperature processing causes dewetting, leading to discontinuous Co metal islands rather than continuous films. Two low temperature (∼200 °C) reduction methods are reported: deuterium atom reduction and the use of an O-scavenging Al metal film. The low temperature methods can suppress dewetting to a large extent, and the resulting metallic cobalt film is smooth and continuous.

Bismuth Islands for Low-Temperature Sodium-Beta Alumina Batteries.

Wetting of the liquid metal on the solid electrolyte of a liquid metal battery controls the operating temperature and performance of the battery. Liquid sodium electrodes are particularly attractive because of their low cost, natural abundance, and geological distribution. However, they wet poorly on a solid electrolyte near its melting temperature, limiting their widespread suitability for low-temperature batteries to be used for large-scale energy storage systems. Herein, we develop an isolated metal-island strategy that can improve sodium wetting in sodium-beta alumina batteries that allows operation at lower temperatures. Our results suggest that in situ heat treatment of a solid electrolyte followed by bismuth deposition effectively eliminates oxygen and moisture from the surface of the solid electrolyte, preventing the formation of an oxide layer on the liquid sodium, leading to enhanced wetting. We also show that employing isolated bismuth islands significantly improves cell performance, with cells retaining 94% of their charge after the initial cycle, an improvement over cells without bismuth islands. These results suggest that coating isolated metal islands is a promising and straightforward strategy for the development of low-temperature sodium-β alumina batteries.

Parity transitions in the superconducting ground state of hybrid InSb\u2013Al Coulomb islands

The number of electrons in small metallic or semiconducting islands is quantised. When tunnelling is enabled via opaque barriers this number can change by an integer. In superconductors the addition is in units of two electron charges (2e), reflecting that the Cooper pair condensate must have an even parity. This ground state (GS) is foundational for all superconducting qubit devices. Here, we study a hybrid superconducting–semiconducting island and find three typical GS evolutions in a parallel magnetic field: a robust 2e-periodic even-parity GS, a transition to a 2e-periodic odd-parity GS, and a transition from a 2e- to a 1e-periodic GS. The 2e-periodic odd-parity GS persistent in gate-voltage occurs when a spin-resolved subgap state crosses zero energy. For our 1e-periodic GSs we explicitly show the origin being a single zero-energy state gapped from the continuum, i.e., compatible with an Andreev bound states stabilized at zero energy or the presence of Majorana zero modes.

On-chip cooling by heating with superconducting tunnel junctions

Heat management and refrigeration are key concepts for nanoscale devices operating at cryogenic temperatures. The design of an on-chip mesoscopic refrigerator that works thanks to the input heat is presented, thus realizing a solid-state implementation of the concept of cooling by heating. The system consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction (N-FI-S) and a series of two normal metal-superconductor tunnel junctions (SINIS). The N-FI-S element converts the incoming heat in a thermovoltage, which is applied to the SINIS, thereby yielding cooling. The cooler's performance is investigated as a function of the input heat current for different bath temperatures. We show that this system can efficiently employ the performance of SINIS refrigeration, with a substantial cooling of the normal metal island. Its scalability and simplicity in the design makes it a promising building block for low-temperature on-chip energy management applications.

Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

An in situ measurement of spin transport in a graphene nonlocal spin valve is used to quantify the spin current absorbed by a small (250 nm $\times$ 750 nm) metallic island. The experiment allows for successive depositions of either Fe or Cu without breaking vacuum, so that the thickness of the island is the only parameter that is varied. Furthermore, by measuring the effect of the island using separate contacts for injection and detection, we isolate the effect of spin absorption from any change in the spin injection and detection mechanisms. As inferred from the thickness dependence, the effective spin current $j_e = \frac{2e}{\hbar} j_s$ absorbed by Fe is as large as $10^8$ A/m$^2$. The maximum value of $j_e$ is limited by the resistance-area product of the graphene/Fe interface, which is as small as 3 $\Omega\mu$m$^2$. The spin current absorbed by the same thickness of Cu is smaller than for Fe, as expected given the longer spin diffusion length and larger spin resistance of Cu compared to Fe. These results allow for a quantitative assessment of the prospects for achieving spin transfer torque switching of a nanomagnet using a graphene-based nonlocal spin valve.

Driving electromagnetic field enhancements in tailored gold surface nanostructures: Optical properties and macroscale simulations

Gold thin films with remarkable Surface Enhanced Raman scattering activity strongly dependent on their surface nanostructure were grown by nanosecond pulsed laser ablation. By changing the number of the laser pulses, keeping fixed all the other deposition parameters, we deliberately modified the surface nanostructure of the films. These nanostructured films consist of metallic islands separated from each other by inter-island channels. We observed differences in both island sizes and inter-island channel sizes as a function of the laser pulse number. The different optical properties of the films such as the position of the localized surface plasmon resonance absorption peak red shifts with increasing the laser pulse number. We performed Finite-Difference Time-Domain calculations to gain insight on how the surface nanostructure of the film affects its optical properties at the macroscale. Results indicate that the strongest localization of the electromagnetic field, and hence the strongest SERS enhancement, occurs at the channel-shaped gaps between adjacent metal islands.Larger enhancements are found in coincidence with narrower and deeper gaps. The number distribution of SERS active sites as a function of the calculated SERS enhancement factors, whose values range between 104 and 105, were obtained. Since the inter-island channels are randomly oriented they show no orientation dependence for linearly polarized radiation on a length scale larger than the typical size of the nanostructures, while larger enhancements were observed for circular polarization.

Photodeposition of metals from inks and their application in photocatalysis

Films of Ag, Pt and Au nanoparticles are photodeposited onto a range of commercial TiO2 based photocatalytic materials, such as paint, tile, awning fabric and glass, as well as a TiO2 sol-gel film, using an aqueous ink containing a sacrificial electron donor, glycerol, and the appropriate metal salt. The photodeposited metal films appear to be comprised of a fine covering of nanoparticular metal islands distributed evenly across the surface of the TiO2 sol-gel film, with some large aggregated particles for Pt (106 nm) and Au (33 nm). The rate of deposition of Ag is particularly fast, since a 10 s exposure to 2 mW cm−2 UVA light produces a very noticeable colour change, which is ca. 60 times that of the other two metal-ion containing inks. When combined with a photomask, the metal inks are used to create finely detailed metal film images on the surfaces of a wide variety of different, mainly commercial, photocatalytic materials; a fine metal mesh photomask is used to make metal micro-patterns. When used to promote the photocatalysed oxidation of organic pollutants dissolved in aqueous solution, such as 4CP and MB, the enhancement in photocatalytic activity (compared to a plain TiO2 film) exhibited by a photodeposited Pt on TiO2 film is modest (16–35%), however, the enhancement is marked (234%) for the photocatalysed oxidation of CO to CO2. In contrast, the Au and Ag films appear to depress the inherent activity of the naked TiO2. The Pt/TiO2 film also enhances significantly the photocatalysed oxidation of a film of soot (179%) but not of stearic acid (−23%); the Ag and Au films appear to impede the former reaction. The Pt/TiO2 film exhibits the most significant enhancement in rate for the photocatalysed reduction of water to H2 by ethanol, i.e. 62 times that of the Ag/TiO2 film, which exhibits only a modest activity, which in turn is better than a naked TiO2 film which shows no activity. The potential of the metal (ion) inks to more easily make photodeposited metal films on semiconductor photocatalyst films is discussed briefly.

Gaussian free field in the iso-height random islands tuned by percolation model

The Gaussian free field (GFF) is considered in the background of random iso-height islands which is modeled by the site percolation with the occupation probability p. To realize GFF, we consider the Poisson equation in the presence of normal distributed white-noise charges, as the stationary state of the Edwards–Wilkinson model. The iso-potential sites (metallic sites in the terminology of the electrostatic problem) are chosen over the lattice with the probability 1 − p in the percolation model, giving rise to some metallic islands and some active (not metallic, nor surrounded by a metallic island) area. We see that the dilution of the system by considering these metallic regions annihilates the spatial correlations and also the potential fluctuations. Some local and global critical exponents of the problem are reported in this work. The GFF, when simulated on the active area show a cross over between two regimes: small (UV) and large (IR) scales. Importantly, by analyzing the change of exponents (in and out of the critical occupation pc) under changing the system size and the change of the cross-over points, we find two fixed points and propose that is unstable towards GFFp=1.

Absorption refrigerators based on Coulomb-coupled single-electron systems

We analyze a simple implementation of an absorption refrigerator, a system that requires heat and not work to achieve refrigeration, based on two Coulomb coupled single-electron systems. We analytically determine the general condition to achieve cooling-by-heating, and we determine the system parameters that simultaneously maximize the cooling power and cooling coefficient of performance (COP) finding that the system displays a particularly simple COP that can reach Carnot's upper limit. We also find that the cooling power can be indirectly determined by measuring a charge current. Analyzing the system as an autonomous Maxwell demon, we find that the highest efficiencies for information creation and consumption can be achieved, and we relate the COP to these efficiencies. Finally, we propose two possible experimental setups based on quantum dots or metallic islands that implement the non-trivial cooling condition. Using realistic parameters, we show that these systems, which resemble existing experimental setups, can develop an observable cooling power.

Nitrogen plasma-induced HfSiON film growth from Hf nanoscale islands on SiO2/Si

HfSiON films for high-k dielectric applications are synthesized from Hf nanoscale islands on SiO2/Si(100) substrates using the exposure of nitrogen inductively coupled plasma (ICP). The process consists of the novel interfacial reaction of nanoscale islands with the substrate induced by plasma-derived radicals and ions. The Hf metal islands nucleate on an SiO2/Si(100) surface by strain-induced engineering during electron beam evaporation. In vacuo AFM measurement shows that the N2 ICP exposure collapses the islands to form films. XPS analysis shows that the ICP exposure initially nitrides Hf islands in 1 min. Then, it induces the intermixing of the base SiO2 with HfN in the next stage. The rapid nitridation of Hf is the unique feature of the process initiated from nanoscale islands. The chemical composition ratio of Hf:Si:N in the film can be roughly controlled to be 1:1:1 with an ICP exposure time of 20 min. The uniformity of the grown films depends on the initial uniformity of the islands and the surface diffusion of materials. The study is a unique example of the film-growth method based on the nanoscale interaction of the surface of nucleated islands and the substrate material induced by plasmas.

Local Conductivity Measurements of Metal-Organic Layers By Conductive AFM on Micropatterned Electrodes Produced By Metal Polymer Blend Lithography

The electronic characterization of thin metal-organic layers like SURMOFs is often difficult, since transport contributions in global measurements may be dominated by pin holes with direct electrical contact of top and bottom electrodes. In addition, these measurements average over a wide surface area and no local information can be derived. Using an AFM with a metalized tip, local measurements of the conductivity and I-V characteristics can be extracted. But these measurements have three important draw backs: (I.) the small contact area in the 20-nm-range restricts the range of measureable conductivity due to a limited sensitivity of the electronic amplifiers, (II.) the electric field underneath the AFM Tip is rather inhomogeneous and current contributions in-plane and off-plane are mixed and (III.) the contact area varies individually from tip to tip and with the applied force. With our approach of polymer blend lithography (PBL) we produce billions of small metal islands with diameters in the range of a few hundred nanometers up to a few microns with a simple process (Metal-PBL) [1]. These Islands serve as planar top electrodes, which we can address individually with the AFM-tip in a highly reproducible way. Using a set of AFM scans in the same region, a manifold of data sets concerning conductivity and I-V characteristic can be extracted, since many individual islands are measured simultaneously. 1.: Polymer blend lithography for metal films: large-area patterning with over 1 billion holes/inch2: C. Huang, A Förste, S. Walheim, and Th. Schimmel: Beilstein Journal of Nanotechnology 2015, 6, 1205–1211

Thermal drag in electronic conductors

We study the electronic thermal drag in two different Coulomb-coupled systems, the first one composed of two Coulomb-blockaded metallic islands and the second one consisting of two parallel quantum wires. The two conductors of each system are electrically isolated and placed in the two circuits (the drive and the drag) of a four-electrode setup. The systems are biased, either by a temperature $\mathrm{\ensuremath{\Delta}}T$ or a voltage $V$ difference, on the drive circuit, while no biases are present on the drag circuit. In the case of a pair of metallic islands we use a master equation approach to determine the general properties of the dragged heat current ${I}_{\mathrm{drag}}^{(\mathrm{h})}$, accounting also for cotunneling contributions and the presence of large biases. Analytic results are obtained in the sequential tunneling regime for small biases, finding, in particular, that ${I}_{\mathrm{drag}}^{(\mathrm{h})}$ is quadratic in $\mathrm{\ensuremath{\Delta}}T$ or $V$ and nonmonotonic as a function of the interisland coupling. Finally, by replacing one of the electrodes in the drag circuit with a superconductor, we find that heat can be extracted from the other normal electrode. In the case of the two interacting quantum wires, using the Luttinger liquid theory and the bosonization technique, we derive an analytic expression for the thermal transresistivity ${\ensuremath{\rho}}_{12}^{(\mathrm{h})}$, in the weak-coupling limit and at low temperatures. ${\ensuremath{\rho}}_{12}^{(\mathrm{h})}$ turns out to be proportional to the electrical transresistivity, in such a way that their ratio (a kind of Wiedemann-Franz law) is proportional to ${T}^{3}$. We find that ${\ensuremath{\rho}}_{12}^{(\mathrm{h})}$ is proportional to $T$ for low temperatures and decreases like $1/T$ for intermediate temperatures or like $1/{T}^{3}$ for high temperatures. We complete our analyses by performing numerical simulations that confirm the above results and allow us to access the strong-coupling regime.

Probing Adhesion of Metallic Nanoparticles on Polymeric Fibrous and Flat Architectures

The exotic characteristics of nanoscopic metallic materials bestows diverse functionalities that are increasingly being utilized for a broad range of applications. Polymer substrates present robust architectures for nanoparticle anchoring as well as modulating attendant size-induced aggregation. However, in principle, interfacial adhesion of a polymer-metal material system is weak, making the susceptibility to delamination a challenge. We have deposited copper particles on model polymer thin film and fibrous architectures to study adhesion behavior on these distinct geometries. The average sizes of copper nanoparticles deposited on electrospun fibers for metallization times of 3 and 5 minutes were 13 and 10 nm, whereas the metal island sizes under same metallization times on thin films was 79nm and 81nm. Scratch tests using a nanoindentation system were unable to generate macroscopic film delamination, but did exhibit apparent removal of individual particles, with adhesion forces of 14.9μN, 36μN, and 28.8μN obtained for films metallized for 1, 3, and 5 minutes respectively. Macroscopic tensile testing of fiber mats showed the metallization maintains conformity with the polymer ligament, albeit, with intermittent fracture of the conformal metal coating, signifying substantial adhesion exists between the metallic layer and the PAN fiber.

Tuning the second harmonic generation of self-generated metallic islands

When crossing the percolation threshold the optical behavior of thin metal films dramatically change within a narrow range. Recording the optical properties in the vicinity of the percolation provides insight into the interplay of those contributions that determine the intensity of the second harmonic signal: the metallic islands and the dielectric gap between the islands. Here we demonstrate the active tuning of the linear and nonlinear optical properties of a thin gold film on a stretchable PDMS substrate. Ellipsometry was combined with nonlinear studies (second-harmonic generation) to describe the optical properties of thin gold films around the percolation. We monitor the metal-insulator transition, characterize the evolution of the permittivity of the layer, and explain the strength variations of the second harmonic generation with respect to the spectral difference between fundamental wavelength and plasmonic resonance, as well as the enhancement of the field in the dielectric gaps as a function of the particle-particle distance. A model reproduces the experimental observations by taking into account both effects at the fundamental and at the generated wavelengths.

Metal-island-film-based plasmonic triple-layer absorber

High-efficiency broadband plasmonic triple-layer absorbers comprised of an ultrathin Ag island layer, a SiO2 spacer, and a reflective Ag substrate are fabricated by magnetron sputtering. Experimental results reveal that surface morphologies of the fabricated Ag island film do not have regular profiles, but appear more like two or more hemi-ellipsoids randomly overlapping each other. Different from previous papers that modeled the metal island film using squared patches or regular hemi-ellipsoids, we construct the numerical model by simplifying the top metal island film as an array of overlapping hemi-ellipsoids aiming to explain the measured efficient absorption over a broad wavelength range more accurately. Through comparison, it is found that the overlapping hemi-ellipsoids can produce an additional absorption peak at the long wavelength range by the excitation of the hot-spot effect with respect to the structure with two isolated hemi-ellipsoids. Other strong absorption peaks at short wavelength ranges are also analyzed systematically. By tuning the geometry of the ellipsoids and the size of overlapping region, the absorption performance of the constructed overlapping model can be tuned. In reality, the geometries of the isolated islands are non-uniform, yielding the broad high-efficiency absorption band witnessed in experiment. Our work contributes to the design of efficient light devices for application in the area of solar energy harvesting and thermal emission tailoring.

Synergistic effect of Fe and Al2O3 layers on the growth of vertically aligned carbon nanotubes for gecko-inspired adhesive applications

This study focused on the application of vertically aligned carbon nanotubes (VACNTs) grown by thermal chemical vapor deposition as a gecko-inspired dry adhesive material. The adhesive properties of VACNTs comprising Fe and Al2O3 layers of different thicknesses were evaluated by scanning electron microscopy, Raman spectroscopy, transmission electron microscopy, and atomic force microscopy. Owing to the surface energy difference between metallic Fe and alumina, the wetting influence of the latter enabled the conversion of the iron film into metal islands (nanoparticles) at high temperature. The bionic morphological characteristics of the produced VACNTs (e.g., alignment, density, diameter, growth rate, and defects) and thus their adhesive properties were determined by the size and distribution of the catalyst particles. With respect to variable adhesion test angles, VACNTs were sensitive to peeling angles and showed high tangential but low normal adhesion strength, thereby providing controlled strong adhesion characteristics in combination with easy peel-off.

Influence of the Geometric Dimensions of a Nanosystem on the Main Characteristics of Charge Exchange with a Negative Ion

The problem of charge exchange between a negative hydrogen ion and nanosystems (thin metal films and island films) is considered. Quantum-mechanical calculations show that the effectiveness of charge exchange is determined by the ion-surface distance and the structure of discrete electron energy levels inside the film. In turn, the structure of the discrete levels depends on the film’s geometric dimensions. The characteristics of charge exchange (the effectiveness of the electron transfer, the probability of charge exchange, the propagation velocity, and the average coordinate of the electron’s position inside the film) are calculated via solution of the nonstationary Schrodinger equation. It is shown that these quantities can also be approximated by a simple linear model of electron propagation.

Electronic Properties of Ag Reconstructions on Si(111): Coulomb Blockade Behavior at Room Temperature

The electronic properties and optical response of Si(111)–Ag reconstructions, with distinct interface structures, are studied with scanning tunneling microscopy (STM), reflectance anisotropy spectroscopy (RAS), and infrared spectroscopic ellipsometry (IRSE). The results are compared to previous studies on the Si(111)‐(3 × 1)–Ag surface. Depending on the preparation conditions, STM shows hybrid surfaces comprising (√3 × √3) areas with semiconducting (3 × 1)–Ag regions or metallic Ag islands. RAS of the (√3 × √3)–Ag reconstruction mostly shows a flat response, confirming the isotropic nature of the surface while Ag islands on this surface produce a steep increase in the infrared as well as a strong optical anisotropy at 3.5 eV. Scanning tunneling spectroscopy (STS) of the islands shows a high density of states with almost no structure. STS of Ag nanodots on the (3 × 1)–Ag surface shows distinct Coulomb resonances within the Si bandgap while the Si(111)‐(3 × 1)–Ag surface reveals an expected semiconducting behavior with a band gap and no sign of resonances. Ag islands on the (3 × 1)–Ag surface also show weak Coulombic oscillations indicating that local transport depends on the electronic properties of the interface.

Nanoliter Sensing for Infrared Bioanalytics.

Nondestructive label-free bioanalytics of microliter to nanoliter sample volumes with low analyte concentrations requires novel analytic approaches. For this purpose, we present an optofluidic platform that combines surface-enhanced in situ infrared spectroscopy with microfluidics for sensing of surface-immobilized ultrathin biomolecular films in liquid analytes. Submonolayer sensitivity down to surface densities of few ng/cm2 is demonstrated for the adsorption of the thiolate tripeptide glutathione and for the recognition of streptavidin on a biotinylated enhancement substrate. Nonfunctionalized and functionalized metal island films on planar oxidized silicon substrates are used for signal enhancement with quantifiable enhancement properties. A single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is used with ordinary planar, IR-transparent windows. The geometry circumvents the strong IR absorption of common polymer materials and of aqueous environments in the IR fingerprint region. This practice enables straightforward quantitative analyses of, e.g., adsorption kinetics as well as chemical and structural properties in dependence of external stimuli.