Interisland Spacing Research Articles

Characterization of switching field distributions in Ising-like magnetic arrays

The switching field distribution within arrays of single-domain ferromagnetic islands incorpo- rates both island-island interactions and quenched disorder in island geometry. Separating these two contributions is important for disentangling the effects of disorder and interactions in the magnetization dynamics of island arrays. Using sub-micron, spatially resolved Kerr imaging in an external magnetic field for islands with perpendicular magnetic anisotropy, we map out the evolution of island arrays during hysteresis loops. Resolving and tracking individual islands across four different lattice types and a range of inter-island spacings, we extract the individual switching fields of every island and thereby determine the relative contributions of interactions and quenched disorder in the arrays. The width of the switching field distribution is well explained by a simple model comprising the sum of an array-independent contribution (interpreted as disorder-induced), and a term proportional to the maximum field the fully polarized array could exert on a single island. We conclude that disorder in these arrays is primarily a single-island property.

Heat transfer in shape memory alloy thin films

The thermal response of periodic, shape memory alloy (SMA) island structures is addressed in this paper. Incorporated into the thermal modeling are several ingredients that are encountered at length scales typical to micron-sized thin films: (i) the microstructure may be heterogeneous, e.g., there may be a mix of an amorphous layer, a non-transforming crystalline layer and a crystalline layer that does undergo phase transformation; (ii) given the typical large surface-to-volume ratios and the attendant thermal losses to the environment, the strength of the heat source as well as its duration during a martensite to austenite transformation become very important. These issues, especially the latter, are investigated for a range of periodic structures with different spacings including the limiting structures of vanishing inter-island spacing on the one hand (infinitely extended thin film) and infinite inter-island spacing on the other (isolated islands). It is seen that a reduced inter-island spacing results in lower heat losses to the substrate, that a heterogeneous microstructure has a minimal effect on the thermal field, and that there appears to be a threshold in the strength of the heat source (represented by a current density, if the heat source is electrical in origin) above which the reduction in the heating time does not improve substantially. Interestingly, the evolution of martensite volume fraction with respect to time seems to be insensitive to the thickness of the film undergoing transformation to austenite.

Simulation of Charge Transport in Disordered Assemblies of Metallic Nano-Islands: Application to Boron-Nitride Nanotubes Functionalized with Gold Quantum Dots

ABSTRACTIn this study, we investigate the charge-transport behavior in a disordered one-dimensional (1D) chain of metallic islands using the newly developed multi-island transport simulator (MITS) based on semi-classical tunneling theory and kinetic Monte Carlo simulation. The 1D chain is parameterized to model the experimentally-realized devices studied by Lee et al. [Advanced Materials25, 4544-4548 (2013)], which consists of nano-meter-sized gold islands randomly deposited on an insulating boron-nitride nanotube. These devices show semiconductorlike behavior without having semiconductor materials. The effects of disorder, device length, temperature, and source-drain bias voltage (VSD) on the current are examined. Preliminary results of random assemblies of gold nano-islands in two dimensions (2D) are also examined in light of the 1D results.At T = 0 K and low source-drain bias voltages, the disordered 1D-chain device shows charge-transport characteristics with a well-defined Coulomb blockade (CB) and Coulomb staircase (CS) features that are manifestations of the nanometer size of the islands and their separations. In agreement with experimental observations, the CB and the blockade threshold voltage (Vth) at which the device begins to conduct increases linearly with increasing chain length. The CS structures are more pronounced in longer chains, but disappear at high VSD. Due to tunneling barrier suppression at high bias, the current-voltage characteristics for VSD > Vth follow a non-linear relationship. Smaller islands have a dominant effect on the CB and Vth due to capacitive effects. On the other hand, the wider junctions with their large tunneling resistances predominantly determine the overall device current. This study indicates that smaller islands with smaller inter-island spacings are better suited for practical applications. Temperature has minimal effects on high-bias current behavior, but the CB is diminished as Vth decreases with increasing temperature.In 2D systems with sufficient disorder, our studies demonstrate the existence of a dominant conducting path (DCP) along which most of the current is conveyed, making the device effectively quasi-1-dimensional. The existence of a DCP is sensitive to the device structure, but can be robust with respect to changes in VSD.

Simulation of charge transport in multi-island tunneling devices: Application to disordered one-dimensional systems at low and high biases

Although devices have been fabricated displaying interesting single-electron transport characteristics, there has been limited progress in the development of tools that can simulate such devices based on their physical geometry over a range of bias conditions up to a few volts per junction. In this work, we present the development of a multi-island transport simulator, MITS, a simulator of tunneling transport in multi-island devices that takes into account geometrical and material parameters, and can span low and high source-drain biases. First, the capabilities of MITS are demonstrated by modeling experimental devices described in the literature, and showing that the simulated device characteristics agree well with the experimental observations. Then, the results of studies of charge transport through a long one-dimensional (1D) chain of gold nano-islands on an insulating substrate are presented. Current-voltage (IV) characteristics are investigated as a function of the overall chain-length and temperature. Under high bias conditions, where temperature has a minimal effect, the IV characteristics are non-Ohmic, and do not exhibit any Coulomb staircase (CS) structures. The overall resistance of the device also increases non-linearly with increasing chain-length. For small biases, IV characteristics show clear CS structures that are more pronounced for larger chain-lengths. The Coulomb blockade and the threshold voltage (Vth) required for device switching increase linearly with the increase in chain length. With increasing temperature, the blockade effects are diminished as the abrupt increase in current at Vth is washed out and the apparent blockade decreases. Microscopic investigations demonstrate that the overall IV characteristics are a result of a complex interplay among those factors that affect the tunneling rates that are fixed a priori (island sizes, island separations, temperature, etc.), and the evolving charge state of the system, which changes as the applied source-drain bias (VSD) is changed. In a system of nano-islands with a broad distribution of sizes and inter-island spacings, the applied bias is divided across the junctions as one would expect of a voltage divider, with larger potential drops across the wider junctions and smaller drops across the narrower junctions. As a result, the tunneling resistances across these wider junctions decrease dramatically, relative to the other junctions, at high VSD thereby increasing their electron tunneling rates. IV behavior at high VSD follows a power-law scaling behavior with the exponent dependent on the length of the chain and the degree of disorder in the system.

Deep etched porous Si decorated with Au nanoparticles for surface-enhanced Raman spectroscopy (SERS)

Au nanoparticles decorated porous Si was fabricated following a two-step process for SERS applications. First, a Si wafer coated with predefined Au nanoisland arrays (∼15nm in average island size and ∼15nm in average inter-island spacing) was etched into porous structure by metal-assisted wet etching with etching time ranging from 0.5 to 40min. Second, Au with thickness ranging from 30 to 300nm was sputtered onto porous Si to activate the SERS effects. By testing 1,2-benzenedithiol with these SERS-active substrates, the influences of etching time and sputtered Au coating thickness on SERS enhancement factor were systematically investigated. Our analysis indicated that deep etching (etching time ≥ 10min) raised SERS enhancement factor by an order of magnitude over shallow etching (etching time ≤ 2min), while the thickness of sputtered Au coating, in the range of 30–300nm, only changed SERS enhancement factor by a factor of 3. The optimal parameters for etching time and sputtered Au coating thickness were experimentally determined to be 40min and 200nm, respectively. At the end of the study, the rationale of using predefined Au nanoisland arrays, instead of plain sputtered Au, to achieve better SERS performance was also experimentally justified.

A. c. conductance of γ-irradiated discontinuous platinum films

Four films (A, B, C and D) of discontinuous platinum films (D(Pt)Fs) whose mass thicknesses (dm) are 10, 20, 30 and 40 A, respectively were deposited onto Corning 7,059 glass substrates at ambient temperature via the thermal evaporation technique. Each film was γ-irradiated by different doses, namely, 100, 200, 300, 500 and 700 Gy; this was done by using 137Cs (0.662 MeV) radiation source of dose rate 0.5 Gy/min. For each dose, the d. c. and total resistance of the Pt films were measured; in that way the a.c. conductance Gac of the films could be determined. It was found that: (1) Gac increases as the dose, dm and the angular frequency ω of the voltage imposed on the film increases (2) the γ-irradiation has modified the shape of islands such that they are elongated parallel to the substrate surface and thus the inter-island spacings have decreased. This elongation has been confirmed via micrographs taken by the atomic force microscope. To account qualitatively for the results of Gac it was assumed that, the a.c. conductance of D(Pt)Fs is due to the hopping of electrons through the sites which exist on the substrate surface between two adjacent islands.

Effect of γ-irradiation on the gauge factor of two-dimensional island platinum films

Four films (A, B, C and D) of two-dimensional island platinum films (2D-I(Pt)Fs) whose mass thicknesses (dm) are 10, 20, 30 and 40Å, respectively were deposited onto Corning 7059 glass substrates via the thermal evaporation technique. The increase in the film resistance with time (aging) in air is monitored until stable values are obtained. Each of the prepared films was γ-irradiated by different doses, namely, 100, 200, 300, 500 and 700Gy; this was done using 137Cs (0.662MeV) radiation source of dose rate 0.5Gy/min. For each dose the relative change in the film resistance was found at different values of strain either in the tensional or compressional mode. The gauge factor (v) of the Pt films was deduced and we found that; for particular dm, the gauge factor decreases as the dose increases. Also, for a particular dose, v decreases as dm increases. Qualitative interpretation for the results was offered on the ground that (i) the transfer of electrons between islands takes place by the thermally activated tunneling mechanism, (ii) the process of γ-irradiation makes the islands spread along the substrate and consequently the inter-island spacing will decrease. The micrographs taken by the Atomic Force Microscope (AFM) confirmed such spreading.

Directional control of cell motility through focal adhesion positioning and spatial control of Rac activation

Local physical interactions between cells and extracellular matrix (ECM) influence directional cell motility that is critical for tissue development, wound repair, and cancer metastasis. Here we test the possibility that the precise spatial positioning of focal adhesions governs the direction in which cells spread and move. NIH 3T3 cells were cultured on circular or linear ECM islands, which were created using a microcontact printing technique and were 1 microm wide and of various lengths (1 to 8 microm) and separated by 1 to 4.5 microm wide nonadhesive barrier regions. Cells could be driven proactively to spread and move in particular directions by altering either the interisland spacing or the shape of similar-sized ECM islands. Immunofluorescence microscopy confirmed that focal adhesions assembled preferentially above the ECM islands, with the greatest staining intensity being observed at adhesion sites along the cell periphery. Rac-FRET analysis of living cells revealed that Rac became activated within 2 min after peripheral membrane extensions adhered to new ECM islands, and this activation wave propagated outward in an oriented manner as the cells spread from island to island. A computational model, which incorporates that cells preferentially protrude membrane processes from regions near newly formed focal adhesion contacts, could predict with high accuracy the effects of six different arrangements of micropatterned ECM islands on directional cell spreading. Taken together, these results suggest that physical properties of the ECM may influence directional cell movement by dictating where cells will form new focal adhesions and activate Rac and, hence, govern where new membrane protrusions will form.

Critical dimensions for YBCO islands grown by pulsed laser deposition

Islands of constant width at half maximum of approximately 100 nm have been observed during the pulsed laser deposition of films of nominal thickness from 0.7 to 3.0 nm of the material YBa2Cu3O7-δ(YBCO) on substrates of SrTiO3. The critical island dimensions of width, height and spacing were analyzed with classical kinetic and thermodynamic theories. Analytically it was calculated that the equilibrium island width for a 29-nm-high island should be 111 nm. The analysis also predicted that islands of smaller height should be wider, and higher islands should be narrower. Islands of height from 3.5 nm to 29 nm were observed with an atomic force microscope to have a constant width at half maximum of approximately 100 nm. There are several possible differences between experiment and analysis that could explain the difference in the results: the islands formed in a non-equilibrium phase of YBCO, the island strain relaxed from the base to the top, and smaller islands may not have reached their equilibrium width. Larger islands had significant roughening at the top. Calculations predict that these islands would be unstable with respect to surface perturbations. Calculations of relaxation strain energy and surface energy showed that there was excess strain energy available for island heights above 9 nm to provide the extra surface energy that would be necessary for surface perturbations to develop. The minimum observed interisland spacing of 36 nm agreed with calculations of the average atom diffusion length (40.6 nm).

Interisland spacing and angle effects on magnetic reversal in permalloy arrays

The switching characteristics of submicron permalloy elements were systematically studied as a function of interisland spacing. The result was an increase of the switching field with reduction in spacing, with a maximum value for near zero spacing. However, in overlapped segments, all elements switched simultaneously at lower fields than the maximum at zero spacing. The switching field depends on the number of connected segments as described by the Jacobs-Bean model, whereas the switching of the separated islands is governed mainly by the field emanating from the nearest-neighbor magnetic poles. The reversal behavior of the branched contiguous array was also studied. The angle between the branch axis and the easy axis played a critical role in establishing the switching field. The reversal field of the branched segment reached a minimum at 45/spl deg/ and a maximum at 90/spl deg/. A coherent rotation model, which includes the effect of the array geometry, appears to explain the trends.

スパッタ法ならびに蒸着法で作製したＣｕ薄膜の初期成長過程

Microstructural changes in the initial growth stage of Cu films fabricated by sputtering and evaporation are discussed. During and after deposition, the amplitude V of surface acoustic waves (SAWs) propagated on the piezoelectric substrate through the films deposited on it, and the electrical sheet resistivity of the films, Rsq, were measured. The surface morphology of the films was examined by using an atomic force microscope. As a result, it was found that (1) the critical thicknesses dp, at which the islands of a film begin to percolate, and dc, at which the film becomes two-dimensionally continuous, are smaller in sputter-deposited films than in evaporated ones; (2) the surface roughness of sputter-deposited films is smaller than that of evaporated films; and (3) microstructural change that increases the average inter-island spacing is harder to achieve in sputter-deposited films than in evaporated ones.

Percolation and Continuation Thickness of Ag Films on the Initial Growth Stage

Growth of vapor-deposited Ag films by the coalescence process in the post-nucleation stage is discussed. During and after deposition, the amplitude V of surface acoustic waves (SAW) propagated on the piezoelectric substrate through the films deposited on it, and the electric sheet resistivity of the films, Rsq, were measured. The microstructures of films with various thicknesses d were examined with a scanning electron microscope. As a result, it was found that (1) the thickness dp at which V reaches a minimum value corresponds to the critical thickness at which the islands of a film begin to percolate, (2) the thickness dc at which 1/Rsq begins to be proportional to the film thickness corresponds to the critical thickness at which the film becomes two-dimensionally continuous, (3) dp and dc become larger with decreasing deposition rate, and (4) in the early coalescence stage (d<dp), the microstructural change progresses, increasing the average inter-island spacing. From these results, we can conclude that a higher deposition rate is necessary to suppress the microstructural change of islands and to allow fabrication of continuous ultra-thin films.

Scaling behavior in the current-voltage characteristic of one- and two-dimensional arrays of small metallic islands.

We have measured the current-voltage ( $I$- $V$) characteristics of one- and two-dimensional arrays of normal metal islands linked by small tunnel junctions. The tunneling resistance is large compared to the resistance quantum, and a ground plane reduces the screening length to much less than the interisland spacing. At temperatures well below the island charging energy, we find a threshold voltage ${V}_{T}$ below which little current flows. For $V&gt;{V}_{T}$, $I$ scales as $({V/V}_{T}\ensuremath{-}1{)}^{\ensuremath{\zeta}}$ where $\ensuremath{\zeta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.36\ifmmode\pm\else\textpm\fi{}0.1$ (1D) and $1.80\ifmmode\pm\else\textpm\fi{}0.16$ (2D). We interpret this behavior as a dynamic critical phenomenon.

Discontinuous gold island films on mica

Epitaxial Au films have been deposited on mica substrates at high temperatures (≳500 °C) and characterized by scanning tunneling microscopy, atomic force microscopy, and scanning electron microscopy. A critical growth temperature between 540 and 580 °C has been identified. Above this critical temperature, discontinuous films of micron-sized Au island are obtained. The islands have the appearance of microcrystals with interisland spacings ranging from 0.2 to 1.5 μm and atomically flat surfaces similar to continuous Au films.

Growth morphology of vicinal hillocks on the {101} face of KH2PO4: From step-flow to layer-by-layer growth.

The growth morphologies of vicinal hillocks on K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ {101} surfaces have been investigated using atomic force microscopy. Growth occurs on monomolecular steps both by step-flow and through layer-by-layer growth on both dislocation induced steps and 2D nuclei. Dislocation induced hillocks exhibit hollow cores. The circular cross sections of these cores are consistent with an isotropic step edge energy and the core radii are in good agreement with theoretical predictions. From the interisland spacing we estimate the surface diffusion length to be 200 to 300 nm at 300 K.

Optical properties of copper island films with consideration of grain size and interisland spacing distribution

Optical properties of discontinuous Cu films were investigated in UV, in the visible range and in the near IR. It was found that the plot of the transmission coefficient in IR differed according to the volume fraction of the film. Optical anomalies were explained in terms of the modified Maxwell-Garnett theory.

Time variation of the interisland spacing at liquid nitrogen temperature for copper and silver island films

Time variation of the interisland spacing at liquid nitrogen temperature for copper and silver island films

Optical properties of gold and aluminum island films with regard to grain size and interisland spacing distributions

In this study, optical properties of gold and aluminum island films are described in terms of a classical macroscopic model. The distributions of island diameters and interisland spacing are assumed to be lognormal. The study also involves the effective permittivity of the surrounding. There is good agreement between theoretical and experimental results, especially for small-thickness and small-grain-size films.

Effect of applied field and temperature on the aging of copper discontinuous films studied by the repeated deposition technique

Presented are the results of investigations on the effect of an applied DC electric field on the post-deposition resistance changes of island copper films on glass. Based on the functional dependence of the film resistance on time, an agglomeration rate is defined with the theory of mobility coalescence being invoked to explain the resistance increase of the films after deposition. Repeated deposition was employed until the film resistance became steady, after which measurements of resistance versus temperature were made. The agglomeration rate for film of a particular resistance decreased as the number of deposition cycles was increased, due to formation of large, less mobile islands. For one of the higher-resistance films, the agglomeration rate showed a drastic fall for a particular value of deposition cycle number indicating that large-scale coalescence (LSC) had occurred. It was found that the presence of a field does not alter the post-deposition coalescence process in island copper films, possibly due to the relative immobility of the copper islands. The initial resistances of the film with and without the field were, however, vastly different. The agglomeration rates of films of different initial resistances showed an oscillatory behaviour corroborating earlier findings on copper films. The resistance-temperature curves showed a minimum below which the film showed a negative temperature coefficient of resistance (TCR) and a positive TCR above the temperature of transition. The transition temperature is found to shift to higher values as the films approach the discontinuous-semi-continuous structure transition. The resistance-temperature behaviour is explained if one considers that the negative TCR region is due to an increase in the number of thermally activated charged islands while the positive TCR region is due to enhanced mobility of the copper islands at elevated temperatures leading to coalescence and thus to an increase in the average inter-island spacing and the resistance of the film.

Influence of residual gases and surface contaminants on the aging behavior of island silver films

The aging of island silver films deposited on glass at room temperature and at a pressure of 2×10−5 Torr was studied by monitoring the dc electrical resistance of the films. Films in the resistance range 1–11 MΩ/⧠ were studied under different conditions to ascertain the role of the residual gases and surface contaminants on the agglomeration rate. It was found that residual gases do not significantly affect the agglomeration rate, but a film of water vapor adsorbed on the glass surface retards the agglomeration to a great extent. An expression is arrived at for the functional dependence of the interisland spacing with time for the different conditions of study, assuming that the conduction in these island films is by quantum mechanical tunneling.