Fabrication of periodic square arrays by angle-resolved nanosphere lithography

We investigate the fabrication of periodic square arrays of solid gold islands by angle-resolved nanosphere lithography (ARNSL) in conjunction with thermal evaporation and etching. By varying θ (the tilt angle between the direction of gold deposition beam and the substrate surface normal) and ? (the substrate rotation angle about the beam axis), adjacent islands on a deposited hexagonal gold array will have a constant and periodic difference in height. Upon etching, this height bias will result in the shorter structures being removed to produce an array with a different symmetry from the original hexagonal symmetry of the parent mask. By depositing at three directions of ? = 0°, 120° and −120° with a constant θ = 20°, experimental results show that deposited two-dimensional gold periodic arrays will have a measurable difference in height between adjacent islands. Etching of the resulting patterns produced periodic near-square arrays with triangular nanostructures. Thus the combination of ARNSL and etching can allow selective periodic nanostructures to be removed, increasing the diversity of array symmetries available through nanosphere lithography.     

Photonic theory of noise

The paper proposes that 1/f noise in materials and devices under non-equilibrium conditions is of electromagnetic origin rather than being related to charge carriers. For samples represented by simple resistors the analysis shows that the noise is due to the discrete nature of photons constituting the impinging electromagnetic flux from the source feeding the resistor. The paper presents detailed analysis of the external and internal electromagnetic fields of the resistor environment, with appropriate interpretation in terms of discrete photons. From quantum theory photons are known to retain their energy under interactions in linear environments. This property implies a departure from macroscopic electromagnetics in that photons cannot be partially transmitted and partially reflected, a phenomenon requiring appropriate modifications of boundary conditions at the resistor surface. These special demands call for inclusion of a supplementary internal resistor mode, serving as a lossless idler which is active only for matching purposes. At the resistor terminals the impinging photons give rise to excitation of RF current and voltage noise with an exact 1/f frequency distribution, which is in agreement with all available experimental measurements. The paper presents detailed formulae for noise spectral densities under general drive conditions from DC and RF sources of arbitrary internal resistance. The presented theoretical noise formulae have the same form as earlier empirical formulae for 1/f noise. With an RF source at frequency f₀ the analysis predicts noise with 1/|f-f₀| frequency distribution, which is compatible with available experimental observations.     

A method to achieve clear‐sky data‐volume download in satellite links affected by tropospheric attenuation

For data downloading from satellites, the traditional approach of considering the complementary probability distribution, P(A_trop) of the total tropospheric attenuation A_trop (dB), at a frequency, polarization, and elevation angle, may be too pessimistic, and it may lead to large overdesign. If the data volume downloaded in a given observation time T_obs (a day, a week, a month, etc.), with a constant probability of symbol error, is more valuable than the instantaneous symbol, or bit rate (as it may be the case in Earth resources observation or in other services that allow long delays in communicating data), another approach can avoid overdesign, namely the various adaptive coding and modulation techniques. We study a particular time integral of A_trop(t): (i) to define the average efficiency of a radio channel faded by the troposphere and (ii) to design a method that can theoretically achieve the same volume of data downloaded in clear‐sky conditions (no troposphere), even if the radio link is faded by the troposphere. The average efficiency and its bounds can be calculated from the complementary probability distribution of A_trop(t). We explicitly apply the theory to radio links faded by rain, by using the experimental rain‐attenuation time series measured with satellite ITALSAT in a 37.8° slant path, at 18.7, 39.6, and 49.5 GHz at Spino d'Adda (Italy), and to those simulated with the synthetic storm technique. Based on the average efficiency, we define the method that can achieve in T_obs, theoretically, the same data volume as in clear sky, directly applicable to quadrature phase‐shift keying, multiple phase‐shift keying, and Shannon capacity theorem. The method requires a fixed increase in power margin, and bandwidth, compared with clear‐sky conditions, and delivers an average symbol rate equal to the maximum symbol rate obtainable when A_trop(t) = 0. The method can also be used in terrestrial links at any frequency. We compare its theoretical performance with the traditional adaptive coding and modulation techniques and show that, even theoretically, these techniques cannot achieve unitary efficiency as, on the contrary, the novel method can do. Copyright © 2015 John Wiley & Sons, Ltd.     

Anisotropic Wettability of Biomimetic Micro/Nano Dual‐Scale Inclined Cones Fabricated by Ferrofluid‐Molding Method

Learning from nature, a series of cone‐shaped structures resembling trichomes of plants are fabricated by ferrofluid molding to understand the influence of geometry on wettability. Experimentally, ferrofluid microdroplets are generated under an external magnetic field, and their shape can be changed from right cones into oblique cones by tilting the external magnetic field. Followed by hard molds made with UV‐curable tri(propylene glycol) diacrylate, polydimethylsiloxane microcones with different inclination angle (θ) are subsequently generated. Nickel thin film is deposited onto the microcones to form micro/nano dual‐scale structures. The largest contact angle (CA) is obtained in nickel‐deposited right cones (CA = 163.1° ± 2.5°). Anisotropic wettability is exhibited in oblique cones and the retention forces in the pin and release directions differ up to 12 μN (cones θ = 50°). As explained by a model as a function of the inclination angle of the cone structures, the contact and retention forces of droplet move in pin and release directions exhibit considerable differences. Results suggest the inclination of the trichomes assist the balance between repellency and retention of water in a direction‐selective manner.     

A Coating that Combines Lotus‐Effect and Contact‐Active Antimicrobial Properties on Silicone

The antimicrobial equipment of materials is of great importance in medicine but also in daily life. A challenge is the antimicrobial modification of hydrophobic surfaces without increasing their low surface energy. This is particularly important for silicone‐based materials. Because most antimicrobial surface modifications render the materials more hydrophilic, methods are needed to achieve antimicrobial activity without changing the high water‐contact‐angle. This is achieved in the present work, where SiO₂ nanoparticles are prepared and functionalized with 3‐(trimethoxysilyl)‐propyldimethyloctadecyl ammonium chloride (QAS) in a one‐pot synthesis. The modified nanoparticles are applied onto a silicone surface from suspension with no need of elaborate pretreatment. The resulting surface exhibits a Lotus‐Effect combined with contact‐active antimicrobial properties. The particle surfaces show self‐organizing micro‐ and nanostructures that afford a water‐contact angle of 144° and a hysteresis below 10°. The particles are self‐adhering on the silicone after solvent evaporation and resistant against immersion into and washing with water for at least 5 d. Thereby, the adhesion of the bacterial strain Staphylococcus aureus to these surfaces is reduced and the remaining bacterial cells are killed within 16 h. This is the first example of a Lotus‐Effect surface with intrinsic contact‐active antimicrobial properties.     

Throughput analysis in mobile ad hoc networks with directional antennas

A major obstacle stunting the application of mobile ad hoc networks (MANETs) is the lack of a general throughput analysis theory for such networks. The available works in this area mainly focus on asymptotic throughput study (order sense) in MANETs with omnidirectional antennas or directional antennas. Although the order sense results can help us to understand the general scaling behaviors, it tells us little about the exact achievable throughput. This paper studies the achievable throughput of a MANET where each node is equipped with a directional antenna of beamwidth θ for transmission. A generalized two hop relay scheme with packet redundancy f is adopted for packet routing. Based on the Markov chain and automatic feedback theory, we explore a general theoretical framework that enables the achievable throughput analysis to be conducted for a directional antenna-based MANET. Based on the results of the achievable per node throughput, we further explore the throughput optimization problem for a fixed beamwidth θ and determine the corresponding optimal setting of f to achieve the optimal throughput. Numerical studies are also conducted to demonstrate the efficiency of these models.It shows that the maximum achievable throughput obtained from our theoretical framework matches nicely (with at most 7% difference) with that obtained from simulation under a realistic model.     

Space communications with variable elevation angle faded by rain: Radio links to the Sun–Earth first Lagrangian point L1

How rain attenuation affects space links with variable elevation angles is not yet fully researched. The aim of this paper is to investigate this topic by simulating rain attenuation at K_a Band, in slant paths with variable elevation angles, with the Synthetic Storm Technique (SST), in links connected with spacecrafts at the Sun–Earth first Lagrangian point L₁, viewed from Spino d'Adda (Italy), Tampa (Florida), White Sands (New Mexico). The input to the SST is a large database of time series of 1‐min rain rate recorded on site, 10 years in Spino d'Adda, 4 years at Tampa and White Sands. After recalling known results on the elevation angle of the Sun (i.e. L₁), θ_s (°), seen from latitude λ (°), I report what seems to be a new result: the mode of the probability density function of θ_s in a year, in the range 0 ≤ λ ≤ 90° ? ε (Earth axis tilt angle ε = 23.44°), coincides with the peak angle found at the day of the Winter solstice at the site, a result valid also for other planets, once their tilt angle is used. Compared to the complementary probability distribution function (pdf) of rain attenuation calculated for a geostationary (GEO) link (fixed elevation angle), the pdf to L₁ depends on the rain‐rate pdf during the contact time with L₁, according to the local climate. I show that, to obtain a good and easier estimate of the rain attenuation pdf in L₁ links, we can consider a GEO link with elevation angle equal to the mean angle and rain rate pdf, both during the contact time, and that the mode angle gives an upper bound to the rain attenuation pdf in the sites considered. Copyright © 2015 John Wiley & Sons, Ltd.     

Surface roughness and slope measurements using polarimetric SARdata

In this paper, the circular polarization coherence, ρ_RRLL , is investigated as a potential estimator of terrain surface roughness and small-scale slopes. The studies utilize microwave backscatter collected from 1) dielectric surfaces in an anechoic chamber and 2) a desert test site using P-, L-, and C-band NASA/JPL AIRSAR data. These experimental studies and supporting theory, indicate a sensitive decrease of |ρ_RRLL| with increasing surface roughness ks over a range 0 ⩽ ks ⩽ 1. For the present studies this decrease is caused largely by the depolarizing effects of small-scale surface slopes in the azimuth direction rather than by volume, or multiple scatter. For cases when the scatter is reflection symmetric, the value of |ρ_RRLL| depends on the surface roughness and on the local incidence angle. The dependence of |ρ_PRRLL| on the local incidence angle is supported by theory and experimental results. For these same scattering cases, however, |ρ_PRRLL| is independent of the surface dielectric constant. Estimation of the functional dependency of |ρ_PRRLL| versus ks, for a mid-range incidence angle, has been carried out using roughness estimates derived from an empirical model     

Profiles of Work Function Shifts and Collective Charge Transfer in Submonolayer Metal–Organic Films

Vacuum level shifts Δ(d) at metal–organic (m–O) interfaces indicate the formation of surface dipoles for film thickness d ≤ d_ML up to a monolayer (ML). Shifts or profiles Δ(θ) of submonolayer films are nonlinear in the coverage θ = d/d_ML ≤ 1, which points to cooperative interactions between adsorbed molecules. Adsorption with weak nonspecific bonding is modeled as charge transfer (CT) between molecules M and localized surface states S of the metal. The dipole μ₀ of ions S^?M⁺ or S⁺M^? gives upper bounds for the vacuum level shift ? ₀ and dipole–dipole repulsion V₀ at θ = 1. Partial CT ρ(θ) < 1 is found self consistently and accounts for published profiles Δ(θ) of representative planar and nonplanar molecules with d_ML～ 4 and ～10 Å. Initial adsorption at θ ～ 0 has considerable ionic character, ρ(0) ～ 1/2, that decreases to ρ(1) ～ 1/10 at θ = 1. Planar molecules with small μ₀ and V₀ have slightly nonlinear profiles while molecules with large μ₀ and V₀ have highly nonlinear Δ(θ). Collective CT is a phenomenological model for m–O interfaces with nonspecific bonding. The CT model is contrasted to fixed dipoles on the surface, to calculations of Δ(1) and to simulations of sub‐ML films.     

Measurement and modeling of the millimeter-wave backscatterresponse of soil surfaces

The millimeter-wave (MMW) backscatter response of bare-soil was examined by conducting experimental measurements at 35 and 94 GHz using a truck-mounted polarimetric scatterometer and by developing appropriate models to relate the backscattering coefficient to the soil's surface and volume properties. The experimental measurements were conducted for three soil surfaces with different roughnesses under both dry and wet conditions. The experimental measurements indicate that in general the backscattering coefficient is comprised of a surface scattering component σ^s and a volume scattering component σ ^v. For wet soil conditions, the backscatter is dominated by surface scattering, while for dry conditions both surface and volume scattering are significant, particularly at 94 GHz. Because theoretical surface scattering models were found incapable of predicting the measured backscatter, a semiempirical surface scattering model was developed that relates the surface scattering component of the total backscatter to the roughness parameter ks, where k=2π/λ and s is the rms height, and the dielectric constant of the soil surface. Volume scattering was modeled using radiative transfer theory with the packed soil particles acting as the host material and the air voids as the scattering particles. The combined contribution of surface and volume scattering was found to provide good agreement between the model calculations and the experimental observations     

Experimental study and empirical evaluation of carrier compression and third order intermodulation products of unequal carriers in a communication satellite transponder

Carrier compression and third order intermodulation products of two and three unequal carriers have been studied experimentally using a 36 MHz wide C-band transponder of INSAT-1B satellite. From the experimental data, empirical relations for carrier compression for two unequal carriers have been derived, which agree with the measured data within ±0.5 dB. The empirical relations for carrier compression derived for two unequal carriers have been extended to three unequal carriers and good correlations with measured data have been obtained. Empirical relations for third order intermodulation products of two and three (f₁+f₂-f₃ type) unequal carriers have also been derived.     

Entropy measures and some distribution approximations

Many information measures are suggested in the literature. Among these measures are the Shannon H_n(θ) and the Awad A_n(θ) entropies. In this work we suggest a new entropy measure, B_n(θ), which is based on the maximum likelihood function.These three entropies were calculated from the gamma distribution and its normal approximation, the binomial and its Poisson approximation, and the Poisson and its normal approximation. The relative losses in these three entropies are used as a criterion for the appropriateness of the approximation.     

一种氮化镓高迁移率晶体管的栅电流噪声模型

This work presents a theoretical and experimental study on the gate current 1/f noise in Al Ga N/Ga N HEMTs. Based on the carrier number fluctuation in the two-dimensional electron gas channel of Al Ga N/Ga N HEMTs, a gate current 1/f noise model containing a trap-assisted tunneling current and a space charge limited current is built. The simulation results are in good agreement with the experiment. Experiments show that, if Vg Vx, gate current 1/f noise comes from not only the trap-assisted tunneling RTS, but also the space charge limited current RTS. This indicates that the gate current 1/f noise of the Ga N-based HEMTs device is sensitive to the interaction of defects and the piezoelectric relaxation. It provides a useful characterization tool for deeper information about the defects and their evolution in Al Ga N/Ga N HEMTs.     

Microstructure-critical current relationships in hard superconductors

The pinning of the flux line lattice (FLL) by crystal lattice defects gives rise to a critical current density J_c for hard superconductors. The volume pinning force density F_p = ‖BXJ_c‖ however, depends both on the elementary interaction force f_p between a single defect and the FLL and on the nature of the summation of these many f_p’s. The latter will depend on the spatial arrangement of defects and on the elastic and plastic deformation properties of the FLL. For localized defects the f_p is a strong function of defect “size”, reaching a maximum when the size is approximately the coherence length, and is approximately proportional to H_c ²(T) (1−h) where H_c is the thermodynamic critical field and h is the reduced magnetic field H/H_c2. The summation model must give a F_p which obeys the following empirical rules: 1) F_p has a maximum at a reduced field h_p which decreases with increasing defect density ρ and f . 2) F_p at h > h_p is structure insensitive while F_p at h < h_p is structure sensitive. 3) A scaling law is obeyed if T is changed, i.e., F_p = H_c2(T)_m .f(h), where m is ∿2.5 andf(h) is only a function of reduced field and microstructure. Experimental evidence for these generalizations is presented and a model which predicts these results is outlined and quantitatively tested. Work supported by the U. S. Energy Research and Development Administration.     

Influence of radiation defects on electrical losses in silicon diodes irradiated with electrons

Silicon diodes with a p ⁺-n junction irradiated with 3.5-MeV electrons (the fluence ranged from 10¹⁵ to 4 × 10¹⁶ cm⁻²) have been studied. It is established that the dependence of the tangent of the angle of electrical losses tanδ on the frequency f of alternating current in the range f = 10²−10⁶ Hz is a nonmonotonic function with two extrema: a minimum and a maximum. Transformation of the dependences tanδ(f) as the electron fluence and annealing temperature are increased is caused by a variation in the resistance of n-Si (the base region of the diodes) as a result of accumulation (as the fluence is increased) or disappearance and reconfiguration (in the course of annealing) of radiation defects. The role of time lag of the defect recharging in the formation of tanδ(f) is insignificant.     

Integral expressions for the numerical evaluation of product form expressions over irregular multidimensional integer state spaces

We consider stochastic systems defined over irregular, multidimensional, integer spaces that have a product form steady state distribution. Examples of such systems include closed and BCMP type of queuing networks, polymerization and genetic models. In these models the system state is a vector of integers, n=[n ₁,...,n _M ] and the steady state solution has product form of the type (n)= _i=1 ^M f _i (n _i ). To obtain useful statistics from such product form solutions, (n) has to be summed over some subset of the space over which it is defined. We consider situations when these subsets are defined by a set of equalities and inequalities with integer coefficients, as is most often the case and provide integral expressions to obtain these sums. Typically, a brute force technique to obtain the sum is computationally very expensive. Algorithmic solutions are available for only specific forms of f _i (n _i ) and shapes of the state space. In this paper we derive general integral expressions for arbitrary state spaces and arbitrary f _i (n _i ). The expressions that we derive here become especially useful if the generating functions f _i (n _i ) can be expressed as a ratio of polynomials in which case, exact closed form expressions can be obtained for the sums. We demonstrate the wide applicability of the integral expressions that we derive here through three examples in which we model finite highway cellular systems, copy networks in multicast packet switches and a BCMP queuing network modeling a multiuser computer system.     

Adaptive orthogonal frequency division multiplexing channel capacity employing diversity in cellular fading environments with symmetric scattering distributions

Gaussian and hyperbolic angle‐of‐arrival probability density functions are used to derive channel capacity of orthogonal frequency division multiplexing transmission employing diversity techniques and adaptive policies in cellular wireless fading environments. The intercarrier interference (ICI) power is quantified and given as a function of Doppler shift f_d, symbol duration T_s, frequency correction ζ and propagation ratio τ. Two scattering distributions, which have been shown to closely fit experimental empirical data, are examined in this paper: (i) Gaussian and (ii) hyperbolic. A new signal‐to‐interference‐and‐noise ratio probability density function is derived as a function of the ICI power using diversity techniques and adaptive policies. From that, effects of f_dT_s, ζ and τ on channel capacity can be discussed. The main contribution of this work is to model ICI as a function of f_d and symbol duration T_s. Two diversity techniques are considered: (i) maximal ratio combining and (ii) selective combining. Three adaptive policies are studied: (i) optimal rate adaptation, (ii) optimal rate and power adaptation and (iii) channel inversion with fixed rate. Closed‐form expressions and bounds on various channel capacity with orthogonal frequency division multiplexing transmission under different scenarios are derived. Copyright © 2012 John Wiley & Sons, Ltd.     

Evolution of Contact Resistance during the Bonding Process of NCA Flip-Chip Interconnections

Non-conductive adhesive (NCA) flip-chip interconnects are emerging as an attractive alternative to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine-pitch applications. The electrical connectivity of an NCA interconnect relies solely on the pure mechanical contact between the integrated circuit (IC) bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extent on the cure shrinkage characteristics of the NCA. Therefore, it is necessary to monitor the evolution of the electrical conductivity which could reflect the impact of cure- and thermal-induced stresses during the curing and cooling process, respectively. In this article, in situ measurement of the development of contact resistance during the bonding process of test chips was developed by using a mechanical tester combined with a four-wire resistance measurement system. A drop of resistance induced by the cure stress during the bonding process is clearly observed. With decreasing bonding temperature, the drop of contact resistance induced by cure shrinkage becomes larger, while the cooling-induced drop of resistance becomes smaller. The evolution of contact resistance agrees well with experimental observations of cure stress build-up. It is found that vitrification transformation during the curing of the adhesive could lead to a large cure stress and result in the reduction of the␣contact resistance. Furthermore, no obvious changes were observed when the applied load was removed at the end of bonding.     

Structure and Energetics of Dislocations at Micro‐Structured Complementary Interfaces Govern Adhesion

Highly enhanced adhesion can be achieved between surfaces patterned with complementary micro‐channel structures. An elastic material, poly(dimethylsiloxane) (PDMS), is used to fabricate such surfaces by molding into a silicon master with micro‐channel profiles patterned by photolithography. For each pair of complementary surfaces, dislocation defects are observed in the form of visible striations, where ridges fail to fully insert into the channels, and the rotational misalignment angle was found to be the key factor controlling the dislocation distribution and adhesion strength. Adhesion between complementary interfaces, as measured by energy release rate required to propagate an interfacial crack, can be enhanced by up to 30 times compared to a flat control depending on the misalignment angle. The ability to control the orientation and periodicity of dislocation patterns by changing misalignment angle makes this system eminently controllable. This system could be a useful experimental tool in assisting research on geometry‐controlled adhesion, while providing a test‐bed for stability theories of interacting dislocations and crack fronts.     

Fine‐Tuning of Superhydrophobicity Based on Monolayers of Well‐defined Raspberry Nanoparticles with Variable Dual‐roughness Size and Ratio

Superhydrophobic surfaces have been extensively investigated for self‐cleaning, low‐adhesion, anti‐corrosion or reduced‐drag applications. Roughness and its characteristics, i.e., morphology, overall roughness and individual feature size, is an essential factor for superhydrophobicity. Several experimental methods and theoretical models strived to predict how the surface wettability is affected by the surface roughness. However, due to the difficulty of making practical surfaces with well‐defined roughness profiles, only limited and arbitrary experimental studies focused on practical superhydrophobic films. Here, the roughness factors which determine the wetting properties of films are reported, based on monolayers of well‐defined raspberry silica‐silica nanoparticles, exhibiting a wide‐range and systematic variation of individual features sizes and ratios (large over small features). The advancing water contact angle does not depend on the feature size or ratio, while the contact angle hysteresis (CAH) is strongly dependent on both. The minimum size and size ratio to reach superhydrophobicity were determined. These new insights into the wetting of rough surfaces can be used to direct the design of practical superhydrophobic materials for advanced applications such as solar panels, microelectronics or microfluidic devices.