Metal-based Structures Research Articles

Second- and third-harmonic generation in metal-based structures

We present a new theoretical approach to the study of second and third harmonic generation from metallic nanostructures and nanocavities filled with a nonlinear material, in the ultrashort pulse regime. We model the metal as a two-component medium, using the hydrodynamic model to describe free electrons, and Lorentz oscillators to account for core electron contributions to both the linear dielectric constant and to harmonic generation. The active nonlinear medium that may fill a metallic nanocavity, or be positioned between metallic layers in a stack, is also modeled using Lorentz oscillators and surface phenomena due to symmetry breaking are taken into account. We study the effects of incident TE- and TM-polarized fields and show that a simple re-examination of the basic equations reveals additional exploitable dynamical features of nonlinear frequency conversion in plasmonic nanostructures.

Metal-Based Photonic Coatings from Electrochemical Deposition

Plasmonic materials offer the remarkable ability to manipulate light by metal–dielectric features significantly smaller than the wavelengths of free space photons. The fabrication of such minuscule features for the desired plasmonic properties, however, remains challenging. Here, we report an economical and versatile bottom-up approach that combines electrodeposition and de- alloying techniques for fabricating elaborate metal-based structures, achieving structural resolutions comparable to the present lithography techniques. We present our studies on the metal-based counterpart of the rugate filter. These metal-based photonic films are porous, magnetic, and feature strong optical responses in visible wavelengths, while their structural periodicities are an order of magnitude smaller than the light’s free space wavelength.

Filling Fraction Dependent Properties of Inverse Opal Metallic Photonic Crystals

Metallic photonic crystals, metal-based structures with periodicities on the scale of the wavelength of light, have attracted considerable attention due to the potential for new properties, including the possibility of a complete photonic bandgap with reduced structural constraints compared to purely dielectric photonic crystals, unique optical absorption, thermally stimulated emission behavior, and interesting plasmonic physics. Photonic applications may include high-efficiency light sources, chemical detection, and photovoltaic energy conversion. Other applications for 3D porous metals, so-called “metal foams”, include acoustic damping, high strength to weight structures, catalytic materials, and battery electrodes. The photonic properties of metallic inverse opal structures have been of significant interest because of the simplicity of fabrication and potential for large-area structures. However, in practice, experiments on metal inverse opals have been inconclusive, presumably because of structural inhomogeneities due to synthetic limitations. In this work, we demonstrate an electrochemical approach for fabricating high-quality metal inverse opals with complete control over sample thickness, surface topography, and, for the first time, the structural openness (metal filling fraction (FF)). Optical measurements conclusively demonstrate that metal inverse opals modulate the absorption and thermal emission of the metal and that these effects only become 3D in nature at high degrees of structural openness. Various metals, for example, Au, Ag, W, Pt, Pd, Co, Ni, and Zn, have been formed into inverse opals. Here, Ni is selected because of its high reflectivity in the IR, temperature stability, and ease of electrochemical processing. Ni inverse opals are fabricated by using electrodeposition through a polystyrene (PS) opal template that was first deposited on a surface-treated Au film evaporated on a Si wafer. PS opals formed from microspheres ranging in diameter from 460 nm to 2.2 lm were used as templates; this paper focuses on metal inverse opals formed using 2.2 lm microspheres. Templated electrodeposition is observed in all systems; this range of microsphere diameters is not an upper or lower limit. The final thickness of the sample is regulated by controlling the total charge. After electrodeposition, the PS microspheres are removed with tetrahydrofuran, resulting in a Ni inverse opal. Although the electrodeposition is quite homogeneous, gradual thickness variations do occur over the sample surface. These variations turn out to be useful, as they generate regions of different number of layers and surface terminations over the same sample (Fig. 1). Scanning electron microscopy (SEM) reveals a direct correspondence between the color, C O M M U N IC A IO N

Experimental Observation of Left-Handed Behavior in an Array of Standard Dielectric Resonators

We demonstrate that by utilizing displacement currents in simple dielectric resonators instead of conduction currents in metallic split-ring resonators and by additionally exciting the proper modes, left-handed properties can be observed in an array of high dielectric resonators. Theoretical analysis and experimental measurements show that the modes, as well as the subwavelength resonance, play an important role in the origin of the left-handed properties. The proposed implementation of a left-handed metamaterial, based on a purely dielectric configuration, opens the possibility of realizing media at terahertz frequencies since scaling issues and losses, two major drawbacks of metal-based structures, are avoided.

Spin-polarized resonant transport in a hybrid ferromagnetic–two-dimensional electron gas structure

We investigated the spin-polarized resonant transport in a hybrid high-electron-mobility transistor (HEMT) structure, with source and drain electrodes made of ferromagnetic (FM) material, while the channel consists of a highly doped ${n}^{++}$ AlGaAs-GaAs two-dimensional electron gas (2DEG). The electron transport in the FM layer is modeled using the spin-drift diffusion model, while across the 2DEG layer, ballistic transport is assumed, given the long mean free path within the 2DEG. By solving the two transport models self-consistently, we found that the transport properties of the device, such as the transmission probability, the spin injection (SI) efficiency, and the magnetoresistance (MR) ratio, all exhibit oscillatory behavior when the 2DEG layer width or the 2DEG Fermi energy is varied. The basis of these oscillations is the resonant transport across the 2DEG, which is reminiscent of the spin-polarized resonant tunneling (SPRT), observed recently in magnetic tunnel junctions (MTJs). The hybrid device has distinct advantages over the metal-based MTJ structures in the practical utilization of the SPRT effect. This is because the ballistic charge conduction through the 2DEG enables easy tunability of the MR ratio and SI efficiency, by varying the doping density and gate bias, while avoiding the exponential suppression of MR with barrier thickness, which occurs in MTJ devices. Numerically, the hybrid HEMT device is predicted to be capable of achieving maximum MR and SI ratios approaching 20% and 40%, respectively, at the crest of their respective oscillations.

Systematic Investigation of the Hydrothermal Syntheses of Pr(III)−PDA (PDA = Pyridine-2,6-dicarboxylate Anion) Metal−Organic Frameworks

A series of novel two-dimensional (2D) and three-dimensional (3D) praseodymium coordination polymers, namely, {[Pr3(PDA)4(HPDA)(H2O)8] x 8H2O}n (2), {[Pr2(PDA)3(H2O)3] x H2O}n (3), {[Pr(PDA)(H2O)4] x ClO4}n (4), and { [Pr2(PDA)2(H2O)5SO4] x 2H2O}n (5) (PDA = pyridine-2,6-dicarboxylic anion), was designed and synthesized under hydrothermal conditions. Complexes 1-3 (chainlike polymer, {[Pr(PDA)(HPDA)(H2O)2] x 4H2O}n (1) was also obtained independently by us, although it has been reported recently by Ghosh et al.) were fabricated successfully by simply tuning the Pr/PDA ratio and exhibited various and intriguing topological structures from a 1D chain to a 3D network. While the synthetic strategy of 5 was triggered and further performed only after 1 was structurally characterized. The complexes were characterized by X-ray single-crystal determination, spectroscopic, and variable-temperature magnetic susceptibility analyses. In complex 2 an unusual nanosized square motif as a building block constructed by eight Pr ions was further assembled into a highly ordered 2D grid compound. In complex 3 the decanuclear Pr metal-based structure as a repeat unit interpenetrated to form a novel 3D polymer. Complex 4 was a 3D network polymer fabricated through a hexanuclear Pr ring as a building block, and ClO4- anions as guests were trapped in the cavity. In complex 5 six Pr atoms, two SO4(2-) anions, and carboxylic oxygen bridges constructed an intriguing rectangle structure as a repeat unit in the grid to form a 2D coordination polymer in which the unique bi-bidentate coordination mode of SO4(2-) anion was observed.

A Remarkable Isostructural Homologous Series of Mixed Lithium−Heavier Alkali Metaltert-Butoxides [(t-BuO)8Li4M4] (M = Na, K, Rb or Cs)

Three new mixed lithium−heavier alkali metal tert-butoxides [(t-BuO)8Li4M4] (M = Na, Rb, Cs) are reported which, added to the previously discovered potassium analogue [(t-BuO)8Li4K4], complete the homologous series. Remarkably, X-ray crystallographic studies reveal that all four heterometallic compounds adopt a common structure. This sixteen-vertex O8Li4M4 “breastplate” motif is built around novel (M+)4 planes (M = Na, K, Rb, Cs), both faces of which support chelating (O4Li2)2- dianions. Each such dianion is positioned approximately normal with respect to the other. Bonding within the breastplate structure involves a combination of μ3-Li, μ4-M, μ3-O, and μ4-O centers. Ab initio MO calculations on model systems predict that formation of the heterometallic breastplate from the exclusively μ3-bonded frameworks of its component homometallic structures is a favorable exothermic process. Best regarded as an inherently stable contacted triple ion sandwich comprising a dianion−tetramonocation−dianion arrangement, the breastplate motif is likely to be more widely applicable within heterometallic structural chemistry than so far recognized. This point is discussed with reference to a previously documented series of heterometallic p-block metal-based imide structures of general formula [(CyN)8M14M24], where M1 = Sb or As and M2 = Ag, Cu, or Na.

Perpendicular hot electron spin-valve effect in a new magnetic field sensor: The spin-valve transistor.

A new magnetic field sensor is presented, based on perpendicular hot electron transport in a giant magnetoresistance (Co/Cu)4 multilayer, which serves as a base region of an n-silicon metal-base transistor structure. A 215% change in collector current is found in 500 Oe (77 K), with typical characteristics of the spin-valve effect. The in-plane magnetoresistance was only 3%. The transistor structure allows the investigation of energy resolved perpendicular transport properties, and in particular spin-dependent scattering of hot electrons in transition-metal as well as rare-earth-based multilayers.

Effect of the metal base structure on the impact ductility of malleable and high-tensile cast irons

1. An increase in the amount of the ferritic phase in cast irons factilitates an increase in impact ductility of both malleable and high-tensile irons. 2. The presence of a notch reduces the impact ductility of irons 3 to 5 times, the hightensile iron with globular graphite being more sensitive to the notch sharpness than the malleable cast iron. 3. Lower sensitivity to the notch sharpness of malleable cast iron is due to the presence of floccular inclusions of graphite which represent sufficiently sharp and effective stress concentrators and also to its lower silicon content.

Effect of metal-base structure on strength of chromium cast irons

Effect of metal-base structure on strength of chromium cast irons