Small Confinement Research Articles

Thermophotovoltaic power conversion from a heat-recirculating micro-emitter

Power generation using a novel configuration of a 1–10W micro-thermophotovoltaic (micro-TPV) device is studied experimentally. A micro-emitter as a thermal heat source is a simple cylinder with an annular-type shield that applies a heat-recirculation concept and an expanded exhaust outlet that facilitates ignition, which provides stable burning in the small confinement and uniform distribution of temperature along the wall. The micro-emitter is surrounded by a chamber with cooling fins, the inner wall of which has an installation of gallium antimonide photovoltaic cells (PVCs). The performance of the micro-TPV device is most favorable at reduced length of the cooling fins unless the temperature on the PVCs is higher than the operating limit temperature for the GaSb cells. The relative position of the micro-emitter to the PVCs also affects the performance of the micro-TPV device. These observations imply that the net amount of heat irradiation onto the PVCs is more dominant than the PVC temperature in determining the TPV performance. Under optimized operating conditions, the micro-TPV device produces 2.4 Wwith an overall efficiency of 2.1%, indicating that the efficiency up to the PVC surface is 21% at least if a PVC efficiency of 10% at most is assumed.

Comparison of dynamic adsorption/desorption characteristics of toluene on different porous materials

Four different types of adsorbents, SBA-15, MCM-41, NaY and SiO2, were used to study the dynamic adsorption/desorption of toluene. To further investigate the influence of pore structure on its adsorption performance, two SBA-15 samples with different microspores were also selected. It is shown that microporous material NaY has the largest adsorption capacity of 0.2873 mL/g, and the amorphous SiO2 exhibits the least capacity of 0.1003 mL/g. MCM-41 also shows a lower break through capacity in spite of the relatively small pore diameter, because it can not provide the necessary small geometric confinement for the tiny adsorbates. However, the mesoporous SBA-15 silica with certain micropore volume shows relatively higher adsorption capacity than that of MCM-41 silica. The presence of micropores directly leads to an increase in the dynamic adsorption capacity of toluene. Although NaY has the highest adsorption capacity for toluene, its complete desorption temperature for toluene is high (< 350°C), which limits its wide application. On the contrary, mesoporous silica materials exhibits a good desorption performance for volatile organic compounds at lower temperatures. Among these materials mesoporous SBA-15 samples, with a larger amount micropores and a lower desorption temperature, are a potentially interesting adsorbent for the removal of volatile organic compounds. This behavior should been related with the best synergetic effect of mesopores and micropores.

Modification of Optical Properties of Porous AIIIBV Layers Produced by Anodic Etching

Morphology investigations (atomic force microscopy (AFM) and scanning electron microscopy (SEM)), study of Raman scattering (RS) and photoluminescence (PL) have been performed to characterize a series of AIIIBV materials (GaAs, GaP, InP) with an electrochemically prepared porous surface layer. It has been shown that the surface morphology of porous AIIIBV compounds strongly depends on various parameters of the anodizationprocess such as the etching time, current density, composition of etching solution, and illumination during the etching procedure. The enhancement of a Raman signal from porous surfaces, which has been observed for almost all samples, is caused mainly by the breaking of selection rules for corresponding phonon modes and a decrease of the reflection at the porous surface. The peculiarities of the PL spectra of porous AIIIBV compounds are studied in a wide temperature range. The small quantum confinement effect has been observed for GaAs and InP porous surfaces.

Water diffusion inside carbon nanotubes: mutual effects of surface and confinement

The mutual effects of two crucial features of carbon nanotubes (CNTs) (surface and confinement) on the temperature-dependent water diffusion are studied through molecular dynamics simulations. A two-stage diffusion mechanism is detected in the CNTs of diameter smaller than 12.2 Å, which becomes obscure as the temperature increases. This peculiar phenomenon can be ascribed to the cooperation of the small confinement and the periodic surface. The diffusion coefficient of the confined water exhibits a nonmonotonic dependence on the confinement size and an unexpected increase inside the large CNTs (compared to that of bulk water). These anomalous behaviors can be attributed to the competition of the smooth surface and the small confinement. Considering the mutual effects, an empirical formula is proposed on the basis of two groups of numerical examples, whose results indicate that the confinement effect will dominate over the surface effect until the CNT diameter increases up to ~16 Å, whereas thereafter the surface effect becomes dominant and finally both of them vanish gradually.

A Novel, Wafer-Level Method to Fabricate Zero-Mode Waveguides for Single Molecule Detection

Zero-mode waveguides (ZMWs) are hollow, metal apertures that are substantially smaller than the wavelength of light typically used in fluorescence spectroscopy. Consequently, ZMWs restrict excitation illumination to sub-diffraction limited (zeptoliter-scale) volumes. Such small confinement volumes enable single molecule detection at micromolar concentrations of fluorescently-labeled target molecules, and hence offer a powerful tool for single molecule observations under near-physiological conditions. The primary limitation to the widespread use of ZMWs in modern biological laboratories is that well-defined nanoapertures currently require expensive fabrication methods to produce. Here, we present a novel method to create ZMWs that employs conventional, low-cost, wafer-level fabrication techniques. We first use conventional photolithography to pattern larger, microapertures (800-1000 nm diameter) into a gold film covering a glass microscope slide. The microapertures are then subjected to metal electrodeposition, where the metal-of-interest is plated onto all accessible metal surfaces. Consequently, the diameter of the nanoaperture array can be controlled by simply varying the electrodeposition time. As a proof-of-principle demonstration of the fabrication method, we fabricated waveguide arrays (300 μm x 300 μm) onto standard microscope cover glasses with diameters ranging from 70±20 nm to 1000±17 nm (mean±std, as determined by SEM). To determine the attenuation of the excitation light within the ZMWs we characterized the fluorescence emission as a function of diameter (D) and found that it is proportional to D3.4 in the zero-mode regime (where no modes can propagate), D2.35 in the transition regime (modes increasingly propagate as a function of diameter), and D∼2 in the super-wavelength regime (all modes exist). In addition, we have directly observed the diffusion of single molecules at a solution concentration of 1 μM. These results demonstrate that our fabrication method can produce usable waveguide geometries using low-cost processing techniques.

Quality Factor of a Nanobowtie Antenna

We present a rigorous formulation based on the Poynting's theorem in dispersive and lossy media to calculate the quality factor, material loss, radiation loss, and radiation pattern of an optical nanobowtie antenna using the finite-difference time-domain method. The quality factor obtained from our theory, 8.73, agrees well with the experimental data. Our results show that the radiation loss and material loss are comparable. The material loss mainly originates from the chromium rather than gold even though the nanobowtie antenna mostly consists of gold. In addition, due to the small gap and plasmonic mode confinement, the effective modal volume is ultrasmall [6.36 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> (λ/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ], which significantly breaks the diffraction limit.

Particle removal from a surface by a bounded vortex flow

A bounded vortex flow consists of an axisymmetric vortex that is confined top and bottom between two plates (the “confinement plate” and “impingement plate”, respectively) and surrounded laterally by a swirling annular slot jet. The bottom of the vortex terminates on the boundary layer along the impingement plate and the top of the vortex is drawn into a suction port positioned at the center of the confinement plate. The circumferential flow within the annular jet is important for supplying circulation to the central wall-normal vortex. This flow field is proposed as a method for mitigation of dust build-up on a surface, where the vortex–jet combination supplements the more traditional vacuum port by enhancing the surface shear stress and related particle transport rate. The paper reports on a computational study of the velocity field and particle transport by a bounded vortex flow. Fluid flow computations are performed using a finite-volume approach for an incompressible fluid and particle transport is simulated using a discrete-element method. Computations are performed for different values of two dimensionless parameters – the ratio of the plate separation distance and the average radial location of the jet inlet (the dimensionless confinement height) and the ratio of flow rate withdrawn at the suction outlet and that injected by the jet (the flow rate ratio). For small values of the flow rate ratio, the impinging jet streamlines pass down to the boundary layer along the bottom surface and then travel up the vortex core. By contrast, for large values of flow rate ratio, the annular jet is quickly entrained into the suction outlet and no wall-normal vortex is formed. Particles are observed to roll along the impingement surface in a direction determined by the fluid shear stress lines. Particles roll outward when they lie beyond a separatrix curve of the surface shear stress lines, where particles within this separatrix curve roll inward, piling up at the center of the flow field. A toroidal vortex ring forms for the small confinement height case with flow rate ratio equal to unity, which yields double separatrix curves in the shear stress lines. The inward rolling particles intermittently lift up due to collision forces and burst away from the impingement surface, eventually to become entrained into the flow out the suction port or resettling back onto the impingement surface.

Quantum confinement in PbI2 nanodisks prepared with cucurbit[7]uril

This work presents an alternative route for the preparation of heavy metal iodide nanoparticles, particularly lead iodide nanodisks (ca. 50-340 × 7 Å, diameter × thickness), using the macrocycle cucurbit[7]uril as a synthetic template and stabilizing agent. These nanoparticles exhibit an optical-gap blue shift consistent with their small size and 1D quantum confinement. Their thicknesses are compatible with an exfoliated single layer of lead iodide, indicating that cucurbit[7]uril, preventing the stacking and formation of tactoids, thus limiting nanoparticles growth in the z direction. The structure, morphology and properties of these disks were analyzed by X-ray powder diffractometry (XRD), UV-Visible spectroscopy, atomic force microscopy (AFM), scanning electron microscopy with analysis of energy dispersive X-ray fluorescence (SEM-EDS) and high resolution transmission electron microscopy (HRTEM).

Ground State Instability in Spin Polarization for Electrons Confined in Two-Dimensional Square Quantum Dots

We present a theoretical study of the ground state electronic structure and the spin polarization for four electrons confined in two-dimensional (2D) square quantum dots (SQDs). We employ standard mean field theory (MFT) approaches using the unrestricted Hartree–Fock (UHF) and density functional theory calculations. The resonant UHF configuration interaction (res-UHF CI) calculation was also performed in order to incorporate the electron correlation more intuitively. The MFT ground state is expected to be spin-polarized when SQDs have a small confinement length L or aspect ratio δ= Lx/Ly = 1, in agreement with Hund's rule. In contrast, the spin-unpolarized ground state singlet is expected in all in other SQDs. Thus, the MFT calculations produce the anti-Hund state, where the spin-density wave forms having the zero of the total spin, even though the SQD has the point group symmetry D4h. However, the res-UHF CI calculation restores the geometrical symmetry in the resulting ground state when the Coulomb interaction is strengthened. Nevertheless, the res-UHF CI ground state maintains the zero total spin. Thus, ground state instability is expected in the spin-polarization of the SQD system, which eventually violates Hund's rule in accordance with the Coulomb interaction.

The Effect of the 17th Amendment on the Party Composition of the Senate: A Counterfactual Analysis

The 17th Amendment is perhaps the most significant change in the American electoral system brought about through amending the Constitution. It changed the theory about who senators represented, shifting the focus from state governments, per se, to the residents of states. Mechanically, it shifted the electorate from the relatively small confines of state legislatures to much larger mass electorates. Lest we lose track of its significance, many supporters of the Tea Party movement blame the 17th Amendment for causing “a failure in the federalist structure, federal deficit spending, inappropriate federal mandates, and federal control over a number of state institutions.” The respectability of this position was demonstrated when a half dozen Republican candidates for the U.S. Senate in 2010 expressed support for repeal, including victorious candidates in Florida, Colorado, and Utah. Leaving aside the question of whether the 17th Amendment led to the destruction of federalism and state institutions, the fact that it so significantly changed the electoral system for Senate candidates naturally raises the question of how candidates, victorious senators, and the Senate itself changed because of the amendment’s ratification. This is not a new thought, as is evidenced by the small-but-vigorous literature on the subject (King and Ellis 1996; Crook and Hibbing 1997; Wirls 1999; Ellis and King 1999; Engstrom and Kernell 2003; Rogers 2009; Gailmard and Jenkins 2009).

The effect of microcavity laser recombination lifetime on microwave bandwidth and eye-diagram signal integrity

Vertical microcavity surface-emitting lasers employing quantum wells and small aperture buried-oxide current and field confinement are demonstrated with wider mode spacing and faster spontaneous carrier recombination (enhanced Purcell factor), lower threshold current, larger side mode suppression ratio (SMSR), and higher photon density and temperature insensitivity. The result is a microcavity laser that achieves higher microwave modulation bandwidth (f−3dB = 15.8 GHz) at ultra-low power consumption (1.5 mW) with a slope for the modulation current efficiency factor (MCEF) = 17.47 GHz/mA−1/2, as well as a better quality eye diagram in high-speed data transmission. The microwave behavior model for the microcavity laser is used to estimate the enhanced recombination and reduced lifetime.

Dynamics of ultracold molecules in confined geometry and electric field

We present a time-independent quantum formalism to describe the dynamics of molecules with permanent electric dipole moments in a two-dimensional confined geometry such as a one-dimensional optical lattice, in the presence of an electric field. Bose/Fermi statistics and selection rules play a crucial role in the dynamics. As examples, we compare the dynamics of confined fermionic and bosonic polar KRb molecules under different confinements and electric fields. We show how chemical reactions can be suppressed, either by a "statistical suppression" which applies for fermions at small electric fields and confinements, or by a "potential energy suppression", which applies for both fermions and bosons at high electric fields and confinements. We also explore collisions that transfer molecules from one state of the confining potential to another. Although these collisions can be significant, we show that they do not play a role in the loss of the total number of molecules in the gas.

Numerical simulations of particle migration in a viscoelastic fluid subjected to Poiseuille flow

In this work we present 2D numerical simulations on the migration of a particle suspended in a viscoelastic fluid under Poiseuille flow. A Giesekus model is chosen as constitutive equation of the suspending liquid. In order to study the sole effect of the fluid viscoelasticity, both fluid and particle inertia are neglected. The governing equations are solved through the finite element method with proper stabilization techniques to get convergent solutions at relatively large flow rates. An Arbitrary Lagrangian–Eulerian (ALE) formulation is adopted to manage the particle motion. The mesh grid is moved along the flow so as to limit particle motion only in the gradient direction to substantially reduce mesh distortion and remeshing. Viscoelasticity of the suspending fluid induces particle cross-streamline migration. Both large Deborah number and shear thinning speed up the migration velocity. When the particle is small compared to the gap (small confinement), the particle migrates towards the channel centerline or the wall depending on its initial position. Above a critical confinement (large particles), the channel centerline is no longer attracting, and the particle is predicted to migrate towards the closest wall when its initial position is not on the channel centerline. As the particle approaches the wall, the translational velocity in the flow direction is found to become equal to the linear velocity corresponding to the rolling motion over the wall without slip.

An electrostatic autoresonant ion trap mass spectrometer

A new method for ion extraction from an anharmonic electrostatic trap is introduced. Anharmonicity is a common feature of electrostatic traps which can be used for small scale spatial confinement of ions, and this feature is also necessary for autoresonant ion extraction. With the aid of ion trajectory simulations, novel autoresonant trap mass spectrometers (ART-MSs) have been designed based on these very simple principles. A mass resolution approximately 60 is demonstrated for the prototypes discussed here. We report also on the pressure dependencies, and the (mV) rf field strength dependencies of the ART-MS sensitivity. Importantly the new MS designs do not require heavy magnets, tight manufacturing tolerances, introduction of buffer gases, high power rf sources, nor complicated electronics. The designs described here are very inexpensive to implement relative to other instruments, and can be easily miniaturized. Possible applications are discussed.

A model of yeast cell‐cycle regulation based on multisite phosphorylation

In order for the cell's genome to be passed intact from one generation to the next, the events of the cell cycle (DNA replication, mitosis, cell division) must be executed in the correct order, despite the considerable molecular noise inherent in any protein-based regulatory system residing in the small confines of a eukaryotic cell. To assess the effects of molecular fluctuations on cell-cycle progression in budding yeast cells, we have constructed a new model of the regulation of Cln- and Clb-dependent kinases, based on multisite phosphorylation of their target proteins and on positive and negative feedback loops involving the kinases themselves. To account for the significant role of noise in the transcription and translation steps of gene expression, the model includes mRNAs as well as proteins. The model equations are simulated deterministically and stochastically to reveal the bistable switching behavior on which proper cell-cycle progression depends and to show that this behavior is robust to the level of molecular noise expected in yeast-sized cells (approximately 50 fL volume). The model gives a quantitatively accurate account of the variability observed in the G1-S transition in budding yeast, which is governed by an underlying sizer+timer control system.

Vibrational Dynamics of Water within Mesoporous Materials at Different Hydration Levels during Adsorption and Desorption Processes

The vibrational dynamics of water within Vycor 7930 under different hydration levels and adsorption and desorption processes has been investigated by Raman measurements. Based on the successfully depressing the background fluorescence of Vycor, a new Gaussian peaking at 3590 cm−1 has been observed in the Raman spectrum of confined water. By comparison with the published Raman data of water within fully filled silica-based pores or reverse micelles, it is confirmed that a small confinement size may conceal the characteristic effect of hydrophilic interface. In addition, a density hysteresis observed within a filling fraction (ϕ) range of 0.25 < ϕ < 0.5 indicates that the cavitation model should be previously considered to explain the adsorption hysteresis of the water/Vycor system.

Noise Concerns and Remedies in VXI and PXI Systems

For optimum performance, a modern, high performance, synthetic instrument must be designed to keep RF energy that is outside the module from leaking in-and not let any of its energy leak out into other modules. Every detail must be addressed. DC power lines, signal lines, air cooling holes, and conventional RF shielding are all important design considerations in the small confines of an SI chassis. Faster digital logic and amplifiers with excessively high gain-bandwidth products increase the design challenge of fielding a clean, high performance synthetic instrument. Each block of an Si-based system connects to another element or subsystem. Removing the mechanisms for spurs to propagate from one module to another is as important as removing the original source of the spur.

Polymeric nanomedicine for cancer MR imaging and drug delivery.

Multifunctional nanomedicine is emerging as a highly integrated platform that allows for molecular diagnosis, targeted drug delivery, and simultaneous monitoring and treatment of cancer. Advances in polymer and materials science are critical for the successful development of these multi-component nanocomposites in one particulate system with such a small size confinement (<200 nm). Currently, several nanoscopic therapeutic and diagnostic systems have been translated into clinical practice. In this feature article, we will provide an up-to-date review on the development and biomedical applications of nanocomposite materials for cancer diagnosis and therapy. An overview of each functional component, i.e. polymer carriers, MR imaging agents, and therapeutic drugs, will be presented. Integration of different functional components will be illustrated in several highlighted examples to demonstrate the synergy of the multifunctional nanomedicine design.

Octagonal photonic crystal fiber of birefringence

In this paper we propose a novel octagonal birefringent photonic crystal fiber，which shows birefringence when the size of several holes near the core is enlarged. The modal field，dispersion，confinement loss，birefringence of the fiber fundamental mode and the influence on birefringence of some parameters are simulated by multipole method. The dispersion，confinement loss，birefringence of the hexagonal photonic crystal fiber having the same parameters were also computed. By comparision，we conclude that the octagonal photonic crystal fiber have larger birefringence，smaller confinement loss and shorter wavelength of zero dispersion than the hexagonal photonic crystal fiber with the same parameters.

A simple classical model for predicting onset crystallization temperatures on curved substrates and its implications for phase transitions in confined volumes

For small confinement volumes, phase transition temperatures are determined by the scarcity of the crystallizing material, rather than the magnitude of the energy barrier, as the supply of molecules undergoing the phase transition can be depleted before a stable nucleus is attained. We show this for the case of crystallization from the melt and from the solution by using a simple model based on an extended classical nucleation theory. This has important implications because it enables a simple and direct measurement of the critical nucleus size in crystallization. It also highlights that predicting the observable melting points of nanoparticles by using the Gibbs-Thomson equation can lead to substantial errors.