Confinement Characteristics Research Articles

Exploring Axial Organic Multiblock Heterostructure Nanowires: Advances in Molecular Design, Synthesis, and Functional Applications

AbstractOrganic multiblock heterostructure nanowires (OMHNs), integrating distinct components in the same axial structure, have attracted much attention due to their excellent properties such as intrinsic good light confinement and rectification characteristics. However, rationally incorporating such features into OMHNs with rigorous 1D morphology control remains a key synthetic objective. It is because that the single components possess fundamentally distinctive properties such as size‐dependent electronic, optical, and magnetic features as well as lattice parameters. In the last decade, researchers made their own efforts to this exciting field, especially from the controlled fabrication to the advanced applications of the multiblock nanowires. As a result, various OMHNs have been developed as very promising candidates for high‐performance optoelectronic applications such as light harvesting, logic gate devices, photonic transistors, and photonic anticounterfeiting. Herein, the recent advances of OMHNs from the aspects of molecular design, synthesis, and advanced applications are reviewed, and an outlook is given providing inspiration for the future development of OMHNs toward integrated optoelectronic at nanoscale.

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO3.

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO3 . Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 μm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO3 permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO3 are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.

Spacer Organic Cation Engineering for Quasi‐2D Metal Halide Perovskites and the Optoelectronic Application

Recently, organic and inorganic halide perovskites are one of the most popular semiconductors owing to their enormous potential for optoelectronic application, such as photovoltaics (PV), light‐emitting diode (LED), photodetector, and so on. The photoelectric conversion efficiency of 3D organic–inorganic hybrid perovskite solar cells has increased rapidly; however, the commercialization of the related devices is largely hampered by the poor stability of perovskite materials under environmental stress. Compared to 3D halide perovskites, quasi‐2D (Q‐2D) perovskites have improved moisture stability and less tendency for ion migration, which offers a new approach to stabilize perovskite‐based optoelectronic devices. Furthermore, Q‐2D hybrid perovskites have diverse structures with different quantum confinement and dielectric confinement characteristics, which enables the fine‐tuning of their optoelectronic properties through structure engineering. Depending on the different structures of spacer organic cations, the Q‐2D perovskite structure mainly includes Ruddlesden−Popper (RP) phase structure, Dion−Jacobson (DJ) phase structure, and alternating cation in the interlayer space (ACI) phase structure. In this review, the state‐of‐the‐art in Q‐2D perovskites is discussed based on their structural engineering, and an overview of PV and LEDs applications is provided. Finally, a brief outlook with respect to the development of Q‐2D perovskite materials, as well as advanced device, is provided.

The Limitation Analysis of Two Dimensional Magnetic Field Configuration in Annular Ion Thruster

In this paper, the limitations of two-dimensional magnetic field configuration for an annular ion thruster are analyzed theoretically and numerically. The two-dimensional magnetic field, which is commonly used in traditional ion thrusters, has been proved to be a very effective way for the confinement of primary electrons and ions. Hence, the confinement characteristics of primary electrons and ions in an annular ion thruster with the two-dimensional magnetic field configuration are first analyzed theoretically. Then, according to the analysis, a two-dimensional magnetic field for an annular ion thruster is designed. On this basis, the limitations of the two-dimensional magnetic field configuration are analyzed and discussed by a three-dimensional (3D) Particle-in-cell Monte Carlo collision (PIC-MCC) model. The results show that if the collision process is not considered, a uniform distribution of primary electrons can be achieved theoretically. However, when considering the collision process, the atom forms a significant impedance to the circumferential motion of primary electrons, thus greatly increasing the loss rate of primary electrons. Finally, it is found that, since primary electrons are not guided and constrained in the circumferential direction under the two-dimensional magnetic field, the discharge uniformity cannot be improved only by increasing the magnetic field intensity.

Two-Dimensional Titanium Dioxide-Surfactant Photoactive Supramolecular Networks: Synthesis, Properties, and Applications for the Conversion of Light Energy.

The desire to harness solar energy to address current global environmental problems led us to investigate two-dimensional (2D) core–shell hybrid photocatalysts in the form of a 2D-TiO2–surfactant, mainly composed of fatty acids. The bulk products, prepared by two slightly different methods, consist of stacked host–guest hybrid sheets held together by van der Waals forces between alkyl carboxylate moieties, favoring the synergistic conjugation of the photophysical properties of the core and the hydrophobicity of the self-assembled surfactant monolayer of the shell. X-ray diffraction and the vibrational characteristics of the products revealed the influence of synthesis strategies on two types of supramolecular aggregates that differ in the core chemical structure, guest conformers of alkyl surfactant tails and type, and the bilayer and monolayer of the structure of nanocomposites. The singular ability of the TiO2 core to anchor carboxylate leads to commensurate hybrids, in contrast to both layered clay and layered double-hydroxide-based ion exchangers which have been previously reported, making them potentially interesting for modeling the role of fatty acids and lipids in bio-systems. The optical properties and photocatalytic activity of the products, mainly in composites with smaller bandgap semiconductors, are qualitatively similar to those of nanostructured TiO2 but improve their photoresponse due to bandgap shifts and the extreme aspect-ratio characteristics of two-dimensional TiO2 confinement. These results could be seen as a proof-of-concept of the potential of these materials to create custom-designed 2D-TiO2–surfactant supramolecular photocatalysts.

Modeling analysis of the growth of a cubic crystal in a finite space.

The applications of semiconductor nanocrystals in optoelectronics are based on the unique characteristic of quantum confinement. There is great interest to tailor the performance of optoelectronic nanodevices and systems through the control of the sizes of nanocrystals. In this work, we develop a general mathematical formulation for the growth of a crystal/particle in a liquid solution, which takes account of the combinational effect of diffusion-limited growth and reaction-limited growth, and formulate the growth equations for the size of a cubic crystal grown under three different scenarios - isothermal and isochoric conditions, isothermal growth with the evaporation and/or extraction of the solvent and isochoric growth with continuous change in temperature. For the growth of a cubic crystal under isothermal and isochoric conditions, there are three growth stages - linear growth, nonlinear growth and plateau, and the growth rate in the stage of linear growth and the final size of the cubic crystal are dependent on the degree of supersaturation. For the growth of multi-crystals with a Gaussian distribution of crystal sizes, the change of the monomer concentration in a liquid solution is dependent on the change rates of average size and the standard deviation of the crystal sizes.

The Transfer Matrix Method and the Theory of Finite Periodic Systems. From Heterostructures to Superlattices

Historically, the physics, energy spectrum, and wave functions of superlattices have had different levels of understanding. Unlike the standard approaches, based on theorems for infinite systems, the theory of finite periodic systems (TFPS) constitutes a genuine quantum approach, with closed analytical results. In this theory the finite number of unit‐cells is an essential condition, while the number of propagating modes, as well as, the potential profiles (or refractive indices) are arbitrary. In this work, the transfer matrix definitions, symmetry properties, group representations, and relations with the scattering amplitudes are reviewed. The derivation of multichannel matrix polynomials (which reduce to Chebyshev polynomials in the one‐propagating mode limit), analytical formulas for resonant states, energy eigenvalues, eigenfunctions, parity symmetries, and discrete dispersion relations for superlattices with different confinement characteristics are reviewed. After showing the inconsistencies and limitations of hybrid approaches that combine the transfer‐matrix method with Floquet's theorem, some applications of the TFPS to multichannel negative resistance, ballistic transistors, coupled channels transport, spintronics, superluminal, and optical antimatter effects are revisited. Two high‐resolution superlattice related experiments are reviewed: the tunneling time in photonic band‐gap and the optical response of blue‐emitting diodes. It is shown that the TFPS accurately predicts the experimental results.

Extension of SAFT equation of state for fluids confined in nano-pores of sedimentary rocks using molecular dynamic simulation

Confined fluids in nano-mineral pores show notably different thermodynamic behavior compared with those of the same fluid in bulk. We have extended an equation of state (EoS) inspired by statistical association fluid theory (SAFT) to incorporate the fluid-mineral effect into the pure fluid properties confined in the nanopore. The established EoS is coupled with a Helmholtz free energy contribution which is a function of confinement characteristics: the potential of fluid-mineral interaction, bulk-like density and CF. The novel fitting parameter CF relies on the local properties of confined fluid and measures the density fraction within the layering and bulk-like zones, i.e., CF=ρlayeringzone/ρbulk-likezone. The first-principle molecular dynamics (MD) simulation was utilized to estimate the confinement parameters. At T = 300 K, 350 K, 400 K, and 450 K, the NVT ensemble was applied to simulate slit-pore calcite with a continuous and homogeneous surface in contact with water, with pore widths ranging from 1.5 nm to 5 nm. After running each NVT ensemble for 6 ns, the energy and density of confined water were analyzed, and a deviation was discovered with different behavior from what had previously been reported. Coupling CF parameter with proposed EoS improves the absolute relative errors (ARE) at low temperatures up about 50%, leading to a good agreement between modified SAFT EoS and MD results. The methodology described here may be extended to include a mixture of associating fluids such as those found in oil composition and implemented in all SAFT versions.

Unified model for fully and partially FRP confined circular and square concrete columns subjected to axial compression

Even though the usage of Fiber-reinforced polymer (FRP) full confinement arrangement is a more reliable and efficient strengthening technique than a partially confining strategy, it might not be cost-effective in real cases of strengthening. Experimental researches have demonstrated that confinement strengthening strategy is more effective for the case of circular columns compared to its application on square columns. This paper is dedicated to introducing a new unified model for determining the concrete confinement characteristics of FRP fully/partially confined circular/square concrete columns subjected to axial compressive loading. Through unification, the variations of the key parameters can be evaluated more-widely based on a unified mathematical framework. Consequently, it leads to the continuity in the predictions of FRP confinement-induced improvements for the different types of columns, contrary to those obtained from models only applicable to a specified cross-section or confining system. The substantial influence of non-homogenous concrete expansion distribution at the horizontal and vertical directions is taken into account in the determination of confinement pressure, besides arching action, by following the concept of confinement efficiency factor. Since the confinement-induced improvement is a function of its confining stress path, a new methodology is proposed to predict global axial stress–strain relation of FRP confined concrete columns considering confinement path effect, based on an extensive set of experimental results including 418 test specimens. The predictive performance of the developed model is assessed by simulating experimental tests reported in the literature.

Prediction of concentration of toxic gases produced by detonation of commercial explosives by thermochemical equilibrium calculations

An adverse effect resulting from explosive mine blasts is the production of toxic nitrogen oxides (NO and NO2) and carbon monoxide (CO). The empirical measurements of the concentration of toxic gases showed that it depends not only on the composition of an explosive and properties of its ingredients but also on several other parameters, such as volume of blasting chamber, explosive charge mass and design, confinement characteristics, surrounding atmosphere, etc. That explains why measured concentrations of toxic gases reported in literature significantly differ.In this paper, we discuss the possibility of theoretical prediction of the concentration of toxic gases by thermochemical equilibrium calculation applying two models: ideal detonation model and deflagration model. It can be demonstrated that thermochemical calculations can provide a good estimation of the measured concentrations and reproduce experimentally obtained effects of additives on the production of toxic gases. It was also found that the ideal detonation model applies to heavily confined explosive charges, while the deflagration model is more suitable for low detonation velocity explosives with light confinement.

Steady-state sustainment of divertor detachment with multi-species impurity seeding in LHD

Multi-species impurity seeding is an advanced operation scenario to mitigate the divertor heat load for the realization of future fusion reactors. In the large helical device, divertor detachment is successfully sustained using higher-Z (krypton, Kr) and lower-Z (neon, Ne) superimposed seeding. Emission from Kr impurities is drastically enhanced if it is followed by Ne seeding. Plasma radiation can be enhanced even at the upstream region in the edge plasma compared with Ne only seeded plasmas with suppression of impurity accumulation toward the central plasma. The characteristics of divertor heat load reduction and energy confinement are comparable between the Kr + Ne seeding and Ne only seeding under the same radiation fraction. However, while the detachment in Ne only seeding is transient, the detachment in Kr + Ne seeding is stable. It indicates that multi-species impurity seeding can be competitive for steady-state operation although further investigation is desired about the balance between divertor heat load reduction, impurity screening, and confinement degradation. The Kr emission enhancement is strongly affected by electron density and temperature at the last closed flux surface resulting in impurity penetration.

Confinement of MnOx@Fe2O3 core-shell catalyst with titania nanotubes: Enhanced N2 selectivity and SO2 tolerance in NH3- SCR process

Surface interface regulation is an important research content in the field of heterogeneous catalysis. To improve the interface interaction between the active component and matrix, tremendous efforts have been dedicated to tailoring the morphology, size, and structure of composite catalysts. In this work, we report a confinement strategy to synthesize a series of core-shell catalysts loaded with metal oxides on titania nanotubes (TNTs), which were applied to the selective catalytic reduction of NOx with ammonia. Interestingly, the core-shell catalyst with confinement of TNTs exhibited the remarkable activity at low temperature region, N2 selectivity and sulfur tolerance. Benefiting from the superior interfacial confinement characteristic of TNTs and Fe2O3, strong component interactions, the surface acid sites and strong oxidizability of MnOx were properly regulated, thus obtained the outstanding activity, N2 selectivity and provide chemical protection to effectively prevent SO2 poisoning. As far as the reaction mechanism, we found that the adsorption and reactivity of Lewis acid sites were the dominant factors affecting the activity in the NH3-SCR process by in situ DRIFT spectra. In general, our work provides an innovative strategy for constructing an TNTs-enwrapped nanocomposite with nano-confinement and core-shell structure to improve the low temperature SCR process.

Analysis of temporal carrier build‐up in reconfigurable field‐effect transistor

Based on the analysis of the carrier density change of a symmetric gate reconfigurable field-effect transistor that can operate p- or n-type transistors in an integrated circuit (IC), its unique limiting factor, carrier build-up time, is quantitatively derived for operating speed in addition to conventional resistance and capacitance (RC) and transit time effect. Originating from the characteristic of carrier confinement in a channel between two Schottky potential barriers, the carrier build-up time for the operation could take up to ∼1000 times longer than the transit time across the channel.

Producing Iron Endohedral Fullerene on Electron Cyclotron Resonance Ion Source

An electron cyclotron resonance (ECR) ion source (ECRIS) can generate an available amount of multicharged ions, thus it is not limited for use in the field of accelerator science, but also in medical/biological fields, such as for heavy ion beam cancer treatment and ion engines. The processes of generating multicharged ions are mainly sequential collisions of a direct ionization process by electrons, and have good ion confinement characteristics. By utilizing this confinement property, we have synthesized iron-encapsulated fullerenes, which are supramolecular and can be expected to have various high functions. Fullerenes and iron ions are vaporized from pure solid materials and introduced into the ECRIS together with the support gas. We investigated conditions under which fullerene ions do not dissociate and iron ions are generated so that both can coexist. Generated ions are extracted from the ECRIS and separated by mass/charge with a dipole magnet, and detected with a Faraday cup. This measurement system is characterized by a wide dynamic range. The charge-state distribution (CSD) of ion currents was measured to investigate the optimum conditions for supramolecular synthesis. As a result, a significant spectrum suggesting the possibility of iron-encapsulated fullerenes was obtained. This paper describes the details of these experimental results.

Amplified sensing of optical phase difference through the phase of the resultant field

A generalized scheme to enhance the sensitivity in the measurement of phase difference in an optical interference by measuring the phase of the resultant field is presented. In the proposed scheme, the weak measurement in scalar optical interferometry is achieved by directly accessing the phase of the output state, rather than looking at the centroid shift in the intensity. The spatial confinement or beam-like characteristics for the optical field, an essential criterion for centroid calculation, is not a requirement for the scheme. The ability to uniquely sense and distinguish contributions to the phase difference from individual optical fields; an aspect with potential to improve the measurement accuracy in optical gyroscopes and gravitational wave sensing, is also presented.

Diverted negative triangularity plasmas on DIII-D: the benefit of high confinement without the liability of an edge pedestal

Diverted discharges at negative triangularity on the DIII-D tokamak sustain normalized confinement and pressure levels typical of standard H-mode scenarios (H 98y2 ≃ 1, β N ≃ 3) without developing an edge pressure pedestal, despite the auxiliary power far exceeding the L → H power threshold expected from conventional scaling laws. The power degradation of confinement is substantially weaker than the ITER-89P scaling, resulting in a confinement factor that improves with increasing auxiliary power. The absence of the edge pedestal is beneficial in several ways, such as eliminating the need for active mitigation or suppression of edge localized modes, low impurity retention and a reconstructed scrape-off layer heat flux width at the mid-plane that exceeds the ITPA multi-machine scaling law by up to 50%. Together with technological advantages granted by placing the divertor at larger radii, plasmas at negative triangularity without an edge pedestal feature both core confinement and power handling characteristics that are potentially suitable for operation in future fusion reactors.

Investigation on the Gas Jet Flow Performance Confined in Round Pipe

In this study, the round jet flow behavior arose by the confinement was investigated experimentally and numerically. The confinement characteristics based on multi-scale characterizations in the atmospheric gas jet flow, which generated from a circular symmetrical subsonic nozzle and flowed into a confined round pipe, was used to studied. The studied region near the nozzle possess really short axial length (within 12d), providing initial conditions and boundaries that affected the flow behaviors in this region. A wide range of inlet velocities (0.98 m/s~84.72 m/s) and confined space sizes (1~20) were involved for simulated and quantitative discussions. The velocity profile evolution was recorded by simulations and characterized by a laser Doppler anemometer (LDA) with a resolution of 0.01 m/s. The confinement characteristics were systematically presented to elucidate the performance under the studied confined conditions with different metrics, such as centerline velocity decay (VR), entrainment rate (MR), pressure coefficient (Cp) and length of recirculation region (LR). The results indicated that the recirculation fluid action mainly contributed to the promoted initial velocity profile evolution with the introduction of the confined space. Theoretically, the confined space could reduce the maximum absolute entrainment mass flux, initiating a peak at the center of the recirculation region, which was controlled by the inlet velocity and the confined space size collectively. The remarkable effect of confined space size further contributes to the confinement characteristics of gas jet flow, representing a shortened flow distance despite similar process as that by reducing the diameter ratio (dR). In some cases, because of the miniaturization of confined space size, the dispersion of initial velocity profile evolution was not significant displayed throughout the variable inlet velocity range, especially when the dR was less than 2. This study gives a new insight in the performance of jet flow confined in round pipe, and such knowledge will be helpful to provide great potential and reference to applied fluid mechanics.

Tunable surface waves supported by graphene-covered left-handed material structures

The active tuning of surface waves supported by left-handed materials (LHMs) is a major challenge for their practical applications and usability, and graphene has been proposed as an agent for the active tuning of surface waves. In this study, tunable surface waves supported by graphene-layered LHM structures are investigated theoretically. The analytical and numerical results are computed for transverse electric (TE) and transverse magnetic (TM)-polarized surface waves. According to the spectral range, two configurations of LHMs (i.e., gigahertz (GHz)-LHM and terahertz (THz)-LHM) are discussed. The physical modeling of graphene is done using the conductivity model based on random phase approximation-based Kubo formalism. The split ring resonator (SRR) model and Kramers–Kronig relations-based causality principle are used for the physical realization of LHMs. To simulate the graphene-layered LHM interface, impedance boundary conditions (IBCs) are applied. Numerical results are computed for dispersion curves, effective wave numbers, and field profiles of surface waves supported by graphene-covered GHz-LHM and THz-LHM. The influence of graphene parameters on the resonance frequency, dispersion curves, and confinement characteristics of surface waves is analyzed for both configurations of LHMs. It is concluded that the graphene-covered GHz-LHM structure supports the tunable surface waves as surface plasmon polaritons (SPPs) support TM-polarized waves. However, with the TE-polarized surface wave, the structure supports the surface polaritons, which do not depend upon the graphene parameters. For the case of the graphene-covered THz-LHM structure, it is concluded that the structure supports the plasmon modes for both polarized waves and the resonance frequency ranges from THz to infrared (IR). The confinement of the TE-polarized surface waves on the graphene layer can be enhanced up to fourfold as compared to the suspended monolayer graphene by using a THz-LHM substrate. Further, the numerical results are found to be consistent with the literature under special conditions. The proposed graphene-layered LHM structure may have potential uses in the active tuning of surface waves, near-field communication and imaging devices, surface waveguide design, and LHM-based metasurfaces.

Confinement characteristic of primary electrons with the variation of channel width in the discharge chamber of annular ion thruster

In this paper, the confinement characteristic of primary electrons in the non-axisymmetric discharge chamber of annular ion thruster is investigated by a three-dimensional (3D) non-self-consistent particle tracking model with Monte Carlo Collision (MCC) method. The results show that: The density of primary electrons upstream of ion optics on cathode axis is about 5–50 times higher than that of the other side, which means that the density of primary electrons is obviously non-axisymmetric. The channel width has a significant effect on the average density and uniformity of primary electrons. The average density can be increased by nearly 1.5 times under the appropriate channel width, meanwhile, the variance of density distribution can be reduced by more than 2 times. This is because that increasing the channel width can improve the average confinement length of primary electrons greatly. Furthermore, there is an infection point in the increase of primary electron average confinement length with the channel width, which caused by the significant change of magnetic field structure. Under the case after the inflection point, primary electron mainly (more than 50%) moves to the channel center, which makes the average confinement length of primary electrons and their number in the discharge chamber increase largely.

Probing embedded topological modes in bulk-like GeTe-Sb2Te3 heterostructures

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and hbox {Sb}_2hbox {Te}_3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb_2Te_3 band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6, 27716 (2016)] where ETSs were seen to couple over thick layers.