Holes In Layer Research Articles

Three-dimensional pore-scale flow and mass transport through composite asymmetric membranes

A three-dimensional Lattice Boltzmann simulation method, is employed to investigate the pore-scale fluid flow and mass transfer in the composite asymmetric membranes. The membrane investigated is composed of three layers: a sponge-like porous layer, a finger-like hole layer, and a skin layer. The former two layers combined are called the porous support layer and they are reconstructed by a modified simulated annealing method. The permeability of the porous support layer is first studied. Then the mass transport in the composite membrane (with the skin layer) is modeled and the effective diffusivity is evaluated. A detailed description of the pore-scale vapor diffusion phenomenon in the membrane is presented. It is found that the mass transfer through the composite membrane can be enhanced by adjusting the membrane structural and chemical properties, but modifying the compositions of the skin layer is the most effective way in mass transfer intensification.

(SMEQ XXXI ECS Best Student Paper Award) Electrosyntesis of nNnostructured Poly- 4,7-di (thien-2’-yl)-2,1,3-benzothiadiazole (p-DTBT) on Gold Electrodes

The organic solar cell is a device capable to transform sunlight into electricity, these are composed of an anode, a cathode, an active layer, sometimes between the active layer and the electrodes, two layers of conductive buffer or holes and electrons are also used [1]. The active layer is made of two different materials an electron-acceptor and an electron-donor, in recent years, efforts to optimize the heterojunction have been reported using donor-acceeptor copolymers synthesized using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) as donor unit and benzothiadiazole (BT), thienopyrroledione (TPD), or thiazolothiazole (TTz) as acceptor units. These materials can be polymerized by electrochemical methods, nevertheless the deposits are irregular. Langmuir-blodgett technique (LB) allow make deposits highly structured, these can be used like a template in order to create substrate with controlled architecture [2]. In this work we use the LB technique in order to create deposits of pDTBT ordered. Gold electrodes of 1*2 cm were used as substrate, on these, five layers of silicon spheres were deposited. Cyclic voltammetry was using in order to deposited pDTBT. 2, 4, 6, 8, 10, 12, 14, 16 cycles were applied, after this the template was removed using HF 15%. Finally the deposits were analyzed by SEM In figure 1 a) shows voltammograms deposit, b) shows the SEM image of eight cycles of deposition. It is possible see the regular deposit of pDTBT (half pore). Figure 1

CDW-Exciton Condensate Competition and a Condensate Driven Force

We examine the competition between the charge-density wave (CDW) instability and the excitonic condensate (EC) in spatially separated layers of electrons and holes. The CDW and the EC order parameters (OPs), described by two different mechanisms and hence two different transition temperatures $T^{CDW}_c$ and $T^{EC}_c$, are self-consistently coupled by a microscopic mean field theory. We discuss the results in our model specifically focusing on the transition-metal dichalcogenides which are considered as the most typical examples of strongly coupled CDW/EC systems with atomic layer separations where the electronic energy scales are large with the critical temperatures in the range $T^{EC}_c \sim T^{CDW}_c \sim 100-200 K$. An important consequence of this is that the excitonic energy gap, hence the condensed free energy, vary with the layer separation resulting in a new type of force ${\cal F}_{EC}$. We discuss the possibility of this force as the possible driver of the structural lattice deformation observed in some TMDCs with a particular attention on the $ 1 {\it T}$-$TiSe_2$ below $200 K$.

Ka-Band Near-Field-Focused Array Antenna with Variable Focal Point

This study presents a Ka-band, multilayer, near-field-focused array antenna combined with the substrate integrated waveguide (SIW) technology. The metallic circular holes are designed as both phase shifters and radiating elements, which are fed by an SIW feeding network. A side-lobe level (SLL) of less than $-18\;{\rm dB}$ is achieved by properly arranging the dimensions of the metallic circular holes. Besides, by changing the thickness of the hole layer, the antenna with the same feeding network can be adjusted to change its focal point. As an example, an $8\times8$ near-field-focused antenna prototype operated at 35 GHz is fabricated and measured.

Design and simulation of high-breakdown-voltage GaN-based vertical field-effect transistor with interfacial charge engineering

A high-breakdown-voltage GaN-based vertical field-effect transistor with negative fixed interfacial charge engineering (GaN ICE-VHFET) is proposed in this work. The negative charge inverts an n-GaN buffer layer along the oxide/GaN interface, inducing a vertical hole layer. Thus, the entire buffer layer consists of a p+-hole inversion layer and an n-pillar buffer layer, and the p-pillar laterally depletes the n-GaN buffer layer, and the electric field distribution becomes more uniform. Simulation results show that the breakdown voltage of the GaN ICE-VHFET increases by 193% and the on-resistance of such a device is still very low when compared with those of conventional vertical FETs. Its figure of merit even exceeds the GaN one-dimensional limit.

Ideal p-n Diode Current Equation for Organic Heterojunction using a Buffer Layer: Derivation and Numerical Study

The equation of p-n diode current-voltage (J-V) of an organic heterojunction (HJ) including a hole and electron buffer layer is derived, and its characteristics are numerically simulated based on a polaron-pair model Giebink et al. (Forrest, Phys. Rev. B 82; 1–12, 2010). In particular, the correlation between a fraction of the potential drop for an electron/hole buffer (δ e − b /δ h − b ) and for a donor (D)/acceptor (A) (δ D /δ A ) is numerically investigated for J-V curves. As a result, the lowest diode current (DC) is obtained for the condition of δ e − b + δ A ≅ 0 or δ D + δ h − b ≅ 1. It is suggested that it is important to characterize the lowest DC curve for the state of D/A blending with a condition of a fraction of the potential drop (δ e − b /δ h − b ). Under these circumstances, the transport of holes (h +) from a DC source at the reverse bias is effectively limited.

Coulomb drag in topological insulator films

We study Coulomb drag between the top and bottom surfaces of topological insulator films. We derive a kinetic equation for the thin-film spin density matrix containing the full spin structure of the two-layer system, and analyze the electron–electron interaction in detail in order to recover all terms responsible for Coulomb drag. Focusing on typical topological insulator systems, with a film thicknesses d up to 6nm, we obtain numerical and approximate analytical results for the drag resistivity ρD and find that ρD is proportional to T2d−4na−3/2np−3/2 at low temperature T and low electron density na,p, with a denoting the active layer and p the passive layer. In addition, we compare ρD with graphene, identifying qualitative and quantitative differences, and we discuss the multi-valley case, ultra thin films and electron–hole layers.

Hybrid organic-inorganic light-emitting diodes based on a polythylenimine interlayer

Implementation of a polyethylenimine ethoxylated (PEIE) interlayer significantly enhances electron injection efficiency from ZnO into light emitting polymers including a blue light emitting polymer with relatively low electron affinity, verified by comparison of current density-voltage characteristics of the electron-only devices with or without a PEIE interlayer. Ultraviolet photoelectron spectroscopy measurements reveal that PEIE forms interfacial dipoles and reduces the work function of ZnO by ca. 1.2 eV, thus facilitating electron injection into light emitting polymers. Neutralized amines rather than protonated amines are responsible for the formation of interfacial dipoles as manifested by N1s core level spectra of the ZnO/PEIE sample. Luminance efficiency of hybrid organic-inorganic light emitting devices employing PEIE interlayer is similar to that of the analogous conventional devices with low work-function metal cathode due to electron injection promoting and hole/exciton-blocking properties of PEIE. Compared to Cs 2 CO 3 , PEIE, which possesses similar surface energy to the organic materials and can form homogeneous film, is expected to collaborate with the adjoining organic layers, without significantly interrupting their morphology, especially for those containing low molecular weight components such as phosphorescent emitters. We envisage that this valuable tribute enables the use of small molecule light emitting materials for device fabrication and is particularly beneficial for the effective introduction of phosphorescent emitters into the emission layer of organic-inorganic light emitting devices. Phosphorescent hybrid organic-inorganic light emitting devices with solution-processed 4,4',4''-tris(carbazol-9-yl)triphenyl-amine (TCTA): 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7): Ir-complex emission layer show the maximum luminance efficiency of 64 cd/A and external quantum efficiency of 19.1%, which represent the best efficiency for organic-inorganic light emitting devices reported so far and are approximately 4 times higher than those of the analoguous devices using Cs 2 CO 3 electron injection layer. The luminance efficiency of the devices is rather independent of Ir-complex concentration, indicating balanced hole and electron transport in the emission layer and broad emission zone due to the use of hole-transporting TCTA and electron-transporting OXD-7 as the co-hosts, together with the confinement of the emission zone inside the emission layer and homogeneous dispersion of Ir-compex into the TCTA: OXD-7 host. Furthermore, the efficiency roll-off at high luminance is rather small, for example, the luminance efficiency at 10000 cd/m 2 is 62 cd/A, maintaining 97% of the maximum luminance efficiency. Our findings enrich the organic materials for the preparation of organic-inorganic light emitting devices and pave the way for the future improvement of device efficiency and stability.

Water-Splitting for Solar Fuel Devices

Social environmental concerns about global warming and CO2 emissions provide the stimulus to find the way to replace fossil fuels by solar fuels. In this context, sustainable energetic schemes should be based on the conversion of abundant energy-poor molecules (such as H2O) into energy-rich molecules (as for example H2) using sunlight as the energy source. This solar fuel can be easily stored, transported and used upon demand in fuel cells, where the reverse reaction takes place to convert chemical energy into electricity. This is one of the key challenges of the twenty-first century and multidisciplinary work is needed to succeed in transforming the theory into practice.To achieve a practical technology, low-cost materials and earth-abundant catalysts are required to be used for both reactions (water splitting and H2 oxidation into water). In the present talk, we report the production of solar fuels with low-cost materials based on organic chemistry (3). The synthetic versatility of these materials and understanding of the energetic aspects provides the design rules to extract the generated photocurrent in the organic blend to the electrolyte solution in the scale of mA·cm-2. For technological application, the rational design of the harvesting light material (the organic blend), the interfacial selective layers (hole and electron selective layers) and the catalyst are equally important. Herein, the study of the electron selective layer and catalyst (noble-metal free) are researched towards optimising both H2 production rate and stability in these devices.

Numerical method to calculate the quantum transmission, resonance and eigenvalue energies: application to a biased multibarrier systems

A novel method to calculate the quantum transmission, resonance and eigenvalue energies forming the sub-bands structure of non-symmetrical, non-periodical semiconducting heterostructure potential has been proposed in this paper. The method can be applied on a multilayer system with varying thickness of the layer and effective mass of electrons and holes. Assuming an approximated effective mass and using Bastard's boundary conditions, Schrödinger equation at each media is solved and then using a confirmed recurrence method, the transmission and reflection coefficients and the energy quantification condition are expressed. They are simple combination of coupled equations. Schrödinger's equation solutions are Airy functions or plane waves, depending on the electrical potential energy slope. To illustrate the feasibility of the proposed method, the N barriers – (N−1) wells structure for N=3, 5, 8, 9, 17 and 35 are studied. All results show very good agreements with previously published results obtained from applying different methods on similar systems.

Conductance Along the Interface Formed by 400 °C Pure Boron Deposition on Silicon

The conductance along the interface between an as-deposited pure boron layer and n-type Si, fabricated at 400 °C, is found to be reliably formed with high-ohmic sheet resistance values going from $\sim 30$ k $\Omega $ /sq to 1 M $\Omega $ /sq, depending on the exact substrate doping and biasing. The temperature coefficient is negative. It is proposed that this behavior is due to an interface monolayer of electron-filled acceptor states attracting an inversion layer of holes that provide ideal p+n junction behavior and lateral carrier transport unperturbed by interface defects.

Highly efficient blue organic light-emitting diodes using various hole and electron confinement layers

In this Letter, blue phosphorescence organic light-emitting diodes (PHOLEDs) employ structures for electron and/or hole confinement; 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene is used as a hole confinement layer and tris-(phenylpyrazole)iridium [Ir(ppz)3] is utilized for an electron confinement layer (ECL). The electrical and optical properties of the fabricated blue PHOLEDs with various carrier-confinement structures are analyzed. Structures with a large energy offset between the carrier confinement and emitting layers enhance the charge-carrier balance in the emitting region, resulting from the effective carrier confinement. The maximum external quantum efficiency of the blue PHOLEDs with the double-ECLs is 24.02% at 1500 cd/m2 and its luminous efficiency is 43.76 cd/A, which is 70.47% improved compared to the device without a carrier-confinement layer.

Contactless measurement of alternating current conductance in quantum Hall structures

We report a procedure to determine the frequency-dependent conductance of quantum Hall structures in a broad frequency domain. The procedure is based on the combination of two known probeless methods—acoustic spectroscopy and microwave spectroscopy. By using the acoustic spectroscopy, we study the low-frequency attenuation and phase shift of a surface acoustic wave in a piezoelectric crystal in the vicinity of the electron (hole) layer. The electronic contribution is resolved using its dependence on a transverse magnetic field. At high frequencies, we study the attenuation of an electromagnetic wave in a coplanar waveguide. To quantitatively calibrate these data, we use the fact that in the quantum-Hall-effect regime the conductance at the maxima of its magnetic field dependence is determined by extended states. Therefore, it should be frequency independent in a broad frequency domain. The procedure is verified by studies of a well-characterized p-SiGe/Ge/SiGe heterostructure.

Collimator for Variable Sensitivity and Spatial Resolution Without the Need for Exchange

A new design of collimator is proposed that has variable sensitivity and spatial resolution, eliminating the need for exchanging collimators in a gamma camera. Using Monte Carlo simulations, the present article evaluates the shielding of undesirable gamma rays in a parallel-hole collimator. It consists of a number of layers of rectangular holes. These layers consist of alternately stacked fixed and movable collimators. In high-resolution mode, the movable collimators are shifted by half the aperture pitch along the diagonal direction. The first collimator (type A) has 50 layers with fixed thicknesses of 1.2 mm. The second collimator (type B) has 25 layers with a thickness of 1.0 mm on the object side and 25 layers with a thickness of 1.4 mm on the opposite side. The third collimator (type C) has 20 layers with non-uniform thicknesses. The ratios of the maximum artificial peak to the main-peak are calculated for point-source responses. The ratios for types A, B, and C collimators are 0.78, 0.08, and 0.03, respectively. The same performance for shielding undesirable gamma rays is achieved in the type C collimator as for a conventional collimator.

Design of a Surface-Plasmon-Resonance Sensor Based on a Microstructured Optical Fiber with Annular-Shaped Holes

A novel design is proposed for highly sensitive surface-plasmon-resonance sensors. The sensor is based on a microstructured optical fiber with two layers of annular-shaped holes. A gold layer is deposited on the inner surface of the second hole-layer, in which the holes have several micrometers thickness in size, facilitating analyte infiltration and metal layer deposition. In the first layer of holes, the sector-ring-shaped arms, used as supporting strips, are utilized to tune the resonance depth of the sensor. Numerical results indicate that the sensor operation wavelength can be tuned across the C+L-band. The spectral sensitivity of 1.0 · 104 nm · RIU−1 order of magnitude and a detection limit of 1.0 · 10−4 RIU order are demonstrated over a wide range of analyte refractive index from 1.320 to 1.335.

Gated Si nanowires for large thermoelectric power factors

We investigate the effect of electrostatic gating on the thermoelectric power factor of p-type Si nanowires (NWs) of up to 20 nm in diameter in the [100], [110], and [111] crystallographic transport orientations. We use atomistic tight-binding simulations for the calculation of the NW electronic structure, coupled to linearized Boltzmann transport equation for the calculation of the thermoelectric coefficients. We show that gated NW structures can provide ∼5× larger thermoelectric power factor compared to doped channels, attributed to their high hole phonon-limited mobility, as well as gating induced bandstructure modifications which further improve mobility. Despite the fact that gating shifts the charge carriers near the NW surface, surface roughness scattering is not strong enough to degrade the transport properties of the accumulated hole layer. The highest power factor is achieved for the [111] NW, followed by the [110], and finally by the [100] NW. As the NW diameter increases, the advantage of the gated channel is reduced. We show, however, that even at 20 nm diameters (the largest ones that we were able to simulate), a ∼3× higher power factor for gated channels is observed. Our simulations suggest that the advantage of gating could still be present in NWs with diameters of up to ∼40 nm.

Supermode resonances in a multi-core holey fiber-based sensor

The propagation characteristics in a new multi-core holey fiber-based sensor are investigated using a finite element method. The fiber is made by an air hole in the center of the structure, six air holes which are placed at the vertices of a hexagon, two layers of air holes arranged in a hexagonal way between which two large air hole on the horizontal line, four or seven small air holes which are infiltrated between some small air holes and are inserted in a silica core which is surrounded by a gold layer, and by a water layer which can be considered infinite for the numerical model. The four supplementary holes impede the resonant interaction (0.595 μm) between one of the pair of twofold degenerate supermode with a plasmon mode from the previously published sensor structure and introduces two new supermodes in resonance with the plasmon modes when the loss matching (0.597 μm and 0.599 μm) conditions are satisfied. By adding only four air holes and enlarge the radius of two existing hole produces a stronger transmission loss (856.2 dB/cm and 937.5 dB/cm) of a guided supermode at the resonant coupling due to efficient interaction with a plasmon mode near the loss matching points. The advantages of the configuration with seven small air holes where there is a resonance between two supermodes, are a better spectral resolution and higher amplitude sensitivity.

Electron-Hole Bilayer TFET: Experiments and Comments

We investigate Si/Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> fully depleted-SOI tunnel FET (TFET) devices operated in the electron-hole bilayer (EHB) mode. The application of negative bias on front gate and positive bias on back gate results in confined hole and electron layers that are expected to enable vertical band-to-band tunneling (BTBT). The idea of the EHB-TFET device is to enhance the tunneling current by expanding the BTBT generation area from the narrow lateral source/channel junction to the entire channel region. Our systematic measurements on a variety of TFETs with variable geometry and channel materials do not offer support to this attractive concept. Self-consistent simulations confirm that the vertical BTBT transitions do not produce an appreciable current in our devices, due to size-and bias-induced quantization, effective mass anisotropy, and incomplete formation of the bilayer. We examine the conditions for efficient vertical BTBT to occur and show that they cannot be met simultaneously, at least in Si or Si/SiGe devices.

The direct insulator-quantum Hall transition

The direct insulator-quantum Hall (I-QH) transition corresponds to a transition from an insulator to a high Landau-level-filling factor ν＞2QH state, which is characterized by an approximately temperature T-independent longitudinal resistance of a few nm thick electron (or hole) layer. In this paper, we review both the experimental and theoretical results on the direct I-QH transition. In particular, we attempt to address several interesting yet unsettled issues in the field of the direct I-Q transition. We suggest that further studies are required for obtaining a thorough understanding of the direct I-QH transition observed in nano-scale charge layers.

Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large-sized quantum dots.

Green CdSe@ZnS quantum dots (QDs) of 9.5 nm size with a composition gradient shell are first prepared by a single-step synthetic approach, and then 12.7 nm CdSe@ZnS/ZnS QDs, the largest among ZnS-shelled visible-emitting QDs available to date, are obtained through the overcoating of an additional 1.6 nm thick ZnS shell. Two QDs of CdSe@ZnS and CdSe@ZnS/ZnS are incorporated into the solution-processed hybrid QD-based light-emitting diode (QLED) structure, where the QD emissive layer (EML) is sandwiched by poly(9-vinlycarbazole) and ZnO nanoparticles as hole and electron-transport layers, respectively. We find that the presence of an additional ZnS shell makes a profound impact on device performances such as luminance and efficiencies. Compared to CdSe@ZnS QD-based devices the efficiencies of CdSe@ZnS/ZnS QD-based devices are overwhelmingly higher, specifically showing unprecedented values of peak current efficiency of 46.4 cd/A and external quantum efficiency of 12.6%. Such excellent results are likely attributable to a unique structure in CdSe@ZnS/ZnS QDs with a relatively thick ZnS outer shell as well as a well-positioned intermediate alloyed shell, enabling the effective suppression of nonradiative energy transfer between closely packed EML QDs and Auger recombination at charged QDs.