Multiple Energy Levels Research Articles

Local charge neutrality condition, Fermi level and majority carrier density of a semiconductor with multiple localized multi-level intrinsic/impurity defects

For semiconductors with localized intrinsic/impurity defects, intentionally doped or unintentionally incorporated, that have multiple transition energy levels among charge states, the general formulation of the local charge neutrality condition is given for the determination of the Fermi level and the majority carrier density. A graphical method is used to illustrate the solution of the problem. Relations among the transition energy levels of the multi-level defect are derived using the graphical method. Numerical examples are given for p-doping of the CdTe thin film used in solar panels and semi-insulating Si to illustrate the relevance and importance of the issues discussed in this work.

A Randomized, Single-Blind, Postmarketing Study of Multiple Energy Levels of High-Intensity Focused Ultrasound for Noninvasive Body Sculpting

High-intensity focused ultrasound (HIFU) is a nonsurgical, noninvasive body sculpting method. To investigate preferences for treatment settings using a HIFU device. HIFU was applied to the anterior abdomen in three passes of decreasing depth (1.6, 1.3, and 1.1 cm) in patients randomized to HIFU energy levels (each of 3 passes [total]) of 47 (141), 52 (156), or 59 (177) J/cm(2). The primary assessment was week 12 post-treatment change from baseline waist circumference at the level of the iliac crest for all treatment groups combined. The primary assessment achieved statistical significance (least squares mean 2.51 cm, 95% confidence interval [CI] = -3.14 to -1.88; p < .001), with no significant differences between groups. At week 12, 69% to 86% of patients and 73% to 79% of investigators rated appearance as improved or much improved. The average worst pain (100-mm visual analog scale) experienced during treatment was mild (47 J/cm(2): 17.1 mm, 95% CI = 4.33-29.81 mm; 52 J/cm(2): 24.6 mm, 95% CI = 12.24-36.95 mm; 59 J/cm(2): 30.9 mm, 95% CI = 18.71-43.17 mm). There were no serious adverse events. HIFU treatment at different energy levels and multiple tissue depths was well tolerated and effective in reducing waist circumference.

Carrier density and compensation in semiconductors with multiple dopants and multiple transition energy levels: Case of Cu impurities in CdTe

Doping is one of the most important issues in semiconductor physics. In many cases, when people describe carrier concentration as a function of dopant density and Fermi energy, they usually assume only one type of dopant with single transition energy level in the system. However, in reality, the situation is often more complicated, that is, in a semiconductor device, it usually contains multidopants and each can have multitransition energy levels. In this paper, using detailed balance theory and first-principles calculated defect formation energies and transition energy levels, we derive formulas to calculate carrier density for semiconductor with multidopants and multitransition energy levels. As an example, we studied CdTe doped with Cu, in which ${\mathrm{V}}_{\mathrm{Cd}}$, ${\mathrm{Cu}}_{\mathrm{Cd}}$, and ${\mathrm{Cu}}_{\mathrm{i}}$ are the dominant defects/impurities. We show that in this system, when Cu concentration increases, the doping properties of the system can change from a poor p-type, to a poorer p-type, to a better p-type, and then to poor p-type again, in good agreement with experimental observations.

Dual- and multi-energy CT: approach to functional imaging

The energy spectrum of X-ray photons after passage through an absorber contains information about its elemental composition. Thus, tissue characterisation becomes feasible provided that absorption characteristics can be measured or differentiated. Dual-energy CT uses two X-ray spectra enabling material differentiation by analysing material-dependent photo-electric and Compton effects. Elemental concentrations can thereby be determined using three-material decomposition algorithms. In comparison to dual-energy CT used in clinical practice, recently developed energy-sensitive photon-counting detectors sample the material-specific attenuation curves at multiple energy levels and within narrow energy bands; the latter allows the detection of element-specific, k-edge discontinuities of the photo-electric cross section. Multi-energy CT imaging therefore is able to concurrently identify multiple materials with increased accuracy. These specific data on material distribution provide information beyond morphological CT, and approach functional imaging. This article reviews the principles of dual- and multi-energy CT imaging, hardware approaches and clinical applications.

Plasmon-induced modulation of the emission spectra of the fluorescent molecules near gold nanorods

Both the excitation and emission processes of a fluorescent molecule positioned near a noble metal nanocrystal can interact strongly with the localized surface plasmon resonance of the metal nanocrystal. While the effects of this plasmon-fluorophore interaction on the intensity, polarization, and direction of the fluorescence emission have been intensively investigated, the plasmonic effect on the emission spectrum has barely been explored. We show, on the single-particle level, that the localized surface plasmon resonance of Au nanorods can strongly alter the spectral profile of the emission from adjacent fluorescent molecules. The fluorescent molecules are embedded in a mesostructured silica shell that is uniformly coated on each Au nanorod. The longitudinal plasmon resonance wavelengths of the nanorods are deliberately shifted away from the intrinsic fluorescence emission peak wavelength by synthetically tuning the nanorod aspect ratio. The resultant emission spectra of the fluorescent molecules are found to be remarkably modulated. Besides the intrinsic fluorescence peak, a plasmon-induced new peak emerges at the plasmon resonance wavelength. The intensity of this plasmon-induced fluorescence peak increases as the size of the Au nanorod is increased. This spectral modulation can be understood by depicting the decay process of the fluorophore with multiple vibrational energy levels. The plasmon with a specific resonance energy will enhance the transition rate between the energy levels that have the transition energy approximately equal to the plasmon energy. This plasmon-enhanced transition rate results in a modulated spectral profile of the fluorescence emission.

Synthesis and optical properties of novel organic–inorganic hybrid UV (R–NH3)2PbCl4semiconductors

We report on the synthesis and the optical properties of several novel ultraviolet (UV) semiconductors (R–NH3)2PbCl4. These semiconductors are two dimensional organic–inorganic perovskite (2DOIP) materials and have multiple quantum-well energy level structures. We systematically varied the organic components (R–NH3+), and discussed their influence on the self-organization ability, excitonic optical features, and long-term photo-stabilities of the 2DOIPs. The trends of selecting the organic groups to tailor the optical features and to improve the self-organization and long-term photo-stabilities of 2DOIPs are obtained by analyzing the experimental results. Self-organization abilities are found to be highly dependent on the interactions among the organic components. The excitonic optical properties of 2DOIPs are found to depend more on the steric structures, and less on the electronic structures and chemical properties of the organic groups (R–NH2) as long as the organic groups are optically inert. However, the long-term photo-stabilities of the (R–NH3)2PbCl4 semiconductors are highly dependent on the thermal and photo stabilities of organic components. We find a new UV semiconductor, (C6H11CH2NH3)2PbCl4 (CMPC), which has better photo-stability than the usually used (C6H5C2H4NH3)2PbCl4 (PEPC).

Nanomaterials for third generation photovoltaics

‘Third generation’ approaches aim to achieve high efficiency for photovoltaic devices but in such a way as to allow large scale implementation. The way to achieve these goals is to tackle one or both of the two major loss mechanisms in solar cells, that of non-absorption of below bandgap photons and thermalisation of carriers generated by photon energies exceeding the bandgap. The theoretical and practical aspects of several approaches to achieving this through the use of multiple energy levels are considered with reference to the limiting detailed balance efficiencies of ideal devices. Nanostructured materials are of great interest in realising some of these approaches, such as quantum dot superlattices for tandem solar cells and intermediate band cells, nanocrystals for multiple exciton generation cells and highly ordered nanostructures for hot carrier cells.

Large-amplitude harmonic driving of highly coherent flux qubits

The device for the Josephson flux qubit can be considered as a solid-state artificial atom with multiple energy levels. When a large-amplitude harmonic excitation is applied to the system, transitions at the energy levels avoided crossings produce visible changes in the qubit population over many driven periods that are accompanied by a rich pattern of interference phenomena. We present a Floquet treatment of the periodically time-dependent Schr\"odinger equation of the strongly driven qubit beyond the standard two-level approach. For low amplitudes, the average probability of a given sign of the persistent current qubit exhibits, as a function of the static flux detuning and the driving amplitude, Landau-Zener-St\"uckelberg (LZS) interference patterns that evolve into complex diamondlike patterns for large amplitudes. In the case of highly coherent flux qubits we show that the higher-order diamonds can not be simply described relying on LZS transitions in each avoided crossing considered separately. In addition we propose a spectroscopic method based on starting the system in the first excited state instead of in the ground state, which can give further information on the energy-level spectrum and dynamics in the case of highly coherent flux qubits. We compare our numerical results with recent experiments that perform amplitude spectroscopy to probe the energy spectrum of the artificial atom.

Slow and Fast Electron Channels in a Coherent Quantum Dot Mixer

We describe a means to realize slow and fast electron channels by coherent mixing of single-particle levels in quantum dots. The underlying physics, which gives insight into state superposition, can potentially be realized in multi-dot structures with complex gate control. However, we employ vertical double dot structures and in our scheme the mixing of single-particle levels arises because of natural perturbations in the confining potential of the high-symmetry dots. Additionally, because of the intrinsic properties of a Fock–Darwin-like spectrum, we utilize a magnetic field to bring multiple single-particle energy levels into close proximity. We determine single-electron resonant tunneling times (effectively dwell times when on resonance) that are either extended in the slow channel or shortened in the fast channel. Most dramatically, for the slow channel, slow-down factors of ∼10 and single-electron resonant tunneling times extended into the µs range are demonstrated in all systems of two, three, and four mixed single-particle states investigated here.

Multiple levels in intermediate band solar cells

The presence of multiple energy levels in the intermediate band solar cell (IBSC) is studied by detailed balance calculations under ideal conditions. Multiple levels are found experimentally in IBSCs made with quantum dots (QDs) which act to reduce the limiting efficiency determined from detailed balance calculations. JL-VOC measurements up to 1000 suns on IBSCs are presented together with their fitting to modified detailed balance calculations. It is found that the introduction of the QDs degrades the performance of the host cell but the sub-bandgap cell operates close to ideality.

Negative Differential Resistance Arising from Tunneling Current through Multiple Quantum Dots

The tunneling current through multiple quantum dots (QDs) sandwiched between metallic contacts is theoretically studied in the framework of Green's function technique. The Anderson model with multiple energy levels is employed to simulate the considered system. We found that the negative differential resistance (NDR) arises from the interdot Coulomb interactions and the QDs under shell-filling conditions. Such NDRs are suppressed by the temperature effect. It is expected that multipeak NDR devices could be realized using a sophisticated layout of quantum dots.

Theory of electron tunneling through a scanning tunneling microscopy-tip/quantum dot junction

Electron transport properties of an isolated quantum dot sandwiched between a metallic contact and a scanning tunneling microscopy tip are theoretically investigated. Keldysh-Green’s function technique is used to calculate the tunneling current of an Anderson model with multiple energy levels. The spectral function of the quantum dot system (with arbitrary number of energy levels) embedded in a tunnel junction is derived and used to calculate the tunneling current spectra. Finally, the authors calculate the emission spectra due to the electron-hole recombination that occurs in the case of bipolar tunneling, where both electrons and holes are allowed to simultaneously tunnel into the quantum dot. The authors find dramatic changes in the emission spectra as the applied bias is varied.

Theory of charge transport in a quantum dot tunnel junction with multiple energy levels

Transport properties of nanoscale quantum dots embedded in a matrix connected with metallic electrodes are investigated theoretically. The Green's function method is used to calculate the tunneling current of an Anderson model with multiple energy levels, which is employed to model the nanoscale tunnel junction of concern. A closed form spectral function of a quantum dot or coupled dots (with arbitrary number of energy levels) embedded in a tunnel junction is derived and rigorously proved via the principle of induction. Such an expression can give an efficient and reliable way for analyzing the complicated current spectra of a quantum dot tunnel junction. Besides, it can also be applied to the coupled dots case, where the negative differential conductance due to the proximity effect is found. Finally, we investigate the case of bipolar tunneling, in which both electrons and holes are allowed to tunnel into the quantum dot, while optical emission occurs. We find dramatic changes in the emission spectra as the applied bias is varied.

Application of X-ray computed tomography to soil science: A literature review

The study of the spatial configuration of soil, in its complexity, requires an understanding of the interrelations and interactions between the diverse soil constituents, at various levels of organization. Investigations of the spatial arrangement of the mineral and organic components of soil have benefited from the development of techniques for structural analysis. X-ray computed tomography (CT) is a non-destructive and non-invasive technique that has been successfully used for three-dimensional (3D) examination of soil. Valuable information has been obtained by the application of CT for the description and quantitative measurements of soil structure elements, especially of soil pores and pore network features. In many studies, X-ray CT has been used to investigate the hydro-physical characteristics of the soil, in a functional and temporal manner. A dynamic approach has also been utilized in the evaluation of the biotic factor influence on soil. The analysis of soil solid phases, by X-ray CT, has been challenging due to the similar X-ray attenuation of different solid constituents. However, the use of multiple X-ray energy levels has facilitated the discrimination of minerals in soil. The aim of this review and synthesis is to offer a perspective on the major issues related to application of the technique, general attempted solutions and possible directions in the utilization of X-ray CT in soil research. Relevant scanning parameters, procedures for CT image reconstruction, algorithms for the quantification of soil characteristics and results are presented for each type of application. Key words: X-ray computed tomography, energy level, spatial resolution, segmentation, soil mineral and organic constituents, soil physical and hydro-physical properties, soil biota

Tunneling Current Spectroscopy of a Nanostructure Junction Involving Multiple Energy Levels

A multilevel Anderson model is employed to simulate the system of a nanostructure tunnel junction with any number of one-particle energy levels. The tunneling current, including both shell-tunneling and shell-filling cases, is theoretically investigated via the nonequilibrium Green's function method. We obtain a closed form for the spectral function, which is used to analyze the complicated tunneling current spectra of a quantum dot or molecule embedded in a double-barrier junction. We also show that negative differential conductance can be observed in a quantum dot tunnel junction when the Coulomb interactions with neighboring quantum dots are taken into account.

Luminescence properties of wurtzite AlN nanotips

The optical properties of aluminum nitride nanotips (AlNNTs) synthesized via vapor transport and condensation process have been studied by cathodoluminescence, photoluminescence (PL), thermoluminescence (TL), and UV absorption measurements. Two defect related transitions around 2.1 and 3.4eV and an excitonic feature at 6.2eV were identified. Compared to the AlN macropowders, the AlNNTs showed a blueshift (+0.2eV) of the ∼3.2eV peak. Analysis of both PL and TL excitation measurements indicated the existence of subband gap multiple energy levels in AlNNTs. A significant TL intensity even at 145°C suggests possible ultraviolet detector and dosimetric applications of these AlNNTs.

Current conduction mechanisms in post-nitridation rapid-thermal-annealed gate oxides on 4H silicon carbide

In this letter we report current-conduction mechanisms in nitrided gate oxides on n-type 4H SiC subjected to various rapid-thermal-annealing temperatures. The experimental results show that by increasing SiC-SiO2 interface trap density, current conduction in the oxide is increased. Fowler-Nordheim (FN) tunneling which is assisted by SiC-SiO2 interface trap is responsible to the current conduction. In contrast, the current conduction through the oxide is significantly reduced when the oxide is having a multiple discrete energy level of electron trap center. This center enables trapping of electrons that are injected from SiC substrate via FN tunneling, causing a reduction in leakage current and an improvement in oxide breakdown strength. Based on the results, a model has been hypothesized and reasons for these observations have been presented.

エネルギー弁別型 X 線カラースキャナー

エネルギー弁別型 X 線カラースキャナー

Multiple discrete energy levels and the bistable state of weak anchoring NLC cells

The weak anchoring nematic liquid crystal (NLC) cell is investigated with regard to energy. Because the Gibbs free energy of liquid crystal system used in theory does not include temperature and entropy, and because the equations and boundary conditions for δG=0 are also the mechanical equilibrium conditions of the continuum, the Gibbs free energy G is equivalent to the energy E of the liquid crystal continuum. There are multiple solutions which satisfy these equations and boundary conditions, each solution corresponding to a certain energy value. We call these discrete energy values and energy levels. Adopting a simple liquid crystal cell model, the energy levels are calculated in detail by means of analytical and numerical methods. The results show that there are three energy levels (or more in certain cases). The values and sequence of the energy levels are related to the external field and anchoring parameters. The relationships between the energy level structure and the bistable. Fréedericksz transition are disussed, together with their influence on the response time. The physical condition for the existence of more than three energy levels is also given.

A new Morse theory and strong resonance problems

Is it possible to establish a new Morse theory if the function $f$ losses the (PS) condition at some isolated values? Yes, it is! In this paper we will recall a such a theory. One of the purposes of establishing such a theory is to consider multiplicity results for strong resonance problems and to deal with multiple resonant energy levels. Both of these questions were not studied much in the past because of the limitation of methods. Using the new Morse theory we can deal with these problems.