Hole Ground State Research Articles

Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering.

Dimeric complexes composed of d8 square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir2 (dimen)4]2+ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d8-d8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.

Effect of External Fields on the Electronic and Optical Properties in ZnTe/CdSe and CdSe/ZnTe Spherical Quantum Dots

A theoretical analysis was conducted to examine the electronic and optical properties of a confined electron and a hole in a type-II core-shell spherical quantum dot composed of CdSe/ZnTe and ZnTe/CdSe. The Schrödinger equation for the electron and the hole was numerically solved using COMSOL-Multiphysics software in the 2D axisymmetric module, which employs the finite element method under the effective mass approximation. A Fortran code was utilized to calculate excitonic energy, specifically designed to solve the Coulomb integral. The calculations encompassed variations in the inner radius (R1), as well as variations in the electric (Fz) and magnetic (B) fields along the z-axis. The absorption coefficients were determined for transitions between the hole and electron ground states, considering z-polarized incident radiation. Including a magnetic field increases the transition energy, consequently causing the absorption peaks to shift toward the blue region of the spectrum. On the other hand, the electric field decreased the overlap of the electron and hole wavefunctions. As a result, the amplitude of the absorption peaks decreased with an increase in the electric field.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

Theoretical and Experimental Studies on Material Gain for Wide Polar InGaN Quantum Well‐Mechanism Leading to Electric Field Screening and Lasing

AbstractThe width of polar InGaN quantum wells (QWs) is usually limited to a few nanometers to ensure a sufficient overlap between the wave functions of the ground electron and hole state. This paper presents the results of material gain measurements for thin (2.6 nm), wide (10.4 nm), and extremely wide (25 nm) InGaN QWs. The comparison of experimental data with theoretical predictions allows for correlation of the current density with the carrier concentration, which contributes to the positive material gain, and determination of the coefficients in the standard ABC model. It is shown that the mechanism in wide and narrow polar QWs that leads to electric field screening and lasing is different. For wide QWs, the optical transitions between excited states become dominant in the gain spectrum. The reason for such a gain mechanism is the built‐in electric field, which must first be screened. The ground electron and hole states play a very important role in the screening; however, their contribution to the material gain and lasing in wide QWs is very weak.

Importance of core and shell sizes in the localization of the electron and hole in the formation of type I or type II excitons in spherical CdSe/ZnTe and CdSe/CdTe quantum dots.

Semiconductor core/shell quantum dots represent a promising class of nanostructure that can provide new techniques of control for performing the electronic and optical properties. Through each combination formed by the core and the shell radii and the confinement effect, the trapped charge carries behave differently, which obviously has a direct effect on the semiconductor properties. Studying the behavior of charge carriers is one of the most important steps in understanding the semiconductor performance when it undergoes certain conditions, which inevitably can offer other benefits for various applications. The electron and hole localizations strongly depend on core and shell sizes, but determining these limit sizes has not been sufficiently discussed in the literature. This paper gives a detailed theoretical study describing the three cases of electron and hole positions in two different type II core/shell quantum dots, CdSe/ZnTe and CdSe/CdTe considering the core and shell sizes. Our numerical calculations show that two significant critical radii of the core allow to switch between the exciton type I and type II quasi-particle. As a function of the shell thickness, the ground state energy of the electron is found to be higher when the core radius is small; contrariwise, the hole ground state energy remains smaller. Afterward, these specific variations influence the ground state binding energy of the correlated electron–hole quasi-particle (exciton), leading to a particular behavior of this energy as a function of the core and shell sizes.

Antibonding ground states in crystal phase quantum dots

Atomistic approaches enable computational studies ofnanostructures with unusual properties. Here, the authors showthat the indium phosphate zinc-blende section embedded in a wurtzitenanowire, thus forming so-called crystal phase quantum dots, mayexhibit a rare antibonding hole ground state. The authors also findthat even a small lattice mismatch at the wurtzite/zinc-blendeinterface can profoundly mix the lowest hole states, resulting inunusual double-peak features in the excitonic optical spectra. Furtherresearch is, however, needed to determine strained band offsets inpolytype homostructures, allowing for better agreement betweenexperiment and theory.

Experimentally-Verified Modeling of InGaAs Quantum Dots.

We develop a model of an epitaxial self-organized InGaAs quantum dot buried in GaAs, which takes into account experimentally determined indium distribution inside the QD, its geometry and crystallography. The problem of solid mechanics was solved to determine the stress-strain field. Then, the parameters of the electron and hole ground states were evaluated by solving the problem of the quantum mechanics on the same mesh. The results of calculations appeared to be reasonably well consistent with experimentally recorded optical emission spectra for the QDs in the same sample. The experimentally-verified modeling reveals a bagel-like shape of the hole wave function at the ground state, which should considerably impact the optical and magnetic properties of the QDs. Such shape of the wave function is beyond the predictions of simplified models with uniform indium distribution.

The biocatalytic activity of the “lantern-like” binuclear copper complex with trisulfide bridges mimicking SOD metallo-proteins

Through a redox process, an in situ transformation of propylthiouracil led to the formation of a trisulfur ligand that coordinates Cu(II) ion forming a lantern-like binuclear copper(II) complex [Cu2(ptu-S-ptu)2]. The complex was characterized by elemental analysis, spectroscopic techniques (FTIR, UV–Vis, EPR, and Hirshfeld surfaces analyses.), single-crystal X-Ray analysis, and its optimized molecular structure was obtained by DFT calculations. It crystallizes as a twin in the space group P-1, with two different, though closely related, centrosymmetric binuclear complexes per unit cell. The (ptu-S-ptu)2− thiouracil rings are nearly perpendicular to each other and their N and O atoms equatorially cis-coordinate the two copper ions in a two-fold Cu-OCN-Cu bridging mode. The four-fold bridging configuration is completed by another (ptu-S-ptu)2− ligand, inversion-related through the middle of the Cu…Cu contact. The elongated octahedral coordination around each copper is completed by the linking S-atom of the ligand at the axial position. The crystallographic results point to a copper d(x2-y2) d-orbital for the electron (or hole) ground state and optimal overlap of this orbital with molecular electronic path bridging the two metal atoms, hence indicating a relatively large antiferromagnetic coupling between them. The designed complex behaves as a promiscuous promising catalyst. It possesses intrinsic peroxidase-like activity catalyzing phenol red bromination in the presence of H2O2 at room temperature, and the oxidation of pyrogallol, and behaves as a superoxide dismutase (SOD) mimic compound acting on superoxide anion. Additionally, insights into their reactivity are analyzed by comparison with a family of other sulfur-containing copper(II) metal complexes in an attempt to explain the differences in catalytic properties and to determine the structural properties that affect the reactivity with substrates. Albumin interactions studies are also incorporated.

Quantum Dot Acceptors in Two-Dimensional Epitaxially Fused PbSe Quantum Dot Superlattices.

Oriented attachment of colloidal quantum dots allows the growth of two-dimensional crystals by design, which could have striking electronic properties upon progress on manipulating their conductivity. Here, we explore the origin of doping in square and epitaxially fused PbSe quantum dot superlattices with low-temperature scanning tunneling microscopy and spectroscopy. Probing the density of states of numerous individual quantum dots reveals an electronic coupling between the hole ground states of the quantum dots. Moreover, a small amount of quantum dots shows a reproducible deep level in the band gap, which is not caused by structural defects in the connections but arises from unpassivated sites at the {111} facets. Based on semiconductor statistics, these distinct defective quantum dots, randomly distributed in the superlattice, trap electrons, releasing a concentration of free holes, which is intimately related to the interdot electronic coupling. They act as acceptor quantum dots in the host quantum dot lattice, mimicking the role of dopant atoms in a semiconductor crystal.

Material Gain in Polar GaInN and AlGaN Quantum Wells: How to Overcome the ‘Dead’ Width for Light Emitters in These QW Systems?

<bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Polar GaInN and AlGaN quantum wells (QWs) are widely used in light emitting diodes and laser diodes (LDs). However, the widths of such QWs are usually limited to a few nanometers in order to ensure a sufficiently large overlap between the wave functions of the ground electron and the ground hole state. By increasing the QW width we enter the area of ‘dead’ width where, the overlap of the electron and hole wave functions decreases almost to zero and the luminescence efficiency drastically deteriorates. Therefore, it is assumed that wide QWs are not suitable for light emitters and very wide (8-15 nm) QWs are not considered as a promising gain medium for LDs. Hence such QWs are very rarely studied both experimentally and theoretically. In this work the material gain is calculated for Ga_0.8In_0.2N/GaN and Al_0.8Ga_0.2N/AlN QWs with a width varying in the range of 2-15 nm. We observed that the material gain at fixed carrier concentration for these QWs drops to zero with the increase in the QW width and reaches negative values in the width range of ∼4-8 nm even for high carrier concentrations, but after exceeding a certain width to ∼8-12 nm it begins to increase rapidly and reaches the values greater than those observed for narrow QWs. This phenomenon is related to the screening of the built-in electric field by carriers, which is easier for wide QWs, and the reduction of the distance between the energy levels for electrons and holes. For the latter reason, optical transitions between higher energy states make a very significant contribution to the positive material gain</b> .

Evolution of a dominant light emission mechanism induced by changes of the quantum well width in InGaN/GaN LEDs and LDs

We examined electroluminescence from In0.17Ga0.83N/GaN quantum wells (QW) of light-emitting diodes (LEDs) and laser diodes (LDs). For increasing QW width we observe transition from electron and hole ground-states recombination to excited states recombination. The effect is accompanied by partial (2.6 nm, 5.2 nm, 7.8 nm QW) or practically complete (10.4 nm QW) screening of the built-in electric field with increasing driving current for both types of emitters. The electric field magnitude was studied using an original high pressure method. The investigations are supported by simulations of the variation with driving current of i) electron and hole wavefunctions overlap affecting the recombination channel, ii) built-in electric field.

Fokker-Planck equation for black holes in thermal potential

We construct a kind of thermal potential and then put the black hole thermodynamic system in it. In this regard, some thermodynamic properties of the black hole are related to the geometric characteristics of the thermal potential. Driven by the intrinsic thermodynamic fluctuations, the behavior of the black hole in the thermal potential is stochastic. With the help of solving the Fokker-Planck equation analytically, we obtain the discrete energy spectrum of Schwarzschild and Banados-Teitelboim-Zanelli (BTZ) black holes in the thermal potential. For Schwarzschild black hole, the energy spectrum is proportional to the temperature of the ensemble, which is an external parameter, and the ground state is non-zero. For BTZ black hole, the energy spectrum only depends on the AdS radius, which is the intrinsic parameter. Moreover, the ground state of BTZ black hole in thermal potential is zero. This also reflects the difference between three-dimensional gravity and four-dimensional gravity.

Excited-State Intramolecular Proton Transfer Reaction and Ground-State Hole Dynamics of 4'-N,N-Dialkylamino-3-hydroxyflavone in Ionic Liquids Studied by Transient Absorption Spectroscopy.

The excited-state intramolecular proton transfer (ESIPT) of 4'-N,N-dialkylamino-3-hydroxyflavone (CnHF) having different alkyl chain lengths (ethyl, butyl, and octyl chains) was investigated in ionic liquids (ILs) by steady-state fluorescence and transient absorption spectroscopy. Upon photoexcitation, CnHF underwent ESIPT from the normal form to the tautomer form, and dual emissions from both states were detected. For C4HF and C8HF, the tautomerization yields determined from the fluorescence intensity ratios increased with the increasing number of alkyl chain carbon atoms in the cation and on reducing the excitation wavelength as reported for C2HF [K. Suda et al., J. Phys. Chem. B. 117, 12567 (2013)]. The transient absorption spectra of CnHF were measured at excitation wavelengths of 360, 400, and 450 nm. The ESIPT rate determined from the induced emission of the tautomer was correlated with the tautomerization yield for C2HF and C4HF. In addition, the recovery of the ground-state bleach was found to be strongly dependent on the excitation wavelength. This result indicates that the solvated state of the molecule before photoexcitation is dependent on the excitation wavelengths. The time constant for the ground-state relaxation was slower than that for the excited state.

Tuning electronic structures and optical properties in CdSe/CdS Dot-in-rod colloidal nanostructures: Atomistic tight-binding theory

Using the atomistic tight-binding theory, in this study, the electronic structures and optical properties of CdSe/CdS dot-in-rod nanocrystals are modified by engineering the core and rod diameters. The ground electron states favor an s-like orbital, while the ground hole states are formed by light hole-like characteristics. The quantum confinement effect is attributed to the reduction in excitonic band gaps as the core and rod diameters increase. Changing the core and rod sizes produces excitonic band gaps across the visible light spectra. The highest oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting are witnessed at a 3.0 nm CdSe core diameter. The oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting decrease as the CdS rod diameters increase. This study highlights the importance of theoretically understanding and controlling the active materials’ structural parameters in optoelectronic technology.

Effects of dissipative electron tunneling manifested in the photocurrent of a GaAs p-i-n photodiode with a double InAs quantum dot layer

We report on the results of experimental studies of the photocurrent (PC) of photodiodes based on GaAs p-i-n structures with InAs/GaAs(001) double asymmetric quantum dot (DAQD) arrays obtained by self-assembling in the process of low-pressure metal-organic vapor phase epitaxy (LP-MOVPE). Three peaks were observed in the dependence of the PC on the reverse bias, measured under photoexcitation with a photon energy equal to the energy of the interband ground state transition in larger InAs QDs. These peaks were attributed to photoexcitation of electrons from the ground hole states in larger QDs into the ground electron states followed by resonant dissipative (with absorption or emission of optical phonons) and conservative tunneling into the GaAs conduction band via the ground electron states in smaller QDs. The PC dependence on the bias voltage agrees qualitatively with the theoretical field dependence of the probability of 1D dissipative tunneling between the QDs.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Диссипативное туннелирование электронов в вертикально связанных двойных асимметричных квантовых точках InAs/GaAs(001)

We report on the results of experimental studies of the photoelectric properties of a GaAs p-i-n photodiode with InAs/GaAs(001) double asymmetric quantum dots (DAQDs) grown by self-assembling in Metal Organic Vapor Phase Epitaxy (MOVPE) process. Three peaks were observed in the dependence of the photocurrent on the reverse bias measured at monochromatic photoexcitation of the DAQDs at the wavelength corresponding to the energy of interband optical transitions between the ground hole and electron states in the bigger QDs. These peaks were related to the tunneling of the photoexcited electrons between the QDs including the dissipative one (with emission and absorption of the optical phonons). The experimental results agree qualitatively with the theoretical field dependence of the 1D dissipative tunneling probability between the QDs.

Stable diagonal stripes in the t\u2013J model at n\u0305h = 1/8 doping from fPEPS calculations

We investigate the two-dimensional t–J model at a hole doping of {bar{n}}_{{rm{h}}}=1/8 using recently developed high accuracy fermionic projected entangled pair states method. By applying the stochastic gradient descent method combined with the Monte Carlo sampling technique, we obtain the ground state hole energy Ehole = −1.621 for J/t = 0.4. We show that the ground state has stable diagonal stripes instead of vertical stripes with a width of 4 unit cells, and stripe filling ρl = 0.5. We further show that the long-range superconductivity order is suppressed at this point.

Effect of quantum well width on the electron and hole states in different single quantum well structures

In this study the electron and hole states in Al0.33Ga0.67As/GaAs single quantum well structures including squared QW, step QW and tilted QW, have been theoretically studied by solving the Schrodinger equation in real space. The energies and wave functions of electron and hole are calculated for different well widths. It is found that energy level of electron and hole decreases with increasing the well width. Adding step or tilted layers gives rise to the decrease of electron and hole energy levels. The ground state energy level in a tilted single quantum well structure is lower than that in a step single quantum well structure. It is also found that the energy of electron and hole ground states do not change as the width of step layer increase. This is because the ground state occupies in a lower well only. The wave functions are symmetric (ground state) and antisymmetric (the first excited state). The maximum of ground state wave function is at the central of the well and the probability of finding electron and hole in excited states are different in each region. The hole levels are lower than the electron levels due to the lower well depth and higher mass of hole compared to electron.

Hole States in Spherical Quantum Nanoheterosystem with Intermediate Spin-Orbital Interaction

The hole energy spectrum has been studied for the spherical semiconductor nanoheterosystem with the cubic symmetry. The exact solutions of the Schrödinger equation for the ground and excited hole states are presented within the framework of the 6-band Luttinger Hamiltonian and the finite gap of bands with the corresponding boundary conditions. Dependence of the holes energies from the radius of the quantum dot has been calculated for the GaAs/AlAs heterostructure. Obtained results where compared with data obtained using the infinite potential well model, as well as the single-band model for heavy and light holes.