Energy Of Electron-hole Pair Research Articles

A monolayer transition-metal dichalcogenide as a topological excitonic insulator.

Monolayer transition-metal dichalcogenides in the T' phase could enable the realization of the quantum spin Hall effect1 at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk2,3. Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T' MoS2, which we investigate from first principles using many-body perturbation theory4. This paradoxical result hints at the instability of the T' phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed 'excitonic insulator' ground state5-7. Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers8-16 where the frustration between the two orders is relieved by breaking time reversal symmetry13,15,16. The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric17,18 as an effect of electron-electron interaction.

Accurate First-Principles Calculation of the Vibronic Spectrum of Stacked Perylene Tetracarboxylic Acid Diimides.

π-stacked organic electronic materials are tunable light absorbers with many potential applications in optoelectronics. The optical properties of such molecules are highly dependent on the nature and energy of electron-hole pairs or excitons formed upon light absorption, which in turn are determined by intra- and intermolecular electronic and vibrational excitations. Here, we present a first-principles approach for describing the optical spectrum of stacked organic molecules with strong vibronic coupling. For stacked perylene tetracarboxylic acid diimides, we describe optical excitations by using the time-dependent density functional theory with a Franck-Condon Herzberg-Teller approximation of vibronic effects and validate our approach with comparison to experimental ultraviolet-visible (UV-vis) absorption measurements of solvated model systems. We determine that for larger macromolecules, unlike for single molecules, the sampling of the ground-state potential energy surface significantly influences the optical absorption spectrum. We account for this effect by applying our analysis to ∼100 structures extracted from equilibrated molecular dynamics simulations and averaging the optical spectrum over the entire ensemble. Additionally, we demonstrate that intermolecular electronic coupling within the stacks results in multiple low-energy electronically excited states that all contribute to the optical spectrum. This study provides a computationally feasible recipe for describing the spectroscopic properties of stacked organic chromophores via first-principles density functional theory.

Photovoltaic effect in the single-junction DBP/PTCBI organic system under low intensity of monochromatic light

Photoelectric properties of the planar ITO/MoO3/DBP/PTCBI/BCP/Ag system were characterized on the basis of short-circuit current, open-circuit voltage and absorption spectra, and current-voltage measurements in the dark and under monochromatic illumination of low intensity. Photovoltaic performance of the system was compared with the performance of ideal semiconductor and excitonic cells of chosen bandgaps. Such analysis shows, that the fabricated cell exhibits quite high value of the open-circuit voltage, in comparison to the SQ limits calculated for semiconductor devices of bandgaps close to the LUMOPTCBI-HOMODBP offset or crystalline silicon cells of the same absorptivity. This confirms that the DBP/PTCBI junction exhibits good properties for conversion of exciton energy to chemical energy of electron-hole pair. Moreover, open-circuit voltage and short-circuit current of the investigated cell practically do not change within the 520 nm–620 nm range, for which they reach the maximum values, making the junction of DBP/PTCBI attractive for use in indoor photovoltaics.

Quasi-Two-Dimensional Electron–Hole Liquid in Si/SiO2 Quantum Wells

To calculate the energy and equilibrium density of electron-hole pairs in SiO2/Si/SiO2 quantum wells (QWs), nonlinear Schrodinger equations for electrons and holes were numerically solved. Calculations were carried out for (100) and (111) silicon surfaces and various values of the QW width. It is shown that the binding energy of electron-hole pairs in a quasi-two-dimensional electron-hole liquid (EHL) is much higher than the binding energy in a three-dimensional EHL. The results of calculations are compared with the experimental results for a wide range of QW widths.

Investigation of surface plasmon coupling with the quantum well for reducing efficiency droop in GaN-based light emitting diodes

The spontaneous emission rate into Surface Plasmon Polariton (SPP) mode for the InGaN/GaN quantum well (QW) with SP coupling is presented taking into account the electron and hole band structures, the photon density of states, and evanescent fields of SPP. The optical properties of SP-enhanced InGaN QW structure with different QW layer number are investigated in detail by using the formula. It is observed that the energy of electron-hole pairs in the InGaN QW can be efficiently transferred into the SPP modes which will induce the significantly enhancement of the internal quantum efficiency (IQE) of SP-enhanced light emitting diodes (LEDs), especially for the original IQE in the range of 6%–25%. Furthermore, the distribution of electron and hole densities in each well layer can evidently affect the Purcell enhancement factor due to the distance dependence of the intensity of SP-QW coupling. The numerical results also indicate that the SP-enhanced LED can suppress the efficiency droop effect as long as the extraction efficiency of SPP mode is enough large.

Monte Carlo simulation for the electron cascade due to gamma rays in semiconductor radiation detectors

A Monte Carlo code was developed for simulating the electron cascade in radiation detector materials. The electron differential scattering cross sections were derived from measured electron energy-loss and optical spectra, making the method applicable for a wide range of materials. The detector resolution in a simplified model system shows dependence on the bandgap, the plasmon strength and energy, and the valence band width. In principle, these parameters could be optimized to improve detector performance. The intrinsic energy resolution was calculated for three semiconductors: silicon (Si), gallium arsenide (GaAs), and zinc telluride (ZnTe). Setting the ionization thresholds for electrons and holes is identified as a critical issue, as this strongly affects both the average electron-hole pair energy w and the Fano factor F. Using an ionization threshold from impact ionization calculations as an effective bandgap yields pair energies that are well matched to measured values. Fano factors of 0.091 (Si), 0.100 (GaAs), and 0.075 (ZnTe) were calculated. The Fano factor calculated for silicon using this model was lower than some results from past simulations and experiments. This difference could be attributed to problems in simulating inter-band transitions and the scattering of low-energy electrons.

Reflection and absorption of light by a wide quantum well with two closely spaced excitation levels

The spectra of reflection and absorption of monochromatic light by semiconductor quantum wells whose width is comparable to the wavelength of exciting radiation are calculated. The case of resonance with two closely spaced excited levels is considered. These levels can arise as a result of splitting of the electron-hole pair energy due to the magnetopolaron effect when the quantum well is placed in a strong magnetic field directed perpendicular to the plane of the quantum well. It is demonstrated that, in wide quantum wells, unlike in narrow quantum wells, the reflectance and absorptance of light depend on the quantum-well width. The theory is applicable at any reciprocal ratio of the radiative lifetime to the nonradiative lifetime of electronic excitations.

Solitonic excitations in linearly coherent channels of bilayer quantum Hall stripes

In some range of interlayer distances, the ground state of the two-dimensional electron gas at filling factor $\ensuremath{\nu}=4N+1$ with $N=0,1,2,\dots{}$ is a coherent stripe phase in the Hartree-Fock approximation. This phase has one-dimensional coherent channels that support charged excitations in the form of pseudospin solitons. In this work, we compute the transport gap of the coherent striped phase due to the creation of soliton-antisoliton pairs using a supercell microscopic unrestricted Hartree-Fock approach. We study this gap as a function of interlayer distance and tunneling amplitude. Our calculations confirm that the soliton-antisoliton excitation energy is lower than the corresponding Hartree-Fock electron-hole pair energy. We compare our results with estimates of the transport gap obtained from a field-theoretic model valid in the limit of slowly varying pseudospin textures.

Site-selective spectroscopy of Er in GaN

We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.

Exciton states and optical transitions in colloidal CdS quantum dots: Shape and dielectric mismatch effects

We present a theoretical investigation of electron-hole and exciton energy spectra as well as oscillator strengths of optical transitions in colloidal CdS quantum dots (QD's) with spherical and tetrahedral shape. The Coulomb potential energy of the electron-hole system is treated taking into account the dielectric mismatch at the QD boundaries. Calculation of electron-hole energy spectrum and Coulomb potential energy in tetrahedral QD's is carried out using the finite difference method. It is shown that the bulk Coulomb potential energy with the dielectric constant of QD leads to lower exciton energy levels as compared to the Coulomb potential, which includes electron-hole interaction and self-action energies. The Coulomb potential changes the electron-hole pair energies without dielectric confinement contributions in such a way that the exciton ground state becomes active for optical transitions in dipole approximation for both tetrahedral and spherical QD's while the lowest electron-hole pair energy level is active for tetrahedral and passive for spherical QD's. The exciton binding energy in both types of QD's is enhanced by a factor of 2 in the presence of the dielectric mismatch. It is proven that the inclusion of the real QD shape and dielectric mismatch is important not only for the quantitative analysis but also for the qualitative description of optical properties of colloidal CdS QD's.

Optical activity of Yb in GaAs and low-dimensional GaAs/GaAlAs structures

It is shown that the optical activation of Yb in GaAs and low-dimensional GaAs/GaAlAs structures can be achieved by forming three-component (Yb+S/Se/Te+O) luminescence centers based on the Yb3+ ion. A correlation between the characteristics of these centers and the parameters of the chalcogen coactivators is discovered. Oxygen is shown to play a decisive role in transferring the energy of electron-hole pairs to the luminescence centers.

Size-Dependent Temperature Variation of the Energy Gap in Lead-Salt Quantum Dots

We observe that the temperature coefficients of electron-hole pair energies $(dE/dT)$ in PbS and PbSe quantum dots depend strongly on the size of the quantum dot. With decreasing size the temperature coefficient of the lowest electron-hole pair energy ${\mathrm{dE}}_{g}/dT$ decreases by more than an order of magnitude from the bulk value. The weak temperature dependence found in the strong-confinement limit is expected for atomiclike levels, but has not been observed previously in a semiconductor.

Quantum-size stark effect in semiconductor microcrystals

The effect of an external electric field on the absorption spectrum of CdS and CdSe microcrystals has been investigated both experimentally and theoretically. It is found that electro-optic properties of the microcrystal doped glasses near the interband edge are governed by the quadratic Stark effect. The value of the Stark shift was observed to decrease in small size microcrystals as the cube of the radius. This dependence is in good agreement with the theoretical considerations taking into account the peculiarities of the size confined electron-hole pair energy spectra.

Brightness degradation and its mechanism in Tb-doped ZnS thin-film electroluminescent devices

We investigated the origin and mechanism of brightness degradation in Tb-doped ZnS thin-film green electroluminescent devices. We concluded that the increased number of TbF bond breaking, acting as nonradiative emission centers for Tb emission, is responsible for the degradation. The recombination energy of electron-hole pairs generated by hot-electron impact at the Tb-doping site breaks the TbF bond, which is vital to efficiency. We found that high-temperature annealing reduces the generation rate of nonradiative recombination centers, resulting in long device life.

Selection rule in the vibrational–electron-hole pair energy transfer for molecules interacting with metal surfaces

In the scattering of a polyatomic molecule from the surface of a metal, there can be exchange of energy between electron-hole pair excitations of the metal and vibrations of the molecule. A possible mechanism for this is electron transfer to an (affinity) orbital of the molecule. We point out that certain vibrations would be selectively excited/deexcited in this mechanism. A similar rule exists for the damping of vibrations of molecules adsorbed on metal surfaces.

Sound Attenuation in Magnetic Metals

The anomalous sound attenuation due to spin fluctuations near the magnetic phase transition temperatures in itinerant magnets and the rare earth metals is theoretically studied. By applying Tsuneto and Kadanoff and Falko's formulas, the attenuation coefficient is calculated. The calculated results explain the critical anomalies observed in magnetic metals such as Ni and Gd. By comparing the results with that for insulating magnets, the expression of the exchange striction is obtained. The effect of magnetic field on the critical attenuation is discussed. ferro­ magnets. In this paper, we formulate the attenuation coefficient clue to spin fluc­ tuations and show that the expression of the coefficient in itinerant electron systems is essentially the same as that in localized spin systems. Through this study, the meaning of the exchange strictive interaction in itinerant magnets is clarified. Tsuneto, 8J and Kadanoff and Falko 9J have given an expression for the attenu­ ation coefficient applicable to the system in which the electronic state is strongly modified from the free electron state by impurity and electron-electron interaction. We apply their formula and obtain the critical attenuation coefficients near the magnetic phase transition temperatures in itinerant magnets (§ 2) and the rare earth metals (§ 3). In our formalism, the attenuation coefficient is expressed in terms of the correlation functions of electron-hole pairs. The excitation energy of electron-hole pair is renormalized due to spin fluctuations and the spectral den­ sities at low energies are extremely enhanced near the critical temperature. This enhancement causes the critical attenuation of sound. vVhen the results obtained in §§ 2 and 3 are compared with that of localized spin systems, the meaning of the exchange striction in magnetic metals becomes clear as will be shown in § 4. In the final section (§ 5), the effect of external magnetic field on the attenuation is discussed. In this paper, we neglect the spin­ orbit coupling.