Energies Of Electronic States Research Articles

Effects of aromatic substituents on the electronic structure and excited state energy levels of diketopyrrolopyrrole derivatives for singlet fission.

Singlet fission (SF) is a spin-allowed process, which is expected to be a feasible strategy to realize photon downward conversion. To achieve a significant increase in the photoelectric conversion efficiency of solar cells, SF molecules should have not only a high SF efficiency, but also suitable energies of the first singlet excited state [E(S1)] and the first triplet excited state [E(T1)] to act as SF sensitizers in solar cells. Aryl-substituted diketopyrrolopyrrole (DPP) is one of the few organic molecules, which can undergo SF efficiently after photoexcitation. In order to find suitable DPP-based SF sensitizers for solar cells, we designed a series of DPP derivatives by varying aromatic substituents, including changing the conjugation and constitution of aromatic substituents, as well as introducing side-substituents on the aromatic substituents. Detailed analysis focused on the molecular structures, the frontier molecular orbitals, multiple diradical characters, and SF relevant excited-state energy levels. The results indicate that introduction of no more than two aromatic rings and modification of the aromatic rings with side-substituents are both practical ways to get suitable SF sensitizers for solar cells. This work would give a deep understanding of DPP-based SF molecules, and pave the way towards the development of new DPP-based SF sensitizers for solar cells.

Complementary version of fermion coupled coherent states method and gram-schmidt algorithm: Theory and applications for electronic states of H2 and H2.

In our previous report, we introduced a new version of Fermion coupled coherent states method (FCCS) which was especially suited for simulating the first symmetric spatial electronic state of two-electron systems. In this manuscript, we report a complementary version for FCCS method to simulate both of the first symmetric and antisymmetric spatial electronic states of two-electron systems. Moreover, the Gram-Schmidt orthogonalization process is employed to reach the excited states of the system. We apply this FCCS method and the original coupled coherent state method to simulate the energy of different electronic states of H2 and H2+, respectively. The results for the energy of computed electronic states of H2 and H2+ show a pretty good consistency with the exact values. © 2017 Wiley Periodicals, Inc.

Quasiparticle and hybrid density functional methods in defect studies: An application to the nitrogen vacancy in GaN

Defects in semiconductors can play a vital role in the performance of electronic devices, with native defects often dominating the electronic properties of the semiconductor. Understanding the relationship between structural defects and electronic function will be central to the design of new high-performance materials. In particular, it is necessary to quantitatively understand the energy and lifetime of electronic states associated with the defect. Here, we apply first-principles density functional theory (DFT) and many-body perturbation theory within the GW approximation to understand the nature and energy of the defect states associated with a charged nitrogen vacancy on the electronic properties of gallium nitride (GaN), as a model of a well-studied and important wide gap semiconductor grown with defects. We systematically investigate the sources of error associated with the GW approximation and the role of the underlying atomic structure on the predicted defect state energies. Additionally, analysis of the computed electronic density of states (DOS) reveals that there is one occupied defect state 0.2 eV below the valence band maximum and three unoccupied defect states at energy of 0.2--0.4 eV above the conduction band minimum, suggesting that this defect in the +1 charge state will not behave as a carrier trap. Furthermore, we compare the character and energy of the defect state obtained from GW and DFT using the HSE approximate density functional and find excellent agreement. This systematic study provides a more complete understanding of how to obtain quantitative defect energy states in bulk semiconductors.

The density-of-states contributions to the negative field charge drift mobility effect in poly(3-hexylthiophene) organic semiconductor

The origin of the empirical linear dependence of Eint the electric field at the charge-injecting metal/poly(3-hexylthiophene), P3HT, interfaces on the externally applied bias, Ea, that governs its room temperature negative field charge drift mobility is investigated. The published electrostatic model is modified in the sense that the energetic shift φ of the Gaussian disordered hole and electron energy states is determined to be a linear function of the externally applied electric field, Ea. On this basis the distorted Gaussian shaped interfacial electric field Eint is obtained and the empirical Eint is identified as the linear interpolation function of positive slope through the inflection point of the calculated curve. The disordered energy states extend throughout the P3HT charge transport gap and the populations of interface charges follow unequal Ea dependences. The coupling between the deduced interfacial electric field Eint and the P3HT effective mobility enables the predictions to be compared to the published time-of-flight room temperature negative field drift mobility data of holes and electrons in various metal/P3HT sandwich-type organic structures.

The energy state of electrons in Mn-ferrite by XRS

The energy state of electrons in Mn-ferrite by XRS

Faraday rotation by the undisturbed bulk and by photoinduced giant polarons in EuTe

A quantum mechanical model is developed for the Faraday effect in europium telluride, for photons of energy within the transparency gap. The model is based on the well known band edge electronic energy states in EuTe. A concise expression for the Verdet constant is obtained, determined by few parameters already available in the literature. The Verdet constant adopted here, defined by the ratio between the Faraday rotation angle and the magnetization, is in effect temperature independent. Its dependence on the photon energy and applied magnetic field is in excellent agreement with published results. Below 3 T the Verdet constant is also nearly independent on field, but above 3 T at low temperatures it increases due to the band gap redshift. The model is used to calculate the photoinduced Faraday rotation associated with photoinduced giant magnetic polarons in EuTe. The theoretical photoinduced Faraday rotation excitation describes quite well the main features seen experimentally. Due to the common band-edge electronic energy structure, the model reported here could be extended to all other europium chalcogenides.

Theoretical study of photodetachment spectroscopy of hydrogenated boron cluster anion H2B7- and its deuterated isotopomer.

Photodetachment spectroscopy of H2B7- and its deuterated isotopomer probing the energetically low-lying electronic states of the respective neutral cluster is theoretically investigated in this paper. The theoretical methodology is based on detailed quantum chemistry calculations of electronic state energies, construction of a vibronic coupling model in the diabatic electronic basis, and nuclear dynamics calculations from first principles using time-dependent and time-independent quantum mechanical methods. The theoretical model consists of five coupled electronic states and fifteen vibrational modes. Several reduced dimensional calculations are performed to identify the relevant vibrational modes contributing to the vibronic structure of electronic bands and the impact of non-adiabatic coupling on them. The low-energy part of the spectrum of both H2B7 and its deuterated analogue is assigned by examining the vibronic wavefunctions and the results are compared with the experimental findings. The nonadiabatic decay dynamics of the electronic excited states of the neutral clusters is examined at length.

Tunable intersubband transitions in ZnO/ZnMgO multiple quantum wells in the mid infrared spectral range

We report on controllable tuning of intersubband transitions in ZnO/Zn0.60Mg0.40O multiple quantum well structures grown by molecular beam epitaxy on sapphire. The transitions from the first to the second electronic energy state within the conduction band are directly observed by infrared spectroscopy. By variation of the quantum well width, the intersubband transition energies are tuned from 290 to 370 meV. The experimental results are in good agreement with theoretical calculations assuming the presence of internal electric fields of 2 MV·cm−1.

The generation of the electrical oscillations in a contact of an AFM probe to an individual Au nanoparticle in a SiO2/Si film

We report on the experimental observation of the electrical oscillations in an oscillating loop connected to a contact of a conductive probe of an atomic force microscope to a tunnel-transparent (≈ 5 nm thick) SiO2/Si(001) film with embedded Au nanoparticles. The oscillations were attributed to the negative differential resistance of the probe-to-sample contact originating from the resonant electron tunnelling between the probe and the Si substrate via the quantum confined electron energy states in small (∼1 nm in size) Au nanoparticles. This observation demonstrates the prospects to build an oscillator nanoelectronic device based on an individual nanometer-sized metal nanoparticle.

Relativistic spinless rotation-vibrational energies of carbon monoxide

We solve the Klein-Gordon equation with the simplified Poschl-Teller potential model by employing the shape invariance technique, and present the relativistic spinless rotation-vibrational energy equation for diatomic molecules with nuclear spin of zero. It is found that the relativistic effects subject to the relative motion of the ions increase slightly the spinless vibrational energies of the ground electronic state of the carbon monoxide molecule in comparison to the nonrelativistic results. We observe that the variation of the relativistic spinless rotation-vibrational energies with respect to the vibrational quantum number in larger rotational quantum numbers holds similar to that with rotational quantum number of zero.

Effect of conduction band non-parabolicity on the optical gain of quantum cascade lasers based on the effective two-band finite difference method

We theoretically investigate the effect of conduction band non-parabolicity (NPB) on the optical gain spectrum of quantum cascade lasers (QCLs) using the effective two-band finite difference method. Based on the effective two-band model to consider the NPB effect in the multiple quantum wells (QWs), the wave functions and confined energies of electron states are calculated in two different active-region structures, which correspond to three-QW single-phonon and four-QW double-phonon resonance designs. In addition, intersubband optical dipole moments and polar-optical-phonon scattering times are calculated and compared without and with the conduction band NPB effect. Finally, the calculation results of optical gain spectra are compared in the two QCL structures having the same peak gain wavelength of 8.55 μm. The gain peaks are greatly shifted to longer wavelengths and the overall gain magnitudes are slightly reduced when the NPB effect is considered. Compared with the three-QW active-region design, the redshift of the peak gain is more prominent in the four-QW active-region design, which makes use of higher electronic states for the lasing transition.

Strain effects on the energy band structure and electronic states of single-layer MoTe2, WTe2 and their heterostructures

ABSTRACTFirst-principle calculations are done to investigate the strain effects on the energy band structure and electronic states of single-layer MoTe2, WTe2 and their vertically stacked heterostructures. It is illustrated that their band gaps decrease with increasing tensile strain, but increase with compressive strain, and have a maximum value at a compressive strain of 2%. The ideal single-layer MoTe2, WTe2 and their hybrid structure are direct band-gap semiconductors, which are maintained up to a tensile strain of 3%, but change into indirect ones at a compressive strain of 2–3%. The band gap of the MoTe2/WTe2 heterostructures is substantially reduced owing to the coupling between MoTe2 and WTe2 sublayers. The band structure is type II alignment, and is advantageous to photoelectric applications. In brief, lattice straining might induce the rearrangement of band structure, and thus could be adopted to tune the physical properties of 2D materials as well as their hybrids.

Electronic properties and aromaticity of substituted diphenylfulvenes in the ground (S0) and excited (T1) states

In the present account, we investigate electronic properties of diphenylfulvene and its derivatives substituted in phenyl rings. The results were compared with the analogous properties of fulvene and its derivatives with the same substituents at the exocyclic carbon atom. All properties were evaluated and compared in the ground electronic S0 state and in the first excited T1 triplet state. These properties are dipole moments, charges, number of π electrons, and aromaticity of the fulvenic, five-membered ring in the two sets of compounds. The latter property was estimated by the harmonic oscillator model for aromaticity (HOMA) index and, for the fulvenes group, by the calculation of aromatic stabilization energy in both electronic states. It was also investigated whether Baird’s rule alone can account for the aromaticity differences in the two electronic states.

Effect of electric field and dot radius on the quantum state energies of an electron confined in a GaAs/GaxAl1−xAs core shell quantum dot

In this paper the quantum state energies of an electron confined in a GaAs/GaxAl1−xAs core shell quantum dot were calculated using W.K.B approximation method, in the frame work of the effective mass approximation under external electric field. The study reveals that the electron energies of 1s and 2p states decreases with increase in applied external electric field at a given dot radius. Furthermore the electron energies of the states are found to decrease with increase in dot radius and become appreciably small <30eV with electric field of strength 5KV/cm. In addition our results show the effect of shell radius on the electronic structure. For a shell radius R1>5nm, the ground and first excited state electron energies are negligibly small and approaches to a constant value. We also display the effect of Al concentration on the electron energy levels. The electron energy increases and shows non linear dependence with the increase in Al concentration. The observed results are in agreement with the theoretical results reported in the previous literature.

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif.

An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for (H2O)n- and (D2O)n-, based on a nonequilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the nonadiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ∼ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200.

Anomalous de Haas-van Alphen Effect in InAs/GaSb Quantum Wells.

The de Haas-van Alphen effect describes the periodic oscillation of the magnetization in a material as a function of an inverse applied magnetic field. It forms the basis of a well established procedure for measuring Fermi surface properties, and its observation is typically taken as a direct signature of a system being metallic. However, certain insulators can show similar oscillations of the magnetization from quantization of the energies of electron states in filled bands. Recently, the theory of such an anomalous dHvAE (AdHvAE) was worked out, but there has not yet been a clear experimental observation. Here, we show that the inverted narrow gap regime of InAs/GaSb quantum wells is an ideal platform for the observation of the AdHvAE. From our microscopic calculations, we make quantitative predictions for the relevant magnetic field and temperature regimes, and we describe unambiguous experimental signatures.

Donor impurity-related photoionization cross section in GaAs cone-like quantum dots under applied electric field

The donor impurity-related electron states in GaAs cone-like quantum dots under the influence of an externally applied static electric field are theoretically investigated. Calculations are performed within the effective mass and parabolic band approximations, using the variational procedure to include the electron–impurity correlation effects. The uncorrelated Schrödinger-like electron states are obtained in quasi-analytical form and the entire electron–impurity correlated states are used to calculate the photoionisation cross section. Results for the electron state energies and the photoionisation cross section are reported as functions of the main geometrical parameters of the cone-like structures as well as of the electric field strength.

Interatomic potentials of van der Waals dimers Hg2 and Cd2: Probing discrepancies between theory and experiment

Results of new all-electron ab initio calculations and revisit of experimental studies of the interatomic potentials of lower-lying ungerade excited and ground electronic energy states of the Hg2 and Cd2 van der Waals complexes are used as probes of discrepancies between theory and experiment. From simulations of the previously and presently measured LIF excitation and dispersed emission spectra new analytical representations of the excited- and the ground-state interatomic potentials are proposed. An inverted perturbation approach was also used to improve the studied interatomic potentials. The comparison of the new ab-initio calculated potentials with the results of the analyses illustrates an improve theory-to-experiment agreement for such a demanding system like Hg2 or Cd2.

Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission.

The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m-2, most likely because of the long delayed fluorescent lifetime (τd). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac-XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (ES) and triplet state (ET) energies. Among them, Ac-3MHPM, with a high ET of 2.95 eV, exhibited a high external quantum efficiency (ηext,max) of 18% and an ηext of 10% at 100 cd m-2 with Commission Internationale de l'Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters.

Ground State Electronic Energy and Ionization Energy of Atoms: Basis Free Density Functional Calculation

Ground State Electronic Energy and Ionization Energy of Atoms: Basis Free Density Functional Calculation