Excitonic Spectra Research Articles

Wannier–Mott excitons in GaSe single crystals

The absorption, reflection and photoluminescence spectra of GaSe crystals with different thicknesses (100 nm–1 mm) were investigated in a wide temperature interval (300 K–10 K). Features due to excitonic states in the spectra were recognized. The contours of the excitonic reflection spectra measured at 10 K were calculated by dispersion equations. On the basis of these calculations, the parameters of the observed excitons were determined. Photoluminescence spectra from the cleaved surface and from the uncleaved facet of a sample were measured at low temperatures.

Excitonic complexes in InAs/InP nanowire quantum dots

InAs quantum dots embedded in InP nanowires form an important platform for basic research studies, as well as for quantum dot applications. Notably, understanding of nanowire quantum dot spectral properties is essential in both cases. Therefore, in this work we use atomistic theory to study spectra of the single exciton $(X)$, the biexciton $(XX)$, the triexciton $(XXX)$, and the positively and negatively charged trions $({X}^{+}$ and ${X}^{\ensuremath{-}})$ confined in these nanostructures. We focus on the role of vertical and lateral confinement, therefore, we systematically study a large family of quantum dots with different heights and diameters, and find the important role of correlations due to presence of higher states. We find that the order of excitonic binding energies is a characteristic feature of InAs/InP nanowire quantum dots being (ordered from negative to positive values): ${X}^{\ensuremath{-}},XX$, and ${X}^{+}$, with strongly bound ${X}^{\ensuremath{-}}$, rather weakly bound $XX$, and typically unbound ${X}^{+}$. Next, we determine the key role of alloy randomness due to intermixing, which turns out to especially important for larger quantum dot heights and phosphorous contents over 40%. In selected cases, the alloying can lead to an unbound biexciton, and can even reverse ordering of excitonic lines.

Origin of Energy States in the Bandgap of Zn1 –xMnxO

We report on the results of analysis of optical absorption, EPR signals under optical excitation and magnetic susceptibility of hydrothermal Zn1 –xMnxO single crystals. In the absorption spectra of polarized light at temperatures of 4.2 and 77.3 K, narrow intense a, b, c, and d lines are observed in the energy range 1.877–1.936 eV of light quanta. The spectrum of these lines differs significantly from the spectra of donor and acceptor excitons for ZnO:Co and ZnO:Ni. The intensity of allowed and forbidden EPR signals of the Mn2+(d5) ions does not change under the action of light in the impurity absorption band, while the EPR signals of uncontrollable Fe3+ (d5) ions under illumination practically disappear. New experimental results for Zn1 –xMnxO lead to the conclusion that the d5/d4 donor level of the Mn2+ ion falls into the valence band, while the bandgap of Zn1 –xMnxO contains several dangling bond hybrid (DBH) states due to hybridization of 3d orbitals of the Mn2+ ion with the p-bonds of the nearest O2– oxygen ions. Electron transitions from the DBH states to the conduction band form a broad impurity absorption band of Zn1 –xMnxO, below the edge of which the a, b, c, and d lines referred to as donor excitons [(hloc + d5)e] and emerging as a result of Coulomb interaction of a free s-electron and a hole localized on DBH states (p + d5) are observed. The detection of donor excitons [(hloc + d5)e] makes it possible to study in detail the DBH states in the bandgap, which is important for photocatalysis in the visible light range.

The two light-harvesting membrane chromoproteins of Thermochromatium tepidum expose distinct robustness against temperature and pressure

An increased robustness against high temperature and the much red-shifted near-infrared absorption spectrum of excitons in the LH1-RC core pigment-protein complex from the thermophilic photosynthetic purple sulfur bacterium Thermochromatium tepidum has recently attracted much interest. In the present work, thermal and hydrostatic pressure stability of the peripheral LH2 and core LH1-RC complexes from this bacterium were in parallel investigated by various optical spectroscopy techniques applied over a wide spectral range from far-ultraviolet to near-infrared. In contrast to expectations, very distinct robustness of the complexes was established, while the sturdiness of LH2 surpassed that of LH1-RC both with respect to temperatures between 288 and 360 K, and pressures between 1 bar and 14 kbar. Subtle structural variances related to the hydrogen bond network are likely responsible for the extra stability of LH2.

Dielectric Confinement Enables Molecular Coupling in Stacked Colloidal Nanoplatelets.

We show theoretically that carriers confined in semiconductor colloidal nanoplatelets (NPLs) sense the presence of neighbor, cofacially stacked NPLs in their energy spectrum. When approaching identical NPLs, the otherwise degenerate energy levels red-shift and split, forming (for large stacks) minibands that are several millielectronvolts in width. Unlike in epitaxial structures, the molecular behavior does not result from quantum tunneling but from changes in the dielectric confinement. The associated excitonic absorption spectrum shows a rich structure of bright and dark states, whose optical activity and multiplicity can be understood from reflection symmetry and Coulomb tunneling. We predict spectroscopic signatures that should confirm the formation of molecular states, whose practical realization would pave the way for the development of nanocrystal chemistry based on NPLs.

Band nesting and exciton spectrum in monolayer MoS2

We discuss here the effect of band nesting and topology on the spectrum of excitons in a single layer of MoS$_2$, a prototype transition metal dichalcogenide material. We solve for the single particle states using the ab initio based tight-binding model containing metal $d$ and sulfur $p$ orbitals. The metal orbitals contribution evolving from $K$ to $\Gamma$ points results in conduction-valence band nesting and a set of second minima at $Q$ points in the conduction band. There are three $Q$ minima for each $K$ valley. We accurately solve the Bethe-Salpeter equation including both $K$ and $Q$ points and obtain ground and excited exciton states. We determine the effects of the electron-hole single particle energies including band nesting, direct and exchange screened Coulomb electron-hole interactions and resulting topological magnetic moments on the exciton spectrum. The ability to control different contributions combined with accurate calculations of the ground and excited exciton states allows for the determination of the importance of different contributions and a comparison with effective mass and $k\cdot p$ massive Dirac fermion models.

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)2PbI4) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.

Excitonic and Vibronic Spectra of a Two-Dimensional Graphene-like Molecular Lattice

Recently, graphene has shown interesting physical properties with promising applications. In this paper, we study the excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional h...

The Wannier-Mott exciton’s dynamic properties without limitations on the magnitudes of the wave vectors of carriers

Most often, the excitation of the Wannier-Mott excitons’ type is generated by electromagnetic action when the electron from the valence band passes into the conduction band. The influence of a magnitude wave vector for the exciton dynamic on the wave vector of the dynamics of the internal relative motion of the carriers is considered. It is shown that due to the effect of the field of intra-crystalline stresses on the exciton movement, a substantial displacement of the exciton spectrum may occur. Estimates of the possible magnitude of such a displacement were made.

Зависимость продольно-поперечного расщепления экситона в квантовой яме от внешнего однородного электрического поля

In our paper theoretical analyzed the decreasing of transverse-longitudinal splitting of an exciton in the GaAs/AlGaAs quantum well in the homogeneous electric field. Also the dependence of the splitting on application field is calculated. The excitonic reflectance spectra of the thick quantum well are calculated for field directed transversal to well layer. The decreasing of reflectance oscillation in the excitonic spectra because of decreasing transverse-longitudinal splitting is demonstrated.

Lévy distributions and disorder in excitonic spectra.

We study analytically the spectrum of excitons in disordered semiconductors like transition metal dichalcogenides, which are important for photovoltaic and spintronic applications. We show that ambient disorder exerts a strong influence on the exciton spectra. For example, in such a case, the well-known degeneracy of the hydrogenic problem (related to Runge-Lenz vector conservation) is lifted so that the exciton energy starts to depend on both the principal quantum number n and orbital l. We model the disorder phenomenologically substituting the ordinary Laplacian in the corresponding Schrödinger equation by the fractional one with Lévy index μ, characterizing the degree of disorder. Our variational treatment (corroborated by numerical results) shows that an exciton exists for 1 < μ≤ 2. The case μ = 2 corresponds to the "ordered" hydrogenic problem, while in the opposite case μ = 1 the exciton collapses. The exciton spectrum is dominated by the sample areas with moderate disorder. Our theory permits controlled predictions to be made of the excitonic properties in semiconductor samples with different degrees of disorder.

Exciton properties and optical spectra of light harvesting complex II from a fully atomistic description.

We present a fully atomistic simulation of linear optical spectra (absorption, fluorescence and circular dichroism) of the Light Harvesting Complex II (LHCII) trimer using a hybrid approach, which couples a quantum chemical description of the chlorophylls with a classical model for the protein and the external environment (membrane and water). The classical model uses a polarizable Molecular Mechanics force field, thus allowing mutual polarization effects in the calculations of the excitonic properties. The investigation is performed both on the crystal structure and on structures generated by a μs long classical molecular dynamics simulation of the complex within a solvated membrane. The results show that this integrated approach not only provides a good description of the excitonic properties and optical spectra without the need for additional refinements of the excitonic parameters, but it also allows an atomistic investigation of the relative importance of electronic, structural and environment effects in determining the optical spectra.

Emission Properties of GaN Planar Hexagonal Microcavities

Fabrication of microcavities based on III‐nitrides is challenging due to difficulties with the coherent growth of heterostructures having a large number of periods, at the same time keeping a good precision in terms of thickness and composition of the alloy. A planar design for GaN microresonators supporting whispering gallery modes is suggested. GaN hexagonal microstructures are fabricated by selective‐area metalorganic vapor phase epitaxy using focused ion beam for mask patterning. Low‐temperature cathodoluminescence spectra measured with a high spatial resolution demonstrate two dominant emission lines in the near bandgap region. These lines merge at room temperature into a broad emission band peaking at ≈3.3 eV, which is shifted toward lower energies compared with the reference excitonic spectrum measured for the GaN layer. A numerical analysis of exciton–polariton modes shows that some strongly localized cavity modes can have high Purcell coefficients and can strongly interact with the GaN exciton.

Composite super-moiré lattices in double-aligned graphene heterostructures.

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.

Exciton Spectra and Energy Transfer in CdTe/ZnTe Double Quantum Wells Grown by Atomic-Layer Epitaxy

Exciton Spectra and Energy Transfer in CdTe/ZnTe Double Quantum Wells Grown by Atomic-Layer Epitaxy

Influence of electron-hole plasma on Rydberg excitons in cuprous oxide

We develop a many-body approach to the behavior of exciton bound states and the conduction electron band edge in a surrounding electron-hole plasma with a focus on the absorption spectrum of Rydberg excitons in cuprous oxide. The interplay of band edge and exciton levels is analyzed numerically, whereby the self-consistent solution is compared to the semiclassical Debye approximation. Our results provide criteria which allow to verify or rule out the different band edge models against future experimental data.

Magneto-excitons in Cu2O: theoretical model from weak to high magnetic fields

Recent experimental and theoretical work on hydrogen-like absorption spectra of excitons in external magnetic fields revealed new effects when the Coulomb interaction becomes comparable to the magnetic perturbation. We present a theoretical approach that allows for calculation of absorption spectra for any value of magnetic field. This approach opens the possibility to compute the optical functions i.e. reflectivity, transmission and absorption including the excitonic effects for various strength of external magnetic field.

Exciton spectra of Cs1–xRbxPbCl3 solid solution thin films

The exciton spectra of Cs1–xRbxPbCl3 solid solution thin films are studied in a range of 2–6 eV. The formation of solid solutions stable at room temperature was detected in range of concentrations 0 ≤ х ≤ 0.7. A linear dependence of the exciton band and bandgap width parameters on the concentration was found. Kinks are observed along the temperature dependences of the spectral position Em(T) (x &amp;gt; 0) of the long-wave exciton band at 310 and 320 K that are characteristic of second-order phase transitions. The exciton excitations in CsPbCl3 are found to have a three-dimensional nature, while in Cs1–xRbxPbCl3 (x &amp;gt; 0) solid solutions they are found to be two-dimensional.

Energy Spectrum of Two-Dimensional Excitons in a Nonuniform Dielectric Medium.

We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the s-type Rydberg series of excitonic states follows a simple energy ladder: ε_{n}=-Ry^{*}/(n+δ)^{2}, n=1,2,…, in which Ry^{*} is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is accounted for only by δ. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe_{2} ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schrödinger equation when approximating the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other MoSe_{2}, WS_{2}, MoS_{2} MLs encapsulated in hBN, we estimate an apparent magnitude of δ for each of the studied structures. Intriguingly, δ is found to be close to zero for WSe_{2} as well as for MoS_{2} monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom.

Generalized Kasha’s Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems

Summary Excitonic organic materials owe their tunable optoelectronic properties to intricate microscopic configurations, challenging for conventional characterization methods to resolve. We generalize Kasha’s model by incorporating the temperature (T)-dependent absorption peak shift, in addition to the monomer-aggregate absorption peak shift that defines J- and H-aggregates, to characterize the microscopic structures. We show that the short-range interactions dominate in determining the direction of the T-dependent peak shift, which accounts for previously observed T-dependent blueshifting J-aggregates (BJ-aggregates) that were not explained and predicts the existence of redshifting H-aggregates (RH-aggregates). This defines four types of excitonic aggregates: RJ-, BH-, BJ-, and RH-aggregates, where the latter two unconventional aggregates are possible because of the two dimensionality of excitonic systems and cannot be understood with the original Kasha’s theory. Our conceptual framework is useful for elucidating structure-function relationships in molecular excitonic systems and is fully compatible with existing tools. Possible extensions to other spectroscopic observables and related systems are discussed.