Optical Properties Of Excitons Research Articles

Tunable interlayer excitons in two-dimensional SiC/MoSSe van der Waals heterostructures

In two-dimensional (2D) van der Waals heterostructures, the excitonic effects are significant enhanced due to the reduction of screening. Accurate description of light-matter interaction of 2D heterostructures is essential to understand their excitonic optical response and radiative lifetime. In this paper, the quasiparticle band structures and excitonic optical properties of SiC/MoSSe heterostructures are investigated by the GW + Bethe-Salpeter equation (GW + BSE) approach. Here the four stable structures 1-AA, 1-A'B, 2-AA and 2-A'B of SiC/MoSSe heterostructures are all type-II band alignment semiconductors, which are favorable for producing interlayer excitons. We find that low energy interlayer excitons of all the four structures posess high binding energies, and the optical dipole oscillator strength and radiative lifetimes can be modulated by several orders of magnitude as a result of the different intrinsic dipole moments of MoSSe. More interestingly, we also find that in 1-A'B and 2-A'B, the lowest-energy exciton is bright interlayer exciton with a strong absorption peak. The detailed analysis of the binding energies, transition contributions and exciton lifetimes of these interlayer excitons will be helpful for SiC/MoSSe heterostructures applications in the optoelectronics.

Strongly Quantum-Confined Blue-Emitting Excitons in Chemically Configurable Multiquantum Wells.

Light matter interactions are greatly enhanced in two-dimensional (2D) semiconductors because of strong excitonic effects. Many optoelectronic applications would benefit from creating stacks of atomically thin 2D semiconductors separated by insulating barrier layers, forming multiquantum-well structures. However, most 2D transition metal chalcogenide systems require serial stacking to create van der Waals multilayers. Hybrid metal organic chalcogenolates (MOChas) are self-assembling hybrid materials that combine multiquantum-well properties with scalable chemical synthesis and air stability. In this work, we use spatially resolved linear and nonlinear optical spectroscopies over a range of temperatures to study the strongly excitonic optical properties of mithrene, that is, silver benzeneselenolate, and its synthetic isostructures. We experimentally probe s-type bright excitons and p-type excitonic dark states formed in the quantum confined 2D inorganic monolayers of silver selenide with exciton binding energy up to ∼0.4 eV, matching recent theoretical predictions of the material class. We further show that mithrene's highly efficient blue photoluminescence, ultrafast exciton radiative dynamics, as well as flexible tunability of molecular structure and optical properties demonstrate great potential of MOChas for constructing optoelectronic and quantum excitonic devices.

Imidazolium Ionic Liquid as Organic Spacer for Tuning the Excitonic Structure of 2D Perovskite Materials

Even though two-dimensional (2D) perovskites have outstanding atmospheric durability, the quantum well (QW) structure of the materials considerably limit their photovoltaic efficiencies far below the three-dimensional (3D) analogues. Herein in this work, we adopt the ionic liquid (IL) 1-butyl-3-methylimidazolium iodide (BMI) as an organic spacer in 2D perovskite to fundamentally alter the QW structures. In comparison to the traditional Ruddlesden–Popper (RP) type perovskites, the new RP-2D perovskite shows drastically decreased interlayer distance to 3.5 Å, which endows stronger interactions between the inorganic slabs and a weaker quantum confinement effect of the material. We further unveil that the combination of BM+ and BA+ cations enables a facile regulation of the excitonic structures and optical properties of 2D perovskite. By this design, the hybrid (BM)2–x(BA)xMAn–1PbnI3n+1 based PSCs achieve an optimized PCE of 17.3%, which is among the highest reported values for 2D PSCs to date. Due to the outstanding heat resilience of IL, the PSCs also demonstrate extraordinary thermal stability under aging at 85 °C.

Regulated Exciton Dynamics and Optical Properties of Single Perovskite CsPbBr3 Quantum Dots by Diluting Surface Ligands

The surface state of all-inorganic cesium lead halide (CsPbX3) perovskite quantum dots (QDs) is a crucial factor for the performance of CsPbX3 QD-based optoelectronic devices. In this research, we ...

Excitonic effects in the optoelectronic properties of graphene-like BC monolayer

Abstract The excitonic effects and optical properties in the graphene-like hexagonal boron carbide (BC) monolayer investigated by ab-initio many-body calculations using GW approximation plus Bethe-Salpeter equation (BSE) approach. BSE calculations have been carried out on the base of G0W0 to integrate excitonic effects. We obtained that the planar BC has direct band gap with metallic character of 3.18 eV. The investigated optical absorption spectrum is dominated by discrete excitonic peaks due to the excitonic states having a noticeably low binding energy of 0.16 eV. The formation of such bound excitons is attributed to enhance electron-hole interaction in low-dimensional materials. By examining the oscillator strength of excitons with imaginary dielectric constant, we confirmed the high accuracy of the calculation. The graphene-like planar BC monolayer exhibit significantly strong and broad optical absorption in the ultraviolet region, which make it a promising candidate for nanoelectronics and optoelectronic applications.

Anisotropic exciton excitations and optical properties of Hittorf's phosphorene

This paper presents a first-principles approach to the quasiparticle electronic structure, exciton, and optical properties in two dimensional Hittorf's phosphorene. The authors show the onset of exciton-enhanced optical absorption and strong polarization-dependent electron-hole excitation.

Observation of excitonic series in monolayer and few-layer black phosphorus

We report the full sequence of excitonic states in monolayer and few-layer black phosphorus (BP). High-quality BP samples are acquired by treating the silicon oxide substrate with a self-assembled monolayer hydrophobic molecule, which facilitates the access to their ground and higher-lying states of intrinsic excitons through the reflection and photoluminescence excitation spectra. Combined with first-principles and model calculations, we further determine that the exciton binding energy shrinks by \ensuremath{\sim}6-fold from 940 meV in monolayer to 160 meV in bilayer BP, yet it only decreases slightly to 108 meV in trilayer. This could be attributed to the strong interlayer coupling and lattice shrinking in multilayer BP samples. Our results shed light on the unique excitonic optical properties of two-dimensional BP.

Excitonic optical properties of cesium trifluoroacetate induced CsPbBr3 thin film with anti-solvent treatment

In this paper, the cesium trifluoroacetate induced CsPbBr3 thin film with anti-solvent chlorobenzene treatment was fabricated where the anti-solvent treatment improved CsPbBr3 photoluminescence emission intensity, thin film surface morphology and crystallization quality. Temperature-dependent photoluminescence and time-resolved photoluminescence from the CsPbBr3 thin film were measured from 10 K to room temperature. Free exciton and bound exciton recombination photoluminescence was identified. The photoluminescence intensity of CsPbBr3 thin film demonstrated a bi-exponentially quenching process with the increase of temperature, yielding two activation energies of 45.54 meV and 9.06 meV for high and low temperature. The photoluminescence lifetime of the CsPbBr3 thin film increased as temperature changed from 20 K to 300 K due to the dissociation effect of excitons. The transient photoluminescence spectra showed an unusual PL peak reverse due to the exciton-exciton scattering and Burstein-Moss effect, which was further observed in the transient absorption spectra. Our results confirm the unique exciton emission profile of the cesium trifluoroacetate induced CsPbBr3 thin film and reveal the complex carriers dynamics property using transient spectroscopy method.

Nonlinear optical characteristics of an exciton in a GaSb-capped InSb heterodot: role of size control

The optical characteristics of a bound electron-hole quasiparticle inside a GaSb-capped InSb nanodot were investigated using a combination between a density-matrix approach and variational calculations. Our theoretical model considers the electric permittivity mismatch between the core and shell materials via the self-energy term obtained by the means of an image charge approach. Furthermore, the core-to-shell conduction and valence band offsets were modeled by finite-depth confinement potentials. To elucidate the impact of the nanoheterodot spatial parameters on the confined exciton optical properties, we have investigated the change of the refractive index, the optical absorption coefficient, and the 1p–1s exciton transition energy with respect to the nanodot shape. Our numerical results exhibit the great potential for the improvement of the exciton optical properties by tailoring the nanodot size. It was also obtained that, for a fixed core radius, the resonance peak position of the optical absorption coefficient was still constant after a certain shell thickness value. Otherwise, decreasing the shell thickness leads to blue-shifting the 1p–1s transition energy.

Excited states of neutral and charged excitons in single strongly asymmetric InP-based nanostructures emitting in the telecom C band

We investigate strongly asymmetric self-assembled nanostructures with one of dimensions reaching hundreds of nanometers. Close to the nanowire-like type of confinement, such objects are sometimes assigned as one-dimensional in nature. Here, we directly observe the spectrum of exciton excited states corresponding to longitudinal quantization. This is based on probing the optical transitions via polarization-resolved microphotoluminescence excitation ($\mu$PLE) measurement performed on single nanostructures combined with theoretical calculation of neutral and charged exciton optical properties. We successfully probe absorption-like spectra for individual bright states forming the exciton ground-state fine structure, as well as for the negatively charged exciton. Confronting the calculated spectrum of excitonic absorption with $\mu$PLE traces, we identify optical transitions involving states that contain carriers at various excited levels related to the longest dimension. Based on cross-polarized excitation-detection scheme, we show very well conserved spin configuration during orbital relaxation of the exciton from a number of excited states comparable to the quasi-resonant pumping via the optical phonon, and no polarization memory for the trion, as theoretically expected.

Excitons in InP, GaP, and GaxIn1−xP quantum dots: Insights from time-dependent density functional theory

Colloidal quantum dots (QDs) of group III-V are considered as promising candidates for next-generation environmentally friendly light emitting devices, yet there appears to be only limited understanding of the underlying electronic and excitonic properties. Using large-scale density functional theory with the hybrid B3LYP functional solving the single-particle states and time-dependent density functional theory accounting for the many-body excitonic effects, we have identified the structural, electronic and excitonic optical properties of InP, GaP and GaInP QDs containing up to a thousand atoms or more. The calculated optical gap of InP QD appears in excellent agreement with available experiments, and it scales nearly linearly with the inverse diameter. The radiative exciton decay lifetime is found to increase surprisingly linearly with increasing the dot size. For GaP QDs, we predict an unusual electronic state crossover at diameter around 1.5 nm whereby the nature of the lowest unoccupied molecular orbital (LUMO) state switches its symmetry from $\Gamma_{5}$-like at larger diameter to $\Gamma_{1}$-like at smaller diameter. After the crossover, the absorption intensity of the band-edge exciton states is significantly enhanced. Finally, we find that Vegard's law holds very well for GaInP random alloyed quantum dots down to ultra-small sizes with less than a hundred atoms. The obtained energy gap bowing parameter of this common-cation compound in QD regime appears positive, size-dependent and much smaller than its bulk parentage. The volume deformation, dominating over the charge exchange and structure relaxation effects, is mainly responsible for the QD energy gap bowing. The present work provides a road map for a variety of electronic and optical properties of colloidal QDs in group III-V that can guide spectroscopic studies.

Optical properties of anisotropic excitons in phosphorene

We study the eigenenergies and optical properties of both direct excitons in a phosphorene monolayer in different dielectric environments, and indirect excitons in heterostructures of phosphorene with hexagonal boron nitride. For these systems, we solve the 2D Schr\"{o}dinger equation using the Rytova-Keldysh (RK) potential for direct, and both the RK and Coulomb potentials for indirect excitons. The results show that excitons formed from charge carriers with anisotropic effective mass exhibit enhanced (suppressed) optical absorption, compared to their 2D isotropic counterparts, under linearly polarized excitations along the crystal axis with relatively smaller (larger) effective carrier masses. This anisotropy leads to dramatically different excited states than the isotropic exciton. The direct exciton binding energy depends strongly on the dielectric environment, and shows good agreement with previously published data. For indirect excitons, the oscillator strength and absorption coefficient increase as the interlayer separation increases. The choice of RK or Coulomb potential does not significantly change the indirect exciton optical properties, but leads to significant differences in the binding energy for small interlayer separation.

The electronic and optical properties of an exciton, biexciton and charged excitons in CdSe/CdTe-based multi-shell type-II quantum dot nanocrystals

It has been recently reported that multi-shell type-II quantum dot nanocrystals (QDNCs) have higher quantum yields. Besides these higher quantum yields of multi-shell type-II QDNCs, additional second layer has been a critical influence on the formation mechanisms of the excitonic structures. Understanding of bound and unbound cases of the excitonic structures in multi-shell type-II QDNCs gives some important information for applications. In this study, we have investigated the electronic and optical properties of a single exciton (X), biexciton (XX), and positively and negatively charged excitons ( $${\mathsf {X^+}}$$ and $${\mathsf {X^-}}$$ ) in CdSe/CdTe-based multi-shell type-II QDNCs. In the study, three different structure compositions, i.e., CdSe/CdTe, CdSe/CdTe/CdS, and CdSe/CdTe/ZnTe, have been considered. We have observed that CdS and ZnTe materials have drastically changed the electronic and optical properties of the bare CdSe/CdTe type-II QDNCs.

Tunable visible-light excitonic absorption and high photoconversion efficiency in two-dimensional group-VI monolayer materials

Recent discovery of tellurene, a two-dimensional (2D) allotrope of tellurium (Te), has effectively extended the realm of 2D materials to group VI. In this paper, we have proceeded to determine the electronic and excitonic optical properties of 2D tellurene and selenene, together with stable monolayer ${\mathrm{TeSe}}_{2}$ in this group, by means of the GW-Bethe-Salpeter equation approach. We find that these materials stabilized in $1\mathrm{T}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$-like structure appear to be indirect semiconductors and possess ultrahigh isotropic carrier mobilities, which outperform their 2D ${\mathrm{MoS}}_{2}$ counterpart. The exciton is found to be strongly bound and thermally stable at room temperature with a binding energy of 0.37, 0.53, and 0.54 eV, respectively, for the three types of monolayers. Strikingly, we find that the optical gaps of monolayer selenene and ${\mathrm{TeSe}}_{2}$ lie in the ideal energy window of $1.0--1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ corresponding to the maximum photoconversion efficiency. The number of layers in the stack further provides an effective means to tune the absorption edge across a wide energy range. These characteristics combined with the strong optical absorbance in the whole visible-light region make both 2D materials promising in the applications of efficient, ultrathin, and flexible photovoltaic devices with upper bounds to the conversion efficiency that rivals organic and dye-sensitized devices.

Electrically driven uniaxial stress device for tuning in situ semiconductor quantum dot symmetry and exciton emission in cryostat

Uniaxial stress is a powerful tool for tuning exciton emitting wavelength, polarization, fine-structure splitting (FSS), and the symmetry of quantum dots (QDs). Here, we present a technique for applying uniaxial stress, which enables us in situ to tune exciton optical properties at low temperature down to 15 K with high tuning precision. The design and operation of the device are described in detail. This technique provides a simple and convenient approach to tune QD structural symmetry, exciton energy and biexciton binding energy. It can be utilized for generating entangled and indistinguishable photons. Moreover, this device can be employed for tuning optical properties of thin film materials at low temperature.

Excitonic nonlinear optical properties in AlN/GaN spherical core/shell quantum dots under pressure

Abstract

Influence of π -conjugated cations and halogen substitution on the optoelectronic and excitonic properties of layered hybrid perovskites

Low-cost chemical engineering of two-dimensional layered hybrid halide perovskite structures allows for the design of hybrid semiconductor quantum wells with tailored room-temperature excitonic optical absorption, emission, and charge carrier transport properties. Here density functional theory and the Bethe-Salpeter equation are used to predict the electronic structure and optical response of layered perovskites with two representative single-ring conjugated organic spacers, ammonium-propyl-imidazole (API) and 2-phenethylammonium (PEA). The inorganic perovskite quantum well properties are further tuned by analyzing the effect of halogen (X = I, Br, Cl) substitution. We found that visible light absorption occurs primarily within the perovskite layer and that UV light absorption induces partial electron-hole separation between layers. In addition, a strong exciton binding energy and influence on absorption spectrum is found by solving the Bethe-Salpeter equation. Our results suggest that further engineering is necessary beyond the single-ring limit, by introducing more conjugated rings and/or heavier nuclei into the organic spacer. This is a promising future direction to achieve photoinduced charge separation and more generally hybrid heterostructures with attractive optoelectronic properties.

Effect of LiNbO3 polarity on the structural, optical and acoustic properties of epitaxial ZnO and MgxZn1−xO films

The effect of LiNbO3 substrate polarization on a charge density at the interface with epitaxial ZnO and MgxZn1−xO films was studied in comparison with ZnO films on dielectric sapphire substrates. An excitonic emission and optical properties such as band gap energy, refraction index, absorption and reflectivity of the ZnO/LiNbO3 structures were studied. The effect of interface charges on the performance of surface acoustic waves with frequency of 2.7 GHz was investigated, as well. The stimulated cavity mode emission was observed due to attained quasi-phase matching conditions for thin ZnO films on sapphire at high laser excitation power. The positive polarization of the ferroelectric substrate induced the band bending of the semiconductor layer, resulting in the accumulation of the negative charges at the ferroelectric/semiconductor interface, which induced an increased reflectivity, a generation of oxygen emission lines in PL spectra under laser excitation, and an increased electromechanical coupling of surface acoustic waves. The negative polarization of the substrate increased atomic peening during sputtering deposition, which induced deteriorated quality of texture and in-plane orientation and increased the residual stresses in ZnO and MgxZn1−xO films.

Optical properties of excitons in buckled two-dimensional materials in an external electric field

We study the binding energies and optical properties of direct and indirect excitons in monolayers and double layer heterostructures of Xenes: silicene, germanene, and stanene. It is demonstrated that an external electric field can be used to tune the eigenenergies and optical properties of excitons by changing the effective mass of charge carriers. The Schr\"{o}dinger equation with field-dependent exciton reduced mass is solved by using the Rytova-Keldysh (RK) potential for direct excitons, while both the RK and Coulomb potentials are used for indirect excitons. It is shown that for indirect excitons, the choice of interaction potential can cause huge differences in the eigenenergies at large electric fields and significant differences even at small electric fields. Furthermore, our calculations show that the choice of material parameters has a significant effect on the binding energies and optical properties of direct and indirect excitons. These calculations contribute to the rapidly growing body of research regarding the excitonic and optical properties of this new class of two dimensional semiconductors.

Effect of disorders on the optical properties of excitons in InAsP/InP quantum wells investigated by magneto-photoluminescence spectroscopy

Disorders acquainted in semiconductor quantum structures can change the crystal potential and thus play an important role in the performance of optoelectronic devices. Such disorders are critical in the performance of InAsP/InP quantum well devices, which are useful for telecommunication applications. These disorders originate due to the difference in adsorption and the sticking coefficient of arsenic and phosphorus. In our previous paper [J. Appl. Phys.119, 095708 (2016)JAPIAU0021-897910.1063/1.4943031], we reported the impact of carrier localizations on the performance of InAsP/InP quantum well detectors. In this work, the magnetic field is used to investigate exciton properties in InAsP/InP quantum wells. Line shape analysis of magneto-photoluminescence (magneto-PL) distinguishes the role of disorders on the excitonic properties that are useful for the identification of sample quality. Furthermore, the blue shift in PL peak energy with an applied magnetic field is used to estimate the effective mass of excitons in InAsP/InP quantum wells. The estimated effective mass of excitons confined in the quantum well is significantly higher than that of the corresponding bulk value, which is explained by considering the non-parabolicity of the conduction band. The non-parabolicity parameter is also estimated and used for obtaining the electron dispersion E-k curve for InAsP/InP quantum wells.