Biexciton Complexes Research Articles

Control of the valley polarization of monolayer WSe2 by Dexter-like coupling

Intervalley scattering mechanisms strongly affect the dynamics of excitonic complexes in transition metal dichalcogenide monolayers. Here, we investigate the excitation energy dependence of the valley polarization of excitons in a WSe2 monolayer. We observe that the valley polarization drastically decreases when the excitation is resonant with the B 1s resonance. This behavior can be explained by a Dexter-like coupling in the momentum space between exciton states residing in opposite valleys but with the same spin configuration. This induces a net transfer of the exciton population from the optically driven valley towards the opposite, undriven valley. We observe the long-term fingerprints of this population transfer as a vanishing valley polarization for the neutral exciton, and a negative valley polarization for biexcitonic complexes, in qualitative agreement with theoretical predictions based on a fully microscopic many-particle approach. This, together with a decrease of the PL energy when the excitation is resonant with the B 1s state, points to the prominent role of the Dexter-like coupling in the exciton dynamics of atomically thin semiconductors.

Determination of exciton binding energy using photocurrent spectroscopy of Ge quantum-dot single-hole transistors under CW pumping

We reported exciton binding-energy determination using tunneling-current spectroscopy of Germanium (Ge) quantum dot (QD) single-hole transistors (SHTs) operating in the few-hole regime, under 405–1550 nm wavelength (λ) illumination. When the photon energy is smaller than the bandgap energy (1.46 eV) of a 20 nm Ge QD (for instance, λ = 1310 nm and 1550 nm illuminations), there is no change in the peak voltages of tunneling current spectroscopy even when the irradiation power density reaches as high as 10 µW/µm2. In contrast, a considerable shift in the first hole-tunneling current peak towards positive VG is induced (ΔVG ≈ 0.08 V at 0.33 nW/µm2 and 0.15 V at 1.4 nW/µm2) and even additional photocurrent peaks are created at higher positive VG values (ΔVG ≈ 0.2 V at 10 nW/µm2 irradiation) by illumination at λ = 850 nm (where the photon energy matches the bandgap energy of the 20 nm Ge QD). These experimental observations were further strengthened when Ge-QD SHTs were illuminated by λ = 405 nm lasers at much lower optical-power conditions. The newly-photogenerated current peaks are attributed to the contribution of exciton, biexciton, and positive trion complexes. Furthermore, the exciton binding energy can be determined by analyzing the tunneling current spectra.

Photon Echo Polarimetry of Excitons and Biexcitons in a CH3NH3PbI3 Perovskite Single Crystal

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Here we specifically address the energy level order of optical transitions and demonstrate photon echos in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times $T_2$ for excitons and biexcitons at cryogenic temperature to be 0.79 ps and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes, but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing.

Electronic and Optical Properties of InAs QDs Grown by MBE on InGaAs Metamorphic Buffer.

We present the optical characterization of GaAs-based InAs quantum dots (QDs) grown by molecular beam epitaxy on a digitally alloyed InGaAs metamorphic buffer layer (MBL) with gradual composition ensuring a redshift of the QD emission up to the second telecom window. Based on the photoluminescence (PL) measurements and numerical calculations, we analyzed the factors influencing the energies of optical transitions in QDs, among which the QD height seems to be dominating. In addition, polarization anisotropy of the QD emission was observed, which is a fingerprint of significant valence states mixing enhanced by the QD confinement potential asymmetry, driven by the decreased strain with increasing In content in the MBL. The barrier-related transitions were probed by photoreflectance, which combined with photoluminescence data and the PL temperature dependence, allowed for the determination of the carrier activation energies and the main channels of carrier loss, identified as the carrier escape to the MBL barrier. Eventually, the zero-dimensional character of the emission was confirmed by detecting the photoluminescence from single QDs with identified features of the confined neutral exciton and biexciton complexes via the excitation power and polarization dependences.

Emission Kinetics of Surface (Bi)Excitons in ZnO Thin Films

The kinetics of near-edge photoluminescence (PL) in ZnO nanofilms prepared by the atomic layer deposition has been investigated. It is established that the kinetics of near-edge PL in 4-nm films is determined to a great extent by surface 2D-exciton (SX) and biexciton (SXX) complexes. The contribution from surface biexcitons is estimated based on a photostimulated change in the surface potential in ZnO films with different thicknesses. Ultrafast dynamics of surface biexcitons in thin films are revealed. It is shown that biexcitons localized near the surface have the shortest radiative lifetime (less than 100 ps) among all bound exciton complexes, which is explained by the large oscillator strength.

Кинетика излучения поверхностных (би) экситонов в тонких пленках ZnO

AbstractThe kinetics of near-edge photoluminescence (PL) in ZnO nanofilms prepared by the atomic layer deposition has been investigated. It is established that the kinetics of near-edge PL in 4-nm films is determined to a great extent by surface 2D-exciton (SX) and biexciton (SXX) complexes. The contribution from surface biexcitons is estimated based on a photostimulated change in the surface potential in ZnO films with different thicknesses. Ultrafast dynamics of surface biexcitons in thin films are revealed. It is shown that biexcitons localized near the surface have the shortest radiative lifetime (less than 100 ps) among all bound exciton complexes, which is explained by the large oscillator strength.

Charge-tuneable biexciton complexes in monolayer WSe2

Monolayer transition metal dichalcogenides have strong Coulomb-mediated many-body interactions. Theoretical studies have predicted the existence of numerous multi-particle excitonic states. Two-particle excitons and three-particle trions have been identified by their optical signatures. However, more complex states such as biexcitons have been elusive due to limited spectral quality of the optical emission. Here, we report direct evidence of two biexciton complexes in monolayer tungsten diselenide: the four-particle neutral biexciton and the five-particle negatively charged biexciton. We distinguish these states by power-dependent photoluminescence and demonstrate full electrical switching between them. We determine the band states of the elementary particles comprising the biexcitons through magneto-optical spectroscopy. We also resolve a splitting of 2.5 meV for the neutral biexciton, which we attribute to the fine structure, providing reference for subsequent studies. Our results unveil the nature of multi-exciton complexes in transitionmetal dichalcogenides and offer direct routes towards deterministic control in many-body quantum phenomena.

Revealing the biexciton and trion-exciton complexes in BN encapsulated WSe2

Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions, and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we resolve the conflict by employing low-temperature photoluminescence spectroscopy to identify the biexciton state in BN-encapsulated single-layer WSe2. The biexciton state only exists in charge-neutral WSe2, which is realized through the control of efficient electrostatic gating. In the lightly electron-doped WSe2, one free electron binds to a biexciton and forms the trion–exciton complex. Improved understanding of the biexciton and trion–exciton complexes paves the way for exploiting the many-body physics in TMDs for novel optoelectronics applications.

Superconducting phase diagrams of cuprates and pnictides as a key to understanding the HTSC mechanism

This paper reviews experimental phase diagrams of cuprates and pnictides to demonstrate that specific features of the superconducting phase diagrams in both HTSC families can be understood within the framework of the proposed approach, which assumes the formation, under heterovalent doping, of localized trion complexes consisting of a doped carrier and charge transfer (CT) excitons. The geometry of such cells containing CT excitons (CT plaquettes) in the basal plane of the crystal is determined by its crystal structure and the type of dopant, so that the dopant concentration range corresponding to the existence of a percolation cluster of CT plaquettes can be readily determined for each particular compound. These dopant concentration ranges coincide with good accuracy with the experimental ranges of superconducting domes in the phase diagrams of the HTSC compounds considered. The generation of free carriers and the mechanism of superconducting pairing in this pattern is related to biexciton complexes (Heitler–London centers) emerging in neighboring CT plaquettes.

Area- and Thickness-Dependent Biexciton Auger Recombination in Colloidal CdSe Nanoplatelets: Breaking the “Universal Volume Scaling Law”

Colloidal nanoplatelets (NPLs) have shown great potentials for lasing applications due to their sharp absorption and emission peaks, large absorption cross sections, large radiative decay rates, and long multiexciton lifetimes. How multiexciton lifetimes depend on material dimensions remains unknown in two-dimensional (2D) materials, despite being a key parameter affecting optical gain threshold and many other properties. Herein, we report a study of room-temperature biexciton Auger recombination time of CdSe NPLs as a function of thickness and lateral area. Comparison of all NPLs shows that the biexciton lifetime does not increase linearly with volume, unlike previously reported "universal volume scaling law" for quantum dots. For NPLs of the same thickness (∼1.8 nm), the biexciton lifetime increase linearly with their lateral area (from 143.7 ± 12.6 to 320.1 ± 17.1 ps when the area increases from 90.5 ± 21.4 to 234.2 ± 41.9 nm2). The biexciton lifetime depends linearly on (1/Ek(e))7/2 (Ek(e) is the electron confinement energy) or nearly linearly on d7 (d is NPL thickness). The observed dependence is consistent with a model in which biexciton Auger recombination rate scales with the product of exciton binary collision frequency and Auger recombination probability in biexciton complexes. The linear increase of Auger lifetimes with NPL lateral areas reflects a 1/area dependence of the binary collision frequency for 2D excitons and the thickness-dependent biexciton Auger recombination time is attributed to its strong dependence on the degree of quantum confinement. This model may be generally applicable to exciton Auger recombination in quantum confined 1D and 2D nanomaterials.

Binding energies of exciton complexes in transition metal dichalcogenide monolayers and effect of dielectric environment

Excitons, trions, biexcitons, and exciton-trion complexes in two-dimensional transition metal dichalcogenide sheets of MoS$_2$, MoSe$_2$, MoTe$_2$, WS$_2$ and WSe$_2$ are studied by means of density functional theory and path integral Monte Carlo method in order to accurately account for the particle-particle correlations. In addition, the effect of dielectric environment on the properties of these exciton complexes is studied by modifying the effective interaction potential between particles. Calculated exciton and trion binding energies are consistent with previous experimental and computational studies, and larger systems such as biexciton and exciton-trion complex are found highly stable. Binding energies of biexcitons are similar or higher than those of trions, but the binding energy of the trion depends significantly stronger on the dielectric environment than that of biexciton. Therefore, as a function of an increasing dielectric constant of the environment the exciton-trion complex "dissociates" to a biexciton rather than to an exciton and a trion.

High temperature stability in non‐polar (11$ \bar 2 $0) InGaN quantum dots: Exciton and biexciton dynamics

AbstractWe report on optical studies of non‐polar InGaN quantum dots grown on the (11$ \bar 2 $2) plane. Excitonic and biexcitonic complexes are identified by their power dependence and show similar binding energies (∼ 36 meV) and recombination dynamics to conventional polar (0001) InGaN quantum dots. Measured lifetimes as low as 300 ps suggest a reduced internal electric field when compared with polar InGaN quantum dots.Temperature dependent micro‐photoluminescence measurements on a single exciton with a lifetime of 327ps reveal no significant exciton linewidth broadening up to 120K, suggesting a reduction in phonon coupling strength when compared to polar quantum dots. This is supported by a measured lifetime of 313 ps for this exciton at 77 K, suggesting the measured exciton decay is almost purely radiative. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiexciton complex from extrinsic centers in AlGaAs epilayers on Ge and Si substrates

The multiexciton properties of extrinsic centers from AlGaAs layers on Ge and Si substrates are addressed. The two photon cascade is found both in steady state and in time resolved experiments. Polarization analysis of the photoluminescence provides clearcut attribution to neutral biexciton complexes. Our findings demonstrate the prospect of exploiting extrinsic centers for generating entangled photon pairs on a Si based device.

Bound and anti-bound biexciton in site-controlled pyramidal GaInAs/GaAs quantum dots

We present a detailed study of biexciton complexes formed in single, site-controlled pyramidal GaInAs/GaAs quantum dots (QDs). By using power dependent measurements and photon correlation spectroscopy, we identify the excitonic transitions of a large number of pyramidal QDs, exhibiting both positive and negative biexciton binding energies. Separation of charges within the QD, caused by piezoelectric fields, is believed to be responsible for the positive to negative crossover of the biexciton binding energy with increasing QD size. In particular, QDs exhibiting vanishing biexciton binding energies are evidenced, with potential applications in quantum information processing.

Theory of highly excited semiconductor nanostructures including Auger coupling: Exciton-biexciton mixing in CdSe nanocrystals

We present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. The Coulomb matrix elements connecting exciton, bi-exciton and tri-exciton complexes are derived and an intuitive picture of breaking neutral multi-exction complexes into positively and negatively charged multi-exciton complexes is given. The general approach is illustrated by analyzing the coupling of biexciton and exciton in CdSe spherical nanocrystals. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting and surface passivation. For each number of electron-hole pairs the many-body spectrum is computed in the configuration-interaction approach. The low-energy correlated biexciton levels are broken into charged complexes: a hole and a negatively charged trion and an electron and a positively charged trion. Out of a highly excited exciton spectrum a subspace coupled to bi-exciton levels via Auger processes is identified. The interaction between correlated bi-exciton and exciton states is treated using exact diagonalization techniques. This allows to extract the spectral function of the biexciton and relate its characteristic width and amplitude to the characteristic amplitude and timescale of the coherent time evolution of the coupled system. It is shown that this process can be described by the Fermi's Golden Rule only if a fast relaxation of the excitonic subsystem is accounted for.

Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals

Theory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells ($s$, $p$, ...), whilst the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the bi-exciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and bi-exciton spectra. The quasi-degenerate nature of the hole spectrum results in a correlated bi-exciton state, which makes it slowly convergent with basis size. We carry out a systematic study of the exciton and bi-exciton emission spectra as a function of the nanocrystal diameter and find that the interplay of repulsion between constituent excitons and correlation effects results in a change of the sign of bi-exciton binding energy from negative to positive at a critical nanocrystal size.

Positive binding energy of a biexciton confined in a localization center formed in a singleInxGa1−xN/GaNquantum disk

We report microphotoluminescence spectroscopy performed on individual and ensemble InGaN/GaN quantum disks (Q-disks). The typical spectrum of a single Q-disk exhibited the contribution of localization centers (LCs) formed in the InGaN active layer of the Q-disks, characterized by sharp lines appearing on the low energy side of the spectra. In addition, a broader emission peak identified as the luminescence of the quasi-two-dimensional (2D) InGaN active layer surrounding the LCs appears systematically at higher energy. Time-resolved photoluminescence experiment performed on single Q-disks exhibited the excitonic transfer, from the 2D InGaN active layer to LCs, at the submicroscopic scale. Excitation power dependence studies and linear polarization analysis allowed us to identify a biexciton complex confined in a LC in a single Q-disk with a surprising positive binding energy of 13 meV. The absence of screening effect by increasing the excitation power density and the fast excitonic radiative lifetime of a few hundred picoseconds that we measured on several individual Q-disks indicate that the absence of internal electric field in the structure can explain the observed positive biexciton binding energy.

A spin-polarized bi-exciton in a semiconductor quantum dot

We discuss the role of spin-polarized multi-exciton complexes in the emission spectra ofsemiconductor quantum dots. The spin-polarized bi-exciton complex composed of twospin-polarized electrons and two spin-polarized holes confined in a single, parabolic,self-assembled quantum dot is discussed in detail. The configuration-interaction approach isused to compute the energies, wavefunctions, and emission spectra as a function of themagnetic field. It is shown that the spin-polarized bi-exciton emission spectrumdiffers from the unpolarized spectrum due to the Pauli blocking and exchangeinteractions. The emission from spin-polarized bi-exciton allows for the measurement ofground and excited single-exciton states, and its magnetic field dependence allowsfor the differentiation of the spin-polarized bi-exciton and tri-exciton spectrum.

Optical properties of single and multi‐layer InGaN quantum dots

AbstractWe discuss the optical properties of single InGaN QDs grown by MOVPE. The samples were structured into mesas by focused‐ion‐beam etching and investigated by micro‐photoluminescence measurements. The QDs are characterized by a high temperature stability of their emission up to 150 K and strong localization of the carriers within the quantum dots, indicated by the observation of antibinding biexcitonic complexes. The influences of piezoelectric fields and spectral diffusion effects were investigated. Furthermore, the polarization of individual QD emission lines was analyzed giving an insight into their geometrical shape. The quantum dots were used for the realization of quantum dot stacks. The photoluminescence of these samples can be traced up to room temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exact biexciton binding energy in carbon nanotubes using a quantum Monte Carlo approach

We performed quantum Monte Carlo (QMC) calculations for a model system of excitons and biexcitons in carbon nanotubes (CNTs) and compared the results with those of a variational approach [T.G. Pedersen, K. Pedersen, H.D. Cornean, P. Duclos, Nano Lett. 5 (2005) 291]. Due to their geometric properties, the shape of a hollow cylinder, CNTs can be treated as 2D objects. With decreasing diameter one expects them even to exhibit quasi-1D properties. In the present study the biexciton in its ground state is found to be more strongly bound than estimated before. Biexcitonic complexes are predicted to remain stable for all diameters even at room temperature. The binding energy grows significantly with decreasing diameter, showing indeed a transition from a quasi-2D system to a quasi-1D system.