Exciton Radiative Lifetime Research Articles

(Invited) Photoluminescence Dynamics in PbS/CdS Core/Shell Nanocrystal Films: Carrier Recombination and Energy Transfer

Colloidal semiconductor nanocrystals (NCs) exhibit highly controllable size-dependent optical properties due to the quantum confinement effect. Among other semiconductor NCs, the lead chalcogenides, especially PbS, are one of the most studied and promising NC systems. They are the material of choice for studies of fundamental phenomena as well as for the development of solution processed infrared optoelectronic devices including LEDs, lasers, solar cells, and photodetectors. Compared to organically passivated NCs, the core-shell approach (more specifically PbS/CdS core-shell NCs) offers not only the improved stability and higher photoluminescence (PL) quantum efficiency but also an opportunity of the core/shell interface nano-engineering resulting in suppression of Auger recombination, reduction of the exciton radiative lifetime and an improved lasing threshold. However, despite more than a decade of detailed studies there are conflicting reports on PL dynamics in PbS/CdS core-shell NCs, and there is no accepted model explaining the observed PL lifetimes ranging from hundred nanoseconds to tens of microseconds. In this work we use time-resolved PL measurements and demonstrate that low-temperature PL dynamics in PbS/CdS core shell NCs show strong dependence on the PL detection wavelengths. The presented experimental results allow us to distinguish between processes of energy transfer and exciton recombination. We propose a model, which considers energy transfer between NCs and various mechanisms of exciton recombination (non-radiative, radiative, spin-flip, diffusion limited, etc.).

Antimony composition impact on band alignment in InAs/GaAsSb quantum dots

We present a theoretical study of the electronic and excitonic states in InAs/GaAsSb quantum dots. We first center our study on the dependence of the antimony composition in the positioning of conduction- and valence-band alignments in InAs/GaAsSb/GaAs heterostructures. We predict a transition from type I to type II quantum dots at critical composition xc=0.128, which describes well the experimental trend. We discuss the influence of the quantum dot size and antimony composition on the spatial distributions of carriers and the exciton binding energy. We find that the ground state exciton binding energy is always significantly smaller ≃4meV for type II than for corresponding type I quantum dots ≃14meV. Finally, we also predict the excitonic radiative lifetime and find 1 ns for type I and 10 ns for type II quantum dots, in agreement with the existing experimental literature.

Strain-controlled electronic transport and exciton radiative lifetime in monolayer germanium sulfide

Strain-controlled electronic transport and exciton radiative lifetime in monolayer germanium sulfide

Effect of environmental screening and strain on optoelectronic properties of two-dimensional quantum defects

Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including the number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that environmental screening causes a lowering of the quasiparticle gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the Bethe–Salpeter equation Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

Exciton radiative lifetime in CdSe quantum dots

Colloidal CdSe quantum dots (QDs) are promising materials for solar cells because of their simple preparation process and compatibility with flexible substrates. The QD radiative recombination lifetime has attracted enormous attention as it affects the probability of photogenerated charges leaving the QDs and being collected at the battery electrodes. However, the scaling law for the exciton radiative lifetime in CdSe QDs is still a puzzle. This article presents a novel explanation that reconciles this controversy. Our calculations agree with the experimental measurements of all three divergent trends in a broadened energy window. Further, we proved that the exciton radiative lifetime is a consequence of the thermal average of decays for all thermally accessible exciton states. Each of the contradictory size-dependent patterns reflects this trend in a specific size range. As the optical band gap increases, the radiative lifetime decreases in larger QDs, increases in smaller QDs, and is weakly dependent on size in the intermediate energy region. This study addresses the inconsistencies in the scaling law of the exciton lifetime and gives a unified interpretation over a widened framework. Moreover, it provides valuable guidance for carrier separation in the thin film solar cell of CdSe QDs.

Advanced tunability of optical properties of CdS/ZnSe/ZnTe/CdSe multi-shell quantum dot by the band edge engineering

In this study, the advanced manipulability of wavefunctions in a type-II multi-shell hetero-nanostructure (MS-HNS) and the tunability of radiative exciton lifetime over a wide range with and/or without changing in transition energies has been demonstrated by the band edge engineering. For this purpose, the electronic and optical properties of exciton (X) and biexciton (XX) in a spherical CdS/ZnSe/ZnTe/CdSe HNS have been explored in detail. In the calculations, effects of all Coulombic interactions between the charges have been taken into account on the wavefunctions. Moreover, in the case of XX, the exchange–correlation potential between the same charged particles has also been considered. The results have been presented as a function of CdS core radius and ZnSe shell thickness and the probable physical reasons have been discussed in detail. • The band edge engineering allows an advanced manipulability of wavefunctions in type-II CdS/ZnSe/ZnTe/CdSe multi-shell quantum dots. • The transition energies and exciton lifetime can be tuned separately and simultaneously by band edge engineering. • These simultaneous tunability properties allow an important degree of freedom for device applications.

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence decay, are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong inter-band coupling.

Dot-Size Dependent Excitons in Droplet-Etched Cone-Shell GaAs Quantum Dots.

Strain-free GaAs quantum dots (QDs) are fabricated by filling droplet-etched nanoholes in AlGaAs. Using a template of nominally identical nanoholes, the QD size is precisely controlled by the thickness of the GaAs filling layer. Atomic force microscopy indicates that the QDs have a cone-shell shape. From single-dot photoluminescence measurements, values of the exciton emission energy (1.58...1.82 eV), the exciton–biexciton splitting (1.8...2.5 meV), the exciton radiative lifetime of bright (0.37...0.58 ns) and dark (3.2...6.7 ns) states, the quantum efficiency (0.89...0.92), and the oscillator strength (11.2...17.1) are determined as a function of the dot size. The experimental data are interpreted by comparison with an atomistic model.

Origin of temperature dependence of exciton radiative lifetime of GaN studied by phononic-excitonic-radiative model

It is theoretically accepted that the temperature (T) dependence of the exciton radiative lifetime is proportional to 3/2 power of the temperature. However, the various experimental temperature dependence of the exciton lifetime has been reported for samples with different crystal qualities, and it is not clear which factor of crystal quality determines the temperature dependence of the exciton radiative lifetime. We calculated the exciton radiative lifetime using the phononic-excitonic-radiative model, which is an analytical model that integrates excitons of various principal quantum numbers, free carriers, and phonons. For the components determining the crystal quality, the effects of nonradiative recombination, carrier trapping, bound excitons, exciton-polaritons, and energy-level broadening on the exciton radiative lifetime are considered. Our calculation model fits the experimental radiative lifetime roughly proportional to T3/2 in a wide range from cryogenic to room temperature and also minor deviation from the T3/2 dependence. We show that the exciton radiative lifetime reflects the effects of crystal quality, whereas conventional theories suggest that it is due to the momentum distribution of excitons.

Twist-Dependent Tuning of Excitonic Emissions in Bilayer WSe2.

Monolayer (ML) transition metal dichalcogenides (TMDCs) have been rigorously studied to comprehend their rich spin and valley physics, exceptional optical properties, and ability to open new avenues in fundamental research and technology. However, intricate analysis of twisted homobilayer (t-BL) systems is highly required due to the intriguing twist angle (t-angle)-dependent interlayer effects on optical and electrical properties. Here, we report the evolution of the interlayer effect on artificially stacked BL WSe2, grown using chemical vapor deposition (CVD), with t-angle in the range of 0 ≤ θ ≤ 60°. Systematic analyses based on Raman and photoluminescence (PL) spectroscopies suggest intriguing deviations in the interlayer interactions, higher-energy exciton transitions (in the range of ∼1.6–1.7 eV), and stacking. In contrast to previous observations, we demonstrate a red shift in the PL spectra with t-angle. Density functional theory (DFT) is employed to understand the band-gap variations with t-angle. Exciton radiative lifetime has been estimated theoretically using temperature-dependent PL measurements, which shows an increase with t-angle that agrees with our experimental observations. This study presents the groundwork for further investigation of the evolution of various interlayer excitons and their dynamics with t-angle in homobilayer systems, critical for optoelectronic applications.

Radiative lifetime of free excitons in hexagonal boron nitride

In a previous PRL, the authors found quantitative evidence that, despite hexagonal boron nitride ($h$-BN) having an indirect band gap, the luminescence efficiency of high-quality crystals reaches the level of the best direct semiconductors. Here, thanks to breakthrough experiments performed using an original time-resolved cathodoluminescence setup, the authors address the root cause of the high quantum yield luminescence efficiency. These results should have practical application for the use of $h$-BN in 2D materials based optoelectronic devices.

Signatures of Dimensionality and Symmetry in Exciton Band Structure: Consequences for Exciton Dynamics and Transport

Exciton dynamics, lifetimes, and scattering are directly related to the exciton dispersion or band structure. Here, we present a general theory for exciton band structure within both ab initio and model Hamiltonian approaches. We show that contrary to common assumption, the exciton band structure contains nonanalytical discontinuities—a feature which is impossible to obtain from the electronic band structure alone. These discontinuities are purely quantum phenomena, arising from the exchange scattering of electron–hole pairs. We show that the degree of these discontinuities depends on materials’ symmetry and dimensionality, with jump discontinuities occurring in 3D and different orders of removable discontinuities in 2D and 1D, whose details depend on the exciton degeneracy and material thickness. We connect these features to the early stages of exciton dynamics, radiative lifetimes, and diffusion constants, in good correspondence with recent experimental observations, revealing that the discontinuities in the band structure lead to ultrafast ballistic transport and suggesting that measured exciton diffusion and dynamics are influenced by the underlying exciton dispersion.

Vacuum Field Photonic Trap for Excitons

AbstractThe spectral and spatial dependence of the resonant Lamb shift of an exciton inside a photonic vacuum field landscape is considered to predict an optical dipole potential generated via virtual photons quantum fluctuations. This trapping potential can be exploited for exciton self‐positioning in electric field hotspots at the nanoscale in view of quantum electrodynamics effects. By non Hermitian decomposition of the electromagnetic Green tensor in terms of quasi normal modes, the resonant part of the Lamb shift is expressed as Fano profile strictly interlinked to the Purcell enhancement of the exciton radiative lifetime. This approach explains how to tailor the depth of the vacuum photonic potential in several possible scenarios.

Substrate effect on excitonic shift and radiative lifetime of two-dimensional materials

Substrates have strong effects on optoelectronic properties of two-dimensional (2D) materials, which have emerged as promising platforms for exotic physical phenomena and outstanding applications. To reliably interpret experimental results and predict such effects at 2D interfaces, theoretical methods accurately describing electron correlation and electron-hole interaction such as first-principles many-body perturbation theory are necessary. In our previous work (2020 Phys. Rev. B 102 205113), we developed the reciprocal-space linear interpolation method that can take into account the effects of substrate screening for arbitrarily lattice-mismatched interfaces at the GW level of approximation. In this work, we apply this method to examine the substrate effect on excitonic excitation and recombination of 2D materials by solving the Bethe–Salpeter equation. We predict the nonrigid shift of 1s and 2s excitonic peaks due to substrate screening, in excellent agreements with experiments. We then reveal its underlying physical mechanism through 2D hydrogen model and the linear relation between quasiparticle gaps and exciton binding energies when varying the substrate screening. At the end, we calculate the exciton radiative lifetime of monolayer hexagonal boron nitride with various substrates at zero and room temperature, as well as the one of WS2 where we obtain good agreement with experimental lifetime. Our work answers important questions of substrate effects on excitonic properties of 2D interfaces.

Exciton radiative lifetime in a monoatomic carbon chain

Linear carbon-based materials such as polyyne and cumulene oligomers provide a versatile platform for nano-physics and engineering. Direct gap quasi-1D polyyne structures are promising for the observation of strong and unusual excitonic effects arising due to the two-dimensional quantum confinement. Recently, we reported on the observation of sharp exciton peaks in low temperature photoluminescence spectra of polyyne chains (Kutrovskaya S et al 2020 Nano Lett. 20 6502–9). Here, we analyze the time-resolved optical response of this system. We extend the non-local dielectric response theory to predict the exciton radiative lifetime dependence on the band-gap value and on the length of the chain. A good agreement between the experiment and the theory is achieved.

Optical properties of excitons in two-dimensional transition metal dichalcogenide nanobubbles.

Strain in two-dimensional transition metal dichalcogenide has led to localized states with exciting optical properties, in particular, in view of designing one photon sources. The naturally formed nanobubbles when the MoS2 monolayer is deposited on an hBN substrate lead to a local reduction in the band gap due to strain developing in the nanobubble. The photogenerated particles are thus confined in the strain-induced potential. Using numerical diagonalization, we simulate the spectra of the confined exciton states, their oscillator strengths, and their radiative lifetimes. We show that a single state of the confined exciton is optically active, which suggests that the MoS2/hBN nanobubbles are a good candidate for the realization of single-photon sources. Furthermore, our calculations show that the localized exciton gains in activation energy and radiative lifetime inside the nanobubble, the latter decreasing toward the one of free excitons when the nanobubble size increases.

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.

First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis

In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine- (gC$_3$N$_4$-t) and heptazine-based (gC$_3$N$_4$-h) forms, as obtained by means of ground- plus excited-state ab initio calculations. By analysing the exciton fine structure, we highlight the presence of dark states and show that the photo-generated electron-hole pairs in gC$_3$N$_4$-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC$_3$N$_4$-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photo-conversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC$_3$N$_4$-t and gC$_3$N$_4$-h layers which shows a type-II band alignment and looks promising for achieving net charge separation.

Twist Angle-Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures.

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe_{2}/WSe_{2} twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1° to 3.5°. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers.

We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.