Temperature-dependent Exciton Research Articles

Temperature-dependent self-trapped exciton emission in Cu(I) doped zinc-based metal halides from well-resolved excited state structures

Temperature-dependent self-trapped exciton emission in Cu(I) doped zinc-based metal halides from well-resolved excited state structures

Direct Visualization of Dark Interlayer Exciton Transport in Moiré Superlattices.

Moiré superlattices have emerged as an unprecedented manipulation tool for engineering correlated quantum phenomena in van der Waals heterostructures. With moiré potentials as a naturally configurable solid-state that sustains high exciton density, interlayer excitons in transition metal dichalcogenide heterostructures are expected to achieve high-temperature exciton condensation. However, the exciton degeneracy state is usually optically inactive due to the finite momentum of interlayer excitons. Experimental observation of dark interlayer excitons in moiré potentials remains challenging. Here we directly visualize the dark interlayer exciton transport in WS2/h-BN/WSe2 heterostructures using femtosecond transient absorption microscopy. We observe a transition from classical free exciton gas to quantum degeneracy by imaging temperature-dependent exciton transport. Below a critical degeneracy temperature, exciton diffusion rates exhibit an accelerating downward trend, which can be explained well by a nonlinear quantum diffusion model. These results open the door to quantum information processing and high-precision metrology in moiré superlattices.

Dual-band photoluminescence mechanism of magnetic doped two-dimensional (PEA)2PbBr4 single crystals

Mn2+ doping effectively realizes white light emission in three-dimensional lead halide perovskite nanocrystals. Meanwhile, research on Mn-doped two-dimensional layered perovskite single crystal is limited. We report centimeter-scale Mn-doped PEA2PbBr4 (C6H5CH2CH2NH3+ and PEA+) single crystals prepared by a slow evaporation method. Mn2+ dopants mainly act as substitutional doping and exhibit paramagnetic properties in the crystal at low doping density, while interstitial doping of Mn2+ prevails and induces antiferromagnetic characteristics at high doping density. Mn:PEA2PbBr4 single crystals exhibit dual-band chromacity-tunable blue-orange photoluminescence originating from excitons and Mn2+ emission. The negative temperature quenching effect is achieved by Mn-doping defects for the temperature-dependent exciton photoluminescence. Upon testing in the low-pressure vacuum chamber, the Mn2+ peak of the single crystal shows a dramatic shift from 610 to 690 nm. These results indicate that Mn:PEA2PbBr4 single crystal can serve as a potential and promising luminescent device material that achieves color tunable properties by regulating the systematic changes in the intensity ratio of exciton emission and Mn2+ emission, which will be very helpful for exploring the application of perovskite in magneto-optical devices in the future.

Role of organic cations in tuning the exciton binding energy of 2D lead iodide perovskite derivatives

Control over the exciton binding energy (Eb) in two-dimensional (2D) perovskites can enable their application in a broad range of optoelectronic technologies. To investigate the effects of organic cations on the excitonic properties of 2D perovskites, films of lead iodide hybrids basing on four structurally similar cations of phenylethylammonium (PEA), 4-florophenylethylammonium (FPEA), 4-chlorophenylethylammonium (ClPEA) and 4-bromophenylethylammonium (BrPEA) were prepared and their optical properties were characterized. Results show that both the perovskite layer distortion and exciton absorption/emission peak do not vary too much by the substitution of halogen atom in the para H of phenylethyl cation. Further examination of the temperature-dependent exciton emission properties reveals that the thermal activation energy Ea, which can also be regarded as exciton binding energy in our work, can be tuned over ca. 170 meV within the set of hybrids. It is found that exciton binding energy is sensitive to the identity of organic cations and to the resulting PbI6 octahedron distortion. Among the compounds considered, FPEA based perovskite has the biggest PbI6 octahedron distortion and the largest calculated Ea of 330 meV, while the Br-PEA based perovskite shows a smaller octahedron distortion and the lowest calculated Ea. We believe that an enhanced polarizability of organic cations may be responsible for the observed decrease in octahedra distortion and Ea for the p-halophenylethylammonium lead iodide perovskite derivatives.

Direct Observation of the Spin Exciton in Andreev Spectroscopy of Iron-Based Superconductors.

Quasiparticle excitations provide viable information on the physics of unconventional superconductors. Higgs and Leggett modes are some of the classic examples. Another important bosonic excitation is the spin exciton originating from the sign-changing superconducting gap structure. Here we report a direct observation of the temperature-dependent spin exciton in the Andreev spectra of iron-based superconductors. Combined with the other experimental evidence, our observation confirms the extended s-wave () order parameter symmetry and indirectly proves the spin-fluctuation mechanism of Cooper pairing.

Ab initio calculations of spin-nonconserving exciton–phonon scattering in monolayer transition metal dichalcogenides

We investigate the spin-nonconserving relaxation channel of excitons by their couplings with phonons in two-dimensional transition metal dichalcogenides using ab initio approaches. Combining GW-Bethe–Salpeter equation method and density functional perturbation theory, we calculate the electron–phonon and exciton–phonon coupling matrix elements for the spin-flip scattering in monolayer WSe2, and further analyze the microscopic mechanisms influencing these scattering strengths. We find that phonons could produce effective in-plane magnetic fields which flip spin of excitons, giving rise to relaxation channels complimentary to the spin-conserving relaxation. Finally, we calculate temperature-dependent spin-flip exciton–phonon relaxation times. Our method and analysis can be generalized to study other two-dimensional materials and would stimulate experimental measurements of spin-flip exciton relaxation dynamics.

Photoluminescence enhancement study in a Bi-doped Cs2AgInCl6 double perovskite by pressure and temperature-dependent self-trapped exciton emission.

Here, we report a halide precursor acid precipitation method to synthesize Cs2AgIn1-xBixCl6 (x = 0, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64, and 1) microcrystals. Cs2AgInCl6 and Bi derivative double perovskites show broadband white light emission via self-trapped excitons (STEs) and have achieved the highest internal quantum efficiency of up to 52.4% at x = 0.08. Synchrotron X-ray diffraction confirmed the linear increase of lattice parameters and cell volume with Bi3+ substitution at In3+ sites. Absorbance, photocurrent excitation, and photoluminescence excitation spectra are used to observe possible transitions from the valence to the conduction band or free exciton (FE) states as well as transitions within local Bi3+ states. The broadband photoluminescence is quenched via a single nonradiative process with an activation energy ΔE = 1490 cm-1 for Cs2AgIn0.92Bi0.08Cl6. Under normal conditions, we observed STE emission, but applying external pressure alters the electronic structure such that at elevated pressure, the only emission via the FE state is observed. We anticipate that structure, temperature and pressure-dependent photoluminescence studies will help the future use of a single-source lead-free double perovskite for white light-emitting diode applications.

Temperature Dependent Exciton Funnel Dynamics in Uniform Strain Gradient Field Observed by Time‐Resolved Photoluminescence

AbstractElastic strain engineering is an important way to reversibly tune the properties of micro/nanoscale semiconductors for promising applications in the emerging advanced nanotechnologies, such as the strain enhanced high‐mobility transistors, nanogenerators, etc. However, direct observation and control of energetic carrier dynamics with precise strain gradient fields remains a challenge. Here, observation of temperature‐dependent exciton funnel dynamics is reported in a precisely controlled strain gradient field by time‐resolved photoluminescence. An efficient exciton hopping process is revealed at picosecond time scale as the donor‐bound excitons in ZnO microwires funnel along the strain gradient, which strongly depends on the temperature. Combined experiments and simulations unravel that, in addition to the exciton funnel by the strain gradient, both the excitation efficiency and emission lifetime of the excitons increase gradually from the compressive side to the tensile side due to the role of local strain regulation. The results give a clear physical picture of the energetic carrier dynamics in the strain gradient field in semiconductors, which provides a promising paradigm for the design of high‐performance optoelectronic devices.

Effect of phononic and electron collisional interaction on temperature dependent exciton radiation dynamics of doped GaN

In ultraviolet light emission devices, excitons are a high-efficiency emission source. However, the mechanism of experimentally observed dependence of excitonic radiative lifetimes on temperature (T) has not been discovered. We present a numerical simulation based on the phonon-exciton-radiation system, which reveals that the dependence of the radiative lifetime of GaN excitons on T is dominated by the population distribution among discrete and continuum energy states. This finding is in contrast to an existing model considering the existence of 1S exciton only. This population distribution is determined by the temperature-dependent integrated effect of background carrier density and exciton energy broadening, which induces a combination of high-order excitonic states and the continuum. Various experimental results on the dependence on temperature, including the functions of T3/2 and higher or lower power of T, are interpreted by a model integrating the interactions with the electron and phononic fields. The proposed model elucidates the corresponding effects of electronic and phononic processes in this complex system and provides a platform for the discussion of Wannier exciton dynamics under various thermal conditions, including nonequilibrium cases beyond the Saha-Boltzmann relation in population distribution.

Temperature Dependent Exciton Dynamics in a Single GaAs Quantum Ring in Comparison to that in a Single GaAs Quantum Dot

Temperature Dependent Exciton Dynamics in a Single GaAs Quantum Ring in Comparison to that in a Single GaAs Quantum Dot

Temperature-Dependent Total Absorption of Exciton Polaritons in Bulk Semiconductors

This paper highlights the temperature factor in the experimental study of total absorption as a method for experimental validation and the study of the exciton–polariton light transfer mechanism near the fundamental absorption edge in semiconductor crystals with spatial dispersion. The results of experimental studies of temperature-dependent total exciton absorption are generalized. The experimentally determined critical temperatures above which total absorption becomes constant, the corresponding critical damping parameter, and longitudinal–transverse splittings for the studied semiconductors CdTe, GaAs, InP, ZnSe, and ZnTe are presented.

Температурно-зависимое интегральное поглощение экситонных поляритонов в полупроводниковых кристаллах

Abstract This paper highlights the temperature factor in the experimental study of total absorption as a method for experimental validation and the study of the exciton–polariton light transfer mechanism near the fundamental absorption edge in semiconductor crystals with spatial dispersion. The results of experimental studies of temperature-dependent total exciton absorption are generalized. The experimentally determined critical temperatures above which total absorption becomes constant, the corresponding critical damping parameter, and longitudinal–transverse splittings for the studied semiconductors CdTe, GaAs, InP, ZnSe, and ZnTe are presented.

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.

High Exciton Diffusion Coefficients in Fused Ring Electron Acceptor Films.

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells; however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 × 10-2 cm2/s, substantially exceeding typical organic semiconductors and consistent with the 20-50 nm domain sizes in optimized blends. Enhanced three-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder, all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology.

Long-Lived Dark Exciton Emission in Mn-Doped CsPbCl3 Perovskite Nanocrystals.

The unusual temperature dependence of exciton emission decay in CsPbX3 perovskite nanocrystals (NCs) attracts considerable attention. Upon cooling, extremely short (sub-ns) lifetimes were observed and were explained by an inverted bright–dark state splitting. Here, we report temperature-dependent exciton lifetimes for CsPbCl3 NCs doped with 0–41% Mn2+. The exciton emission lifetime increases upon cooling from 300 to 75 K. Upon further cooling, a strong and fast sub-ns decay component develops. However, the decay is strongly biexponential and also a weak, slow decay component is observed with a ∼40–50 ns lifetime below 20 K. The slow component has a ∼5–10 times stronger relative intensity in Mn-doped NCs compared to that in undoped CsPbCl3 NCs. The temperature dependence of the slow component resembles that of CdSe and PbSe quantum dots with an activation energy of ∼19 meV for the dark–bright state splitting. Based on our observations, we propose an alternative explanation for the short, sub-ns exciton decay time in CsPbX3 NCs. Slow bright–dark state relaxation at cryogenic temperatures gives rise to almost exclusively bright state emission. Incorporation of Mn2+ or high magnetic fields enhances the bright–dark state relaxation and allows for the observation of the long-lived dark state emission at cryogenic temperatures.

Evidence for line width and carrier screening effects on excitonic valley relaxation in 2D semiconductors

Monolayers of transition metal dichalcogenides (TMDC) have recently emerged as excellent platforms for exploiting new physics and applications relying on electronic valley degrees of freedom in two-dimensional (2D) systems. Here, we demonstrate that Coulomb screening by 2D carriers plays a critical role in excitonic valley pseudospin relaxation processes in naturally carrier-doped WSe2 monolayers (1L-WSe2). The exciton valley relaxation times were examined using polarization- and time-resolved photoluminescence spectroscopy at temperatures ranging from 10 to 160 K. We show that the temperature-dependent exciton valley relaxation times in 1L-WSe2 under various exciton and carrier densities can be understood using a unified framework of intervalley exciton scattering via momentum-dependent long-range electron–hole exchange interactions screened by 2D carriers that depend on the carrier density and the exciton linewidth. Moreover, the developed framework was successfully applied to engineer the valley polarization of excitons in 1L-WSe2. These findings may facilitate the development of TMDC-based opto-valleytronic devices.

Characterization of the influences of morphology on the intrinsic properties of perovskite films by temperature-dependent and time-resolved spectroscopies.

Organic-inorganic halide perovskites have attracted enormous attention owing to their promising application in photovoltaic devices. The morphology of the perovskites is the key to driving the performance of perovskite devices, which necessitates a systematic study. In this work, two typical morphologies, i.e., flake and cube, of perovskite films are fabricated, and the temperature-dependent optical absorption and photoluminescence properties of the two types of perovskite film are systematically investigated. From the temperature-dependent spectra, both exciton and phase transition temperatures of the flake film are found to be about 10 K lower than those of the cube one. Meanwhile, the influences of the morphology on the exciton binding energy, optical phonon energy and polaron binding energy are quantitatively characterized. The exciton binding of the flake film is nearly three times smaller than that of the cube one, while the phonon coupling energy and the polaron binding energy of the former are about 5 meV and 2 meV larger than those of the latter. Furthermore, the results of photoluminescence lifetime and charge separation efficiency further reveal that the charge carrier kinetics in the two kinds of perovskite films is significantly different. The current study provides a theoretical framework to understand the fundamental physics of perovskites and to promote the design and enhancement of active materials for improved optoelectronic devices.

Straining effects in MoS2 monolayer on nanostructured substrates: temperature-dependent photoluminescence and exciton dynamics.

Strain-engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) has great potential to alter their electronic and optical properties. Thus far, experimental studies of the straining effects in 2D TMDs primarily focused on the static property measurements at room temperature. However, low-temperature and temperature-dependence studies are essential in understanding the underlying mechanisms of the unique properties of monolayer TMDs. Herein, the temperature-dependent dynamic properties of laser shock strain-engineered monolayer MoS2 were studied using temperature-dependent photoluminescence (PL) and pump-probe spectroscopy. Both the photoluminescence spectra and exciton dynamics exhibit the differences between the MoS2 monolayers transferred on the flat and nanostructured surfaces by laser shock strain engineering and display a strong temperature dependence. The laser-induced straining effect and temperature-dependent dynamic behavior of MoS2 were studied through molecular dynamics simulation. The observed behaviors can be explained by the thermally induced strain in the monolayer MoS2 due to the mismatching thermal expansion coefficients of the monolayer and the substrate, which are coupled by the van der Waals forces. The ultrafast pump-probe experiments were performed to investigate the effect of strain on the exciton dynamics upon optical excitation. The results from the pump-probe measurements indicate that the effects of strain extend beyond that of the static properties and profoundly influence the valley carrier dynamics. This report extends the understanding of the substrate-induced straining effect to temperature-dependent luminescence behaviors and dynamic behaviors of the TMD materials.

Photoluminescence Temperature Dependence, Dynamics, and Quantum Efficiencies in Mn2+-Doped CsPbCl3 Perovskite Nanocrystals with Varied Dopant Concentration

A series of Mn2+-doped CsPbCl3 nanocrystals (NCs) was synthesized using reaction temperature and precursor concentration to tune Mn2+ concentrations up to 14%, and then studied using variable-temperature photoluminescence (PL) spectroscopy. All doped NCs show Mn2+ 4T1g → 6A1g d–d luminescence within the optical gap coexisting with excitonic luminescence at the NC absorption edge. Room-temperature Mn2+ PL quantum yields increase with increased doping, reaching ∼60% at ∼3 ± 1% Mn2+ before decreasing at higher concentrations. The low-doping regime is characterized by single-exponential PL decay with a concentration-independent lifetime of 1.8 ms, reflecting efficient luminescence of isolated Mn2+. At elevated doping, the decay is shorter, multiexponential, and concentration-dependent, reflecting the introduction of Mn2+–Mn2+ dimers and energy migration to traps. A large, anomalous decrease in Mn2+ PL intensity is observed with decreasing temperature, stemming from the strongly temperature-dependent exciton l...

Low temperature exciton dynamics and structural changes in perylene bisimide aggregates

The temperature dependent exciton dynamics of J-aggregates formed by a perylene bisimide dye is investigated down to liquid nitrogen temperature (77 K) by femtosecond pump–probe spectroscopy. The analysis of the transient absorption data using a diffusion model for the excitons does not only reveal an overall decrease of the exciton mobility, but also a change in the dimensionality of the exciton transport at low temperatures. This change in dimensionality is further investigated by kinetic Monte Carlo simulations, identifying weakly interlinked one-dimensional aggregate chains as the most likely structure at low temperatures. This causes the exciton transport to be highly anisotropic.