Exciton Density Research Articles

Dynamic Generation of Spin Spirals of Moiré Trapped Carriers via Exciton Mediated Spin Interactions.

Stacking transition metal dichalcogenides (TMDs) to form moiré superlattices has provided exciting opportunities to explore many-body correlation phenomena of the moiré trapped carriers. TMD bilayers, on the other hand, host long-lived interlayer exciton (IX), an elementary excitation of long spin-valley lifetime that can be optically or electrically injected. Here we find that, through the Coulomb exchange between mobile IXs and carriers, the IX bath can mediate both Heisenberg and Dzyaloshinskii-Moriya type spin interactions between moiré trapped carriers, controllable by exciton density and exciton spin current, respectively. We show the strong Heisenberg interaction and the extraordinarily long-ranged Dzyaloshinskii-Moriya interaction here can jointly establish robust spin spiral magnetic orders in Mott-Wigner crystal states at various filling factors, with the spiral direction controlled by the exciton current.

Chester Supersolid of Spatially Indirect Excitons in Double-Layer Semiconductor Heterostructures.

A supersolid, a counterintuitive quantum state in which a rigid lattice of particles flows without resistance, has to date not been unambiguously realized. Here we reveal a supersolid ground state of excitons in a double-layer semiconductor heterostructure over a wide range of layer separations outside the focus of recent experiments. This supersolid conforms to the original Chester supersolid with one exciton per supersolid site, as distinct from the alternative version reported in cold-atom systems of a periodic density modulation or clustering of the superfluid. We provide the phase diagram augmented by the supersolid. This new phase appears at layer separations much smaller than the predicted exciton normal solid, and it persists up to a solid-solid transition where the quantum phase coherence collapses. The ranges of layer separations and exciton densities in our phase diagram are well within reach of the current experimental capabilities.

High performance solution-processed red phosphorescent organic light-emitting diodes by co-doping europium complex as sensitizer

In this work, efficient red phosphorescent OLEDs (PHOLEDs) were designed and fabricated by solution processing method. Based on well optimized doping concentrations of emitter and ratio of mixed host materials, Eu(DBM) 3 Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) was doped into the light-emitting layer (EML) which plays the role of sensitizer. Experimental results indicated that the incorporation of Eu(DBM) 3 Phen molecules can effectively trap the superfluous electrons within EML, which is helpful for balancing charges’ distribution and widening the recombination zone. Besides, the presence of Eu(DBM) 3 Phen molecules can accelerate the transfer of triplet energy from hosts to red emitters, therefore remarkably enhances device performance. By optimizing the doping concentrations of emitter and sensitizer, ratio of co-hosts and the functional layer thickness, recombination center was accurately adjusted to further broaden recombination zone, therefore causing decreased excitons density and suppressed excitons quenching. Eventually, the optimal solution-processed red PHOLED obtained the maximum current efficiency (47.12 cd/A) and external quantum efficiency (27.3%). High performance solution-processed red phosphorescent organic light-emitting diodes by incorporating trivalent rare earth europium complex Eu(DBM) 3 Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) into light-emitting layer as sensitizer. • High performance solution-processed red phosphorescent OLED was designed by utilizing trivalent rare earth europium complex Eu(DBM) 3 Phen into light-emitting layer as sensitizer. • Co-doped sensitizer molecules function as electron trappers, which is helpful in balancing carriers' distribution and broadening recombination zone. • The optimal solution-processed red PHOLED obtained the maximum current efficiency up to 47.12 cd/A and external quantum efficiency up to 27.3%. • This work firstly demonstrated the efficacy of utilizing rare earth complex as sensitizer in realizing high performance solution-processed OLEDs.

Non‐Equilibrium Bose–Einstein Condensation of Exciton‐Polaritons in Silicon Metasurfaces

AbstractExciton‐polaritons (EPs) are hybrid light–matter quasi‐particles with bosonic character formed by the strong coupling between excitons in matter and photons in optical cavities. Their hybrid character offers promising prospects for the realization of non‐equilibrium Bose–Einstein condensates (BECs), and room‐temperature BECs are possible with organic materials. However, the thresholds required to create BECs of organic EPs remain still high to allow condensation with electrical injection of carriers. One of the factors behind these high thresholds is the very short cavity lifetimes, leading to a fast EP decay and the need to inject higher exciton densities in the reservoir to form the condensate. Here a BEC of EPs in organic dyes and all‐dielectric metasurfaces at room temperature is demonstrated. By using dielectric metasurfaces that exhibit very low losses it is possible to achieve cavity lifetimes long enough to allow an efficient population of EP states via vibrational relaxation and radiative pumping. It is shown how polariton lasing or non‐equilibrium Bose–Einstein condensation is achieved in several cavities, and one of the lowest reported thresholds for BECs in organic materials is observed.

Exciton density waves in Coulomb-coupled dual moiré lattices.

Strongly correlated bosons in a lattice are a platform that can realize rich bosonic states of matter and quantum phase transitions1. While strongly correlated bosons in a lattice have been studied in cold-atom experiments2-4, their realization in a solid-state system has remained challenging5. Here we trap interlayer excitons-bosons composed of bound electron-hole pairs, in a lattice provided by an angle-aligned WS2/bilayer WSe2/WS2 multilayer. The heterostructure supports Coulomb-coupled triangular moiré lattices of nearly identical period at the top and bottom interfaces. We observe correlated insulating states when the combined electron filling factor of the two lattices, with arbitrary partitions, equals [Formula: see text] and [Formula: see text]. These states can be interpreted as exciton density waves in a Bose-Fermi mixture of excitons and holes6,7. Because of the strong repulsive interactions between the constituents, the holes form robust generalized Wigner crystals8-11, which restrict the exciton fluid to channels that spontaneously break the translational symmetry of the lattice. Our results demonstrate that Coulomb-coupled moiré lattices are fertile ground for correlated many-boson phenomena12,13.

Supramolecular Tuning of Exciton Transport in Pi-Peptide Assemblies

We demonstrate the use of sequence-dependent interactions between peptide-functionalized pi-conjugated pigments to tune exciton transport behavior in their supramolecular assemblies in aqueous solutions. Peptide sequences attached to a perylene diimide (PDI) core were selected based on their ability to foster a wide range of excitonic coupling strengths within organized assemblies, as reflected by experimentally measured steady-state absorption and emission spectra. Photoresponses of organized assemblies of weakly interacting PDIs closely resemble those observed for weakly assembled chromophores. In contrast, organized assemblies that support strong intermolecular coupling exhibit significant excitonic delocalization and transport, as observed through distinct transient signatures of fluence-dependent singlet–singlet annihilation and a short-lived biexcitonic state at high initial exciton densities. A one-dimensional (1D) diffusion model appropriately accounts for exciton–exciton encounters in assemblies, with effective exciton diffusion constants that scale with interchromophore coupling strength, ranging between 0 and 7 sites2/ps (0–1 nm2/ps or LD ranging from 0 to 40 nm). Variations in effective diffusion constant arise from peptide-tuned variations in interchromophore alignment and wavefunction overlap, with mixed Frenkel-CT exciton coupling identified as the key interaction that facilitates site-to-site exciton diffusion within PDI stacks. This work demonstrates the potential to use simple peptide sequence variation as a tool for rational engineering of exciton transport and other excited-state behaviors in supramolecular materials.

Threshold-like Superlinear Accumulation of Excitons in a Gated Monolayer Transition Metal Dichalcogenide

Coexistence and mutual conversion of various excitonic species in semiconductors are the essence of Mott physics and underpin important device applications such as excitonic or polaritonic lasers. The emergence of monolayer semiconductors provides unprecedented opportunities to study these fundamental issues due to the much larger exciton binding energies. In this paper, we study the evolution of the coupled exciton–trion system in electrically gated monolayer MoTe2 devices. Contrary to the conventional linear scaling, we found that exciton density exhibits an abnormal three-stage scaling behavior: a conventional linear scaling at low pumping levels, followed by a superlinear behavior accompanied by a strong saturation of trion emission. In the third stage, the exciton emission returns to the linear scaling with the further increase of pumping. Although such behavior has a rare similarity in other physical systems, surprisingly we discovered a complete analogy of this behavior with the threshold of a conventional laser and proved mathematically that the exciton–trion equations are identical to the laser equations. We further showed that the power-law index increases with the charge density experimentally and can be as high as 40 at a density of ∼1 × 1012/cm2 in principle, leading to extremely nonlinear behavior of exciton accumulation. Our results reveal a new behavior of exciton accumulation and provide an alternative mechanism for multiplying exciton population, in analogy to a condensate. The intricate dynamics between excitons and trions will also enrich our understanding of the complex Mott physics in 2D semiconductors and may lead to new devices based on the exciton nonlinearity.

Design of the Smallest Intramolecular Singlet Fission Chromophore with the Fastest Singlet Fission.

We designed an intramolecular singlet fission (iSF) chromophore, pyrazino[2,3-g]quinoxaline-1,4,6,9-tetraoxide. Appropriate substitutions into anthracene enhance the tetraradical character, so that the molecule accommodates a pair of triplet excitons in its lowest singlet excited state. Our simulation showed a 16 fs fast iSF of the design, which is a new record. The design also sets a new record of small size iSF chromophore and high exciton density. The design can be synthesized by oxidizing the tertiary N centers of the existent pyrazino[2,3-g]quinoxaline.

Investigation of self-trapped excitonic dynamics in hematite nanoforms through non-degenerate pump–probe transmission spectroscopy

Non-degenerate pump–probe transmission spectroscopy is used to examine the ultrafast dynamics of photo-excited carriers in hematite nanoforms at various pump fluences. Using coupled rate equations, the kinetics of self-trapped exciton (STE) formation and its interaction with free excitons resulting in exciton annihilation were studied. It is shown from this model that the majority of the excitons were trapped by polaronic trap states to form self-trapped excitons within ∼3.5 ps. The findings indicate that free excitons and STEs interact non-linearly, similar to trap-assisted bi-molecular Auger recombination to annihilate one another. It is observed that there is substantial dependence of kinetics of STE formation and exciton decay on photo-excited exciton density, and the nature of this dependence indicates the reduced screening of electron–phonon interaction. Using the screening model applied to the rate constants of STE formation and decay, we estimate the saturation exciton density to be ∼3.3 × 1017 cm−3 and the average STE density to be ∼3.8 × 1018 cm−3 in the hematite nanoforms. We also noticed that doping K and Ni to hematite nanoforms up to 5% did not remarkably change the nature of the exciton dynamics.

Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence.

Phosphorescent and thermally activated delayed fluorescence (TADF) blue organic light-emitting diodes (OLEDs) have been developed to overcome the low efficiency of fluorescent OLEDs. However, device instability, originating from triplet excitons and polarons, limits blue OLED applications. Here, we develop a phosphor-sensitized TADF emission system with TADF emitters to achieve high efficiency and long operational lifetime. Peripheral carbazole moieties are introduced in conventional multi-resonance–type emitters containing one boron atom. The triplet exciton density of the TADF emitter is reduced by facilitating reverse intersystem crossing, and the Förster resonant energy transfer rate from phosphor sensitizer is enhanced by high absorption coefficient of the emitters. The emitter exhibited an operational lifetime of 72.9 hours with Commission Internationale de L’Eclairage chromaticity coordinate y = 0.165, which was 6.6 times longer than those of devices using conventional TADF emitters.

Direct observation of ultrafast singlet exciton fission in three dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Tuning moiré excitons in Janus heterobilayers for high-temperature Bose-Einstein condensation.

Using first-principles calculations, we predict that moiré excitons in twisted Janus heterobilayers could realize tunable and high-temperature Bose-Einstein condensation (BEC). The electric dipole in the Janus heterobilayers leads to charge-transfer interlayer and intralayer moiré excitons with exceptionally long lifetimes, in the absence of spacer layers. The electric dipole is also expected to enhance exciton-exciton repulsions at high exciton densities and can modulate moiré potentials that trap excitons for their condensation. The key parameters for exciton condensation, including exciton Bohr radius, binding energy, effective mass, and critical Mott density, are examined as a function of the twist angle. Last, exciton phase diagrams for the Janus heterobilayers are constructed from which one can estimate the BEC (>100 K) and superfluid (~30 K) transition temperatures. In addition to indirect interlayer excitons, we find that direct intralayer excitons can also condense at high temperatures, consistent with experiments.

Exciton optics, dynamics, and transport in atomically thin semiconductors

Atomically thin semiconductors such as transition metal dichalcogenide (TMD) monolayers exhibit a very strong Coulomb interaction, giving rise to a rich exciton landscape. This makes these materials highly attractive for efficient and tunable optoelectronic devices. In this Research Update, we review the recent progress in the understanding of exciton optics, dynamics, and transport, which crucially govern the operation of TMD-based devices. We highlight the impact of hexagonal boron nitride-encapsulation, which reveals a plethora of many-particle states in optical spectra, and we outline the most novel breakthroughs in the field of exciton-polaritonics. Moreover, we underline the direct observation of exciton formation and thermalization in TMD monolayers and heterostructures in recent time-resolved, angle-resolved photoemission spectroscopy studies. We also show the impact of exciton density, strain, and dielectric environment on exciton diffusion and funneling. Finally, we put forward relevant research directions in the field of atomically thin semiconductors for the near future.

Performance improvement for organic light emitting diodes by changing the position of mixed-interlayer

Organic Light-Emitting Diode (OLED) is presently the most sought-after display technology. It provides low-cost, flexible, rollable displays in addition to wide viewing angles and excellent colour qualities. Still, the organic displays have not reached at their best performance and there is a lot of scope for improvement in their performance. In addition to the injection layer, emission layer, transport layer, etc, researchers are looking forward to the charge carrier transport layer, spacer layer, mixed interlayer, etc. to further enhance the device performance. In this article, a depth analysis related to the impact of the position of the mixed interlayer is performed to analyze the impact on device performance. It is observed that on shifting mixed interlayer (MI) towards the cathode; luminescence and current density depict depreciation. However, on shifting MI towards anode there is a significant performance improvement. The complete analysis includes seven device structures, wherein the position of MI is varied. The best performing device depicts luminescence of 17139 cd/m2 and a current density of 84.6 mA/cm2, which is 40.05% higher for luminescence and 111.5% for current density than that of reference device. Additionally, the internal analysis of device structure is thoroughly evaluated using the cut line method to better understand the internal device physics in terms of the electric field, electron concentration, total current density, Langevin’s recombination rate, and Singlet exciton density.

Self-Kerr Effect across the Yellow Rydberg Series of Excitons in Cu_{2}O.

We investigate the nonlinear refraction induced by Rydberg excitons in Cu_{2}O. Using a high-precision interferometry imaging technique that spatially resolves the nonlinear phase shift, we observe significant shifts at extremely low laser intensity near each exciton resonance. From this, we derive the nonlinear index n_{2}, present the n_{2} spectrum for principal quantum numbers n≥5, and report large n_{2} values of order 10^{-3} mm^{2}/mW. Moreover, we observe a rapid saturation of the Kerr nonlinearity and find that the saturation intensity I_{sat} decreases as n^{-7}. We explain this with the Rydberg blockade mechanism, whereby giant Rydberg interactions limit the exciton density, resulting in a maximum phase shift of 0.5rad in our setup.

Dark and thermal reservoir contributions to polariton sound velocity

Exciton-polaritons in an optical microcavity can form a macroscopically coherent state despite being an inherently driven-dissipative system. In comparison with equilibrium bosonic fluids, polaritonic condensates possess multiple peculiarities that make them behave differently from well-known textbook examples. One such peculiarity is the presence of dark excitons which are created by the pump together with optically active particles. They can considerably affect the spectrum of elementary excitations of the condensate and hence change its superfluid properties. Here, we theoretically analyze the influence of the bright and dark ``reservoir'' populations on the sound velocity ${c}_{s}$ of incoherently driven polaritons. Both pulsed and continuous-wave pumping schemes characterized by essentially different condensate-to-reservoir ratios are considered. We show that the dark exciton contribution leads to considerable lowering of ${c}_{s}$ and to its deviation from the square-root-like behavior on the system's chemical potential (measurable condensate blueshift). Importantly, our model allows us to unambiguously define the density of dark excitons in the system by experimentally tracking ${c}_{s}$ against the condensate blueshift and fitting the dependence at a given temperature.

Excitonic Tonks-Girardeau and charge density wave phases in monolayer semiconductors

Excitons in two-dimensional semiconductors provide a novel platform for fundamental studies of many-body interactions. In particular, dipolar interactions between spatially indirect excitons may give rise to strongly correlated phases of matter that so far have been out of reach of experiments. Here, we show that excitonic few-body systems in atomically thin transition-metal dichalcogenides undergo a crossover from a Tonks-Girardeau to a charge-density-wave regime. To this end, we take into account realistic system parameters and predict the effective exciton-exciton interaction potential. We find that the pair correlation function contains key signatures of the many-body crossover already at small exciton numbers and show that photoluminescence spectra provide readily accessible experimental fingerprints of these strongly correlated quantum many-body states.

Role of Thermally Occupied Hole States in Room‐Temperature Broadband Gain in CdSe/CdS Giant‐Shell Nanocrystals

AbstractGrowing CdSe/CdS nanocrystals from a large CdSe core, and employing a giant CdS shell, a continuous, broadband gain spectrum, covering the spectral range between the CdSe and the CdS band edge, is induced. As revealed by k·p calculations, this feature is enabled by a set of closely spaced S‐, P‐ and, for larger CdSe cores, D‐state hole levels, which are thermally occupied at room temperature, combined with a sparse density of electron states. This leads to a range of bleach signals in the transient absorption spectra that persist up to a microsecond. By extending a state‐filling model including relevant higher‐energy states and a Fermi–Dirac distribution of holes at finite temperature, it is shown that thermal occupancy can lower the gain threshold for excited states. Inclusion of Gaussian broadening of discrete transitions also leads to a smoothening of the gain threshold spectrum. Next to a direct measurement of the gain threshold, a method is also developed to extract this from the gain lifetime, taking advantage that population inversion is limited by Auger recombination and recombination rates scale with the exciton density as 〈N〉·(〈N〉 − 1). The results should be readily extendable to other systems, such as perovskite or III–V colloidal nanocrystals.

Ultrafast Studies of ZrTe3 by Transient Absorption Spectrometer.

Two-dimensional (2D) tri-TMDCs carrier dynamics provide a platform for studying excitons through Ultrafast Pump-Probe Transient Absorption Spectroscopy. Here we studied the ZrTe3 nanosheets (NTs) exciton dynamics by transient absorption (TA) spectrometer. We observed different carrier dynamics in the ZrTe3 NTs sample at different pump powers and with many wavelengths in the transient absorption spectrometer. The shorter life decay constant is associated with electron-phonon relaxation. Similarly, the longer-life decay constant represents the long live process that is associated with charge separation. The interactions between carrier-phonons at nanoscale materials can be changed by phonons quantum confinements. The hot carrier lifetime determined the strength of carrier phonon interactions. The value of fast decay in the conduction band is due to carrier relaxation or the carrier gets trapped due to surface states or localized defects. The value of slow decay is due to the recombination of surface state and localized defects processes. The lifetime declines for long wavelengths as size decreases. Whereas, during short wavelength-independent decay, carrier characteristics have been observed. TA spectroscopy is employed to investigate insight information of the carrier’s dynamical processes such as carrier lifetime, cooling dynamics, carrier diffusion, and carrier excitations. The absorption enhanced along excitons density with the increase of pump power, which caused a greater number of carriers in the excited state than in the ground state. The TA signals consist of trap carriers and (electron-hole) constituents, which can be increased by TA changes that rely on photoexcitation and carrier properties.

The Role of Spin-Flip Collisions in a Dark-Exciton Condensate

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin-flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under an in-plane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate-equations model, which well reproduces the observed temperature, power, and magnetic-field dependence of the exciton density.