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We studied the interaction between the neutral and charged excitons of monolayer MoSe2 at various temperatures via quantum beat spectroscopy in the time domain. We introduced temperature as an efficient control knob to regulate the relative photoluminescence intensities of the neutral and charged excitons to obtain maximum quantum beat resolution. Furthermore, our quantum beat measurements under different temperatures indicate that the decoherence time of the coupled exciton-trion state slightly decreases from 530 fs at 3.5 K to 420 fs at 63 K with increased temperature due to the low-energy acoustic phonon-induced dephasing. [ABSTRACT FROM AUTHOR]

We studied the interaction between the neutral and charged excitons of monolayer MoSe2 at various temperatures via quantum beat spectroscopy in the time domain. We introduced temperature as an efficient control knob to regulate the relative photoluminescence intensities of the neutral and charged excitons to obtain maximum quantum beat resolution. Furthermore, our quantum beat measurements under different temperatures indicate that the decoherence time of the coupled exciton-trion state slightly decreases from 530 fs at 3.5 K to 420 fs at 63 K with increased temperature due to the low-energy acoustic phonon-induced dephasing. [ABSTRACT FROM AUTHOR]

In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr‐ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge‐transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr‐ESFG especially sensitive to the trion formation dynamics. The presence of charge‐transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers. [ABSTRACT FROM AUTHOR]

Nanolayers of MoS2 can be grown to be used as active elements in nano-optoelectronic devices such as two-dimensional (2D) light emitters and optical detectors. The growth of 2D flakes might result in the formation of not only isolated triangles but also complex polycrystal flakes when different flakes interact during their in-plane expansion. In this paper, we investigate how monolayer MoS2 flakes of different shapes are affected by the biaxial strain resulting from the cooling process after chemical vapor deposition growth. The single- and polycrystal flakes are characterized at the nanoscale level by correlating morphological, electrical, and optical measurements and imaging. The main focus is given to the analysis of the exciton/trion photoluminescence (PL) components extracted from the spectra at different areas of the flake surface. According to the Raman imaging, the whole flake has built-in heterogeneous tensile strain, with the perimeter and the grain boundaries between the single-crystal parts of the poly flakes exhibiting a lower strain level (0.2–0.3%) and the central area being more strained (∼0.4%). At the perimeter and grain boundaries, the PL undergoes the strain-related blueshift accompanied by a weakening of the contribution of the long-wave trion to the spectrum and the trion binding energy. The trion formation is known to be proportional to a local electron concentration. The trion PL imaging compared to the surface potential mapping confirms a decrease in n-doping at the perimeter and grain boundaries, leading to the trion weakening. To confirm the results of electron drift to strained areas of the flake, creating the trion, the band bending at the tensile-strained flake has been theoretically calculated and modeled. The effect of edge defects at the perimeter and grain boundaries on the doping, which leads to the enhancement or inhibition of the trion formation depending on the edge and grain boundary interface type, is also discussed. [ABSTRACT FROM AUTHOR]

We present femtosecond pump-probe measurements of neutral and charged exciton optical response in monolayer MoSe2 to resonant photoexcitation of a given exciton state in the presence of 2D electron gas. We show that creation of charged exciton (X−) population in a given K+, K− valley requires the capture of available free carriers in the opposite valley and reduces the interaction of neutral exciton (X) with the electron Fermi sea. We also observe spectral broadening of the X transition line with the increasing X− population caused by efficient scattering and excitation induced dephasing. From the valley-resolved analysis of the observed effects we are able to extract the spin-valley relaxation times of free carriers as a function of carrier density. Moreover, we analyze the oscillator strength and energy shift of X in the regime of interaction with electron Fermi sea under resonant excitation. From this we can observe the process of X decay by radiative recombination paired with trion formation. We demonstrate an increase of neutral exciton relaxation rate with the introduction of Fermi sea of electrons. We ascribe the observed effect to the increased efficiency of the trion formation, as well as the radiative decay caused by the screening of disorder by the free carriers.

We study theoretically fundamental Coulomb-correlated complexes: neutral and charged excitons, also known as trions, in transition metal dichalcogenides monolayers. We focus on the situation where one of the electrons occupies an excited, high-lying, conduction band characterized by a negative effective mass. We develop the theory of such high-lying excitons and trions with negative effective mass and demonstrate the key role of the non-parabolicity of the high-lying conduction band dispersion in the formation of the bound exciton and trion states. We present simple, accurate and physically justified trial wavefunctions for calculating the binding energies of Coulomb-bound complexes and compare the results of variational calculations with those of a fully numerical approach. Within the developed model, we discuss recent experimental results on the observation of high-lying negative effective mass trions. [ABSTRACT FROM AUTHOR]

Because of their bandgap tunability and strong light–matter interactions, two-dimensional (2D) semiconductors are considered promising candidates for next-generation optoelectronic devices. However, their photophysical properties are greatly affected by their surrounding environment because of their 2D nature. In this work, we report that the photoluminescence (PL) of single-layer WS2 is substantially affected by interfacial water that is inevitably present between it and the supporting mica substrates. Using PL spectroscopy and wide-field imaging, we show that the emission signals from A excitons and their negative trions decreased at distinctively different rates with increasing excitation power, which could be attributed to the more efficient annihilation between excitons than between trions. By gas-controlled PL imaging, we also prove that the interfacial water converted the trions into excitons by depleting native negative charges through an oxygen reduction reaction, which rendered the excited WS2 more susceptible to nonradiative decay via exciton–exciton annihilation. Understanding the role of nanoscopic water in complex low-dimensional materials will eventually contribute to devising their novel functions and related devices. [ABSTRACT FROM AUTHOR]

The extraordinarily large exciton binding energies of transition metal dichalcogenides (TMDs) have attracted significant attention in both fundamental scientific research and innovative technology development. TMD-metal nanostructures have been fabricated to control excitonic behaviors as well as improve the light-matter interaction in TMDs. WS 2 monolayers are integrated with periodic Ag nanohole (AgNH) arrays prepared using a template stripping method. The ultrasmooth and contamination-free surface of the template-stripped Ag layer allows for the direct exfoliation of WS 2 flakes on the AgNHs. Surface plasmon excitation increases optical absorption in WS 2 /AgNH, as evidenced by reflectance, PL, and Raman measurements, along with numerical calculations. Furthermore, the AgNH structures significantly increase the trion-to-exciton emission ratios from the WS 2 monolayers on them. Nanoscopic surface photovoltage mapping of WS 2 /AgNH using Kelvin probe force microscopy can visualize electron accumulation at the suspended WS 2 region under light illumination, which triggers the formation of trions. All the results highlight the capability of WS 2 /AgNH to boost trion emission through plasmon-induced concentrated light and a built-in potential gradient. This work introduces a simple and efficient strategy for fabricating TMD-metal integrated systems to control their excitonic behaviors. [Display omitted] • Au-nanohole arrays fabricated using a template stripping technique. • WS 2 monolayer flakes directly exfoliated on Au-nanohole arrays. • Electron accumulation revealed by nanoscale surface potential maps. • Trion emission from WS 2 promoted by the plasmon-enhanced absorption and the inherent potential gradient at the surface. [ABSTRACT FROM AUTHOR]

The interaction of high-frequency surface acoustic waves (SAWs) and excitons in van der Waals heterostructures (vdWHs) offers challenging opportunities to explore novel quantum effects and functionalities. We probe the interaction of neutral excitons, trions, and biexcitons with SAWs in a hBN/WSe 2 /hBN vdWH. We show that neutral excitons respond weakly to the SAW stimulus at 5 K. The remaining excitonic complexes, because of their lower binding energy or charged character, interact much more efficiently with the SAW piezoelectric field, particularly intra- and intervalley trions. At room temperature, the SAW can play a dual role (sometimes dissociating excitons and sometimes increasing the vdWH local doping density) which depends of the laser-induced photodoping of the vdWH prior to the SAW generation and the role of metastable energy states in the SAW-induced carrier dynamics. Our results shed light in the unexplored biexciton modulation with SAWs, important for 2D materials-based optoelectronic and energy harvesting devices.

Controlling excitons and their transport in two-dimensional (2D) transition metal dichalcogenide heterostructures is central to advancing photonics and electronics on-chip integration. We investigate the controlled generation and manipulation of excitons and their complexes in monolayer MoSe 2 -WSe 2 lateral heterostructures (LHSs). Incorporating graphene as a back gate and edge contact in a field-effect transistor geometry, we achieve the precise electrical tuning of exciton complexes and their transfer across interfaces. Photoluminescence and photocurrent maps at 4 K reveal the synergistic effect of the local electric field and interface phenomena in the modulation of excitons, trions, and free carriers. We observe spatial variations in the exciton and trion densities driven by exciton-trion conversion under electrical manipulation. Additionally, we demonstrate controlled narrow-band emissions within the LHS through carrier injection and electrical biasing. Density functional theory calculation reveals significant band modification at the lateral interfaces. This work advances exciton manipulation in LHS and shows promise for next-generation 2D quantum devices.

Based on measurements of low-temperature (5 K) photoluminescence and reflectance spectra, changes in optical properties of a WSe2 monolayer resulting from vacuum annealing at a temperature of 300°С are shown. Initial monolayers were obtained by gold-assisted mechanical exfoliation from a bulk material. It is shown that annealing results in narrowing the resonance associated with the A-exciton, trion luminescence suppression, and the appearance of polarized impurity—defect luminescence. [ABSTRACT FROM AUTHOR]

Interaction of excitons with resident charge carriers in semiconductors gives rise to bound three-particle complexes, trions, whose optical response is conveniently described in the framework of many-body correlated Fermi polaron states. These states are formed as a result of correlation of photocreated trion with the Fermi sea hole and possess the angular momentum component of ±1 depending on the helicity of the photon. We study theoretically the energy spectrum fine structure of Fermi polarons in two-dimensional semiconductors based on transition metal dichalcogenides. We demonstrate both by the symmetry analysis and microscopic calculation that the Fermi polarons with nonzero in-plane wavevector k are split, similarly to the neutral exciton states, into the linearly polarized longitudinal and transverse, with respect to the k , states. The origin of this longitudinal-transverse splitting is the long-range electron-hole exchange interaction that can be also described as the interaction of Fermi polarons with their induced electromagnetic field. The effective Hamiltonian describing the Fermi polaron fine structure is derived, and its parameters are determined from the microscopic model. • Fermi polaron (Suris tetron) states are split by the electron-hole exchange interaction. • The Fermi polaron eigenstates in two-dimensional semiconductors are the longitudinal and transverse ones. • The mixing of inter- and intravalley Fermi polarons in tungsten based transition metal dichalcogenide monolayers is possible. [ABSTRACT FROM AUTHOR]

Time-resolved and time-integrated circularly polarized photoluminescence of excitons and trions in external magnetic fields up to 10 T has been studied in undoped and n-type doped quantum well structures based on ZnSe. In an undoped structure, a circular polarization of photoluminescence induced by magnetic fields corresponded to the Boltzmann distribution of excitons on Zeeman sublevels. The inverse spin orientation of excitons is observed in doped samples with a carrier density of 3 × 1 0 10 cm−2 and higher. Model calculations show that the reason for the inverse spin orientation is the effective depletion of the lowest-exciton Zeeman sublevel as a result of the spin-dependent formation of trions. The trion formation time as a result of exciton-electron binding was determined as 2 ps. This is noticeably shorter than the characteristic time of the exciton-photon interaction. The observed effect can be considered as a way of spin manipulation by electric fields. • The presented work opens a new direction in spintronics, shows the possibility to control the population of exciton Zeeman sublevels by changing the electron density in QWs. • The founded effect is due to the depletion of the lower energy sublevel of the exciton during the trion formation. • The effect can be used to control the exciton's angular momentum using an electric field or additional ultraviolet illumination. • New spin control method and new perspectives for exciton angular momentum control are presented. [ABSTRACT FROM AUTHOR]

At cryogenic temperatures, the photoluminescence spectrum of CdSe nanoplatelets (NPLs) usually consists of multiple emission lines, the origin of which is still under debate. While there seems to be consensus that both neutral excitons and trions contribute to the NPL emission, the prominent role of trions is rather puzzling. In this work, we demonstrate that Förster resonant energy transfer in stacks of NPLs combined with hole trap states in specific NPLs within the stack trigger trion formation, while single NPL spectra are dominated by neutral excitonic emission. This interpretation is verified by implementing copper (Cu + ) dopants into the lattice as intentional hole traps. Trion emission gets strongly enhanced, and due to the large amount of hole trapping Cu + states in each single NPL, trion formation does not necessarily require stacking of NPLs. Thus, the ratio between trion and neutral exciton emission can be controlled by either changing the amount of stacked NPLs during sample preparation or implementing copper dopants into the lattice which act as additional hole traps.

Photoluminescence (PL) blinking of nanoparticles, while detrimental to their imaging applications, may benefit next-generation displays if the blinking is precisely controlled by reversible electron/hole injections from an external source. Considerable efforts are made to create well-characterized charged excitons within nanoparticles through electrochemical charging, which has led to enhanced control over PL-blinking in numerous instances. Manipulating the photocharging/discharging rates in nanoparticles by surface engineering can represent a straightforward method for regulating their blinking behaviors, an area largely unexplored for perovskite nanocrystals (PNCs). This work shows facet engineering leading to different morphologies of PNCs characterized by distinct blinking patterns. For instance, examining the PL intensity trajectories of single PNCs, representing the instantaneous photon count rate over time, reveals that the OFF-state population significantly increases as the number of facets increases from six to twenty-six. This study suggests that extra-faceted PNCs, owing to their polar facets and expanded surface area, render them more susceptible to photocharging, which results in larger OFF-state populations. Furthermore, the fluorescence correlation spectroscopy (FCS) study unveils that the augmented propensity for photocharging in extra-faceted PNCs can also originate from their greater tendency to form complexes with neighboring molecules., (© 2024 Wiley‐VCH GmbH.)

In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr-ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge-transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr-ESFG especially sensitive to the trion formation dynamics. The presence of charge-transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Monolayer transition metal dichalcogenides have emerged as promising materials for optoelectronic and nanophotonic devices. However, the low photoluminescence (PL) quantum yield (QY) hinders their various potential applications. Here we engineer and enhance the PL intensity of monolayer WS2 by femtosecond laser irradiation. More than two orders of magnitude enhancement of PL intensity as compared to the as-prepared sample is determined. Furthermore, the engineering time is shortened by three orders of magnitude as compared to the improvement of PL intensity by continuous-wave laser irradiation. Based on the evolution of PL spectra, we attribute the giant PL enhancement to the conversion from trion emission to exciton, as well as the improvement of the QY when exciton and trion are localized to the new-formed defects. We have created microstructures on the monolayer WS2 based on the enhancement of PL intensity, where the engineered structures can be stably stored for more than three years. This flexible approach with the feature of excellent long-term storage stability is promising for applications in information storage, display technology, and optoelectronic devices. [ABSTRACT FROM AUTHOR]

InP-based colloidal nanocrystals are being developed as an alternative to cadmium-based materials. However, their optical properties have not been widely studied. In this paper, the fundamental magneto-optical properties of InP/ZnSe/ZnS nanocrystals are investigated at cryogenic temperatures. Ensemble measurements using two-photon excitation spectroscopy revealed the band-edge hole state to have 1 S symmetry, resolving some controversy on this issue. Single nanocrystal microphotoluminescence measurements provided increased spectral resolution that facilitated direct detection of the lowest energy confined acoustic phonon mode at 0.9 meV, which is several times smaller than the previously reported values for similar nanocrystals. Zeeman splitting of narrow spectral lines in a magnetic field indicated a bright trion emission. A simple trion model was used to identify a positive trion charge. Furthermore, the Zeeman split spectra allowed the direct measurement of both the electron and hole h symmetry, resolving some controversy on this issue. Single nanocrystal microphotoluminescence measurements provided increased spectral resolution that facilitated direct detection of the lowest energy confined acoustic phonon mode at 0.9 meV, which is several times smaller than the previously reported values for similar nanocrystals. Zeeman splitting of narrow spectral lines in a magnetic field indicated a bright trion emission. A simple trion model was used to identify a positive trion charge. Furthermore, the Zeeman split spectra allowed the direct measurement of both the electron and hole g -factors, which match existing theoretical predictions.

We propose a generalized model of electronic structure modification in HTSC cuprates and ferropnictides under doping. In this model, the role of doping consists in only a local change in the electronic structures of the parent phases of cuprates and ferropnictides due to the formation of trion complexes comprising a doped carrier localized in unit cell and charge transfer (CT) excitons around it. These CT excitons emerge in CuO4 or AsFe4 plaquettes in the CuO2 or FeAs basal planes (CT plaquettes) under the influence of the doped carrier, restricting its itinerancy. As the dopant concentration is increased, CT plaquettes combine into clusters of the so-called CT phase. It is this CT phase that is related in the model to the HTSC phase. In support of this assumption, we determined the ranges of dopant concentrations conforming to the existence of percolation clusters of the CT phase; these ranges were shown to coincide with the positions of the superconducting domes on the phase diagrams of these compounds. The model also perfectly describes subtle features of the phase diagrams of various cuprates and ferropnictides including the "1/8" anomaly, narrow peaks in the dependences of the London penetration depth on the concentration of the dopant, and other specific features. The mechanism of the generation of free carriers in the CT phase, provided by intrinsic self-doping, was considered. The mechanism is not directly related to external doping, but is due to the interaction of band electrons with so-called Heitler–London (HL) centres inherently existing in the percolation cluster of the CT phase and representing pairs of adjacent CuO4 or AsFe4 CT plaquettes in the CuO2 or FeAs basal planes. Material in the CT phase was shown to represent a medium, in which the mechanism of excitonic superconductivity, specified by the interaction of band electrons with HL centres, can be realized. [ABSTRACT FROM AUTHOR]

We report on the magneto-photoluminescence of (Cd,Mn)Te/(Cd,Mg)Te quantum wells excited by photons with varied energy. We observe that laser illumination modifies the carrier density and Coulomb disorder in the quantum wells. Three different regimes are analyzed, corresponding to low hole density with low disorder, low hole density with significant disorder, and high hole density. By using the diluted magnetic semiconductor as a quantum well material, we can induce the spin singlet–triplet transition of charged excitons in a magnetic field. This transition is then used as a tool to determine the charged exciton dissociation energy. With this approach, we find the same value for the dissociation energy in all the regimes of hole density and disorder. Our result is compared with the dissociation energy obtained from the PL splitting between the X and X + lines at zero field, which exhibits significantly greater variation. [ABSTRACT FROM AUTHOR]

Mixed dimensional van der Waals (vdW) heterostructures constructed by one-dimensional (1D) and two-dimensional (2D) materials exhibit extra degree of freedom to modulate the electronic and optical properties due to the combination of different dimensionalities. The charge transfer at the interface between 1D and 2D materials plays a crucial role in the optoelectronic properties and performance of the heterostructure-based devices. Here, we stacked single-walled carbon nanotubes (SWNTs) on monolayer WS2 for a mixed dimensional vdW heterostructure, and investigated the local modulation of excitions and trions in WS2 by SWNTs. Different directions of charge transfer between SWNTs and WS2 are evidenced by the photoluminescence (PL) spectra of WS2. The PL intensity can be either enhanced or weakened by individual SWNTs. In our work, the PL intensity of WS2 is enhanced and the exciton peak position heterostructure is red-shifted about 3 meV due to the charge transfer from WS2 to an individual SWNT (SWNT#1). The change of PL by another SWNT (SWNT#2) can not be well-resolved in far-field, but scanning near-field optical microscope (SNOM) measurements show that the PL intensity of WS2 is weakened by the SWNT. The peak position of exciton is blue-shifted by ~ 1 meV while that of trion is redshifted by ~ 1 meV due to the charge transfer from the SWNT to WS2. These results give insight into the charge transfer at the interface of SWNT/WS2 heterostructure, and can be useful for design of optoelectronic devices based on mixed dimensional vdW heterostructures. [ABSTRACT FROM AUTHOR]

We report the room temperature tuning of excitonic lifetime in pristine and hole-doped monolayer MoS2 based field effect transistor (FET) devices by systematically controlling the free carrier density. We observed that in pristine MoS2 devices, with intrinsic electron doping, an exciton dominant regime with an exciton lifetime of 3 ns exists, when doped electrostatically with holes. Interestingly we observe a sharp decrease in exciton lifetime and population with an increase of the electron density by electrostatic doping, with a corresponding increase in negative trion population. With increased hole doping by a chemical method, the exciton lifetime decreases, but it remains almost constant with electrostatic carrier density tuning. This decrease in lifetime, compared to that of the pristine case, might be due to the exciton–exciton annihilation mechanism which is proposed to be existent in a high exciton density regime. Further hole doping by a chemical method leads to a transition to a positive trion dominated regime, in which the exciton lifetime decreases further due to nonradiative energy transfer to the positive trions. We observe a slight increase in exciton lifetime due to partial neutralization of positive trions at high electrostatic electron doping and a corresponding increase in the probability of excitons. We suggest that when calculating the lifetime of excitons, the exciton-to-trions formation and exciton–exciton annihilation mechanisms should be considered. These fine-tunings of excitons in monolayer MoS2 can provide a platform for probing the excitonic physics and photonic applications. [ABSTRACT FROM AUTHOR]

Lead halide perovskite nanocrystals (NCs) have attracted much attention as light-source materials for light-emitting diodes, lasers, and quantum light emitters. The luminescence properties of perovskite NCs and the performance of NC-based light-source devices depend on trion and biexciton dynamics. Here, we examined the size dependence of trion and biexciton binding energies by conducting low-temperature single-dot spectroscopy on three different perovskite NCs: CsPbBr 3 , CsPbI 3 , and FAPbBr 3 . While the photoluminescence spectral widths of the all-inorganic CsPbBr 3 and CsPbI 3 NCs were narrow, compared with those of the organic-inorganic hybrid FAPbBr 3 NCs, the binding energies of trions and biexcitons of all three samples showed similar size dependences, independent of the A-site cation and halogen. The effective-mass approximation calculations implied the importance of dynamical dielectric screening on the formation of trions and biexcitons.

The diverse nature of optoelectronic properties of few-layer or monolayer MoS 2 is generally dominated by A and B excitons. Occasionally, strong Coulombic interactions within the 2D monolayer led to the creation of hydrogen-like Rydberg states of excitons in MoS 2 similar to other 2D monolayers. In this paper, a simple process is used to convert trilayer MoS 2 films to a monolayer by introducing H 2 gas during chemical vapor deposition. Remarkably, alongside the usual A, B excitons, and A - trion, the appearance of the Rydberg states is evidenced by photoluminescence spectra even at room temperature; also, there is an increase in their areal percentage with an increase in H 2 content. The s-type excited Rydberg states up to the fourth order ( n = 5) and third order ( n = 4) of A and B excitons, respectively, have been probed from the photoluminescence spectra at 93 K. Unprecedentedly, the first-order derivative of room-temperature photocurrent spectrum reveals the Rydberg states concurrently and elaboratively. Furthermore, the large-area MoS 2 films exhibit photoresponse in a broad UV to visible region with excellent photosensitivity (∼10 2 ) toward both UV and visible lights. Not only does this provide a profound understanding of the excitonic Rydberg states but also highlights the considerable potential of large-area monolayer MoS 2 overcoming the difficulty of tiny flake-related 2D device endeavors.

Abstract The biexciton, negative and positive trion states in strongly oblate ellipsoidal quantum dot are investigated in the framework of variational method and Heisenberg's uncertainty principle. Because of the oblate geometry of the quantum dot, the exciton complexes are considered as quasi-two-dimensional. The trial wave function for the biexciton and trions are constructed on the base of one-particle wave functions, which are obtained within the framework of adiabatic approximation. The dependencies of the energies and binding energies for the biexciton, negative and positive trions on the geometrical parameters of ellipsoidal quantum dot and ratio of the effective masses of the electron and hole are obtained. The recombination energies for the biexciton, negative and positive trions are considered as a function of the strongly oblate ellipsoidal quantum dot's small semiaxis. For the crossover states, it is shown that for the decreasing small semiaxis the photoluminescence emission peaks of exciton and biexciton shift to the higher energies, but the intensities remain constant. Finally, with the increase of temperature, the photoluminescence emission peaks and intensities become lower. The radiative lifetime of biexciton in ellipsoid is estimated. Graphical abstract Image 1 Highlights • The dependence of binding energies of exciton complexes on semiaxes are calculated. • The recombination energies of exciton complexes are revealed. • The lifetime of the biexciton in ellipsoidal quantum dot is estimated. • The dependence of the PL spectra of exciton and biexciton is obtained. [ABSTRACT FROM AUTHOR]

Raman and photoluminescence (PL) hyperspectral mapping of a bilayer MoS 2 flake grown on SiO 2 /Si substrate by chemical vapor deposition is studied focusing on the surface variation of the exciton and trion PL characteristics. The Raman modes and the excitonic PL in the flake interior are redshifted compared to those at the perimeter. This is attributed to tensile strain present after the post-growth cooling so that the flake perimeter is less strained. At the perimeter, the PL blueshift is also enhanced due to a reduction of the trion (a negatively charged exciton) peak, being attributed to an outflow of electrons (p -doping) induced by a weakening of tensile strain towards the flake edge. This occurs because tensile strain bends down the conduction band of MoS 2 , causing a drift of electrons towards the strained interior region and, in turn, creating there an increased electron density which fosters the creation of trion. [Display omitted] • The surface variation of the exciton/trion photoluminescence (PL) and Raman of bilayer MoS 2 on SiO 2 /Si. • The Raman modes and the excitonic PL in the flake interior are redshifted compared to the perimeter. • Tensile strain present after the post-growth cooling so that the flake perimeter is less strained. • At the perimeter, the PL blueshift is also enhanced due to a reduction of the trion peak. • Tensile strain causes a drift of electrons towards the strained interior region and the creation of trion. [ABSTRACT FROM AUTHOR]

Formation of charged trions is detrimental to the luminescence quantum efficiency of colloidal quantum dot (QD) thin films as they predominantly undergo nonradiative recombination. In this regard, control of charged trion formation is of interest for both fundamental characterization of the quasi-particles and performance optimization. Using CdSe/CdS QDs as a prototypical material system, here we demonstrate a metal-oxide-semiconductor capacitor based on QD thin films for studying the background charge effect on the luminescence efficiency and lifetime. The concentration ratio of the charged and neutral quasiparticles in the QDs is reversibly controlled by applying a gate voltage, while simultaneous steady-state and time-resolved photoluminescence measurements are performed. Notably, the photoluminescence intensity is modulated by up to 2 orders of magnitude with a corresponding change in the effective lifetime. In addition, chip-scale modulation of brightness is demonstrated, where the photoluminescence is effectively turned on and off by the gate, highlighting potential applications in voltage-controlled electrochromics.

Quasiparticles consisting of correlated electron(s) and hole(s), such as excitons and trions, play important roles in the optical phenomena of van der Waals semiconductors and serve as unique platforms for studies of many-body physics. Herein, we report a gate-tunable exciton-to-trion transition in pressurized monolayer MoSe 2 , in which the electronic band structures are modulated continuously within a diamond anvil cell. The emission energies of both the exciton and trion undergo large blueshifts over 90 meV with increasing pressure. Surprisingly, the trion binding energy remains constant at 30 meV, regardless of the applied pressure. Combining ab initio density functional theory calculations and quantum Monte Carlo simulations, we find that the remarkable robustness of the trion binding energy originates from the spatially diffused nature of the trion wave function and the weak correlation between its constituent electron-hole pairs. Our findings shed light on the optical properties of correlated excitonic quasiparticles in low-dimensional materials.

Abstract We investigated the optical behaviour of excitons in monolayer MoS 2 prepared on a 60 nm-thick metal (Au) ring structure patterned on SiO 2. CVD-grown monolayer MoS 2 was transferred to the metal structure and placed in contact with three different environments simultaneously: SiO 2 , Au metal and air. The optical properties of the transferred MoS 2 on the substrates were characterized through micro-photoluminescence (μ-PL) measurements. PL peaks were observed near ∼1.82 eV for defect-related emission, ∼1.92 eV for an A exciton, which includes a neutral exciton and a trion, and ∼2.1 eV for a B exciton. The optical band gap and exciton-phonon interaction of the samples on the three substrates were investigated by measuring the temperature-dependent photoluminescence. Highlights • A chemical vapour deposition (CVD)-grown MoS 2 monolayer is transferred to the Au-patterned structure. • The monolayer MoS 2 is placed in contact with three different environments simultaneously: SiO 2 , Au, and air (free standing). • The excitonic emissions from the different contact regions were characterized by temperature-dependent μ-PL. [ABSTRACT FROM AUTHOR]

The origin of the variation of photoluminescence (PL) spectra of monolayer tungsten disulfide (WS 2 ) is investigated systematically. Dependence of the PL spectrum on the excitation power show that the relatively sharp component corresponds to excitons whereas the broader component at slightly lower energy corresponds to negatively charged trions. PL imaging and second harmonic generation measurements show that the trion signals are suppressed more than the exciton signals near the edges, thereby relatively enhancing the excitonic feature in the PL spectrum and that such relative enhancement of the exciton signals is more pronounced near approximately armchair edges. This effect is interpreted in terms of depletion of free electrons near the edges caused by structural defects and adsorption of electron acceptors such as oxygen atoms. [ABSTRACT FROM AUTHOR]

Excitons complexes in ZnO/(Zn, Mg)O non-polar quantum well have been studied by using continuous-wave photoluminescence according to temperature and to the density of excitation. The spectrum of photoluminescence of the quantum well consists of three different lines. The ratio between the intensities of these lines depends of the temperature but also of the density of excitation. The lines are attributed to the free exciton, the negatively charged trion and the biexciton. The binding energy of trion and biexciton are found to be respectively 19.5 ± 2 meV and 45 ± 5 meV. [ABSTRACT FROM AUTHOR]

The recombination energy of isolated neutral exciton and that of isolated negatively charged exciton inside a type-II core/shell spherical quantum dot are studied. Our investigation considers the charge-carriers effective mass discontinuity at the surface contact between the core and shell materials. Although our model omits the effect of the surface polarization, the dielectric-constant mismatch at the nanodot boundaries was taken into account. In order to achieve the exciton and negative trion energies, we proceed by a variational calculation in the framework of the envelope approximation. Our results reveal a strong correlation between the nanodot morphology and the energy spectrum of the neutral and negatively charged exciton. [ABSTRACT FROM AUTHOR]

Formation of dressed light-matter states in optical structures, manifested as Rabi splitting of the eigen energies of a coupled system, is one of the key effects in quantum optics. In pursuing this regime with semiconductors, light is usually made to interact with excitons, electrically neutral quasiparticles of semiconductors; meanwhile interactions with charged three-particle states, trions, have received little attention. Here, we report on strong interaction between localized surface plasmons in silver nanoprisms and excitons and trions in monolayer tungsten disulfide (WS2). We show that the plasmon–exciton interactions in this system can be efficiently tuned by controlling the charged versus neutral exciton contribution to the coupling process. In particular, we show that a stable trion state emerges and couples efficiently to the plasmon resonance at low temperature by forming three bright intermixed plasmon–exciton–trion polariton states. Our findings open up a possibility to exploit electrically charged polaritons at the single nanoparticle level. [ABSTRACT FROM AUTHOR]

The trion, a three-body charge-exciton bound state, offers unique opportunities to simultaneously manipulate charge, spin, and excitation in one-dimensional single-walled carbon nanotubes (SWNTs) at room temperature. Effective exploitation of trion quasi-particles requires fundamental insight into their creation and decay dynamics. Such knowledge, however, remains elusive for SWNT trion states, due to the electronic and morphological heterogeneity of commonly interrogated SWNT samples, and the fact that transient spectroscopic signals uniquely associated with the trion state have not been identified. Here, we prepare lengthsorted SWNTs and precisely control charge-carrier-doping densities to determine trion dynamics using femtosecond pump-probe spectroscopy. Identification of the trion transient absorptive hallmark enables us to demonstrate that trions (i) derive from a precursor excitonic state, (ii) are produced via migration of excitons to stationary hole-polaron sites, and (iii) decay in a first-order manner. Importantly, under appropriate carrier-doping densities, exciton-to-trion conversion in SWNTs can approach 100% at ambient temperature. Our findings open up possibilities for exploiting trions in SWNT optoelectronics, ranging from photovoltaics and photodetectors to spintronics. [ABSTRACT FROM AUTHOR]

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured, and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble. [ABSTRACT FROM AUTHOR]