Exciton Bleaching Research Articles

Gain roll-off in cadmium selenide colloidal quantum wells under intense optical excitation

Colloidal quantum wells, or nanoplatelets, show among the lowest thresholds for amplified spontaneous emission and lasing among solution-cast materials and among the highest modal gains of any known materials. Using solution measurements of colloidal quantum wells, this work shows that under photoexcitation, optical gain increases with pump fluence before rolling off due to broad photoinduced absorption at energies lower than the band gap. Despite the common occurrence of gain induced by an electron–hole plasma found in bulk materials and epitaxial quantum wells, under no measurement conditions was the excitonic absorption of the colloidal quantum wells extinguished and gain arising from a plasma observed. Instead, like gain, excitonic absorption reaches a minimum intensity near a photoinduced carrier sheet density of 2 × 1013 cm−2 above which the absorption peak begins to recover. To understand the origins of these saturation and reversal effects, measurements were performed with different excitation energies, which deposit differing amounts of excess energy above the band gap. Across many samples, it was consistently observed that less energetic excitation results in stronger excitonic bleaching and gain for a given carrier density. Transient and static optical measurements at elevated temperatures, as well as transient X-ray diffraction of the samples, suggest that the origin of gain saturation and reversal is a heating and disordering of the colloidal quantum wells which produces sub-gap photoinduced absorption.

The low temperature limit of the excitonic Mott density in GaN: an experimental reassessment

The research on GaN lasers aims for a continuous reduction of the lasing threshold. An approach to achieve it consists in exploiting stimulated polariton scattering. This mechanism, and the associated polariton lasers, requires an in-depth knowledge of the GaN excitonic properties, as polaritons result from the coupling of excitons with photons. Under high excitation intensities, exciton states no longer exist due to the Coulomb screening by free carriers; this phenomenon occurs at the so-called Mott density. The aim of this work is to study the bleaching of excitons under a quasi-continuous optical excitation in a bulk GaN sample of high quality through power dependent micro-photoluminescence and time-resolved experiments at 5 K. Time-resolved photoluminescence allows to measure the carrier lifetime as a function of excitation intensity, which is required for a reliable evaluation of the injected carrier density. The vanishing of excitonic lines together with the red-shift of the main emission evidences the occurrence of the Mott transition for a carrier concentration of (6 ± 3) × 1016 cm−3. This value is more than an order of magnitude smaller than previous determinations published in the literature and is in accordance with many-body calculations.

Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots.

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% ± 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs.

Atomlike interaction and optically tunable giant band-gap renormalization in large-area atomically thin MoS2

Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\ensuremath{\sim}1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_{2}$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.

Exciton-Dominated Ultrafast Optical Response in Atomically Thin PtSe2.

Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light-matter interactions and novel optical applications. Platinum diselenide (PtSe2 ) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe2 is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe2 have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe2 single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking band-gap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.

Excitonic nonlinear shifts in photonic crystal nanocavities with buried multiple quantum wells

We investigate strong excitonic absorption and large nonlinear wavelength shifts in buried multiple-quantum-well photonic crystal L3 nanocavities that well confine the carriers. The confined carriers can survive as long as the radiative lifetime because carrier diffusion is negligible and surface recombination is well suppressed. In addition, strong optical confinement and small mode volume provided by photonic crystals can enhance excitonic nonlinearity. Therefore, our structure shows a strong excitonic effect that induces large refractive index changes by exciton bleaching and exhibits a large cavity-frequency shift. Our results will contribute to the development of ultralow energy all-optical switching devices in the future.

Two‐Step Anti‐Stokes Photoluminescence of CsPbX3 Nanocrystals

AbstractCesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) exhibit efficient anti‐Stokes photoluminescence (ASPL) upon sub‐gap single‐photon excitation, which has great potential for all‐solid‐state laser cooling. However, the mechanism responsible for the ASPL still remains elusive. Here, ASPL properties of CsPb(BrI)3 NCs and CsPbI3 NCs in inorganic solid matrix are investigated. Excitation wavelength‐ and power‐dependent ASPL demonstrates that it is a phonon‐assisted single‐photon process. Femtosecond transient absorption spectra show that ASPL is mediated by the Urbach tail states, where localized excitons are generated upon sub‐gap excitation. Through electron–phonon coupling, these localized excitons change into free exciton to generate the exciton bleaching, and then thermally dissociate into free carriers accompanied by band edge bleaching, leading to the efficient ASPL. Such a two‐step up‐conversion process is further confirmed by the carrier temperature extracted from the band edge bleaching band. This work deepens the understanding of ASPL in CsPbX3 NCs, and is valuable for the development of materials for solid‐state laser cooling.

Manipulating Exciton Transfer between Colloidal Quantum Dots and Graphene Oxide

Colloidal quantum dots and graphene oxide composite are promising semiconductor nanomaterials for optoelectronic devices due in part to their tunable light absorption and extraordinary carrier transport properties. We investigate the exciton interaction between them during laser irradiation by transient absorption spectroscopy. In the transient measurement, an exciton bleach buildup signal is exhibited before its recovery at the long probe wavelength. It can be attributed that the trapped electron of graphene oxide backward transfers to the conduction band of quantum dots. With the reduction degree of graphene oxide being increased by laser irradiation, backward electron transfer would take place at both longwave and shortwave, resulting in a broader and stronger exciton bleach feature. Subsequently, exciton bleach disappeared at longwave because of hole trapping by graphene oxide. Exciton transfer of quantum dots and graphene oxide composite has been efficiently manipulated by laser irradiation.

Colloidal formamidinium lead bromide quantum dots for photocatalytic CO2 reduction

With the target of bringing out a sustainable carbon–neutral system, halide perovskite materials have been explored to fulfil the requirements of solar-to-fuel conversion. Herein, formamidinium (CH(NH2)2+) lead bromide quantum dot (FAPbBr3 QD) as a novel of photocatalyst is researched, which exhibits an excellent photostability and the optimum CO yield rate of 181.25 μmol·g−1·h−1 towards photocatalytic CO2 reduction in the mixed solvent of deionized water/ethyl acetate. Furthermore, the kinetic features of exciton bleach and photoluminescence spectra infer that FAPbBr3 QDs have an enough lifetime to separate and transfer charges for adsorbing CO2 molecules, thus facilitating the photocatalytic CO2 reduction. This study puts forward a reliable avenue of organic–inorganic hybrid perovskite materials as efficient photocatalysts to convert CO2 into valuable chemical fuels.

Slow carrier cooling in methylammonium lead bromide quantum dots: Evidence of hot phonon bottleneck

Photoinduced charge carrier dynamics studies in lead halide perovskites helps in explaining key processes like carrier trapping, diffusion and multiexciton dynamics etc. which are of importance to understand the fate of excitons. In the present study, we investigate the dynamics in methylammonium lead bromide perovskite nanocrystals under low and high pump intensity to study hot carrier cooling mechanisms. The single exciton studies conducted under low pump fluences (<N> = 0.3) revealed slow carrier decay with a significant fraction of >1 ns decay times as expected due to long radiative carrier recombination times. This also indicated that fast carrier trapping does not occur as evidenced by absence of any fast decay components. The hot carrier (HC) cooling mechanisms were verified by studying carrier density dependence of carrier temperature decays as extracted from the higher energy part of exciton bleach. These studies revealed a very fast cooling of ∼150 fs under low carrier density 1.5 × 1017/ cm3. However additional experiments under higher carrier densities of ∼1018-1019/cm3 showed a very slow carrier temperature decay with decay time ∼100 ps for the highest intensity investigated. The analysis of hot carrier lifetimes revealed Auger heating and acoustic phonon up-conversion as reason for hot carrier relaxations at higher intensities. Additionally, we carried out experiments under hole quencher, we found lower initial carrier temperatures and faster decay indicating highly efficient hole transfer to the quencher in a time scale faster than cooling of carrier at higher intensities. These results hint at possibility of molecular design for enabling efficient hot carrier extraction from these Quantum Dots (QDs).

Ultrafast Charge Separation in Two-Dimensional CsPbBr3 Perovskite Nanoplatelets.

Two-dimensional (2D) cesium lead halide perovskite colloidal nanoplatelets show sharper excitonic absorption/emission peaks and larger absorption crosssection in comparison to bulk materials and quantum dots. It remains unclear how 2D exciton and charge separation properties can be utilized to further enhance the performance of perovskite materials for optoelectrical applications. Herein, we report a study of exciton and interfacial charge-transfer dynamics of CsPbBr3 nanoplatelets via transient absorption spectroscopy. The exciton binding energy (∼260 meV) is determined via detailed spectral analysis. The exciton bleach is caused by band-edge exciton state-filling with negligible single carrier (electron or hole) contributions. Efficient charge separation can be achieved by selective electron and hole transfers to adsorbed molecular acceptors (benzoquinone and phenothiazine, respectively), and the half-life of the charge-separated state (≫100 ns) in nanoplatelet-phenothiazine complexes is >100 fold longer than that in quantum dot-phenothiazine complexes. Our results suggest that CsPbBr3 nanoplatelets are promising materials for photocatalysis and photovoltaic applications.

Ultrafast multiexciton Auger recombination of CdSeS

Multiexciton generation is a process where multiple excitons are generated by absorbing single photons. Efficient multiexciton generation in quantum dots may be a revolutionary discovery, because it provides a new method to improve the solar-to-electric power conversion efficiency in quantum dots-based solar cells and to design novel quantum dots-based multielectron or hole photocatalysts. However, the mechanism of ultrafast multiexciton generation and recombination remain unclear. In this paper, alloy-structured quantum dots, CdSeS, are prepared by the hot injection method. The generation and recombination mechanism of charge carriers in quantum dots samples are discussed in detail. The bivalent band structure of alloy-structured quantum dots is determined by ultraviolet-visible absorption spectra. It is found that the 1S&lt;sub&gt;3/2&lt;/sub&gt;(h)-1S(e) (or 1S), 2S&lt;sub&gt;3/2&lt;/sub&gt;(h)-1S(e) (or 2S) and 1P&lt;sub&gt;3/2&lt;/sub&gt;(h)-1P(e) (or 1P) exciton absorption bands of these quantum dots are at 510 nm, 468 nm and 430 nm, respectively. Femtosecond transient absorption spectroscopy and nanosecond time-resolved photoluminescence spectroscopy are used to investigate the ultrafast exciton generation and recombination dynamics in the alloy-structured quantum dots. By fitting the transient kinetics of 1S exciton bleach, an average biexciton decay time is obtained to be about 80 ps, which is almost twice the decay time of traditional quantum dots (less than 50 ps). Combined with the recently developed ultrafast interface charge separation technology that can extract multiple excitons before their annihilation, it will have a promising application prospect. Moreover, there is a hole relaxation on a the time scale of 5-6 ps via a phonon coupling pathway to lower-energy hole states in addition to the above-described ultrafast exciton-exciton annihilation process in 2S and 1P excitons. Furthermore, by nanosecond time-resolved photoluminescence spectroscopy, it can be concluded that the charge separated state is long-lived (200 ns). Our findings provide a valuable insight into the understanding of ultrafast multiexciton generation and recombination in quantum dots. These results are helpful to understand the intrinsic photo-physics of multiexciton generation in quantum dots, to implement the photovoltaic and optoelectronic applications, and to ascertain the exciton relaxation dynamics of quantum dots.

N-Type PbSe Quantum Dots via Post-Synthetic Indium Doping.

We developed a postsynthetic treatment to produce impurity n-type doped PbSe QDs with In3+ as the substitutional dopant. Increasing the incorporated In content is accompanied by a gradual bleaching of the interband first-exciton transition and concurrently the appearance of a size-dependent, intraband absorption, suggesting the controlled introduction of delocalized electrons into the QD band edge states under equilibrium conditions. We compare the optical properties of our In-doped PbSe QDs to cobaltocene treated QDs, where the n-type dopant arises from remote reduction of the PbSe QDs and observe similar behavior. Spectroelectrochemical measurements also demonstrate characteristic n-type signatures, including both an induced absorption within the electrochemical bandgap and a shift of the Fermi-level toward the conduction band. Finally, we demonstrate that the In3+ dopants can be reversibly removed from the PbSe QDs, whereupon the first exciton bleach is recovered. Our results demonstrate that PbSe QDs can be controllably n-type doped via impurity aliovalent substitutional doping.

Investigations of intensity dependant nonlinear optical properties of betanin/ZnO composites embedded in PVA

In this report, we report the intensity dependant nonlinear absorption properties of bio-inspired hybrid materials (betanin-ZnO) embedded in polymeric matrices through the Z-scan technique using an Nd: YAG laser (532nm, 7ns, 10Hz). We observed a change over in the sign of nonlinearity due to the interplay of exciton bleaching and optical limiting mechanisms. Light confinement effect and ship-in-a bottle effect play crucial roles. Theoretical analysis has been performed using a model based on nonlinear absorption coefficient and saturation intensity. The result of present study gives an additional mechanism for the gain enhancement in dye doped ZnO matrix.

The direct observation of charge separation dynamics in CdSe quantum dots/cobaloxime hybrids.

In this work, we investigated photoinduced charge separation dynamics in a CdSe quantum dot/cobaloxime molecular catalyst hybrid using the combination of transient optical (OTA) and X-ray absorption (XTA) spectroscopy. We show that ultrafast charge separation occurs through electron transfer (ET) from CdSe QDs to cobaloxime. In addition to the enhanced 1S exciton bleach recovery in CdSe QDs due to the presence of cobaloxime, the direct evidence for ET process, i.e. the formation of the transient charge separated state, is captured by XTA. These results not only demonstrate the capability of XTA to capture the transient species during the photoinduced reactions in hybrid nanostructures but also enhance our understanding of charge separation dynamics in semiconductor nanocrystal/molecular catalyst hybrid.

Interfacial band bending in Au-tipped Cd-chalcogenides hybrid nanostructures

The interfacial electronic structure of Au-tipped CdX (X = S, Se and Te) hybrid nanostructures (HNSs) have been studied by UHV scanning tunneling spectroscopy (STS) at room temperature. Au-tipped CdX (X = S, Se and Te) HNSs were synthesized by phase transfer chemical methods. The dimensions were determined by the scanning tunneling microscope (STM), the high resolution transmission electron microscope (HRTEM) and the optical absorption. The measured dimensions using these techniques are consistent, giving Au tip average size within the range of [2.5, 3.5] nm and Cd-chalcogenides quantum dots (QDs) nanoparticles (NPs) average size within the range of [3.5, 4] nm. The STS at the interface of Au-tipped CdS and CdSe HNSs detects a downward band bending towards the interface ∼0.25 ± 0.01 eV and 0.18 ± 0.03 eV respectively, indicating an electron accumulation at these interfaces. However, an upward band bending towards the interface of ∼0.78 ± 0.01 eV is measured by the STS at the interface of Au-tipped CdTe HNS, indicating electron depletion at the interface. The band bending values were also confirmed using the corresponding calculated models of the energy band diagrams. These different behaviors were also observed in the UV–vis absorption of Au-tipped CdS and CdSe HNSs, which shows exciton bleaching features, but an exciton increasing feature is observed in the case of Au-tipped CdTe HNS. These later results are explained as a result of the presence of electron accumulation at the interface of Au-tipped CdS and CdSe HNSs, and also an electron depletion at the interface of Au-tipped CdTe HNS. Such determinations of the interfacial band bending in Au-tipped Cd-chalcogenides HNSs have significant consequences on the charge separation efficiency and the photo-response behavior. Furthermore, the presence of these bands has a direct impact in the field of applying plasmonics for improved photovoltaic solarcells.

Ultrafast Transient Absorption Study of the Nature of Interaction between Oppositely Charged Photoexcited CdTe Quantum Dots and Cresyl Violet

Understanding the dynamics of exciton quenching of the quantum dots (QDs) is of great importance considering the fact that the applications of these substances are based mainly on luminescence. In this work, we have studied exciton quenching of the CdTe QDs by cresyl violet (CV) employing steady-state and time-resolved absorption and emission techniques. Efficient luminescence quenching of these QDs is observed in the presence of CV. Interestingly, despite an excellent overlap of the absorption and emission spectra of CV and QDs, respectively, the emission quenching of the QDs is not accompanied by an increase in the steady-state fluorescence intensity of CV or a rise in its fluorescence time profile that can be considered as evidence for the Förster resonance energy transfer process. The time-resolved fluorescence measurements suggest that the quenching is partially due to static interaction of the two species. The transient absorption studies in the 0–100 ps time scale show that recovery of the 1S exciton bleach of the QDs is much faster in the presence of CV when compared with that in its absence, indicating that ultrafast photoinduced electron transfer from the conduction band of the QDs to CV is responsible for the emission quenching of the former. The recombination of the charge-separated species is found to occur in ∼2 ps.

Modeling of optically controlled reflective bistability in a vertical cavity semiconductor saturable absorber

Bistability switching between two optical signals has been studied theoretically utilizing the concept of cross absorption modulation in a vertical cavity semiconductor saturable absorber (VCSSA). The probe beam is fixed at a wavelength other than the low power cavity resonance wavelength, which exhibits bistable characteristic by controlling the power of a pump beam (λpump≠λprobe). The cavity nonlinear effects that arises simultaneously from the excitonic absorption bleaching, and the carrier induced nonlinear index change has been considered in the model. The high power absorption in the active region introduces thermal effects within the nonlinear cavity due to which the effective cavity length changes. This leads to a red-shift of the cavity resonance wavelength, which results a change in phase of the optical fields within the cavity. In the simulation, the phase-change due to this resonance shifting is considered to be constant over time, and it assumes the value corresponding to the maximum input power. Further, an initial phase detuning of the probe beam has been considered to investigate its effect on switching. It is observed from the simulated results that, the output of the probe beam exhibits either clockwise or counter-clockwise bistability, depending on its initial phase detuning.

Effect of betanin natural dye extracted from red beet root on the non linear optical properties ZnO nanoplates embedded in polymeric matrices

In this article, we have investigated the effect of betanin natural dye extracted from red beetroot on nonlinear optical properties of ZnO nanoplates embedded in polymeric matrices through the Z-scan technique using an Nd: YAG laser (532 nm, 7 ns, 10 Hz). We observed reverse saturable absorption (RSA) at 532 nm for dye and ZnO nanoplates. A strong influence on RSA behavior of nanoplates-PVA matrix was observed by adding betanin natural dye. The influence of betanin on the nonlinear character of ZnO-PVA system leads to saturable absorption and again to RSA on increasing input fluence. Such a change over in the sign of the nonlinearity is due to the interplay of the exciton bleaching and optical limiting mechanisms, and probably due the presence of sucrose. Theoretical analysis has been performed using a model based on nonlinear absorption coefficient and saturation intensity. The result of present study gives an additional mechanism for the gain enhancement in dye doped ZnO matrix.

Ultrafast Charge Transfer Dynamics in Photoexcited CdTe Quantum Dot Decorated on Graphene

We report synthesis and ultrafast charge transfer dynamics of photoexcited CdTe quantum dots (QDs) decorated on graphene. We have synthesized CdTe QD particles of 2.2 nm sizes with first exciton (1S3/2-1Se) band ∼450 nm and then decorated the QD particles on graphene which has been confirmed by HRTEM studies. The CdTe QD decorated graphene has been named as G-CdTe. Steady state emission studies revealed that on the graphene surface CdTe emission gets quenched drastically which indicates the charge transfer from photoexcited CdTe to graphene. To unravel the charge transfer dynamics in ultrafast time scale we have carried out femtosecond transient absorption studies by exciting the CdTe QD particles and monitoring the transients in the visible to near-IR region. Transient absorption studies indicate that exciton recombination time (as monitored the exciton bleach) of pure CdTe QD takes place within 50 ps; however, on graphene the surface exciton recombination time was found to be much longer (>1 ns). Our st...