Photoexcitation Density Research Articles

Coexistence of the Band Filling Effect and Trap-State Filling in the Size-Dependent Photoluminescence Blue Shift of MAPbBr3 Nanoparticles.

The size-dependent photoluminescence (PL) blue shift in organometal halide perovskite nanoparticles has traditionally been attributed to quantum confinement effects (QCEs), irrespective of nanoparticle size. However, this interpretation lacks rigor for nanoparticles with diameters exceeding the exciton Bohr radius (rB). To address this, we investigated the PL of MAPbBr3 nanoparticles (MNPs) with diameters ranging from ~2 to 20 nm. By applying the Brus equation and Burstein-Moss theory to fit the PL and absorption blue shifts, we found that for MNPs larger than rB, the blue shift is not predominantly governed by QCEs but aligns closely with the band filling effect. This was further corroborated by a pronounced excitation-density-dependent PL blue shift (Burstein-Moss shift) at high photoexcitation densities. Additionally, trap-state filling was also found to be not a negligible origin of the PL blue shift, especially for the smaller MNPs. The time-resolved PL spectra (TRPL) and excitation-density-dependent TRPL are collected to support the coexistence of both filling effects by the high initial carrier density (~1017-1018 cm-3) and the recombination dynamics of localized excitons and free carriers in the excited state. These findings underscore the combined role of the band filling and trap-state filling effects in the size-dependent PL blue shift for solution-prepared MNPs with diameters larger than rB, offering new insights into the intrinsic PL blue shift in organometal halide perovskite nanoparticles.

Intrinsic Self-Trapped Excitons in Graphitic Carbon Nitride.

Graphitic carbon nitrides (g-C3N4) as low-cost, chemically stable, and ecofriendly layered semiconductors have attracted rapidly growing interest in optoelectronics and photocatalysis. However, the nature of photoexcited carriers in g-C3N4 is still controversial, and an independent charge-carrier picture based on the band theory is commonly adopted. Here, by performing transient spectroscopy studies, we show characteristics of self-trapped excitons (STEs) in g-C3N4 nanosheets including broad trapped exciton-induced absorption, picosecond exciton trapping without saturation at high photoexcitation density, and transient STE-induced stimulated emissions. These features, together with the ultrafast exciton trapping polarization memory, strongly suggest that STEs intrinsically define the nature of the photoexcited states in g-C3N4. These observations provide new insights into the fundamental photophysics of carbon nitrides, which may enlighten novel designs to boost energy conversion efficiency.

New insight into the visible-light-driven photoinduced electron transfer mechanism for ozone activation over defective sites of ceria for organic compounds destruction

Herein, we prepared CeO2 with varying concentrations of oxygen vacancies (Ov) and investigated the effects of the Ov concentration and photoinduced electron transfer (PET) on O3 activation under visible-light-driven (VLD) photocatalytic-ozonation (PO) for removal of oxalic acid (OA). Density functional theory calculations showed the formation of Ov reduced the bandgap of CeO2, leading to region around the Ce atom with a low adsorption energy barrier toward O3. Notably, no distinct linear correlation was found between the Ov concentration and VLD-PO activity. This implies that CeO2 with a lower amount of Ov (14.64%) exhibits better PET and higher photoexcited electron density than CeO2 with a relatively higher Ov (17.62%), suggesting that the PET properties of CeO2 are crucial for the Ov regeneration and O3 activation and conversion for OA removal. Electric energy per order (EEO) for VLD-PO using CeOx(700) was 6.94 kWh m−3 order−1. Additionally, bioassay experiments with Daphnia Magna showed toxicity reduction in the PO-treated effluent and continuous-flow experiments (CFEs) proved that VLD-PO could effectively remediate the toxic and refractory micropollutants containing water. The study findings can promote the application of PO systems in water treatment for treating organic pollutants.

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.

Long-range charge carrier mobility in metal halide perovskite thin-films and single crystals via transient photo-conductivity

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA0.83Cs0.17Pb(I0.9Br0.1)3, (FA0.83MA0.17)0.95Cs0.05Pb(I0.9Br0.1)3 and CH3NH3PbI3-xClx polycrystalline films in the range of 0.3 to 6.7 cm2 V−1 s−1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.

The influence of Ce3+ codoping on upconversion in nanocrystalline NaYF4:Yb3+,Tm3+

Usually, to obtain multicolor upconversion in lanthanides, materials with different codopants, different size, chemical composition or compositional architecture are required. However, designing a material whose emission color can be tuned in-situ by an external stimulus, such as temperature, excitation pulse duration or photoexcitation density, is of great interest for many applications, but still remains a challenge. The possible solution for this issue can be achieved by intentional codoping the luminescent materials with optically active ions which can disturb or compete with other energy transfer processes, and in consequence lead to changes in relative emission intensity, thus the dominant emission color. Here we synthesized and investigated colloidal luminescent Yb3+ and Tm3+ doped NaYF4 nanoparticles which were codoped with optically active Ce3+ ions, whose relative emission intensity ratios between 1D2→3F4 (at 450 nm), 1G4→3H6 (at 475 nm), 1G4→3F4 (at 650 nm), and 3H4→3H6 (at 800 nm) were significantly dependent from external stimulus – namely temperature and/or excitation pulse length. Our results shows two fold increase in 450/800, 470/800 and 650/800 luminescence intensity bands ratios (LIR) only after incorporation of Ce3+ ions to the material, and even three-fold increase in those LIR after changing the excitation conditions (i.e. pulse width). Moreover, temperature dependent luminescence properties revealed relative sensitivity of 4%/K for the 450/800 and 470/800 bands ratio and almost 5.5%/K for the 650/800 band ratio close to 0 °C. By switching to physiological range of temperatures, the thermal sensitivity decreased to around 2%/K for the 450/800 and 470/800 bands ratio and to over 2.5%/K for 650/800 bands ratio, which should be considered as significant values and suitable for temperature sensing in practical applications. It is worth noting that the thermal sensitivity values can be further enhanced by applying different excitation pulse width. The proposed materials allows to tune its emissions as well as thermal properties by changing the dopant concentration or more importantly also in situ, through excitation scheme modifications.

Nonequilibrium quasistationary spin disordered state in α−RuCl3

The authors use optical excitation as a way to alter the magnetic free energy landscape and the resulting phase diagram of the frustrated honeycomb magnet $\ensuremath{\alpha}$-RuCl${}_{3}$. They time-track experimentally the magneto-optical response from the $\ensuremath{\alpha}$-RuCl${}_{3}$'s zigzag-ordered ground state after photoexcitation and they reproduce the observed dynamics within a Ginzburg-Landau model. A quasistationary spin disordered state is induced above a critical photoexcitation density, suggesting a new route to reach a nontrivial spin disordered state in Kitaev-like magnets such as $\ensuremath{\alpha}$-RuCl${}_{3}$.

Spatiotemporal spin dynamics of two-dimensional electron gas with ballistic motion in persistent spin helix state

The spatiotemporal spin dynamics of two-dimensional electron spins with ballistic motion in a GaAs quantum well is investigated using time- and spatially resolved magneto-optical Kerr rotation measurements. The ballistic movement of photoexcited spins, which maintains the width of the excitation spot size, results in a wave-packet-like motion of the spin distribution. The time evolution of the spin distribution measured at different excitation intensities reveals that the spin diffusion slows down and the wave-packet-like motion disappears at higher photoexcited spin densities, at which the spin distribution converges to the helical spin mode observed in the diffusive regime. The variation in the spatiotemporal spin distribution can be controlled by spin diffusion, while the spin precession length is determined by the strength of the spin-orbit interaction.

Imaging the effect of high photoexcited densities on valley polarization and coherence in MoS2 monolayers

We have investigated the laser-induced valley polarization and coherence of encapsulated MoS2 monolayer as a function of temperature, power density, and spatial position. Besides a non-monotonic dependence on temperature, recently attributed to a dependence of the valley relaxation time on the momentum scattering rate, we observe a two-fold increase of the valley polarization when increasing the laser excitation power. We attribute this effect to a local heating induced by the energy relaxation of photoexcited excitons and to an increase of the exciton-exciton scattering rate. In contrast, only a moderate enhancement of valley coherence is observed, which exhibits a dramatic drop after further increasing the excitation power. We attribute this behaviour to the detrimental role of exciton-exciton interactions on the pure dephasing rate responsible for the loss of coherence between the valleys. This manifests itself by a strong dip in the spatial profile of the valley coherence at high photoexcited densities.

Nonequilibrium dynamics of α-RuCl3 - a time-resolved magneto-optical spectroscopy study.

We present time-resolved magneto-optical spectroscopy on the magnetic Mott-Hubbard-insulating Kitaev spin liquid candidate α-RuCl3 to investigate the nonequilibrium dynamics of its antiferromagnetically ordered zigzag groundstate after photoexcitation. A systematic study of the transient magnetic linear dichroism under different experimental conditions (temperature, external magnetic field, photoexcitation density) gives direct access to the dynamical interplay of charge excitations with the zigzag ordered state on ultrashort time scales. We observe a rather slow initial demagnetization (few to 10s of ps) followed by a long-lived non-thermal antiferromagnetic spin-disordered state (100-1000s of ps), which can be understood in terms of holons and doublons disordering the antiferromagnetic background after photoexcitation. Varying temperature and fluence in the presence of an external magnetic field reveals two distinct photoinduced dynamics associated with the zigzag and quantum paramagnetic disordered phases. The photo-induced non-thermal spin-disordered state shows universal compressed-exponential recovery dynamics related to the growth and propagation of zigzag domains on nanosecond time scales, which is interpreted within the framework of the Fatuzzo-Labrune model for magnetization reversal. The study of nonequilibrium states in strongly correlated materials is a relatively unexplored topic, but our results are expected to be extendable to a large class of Mott-Hubbard insulator materials with strong spin-orbit coupling.

Quantifying Exciton Annihilation Effects in Thermally Activated Delayed Fluorescence Materials

AbstractImportant parameters for the design and performance of thermally activated delayed fluorescence (TADF) emitters are the forward and reverse intersystem crossing rates between singlet and triplet states. The magnitude of these rates is determined from the prompt and delayed transient photoluminescence decay. It is demonstrated that this photoluminescence decay strongly depends on the initial photoexcited population density due to exciton–exciton annihilation processes. By kinetic modeling of the power‐dependent time‐resolved photoluminescence of the TADF emitter 9,10‐bis(4‐(9H‐carbazol‐9‐yl)‐2,6‐dimethylphenyl)‐9,10‐diboraanthracene (CzDBA), singlet–triplet annihilation and triplet–triplet annihilation are identified as the main loss processes with rate constants in the order of 10−17 m3 s−1. Neglecting these quenching processes leads to erroneous estimates of the (reverse) intersystem crossing rates.

Excitonic terahertz emission from Si at intense interband photoexcitation

Terahertz photoluminescence from silicon crystals due to radiative transitions between the energy levels of free excitons has been studied up to intensities of interband photoexcitation significantly exceeding the threshold for the formation of electron-hole liquid droplets. At temperatures above 20 K and a photoexcitation density in excess of 7 W/cm2, the appearance of stimulated intraexciton terahertz radiation was clearly demonstrated. Terahertz gain spectrum was obtained. The gain spectrum indicates that population inversion arises between the highly excited states of excitons and sublevels of the ground state of free excitons, as well as possibly between two-exciton and biexciton states. The performed calculations of the times of energy relaxation of exciton states show the fundamental possibility of the appearance of population inversion between the states of excitons in the process of their energy relaxation.

Renormalization of the electron g factor in the degenerate two-dimensional electron gas of ZnSe- and CdTe-based quantum wells

The effective electron $g$ factor, ${g}_{\text{eff}}$, is measured in a two-dimensional electron gas (2DEG) in modulation-doped ZnSe- and CdTe-based quantum wells by means of time-resolved pump-probe Kerr rotation. The measurements are performed in magnetic fields applied in the Voigt geometry, i.e., normal to the optical axis parallel to the quantum well plane, in the field range $0.05--6$ T at temperatures $1.8--50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The ${g}_{\text{eff}}$ absolute value considerably increases with increasing electron density ${n}_{e}$. ${g}_{\text{eff}}$ changes in the ZnSe-based QWs from $+1.1$ to $+1.9$ in the ${n}_{e}$ range $3\ifmmode\times\else\texttimes\fi{}{10}^{10}\ensuremath{-}1.4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ and in the CdTe-based QWs from $\ensuremath{-}1.55$ down to $\ensuremath{-}1.76$ in the ${n}_{e}$ range $5\ifmmode\times\else\texttimes\fi{}{10}^{9}\ensuremath{-}3\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The modification of ${g}_{\text{eff}}$ reduces with increasing magnetic field, increasing temperature of lattice and 2DEG, the latter achieved by a higher photoexcitation density. A theoretical model is developed that considers the renormalization of the spin-orbit coupling constant of the two-dimensional electrons by the electron-electron interaction and takes into account corrections to the electron-electron interaction in the Hubbard form. The model results are in good agreement with experimental data.

Biexcitons in highly excited CdSe nanoplatelets

We present the phase diagram of free charges (electrons and holes), excitons, and biexcitons in highly excited CdSe nanoplatelets that predicts a crossover to a biexciton-dominated region at easily attainable low temperatures or high photoexcitation densities. Our findings extend previous work describing only free charges and excitons by introducing biexcitons into the equation of state, while keeping the exciton and biexciton binding energies constant in view of the relatively low density of free charges in this material. Our predictions are experimentally testable in the near future and offer the prospect of creating a quantum degenerate, and possibly even superfluid, biexciton gas. Furthermore, we also provide simple expressions giving analytical insight into the regimes of photoexcitation densities and temperatures in which excitons and biexcitons dominate the response of the nanoplatelets.

Photoluminescence efficiency of Al-rich AlGaN heterostructures in a wide range of photoexcitation densities over temperatures up to 550 K

Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.

Mahan excitons in room-temperature methylammonium lead bromide perovskites

In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to reveal that organic-inorganic lead-bromide perovskites host Mahan excitons at room temperature. Persistence of the Wannier exciton peak and the enhancement of the above-bandgap absorption are observed at all achievable photoexcitation densities, well above the Mott density. This is supported by the solution of the semiconductor Bloch equations, which confirms that no sharp transition between the insulating and conductive phase occurs. Our results demonstrate the robustness of the bound states in a regime where exciton dissociation is otherwise expected, and offer promising perspectives in fundamental physics and in room-temperature applications involving high densities of charge carriers.

Ultrafast Control of the Dimensionality of Exciton-Exciton Annihilation in Atomically Thin Black Phosphorus.

Using microtransient absorption spectroscopy, we show that the dynamical form of exciton-exciton annihilation in atomically thin black phosphorous can be made to switch between time varying 1D scattering and time-independent 2D scattering. At low carrier densities, anisotropy drives the 1D behavior, but as the photoexcitation density approaches the exciton saturation limit, the 2D nature of exciton-exciton scattering takes over. Furthermore, lowering the temperature provides a handle on the ultrafast timescale at which the 1D to 2D transition occurs. We understand our results quantitatively using a diffusion based model of exciton-exciton scattering.

Comparative studies of optoelectrical properties of prominent PV materials: Halide perovskite, CdTe, and GaAs

We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination and passivation, over multiple orders of photo-excitation density or carrier density appropriate for different applications. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley–Read–Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.

Explaining observed stability of excitons in highly excited CdSe nanoplatelets

Two-dimensional electron-hole gases in colloidal semiconductors have a wide variety of applications. Therefore, a proper physical understanding of these materials is of great importance. In this paper we present a detailed theoretical analysis of the recent experimental results by Tomar et al. [J. Phys. Chem. C 123, 9640 (2019)] that show an unexpected stability of excitons in CdSe nanoplatelets at high photoexcitation densities. Including the screening effects by free charges on the exciton properties, our analysis shows that CdSe nanoplatelets behave very differently from bulk CdSe, and in particular do not show a crossover to an electron-hole plasma in the density range studied experimentally, even though there is substantial overlap between the excitons at the highest densities achieved. From our results we also conclude that a quantum degenerate exciton gas is realized in the experiments, which opens the prospect of observing superfluidity in CdSe nanoplatelets in the near future.

Charge-Carrier Cooling and Polarization Memory Loss in Formamidinium Tin Triiodide.

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain. Here we present ultrafast time-resolved photoluminescence measurements on formamidinum tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. Further, we do not find any persistent polarization anisotropy in FASnI3 within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice.