Transient Grating Technique Research Articles

Investigation of the Effect of the Passivation on the Charge Transfer from MAPbI3 to Electron Transport Layer Using a Heterodyne Transient Grating Spectroscopic Technique.

We fabricated MAPbI3 perovskite thin films with ZnO on a glass substrate, in which a passivation layer (Phenethylammonium iodide (PEAI); p-methoxyphenethylammonium iodide (CH3O-PEAI); 2-methoxyethylammonium iodide (MEAI)) was inserted between two layers. In order to understand the effect of the insertion of each passivation material on the transfer efficiency of the photo-generated electrons from MAPbI3 to ZnO, we observed the near-field heterodyne transient grating (NF-HD-TG) responses of each film and investigated the component arising from the recombination of the trapped electrons at the ZnO surface. Based on the accelerated recombination between photo-generated holes remaining in the MAPbI3 layer and surface-trapped electrons in ZnO and the increase in the number of the trapped electrons in ZnO when either CH3O-PEAI or PEAI was applied, we successfully revealed that the charge transfer efficiency was enhanced by the insertion of the passivation materials including a benzene ring stabilizing the defect states. Particularly, it was demonstrated that CH3O-PEAI showed the highest increase in the charge transfer efficiency, which could be attributed to the high electron density in the benzene ring, resulting from the existence of the electron donating group, CH3O, and its role in the effective transition from 3D to 2D perovskite phases.

Temperature dependence of the diffusion, thermodiffusion and Soret coefficient of a dilute polystyrene/toluene solution as measured by means of a transient holographic grating technique

We have measured diffusion, thermodiffusion and Soret coefficients of a dilute solution of polystyrene in toluene by means of the transient holographic grating technique of thermal diffusion forced Rayleigh scattering over a temperature range from 10 to 50 °C. The results show a systematic deviation from optical beam deflection measurements over a narrower temperature interval of the order of ten percent but the identical temperature dependence. A comparison with previous measurements from our laboratory, new control measurements and literature data does not lead to a clear conclusion about the correct values. From the diffusion coefficient a temperature independent hydrodynamic radius is obtained. Both the Soret coefficient and the viscosity-scaled thermodiffusion coefficient ηDT, for which a universal value has been reported in the case of asymmetric systems with large solute entities, decrease with temperature.

Investigation of the charge injection from Organometal halide perovskite to a hole transport layer using a heterodyne transient grating spectroscopic technique

The responses of MAPbI3 coated by ZnO with and without CuSCN were observed using near-field heterodyne transient grating spectroscopic technique for the investigation of the effect of the insertion of CuSCN, the hole transport layer, on the hole transfer efficiency in the perovskite-based solar cell. We revealed that the recombination arising from the surface trapped electrons in ZnO, the electron transport layer, was retarded when CuSCN was inserted in the film, which could be attributed to the spatially separated charge carriers more sufficiently, caused by the increase in the distance between the electron transport layer and the conducting substrate.

Uncovering Energetic Positions of Surface Trap States in α-Fe2O3 Treated with Cobalt Phosphate (Co-Pi) Using Charge Carrier-Selective Heterodyne Transient Grating Technique.

Near-field heterodyne transient grating (NF-HD-TG) responses of hematite (α-Fe2O3) treated with cobalt phosphate (Co-Pi) were measured with the burn lasers inducing the depletion of the response by the removal of the trapped charge carriers in the target state, which is called charge carrier-selective heterodyne transient grating (CS-HD-TG) spectroscopic technique. We found that two distinct trap states co-existed in Co-Pi loaded on the surface of α-Fe2O3. One of them named r-SS2, of which potential was similar to that of r-SS1 in the surface of α-Fe2O3, acted as a recombination centre but could increase the lifetime of the trapped holes by the charge separation. We also revealed that the energetic position of the other (i-SS2), which has been regarded as the intermediate state for oxygen evolution reaction with low overpotential, was higher than that of i-SS1 in α-Fe2O3 but lower than those of r-SS1 and r-SS2.

Ultrafast electron diffuse scattering as a tool for studying phonon transport: Phonon hydrodynamics and second sound oscillations.

Hydrodynamic phonon transport phenomena, like second sound, have been observed in liquid helium more than 50 years ago. More recently second sound has been observed in graphite at over 200 K using transient thermal grating (TG) techniques. In this work, we explore signatures of phonon hydrodynamic transport and second sound oscillations in ultrafast electron diffuse scattering patterns, which can provide time, momentum, and branch resolved information on the state-of-excitation of the phonon system beyond that available through TG experiments. We use the density functional theory and solve the Boltzmann transport equation to determine time-resolved non-equilibrium phonon populations and model phonon transport in graphite. This model also provides the information necessary to calculate the time evolution of one-phonon structure factors and diffuse scattering patterns during thermal transport covering ballistic, diffusive, and hydrodynamic regimes where the effect of a second sound oscillation on the phonon distribution is observed. Direct measurements of how the phonon distribution varies in time and space in various thermal transport regimes should yield new insights into the fundamental physics of the underlying processes.

The effect of echoes interference on phonon attenuation in a nanophononic membrane

Nanophononic materials are characterized by a periodic nanostructuration, which may lead to coherent scattering of phonons, enabling interference and resulting in modified phonon dispersions. We have used the extreme ultraviolet transient grating technique to measure phonon frequencies and lifetimes in a low-roughness nanoporous phononic membrane of SiN at wavelengths between 50 and 100 nm, comparable to the nanostructure lengthscale. Surprisingly, phonon frequencies are only slightly modified upon nanostructuration, while phonon lifetime is strongly reduced. Finite element calculations indicate that this is due to coherent phonon interference, which becomes dominant for wavelengths between ~ half and twice the inter-pores distance. Despite this, vibrational energy transport is ensured through an energy flow among the coherent modes created by reflections. This interference of phonon echos from periodic interfaces is likely another aspect of the mutual coherence effects recently highlighted in amorphous and complex crystalline materials and, in this context, could be used to tailor transport properties of nanostructured materials.

Monitoring Charge Transfer Dynamics of Surface Trapped Holes in ZnO Surface Utilized for Ethanol Oxidation Using Heterodyne Transient Grating Spectroscopic Techniques

Monitoring Charge Transfer Dynamics of Surface Trapped Holes in ZnO Surface Utilized for Ethanol Oxidation Using Heterodyne Transient Grating Spectroscopic Techniques

Evaluation of ambipolar diffusion coefficient in AlxGa1−xN semiconductor

Numerical approach to account for the influence of randomly fluctuating potential on carrier mobility in compound semiconductors with compositional disorder is developed and exploited for calculating the heavy hole-defined ambipolar diffusion coefficient in AlxGa1-xN layers. The rates of inelastic heavy hole-phonon scattering were obtained by integrating the quasi-classical hole-phonon scattering rates for the virtual crystal over the semiclassical confining potential obtained by using the local landscape method. The calculated dependence of the heavy-hole diffusion coefficient on the aluminum content in AlxGa1-xN crystal in a wide compositional range (x = 0 – 1) evidences a substantial influence of localization on the diffusivity of nonequilibrium carriers and is in a good quantitative correspondence with the experimental data obtained in this work by using the light-induced transient grating technique. The adopted Monte Carlo simulation technique also enables diffusivity calculations in other multicomponent compounds (e.g., high-In-content InGaN) and their structures, in particular, those for LEDs and µ-LEDs.

Development of charge carrier-selective heterodyne transient grating spectroscopic technique and its application in the distinction of surface trap states in hematite.

We developed a new spectroscopic technique, which is named the charge carrier-selective heterodyne transient grating (CS-HD-TG) method, for the distinction of surface trap states existing in photocatalysts and photovoltaic materials by employing a burn laser inducing depletion of the number of surface trapped charge carriers. In addition, as a case study, we measured the heterodyne transient grating responses of hematite under bias conditions with and without the burn laser and revealed that two distinct trap states co-exist at the surface of the hematite film and only one of them could act as the reaction intermediate for the oxygen evolution reaction (OER), which is consistent with former studies.

A new strategy for monitoring the charge transfer from perovskite thin films to electron transport layers using a heterodyne transient grating technique.

We fabricated CH3NH3PbI3 (MAPbI3) thin films with and without a ZnO layer and measured the heterodyne transient grating (HD-TG) responses of each film to investigate the charge injection dynamics from MAPbI3 to ZnO, based on the component arising from the recombination of the surface trapped electrons in the ZnO layer with the remaining holes in MAPbI3. In addition, we observed the HD-TG response of the MAPbI3 thin film coated with the ZnO layer in which phenethyl ammonium iodide (PEAI) was inserted between two layers as a passivation layer and confirmed that the charge transfer was enhanced when PEAI existed, based on the increase in the amplitude of the component arising from the recombination and its acceleration.

High ambipolar mobility in cubic boron arsenide.

Semiconductors with high thermal conductivity and electron-hole mobility are of great importance for electronic and photonic devices as well as for fundamental studies. Among the ultrahigh-thermal conductivity materials, cubic boron arsenide (c-BAs) is predicted to exhibit simultaneously high electron and hole mobilities of >1000 centimeters squared per volt per second. Using the optical transient grating technique, we experimentally measured thermal conductivity of 1200 watts per meter per kelvin and ambipolar mobility of 1600 centimeters squared per volt per second at the same locations on c-BAs samples at room temperature despite spatial variations. Ab initio calculations show that lowering ionized and neutral impurity concentrations is key to achieving high mobility and high thermal conductivity, respectively. The high ambipolar mobilities combined with the ultrahigh thermal conductivity make c-BAs a promising candidate for next-generation electronics.

Unsteady ballistic heat transport in two-dimensional harmonic graphene lattice

We study the evolution of initial temperature profiles in a two-dimensional isolated harmonic graphene lattice. Two heat transfer problems are solved analytically and numerically. In the first problem, the evolution of a spatially sinusoidal initial temperature profile is considered. This profile is usually generated in real experiments based on the transient thermal grating technique. It is shown that at short times the amplitude of the profile decreases by an order magnitude and then it performs small decaying oscillations. A closed-form solution, describing the decay of the amplitude at short times is derived. It shows that due to symmetry of the lattice, the anisotropy of the ballistic heat transfer is negligible at short times, while at large times it is significant. In the second problem, a uniform spatial distribution of the initial temperature in a circle is specified. The profile is the simplest model of graphene heating by an ultrashort localized laser pulse. The corresponding solution has the symmetry of the lattice and many local maxima. Additionally, we show that each atom has two distinct temperatures corresponding to motions in zigzag and armchair directions. Presented results may serve for proper statement and interpretation of laboratory experiments and molecular dynamics simulations of unsteady heat transfer in graphene.

Observation of second sound in graphite over 200\u2009K

Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime. We directly observe second sound in graphite at temperatures of over 200 K using a sub-picosecond transient grating technique. The experimentally determined dispersion relation of the thermal-wave velocity increases with decreasing grating period, consistent with first-principles-based solution of the Peierls-Boltzmann transport equation. Through simulation, we reveal this increase as a result of thermal zero sound—the thermal waves due to ballistic phonons. Our experimental findings are well explained with the interplay among three groups of phonons: ballistic, diffusive, and hydrodynamic phonons. Our ab initio calculations further predict a large isotope effect on the properties of thermal waves and the existence of second sound at room temperature in isotopically pure graphite.

Radiative contribution to thermal grating decay

The decay of a spatially sinusoidal temperature perturbation (thermal grating) via thermal transport by both conduction and radiation is analyzed theoretically. The intrinsic non-radiative conduction is described by the heat diffusion equation while radiative transport is described by the radiation transport equation for a non-scattering absorbing medium. We solve the coupled equations analytically and obtain the thermal grating decay rate, which leads to an explicit expression for the radiative contribution to the effective thermal conductivity measured with the laser-induced transient thermal grating technique. We find that at a given thermal grating period, the radiative contribution to thermal transport is maximized when the absorption length of thermal radiation is about 1/4 of the period. We present the expression for an upper bound for the radiative contribution and discuss the results for representative materials. We conclude that in a typical transient grating measurement with the grating period in the micrometer range, the radiative contribution is negligible, but it may be significant if the grating period is in the millimeter or centimeter range. Our analysis also provides a Fourier-domain Green's function that can be used to find the temperature field produced by an arbitrary spatiotemporal distribution of heat sources.

Time-resolved detection of association/dissociation reactions and conformation changes in photosensor proteins for application in optogenetics.

Photosensor proteins are important not only because of their biological functions but also because of their applications in optogenetics. To understand the molecular mechanism behind their biological functions and consequently seek possible applications to optogenetics, the dynamics of their intermolecular interaction (for example, association/dissociation reaction and conformational changes) upon photoexcitation need to be elucidated. Although it has been difficult to trace such reactions in the time domain using traditional spectroscopic techniques, the time-resolved diffusion method based on the transient grating technique has been demonstrated to possess a significant advantage in detecting such spectrally silent dynamics in a time-resolved manner. In this paper, the principle and studies on blue light sensor proteins, phototropins, are described. Reaction kinetics of dimerization, dissociation reactions, and conformational changes were measured, and reaction schemes were determined. This method can be employed to study protein reactions from the viewpoint of diffusion and to elucidate the reaction schemes and kinetics that cannot be detected by other spectroscopic methods.

Investigation of the photo-excited charge carrier dynamics in Bismuth vanadate (BiVO4) photoanode by the heterodyne transient grating technique

We measured the heterodyne transient grating (HD-TG) responses of the BiVO4 photoanode for the first time. We demonstrated that the responses observed in acetonitrile, an inactive solvent, consisted of the recombination arising from the surface trapped holes and the thermal diffusion component. In addition, we found two components in the responses observed in aqueous solution under the applied bias condition, which were assigned to the recombination arising from the surface trapped holes and mixed component corresponding to the competition between back electron/hole recombination and water oxidation, respectively. The observed responses showed the strong dependence of the signal intensities on the applied voltage and the retardation of the recombination, resulting from the band bending.

Time-resolved detection of SDS-induced conformational changes in α-synuclein by a micro-stopped-flow system.

An intrinsically disordered protein, α-synuclein (αSyn), binds to negatively charged phospholipid membranes and adopts an α-helical structure. This conformational change is also induced by interaction with sodium dodecyl sulfate (SDS), which is an anionic surfactant used in previous studies to mimic membrane binding. However, while the structure of the αSyn and SDS complex has been studied widely by various static measurements, the process of structural change from the denatured state to the folded state remains unclear. In this study, the interaction dynamics between αSyn and SDS micelles was investigated using time-resolved measurements with a micro-stopped-flow system, which has been recently developed. In particular, the time-resolved diffusion based on the transient grating technique in combination with a micro-stopped-flow system revealed the gradual change in diffusion triggered by the presence of SDS micelles. This change is induced not only by binding to SDS micelles, but also by an intramolecular conformational change. It was interesting to find that the diffusion coefficient decreased in an intermediate state and then increased to the final state in the binding reaction. We also carried out stopped-flow-kinetic measurements of circular dichroism and intramolecular fluorescence resonance energy transfer, and the D change was assigned to the formation of a compact structure derived from the helix bending on the micelle.

Time-resolved spectroscopic study of photo-excited charge carrier dynamics in hematite (α-Fe2O3): Effect of re-growth treatment

In order to figure out the origin of the improvement of the photo-electrochemical performance of the re-growth treated hematite, in which the deposition process was repeatedly performed, resulting in the reduction of the amount of the defect states, we investigated the photo-excited charge carrier dynamics in both bare and re-growth treated hematite films under the bias condition using heterodyne transient grating technique. In this study, we were able to observe the strong dependence of the signal intensities of the components appearing in the responses of the re-growth treated hematite film on the applied bias voltage, which was attributed to the Fermi level unpinning, caused by the reduction of the surface defect states.

Direct observation of large electron\u2013phonon interaction effect on phonon heat transport

As a foundational concept in many-body physics, electron–phonon interaction is essential to understanding and manipulating charge and energy flow in various electronic, photonic, and energy conversion devices. While much progress has been made in uncovering how phonons affect electron dynamics, it remains a challenge to directly observe the impact of electrons on phonon transport, especially at environmental temperatures. Here, we probe the effect of charge carriers on phonon heat transport at room temperature, using a modified transient thermal grating technique. By optically exciting electron-hole pairs in a crystalline silicon membrane, we single out the effect of the phonon–carrier interaction. The enhanced phonon scattering by photoexcited free carriers results in a substantial reduction in thermal conductivity on a nanosecond timescale. Our study provides direct experimental evidence of the elusive role of electron–phonon interaction in phonon heat transport, which is important for understanding heat conduction in doped semiconductors. We also highlight the possibility of using light to dynamically control thermal transport via electron–phonon coupling.

Impact of Alloy-Disorder-Induced Localization on Hole Diffusion in Highly Excited c -Plane and m -Plane ( In , Ga ) N Quantum Wells

The diffusion coefficient of holes can provide knowledge about carrier localization in ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$, where the carrier dynamics are altered by randomly fluctuating potential landscape. In group-III nitrides, the diffusivity of holes is difficult to measure by electrical methods but it can be studied using optical techniques. Here, we investigate the dependence of the hole diffusion coefficient on direction and carrier density in $c$-plane and $m$-plane ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$ structures by employing the light-induced transient-grating technique. We show that the hole diffusion coefficient is anisotropic in the $m$-plane structure, where it is several times larger along the $a$ crystallographic direction than along the $c$ direction. Such anisotropy is observed within the broad range of carrier densities from ${10}^{18}$ to ${10}^{20}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The diffusivity changes nonmonotonously with increasing photoexcitation, this dependence being different in thick and thin layers. We argue that an unexpectedly high diffusion coefficient at low carrier densities in thick quantum wells can be a signature of efficient hole transport via percolative paths occurring due to compositional disorder. In turn, a decrease of diffusivity with the excitation can reflect the effect of Coulomb blockade of these paths. Finally, we demonstrate that disorder impacts carrier diffusivity even at carrier densities above ${10}^{19}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, where the overflow of localized states must be included to explain the observed increase of the diffusion coefficient with the carrier density.