Slow Carrier Research Articles

A facile approach to synthesize CoO-Co3O4/TiO2 NAs for reinforced photoelectrocatalytic water oxidation

Photoelectrocatalytic (PEC) water splitting technology is used to convert solar energy into hydrogen energy, and has the potential to alleviate energy crisis problem. TiO2 nanowire arrays (NAs), as a promising PEC photoanode, have the advantages of low cost and stable mechanical properties. However, due to its wide energy band, poor conductivity, and slow carrier transfer kinetics, modifying TiO2 NAs with metal is a feasible way to achieve high PEC performance, especially cobalt-based compounds. Different from the fussy and polluted synthesizing methods, here, we use a facile method to successfully load Co3O4 and CoO mixtures on TiO2 NAs. A series of characterization methods are conducted to study the morphology, structure, and PEC water oxidation performance. It is found that the photocurrent density of the obtained CoO-Co3O4/TiO2 photoelectrode is enhanced and good stability can be kept up for 13 h or more. The prominent reason for enhanced PEC performance of CoO-Co3O4/TiO2 NAs is that loading CoO and Co3O4 can increase the light absorption, reduce the charge transfer resistance, and improve the charge separation efficiency of TiO2 NAs. Our work introduces an alternative designing way to load both of CoO and Co3O4 on photoelectrode in a particularly eco-benign way, which provides efficient and stable cobalt-based TiO2 photoanodes, and it can be applied to wider research fields, such as solar cells and photo-responsive devices.

Hot Carriers in Halide Perovskites: How Hot Truly?

Slow hot carrier cooling in halide perovskites holds the key to the development of hot carrier (HC) perovskite solar cells. For accurate modeling and pragmatic design of HC materials and devices, it is essential that HC temperatures are reliably determined. A common approach involves fitting the high-energy tail of the main photobleaching peak in a transient absorption spectrum with a Maxwell-Boltzmann distribution. However, this approach is problematic because of complications from the overlap of several photophysical phenomena and a lack of consensus in the community on the fitting procedures. Herein, we propose a simple approach that circumvents these challenges. Through tracking the broadband spectral evolution and accounting for bandgap renormalization and spectral line width broadening effects, our method extracts not only accurate and consistent carrier temperatures but also other important parameters such as the quasi-Fermi levels, bandgap renormalization constant, etc. Establishing a reliable method for the carrier temperature determination is a step forward in the study of HCs for next-generation perovskite optoelectronics.

Graft and crosslinked polymerization of polysaccharide gum to form hydrogel wound dressings for drug delivery applications

The present article discusses the exploration of the potential of moringa gum (MOG) polysaccharides for developing a hydrogel dressing as a slow drug carrier to the enhanced wound healing. The polymer films were formed by grafting of carbomer onto MOG by radiation induced crosslinking technique. The polymers were characterized by cryo-SEM, AFM, FTIR, 13C NMR spectroscopy, and swelling studies. Drug delivery and biomedical properties of the dressings were also determined. Polymer dressing absorbed 4.20 ± 0.09 g/g simulated wound fluid. The release of levofloxacin was observed without burst effect and followed non-Fickian diffusion mechanism and best fitted in Higuchi kinetic model. The hydrogel films were permeable to O2 and H2O vapour and impermeable to microbes and showed antioxidant activity.

Anion Engineering Enhanced Response Speed and Tunable Spectral Responsivity in Gallium-Oxynitrides-Based Ultraviolet Photodetectors

Most of the currently developed Ga2O3-based solar blind photodetectors exhibit unexpected high persistent photoconductive gain at the expense of low response speed, and thus, the suppression of carrier trapping remains challenging. In this work, we demonstrated amorphous gallium oxynitride (GaON) based ultraviolet photodetectors with tunable spectral response and enhanced response speed by in-situ anion engineering with reactive sputtering technique. The tunable spectral response from 4.95 to 4.37 eV is a result of bandgap narrowing effect, attributed to the elevation of the valence band maximum (VBM) by the hybridization of N 2p and O 2p states and the enhanced p-d repulsion. The constructed GaON PDs with a proper nitrogen composition exhibit remarkably reduced dark current and a fast response time of about 100 µs. Oxygen vacancies are deactivated by the lift-up of VBM so that slow carrier detrapping processes are suppressed, resulting in the reduced persistent photoconductivity and improved response spe...

Light-Induced Defect Healing and Strong Many-Body Interactions in Formamidinium Lead Bromide Perovskite Nanocrystals.

Organic lead halide perovskite (OLHP) nanocrystals (NCs) have paved the way to advanced optoelectronic devices through their extraordinary electrical and optical properties. However, understanding of the light-induced complex dynamic phenomena in OLHP NCs remains a subject of debate. Here we used wide field microscopy and time-resolved spectroscopy to correlate the local changes in photophysics and the dynamical behavior of photocarriers. We demonstrate that light-induced brightening of the photoluminescence from the formamidinium lead bromide NC films is related to the film preparation condition and reduction of trap density. The density of trap states is reduced via halide ion migration from interstitial position. Our femtosecond transient absorption study identifies transient Stark effect due to the generation of hot carriers. Because of slow carrier trapping, Auger recombination through many-body carrier-carrier interactions dominates over trion recombination. This work presents unprecedented insights into the light-driven processes enabling better device design in the future.

Hot Phonon and Auger Heating Mediated Slow Intraband Carrier Relaxation in Mixed Halide Perovskite

A clear understanding of excited carrier relaxation to their equilibrium state via carrier-phonon, carrier-carrier interaction etc. is always required for the advancement of perovskite based device performance. Organic/inorganic lead halide perovskite have shown remarkable slow hot carrier cooling properties. Herein, we investigate the effect of hot phonon and Auger heating on hot carrier relaxation dynamics in Methyl-ammonium lead iodide-chloride (CH3NH3PbI3-XClX or mixed halide) perovskite using transient absorption spectroscopy. The reduced energy-loss rate on sub-picosecond time scale clearly shows the non-equilibrium build-up of longitudinal optical and acoustic phonon modes (carrier-phonon interaction) at low pump excitation (1018 cm-3). However at high pump excitation (1019 cm-3), dominating Auger heating process (carrier-carrier interaction) further reduces the energy-loss rate by more than two order of magnitude and delays the carrier relaxation process to tens of picoseconds which makes perovskite as a promising candidates in hot carrier photovoltaic.

Characteristics of Corn Cobs Waste Activated Carbon for Slow Release Micro Fertilizer Carrier

<span>The problem of Fertilization, especially micro fertilizers</span><span lang="IN">,</span><span> is </span><span lang="IN">toxication </span><span>due to the excessive application. Using the concept of slow release fertilizer is a very possible solution to overcome the problem. The objectives of this study are 1) </span><span lang="IN">to characterize </span><span>corn cobs activated carbon for slow release micro fertilizer carrier, </span><span>2) </span><span lang="IN">to produce </span><span>and </span><span>t</span><span lang="IN">o test</span><span> of slow release micro fertilizer.</span><span>The research was carried out by converting corn cobs into activated carbon with an activation temperature of 600 ˚C and water vapor for 90 minutes. Production</span><span lang="IN"> of</span><span> slow release fertilizer was carried out by soaking activated carbon in a solution of CuSO<sub>4</sub>, FeSO<sub>4</sub> and ZnSO<sub>4</sub> 1N for 24 hours. The results of micro fertilizer were then characterized</span><span lang="IN">, then</span><span> the solubility test </span><span lang="IN">was </span><span>carried out. </span><span>The results of the characteristic analysis showed <span>that some parameters that could be used as fertilizer carriers include, iodine adsorption 404.21 mg g<sup>-1</sup>,</span> <span>adsorption of methylene blue 16.88 mg g<sup>-1</sup>, the pore volume of 0.19 cc mg<sup>-1</sup> and surface area of 315.77 m<sup>2</sup> g<sup>-1</sup>.</span> While</span><span lang="IN">,</span><span> based on the results of micronutrient solubility test the highest nutrient content that can be absorbed by activated carbon (AA) is found in Cu, followed by Zn and Fe. </span><span lang="IN">It</span><span> relates to the characteristics possessed by activated carbon namely specific surface area, pore volume, and nutrient diameter size.</span><p> </p>

Novel 2D hybrids composed of SnIn4S8 nanoplates on BiOBr nanosheets for enhanced photocatalytic applications

In the last ten years, bismuth oxybromide (BiOBr) has attracted wide attention due to its superior optoelectronic property. However, its practical application still suffers from slow carrier transport and high carrier recombination. Here we report a kind of novel tannum indium sulfide (SnIn4S8)/BiOBr hybrid prepared by a two-step hydrothermal method. The results showed that small amount of SnIn4S8 had no influence on the crystal phase of BiOBr, but the morphology could be regulated from nanosheet to nanoflower. Specially, SnIn4S8 exerted a slight effect on the light absorption and band gap of BiOBr. Importantly, SnIn4S8/BiOBr hybrids exhibited remarkable enhancement of the photocatalytic activity towards the degradation of rhodamin B (RhB) dye molecules. SnIn4S8/BiOBr-0.20 with 99.8% degradation efficiency had the highest photocatalytic activity within 40 min, while it was only 71.1% for pure BiOBr. The enhanced photocatalytic activity was mainly attributed to efficient interfacial transfer and low carrier recombination. This work will help us understand the photocatalytic mechanism of bismuth oxyhalide hybrids.

Impact of one step alloying on the carrier relaxation and charge separation dynamics of CdxZn1-xSe graded nanocrystals

Slow carrier relaxation and extent of charge separation in semiconductor nanocrystals (NCs) are the desired parameters, which control the power conversion efficiency in photovoltaic. Although several synthesis approaches have explored, the spectroscopic analysis of the fundamental understanding of charge carrier dynamics in the quantum confined states of cadmium zinc selenide (CdxZn1-xSe) graded alloy NCs remain relatively unexplored. Here, we study the charge carrier dynamics of a series of CdxZn1-xSe alloy NCs by changing the metal (Cd and Zn; M) to selenide ratio adopting one-step synthesis. Our findings underline that both excitonic absorption and band edge photoluminescence (PL) of all the alloys are red shifted concerning the binary analogs (CdSe and ZnSe), which implies the formation of gradient alloy structure where the core is CdSe rich that leads to increasing light absorption cross-section in the solar spectrum. Femtosecond transient absorption (fs-TA) measurements of the alloy NCs reveal that the CdxZn1-xSe alloy show three distinct bleaches due to the different electronic transition like 1S (1Se-1S3/2), 2S (1Se-2S3/2), and 1P (1Pe-1P3/2) while only CdSe shows two bleaches due to 1S, and 1P. The distinctive band structure (quasi type-II) of the alloy leads to slow down the intraband carrier cooling time and prolonging the charge separation due to specially decoupled photo-excited electron and hole. This finding offers new insights to understand the photophysical behavior and suggest that CdxZn1-xSe graded NCs can be used as efficient light harvesting materials in quantum dot solar cell.

Overlap Between Apolipoprotein Eε4 Allele and Slowing Gait Results in Cognitive Impairment.

Background: Although apolipoprotein E polymorphism ε4 allele (ApoE4) and slow gait are well-known risk factors for cognitive impairment, examination of their combined effect on cognitive function is lacking. Our objective was to elucidate whether a combination of ApoE4 phenotyping and slow gait resulted in greater cognitive impairment.Methods: Overall, 1,085 community-dwelling older adults, either ApoE4 carriers (n = 167, 15.4%) or non-ApoE4 carriers, were included from the “Takashimadaira study.” Gait speed was assessed with an electronic walkway and slow gait was defined as <1 m/s. Cognitive performance was also assessed using the Mini-Mental State Exam (MMSE) and the Trail Making Test (TMT)-A and -B. A two-way analysis of covariance (ANCOVA; ApoE and gait velocity factors) adjusted for covariates was performed for each analysis.Results: Gait and cognitive performances were similar for ApoE4 and non-ApoE4 carriers. A two-way ANCOVA of the MMSE showed a significant interaction between the two factors. ApoE4 carriers with slow gait had lower MMSE scores than ApoE4 carriers without slow gait and non-ApoE4 carriers with slow gait. Also, a significant main effect of gait velocity on TMT-A was observed, indicating that slow gait is associated with lower scores irrespective of the presence of ApoE4. There was no main effect or interaction observed on the TMT-B.Conclusions: Our results suggest that the concurrent presence of at least one copy of ApoE4 and slow gait can define a subgroup with the lowest cognition. Elucidating the mechanisms underlying these associations may point out modifiable factors in populations at risk of dementia.

Polaron-Mediated Slow Carrier Cooling in a Type-1 3D/0D CsPbBr3@Cs4PbBr6 Core-Shell Perovskite System.

Rapid hot carrier cooling is the key loss channel overriding all possible energy loss pathways that limit achievable solar conversion efficiency. Thus, delayed hot carrier cooling in the cell absorber layer can make hot carrier extraction a less cumbersome task, assisting in the realization of hot carrier solar cells. There have been plentitude of reports concerning the slow carrier cooling in perovskite materials, which eventually triggered interest in radical understanding of the native photophysics driving the device design. Here in this finding, a further dramatic dip in the cooling rate has been discerned upon a growing Cs4PbBr6 shell over CsPbBr3 core nanocrystals (NCs), in contrast to the bare CsPbBr3 core NCs. Using transient absorption spectroscopy, we investigated the disparity in the hot carrier thermalization pathways in the CsPbBr3 and CsPbBr3@Cs4PbBr6 core-shell NCs under the same laser fluence, which can be validated as a corollary of polaron formation in the later NCs.

Excited-State Dynamics of a CsPbBr3 Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin-Orbit Coupling Suppresses Carrier Cooling.

Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and light-emitting devices due to optoelectronic properties that can be tuned/optimized via halide composition, surface passivation, doping, and confinement. Compared to bulk materials, certain excited-state properties in NCs can be adjusted by electronic confinement effects such as suppressed hot carrier cooling and enhanced radiative recombination. Here we use spinor Kohn-Sham orbitals (SKSOs) with spin-orbit coupling (SOC) interaction as a basis to compute excited-state dissipative dynamics simulations on a fully passivated CsPbBr3 NC atomistic model. Redfield theory in the density matrix formalism is used to describe electron-phonon interactions which drive hot carrier cooling and nonradiative recombination ( knonrad). Radiative recombination ( krad) is calculated through oscillator strengths using SKSO basis. From krad and krad + knonrad, we compute a theoretical photoluminescence quantum yield (PLQY) of 53%. Computed rates of hot carrier cooling ( kcooling ≈ 10-1 1/ps) compare favorably with what has been reported in the literature. Interestingly, we observe that hot electron cooling slows down near the band edge, which we attribute to large SOC in the conduction band combined with strong confinement, which creates subgaps above the band edge. This slow carrier cooling could potentially impact hot carrier extraction before complete thermalization in photovoltaics (PVs). Implications of this work suggest that strong/intermediate confined APbX3 NCs are better suited to applications in PVs due to slower carrier cooling near the conduction band edge, while intermediate/weak confined NCs are more appropriate for light-emitting applications, such as LEDs.

Tailoring the thermoelectric properties of sol-gel grown CZTS/ITO thin films by controlling the secondary phases

In this work, we have demonstrated the effect of secondary phases on the thermoelectric properties of Cu2ZnSnS4 thin films (TF). The samples were grown on ITO substrate by chemical solution method and sulfurized in tube furnace having constant annealing temperature (500 °C) for different time duration (20, 40, 60 and 80 min). The temperature dependent (25–100 °C) thermoelectric properties (Seebeck coefficient, electrical conductivity and power factor) were measured using Seebeck effect and Hall effect. It was found that the value of Seebeck coefficient was increased from 70 to 753 μV/°C as the sulpurization time decreased from 80 to 20 min. The enhancement in Seebeck coefficient with decreasing sulpurized time duration is related with emergence of secondary phases which cause the filtering of slow carriers at the interface of secondary phases. The presence of secondary phases in the grown samples was confirmed by various characterization techniques. Raman spectroscopy data suggested CZTS/ITO sample sulpurized for 80 min has less secondary phase but as the sulpurized time duration decreased to 20 min, secondary phases such as Cu2S, SnS, SnS2 and ZnS were emerged and enhanced the thermoelectric properties. The XRD measurements were also verified that CZTS/ITO sample sulpurized for 20 min has poor crystal quality as compared to other samples due to the presence of secondary phases. SEM and UV/Visible spectroscopy measurements were also performed to support our argument.

Dynamic Screening and Slow Cooling of Hot Carriers in Lead Halide Perovskites.

Among the exceptional properties of lead halide perovskites (LHPs) is the ultraslow cooling of hot carriers. Carrier densities below the Mott density for large polarons (≤ ≈1018 cm-3 ) are focused on here. As in other semiconductors, a nascent hot electron distribution initially cools down via emission of longitudinal optical (LO) phonons on the 10-14 -10-13 s timescale. What distinguishes LHPs from conventional semiconductors is the exceptionally efficient screening in the former. The dielectric screening in LHPs on the 10-13 s timescale results in an order-of-magnitude reduction in the Coulomb potential upon the formation of a large polaron, likely with ferroelectric-like local ordering. Further LO-phonon emission is inhibited, and this leads to partial retention of hot electron energy on the 10-12 s timescale, more so in hybrid LHPs than in their all-inorganic counterparts. Further cooling of hot polarons occurs on the 10-10 s timescale, and this can be attributed to the slow diffusion of heat out of the large polaron volume due to the low thermal conductivity of LHPs. Like other carrier properties, slow hot carrier cooling in LHPs can be intimately related to efficient screening in a soft, anharmonic, and dynamically disordered lattice.

Optical Properties of Quantum Dots with a Core–Multishell Structure

In the last decade, colloidal semiconductor nanocrystals (quantum dots) have been not only studied fundamentally but also applied in photovoltaics, optoelectronics, and biomedicine. Beginning with simple approaches to the deposition of protective shells, e.g., ZnS on CdSe cores, searches for ways to increase the quantum yield of photoluminescence of quantum dots have resulted now in the development of new types of quantum dots characterized not only by record high extinction coefficients but also by high photoluminescence quantum yields. In this work, the optical properties of core–multishell quantum dots have been analyzed. These quantum dots have been specially designed to reach the maximum possible localization of excited charge carriers inside luminescent cores, which makes it possible to reach a photoluminescence quantum yield close to 100%. Core–multishell quantum dot samples with a shell thickness of 3–7 monolayers have been fabricated. Changes in the characteristics of optical transitions in such quantum dots with an increase in the number of layers of the shell have been studied. The effect of the thickness of the shell on the optical properties of prepared quantum dots has been analyzed. In particular, analysis of photoluminescence lifetimes of such quantum dots has revealed a possible alternative mechanism of radiation of core–multishell quantum dots based on the slow charge carrier transfer from the excited outer layer of the CdS shell to the CdSe core.

Nanoporous Distorted TiO2 Ribbons Combined with CdS for Photocatalytic Conversion of CO2 under Visible Light

Photoreduction of CO2 into fuels via TiO2 is conducive to mitigating the worsening global climate and the increasing energy crisis, while the low energy efficiency hinders its large-scale applications. In this work, nanoporous distorted TiO2 ribbons combined with CdS are prepared to optimize the CO2 photoreduction processes through the fast transmission of photoinduced charge carriers and the slow charge carriers’ recombination in nanoporous distorted TiO2 ribbons. The results showed that the formation rate of methanol in nanoporous distorted TiO2 ribbons combined with CdS was 20.6 μmol g−1h−1, which was 1.82 times higher than that of TiO2 nanoparticles combined with CdS.

Enhanced photoconductivity of semiconductor polymers using graphene nanoflakes

Abstract We have examined the effect of the addition of graphene nanoflakes (GNs) to improve the field-effect mobility of poly(3-hexylthiophene) (P3HT)-based field-effect transistors (FETs). We observe that the FET mobility increases up to 0.02 cm2/Vs at GNs concentration of 0.06 mg/mL. The remarkable increase in FET mobility can be ascribed to the incorporation of highly conducting and highly ordered graphene flakes, which acts like conducting bridges between the P3HT molecules. With further increasing the concentration of GNs, the mobility and Ion/Ioff ratio starts to decrease, due to mismatching of the energy levels of graphene and P3HT. The effect is further evidenced from the time-of-flight photocurrent (TOFP) measurements, in which the transit time (ttr) of the charge carriers are shifted to shorter times in blended layers compared to pristine P3HT. Further, we have analyzed time-resolved photocurrent variation on these samples in terms of hopping transport model using Monte-Carlo (MC) simulations within the framework of Gaussian disorder. These studies reveal that the addition of GNs causes improvement in mobility and reduces the extremely slow carriers and uniform arrival of the charge carriers.

Temperature/pH-Responsive Hexagonal Liquid Crystal for Curcumin Release.

The temperature/pH-responsive hexagonal liquid crystal constructed in a polyoxyethylene (20) sorbitan monooleate (Tween 80)/water-soluble chitosan (WCS)/ethyl oleate (EtOL)/H2O system was used to encapsulate curcumin. We discuss the effect of temperature and pH on the in vitro release of curcumin and the rheological properties of a curcumin-encapsulated liquid crystal. The drug-encapsulated hexagonal phase containing relatively high WCS content (9.9 wt %) exhibited good temperature response, as indicated by three phase transformation temperature obtained by temperature scanning. The viscosity of the drug-encapsulated hexagonal phase decreased as the temperature rose, which exhibited diverse constancy and temperature sensitivity. Also, the in vitro release rate of curcumin increased as the temperature increased. After introducing WCS, the release rate of curcumin decreased obviously the pH dependence, where the release of curcumin was slower as the pH was higher. All of the results indicated that the temperature/pH-responsive hexagonal liquid crystals were proposed to be potential slow release carriers to deliver curcumin.

Hole accumulation effect on Laser-assisted field evaporation of insulators

Current issues associated with laser-assisted atom probe tomography of insulators are addressed by investigating laser-induced carrier dynamics and field evaporation kinetics. It is shown that for typical insulators with slow carrier recombination compared to the sub-picosecond laser pulse, hole accumulation at the surface plays a key role. By carrying out density functional theory calculations on a MgO cluster, it is found that the critical evaporation field strength decreases linearly as the surface hole density increases. This phenomenon can be explained by the hole-induced electric field. The evaporation of neutral oxygen is enhanced at low electrostatic field strength and high laser intensity. Theoretical insight is also provided for the non-stoichiometry problem in the mass spectra measured in atom probe tomography of compounds.

Nanoclay polymer composites loaded with urea and nitrification inhibitors for controlling nitrification in soil

ABSTRACTNanoclay polymer composites (NCPCs) were synthesized with partially neutralized acrylic acid and bentonites and loaded with urea and nitrification inhibitors (NIs) to act as a slow release carrier of nitrogen (N). The resulting product was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD of NCPCs revealed that the bentonite layers were completely exfoliated and dispersed in the composite after the polymerization. The water absorbency of pure polymer (Acrylic acid + Acrylamide) was 197.53 g g−1 and 137.75 g g−1 by nanocomposite (8% nanobentonite) in distilled water. The nitrification inhibition ability of these NCPCs was evaluated by incubation study for 60 days in laboratory at 28°C and 50% water-holding capacity. The Schiff base -NCPCs were most effective at inhibiting nitrification (30–87%) compared to dicyandiamide and Neem oil. A column study was performed to know the movement of NH4-N and NO3-N at three different depths. Result showed that the Schiff base -NCPC decreased nitrate movement by 78.5% at the depth of 5 cm in soil column. The slow release of nitrogen and good water retention capacity confirmed that these NCPCs can be viably exploited for application in agriculture.