Free Lifetime Research Articles

Photocurrent decay from the steady-state in thin film hydrogenated amorphous silicon: Numerical simulation analysis of experimental results

Starting from the multiple trapping rate equations that define the non-equilibrium concentrations of electrons and holes in extended states, the experiment of photocurrent decay from the steady-state is examined. A system of non-linear coupled differential equations is solved to get the temporal evolution of the occupation functions and the carrier concentrations after cessation of the illumination. Different expressions proposed in the literature to evaluate the carrier lifetimes from the photocurrent decay data are critically examined. Measurements performed on a series of hydrogenated amorphous silicon samples deposited at different substrate temperatures are reproduced by the simulations. It is found that the response time determined from the photocurrent initial rate-of-decay provides an excellent estimation of the free lifetime of the majority carrier, provided the decay is recorded from sufficiently short times. It is also found that the common recombination lifetime can also be estimated from the photocurrent decay data.

Fluorine Tuning of Morphology, Energy Loss, and Carrier Dynamics in Perylenediimide Polymer Solar Cells

We investigate backbone fluorination effects in bulk-heterojunction (BHJ) polymer solar cells (PSCs) with the fluorine-poor PBDTT-FTTE and fluorine-rich PBDTTF-FTTE donor polymers, paired with the perylenediimide (PDI) 3D “propeller acceptor” Ph(PDI)3. The PBDTTF-FTTE:Ph(PDI)3 devices exhibit a >50% power conversion efficiency (PCE, up to 9.1%) increase versus PBDTT-FTTE:Ph(PDI)3. This enhancement reflects structurally optimized phase separation due to templating effects, affording reduced energy loss, higher electron mobility, greater free charge lifetimes and yields, and lower bimolecular recombination, as quantified by UPS, AFM, TEM, GIWAXS, SCLC, light intensity dependence measurements, and fs/ns transient absorption (TA) spectroscopy. In PBDTTF-FTTE, the DFT-computed dipole orientations of the ground and excitonic states are nearly antiparallel, explaining the longer free charge lifetimes, minimized recombination, and lowered exciton binding energy. The PBDTTF-FTTE:Ph(PDI)3 performance enhancement vs that of the fluorine-poor PBDTT-FTTE:Ph(PDI)3 analogue as well as the overall PSC performance exceeds that of the corresponding PC71BM- and ITIC-Th-based cells.

Determination of the metabolic index using the fluorescence lifetime of free and bound nicotinamide adenine dinucleotide using the phasor approach.

The fluorescence lifetime of nicotinamide adenine dinucleotide (NADH) is commonly used in conjunction with the phasor approach as a molecular biomarker to provide information on cellular metabolism of autofluorescence imaging of cells and tissue. However, in the phasor approach, the bound and free lifetime defining the phasor metabolic trajectory is a subject of debate. The fluorescence lifetime of NADH increases when bound to an enzyme, in contrast to the short multiexponential lifetime displayed by NADH in solution. The extent of fluorescence lifetime increase depends on the enzyme to which NADH is bound. With proper preparation of lactate dehydrogenase (LDH) using oxalic acid (OA) as an allosteric factor, bound NADH to LDH has a lifetime of 3.4 ns and is positioned on the universal semicircle of the phasor plot, inferring a monoexponential lifetime for this species. Surprisingly, measurements in the cellular environments with different metabolic states show a linear trajectory between free NADH at about 0.37 ns and bound NADH at 3.4 ns. These observations support that in a cellular environment, a 3.4 ns value could be used for bound NADH lifetime. The phasor analysis of many cell types shows a linear combination of fractional contributions of free and bound species NADH.

Highlights of the production of (anti-)(hyper-)nuclei and exotica with ALICE at the LHC

Measurements of the production of light (anti-)nuclei and (anti-)hypernuclei have been performed in different colliding systems at the LHC, namely pp, p–Pb and Pb–Pb. The results of the production in Pb–Pb collisions of light (anti-)(hyper-)nuclei follows the expectation of the thermal model, whereas the production of nuclei in pp and p–Pb collisions shows a closer agreement with the expectations from coalescence models. Both models can give predictions for the production yields of hypothetical states such as bound states of two Λ hyperons or of a Λ and a neutron, which are expected to decay weakly. These states are searched for with the ALICE apparatus. Since no signal is found, upper limits, which are significantly below the model expectations, are set. Furthermore, the most recent measurement of the lifetime of the hypertriton determined in Pb–Pb collisions at at the LHC gives a value exactly between the world average and the free lifetime. In addition, the new result is in agreement with other recent measurements in heavy-ion collisions.

Decay of muons generated by laser-induced processes in ultra-dense hydrogen H(0)

This work reports identification of muons by their characteristic life-time of 2.20 μs after laser-induction of their precursor mesons, both kaons K± and KL0 and pions π± in ultra-dense hydrogen H(0). The pair-production signal from scattered muons at a metal converter in front of a photo-multiplier detector is observed with its decay. The observed signal intensity is decreased by a metal beam-flag which intercepts the meson and muon flux to the detector. Using D(0), the observed decay time is (2.23 ± 0.05) μs in agreement with the free muon lifetime of 2.20 μs. This signal is apparently due to the preferential generation of positive muons. Using p(0), the observed decay time is in the range 1–2 μs, thus shorter than the free muon lifetime, as expected when the signal is mainly caused by negative muons which interact with matter by muon capture.

A new approach for determination of free carriers lifetime and density of localised states in disordered semiconductors

ABSTRACTA new method for measuring the free carriers lifetime () and the density of localised states (DOS) in amorphous semiconductors is described. The method is based on the analysis of transient photoconductivity (TPC) data using Laplace transform technique. This technique involves Laplace transform of TPC data, it is a simple analysis of the solution of the basic linearised multiple trapping equations for free and trapped electrons. It is demonstrated by application to simulated and experimental TPC data measured on a typical disordered semiconductor, the hydrogenated amorphous silicon (a-Si:H). An introduced in the computing of the TPC using an arbitrarily proposed DOS model is recovered with high accuracy. For the experimental case, the determination of the exact DOS and the estimated from the experimental TPC data allow to reconstruct accurately this last. The performance and limitations of the technique are studied by means of computer simulations. Limitations are essentially the low temperatures of measurement of TPC when the recombination phenomenon is not observed at short times.

МЕТОДИ ВИЗНАЧЕННЯ КОНЦЕНТРАЦІЙ АЕРОІОНІВ У ПРИМІЩЕННЯХ ТА МОДЕЛЮВАННЯ ЇХ ЗМІН

На основі аналізу рівняння неперервності потоку повітря при наявності джерела його іонізації розглянуто можливі закономірності розповсюдження аероіонів у приміщенні. Надано розв’язок рівняння за наявності спрямованого руху повітря. Показано, що процеси дифузії мало впливають на розповсюдження аероіонів. Проведено моделювання поширення аероіонів при неперервному функціонуванні джерела іонізації повітря за різних його продуктивностей. Розраховано параметри для визначення динаміки аероіонів – середні довжини вільного пробігу та час життя. Визначено час життя аероіонів при їх різних концентраціях. Розраховано середню генерацію аероіонів у залежності від рівнів природної радіоактивності у приміщенні. Надано рекомендації щодо використання результатів у практичній діяльності.

Sacrificing the Fukushima 50 again?

In the aftermath of the 2011 Fukushima nuclear plant accident, many workers helped restore the contaminated site, exposing themselves to a highly radioactive environment. They were referred to as the 'Fukushima 50' and applauded as heroes who saved Japan. A cohort study targeting those emergency workers is, currently, underway. We object to the study on ethical grounds. Ethical and content analyses. First, the low participation rate raises ethical questions about why potential participants declined. Content analyses of nuclear power plant workers' narratives from a television broadcast extracted eight recurrent themes: disposable, treated like a sacrificial pawn, taboo, fear of contamination, readiness to risk one's life, distrust and dissatisfaction with the nation's response, regret over participating and uncertainty about the future. Second, the unscientific nature of the cohort design undermines the ethical basis for conducting it. Third, public resources were allocated in a way that compromises justice. We urge re-considering the current Fukushima 50 research study. We also urge applying the public funds now invested in this research project to activities that would directly benefit the Fukushima 50, such as offering free lifetime healthcare and direct financial compensation.

Spectroscopic investigations of thulium doped YAG and YAP crystals between 77 K and 300 K for short-wavelength infrared lasers

We present detailed measurements of laser relevant cross sections of thulium doped yttrium-aluminum-garnet (Tm:YAG) and yttrium-aluminum-perovskite (Tm:YAP), including the absorption cross sections for the H63 to H43 transition near 800 nm, and the absorption and emission cross sections for the transitions between the H63 and F43 manifolds in the short-wavelength infrared region. For Tm:YAP we present data for all polarization axes. The measurements were carried out at temperatures ranging from 80 K to 300 K. Furthermore, re-absorption free fluorescence lifetimes of the F43 to H63 transition at 77 K, 200 K and 295 K were obtained using the pinhole method. We observed a significant enhancement of the fluorescence lifetime when cooling from room temperature to 77 K. The lifetime was increased from 9.42 ms to 15.22 ms in Tm:YAG and from 3.81 ms to 4.93 ms in Tm:YAP. This indicates that lifetime quenching is present at room temperature, which can be overcome, at least partially, by cryogenic cooling. These data are presented with the scope to qualify these materials for their use in a new generation of cryogenically cooled, short-wavelength infrared, high-energy class diode pumped solid state lasers utilizing the cross relaxation mechanism for pumping.

Neutron lifetime measurements with a large gravitational trap for ultracold neutrons

Neutron lifetime is one of the most important physical constants which determines parameters of the weak interaction and predictions of primordial nucleosynthesis theory. There remains the unsolved problem of a 3.9{\sigma} discrepancy between measurements of this lifetime using neutrons in beams and those with stored neutrons (UCN). In our experiment we measure the lifetime of neutrons trapped by Earth's gravity in an open-topped vessel. Two configurations of the trap geometry are used to change the mean frequency of UCN collisions with the surfaces - this is achieved by plunging an additional surface into the trap without breaking the vacuum. The trap walls are coated with a hydrogen-less fluorine-containing polymer to reduce losses of UCN. The stability of this coating to multiple thermal cycles between 80 K and 300 K was tested. At 80 K, the probability of UCN loss due to collisions with the trap walls is just 1.5% of the probability of beta-decay. The free neutron lifetime is determined by extrapolation to an infinitely large trap with zero collision frequency. The result of these measurements is 881.5 +/- 0.7_stat +/- 0.6_syst s which is consistent with the conventional value of 880.2 +/- 1.0 s presented by the Particle Data Group. Future prospects for this experiment are in further cooling to 10 K which will lead to an improved accuracy of measurement. In conclusion we present an analysis of currently-available data on various measurements of the neutron lifetime.

New Neutron Lifetime Measurements with the Big Gravitational Trap and Review of Neutron Lifetime Data

Neutron lifetime is one of the most important physical constants which determines parameters of the weak interaction and predictions of primordial nucleosynthesis theory. In our experiment we measure the storage time of UCN in the material trap coated with a hydrogen-free fluorine-containing polymer (Fomblin grease UT-18). The stability of this coating to multiple thermal cycles between 80 K and 300 K was tested. The achieved storage time is only 1.5% less than free neutron lifetime. Using additional surface, which can be plunged into the trap to change the collision frequency of UCN with walls, we calculate free neutron lifetime by extrapolation to zero collision frequency. The result of the measurements with this new experimental setup is

Free dialysis in Nepal: Logistical challenges explored.

Nepal's Ministry of Health began offering free lifetime hemodialysis (HD) in 2016. There has been a large growth in renal replacement therapy (RRT) services offered in Nepal since 2010, when the last known data on the subject was published. In 2016, 42 HD centers existed (223% increase since 2010) serving 1975 end stage renal disease patients (303% increase since 2010); 36 nephrologists were registered (200% increase since 2010), 12 were trained in transplantation, and 790 transplants had been performed to date. We estimate the incidence of end stage renal disease to be 2900 patients (100 per million population). With an annual cost of approximately US$2300 per dialysis patient, offering free dialysis could potentially cost the government US$6.7 million per year, suggesting that 2.1% of the annual health budget would be allocated to 0.01% of the population. The geographic zone surrounding the capital city, Kathmandu, contains 50% of HD centers, but only 14.5% of Nepal's population. Forty-eight percent of the population lives within zones without HD service, therefore infrastructure challenges exist in providing equitable access to RRT. The aim of this article is to summarize the current statistics of RRT in Nepal.

Inhibitory effects of curcumin and cyclocurcumin in 1-methyl-4-phenylpyridinium (MPP+) induced neurotoxicity in differentiated PC12 cells

Development and progression of neurodegenerative diseases like Parkinson’s disease (PD) involve multiple pathways. Thus, effective therapeutic treatments should intervene to address all these pathways simultaneously for greater success. Most of the current pharmacotherapeutic approaches just supplement striatal dopamine. Hence, natural extracts of plants with therapeutic potential have been explored. Curcuminoids belong to one such group of polyphenol which show immense therapeutic effects. Here, we have used intracellular reactive oxygen species (ROS) measurement, and two-photon fluorescence lifetime imaging microscopy (2P-FLIM) of cellular autofluorescent co-enzyme reduced nicotinamide adenine dinucleotide (NADH) to study the inhibitory effects of curcumin and cyclocurcumin in alleviating PD like neurotoxicity of 1-methyl-4-phenylpyridinium (MPP+) in neuronal growth factor (NGF) induced differentiated PC12 cells. Our results showed that both cyclocurcumin and curcumin reduced the level of ROS caused by MPP+ treatment. Moreover, a significant increase in the free, protein-bound, and average NADH fluorescence lifetimes along with a decrease in the relative contribution of free- vs. protein-bound NADH components in curcuminoids treated cells (pretreated with MPP+) were observed compared with those treated with MPP+ only. This study, which indicates that cyclocurcumin offers higher neuronal protection than curcumin, may initiate further studies of these compounds in the cure of neurodegenerative diseases.

Lattice Thermal Conductivity of MgSiO3 Perovskite from First Principles

We investigate lattice thermal conductivity κ of MgSiO3 perovskite (pv) by ab initio lattice dynamics calculations combined with exact solution of linearized phonon Boltzmann equation. At room temperature, κ of pristine MgSiO3 pv is found to be 10.7 W/(m · K) at 0 GPa. It increases linearly with pressure and reaches 59.2 W/(m · K) at 100 GPa. These values are close to multi-anvil press measurements whereas about twice as large as those from diamond anvil cell experiments. The increase of k with pressure is attributed to the squeeze of weighted phase-spaces phonons get emitted or absorbed. Moreover, we find κ exhibits noticeable anisotropy, with κzz being the largest component and ({{boldsymbol{kappa }}}_{{rm{max }}}-{{boldsymbol{kappa }}}_{{rm{min }}})/bar{{boldsymbol{kappa }}} being about 25%. Such extent of anisotropy is comparable to those of upper mantle minerals such as olivine and enstatite. By analyzing phonon mean free paths and lifetimes, we further show that the weak temperature dependence of κ observed in experiments should not be caused by phonons reaching ‘minimum’ mean free paths. These results clarify the microscopic mechanism of thermal transport in MgSiO3 pv, and provide reference data for understanding heat conduction in the Earth’s deep interior.

Edge localized modes of cold neutrons in periodic condensed media

It is found that for certain energies of discreet cold neutrons, quasi-stationary eigen solutions of the corresponding Schrodinger equation, which are localized in the layer of a periodic medium, exist. The localization time of these solutions is strongly dependent on the layer thickness, being finite for a finite layer thickness and increasing indefinitely upon a infinite growth of the layer thickness as the third power of the layer thickness. The problem has been solved in the two-wave approximation of the dynamic diffraction theory for the neutron propagation direction coinciding with the periodicity axes (normal incidence of the neutron beam on the layer). The expressions for neutron eigenwave functions in a periodic medium, the reflection and transmission coefficients, and the neutron wavefunction in the layer as a function of the neutron energy incident on the layer have been determined. It turns out that for the certain discrete neutron energies, the amplitudes of the neutron wavefunction in the layer reach sharp maxima. The corresponding energies are just outside of the neutron stop band (energies forbidden for neutron propagation in the layer) and determine the energies of neutron edge modes (NEMs) localized in the layer, which are direct analogs of the optical edge modes for photonic crystals. The dispersion equation for the localized neutron edge modes has been obtained and analytically solved for the case of thick layers. A rough estimate for the localization length L is L ~(db N)–1, where b is the neutron scattering length, d is the crystal period, and N is the density of nuclei in the crystal. The estimates of the localized thermal neutron lifetime show that acheaving of a lifetime close to the free neutron lifetime seems nonrealistic due to absorption of thermal neutrons and requires a perfect large size crystal. Nevertheless, acheaving the localized neutron lifetime exceeding by ~104 times the neutron time of flight through the layer appears as experimentally attainable. The perspectives of the NEM observation are briefly discussed. It is proposed to use NEM for ultrahigh thermal neutron monochromatization by means of NEM excitation in perfect single crystals.

Isolating lattice from electronic contributions in thermal transport measurements of metals and alloys above ambient temperature and an adiabatic model

From femtosecond spectroscopy (fs-spectroscopy) of metals, electrons and phonons reequilibrate nearly independently, which contrasts with models of heat transfer at ordinary temperatures ([Formula: see text] K). These electronic transfer models only agree with thermal conductivity [Formula: see text] data at a single temperature, but do not agree with thermal diffusivity [Formula: see text] data. To address the discrepancies, which are important to problems in solid state physics, we separately measured electronic (ele) and phononic (lat) components of [Formula: see text] in many metals and alloys over [Formula: see text]290–1100 K by varying measurement duration and sample length in laser-flash experiments. These mechanisms produce distinct diffusive responses in temperature versus time acquisitions because carrier speeds [Formula: see text] and heat capacities [Formula: see text] differ greatly. Electronic transport of heat only operates for a brief time after heat is applied because [Formula: see text] is high. High [Formula: see text] is associated with moderate [Formula: see text], long lengths, low electrical resistivity, and loss of ferromagnetism. Relationships of [Formula: see text] and [Formula: see text] with physical properties support our assignments. Although [Formula: see text] reaches [Formula: see text] near 470 K, it is transient. Combining previous data on [Formula: see text] with each [Formula: see text] provides mean free paths and lifetimes that are consistent with [Formula: see text] K fs-spectroscopy, and new values at high [Formula: see text]. Our findings are consistent with nearly-free electrons absorbing and transmitting a small fraction of the incoming heat, whereas phonons absorb and transmit the majority. We model time-dependent, parallel heat transfer under adiabatic conditions which is one-dimensional in solids, as required by thermodynamic law. For noninteracting mechanisms, [Formula: see text]. For metals, this reduces to [Formula: see text] above [Formula: see text]20 K, consistent with our measurements, and shows that Meissner’s equation [Formula: see text] is invalid above [Formula: see text]20 K. For one mechanism with multiple, interacting carriers, [Formula: see text]. Thus, certain dynamic behaviors of electrons and phonons in metals have been misunderstood. Implications for theoretical models and technological advancements are briefly discussed.

Excitonic Properties and Ultrafast Carrier Dynamics in 2D Hybrid Organic-Perovskites

Due to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites (HOPs) has emerged since 2012. HOPs can be arranged in such a way that carriers are free to move at 3D or at 2D. At 3D, the exciton binding energy has been shown to be of the order of tenth of meV. When they are arranged in two dimensions, these materials can be considered as hybrid multi-quantum wells [1], that lead to strong light-matter coupling regime when they are embedded in an optical microcavity [2-9]. Moreover, both quantum and dielectric confinement lead to a huge exciton binding energy of several hundreds of meV [1]. In this talk, we will focus on the excitonic properties of 2D HOP. In particular, we will report on ultrafast pump-probe experiments performed on (C6H5C2H4NH3)2PbI4 thin layers at room temperature [10]. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ∼100 ps. The presence of a long tail in the pump/probe signal is attributed to trapped excitons on dark states, while an ultrafast intraband relaxation (tintra ≤ 150 fs) is reported. The analysis of the transient broadening and loss of oscillator strength demonstrates that 2D-HOPs share common behaviours with standard semiconductors quantum wells despite their huge exciton binding energy that is closer to the one reported for organic semiconductors.

Polystyrene CuO/Cu2O uniform films inducing MB-degradation under sunlight

This study reports on a Cu-sputtered film on polystyrene (PS) leading to the discoloration/degradation of methylene blue (MB) under low intensity solar simulated irradiation. Direct current magnetron sputtering (DCMS) was used to graft uniform, adhesive Cu/Cu oxides on the polystyrene substrate. The kinetics of Cu-PS mediated MB-discoloration adding H2O2 was observed to take place within 90–120min. The surface potential and pH variation was followed on the Cu-PS surface during MB-discoloration. Insight is provided for the observed changes relating them to the dye discoloration mechanism. The concentration, mean- free path and lifetime of the oxidative radical leading to MB-degradation were estimated. The Cu/Cu-oxides on the PS were characterized by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) evidence for redox catalysis involving Cu(I)/Cu(II)-species was detected during MB-discoloration. Also by XPS the surface atomic percentage concentration was determined for the topmost Cu-PS layers. The Cu-PS coatings were also investigated for their optical and crystallographic properties.

Exciton dynamics and non-linearities in two-dimensional hybrid organic perovskites

Due to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites has emerged for few years. When they are arranged in two dimensions, these materials can be considered as hybrid quantum wells. One consequence of the unique structure of 2D hybrid organic perovskites is a huge exciton binding energy that can be tailored through chemical engineering. We present experimental investigations of the exciton non-linearities by means of femtosecond pump-probe spectroscopy. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ∼100 ps. Moreover, an ultrafast intraband relaxation (<150 fs) is also reported. Finally, the transient modification of the excitonic line is analyzed through the moment analysis and described in terms of reduction of the oscillator strength and linewidth broadening. We show that excitonic non-linearities in 2D hybrid organic perovskites share some behaviours of inorganic semiconductors despite their high exciton binding energy.

Exciton Dynamics in Anthracene Nanoaggregates

Anthracene nanoaggregates (NA), synthesized using a well-known reprecipitation method, have shapes of nanodisks with the average diameter and height of about 260 and 50 nm, respectively. Following photoexcitation of the anthracene nanoaggregate, we have observed population of two different types of singlet excitonic states, namely, free exciton and self-trapped exciton or excimeric state. Our investigation reveals that the presence of defect conditions in the molecular aggregates reduces the free exciton lifetime and slows the exciton diffusion process marginally. Both the exciton diffusion coefficient and exciton diffusion length in the nanoaggregate and in crystals are comparable. Comparable values of the diffusion lengths in the case of the singlet exciton in the nanoaggregate and crystals suggest that the nanoaggreagates may also be considered as materials for efficient optoelectronic and photovoltaic devices. However, drastic reduction in triplet exciton diffusion coefficient in the nanoaggregate as ...