Low-temperature Lifetime Research Articles

Fundamental Study on the Development of Pure Magnesium Parts by Additive Manufacturing: An Experimental and Computational Analysis

The low density and high biocompatibility of Mg-based materials make them suitable for lightweight structural and biomedical applications. In this study, we explored the use of selective laser melting (SLM)–an additive manufacturing process wherein metal powders are consolidated in a layer-by-layer manner, allowing the fabrication of complex components. SLM typically involves complex physicochemical phenomena and results in laser-processing defects, which makes it difficult to predict the densification mechanisms of the melt pool. Therefore, a full-scale model was developed to investigate the thermal behavior of the melt pool (e.g., temperature gradient distribution, melt pool dimensions, and cooling rate) and the resultant densification activity under various laser energy density (η) values. In parallel, experimental investigations of the densification behavior and microstructural evolution were undertaken with the same SLM processing parameters. The challenges associated with the SLM processability of Mg were comprehensively addressed. Both the peak temperature gradients within the molten pool and molten pool dimensions increased with increasing η, and an opposite trend was observed for the cooling rate. A low η (i.e., high scanning speed) results in a low operating temperature and short liquid lifetime, which in turn lead to poor wettability and many pore-chain and balling defects. However, high η values generated melt pool instability, which resulted in extensive evaporation, cracks, and porosity. The SLM-processed samples had fine twin-like microstructures as a result of rapid solidification. The experimental and simulation results agreed well, validating the thermal behavior of the molten pool and underlying physical mechanism.Graphic Abstract

(INVITED) Reduced low temperature lifetimes of Bi3+ luminescence in magnetically ordered host lattices

This paper is specifically concerned with the radiative lifetime of the spin and orbitally forbidden Bi3+3P0 →1S0 emission transition. In diamagnetic host lattices, the lifetime of this strongly forbidden transition is long (∼1 ms). Experimental data in the literature show a drastic lifetime shortening in lattices containing Gd3+ compared to the diamagnetic lattices. Here, we propose that the lifetime shortenings have their origin in an internal magnetic field, the so-called molecular field, which is created at the Bi3+ site by the magnetically ordered Gd3+ sublattice. This molecular field mixes the 3P0 and 3P1 states to break the selection rule forbidding the 3P0 →1S0 emission transition.

Ingenious golden section search MPPT algorithm for PEM fuel cell power system

Among several types of fuel cells available in the market, proton exchange membrane fuel cell (PEMFC) is characterized by low operating temperature, high efficiency and long lifetime. These advantages have prompted the PEM fuel cell to enter into several applications such as vehicle power sources, portable power and backup power applications. However, the PEM fuel cell faces several challenges due to the dependency of the output power on the operating condition like cell temperature and membrane water content. Under changing operating conditions, there is only one unique operating point for the fuel cell system with maximum output. Therefore, for better operation and optimal exploitation, the extraction of maximum power from PEM fuel cell is indispensable. This paper deals with the development golden section search (GSS)-based maximum power point tracking (MPPT) controller for PEMFC power system. To our knowledge, this paper is a first, if modest, attempt to develop a fuel cell MPPT controller based on golden section algorithm. The proposed GSS-based MPPT has been implemented and validated on fuel cell power system composed of 7 kW PEMFC supplying a resistive load via a DC/DC boost converter controlled using the proposed MPPT. Simulation results obtained using MATLAB/Simulink show that the proposed GSS MPPT outperforms the variable step size incremental conductance one in all test cases including cell temperature and membrane water content variations in terms of static as well as dynamic performances regarding all used metrics reducing by the way the response time by 34.33%, the overshoot and ripple by around 100% and with neglect oscillation around MPP improving as a consequence the fuel cell efficiency.

Lifetime instabilities in gallium doped monocrystalline PERC silicon solar cells

Gallium doped silicon is an industrially viable alternative to boron doped silicon for photovoltaics, and is assumed to be immune from light-induced degradation. We have studied light soaking for >1000 h of industrially fabricated passivated emitter and rear cell (PERC) devices formed from monocrystalline gallium and boron doped substrates, with cell properties monitored using a non-contact photoluminescence imaging proxy method. As-fabricated stabilised boron doped cells did not degrade or underwent a slight improvement, whereas as-fabricated gallium doped cells which had not been intentionally stabilised experienced a slight (~5%) deterioration which then recovered. When PERC devices were subjected to a 200–300 °C dark anneal before light soaking, significant differences in the cell degradation signatures were observed. Degradation characteristic of light and elevated temperature induced degradation (LeTID) was observed for boron and gallium PERC solar cells, with the onset of degradation taking longer, and the severity being less, with gallium. Investigation of stripped gallium PERC devices with room temperature surface passivation revealed bulk lifetime degradation correlates with the cell-level degradation. When the cells were stripped and passivated with aluminium oxide, a complete change in the degradation behaviour was observed, with no degradation occurring in the gallium case and boron-oxygen-like degradation observed for boron. This indicates that dielectric passivation is not suitable for lifetime degradation diagnosis in stripped cells. Gallium retains advantages over boron doping with stabilisation processes not generally required, but manufacturers need to be aware of possible low-temperature lifetime instabilities when developing future fabrication processes, such as for passivated contact structures.

Local excitation and emission dynamics of an isolated single basal-plane stacking-fault in GaN studied by spatio-time-resolved cathodoluminescence

Local excitation and emission dynamics of an isolated “Type-I1” basal-plane stacking-fault (BSF) in very low dislocation density GaN were studied using spatio-time-resolved cathodoluminescence. The low temperature lifetime of the BSF emission was quantified to be 640 ps. The carrier diffusion length was estimated by observing the temporal delay of the BSF peak relative to the free-exciton signal as a function of distance from the BSF. The results indicate that the near-band-edge emission leads to subsequent optical excitation of the BSF that increases the apparent diffusion length. Limiting the observation volume can improve the spatial resolution.

Low temperature and decay lifetime photoluminescence of Eu and Tb nanoparticles embedded into SiO2

In the present work, we have studied the photoluminescence (PL) and decay lifetime of Tb and Eu nanoparticles (NPs) at low temperatures. The NPs were obtained by ion implantation into a SiO2 matrix. Concerning the PL emission of Tb NPs (from 370 to 700nm), the shape does not change with the sample temperature and the PL yield has a maximum at 12K and decreases with increasing temperature, reaching a minimum at 300K. The PL lifetime is wavelength independent and remains almost constant at a value of 1.5ms. Regarding Eu NPs emission, two spectral regions were identified, one with narrow emission bands (from 570 to 750nm) and the other with a broad emission band (from 400 to 550nm). Both PL regions show a minimum yield at 12K, and next it rises with increasing temperatures, reaching the maximum at around 100K. Then, the PL yields start to decrease, reaching at 300K a value similar to the one obtained at 12K. For the Eu NPs PL lifetime, two different results were obtained. The long wavelength spectral region shows a lifetime of the order of 1.0ms independent of the temperature. Conversely, the short wavelength one is strongly temperature dependent, being of the order of several milliseconds for temperatures lower than 100K down to 0.05ms at 300K.

Metastability for the Blume-Capel model with distribution of magnetic anisotropy using different dynamics

We investigate the relaxation time of magnetization or the lifetime of the metastable state for a spin S=1 square-lattice ferromagnetic Blume-Capel model with distribution of magnetic anisotropy (with small variances), using two different dynamics such as Glauber and phonon-assisted dynamics. At each lattice site, the Blume-Capel model allows three spin projections (+1, 0, -1) and a site-dependent magnetic anisotropy parameter. For each dynamic, we examine the low-temperature lifetime in two dynamic regions with different sizes of the critical droplet and at the boundary between the regions, within the single-droplet regime. We compute the average lifetime of the metastable state for a fixed lattice size, using both kinetic Monte Carlo simulations and the absorbing Markov chains method in the zero-temperature limit. We find that for both dynamics the lifetime obeys a modified Arrhenius-like law, where the energy barrier of the metastable state depends on the temperature and standard deviation of the distribution of magnetic anisotropy for a given field and magnetic anisotropy and that an explicit form of this dependence differs in different dynamic regions for different dynamics. Interestingly, the phonon-assisted dynamic prevents transitions between degenerate states, which results in a large increase in the energy barrier at the region boundary compared to that for the Glauber dynamic. However, the introduction of a small distribution of magnetic anisotropy allows the spin system to relax via lower-energy pathways such that the energy barrier greatly decreases. In addition, for the phonon-assisted dynamic, even the prefactor of the lifetime is substantially reduced for a broad distribution of magnetic anisotropy in both regions considered, in contrast to the Glauber dynamic. Our findings show that overall the phonon-assisted dynamic is more significantly affected by the distribution of magnetic anisotropy than the Glauber dynamic.

Intense Blue-Green Cooperative Luminescence from Yb3+ Pairs within CaNb2O6 Matrix

An intense and rather broad emission band around 500 nm is observed in Yb3+ doped CaNb2O6 phosphor with near-infrared (NIR) laser excitation. On the basis of measurements of emission spectra, excitation intensity dependence and decay times, we conclude that the visible emission is ascribed to cooperative luminescence, i.e., the emission of one photon due to simultaneous radiative relaxation of a pair of interacting excited Yb3+ ions. The concentration dependence of the Yb3+ cooperative luminescence has been studied. Competition with a nonradiative energy transfer by upconversion process is observed due to the presence of Er3+ and Tm3+ ions as unexpected impurities. Low temperature spectroscopy and lifetime measurements evidence the existence of Yb3+ pairs formed at two types of minor Yb3+ centers which have been identified within CaNb2O6 matrix. The experimental results indicate that the cooperative luminescence mainly directly originates from the minority Yb3+ ion-pair centers while energy transfer from the main center to the minor centers of Yb3+ exists. In addition, cooperative luminescence exhibits remarkable thermal quenching at high temperature.

Temperature dependentE2Raman modes in theZnCoOternary alloy

The anharmonic properties of low and high frequency ${\mathrm{E}}_{2}$ modes of $\mathrm{ZnO}$ and Co doped $\mathrm{ZnO}$ were investigated using Raman scattering spectroscopy. We have determined the behavior of frequency, linewidths, and lifetime of ${\mathrm{E}}_{2}$ modes in the temperature range from 80 to $800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the case of ${\mathrm{E}}_{2}^{\mathrm{high}}$ mode the frequency shift towards the lower energy side was analyzed in light of the theory of anharmonic phonon-phonon interaction and thermal expansion of the lattice, and the linewidth behavior was analyzed in terms of anharmonic effect of three-phonon decay mechanism. But in the case of ${\mathrm{E}}_{2}^{\mathrm{low}}$, the linewidth and frequency behaved practically harmonic with respect to temperature and independent of Co substitutions. It is found that the ${\mathrm{E}}_{2}^{\mathrm{high}}$ phonon anharmonicity is higher for $\mathrm{ZnCoO}$ alloys than in pure $\mathrm{ZnO}$ and it increases with the compositional disorder. The low temperature lifetime of ${\mathrm{E}}_{2}$ phonon in $\mathrm{ZnO}$, 1% and 3% Co doped $\mathrm{ZnO}$ were found to be 1.82, 1.74, and $1.54\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, respectively.

Efficient Low-Temperature Data Retention Lifetime Prediction for Split-Gate Flash Memories Using a Voltage Acceleration Methodology

In developing a fast statistical testing methodology to predict the postcycling low-temperature data-retention lifetime of split-gate Flash memories, word-line stress is used to accelerate the charge-gain effect responsible for bit-cell-current reduction among the tail-bits. To find out the voltage dependence on data-retention lifetime, various word-line stress voltages are performed to enhance the charge-gain effect of the erase-state cells. At an accelerated state, word-line stress lifetime tests can be completed within a much shorter test period and still provide accurate lifetime prediction for embedded Flash-memory products

Vibrational lifetimes of light impurities in silicon

Light impurities in covalent materials, such as H or O in Si, are characterized by high-frequency local vibrational modes (LVMs) far above the phonon frequencies of the host crystal. Upon excitation, such LVMs are expected to have long lifetimes since their decay involve multi-phonon processes. Yet, time-resolved infrared absorption spectroscopy reveals that the vibrational lifetimes of almost identical LVMs sometimes differ by up to two orders of magnitude. Indeed, the low-temperature lifetimes of the 2062cm−1 mode of the H2∗ complex, the 1998cm−1 mode of bond-centered hydrogen HBC+ and the 2072cm−1 mode of the di-vacancy di-hydrogen complex VH·HV are 4, 8 and 295ps, respectively. More surprising still, the measured lifetime of the asymmetric stretch of interstitial O in Si changes by almost an order of magnitude with the isotope of O or one of its Si neighbors. In this paper, ab-initio molecular dynamic simulations in periodic supercells are used to calculate vibrational lifetimes. The calculations predict the correct lifetimes of the various defects in the range 50<T<200K and provide critical insight into the decay processes.

Photoluminescence of Di-Ureasil Hybrids Doped with Silica Coated Ag Nanoparticles

Photoluminescence of di-ureasil hybrids containing homogeneously dispersed Ag nanoparticles, covered with silica shells was investigated at room temperature and compared with emission of undoped hybrids. Metal-doped di-ureasils show a broad emission band in the blue spectral region similar to what has been observed in pure di-ureasils but with lower relative intensity. A broad minimum arises around 438 nm for Ag@SiO2 - containing di-ureasils which coincides with the peak of plasmon resonance band in the absorption spectrum. This behaviour can be attributed to inner filtering effect, which manifests itself as absorption by metal nanoparticles of light emitted from the matrix. Low-temperature time-resolved spectroscopy and lifetime measurements demonstrate that changes are induced in the time-scale behind the di-ureasil host emission when the nanoparticles are incorporated. In particular, the lifetime values for Ag@SiO2- doped di-ureasils are smaller than those measured for the undoped host, suggesting that energy transfer might exist between the hybrid's emitting centres (NH groups and siliceous nanodomains) and the nanoparticles.

Luminescence from the trans-Dioxotechnetium(V) Chromophore

The luminescence of trans-[TcO2(L)4]+ (L = pyridine (py) or picoline (pic)) and trans-[TcO2(CN)4]3- at room and low temperature is described and represents the first example of room temperature excited-state luminescence observed for Tc complexes. At room temperature, the complexes exhibited broad luminescence with emission maxima ranging from 745 to 780 nm. Analogous to the Re complexes (emission at 635-655 nm), the low-temperature emission spectra of microcrystalline samples of [TcO2(py)4]BPh4 and [TcO2(pic)4]BPh4 display the characteristic progressions of the symmetric O=Tc=O and Tc-L stretching modes. DFT/TDDFT calculations were performed on the trans-[MO2(L)4]+ (M = Re, Tc) congeners and predicted the dioxotechnetium emission to be 0.41 eV lower in energy than its Re analogue. Low-temperature lifetimes (8 K) ranging from 15 to 1926 mus for the series of Tc complexes are consistent with the Re analogues.

Positron annihilation spectroscopic studies on Nd-doped ceria

We report a new result on the characterization of Nd-doped ceria, Ce 1− x Nd x O 2− x /2 ( x = 0.075 – 0.675 ) using positron lifetime spectroscopy (LTS) and coincidence Doppler broadening (CDB) measurements. A systematic increase in lifetime that is attributed to formation of Nd-oxygen vacancy associates is seen from x = 0.075 –0.4 followed by a sharper increase up to x = 0.5 . The change in profile of lifetime around x = 0.4 suggests drastic increase in the concentration of these associates. Discontinuity in lifetime around x = 0.5 is ascribed to ordering of oxygen vacancies. Coincidence Doppler broadening measurements indicate reduction in the overlap of positron wavefunction with oxygen core electrons due to trapping of positrons. Low-temperature (50–300 K) lifetime measurements indicate the presence of Rydberg-like positron states associated with Nd 3+ sites.

Time-resolved photoluminescence in $alpha;-PTCDA single crystals: evidence for recombination via Frenkel excitons, charge transfer states, and excimers

The radiative recombination channels in α -PTCDA are analysed with time-resolved PL techniques in the 50 ns range between temperatures of T =10 and T =300 K. The resulting PL spectra are interpreted with calculations based on the transfer of Frenkel excitons and with time-dependent density functional theory applied to different deformed geometries of molecular dimers as microscopic models for self-trapped excitons. At low temperature, the lineshape and radiative lifetime of the most important PL channel can be assigned quantitatively to a vertical transition from the indirect minimum of the Frenkel exciton dispersion towards the electronic ground state. In an intermediate regime below about T =100 K, charge transfer states involving an anionic and a cationic molecule dominate the PL spectra. Radiative recombination from an excimer state in a stack geometry has a rather weak temperature dependence, and due to non-radiative quenching of the other PL channels, this excimer PL band predominates above about T =200 K.

Time-resolved photoluminescence in $alpha;-PTCDA single crystals: evidence for recombination via Frenkel excitons, charge transfer states, and excimers

The radiative recombination channels in α -PTCDA are analysed with time-resolved PL techniques in the 50 ns range between temperatures of T =10 and T =300 K. The resulting PL spectra are interpreted with calculations based on the transfer of Frenkel excitons and with time-dependent density functional theory applied to different deformed geometries of molecular dimers as microscopic models for self-trapped excitons. At low temperature, the lineshape and radiative lifetime of the most important PL channel can be assigned quantitatively to a vertical transition from the indirect minimum of the Frenkel exciton dispersion towards the electronic ground state. In an intermediate regime below about T =100 K, charge transfer states involving an anionic and a cationic molecule dominate the PL spectra. Radiative recombination from an excimer state in a stack geometry has a rather weak temperature dependence, and due to non-radiative quenching of the other PL channels, this excimer PL band predominates above about T =200 K.

Binding energy and dynamics of Be acceptor levels in AlAs/GaAs multiple quantum wells

We report an infrared study of the effect of quantum well confinement on the binding energy and dynamics of shallow Be acceptors in both bulk GaAs and a series of delta-doped AlAs/GaAs multiquantum well samples with well thicknesses of 20,15 and 10 nm. Low temperature far-infrared absorption measurements clearly show the three principal absorption lines due to the 1s–2p transitions of the acceptors for all four samples. The dynamics of these transitions were studied by the pump-probe technique using a free electron laser as a source of far-infrared radiation. It was found that the 2p acceptor state has a low-temperature lifetime of 360 ps in the bulk, which reduces with increasing confinement, falling to 55 ps in the case of the 10 nm well. We suggest that the increase of phonon emission rate with decreasing well width is due to the breaking of translational symmetry in the quantum well and a subsequent increase in the single-phonon emission probability

Solid-state structure dependence of the molecular distortion and spectroscopic properties of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion.

The relation between the geometry and spectroscopic properties of a series of salts of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion, (Cu((I))(dmp)(2))(+), is explored. The distortions from the idealized D(2)(d)() geometry, which include flattening, rocking of the dmp ligands, and displacement of the Cu atoms out of the dmp planes, show considerable variation, indicating the importance of packing forces in the crystalline environment. The change in the absorption spectra upon flattening of the complex, expressed as the variation of the angle between the dmp planes, which varies from 88 degrees in the BF(4) and tosylate salts to 73 degrees in the picrate, agrees qualitatively with parallel DFT calculations. No correlation is found between ground state geometry and luminescence lifetimes, recorded both at room temperature and at 16 K. The low temperature lifetimes vary by a factor of 8 among the (Cu((I))(dmp)(2))(+) salts examined, the longest lifetime (2.4 micros at 16 K) being observed for the tosylate salt.

Anomalous LO phonon lifetime in AlAs.

The temperature dependence of the frequencies and linewidths of the Raman-active longitudinal optical (LO) phonons in GaAs and AlAs have been measured. The low-temperature lifetime of the LO phonon in AlAs is found to be 9.7 ps, very close to the corresponding GaAs value of 9.5 ps. This contradicts early theoretical predictions. The agreement between theory and experiment can be restored when the accidental degeneracy between the AlAs LO phonon frequency and a feature in the two-phonon density of states is taken into account.

Triplet state studies of β-carbolines

Low temperature phosphorescence spectra, quantum yields and lifetimes, and room temperature triplet–triplet absorption spectra and lifetimes have been obtained for the neutral and cationic forms of the three β-carbolines: harmine (Hi), harmane (Ha) and norharmane (NorH). Based on the phosphorescence spectra, the energies of the lowest triplet states are 25 250 (Hi), 24 750 (Ha) and 24 400 cm−1 (NorH) for the neutral forms and 22 900 (Hi), 21 750 (Ha) and 21 850 cm−1 (NorH) for the corresponding cations. Phosphorescence quantum yields are in the range 0.4 and 0.6 for the neutral forms, and are found to decrease on protonation, while phosphorescence lifetimes vary from 3.15 s for the neutral form of Hi to 6.89 s for the cationic form of NorH. In all the cases, the lowest excited triplet state is suggested to have 3(π, π∗) character. For several of the systems, the sum of emission yields is less than 1, indicating that internal conversion may also be an important deactivation route for the lowest excited singlet state. Microsecond flash photolysis studies show that the triplet states of all three carbolines undergo acid–base equilibria in water with pKa values in the region 4.4–5.0 at room temperature. In water and ethanolic solutions, the cationic forms show two absorption maxima at ca. 450 and 580 nm, while the neutral forms have only one absorption maximum in the visible spectral region at ca. 510 nm. In nitrogen purged solutions, triplet state lifetimes are 530 (Hi), 380 (Ha) and 600 μs (NorH), and the triplet states are quenched by β-carotene with rate constants k(av)=3.3(±0.9)×109M−1s−1 close to but slightly lower than those expected for a diffusion controlled process.