Thermal Excitation Of Electrons Research Articles

Ab initio quasi-harmonic thermoelasticity of molybdenum at high temperature and pressure.

We present the abinitio thermoelastic properties of body-centered cubic molybdenum under extreme conditions obtained within the quasi-harmonic approximation including both the vibrational and electronic thermal excitation contributions to the free energy. The quasi-harmonic temperature-dependent elastic constants are calculated and compared with existing experiments and with the quasi-static approximation. We find that the quasi-harmonic approximation allows for a much better interpretation of the experimental data, confirming the trend found previously in other metals. Using the Voigt-Reuss-Hill average, we predict the compressional and shear sound velocities of polycrystalline molybdenum as a function of pressure for several temperatures, which might be accessible in experiments.

MesoHOPS: Size-invariant scaling calculations of multi-excitation open quantum systems.

The photoexcitation dynamics of molecular materials on the 10-100nm length scale depend on complex interactions between electronic and vibrational degrees of freedom, rendering exact calculations difficult or intractable. The adaptive Hierarchy of Pure States (adHOPS) is a formally exact method that leverages the locality imposed by interactions between thermal environments and electronic excitations to achieve size-invariant scaling calculations for single-excitation processes in systems described by a Frenkel-Holstein Hamiltonian. Here, we extend adHOPS to account for arbitrary couplings between thermal environments and vertical excitation energies, enabling formally exact, size-invariant calculations that involve multiple excitations or states with shared thermal environments. In addition, we introduce a low-temperature correction and an effective integration of the noise to reduce the computational expense of including ultrafast vibrational relaxation in Hierarchy of Pure States (HOPS) simulations. We present these advances in the latest version of the open-source MesoHOPS library and use MesoHOPS to characterize charge separation at a one-dimensional organic heterojunction when both the electron and hole are mobile.

Thermodynamic properties of rhodium—A first principle study

The high-pressure and high-temperature thermodynamic properties of rhodium (up to 2000 K and 300 GPa) are presented using the first principle approach within the quasi-harmonic approximation. The thermal Helmholtz free energy includes the contribution of both phonon vibrations and electronic excitations. The performance of three popular exchange-correlation functionals—local density approximation [Perdew et al., Phys. Rev. B 23, 5048 (1981)], Perdew–Burke–Ernzerhof generalized gradient approximation (PBE) [Perdew et al., Phys. Rev. Lett. 77, 3865 (1996)], PBE modified for dense solids [Perdew et al., Phys. Rev. Lett. 100, 136406 (2008)] are shown. The simulated thermal expansion coefficient, isobaric heat capacity, mode-Grüneisen parameter, thermodynamic average Grüneisen parameter, and bulk modulus are compared with the available experimental and theoretical reports. The contribution of thermal electronic excitations to the obtained thermodynamic parameters is significant at low pressure and high temperatures, except in bulk modulus, where it is small. The pressure-dependent elastic constant coefficient (Cij) and the Debye temperature are computed at 0 K. The Pugh ratio calculated from Cij indicates that rhodium undergoes brittle to ductile transitions at an average pressure of 7.45 GPa.

Extension of Mott Formula in the Linearized Boltzmann Transport Equation to the Study of Thermoelectric Power of Electron in Metal

The focus of this paper is to compute and study thermoelectric power of electron in metals using Mott formula based on free electron theory to get an insight on the effect of strain on electron thermoelectric power in metals. Electron density parameter of strained metals and Poisson ratio of metals is obtained and used in this work unlike some researchers that assumed a value for Poisson ratio of all metals. This assumption make the electron density parameter of strained and unstrained metals to be equal and then reduce the computation to only one input parameter. The experimental and simulated value are in good agreement. This agreement is due to the usefulness of free electron theory in theoretical forecast of some properties of metals. The Poisson ratio is involved in simulation. Result obtained revealed that thermoelectric power of electron in metals goes up as the electron density parameter rises due to high-order contributions from electron scattering theory. Electron thermoelectric power in metals increases as the temperature increases due to electron thermal excitation from their mean position. The electron thermoelectric power rises as strain rises for metals.

High-temperature efficient luminescence of dilute-nitride InGaAsN quantum dots with deep electron potential

The temperature dependence of the optical properties of In0.4Ga0.6As0.98N0.02 quantum dots (QDs) was investigated using continuous-wave and time-resolved photoluminescence (PL). Significant increases in the PL peak energy and the PL linewidth were observed at temperatures above 200 K, which reflected the high luminescence efficiency of ground and excited states at high temperatures. The PL decay times of the ground state were almost constant between 200 and 300 K at 200–220 ps, which were significantly longer than that of 38 ps for the In0.4Ga0.6As QDs at 300 K. The temperature independence of the PL decay time represents significant suppression of the thermal escape and the thermal excitation of electrons because the electron ground-state localization energy is much larger than the thermal energy. The PL intensity of the In0.4Ga0.6As0.98N0.02 QDs was seven times stronger than that of the In0.4Ga0.6As QDs at 300 K, and this tendency was maintained up to 400 K with a PL intensity one order of magnitude stronger. These findings demonstrate that lowering the QD conduction band by nitrogen incorporation is an effective approach for achieving strong QD luminescence above room temperature.

Development of a flexible quasi-harmonic-based approach for fast generation of self-consistent thermodynamic properties used in computational thermochemistry

We present in this work a new formalism for the rapid and accurate evaluation of the thermodynamic properties of compounds and solid solution end-members which are required in the high-throughput computational thermochemistry discovery of new materials. The method is based on the Debye approximation and is fully thermodynamically self-consistent. It considers several energetic contributions, such as the vibrational free energy (including both implicit and explicit anharmonicity), the defect-free energy (via the introduction of thermal vacancies), the energetic effect of thermal excitation of electrons (for metallic solids) as well as other excess terms to precisely reproduce experimental data. Sets of fully self-consistent pressure- and temperature-dependent thermodynamic properties are evaluated from the exact differentiation of this complete free energy formalism. This strategy allows the formalism to be fully integrated in the CALPHAD framework for the development of large thermodynamic databases of both stable and metastable compounds which may form in multicomponent/high-order systems. An emphasis is put in this work on the analysis and precise description of anharmonic vibrational contributions. Finally, sets of self-consistent thermodynamic and thermo-physical properties have been predicted as a function of temperature and compared with both experiments and classical QHA for pure elements (i.e. Ag, Li, V, Al, and C) and stoichiometric compounds of different chemical nature (i.e. MgO, CaO, Al2O3, NaCl, LiFePO4 and InP). The impact of various input parameters of the formalism on the free energy and the derived thermodynamic properties for these solids is analyzed. A better predictive capability and the enhanced flexibility of the proposed formalism are demonstrated.

CEEM: a Chemically Explainable Deep Learning Platform for Identifying Compounds with Low Effective Mass.

The semiconductor industry occupies a crucial position in the fields of integrated circuits, energy, and communication systems. Effective mass (mE ), which is closely related to electron transition, thermal excitation, and carrier mobility, is a key performance indicator of semiconductor. However, the highly neglected mE is onerous to measure experimentally, which seriously hinders the evaluation of semiconductor properties and the understanding of the carrier migration mechanisms. Here, a chemically explainable effective mass predictive platform (CEEM) is constructed by deep learning, to identify n-type and p-type semiconductors with low mE . Based on the graph network, a versatile explainable network is innovatively designed that enables CEEM to efficiently predict the mE of any structure, with the area under the curve of 0.904 for n-type semiconductors and 0.896 for p-type semiconductors, and derive the most relevant chemical factors. Using CEEM, the currently largest mE database is built that contains 126335 entries and screens out 466 semiconductors with low mE for transparent conductive materials, photovoltaic materials, and water-splitting materials. Moreover, a user-friendly and interactive CEEM web is provided that supports query, prediction, and explanation of mE . CEEM's high efficiency, accuracy, flexibility, and explainability open up new avenues for the discovery and design of high-performance semiconductors.

Pgm: A Python package for free energy calculations within the phonon gas model

The quasi-harmonic approximation (QHA) is a powerful method that uses the volume dependence of non-interacting phonons to compute the free energy of materials at high pressures (P) and temperatures (T). However, anharmonicity, electronic excitations in metals, or both, introduce an intrinsic T-dependence on phonon frequencies, rendering the QHA inadequate. Here we present a Python code, pgm, to compute the free energy and thermodynamic property within the phonon gas model (PGM) that uses T-dependent phonon quasiparticle frequencies. In this case, the vibrational contribution to the Helmholtz free energy is obtained by integrating the vibrational entropy, which can be readily calculated for a system of phonon quasiparticles. Other thermodynamic properties are then obtained from standard thermodynamic relations. We demonstrate the successful applications of pgm to two cases of geophysical significance: cubic CaSiO3-perovskite (cCaPv), a strongly anharmonic insulator and the third most abundant phase of the Earth's lower mantle, and NiAs-type (B8) FeO, a partially covalent-metallic system. This is the oxide endmember of a recently discovered iron-rich FenO alloy phase likely to exist in the Earth's inner core. Program summaryProgram Title:pgmCPC Library link to program files:https://doi.org/10.17632/8rfw6syvzp.1Developer's repository link:https://github.com/MineralsCloud/pgmLicensing provisions: GNU General Public License 3Programming language: Python3Nature of problem: The classic quasi-harmonic approximation (QHA) method to compute the vibrational free energy does not apply to physical systems when phonon frequencies have an intrinsic and non-negligible temperature (T) dependence. Examples are anharmonic systems or metals with abundant electronic thermal excitations. Both cases introduce an intrinsic T-dependence on phonon frequencies.Solution method: The method implemented in pgm is based on the phonon gas model where the entropy is well defined for T-dependent phonon quasiparticle. The free energy is calculated by integrating the entropy, making it suitable for anharmonic systems or systems with extensive thermal electronic excitations affecting phonon frequencies. The static free energy, the vibrational density of states (VDoS), and the entropy are first computed on sparse T- and P-grids. The entropy is then suitably interpolated on denser user-specified grids for integration.Additional comments, including restrictions and unusual features: The package allows it to be run directly in the command line. It can also be incorporated into other programs. We implemented Just-in-time (JIT) compiling and parallel computing techniques [1] in pgm Python code to speed up the numerical calculation.

PBE-GGA predicts the B8↔B2 phase boundary of FeO at Earth’s core conditions

FeO is a crucial component of the Earth's core, and its thermodynamic properties are essential to developing more accurate core models. It is also a notorious correlated insulator in the NaCl-type (B1) phase at ambient conditions. It undergoes two polymorphic transitions at 300 K before it becomes metallic in the NiAs-type (B8) structure at ~100 GPa. Although its phase diagram is not fully mapped, it is well established that the B8 phase transforms to the CsCl-type (B2) phase at core pressures and temperatures. Here, we report a successful ab initio calculation of the B8↔B2 phase boundary in FeO at Earth's core pressures. We show that fully anharmonic free energies computed with the Perdew-Burke-Ernzerhof-generalized gradient approximation coupled with thermal electronic excitations reproduce the experimental phase boundary within uncertainties at P > 255 GPa, including the largely negative Clapeyron slope of -52 MPa/K. This study validates the applicability of a standard density functional theory functional to FeO under Earth's core conditions and demonstrates the theoretical framework that enables complex predictive studies of this region.

DFT calculations of structural, magnetic, and stability of FeNiCo-based and FeNiCr-based quaternary alloys

As the Cantor-derived medium-entropy alloys (MEAs), FeNiCoMn and FeNiCrMn quaternaries in both equiatomic and non-equiatomic compositions were investigated by density functional theory combined with the quasiharmonic Debye–Grüneisen approximation using the special-quasirandom structure model. The structural properties, magnetic properties, and thermodynamics and phase stability were explored in detail. The temperature stabilization effect of lattice vibration, configurational mixing entropy, and thermal electronic excitation was discussed. Also FeNiCoPd and FeNiCrPd quaternaries, in which Mn was replaced by Pd, were considered in the same framework in order to highlight the similarities and differences between these Mn- and Pd-MEAs. The phase stability competition between homogeneous and inhomogeneous states in terms of both size and chemical ordering was revealed for four groups of FeNiCoMn, FeNiCoPd, FeNiCrMn, and FeNiCrPd MEAs.

Surface Plasmon-Induced Hot Carriers: Generation, Detection, and Applications.

ConspectusDuring surface plasmon-mediated light-matter interactions, external energies on plasmonic metal nanostructures undergo energy dissipation via elastic e-e scattering, radiative luminescence, and nonradiative processes such as thermal relaxation (phonon) and electronic excitation (electron-hole pairs). In this process, surface plasmon decays dominantly through nonradiative recombination when the metal is smaller than 25 nm, forming hot carriers, including hot electrons and hot holes, with high kinetic energy of 1-3 eV. Although the ultrafast dynamics of hot carriers are on time scales ranging from femtoseconds to picoseconds, these fast-disappearing hot carriers can be collected as the steady-state photocurrent or chemicurrent by adopting the metal-semiconductor (M-S)-based platform for detecting hot carrier flow. Plasmonic hot carriers, especially as they convert to an electrochemical signal, are a promising topic, and their energy conversion mechanisms are being actively studied in the fields of renewable energy, optoelectronics, and photocatalysis. Recent studies have demonstrated that these hot carriers can both improve the performance of solar energy conversion and control the catalytic activity or selectivity by directly participating in the photoelectrochemical (PEC) reaction.In this Account, we describe the inherent relationship between hot carriers and surface plasmon as well as what role hot carriers play throughout the catalytic reaction. The recent experimental work and the theoretical analysis of in situ hot carrier generation on Au nanostructures were conducted with photoconductive atomic force microscopy and finite-difference time-domain (FDTD) simulations, respectively. We highlight the recent nanoscale visualization of hot carrier flow occurring through light-matter interactions and that the localized surface plasmon field surrounding the Au nanostructure leads to boosted hot carrier generation. In addition, we highlight the recent demonstration that plasmonic hot carriers prolong the lifetime of photoexcited carriers in the MAPbI3/Au/TiO2 hybrid nanodiode by the synergistic effect between plasmonic Au and perovskites. From this work, the solar-to-electron conversion performance of this nanodiode significantly increases due to the amplification of light absorption, which helps to design hybrid platforms for efficient hot carrier photovoltaics. We discuss the application of surface plasmon-driven hot electron generation, including hot electron-based photovoltaic devices and photocatalysts. We highlight the recent photoelectrochemical measurements on the Au/p-GaN heterostructures that are controlled by participating plasmonic hot carriers in the water splitting reaction. Furthermore, controlling the flow of both hot electrons and holes by developing hybrid platform configurations for hot carrier applications has promising opportunities for regulating the catalytic activities of hot carrier-based photocatalysis and improving the photoconversion efficiency of hot carrier-based optoelectronic devices.

Polaron induced ferromagnetism in ZnO

The odd phenomenon of the ferromagnetic moment of n-type ZnO increasing with temperature has been observed in experiments. We proposed an impurity model accompanying the polaron theory to explain this result because the electron-phonon interaction plays a critical role in inducing ferromagnetism in n-type ZnOs. Thus, we propose that the ferromagnetism of ZnOs comes from the polarons in the impurity states, and the strong Coulomb interaction in these states causes the spin of electrons to split. In addition, the proper hybridization interaction between the conduction band and the impurity states promotes the increase of electron density in the impurity states as well as the ferromagnetism. In contrast, a too strong hybridization interaction will delocalize the electrons in the impurity states to suppress the ferromagnetism. The polarons induce the ferromagnetism by the Coulomb excitation in the impurity states. The electron thermal excitation produces the polarons in the impurity states near the ZnO conduction band edge coupling with the phonon. Based on the model, the ferromagnetism increases with the temperature consistent with the experimental results.

Comparison of Temperature and Light Distributions Around Nano-Resistors in SSI-LEDs

A recent optoelectronic device termed the Solid-State Incandescent LED (SSI-LED) was discovered by Kuo’s group in 2013 [1]. The device is made by subjecting a MOS Capacitor of transparent gate material and appropriate high-k properties to dielectric breakdown, which results in the formation of numerous nano-scale conduction paths called nano-resistors (NRs). Due to its extremely small cross-sectional area, NRs display high current densities for relatively small voltages applied across the device. Consequently, the high current densities yield high local temperatures that approach black body radiation and allow NRs to undergo incandescence, where white light is emitted from the top surface through the transparent gate electrode by thermal excitation of electrons. Since the heat generated from Joule heating is contained within the NRs, a large temperature drop occurs over the surrounding volume as the heat flux transitions from nanoscale to microscale, keeping the whole device at a much lower temperature. The light emission from the SSI-LED is similar to that of an incandescent lamp where a spectrum of a wide range of wavelengths is produced in contrast to the narrow band light emitted from the conventional pn junction-structured LED. Therefore, SSI-LEDs are suitable for natural light or white light applications [2].It is desirable to have a direct comparison of emitted light intensity, temperature, and current density profiles of not just single NRs, but of NRs with various layouts to quantify the interaction of these variables. Earlier simulations using COMSOL Multiphysics have produced temperature and current density distributions without any accompanying light intensity study [3]. Using the python simulation framework developed earlier, these variables were studied in COMSOL Multiphysics for 3 NRs arranged as a right-angled triangle, 4 NRs as a square, and 5 NRs in the shape of a '+' sign. The effect of mesh quality on the temperature and current density distributions was also quantified. Since the current density profile is highly dependent on the relative positions of the NRs, the mesh quality has a noticeable effect on the temperature surrounding the NRs. In accordance with theoretical calculations, there is a direct relationship between the current density as well as the temperature and light intensity. The resultant light intensity profiles qualitatively match experimental observations and offers a degree of validation at near screen distances. Near the circumference at the surfaces of the NRs, the current density is high [4].Figure 1(a) shows the arrangement of NRs which are all of 10 nm radius with NR centers separated from adjacent NR centers by 40 nm. Fig. 1(b) shows the light intensity (in log10 scale) of the same patterns at a distance of 1 nm above the gate electrode surface, where the light is emitted from the blackbody radiation with corresponding the CCT to the NR at the top surface. Fig. 1(c) is the temperature of the gate electrode surface for the same set of NR patterns. Blue regions have higher temperature and white regions are close to room temperature. The temperature on the gate electrode surface blends to form a distribution that resembles individual NRs, when in fact, the observed temperature is for the pattern as a whole. The possible reason for this is that temperature is distributed in terms of sinusoidal waves according to the solution of the heat equation and is largely diffused when measured at the gate electrode surface, while the light intensity distribution obtained from the Stefan-Boltzmann law and inverse square law, varies with the 4th power of temperature close to the gate and shows a significantly steeper curve with individual NRs still distinct.More detailed comparisons of the temperature and light distributions will be shown in the presentation.1. Y. Kuo and C.-C. Lin, Appl. Phys. Lett., 102, 031117 (2013).2. Y. Kuo, 2014 IEEE International Electron Devices Meeting, 4.7.1-4.7.4 (2014).3. A. Shukla and Y. Kuo, ECS J. Solid State Sci. Technol., 9, 065017 (2020).4. K. Natarajan and Y. Kuo (To be published). Figure 1

The Role of Local DFT+U Minima in the First-Principles Modeling of the Metal-Insulator Transition in Vanadium Dioxide.

The DFT+U method is frequently employed to improve the first-principles description of strongly correlated materials. However, it is prone to deliver metastable electronic minima. While these local minima of the DFT+U method are often considered to be computational artifacts, their physical meaning and relationship to true excited states remains unclear. In this work, the possibility of theoretically modeling transformations in the solid state that require thermal or optical excitations of electrons is explored, taking into account the metastable states of the computationally undemanding DFT+U formalism. For this purpose, we choose to examine the example of the VO2 metal-insulator transition. Metastable states that are located on different electronic potential energy surfaces are found to correspond to experimentally observed VO2 phases. The identified metastable electronic states can be used to model the collapse of the VO2 band gap at elevated temperatures and upon photoexcitation as well as other monoclinic-monoclinic phase transformations. The results suggest that local DFT+U minima can indeed carry physical meaning, while they remain under-reported in theoretical literature on transition metal oxides like VO2.

Intermediate spin state and the B1−B2 transition in ferropericlase

Ferropericlase (fp), (Mg$_{1-x}$Fe$_x$)O, the second most abundant mineral in the Earth's lower mantle, is expected to be an essential component of super-Earths' mantles. Here we present an ab initio investigation of the structure and magnetic ground state of fp up to $\sim$ 3 TPa with iron concentrations (x$_{Fe}$) varying from 0.03 to 0.12. Calculations were performed using LDA+U$_{sc}$ and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard U (U$_{sc}$) is required. Similar to the end-members FeO and MgO, fp also undergoes a B1 to B2 phase transition that should be essential for modeling the structure and dynamics of super-Earths' mantles. This structural transition involves a simultaneous change in magnetic state from a low spin (LS) B1 phase with iron total spin S=0 to an intermediate spin (IS) B2 phase with S=1. This is a rare form of pressure/strain-induced magnetism produced by local cation coordination changes. Phonon calculations confirm the dynamical stability of the iron B2-IS state. Free energy calculations are then carried out including vibrational effects and electronic and magnetic entropy contributions. The phase diagram is then obtained for low concentration fp using a quasi-ideal solid solution model. For x$_{Fe}$ > 0.12 this approach is no longer valid. At ultra-high pressures, there is an IS to LS spin state change in Fe in the B2 phase, but the transition pressure depends sensitively on thermal electronic excitations and on x$_{Fe}$.

Electron–phonon coupling factor and electron heat capacity of 6H-SiC

ABSTRACT For high-energy single ion events, the relationship between electronic energy loss mechanism and atomic processes is generally described by the inelastic thermal spike model. However, the parameters required for the model are not accurately known and are often estimated from a free electron gas model. To ensure this model is more reliable and predictive, a more accurate calculation for the parameters should be taken. In this paper, the temperature dependence of the electronic heat capacity and electron–phonon coupling factor (e-ph) has been calculated, using density functional theory (DFT). The calculation results demonstrate that the effect of electron thermal excitation on the thermodynamic parameters is sensitive to the bandgap, and these parameters vary immensely with the electron temperature. Besides, to verify the accuracy of obtained parameters, both the swift heavy ions (SHI) irradiation in crystalline 6H-SiC and amorphous 6H-SiC are simulated, using the inelastic thermal spike model. And the results are more consistent with the experiment conclusion, for the obtained parameters by DFT. Hence, the calculated temperature-dependent parameters are more suitable for the inelastic thermal spike model.

Fluorescence thermometry application of photoluminescence image from Cr-doped YAG.

The temperature dependence of a photoluminescence (PL) image from a Cr-doped Y3Al5O12 (YAG:Cr) crystal was evaluated for a fluorescence thermometry application. The brightness of a red PL image of YAG:Cr was found to be increased by increasing the temperature. Temperature variations of the peak intensity and peak area of red PL related to the PL peaks at λ = 680, 690, 708, and 725nm were evaluated to reveal the cause of the increase in brightness of the PL image. While intensities of the PL peaks at λ = 690, 690, and 708nm decreased with temperature due to thermal quenching, the peak intensity at λ = 725nm and the peak area increased with temperature up to 250°C. The latter factor greatly affects the temperature variation of the brightness of PL images. The increase in the brightness of the PL image with temperature is evidently linked with the thermal excitation of electrons at the 2E level responsible for the increase in the PL peak area. Cr concentration suitable for the temperature sensor is suggested based on the temperature variation of the PL peak area.

Thermoactivated Dislocation Motion in Rolled and Extruded Magnesium: Data of the Low-Temperature Acoustic Experiment

Acoustic properties (logarithmic decrement and dynamic Young’s modulus) of commercial grade magnesium have been measured in the temperature range 51–310 K. Two types of magnesium samples have been studied: polycrystalline magnesium rolled at room temperature and subjected to hot extrusion. It is shown that the amplitude dependences of the acoustic properties are due to the thermally activated breakaway of dislocations from weak pinning centers. Within the framework of the Indenbom-Chernov theory of thermally activated dislocation hysteresis, the binding energy of the interaction between dislocations and defects was estimated. Furthermore, dependences of the activation energy and activation volume on the applied stress were obtained in the microplastic region. The temperature dependences of the dynamic Young’s modulus are obtained in the amplitude independent region in the temperature range of 51–310 K. Functional form of the Young’s modulus temperature dependences corresponds to the classical concepts of the effect of thermal excitation of electrons and phonons on the elastic properties of crystals.

(INVITED) On the mechanism leading to afterglow in Gd2O2S:Pr

The performance of scintillator materials and phosphors for x-ray imaging is adversely affected by afterglow, giving rise to ghost images and thus compromising image quality. Here, the afterglow of the well-known computed tomography (CT) phosphor Gd2O2S:Pr is investigated with time-delayed afterglow spectroscopy. It is shown that these afterglow spectra mainly show Eu3+ emission. This points to a mechanism, during excitation across the bandgap, in which holes are captured by Pr3+ ions, resulting in Pr4+ and electrons are captured by Eu3+ ions, creating Eu2+. Afterglow is due to thermal excitation of electrons from the valence band to Pr4+ ions (detrapping of holes from Pr4+), restoring Pr3+ ions. Capture of the resulting holes in the valence band by Eu2+ ions leads to excited Eu3+ ions that generate Eu3+ emission thus causing the undesired afterglow. Time delayed Eu3+ afterglow spectra are observed already for very low Eu3+ concentrations (<10 ppm). These observations provide insight in the dominant afterglow mechanism in Gd2O2S:Pr3+ phosphors and allow to optimize the performance and improve the CT image quality by selection of starting materials with the lowest possible Eu3+ contamination levels to effectively reduce afterglow without loss of light yield.

The Critical Number of Gold Atoms for a Metallic State Nanocluster: Resolving a Decades-Long Question.

Probing the transition from a metallic state to a molecular state in gold nanoparticles is fundamentally important for understanding the origin of surface plasmon resonance and the nature of the metallic bond. Atomically precise gold nanoclusters are desired for probing such a transition based upon a series of precise sizes with X-ray structures. While the definition of the metallic state in nanoclusters is simple, that is, when the HOMO-LUMO gap (Eg) becomes negligibly small (Eg < kBT, where kB is the Boltzmann constant and T the temperature), the experimental determination of ultrasmall Eg (e.g., of kBT level) is difficult, and the thermal excitation of valence electrons apparently comes into play in ultrasmall Eg nanoclusters. Although a sharp transition from nonmetallic Au246(SR)80 to metallic Au279(SR)84 (SR: thiolate) has been observed, there is still uncertainty about the transition region. Here, we summarize several criteria on determining the metallic state versus the molecular (or nonmetallic) state in gold nanoclusters, including (1) Eg determined by optical and electrochemical methods, (2) steady-state absorption spectra, (3) cryogenic optical spectra, (4) transient absorption spectra, (5) excited-state lifetime and power dependence, and (6) coherent oscillations in ultrafast electron dynamics. We emphasize that multiple analyses should be performed and cross-checked in practice because no single criterion is definitive. We also review the photophysics of several gold nanoclusters with nascent surface plasmon resonance. These criteria are expected to deepen the understanding of the metallic to molecular state transition of gold and other metal nanoclusters and also promote the design of functional nanomaterials and their applications.