Energy Mismatch Research Articles

Energy-transfer from Gd(iii) to Tb(iii) in (Gd,Yb,Tb)PO4 nanocrystals

The photoluminescence properties of (Gd,Yb,Tb)PO4 nanocrystals synthesized via a hydrothermal route at 150 °C are reported. Energy-transfer from Gd(3+) to Tb(3+) is witnessed by the detailed analyses of excited-state lifetimes, emission quantum yields, and emission and excitation spectra at room temperature, for Tb(3+) concentrations ranging from 0.5 to 5.0 mol%. Absolute-emission quantum yields up to 42% are obtained by exciting within the (6)I7/2-17/2 (Gd(3+)) manifold at 272 nm. The room temperature emission spectrum is dominated by the (5)D4 → (7)F5 (Tb(3+)) transition at 543 nm, with a long decay-time (3.95-6.25 ms) and exhibiting a rise-time component. The (5)D3 → (7)F6 (Tb(3+)) rise-time (0.078 ms) and the (6)P7/2 → (8)S7/2 (Gd(3+)) decay-time (0.103 ms) are of the same order, supporting the Gd(3+) to Tb(3+) energy-transfer process. A remarkably longer lifetime of 2.29 ms was measured at 11 K for the (6)P7/2 → (8)S7/2 (Gd(3+)) emission upon excitation at 272 nm, while the emission spectrum at 11 K is dominated by the (6)P7/2 → (8)S7/2 transition line, showing that the Gd(3+) to Tb(3+) energy-transfer process is mainly phonon-assisted with an efficiency of ~95% at room temperature. The Gd(3+) to Tb(3+) energy transfer is governed by the exchange mechanism with rates between 10(2) and 10(3) s(-1), depending on the energy mismatch conditions between the (6)I7/2 and (6)P7/2 levels of Gd(3+) and the Tb(3+ 5)I7, (5)F2,3 and (5)H5,6,7 manifolds and the radial overlap integral values.

Lattice dynamics in Bi2Te3and Sb2Te3: Te and Sb density of phonon states

The lattice dynamics in Bi${}_{2}$Te${}_{3}$ and Sb${}_{2}$Te${}_{3}$ were investigated both microscopically and macroscopically using ${}^{121}$Sb and ${}^{125}$Te nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi${}_{2}$Te${}_{3}$. A prominent peak in the Te specific density of phonon states at $13\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a $\ensuremath{\sim}$$20%$ force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at $20\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in Bi${}_{2}$Te${}_{3}$, $1.75(1)\phantom{\rule{0.28em}{0ex}}\mathrm{km}/\mathrm{s}$, compared to Sb${}_{2}$Te${}_{3}$, $1.85(4)\phantom{\rule{0.28em}{0ex}}\mathrm{km}/\mathrm{s}$, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at $295\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, $2.4\phantom{\rule{4.pt}{0ex}}{\text{Wm}}^{\ensuremath{-}1}{\text{K}}^{\ensuremath{-}1}$ for Sb${}_{2}$Te${}_{3}$ and $1.6\phantom{\rule{4.pt}{0ex}}{\text{Wm}}^{\ensuremath{-}1}{\text{K}}^{\ensuremath{-}1}$ for Bi${}_{2}$Te${}_{3}$, cannot be explained by anharmonicity alone given the rather modest Gr\uneisen parameters, $1.7(1)$ for Sb${}_{2}$Te${}_{3}$ and $1.5(1)$ for Bi${}_{2}$Te${}_{3}$, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy.

Vacuum-induced coherence in quantum dot systems

We present a theoretical study of vacuum-induced coherence in a pair of vertically stacked semiconductor quantum dots. The process consists in a coherent excitation transfer from a single-exciton state localized in one dot to a delocalized state in which the exciton occupation gets trapped. We study the influence of the factors characteristic of quantum dot systems (as opposed to natural atoms): energy mismatch, coupling between the single-exciton states localized in different dots, and different and nonparallel dipoles due to sub-band mixing, as well as coupling to phonons. We show that the destructive effect of the energy mismatch can be overcome by an appropriate interplay of the dipole moments and coupling between the dots which allows one to observe the trapping effect even in a structure with technologically realistic energy splitting of the order of milli--electron volts. We also analyze the impact of phonon dynamics on the occupation trapping and show that phonon effects are suppressed in a certain range of system parameters. This analysis shows that the vacuum-induced coherence effect and the associated long-living trapped excitonic population can be achieved in quantum dots.

Influence of surface oxidation on plasmon resonance in monolayer of gold and silver nanoparticles

Surface plasmon resonance of gold and silver nanoparticle (NP) layers is investigated by the experiment as well as simulations. Although the good agreement was found for gold NP film, a significant mismatch in the resonance energy for silver NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in silver NPs. As an alternative to the NP size-dependent Drude model, the analysis based on effective medium approximation for refractive index of Ag-Ag2O material system is carried out and compared with the core-shell model. Both Mie's model and numerical simulation results illustrate shift of the surface plasmon resonance due to silver NP surface oxidation.

Blood–brain barrier dysfunction and epilepsy: Pathophysiologic role and therapeutic approaches

The blood-brain barrier (BBB) is located within a unique anatomic interface and has functional ramifications to most of the brain and blood cells. In the past, the BBB was considered a pharmacokinetic impediment to antiepileptic drug penetration into the brain; nowadays it is becoming increasingly evident that targeting of the damaged or dysfunctional BBB may represent a therapeutic approach to reduce seizure burden. Several studies have investigated the mechanisms linking the onset and sustainment of seizures to BBB dysfunction. These studies have shown that the BBB is at the crossroad of a multifactorial pathophysiologic process that involves changes in brain milieu, altered neuroglial physiology, development of brain inflammation, leukocyte-endothelial interactions, faulty angiogenesis, and hemodynamic changes leading to energy mismatch. A number of knowledge gaps, conflicting points of view, and discordance between clinical and experimental data currently characterize this field of neuroscience. As more pieces are added to this puzzle, it is apparent that each mechanism needs to be validated in an appropriate clinical context. We now offer a BBB-centric view of seizure disorders, linking several aspects of seizures and epilepsy physiopathology to BBB dysfunction. We have reviewed the therapeutic, antiseizure effect of drugs that promote BBB repair. We also present BBB neuroimaging as a tool to correlate BBB restoration to seizure mitigation. Add-on cerebrovascular drug could be of efficacy in reducing seizure burden when used in association with neuronal antiepileptic drugs.

The Aharonov-Bohm effect in graphene rings with metal mirrors

We measured the Aharonov-Bohm (AB) effect in graphene rings with superconducting-(Al) and normal-metal (Au) mirrors. The mirrors were deposited either on additional stubs connected to the rings in the transverse direction or on the ring bias lines. A significant enhancement of the visible phase coherence was observed in the latter case, in which we observed even the third harmonic of the AB oscillations. The superconductivity of the mirrors appears to be unimportant for the improved coherence in graphene. A large Fermi energy mismatch between graphene and the mirror material is sufficient for this effect. In addition, a transport gap was observed in our graphene structures at the gate voltage close to the Dirac point. The value of the gap can be reproduced by assuming the occurrence of Coulomb blockade effects in graphene.

Microscopic understanding of the orbital splitting and its tuning at oxide interfaces

By means of a Wannier projection within the framework of density functional theory, we are able to identify the modified c-axis hopping and the energy mismatch between the cation bands as the main source of the t2g splitting around the Γ-point for oxide heterostructures, excluding previously proposed mechanisms such as Jahn-Teller distortions or electric-field asymmetries. Interfacing LaAlO3, LaVO3, SrVO3 and SrNbO3 with SrTiO3 we show how to tune this orbital splitting, designing heterostructures with more dxy electrons at the interface. Such an “orbital engineering” is the key for controlling the physical properties at the interface of oxide heterostructures.

Discontinuous Molecular Dynamics (DMD) Study of Heteropolymer Collapse in an Explicit Solvent

In this study, we employ the discontinuous molecular dynamics simulation method to investigate the collapse properties of a single heteropolymer chain in an explicit solvent. Solvent density ρ, fraction of hydrophobic monomers n H (defined as the ratio of the number of hydrophobic monomers to the total number of monomers) and a hydrophobicity parameter λ (which controls the energy mismatch between the monomers and solvent particles) were systematically varied to examine their role in polymer collapse. The average static structure factor of the polymer was used to find the so-called θ-point characterizing the state of an ideal chain. Phase diagrams of ρ versus λ for the coil-globule transition were mapped out for different values of n H. Increasing the fraction of hydrophobic monomers n H, solvent density ρ, and hydrophobicity parameter λ were all shown to aid in stabilizing the globule phase. In an effort to explore scaling behaviour of the coil-globule phase diagram as a function of n H, and to investigate whether the phase boundaries for different n H collapsed on to one universal curve, we rescaled λ by n H (δ) λ; we determined δ = 1.72, in contrast to mean-field predictions of δ = 2.0.

Optimization of electroluminescence from n-ZnO/AlN/p-GaN light-emitting diodes by tailoring Ag localized surface plasmon

The localized surface plasmon (LSP)-enhanced n-ZnO/AlN/p-GaN light-emitting diodes (LEDs) were fabricated by inserting Ag nanoparticles (NPs) into the ZnO/AlN interface. To investigate the effects of morphology of Ag NPs on the electroluminescence (EL) of device, the Ag NPs with various sizes were prepared by annealing Ag thin films with different deposition times. It is found that the insertion of Ag NPs with suitable size and surface coverage is favorable for the effective resonant coupling between excitons in ZnO and LSP of Ag NPs, and thereby significantly improve the EL performance of the device. For the n-ZnO/AlN/p-GaN LED with 10 nm Ag NPs, a maximum EL enhancement factor of 3.7 was observed at 420 nm at an injection current of 10 mA. For the device with the smaller Ag NPs, only the weaker enhancement is observed due to the smaller scattering cross section. On the other hand, in the case of the larger Ag NPs, the energy mismatch between the LSP of Ag NPs and the near band-edge emission of ZnO, as well as the poor crystalline quality of the ZnO film, leads to degradation in device performance.

Spin-dependent transport properties through gapless graphene-based ferromagnet and gapped graphene-based superconductor junction

By depositing a superconductor on gapped graphene (graphene grown on SiC substrate), the motion of quasiparticles in this superconductor is explained by the massive Dirac equation. In this paper, we study the spin dependent transport properties of graphene-based ferromagnetic/insulator/superconductor (FIS) junction and graphene-based ferromagnetic/ferromagnetic barrier/superconductor (FFBS) junction in which only the superconducting region is deposited on the gapped graphene and the other graphene regions are gapless. We found that in the graphene-based FIS junction and in the thin barrier approximation, by opening the energy gap in the superconducting region, the charge conductance is an oscillatory function of barrier strength (χG), despite the large Fermi energy mismatch between ferromagnetic and superconductor regions. As an important result, we analytically obtained that for the normal incident of charge carriers, this junction is not totally transparent. This means that the second characteristic of Klein tunneling is not satisfied due to the massive Dirac fermions carrying the current in the superconductor region. For the graphene-based FFBS junction, opening the energy gap causes a phase shift as large as π2 to appear between the peaks of charge conductance for parallel and antiparallel configuration versus χG. Interestingly, we obtained that by increasing the energy gap in this junction, the magnetoresistance increases and by approaching the energy gap to the Fermi energy of the superconductor, it reaches its maximum value (more than −150%). This characteristic shows that this junction can be a suitable candidate for application in the graphene-based spintronics devices.

Tunneling conductance in topological insulator ferromagnet/p-wave superconductor junctions

The tunneling conductance in topological insulator (TI) ferromagnet/p-wave superconductor (FM/pS) junction is studied based on the Blonder–Tinkham–Klapwijk (BTK) theory. The Fermi energy mismatch between FM and pS as well as the finite quasiparticle lifetime are considered. Three kinds of pairings px, py, and px+ipy-waves for pS are chosen. It is found that the spectrum strongly depend on the magnetic gap, the gate potential, the quasiparticle lifetime as well as the type of the pair potential symmetry. The pair potential symmetry drastically affects the formation of the zero-energy bound states dependent on the magneto effect or the Fermi energy mismatch effect. The finite quasiparticle lifetime effect can suppress the Andreev resonant scattering process at eV=Δ0 and smear the dips in the conductance.

Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby

The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.

Growth of p-Type Hematite by Atomic Layer Deposition and Its Utilization for Improved Solar Water Splitting

Mg-doped hematite (α-Fe(2)O(3)) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 10(15) cm(-3). When grown on n-type hematite, the p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions. A nominal 200 mV turn-on voltage shift toward the cathodic direction was measured, which is comparable to what has been measured using water oxidation catalysts. This result suggests that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations. Similar approaches may be used to mitigate problems caused by energy mismatch between water redox potentials and the band edges of hematite and many other low-cost metal oxides, enabling practical solar water splitting as a means for solar energy storage.

Atomic structure variations of mechanically stable fcc-bcc interfaces

It has recently been shown that under severe plastic deformation processing bi-metal fcc/bcc composites develop a mechanically stable heterophase interface that joins the {112}fcc//{112}bcc planes in the Kurdjumov-Sachs orientation relationship. In this article, we study variations in the relaxed equilibrium atomic structure of this interface with changes in fcc stacking fault energy (SFE) and lattice mismatch between the two crystals. Using molecular statics/dynamics simulations for three fcc/bcc systems, Cu-Nb, Al-Fe, and Al-Nb, we find that the number of distinct sets of intrinsic interfacial dislocations and their core structures vary significantly among these three systems. The impact of these atomic-scale structural differences on interfacial properties is demonstrated through their interactions with point defects. The interfaces studied here are shown to exhibit a wide variation in ability, ranging from being a poor to an excellent sink for vacancies.

Phase-field Simulation of Microstructural Evolution of γ Precipitate in γ′ Matrix in Binary Ni-Al Alloys

The microstructural evolution of γ precipitates (Ni with fcc_A1 structure) in γ′ matrix (Ni3Al with L12 structure) in the binary Ni-Al alloys was studied by means of the multi-phase-field (MPF) approach. The thermodynamic driving force and diffusivities were taken from the CALPHAD databases via the TQ interface. The thermophysical parameters, interfacial energy, elastic constants, and lattice mismatch were all set to be experimental values. The morphology evolution of one, two and multi γ precipitates in γ′ matrix were analyzed via the present phase-field simulations. A splitting behavior of two close precipitates was also observed. In the case of multi γ precipitates, the shape of the precipitates is spherical at the stage of nucleation, then cuboidal during growth stage, and becomes plate like at the stage of coarsening, which shows good agreement with the experimental results.

Highly Flexible, Transparent and Electrically Conducting Silver Nanoparticles Films Enabled by Controlled Sedimentation

ABSTRACTOften, silver nanoparticles (AgNps) are looked at in the realm of their plasmonic effects that are characterized by unique absorptional bands in the visible spectrum. Herein, the kinetics of a simple gravity mediated sedimentation process of AgNps, in aqueous suspensions. The surface energy mismatch between the AgNps and the receiver substrate allow for the formation of irregularly shaped AgNp microclusters with interconnected microchannels with dimensions and particle density controlled by the distance between the exposed substrate surface and the water/suspension interface. An investigation of the interplay between these properties and the films’ nanoparticle density is presented.

Suppressing H2 Evolution by Silicon Powder Dispersions

Silicon dispersions in water are used to produce pyrotechnic time delay compositions. The propensity of the silicon to react with water and to produce hazardous hydrogen gas must be suppressed. To this end, the effect of surface modifications and medium pH on the rate of corrosion of silicon was studied at ambient temperature. It was found that the rate of hydrogen evolution increased with increasing pH. Silanes proved to be more effective silicon corrosion inhibitors than alcohols, with vinyl tris (2-methoxyethoxy) silane producing the best results. Differential thermal analysis (DTA) studies were performed using a near-stoichiometric amount of lead chromate as oxidant. Comparable combustion behavior was observed when both the fuel and the oxidant powders were either uncoated or silane modified. Mixtures of neat oxidant with silane-coated silicon showed poor burn behavior and this was attributed to poor particle–particle mixing due to the mismatch in surface energies.

Nonresonant and Resonant Mode-Specific Intermolecular Vibrational Energy Transfers in Electrolyte Aqueous Solutions

The donor/acceptor energy mismatch and vibrational coupling strength dependences of interionic vibrational energy transfer kinetics in electrolyte aqueous solutions were investigated with ultrafast multiple-dimensional vibrational spectroscopy. An analytical equation derived from the Fermi's Golden rule that correlates molecular structural parameters and vibrational energy transfer kinetics was found to be able to describe the intermolecular mode specific vibrational energy transfer. Under the assumption of the dipole-dipole approximation, the distance between anions in the aqueous solutions was obtained from the vibrational energy transfer measurements, confirmed with measurements on the corresponding crystalline samples. The result demonstrates that the mode-specific vibrational energy transfer method holds promise as an angstrom molecular ruler.

Influence of Hole‐Transport Layers and Donor Materials on Open‐Circuit Voltage and Shape of I–V Curves of Organic Solar Cells

AbstractThe effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (Voc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, Voc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which Voc is independent of the HTL.

Coherent and incoherent dynamics in excitonic energy transfer: Correlated fluctuations and off-resonance effects

We study the nature of the energy transfer process within a pair of coupled two-level systems (donor and acceptor) subject to interactions with the surrounding environment. Going beyond a standard weak-coupling approach, we derive a master equation within the polaron representation that allows for investigation of both weak and strong system-bath couplings, as well as reliable interpolation between these two limits. With this theory, we are then able to explore both coherent and incoherent regimes of energy transfer within the donor-acceptor pair. We elucidate how the degree of correlation in the donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the range of the environment frequency distribution impact upon the energy transfer dynamics. In the resonant case (no energy mismatch) we describe in detail how a crossover from coherent to incoherent transfer dynamics occurs with increasing temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also explore how fluctuation correlations are able to protect coherence in the energy transfer process. We show that a strict crossover criterion is harder to define when off-resonance, though we find qualitatively similar population dynamics to the resonant case with increasing temperature, while the amplitude of coherent population oscillations also becomes suppressed with growing site energy mismatch.