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We use a master equation model for numerical simulations of triplet loss mechanisms in phosphorescent organic light-emitting diodes (OLEDs). In addition to the IV characteristics, which were successfully simulated in our previous work, the measurements of luminous efficacy (LE) for the OLEDs with various doping profiles of the emitter fac-tris(2-phenylpyridine)iridium (III) (Ir(ppy) 3) can be reproduced based on a single set of excitonic parameters, which is a strong indication of the validity of the model. In order to minimize the strong Dexter diffusion of triplets out of the emission layer (EML) and the nonradiative decays, an additional exciton blocking layer and stronger exciton confinement on guest molecules are investigated in the simulations to improve the device efficiency. With the modifications, the LE of the lowest-doped OLED with 4% Ir(ppy) 3 can be more than twofold increased at a luminance up to 20 000 cd / m 2 . However, due to the enhanced triplet-triplet annihilation, the roll-off in electrophosphorescence efficiencies under high current injections of the upgraded devices becomes stronger, which can nevertheless be suppressed by a higher emitter concentration in the EML. [ABSTRACT FROM AUTHOR]

The exciton bound to a pair of nitrogen atoms situated at nearby lattice sites in dilute GaAs:N provides an energetically uniform electronic system, spectrally distinct from pairs with larger or smaller separations, and can even be grown with a uniform pair orientation in the crystal. We use photoluminescence excitation spectroscopy on an ensemble of N pairs to study the narrow continuous energy distribution within two of the individual exchange- and symmetry-split exciton states. Inhomogeneous linewidths of 50–60 μeV vary across the crystal on a mesoscopic scale and can be 30 μeV at microscopic locations indicating that the homogeneous linewidth inferred from previous time-domain measurements is still considerably broadened. While excitation and emission linewidths are similar, results show a small energy shift between them indicative of exciton transfer via phonon-assisted tunneling between spatially separated N pairs. We numerically simulate the tunneling in a spatial network of randomly distributed pairs having a normal distribution of bound exciton energies. Comparing the ensemble excitation-emission energy shift with the measured results shows that the transfer probability is higher than expected from the dilute pair concentration and what is known of the exciton wavefunction spatial extent. Both the broadening and the exciton transfer have implications for the goal of pair-bound excitons as a single- or multi-qubit system. [ABSTRACT FROM AUTHOR]

We studied the light-induced effects in BiFeO 3 single crystals as a function of temperature by means of optical spectroscopy. Here, we report the observation of several light-induced absorption features, which are discussed in terms of the photostriction effect and are interpreted in terms of excitons. The temperature dependence of their energy position suggests a possible coupling between the excitons and the lattice vibrations. Moreover, there are hints for anomalies in the temperature evolution of the excitonic features, which might be related to the temperature-induced magnetic phase transitions in BiFeO 3. Our findings suggest a coupling between light-induced excitons and the lattice and spin degrees of freedom, which might be relevant for the observed ultrafast photostriction effect in multiferroic BiFeO 3. [ABSTRACT FROM AUTHOR]

We report Raman and photoluminescence (PL) emission spectroscopy studies on monolayer MoS 2 /gold micro-disc hybrid structures. The observed frequency shift and broadening of phonon modes of the monolayer MoS 2 are mainly the manifestations of the mechanical strain rather than the plasmonic field induced electron doping. We also observe considerable enhancement of the phonon mode intensities as a consequence of the surface enhanced Raman scattering caused by the plasmonic fields. The interaction of localized surface plasmon resonances (LSPRs) excited near the edge of the micro-discs with excitons in the monolayer MoS 2 enhances A exciton emission, while suppressing that from B excitons and the trions. The changes in the PL emission near the edge are attributed to the modification of the emission rate via the Purcell effect, whereas the PL quenching on a relatively flat disc surface is due to lower density and the non-radiative nature of LSPRs. [ABSTRACT FROM AUTHOR]

Photoluminescence excitation (PLE) spectroscopy was used to investigate the optical properties of the recently discovered 3364.3 meV antimony-related donor bound exciton (D 0 X) in zinc oxide. By suitable control of the growth conditions, we achieved samples in which the Sb-related donor emission was the dominant near-bandgap luminescence feature. Resonant excitation using a tunable UV source enabled the observation of the two electron transitions of the donor bound exciton, enabling the direct determination of the binding energy of the Sb donor at 42.2 ± 0.5 meV, the lowest value yet reported for a donor in this material. The two electron transitions exhibit an unusual blue shift with increasing temperature, which is explained in terms of thermalization of excited states of the D 0 X initial state. The D 0 X excited states were probed by PLE and follow the trends of conventional shallow group III donors such as Al, Ga, and In. [ABSTRACT FROM AUTHOR]

Multiple exciton generation (MEG) in semiconductor nanostructures is of great interest for the enhancement of related performances in optoelectronic devices and for the shape dependence of conversion efficiency with which absorbed photons are converted into electron-hole pairs. However, theoretical insight into the coupling effects from the size and shape gradient on the MEG and related optoelectronic properties at the atomic level remains unclear. Here, we investigate the MEG and optoelectronic properties in PbSe nanostructures with different morphologies (nanocrystals, nanowires, and nanocones) based on the bond relaxation correlation mechanism, detailed balance principle, and Fermi statistical theory. It is found that size reduction of nanostructures can increase the bandgap, suppress the threshold energy, and enhance the MEG efficiency. Moreover, optimal conversion efficiency of PbSe nanostructures can be achieved by modulating the geometrical parameters. [ABSTRACT FROM AUTHOR]

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice. [ABSTRACT FROM AUTHOR]

Little is known about the role of network rigidity in light-induced structural rearrangements in network glasses due to a lack of supporting experiments and theories. In this article, we demonstrate for the first time the ultrafast structural rearrangements manifested as induced absorption (IA) over a broad spectral range in a-GexAs35-xSe65 thin films above the mean-field rigidity percolation transition, quantified by the mean coordination number ⟨r⟩ = 2.40. The IA spectrum arising from self-trapped excitons induced structural rearrangements by softening the glass network that strikingly reveals two relaxation mechanisms which differ by one order of magnitude. The fast kinetics of electron-lattice interaction occurs within 1 ps, exhibits a weak dependence on rigidity, and dominates in the sub-bandgap region. In a stark contrast, the slow kinetics is associated with the structural changes in the bandgap region and depends strongly on network rigidity. Our results further demonstrate that amplitude of IA scales a linear relationship with excitation fluence which provides a unique way to induce structural rearrangements in an over-coordinated network to exploit it for practical purposes. Our results modify the conventional concept of rigidity dependence of light-induced effects in network glasses, when excited with an ultrafast laser. [ABSTRACT FROM AUTHOR]

In order to inhibit high carrier recombination rates in Ge-on-Si nanostructures, GexSi1–x nanoislands were covered by a thin amorphous silicon layer via a low pressure CVD process. It is demonstrated that the surface photovoltage (SPV) signal in capped GexSi1–x/Si is increased by an order of magnitude compared to that of bare GexSi1–x islands, which can be due to the effective passivation of recombination centers at the a-Si/GexSi1–x interface. The effect is even more enhanced after subsequent annealing at 400 °C in an O2 ambient environment, with the signal increases ranging from 5 to 10 times. The observed increased photovoltage is accompanied by longer time decays in the SPV transients, being most increased after the annealing step. These results show that the photoexcited electron-hole pairs can be efficiently separated by the internal electric field at the a-Si/GexSi1–x/c-Si interfaces and can contribute to the photovoltage with decreasing recombination in GexSi1–x islands or at the interfaces. This work can facilitate the photovoltaic applications of Ge/Si heterostructures. [ABSTRACT FROM AUTHOR]

The effect of the magnetic field on radiative and non-radiative mechanisms of charge carriers in GaAs/AlGaAs quantum wells (QWs) is investigated via quasi-simultaneous magneto-photoluminescence (PL) and magneto-surface photo-voltage (SPV) spectroscopy. At low-temperature, the luminescence intensity of ultra-low disordered GaAs/AlGaAs QWs generally increases under strong magnetic perturbation. Even at relatively high-temperature (100K), the magnetic field driven enhancement of PL intensity is observed for thick QWs. On the other hand, it is found that the PL intensity of narrow QWs gradually decreases under a strong magnetic field at 100 K. The magnetic field driven enhancement (suppression) of radiative recombination efficiency for wide (narrow) QWs is investigated by considering the oscillator strength, thermal effects, and carrier re-distribution in energy states. Also, the charge carriers which escape from narrow QWs or are captured by interface defects are probed via magneto-SPV measurements. Radiative recombination and thermionic emission of charge carriers, investigated by magneto-PL and magneto-SPV spectroscopy, provide a clear guideline of the critical QW width which would be essential for magnetic field driven hightemperature operation of advanced emission based-devices. [ABSTRACT FROM AUTHOR]

Here, we report on the strong temperature dependence of PL properties in CdSe/ZnS core/shell quantum dots, within a broad temperature range of 80–500 K. Our results demonstrate different PL quenching mechanisms in high and low temperature ranges. In the low temperature range of below 320 K, PL quenching is proved to be dominated by thermally activated carrier escape. While in the high temperature range of above 320 K, PL undergoes additional quenching because of surface trapping by the thermally created surface defects, which is calculated and proved by the rapidly decreasing PL lifetime. Our calculating result proves that the model of thermally activated carrier escape is also responsible for PL quenching in the high temperature range. However, the red-shifted PL peak energy and the broadened FWHM with increasing temperature follow the same mechanisms in the whole temperature range. The former is due to the temperature-dependent band-gap shrinkage and the latter is due to the exciton scattering with acoustic and longitudinal-optical phonons. [ABSTRACT FROM AUTHOR]

We theoretically investigate the energy of the ground state exciton confined to two-dimensional (2D) monolayers with circular shape. Within an effective mass approach employing a nonlocal screening effect on the Coulomb potential energy, we demonstrate how the exciton energy is correlated with the radius of the circle, electron-hole reduced mass, and 2D susceptibility. In addition, we show that a dead layer around the circle edge, into which the electron-hole pair cannot penetrate, is necessary for understanding the energy shift recently observed in monolayer WSe2 quantum dots. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the formation of a macroscopically occupied state (condensation) of exciton-polaritons in a tunable 3D open-access microcavity system under resonant excitation. We observe a high conversion efficiency of polaritons from the pumped state to the ground mode, leading to the population of the condensate 1.5 times greater than that of the driven state for a total number of polaritons of ∼110. Comparative numerical simulations show that coherent polariton-polariton scattering mechanisms alone cannot explain the observed experimental results and that other relaxation channels play an important role. [ABSTRACT FROM AUTHOR]

A refractive index sensor is proposed comprising one-dimensional photonic bandgap geometry in conjunction with a thin metal film. The refractive index sensor is based on the cavity mode formed in a thin cavity layer sandwiched between multilayers of SiO2/Ta2O5 and a metal film. The cavity structure traps light through excitation of the cavity mode and hence sharp resonance minima (maxima) are obtained in the reflection (transmission) spectrum. Our theoretical model investigates the impact of cavity thickness, nature of plasmon-active metal, the angle of incidence, and polarization of light on the sensitivity. It is found that the resonance wavelength increases linearly as the analyte refractive index increases. Additionally, the enhancement in the interaction between the analyte medium and micro-cavity mode-field further enhances the sensitivity of the proposed sensor. It is observed that as the cavity thickness increases from 200 nm to 900 nm, the sensitivity increases from 200 nm/RIU to 350 nm/RIU. The performance of the proposed photonic crystal based sensor exhibits strong polarization and angular dependences. Also, the sharp micro-cavity resonance results in improved detection accuracy as compared to surface plasmon resonance based schemes. The proposed structure advantage is an alternative to the conventional surface plasmon refractive index sensor. [ABSTRACT FROM AUTHOR]

The expansion behavior of a single Shockley stacking fault (SSSF) originating from a basal plane dislocation in a 4H-SiC epitaxial layer on the (11 2 ¯ 0) a-plane under electron beam (EB) (//[11 2 ¯ 0]) irradiation was observed. The width of the SSSF was proportional to the EB current. EB irradiation at a fixed spot outside an SSSF can expand the SSSF as effectively as direct SSSF irradiation. It was found that the selective excitation of an SSSF and/or a Si-core partial dislocation (PD) is possible by appropriately setting the EB irradiation position because the cathodoluminescence spectrum varies with the irradiation position around an SSSF. The rate of SSSF expansion upon the indirect excitation of a Si-core PD is much larger than that upon direct SSSF excitation. However, the expansion rate under both indirect SSSF excitation and indirect Si-core PD excitation is smaller than that under indirect Si-core excitation. The C-core PD became mobile after supplying a threshold number of electron-hole pairs. [ABSTRACT FROM AUTHOR]

We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygen—the key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200 μeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task. [ABSTRACT FROM AUTHOR]

The excitonic optical properties of an Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum well structure were studied by photoluminescence (PL) spectroscopy at various temperatures. An analysis of the temperature dependence of PL peak energy revealed the stronger localization of excitons than biexcitons, consistent with the trend predicted from the difference between the spatial extents of excitons and biexcitons. The PL linewidth of the excitons increased linearly with increasing temperature up to 300 K and then more rapidly above 300 K. This result indicated that the exciton-longitudinal optical phonon interaction became prominent as the broadening mechanism of the exciton linewidth above 300 K. The luminescence of both excitons and biexcitons exhibited low thermal quenching, with their intensities at 300 K remaining at 37% and 47%, respectively, of their intensities at 4 K. These observations reflected the high thermal stability of the excitons and biexcitons in this multiple quantum well structure. In addition, the ratio of the PL intensity of the exciton-biexciton scattering to the product of the exciton and biexciton luminescence intensities was independent of temperature up to 200 K and increased rapidly with increasing temperature above 250 K. The temperature-independent behavior of this ratio up to 200 K originated from the localization of excitons and biexcitons, and the rapid increase in this ratio with temperature above 250 K reflected an increase in the frequency of interactions between the excitons and biexcitons due to the gradual delocalization of the excitons and biexcitons with increasing temperature. [ABSTRACT FROM AUTHOR]

We study the effects of an applied in-plane uniaxial strain on the plasmon dispersions of monolayer, bilayer, and double-layer black phosphorus structures in the long-wavelength limit within the linear elasticity theory. In the low-energy limit, these effects can be modeled through the change in the curvature of the anisotropic energy band along the armchair and zigzag directions. We derive analytical relations of the plasmon modes under uniaxial strain and show that the direction of the applied strain is important. Moreover, we observe that along the armchair direction, the changes of the plasmon dispersion with strain are different and larger than those along the zigzag direction. Using the analytical relations of two-layer phosphorene systems, we found that the strain-dependent orientation factor of layers could be considered as a means to control the variations of the plasmon energy. Furthermore, our study shows that the plasmonic collective modes are more affected when the strain is applied equally to the layers compared to the case in which the strain is applied asymmetrically to the layers. We also calculate the effect of strain on the drag resistivity in a double-layer black phosphorus structure and obtain that the changes in the plasmonic excitations, due to an applied strain, are mainly responsible for the predicted results. This study can be readily extended to other anisotropic two-dimensional materials. [ABSTRACT FROM AUTHOR]

Fundamental device physics of exciton dynamics is crucial to the design and fabrication of organic light-emitting diodes (OLEDs) with a long lifetime at high brightness. In this paper, we report a set of analytical equations which describe how and where defects form during exciton-driven degradation of an OLED and their impact on device operation. This set of equations allows us to quantify changes in the exciton and defect populations as a function of time in neat layers of 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) in simple bilayer OLEDs. CBP produces luminescent defects which present a unique opportunity to quantify the exciton capturing dynamics of the defects. Through modeling of the time and current density dependence of both the CBP and defect emission, we clearly identify CBP singlet excitons as the source of OLED degradation. Further analysis of experimental data on devices with precisely positioned exciton capturing layers suggests that defects are formed near organic heterojunctions. [ABSTRACT FROM AUTHOR]

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers--depending on the balance between spin-orbit interaction and interlayer hopping. This balance depends on the layer thickness, momentum space symmetry points, and applied gate fields. The design range of this balance, the effective Fermi levels, and all relevant effective masses is analyzed in great detail. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as the major source for the observed photoluminescence. [ABSTRACT FROM AUTHOR]

Inverted type-I heterostructure core/crown quantum rings (QRs) are quantum-efficient luminophores, whose spectral characteristics are highly tunable. Here, we study the optoelectronic properties of type-I core/crown CdS/CdSe QRs in the zincblende phase—over contrasting lateral size and crown width. For this, we inspect their strain profiles, transition energies, transition matrix elements, spatial charge densities, electronic bandstructures, band-mixing probabilities, optical gain spectra, maximum optical gains, and differential optical gains. Our framework uses an effective-mass envelope function theory based on the 8-band k · p method employing the valence force field model for calculating the atomic strain distributions. The gain calculations are based on the density-matrix equation and take into consideration the excitonic effects with intraband scattering. Variations in the QR lateral size and relative widths of core and crown (ergo the composition) affect their energy levels, band-mixing probabilities, optical transition matrix elements, emission wavelengths/intensities, etc. The optical gain of QRs is also strongly dimension and composition dependent with further dependency on the injection carrier density causing the band-filling effect. They also affect the maximum and differential gain at varying dimensions and compositions. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically investigate plasmon propagation characteristics in AB and AA stacked bilayer graphene (BLG) in the presence of energy asymmetry due to an electrostatic field oriented perpendicularly to the plane of the graphene sheet. We first derive the optical conductivity of BLG using the Kubo formalism incorporating energy asymmetry and finite electron scattering. All results are obtained for room temperature (300K) operation. By solving Maxwell’s equations in a dual gate device setup, we obtain the wavevector of propagating plasmon modes in the transverse electric (TE) and transverse magnetic (TM) directions at terahertz frequencies. The plasmon wavevector allows us to compare the compression factor, propagation length, and the mode confinement of TE and TM plasmon modes in bilayer and monolayer graphene sheets and also to study the impact of material parameters on plasmon characteristics. Our results show that the energy asymmetry can be harnessed to increase the propagation length of TM plasmons in BLG. AA stacked BLG shows a larger increase in the propagation length than AB stacked BLG; conversely, it is very insensitive to the Fermi level variations. Additionally, the dual gate structure allows independent modulation of the energy asymmetry and the Fermi level in BLG, which is advantageous for reconfiguring plasmon characteristics post device fabrication. [ABSTRACT FROM AUTHOR]

When a Metal-Insulator-Metal waveguide is coupled with a ring resonator, one expects, for a given integer referring to cavity mode number, exactly one resonance phenomenon. However, numerical simulations of the spectrum of the transmission of the structure highlight two resonances around certain values of the resonance wavelength. This phenomenon was numerically studied thanks to a Finite Difference Time Domain method. Until now, and to my knowledge, this phenomenon is linked to a correlation between the standing waves forming in the ring at the resonance wavelength and the faces and corners of the ring. In this document, I present a new physical analysis and interpretation of the transmission spectrum anomaly of the square ring resonator. By using an efficient polynomial modal method, I show that this phenomenon is due to the interference of hybrid modes of the superstructure. [ABSTRACT FROM AUTHOR]

The energy theorem of electrodynamics is extended so as to apply to the plasmonic waves on single-walled carbon nanotubes which propagate parallel to the axial direction of the system and are periodic waves in the azimuthal direction. Electronic excitations on the nanotube surface are modeled by an infinitesimally thin layer of free-electron gas which is described by means of the linearized hydrodynamic theory. General expressions of energy and power flow associated with surface waves are obtained by solving Maxwell and hydrodynamic equations with appropriate boundary conditions. Numerical results for the transverse magnetic mode show that energy, power flow, and energy transport velocity of the plasmonic waves strongly depend on the nanotube radius in the long-wavelength region. [ABSTRACT FROM AUTHOR]

Post-implantation annealing is typically used to remove structural defects and electrically activate implanted dopants in semiconductors. However, ion-induced defects and their interaction with dopants in group II oxide semiconductors are not fully understood. Here, we study defect evolution in the course of annealing in CdO and ZnO materials implanted with nitrogen which is one of the most promising candidates for p-type doping. The results of photoluminescence and ion channeling measurements revealed a striking difference in defect behavior between CdO and ZnO. In particular, the defect annealing in CdO exhibits a two stage behavior, the first stage accounting for efficient removal of point defects and small defect clusters, while the second one involves gradual disappearance of extended defects where the sample decomposition can play a role. In contrast, a strong reverse annealing occurs for ZnO with a maximum defect concentration around 900 °C. This effect occurs exclusively for nitrogen ions and is attributed to efficient growth of extended defects promoted by the presence of nitrogen. [ABSTRACT FROM AUTHOR]

Knowledge of acceptors is critical to the understanding of the p-type doping mechanism of ZnO. Here, we report the influence of annealing temperature and polarity on the acceptor formation in Na-implanted a-plane ZnO bulk crystals, which has been studied by low temperature photoluminescence spectroscopy. The formation of a Na acceptor is evidenced by the acceptor bound exciton emission around 3.35 eV, which depends strongly on the annealing temperature. Unlike the reported results in ZnO thin films, the conditions for Na acceptor formation are harsher in nonpolar ZnO than in the polar one. The acceptors gradually transform into donors when the annealing temperature is elevated or when the crystals are aged, thus calling for effective control over the stability. [ABSTRACT FROM AUTHOR]

In this paper, solution-processed nickel oxide (NiOx) is used as hole-injection layers (HILs) in solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). Serious exciton quenching is verified at the NiOx/emitting layer (EML) interface, resulting in worse device performance. The device performance is significantly improved by inserting a layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) between the EML and NiOx. The solution-processed blue PhOLED with the double-stacked NiOx/PEDOT:PSS HILs shows a maximum current efficiency of 30.5 cd/A, which is 75% and 30% higher than those of the devices with a single NiOx HIL and a PEDOT:PSS HIL, respectively. Improvement of device efficiency can be attributed to reducing exciton quenching of the PEDOT:PSS layer as well as the electron blocking effect of the NiOx layer. [ABSTRACT FROM AUTHOR]

Multi-quantum well organic-inorganic perovskites offer an approach to tunable exciton binding energies based on well-barrier dielectric mismatches, that is, the image-charge effect. As with other two-dimensional materials, ultrathin exfoliated perovskites are highly sensitive to their dielectric environments. We investigate ultrathin crystalline two-dimensional van der Waals layers of organic-inorganic perovskite crystals adjacent to the surface of a substrate. The exciton binding energies are strongly influenced by the surrounding dielectric environment. The Keldysh model describes this dependence. We compare our binding-energy results with data in the (C6H13NH3)2PbI4 perovskite and estimate binding-energy values for (C4H9NH3)2PbBr4. [ABSTRACT FROM AUTHOR]

We perform a theoretical study of the optical transitions for different excitonic complexes in highly symmetric strain-free GaAs quantum dots (QDs) fabricated by epitaxially filling nanoholes (NHs) in an AlGaAs surface. NHs are formed by local droplet etching. As a first step, we propose a QD shape modeling consistent with atomic force microscopy (AFM) profiles and an experimental growth procedure. We investigate the QD height dependence of s- and p- shell exciton recombination energies in the framework of the effective mass approximation with an exact numerical diagonalization method. A comparison between theoretical results and available spectroscopic data is carried out. Systematic evolution of the binding energies of neutral (X), charged excitons (X-, X+) and the biexciton (XX), with QD height, is interpreted in terms of a balance between the Coulomb interactions and charge carrier correlation effects. Our calculations demonstrate the important role of the correlation energies in elucidating the bound character of all few-particle states especially the biexciton. [ABSTRACT FROM AUTHOR]

A graphical representation of current-voltage (IV) measurements of typical memristive interfaces at constant temperature is presented. This is the starting point to extract relevant microscopic information of the parameters that control the electrical properties of a device based on a particular metal-oxide interface. The convenience of the method is illustrated presenting some examples where the IV characteristics were simulated in order to gain insight into the influence of the fitting parameters. [ABSTRACT FROM AUTHOR]

Osmium (Os) is a hexagonal-close-packed metal with a non-spherical Fermi surface that seriously deviates from the assumption in the Mayadas-Shatzkes electrical-resistivity model (MS model) for the size effects of polycrystalline films of metals due to electron scattering by grain boundaries. In this work, we studied the resistivity of the Os films with different thicknesses as a function of temperature in the range of 20 to 296 K. The electron scattering by the surface was found to be unimportant in the contributions to the size effects of resistivity of Os films with a sufficient thickness. Based on the first-principles calculations, an analytical equation was suggested for correction to the MS model and used for fitting the temperature-dependent resistivity of the Os films. The results show that correction to the MS model is necessary and the residual resistivity caused by the defects and impurities cannot be neglected. In addition, the inhomogeneity of resistivity in the direction perpendicular to the film surface was discussed under an assumption of parallel circuits. [ABSTRACT FROM AUTHOR]

The bottom-gated graphene field-effect transistors (GFETs) with HfO2 as the dielectric layer are fabricated and the transport behaviors with double Dirac points defined as the right conductance minima (VDirac+) and the left one (VDirac-) are obtained, which exhibit the unique W-shaped transform characteristics rather than the typical V-shape. This observation indicates that the graphene in the channel region shows the different doping behaviors from that underlying the metal contact region. The W-shaped characteristics of GFETs are affected by the ambient environment and the time-dependence of double Dirac points has been also found. After being treated by Poly (ethylene imine) (PEI), the transport behaviors of GFETs could be modulated, especially for VDirac+ showing more obvious negative shift, indicating that PEI has a remarkable n-doping effect on the graphene in the channel region. PEI also screens the environmental influence and could protect the graphene from being p-doped to some extent. At the same time, the annealing effect has been also studied and it is found that the proper annealing process could contribute to the recovery of the graphene properties. [ABSTRACT FROM AUTHOR]

We demonstrate negative capacitance (NC) dispersion in ZnO by doping lesser electronegative chalcogen (Ch=S, Se, Te) elements at the oxygen (O)-site. Approximately 4.00×0.15 atomic percentage of ChOX is obtained from Rietveld refinement. Using ab initio and dielectric spectroscopy, the tailoring of charge localization around dopants and consequent charge recombination are observed to have a systematic dependence on the stabilization of NC. With the increase of dopant electronegativity difference from SOX to TeOX, the low frequency (<100Hz) dispersion of NC gradually extends to a larger frequency under lower external bias. Universal Debye response is found to govern the NC dispersion with calculated relaxation time indicating to trap mediated charge recombination. [ABSTRACT FROM AUTHOR]

A series of plate-impact experiments were conducted to investigate the influences of impact stress and microstructure on the shock and spall behaviors of a high specific strength steel (HSSS). The HSSS shows a strong positive strain rate sensitivity on the yield strength. With increasing impact stress up to about 6 GPa, the spall strength is found to decrease significantly and then levels off with further increasing impact stress. This trend is proposed to be attributed to the accumulation damage within the target as the initial shock-induced compression wave propagates through the target. The microcracks are clearly observed to nucleate from the interfaces between c-austenite and B2 phase and propagate along the interfaces or cut through the B2 phase in the HSSS during the spalling process. The Hugoniot elastic limit and the spall strength were found to be highly dependent on the microstructure. The spall strength was found to be higher when the density of the void nucleation sites is lower, indicating that the spall strength should be a microstructure parameter of the HSSS under impact tensile conditions depending on the density of phase interfaces. It was also found that there is a tradeoff between the specific yield strength and the spall strength for this HSSS; thus, the current findings should provide insights for achieving an optimal combination of both mechanical properties for impact-resistant applications by tailoring the microstructure. [ABSTRACT FROM AUTHOR]

The effects of the type of background gas and pressure on the spatial distributions of plume species have been investigated by time and space resolved imaging in vacuum, 1×10-2 mbar and 1×10-1 mbar O2 and Ar. The ablation of La0.4Ca0.6MnO3 in vacuum shows dissimilar arrival times for the different neutral species and a backscattering of the impinging species from the substrate. At 1×10-2 mbar, a species-dependent plume splitting appears and preferential scattering of the lighter elements is detected generating a cation off-stoichiometry along the plume axis. In addition at 1×10-1 mbar the plume expansion in this relatively high pressure traps a portion of the background gas against the substrate holder, thereby creating a transient high local pressure with remarkable effects once the plume reaches the substrate. In an Ar background, a rebound wave is seen, which travels backwards and recoats/contaminates the target with a different composition than the original target. In O2, in addition to the rebound, a long-lived volume of excited species is created, which consists mainly of LaO I. The rebound has important effects on the film composition and is background gas dependent. The same effects are also detected during Ag ablation and are probably valid for most target materials. [ABSTRACT FROM AUTHOR]

A detailed study of the electrical properties of planar AlGaN/GaN Schottky diodes is presented, the focus being on the role of the two dimensional electron gas (2DEG) depletion and the diodes non-idealities in different voltage regimes. The 2DEG depletion behavior is inferred from the analysis of capacitance and current measurements with transition from vertical to lateral diode operation occurring at Vpinch-off=4V. In particular, the sub-micrometer depletion width, laterally extending from the edge of the Schottky contact under high reverse voltages, is evaluated on the basis of a simple fringe capacitance model. Current transport mechanisms are discussed, investigating the interrelation between 2DEG, Poole-Frenkel effect, and defects. With regard to defects, the role of dislocations in the AlGaN/GaN diode non-idealities, usually interpreted in terms of Schottky barrier inhomogeneities, is critically addressed. Photocurrent spatial mapping under high reverse voltage points out the not uniform electric field distribution around the Schottky contact and highlights the presence of local photo-conductive paths, likely associated with the dislocations near the edge of the Schottky contact. [ABSTRACT FROM AUTHOR]

We obtained metallic SrTiO3 with an atomically smooth surface, where the step height is close to the unit cell height of the crystal. After the surface treatments for generating the TiO2 terminated layer, the optimal conditions for vacuum annealing were found. The atomically smooth surface was verified by atomic force microscopy and lateral force microscopy. The temperature dependent resistance R(T) measured down to 52 mK indicates the metallic behavior, and its physical origin of the conduction was analyzed by fitting an equation involving electron-phonon and electronelectron scattering mechanisms. Our results are similar to R(T) reported for LaAlO3/SrTiO3 interfaces. In addition, the Hall effect measurement shows a clear resemblance between our reduced SrTiO3 and LaAlO3/SrTiO3 interfaces with sheet carrier density and Hall mobility. We expect that our treatments not only promote the usage of conducting SrTiO3 substrates for subsequent thin film growth but also contribute to the current research interest in two dimensional electron gas (2DEG) SrTiO3 and the interfaces between insulating oxides of LaAlO3/SrTiO3. [ABSTRACT FROM AUTHOR]

We report on a novel technique of wireless actuation of a micromembrane mounted on a piezoelectric stack using radio frequency magnetic fields. The magnetic field component of the radio frequency field induces time varying voltage across the leads of the piezoelectric stack which results in vibrations of the piezoelectric stack which are eventually transferred to a micromembrane of silicon nitride mounted on top of it. Thus, wireless actuation of micromembranes is achieved which is measured using a laser-photodetector system. Wireless actuation of micromembranes has applications in controlled drug delivery with rates of the order of tens of nanolitres per second. It can also be used in controlling capsule endoscopes, in vivo sensors, and micromachines for biomedical applications. [ABSTRACT FROM AUTHOR]

Aluminum nitride (AlN) film coatings on flexible substrates (polymeric Teflon, Mylar) have been obtained using a hybrid helicon-arc ion-plasma deposition technique with high adhesion of coatings. Studies of optical, morphological, and structural properties of AlN films have been carried out. It was found that AlN coatings on Teflon and Mylar thin-film substrates substantially suppress transmission of infrared (IR) radiation within the spectral range λ∼5-20 μm at certain technological parameters and thickness of AlN. Transmission in THz regions by using quasioptics attains T≈79%-95%, and losses measured in the channels within the microwave region 2 to 36 GHz are <0.06 dB. The obtained composite structures (AlN coatings on Teflon and Mylar thin-film substrates), due to a high thermal conductivity of AlN, could be used as efficient blocking structures in the infrared spectral range ("infrared stealth") withdrawing the heat from filters warmed by IR radiation. At the same time, they can be used as the transparent ones in the microwave and THz regions, which can be important for low-temperature detector components of navigation, positioning, and telecommunication systems due to reducing the background noise. [ABSTRACT FROM AUTHOR]

In this paper, strain transfer efficiencies from a single crystalline piezoelectric lead magnesium niobatelead titanate substrate to a GaAs semiconductor membrane bonded on top are investigated using state-of-the-art x-ray diffraction (XRD) techniques and finite-element-method (FEM) simulations. Two different bonding techniques are studied, namely, gold-thermo-compression and polymer-based SU8 bonding. Our results show a much higher strain-transfer for the "soft" SU8 bonding in comparison to the "hard" bonding via gold-thermo-compression. A comparison between the XRD results and FEM simulations allows us to explain this unexpected result with the presence of complex interface structures between the different layers. [ABSTRACT FROM AUTHOR]

Solid-state reactions between Ni1-uPtu (0

The recombination of electric charge carriers at semiconductor surfaces continues to be a limiting factor in achieving high performance optoelectronic devices, including solar cells, laser diodes, and photodetectors. The theoretical model and a solution algorithm for surface recombination have been previously reported. However, their successful application to experimental data for a wide range of both minority excess carrier concentrations and dielectric fixed charge densities has not previously been shown. Here, a parametrisation for the semiconductor-dielectric interface charge Qit is used in a Shockley-Read-Hall extended formalism to describe recombination at the c-Si/SiO2 interface, and estimate the physical parameters relating to the interface trap density Dit, and the electron and hole capture cross-sections rn and rp. This approach gives an excellent description of the experimental data without the need to invoke a surface damage region in the c-Si/SiO2 system. Band-gap tail states have been observed to limit strongly the effectiveness of field effect passivation. This approach provides a methodology to determine interface recombination parameters in any semiconductor-insulator system using macro scale measuring techniques. [ABSTRACT FROM AUTHOR]

A double Dirac cone plays a significant role in the design of zero-refractive-index metamaterials without phase variation and topological insulators with pseudospin states. We present a study on the formation of a double Dirac cone in two-dimensional phononic crystals consisting of either hexagonal or triangular columns in air. We arranged hexagonal and triangular columns separately in a honeycomb lattice to explore the influence of phononic crystal symmetry on the formation of the double Dirac cone. The results show that phononic crystals forming a honeycomb lattice with C6v or C6 symmetry induce an accidental degeneracy, but C3v and C3 cannot. We also demonstrate that by varying the filling ratio of the hexagonal columns, a topological phase transformation induced by energy band inversion with dipolar and quadrupolar states occurs near the double Dirac cone. Transmission properties for acoustic tunneling and waveform shaping are confirmed in two numerical simulation examples. A discussion is given on the formation of the double Dirac cone in different phononic crystal symmetries in a honeycomb lattice. The conclusions suggest a new route for designing topological and zero-refractive-index acoustic devices. [ABSTRACT FROM AUTHOR]

We have studied the structure and electrical properties of La2Mo2O9 thin films of different thicknesses prepared by the laser deposition technique at different substrate temperatures. The structural properties of the thin films have been investigated using XRD, XPS, AFM, TEM, SEM, and Raman spectroscopy. The electrical transport properties of the thin films have been investigated in wide temperature and frequency ranges. The cubic nature of the thin films has been confirmed from structural analysis. An enhancement of the oxygen ion conductivity of the films up to five orders of magnitude is obtained compared to that of the bulk La2Mo2O9, suggesting usefulness of the thin films as electrolytes in micro-solid oxide fuel cells. The enhanced dc ionic conductivity of the thin films has been interpreted using the rule of the mixture model, while a power law model has been used to investigate the frequency and temperature dependences of the conductivity. The analysis of the results predicts the three-dimensional oxygen ion conduction in the thin films. [ABSTRACT FROM AUTHOR]

Transparent conducting oxides such as ZnO doped with Al or Mg are commonly used in solar cells, light emitting diodes, photodetectors, and ultraviolet laser diodes. In our work, we focus on a comparative study of the structural, optical, and electrical properties of ZnO films highly doped with Al (AZO) and Mg (MZO). These films are deposited on glass substrates by the sol-gel spin coating method. The doping concentrations for Al and Mg are fixed to 5%-30%. The XRD spectra indicate that all the samples are polycrystalline with hexagonal wurtzite structures, exhibiting a preferred orientation along the (002) plane. Low degradation in crystallinity was observed for MZO even at a Mg concentration of 30%. The MgO phase started to appear compared to Al-doped layers where smaller grains are formed inducing a deterioration in the films just after doping but no new phase appeared. This result is in agreement with other experimental results [J. K. Rath, Sol. Energy Mater. Sol. Cells 76, 431-487 (2003); Morris et al., J. Appl. Phys. 67, 1079-1087 (1990)]. By AFM analysis, the results indicate a significantly rough surface for MZO compared to AZO films. For equal Al and Mg dopant concentrations, we observe that the transmittance spectra of MZO thin films are wider than those of AZO, indicating a shift toward shorter wavelengths with an optical gap energy equal to 3.67 eV. The electrical measurements of AZO and MZO thin films were made using the I-V characteristic obtained by the four probe method. All the films present an ohmic behavior. The conductivity and the mobility of AZO films were found to be better than those of MZO. [ABSTRACT FROM AUTHOR]

Substantial efforts in theoretical and experimental studies have demonstrated that the dynamics in supercooled liquids is spatially heterogeneous. However, a complete description concerning the dynamic heterogeneity evolution from liquid to rigid glass is still lacking. Here, by a combining study of the dynamic and static mechanical responses, we quantify the characterization of dynamic heterogeneity and its temperature evolution spanning an unprecedented broad temperature range in metallic glass. We show that the dynamic heterogeneity persists from the warm liquid state into the rigid glassy state and becomes progressively pronounced with cooling, accompanied by increasing breadth of the relaxation rate dispersion. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is an attractive approach for the design and fabrication of structures capable of achieving controlled mechanical response of the underlying deformation mechanisms. While there are numerous examples illustrating how the quasi-static mechanical responses of polymer foams have been tailored by additive manufacturing, there is limited understanding of the response of these materials under shockwave compression. Dynamic compression experiments coupled with time-resolved X-ray imaging were performed to obtain insights into the in situ evolution of shockwave coupling to porous, periodic polymer foams. We further demonstrate shock wave modulation or "spatially graded-flow" in shock-driven experiments via the spatial control of layer symmetries afforded by additive manufacturing techniques at the micron scale. [ABSTRACT FROM AUTHOR]

A continuum model for calculating the time-dependent hydrogen pickup fractions in various Zirconium alloys under steam and pressured water oxidation has been developed in this study. Using only one fitting parameter, the effective hydrogen gas partial pressure at the oxide surface, a qualitative agreement is obtained between the predicted and previously measured hydrogen pickup fractions. The calculation results therefore demonstrate that H diffusion through the dense oxide layer plays an important role in the hydrogen pickup process. The limitations and possible improvement of the model are also discussed. [ABSTRACT FROM AUTHOR]

In this paper, different from Bragg scattering or local resonance mechanisms, a novel mechanism of an ultra-low-frequency broadband for flexural waves propagating in a one-dimensional elastic metamaterial beam with zero bending stiffness is proposed, which consists of periodic hinge-linked blocks. The dispersion relationship of this kind of metamaterial beam is derived and analyzed, from which we find that these hinge-linked blocks can produce the zero bending stiffness. Thus, the flexural waves within the metamaterial beam can be suppressed, and an ultra-low-frequency wide band-gap is formed in which the first branch is generated by the zero bending spring and the second branch by the negative velocity of the metamaterial beam. Numerical results show that the elastic metamaterial beams with zero bending stiffness can indeed generate an ultra-low-frequency wide band gap even starting from almost zero frequency, such as from 0Hz to 525 Hz in our structure. Therefore, the puzzle of realizing an ultra-low-frequency broadband of flexural waves may have been better solved, which could be applied in controlling ultra-low-frequency elastic waves in engineering. [ABSTRACT FROM AUTHOR]

This work is devoted to the investigation of the broad band dielectric spectra of 0.83PbMg1/3Nb2/3O3-0.17PbTiO3 (PMN-17PT) single crystals which have intermediate random fields. The necessity to understand the impact of random fields for the phase transition in heterogeneous perovskite oxides is of central importance. The thorough studies of dielectric properties revealed that the structural phase transition in PMN-17PT has a very complex dynamics. The temperature dependences of dielectric permittivity show that this material has features characteristic of polar nano regions and order-disorder phase transition dynamics. We speculate that this is the fingerprint of the phase transition in such heterogeneous perovskite oxides. [ABSTRACT FROM AUTHOR]