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Fiber coupling is difficult due to the uneven beam parameter product between the vertical and horizontal axes of semiconductor lasers. A beam shaping technique based on the combination of the internal total reflection and polarization surface of a stepped prism is proposed to achieve filling the dark area of the beam and polarization merging, as well as enabling the polarization plane to be multiplexed and the utilization rate of the polarization plane to be increased. The proposed technology can couple three groups of stacked array semiconductor lasers into a single fiber, in which the center diameter is 200 µm, and the numerical aperture is 0.22. The simulation results indicated that the output power of 1099 W and optical-optical conversion productivity of 85.8% were achieved.

We report an ultra-broadband metasurface perfect absorber from the UV to NIR region based on TiN nanostructures. A polarization-independent experimental average absorption of 0.900 (0.921 in simulation) at the wavelength band from 300 nm to 1500 nm is realized with only an 82 nm-thick TiN layer with TiO

Organic light-emitting field-effect transistors (OLEFETs) are regarded as an ideal device platform to achieve electrically pumped organic semiconductor lasers (OSLs). However, the incorporation of a high-quality resonator into OLEFETs is still challenging since the process usually induces irreparable deterioration to the electric-related emission performance of the device. We here propose a dual distributed Bragg reflector (DBR)-based planar microcavity, which is verified to be highly compatible with the OLEFETs. The dual DBR planar microcavity shows the great advantage of simultaneously promoting the quality (Q) factor and outcoupling efficiency of the device due to the reduced optical loss. As a result, a moderately high Q factor of ∼160, corresponding to EL spectrum linewidth as narrow as 3.2 nm, concomitantly with high outcoupling efficiency (∼7.1%) has been successfully obtained. Our results manifest that the dual DBR-based planar microcavity is a promising type of resonator, which might find potential applications in improving the spectra and efficiency performance of OLEFETs as well as in OLEFET-based electrically pumped OSLs.

In this article, with the objective of reducing the emission loss, enhancing the emission directionality and scale-scanning range, without increasing the complexity of the LIDAR system, a CMOS-compatible silicon nitride taper-tip double-layer auxiliary grating antenna is proposed and investigated systematically. Compared to conventional double layer grating, the directionality exceeds 72% for wavelengths from 1450 nm to 1600 nm, with a maximum directionality of 97.5% at 1550 nm. Therefore, the far-field divergence angle of 0.0081° × 0.03° and a large field of view of 108.45° × 23.43° can be achieved when the antenna aperture is 1.6 cm × 2.5 cm. These improvements come mainly from the increasing unidirectional emission performance of the phased array by adjusting the key structural parameters of the inverted taper-tip double-layer structure.

Objective: The aim of the study was to check on the reliability and validity of the translated version of Nociception Coma Scale–Revised. Design: Prospective psychometric study. Setting: Rehabilitation and neurology unit in hospital. Subjects: Patients with prolonged disorders of consciousness. Interventions: None. Main measures: The original English version of the Nociception Coma Scale–Revised was translated into Chinese. The reliability and validity were undertaken by trained raters. Intraclass correlation coefficients were used to assess inter-rater reliability and test–retest reliability. Cronbach’s alpha test was used to investigate internal consistency. Spearman’s correlation was used to calculate concurrent validity. The Coma Recovery Scale–revised was used to assess the consciousness of patients. Results: Eighty-four patients were enrolled in the study. Inter-rater reliability of the Chinese version of Nociception Coma Scale–Revised was high for total scores and motor and verbal subscores and good for facial subscores. Test–retest reliability was high for total score and for all subscores. Analysis revealed a moderate internal consistency for subscores. For the concurrent validity, a strong correlation was found between the Nociception Coma Scale–Revised and the Face, Legs, Activity, Cry, and Consolability behavioral scale for all patients. A moderate correlation was found between the Nociception Coma Scale–Revised and the Coma Recovery Scale–revised scores for all patients. Conclusion: The Chinese version of Nociception Coma Scale–Revised has good reliability and validity data for assessing responses to pain in patients with prolonged disorders of consciousness.

All-inorganic cesium lead halide perovskite nanostructure CsPbBr3 quantum dots have brought extensive attention in photoelectric applications. In this paper, CsPbBr3 quantum dots were synthesized with the diameter ranges from 8.7 to 11.8 nm by adjusting reaction temperature from 140 to 180 °C. In addition, for the monodisperse CsPbBr3 nanocrystals, it was found that radiative lifetime exists between 5 and 20 ns and PL decay lifetimes ranges from 4 to 11 ns, and their PL mechanism was studied thoroughly.

We investigate theoretically the coherent spin dynamics of gate control of quantum dot-based electron spin–orbit qubits subjected to a tilted magnetic field under electric-dipole spin resonance (EDSR). Our results reveal that Rabi oscillation of qubit states can be manipulated electrically based on rapid gate control of SOC strength. The Rabi frequency is strongly dependent on the gate-induced electric field, the strength and orientation of the applied magnetic field. There are two major EDSR mechanisms. One arises from electric field-induced spin–orbit hybridization, and the other arises from magnetic field-induced energy-level crossing. The SOC introduced by the gate-induced electric field allows AC electric fields to drive coherent Rabi oscillations between spin-up and -down states. After the crossing of the energy-levels with the magnetic field, the spin-transfer crossing results in Rabi oscillation irrespective of whether or not the external electric field is present. The spin–orbit qubit is transferred into the orbit qubit. Rabi oscillation is anisotropic and periodic with respect to the tilted and in-plane orientation of the magnetic field originating from the interplay of the SOC, orbital, and Zeeman effects. The strong electrically-controlled SOC strength suggests the possibility for scalable applications of gate-controllable spin–orbit qubits.

We theoretically investigate the effects of the unintentional background concentration, indium composition and defect density of intrinsic layer (i-layer) on the photovoltaic performance of InGaN p-i-n homojunction solar cells by solving the Poisson and steady-state continuity equations. The built-in electric field and carrier generation rate depend on the position within the i-layer. The collection efficiency, short circuit current density, open circuit voltage, fill factor, and conversion efficiency are found to depend strongly on the background concentration, thickness, indium composition, and defect density of the i-layer. With increasing the background concentration, the maximum thickness of field-bearing i-layer decreases, and the width of depletion region may become even too small to cover the whole i-layer, resulting in a serious decrease of the carrier collection. Some oscillations as a function of indium composition are found in the short circuit current density and conversion efficiency at high indium composition and low defect density due to the interference between the absorbance and the generation rate of carriers. The defect density degrades seriously the overall photovoltaic performance, and its effect on the photovoltaic performance is roughly seven orders of magnitude higher than the previously reported values [Feng et al., J. Appl. Phys. 108 (2010) 093118]. As a result, the high crystalline quality InGaN with high indium composition is a key factor in the device performance of III-nitride based solar cells.

The magnetic field dependence of the optical properties of CdSe nanocrystal quantum dots (NQDs) is investigated theoretically using a perturbation method within the effective-mass approximation. The results show that the magnetic field lifts the degeneracy of the electron (hole) states. A blue-shift in the absorption spectra of m ≥ 0 exciton states is observed while the absorption peak of m

Single-longitudinal-mode, gain-coupled distributed feedback (DFB) lasers based on high order surface gain-coupled gratings are achieved. Periodic surface metal p-contacts with insulated grooves realize gain-coupled mechanism. To enhance gain contrast in the quantum wells without the introduction of effective index-coupled effect, groove length and depth were well designed. Our devices provided a single longitudinal mode with the maximum CW output power up to 48.8 mW/facet at 971.31 nm at 250 mA without facet coating, 3dB linewidth ( 3.2 pm) and SMSR ( > 39 dB). Optical bistable characteristic was observed with a threshold current difference. Experimentally, devices with different cavity lengths were contrasted on power–current and spectrum characteristics. Due to easy fabrication technique and stable performance, it provides a method of fabricating practical gain-coupled distributed feedback lasers for commercial applications.

In this study, an InGaN laser diode (LD) with InGaN–GaN–InGaN quantum barriers was proposed and studied systematically. The energy band diagrams, stimulated recombination rate, optical field distribution, current distribution near the active region, and power–current–voltage performance curves were investigated. The simulation results suggest that the LD with InGaN–GaN–InGaN quantum barriers has better performance than the LD with conventional GaN and InGaN quantum barriers because of the properly adjusted refraction index profile and energy band diagrams, which are advantageous to both the suppressed leakage of the optical field and electrons out of and the enhanced injection of holes into the active region.

A low-cost regrowth-free narrow-strip single-longitudinal-mode diode laser, based on periodic anodes defined only by i-line lithography is proposed. With the periodic anodes separated by surface insulated grooves acting as a 44th-order gain grating, a high gain contrast is achieved without introducing effective index coupling. The CW output power of the device without facet coatings is up to 100.9 mW/facet at 980.63 nm in single-longitudinal-mode operation with a single facet having a power conversion efficiency of up to 18.6% and a slope efficiency of 0.53 W/A. The highest side-mode suppression ratio is 45 dB with a linewidth below 889 MHz, at 350 mA. The beam parameter product, M2, in the slow axis is 1.85, achieving a near diffraction-limited beam quality. A novel method for easy surface gratings fabrication is first presented, wherein, 4-μm-width periodic surface grooves, acting as waveguide ridges, are defined once by i-line lithography only without regrowth, nanoscale gratings fabrication, or facet coating. Owing to the low-cost fabrication methods and good performances, our single-longitudinal-mode devices can be used for widespread practical applications.

In order to improve the reliability of InGaN light-emitting diodes (LEDs), the size-dependence of V-shaped pits on the electric and luminous properties was investigated. By adjusting the thickness of the barrier layer of the stress buffer layer, thereby increasing the size of the V-pits, the electro-static discharge (ESD) yield of the InGaN LED can be increased from 0 % to 98 % at negative 4000 V. At the same time, the aging attenuation of the device is significantly reduced. The results show that increasing the size of V-pits is beneficial to the reliability of InGaN LEDs. As the V-pits provide an additional current channel, which effectively reduces the current distribution density of the device's C-plane quantum well (light emitting region) under the impact of large current, thus avoiding the thermal breakdown effect caused by the excessive local current, and the ESD yield of the device is significantly increased. At the operating voltage, the difference of the band gap generated by the V-pits is a barrier to the carriers near the dislocations, which effectively avoids the non-radiation recombination of the carriers on the dislocation, resulting in a reduction in the aging attenuation.

We report computational and experimental studies (i) on the Fermi level (EF) pinning phenomena in weakly interacting electrode-organic semiconductor systems with pentacene, N, N-Di(naphthalene-l-yl)-N, N’-diphenyl-benzidine (α-NPD) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)], and (ii) to elucidate a common reason of the pinning phenomena which have been widely observed for various organic materials on inert electrode surfaces. For (i), the computed results on the electrode-dependence of EF-HOMO and EF-LUMO distance agreed excellently with UPS and Kelvin-probe results. For (ii) we found theoretically that the pinning phenomena occur at systems even without any electronic states in the HOMO-LUMO gap, and thereby indicate that this is a universal Fermi-level pinning-like phenomena for various band gap materials, even if specific interface states do not exist upon contact. We further obtained for Gaussian-distributed HOMO and LUMO that the minimal hole and electron injection barriers are quantitatively determined by degree of standard deviations of HOMO and LUMO bands, which are in excellent agreement with the experimental results. Furthermore, our results provide a guideline for approaching the “zero” injection barriers.

The role of transition metal oxides (TMOs) based intermediate connectors in tandem organic light emitting diodes (OLEDs) has been studied via capacitance-voltage and current-voltage characteristics, in order to elucidate the dynamic processes of charges generation and transport within externally applied voltages. The TMO-based intermediate connectors are composed of molybdenum trioxide (MoO3) and cesium azide (CsN3)-doped-4, 7–diphenyl-1, 10-phenanthroline (BPhen) layers, where MoO3 and CsN3 are used due to low deposition temperatures. From the obtained results of capacitance and current density, charges generation in CsN3:BPhen/MoO3/NPB is proposed to the defect states in thermally evaporated MoO3, which offers a minimal energy offset for charges generation. Moreover, our results clearly indicate that charges generation efficiency is not only relying on the MoO3-NPB interface, but also influenced by CsN3:BPhen-MoO3 interface. CsN3 doped BPhen layer further improves charges separation efficiency, which finally results in favorable charges transport into the adjacent layers and ensure to function efficiently for tandem OLEDs.

The comprehensive analyses of exciton states and absorption spectra from freestanding monolayer transition metal dichalcogenides such as ${\mathrm{MoS}}_{2}, {\mathrm{MoSe}}_{2}, {\mathrm{MoTe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ are theoretically presented in the Wannier\ensuremath{-}Mott exciton theory. We propose an efficient and simple exact diagonalization method based on a variationally optimized two-dimensional (2D) modified Laguerre basis set, which has been successfully applied to calculate both the ground and excited states. The convergence of the method as a function of the basis set size was analyzed in detail. The modified Laguerre basis functions were very closely concentrated in space, and they led to a rather rapid convergence. Moreover, the convergence rate was faster under variational optimization. Our results regarding the exciton binding energies of a ground state with even a small number of basis states are in excellent agreement with those of the density-function theory calculations in the literature [I. Kyl\"anp\"a\"a and H.-P. Komsa, Phys. Rev. B 92, 205418 (2015)]. The exciton states appeared in an anomalous energy-level ordering of the orbital angular momentum caused by the dielectric screening effect. The exciton absorption spectra exhibited strong absorption lines of the nonhydrogenic exciton Rydberg series, which was dominated by the s-type exciton states. There were two prominent low-energy peaks of the A and B excitons, and the A exciton was slightly higher in intensity than the B exciton, which is in good agreement with the experimental observations. In general, our presented exact diagonalization method based on a variationally optimized 2D modified Laguerre basis set has proven to be applicable to more general cases of screened potentials in 2D materials and atomic physics, as well as providing a simple approach for investigating exciton properties.

We report on an ultra-broadband metasurface perfect absorber in the visible and NIR region based on the triple Mie resonances generated from the three sets of nanopillars grown on SiO2 substrate. The polarization-independent and wide-angular-insensitive (up to 70°) high absorption is realized within a surface layer that is only 284 nm thick. Double-beam UV interference lithography and followed sputter coating depositions are utilized to efficiently fabricate the proposed metasurface structures. Experimental results demonstrate that the average absorption is 0.961 (0.970 in simulation) at the wavelength band ranging from 400 nm to 1800 nm, and works well even the incident angle is large as 70°. The demonstrated device suggests an efficient way of designing and fabricating ultra-broadband perfect absorber with metasurface structures, which has huge application prospects in advanced nanophotonic devices and integrated photonic systems.

Hierarchical SnO 2 @graphene nanocomposites were synthesized by impregnating different weight percentages of Sn 2+ with graphene oxide nanosheets using a simple hydrothermal method. The precursor ratio of Sn 2+ to graphene oxide plays a decisive role in tailoring the phase structure and composition, which thereby influences the photocatalytic performance to degrade the organic contaminants. The SnO 2 @graphene nanocomposites exhibited greatly enhanced abilities for photocatalytic degradation of MO dye compare with the pure SnO 2 . It is believed that the tight heterojunction structure and efficient charge separation play important roles in facilitating interfacial electron transfer and reducing self-radiative recombination of charges, which accordingly enhance the photocatalytic performance.

In this paper, we proposed and numerically investigated InGaN/GaN light-emitting diodes (LEDs) with a last barrier of GaN-InGaN. Through simulations, we investigated their voltage–current curves, performance curves, carrier concentrations, radiative recombination rates, and energy band diagrams. The simulation results show that the InGaN/GaN LEDs with the GaN-InGaN last barrier have lower turn-on voltage, higher output light power, and better efficiency droop than LEDs with the conventional GaN last barrier. This improvement comes from the appropriately modified energy band diagram, which aids in electron confinement and hole injection, as well as in uniformly distributing these carriers in the quantum wells.

To obtain the deviation between the actual and designed thickness of distributed Bragg reflectors (DBRs) and resonant cavities in vertical cavity surface emitting lasers (VCSELs), the factors that affect the Fabry–Perot dip (F–P dip) shift were analyzed in detail. A basic VCSEL structure was designed and grown as a reference, and the thicknesses of the DBRs and the resonant cavity were varied. The near-infrared reflection spectra of VCSELs were measured. The results reveal that the F–P dip is influenced not only by the thickness of the resonant cavity, but also by the thickness of p-/n-DBRs. Additionally, when the thickness of either a p- or n-DBR is increased while that of the other component is decreased, the main differences in the characteristics can be observed. Finally, an effective method is proposed to obtain the layer thickness deviation by analyzing the change in the F–P dip and the central wavelength in photoluminescence near-infrared reflection spectra.

The highly magnesium (Mg)-doped wide-bandgap AlGaN electron blocking layer (EBL) is widely used in InGaN/GaN light-emitting diodes (LEDs) for electron leakage suppression. However, the EBL also reduces the hole injection efficiency and high Mg doping in AlGaN is a challenging. In this work, InGaN/GaN LEDs with a graded-composition AlGaN last quantum barrier and without the conventional AlGaN electron blocking layer were proposed and investigated numerically. Using the proposed structure, the optical output power and the efficiency droop are improved significantly when compared with the corresponding properties of conventional LEDs. The simulation results indicate that these improvements can be attributed to increments in both the electron confinement and the hole injection efficiency because of the appropriate energy band structure design of the LEDs.

The GaN-based light-emitting diodes (LEDs) with hemisphere patterned sapphire substrate (PSS) have been investigated numerically using a combined method of ray tracing and finite-difference time-domain techniques based on Poynting vector. Our method has been verified using unpatterned sapphire substrate for which analytical formulas exist. The simulated results show that the hemisphere PSS can improve the light extraction efficiency by increasing the reflection angle, decreasing the transmission angle, enhancing the reflectance of light incident on it, and enlarging the escape cone at its interface. Moreover, it is found that the hemisphere pattern size has significant influence on the improvement of the LED emission efficiency. The optimal radius of the hemisphere is about the emission wavelength of GaN-based LEDs, which can be attributed to the strong effect of wave properties of light at the PSS interface.

Blue InGaN light-emitting diodes with a GaN-AlGaN-GaN last barrier (GAGLB) and without an AlGaN electron blocking layer (EBL) are investigated numerically. When the conventional last GaN barrier is replaced by a GAGLB and the most commonly used AlGaN EBL is removed, the forward voltage is reduced from 3.39 to 2.98 V, and the efficiency droop is improved from 30.1% to 14.2%. Simulation results indicate that these improvements can be attributed to the effective barrier height increase for electron confinement and to hole injection efficiency enhancement due to tunneling effect in the GAGLB.

In order to study the distribution of sea ice in the Arctic Channel, sea ice along the Arctic Channel was extracted based on MODIS remote sensing data. Supervised classification method is used to establish a noise removal method for short time series to obtain the sea ice distribution boundary. From this, the navigability of the Arctic waterway is analyzed. The results show that although MODIS data contain more noise effects, more accurate sea ice distribution information can be obtained by effective noise removal methods, which provides support for the judgment of navigation capacity of Arctic waterway.

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) with various kinds of electron blocking layers (EBLs) are analyzed numerically. The results indicate that an enhanced hole injection efficiency and a reduced electron leakage could be achieved with the GaN-based LED where a p-InAlGaN/GaN superlattice (SL) was employed as EBL as compared with the conventional GaN-based LEDs using rectangular p-AlGaN EBL or p-AlGaN/GaN SL EBL. Moreover, it was found that the efficiency droop could be significantly improved at high injection current density for GaN-based LEDs with p-InAlGaN/GaN SL EBL.

Thermally evaporated molybdenum trioxide (MoO3) doped C60 films, which could change n type features of pristine C60 to form a p type mixed C60 layer, are investigated by x-ray and ultraviolet photoelectron spectroscopy. It is found that C60 HOMO progressively shifts closer to the Fermi level after increased MoO3 doping concentration, and final onset of C60 HOMO is pinned at binding energy of 0.20 eV, indicating the formation of p type C60 films. It is proposed that in charge transfer induced p type C60 formation, due to large electron affinity of MoO3 (6.37 eV), electrons from HOMO of C60 could easily transfer to MoO3 to form cations and therefore increase hole concentration, which could gradually push C60 HOMO to the Fermi level and finally form p type C60 films. Moreover, clear different types of C60 species have been confirmed from UPS spectra in highly doped films.

Acetonitrile confined in silica nanopores with surfaces of varying functionality is studied by means of molecular dynamics simulation. The hydrogen-bonding interaction between the surface and the liquid is parametrized by means of first-principles molecular dynamics simulations. It is found that acetonitrile orders into bilayer like structures near the surface, in agreement with prior simulations and experiments. A newly developed method is applied to calculate relevant time correlation functions for molecules in different layers of the pore. This method takes into account the short lifetimes of the molecules in the layers. We compare this method with prior techniques that do not take this lifetime into account and discuss their pitfalls. We show that in agreement with experiment, the dynamics of the system may be described by a two population model that accounts for bulk-like relaxation in the center and frustrated dynamics near the surface of the pore. Specific hydrogen-bonding interactions are found to...

In this study, we discussed the validity of an inorganic semiconductor-based model on energy level alignment at weakly interacting substrate-organic film interfaces, in which the influence of Gaussian distributed density of states on injection barrier based on the thermal equilibrium condition after contacted with substrates was quantitatively investigated. A “Z”-like shape of the energy level alignment has been quantitatively obtained: a slope “S = 1” is clearly given, when the substrate work function is located approximately in between the threshold of ionization energy (IEth) and electron affinity (EAth); on the other hand, “S ∼ 0” is given while the substrate work function is lower than EAth or larger than IEth. Although different calculated results compared with experimental data can also be found, in which the calculated minimal injection barrier is ∼0.2 eV smaller than experimental data, this simplified practical method can be easily applied to calculate the device-related electrostatic potential inside dielectric films.

In this study, an InGaN lighting-emitting diode (LED) containing GaN/AlGaN/GaN triangular barriers is proposed and investigated numerically. The simulation results of output performance, carrier concentration, and radiative recombination rate indicate that the proposed LED has a higher output power and an internal quantum efficiency, and a lower efficiency droop than the LED containing conventional GaN or AlGaN barriers. These improvements mainly arise from the modified energy bands, which is evidenced by analyzing the LED energy band diagram and electrostatic field near the active region. The modified energy bands effectively improve carrier injection and confinement, which significantly reduces electron leakage and increases the rate of radiative recombination in the quantum wells.

In this study, with the objective of lowering the polarization without degrading the carrier confinement in the active region, an InGaN light-emitting diode with GaN/InGaN/GaN triangular (GIGT) quantum barriers (QBs) was proposed and studied systematically. When traditional GaN QBs were replaced by the GIGT QBs, the output power at 150 mA rose from 82.05 to 155.99 mW, and the efficiency droop was decreased from 51.0% to 16.5%. Simulation results indicated that these improvements could result from the effectively suppressed polarization field in the quantum wells and the markedly enhanced carrier confinement in the active region because of the appropriately modified energy band structure.

Because the polarization effect influences the distribution of the carriers in the multiple quantum wells of the light-emitting diodes (LEDs), the light-emitting efficiency is also affected. The influence of the polarization effect on GaN-based LEDs’ performance is simulated. By simulating four different types of electrode shapes, it’s found that the electrode shape influences not only the photoelectric characteristics but also the optical absorption by the semiconductor. Through the optimization of the electrode shape, the I–V characteristic is improved, and the series resistance is lowered. The optical absorption by the semiconductor is decreased and then the heat generated in the LEDs is lowered. As a result, both the photoelectric conversion efficiency and the stability are improved.

InGaN/GaN light-emitting diodes (LEDs) with graded-composition multiple quantum wells (GQWs) were designed and theoretically investigated. In the GQW LED structure, the indium composition in In x Ga1−x N QW was decreased sequentially in increments, $$\Delta x$$ , along the [0001] orientation. Across the composition range, the GQW LEDs have high carrier injection efficiencies and enhanced uniform carrier distributions in the multiple quantum wells because of the band structure. When $$\Delta x = 0.01$$ , the light output power the GQW LED was enhanced by 4.2 % compared with that of the original LED, which corresponds to an increase of IQE of 4.5 % at 180 mA. However, when $$\Delta x$$ increased to 0.02, the light output power of the GQW LEDs was reduced by 3.1 % compared with that of original LED, which corresponds to a decrease of IQE of 3.3 % at 180 mA. Results from the simulations indicated that the device performance was not only related to the carrier injection efficiency and the carrier distribution, but was also related to the balance of electrons and holes in the QWs.

The role of many-body effects in modeling silica was investigated using self-consistent force matching. Both pairwise and polarizable classical force fields were developed systematically from ab initio density functional theory force calculations, allowing for a direct comparison of the role of polarization in silica. It was observed that the pairwise potential performed remarkably well at reproducing the basic silica tetrahedral structure. However, the Si-O-Si angle that links the silica tetrahedra showed small but distinct differences with the polarizable potential, a result of the inability of the pairwise potential to properly account for variations in the polarization of the oxygens. Furthermore, the transferability of the polarizable potential was investigated and suggests that additional forces may be necessary to more completely describe silica annealing.

GaN-based light emitting diodes (LEDs) with graded-thickness quantum barriers (GQB) is studied. Simulation results show that GaN-based LEDs with GQB has better performance than conventional GaN-based LEDs with uniform thickness barriers, the major physical cause is attributed to the superior hole distribution in the quantum wells due to the appropriate energy band diagram.

The size- and dimensionality-dependence of excitonic effects and related properties in semiconductor nanostructures are theoretically studied in detail within the effective-mass approximation. When nanostructure sizes become smaller than the bulk exciton Bohr radius, excitonic effects are significantly enhanced with reducing size or dimensionality. This is as a result of quantum confinement in more directions leading to larger exciton binding energies and normalized exciton oscillator strengths. These excitonic effects originate from electron–hole Coulombic interactions, which strongly enhance the oscillator strength between the electron and hole. It is also established that the universal scaling of exciton binding energy versus the inverse of the exciton Bohr radius follows a linear scaling law. Herein, we propose a stretched exponential law for the size scaling of optical gap, which is in good agreement with the calculated data. Due to differences in the confinement dimensionality, the radiative lifetime of low-dimensional excitons becomes shorter than that of bulk excitons. The size dependence of the exciton radiative lifetimes is in good agreement with available experimental data. This strongly enhanced electron–hole exchange interaction is expected in low-dimensional structures due to enriched excitonic effects. The main difference in nanostructures compared to the bulk can be interpreted in terms of the enhanced excitonic effects induced by exciton localization. The enhanced excitonic effects are expected to be of importance in developing stable and high-efficiency nanoscale excitonic optoelectronic devices.