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Comparison of size-dependent characteristics of blue and green InGaN microLEDs down to 1a?? <b> <i>??</i> </b> m in diameter
摘要： There is growing interest in microLED devices with lateral dimensions between 1 and 10 lm. However, reductions in external quantum ef?ciency (EQE) due to increased nonradiative recombination at the surface become an issue at these sizes. Previous attempts to study size-dependent EQE trends have been limited to dimensions above 5 lm, partly due to fabrication challenges. Here, we present size-dependent EQE data for InGaN microLEDs down to 1 lm in diameter fabricated using a process that only utilizes standard semiconductor processing techniques (i.e., lithography and etching). Furthermore, differences in EQE trends for blue and green InGaN microLEDs are compared. Green wavelength devices prove to be less susceptible to reductions in ef?ciency with the decreasing size; consequently, green devices attain higher EQEs than blue devices below 10 lm despite lower internal quantum ef?ciencies in the bulk material. This is explained by smaller sur- face recombination velocities with the increasing indium content due to enhanced carrier localization.
关键词： InGaN,microLEDs,size-dependent characteristics,external quantum efficiency,blue and green wavelengths
更新于2025-09-23 15:19:57
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Selection of the magnetic quantum number in resonant ionization of neon using an XUV-IR two-color laser field
摘要： A recent experiment [Villeneuve et al. Science 356, 1150 (2017)] has shown that two-color photoionization of neon by the combination of an attosecond XUV pulse train and a moderately strong, linearly polarized IR pulse can preferentially produce photoelectrons with orbital angular quantum number l = 3 (f-wave) and magnetic quantum number m = 0. This result was rationalized by the occurrence of different Stark shifts of m = 0 and |m| = 1 sub-levels in the IR laser field. Here we perform 3D time-dependent Schrodinger equation calculations with a neon effective potential to identify the mechanism for the selective excitation and ionization of m = 0 sub-levels. Calculations of the ionization and excitation yields as a function of the IR intensity and the XUV and IR photon energy reveal that a coupling between two dominant ionization channels involving 3p and 3d intermediate excitations is responsible for the observed m-level selectivity. We compare calculated and measured photoelectron velocity map images and ionization yields over a range of IR intensities and XUV and IR photon energies, and confirm that the m = 0 or |m| = 1 channel, and thus a single set of the quantum numbers, (Jion, l, m), can be selected by an appropriate choice of these parameters.
关键词： photoelectron velocity map images,XUV-IR laser field,neon,time-dependent Schrodinger equation,two-color photoionization,magnetic quantum number,Stark shifts
更新于2025-09-23 15:19:57
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Theoretical study on nonlinear optical properties of N-(6-hydroxyhexyl)-5-nitroazophenyl carbazole
摘要： The first-, second- and third-order nonlinear optical properties of N-(6-hydroxyhexyl)-5-nitroazophenyl carbazole in gas phase employing sum-over-states (SOS) method have been calculated for the first time. The ground state molecular structure of N-(6-hydroxyhexyl)-5-nitroazophenyl carbazole was obtained by the geometrical optimizations based on the B3LYP/6-31+G(d) level. The energy of excited states and transition dipole moments between different excited states were obtained by using the time-dependent density functional theory (TDDFT) based on the CAM-B3LYP/Sadlej POL level. Charge transfer during electron excitation was analyzed based on hole and electron distributions. Our calculations showed that the N-(6-hydroxyhexyl)-5-nitroazophenyl carbazole has good nonlinear optical properties and its nonlinear optical properties arise from charge-transfer excitation and local excitation but charge transfer plays the leading role.
关键词： N-(6-hydroxyhexyl)-5-nitroazophenyl carbazole,the time-dependent density functional theory (TDDFT),charge transfer,sum-over-states (SOS) method,nonlinear optical properties
更新于2025-09-19 17:15:36
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Comparing blends and blocks: Synthesis of partially fluorinated diblock polythiophene copolymers to investigate the thermal stability of optical and morphological properties
摘要： The microstructure of the active blend layer has been shown to be a critically important factor in the performance of organic solar devices. Block copolymers provide a potentially interesting avenue for controlling this active layer microstructure in solar cell blends. Here we explore the impact of backbone fluorination in block copolymers of poly(3-octyl-4-fluorothiophene)s and poly(3-octylthiophene) (F-P3OT-b-P3OT). Two block co-polymers with varying block lengths were prepared via sequential monomer addition under Kumada catalyst transfer polymerisation (KCTP) conditions. We compare the behavior of the block copolymer to that of the corresponding homopolymer blends. In both types of system, we find the fluorinated segments tend to dominate the UV–visible absorption and molecular vibrational spectral features, as well as the thermal behavior. In the block copolymer case, non-fluorinated segments appear to slightly frustrate the aggregation of the more fluorinated block. However, in situ temperature dependent Raman spectroscopy shows that the intramolecular order is more thermally stable in the block copolymer than in the corresponding blend, suggesting that such materials may be interesting for enhanced thermal stability of organic photovoltaic active layers based on similar systems.
关键词： fluorination,conjugated block-copolymer synthesis,microphase stabilization,polythiophene,temperature-dependent Raman spectroscopy
更新于2025-09-19 17:15:36
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Zero-Forcing Aided Polarization Dependent Loss Elimination for Polarization Modulation Based Dual-Polarized Satellite Systems
摘要： To improve the robustness of the polarization modulation (PM) technique applied in dual-polarized satellite systems, a zero-forcing aided demodulation (ZFAD) method is proposed to eliminate the impairment to the PM from the depolarization effect (DE). The DE elimination is traditionally dependent on the pre-compensation method, which is based on the channel state information (CSI). While the distance between communication partners in satellite systems is so long that the CSI can not be always updated in time at the transmitter side. Therefore, the pre-compensation methods may not perform well. In the ZFAD method, the CSI is estimated at the receiver side and the zero forcing matrix is constructed to process the received signal before demodulating the PM signal. In this way, the DE is eliminated. In addition, we derive the received signal-to-noise ratio expression of the PC and ZFAD methods with the statistical channel model for a better comparison. Theoretical analysis and simulation results demonstrate the ZFAD method can eliminate the DE effect effectively and achieve a better symbol error rate performance than the pre-compensation method.
关键词： polarization modulation,dual-polarized satellite,polarization dependent loss effect
更新于2025-09-19 17:15:36
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Investigation on the photophysical properties of a series of promising phosphorescent iridium (III) complexes with modified cyclometalating ligands
摘要： Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were applied to investigate the electronic structures and photophysical properties of a series of phosphorescent iridium (III) complexes, [(C^N)2Ir(N^N)]+(PF6)(cid:2), in which C^N = 4-aryl-1-benzyl-1H-1, 2, 3-triazoles. Herein, aryl = phenyl, biphenyl, three-phenyl aromatic for complexes 1a, 2a, 3a (N^N = 2, 20-bipyridine) and aryl = 40, 60-di?uorophenyl, 60-?uoro-biphenyl, 60-?uoro-three-phenyl aromatic for complexes 1b, 2b, 3b (N^N = 4, 40-di-tert-butyl-2, 20-bipyridine), respectively. The geometric/electronic con?gurations, absorption/emission properties and phosphorescent performances have been outlined for each of the complexes. Based on the two simpli?cations presented in this paper and the available experimental data, the phosphorescent radiative rates for complexes 1a-3b were approximately obtained to be: 1.20 (cid:3) 106 s(cid:2)1, 1.68 (cid:3) 105 s(cid:2)1, 2.19 (cid:3) 107 s(cid:2)1, 3.85 (cid:3) 106 s(cid:2)1, 1.85 (cid:3) 107 s(cid:2)1 and 1.50 (cid:3) 107 s(cid:2)1, respectively. In view of the electroluminescent applications in OLED, our present research work is of great importance for the design and synthesis of organo-transition metal complexes with improved phosphorescent performances.
关键词： Phosphorescence,Photophysical properties,Time-dependent DFT,Iridium (III) complexes,Density functional theory (DFT)
更新于2025-09-19 17:15:36
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Electronic structure of CuWO<sub>4</sub>: dielectric-dependent, self-consistent hybrid functional study of a Mott-Hubbard type insulator
摘要： Copper tungstate, CuWO4, is a semiconducting oxide with interesting applications in photocatalysis. In this paper we present an accurate study of the electronic properties of stoichiometric and oxygen deficient CuWO4 based on a dielectric dependent hybrid density functional. In CuWO4 the Cu ions (Cu2+) are in a 3d9 configuration, so that the material must be classified as a magnetic insulator. Various magnetic configurations of CuWO4 have been considered, the most stable one being anti-ferromagnetic. The band structure, described in terms of density of states, exhibit the presence of a wide band dominated by W 5d states, separated by about 5 eV from the top of the valence band, consisting of O 2p states partly mixed with Cu 3d states. The empty component of the Cu 3d orbitals forms a narrow band 3.6 eV above the valence band maximum. The electronic structure emerging from the DOS curves and the Kohn-Sham energies is hard to reconcile with an experimental band gap of 2.1-2.3 eV. This gap can be rationalized within the Mott-Hubbard model of magnetic insulators, and has been computed from the total energies of the system with one electron removed from the O 2p band and one electron added to the Cu 3d states. Computing the charge transition levels for CuWO4, we come to a theoretical band gap of 2.1 eV, in excellent agreement with the experimental observations. We also studied the nature of the oxygen vacancy in CuWO4 with particular attention to the electron redistribution following the oxygen removal. The excess electrons, in fact, can occupy the localized 3d states of Cu or the localized 5d states of W. The resulting solution depends on various factors, including the concentration of oxygen vacancies.
关键词： CuWO4,Mott-Hubbard model,Dielectric-dependent hybrid functionals,semiconducting oxide,oxygen vacancy,density functional theory (DFT)
更新于2025-09-19 17:15:36
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Theory of coherent optical transients with quantized atomic motion
摘要： A theory of coherent transients is developed in which a sequence of optical pulses is incident on a sample of trapped atoms and gives rise to phase-matched emission from the sample. The trapping potential for the atoms can be state dependent, necessitating a quantum treatment of the center-of-mass motion. A source-field approach is followed, modified to account for the quantized motion of the atoms. The theory is illustrated with two examples, one involving the creation of ground-Rydberg level coherence in an optical lattice and the second Raman coherence between two ground-state sublevels of atoms in a dipole trap. For state-independent potentials, a comparison is made with a theory in which the center-of-mass motion is treated classically.
关键词： phase-matched emission,trapped atoms,quantized atomic motion,coherent optical transients,state-dependent potentials
更新于2025-09-19 17:15:36
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Facet-Dependent Optical Properties of Semiconductor Nanocrystals
摘要： Recent observations of facet-dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet-dependent surface plasmon resonance absorption of metal–Cu2O core–shell nanocrystals with tunable sizes and shapes is discussed. The Cu2O shells also exhibit a facet-specific optical absorption feature. The facet-dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu2O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu2O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu2O can be constructed to incorporate these optical effects. Literature also provides examples of facet-dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet-dependent. Some applications of semiconductor optical size and facet effects are considered.
关键词： surface band bending,semiconductors,facet-dependent properties,cuprous oxide,nanocrystals
更新于2025-09-19 17:15:36
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Speed-dependent line mixing in the ν3 band Q branch of methane
摘要： In 1992, we reported high-resolution spectra of the Q branch of the CH4 ν3 band from the Doppler limit to pressures of 500 Torr for various buffer gases recorded with a tunable difference-frequency laser. For P ≤ 100 Torr, the individual transitions were well enough resolved to be fit with simple Dicke-narrowed lineshapes, but at higher pressures, line mixing among the severely blended lines prevented further analysis. Subsequently, using a simultaneous multispectrum analysis, we were able to fit the full range of pressures quite satisfactorily with Dicke-narrowed first-order line-mixing profiles. We also tested scaled model relaxation W-matrix mixing and hypergeometric speed-dependent broadening, but found only minimal improvement not worth the computational complications and increased processing time. Since then, fast quadratic speed-dependence algorithms have been developed, and simplified W-matrix methods involving selected-coupled-doublets have been demonstrated, recently for this same Q branch. Therefore, we have reexamined these newer developments on our higher resolution spectra and found that the improved efficiency is tempered by a strong competition between Dicke narrowing and speed dependence and by a deteriorated fit using only selected-coupled-doublets. A detailed comparative study is reported here, along with some new models proposed and tested for truncating the W-matrix and for a hybrid combination of first-order mixing with selected W-matrix elements.
关键词： Dicke narrowing,Speed-dependent broadening,Non-Voigt effects,Line mixing,Self and foreign-gas-broadened methane
更新于2025-09-19 17:15:36