Bioinspired Synaptic Branched Network within Quasi-Solid Polymer Electrolyte for High-Performance Microsupercapacitors.

The branched network-driven ion solvating quasi-solid polymer electrolytes (QSPEs) are prepared via one-step photochemical reaction. A poly(ethylene glycol diacrylate) (PEGDA) is combined with an ion-conducting solvate ionic liquid (SIL), where tetraglyme (TEGDME), which acts like interneuron in the human brain and creates branching network points, is mixed with EMIM-NTf2 and Li-NTf2. The QSPE exhibits a unique gyrified morphology, inspired by the cortical surface of human brain, and features well-refined nano-scale ion channels. This human-mimicking method offers excellent ion transport capabilities through a synaptic branched network with high ionic conductivity (σDC ≈ 1.8 mS cm-1 at 298 K), high dielectric constant (εs ≈ 125 at 298 K), and strong ion solvation ability, in addition to superior mechanical flexibility. Furthermore, the interdigitated microsupercapacitors (MSCs) based on the QSPE present excellent electrochemical performance of high energy (E  =  5.37 µWh cm-2) and power density (P  =  2.2 mW cm-2), long-term cycle stability (≈94% retention after 48 000 cycles), and mechanical stability (>94% retention after continuous bending and compressing deformation). Moreover, these MSC devices have flame-retarding properties and operate effectively in air and water across a wide temperature range (275 to 370 K), offering a promising foundation for high-performance, stable next-generation all-solid-state energy storage devices.

Partial Edge Dislocations Comprised of Metallic Ga Bonds in Heteroepitaxial GaN.

We investigated the atomic structure of inclined threading edge dislocation (TED) typically observed in GaN grown on Si(111) through (scanning) transmission electron microscopy. Atomic observations verified that the inclined TED consisted of two partial dislocations. These results imply that the inclined TED possesses a Ga-Ga atomic configuration that is energetically unfavorable. However, the introduction of such structures is considered unavoidable because the TEDs should climb regularly to mediate the applied stress or the increasing surface due to the buffer layer. This Ga-Ga configuration is highly likely to form metallic bonds and appears to be the primary reason for the inferior efficacy of a GaN light-emitting diode grown on Si(111).

Dimension of discrete variable representation for mixed quantum/classical computation of three lowest vibrational states of OH stretching in liquid water.

Three low-lying vibrational states of molecular systems are responsible for the signals of linear and third-order nonlinear vibrational spectroscopies. Theoretical studies based on mixed quantum/classical calculations provide a powerful way to analyze those experiments. A statistically meaningful result can be obtained from the calculations by solving the vibrational Schrödinger equation over many numbers of molecular configurations. The discrete variable representation (DVR) method is a useful technique to calculate vibrational eigenstates subject to an arbitrary anharmonic potential surface. Considering the large number of molecular configurations over which the DVR calculations are repeated, the calculations are desired to be optimized in balance between the cost and accuracy. We determine a dimension of the DVR method which appears to be optimum for the calculations of the three states of molecular vibrations with anharmonic strengths often found in realistic molecular systems. We apply the numerical technique to calculate the local OH stretching frequencies of liquid water, which are well known to be widely distributed due to the inhomogeneity in molecular configuration, and found that the frequencies of the 0-1 and 1-2 transitions are highly correlated. An empirical relation between the two frequencies is suggested and compared with the experimental data of nonlinear IR spectroscopies.

In-vitro cytotoxicity assessment of carbon-nanodot-conjugated Fe-aminoclay (CD-FeAC) and its bio-imaging applications.

We have investigated the cytotoxic assay of Fe-aminoclay (FeAC) nanoparticles (NPs) and simultaneous imaging in HeLa cells by photoluminescent carbon nanodots (CD) conjugation. Non-cytotoxic, photostable, and CD NPs are conjugated with cationic FeAC NPs where CD NPs play a role in bio-imaging and FeAC NPs act as a substrate for CD conjugation and help to uptake of NPs into cancer cells due to positively charged surface of FeAC NPs in physiological media. As increase of CD-FeAC NPs loading in HeLa cell in vitro, it showed slight cytotoxicity at 1000 µg/mL but no cytotoxicity for normal cells up to concentration of 1000 µg/mL confirmed by two 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and neutral red (NR) assays, with further observations by 4',6-diamidino-2-phenylindole (DAPI) stained confocal microscopy images, possessing that CD-FeAC NPs can be used as potential drug delivery platforms in cancer cells with simultaneous imaging. Graphical abstract CD conjugation with organo-building blocks of delaminated FeAC NPs.

Effects of excluded volume and correlated molecular orientations on Förster resonance energy transfer in liquid water.

Förster theory for the survival probability of excited chromophores is generalized to include the effects of excluded volume and orientation correlation in the molecular distribution. An analytical expression for survival probability was derived and written in terms of a few simple elementary functions. Because of the excluded volume, the survival probability exhibits exponential decay at early times and stretched exponential decay at later times. Experimental schemes to determine the size of the molecular excluded volume are suggested. With the present generalization of theory, we analyzed vibrational resonance energy transfer kinetics in neat water. Excluded volume effects prove to be important and slow down the kinetics at early times. The majority of intermolecular resonance energy transfer was found to occur with exponential kinetics, as opposed to the stretched exponential behavior predicted by Förster theory. Quantum yields of intra-molecular vibrational relaxation, intra-, and intermolecular energy transfer were calculated to be 0.413, 0.167, and 0.420, respectively.

Spatial distribution of dislocations in relation to a substructure in high-quality GaN film.

The dislocation distribution of high-quality single-crystal gallium nitride (GaN) films grown by the hybrid vapor phase epitaxy was analyzed. This study examined the domain structure of GaN from the dislocation distribution on the macroscale by optical microscopy. The surface structure of GaN consisted of domains with microcolumns as the substructure. The inner domains contained a lower density of dislocations but a large number of these dislocations were observed along the domain boundaries. The existence of a domain boundary structure doubly increased the total dislocation density.

Meso-scale transmission electron microscope tomography applied for wax distribution in toner particles.

Distribution of wax in laser printer toner was observed using an ultra-high-voltage (UHV) and a medium-voltage transmission electron microscope (TEM). As the radius of the wax spans a hundred to greater than a thousand nanometers, its three-dimensional recognition via TEM requires large depth of focus (DOF) for a volumetric specimen. A tomogram with a series of the captured images would allow the determination of their spatial distribution. In this study, bright-field (BF) images acquired with UHV-TEM at a high tilt angle prevented the construction of the tomogram. Conversely, the Z-contrast images acquired by the medium-voltage TEM produced a successful tomogram. The spatial resolution for both is discussed, illustrating that the image degradation was primarily caused by beam divergence of the Z-contrast image and the combination of DOF and chromatic aberration of the BF image from the UHV-TEM.

Environmentally Stable and Reconfigurable Ultralow-Power Two-Dimensional Tellurene Synaptic Transistor for Neuromorphic Edge Computing.

While neuromorphic computing can define a new era for next-generation computing architecture, the introduction of an efficient synaptic transistor for neuromorphic edge computing still remains a challenge. Here, we envision an atomically thin 2D Te synaptic device capable of achieving a desirable neuromorphic edge computing design. The hydrothermally grown 2D Te nanosheet synaptic transistor apparently mimicked the biological synaptic nature, exhibiting 100 effective multilevel states, a low power consumption of ∼110 fJ, excellent linearity, and short-/long-term plasticity. Furthermore, the 2D Te synaptic device achieved reconfigurable MNIST recognition accuracy characteristics of 88.2%, even after harmful detergent environment infection. We believe that this work serves as a guide for developing futuristic neuromorphic edge computing.

Reinforcing Synaptic Plasticity of Defect-Tolerant States in Alloyed 2D Artificial Transistors.

While two-dimensional (2D) materials possess the desirable future of neuromorphic computing platforms, unstable charging and de-trapping processes, which are inherited from uncontrollable states, such as the interface trap between nanocrystals and dielectric layers, can deteriorate the synaptic plasticity in field-effect transistors. Here, we report a facile and effective strategy to promote artificial synaptic devices by providing physical doping in 2D transition-metal dichalcogenide nanomaterials. Our experiments demonstrate that the introduction of niobium (Nb) into 2D WSe2 nanomaterials produces charge trap levels in the band gap and retards the decay of the trapped charges, thereby accelerating the artificial synaptic plasticity by encouraging improved short-/long-term plasticity, increased multilevel states, lower power consumption, and better symmetry and asymmetry ratios. Density functional theory calculations also proved that the addition of Nb to 2D WSe2 generates defect tolerance levels, thereby governing the charging and de-trapping mechanisms of the synaptic devices. Physically doped electronic synapses are expected to be a promising strategy for the development of bioinspired artificial electronic devices.

Time-averaging approximation in the interaction picture: anisotropy of vibrational pump-probe experiments for coupled chromophores with application to liquid water.

A time-averaging approximation method developed to efficiently calculate the short-time dynamics of coupled vibrational chromophores using mixed quantum/classical theories is extended in order to be applicable to the study of vibrational dynamics at longer time scales. A quantum mechanical time propagator for long times is decomposed into the product of short-time propagators, and a time-averaging approximation is then applied to each of the latter. Using the extended time-averaging approximation, we calculate the anisotropy decay of the data obtained from impulsive vibrational pump-probe experiments on the OH stretching modes of water, which is in excellent agreement with numerically exact results.

Visualizing Line Defects in non-van der Waals Bi2O2Se Using Raman Spectroscopy.

Atomic-layered materials, such as high-quality bismuth oxychalcogenides, which are composed of oppositely charged alternate layers grown using chemical vapor deposition, have attracted considerable attention. Their physical properties are well-suited for high-speed, low-power-consumption optoelectronic devices, and the rapid determination of their crystallographic characteristics is crucial for scalability and integration. In this study, we introduce how the crystallographic structure and quality of such materials can be projected through Raman spectroscopy analysis. Frequency modes at ∼55, ∼78, ∼360, and ∼434 cm-1 were detected, bearing out theoretical calculations from the literature. The low-frequency modes positioned at 55 and 78 cm-1 were activated by structural defects, such as grain boundaries and O-rich edges in the Bi2O2Se crystals, accompanied by sensitivity to the excitation energy. Furthermore, the line defects at ∼55 cm-1 exhibited a strong 2-fold polarization dependence, similar to graphene/graphite edges. Our results can help illuminate the mechanism for activating the Raman-active mode from the infrared active mode by defects, as well as the electronic structures of these two-dimensional layered materials. We also suggest that the nanoscale width line defects in Bi2O2Se can be visualized using Raman spectroscopy.

Time-averaging approximation in the interaction picture: absorption line shapes for coupled chromophores with application to liquid water.

The time-averaging approximation (TAA), originally developed to calculate vibrational line shapes for coupled chromophores using mixed quantum/classical methods, is reformulated. In the original version of the theory, time averaging was performed for the full one-exciton Hamiltonian, while herein the time averaging is performed on the coupling (off-diagonal) Hamiltonian in the interaction picture. As a result, the influence of the dynamic fluctuations of the transition energies is more accurately described. We compare numerical results of the two versions of the TAA with numerically exact results for the vibrational absorption line shape of the OH stretching modes in neat water. It is shown that the TAA in the interaction picture yields theoretical line shapes that are in better agreement with exact results.

Vibrational energy transfer and anisotropy decay in liquid water: is the Förster model valid?

Ultrafast pump-probe anisotropy experiments have been performed on liquid H(2)O and D(2)O. In both cases, the anisotropy decay is extremely fast (on the order of 100 or 200 fs) and is presumed due to resonant vibrational energy transfer. The experiments have been interpreted in terms of the Förster theory, wherein the rate constant for intermolecular hopping transport is proportional to the inverse sixth power of the distance between the vibrational chromophores. In particular, the anisotropy decay is assumed to be simply related to the survival probability as calculated with the Förster theory. While the theory fits the data well, and is a reasonable model for these systems, there are several assumptions in the theory that might be suspect for water. Using our mixed quantum/classical model for vibrational spectroscopy and dynamics in liquid water, which agrees well with anisotropy decay experiments on the pure liquids as well as H(2)O/D(2)O mixtures, we critically analyze both the survival probability and anisotropy decay, in order to assess the applicability of the Förster theory.

Signatures of coherent vibrational energy transfer in IR and Raman line shapes for liquid water.

We calculate theoretical IR and Raman line shapes for the OH stretch region of liquid water, using mixed quantum/classical and electronic-structure/molecular-dynamics methods. Our approach improves upon the time-averaging approximation used earlier for the same problem, and our results are in excellent agreement with experiment. Previous analysis of theoretical results for this problem considered the extent of delocalization (over local OH stretch excitations) of the instantaneous vibrational eigenstates. In this work we present a complementary analysis in the time-domain, by decomposing the appropriate response functions into diagonal and off-diagonal contributions (in the local mode basis). Our analysis indicates that all vibrational spectra show signatures of coherent vibrational energy transfer. This is manifest in different (IR, isotropic and depolarized Raman) experiments to different extents, because of the competition between coherent energy transfer and rotational disorder.

Two-dimensional infrared spectroscopy and ultrafast anisotropy decay of water.

We introduce a sparse-matrix algorithm that allows for the simulation of two-dimensional infrared (2DIR) spectra in systems with many coupled chromophores. We apply the method to bulk water, and our results are based on the recently developed ab initio maps for the vibrational Hamiltonian. Qualitative agreement between theory and experiment is found for the 2DIR spectra without the use of any fitting or scaling parameters in the Hamiltonian. The calculated spectra for bulk water are not so different from those for HOD in D(2)O, which we can understand by considering the spectral diffusion time-correlation functions in both cases. We also calculate the ultrafast anisotropy decay, which is dominated by population transfer, finding very good agreement with experiment. Finally, we determine the vibrational excitation diffusion rate, which is more than two orders of magnitude faster than the diffusion of the water molecules themselves.

On the calculation of rotational anisotropy decay, as measured by ultrafast polarization-resolved vibrational pump-probe experiments.

Polarization-resolved vibrational pump-probe experiments are useful for measuring the dynamics of molecular reorientation. The rotational anisotropy observable is usually modeled by the second-Legendre-polynomial time-correlation function of the appropriate molecule-fixed unit vector. On the other hand, more elaborate calculations that include non-Condon effects, excited-state absorption, and spectral diffusion, can be performed using the infrastructure of the nonlinear response formalism. In this paper we present "exact" (within the impulsive limit) results from the nonlinear response formalism, and also a series of approximations that ultimately recover the traditional result mentioned above. To ascertain the importance of these effects not included in the traditional approach, we consider the specific case of dilute HOD in H(2)O. We find that for the frequency-integrated anisotropy decay, it is important to include non-Condon effects. For the frequency-resolved anisotropy decay, non-Condon effects, excited-state absorption, and spectral diffusion are all important. We compare our results with recent experiments.

Highly Efficient Excitonic Recombination of Non-polar ([Formula: see text]) GaN Nanocrystals for Visible Light Emitter by Hydride Vapour Phase Epitaxy.

While non-polar nanostructured-GaN crystals are considered as a prospective material for the realization of futuristic opto-electronic application, the formation of non-polar GaN nanocrystals (NCs) with highly efficient visible emission characteristics remain unquestionable up to now. Here, we report the oxygen-incorporated a-plane GaN NCs with highly visible illumination excitonic recombination characteristics. Epitaxially aligned a-plane NCs with average diameter of 100 nm were formed on r-plane sapphire substrates by hydride vapor phase epitaxy (HVPE), accompanied by the oxygen supply during the growth. X-ray photoemission spectroscopy measurements proved that the NCs exhibited Ga-O bonding in the materials, suggesting the formation of oxidized states in the bandgap. It was found that the NCs emitted the visible luminescence wavelength of 400‒500 nm and 680‒720 nm, which is attributed to the transition from oxygen-induced localized states. Furthermore, time-resolved photoluminescence studies revealed the significant suppression of the quantum confined Stark effect and highly efficient excitonic recombination within GaN NCs. Therefore, we believe that the HVPE non-polar GaN NCs can guide the simple and efficient way toward the nitride-based next-generation nano-photonic devices.

Combined theoretical modeling of photoexcitation spectrum of an isolated protonated tyrosine.

A photodepletion spectrum of gaseous protonated tyrosine molecules was obtained at 150 K by a UV laser spectroscopic technique in conjunction with mass spectrometry and interpreted by theoretical methods. The spectrum exhibits three distinct bands separated each other by about 800 cm(-1). Stable conformers of the molecular ion were determined by quantum mechanical density functional theory, and their electronic transition energies were obtained by a semiempirical quantum chemistry calculation. The whole pattern of the spectrum was reasonably reproduced by a combination of theoretical methods, the second-order cumulant expansion, a semiempirical quantum chemistry method, molecular dynamics simulation, and a semiclassical time-correlation function approach. The three spectral bands turned out to arise from the vibronic transition of two vibrational modes constituted by the "benzene breathing" mode and a torsional mode of the amino acid backbone. It is suggested that the major factor in spectral broadening is not conformational disorder or lifetime broadening but thermal fluctuation of the stable conformers. The good agreement between experimental and theoretical spectra exemplifies the validity of the theoretical methods applied for the present molecular system.

Determination of complex dielectric functions at HfO(2)/Si interface by using STEM-VEELS.

The complex dielectric functions and refractive index of atomic layer deposited HfO(2) were determined by the line scan method of the valence electron energy loss spectrum (VEELS) in a scanning transmission electron microscope (STEM). The complex dielectric functions and dielectric constant of monoclinic HfO(2) were calculated by the density functional theory (DFT) method. The resulting two dielectric functions were relatively well matched. On the other hand, the refractive index of HfO(2) was measured as 2.18 by VEELS analysis and 2.1 by DFT calculation. The electronic structure of HfO(2) was revealed by the comparison of the inter-band transition strength, obtained by STEM-VEELS, with the density of states (DOS) calculated by DFT calculation.

The investigation of stress in freestanding GaN crystals grown from Si substrates by HVPE.

We investigate the stress evolution of 400 µm-thick freestanding GaN crystals grown from Si substrates by hydride vapour phase epitaxy (HVPE) and the in situ removal of Si substrates. The stress generated in growing GaN can be tuned by varying the thickness of the MOCVD AlGaN/AlN buffer layers. Micro Raman analysis shows the presence of slight tensile stress in the freestanding GaN crystals and no stress accumulation in HVPE GaN layers during the growth. Additionally, it is demonstrated that the residual tensile stress in HVPE GaN is caused only by elastic stress arising from the crystal quality difference between Ga- and N-face GaN. TEM analysis revealed that the dislocations in freestanding GaN crystals have high inclination angles that are attributed to the stress relaxation of the crystals. We believe that the understanding and characterization on the structural properties of the freestanding GaN crystals will help us to use these crystals for high-performance opto-electronic devices.