Alkali hydroxide (LiOH, NaOH, KOH) in water: Structural and vibrational properties, including neutron scattering results.

Structural and vibrational properties of aqueous solutions of alkali hydroxides (LiOH, NaOH, and KOH) are computed using quantum molecular dynamics simulations for solute concentrations ranging between 1 and 10M. Element-resolved partial radial distribution functions, neutron and x-ray structure factors, and angular distribution functions are computed for the three hydroxide solutions as a function of concentration. The vibrational spectra and frequency-dependent conductivity are computed from the Fourier transforms of velocity autocorrelation and current autocorrelation functions. Our results for the structure are validated with the available neutron data for 17M concentration of NaOH in water [Semrouni et al., Phys. Chem. Chem. Phys. 21, 6828 (2019)]. We found that the larger ionic radius [rLi+

Hydrogen Bonding in Liquid Ammonia.

The nature of hydrogen bonding in condensed ammonia phases, liquid and crystalline ammonia has been a topic of much investigation. Here, we use quantum molecular dynamics simulations to investigate hydrogen bond structure and lifetimes in two ammonia phases: liquid ammonia and crystalline ammonia-I. Unlike liquid water, which has two covalently bonded hydrogen and two hydrogen bonds per oxygen atom, each nitrogen atom in liquid ammonia is found to have only one hydrogen bond at 2.24 Å. The computed lifetime of the hydrogen bond is t ≅ 0.1 ps. In contrast to crystalline water-ice, we find that hydrogen bonding is practically nonexistent in crystalline ammonia-I.

Electric-Field-Induced Room-Temperature Antiferroelectric-Ferroelectric Phase Transition in van der Waals Layered GeSe.

Searching van der Waals ferroic materials that can work under ambient conditions is of critical importance for developing ferroic devices at the two-dimensional limit. Here we report the experimental discovery of electric-field-induced reversible antiferroelectric (AFE) to ferroelectric (FE) transition at room temperature in van der Waals layered α-GeSe, employing Raman spectroscopy, transmission electron microscopy, second-harmonic generation, and piezoelectric force microscopy consolidated by first-principles calculations. An orientation-dependent AFE-FE transition provides strong evidence that the in-plane (IP) polarization vector aligns along the armchair rather than zigzag direction in α-GeSe. In addition, temperature-dependent Raman spectra showed that the IP polarization could sustain up to higher than 700 K. Our findings suggest that α-GeSe, which is also a potential ferrovalley material, could be a robust building block for creating artificial 2D multiferroics at room temperature.

Robust finite-time and fixed-time chaos synchronization of PMSMs in noise environment.

This paper addresses the robust stochastic finite-time and fixed-time chaos synchronization of two permanent magnet synchronous motors (PMSMs) in noise environment. The novel adaptive finite-time and fixed-time control schemes are implemented, respectively, which can not only ensure that the stochastic chaos synchronization of PMSMs can be achieved in a fast rate, but also determine the control gains successfully(not necessary to set them in advance). The sufficient conditions are derived in the light of the stochastic finite-time and fixed-time stability theories, where the upper bound of synchronization time can be estimated. Furthermore, the stochastic fixed-time synchronization can get rid of the dependence of initial conditions in PMSMs, which overcomes the critical deficiency of stochastic finite-time synchronization of PMSMs. Finally, simulation results demonstrate the validity of proposed theoretical analysis with comparisons.

Strain-engineering on GeSe: Raman spectroscopy study.

Among the IV-VI compounds, GeSe has wide applications in nanoelectronics due to its unique photoelectric properties and adjustable band gap. Even though modulation of its physical characteristics, including the band gap, by an external field will be useful for designing novel devices, experimental work is still rare. Here, we report a detailed anisotropic Raman response of GeSe flakes under uniaxial tension strain. Based on theoretical analysis, the anisotropy of the phonon response is attributed to a change in anisotropic bond length and bond angle under in-plane uniaxial strain. An enhancement in anisotropy and band gap is found due to strain along the ZZ or AC directions. This study shows that strain-engineering is an effective method for controlling the GeSe lattice, and paves the way for modulating the anisotropic electric and optical properties of GeSe.

Ion adsorption-induced reversible polarization switching of a van der Waals layered ferroelectric.

Solid-liquid interface is a key concept of many research fields, enabling numerous physical phenomena and practical applications. For example, electrode-electrolyte interfaces with electric double layers have been widely used in energy storage and regulating physical properties of functional materials. Creating a specific interface allows emergent functionalities and effects. Here, we show the artificial control of ferroelectric-liquid interfacial structures to switch polarization states reversibly in a van der Waals layered ferroelectric CuInP2S6 (CIPS). We discover that upward and downward polarization states can be induced by spontaneous physical adsorption of dodecylbenzenesulphonate anions and N,N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium cations, respectively, at the ferroelectric-liquid interface. This distinctive approach circumvents the structural damage of CIPS caused by Cu-ion conductivity during electrical switching process. Moreover, the polarized state features super-long retention time (>1 year). The interplay between ferroelectric dipoles and adsorbed organic ions has been studied systematically by comparative experiments and first-principles calculations. Such ion adsorption-induced reversible polarization switching in a van der Waals ferroelectric enriches the functionalities of solid-liquid interfaces, offering opportunities for liquid-controlled two-dimensional ferroelectric-based devices.

FIB-Assisted Fabrication of Single Tellurium Nanotube Based High Performance Photodetector.

Nanoscale tellurium (Te) materials are promising for advanced optoelectronics owing to their outstanding photoelectrical properties. In this work, high-performance optoelectronic nanodevice based on a single tellurium nanotube (NT) was prepared by focused ion beam (FIB)-assisted technique. The individual Te NT photodetector demonstrates a high photoresponsivity of 1.65 × 104 AW-1 and a high photoconductivity gain of 5.0 × 106%, which shows great promise for further optoelectronic device applications.

Enhanced and Balanced Charge Transport Boosting Ternary Solar Cells Over 17% Efficiency.

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB-T-C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so-called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB-T-C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

Phonon-Suppressed Auger Scattering of Charge Carriers in Defective Two-Dimensional Transition Metal Dichalcogenides.

Two-dimensional transition metal dichalcogenides (TMDs) draw strong interest in materials science, with applications in optoelectronics and many other fields. Good performance requires high carrier concentrations and long lifetimes. However, high concentrations accelerate energy exchange between charged particles by Auger-type processes, especially in TMDs where many-body interactions are strong, thus facilitating carrier trapping. We report time-resolved optical pump-THz probe measurements of carrier lifetimes as a function of carrier density. Surprisingly, the lifetime reduction with increased density is very weak. It decreases only by 20% when we increase the pump fluence 100 times. This unexpected feature of the Auger process is rationalized by our time-domain ab initio simulations. The simulations show that phonon-driven trapping competes successfully with the Auger process. On the one hand, trap states are relatively close to band edges, and phonons accommodate efficiently the electronic energy during the trapping. On the other hand, trap states localize around defects, and the overlap of trapped and free carriers is small, decreasing carrier-carrier interactions. At low carrier densities, phonons provide the main charge trapping mechanism, decreasing carrier lifetimes compared to defect-free samples. At high carrier densities, phonons suppress Auger processes and lower the dependence of the trapping rate on carrier density. Our results provide theoretical insights into the diverse roles played by phonons and Auger processes in TMDs and generate guidelines for defect engineering to improve device performance at high carrier densities.

Optical Control of Non-Equilibrium Phonon Dynamics.

The light-induced selective population of short-lived far-from-equilibrium vibration modes is a promising approach for controlling ultrafast and irreversible structural changes in functional nanomaterials. However, this requires a detailed understanding of the dynamics and evolution of these phonon modes and their coupling to the excited-state electronic structure. Here, we combine femtosecond mega-electronvolt electron diffraction experiments on a prototypical layered material, MoTe2, with non-adiabatic quantum molecular dynamics simulations and ab initio electronic structure calculations to show how non-radiative energy relaxation pathways for excited electrons can be tuned by controlling the optical excitation energy. We show how the dominant intravalley and intervalley scattering mechanisms for hot and band-edge electrons leads to markedly different transient phonon populations evident in electron diffraction patterns. This understanding of how tuning optical excitations affect phonon populations and atomic motion is critical for efficiently controlling light-induced structural transitions of optoelectronic devices.

The First Examples of Lithium-Containing Mixed-Alkali Strontium Borates with Different Dimensional Anionic Architectures and Short Cutoff Edges.

Three new mixed-alkali strontium borates, Li2 KSr6 (BO3 )5 , Li2 Rb7 Sr24 (BO3 )19 , and Li2 Rb2 SrB18 O30 , have been synthesized by the high-temperature solution method by adjusting the molar ratios of the reactants. They represent the first examples of lithium-containing mixed-alkali strontium borates. The structures of the three compounds show different dimensional anionic architectures, with Li2 KSr6 (BO3 )5 and Li2 Rb7 Sr24 (BO3 )19 possessing isolated BO3 units, whereas Li2 Rb2 SrB18 O30 has three-dimensional (3D) ∞ 3 [ B 9 O 15 ] 3 - open frameworks with three types of channels occupied by the strontium and rubidium atoms. Interestingly, Li2 KSr6 (BO3 )5 is the first example of a ∞ 1 [ Li 2 O 7 ] 12 - chain composed of LiO5 linked through the sharing of vertexes and edges. However, in Li2 Rb7 Sr24 (BO3 )19 , isolated LiO3 units can be observed, which is also rare in borates. Detailed analysis and comparisons of the structures of the anhydrous mixed-alkali- and alkaline-earth-metal borates and nine-polymeric fundamental building blocks have been made. Furthermore, Li2 Rb7 Sr24 (BO3 )19 and Li2 Rb2 SrB18 O30 present short cutoff edges (<190 nm), which was confirmed by the UV/Vis/NIR diffuse reflectance spectra. These attributes make them attractive for deep-UV optical materials.

LiCs2La(BO3)2 and Li3K9La3(BO3)7: new mixed alkali metal lanthanum borates with three-dimensional open frameworks and short cut-off edges.

Two new mixed alkali metal lanthanum borates, LiCs2La(BO3)2 and Li3K9La3(BO3)7, were synthesized through a high-temperature solution method and millimeter-sized colorless crystals were obtained. The structures of LiCs2La(BO3)2 and Li3K9La3(BO3)7 have an intricate three-dimensional (3D) open framework with one-dimensional (1D) infinite channels surrounded by cation polyhedra and isolated BO3 units. In particular, LiCs2La(BO3)2 exhibits a short cut-off edge <190 nm and a large experimental band gap of 6.20 eV, evidenced by diffuse reflection spectroscopy. Moreover, to better understand the relationship between the electronic structures and optical properties of LiCs2La(BO3)2, theoretical calculations using density functional theory were performed. The result indicated that LiCs2La(BO3)2 has a calculated birefringence of 0.036@1064 nm and the direct band gap is 4.54 eV. In addition, the effects of ion substitution on structural transition and the metal ion/boron ratio on dimensions of the B-O frameworks are also discussed in detail.

ScMO(BO3) (M = Ca and Cd): new Sc-based oxyborates featuring interesting edge-sharing sandwich-like chains and UV cut-off edges.

Two new isostructural rare-earth oxyborates ScMO(BO3) (M = Ca and Cd) with a three-dimensional (3D) cationic framework and parallel arranged [BO3] triangles have been synthesized by the flux method. In the 3D cationic framework, an interesting sandwich-like basic building unit (BBU) is constructed by two [Ca(1)O4]6- chains and two [Sc(1)O4]5- chains. ScMO(BO3) melt incongruently, which shows that title compounds can be grown by the flux method. The UV cut-off edges for ScCaO(BO3) and ScCdO(BO3) are 230 and 249 nm, respectively. In addition, the first-principles calculations are performed to gain further insights into the relationship between the microscopic electronic structures and associated optical properties.