Electronic Contribution Research Articles

Pivotal Role of A-Site Cations in Tailoring the Band-Edge States, Optical Properties, and Stability of 0D Hybrid Indium Chlorides

Optoelectronic properties of organic–inorganic hybrid perovskites (ABX3) or metal halides are governed primarily by the B and X-site ions due to their electronic contribution to the band edge. The A-site cations do not directly participate in the band edge; instead, they stabilize the crystal structures. Facilitating these A-site cations to contribute to the band edge can enable an additional degree of tunability to properties and expand the opportunities beyond B and X ions. Here, we rationally employ different A-site cations based on anilinium cations in Sb3+-doped zero-dimensional (0D) indium chlorides and illustrate their contribution to the band-edge states using experimental data and theoretical calculations. We demonstrate that a change in the A-site cation results in a change in the octahedral distance in the crystal structure. In addition to this, we show that the frontier orbitals of hybrid indium chlorides are delocalized across the inorganic and organic moieties, leading to significant changes in their optical properties. Consequently, Sb3+-doped hybrid indium chlorides show a broadband emission with a PLQY over 93% and remarkable spectral and structural stability in five different solvents, including water, for 21 days.

Hyperpolarizabilities of Push-Pull Chromophores in Solution: Interplay between Electronic and Vibrational Contributions.

Contemporary design of new organic non-linear optical (NLO) materials relies to a large extent on the understanding of molecular and electronic structure-property relationships revealed during the years by available computational approaches. The progress in theory-hand-in-hand with experiment-has enabled us to identify and analyze various physical aspects affecting the NLO responses, such as the environmental effects, molecular vibrations, frequency dispersion, and system dynamics. Although it is nowadays possible to reliably address these effects separately, the studies analyzing their mutual interplay are still very limited. Here, we employ density functional theory (DFT) methods in combination with an implicit solvent model to examine the solvent effects on the electronic and harmonic as well as anharmonic vibrational contributions to the static first hyperpolarizability of a series of push-pull α,ω-diphenylpolyene oligomers, which were experimentally shown to exhibit notable second-order NLO responses. We demonstrate that the magnitudes of both vibrational and electronic contributions being comparable in the gas phase significantly increase in solvents, and the enhancement can be, in some cases, as large as three- or even four-fold. The electrical and mechanical anharmonic contributions are not negligible but cancel each other out to a large extent. The computed dynamic solute NLO properties of the studied systems are shown to be in a fair agreement with those derived from experimentally measured electric-field-induced second-harmonic generation (EFISHG) signals. Our results substantiate the necessity to consider concomitantly both solvation and vibrational effects in modeling static NLO properties of solvated systems.

Comprehensive Model for the Thermoelectric Properties of Two-Dimensional Carbon Nanotube Networks

Networks of semiconducting single-walled carbon nanotubes (SWCNTs) are interesting thermoelectric materials due to the interplay between CNT and network properties. Here, we present a unified model to explain charge and energy transport in SWCNT networks. We use the steady-state master equation for the random resistor network containing both intra- and intertube resistances, as defined through their one-dimensional density of states that are modulated by static Gaussian disorder. The tube-resistance dependence on the carrier density and disorder is described through the Landauer formalism. Electrical and thermoelectric properties of the network are obtained by solving Kirchhoff's laws through a modified nodal analysis, where we use the Boltzmann-transport formalism to obtain the conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The model provides a consistent description of a wide range of previously published experimental data for temperature and charge-carrier-density-dependent conductivities and Seebeck coefficients, with energetic disorder being the main factor to explain the experimentally observed mobility upswing with carrier concentration. Moreover, we show that, for lower disorder energies, the Lorentz factor obtained from the simulation is in accordance with the Wiedemann-Franz law for degenerate-band semiconductors. At higher disorder, deviations from simple band behavior are found. Suppressed disorder energy and lattice thermal conductivity can be the key to higher thermoelectric figures of merit, zT, in SWCNT networks, possibly approaching or even exceeding zT = 1. The general understanding of transport phenomena will help the selection of chirality, composition, and charge-carrier density of SWCNT networks to improve their efficiency of thermoelectric energy conversion.

Positive temperature coefficient of the thermal conductivity above room temperature in a perovskite cobaltite

ABSTRACT The thermal conductivity above room temperature is investigated for LaCoO3-based materials showing spin-state and insulator-metal crossovers. A positive temperature coefficient (PTC) of the thermal conductivity is observed during the insulator-metal crossover around 500 K. Our analysis indicates that the phononic thermal transport is also enhanced in addition to the electronic contribution as the insulator-metal crossover takes place. The enhancement of the phononic component is ascribed to the reduction of the incoherent local lattice distortion coupled with the spin/orbital state of each Co3+ ion, which is induced by the enhanced spin-state fluctuation between low and excited spin-states. Moreover, fine tunability for the PTC of the thermal conductivity is demonstrated via doping hole-type carriers into LaCoO3. The observed enhancement ratio of the thermal conductivity κ T (773 K) / κ T (323 K) = 2.6 in La0.95Sr0.05CoO3 is the largest value among oxide materials which exhibit a PTC of their thermal conductivity above room temperature. The thermal rectification ratio is estimated to reach 61% for a hypothetical thermal diode consisting of La0.95Sr0.05CoO3 and LaGaO3, the latter of which is a typical band insulator. These results indicate that utilizing spin-state and orbital degrees of freedom in strongly correlated materials is a useful strategy for tuning thermal transport properties, especially for designing thermal diodes.

Disentangling electronic and thermal contributions to light-induced resistance switching in BaTiO3 ferroelectric tunnel junction

In the presence of asymmetric potential barriers, such as those created by imprint fields, ferroelectric polarization can be reversed by light due to the photoinduced suppression of polarization. Both thermal effects and photocarrier-induced polarization screening may agree with this experimental observation, challenging its understanding. Here, we explore light-induced ferroelectric polarization switching in BaTiO3 thin films. Time-dependent photocurrent and photoresistance experiments at different wavelengths indicate that the optical switch of polarization is mainly driven by photocarriers rather than thermal effects. The effect of light on sample polarization is found to be relatively slow and that an illumination period as long as ≈100 s is required to achieve complete switching when using a 405 nm light wavelength and 1.4 W/cm2 power density. It is shown that this response is governed by the concentration of photo-generated charges, which is low due to the reduced light absorption of BaTiO3 films at the explored wavelengths. Our conclusions can help us to better design optically switching devices based on ferroelectric materials.

Packing fraction induced phase separation in A-site doped antiperovskites

Local phase separation displayed by the A site doped Mn 3 Sn 1 − x A x C antiperovskites is investigated by studying a series of compounds wherein, the doped A site atoms inflicts changes in electronic contribution to the valence band while keeping the average size of the A site similar (In for Sn), or the size of the A-site atom is varied while maintaining the electronic density constant (Ge for Sn) or both, size and electronic density are varied (Zn for Sn). The study shows that the phase separation is a result of varied distortion of Mn 6 C octahedra due to the difference in the packing fractions of the unit cells. This distortions of the Mn 6 C octahedra are responsible for the observed magnetic properties of these antiperovskite compounds. • All A site substituted antiperovskite compounds, though have a cubic structure, display a local phase separation. • The Mn 6 C octahedra are distorted and the extent of distortion depends on the packing fraction of the unit cell. • Local phase separation and the distortions of the Mn 6 C octahedra play a decisive role in magnetic ground state of these antiperovskite compounds.

Operando AC In-Plane Impedance Spectroscopy of Electrodes for Energy Storage Systems

Characterization of battery and supercapacitor materials and devices is typically performed using various methods such as cyclic voltammetry, electrochemical impedance spectroscopy, galvanostatic cycling, etc. However, none of these techniques allow for operando tracking of changes in the electrode material’s in-plane ionic and electronic percolation under polarization, which plays a key-role in the electrochemical performance of the material. Here, we report an experimental set-up devoted to measuring the operando in-plane AC impedance and DC resistance of porous activated carbon and Ti3C2 MXene electrodes during electrochemical cycling. These impedance measurements allow for the deconvolution and tracking of the ionic and electronic contributions of the total impedance and the change in these components under polarization. Operando tracking of the in-plane electrode impedance under polarization brings insights regarding electronic and ionic transport mechanisms of electrodes during operation. This set-up serves as a complementary tool to further evaluate and improve the performance of electrode materials for energy storage.

Influence of the metal - support and metal - metal interactions on Pd nucleation and NO adsorption in a Pd4/γ-Al2O3 (110D) model.

The role played by the metal - support (MSI) and metal - metal (MMI) interactions on two important processes in controlling the catalyst performance - nucleation and molecular adsorption - has been investigated using density functional theory (DFT), by means of B3LYP functional, combined with localized molecular orbital energy decomposition analysis (LMOEDA), and natural bond orbital (NBO) calculations, with aid of a Pd4/γ-alumina (110D) model (Pd4/Al13O23H7). Our results indicate the occurrence of an electronic metal - support interaction (EMSI) which induces a most intense charge transfer in the Pd4 → γ-alumina backdonation direction, most expressive in Pd → Al, promoting an electronic redistribution within the units and attenuating the MMI. Nevertheless, the MSI/MMI synergistic effect seems to favor slightly the nucleation of a fifth palladium atom, leading to a distorted square pyramidal arrangement for Pd5. The LMOEDA analysis points to a mostly covalent character in the Pd - Al bonds, whereas the Pd - O bonds are mainly electrostatic in nature. The palladium atoms deposited on oxygen anions are the acid centers, where both NO molecule and an additional palladium atom anchor more strongly. In addition, the MSI/MMI effect, through the electronic and geometric contributions, drives the adsorption of the NO molecule to the mode which most favors the Pd → NO (4dz2 → 2π*) backdonation (bridge mode). MSI and MMI effects on the nature of the Pd - O (electrostatic) and Pd - Al (covalent) bonds, charge transfer into Pd4/γ-Al2O3 (110D) interface (back donation) and preferential site for adsorption of a single NO molecule and an additional Pd atom (Pd - O).

Chirality Induced Spin Selectivity – the Photoelectron View

AbstractIn this short overview we discuss the manifestation of the chirality‐induced spin selectivity (CISS) effect in photoelectron spectroscopy experiments with direct spin analysis. Various systems, from the initially investigated monolayers of molecular biosystems and organic hepta‐helicene to recent chirally grown solid oxide films, are evaluated. High spin polarization values of up to P=60 % have been observed for samples at room temperature. For all classes it was shown that the longitudinal spin orientation changes sign with a change of the enantiomeric form. Recent advances allow to distinguish the contributions of electrons with different kinetic energies to the electron spin polarization, and thus to distinguish the CISS effect from bulk contributions to the spin originating from different mechanisms.

On the Theory of Magnetoelectric Coupling in Fe2Mo3O8.

In the last decade, Fe2Mo3O8 was recognized for a giant magnetoelectric effect, the origin of which is still not clear. In the present paper, we contribute to the microscopic theory of the magnetoelectric coupling in this compound. Using crystal field theory and the molecular field approximation, we calculated the low-lying energy spectrum for iron ions and their interaction with electric and magnetic fields. Classical ionic contribution to the electric polarization related to the ionic shifts is also estimated. It is found that the electronic and ionic contributions to the electric polarization are comparable and these mechanisms support each other at T<TN. The suggested electronic mechanism provides insight into the nature of huge jumps in polarization upon phase transitions from paramagnetic (PM) to antiferromagnetic (AFM) and then to ferrimagnetic (FRM) states under an applied external magnetic field as well as the large differential magnetoelectric coefficient.

Hydrogen diffusion and its electrical properties variation as a function of the IGZO stacking structure

The oxygen vacancies and hydrogen in oxide semiconductors are regarded as the primary sources of charge carriers and various studies have investigated the effect of hydrogen on the properties of oxide semiconductors. However, the carrier generation mechanism between hydrogen and oxygen vacancies in an a-IGZO semiconductor has not yet been clearly examined. In this study we investigated the effect of hydrogen and the variation mechanisms of electrical properties of a thin film supplied with hydrogen from the passivation layer. SiOx and SiNx, which are used as passivation or gate insulator layers in the semiconductor process, respectively, were placed on the top or bottom of an a-IGZO semiconductor to determine the amount of hydrogen penetrating the a-IGZO active layer. The hydrogen diffusion depth was sufficiently deep to affect the entire thin semiconductor layer. A large amount of hydrogen in SiNx directly affects the electrical resistivity of a-IGZO semiconductor, whereas in SiOx, it induces a different behavior from that in SiNx, such as inducing an oxygen reaction and O–H bond behavior change at the interface of an a-IGZO semiconductor. Moreover, the change in electrical resistivity owing to the contribution of free electrons could be varied based on the bonding method of hydrogen and oxygen.

Effect of Composition and Local Environment on CO2 Adsorption on Nickel and Magnesium Oxide Solid Solutions

CO2 adsorption energies on the (100) surfaces of a nickel oxide doped with Mg, magnesium oxide doped with Ni, and their 50:50 solid solution were calculated using density functional theory. The composition and atomic arrangement of the adsorption site were varied to understand how the local environment affects CO2 adsorption and the basicity of the surfaces at the atomic level. The dispersive and electronic contributions to the adsorption energies were quantified, and the results indicate that the variation of the adsorption energy with adsorption site configuration and metal composition is dominated by electronic interactions. Interestingly, for magnesium oxide doped with nickel, a single substitution can create stronger CO2 binding sites, which implies stronger basic sites, even though nickel oxide is less basic than magnesium oxide. The effect of double substitution at the binding site can be reasonably approximated by summing the effects of single substitutions. This work provides guidance for the preparation of metal oxides with tailored Lewis basicity.

Electron self-energy corrections using the Welton concept for atomic structure calculations

The high level of accuracy achieved by atomic experiments in recent time has shone a spotlight on the need for a similarly high level of accuracy in atomic structure calculations, and in particular, QED prediction. A method of electron self-energy correction originally derived from the Welton idea by Lowe et al. (2013) (LCG-Welton method) has now been fully incorporated into the popular atomic structural package, GRASP2K, which we have introduced in this paper. A series of benchmark tests and results are presented, which enables the comparison of the implementations of different versions of GRASP2K, and the implementations on different platforms or operating systems. Test results presented in this paper demonstrate that these new implementations maintain the overall consistency and stability of the program across various platforms, while at the same time improve the accuracy of final energies. Our calculations for hydrogenic Lyα1,2 transitions show excellent agreement with experiment, to within less than 0.5 eV. On helium-like systems, our calculations show an improvement from the previous GRASP2K screening method. The new results from electronic self-energy contribution using the LCG-Welton method is more consistent with current standards in the literature, where they now fit within experimental variability of up to 0.1 eV. An option for users to adjust the gauge factor in the electric component of the transition rate has also been added to facilitate further investigation of this particular topic.

On the O-mode instability generated by dual core-halo solar wind electrons

Abstract In situ measurements reported with different spacecraft reveal the temperature anisotropy constraints of solar wind species. Micro-instabilities driven by the temperature anisotropy are investigated as key mechanism which limit an unchecked rise in the temperature anisotropy in dilute space plasmas. The present study considers the excitation of O-mode instability under the excessive electrons parallel temperature condition i.e., T ‖e > T ⊥e . Looking at the recent developments made by different satellite missions, a dual core-halo electron distribution is employed that represents a true picture of solar wind electrons. We employ Vlasov–Maxwell’s model equations and derive the dispersion relation of the O-wave whose analytical solution characterizes the growth rate of O-mode instability. We take different sets of temperature anisotropy and beta combinations associated with core-halo electron components that highlight the instability characteristics e.g. growth rate and the range of wave number of unstable mode. We note an increment in the growth rate with the increase of anisotropic ratios associated either with core electrons or halo electrons and or the combination thereof. In view of a global perspective of solar wind, it may be a favorable study with the contribution of non-thermal solar wind electrons.

Modeling Luminescence Spectrum of BaZrO3:Ti Including Vibronic Coupling from First Principles Calculations.

Herein, we present a methodology based on constrained density functional theory and vibrational mode computations to simulate and interpret the luminescence spectra of periodic solids. A multi-dimension harmonic model is used to combine electronic and vibrational contributions into an overall vibrationally resolved emission spectrum. We applied it to Ti-doped BaZrO3 to accurately reproduce its blue luminescence and unambiguously assign the observed luminescence to a Ti3+ + O- → Ti4+ + O2- charge transfer.

Quantitative analysis of free-electron dynamics in InSb by terahertz shockwave spectroscopy

The contribution of free electrons to the dielectric function is largely determined by their plasmonic resonance, a collective density oscillation. We studied this broadband and carrier-density-dependent response in the narrow-gap semiconductor InSb with THz shockwave spectroscopy. A synthesized waveform, with a steep onset followed by a short electric-field plateau, gives access to a multioctave spectrum of frequencies from 100 GHz up to the mid infrared. By measuring the entire spectral characteristics of the plasma, we analyze the dynamics of photogenerated free electrons for a wide range of excitation fluences. Thanks to this analysis, we were able to quantify the coefficients of both electron trapping and the Auger process from cryogenic to room temperature.

Prediction of Multiple Hydrogen Ligation at a Vanadium(II) Site in a Metal-Organic Framework.

Densifying hydrogen in a metal-organic framework (MOF) at moderate pressures can circumvent challenges associated with high-pressure compression. The highly tunable structural and chemical composition in MOFs affords vast possibilities to optimize binding interactions. At the heart of this search are the nanoscale characteristics of molecular adsorption at the binding site(s). Using density functional theory (DFT) to model binding interactions of hydrogen to the exposed metal site of cation-exchanged MFU-4l, we predict multiple hydrogen ligation of H2 at the first coordination sphere of V(II) in V(II)-exchanged MFU-4l. We find that the strength of this binding between the metal site and H2 molecules can be tuned by altering the halide counterion adjacent to the metal site and that the fluoride containing node affords the most favorable interactions for high-density H2 storage. Using energy decomposition analysis, we delineate electronic contributions that enable multiple hydrogen ligation and demonstrate its benefits for hydrogen adsorption and release at modest pressures.

P-n heterojunctions of Si@WO3 mimicking thylakoid for photoelectrocatalytic CO2 reduction to C2+ products — Morphology control

The photoelectrocatalytic reduction of CO2 to high value-added chemicals is considered as one of the most promising technologies for solving both environmental and energy issues of the planet. Here, p-n heterojunction nanowires Si@WO3-x derived from p-silicon wafer modified by tungsten trioxide with different morphologies, including nanosheets (NS), nanobulks (NB) and nanoneedles (NN), were designed and fabricated. The Si@WO3-NS heterojunction gives the highest apparent quantum efficiency of light (0.49 % AQE) excluding the contribution of electrons from anode (>0.4 % QE of nature plant), which is nearly 25 times than that of pure Si NW (0.02 % AQE). The selectivity of multicarbon products (C2+) for Si@WO3-NS catalyst reaches 62.7 %, benefiting from the morphology simulated to the structure of thylakoid in plants. Moreover, the mechanism was proposed and confirmed by operando FT-IR experiments indicating the existence of active species COO−, HCOO−, C−O and C−C, respectively. This engineering design for Si-based material simulated plant cell can firstly produce C2+ chemicals without assistance of copper particles known as good catalyst or co-catalyst for C−C coupling.

Enhanced thermoelectric performance in n-type Mg3.2Sb1.5Bi0.5 doping with lanthanides at the Mg site

• Gd and Ho have increased the DOS of the valence band near the Fermi level, and the d electrons of Gd and Ho have made extraordinary contributions. • Gd and Ho are effective n-type cation site dopants with higher doping efficiency and better carrier concentration than the anion site dopants chalcogenide elements. • For Mg 3.2 Gd 0.02 Sb 1.5 Bi 0.5 and Mg 3.2 Ho 0.02 Sb 1.5 Bi 0.5 samples, we obtain high zT values of 1.61 and 1.55 at 725 K, respectively. • Gd as well as Ho are an alternative effective n-type doping element for higher thermoelectric performance. Mg-based thermoelectric materials have attracted more and more attention because of their rich composition elements, green environmental protection, and lower price. In recent years, the thermoelectric properties of n-type Mg 3 Sb 2 materials have been optimized by doping chalcogenide elements (S, Se, and Te) at the anionic position. In this work, n-type Mg 3.2 A x Sb 1.5 Bi 0.5 ( A = Gd, Ho; x = 0.01, 0.02, 0.03, and 0.4) samples were prepared by the cation site doping of lanthanide elements (Gd and Ho). The research results show that Gd and Ho doped n-type Mg 3.2 Sb 1.5 Bi 0.5 samples are entirely comparable to the S, Se, and Te doped n-type Mg 3.2 Sb 1.5 Bi 0.5 samples, demonstrating more excellent thermoelectric properties. Doping with lanthanides (Gd and Ho) at the Mg site increases the carrier concentration of the material to 8.161 × 10 19 cm −3 . Doping induces the contribution of more electron, thus obtaining higher conductivity. The maximum zT value of the Mg 3.2 Gd 0.02 Sb 1.5 Bi 0.5 and the Mg 3.2 Ho 0.02 Sb 1.5 Bi 0.5 samples reaches 1.61 and 1.55, respectively. This work theoretically and experimentally demonstrates Gd and Ho are efficient n-type dopants for Mg 3.2 Sb 1.5 Bi 0.5 thermoelectric material.

Tuning phononic and electronic contributions of thermoelectric in defected S-shape graphene nanoribbons

Thermoelectrics as a way to use waste heat, is essential in electronic industries, but its low performance at operational temperatures makes it inappropriate in practical applications. Tailoring graphene can change its properties. In this work, we are interested in studying the transport properties of S-shape graphene structures with the single vacancy (SV) and double vacancy (DV) models. The structures are composed of a chiral part, which is an armchair graphene nanoribbon, and two zigzag graphene ribbons. We investigate the changes in the figure of merit by means of the Seebeck coefficient, electronic conductance, and electronic and phononic conductances with the vacancies in different device sizes. The transport properties of the system are studied by using the non-equilibrium Green’s function method, so that the related Hamiltonians (dynamical matrices) are obtained from the tight-binding (force constant) model. The maximum figure of merit (ZT) obtains for the DVs in all lengths. Physical properties of such a system can be tuned by controlling various parameters such as the location and the type of the defects, and the device size. Our findings show that lengthening the structure can reduce phononic contribution, and single vacancies than double vacancies can better distinguish between electronic thermal conductance behavior and electronic conductance one. Namely, vacancy engineering can significantly increase thermoelectric performance. In the large devices, the SVs can increase the ZT up to 2.5 times.