Free Charge Research Articles

Manipulating Photoconduction in Supramolecular Networks for Solar-Driven Nitrate Conversion to Ammonia and Oxygen.

For photoelectrodes to be used in practical catalytic applications, challenges exist in achieving the efficient production and transport of photogenerated charge-separated states. Analogous concepts in traditional inorganic photoelectrodes can be applied to their organic-polymer counterparts with improved charge-separation efficiencies. In this work, we develop photoconductive organic networks to form a high-performance photoelectrode for NO3- reduction to NH3. In the integrated network, interfaces between the organic electron-donating photoconductor and electron-accepting catalyst can generate charge carriers efficiently upon illumination, leading to enhanced charge separation for photoelectrocatalysis. The photoelectrode network is capable of converting NO3- to NH3 at an external quantum efficiency of 13%. By coupling with a BiVO4 photoanode in tandem, the system reduces NO3- to NH3 and oxidizes H2O to O2 simultaneously at Faradaic efficiencies of 95-98% with sustained photocurrents and production yields. Investigation of the photoconductive network by steady-state/time-resolved spectroscopies reveals the efficient generation and transport of free charge carriers in the photoelectrode, providing a basis for high photoelectrocatalytic performances.

Mechanism of sinuous and varicose modes in electrokinetic instability.

The flow of miscible electrolyte streams with mismatched electrical conductivities in the presence of a parallel applied electric field is known to exhibit electrokinetic instability (EKI). This paper deals with EKI in a configuration where the base state is established by electro-osmotic flow (EOF) of three coflowing streams, with the center stream having different conductivity than the sheath streams. All reported experiments of this EKI have shown that the instability exhibits either sinuous or varicose modes depending upon whether the center stream has higher or lower conductivity than the sheath streams, respectively. In this paper we elucidate the physical mechanism underlying the selection of these unstable modes in EKI using linear stability analysis. The stability analysis shows that the EOF simply convects the unstable modes besides establishing the base state. The instability occurs due to stationary convection cells, in the reference frame moving with the EOF, resulting from the coupling of the applied electric field with free charge in the regions with conductivity gradients. Importantly, we show that the unstable and stable disturbances for the configuration with a higher conductivity center stream have opposite stability characteristics when the center stream has lower conductivity than the sheath streams. Our analysis correctly explains the numerous experimental observations showing the consistent appearance of sinuous modes for higher conductivity and varicose modes for lower conductivity center streams.

Gleditsia sinensic extract as green corrosion inhibitor for N80 steel in 1 M HCl

The inhibition efficiency of gleditsia sinensic extract (GSE) as green corrosion inhibitor for N80 steel has been evaluated by weightlessness test, electrochemical impedance spectroscopy (EIS) study, Potentiodynamic polarization (PDP) measurement and Density Functional Theory (DFT) calculations in 1M HCl solution. Consequently, the inhibition efficiency of GSE is increased with the concentrations, immersion time and corrosion temperature (except 60 °C). After 144 h of immersion at 30 °C, the inhibition efficiency of GSE at a concentration of 500 mg/L was as high as 92.34 %. The adsorption process was spontaneous exothermic and conformed to Langmuir's law of adsorption, while Tafel's analysis showed that GSE is a mixed type of inhibitor. In addition, the corrosion products have been examined by scanning electron microscopy (SEM) analysis, X-Ray Diffractomer (XRD) testing, Fourier transform infrared spectroscopy (FTIR-ATR), UV-Vis spectroscopy and contact angle (CA) measurements. Furthermore, the corrosion inhibition mechanism has also been investigated.

The influence of the dopant concentration and sintering parameters on properties of antimony doped barium stannate ceramics

In this paper, the influence of antimony concentration and different sintering techniques on the structural, microstructural, and electrical properties of antimony-doped barium stannate, BaSn1-xSbxO3 (BSSO, x = 0.00, 0.04, 0.06, 0.08 and 0.10) was investigated. BSSO-based ceramic samples were obtained by conventional and spark plasma sintering. The XRD analysis confirmed the single-phase, cubic BaSnO3 lattice system in all conventionally sintered samples. Apart from the dominant cubic phase, the spark plasma sintering conditions led to the formation of a secondary phase, Ba2SnO4, in all samples. FESEM analysis revealed the presence of low angle grain boundaries (LAGBs) in BSSO samples with high antimony concentration (x = 0.08), independently of the sintering technique. However, the fraction of LAGBs is significantly higher in the BaSn0.92Sb0.08O3-SPS sample due to the simultaneous exposure of the conductive sample to the effects of high temperature and pressure during sintering process. These boundaries have low activation energy and allow free charge carrier transport through the grain boundary region. The high dopant concentration and the presence of large fraction of LAGBs in BaSn0.92Sb0.08O3-SPS sample reflected on its electrical properties through low and almost temperature-independent electrical resistivity in the temperature range of 25–150 °C.

Breaking the size limitation of nonadiabatic molecular dynamics in condensed matter systems with local descriptor machine learning

Nonadiabatic molecular dynamics (NA-MD) is a powerful tool to model far-from-equilibrium processes, such as photochemical reactions and charge transport. NA-MD application to condensed phase has drawn tremendous attention recently for development of next-generation energy and optoelectronic materials. Studies of condensed matter allow one to employ efficient computational tools, such as density functional theory (DFT) and classical path approximation (CPA). Still, system size and simulation timescale are strongly limited by costly ab initio calculations of electronic energies, forces, and NA couplings. We resolve the limitations by developing a fully machine learning (ML) approach in which all the above properties are obtained using neural networks based on local descriptors. The ML models correlate the target properties for NA-MD, implemented with DFT and CPA, directly to the system structure. Trained on small systems, the neural networks are applied to large systems and long timescales, extending NA-MD capabilities by orders of magnitude. We demonstrate the approach with dependence of charge trapping and recombination on defect concentration in MoS2. Defects provide the main mechanism of charge losses, resulting in performance degradation. Charge trapping slows with decreasing defect concentration; however, recombination exhibits complex dependence, conditional on whether it occurs between free or trapped charges, and relative concentrations of carriers and defects. Delocalized shallow traps can become localized with increasing temperature, changing trapping and recombination behavior. Completely based on ML, the approach bridges the gap between theoretical models and realistic experimental conditions and enables NA-MD on thousand-atom systems and many nanoseconds.

Kesterite‐Type Narrow Bandgap Piezoelectric Catalysts for Highly Efficient Piezocatalytic Fenton System

AbstractPiezocatalytic Fenton (PF) system emerges as a promising approach to wastewater treatment by leveraging piezocatalysis to enhance Fenton‐like reactions. However, conventional piezocatalysts encounter challenges because they often compromise catalytic properties in biased favor of superior piezoelectricity, resulting in sluggish catalytic kinetics. To tackle this trade‐off, here a novel class of kesterite‐type narrow bandgap piezoelectrics, Cu2XSnS4 (CXTS, X = Zn, Ni, Co), is developed for PF reactions, which exhibit a unique combination of physicochemical attributes favorable for catalysis such as narrow bandgap (1.2–1.5 eV), high free charge density (1 × 1018 cm−3), mobility, and redox activity while retaining excellent piezoelectricity (62–142 pm V−1). With the well‐balanced piezoelectric, semiconducting, and catalytic properties, CXTS‐based PF systems demonstrate outstanding performance for tetracycline degradation, delivering a notable reaction kinetics of 0.34 min−1 only with a minor H2O2 dosage (1.2 mm), outperforming most of the conventional Fenton‐like reactions requiring a large amount H2O2 dosage by a factor up to 10. Such a remarkable performance is fulfilled by the simultaneously effective H2O2 activation and in situ generation of reactive oxygen species from oxygen and water via piezocatalysis. Additionally, the distinctive hierarchical morphology consisting of 2D nanosheets enables easy crystal domain deformation to trigger the piezoelectric effect, thereby drastically reducing the mechanical energy input required to drive redox reactions. Rigorous testing has validated the viability and practical feasibility of this system. The study offers a new design strategy for highly efficient piezocatalysts in the PF systems, enabling a cost‐effective and sustainable water treatment approach.

Molecular Control of the Donor/Acceptor Interface Suppresses Charge Recombination Enabling High-Efficiency Single-Component Organic Solar Cells.

Single-component organic solar cells based on double cable polymers have achieved remarkable performance, with DCPY2 reaching a high efficiency of over 13%. In this study, DCPY2 is further optimized with an efficiency of 13.85%, maintaining a high fill factor (FF) without compromising the short circuit current. Despite its intermixed morphology, DCPY2 shows a reduced recombination rate compared to their binary counterpart (PBDB-T:Y-O6). This slower recombination in DCPY2 is attributed to the reduced wavefunction overlap of delocalized charges, achieved by spatially separating the donor and acceptor units with an alkyl linker, thereby restricting the recombination pathways. Adding 1,8-diiodooctane (DIO) into DCPY2 further reduced the recombination rate by facilitating acceptor aggregation, allowing free charges to become more delocalized. The DIO-assisted aggregation in DCPY2 (5% DIO) is evidenced by an increased pseudo-pure domain size of Y-O6. Fine molecular control at the donor/acceptor interface in the double-cable polymer achieves reduced non-geminate recombination under efficient charge generation, increased mobility, and charge carrier lifetime, thereby achieving superior performance. Nevertheless, the FF is still limited by relatively low mobility compared to the blend, suggesting the potential for further mobility improvement through enhanced higher-dimensional packing of the double-cable material.

Crystal Structure and Microwave Dielectric Characteristics of Novel Ba(Eu1/5Sm1/5Nd1/5Pr1/5La1/5)2Ti4O12 High-Entropy Ceramic

High-permittivity Ba(Eu1/5Sm1/5Nd1/5Pr1/5La1/5)2Ti4O12 (BESNPLT) high-entropy ceramics (HECs) were synthesized via a solid-state route. The microstructure, sintering behavior, phase structure, vibration modes, and microwave dielectric characteristics of the BESNPLT HECs were thoroughly investigated. The phase structure of the BESNPLT HECs was confirmed to be a single-phase orthorhombic tungsten-bronze-type structure of Pnma space group. Permittivity (εr) was primarily influenced by polarizability and relative density. The quality factor (Q×f) exhibited a significant correlation with packing fraction, whereas the temperature coefficient (TCF) of the BESNPLT HECs closely depended on the tolerance factor and bond valence of B-site. The BESNPLT HECs sintered at 1400 °C, demonstrating high relative density (>97%) and optimum microwave dielectric characteristics with TCF = +38.9 ppm/°C, Q×f = 8069 GHz (@6.1 GHz), and εr = 87.26. This study indicates that high-entropy strategy was an efficient route in modifying the dielectric characteristics of tungsten-bronze-type microwave ceramics.

Enhanced electrocatalysis of oxygen reduction on Se-Modified platinum single crystal electrodes

Se-modified platinum single-crystal surfaces in the [011¯] zone have been used as model electrocatalysts for the oxygen reduction reaction (ORR). The results show that: (1) the Pt(hkl)-[n(111)×(100)] electrodes display enhanced ORR electrocatalytic activity with Se modification. (2) Se adatoms have a double effect on the ORR kinetics at model Pt electrodes: they enhance the activity on the neighboring sites and inhibit the reaction on the sites in direct contact with the Se adatoms. (3) The hydrophobicity and the altered electronic properties are probably the reasons for the enhanced electrocatalysis for ORR with Se modification. The positive shift of the potential of zero free charge of Pt(hkl)@Se electrodes compared to that of un-modified Pt(hkl) are in agreement with these interpretations. (4) At very low coverages, the promotion effect of Se modification on ORR activity at (100) sites is stronger than that at (111) sites, which is probably due to the more pronounced hydrophobicity of the former.

Strong absorption of silica over full solar spectrum boosted by interfacial junctions and light–heat–storage of Mg(OH)2–(CrOx–SiO2)

Strong absorption of near-infrared (NIR) light is essential for efficient solar energy applications. NIR absorption primarily depends on the surface plasmon resonance of incident light with free charge carriers (FCCs). We demonstrated that S-scheme interfacial junctions (IJs) can substantially enhance FCC density, light-harvesting, photocurrent intensity, long lifetime of the charge carriers and photothermal effects, paving a way for novel light-material design. Abundant S–scheme IJs are constructed in CrOx–SiO2 nanoparticles, which increase the FCCs outstandingly and enhance light absorption exceptionally over the entire solar spectrum. IJs’ construction enables silica to harvest solar energy strongly. Doped with multifunctional CrOx–SiO2 nanoparticles, the photothermal-dehydration conversion and the stored heat of Mg(OH)2 can be improved by 7.0- and 5.1-times upon 30-min irradiation, respectively. The reversibility of the photothermal charge–discharge cycles of Mg(OH)2 improves 19.3 times. The thermal-storage kinetics is substantially improved owing to the reduction of the dehydration activation energy (−15.2 %). Mg(OH)2–CrOx–SiO2 composite is an excellent one-step system of light–heat–storage.

THE COMPOSITION OF IMPURITIES AND DEFECTS IN Cd1-XMgXTe:In, NECESSARY TO ENSURE STABLE DETECTOR PROPERTIES

A model study of the promising new material Cd0.92Mg0.08Te:In, intended for X-ray and gamma radiation detectors operating at room temperature, was carried out. The paper highlights the results of quantitative studies of the influence of the content of impurities and structural defects on the electrophysical and detector properties of Cd0.92Mg0.08Te:In. An analysis of the calculated values of resistivity ρ and concentrations of free charge carriers, life time of non-equilibrium electrons τn, and holes τp, charge collection efficiency η with different composition of impurities and defects in this material at temperature T = 298 K was carried out. The optimal ranges of energy and concentration of alloying deep donor, which ensure a stable high-resistive state and acceptable values of η, are established. Compensation of cadmium vacancies with indium admixture was studied. Assumption was made regarding possibility of increasing the operating time of the detector having semi-insulating properties and great charge collection efficiency. A direction for further research has been formulated in order to clarify the nature of a suitable doping deep donor that ensures stable properties of the detector.

Performance Improvement of Wireless Power Transfer System for Sustainable EV Charging Using Dead-Time Integrated Pulse Density Modulation Approach

The recent developments in electric vehicle (EV) necessities the requirement of a human intervention free charging system for safe and reliable operation. Wireless power transfer (WPT) technology shows promising options to automate the charging process with user convenience. However, the operation of the WPT system is designed to operate at a high-frequency (HF) range, which requires proper control and modulation technique to improve the performance of power electronic modules. This paper proposes a dead-time (DT) integrated Pulse Density Modulation (PDM) technique to provide better control with minimal voltage and current ripples at the switches. The proposed technique is investigated using a LCC-LCL compensated WPT system, which predominantly affects the high-frequency voltage and current ripples. The performance analysis is studied at different density conditions to explore the impact of the integrated PDM approach. Moreover, the PDM technique gives better control over the power transfer at different levels of load requirement. The simulation and experimental analysis was performed for a 3.7 kW WPT prototype test system under different modes of operation of the high-frequency power converters. Both the simulated and experimental results demonstrate that the proposed PDM technique effectively enhances the efficiency of the HF inverter while significantly reducing output current ripples, power dissipation and improving the overall WPT system efficiency to 92%, and leading to a reduction in the power loss in the range of 10% to 20%. This leads to improved overall system control and performance.

Rearrangement collision theory of phonon-driven exciton dissociation

Understanding the processes governing the dissociation of excitons to free charge carriers in semiconductors and insulators is of central importance for photovoltaic applications. Dyson's S-matrix formalism provides a framework for computing scattering rates between quasiparticle states derived from the same underlying Hamiltonian, often reducing to familiar Fermi's “golden rule” like expressions at first order. By presenting a rigorous formalism for multichannel scattering, we extend this approach to describe scattering between composite quasiparticles and, in particular, the process of exciton dissociation mediated by the electron-phonon interaction. Subsequently, we derive rigorous expressions for the exciton dissociation rate, a key quantity of interest in optoelectronic materials, which enforce correct energy conservation and may be readily used in calculations. We apply our formalism to a three-dimensional model system to compare temperature-dependent exciton rates obtained for different scattering channels. Published by the American Physical Society 2024

(Invited) Ultrafast Spectroscopy of Nanostructured Plasmonic and Energy Conversion Materials

Ultrafast processes that occur upon the absorption of photons by a material frequently play important roles in the efficiency of a desired energy conversion or photocatalytic outcome. For example, energetic carriers produced upon photoexcitation of plasmonic systems can quickly dissipate energy through electron scattering processes on femtosecond timescales, followed by electron-phonon coupling on picosecond timescales. Once the carriers are thermalized, less energy is available to drive a charge separation or photocatalytic process. At the same time, there are number of handles with which to improve the efficiency of photoinduced processes, and ultrafast spectroscopy can be used to characterize that improvement. Nanostructuring, for example, can produce higher efficiency of photocatalytic processes as free charges are more likely to reach a surface following photoexcitation, and local anisotropies at the surface can drive electromagnetic field enhancements and the greater production of “hot” electrons. Hybridization of differing materials is also a powerful handle with which to manipulate or improve charge separation and photocatalytic outcomes. New materials can lead to greater robustness under illumination, as well as lead to changes in dissipation processes. In this talk, I describe recent efforts to characterize novel nanostructures of interest for plasmonics and energy conversion via ultrafast spectroscopy, as well as explore the ultrafast dissipation processes in refractory plasmonic nanomaterials. Particular attention is paid to dissipation of energy to the local environment of the nanoparticles as well as to ultrafast electron scattering processes. Hybrid nanostructures are also explored. Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Fe-MOF-74 Reactivity Explored: Comprehensive Computational Assessment of O and S Atom Influence on CO2 and CO Reduction Reactions

The conversion of carbon dioxide (CO2) and carbon monoxide (CO) into higher-value materials is crucial for addressing escalating levels of greenhouse gases in Earth’s atmosphere. Employing first-principles methods, we investigated the electrocatalytic reduction of CO2 and CO on two Fe-based MOFs: Fe2DOBDC and Fe2DSBDC. By directly comparing these MOFs, we aimed to discern the impact of introducing S atoms into the framework without changing the topological framework. Several chemical properties such as electronegativity, atomic radius, polarizability, and charge density are expected to lead to significant changes for the reduction reactions upon the replacement of O atoms with S atoms. CM5 atomic charge analysis highlights some of these differences by showing the equatorial Fe-O/S bonds of Fe2DSBDC are less polarized and result in smaller positive and negative charges on the Fe and O/S atoms, respectively. Additionally, the larger S atoms are expected to weaken adsorbate binding due to less favorable van der Waals interactions near the open-metal Fe site. Consequently, the less electropositive Fe site and the larger S atoms of Fe2DSBDC impede the adsorption of reduced CO2 and CO products, while the more electropositive Fe site and smaller O atoms of Fe2DOBDC strongly favor product adsorption. This implies that CO2 and CO reduction on Fe2DSBDC is likely to yield only 2e- products (HCOOH and CH2O, respectively), whereas Fe2DOBDC is expected to produce deeper reduction products (CH2O and CH4, respectively). This insight offers a foundation for constructing novel MOFs with tunable reaction behaviors by strategically replacing O atoms with heavier S atoms in the MOF scaffold.

Photoluminescence of Chemically and Electrically Doped Two-Dimensional Monolayer Semiconductors.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers exhibit unique physical properties, such as self-terminating surfaces, a direct bandgap, and near-unity photoluminescence (PL) quantum yield (QY), which make them attractive for electronic and optoelectronic applications. Surface charge transfer has been widely used as a technique to control the concentration of free charge in 2D semiconductors, but its estimation and the impact on the optoelectronic properties of the material remain a challenge. In this work, we investigate the optical properties of a WS2 monolayer under three different doping approaches: benzyl viologen (BV), potassium (K), and electrostatic doping. Owing to the excitonic nature of 2D TMDC monolayers, the PL of the doped WS2 monolayer exhibits redshift and a decrease in intensity, which is evidenced by the increase in trion population. The electron concentrations of 3.79×1013 cm-2, 6.21×1013 cm-2, and 3.12×1012 cm-2 were measured for WS2 monolayers doped with BV, K, and electrostatic doping, respectively. PL offers a direct and versatile approach to probe the doping effect, allowing for the measurement of carrier concentration in 2D monolayer semiconductors.

Discovery of molecular ferroelectric catalytic annulation for quinolines

Ferroelectrics as emerging and attractive catalysts have shown tremendous potential for applications including wastewater treatment, hydrogen production, nitrogen fixation, and organic synthesis, etc. In this study, we demonstrate that molecular ferroelectric crystal TMCM-CdCl3 (TMCM = trimethylchloromethylammonium) with multiaxial ferroelectricity and superior piezoelectricity has an effective catalytic activity on the direct construction of the pharmacologically important substituted quinoline derivatives via one-pot [3 + 2 + 1] annulation of anilines and terminal alkynes by using N,N-dimethylformamide (DMF) as the carbon source. The recrystallized TMCM-CdCl3 crystals from DMF remain well ferroelectricity and piezoelectricity. Upon ultrasonic condition, periodic changes in polarization contribute to the release of free charges from the surface of the ferroelectric domains in nano size, which then quickly interacts with the substrates in the solution to trigger the pivotal redox process. Our work advances the molecular ferroelectric crystal as a catalytic route to organic synthesis, not only providing valuable direction for molecular ferroelectrics but also further enriching the executable range of ferroelectric catalysis.

Formation and annihilation of bulk recombination-active defects induced by muon irradiation of crystalline silicon

Muons are part of natural cosmic radiation but can also be generated at spallation sources for material science and particle physics applications. Recently, pulsed muons have been used to characterize the density of free charge carriers in semiconductors and their recombination lifetime. Muon beam irradiation can also result in the formation of dilute levels of crystal defects in silicon. These crystal defects are only detected in high carrier lifetime silicon samples that are highly sensitive to defects due to their long recombination lifetimes. This work investigates the characteristics of these defects in terms of their formation, recombination activity, and deactivation. Charge carrier lifetime assessments and photoluminescence imaging have great sensitivity to measure the generated defects in high-quality silicon samples exposed to ∼4 MeV (anti)muons and their recombination activity despite the extremely low concentration. The defects reduce the effective charge carrier lifetime of both p- and n-type silicon and appear to be more detrimental to n-type silicon. Defects are created by transmission of muons through the wafer, and there are indications that slowed or implanted muons may create additional defects. In a post-exposure isochronal annealing study, we observe that annealing at temperatures of up to 450 °C does not by itself fully deactivate the defects. A recovery of charge carrier lifetime was observed when the annealing was combined with Al2O3 surface passivation, probably due to passivation of bulk defects from hydrogen from the dielectric film.

Muonium state exchange dynamics in n -type GaAs

Muonium (Mu), a pseudoisotope of hydrogen with a positively charged muon in place of the proton, takes the form Mu+,0,− (like isolated hydrogen, i.e., H+,0,−) in semiconductors. The specifics of the charge state depend on the local electronic environment of the host. After initial muon implantation, generated Mu centers interact with free charge carriers and electronic spins, transition between sites, and form a dynamic network of state exchange. We identified the model of Mu dynamics in n-type GaAs using the density matrix simulation method and photoexcited muon spin spectroscopy technique. Fitting to the dark and illuminated μSR data provided transition rates between Mu states, revealing the underlying mechanisms at play between the Mu centers (cf. isolated H) and the host. Deduced capture/scattering cross sections of the Mu states reflected the microscopic dynamics of Mu. Illumination studies enabled us to measure interactions between Mu and minority carriers, which would be unavailable in dark measurements without heating. The methodology developed in this study may be applied to other semiconductor systems for a deeper understanding of the Mu state exchange dynamics. Published by the American Physical Society 2024

Emergence of polar skyrmions in 2D Janus CrInX3 (X=Se, Te) magnets

In the realm of multiferroicity in 2D magnets, whether magnetic and polar skyrmions can coexist within a single topological entity has emerged as an important question. Here, we study Janus 2D magnets CrInX3 (X=Se, Te) for a comprehensive investigation of the magnetic ground state, magnetic excited state, and corresponding ferroelectric polarization by first-principles electronic structure calculations and Monte Carlo simulations. Specifically, we have thoroughly elucidated the magnetic exchange mechanisms, and have fully exemplified the magnetic field dependence of the magnon spectrum. More importantly, our study reveals a previously unrecognized, remarkably large spin-spiral-induced ferroelectric polarization (up to 194.9 μC/m2) in both compounds. We propose an approach to identify polar skyrmions within magnetic skyrmions, based on the observed direct correlation between spin texture and polarization density. Elucidating this correlation not only deepens our understanding of magnetic skyrmions but also paves the way for innovative research in the realm of multiferroic skyrmions.