Positive Charge Generation Research Articles

The friction pair composed of polymers and Bi12TiO20 facilitates the tribocatalytic degradation of organic pollutants

In recent years, friction-induced tribocatalysis has garnered significant attention in the fields of environmental pollution and clean energy utilization. Despite its efficacy in converting mechanical energy into chemical energy, the degradation process is time-consuming, which limits its practical applications. In this study, we optimized the morphology of Bi12TiO20 catalyst and employed the most active Bi12TiO20 combined with four kinds of triboelectric polymer powders to form an efficient friction pair that enhances the tribocatalytic efficiency for organic pollutant degradation. The amount of polymer added, stirring rate, number and shape of stirring bars, as well as the materials of stirring bars and beakers were investigated. The recycled tribocatalyst exhibited recyclability and stability in degrading organic pollutants. The hydroxyl and superoxide radicals were quantitatively determined, and the generation and mechanism of positive and negative charges in the process of tribocatalysis were analysed. The potential mechanisms underlying charge generation during tribocatalysis were investigated, while also exploring the degradation pathway of Rhodamine B through high-efficiency tribocatalysis.

Boosting Kesterite Solar Cell Performance with Tantalum Oxide Passivation Layer and In Situ Formed Point Contacts

Kesterite solar cells often suffer from low open‐circuit voltage due to interface recombination. To address this, a passivation approach is explored using a dielectric layer between the absorber and buffer layer, coupled with a point contact structure. In this method, carrier transport is enabled through point contacts while effectively passivating regions covered by oxide. In this study, although forming point contacts on the front surface poses processing challenges, a novel solution is presented. Herein, in situ formed ZnS nanoparticles (NPs) during Cu2ZnSn(S, Se)4 (CZTSSe) absorber fabrication is utilized as a nanomask for front surface point contacts. A thin TaOx layer, acting as the passivation layer, is deposited over the CZTSSe surface with ZnS NPs by using atomic layer deposition (ALD). The selective removal of these NPs by deionized water soaking creates size‐controlled point contacts without damaging the absorber surface. In this approach, the first successful implementation of TaOx in enhancing CZTSSe solar cell efficiency, which jumps from 6.17% to 9.02% due to this strategy, is marked. The passivation mechanism involves sodium attraction during ALD, reduced interface defects, and the generation of positive fixed oxide charges. In this innovative method, not only solar cell efficiency is improved but new insights are also provided into using secondary phases as nanomasks in interface engineering.

Surface charge dynamics on air-exposed ferroelectric Pb(Zr,Ti)O3(001) thin films.

Probing of the free surface ferroelectric properties of thin polar films can be achieved either by estimating the band bending variance under the top-most layer or by studying the extent of the extrinsic charge accumulated outside the surface. Photoemitted or incoming low-energy electrons can be used to characterize locally both properties in a spectromicroscopic approach. Thin ferroelectric lead zirco-titanate (PZT) is investigated by combining low energy/mirror electron microscopy (LEEM/MEM) with photoemission electron microscopy (PEEM) and high-resolution photoelectron spectroscopy (XPS). Significant extrinsic negative compensation charge is proven to accumulate on the surface of the outward polarized thin film, indicated by high MEM-LEEM transition values, up to 15.3 eV, and is correlated with the surface electrostatic potential, which can be partially screened either by electrons interacting with the sample or by soft X-rays through the ejection of secondary electrons and generation of positive charge under the surface. A radiation-induced surface charge compensation effect is observed. The study indicates that air-exposed high quality ferroelectric thin films show large negative surface potentials, determined locally on the surface, which are nevertheless sensitive to beam damage and molecular desorption. These values represent a confirmation of previously estimated surface potential energy values determined from the LEED data on clean surfaces.

Kinetics of flocculated illite suspensions affected by ionic strength, pH, and hydrodynamic shearing

An experimental investigation is presented of the kinetics of particle size distributions (PSDs) of flocculated illite suspensions subject to varying ionic strength, pH, and hydrodynamic shearing. A total of 21 PSD measurements were conducted by analyzing the high-resolution optical images of particle flocs extracted from illite suspensions with a concentration of 0.4 g/L prepared at four ionic strengths (i.e., 0, 0.09, 0.17, 0.60 M NaCl solution), three pHs (i.e., 8.61, 4.51, 2.25), and three vibrational shaking speeds (i.e., 150, 225, 300 oscillation/min) at four shaking durations (i.e., 10, 30, 60, 1440 min). Experimental PSD histograms were constructed using a new bin size index (BSI) data binning method involving the conversion of original multimodal lognormal distributions into regular Gaussian distributions, followed by the probability density function (PDF)-based deconvolution to extract the number, mean, standard deviation, and fraction of different characteristic particle size groups: primary particles, flocculi, microflocs, and macroflocs. Results show that flocculation is enhanced by the high ionic strength and low pH, and more microflocs and macroflocs form at higher ionic strengths (i.e., 0.60 M) and more acidic pHs (i.e., 2.25, 4.51). Moreover, higher hydrodynamic shearing causes more breakdown of weaker flocs, but promotes stronger and smaller ones. The new BSI data binning method can better reveal the smaller-sized particle groups due to the logarithmic conversion. The PSD kinetics of illite suspensions is chiefly controlled by the face-to-face and edge-to-face interparticle interactions, caused by the suppression of the electrical double layer and generation of positive edge charges, respectively. Finally, the implications and significance of PSD kinetics of clay suspensions for engineering practices and environmental processes are discussed.

Study of Contact Electrification at Liquid-Gas Interface.

It is known that the suspended liquid droplets in clouds can generate electrostatic charges, which finally results in the lightning. However, the detailed mechanism related to the contact-electrification process on the liquid-gas (L-G) interfaces is still poorly understood. Here, by introducing an acoustic levitation method for levitating a liquid droplet, we have studied the electrification mechanism at the L-G interface. The tribo-motion between water droplets and air induced by the ultrasound wave leads to the generation of positive charges on the surface of the droplets, and the charge amount of water droplets (20 μL) gradually reaches saturation within 30 s. The mixed solid particles in droplets can increase the amount of transferred charge, whereas the increase of ion concentration in the droplet can suppress the charge generation. This charge transfer phenomenon at L-G interfaces and the related analysis can be a guidance for the study in many fields, including anti-static, harvesting rainy energy, micro/nano fluidics, triboelectric power generator, surface engineering, and so on. Moreover, the surface charge generation due to L-G electrification is an inevitable effect during ultrasonic levitation, and thus, this study can also work for the applications of the ultrasonic technique.

Positive and negative charge trapping GaN HEMTs: Interplay between thermal emission and transport-limited processes

This paper investigates the kinetics of buffer trapping in GaN-based normally-off high-voltage transistors. The analysis was carried out on transfer-length method (TLM) structures.By means of a custom setup, (i) we investigated the trapping and de-trapping processes induced by a large vertical bias and identified different mechanisms, responsible for the storage of negative and positive charge in the buffer. (ii) temperature-dependent analysis was carried out to evaluate the time constants associated to negative and positive charge build-up. Remarkably, the results indicate that the activation energy for negative charge trapping is ~0.3 eV, which is much lower than the ionization energy of carbon acceptors (0.8–0.9 eV). This result is explained by considering that trapping and de-trapping are not dominated by thermal processes (thermal emission from acceptors), but by transport mechanisms, that limit the transfer of charge to trap states. (iii) in the recovery experiments, after low stress bias negative charge trapping dominates. After high stress bias, also the effect of positive charge generation is detected, and the related activation energy is evaluated.The results presented within this paper clearly indicate that the trapping and de-trapping kinetics of normally-off GaN HEMTs are the results of the interplay of transport-limited conduction processes, that result in a low thermal activation (Ea ~ 0.3 eV), compared to that of CN acceptors.

Azathioprine Electrochemical Adsorption onto the Reduced Graphene Oxide Sheets in Absence and in the Presence of Polyaniline

The azathioprine (AZA) electrochemical adsorption onto the screen-printed carbon electrodes (SPCE) modified with the reduced graphene oxide (RGO) sheets in the absence and in the presence of polyaniline-emeraldine salt (PANI-ES) is reported in this work. Using cyclic voltammetry (CV), in the case of the SPCE modified with the RGO sheets non-functionalized and functionalized with PANI-ES, respectively, an irreversible process at the electrode/electrolyte interface is highlighted to take place. In the case of the SPCE modified with the non-functionalized RGO sheets (SPCERGO), the oxidation-reduction processes induce an up-shift of the AZA Raman lines from 856 and 1011 cm-1 to 863 and 1020 cm-1, respectively. These variations indicate an AZA adsorption onto the surface of the SPCE modified with the RGO sheets that takes place throught the imidazole and pyrimidine cycles of mercaptopurine, when the generation of the π-π* bonds between the mercaptopurine structure and hexagonal carbon cycles of RGO occurs. The electrochemical functionalization of the RGO sheets with PANI-ES is proved by the appearance of the Raman lines at 1165, 1332-1371, 1496 and 1585 and 1616 cm-1. The oxidation-reduction processes induced at the interface of the SPCE modified with PANI-ES functionalized RGO sheets and the electrolyte consisting into a phosphate buffer (PB) and AZA lead to the generation of new positive charges onto the PANI macromolecular chain and the adsorption of the drug on the working electrode surface that takes place via the π-π* bonds established between the benzene/quinoide rings of PANI and the imidazole/ purine cycles of AZA. These results indicate that the SPCE modified with the PANI-ES functionalized RGO sheets shows potential applications in the field of sensors for AZA detection.

Tumor microenvironment-responsive and oxygen self-sufficient oil droplet nanoparticles for enhanced photothermal/photodynamic combination therapy against hypoxic tumors

The combination of photothermal and photodynamic therapy (PTT/PDT) shows pronounced potential as a prominent therapeutic strategy for tumor treatment. However, the efficacy is limited by insufficient tumor-targeted delivery of PTT and PDT reagents and the hypoxic nature of the tumor microenvironment. To overcome these limitations, tumor acidity-responsive lipid membrane-enclosed perfluorooctyl bromide oil droplet nanoparticles (NPs) surface modified with N-acetyl histidine-modified D-α-tocopheryl polyethylene glycol 1000 succinate (PFOB@IMHNPs) were developed, capable of co-delivering oxygen, IR780 (a photothermal agent) and mTHPC (a photodynamic sensitizer) into tumors. Through self-sufficient oxygen transportation in combination with promotion of cellular uptake upon acid-triggered generation of surface positive charge, the PFOB@IMHNPs effectively delivered IR780 and mTHPC and produced singlet oxygen within hypoxic TRAMP-C1 cells following exposure to irradiation at 660 nm. This led to effective killing of hypoxic cancer cells in vitro. Importantly, when irradiation at 808 and 660 nm was carried out, PT/PD combination therapy utilizing PFOB@IMHNPs dramatically suppressed the growth of TRAMP-C1 tumors through effective tumor-targeted cargo delivery and relief of tumor hypoxia. Our results suggest the high potential of the PFOB@IMHNPs developed in this study in clinical application for cancer treatment.

Investigation of Quantization Effects on RTS Due to Oxide Traps Induced by Channel Hot-Carrier-Stressing in pMOSFETs

Although it is the worst degradation mechanism, the effect of channel hot carrier (CHC) stressing on random telegraph signals (RTS) has not been given enough attention in pMOSFETs. We report on the effect of CHC stressing on different RTS trap parameters namely screened scattering coefficient which controls the amount of charge carrier mobility fluctuations due to remote Coulomb scattering by the trap, RTS fluctuation amplitude, average capture time and capture cross-section. The generation of positive fixed oxide charge with stressing influences the screened Coulomb scattering of the channel carriers and therefore their mobility, in addition to the commonly accepted self-screening of the channel carriers. The two-dimensional mobility fluctuations model is adopted for analyzing the CHC effect on pMOSFETs. The comparison between theoretically and experimentally obtained screened scattering coefficients points toward the impact of the newly generated positive fixed oxide charge due to stressing. The decrease of the screened scattering coefficient results in increased RTS amplitude and decreased contribution of mobility fluctuations to RTS amplitude. Capture time and capture cross-section also change as the capture cross-section pre-factor has been impacted by the reduction of relaxation energy with stressing.

A Tutorial on Theoretical and Computational Techniques for Gas Breakdown in Microscale Gaps

Paschen’s law (PL), derived based on the Townsend avalanche (TA) condition, is commonly used to predict gas breakdown. For microscale gaps near atmospheric pressure, TA is insufficient to drive breakdown and ion-enhanced field emission (FE) dominates. Accurately predicting breakdown voltages for these gaps is critical for numerous applications, including environmental remediation, medicine, combustion, and propulsion. This tutorial summarizes theoretical and computational approaches for predicting this behavior and demonstrating the transition between the TA and FE mechanisms. It focuses on the derivation of closed-form solutions from a theory that accounts for the generation of additional positive space charge at the cathode due to electrons generated by the strong FE-induced electric fields. Appropriate simplifications using a matched asymptotic analysis in terms of the total ionization in the gap agree well with simulations and experiments and show the transition from FE for small gaps to PL at larger gaps. Specifically, this theory shows that the breakdown voltage varies linearly with gap distance when FE dominates, agreeing with both the experimental and simulation results. The particle-in-cell Monte Carlo collision (PIC/MCC) simulations used to predict the ionization coefficient provide additional insight into the mechanisms involved. Future benefits of extended theoretical and computational research for examining microscale and nanoscale breakdown and electron emission, particularly assessing the impact of electrode surface structure and device design and coupling with additional emission mechanisms, will be discussed.

Charge Effects in the Dielectric Films of MIS Structures under the Concurrent Influence of Radiation and High-Field Electron Injection

Based on the obtained experimental data, a model is developed for the processes of a variation in the charge state of MIS (metal–insulator–semiconductor) structures under the concurrent influence of high-field tunneling electron injection and radiation. The model takes into account the interaction between injected electrons and charges appearing in the dielectric film due to radiation and high-field ionization. It is shown that some holes may be annihilated during the interaction between injected electrons and holes trapped in a SiO2 film, thus leading to the formation of surface states at the interface with silicon. The effect of the electric-field intensity and injection current density on the generation and annihilation of positive charge and the formation of surface states under radiation is studied. The effect of charge processes occurring in the insulator film of a MIS structure under the concurrent action of radiation and high-field electron injection on a change in the threshold voltage of MIS devices and radiation sensors based on them is considered.

Polyethylenimine modified with 2,3-dimethylmaleic anhydride potentiates the antitumor efficacy of conventional chemotherapy

Conventional chemotherapy is a standard care for many cancers at present. However, their severe dose-dependent side effects are the major impediment for successful cancer therapy. Herein nanoparticles were used as a potentiator to enhance the uptake of free chemotherapeutic agents by cancer cells during chemotherapy. A pH-sensitive β-carboxylate amide group-containing polymer, bPEI-DMA, was obtained by a one-step chemical reaction of commercially available branched polyethyleneimine with 2,3-dimethylmaleic anhydride. The obtained single-macromolecule nanoparticles with a size of 6.4 nm possessed zwitterions and a slight net negative charge at neutral pH, and thereby showed low cytotoxicity. Incubation of MCF-7 cells with bPEI-DMA at tumor acidic pHs led to leakage of lactate dehydrogenase from the cells. Sequential incubation of bPEI-DMA and doxorubicin with MCF-7 cells at tumor acidic pHs caused enhanced uptake of doxorubicin by the cells. These results can be attributed to the tumor pH-triggered positive charge generation on the nanoparticles due to the hydrolysis of the β-carboxylate amide groups, and subsequently the positive charge caused an increase in cell membrane permeability. Sequential injection of bPEI-DMA and free doxorubicin or free cisplatin into nude mice bearing human tumors markedly inhibited the tumor growth, leading to a ~ 68% decrease in tumor volumes compared to injection of the free drugs alone. Sequential injection of bPEI-DMA and a half dose of free doxorubicin resulted in even greater tumor inhibition but less side effects than injection of a full dose of doxorubicin alone.

Simulation of charge processes in dielectric films of MIS structures at simultaneous influence by ionization and high-field injection of electrons

The paper presents model of processes of charge state changing of gate dielectric of MIS structure under conditions of simultaneous influence of radiation and high-field injection of electrons. The model takes into account the generation of positive charge by both radiation and high-field ionization and, besides, it takes into consideration the interaction of injected electrons with charges taken place in the dielectric film because of concurrent influence by ionizing radiation and high fields. We model an influence of radiation on charge state of the dielectric films of MIS structures in a wide range of influences by radiation and high-field injection of electrons. We study an influence of electric field strength and intensity of radiation on processes of generation and annihilation of positive charge accumulated in the gate dielectric because of both radiation and high-field ionization. We make suggestions on use of radiation MIS sensors which are utilized under conditions of high-field injection of electrons into the gate dielectric.

Micro-Raman spectroscopy and electrical conductivity of graphene layer on SiO2 dielectric subjected to electron beam irradiation

The effect of electron beam irradiation on the structure and doping of the graphene film located on the silicon dioxide substrate is investigated, and its influence on carrier transport mechanism is discussed. Local potential measurements have shown that the graphene layer synthesized by CVD method on Cu foil has a granular structure with the grain size of about 3–5 μm. Micro-Raman analysis along the graphene film between the metal contacts demonstrates that at low doses electron beam irradiation leads to the generation of an additional positive charge in the silicon dioxide layer, which results in fluctuations of the graphene film doping that, in turn, considerably decreases electrical conductivity of graphene. Further increase of electron beam irradiation dose leads to formation of structural defects directly in the graphene layer in addition to doping fluctuations.

Al2O3/HfO2 Multilayer High‐k Dielectric Stacks for Charge Trapping Flash Memories

Dielectric and electrical properties of Al2O3/HfO2 multilayer stacks deposited by atomic layer deposition (ALD) in dependence on the thickness of Al2О3 and HfO2 layers and the post‐deposition annealing (PDA) in different ambient (O2, N2, air) are investigated in terms of their application as trapping layers in emerging charge‐trapping non‐volatile flash memories. Differentiation is made between different processes giving rise to hysteresis effects: trapping of electrons and holes under positive and negative gate bias, respectively (as useful processes which define the memory window), and the generation of positive charge under high electric field stress (which degrades the stacks and results in permanent damage and ultimately in breakdown). Dependence of these processes on HfO2 and Al2O3 thickness as well as annealing ambient is analyzed. It is established that electrically active defects and trapping phenomena are most strongly affected by the annealing ambient. Oxygen annealing is favorable in terms of charge storage ability as it increases electron trapping and improves the electric field stress stability of Al2О3/HfO2 multilayer stacks.

A deep-level transient spectroscopy study of gamma-ray irradiation on the passivation properties of silicon nitride layer on silicon

Plasma-enhanced chemical vapor deposited silicon nitride (SiNx) films are extensively used as passivation material in the solar cell industry. Such SiNx passivation layers are the most sensitive part to gamma-ray irradiation in solar cells. In this work, deep-level transient spectroscopy has been applied to analyse the influence of gamma-ray irradiation on the passivation properties of SiNx layer on silicon. It is shown that the effective carrier lifetime decreases with the irradiation dose. At the same time, the interface state density is significantly increased after irradiation, and its energy distribution is broadened and shifts deeper with respect to the conduction band edge, which makes the interface states becoming more efficient recombination centers for carriers. Besides, C–V characteristics show a progressive negative shift with increasing dose, indicating the generation of effective positive charges in SiNx films. Such positive charges are beneficial for shielding holes from the n-type silicon substrates, i. e. the field-effect passivation. However, based on the reduced carrier lifetime after irradiation, it can be inferred that the irradiation induced interface defects play a dominant role over the trapped positive charges, and therefore lead to the degradation of passivation properties of SiNx on silicon.

Protection of Endogenous Thiols against Methylmercury with Benzimidazole-Based Thione by Unusual Ligand-Exchange Reactions.

Organomercurials, such as methylmercury (MeHg+ ), are among the most toxic materials to humans. Apart from inhibiting proteins, MeHg+ exerts its cytotoxicity through strong binding with endogenous thiols cysteine (CysH) and glutathione (GSH) to form MeHgCys and MeHgSG complexes. Herein, it is reported that the N,N-disubstituted benzimidazole-based thione 1 containing a N-CH2 CH2 OH substituent converts MeHgCys and MeHgSG complexes to less toxic water-soluble HgS nanoparticles (NPs) and releases the corresponding free thiols CysH and GSH from MeHgCys and MeHgSG, respectively, in solution by unusual ligand-exchange reactions in phosphate buffer at 37 °C. However, the corresponding N-substituted benzimidazole-based thione 7 and N,N-disubstituted imidazole-based thione 3, in spite of containing a N-CH2 CH2 OH substituent, failed to convert MeHgX (X=Cys, and SG) to HgS NPs under identical reaction conditions, which suggests that not only the N-CH2 CH2 OH moiety but the benzimidazole ring and N,N-disubstitution in 1, which leads to the generation of a partial positive charge at the C2 atom of the benzimidazole ring in 1:1 MeHg-conjugated complex of 1, are crucial to convert MeHgX to HgS NPs under physiologically relevant conditions.

Positive charge trapping phenomenon in n-channel thin-film transistors with amorphous alumina gate insulators

In this work, we investigate the charge trapping behavior in InGaZnO4 (IGZO) thin-film transistors with amorphous Al2O3 (alumina) gate insulators. For thicknesses ≤10 nm, we observe a positive charge generation at intrinsic defects inside the Al2O3, which is initiated by quantum-mechanical tunneling of electrons from the semiconductor through the Al2O3 layer. Consequently, the drain current shows a counter-clockwise hysteresis. Furthermore, the de-trapping through resonant tunneling causes a drastic subthreshold swing reduction. We report a minimum value of 19 mV/dec at room temperature, which is far below the fundamental limit of standard field-effect transistors. Additionally, we study the thickness dependence for Al2O3 layers with thicknesses of 5, 10, and 20 nm. The comparison of two different gate metals shows an enhanced tunneling current and an enhanced positive charge generation for Cu compared to Cr.

Zeta potential changing phosphorylated nanocomplexes for pDNA delivery

The objective of this study was to evaluate the suitability of a zeta potential changing system as gene delivery system. The phosphate ester bearing ligand 6-phosphogluconic acid (6-PGA) was attached to linear and branched polyethyleneimine (PEI) via a carbodiimide-mediated reaction whereby 287 μmol and 413 μmol 6-PGA could be coupled per gram polymer. Nanocomplexes of these modified polymers with pDNA showed a zeta potential of +12mV for nanocomplexes with the linear PEI-6PGA and +16mV in case of the branched derivative. By the addition of carboxymethylcellulose (CMC), zeta potentials of the complexes were reduced to +2.86 and +3.25, respectively. Phosphate release studies on Caco 2 cells and HEK-293 cells demonstrated the ability to cleave the phosphate ester. Compared to HEK-293 cells, enzymatic degradation of the phosphate ester in Caco 2 cells was 2.3-fold higher from nanocomplexes comprising linear PEI and 4.3-fold higher from those with branched PEI. Furthermore, incubation with alkaline phosphatase led to an increase in the zeta potential of nanocomplexes based on linear PEI-6PGA to +6.96mV and +8.26mV in nanocomplexes comprising branched PEI-6PGA. Studying transfection efficiency in Caco 2 cells and HEK-293 cells, a higher expression of the green fluorescent protein (GFP) could be detected in HEK-293 cells. In presence of a phosphate inhibitor, transfection efficiencies were decreased in both cells lines, due to a lacking shift of the zeta potential of the tested pDNA complexes. According to these results, zeta potential changing systems seem to be a promising strategy for future gene delivery systems, as this concept allows the in situ generation of positive charges in close proximity to the cellular surface.

Influence of CO annealing in metal-oxide-semiconductor capacitors with SiO2 films thermally grown on Si and on SiC

Understanding the influence of SiC reaction with CO, a by-product of SiC thermal oxidation, is a key point to elucidate the origin of electrical defects in SiC metal-oxide-semiconductor (MOS) devices. In this work, the effects on electrical, structural, and chemical properties of SiO2/Si and SiO2/SiC structures submitted to CO annealing were investigated. It was observed that long annealing times resulted in the incorporation of carbon from CO in the Si substrate, followed by deterioration of the SiO2/Si interface, and its crystallization as SiC. Besides, this incorporated carbon remained in the Si surface (previous SiO2/Si region) after removal of the silicon dioxide film by HF etching. In the SiC case, an even more defective surface region was observed due to the CO interaction. All MOS capacitors formed using both semiconductor materials presented higher leakage current and generation of positive effective charge after CO annealings. Such results suggest that the negative fixed charge, typically observed in SiO2/SiC structures, is not originated from the interaction of the CO by-product, formed during SiC oxidation, with the SiO2/SiC interfacial region.