Contact electrification (CE) (or triboelectrification) is a well‐known phenomenon, and the identity of the charge carriers and their transfer mechanism have been discussed for decades. Recently, the ...species of transferred charges in the CE between a metal and a ceramic was revealed as electron transfer and its subsequent release is dominated by the thermionic emission process. Here, the release of CE‐induced electrostatic charges on a dielectric surface under photon excitation is studied by varying the light intensity and wavelength, but under no significant raise in temperature. The results suggest that there exists a threshold photon energy for releasing the triboelectric charges from the surface, which is 4.1 eV (light wavelength at 300 nm) for SiO2 and 3.4 eV (light wavelength at 360 nm) for PVC; photons with energy smaller than this cannot effectively excite the surface electrostatic charges. This process is attributed to the photoelectron emission of the charges trapped in the surface states of the dielectric material. Further, a photoelectron emission model is proposed to describe light‐induced charge decay on a dielectric surface. The findings provide an additional strong evidence about the electron transfer process in the CE between metals and dielectrics as well as polymers.
The decay of contact electrification (CE)‐induced electrostatic charges under light irradiation is investigated at the nanoscale. The results suggest that there exists a threshold photon energy for releasing the triboelectric charges from the surface, and the charge decay is suggested to be caused by photoemission. The photon excitation effect provides strong evidence for electron transfer in CE.
Constructing heterojunctions is an efficient approach for enhancing charge separation to optimize photoreactivity. Although the aligned built‐in electric fields across the heterointerface are ...generally considered as the main driving force for charge separation, diffusion‐controlled charge separation also happens, which is poorly investigated in photocatalytic heterojunctions. Here, a perylene‐3,4,9,10‐tetracarboxylic diimide (PDI)–bismuth oxyiodide (BiOI) heterojunction is elaborately fabricated by in situ successive ion layer adsorption and reaction (SILAR) methods. Utilizing Kelvin probe force microscopy (KPFM), the local separation of photogenerated charge carriers across the heterointerface is directly mapped, which obeys a Z‐scheme mechanism. Experimental results and theoretical simulations reveal that the differences of electron densities between PDI and BiOI enable a diffusion‐controlled charge separation process, which overwhelm that of built‐in electric fields across heterointerfaces. Benefiting from the effective charge separation driven by a diffusion‐controlled driving force, this PDI/BiOI heterojunction exhibits superior photocatalytic activities even under infrared (IR)‐light irradiation. These findings highlight the importance of diffusion‐controlled charge separation, and also offer useful roadmaps for the design of high‐performance heterojunction photocatalysts for down‐to‐earth applications.
Swimming upstream driven by the transient diffusion electric field, the photoinduced electrons of perylene‐3,4,9,10‐tetracarboxylic diimide (PDI) swim toward bismuth oxyiodide (BiOI) across the PDI/BiOI heterointerfaces under illumination, overcoming the aligned built‐in electric field.
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•Air-stable and water-stable halide perovskite photocatalysts.•Core-shell CsPbBr3@monolayer carbon nitride heterojunction.•Visible light harvesting photoanode for water-splitting.•CO2 ...reduction photocatalyst.•Unusual 30 nm redshift after wrapping CsPbBr3 QDs with CN nanosheets.
Halide perovskites are exciting candidates for broad-spectrum photocatalysts but have the problem of ambient stability. Protective shells of oxides and polymers around halide perovskite nano- and micro-crystals provide a measure of chemical and photochemical resilience but the photocatalytic performance of perovskites is compromised due to low electron mobility in amorphous oxide or polymer shells and rapid charge carrier recombination on the surface. Herein an in situ surface passivation and stabilization of CsPbBr3 nanocrystals was achieved using monolayered graphenic carbon nitride (CNM). Extensive characterization of carbon nitride protected CsPbBr3 nanocrystals (CNMBr) indicated spherical CsPbBr3 nanoparticles encased in a few nm thick g-C3N4 sheets facilitating better charge separation via percolation/tunneling of charges on conductive 2D nanosheets. The CNMBr core-shell nanocrystals demonstrated enhanced photoelectrochemical water splitting performance and photocurrent reaching up to 1.55 mA cm−2. The CNMBr catalyst was successfully deployed for CO2 photoreduction giving carbon monoxide and methane as the reaction products.
Nanoscale engineered materials such as nanocomposites can display or be designed to enhance their material properties through control of the internal interfaces. Here, we unveil the nanoscale origin ...and important characteristics of the enhanced dielectric breakdown capabilities of gold nanoparticle/polymer nanocomposites. Our multiscale approach spans from the study of a single chemically designed organic/inorganic interface to micrometer-thick films. At the nanoscale, we relate the improved breakdown strength to the interfacial charge retention capability by combining scanning probe measurements and density functional theory calculations. At the meso- and macroscales, our findings highlight the relevance of the nanoparticle concentration and distribution in determining and enhancing the dielectric properties, as well as identifying this as a crucial limiting factor for the achievable sample size.
Here, we report on nm-scale electrical potential imaging throughout As-doped and Cu-doped CdSeTe absorbers using Kelvin probe force microscopy (KPFM). The potential imaging was conducted both ...laterally and vertically on beveled films using ion milling at small glancing angle. KPFM images electrical potential on the beveled surface and assesses defect charging in the subsurface region within a screening length from the beveled surface. We found that the grain boundaries were positively charged and that there were significant potential fluctuations in both grain boundary versus grain interior and intragrain. We further found that these potential fluctuations decreased significantly toward the front interface. Time of flight secondary ion mass spectrometry imaging shows that Se content increased toward the front interface, consistent with Se passivation of defects. The potential fluctuation was induced by defect charging, and the results elucidate different details of the defect configurations and grain structures of the films with different CdCl2 treatment temperatures in the As-doped CdSeTe. The defect configurations in the region near the front interface can be a main factor contributing to the device performance difference. Our potential imaging provides insights about the defects throughout the absorber films, and shows that the potential fluctuation has a direct correlation to the Voc deficit.
Through detailed device characterization using cross‐sectional Kelvin probe force microscopy (KPFM) and trap density of states measurements, we identify that the J–V hysteresis seen in planar ...organic–inorganic hybrid perovskite solar cells (PVSCs) using SnO2 electron selective layers (ESLs) synthesized by low‐temperature plasma‐enhanced atomic‐layer deposition (PEALD) method is mainly caused by the imbalanced charge transportation between the ESL/perovskite and the hole selective layer/perovskite interfaces. We find that this charge transportation imbalance is originated from the poor electrical conductivity of the low‐temperature PEALD SnO2 ESL. We further discover that a facile low‐temperature thermal annealing of SnO2 ESLs can effectively improve the electrical mobility of low‐temperature PEALD SnO2 ESLs and consequently significantly reduce or even eliminate the J–V hysteresis. With the reduction of J–V hysteresis and optimization of deposition process, planar PVSCs with stabilized output powers up to 20.3% are achieved. The results of this study provide insights for further enhancing the efficiency of planar PVSCs.
Through detailed characterizations, it is identified that the current density‐voltage hysteresis of planar perovskite solar cells using low‐temperature atomic‐layer deposited SnO2 electron selective layers originates from the poor‐electrical conductivity of the SnO2 layers. A facile low‐temperature thermal annealing in ambient air can effectively reduce the degrees of the hysteresis and improve the power conversion efficiency of planar perovskite solar cells.
Kelvin Probe Force Microscopy
In article number 2102495, Xiaoji G. Xu and co‐workers introduce a novel approach to surface potential imaging with tapping mode Kelvin probe force microscopy without ...the need for an external AC driving voltage – fully compatible with photoinduced force microscopy for multimodal nano‐imaging.
The fracture mechanism of hydrogen charged Ti–6Al–4V has been investigated through a multianalytical approach. The difference in hydrogen solubility between β phase (high solubility) and α phase ...(low) governs the formation and growth pattern of titanium hydrides, and determine the fracture mode of Ti–6Al–4V. Depending on the hydrogen charging extent, the penetration of hydrogen and distribution of hydrides can be divided into three stages. In the initial stage hydrogen diffuses mainly into the β phase, as judged from its increase in Volta potential, and with no hydrides formed. Failure analysis after tensile tests exhibits plastic behavior and a fracture surface with mainly dimples. In the subsequent transition stage, hydrides are formed at the α/β interfaces and along α grain boundaries. More initial cracks occur in the brittle hydrides and the fracture surface transforms from dimple to quasi-cleavage. In the final stage a layer of uniformly distributed hydride is produced on the surface and within the α phase. Supported by nanoindentation measurements, the plasticity of the charged sample diminishes with hydrogen charging time, and an intergranular-transgranular mixed fracture is observed. Overall, the study forms clear evidence that the distribution and cracks of hydrides influence the fracture mode of the Ti-6A-4V alloy.
•Hydrides form at α/β interface, grow along α grain boundaries, and occupy α grains.•Cracks initiate and propagate through soft-brittle hydrides.•The presence and distribution of hydrides determine the mode of fractures.•Hydrogen permeation increases the Volta potential difference between β and α phases.
Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction, and interfacial trap states can ...prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on time scales of seconds cause hysteresis in the current–voltage characteristics. In this work, we localized and investigated these slow processes using frequency-modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers formed at the interfaces of the selective contacts with the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at the interfaces of MAPI with adjacent layers. Our results indicate that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states causes the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.
Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells have been fabricated by various methods. Sputtering is one of the vacuum processes that can be used for the growth of the precursors. In this study, ...CZTSSe thin-films that were fabricated using metal-precursors and with a high efficiency of 12.3% were examined in a comparison with CZTSSe thin-films that were fabricated using compound-precursors, whose efficiency is 9.1%. Especially, the Kelvin probe force microscopy (KPFM) analysis and local current measurement by conductive atomic force microscopy (c-AFM) show that their local electrical properties indicate completely opposite results. The grain boundaries (GBs) has a downward surface potential bending, and this repelled the minority carriers into the intragrains (IGs) in the sample from the metal-precursors. Therefore, we originally verified the differences between the carrier behaviors and the current flows on the surface.
•Using different precursor materials and stacks, Cu2ZnSn(S,Se)4 thin-film solar cells were fabricated.•The PCE of the metal-precursor CZTSSe solar cell is 12.3%, while that of compound-precursor's is 9.1%.•Downward potential bending was observed in the metal-precursor CZTSSe thin-film.•Surface current was formed on the intra-grains on the metal-precursor CZTSSe thin-film.•The compound-precursor CZTSSe thin-film indicated inversed results from the metal-precursor one.
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