Band Bending Research Articles

Plasma treatment enhanced piezo-photocatalytic performance of Bi2WO6 for efficient degradation of tetracycline hydrochloride

Photocatalysis is limited by the high recombination rate of photogenerated e- and h+, but the piezoelectric effect can enhance the separation efficiency of charge carriers, so that the combining piezoelectric catalysis with photocatalysis is a highly promising approach for improving catalytic performance. In this study, aim to enhance piezo-photocatalytic performance, plasma treatment was applied to modify the surface of Bi2WO6 to form oxygen vacancies. After plasma treatment, Bi2WO6 exhibited a larger specific surface area, thinner thickness, narrower band gap, etc. These improvements have been confirmed to enhance the piezoelectric response of Bi2WO6, facilitate band bending and further improve the absorption of visible light, accelerate the separation of charge carriers, thus enhanced the piezo-photocatalytic activity. Consequently, utilization of plasma treatment had improved greatly the piezo-photocatalytic performance of Bi2WO6 for the degradation of tetracycline hydrochloride. Impressively, plasma treatment promoted the generation of 1O2 during the piezo-photocatalytic reaction, emphasizing the advantages of utilizing plasma treatment for improving the performance of piezo-photocatalysts. This work highlights the use of plasma treatment to increase active sites and induce vacancy defects on the surface of catalysts, which exhibit excellent piezo-photocatalytic degradation performance and show great potential in the field of environmental remediation.

Surface p-type band bending and energy level alignment minimizes voltage loss in perovskite solar cells

Due to the significant energy level mismatch and contact energy loss between the perovskite absorber layer and p-type hole-transporting layer (HTL), there exists a large hole transport barrier between the perovskite absorber layer and HTL, which hinders hole transport and leads to lower open circuit voltage (VOC) in perovskite devices. Therefore, the surface contact process between the perovskite film and the hole-transporting layer becomes particularly important. Here, we doped cesium silver bismuth bromide (Cs2AgBiBr6) quantum dots (QDs) into ethyl acetate (EA) as an anti-solvent to achieve the formation of a bulk heterojunction structure with quantum dot solution on the surface of the MAPbI3 perovskite film. The perovskite film exhibits appropriate band edge bending and forms a p-type semiconductor. This facilitates the directed transport of photo-induced charge carriers to the hole-transporting layer, reducing carrier recombination losses and enhancing the collection efficiency of holes by the HTL. Through characterization experiments, we have found that this method significantly improves the VOC and photovoltaic conversion efficiency (PCE) of perovskite solar cells. The perovskite solar cells fabricated using this method show a better PCE with a VOC of 1.06 V.

Enhanced photocatalytic performance of NH2-MIL-125(Ti)/SrTiO3 heterostructures by the piezoelectric effect of SrTiO3

Applying piezoelectric effect on modulating charge transport behavior was considered as one efficient strategy to make feasible charge separation in photocatalysts towards boosting hydrogen evolution performance. Here, with employing corona-poling treatment, SrTiO3 is converted to be a piezoelectric component in the constructed type II NH2-MIL-125(Ti)/SrTiO3 heterojunction. Since the piezoelectric field could tilt the band bending to a degree to be convenient for the charge transfer from SrTiO3 to NH2-MIL-125(Ti), it is also favorable to offer additional active sites on SrTiO3 to form dual reduction sites for the following piezo-photocatalytic reaction. Both the ameliorated surficial polarization and charge transfer trajectory in NH2-MIL-125(Ti)/SrTiO3 type II heterojunction are devoted to the boosted hydrogen production rate for NH2-MIL-125(Ti)/SrTiO3-0.1 (NM/STO-0.1), that is up to 1562.3 μmol h−1 g−1, 1.79 times higher than that of unpolarized NM/STO-0.1 (871.6 μmol h−1 g−1) and 2.35 times higher than that of NH2-MIL-125(Ti) (663.8 μmol h−1 g−1), respectively. This research reported initially the assistant of SrTiO3 with piezoelectric feature for enhanced performance photocatalytic hydrogen evolution, which could be one prominent strategy to achieve the reasonable design of efficient photocatalytic system.

Low-frequency noise behaviors of quasi-two-dimensional electron systems based on complex oxide heterostructures

We report the low-frequency noise behaviors in quasi-two-dimensional (quasi-2D) electron systems based on complex oxide heterostructures. First, the surface 2D electron gas (2DEG) on SrTiO3 (STO) exhibits the 1/fα-type current power spectral density (PSD) with α∼1.39. The non-unity exponent α indicates the discrepancy between the depth distributions of electrons and oxygen vacancies in the STO substrate. Second, the amorphous LaAlO3/KTaO3 (LAO/KTO) interface, another quasi-2D electron system, shows Lorentzian components of PSD at a high-frequency region around 1 kHz, implying that the amorphous overlayer can provide additional shallow charge-trapping sites to the quasi-2D electrons in the crystalline KTO substrate. Lastly, ultrathin SrRuO3 (SRO) film grown on STO substrate exhibits the Lorentzian components of PSD at a low-frequency region around 200 Hz. The slight suppression of the fast charge trapping is attributed to the intrinsic band bending at the interface between film and substrate. These results will provide a guideline for understanding the defect-induced charge trapping and the relevant electron dynamics in the quasi-2D electron systems as well as the oxide-based electronic materials in general.

Modeling the Conduction Mechanism in Chemoresistive Gas Sensor Based on Single-Crystalline Sn3O4 Nanobelts: A Phenomenological In Operando Investigation.

Investigating the sensing mechanisms in semiconducting metal oxide (SMOx) gas sensors is essential for optimizing their performance across a wide range of potential applications. Despite significant progress in the field, there are still many gaps in comprehending the phenomenological processes occurring in one-dimensional (1D) nanostructures. This article presents the first insights into the conduction mechanism of chemoresistive gas sensors based on single-crystalline Sn3O4 nanobelts using the operando Kelvin Probe technique. From this approach, direct current (DC) electrical resistance and work function changes were simultaneously measured in different working conditions, and a correlation between the conductance and the surface band bending was established. Appropriate modeling was proposed, and the results revealed that the conduction mechanism in the single-crystalline one-dimensional nanostructures closely aligns with the behavior observed in single-crystalline epitaxial layers rather than in polycrystalline grains. Based on this assumption, relevant parameters were further estimated, including Debye length, concentration of free charge carriers, effective density of states in the conduction band, and position of the Fermi level. Overall, this study provides an effective contribution to understanding the role of surface chemistry in the transduction of the electrical signal generated from gas adsorption in single-crystalline one-dimensional nanostructures.

Effect of the energy band bending on surface by Cu-poor layer of Cu(In, Ga)Se2-related photoelectrode for water splitting

To achieve the field of photoelectrochemical water splitting, the modulating of the valence band maximum (VBM) grading on the surface of Cu(In, Ga)Se2 (CIGS) photoelectrodes via the deposition of additional In and Se (forming Cu-poor-CIGS layer) following CIGS growth was investigated. The resulting downshift of the Fermi level was likely to occur around the surface of the CIGS films for modulating the VBM grading, increasing the onset potential of the CIGS-related photoelectrodes. Moreover, a hole-blocking layer was obtained via modulated VBM grading of the CIGS-related photoelectrode, which increased the photocurrent density of the CIGS-related photoelectrode. Subsequently, a Cu-poor-CIGS/CIGS interface was formed for modulating the VBM grading using Cu-poor-CIGS, which contributed to increasing the photocurrent density owing to the enhancement of charge separation by the depletion layer in the Cu-poor-CIGS/CIGS interface. This study shows that water splitting can be effectively improved by modulating the VBM grading on the surface of CIGS-related photoelectrodes.

Vertical plane depth‐resolved surface potential and carrier separation characteristics in flexible CZTSSe solar cells with over 12% efficiency

AbstractCu2ZnSn(S,Se)4 (CZTSSe) solar cells have resource distribution and economic advantages. The main cause of their low efficiency is carrier loss resulting from recombination of photo‐generated electron and hole. To overcome this, it is important to understand their electron‐hole behavior characteristics. To determine the carrier separation characteristics, we measured the surface potential and the local current in terms of the absorber depth. The elemental variation in the intragrains (IGs) and at the grain boundaries (GBs) caused a band edge shift and bandgap (Eg) change. At the absorber surface and subsurface, an upward Ec and Ev band bending structure was observed at the GBs, and the carrier separation was improved. At the absorber center, both upward Ec and Ev and downward Ec‐upward Ev band bending structures were observed at the GBs, and the carrier separation was degraded. To improve the carrier separation and suppress carrier recombination, an upward Ec and Ev band bending structure at the GBs is desirable.

Approach to determining the limiting recombination mechanism in CdTe-based solar cells

Determining exactly how the performance of a thin-film photovoltaic device is limited by a particular recombination mechanism can be difficult, particularly in the case of CdTe solar cells. As a result, efforts are being made to improve all parts of the device without good knowledge of which improvements are necessary. To understand where the device limitation is, the recombination current densities of at least one on of the interfaces must be known. Here, we present a method to determine which recombination mechanisms is limiting. First, back illuminated quantum efficiency measurements are used to determine the key parameters of the back interface – the back surface recombination velocity and the band bending near the back surface. Once these back interface parameters are determined, the recombination current densities can be calculated for a front illuminated device to determine the limiting mechanism. The validity of the approach is tested using previously reported data.

Enhancing minority carrier lifetime in Ge: Insights from HF and HCl cleaning procedures

Efficiently passivating germanium (Ge) surfaces is crucial to reduce the unwanted recombination current in high-performance devices. Chemical surface cleaning is critical to remove surface contaminants and Ge oxides, ensuring effective surface passivation after dielectric deposition. However, Ge oxides can rapidly regrow upon air exposure. To understand the surface evolution after wet cleaning, we present a comprehensive study comparing HF and HCl deoxidation steps on p-type Ge surfaces and monitor the surface as a function of air exposure time. Distinct oxide regrowth dynamics are observed: HF-treated samples exhibit swift regrowth of all Ge oxide states, whereas HCl-treated Ge surfaces exhibit a lower concentration of low degrees of oxidation and slower or no regrowth of high oxide states even after 110 min of air exposure. In addition, the presence of Ge–Cl bonds induces different oxidation dynamics compared to the Ge–OH bonds resulting from HF cleaning. This leads to varying surface electronic band structures, with HF-treated Ge exhibiting a strong positive band bending (+0.20 eV). Conversely, HCl-treated samples display a lower band curvature (+0.07 eV), mostly due to the presence of Ge–Cl bonds on the Ge surface. During air exposure, the increased GeOx coverage significantly reduces the band bending after HF, while a constant band bending is observed after HCl. Finally, these factors induce a reduction in the surface recombination velocity after wet etching. Combining both chemical and field-induced passivation, HF-treated Ge without rinsing exceeds 800 μs.

Towards 26% efficiency in inverted perovskite solar cells via interfacial flipped band bending and suppressed deep-level traps

Minimizing interfacial recombination loss in inverted perovskite solar cells is achieved by introducing piperazinium diiodide (PDI) as a surface modifier to passivate deep surface defects and adjust the interface band bending.

Van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device.

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Physical modeling for photo-capacitance characteristics of metal oxide TFTs

We propose an amorphous metal oxide thin film transistor photo-capacitance model in the depletion region that takes Fermi level splitting and band-bending rearrangement into consideration. The split Fermi level is used to characterize the variation in trapped electrons under illumination. Those trapped electrons are excited by optical energy transport under the electric field induced by gate voltage, changing the charge density in the space charge and inducing the rearrangement of band bending. By comparing the data calculated from the model with the test data under three different illumination conditions, that is, 808, 635, and 520 nm, we verify the correctness of this model. Furthermore, the fitting results were in accordance with the general law: the higher the photon energy, the higher the energy level splitting.

Facile engineering of TiO2@Pd based hybrid heterogeneous nanostructures for enhanced NO2 gas sensing at room temperature under UV activation.

Highly sensitive sub-ppb level NO2 gas sensors based on hybrid heterostructures made out of metal nanoparticles (NPs) and porous metal oxide semiconductors are of great significance in environmental monitoring and medical diagnosis. Herein, we report the successful synthesis of a Pd nanoparticle decorated porous TiO2 nanostructure-based hybrid Schottky heterostructure and its application in room temperature (RT ∼ 30 °C) low-ppb level NO2 detection under UV activation. The optimized TiO2@Pd 50 hybrid heterogeneous nanostructures (HNs) showed an excellent NO2 (100 ppm) gas sensing response of 175.42%, excellent cycling stability, outstanding selectivity, low detection limit (∼82 ppb) and fast response/recovery speed (48/72 s) under UV illumination. Moreover, the as-prepared hybrid sensor demonstrates reliable reproducibility and robust sensing responses under various elevated temperatures (30-200 °C) and relative humidity conditions (0-80% RH) towards NO2. The excellent RT NO2 gas sensing properties of TiO2@Pd 50 HNs are primarily attributed to the improved surface area, enhanced oxygen vacancies, low charge carrier recombination, and catalytic effect triggered by the hot electrons induced, from the localized plasmonic resonance effect of Pd NPs, resulting in effective NO2 adsorption/desorption dynamics. The Schottky junction formation at the hybrid TiO2@Pd 50 HNs interface effectively aids in the separation of charge carriers via band bending, leading to excellent charge transfer between the NO2 molecules and the heterointerface under UV light, yielding substantial sensitivity and selectivity to NO2. The present work is expected to provide a strategic route for designing and fabricating sub-ppb level high-performance NO2 sensors based on hybrid heterostructures.

Contact evaluation of the penta-PdPSe/graphene vdW heterojunction: tuning the Schottky barrier and optical properties.

In this work, we design a van der Waals heterojunction composed of semiconducting penta-PdPSe and semi-metallic graphene (G) monolayers based on state-of-the-art theoretical calculations. Our results show that both monolayers well preserve their intrinsic features and possess an n-type near Ohmic Schottky contact with a low Schottky barrier height of 0.085 eV for the electrons at the vertical interface. The electronic band alignment suggests a negative band bending of -1.47 eV at the lateral interface, implying electrons as the major transport carriers. Moreover, the transmission gap closely mirrors the heterojunction's band gap, indicating a subtle yet profound interaction between graphene and penta-PdPSe monolayers, which leads to enhanced optical absorption coefficient reaching 106 cm-1 and strong conductivity spanning the visible to ultraviolet region. In addition, our study demonstrates the ability to modify the penta-PdPSe/G heterojunction interface, switching between p-type as well as Ohmic contacts by applying external electric fields. These properties render the penta-PdPSe/G heterojunction promising for optoelectronic applications.

Effect of fuel dosage in self-propagation combustion: evolution of BiOCl to BiOCl/Bi24O31Cl10 heterojunctions enriched with oxygen vacancies

BiOCl/Bi24O31Cl10 heterojunction: BiOCl and Bi24O31Cl10 contact to form a type-Z heterojunction. The band bending facilitates the transfer and recombination of charge carriers of low redox potential, leading to improved degradation efficiency.

Design and implementation of an infrared artificial visual neural synapse based on a p-WSe2/n-Ta2NiS5 van der Waals heterojunction

Inspired by human visual synapses, high performance and versatile infrared vision synapses can be achieved in the p-WSe2/n-Ta2NiS5 van der Waals heterojunction by introducing narrow band gap materials and bias-induced band bending.

Enhanced photocatalytic activity in Mn-doped multiferroic BiFeO3

Mn doping in ferroelectric BiFeO3 enhanced photocatalytic activity through the synergistic effect of the enhanced band bending on the surface as a result of internal screening of polarization bound charges and formation of inter-band energy levels while shifting the energy bands closer together.

In situ fabrication of a 2D/2D WO3/Bi5O7I S-scheme heterojunction with enhanced spatial charge separation and tetracycline degradation

Schematic illustration of the formation of energy band bending and interface electric field (IEF) from Bi5O7I (+) to WO3 (−) before and after the contact of WO3 and Bi5O7I, and the possible S-scheme carrier transfer mechanism.

Different spectral features near the energy bandgaps of normal and inverse heterostructures of In<sub>0.52</sub>Al<sub>0.48</sub>As/InP

Photoconductivity (PC) spectroscopy and photoluminescence (PL) spectroscopy were used to characterize two heterostructure configurations of InAlAs/InP grown by molecular beam epitaxy (MBE) on the InP (100) substrate. The sample A is the type called normal heterostructure, which has an In<sub>0.52</sub>Al<sub>0.48</sub>As layer grown on InP, while sample B is called the inverse type formed by an InP cap layer on In<sub>0.52</sub>Al<sub>0.48</sub>As. The front excitation was employed in both PC experiment and PL experiment and the measurements were conducted at 77 K. The PC spectrum of sample A shows an abnormal step-like drop when the photon energy is larger than the energy band gap of In<sub>0.52</sub>Al<sub>0.48</sub>As. The phenomenon implies that the conductance of sample is a multilayer effect including the contribution of interfacial two-dimensional electron gas (2DEG). Moreover, a conductance peak is observed at 916 nm below the bandgap of InP. Accordingly, an intense luminescent peak at the wavelength manifests in the PL spectrum. The origin of the 916 nm peak is attributed to the recombination of 2DEG electrons with the valence band holes excited near the interface. However, the spectral feature of the above energy does not exist in both PC and PL spectra of sample B. This difference may be explained by the different interface electronic structures of the inverse interface. For the latter case, considering that a graded variation in In-As-P composition is related to the inverse interface of InP/InAlAs, the band bending effect should be weak. In such a case, the bound energy of 2DEG in the interface potential well is raised closer to the conductance band of the bulk. Consequently, the recombination energy of 2DEG at the inverse interface with the holes in the valence band is close to the band-to-band transition of InP bulk and the luminescence is difficult to be distinguished from that of bulk InP. The work also demonstrates that the comparative study with both PC technique and PL technique is helpful to provide a full insight into the interface electronic property.

V-doped MoS2 nanozymes providing reactive oxygen species and depleting glutathione for photothermally-enhanced nanocatalytic therapy.

Introduction: The tumor microenvironment and multidrug resistance of tumor cells seriously impair the activity of the nanozymes. Methods: Herein, a polyethylene glycol (PEG)-modified vanadium-doped molybdenum disulfide (V-MoS2@PEG) nanozymes were constructed to enhance anti-tumor activity through multi-enzymatic catalysis and photothermal effect with simultaneous reactive oxygen species replenishment and glutathione depletion. Results and discussion: V-MoS2@PEG nanosheets exerted peroxidase activity by causing molybdenum ion (Mo4+) to react with hydrogen peroxide to form toxic hydroxyl radicals (·OH). Meanwhile, the V-doping can deplete glutathione avoiding ·OH consumption. In addition, the high heat generated by V-MoS2@PEG nanozymes under near-infrared laser irradiation brought about a desirable local temperature gradient, which produced an enhanced catalytic effect by promoting band bending. Furthermore, the photothermally inspired polarized charge increased the permeability of the tumor cell membrane and promoted further aggregation of the nanozymes, which realized the combination of photothermal therapy with multi-enzymatic catalysis, solved the problem of multi-enzyme catalysis, and improved the anti-tumor efficiency.