Interface Band Bending Research Articles

Calibration of binding energy and clarification of interfacial band bending for the Al2O3/diamond heterojunction

Due to the presence of an intrinsic C 1s peak in diamond, it is impossible to calibrate its binding energies using the adventitious C 1s peak (284.8 eV) during x-ray photoelectron spectroscopy measurement. The absence of accurate binding energy measurement makes it challenging to determine the interfacial band bending for the oxide/diamond heterojunction. To overcome this issue, a net-patterned gold (Au) mask is applied to the boron-doped diamond (B-diamond) to suppress the charge-up effect and calibrate the binding energy using the standard Au 4f peak (83.96 eV). The B-diamond epitaxial layer shows downward band bending toward the surface with a valence band maximum of 0.85 eV. Upon the formation of Al2O3 using an ozone precursor through the atomic layer deposition technique, the B-diamond continues to exhibit downward band bending toward the Al2O3/B-diamond interface. However, the bending energy has reduced, potentially attributed to the modification of the oxygen vacancies on the B-diamond surface by the ozone precursor during the Al2O3 deposition.

Role of Interfacial Morphology in Cu2O/TiO2 and Band Bending: Insights from Density Functional Theory.

Photocatalysis, a promising solution to environmental challenges, relies on the generation and utilization of photogenerated charge carriers within photocatalysts. However, the recombination of these carriers often limits efficiency. Heterostructures, especially Cu2O/TiO2, have emerged as effective solutions to enhance charge separation. This study systematically explores the effect of interfacial morphologies on the band bending within Cu2O/TiO2 anatase heterostructures by employing density functional theory. Through this study, eight distinct interfaces are identified and analyzed, revealing a consistent staggered-type band alignment. Despite variations in band edge positions, systematic charge transfer from Cu2O to TiO2 is observed across all interfaces. The proposed band bending configurations would suggest enhanced charge separation and photocatalytic activity under ultraviolet illumination due to a Z-scheme configuration. This theoretical investigation provides valuable insights into the interplay between interfacial morphology, band bending, and charge transfer for advancing the understanding of fundamental electronic mechanisms in heterostructures.

A Broadband Photodetector Based on Non-Layered MnS/WSe2 Type-I Heterojunctions with Ultrahigh Photoresponsivity and Fast Photoresponse.

The separation of photogenerated electron-hole pairs is crucial for the construction of high-performance and wide-band responsive photodetectors. The type-I heterojunction as a photodetector is seldomly studied due to its limited separation of the carriers and narrow optical response. In this work, we demonstrated that the high performance of type-I heterojunction as a broadband photodetector can be obtained by rational design of the band alignment and proper modulation from external electric field. The heterojunction device is fabricated by vertical stacking of non-layered MnS and WSe2 flakes. Its type-I band structure is confirmed by the first-principles calculations. The MnS/WSe2 heterojunction presents a wide optical detecting range spanning from 365 nm to 1550 nm. It exhibits the characteristics of bidirectional transportation, a current on/off ratio over 103, and an excellent photoresponsivity of 108 A W-1 in the visible range. Furthermore, the response time of the device is 19 ms (rise time) and 10 ms (fall time), which is much faster than that of its constituents MnS and WSe2. The facilitation of carrier accumulation caused by the interfacial band bending is thought to be critical to the photoresponse performance of the heterojunction. In addition, the device can operate in self-powered mode, indicating a photovoltaic effect.

Effective redox reaction in a three-body smart photocatalyst through multi-interface modulation of organic semiconductor junctioned with metal and inorganic semiconductor

Forming metal/semiconductor heterojunctions is a well-known strategy to enhance photo-reaction through interface band bending. However, challenges remain in simultaneously minimizing fast charge carrier recombination and avoiding potential photo-corrosion in photocatalysts for high efficiency and long lifetime application in energy, environment, and green chemistry. We here design and propose a new strategy by which multi-interface band structure modulation occurs in a three-body photocatalyst, exemplified by Ag/PDA@Ag2S (PDA: polydopamine). Experimental analyses including in-situ x-ray photoelectron spectroscopy (XPS) and time-resolved photoluminescence (TRPL) spectroscopy together with projected density of states studies suggest an S-scheme heterojunction formation in the PDA@Ag2S, which greatly enhances the separation of photogenerated carriers. Consequently, we demonstrate that this multibody photocatalyst exhibits superior stability and selective oxidation of a series of ionic dyes under solar-simulated irradiation. The hydrogen generation efficiency (reduction) is also remarkably enhanced compared to virgin Ag/Ag2S photocatalysts. We believe this work brings a new insight into catalyst design.

Dihydropyrazine-reductant effects on band bending at PEDOT:PSS/perovskite interfaces in tin halide perovskite solar cells

This study investigates the effects of reducing treatment by 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (TM-DHP) additives on band bending in the perovskite surface near poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-selective contacts in tin-based-perovskite solar cells. We took electron spin resonance (ESR) spectroscopy measurements of PEDOT:PSS/tin perovskite stacks in the dark and under one-sun illumination. The findings indicate that downward band bending is formed in the tin perovskite layer near the PEDOT:PSS layer. This downward bending is not favorable in terms of surface passivation and hole selectivity. On the other hand, upward band bending occurs in stacks including tin perovskite layers with TM-DHP additives, indicating that TM-DHP prevents oxidation of tin perovskite, thus unfavorable downward band bending. ESR measurements of PEDOT:PSS/tin perovskite stacks without TM-DHP under illumination suggest reduction in the number of polarons caused by electron transport from perovskite layers toward PEDOT:PSS, which is driven by the unfavorable downward band bending. However, such electron transport toward PEDOT:PSS is prevented in PEDOT:PSS/tin perovskite stacks with TM-DHP. These findings, which demonstrate TM-DHP effects on interface band bending, are important for realizing highly efficient and stable tin perovskite solar cells.

Direct Z-scheme GeH/InSe heterostructure with high solar-to-hydrogen efficiency for photocatalytic water splitting

The stability, electronic structures and photocatalytic applications of GeH/InSe heterostructure are studied through first-principles calculations. The GeH/InSe heterostructure can be used as a direct Z-scheme photocatalyst for overall water splitting due to the influence of the built-in electric field and the interface band bending. The heterostructure exhibits a lower exciton binding energy than their individual constituents, indicating the effective separation of electron-hole pairs. Electron mobilities along transport directions are two orders of magnitude greater than hole mobilities, which further prompts the carriers separation rate. The heterostructure’s solar-to-hydrogen efficiency is 24.72 % at pH = 0, indicating great application prospects in photocatalysis.

2D Lateral Heterojunction Arrays with Tailored Interface Band Bending.

Two-dimensional (2D) lateral heterojunction arrays, characterized by well-defined electronic interfaces, hold significant promise for advancing next-generation electronic devices. Despite this potential, the efficient synthesis of high-density lateral heterojunctions with tunable interfacial band alignment remains a challenging. Here, a novel strategy is reported for the fabrication of lateral heterojunction arrays between monolayer Si2Te2 grown on Sb2Te3 (ML-Si2Te2@Sb2Te3) and one-quintuple-layer Sb2Te3 grown on monolayer Si2Te2 (1QL-Sb2Te3@ML-Si2Te2) on a p-doped Sb2Te3 substrate. The site-specific formation of numerous periodically arranged 2D ML-Si2Te2@Sb2Te3/1QL-Sb2Te3@ML-Si2Te2 lateral heterojunctions is realized solely through three epitaxial growth steps of thick-Sb2Te3, ML-Si2Te2, and 1QL-Sb2Te3 films, sequentially. More importantly, the precisely engineering of the interfacial band alignment is realized, by manipulating the substrate's p-doping effect with lateral spatial dependency, on each ML-Si2Te2@Sb2Te3/1QL-Sb2Te3@ML-Si2Te2 junction. Atomically sharp interfaces of the junctions with continuous lattices are observed by scanning tunneling microscopy. Scanning tunneling spectroscopy measurements directly reveal the tailored type-II band bending at the interface. This reported strategy opens avenues for advancing lateral epitaxy technology, facilitating practical applications of 2D in-plane heterojunctions.

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.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids.

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

BiOI/AgI/Ag plasmonic heterostructure for efficient photoelectrochemical water splitting

BiOI-based photocathode has attracted much attention for photoelectrochemical (PEC) water splitting application. Here, we have designed a BiOI/AgI/Ag plasmonic heterostructure through a facile electrodeposition combined with ion-exchange and illumination approach. The work function difference between BiOI and AgI brings favorable interfacial band bending, and the Ag nanoparticle exhibits localized surface plasmonic effect to boost the carrier transfer. Thus, the BiOI/AgI/Ag photocathode achieves superior current density of −1.02 mA cm−2 at 0 V vs. RHE for 200 min, which is 50 times than that of pristine BiOI. These results pave a way for promising PEC water splitting using BiOI-based heterostructure photocathodes.

Successive performance enhancement on a Ge-Ti codoped α-Fe2O3 with AlOOH modification photoanode for photoelectrochemical water splitting

The poor charge separation efficiency is the main limiting factor of the α-Fe2O3 photoanode for photoelectrochemical water splitting. In this study, we have discovered that the incorporation of Ge-Ti codoping and AlOOH modification strategies can greatly enhance the charge separation efficiency of the α-Fe2O3 photoanode. By introducing the Ti element, which possesses high conductivity, and further enhancing the charge transfer through Ge doping, we have achieved a synergistic inhibition of charge recombination in the Ge-Ti codoped α-Fe2O3. Additionally, the introduction of AlOOH through modification has led to an increase in the interface band bending, thereby enhancing charge separation by elevating the Fermi band level. Furthermore, through the secondary growth (shorted as SG) of undoped α-Fe2O3 and the incorporation of a NiFeOx cocatalyst, we have successfully adjusted the surface states and reduced the onset potential. Through the collaborative efforts of Ge-Ti codoping, SG, AlOOH modification, and NiFeOx catalysis, we have sequentially optimized the performance of the α-Fe2O3-based photoanode, resulting in an impressive photocurrent density of 3.46 mA cm−2 at 1.23 VRHE, as well as a low onset potential of 0.65 VRHE.

Exciton-Scissoring Perfluoroarenes Trigger Photomultiplication in Full Color Organic Image Sensors.

We report an unprecedented but useful functionality of perfluoroarenes to enable exciton scissoring in photomultiplication-type organic photodiodes (PM-OPDs). Perfluoroarenes that are covalently connected to polymer donors via photochemical reaction enable us to demonstrate high external quantum efficiency and B-/G-/R-selective PM-OPDs without the use of conventional acceptor molecules. We investigated the operation mechanism of the suggested perfluoroarene-driven PM-OPDs, "How can covalently bonded polymer donor:perfluoroarene PM-OPDs perform as effectively as polymer donor:fullerene blend-based PM-OPDs?". By employing a series of arenes and conducting steady-state/time-resolved photoluminescence and transient absorption spectroscopy analyses, we find that interfacial band bending between the perfluoroaryl group and polymer donor is responsible for exciton scissoring and subsequent electron trapping, which induces photomultiplication. Owing to the acceptor-free and covalently interconnected photoactive layer in the suggested PM-OPDs, superior operational and thermal stabilities were observed. Finally, we demonstrated finely patterned B-/G-/R-selective PM-OPD arrays that enable the construction of highly sensitive passive matrix-type organic image sensors. This article is protected by copyright. All rights reserved.

Interfacial electronic properties of hybrid interfaces

A detailed understanding of interfacial electronic properties of organic–inorganic hybrid junctions is crucial for their technological applications. In case of organic–inorganic hybrid junctions information about interfacial charge transfer, band bending and work function is crucial for efficient charge carrier movement across interfaces. In this context, investigation of filled and empty starts near Fermi level is relevant in molecular electronics to know electron affinity, and charge injection barriers. A multi-technique approach using ultraviolet photoemission and Inverse photoemission spectroscopy provides quantitative information about frontier occupied and unoccupied states near Fermi level, thereby providing crucial information about energy level alignment. Inverse photoemission spectroscopy technique is particularly helpful for the study of the electronic properties at metal–molecule interfaces and thus provides information about transport gap for conjugated organic molecules. Organic molecular beam epitaxy has been extensively utilized to deposit conjugated organic molecular thin films on various oxide, transition metal dichalcogenides and metallic substrates. Novel approaches have been investigated explored in order to control the interfacial energy level alignment by introducing intermediate electron donor and electron acceptor organic molecular layers across interfaces to tune the work function of electrodes. This review summarizes recent advances in exploiting combined approach using photoemission and inverse photoemission spectroscopy for the investigation of electronic properties of conjugated organic molecular interfaces.

Direct z-scheme ZnCo2S4/MOF-199 constructed by bimetallic sulfide modified MOF for photocatalytic hydrogen evolution

This work reports a simple ultrasonic impregnation method, using MOF-199 as the carrier platform to construct direct Z-Scheme ZnCo2S4/MOF-199 (ZCS/M) for stable and efficient photocatalytic hydrogen (H2) evolution. Under simulated sunlight, ZCS/M shows high photocatalytic activity, with H2 evolution amount of 11.6 mmol·g−1·h−1, which is 48.4 and 83.3 times of MOF-199 and ZCS respectively, and the apparent quantum efficiency (AQY) reaches 4.92 % at 420 nm. The successful construction of direct Z-Scheme heterojunction greatly improves the separation of electron-hole pairs to promote photocatalytic H2 evolution. In addition, ZCS is evenly anchored on the surface of MOF-199, which effectively solves the problem that ZCS is easy to agglomerate, and exposes more active sites. In-situ XPS, VB, band gap, work function and Fermi level is used to analyze the Fermi level alignment, interfacial built-in electric field and band bending, and photocatalysis mechanism and path of electron migration is speculated. This work provides a new strategy for the design and construction of direct Z-Scheme heterojunction with transition metal sulfide on the platform of MOF to enhance photocatalytic H2 evolution during water splitting.

Ferroelectric Modulation in Flexible Lead-Free Perovskite Schottky Direct-Current Nanogenerator for Capsule-Like Magnetic Suspension Sensor.

The tribovoltaic nanogenerator (TVNG), a promising semiconductor energy technology, displays outstanding advantages such as low matching impedance and continuous direct-current output. However, the lack of controllable and stable performance modulation strategies is still a major bottleneck that impedes further practical applications of TVNG. Herein, by leveraging the ferroelectricity-enhanced mechanism and the control of interfacial energy band bending, a lead-free perovskite-based (3,3-difluorocyclobutylammonium)2 CuCl4 ((DF-CBA)2 CuCl4 )/Al Schottky junction TVNG is constructed. The multiaxial ferroelectricity of (DF-CBA)2 CuCl4 enables an excellent surface charge modulating capacity, realizing a high work function regulation of ≈0.7eV and over 15-fold current regulation (from 6 to 93µA) via an electrical poling control. The controllable electrical poling leads to elevated work function difference between the Al electrode and (DF-CBA)2 CuCl4 compared to traditional semiconductors and halide perovskites, which creates a stronger built-in electric field at the Schottky interface to enhance the electrical output. This TVNG device exhibits outstanding flexibility and long-term stability (>20000 cycles) that can endure extreme mechanical deformations, and can also be used in a capsule-like magnetic suspension device capable of detecting vibration and weights of different objects as well as harvesting energy from human motions and water waves.

The Role of Electrolytes in the Relaxation of Near-Surface Spin Defects in Diamond.

Quantum sensing with spin defects in diamond, such as the nitrogen vacancy (NV) center, enables the detection of various chemical species on the nanoscale. Molecules or ions with unpaired electronic spins are typically probed by their influence on the NV center's spin relaxation. Whereas it is well-known that paramagnetic ions reduce the NV center's relaxation time (T1), here we report on the opposite effect for diamagnetic ions. We demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte solutions increase the T1 time of near-surface NV center ensembles compared to pure water. To elucidate the underlying mechanism of this surprising effect, single and double quantum NV experiments are performed, which indicate a reduction of magnetic and electric noise in the presence of diamagnetic electrolytes. In combination with ab initio simulations, we propose that a change in the interfacial band bending due to the formation of an electric double layer leads to a stabilization of fluctuating charges at the interface of an oxidized diamond. This work not only helps to understand noise sources in quantum systems but could also broaden the application space of quantum sensors toward electrolyte sensing in cell biology, neuroscience, and electrochemistry.

Manipulating the Interfacial Band Bending For Enhancing the Thermoelectric Properties of 1T'-MoTe2 /Bi2 Te3 Superlattice Films.

Interfacial charge effects, such as band bending, modulation doping, and energy filtering, are critical for improving electronic transport properties of superlattice films. However, effectively manipulating interfacial band bending has proven challenging in previous studies. In this study, (1T'-MoTe2 )x (Bi2 Te3 )y superlattice films with symmetry-mismatchweresuccessfully fabricated via the molecular beam epitaxy. This enables to manipulate the interfacial band bending, thereby optimizing the corresponding thermoelectric performance. These results demonstrate that the increase of Te/Bi flux ratio (R) effectively tailored interfacial band bending, resulting in a reduction of the interfacial electric potential from ≈127 meV at R= 16 to ≈73 meV at R= 8. It is further verified that a smaller interfacial electric potential is more beneficial for optimizing the electronic transport properties of (1T'-MoTe2 )x (Bi2 Te3 )y . Especially, the (1T'-MoTe2 )1 (Bi2 Te3 )12 superlattice film displays the highest thermoelectric power factor of 2.72 mW m-1 K-2 among all films, due to the synergy of modulation doping, energy filtering, and the manipulation of band bending. Moreover, the lattice thermal conductivity of the superlattice films is significantly reduced. This work provides valuable guidance to manipulate the interfacial band bending and further enhance the thermoelectric performances of superlattice films.

Capping-layer-mediated lattice mismatch and redox reaction in SrTiO3-based bilayers

It is well known that the traditional two-dimensional electron system (2DES) hosted by the SrTiO3 substrate can exhibit diverse electronic states by modifying the capping layer in heterostructures. However, such capping layer engineering is less studied in the SrTiO3-layer-carried 2DES (or bilayer 2DES), which is different from the traditional one on transport properties but more applicable to the thin-film devices. Here, several SrTiO3 bilayers are fabricated by growing various crystalline and amorphous oxide capping layers on the epitaxial SrTiO3 layers. For the crystalline bilayer 2DES, the monotonical reduction on the interfacial conductance, as well as carrier mobility, is recorded on increasing the lattice mismatch between the capping layers and epitaxial SrTiO3 layer. The mobility edge raised by the interfacial disorders is highlighted in the crystalline bilayer 2DES. On the other hand, when increasing the concentration of Al with high oxygen affinity in the capping layer, the amorphous bilayer 2DES becomes more conductive accompanied by the enhanced carrier mobility but almost constant carrier density. This observation cannot be explained by the simple redox-reaction model, and the interfacial charge screening and band bending need to be considered. Moreover, when the capping oxide layers have the same chemical composition but with different forms, the crystalline 2DES with a large lattice mismatch is more insulating than its amorphous counterpart, and vice versa. Our results shed some light on understanding the different dominant role in forming the bilayer 2DES using crystalline and amorphous oxide capping layer, which may be applicable in designing other functional oxide interfaces.

Surface carrier dynamics and diffusion in inorganic metal-halide perovskite films

Carrier dynamics in metal-halide perovskites, in particular diffusion, has typically been investigated on micrometer-length scales and above. However, surfaces and interfaces modify the carrier dynamics, e.g., by interface band bending and recombination. To investigate the carrier dynamics and diffusion at the surface, we use time-resolved photoelectron spectroscopy which is sensitive to the photoexcited carriers in the topmost few nm of the material. We extract electron mobilities in well-ordered ${\mathrm{CsPbBr}}_{3}$(001) and ${\mathrm{CsSnBr}}_{3}$(001) films at room temperature of $31\ifmmode\pm\else\textpm\fi{}6$ and $13\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. The mobility in ${\mathrm{CsPbBr}}_{3}$(001) increases to $200\ifmmode\pm\else\textpm\fi{}8\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 90 K, indicating strong electron-phonon coupling for electrons at the conduction band minimum. Temperature-dependent photoemission experiments find different phonon coupling mechanisms for high-energetic electrons and holes at the valence band maximum and in the main valence bands.

Evolution of Local Structural Motifs in Colloidal Quantum Dot Semiconductor Nanocrystals Leading to Nanofaceting.

Colloidal nanocrystals (NCs) have shown remarkable promise for optoelectronics, energy harvesting, photonics, and biomedical imaging. In addition to optimizing quantum confinement, the current challenge is to obtain a better understanding of the critical processing steps and their influence on the evolution of structural motifs. Computational simulations and electron microscopy presented in this work show that nanofaceting can occur during nanocrystal synthesis from a Pb-poor environment in a polar solvent. This could explain the curved interfaces and the olivelike-shaped NCs observed experimentally when these conditions are employed. Furthermore, the wettability of the PbS NCs solid film can be further modified via stoichiometry control, which impacts the interface band bending and, therefore, processes such as multiple junction deposition and interparticle epitaxial growth. Our results suggest that nanofaceting in NCs can become an inherent advantage when used to modulate band structures beyond what is traditionally possible in bulk crystals.