Passivation Of Surface States Research Articles

Dual-emissive carbon dots with high-color purity from sweet basil leaves: synthesis, characterization and optical properties

Abstract In this study, we synthesized dual-emission carbon dots (CDs) from sweet basil leaves dissolved in hexane using the hydrothermal method. Extensive analyses were carried out on their morphology, structure, and optical properties. The CDs show a spherical shape and highly disordered structure with an average diameter of 2 nm. They predominantly comprise carbon surrounded by a dense shell layer of oxygen and nitrogen-related functional groups. Under excitation at a single wavelength of 380 nm, the CDs emit two distinct peaks at 450 and 675 nm demonstrating a narrow bandwidth emission with full width at half maximum (FWHM) of 72 and 27 nm, respectively. The emission characteristics of the CDs are ascribed to the combined effects of radiative recombination of the carbon-core and fully passivated surface states, resulting in two distinct emission peaks and excitation-independent emission property. We present a highly effective and eco-friendly approach approach to fabricate luminescent CDs exhibiting dual emission properties derived from sustainable resources, holding promise for utilization in bioimaging.

Nanoheteroepitaxy of Ge and SiGe on Si: role of composition and capping on quantum dot photoluminescence.

We investigate the nanoheteroepitaxy of SiGe and Ge quantum dots (QDs) grown on nanotips substrates realized in Si(001) wafers. Due to the lattice strain compliance, enabled by the nanometric size of the tip and the limited dot/substrate interface area, which helps to reduce dot/substrate interdiffusion, the strain and SiGe composition in the QDs could be decoupled. This demonstrates a key advantage of the nanoheteroepitaxy over the Stranski-Krastanow growth mechanism. Nearly semi-spherical, defect-free, ∼100 nm wide SiGe QDs with different Ge contents were successfully grown on the nanotips with high selectivity and size uniformity. On the dots, thin dielectric capping layers were deposited, improving the optical properties by the passivation of surface states. Intense photoluminescence was measured from all samples investigated with emission energy, intensity, and spectral linewidth dependent on the SiGe composition of the QDs and the different capping layers. Radiative recombination occurs in the QDs, and its energy matches the results of band-structure calculations that consider strain compliance between the QD and the tip. The nanotips arrangement and the selective growth of QDs allow to studying the PL emission from only 3-4 QDs, demonstrating a bright emission and the possibility of selective addressing. These findings will support the design of optoelectronic devices based on CMOS-compatible emitters.

NiO@GaN nanorods-based core-shell heterostructure for enhanced photoelectrochemical water splitting via efficient charge separation

Remarkable properties of III-V semiconductors, particularly GaN nanostructured based photoelectrodes offers a potential attention in the field of photoelectrochemical water splitting (PEC-WS) for clean and sustainable hydrogen production, due to its wide bandgap, magnificent optoelectrical properties. However, the presence of inevitable surface states in GaN nanorods (NRs) leads to low solar-to-hydrogen (STH) conversion efficiency with poor stability, thereby severely limiting their practical application in PEC-WS, which can be effectively alleviated by constructing a hybrid heterostructures. Herein, we present the development of interfacial engineering of a type-II core-shell heterostructure based on p-NiO nanoparticles (NPs) loaded on n-GaN NRs photoelectrodes for PEC-WS. We assessed the impact of the NiO NPs shell density on core GaN NRs, finding that the optimized NiO@GaN NRs photoelectrode achieved a photocurrent density (Jph) of 1.38 mA/cm² at 1.23 V versus RHE and an excellent applied bias photo-to-current conversion efficiency (ABPE) of ∼0.39 %, which was 3.8 (Jph) and 4.8 (ABPE)-fold times higher than the pristine GaN NRs photoanode under 1-Sun illumination. The type-II p-n heterojunction band alignment between core-shell NiO@GaN NRs photoelectrode effectively reduced the photogenerated carrier recombination rate through surface states passivation and boost the light absorption and harvesting capacity. This facilitates a significant charge separation and transfer at the photoanode/electrolyte interface leading to enhanced redox reactions, resulting in improved PEC-WS and STH performances. These findings offer a promising strategy to design and fabricate highly efficient III-V hybrid heterostructure photoelectrodes for futuristic PEC-WS-based green energy applications.

Bulk and interface engineering with the combined addition of Na+ and E5+ (E = Nb5+, Ta5+) in the design of hematite photoanodes

In this letter, the synergistic effects of multiple element modification through a single polymeric precursor solution in hematite-based photoanodes using Na/Nb and Na/Ta as dopants are evaluated. Linear sweep voltammetry curves showed that the photoelectrochemical response was improved after Na/Nb (or Ta) addition. In both circumstances, an anodic shift (∼200 mV) in the onset potential was noticeable due to surface states created after Nb5+/Ta5+ segregation. The energetic loss caused by Nb (and Ta) segregation was partially restituted by NiFeOx addition due to the passivation of surface states, acting as recombination centers created by pentavalent modifiers. These combined modifications brought a three and four-fold increase for Na0.1HemNb1.5 NiFeOx and Na0.1HemTa1.0NiFeOx respectively. Charge carrier dynamics analysis studied by intensity modulated photocurrent spectroscopy revealed that the PEC enhancement in both systems is mainly associated with improved charge separation efficiency.

First-principles study on the electronic properties of atom passivating CsPbI2Br surface

The electronic properties of the bulk CsPbI2Br and the passivation of CsPbI2Br (110) surface states are calculated by using first-principles calculations. It is found that the band gap of CsPbI2Br is 1.42[Formula: see text]eV by using the generalized gradient approximation of the Perdew–Burke–Ernzerhof function. The band gap is about 1.96[Formula: see text]eV by using the more complex Heyd–Scuseria–Ernzerhof mixed functional, which is closer to the experimental value of 1.92[Formula: see text]eV. The valence band top of CsPbI2Br bulk is mainly contributed by I-5p orbital and Br-4p orbital, and the conduction band bottom is mainly contributed by Pb-6p orbital. Through the calculation of CsPbI2Br (110) surface states passivated by Cl, F and H atoms, it is found that H atom has the best passivation effect. Its adsorption energy value fluctuates less, it is less sensitive to the adsorption position, and the adsorption is stable. Followed by F atom, its passivation effect is worse than that of H atom, but better than that of Cl atom. Although the passivation position has a certain influence on it, it has little effect. The Cl atom is most affected by the passivation position. The different positions of passivated atoms have a significant impact on adsorption energy. The adsorption stability is poor, and the passivation effect is also poor. Through analyzing the charge density difference and Bader charges, it is found that the H atom gets more electrons from the I atom, which is beneficial to passivate surface states. The electron-acquiring ability of the F atom is inferior to that of the H atom and is superior to that of the Cl atom. The electron-acquiring ability and passivation ability of Cl atom are the weakest among the three elements.

Passivating surface states of graphitic carbon nitride for improved photocatalytic Fenton-like decontamination

Graphitic carbon nitride (g-C3N4) is a promising photocatalyst for peroxymonosulfate (PMS) activation towards water treatment, but its inherent surface defects always lead to serious recombination wasting charge carriers. Herein, we constructed cobalt doped aluminum oxide overlayer (AlOx-Co) on g-C3N4 nanosheets, passivating surface states for accelerated photocatalytic PMS activation. The AlOx coating prevents dangling bonds associated with surface recombination, and the Co doping reduces dynamic barrier. Thus, g-C3N4/AlOx-Co (0.0745 min−1) exhibits a 3.5 times higher activity than bare g-C3N4 (0.0168 min−1) towards iohexol degradation. Systemic spectroscopic and photoelectrochemical measurements reveal that AlOx-Co overlayer could weaken in-band recombination and relieve Fermi level pinning, which facilitate the charge transfer. Meanwhile, this overlayer has suitable thickness (3 ∼ 5 nm) for feasible tunneling effects to generate surface-bound radicals. Our work attempts to overcome inefficient surface charge separation and transfer simultaneously for improved contaminant removal via photocatalytic Fenton-like catalysis.

Improved stability and luminescent efficiency of AgInSe2 nanocrystals by shelling with ZnS

In this paper, AgInSe2 (AISe) core and AgInSe2/ZnS (AISe/ZnS) core/shell nanocrystals (NCs) were synthesised by a one-pot method in an organic solvent. Firstly, the synthesis of AIS core NCs with different sizes was performed by hot-injection of Se precursor into the Ag and In complexes at different temperatures from 100 °C to 180 °C for a reaction time of 20 min Then, the ZnS was grown on the surface of AISe NCs at 150 °C for 60 min to produce the AISe/ZnS core/shell structures. The as-synthesised AISe core and AISe/ZnS core/shell NCs were characterised by using x-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and optical spectroscopies (UV–vis absorption and photoluminescence (PL)). After shelling ZnS layer, AISe/ZnS core/shell NCs become more stable (12 months) in ambient air and emit strong luminescence with a high quantum yield (QY) of 40% in the range from 610 nm to 762 nm by varying the reaction temperature of AISe core synthesis from 100 °C to 180 °C. The observed increase of QY and blue-shift in photoluminescence spectra after coating ZnS on surface AISe core NCs are rationalised by the formation of the alloyed structure and passivation of surface states. With their outstanding luminescent properties, AISe core and AISe/ZnS core/shell NCs are very promising in designing emitters for solid-state lighting sources in the greenhouse and in-door farming and bio-related devices.

Surface states passivation in GaN single crystal by ruthenium solution

GaN single crystal samples were cleaned and passivated with ruthenium solution. Photoluminescence (PL) and scanning tunneling spectroscopy (STS) were used to characterize the passivated surface. PL study showed an effective increase in band edge emission after passivation. I–V (current–voltage) and dI/dV (differential conductance) spectra measurements of GaN single crystal samples using ambient STS revealed the variation in the density of states (local), shifting of Fermi-level position, and onset/offset of valence and conduction bands. We found a significant change in I–V and dI/dV measurements after surface treatment, which means modification in surface electronic properties. The ruthenium solvent passivates the surface states, converting the surface into a highly ordered and air oxidation-resistant state. Finally, Ni/GaN Schottky diodes were fabricated to demonstrate improved device characteristics after passivation, which was a direct indication of improved GaN interface due to ruthenium passivation.

Hydrothermal Synthesized CoS2 as Efficient Co-Catalyst to Improve the Interfacial Charge Transfer Efficiency in BiVO4

The bare surface of BiVO4 photoanode usually suffers from extremely low interfacial charge transfer efficiency which leads to a significantly suppressed photoelectrochemical water splitting performance. Various strategies, including surface modification and the loading of co-catalysts, facilitate the interface charge transfer process in BiVO4. In this study, we demonstrate that CoS2 synthesized from the hydrothermal method can be used as a high-efficient co-catalyst to sufficiently improve the interface charge transfer efficiency in BiVO4. The photoelectrochemical water splitting performance of BiVO4 was significantly improved after CoS2 surface modification. The BiVO4/CoS2 photoanode achieved an excellent photocurrent density of 5.2 mA/cm2 at 1.23 V versus RHE under AM 1.5 G illumination, corresponding to a 3.7 times enhancement in photocurrent compared with bare BiVO4. The onset potential of the BiVO4/CoS2 photoanode was also negatively shifted by 210 mV. The followed systematic combined optical and electrochemical characterization results reveal that the interfacial charge transfer efficiency of BiVO4 was largely improved from less than 20% to more than 70% due tor CoS2 surface modification. The further surface carrier dynamics study performed using an intensity modulated photocurrent spectroscopy displayed a 6–10 times suppression in surface recombination rate constants for CoS2 modified BiVO4, which suggests that the key reason for the improved interfacial charge transfer efficiency possibly originates from the passivated surface states due to the coating of CoS2.

Surface selective passivation and FexNi1-xOOH co-modified Fe2O3 photoanode toward high-performance water oxidation

Improving the water-splitting performance of hematite (α-Fe2O3) is still hindered due to its severe charge recombination and poor water oxidation kinetics. Herein, borate-treated Ti–Fe2O3 combined with a FexNi1-xOOH cocatalyst (FexNi1-xOOH/B/Ti–Fe2O3) greatly improved the performance of Ti–Fe2O3, and reached a notable photocurrent density of 3.39 mA/cm2 at 1.23 V vs. RHE. Transient surface photovoltage spectroscopy (TPV) directly reveals that [B(OH)4]− as a Lewis base can selectively passivate acceptor surface states on Ti–Fe2O3 photoanode surface, efficiently enhancing the charge separation efficiency. Moreover, the FexNi1-xOOH thin layer is devoted to further facilitate holes injection into the electrolyte, accelerating the water oxidation kinetics of Ti–Fe2O3 photoanode. The synergetic integration of acceptor surface states passivation and FexNi1-xOOH cocatalyst provides a novel strategy for the construction of efficient photoanodes by surface engineering.

Passivation of perovskite layer surface states with pyridine in flexible and printed perovskite solar cells

Perovskite solar cells (PSCs), prepared by using solution-processed printing techniques, gained much attention over the past few years and a considerable progress has been achieved in improving the power conversion efficiencies of these devices. Nevertheless, there are still some advancements that can be implemented, especially in terms of passivation of surface defects in the perovskite photoactive layer. Passivation can afford considerable reduction in surface recombination of charge carriers in the photoactive layer and help to obtain devices with better performance. In this work, poly(3-hexylthiophene-2,5-diyl)-based inks with small amount of pyridine as an additive are used to deposit the hole transport layer and simultaneously passivate the surface defects of the perovskite layer in flexible and printed PSCs. The devices are fabricated on flexible conductive plastic substrates using a slot-die coating method. It is found that 2.5 wt.% pyridine-containing inks for preparing hole transport layer have a positive effect on the performance of resulting PSCs. On average, around 13% improvement in the power conversion efficiency is observed for the devices with passivation as opposed to the reference devices without passivation. The effect of pyridine passivation on the structural and electronic properties of the perovskite layer on a flexible substrate is studied using experimental and analytical techniques, whereas the computer simulation methods are employed to rule out the possible mechanisms for the performance improvements in the devices with passivation. The approach presented here can be useful for developing simplified protocols for printing of flexible PSCs with the passivated perovskite layer and improved device efficiency.

Suppressing Non-Radiative Relaxation through Single-Atom Metal Modification for Enhanced Fluorescence Efficiency in Molybdenum Disulfide Quantum Dots.

To enhance the fluorescence efficiency of semiconductor nanocrystal quantum dots (QDs), strategies via enhancing photo-absorption and eliminating non-radiative relaxation have been proposed. In this study, we demonstrate that fluorescence efficiency of molybdenum disulfide quantum dots (MoS2 QDs) can be enhanced by single-atom metal (Au, Ag, Pt, Cu) modification. Four-fold enhancement of the fluorescence emission of MoS2 QDs is observed with single-atom Au modification. The underlying mechanism is ascribed to the passivation of non-radiative surface states owing to the new defect energy level of Au in the forbidden band that can trap excess electrons in n-type MoS2 , increasing the recombination probability of conduction band electrons with valence band holes of MoS2 . Our results open an avenue for enhancing the fluorescence efficiency of QDs via the modification of atomically dispersed metals, and extend their scopes and potentials in a fundamental way for economic efficiency and stability of single-atom metals.

Metal-organic framework-enabled surface state passivation integrating with single-nuclease-propelled multistage amplification for ultrasensitive lab-on-paper photoelectrochemical biosensing

The surface states and photoinduced corrosion in the photoelectrode can severely reduce the photocurrent signal and stability of photoelectrochemical (PEC) sensor, which imposes a great challenge on the improvement of analytical performance. Herein, metal–organic framework (MOF)-enabled surface passivation strategy is proposed to passivate surface states, suppress photocorrosion and facilitate interfacial charge transport. In the concrete, zeolitic imidazolate framework-8 (ZIF-8), which serves as the passivation layer, is in-situ assembled on the surface of paper-based ZnO nanorods (NRs) array to restrain the charge trapping at the surface states and keep ZnO NRs from photocorrosion, thereby substantially enhancing the photocurrent signal and durability. Moreover, the unique single-nuclease-propelled multistage amplification (SNMA) and triple-helix molecular switch (THMS)-conformation-transition-mediated signal quenching strategies are also designed to further improve the sensitivity. The SNMA process can convert target microRNA-141 into quadruple amount of useful output DNA strands (trigger DNA) in one duplex-specific nuclease (DSN) cleavage cycle assisted by succedent DSN cleavage of DNA duplexes, guaranteeing a higher target amplification efficiency. Subsequently, the THMS structure which anchors double amount of signal enhancers to the photoelectrode surface can be disassembled by the resulted trigger DNA, therefore achieving powerful signal amplification. Profiting from the MOF-enabled surface state passivation and elegantly designed signal amplification strategies, the ultrasensitive detection of microRNA-141 is realized with a liner range from 0.2 fM to 2 nM and detection limit of 72 aM, which provides a promising paradigm for developing high-performance lab-on-paper PEC biosensing platform.

Highly luminescent air-stable AgInS2/ZnS core/shell nanocrystals for grow lights

Plants need lights in the blue (400–500 nm) and red (600–700 nm) spectral regions for the photosynthesis to grow. Lights to match the mentioned spectral ranges can be provided from a blue LED and red luminescence materials. This article presents the high yield solvothermal syntheses of AgInS2 (AIS) core nanocrystals (NCs) and AIS/ZnS core/shell NCs that emit stably strong luminescence peaking around 665 nm under the excitation of a blue LED. The solvothermal syntheses of AIS core NCs were performed in oleylamine solvent added with 1,3-dimethylthiourea at 200 °C for a relatively short time of 20 min, in a closed Teflon vessel. Then, the ZnS shells with different thicknesses of 1, 3, 5, and 7 monolayers were coated on the same AIS core NCs at 150 °C for 60 min under open air. The synthesised AIS core and AIS/ZnS core/shell NCs were characterised by using X-ray diffraction (XRD), transmission electron microscopy (TEM), and optical spectroscopies (UV–Vis absorption and photoluminescence (PL), including steady state and time-resolved PL). With increasing the thickness of the ZnS shell, AIS/ZnS core/shell NCs become more stable under open air to emit the strong luminescence in the 600–700 nm range, peaking at ∼650 nm with the high luminescence quantum yield (QY) up to ∼60%. The blue-shift of the emission and increase of the QY after shelling AIS core NCs with ZnS are rationalized by the formation of the partially alloyed structure and passivation of surface states. In addition, time-resolved PL spectra exhibit to be red-shifted with the delay time after the excitation that is evident for the nature of donor–acceptor pairs recombination in AIS/ZnS core/shell NCs. Coating of layers of AIS/ZnS core/shell NCs dispersed in transparent epoxy on the 460-nm LED as assays for the grow light sources are also illustrated.

A sol-gel route to prepare CeOx dot-decorated TiO2 pigment with improved weatherability

Conventional surface treatment of TiO2 pigment aims at depositing uniform and continuous inorganic shells on the surface of TiO2 nanoparticles via precipitation methods, however, requiring strictly controlled conditions. Here we demonstrate the preparation of the CeOx dot-decorated TiO2 pigment via a sol-gel route. The CeOx dots were directly deposited onto TiO2 particles by adding CeOx sol solution into TiO2 slurry, which has been proved to improve both the weatherability and dispersibility in the water of the TiO2 pigment. Besides, the suppression mechanism of the TiO2 photoactivity by CeOx was investigated in detail using photoelectrochemical techniques. CeOx has been found to passivate surface states of TiO2 and hinder the hole transport rather than hole interfacial transfer. Moreover, an extra SiO2 layer was subsequently deposited onto the CeOx dot-decorated TiO2, further enhancing the weatherability and pigment properties of the product.

Passivation of Surface States in GaN by NiO Particles

GaN and NiO/GaN electrodes were characterized by impedance spectroscopy measurements in 0.1 M NaOH. We observed the suppression of the surface states capacitance due to the modification of the chemical state of superficial Ga atoms by NiO. This result suggests that the carriers involved in the photocorrosion of GaN in alkaline conditions originate in its surface states. In addition, we characterized the epitaxial relationship between the NiO particles deposited on GaN by transmission electron microscopy, finding the NiO{111}||GaN{0002} and NiO[220] ||GaN[112¯0] symmetry constraints.

Optical investigations of porous InP for ferroelectric application

Photoluminescence properties of porous InP are found to be strongly affected by infilling ferroelectric polymers. Based on the temperature- and excitation-power- dependent photoluminescence, the intensity suppression and blue shift of the near-band-edge emission are supposed to result from the passivation of surface states by introducing ferroelectric polymers. On the other hand, the significant enhancement of deep-level emission is caused by the increased concentration of phosphorus vacancies due to ion exchange when infilling the ferroelectric polymers into porous InP. The surface passivation of porous InP by ferroelectric polymers is useful for improving the performances of InP-based electronic and optoelectronic devices.

An ultrathin amorphous defective co-doped hematite passivation layer derived via an in situ electrochemical method for durable photoelectrochemical water oxidation

Owing to the passivation of surface states, low surface-potential fluctuations, and low charge-transfer resistance, an in situ electrochemically passivated photoanode shows higher photoelectrochemical performance and outstanding stability for ∼100 h.

Polydopamine and Nafion bi‐layer passivation modified CdS photoanode for photoelectrochemical hydrogen evolution

Photoelectrochemical (PEC) hydrogen production, using sunlight, is a clean, renewable, and eco-friendly strategy for solar fuel production. To reduce the rate of photogenerated carriers' recombination and chemically corrosion of CdS photoanode, a composite component surface state passivation layer was introduced. The polydopamine (PDA) and Nafion co-modified CdS nanorods were successfully prepared by simple electropolymerization and dipping methods. The inherent adhesion of PDA can make it adhere well to CdS nanorods to improve the surface defects of CdS, and it can also promote the assembly of Nafion coating. The mechanism of PDA/Nafion bi-layer passivation on CdS was analyzed by steady-state/time-resolved photoluminescence spectra and electron paramagnetic resonance. The optimized CdS/PDA/Nafion photoanode exhibits highly improved PEC water splitting activity and stability under visible light. It is highly anticipated that this multifunctional polymer passivation layer will provide new routes to practically engineer photoanodes in PEC hydrogen production devices.

Surprising stability of polar (001) surfaces of the Mott insulator GdTiO3

Using first-principles techniques based on hybrid density functional calculations, we study the stability, energetics, and electronic structure of the (001) surface of the Mott insulator GdTiO3 (GTO), which has an orthorhombic perovskite structure. Interestingly, we find the bare unreconstructed (but relaxed) polar surface terminated by a TiO2 plane to be very stable with a low surface energy (71 meV/Å2). As a test for stability of the TiO2 termination against reconstructions, we studied the influence of an H adatom. Hydrogen is known to form strong bonds with surface O atoms and passivate surface states, but contrary to expectations, hydrogen does not lead to a lowering of the GTO surface energy. We explain the energetics based on the surface electronic structure. We also address the interaction between the TiO2-terminated GTO surface and the high-density two-dimensional electron gas (2DEG) that can be formed at an SrTiO3 (STO)/GTO heterointerface. Unlike the situation in STO/LaAlO3 (LAO) heterostructures, where the LAO surface acts as a sink for electrons, the GTO surface does not drain electrons away from the 2DEG.