Tunable Energy Levels Research Articles

Predicting and analyzing the fill factor of non-fullerene organic solar cells based on material properties and interpretable machine-learning strategies

In recent years, encouraging progress in non-fullerene acceptors-based organic solar cells (NFAs-OSCs) has been made in the field of clean-energy technologies. Nevertheless, achieving a high fill factor (FF) for NFAs-OSCs is a great challenge due to the FF value can be dramatically affected by various factors. An accurate prediction (based on empirical data) of how inherent characteristics of polymer-NFA combinations (e.g., frontier molecular orbitals (FMO) and charge-carrier mobilities) affects the FF in NFAs-OSCs is therefore urgently needed for the future commercialization prospect. The presented work demonstrates an outperforming predictive model using the optimized Gradient-boosting decision tree (GBDT) machine learning (ML) algorithm to accurately predict the FF value, which is based on a dataset consisting of > 180 unique donor/NFA blends with reported FF from previously published publications and experimental physical descriptors (FMO and charge-carrier mobilities). In addition, the SHapley Additive exPlanations (SHAP) strategy is further applied to not only gain the interpretability of the GBDT model prediction but also directly map the relationship between the FF value and experimental physical descriptors by systematic interpretable explanations (e.g., global and local). The SHAP analysis qualitatively reveals that the highest occupied molecular orbital (HOMO) of the donor and the HOMO of the acceptor are the two most important features associated with FF value prediction, which is in good agreement with previous experimental facts. Furthermore, the ML strategies explore the key requirements (e.g., small HOMO offset (<0.3 eV), high hole and electron mobilities (>1.0 × 10–4 cm2 V−1 s−1), and desired μh/μe ratio (in the range of 1.8–3.3)) for enhancing the FF values of NFAs-OSCs. This study provides novel insights into the possible physical mechanisms of FF enhancement, thus facilitating research efforts (e.g., rational design of energy level tuning in donor/NFA blend) for high-performance NFAs-OSCs.

Unveiling the impact of exchange-correlation functionals on the description of key electronic properties of non-fullerene acceptors in organic photovoltaics.

Non-fullerene electron acceptors have emerged as promising alternatives to traditional electron-acceptors in the active layers of organic photovoltaics. This is due to their tunable energy levels, optical response in the visible light spectrum, high electron mobility, and photochemical stability. In this study, the electronic properties of two representative non-fullerene acceptors, ITIC and Y5, have been calculated within the framework of density functional theory using a range of hybrid and non-hybrid density functionals. Screened range-separated hybrid (SRSH) approaches were also tested. The results are analyzed in light of the previously reported experimental outcomes. Specifically, we have calculated the oxidation and reduction potentials, fundamental and optical gaps, the highest occupied molecular orbital and lowest unoccupied molecular orbital energies, and exciton binding energies. Additionally, we have investigated the effects of the medium dielectric constant on these properties employing a universal implicit solvent model. It was found that hybrid functionals generally perform poorly in predicting oxidation potentials, while non-hybrid functionals tend to overestimate reduction potentials. The inclusion of a large Hartree-Fock contribution to the global or long range was identified as the source of inaccuracy for many hybrid functionals in predicting both redox potentials and the fundamental and optical gaps. Corroborating with the available literature, ∼50% of all tested functionals predicted very small exciton binding energies, within the range of ±0.1eV, that become even smaller by increasing the dielectric constant of the material. Finally, the OHSE2PBE and tHCTHhyb functionals and the optimal tuning SRSH approach emerged as the best-performing methods, with good accuracy in the description of the electronic properties of interest.

Simultaneous inclusion of quantum dots in multi-functional layers of thin film organic solar cells

The role of quantum dot (QD) decoration in the hole transport buffer layer and the photoactive medium on the photovoltaic parameters of thin film organic solar cells (TFOSCs) was investigated. A cadmium–tellurium-based QD was synthesized successfully and embedded in two of the functional layers of a TFOSC to improve its overall power conversion efficiency. The experimentally determined optimum concentration of the QD was maintained in the interfacial layer to investigate the effect of QD concentration in the active layer. The observed increased short-circuit current density (Jsc) and open circuit voltage (Voc) are attributable to the enhanced energy level tuning, broadened optical absorption, and charge transport process facilitated by the integration of QDs inside the media. Moreover, an improved device efficiency was obtained when the solvent additive was introduced into the bulk heterojunction photoactive layer films to facilitate QD dispersion and increase the interpenetrating network of the active layer blend that reduces the occurrence of trap sites, which, in turn, limits the Auger recombination rates. The QD-doped TFOSCs catalyzed with solvent additives displayed an enhanced overall photovoltaic parameter, which is quite appreciable in comparison with that of the pristine devices.

(Invited) Tuning Photocatalytic Activity with Energetic and Kinetic Asymmetry at Coating-Stabilized Particulate Semiconductors

Semiconductor photocatalysts coevolve reductive and oxidative reactions at nanoscale proximity. The local charge separation and local coevolution have unique advantages: i) photocatalysts can operate in neutral pH water or water vapor instead of strong acid or base; ii) most semiconductors form a self-passivating layer at corroded surfaces to prevent corrosion propagation; and iii) nanoparticulate photoabsorbers or nanocrystalline films have achieved near-unity quantum efficiency, promising scale-up solar-to-chemical conversion. Despite the near-unity quantum efficiency achieved for SrTiO3 particles, the Edisonian approach is applied to the vast majority of photocatalysts, thus limiting progress. Technologically important semiconductors of 1.1 – 2.3 eV bandgaps and properly adjusted band edges, such as GaP and Si, can be ball milled into powder, followed by chemical passivation and protective coating processes. Thus, we introduce the Hu-lab-invented charge-transport coatings to protect semiconductors, such as silicon and gallium indium phosphide. We tune their photocatalytic activity by varying and measuring the energetics and kinetics at local reactive sites, in the following case studies.First, we combine photoelectrochemical cells with local potential-sensing probe microscopy to elucidate photocatalytic processes. In particular, we focus on local energetics and kinetics of multiple concurrent redox processes, i.e., two-redox chemistry. Figure 1A illustrates such a three-terminal photo-electro-catalytic cell setup, where the photoelectrode materials are photocatalysts by themselves. We showcase the efficient charge-separation design by fabricating photocatalysts with particulate semiconductors (e.g., commercial CdS powders, GaInP2 epitaxial layers, and ball-milled GaP nanoparticles), TiO2 coatings, and Rh cocatalysts for the concurring reduction of water and oxidation of reversible redox mediators or sacrificial reagents.We discovered that nanoscale site energetics are mutually dependent on local charge-transfer kinetics.[1] Besides, all known visible-light active photocatalysts of 1.1 – 2.3 eV bandgap photo-oxidize or corrode in water.[2] We discovered multi-functional protective coatings that i) allow for both electrons and holes to transport at tunable energy levels;[3] ii) shift band edges favorably, [4]; iii) reduce charge trapping and recombination,[5] and iv) tune water-oxidation selectivity between O2 and hydrogen peroxide production. [6] We show that in the presence of quinone redox couples in a bicarbonate solution, CO is produced with a 1-atm CO2-free Ar-purged headspace where the only source of CO2 is the (bi)carbonate anions, whilst the quinone redox couples can systematically vary the band edge positions of GaInP photoabsorbers.As shown in Figure 1B, CO evolution rate for both Fe(CN)6 3-/4- and H2BQ/BQ are higher at pH 7 than pH 8.5. The local CO2 concentration at pH 8.5 limited CO production rates. This observation corroborates our hypothesis that photocatalytic redox reactions locally drive pH swing to release CO2 at the oxidative sites, whereas the nearby reductive sites reduce in-situ generated CO2 into CO2R products. We envision direct solar fuel production from natural resources such as sunlight, bicarbonates from the ocean, or moisture in the air in a durable particle reactor.[7] We also processed wood and leaves into pulps and performed photocatalytic H2 production from the CdS/TiO2/Rh/CoOx panel (Figure 1C) using bottom LED illumination, as the glucose solution became colored.In summary, we show the rational design of photocatalysts. These coating-stabilized semiconductor particles present a scalable pathway for making H2 while effectively activating water and biomass as the two most abundant oxidants. We address the timescale mismatch between light absorption and catalysis by effectively accumulating electrons and holes at the respective reductive and oxidative sites.[8] The study paves the way for utilizing biomass to obtain bioenergy with carbon capture and storage.

Conductivity in Thin-Films of Transition Metal Coordination Complexes

In electronic commercial devices silicon remains the dominant material. Although it shows many advantages it requires usually high energy demand processes. Moreover, modifications of the electronic and physical properties are usually coupled with those processes.[ 1 ] While organic semiconductors usually show low energy processes and deposition with the desirable tunability reached with well-established synthesises.[ 2 ] Coordination complexes can even have an easier way of tunability by simply change the metal cores involved in the formation of the compounds. We aimed to design two coordination compounds formed by two complexes with equal but opposite charge and checking how the conductivity changes upon the metal substitution. They were synthetised by metathesis reaction by combining dithiolene complexes [M(mnt)2]2- (mnt = maleonitriledithiolate) M = Ni2+, Cu2+ with a copper coordination complexes [Cu(Stetra)]2+ (Stetra = 6,6’-bis(4,5-dihydrothiazol-2-yl)-2,2’-bipyridine). Upon the substitution of the metal core the system passed from insulator (Ni) to semiconductor behaviour (Cu) with a value of 10-8 (S/cm) and an activation energy of 0.9 eV. Their properties were investigated by using UV-Vis absorption, cyclic voltammetry, Raman spectroscopy, powder XRD, and conductivity. UV-Vis absorption and cyclic voltammetry were performed, in solid state, for the approximation of the energy levels. To support our investigations and experimental results computational modelling was performed. It revealed the central role that the copper has upon the conductivity compared to nickel. The lower reorganisation energy showed by the copper complex Cu(mnt)2 2- in solid state results in a lower energy barrier and a more favourable hopping mechanism through the molecules. Segal, M. Material history: Learning from silicon. Nat. 2012 4837389 483, S43–S44 (2012).Koch, N. Opportunities for energy level tuning at inorganic/organic semiconductor interfaces. Appl. Phys. Lett. 119, 260501 (2021). Figure 1

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells.

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

Calix[4]arene-Functionalized Titanium-Oxo Compounds for Perceiving Differences in Catalytic Reactivity Between Mono- and Multimetallic Sites.

While the difference in catalytic reactivity between mono- and multimetallic sites is often attributed to more than just the number of active sites, still few catalyst model systems have been developed to explore more underlying causal factors. In this work, we have elaborately designed and constructed three stable calix[4]arene (C4A)-functionalized titanium-oxo compounds, Ti-C4A, Ti4-C4A, and Ti16-C4A, with well-defined crystal structures, increasing nuclearity, and tunable light absorption capacity and energy levels. Among them, Ti-C4A and Ti16-C4A can be taken as model catalysts to compare the differences in reactivity between mono- and multimetallic sites. Taking CO2 photoreduction as the basic catalytic reaction, both compounds can achieve CO2-to-HCOO- conversion with high selectivity (close to 100%). Moreover, the catalytic activity of multimetallic Ti16-C4A is up to 2265.5 μmol g-1 h-1, which is at least 12 times higher than that of monometallic Ti-C4A (180.0 μmol g-1 h-1), and is the best-performing crystalline cluster-based photocatalyst known to date. Catalytic characterization combined with density functional theory calculations shows that in addition to the advantage of having more metal active sites (for adsorption and activation of more CO2 molecules), Ti16-C4A can effectively reduce the activation energy required for the CO2 reduction reaction by completing the multiple electron-proton transfer process rapidly with synergistic metal-ligand catalysis, thus exhibiting superior catalytic performance to that of monometallic Ti-C4A. This work provides a crystalline catalyst model system to explore the potential factors underlying the difference in catalytic reactivity between mono- and multimetallic sites.

HOMO/LUMO Energy level tuning of bithiopheneimide and its homopolymer by heteroatom relative position engineering

HOMO/LUMO Energy level tuning of bithiopheneimide and its homopolymer by heteroatom relative position engineering

Revealing the Role of Band Offsets on the Charge Carrier Dynamics of CsSn0.5Ge0.5I3‐Based Perovskite Solar Cell: A Theoretical Study

A computational method using a unidimensional device simulator to theoretically study the CsSn0.5Ge0.5I3‐based perovskite solar cell (PSC) is adopted. The results provide a detailed understanding of energy level tuning between the transport and absorber layer in order to attain high‐performance parameters. The influence of band offset (BO) on the charge carrier dynamics of the PSCs is explained in terms of cliff and spike structures for various levels of conduction band offsets (CBOs) and valence band offsets (VBOs). The ideality factor (IF) is calculated with variable BOs, and its variation with the applied voltage is examined. IF value ranges from 1.95 to 1.55 and 1.57 to 1.51 for different levels of VBO and CBO, respectively. The study on absorber layer thickness is conducted at different levels of BOs, revealing the importance of optimized absorber thickness on the performance of the PSCs. The capacitance–voltage measurements under illumination and in the dark signify the importance of optimized BO on the built‐in potential and maximum capacitances of the PSC. The Nyquist plot of the PSC at different levels of BO reveals that the highest recombination resistance is obtained at the optimum BO levels, rendering device to attain superior performance.

An Isomeric Solid Additive Enables High-Efficiency Polymer Solar Cells Developed Using a Benzo-Difuran-Based Donor Polymer.

Currently, nearly all high-efficiency organic photovoltaic devices use donor polymers based on the benzo-dithiophene (BDT) unit. To diversify the choices of building blocks for high-performance donor polymers, the use of benzo-difuran (BDF) units is explored, which can achieve reduced steric hindrance, stronger molecular packing, and tunable energy levels. In previous research, the performance of BDF-based devices lagged behind those of BDT-based devices. In this study, a high efficiency (18.4%) is achieved using a BDF-based polymer donor, which is the highest efficiency reported for BDF donor materials to date. The high efficiency is enabled by a donor polymer (D18-Fu) and the aid of a solid additive (2-chloronaphthalene), which is the isomer of the commonly used additive 1-chloronaphthalene. These results revealed the significant effect of 2-chloronaphthalene in optimizing the morphology and enhancing the device parameters. This work not only provides a new building block that can achieve an efficiency comparable to dominant BDT units but also proposes a new solid additive that can replace the widely used 1-chloronaphthalene additive.

BAr2 -Bridged Azafulvene Dimers with Tunable Energy Levels for Photostable Near-Infrared Dyes.

Organic dyes with strong absorption in the near-infrared (NIR) region are potentially useful in medical applications, such as tumor imaging and photothermal therapy. In this work, new NIR dyes combining BAr2 -bridged azafulvene dimer acceptors with diarylaminothienyl donors in a donor-acceptor-donor configuration were synthesized. Surprisingly, it was found that in these molecules the BAr2 -bridged azafulvene acceptor adopts a 5-membered, rather than 6-membered ring structure. The influence of the aryl substituents on the HOMO and LUMO energy levels of the dye compounds was assessed from electrochemical and optical measurements. Strong electron-withdrawing fluorinated substituents (Ar=C6 F5 , 3,5-(CF3 )2 C6 H3 ) lowered the HOMO energy while preserving the small HOMO-LUMO energy gap, resulting in promising NIR dye molecules that combine strong absorption bands centered around 900 nm with good photostability.

Two-Dimensional Silver-Metalated Graphdiyne Nanosheets for Photocatalytic Degradation of Antibiotics

The two-dimensional layered carbon allotrope graphdiyne (GDY) is formed by combining hybridized sp–sp2 carbon atoms. This alkynyl-bonded 2D carbon network with tunable energy levels and high carrier-mobility has excellent potential in solar energy utilization. However, the standard but complex synthesis method of GDYs by epitaxial growth on a metal-crystal surface and the poor activity of metal-free materials limit applications beyond the lab. Here, we produce a silver-metalated GDY (Ag-GDY) through a facile and scalable wet-chemistry strategy, featured by an appropriate band gap at 2.61 eV and good e–/h+ separation ability. These unique characteristics facilitate the reaction between Ag-GDY and oxygen to form a variety of active free radicals. The Ag-GDY photocatalyst displays an excellent ability for the light-driven decomposition of antibiotics. Tetracycline with a concentration as low as 5.0 × 10–5 M is degraded within 30 min by Ag-GDY, showing excellent performance stability of up to five reuse cycles.

Tuning Hole Transport Properties via Pyrrole Derivation for High‐Performance Perovskite Solar Cells

AbstractHole transport materials (HTMs) with high hole mobility, good band alignment and ease of fabrication are highly desirable for perovskite solar cells (PSCs). Here, we designed and synthesized novel organic HTMs, named T3, which can be synthesized in high yields with commercially available materials, featuring a substituted pyrrole core and triphenylamine peripheral arms. The capability of functionalization in the final synthetic step provides an efficient way to obtain a variety of T3‐based HTMs with tunable energy levels and other properties. Among them, fluorine‐substituted T3 (T3‐F) exhibits the best band alignment and hole extraction properties, leading to PSCs with outstanding PCEs of 24.85 % and 24.03 % (certified 23.46 %) for aperture areas of 0.1 and 1 cm2, respectively. The simple structure and tunable performance of T3 can inspire further optimization for efficient PSCs.

Tuning Hole Transport Properties via Pyrrole Derivation for High-Performance Perovskite Solar Cells.

Hole transport materials (HTMs) with high hole mobility, good band alignment and ease of fabrication are highly desirable for perovskite solar cells (PSCs). Here, we designed and synthesized novel organic HTMs, named T3, which can be synthesized in high yields with commercially available materials, featuring a substituted pyrrole core and triphenylamine peripheral arms. The capability of functionalization in the final synthetic step provides an efficient way to obtain a variety of T3-based HTMs with tunable energy levels and other properties. Among them, fluorine-substituted T3 (T3-F) exhibits the best band alignment and hole extraction properties, leading to PSCs with outstanding PCEs of 24.85 % and 24.03 % (certified 23.46 %) for aperture areas of 0.1 and 1 cm2 , respectively. The simple structure and tunable performance of T3 can inspire further optimization for efficient PSCs.

Energy Level, Crystal Morphology and Fluorescence Emission Tuning in Cocrystals via Molecular-Level Engineering.

Organic donor-acceptor complexes as new organic semiconductor class have attracted wide attention, due to their potential applications in functional optoelectronics. Herein, we present two new charge transfer cocrystals of di-cyanodiazafluorene -perylene (DCPE) and di-cyanodiazaflfluorene-pyrene (DCPY) through a rational cocrystal-engineering strategy. Although they are both 1 : 1 mixed stacking cocrystals with similar chemical structures, the DCPE cocrystal possesses a non-centrosymmetric space group and narrower band gap compared to DCPY cocrystal, because of the non-covalent bonding variation. The electrostatic potential accumulated in the lateral facets leads to highly twisted DCPE nanobelts, and the small band gap causes near infrared fluorescence. Meanwhile, the DCPY crystals with centrosymmetric space groups and weaker intermolecular interactions exhibited an untwisted morphology and red emission. This study will be helpful for the design and understanding of functional cocrystal materials that can be used in flexible micro/nano-mechanics, mechanical energy, and optical devices.

Energy level tuning of push-pull porphyrin sensitizer by trifluoromethyl group for dye-sensitized solar cells

We have introduced trifluoromethyl groups into meta-positions of two meso-phenyl rings of a push-pull-type porphyrin dye (ZnP-CF[Formula: see text] to modulate the energy levels of the porphyrin dye for dye-sensitized solar cells (DSSCs). The light-harvesting ability of ZnP-CF[Formula: see text] is almost comparable to those of reference porphyrins where the trifluoromethyl groups are replaced with methyl (ZnP-CH[Formula: see text] or tert-butyl groups (YD2). We revealed that the introduction of the electron-withdrawing trifluoromethyl groups is effective to lower the excited-state oxidation potential of the porphyrin (–0.80 V vs NHE), which is consistent with the theoretical calculation. Meanwhile, DSSCs with ZnP-CF[Formula: see text] exhibited a lower power conversion efficiency ([Formula: see text] of 5.95% than DSSCs with ZnP-CH[Formula: see text] ([Formula: see text] = 7.33%) and YD2 ([Formula: see text] = 8.97%) because of the severe aggregation tendency of ZnP-CF[Formula: see text] arising from the trifluoromethyl groups. In addition, the insufficient steric bulkiness of the trifluoromethyl and methyl groups relative to tert-butyl groups would result in the lower short circuit current and open circuit voltage for ZnP-CF[Formula: see text] and ZnP-CH[Formula: see text] due to fast charge recombination between electrons in the conduction band of TiO2 and I[Formula: see text]in the electrolyte solution. Overall, introducing both trifluoromethyl groups and bulky substituents into a porphyrin core would be necessary to boost cell performance.

Base-metal oxide semiconductor electrodes for PPCP degradation: Ti-doped α-Fe2O3 for sulfosalicylic acid oxidation as an example

Sulfosalicylic acid is a typical pharmaceutical and personal care product with high toxicity, environmental persistence, and low biodegradability. Electrochemical oxidation has been demonstrated to be a promising way for hazardous organics treatment, but it is severely limited by the high cost and resource shortage of electrode materials. Base-metal oxide semiconductor anodes have the merits of low cost, diversity, and tunable energy levels for charge transfer, and thus may be alternatives to the electrodes for wastewater treatment. Herein, we found that Ti-doped α-Fe2O3, as an example, could be efficient for sulfosalicylic acid oxidation, reaching comparable faraday efficiency of sulfosalicylic acid to that of the boron-doped diamond electrode. Ti-doped electrodes exhibited both higher removal rates and current efficiency compared to the undoped. This could be mainly ascribed to the enhanced charge transfer rate constant. Kinetic analysis shows that the apparent reaction order, in terms of sulfosalicylic acid in bulk solution, depended on applied potential and pollutant concentration. Mechanism study shows that the oxidation of sulfosalicylic acid was mainly through indirect pathway. Moreover, the oxidation products were determined and the oxidation mechanism was proposed. This study may open a door to employ base-metal oxide semiconductor anodes for the efficient treatment of organic wastewater.

The Electronic Properties of a 2D Ruddlesden‐Popper Perovskite and its Energy Level Alignment with a 3D Perovskite Enable Interfacial Energy Transfer

AbstractThe success of using 2D Ruddlesden‐Popper metal halide perovskites (MHPs) in optoelectronic devices has ignited great interest as means for energy level tuning at the interface with 3D MHPs. Inter alia, the application of 2D phenylethylammonium lead quaternary iodide (PEA2PbI4)/3D MHPs interfaces has improved various optoelectronic devices, where a staggered type‐II energy level alignment is often assumed. However, a type‐II heterojunction seems to contradict the enhanced photoluminescence observed for 2D PEA2PbI4/3D MHP interfaces, which raises fundamental questions about the electronic properties of such junctions. In this study, using direct and inverse photoelectron spectroscopy, it is revealed that a straddling type‐I energy level alignment is present at 2D PEA2PbI4/3D methylammonium lead triiodide (MAPbI3) interfaces, thus explaining that the photoluminescence enhancement of the 3D perovskite is induced by energy transfer from the 2D perovskite. These results provide a reliable fundamental understanding of the electronic properties at the investigated 2D/3D MHP interfaces and suggest careful (re)consideration of the electronic properties of other 2D/3D MHP heterostructures.

Tunable energy level induced fluorescence enhancement in copper functionalized silicon quantum dots for highly selective detection of bisulfite

Bisulfite plays crucial roles in various environmental pollution and toxicological effects of atmospheric particles events. Selective and highly sensitive sensing of bisulfite in environment is of great significance for particulate matter formation research. In this work, we purposely synthesized copper functionalized silicon quantum dots (Cu-SiQDs) with small energy level, which can specially react with bisulfite resulting significant fluorescence enhancement, and a direct sensing platform for bisulfite was constructed in atmospheric particulate matters. Additionally, the fabricated assay had a broad linear range of 1–80 μM and low detection limit (0.82 μM). Through amounts of experiments and analysis, the interaction mechanisms between Cu-SiQDs and the enhancer bisulfite was proposed that a complex shell was formed on the surface of Cu-SiQDs, which modulated its energy band gap and accelerated electron transfer. The study provides a new sight of modified metal-free QDs nanostructure for highly selective luminescence detection in complex samples. • Cu-SiQDs with superior fluorescence property was synthesized by modulating its energy banding energy. • A highly sensitive and selective fluorescent sensing platform with real time monitoring for bisulfite was established. • The sensing mechanism accounted for the formation of complex shell and enhanced electron transfer efficiency of Cu-SiQDs.

Deciphering competing radiative relaxation pathways observed in Pr3+-Activated yttrium-based compounds: UV emission versus visible emission

Pr3+-activated yttrium-based compounds have garnered significant research interest as the electronic transition of Pr3+ dopant incorporated in host lattices are quite versatile, giving rise to the production of either UV or visible electromagnetic radiation. The tunable 5d orbital energy level of Pr3+ with an electronic configuration [Xe]4f15d1 depending on the external crystal fields, coordinating anion ligands (mainly determined by covalency of anion ligands), and symmetry of crystal structures can indeed control the radiative decay processes whether to generate UV or self-trapped exciton-mediated visible emission. We have investigated three different Pr3+-activated crystals of YPO4, YBO3, and Y2SiO5 using two different excitation processes including direct host lattice excitation (cathodoluminescence: CL) and f-d excitation of Pr3+ dopants (photoluminescence: PL). Such radiative decay mechanism observed in Pr3+-doped yttrium-based compounds has been deciphered that photoexcitation of Pr3+ ions results in 4f15d1→4f2 electronic transition with UV emission (PL) whereas above-bandgap excitation of host lattices brings about the efficient production of visible emission mediated through self-trapped excitons (CL). When changing the anion sublattices, the emission peak corresponding to 4f15d1→4f2 transition shifted to longer wavelength as a result of modulated covalency and crystal field strength. As the host lattices changes as a function of anion sublattices, the bandgap reduced from 8.6 eV (for YPO4) to 7.6 eV (for YBO3) and to 6.1 eV (for Y2SiO5), directly influencing rate of trapping of exciton and their formation of self-trapped exciton, which further give rise to production of the self-trapped exciton mediated visible emission. The fundamental understanding of the multiple, radiative relaxation pathways provides a designing principle for achieving more efficient UV emitting phosphors (at the same time by suppressing visible emission or vice versa) for germicidal and medical applications.