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A novel photoelectrochemical biosensing platform for the detection of biomolecules at relatively low applied potentials was constructed using porphyrin-functionalized Ti[O.sub.2] nanoparticles. The functional Ti[O.sub.2] nanoparticles were prepared by dentate binding of Ti[O.sub.2] with sulfonic groups of water-soluble [meso-tetrakis(4-sulfonatophenyl)porphyrin] iron(III) monochloride (FeTPPS) and characterized by transmission electron microscopy; contact angle measurement; and Raman, X-ray photoelectron, and ultraviolet-visible absorption spectroscopies. The functional nanoparticles showed good dispersion in water and on indium fin oxide (ITO) surface. The resulting FeTPPS-Ti[O.sub.2]-modified ITO electrode showed a photocurrent response at +0.2 V to a light excitation at 380 nm, which could be further sensitized through an oxidation process of biomolecules by the hole-injected FeTPPS. Using glutathione as a model, a methodology for sensitive photoelectrochemical biosensing at low potential was thus developed. Under optimal conditions, the proposed photoelectrochemical method could detect glutathione ranging from 0.05 to 2.4 mmol [L.sup.-1] with a detection limit of 0.03 mmol [L.sup.-1] at a signal-to-noise ratio of 3. The photoelectrochemical biosensor had an excellent specificity against anticancer drugs and could be successfully applied to the detection of reduced glutathione in gluthion injection, showing a promising application in photoelectrochemical biosensing. 10.1021/ac102070f

Perovskite quantum dots (QDs) with high room-temperature luminescence efficiency have been applied in single-photon sources. While the optical properties of large, weakly confined perovskite nanocrystals have been extensively explored at the single-particle level, few studies have focused on single-perovskite QDs with strong quantum confinement. This is mainly due to their poor surface chemical stability. Here we demonstrate that strongly confined CsPbBr 3 perovskite QDs (SCPQDs) embedded in a phenethylammonium bromide matrix exhibit a well-passivated surface and improved photostability under intense photoexcitation. We find that in our SCPQDs, photoluminescence blinking is suppressed at moderate excitation intensities, and increasing the excitation rates leads to weak photoluminescence intensity fluctuations accompanied by an unusual spectral blue-shift. We attribute this to a biexciton-like Auger interaction between excitons and trapped excitons formed by surface lattice elastic distortions. This hypothesis is corroborated by the unique repulsive biexciton interaction observed in the SCPQDs.

Perovskite quantum dots (PQDs) are applicable in light-emitting diodes (LEDs) owing to their color tunability, high color purity, and excellent photoluminescence quantum yield (PLQY) in the solution state. However, a PQD film obtained through nonradiative recombination by concentration quenching and the formation of surface defects exhibited a low PLQY. In this study, we focused on the energy transfer between PQDs with different energy gaps ( E g ) to reduce nonradiative recombination in the film state and consequently achieve high device performance. We prepared size-controlled PQDs measuring 10.7 nm (large-size QD; LQD) and 7.9 nm (small-size QD; SQD) with different E g values and observed a spectral overlap between SQD emission and LQD absorption. To investigate the Förster resonance energy transfer (FRET) from SQDs to LQDs, we prepared SQD-LQD mixed QDs (MQDs). The MQD film enhanced LQD emission and exhibited a higher PLQY (52%) with a longer PL decay time (7.4 ns) than those exhibited by the neat LQD film (38% and 6.2 ns). This energy transfer was determined to be FRET by photoluminescence excitation and PL decay times. Moreover, the external quantum efficiency of an MQD-based LED increased to 15%, indicating that the FRET process can enhance the PLQY of the film and LED efficiency.

The epitaxial growth of a perovskite matrix on quantum dots (QDs) has enabled the emergence of efficient red light-emitting diodes (LEDs) because it unites efficient charge transport with strong surface passivation. However, the synthesis of wide-band gap ( E g ) QD-in-matrix heterostructures has so far remained elusive in the case of sky-blue LEDs. Here, we developed CsPbBr 3 QD-in-perovskite matrix solids that enable high luminescent efficiency and spectral stability with an optical E g of over 2.6 eV. We screened alloy candidates that modulate the perovskite E g and allow heteroepitaxy, seeking to implement lattice-matched type-I band alignment. Specifically, we introduced a CsPb 1- x Sr x Br 3 matrix, in which alloying with Sr 2+ increased the E g of the perovskite and minimized lattice mismatch. We then developed an approach to passivation that would overcome the hygroscopic nature of Sr 2+ . We found that bis (4-fluorophenyl)phenylphosphine oxide strongly coordinates with Sr 2+ and provides steric hindrance to block H 2 O, a finding obtained by combining molecular dynamics simulations with experimental results. The resulting QD-in-matrix solids exhibit enhanced air- and photo-stability with efficient charge transport from the matrix to the QDs. LEDs made from this material exhibit an external quantum efficiency of 13.8% and a brightness exceeding 6000 cd m -2 .

Light-emitting diodes (LEDs) based on metal halide perovskite quantum dots (QDs) have achieved impressive external quantum efficiencies; however, the lack of surface protection of QDs, combined with efficiency droop, decreases device operating lifetime at brightnesses of interest. The epitaxial incorporation of QDs within a semiconducting shell provides surface passivation and exciton confinement. Achieving this goal in the case of perovskite QDs remains an unsolved challenge in view of the materials' chemical instability. Here, we report perovskite QDs that remain stable in a thin layer of precursor solution of perovskite, and we use strained QDs as nucleation centers to drive the homogeneous crystallization of a perovskite matrix. Type-I band alignment ensures that the QDs are charge acceptors and radiative emitters. The new materials show suppressed Auger bi-excition recombination and bright luminescence at high excitation (600 W cm -2 ), whereas control materials exhibit severe bleaching. Primary red LEDs based on the new materials show an external quantum efficiency of 18%, and these retain high performance to brightnesses exceeding 4700 cd m -2 . The new materials enable LEDs having an operating half-life of 2400 h at an initial luminance of 100 cd m -2 , representing a 100-fold enhancement relative to the best primary red perovskite LEDs.

Increasing the power conversion efficiency (PCE) of colloidal quantum dot (CQD) solar cells has relied on improving the passivation of CQD surfaces, enhancing CQD coupling and charge transport, and advancing device architecture. The presence of hydroxyl groups on the nanoparticle surface, as well as dimers-fusion between CQDs-has been found to be the major source of trap states, detrimental to optoelectronic properties and device performance. Here, we introduce a CQD reconstruction step that decreases surface hydroxyl groups and dimers simultaneously. We explored the dynamic interaction of charge carriers between band-edge states and trap states in CQDs using time-resolved spectroscopy, showing that trap to ground-state recombination occurs mainly from surface defects in coupled CQD solids passivated using simple metal halides. Using CQD reconstruction, we demonstrate a 60% reduction in trap density and a 25% improvement in charge diffusion length. These translate into a PCE of 12.5% compared to 10.9% for control CQDs.

Metal halide perovskites show promise for light-emitting diodes (LEDs) owing to their facile manufacture and excellent optoelectronic performance, including high color purity and spectral stability, especially in the green region. However, for blue perovskite LEDs, the emission spectrum line width is broadened to over 25 nm by the coexistence of multiple reduced-dimensional perovskite domains, and the spectral stability is poor, with an undesirable shift (over 7 nm) toward longer wavelengths under operating conditions, degradation that occurs due to phase separation when mixed halides are employed. Here we demonstrate chloride insertion-immobilization, a strategy that enables blue perovskite LEDs, the first to exhibit narrowband (line width of 18 nm) and spectrally stable (no wavelength shift) performance. We prepare bromide-based perovskites and then employ organic chlorides for dynamic treatment, inserting and in situ immobilizing chlorides to blue-shift and stabilize the emission. We achieve sky-blue LEDs with a record luminance over 5100 cd/m 2 at 489 nm, and an operating half-life of 51 min at 1500 cd/m 2 . By device structure optimization, we further realize an improved EQE of 5.2% at 479 nm and an operating half-life of 90 min at 100 cd/m 2 .

We report the strong light-induced activation of forbidden exciton transition in CsPbBr 3 perovskite quantum dots mediated by the symmetry-breaking polaron that modifies the optical selection rule of the confined exciton transition. The activated forbidden transition results in an intense pump-induced absorption in the transient absorption spectra above the bandgap, where the original parity-forbidden transition was located. In contrast to many other semiconductor quantum dots, photoexcitation of an exciton in CsPbBr 3 quantum dots creates a sufficiently large perturbation via a lattice-distorting polaron, which turns on the formally forbidden transition. Compared to the bulk or weakly confined CsPbBr 3 , the activation of the forbidden transition in strongly confined quantum dots is much more prominent due to the stronger influence of the polaron on exciton transitions in the confined space. This nonlinear optical property highlights the intimate coupling of the photoexcited charge carriers with the lattice in the CsPbBr 3 quantum dots, allowing access to the forbidden exciton transitions with light.

Cesium lead halide (CsPbX 3 ) nanocrystals have emerged as a new family of materials that can outperform the existing semiconductor nanocrystals due to their superb optical and charge-transport properties. However, the lack of a robust method for producing quantum dots with controlled size and high ensemble uniformity has been one of the major obstacles in exploring the useful properties of excitons in zero-dimensional nanostructures of CsPbX 3 . Here, we report a new synthesis approach that enables the precise control of the size based on the equilibrium rather than kinetics, producing CsPbX 3 quantum dots nearly free of heterogeneous broadening in their exciton luminescence. The high level of size control and ensemble uniformity achieved here will open the door to harnessing the benefits of excitons in CsPbX 3 quantum dots for photonic and energy-harvesting applications.

Cesium lead halide (CsPbX 3 ) perovskite nanocrystals (NCs) possess the unique capability of post-synthesis anion exchange providing facile tunability of the optical properties, which is usually achieved by mixing NCs with reactive anion precursors. In this work, we show that the controllable anion exchange can be achieved in a dihalomethane solution of CsPbX 3 NC in the absence of any spontaneously reacting anion source using photoexcitation of CsPbX 3 NCs as the triggering mechanism for the halide ion exchange. The reaction begins with the photoinduced electron transfer from CsPbX 3 NCs to dihalomethane solvent molecules producing halide ions via reductive dissociation, which is followed by anion exchange. The reaction proceeds only in the presence of excitation light and the rate and extent of reaction can be controlled by varying the light intensity. Furthermore, the asymptotic extent of reaction under continuous excitation can be controlled by varying the wavelength of light that self-limits the reaction when light becomes off-resonance with the absorption of NCs. The light-controlled anion exchange demonstrated here can be utilized to pattern the post-synthesis chemical transformation of CsPbX 3 NCs, not readily achievable using typical methods of anion exchange.

We report the one-pot synthesis of colloidal Mn-doped cesium lead halide (CsPbX 3 ) perovskite nanocrystals and efficient intraparticle energy transfer between the exciton and dopant ions resulting in intense sensitized Mn luminescence. Mn-doped CsPbCl 3 and CsPb(Cl/Br) 3 nanocrystals maintained the same lattice structure and crystallinity as their undoped counterparts with nearly identical lattice parameters at ∼0.2% doping concentrations and no signature of phase separation. The strong sensitized luminescence from d-d transition of Mn 2+ ions upon band-edge excitation of the CsPbX 3 host is indicative of sufficiently strong exchange coupling between the charge carriers of the host and dopant d electrons mediating the energy transfer, essential for obtaining unique properties of magnetically doped quantum dots. Highly homogeneous spectral characteristics of Mn luminescence from an ensemble of Mn-doped CsPbX 3 nanocrystals and well-defined electron paramagnetic resonance spectra of Mn 2+ in host CsPbX 3 nanocrystal lattices suggest relatively uniform doping sites, likely from substitutional doping at Pb 2+ . These observations indicate that CsPbX 3 nanocrystals, possessing many superior optical and electronic characteristics, can be utilized as a new platform for magnetically doped quantum dots expanding the range of optical, electronic, and magnetic functionality.

The benefits of the hot electrons from semiconductor nanostructures in photocatalysis or photovoltaics result from their higher energy compared to that of the band-edge electrons facilitating the electron-transfer process. The production of high-energy hot electrons usually requires short-wavelength UV or intense multiphoton visible excitation. Here, we show that highly energetic hot electrons capable of above-threshold ionization are produced via exciton-to-hot-carrier up-conversion in Mn-doped quantum dots under weak band gap excitation (∼10 W/cm 2 ) achievable with the concentrated solar radiation. The energy of hot electrons is as high as ∼0.4 eV above the vacuum level, much greater than those observed in other semiconductor or plasmonic metal nanostructures, which are capable of performing energetically and kinetically more-challenging electron transfer. Furthermore, the prospect of generating solvated electron is unique for the energetic hot electrons from up-conversion, which can open a new door for long-range electron transfer beyond short-range interfacial electron transfer.

The study of behaviors of ionic current rectification (ICR) in organic solutions with quartz nanopipettes is reported. ICR can be observed even in organic solutions using quartz pipettes with diameters varied from several to dozens of nanometers, and the direction of ICR is quite different from the ICR observed in aqueous phase. The influences of pore size, electrolyte concentration, and surface charge on the ICR have been investigated carefully. Water in organic solutions affects the direction and extent of ICR significantly. Mechanisms about the formation of an electrical double layer (EDL) on silica in organic solutions with different amount of water have been proposed. An improved method, which can be employed to detect trace water in organic solutions, has been implemented based on Au ultramicroelectrodes with cathodic differential pulse stripping voltammetry.

We show that hot electrons exhibiting the enhanced photocatalytic activity in H2 production reaction can be efficiently generated in Mn-doped quantum dots via the "upconversion" of the energy of two excitons into the hot charge carriers. The sequential two-photon-induced process with the long-lived Mn excited state serving as the intermediate state is considered as the pathway generating hot electrons. H2 production rate from doped quantum dots is significantly higher than that from undoped quantum dots and also exhibited the quadratic increase with the light intensity, demonstrating the effectiveness of the hot electrons produced in doped quantum dots in photocatalytic reaction. Due to the very long lifetime of Mn excited state (∼6 ms) in the doped quantum dots, the sequential two-photon excitation requires relatively low excitation rates readily achievable with a moderately concentrated solar radiation, demonstrating their potential as an efficient source of hot electrons operating at low excitation intensities.

The dependence of the energy transfer rate on the content of sp(2)-hybridized carbon atoms in the hybrid structures of reduced graphene oxide (RGO) and Mn-doped quantum dot (QD(Mn)) was investigated. Taking advantage of the sensitivity of QD(Mn)'s dopant luminescence lifetime only to the energy transfer process without interference from the charge transfer process, the correlation between the sp(2) carbon content in RGO and the rate of energy transfer from QD(Mn) to RGO was obtained. The rate of energy transfer showed a strongly superlinear increase with increasing sp(2) carbon content in RGO, suggesting the possible cooperative behavior of sp(2) carbon domains in the energy transfer process as the sp(2) carbon content increases.

The modification of glass nanopipettes with polyethyleneimines (PEIs) has been successfully achieved by a relatively simple method, and the smallest tip opening is around 3 nm. Thus, in a much wider range of glass pipettes with radii from several nanometers to a few micrometers, the ion current rectification (ICR) phenomenon has been observed. The influences of different KCl concentrations, pH values, and tip radii on the ICR are investigated in detail. The sizes of PEIs have been determined by dynamic light scattering, and the effect of the sizes of PEIs for the modification, especially for a few nanometer-pipettes in radii, is also discussed. These findings systemically confirm and complement the theoretical model and provide a platform for possible selectively molecular detection and mimic biological ion channels.