High-Sensitivity Photoelectrochemical Ultraviolet Photodetector with Stable pH-Universal Adaptability Based on Whole Single-Crystal Integrated Self-Supporting 4H-SiC Nanoarrays.

Emerging photoelectrochemical (PEC) photodetectors (PDs) have notable advantages over conventional PDs and have attracted extensive attention. However, harsh liquid environments, such as those with high corrosivity and attenuation, substantially restrict their widespread application. Moreover, most PEC PDs are constructed by assembling numerous nanostructures on current collector substrates, which inevitably contain abundant interfaces and defects, thus greatly weakening the properties of PDs. To address these challenges, a high-performance pH-universal PEC ultraviolet (UV) PD based on a whole single-crystal integrated self-supporting 4H-SiC nanopore array photoelectrode is constructed, which is fabricated using a two-step anodic oxidation approach. The PD exhibits excellent photodetection behavior, with high responsivity (218.77 mA W-1 ), detectivity (6.64 × 1013  Jones), external quantum efficiency (72.47%), and rapid rise/decay times (17/48 ms) under 375 nm light illumination with a low intensity of 0.15 mW cm-2 and a bias voltage of 0.6 V, which is fall in the state-of-the-art of the wide-bandgap semiconductor-based PDs reported thus far. Furthermore, the SiC PEC PD exhibits excellent photoresponse and long-term operational stability in pH-universal liquid environments. The improved photodetection performance of the SiC PEC PD is primarily attributed to the synergistic effect of the nanopore array structure, integrated self-supporting configuration, and single-crystal structure of the whole photoelectrode.

Reducing Emission Linewidth of Pure-Blue ZnSeTe Quantum Dots through Shell Engineering toward High Color Purity Light-Emitting Diodes.

High color purity blue quantum dot light-emitting diodes (QLEDs) have great potential applications in the field of ultra-high-definition display. However, the realization of eco-friendly pure-blue QLEDs with a narrow emission linewidth for high color purity remains a significant challenge. Herein, a strategy for fabricating high color purity and efficient pure-blue QLEDs based on ZnSeTe/ZnSe/ZnS quantum dots (QDs) is presented. It is found that by finely controlling the internal ZnSe shell thickness of the QDs, the emission linewidth can be narrowed by reducing the exciton-longitudinal optical phonon coupling and trap states in the QDs. Additionally, the regulation of the QD shell thickness can suppress the Förster energy transfer between QDs in the QLED emission layer, which will help to reduce the emission linewidth of the device. As a result, the fabricated pure-blue (452 nm) ZnSeTe QLED with ultra-narrow electroluminescence linewidth (22 nm) exhibit high color purity with the Commission Internationale de l'Eclairage chromatic coordinates of (0.148, 0.042) and considerable external quantum efficiency (18%). This work provides a demonstration of the preparation of pure-blue eco-friendly QLEDs with both high color purity and efficiency, and it is believed that it will accelerate the application process of eco-friendly QLEDs in ultra-high-definition displays.

Component Engineering to Tailor the Structure and Optical Properties of Sb-Doped Indium-Based Halides.

Controlling the structure of halide perovskites through component engineering, and thus revealing the changes in luminescence properties caused by the conversion of crystal structure, is of great significance. Herein, we report a controllable synthetic strategy of three-dimensional (3D) Cs2KInCl6 and zero-dimensional (0D) (Cs/K)2InCl5(H2O) halide perovskites by changing the Cs/K feed ratio. 3D Cs2KInCl6 double perovskites are obtained at the Cs/K feed ratio of 1:1, while 0D (Cs/K)2InCl5(H2O) perovskites are formed at the Cs/K feed ratio of 2:1. Further, a reversible crystal structure transformation between 3D Cs2KInCl6 double perovskites and 0D (Cs/K)2InCl5(H2O) perovskites can be achieved by subsequent addition of metal-salt precursors. In addition, the emission efficiency of two perovskite structures can be greatly boosted by breaking the forbidden transition through Sb doping, and as a result, a novel green/yellow reversible emission switch is generated. Meanwhile, the relationship between perovskite structure and luminescence mechanism has been systematically revealed. These environmentally stable halide perovskites have great potential to be applied in optoelectronic devices.

Mn-Doped AgZnInS/ZnS Nanocrystals (NCs): Effects of Zn Etching on the NC Optical Properties.

Mn-doped I(II)-III-VI NCs (e.g., Mn-doped AgZnInS/ZnS NCs) possessing low-energy excitation, high brightness and long fluorescence lifetimes have been desired for time-gated fluorescence biosensing/imaging. In this type of NCs, their optical properties are significantly affected by the microscopic interactions between Mn and Mn and between Mn and host NC, the compositions of NCs, and the defects in NCs. On the other hand, it is known that Zn etching to core I(II)-III-VI NCs in NC synthesis can significantly enhance the NC brightness because Zn can exchange surface atoms (e.g., Ag and In) in NCs to minimize NC surface-defects. But for Mn-doped I(II)-III-VI NCs, Zn etching could etch out not only surface-atoms of host NCs (e.g., Ag and In) but also Mn in NCs. As a result, it could significantly affect the NC compositions and the microscopic interactions between Mn and Mn as well as between Mn and host NC, and thus the optical properties of NCs (like lifetime and absorption/emission spectra). Therefore, it is needed to investigate how Zn etching would affect the optical properties of such Mn-doped NCs. In this study, a series of Mn-doped AgZnInS NCs with different Mn doping levels were prepared through nucleation doping, and then Zn etching was applied to etch these core NCs. To identify the effects of Zn etching on NC optical properties, ZnS coating (a different ZnS shelling approach by injecting Zn precursor and S precursor alternately in synthesis) was performed on the same Mn:AgZnInS NCs, and the optical properties of NCs with these two different ZnS shelling approaches were compared. Experimental results showed that under appropriate Mn doping levels in synthesis, Zn etching instead of ZnS coating can produce low-energy excitable NCs with higher QYs and longer lifetimes, which would further facilitate the use of such NCs in time-gated fluorescence measurement. To understand the reasons for the different optical properties under different ZnS shelling approaches, the material characteristics of the prepared NCs were further measured/analyzed and the possible fluorescence mechanisms were discussed.

Thermal and photo stability of all inorganic lead halide perovskite nanocrystals.

Inorganic lead halide perovskite (ILHP) nanocrystals (NCs) show great potential in solid state lighting and next generation display technology due to their excellent optical properties. However, almost all ILHP NCs are still facing the problem of unstable luminescence properties caused by heating and/or UV illumination. Further improving the thermal and photo stability of ILHP NCs has become the most urgent challenge for their practical application. This Perspective review specifically focuses on the thermal and photo stability of ILHP NCs, discusses and analyzes the factors that affect the thermal and photo stability of ILHP NCs from the perspective of surface ligands and structure composition, summarizes the current strategies to improve the thermal and photo stability of ILHP NCs, and presents the key challenges and perspectives on the research for the improvement of thermal and photo stability of ILHP NCs.

Exploring the emission mechanism of dichromatic white-light quantum-dot light-emitting diodes using wavelength-resolved transient electroluminescence analysis.

Exploring electroluminescence (EL) processes is extremely vital to fabricate efficient white-light quantum-dot light-emitting diodes (QLEDs). A model white QLED consisting of a bilayer CdSe/ZnSeS quantum-dot (QD)//CuInS2/ZnS QDs emissive layer has been used to analyze the white-light emission mechanism. In this design, the CdSe/ZnSeS QDs and CuInS2/ZnS QDs contribute to the blue and yellow emissions, respectively, in the dichromatic white QLED. Wavelength-resolved transient EL (TrEL) results demonstrate that the excitons are mainly formed on the CuInS2/ZnS QDs in the QLED operated at low biases due to the low barrier to hole injection and energy transfer from the CdSe/ZnSeS QDs to the CuInS2/ZnS QDs. Further, the TrEL decays of both white and monochromic devices reveal that the emission behavior of the white QLED is closely related to that of the monochromic device, but is minimally affected by the interactions between different emission units. The simulation results performed by the solar cell capacitance simulator model agree well with the experimental data. Our results show an insight into the EL processes in the white device QLED and demonstrate a powerful tool to investigate emission behavior of the white QLEDs.

C-O bond activation and splitting behaviours of CO2 on a 4H-SiC surface: a DFT study.

Conversion of CO2 into valuable chemicals can not only reduce the amount of CO2 in the atmosphere, but also realize the reuse of resources. It's well known that C-O bond activation and splitting are critical steps in the CO2 conversion process and it's crucial to employ an appropriate catalyst. Here, the adsorption and activation behaviors of a CO2 molecule on 4H-SiC surfaces were systematically investigated based on DFT calculations. Calculation results show that the CO2 molecule can anchor on 4H-SiC(0001) and (0001[combining macron]) surfaces. On the 4H-SiC(0001) surface, the adsorbed CO2 molecule prefers to dissociate with an energy barrier of 0.52-0.70 eV, producing an O adatom and a CO molecule on the surface. Further dissociation of the CO is hindered due to a large energy barrier of 2.12 eV. However, if a H atom is introduced, the CO molecule may combine with H into a CHO group and the reaction energy barrier is 1.69 eV. Moreover, the CHO group tends to transform into a CH group and an O adatom, a reaction in which a relatively low energy barrier of 0.09 eV needs to be surmounted. For the 4H-SiC(0001[combining macron]) case, the direct C-O bond dissociation energy barrier for CO2 is only 0.37 eV while further breaking of the C-O bond in CO is energetically unfavorable even with the help of a H atom. So the final products are an O adatom and CO chemisorbed on the 4H-SiC(0001[combining macron]) surface. All the calculation results demonstrate that the inert CO2 molecule can be effectively activated on both the 4H-SiC(0001) and (0001[combining macron]) surfaces and different splitting products could be obtained on the two different surfaces, implying that SiC is an applicable catalyst material for CO2 conversion with high efficiency and product selectivity.

Mn Doped AZIS/ZnS Nanocrystals (NCs): Effects of Ag and Mn Levels on NC Optical Properties.

In this work, Mn-doped AZIS/ZnS NCs were prepared using a nucleation doping approach with the tuning of Mn and Ag levels in their synthesis. The optical properties of Mn:AZIS/ZnS NCs are found to be significantly affected by Ag and Mn levels. Specifically, more Ag and Mn atoms in Mn:AZIS/ZnS NCs cause their fluorescence red-shift, and as the Ag or Mn level reaches a high threshold, the fluorescence lifetime of Mn:AZIS/ZnS NC has a significant drop. The reasons for the effects of Mn and Ag levels on NC optical properties were explored and discussed. Through this study, it is also found that with certain Ag and Mn levels in synthesis, some Mn:AZIS/ZnS NCs present optimal optical properties including high brightness (QY > 40%), long fluorescence lifetime (> 1.2 ms), low energy for excitation (excitable at 405 nm), and no reabsorption. The feasibility of the optimized NCs for time-gated fluorescence measurement using a portable/compact instrument was further demonstrated, which indicates the application potential of the NCs in time-gated biosensing including point-of-care testing. Notably, this study also discloses that Mn:AZIS/ZnS NCs with different lifetimes can be achieved by tuning Mn and Ag levels in synthesis, which may further broaden the applications of Mn:AZIS/ZnS NCs in multiplexing detection/measurement.

Inverted structural quantum dot light-emitting diodes based on Al-doped ZnO electrode.

As an indium-free transparent conducting film, Al-doped zinc oxide (AZO) was prepared by magnetron sputtering technique, exhibiting good electrical, optical and surface characteristics. UPS/XPS measurements show that AZO and zinc oxide nanoparticles (ZnO NPs) have matched energy level that can facilitate the electron injection from AZO to ZnO NPs. Inverted structural green quantum dot light-emitting diodes based on AZO cathode were fabricated, which exhibits a maximum luminance up to 178 000 cd m-2, and a maximum current efficiency of 10.1 cd A-1. Therewith, combined with the simulated space-charge-limited current (SCLC) theory, the measured current density-voltage characteristics of charge-only devices were analyzed. It demonstrated that AZO and ZnO NPs had much better electron injection efficiency than ITO, showing a electron injection efficiency close to 100%. By studying the relationship between the electric field and the current density, the measured curve of AZO-based devices nearly fits the theoretical curve of SCLC and the AZO electrode has a better ohmic contact with ZnO NPs than ITO.

Thermal degradation of luminescence in inorganic perovskite CsPbBr3 nanocrystals.

The luminescence properties of inorganic perovskite CsPbBr3 nanocrystals (NCs) with emissions of 492 and 517 nm under thermal annealing treatment were studied by temperature-dependent photoluminescence (PL) spectroscopy. The CsPbBr3 NCs were annealed in vacuum at various temperatures. It was found that the NCs exhibited significant thermal degradation of PL at thermal annealing temperatures above 320 K. The transmission electron microscopy, X-ray diffraction and PL spectroscopy demonstrated that the size of NCs almost kept constant at thermal annealing temperatures below 360 K while it significantly enlarged at higher thermal temperatures above 380 K. The PL intensities, peak energies and linewidths of the annealed NCs, as a function of temperature, are discussed in detail. The PL degradation of the NCs was related to the formation of nonradiative recombination centers due to the loss of ligands and growth of NCs under thermal annealing.

Mn Doped AIZS/ZnS Nanocrystals: Synthesis and Optical Properties.

In this work, Mn doped AIZS/ZnS (Mn:AIZS/ZnS) nanocrystals (NCs) have been synthesized in an approach using heat-up and drop-wise addition of precursors. On the basis of the characterization of these doped NCs on their optical properties and materials, it is found that: (1) as more Mn atoms are doped into NCs, the doped NCs present photoluminescence (PL) red-shift and quantum yield quenching; (2) the doped NCs possess a short PL lifetime in tens of microseconds and a long PL lifetime in hundreds of microseconds, and the short lived PL is more dominant than the long lived one; and (3) the doped NCs present a reversible PL thermal quenching in a range from room temperature to 170°C. Possible PL mechanisms of these NCs were discussed by analyzing their time-resolved PL spectra and thermal stability.

Ammonia reduced graphene oxides as a hole injection layer for CdSe/CdS/ZnS quantum dot light-emitting diodes.

In this study, we report quantum-dot light-emitting devices (QD-LEDs) using ammonia reduced graphene oxide (rGO) as a hole injection layer (HIL). Compared with pristine GO, QD-LEDs employing rGO as a HIL show higher maximum luminance (936 cd m(-2) versus 699 cd m(-2)) and lower turn-on voltage (V th, 5.0 V versus 7.5 V). The improved performance can be attributed to the synergistic effect of the improved conductivity (1.27 µS cm(-1) versus 0.139 µS cm(-1)) and decreased work function (5.27 eV versus 5.40 eV) of the GO after the reduction process. The above results indicate that ammonia functionalized graphene may be a promising hole injection material for QD-LEDs.

Thermal stability of photoluminescence in Cu-doped Zn-In-S quantum dots for light-emitting diodes.

The photoluminescence (PL) properties of the Cu:Zn-In-S core quantum dots (QDs) and core-shell QDs were systematically investigated by using steady-state and time-resolved PL spectra at temperatures ranging from 80 to 400 K. The effects of the shell structure and the host bandgap on the thermal stability of Cu dopant emissions were studied by measuring the change in the PL intensity and the lifetime. It was found that the PL intensities and lifetimes of the core and core/shell QDs with green, yellow, and red emissions almost decrease with increasing temperatures while their PL was quenched at 300 K and 400 K, respectively, indicating the shell-enhanced thermal stability of the PL. The emission wavelength of the QDs as a function of temperature was also provided. The mechanisms of Cu dopant emission and thermal quenching were discussed. Finally, the green, yellow, red, and white light emitting light emitting diodes (LEDs) were fabricated based on Cu:Zn-In-S QDs.

Heat-up Synthesis of Ag-In-S and Ag-In-S/ZnS Nanocrystals: Effect of Indium Precursors on Their Optical Properties.

Cadmium-free I-III-VI nanocrystals (NCs) have recently attracted much research interests due to their excellent optical properties and low toxicity. In this work, with a simple heat-up synthetic system to prepare high quality Ag-In-S (AIS) NCs and their core/shell structures (AIS/ZnS NCs), we investigated the effect of different indium precursors (indium acetate and indium chloride) on NC optical properties. The measurements on photoluminescence spectra of AIS NCs show that the photoluminescence peak-wavelength of AIS NCs using indium acetate is in the range from 596 to 604 nm, and that of AIS NCs using indium chloride is from 641 to 660 nm. AIS and AIS/ZnS NCs using indium acetate present around 15% and 40% QYs, and both AIS and AIS/ZnS NCs using indium chloride present around 31% QYs. The photoluminescence decay study indicates that the lifetime parameters of AIS and AIS/ZnS using indium chloride are 2 ~ 4 times larger than those of AIS and AIS/ZnS NCs using indium acetate. Moreover, AIS NCs using indium chloride have a slower photobleaching dynamics than AIS NCs using indium acetate, and ZnS shell coating on both types of AIS NCs significantly enhances their photostability against UV exposure. We believe that the unique optical properties of AIS and AIS/ZnS NCs will open an avenue for these materials to be employed in broad electronic or biomedical applications.

Photocarrier recombination dynamics in ternary chalcogenide CuInS2 quantum dots.

Photocarrier recombination dynamics in ternary chalcogenide CuInS2 quantum dots (CIS QDs) was studied by means of femtosecond transient-absorption (TA) and nanosecond time-resolved photoluminescence (PL) spectroscopy. Under strong excitation, the TA dynamics in CIS QDs is well described by a simple rate equation including single-carrier trapping, free-to-bound recombination, and trap-assisted Auger recombination. Under weak excitation, on the other hand, the PL decays of the QDs are composed of a short-lived component caused by surface trapping and a long-lived one caused by free-to-bound recombination. It is found that the surface trapping accelerates markedly with decreasing QD size while the free-to-bound radiative recombination hardly depends on the QD size. Besides this, we observed both a decrease in the PL lifetimes and a dynamic spectral redshift, which are attributed to the surface trapping and the coexistent inhomogeneous broadening in CIS QDs. The spectral redshift becomes less pronounced in CIS/ZnS core/shell QDs because of the suppression of the fast nonradiative recombination caused by the passivation of the surface traps. These results give clear evidence that the free-to-bound model is appropriate for interpreting the optical properties of CIS QDs.

Towards efficient photoinduced charge separation in carbon nanodots and TiO2 composites in the visible region.

In this work, photoinduced charge separation behaviors in non-long-chain-molecule-functionalized carbon nanodots (CDs) with visible intrinsic absorption (CDs-V) and TiO2 composites were investigated. Efficient photoinduced electron injection from CDs-V to TiO2 with a rate of 8.8 × 10(8) s(-1) and efficiency of 91% was achieved in the CDs-V/TiO2 composites. The CDs-V/TiO2 composites exhibited excellent photocatalytic activity under visible light irradiation, superior to pure TiO2 and the CDs with the main absorption band in the ultraviolet region and TiO2 composites, which indicated that visible photoinduced electrons and holes in such CDs-V/TiO2 composites could be effectively separated. The incident photon-to-current conversion efficiency (IPCE) results for the CD-sensitized TiO2 solar cells also agreed with efficient photoinduced charge separation between CDs-V and the TiO2 electrode in the visible range. These results demonstrate that non-long-chain-molecule-functionlized CDs with a visible intrinsic absorption band could be appropriate candidates for photosensitizers and offer a new possibility for the development of a well performing CD-based photovoltaic system.

Efficient inverted quantum-dot light-emitting devices with TiO2/ZnO bilayer as the electron contact layer.

We have demonstrated an efficient inverted CdSe/CdS/ZnS core/shell quantum-dot light-emitting device (QD-LED) using a solution-processed sol-gel TiO2 and ZnO nanoparticle composite layer as an electron-injection layer with controllable morphology and investigated the electroluminescence mechanism. The introduction of the ZnO layer can lead to the formation of spin-coated uniform QD films and fabrication of high-luminance QD-LEDs. The TiO2 layer improves the balance of charge injection due to its lower electron mobility relative to the ZnO layer. These results offer a practicable platform for the realization of a trade-off between the luminance and efficiency in the inverted QD-LEDs with TiO2/ZnO composites as the electron contact layer.

Efficient white light emitting diodes based on Cu-doped ZnInS/ZnS core/shell quantum dots.

We report the fabrication of efficient white light-emitting diodes (WLEDs) based on Cu : ZnInS/ZnS core/shell quantum dots (QDs) with super large Stokes shifts. The composition-controllable Cu : ZnInS/ZnS QDs with a tunable emission from deep red to green were prepared by a one-pot noninjection synthetic approach. The high performance Cu : ZnInS QD-WLEDs with the colour rendering index up to 96, luminous efficacy of 70-78 lm W(-1), and colour temperature of 3800-5760 K were successfully fabricated by integration of red and green Cu-doped QDs. Negligible energy transfer between Cu-doped QDs was clearly found by measuring the photoluminescence lifetimes of the QDs, consistent with the small spectral overlap between QD emission and absorption. The experimental results indicated low toxic Cu : ZnInS/ZnS QDs could be suitable for solid state lighting.

Repercussions of the Inner Shell Layer on the Performance of Cd-Free Quantum Dots and Their Light-Emitting Diodes.

Indium phosphide (InP) and zinc selenium tellurium (ZnSeTe) quantum dots (QDs) as less toxic alternatives have received substantial attention. The structure of QDs generally consists of a QD core, inner shell layer, and outer shell layer. We reckon that the inner shell layer, especially its components and thickness, have a significant influence on the optical and electronic performances of QDs. In this Perspective, we compare optical properties of these QDs with different inner shells and summarize how typical inner shell components and thickness influence their optical properties. The impact of the inner shell on the performance of QD light-emitting diodes (QLEDs) has also been discussed. The appropriate components and thickness of the inner shell both contribute to alleviate valence or lattice mismatch, thereby enhancing the performance of QDs. We expect that this Perspective could heighten awareness of the significance and impact of the inner shell layer in QDs and facilitate further development of QDs and QLEDs.

Synergistic photoinduced charge transfer resonance from porous ZIF-67 decorated violet phosphorus array for SERS immunoassay of SARS-CoV-2 spike protein.

As a rapid, highly sensitive, and user-friendly technique, surface-enhanced Raman scattering (SERS) has an extraordinary appeal to home self-test of COVID-19 during the post pandemic era. However, most of the existing SERS substrates have been still criticized in stability, repeatability, and sample enrichment. To address these obstacles, a novel non-metallic SERS substrate with porous surfaces and array geometry was developed by in-situ growing ZIF-67 particles on two-dimensional violet phosphorus (VP) matrix. Chemical enhancement was prominently promoted by the synergistic photoinduced charge transfer resonance in the hybrid band structure of the ZIF-67@VP substrate, facilitating a noble metal-similar enhancement factor of 6.11 × 107. The biocompatible ZIF-67@VP porous array with attractive enhancement capability and high anchoring efficiency was further utilized to monitoring SARS-CoV-2 spike protein in practical saliva samples based on a sandwich immunostructure, achieving a limit of detection of 1.7 ng/mL assisted by black phosphorus nanosheets. This nonmetallic immunoassay strategy with exceptional sensitivity and specificity is predicted to extend the utilization of SERS obstacle in daily infectious disease screening.