Artificial Optoelectronic Synapse Based on Violet Phosphorus Microfiber Arrays.

Memristor-based artificial synapses are regarded as the most promising candidate to develop brain-like neuromorphic network computers and overcome the bottleneck of Von-Neumann architecture. Violet phosphorus (VP) as a new allotrope of available phosphorus with outstanding electro-optical properties and stability has attracted more and more attention in the past several years. In this study, large-scale, high-yield VP microfiber vertical arrays have been successfully developed on a Sn-coated graphite paper and are used as the memristor functional layers to build reliable, low-power artificial synaptic devices. The VP devices can well mimic the major synaptic functions such as short-term memory (STM), long-term memory (LTM), paired-pulse facilitation (PPF), spike timing-dependent plasticity (STDP), and spike rate-dependent plasticity (SRDP) under both electrical and light stimulation conditions, even the dendritic synapse functions and simple logical operations. By virtue of the excellent performance, the VP artificial synapse devices can be conductive to building high-performance optic-neural synaptic devices simulating the human-like optic nerve system. On this basis, Pavlov's associative memory can be successfully implemented optically. This study provides a promising approach for the design and manufacture of VP-based artificial synaptic devices and outlines a direction with multifunctional neural devices.

TiO2 nanotube arrays-based photoelectrocatalyst: Tri-Doping engineering and carbon coating engineering boosting visible activity, and stable hydrogen evolution.

The integration of non-metallic doping and carbon coating for TiO2-based photoelectrocatalysts can be recognized as a promising strategy to enhance their hydrogen production performance. To this end, this study explored the carbon coating engineering to induce stable multi-element doping with an aim to develop high-performance TiO2 nanotube array-based photoelectrocatalysts. The resulting structures consisted of carbon-nitrogen-sulfur-tri-doped TiO2 nanotube arrays with a nitrogen-sulfur-codoped carbon coating (CNS-TNTA/NSC). The fabrication process involved a one-step, low-cost strategy of the carbon-coated tridoped reaction confined in vacuum space, utilizing polymer thiourea sealed in a controlled environment. Compared the photocurrent density of CNS-TNTA/NSC with pristine TNTA, the photocurrent enhancement of approximately 18.3-fold under simulated sunlight and a remarkable increase of 32.8-fold under simulated visible light conditions. The enhanced photocatalytic activity under visible light was ascribed to two factors: First, C, N, and S tri-doping and Ti3+ created a diverse array of impurity energy levels within the band gap, which synergistically narrowed the band gap and further enhanced response to the visible light range. Second, the presence of a carbon coating shell doped with N and S can greatly promote electron transfer and efficient electron-hole pair separation. This study could provide significant insights concerning the design of sophisticated photoanodes.

Erbium Single Atom Composite Photocatalysts for Reduction of CO2 under Visible Light: CO2 Molecular Activation and 4f Levels as an Electron Transport Bridge.

It is still challenging to design a stable and efficient catalyst for visible-light CO2 reduction. Here, Er3+ single atom composite photocatalysts are successfully constructed based on both the special role of Er3+ and the special advantages of Zn2 GeO4 /g-C3 N4 heterojunction in the photocatalysis reduction of CO2 . Especially, Zn2 GeO4 :Er3+ /g-C3 N4 obtained by in situ synthesis is not only more conducive to the tight junction of Zn2 GeO4 and g-C3 N4 , but also more favorable for g-C3 N4 to anchor rare-earth atoms. Under visible-light irradiation, Zn2 GeO4 :Er3+ /g-C3 N4 shows more than five times enhancement in the catalytic efficiency compared to that of pure g-C3 N4 without any sacrificial agent in the photocatalytic reaction system. A series of theoretical and experimental results show that the charge density around Er, Ge, Zn, and O increases compared with Zn2 GeO4 :Er3+ , while the charge density around C decreases compared with g-C3 N4 . These results show that an efficient way of electron transfer is provided to promote charge separation, and the dual functions of CO2 molecular activation of Er3+ single atom and 4f levels as electron transport bridge are fully exploited. The pattern of combining single-atom catalysis and heterojunction opens up new methods for enhancing photocatalytic activity.

Enhanced luminescence through interface energy transfer in hierarchical heterogeneous nanocomposites and application in white LEDs.

Highly efficient light-emitting materials are essential for achieving high-performance devices. Here, a novel composite system, as well as enhanced luminescence processes, was designed, where NaLn(MoO4)2 ultra-small nucleus can be effectively isolated by In(OH)3 to form NaLn(MoO4)2@In(OH)3 composite nanoclusters due to the different nucleation rate between NaLn(MoO4)2 and In(OH)3, and then these small composite clusters gradually self-assemble into hierarchical structures. As we expected, the enhanced luminescence was achieved from hierarchical NaLn(MoO4)2 nanostructures with adjusting the distance among NaLn(MoO4)2 ultra-small nucleus by inserting In(OH)3. A series of spectroscopy results show that the In(OH)3 not only acts as an energy transfer bridge from CTB Eu3+ → O2- (or MoO42- absorption) to Eu3+, but also can effectively alleviate the concentration quenching of Ln3+ and change the J-O parameters. The Raman peak at 134 cm-1 is helpful to populate the 5D0 level of Eu3+ or the excited states of Er3+, resulting in stronger up/down-conversion emissions. The use of NaLn(MoO4)2@In(OH)3 in white light-emitting diodes (LEDs) has been demonstrated. The combination of red emission from NaLn(MoO4)2@In(OH)3 with blue, green, and yellow emission from halide perovskites could achieve white light with excellent vision performance (an LER of 376 lm/W) and superior color quality (CRI > 92). The findings of this experiment provide a new idea for the design of composite interface materials.

Dual-Mode Light-Emitting Lanthanide Metal-Organic Frameworks with High Water and Thermal Stability and Their Application in White LEDs.

It is well known that the upconversion luminescence from lanthanide metal-organic frameworks (Ln-MOFs) is difficult to achieve, and thus, there are few reports on dual luminescence-based MOFs. Here, dual-mode light-emitting Ln-MOFs are synthesized using a low-cost hydrothermal method. Our results show that the obtained Ln-MOFs not only have high thermal stability (up to 420°) but also are stable in deionized water. The dual-mode up- and downconversion luminescence is simultaneously observed from Er-Eu-MOFs. The temperature-dependent fluorescence decay time is calculated to be ranging from 0.46 to 0.36 ms for temperatures from 100 to 300 K. We suggested that this phenomenon was because the number of phonons participating in the MOF matrix increases with temperature during the luminescence process, and the phonons interact with the electrons in the material. The values of the J-O parameters calculated from the emission spectra indicated that the symmetry around Eu3+ ions in Eu-MOF is the highest, which was also higher than that of Er-Eu-MOF. To explore the potential applications of Eu-MOFs in white light-emitting diodes (LEDs), red emission from Eu-MOFs was combined with blue, green, and yellow emissions from metal halide perovskites to achieve white light emission. White light with excellent color quality and vision performance was obtained. These findings demonstrate that Ln-MOFs are potentially successful materials for applications in white LEDs.

Zn-Alloyed All-Inorganic Halide Perovskite-Based White Light-Emitting Diodes with Superior Color Quality.

Recently, lead halide perovskite nanocrystals (NCs) have gained tremendous attention in optoelectronic devices due to their excellent optical properties. However, the toxicity of lead limits their practical applications. Here, the synthesis of Zn2+-alloyed CsZnxPb1-xX3 (up to 15%) NCs is reported to achieve lead-reduced white light-emitting diodes (WLEDs). The incorporation of Zn2+ into CsPbX3 host NCs results in a lattice contraction, without altering the structure and morphology, which has a direct effect on the optical properties. The blue-shifts in the photoluminescence emission and increase in bandgap is observed while retaining high photoluminescence quantum yield. Then by engineering the different compositions of halides for 15% Zn2+-alloyed CsZnxPb1-xX3 NCs, tunable emission (411-636 nm) is obtained. Notably, the WLEDs are experimentally demonstrated employing the lead-reduced NCs (blue, green, yellow, and red). By varying the ratios of the amount of NCs, white lights with a tunable correlated-color temperature (2218-8335 K), an exemplary color-rendering index (up to 93) and high luminous efficacy of radiation (268-318 lm·W-1) are obtained. Best of our knowledge, these are superior to other reported WLEDs based on CsPbX3 NCs doped with transition metal ions. This work places the halide perovskite NCs one-step closer in designing the environmentally benign and energy-efficient WLEDs.

Design rules for white light emitters with high light extraction efficiency.

The finite-difference time-domain method is employed to study the light extraction efficiency of white light emitters. The cone arrays designed on top of white light emitters eliminate the dependency of light extraction on the wavelength and the cavity thickness and that leads to significant enhancement in light extraction efficiency for whole visible light spectrum. The light extraction efficiency of 81% has been achieved. Most importantly, the high extraction efficiency is achieved for the whole visible spectrum from 400 nm to 700 nm. This work will provide guidelines for designing highly efficient white light emitters for general illumination and display purpose.

Multifunctional NaLnF4@MOF-Ln Nanocomposites with Dual-Mode Luminescence for Drug Delivery and Cell Imaging.

Multifunctional nanomaterials for bioprobe and drug carrier have drawn great attention for their applications in the early monitoring the progression and treatment of cancers. In this work, we have developed new multifunctional water-soluble NaLnF4@MOF-Ln nanocomposites with dual-mode luminescence, which is based on stokes luminescent mesoporous lanthanide metal-organic frameworks (MOFs-Y:Eu3+) and anti-stokes luminescent NaYF4:Tm3+/Yb3+ nanoparticles. The fluorescence mechanism and dynamics are investigated and the applications of these nanocomposites as bioprobes and drug carriers in the cancer imaging and treatment are explored. Our results demonstrate that these nanocomposites with the excellent two-color emission show great potential in drug delivery, cancer cell imaging, and treatment, which are attributed to the unique spatial structure and good biocompatibility characteristics of NaLnF4@MOF-Ln nanocomposites.

Tetradic phosphor white light with variable CCT and superlative CRI through organolead halide perovskite nanocrystals.

In this work, the emission spectral range of halide perovskite nanocrystals is extended from violet to infrared, the widest emission range for halide perovskites to date. This range extension was made possible by a cost-effective solution-based synthesis process that only involves two halides [MAPb(Br x I1-x )3 and MA = CH3NH3]. Furthermore, the correlated-color temperature (CCT) of white light is tuned by blending an appropriate fraction of the as-synthesized blue, green, yellow, and red emitting nanocrystals. This represents one of the first applications of a tetradic phosphor system for maximizing the color rendering index (CRI) for this material. The CCT ranges from warm to cool white (2759-6398 K) and the CRI has a maximum value of 93.95. Thus, this fourfold phosphor approach demonstrates that halide perovskites are promising alternatives to conventional phosphors in the search for low-cost and high-quality white light sources in the next generation of white lighting technology.

Red photoluminescent Eu3+-doped Y2O3 nanospheres for LED-phosphor applications: Synthesis and characterization.

Y2O3:Eu3+ nanospheres with sizes of 40-334 nm in diameter were obtained using a low-cost co-precipitation method followed by a thermal annealing process. The sizes of the nanospheres were controlled by tuning the synthesis time and annealing temperature. X-ray diffraction patterns and scanning electron microscopy images were used to determine the structure, shape, and size of the obtained nanoparticles. The optical properties of the nanospheres were investigated with measurements of the photoluminescence excitation and emission spectra. A phase transformation of the nanospheres from an amorphous structure to a cubic crystalline Y2O3 structure was observed when the annealing temperature was higher than 500 °C. Intense red photoluminescence emission and UV excitation of the nanospheres with a crystal structure were identified. In addition, the optimal concentration of dopant (Eu3+) for the red emission was determined to be ~8 mol%. The unique structural and optical properties of the Y2O3:Eu3+ nanospheres could lead to efficient red LED-phosphors for use in generating white light with GaAlN-based UV LEDs.

High-Pressure Studies of 4-Acetamidobenzenesulfonyl Azide: Combined Raman Scattering, IR Absorption, and Synchrotron X-ray Diffraction Measurements.

We have reported the high-pressure behavior of 4-acetamidobenzenesulfonyl azide (C8H8N4O3S, 4-ABSA) by in situ Raman scattering, IR absorption, and synchrotron angle-dispersive X-ray diffraction (ADXRD) measurements in diamond anvil cells with the pressure up to ∼13 GPa at room temperature. All of the fundamental vibrational modes of 4-ABSA at ambient pressure were analyzed by combination of experimental measurements and theoretical calculations using the density functional theory method. Detailed Raman and IR spectroscopic analyses reveal two phase transitions in the pressure region of 0.8-2 and 4.2 GPa, respectively, which are confirmed by the changes in the ADXRD patterns. The first phase transition in the pressure region of 0.8-2 GPa is attributed to the ring distortion and the rotation of CH3 group, whereas the second phase transition at 4.2 GPa might be induced by the rearrangement of azide group and hydrogen bonds. The analyses of the N3 vibrational modes suggest that the bent azide group rotates progressively upon compression, which is ascribed to the compression of the unit cell along the b axis. This study is helpful to understand the behavior of azide group and structural evolution of 4-ABSA under high pressure.

Resonant cavity effect optimization of III-nitride thin-film flip-chip light-emitting diodes with microsphere arrays.

Comprehensive studies were carried out to investigate the light extraction efficiency of thin-film flip-chip (TFFC) light-emitting diodes (LEDs) with anatase TiO(2) microsphere arrays by employing the finite-difference time-domain method. The quantum well position and the resonant cavity effect were studied to obtain optimum light extraction for the planar TFFC LED. Further enhancement in light extraction was achieved by depositing microsphere arrays on the TFFC LED. The calculation results showed that the sphere diameter, packing density, and packing configuration have significant effects on the light extraction efficiency. A maximum light extraction efficiency of 75% in TFFC LEDs with microsphere arrays has been achieved. This study demonstrates the importance of optimizing the quantum well position, cavity thickness, sphere diameter, sphere packing density, and packing configuration for enhancing the light extraction efficiency of TFFC LEDs with microsphere arrays.

Aspect ratio engineering of microlens arrays in thin-film flip-chip light-emitting diodes.

Light extraction efficiency of thin-film flip-chip InGaN-based light-emitting diodes (LEDs) with a TiO2 microlens arrays was calculated by employing the finite-difference time-domain method. The microlens arrays, formed by embedding hexagonal close-packed TiO2 sphere arrays in a polystyrene (PS) layer, were placed on top of the InGaN LED to serve as an intermediate medium for light extraction. By tuning the thickness of the PS layer, in-coupling and out-coupling efficiencies were optimized to achieve maximum light extraction efficiency. A thicker PS layer resulted in higher in-coupling efficiency, while a thinner PS layer led to higher out-coupling efficiency. Thus, the maximum light extraction efficiency becomes a trade-off between in-coupling and out-coupling efficiency. In addition, the cavity formed by the PS layer also affects light extraction from the LED. Our study reveals that a maximum light extraction efficiency of 86% was achievable by tuning PS thickness to 75 nm with maximized in-coupling and out-coupling efficiency accompanied by the optimized resonant cavity condition.

Synthesis and luminescence properties of Er3+ doped Y(OH)3, NH4Y3F10, and YF3 nanocrystals.

Y(OH)3:Er3+ nanowires were synthesized by a hydrothermal method. Y(OH)3:Er3+ can convert into NH4Y3F10:Er3+ after fluorization, and NH4Y3F10:Er3+ can convert into YF3:Er3+ after being annealed. The structures of obtained Y(OH)3:Er3+, NH4Y3F10:Er3+, and YF3:Er3+ samples were pure hexagonal, cubic, and orthorhombic phase, respectively. Under 378-nm excitation, the three samples showed similar features. The 2H9/2, --> 4I15/2, 4F3/2(4F5/2) --> 4I15/2, and 4S3/2 --> 4I15/2 were observed, and the most intense peak was centered at 436 nm [4F3/2(4F5/2) --> 4I15/2]. Under 980-nm excitation, only the upconversion emissions from NH4Y3F10:Er3+ and YF3:Er3+ were observed. These emissions come from the following transitions: 2H11/2 --> 4I15/2, 4S3/2 --> 4I15/2, and 4F9/2 --> 4I15/2. The upconversion mechanism is discussed in detail.

Large-scale synthesis and photoluminescence properties of aligned multicore SiC-SiO2 nanocables.

By using a simple and low-cost microwave method, aligned multicore SiC-SiO2 nanocables have been successfully synthesized on a large scale. The composition and structural features of the products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results revealed that each of the nanocables was composed of several 3C-SiC nanowires encapsulated in a single amorphous SiO2 shell. The cores were 10-50 nm in diameter and up to hundreds of microns in length. The photoluminescence properties of the nanocables were studied, and strong violet blue light emission was observed at wavelengths of about 339 and 390 nm under 325-nm excitation. The origin of the photoluminescence from the nanocables can be attributed to the central SiC nanowires and defects in silicon oxide or the SiC/SiO2 interface boundary. Based on experimental characterizations, an oxide-assisted vapor-liquid-solid (VLS) growth mechanism was used to elucidate the growth process of the multicore SiC-SiO2 nanocables.

Synthesis and upconversion luminescence of YF3:Yb3+ Tm3+ and TiO2-coated YF3:Yb3+, Tm3+ microcrystals.

YF3:Yb3+, Tm3+ microcrystals were prepared by a microemulsion method. The microcrystals were coated with TiO2 by hydrolysis of titanium n-butoxide (TBOT). Transform electron microscopy and X-ray diffraction were used to characterize the core/shell materials. The results indicated that the low TBOT/YF3 molar ratio was favorable to preparing the uniform TiO2 coatings. The upconversion luminescence properties of YF3:Yb3+, Tm3+ and TiO2-coated YF3:Yb3+, Tm3+ microcrystals were studied under 980-nm excitation. The 1I6 --> 3H6, 1I6 --> 3F4, 1D2 --> 3H6, 1D2 --> 3F4, 1G4 --> 3H6, and 1G4 --> 3F4 emissions were observed. The upconversion mechanisms were discussed in detail.

Synthesis of ZnO nanosheets by microwave thermal vapor method.

Two-dimensional single-crystalline ZnO nanosheets have been successfully synthesized in high yield from a mixture of ZnO and graphite powders via a microwave thermal vapor process. Morphological analysis indicates that the widths and thickness of the nanosheets are in the range of several to tens of microm and 20-80 nm, respectively. ZnO nanosheets grew along the [0001] and [2110] directions, and the width-to-thickness ratio of the nanosheets could reach up to 1,000. The room temperature photoluminescence revealed that the ZnO nanosheets exhibited a weak ultraviolet (UV) emission at approximately 378 nm and a strong green emission at approximately 505 nm.

Ultraviolet upconversion fluorescence from 6D(J) of Gd3+ induced by 980 nm excitation.

Ultraviolet upconversion emissions of 246.2 and 252.8 nm from (6)D(J) levels of Gd(3+) ions were observed in GdF(3): 10% Yb(3+), 0.7% Tm(3+) nanocrystals under 980 nm excitation from a laser diode. The (6)D(J) levels of Gd(3+) ions can be efficiently populated by energy transfer processes of Yb-->Tm-->Gd and Yb-->Gd. A six-photon upconversion process was confirmed by the dependence of 252.8 nm emission intensity on the pumping power. The upconversion mechanism in the six-photon process was discussed based on excited state absorption of Gd(3+) ions, cross relaxation energy transfer between two excited Gd(3+) ions, and energy transfer between Gd(3+) and Yb(3+) or Tm(3+) ions.

Intense ultraviolet upconversion luminescence from hexagonal NaYF4:Yb3+/Tm3+ microcrystals.

Under 980 nm excitation, unusual 3P2-->3H6 (approximately 264 nm) and 3P2-->3F4 (approximately 309 nm) emissions from Tm3+ ions were observed in hexagonal NaYF4:Yb3+ (20%)/Tm3+ (1.5%) microcrystals. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished due to the efficient cross-relaxation of 1G4 + 3H4-->3F4 + 1D2(Tm3+). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation.

Europium(III) complexes/silica hybrid nanospheres synthesized in microemulsion.

Eu(DBM)3Phen/silica nanospheres with a uniform diameter of approximately 40 nm and the characteristic fluorescence of Eu3+ ions have been synthesized by a microemulsion method. SEM and TEM analysis indicate that the hybrid nanospheres are core/shell structures with fine spherical surfaces and that Eu(DBM)3Phen has been successfully enveloped in the SiO2 spheres as chromophore cores. IR absorption spectra and photoluminescence spectra suggest that the hybrid nanoparticles are promising materials for bioassay.