Sn-doped ZnO for efficient and stable quantum dot light-emitting diodes via a microchannel synthesis strategy.

ZnO nanocrystals (NCs) are widely employed as an electron transport layer (ETL) in quantum-dot light-emitting diodes (QLEDs). However, the excessive electron mobility, abundant surface defects and poor reproducibility of ZnO NC synthesis are currently the primary restrictive factors influencing the development of QLEDs. In this study, we developed Sn(IV)-doped ZnO NCs as the ETL for constructing highly efficient and long lifetime QLEDs. The introduction of Sn can reduce the surface hydroxyl oxygen defects and alter the electron transport properties of NCs, and thus is beneficial for improving the efficiency of hole-electron recombination in the emitting layer. Meanwhile, a microchannel (MC) reactor is utilized to finely control the synthesis of Zn0.96Sn0.04O NCs, enabling us to achieve uniform size distribution and consistent production reproducibility. Using the Sn(IV)-doped ZnO NCs as the ETL has led to a remarkable enhancement of external quantum efficiency (EQE) for the fabricated red QLED, from 9.2% of the ZnO only device to 15.5% of the Zn0.96Sn0.04O device. Furthermore, the T70 (@1000 cd m-2) of the Zn0.96Sn0.04O device reached 78 h, which is 1.77-fold higher than that of the ZnO only device (44 h). The present work provides an alternative ETL for efficient and stable QLEDs.

Molecular Design of Diazo Compound for Carbene-Mediated Cross-Linking of Hole-Transport Polymer in QLED with Reduced Energy Barrier and Improved Charge Balance.

Polymeric hole-transport materials (HTMs) have been widely used in quantum-dot light-emitting diodes (QLEDs). However, their solution processability normally causes interlayer erosion and unstable film state, leading to undesired device performance. Besides, the imbalance of hole and electron transport in QLEDs also damages the device interfaces. In this study, we designed a bis-diazo compound, X1, as carbene cross-linker for polymeric HTM. Irradiated by ultraviolet and heating, a poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl))] (TFB)/X1 blend can achieve fast "electronically clean" cross-linking with ∼100% solvent resistance. The cross-linking reduced the stacking behaviors of TFB and thus led to a lower hole-transport mobility, whereas it was a good match of electron mobility. The carbene-mediated TFB cross-linking also downshifted the HOMO level from -5.3 to -5.5 eV, delivering a smaller hole-transport energy barrier. Benefiting from these, the cross-linked QLED showed enhanced device performances over the pristine device, with EQE, power efficiency, and current efficiency being elevated by nearly 20, 15, and 83%, respectively. To the best of our knowledge, this is the first report about a bis-diazo compound based carbene cross-linker built into a polymeric HTM for a QLED with enhanced device performance.

SERS-active substrate assembled by Ag NW-embedded porous polystyrene fibers.

Here we demonstrate a novel SERS-active substrate assembled by silver nanowire (Ag NW)-embedded porous polystyrene (PS) fibers. Ag NWs are synthesized through a glycerol-mediated solvothermal method firstly, then electrospun into PS porous fibers. The as-synthesized Ag NWs are embedded in PS fiber and aligned orderly along the axial direction. Porous structure appears in PS fiber due to the phase separation induced by rapid evaporation of solvents. Large amounts of holes not only greatly improve the sample collection efficiency of the SERS-active substrate, but also significantly facilitate the adsorption of target molecules on the surface of Ag NWs, thus increasing the probability of enhancement of target molecules. In addition, compared with polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP), PS has better solvent resistance. The detection limit of 4-aminothiophenol (4-ATP) on our fabricated electrospun fiber mats is 10-7 M, and the electrospun fiber mats showed good reproducibility of SERS signal detection. This study proposes a feasible strategy for the large-scale preparation of flexible SERS-active substrate assembled by Ag NW-embedded porous PS fibers. The produced flexible SERS substrates may have potential application in wearable sensors for the trace detection of chemical and biological molecules.

Observation of High-Frequency Raman Modes in FeCl3- and Zn-Intercalated MoS2 Flakes.

In place of the widely studied graphene, monolayer or few-layer MoS2 flakes are promising materials for next-generation optoelectronic devices. MoS2 has attracted increasing attention in physics and its applications because of its capacity to undergo indirect-to-direct band gap transition. Raman spectroscopy is a useful and versatile tool to probe the physical properties of pristine and intercalated MoS2. This study investigates for the first time the multiphoton modes of FeCl3- and Zn-intercalated few-layer MoS2 at high frequencies of 1513 and 1732 cm-1 for FeCl3-MoS2 and 1341 and 1604 cm-1 for Zn-MoS2. The substrates interact with MoS2 during intercalation. The Raman peak intensities of the intercalated samples vary with intercalation time while keeping the peak position nearly constant. This finding is interesting and suitable for studying other 2D layered materials.

Band Engineering by Controlling vdW Epitaxy Growth Mode in 2D Gallium Chalcogenides.

Atomically thin quasi-2D GaSe flakes are synthesized via van der Waals (vdW) epitaxy on a polar Si (111) surface. The bandgap is continuously tuned from its commonly accepted value at 620 down to the 700 nm range, only attained previously by alloying Te into GaSe (GaSex Te1- x ). This is accomplished by manipulating various vdW epitaxy kinetic factors, which allows the choice bet ween screw-dislocation-driven and layer-bylayer growth, and the design of different morphologies with different material-substrate interaction (strain) energies.

Exciton pumping across type-I gallium chalcogenide heterojunctions.

Quasi-two-dimensional gallium chalcogenide heterostructures are created by transferring exfoliated few-layer GaSe onto bulk GaTe sheets. Luminescence spectroscopy measurements reveal that the light emission from underlying GaTe layers drastically increases on heterojunction regions where GaSe layers make contact with the GaTe. Density functional theory (DFT) and band offset calculations show that conduction band minimum (CBM) (valance band maximum (VBM)) values of GaSe are higher (lower) in energy compared to GaTe, forming type-I band alignment at the interface. Consequently, GaSe layers provide photo-excited electrons and holes to GaTe sheets through relatively large built-in potential at the interface, increasing overall exciton population and light emission from GaTe. Observed results are not specific to the GaSe/GaTe system but observed on GaS/GaSe heterolayers with type-I band alignment. Observed experimental findings and theoretical studies provide unique insights into interface effects across dissimilar gallium chalcogenides and offer new ways to boost optical performance by simple epitaxial coating.

Site Selective Doping of Ultrathin Metal Dichalcogenides by Laser-Assisted Reaction.

Laser-assisted phosphorus doping is demonstrated on ultrathin transition-metal dichalcogenides (TMDCs) including n-type MoS2 and p-type WSe2 . Temporal and spatial control of the doping is achieved by varying the laser irradiation power and time, demonstrating wide tunability and high site selectivity with high stability. The laser-assisted doping method may enable a new avenue for functionalizing TMDCs for customized nanodevice applications.

Engineering excitonic dynamics and environmental stability of post-transition metal chalcogenides by pyridine functionalization technique.

Owing to their strong photon emission, low excitonic binding energies, and nearly-ideal band offset values for water splitting reactions, direct gap quasi-2D gallium chalcogenides are potential candidates for applications in energy harvesting, optoelectronics, and photonics. Unlike other 2D materials systems, chemical functionalization of gallium chalcogenides is still at its seminal stages. Here, we propose vapor phase pyridine intercalation technique to manipulate optical properties of gallium chalcogenides. After functionalization, the excitonic dynamics of quasi-2D GaSe change significantly as evidenced by an increase in integrated PL intensity and emergence of a new emission feature that is below the band edge. Based on our DFT calculations, we attribute these to formation of bound exciton complexes at the trap sites introduced by chemical reaction between pyridine and GaSe. On the contrary, pyridine functionalization does not impact the optical properties of GaTe, instead treats GaTe surface to prevent oxidization of tellurium atoms. Overall, results suggest novel ways to control properties of gallium chalcogenides on demand and unleash their full potential for a range of applications in photonics and optoelectronics.

Mo doping-enhanced dye absorption of Bi2Se3 nanoflowers.

A simple solvothermal approach is explored to prepare Bi2-xMoxSe3 nanostructures by employing N,N-dimethylformamide (DMF) as the solvent. Mo plays an important role in the assembly of the Bi2-xMoxSe3 nanostructures from nanoplates to nanoflowers. Structural and morphological studies indicate that the resulting products are large specific surface area single-crystalline Bi2-xMoxSe3 nanoflowers self-assembled from thin nanoplates during the reaction process. The absorption properties of the as-prepared samples are investigated with Rhodamine B (RhB) as dye, and it is found that the Bi1.85Mo0.15Se3 nanoflowers show an optimal adsorption capacity, implying that Mo doping not only changes the morphologies of the nanostructures but also enhances their absorption behaviors.

Strong up-conversion emissions in ZnO:Er3+, ZnO:Er(3+)-Yb3+ nanoparticles and their surface modified counterparts.

ZnO:Er(3+) and ZnO:Er(3+)-Yb(3+) nanoparticles (NPs) are fabricated by a sol-gel method, afterwards parts of which are separated and surface modified in Mo(NO(3))(3) solution. Analyses on phase and structure based on X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques indicate that Er(3+) and Yb(3+) are incorporated into the ZnO lattice successfully and after Mo treatment, a thin layer of MoO(3) forms on the NPs surface, forming core/shell structures. Raman scattering spectra reveal the existence of ZnMoO(4) in the shell part. Visible up-conversion (UC) is observed in all the samples, with Mo treated and untreated ZnO:Er(3+) emitting dominant but relatively weak red light, corresponding to (4)F(9/2)-(4)I(15/2) transition of Er(3+). In Yb(3+)-codoping systems, the integral UC intensity is enhanced obviously though red emission still dominates the UC spectra before surface modification. In the Mo treated system, ZnO:Er(3+)-Yb(3+)/MoO(3), green emission is increased while the red is suppressed in comparison to ZnO:Er(3+)-Yb(3+), with the intensity of green to red ratio (GRR) changing from 0.25 to 8. A novel phenomenon is discovered that the green emissions in our samples involve three-photon processes.

Enhanced ultraviolet emission from ZnS-coated ZnO nanowires fabricated by self-assembling method.

A simple chemical route for ZnS-coated ZnO nanowires with preferential (002) orientation is reported. Sodium sulfide and zinc nitrate were employed to supply S and Zn atoms at 60 degrees C to form ZnS-coated ZnO nanowires structures. Electron diffraction measurement shows that the ZnO/ZnS core-shell nanostructure is single crystalline. Interesting features are found in the photoluminescence (PL) spectra of ZnS-coated ZnO nanostructures. After coating, the UV emission of nanorods is dramatically enhanced at the expense of the green emission. The core/shell structure with higher band gap shell material and reduced surface states should be responsible for this PL enhancement.