Extremely Enhanced Photoluminescence in MoS2-Derived Quantum Sheets.

Molybdenum disulfide (MoS2) quantum sheets (QSs) are attractive for applications due to their tunable energy band structures and optical and electronic properties. The photoluminescence quantum yield (PLQY) of MoS2 QSs achieved by mechanical and liquid exfoliation and chemical vapor deposition is low. Some studies have reported that chemical treatment and elemental doping can improve the PLQY of transition metal dichalcogenides (TMDs), but this is limited by complex instruments and reactions. In this study, a heat treatment method based on a polar solvent is reported to improve the PLQY and photoluminescence (PL) intensity of MoS2 QSs at room temperature. The absolute PLQY of treated MoS2 QSs is increased to 18.5%, and the PL intensity is increased by a factor of 64. This method is also effective for tungsten disulfide (WS2) QSs. The PL enhancement of QSs is attributed to oxidation of the edges. Such passivation/deformation of MoS2 QSs facilitates the radiative route rather than the nonradiative route, resulting in extreme enhancement of the PL. Our work could provide novel insights/routes toward the PL enhancement of TMD QSs.

Tailoring Graphite into Subnanometer Graphene.

Within the intersection of materials science and nanoscience/technology, extremely downsized (including quantum-sized and subnanometer-sized) materials attract increasing interest. However, the effective and controllable production of extremely downsized materials through physical strategies remains a great challenge. Herein, an all-physical top-down method for the production of sub-1 nm graphene with completely broken lattice is reported. The graphene subnanometer materials (GSNs) with monolayer structures and lateral sizes of ≈0.5 nm are obtained. Compared with their bulk, nanosheets, and quantum sheets, the intrinsic GSNs present extremely enhanced photoluminescence and nonlinear saturation absorption performances, as well as unique carrier behavior. The non-equilibrium states induced by the entirely exposed and broken, intrinsic lattices in sub-1 nm graphene can be determinative to their extreme performances. This work shows the great potential of broken lattice and provides new insights toward subnanometer materials.

Quantum-sized topological insulators/semimetals enable ultrahigh and broadband saturable absorption.

Two-dimensional topological insulators/semimetals have recently attracted much attention. However, quantum-sized topological insulators/semimetals with intrinsic characteristics have never been reported before. Herein, we report the high-yield production of topological insulator (i.e., Bi2Se3 and Sb2Te3) and semimetal (i.e., TiS2) quantum sheets (QSs) with monolayer structures and sub-4 nm lateral sizes. Both linear and nonlinear optical performances of the QSs are investigated. The QS dispersions present remarkable photoluminescence with excitation wavelength-, concentration-, and solvent-dependence. The solution-processed QSs-poly(methyl methacrylate) (PMMA) hybrid thin films demonstrate exceptional nonlinear saturation absorption (NSA). Particularly, Bi2Se3 QSs-PMMA enables record-high NSA performance with a broadband feature. Specifically, the (absolute) modulation depths up to 71.6 and 72.4% and saturation intensities down to 1.52 and 0.49 MW cm-2 are achieved at 532 and 800 nm, respectively. Such a phenomenal NSA performance would greatly facilitate their applications in mode-locked lasers and related fields.

Ti3AlC2 MAX and Ti3C2 MXene Quantum Sheets for Record-High Optical Nonlinearity.

Two-dimensional (2D) transition-metal carbides (MXenes) have attracted great interest owing to their unique structures and superior properties compared to those of traditional 2D materials. The transformation of 2D MXenes into sub-5-nm quantum sheets (QSs) is urgently required but rarely reported. Herein, the Ti3AlC2 MAX and Ti3C2 MXene QSs with monolayer structures and sub-5-nm lateral sizes are demonstrated. Exceptionally high yields (>15 wt %) are obtained through an all-physical top-down method. The QS dispersions present unique photoluminescence, and the QSs-poly(methyl methacrylate) (PMMA) hybrid thin films demonstrate remarkable nonlinear saturation absorption (NSA). Absolute modulation depths of 30.6 and 49.9% and saturation intensities of 1.16 and 1.25 MW cm-2 (i.e., 116 and 125 nJ cm-2) are achieved for Ti3AlC2 QSs and Ti3C2 QSs, respectively. Such record-high NSA performances of MXene QSs would boost the application of MAX/MXene materials in nonlinear optics.

Scalable production of intrinsic WX2(X = S, Se, Te) quantum sheets for efficient hydrogen evolution electrocatalysis.

Mass production of transition-metal dichalcogenides has attracted much attention to replace platinum-based catalysts for the hydrogen evolution reaction (HER). Herein, we demonstrate a general strategy for the scalable production of the intrinsic tungsten dichalcogenide (WX2(X = S, Se, Te)) quantum sheets (QSs) by an all-physical top-down method. The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation and thus enables production of WS2QSs, WSe2QSs, and WTe2QSs in exceedingly high yields of 28.2, 21.3, 19.9 wt%, respectively. The WX2QSs are confirmed as intrinsic and defect-free, which could be determinative to their improved HER performance. The overpotentials of 285, 331, 435 mV at the current density of 10 mA cm-2and Tafel slopes of 116, 78, 162 mV dec-1in acidic media, as well as charge transfer resistance values of 171, 242, 1973 Ω, are derived for WS2QSs, WSe2QSs, and WTe2QSs, respectively, which are much better than those of bulk materials. The WX2QSs exhibit high stability during the electrocatalysis as well. This work offers a powerful approach for fabrication of intrinsic QSs as efficient and robust electrocatalysts.

A general strategy for semiconductor quantum dot production.

Mass production of semiconductor quantum dots (QDs) from bulk materials is highly desired but far from being satisfactory. Herein, we report a general strategy to mechanically tailor semiconductor bulk materials into QDs. Semiconductor bulk materials are routinely available via simple chemical precipitation. From their bulk materials, a variety of semiconductor (e.g., lead sulfide (PbS), cadmium sulfide (CdS), copper sulfide (CuS), ferrous sulfide (FeS), and zinc sulfide (ZnS)) QDs are successfully produced in high yields (>15 wt%). This is achieved by a combination of silica-assisted ball-milling and sonication-assisted solvent treatment. The as-produced QDs show intrinsic characteristics and outstanding water solubility (up to 5 mg mL-1), facilitating their practical applications. The QD dispersions present remarkable photoluminescence (PL) with exciton-dependence and nanosecond (ns)-scale lifetimes. The QDs-poly(methyl methacrylate) (PMMA) hybrid thin films demonstrate exciting solid-state fluorescence and exceptional nonlinear saturation absorption (NSA). Absolute modulation depths of up to 58% and saturation intensities down to 0.40 MW cm-2 were obtained. Our strategy could be applied to any semiconductor bulk materials and therefore paves the way for the construction of the complete library of semiconductor QDs.

Inch-Scale Ball-in-Bowl Plasmonic Nanostructure Arrays for Polarization-Independent Second-Harmonic Generation.

Second-harmonic generation (SHG) in plasmonic nanostructures has been investigated for decades due to their wide applications in photonic circuit, quantum optics and biosensing. Development of large-scale, uniform, and efficient plasmonic nanostructure system with tunable modes is desirable for their feasible utilizations. Herein, we design an efficient inch-scale SHG source by a solution-processed method instead of traditional high-cost processes. By assembling the gold nanoparticles with the porous anodic alumina templates, multiresonance in both visible and near-infrared regions can be achieved in hexagonal plasmonic nanostructure arrays, which provide strong electric field enhancement at the gap region. Polarization-independence SHG radiation has been realized owing to the in-plane isotropic characteristic of assembled unit. The tilt-angle dependent and angle-resolved measurement showed that wide-angle nonlinear response is achieved in our device because of the gap geometry of ball-in-bowl nanostructure with nonlinear emission electric dipoles distributed on the concave surface, which makes it competitive in practical applications. Our progress not only makes it possible to produce uniform inch-scale nonlinear arrays through low-cost solution process; and also advances the understanding of the SHG radiation in plasmonic nanostructures.

Cobalt/nitrogen codoped carbon nanosheets derived from catkins as a high performance non-noble metal electrocatalyst for oxygen reduction reaction and hydrogen evolution reaction.

Novel energy devices which are capable of alleviating and/or solving the energy dilemma such as overall water splitting and fuel cells require the development of highly efficient catalysts, especially cheap high performance non-precious metal (NPM) catalysts. Here, we prepare highly efficient NPM catalysts of cobalt and nitrogen codoped carbon nanosheets (Co/N-CNSs) for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) using harmful environment-polluting waste of biomass catkins as carbon precursors via a mild mechanical exfoliation and chemical process which is facile, low-cost, environmentally friendly and up-scalable. Compared with a commercial platinum-based catalyst (commercial 20% Pt/C), the Co/N-CNS electrocatalysts show outstanding ORR activity, acceptable HER activity and long term stability with an onset potential of 0.92 V versus the reversible hydrogen electrode (vs. RHE) and a half-wave potential of 0.83 V vs. the RHE in alkaline electrolytes. The excellent performance is closely related to the presence of abundant CoN x active sites. This work offers a novel and effective approach for preparing highly efficient ORR and HER NPM electrocatalysts from waste biomass materials.

Robust Assembly of Colloidal Nanoparticles for Controlled-Reflectance Surface Construction.

Controlled placement of nanoscale particles with nanometer precision on substrates/surfaces is highly desired toward functional nanodevices. Herein, we report the robust assembly of colloidal nanoparticles onto nanostructured aluminum surfaces. The surfaces are configured by porous anodic alumina (PAA) membranes on top of textured aluminum substrates. Capillary force and geometry confinement enable rapid and precise transfer of colloidal nanoparticles from solutions into PAA templates. Such top-down control of bottom-up assembly demonstrates large-area (>1 × 1 cm2) integration of nanoscale particles with exceedingly high yield (>95%) and exceptionally high density (>1010 particles/cm2). The plasmonic coupling between gold nanoparticles and aluminum surfaces, as well as between adjacent nanoparticles, is responsible for the unique reflectance from the assembled surfaces. The reflectance minimum (resonant absorption) can be readily shifted from visible to near-infrared by simple structural variation. The apparent surface colors are thus broadly manipulated. Our work offers a straightforward platform toward construction of surfaces with controlled reflectance.