Interfacial Fabrication of Single-Crystalline ZnTe Nanorods with High Blue Fluorescence

Synthesis of nanoparticles via microfluidic devices and integrated applications

In recent years, nanomaterials have attracted the research intervention of experts in the fields of catalysis, energy, biomedical testing, and biomedicine with their unrivaled optical, chemical, and biological properties. From basic metal and oxide nanoparticles to complex quantum dots and MOFs, the stable preparation of various nanomaterials has always been a struggle for researchers. Microfluidics, as a paradigm of microscale control, is a remarkable platform for online stable synthesis of nanomaterials with efficient mass and heat transfer in microreactors, flexible blending of reactants, and precise control of reaction conditions. We describe the process of microfluidic preparation of nanoparticles in the last 5 years in terms of microfluidic techniques and the methods of microfluidic manipulation of fluids. Then, the ability of microfluidics to prepare different nanomaterials, such as metals, oxides, quantum dots, and biopolymer nanoparticles, is presented. The effective synthesis of some nanomaterials with complex structures and the cases of nanomaterials prepared by microfluidics under extreme conditions (high temperature and pressure), the compatibility of microfluidics as a superior platform for the preparation of nanoparticles is demonstrated. Microfluidics has a potent integration capability to combine nanoparticle synthesis with real-time monitoring and online detection, which significantly improves the quality and production efficiency of nanoparticles, and also provides a high-quality ultra-clean platform for some bioassays.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

Controllable preparation and rapid photoelectric response of homogeneous ZnTe microspheres

Uniform ZnTe microspheres with 1.7 μm diameter were prepared by a PVP-assisted solvothermal process. By assembling ZnTe microspheres into photodetectors, the rise time and decay time of the photodetector were 96.93 ms and 103.57 ms, respectively.

Hydrogen production via water splitting over graphitic carbon nitride (g-C3N4 )-based photocatalysis

Photocatalytic splitting of water into hydrogen and oxygen using semiconductor photocatalysts and light irradiation has been attracted much attention and considered to be an alternative for nonrenewable fossil fuel to solve environmental problems and energy crisis and also an as promising approach to produce clean, renewable hydrogen fuel. Owing to their various advantages such as low cost and environmental friendly, chemical, and thermal stability, appropriate band structure, graphitic carbon nitride (g-C3N4 ) photocatalysts have gained multitudinous attention because of their great potential in solar fuels production and environmental remediation. However, due to its fast charge carrier’s recombination, low surface, and limited absorption of the visible light restrict their activity toward hydrogen evolution and numerous modification techniques were applied to solve these problems such as structural modification, metal/nonmetal doping, and noble metal loading, and coupling semiconductors. In this chapter, we summarize recent progress in the synthesis and characterization of the g-C3N4-based photocatalyst. Several modification methods used to enhance the photocatalytic hydrogen production of g-C3N4-based photocatalyst were also highlighted. This chapter ends with the future research and challenges of hydrogen production over g-C3N4-based photocatalyst.

Facile Solvothermal Synthesis of Hollow BiOBr Submicrospheres with Enhanced Visible-Light-Responsive Photocatalytic Performance

In this work, hierarchical hollow BiOBr submicrospheres (HBSMs) were successfully prepared via a facile yet efficient solvothermal strategy. Remarkable effects of solvents upon the crystallinities, morphologies, and microstructures of the BiOBr products were systematically investigated, which revealed that the glycerol/isopropanol volumetric ratio played a significant role in the formation of hollow architecture. Accordingly, the underlying formation mechanism of the hollow submicrospheres was tentatively put forward here. Furthermore, the photocatalytic activities of the resulting HBSMs were evaluated in detail with photocatalytic degradation of the organic methyl orange under visible light irradiation. Encouragingly, the as-obtained HBSMs with striking recyclability demonstrated excellent visible-light-responsive photocatalytic performance, which benefits from their large surface area, effective visible light absorption, and unique hollow feature, highlighting their promising commercial application in waste water treatment.

Multifunctional Micro/Nanoscale Fibers Based on Microfluidic Spinning Technology

Superfine multifunctional micro/nanoscale fibrous materials with high surface area and ordered structure have attracted intensive attention for widespread applications in recent years. Microfluidic spinning technology (MST) has emerged as a powerful and versatile platform because of its various advantages such as high surface-area-to-volume ratio, effective heat transfer, and enhanced reaction rate. The resultant well-defined micro/nanoscale fibers exhibit controllable compositions, advanced structures, and new physical/chemical properties. The latest developments and achievements in microfluidic spun fiber materials are summarized in terms of the underlying preparation principles, geometric configurations, and functionalization. Variously architected structures and shapes by MST, including cylindrical, grooved, flat, anisotropic, hollow, core-shell, Janus, heterogeneous, helical, and knotted fibers, are emphasized. In particular, fiber-spinning chemistry in MST for achieving functionalization of fiber materials by in situ chemical reactions inside fibers is introduced. Additionally, the applications of the fabricated functional fibers are highlighted in sensors, microactuators, photoelectric devices, flexible electronics, tissue engineering, drug delivery, and water collection. Finally, recent progress, challenges, and future perspectives are discussed.

Heavy-Metal-Free Colloidal Semiconductor Nanorods: Recent Advances and Future Perspectives

Quasi-1D colloidal semiconductor nanorods (NRs) are at the forefront of nanoparticle (NP) research owing to their intriguing size-dependent and shape-dependent optical and electronic properties. The past decade has witnessed significant advances in both fundamental understanding of the growth mechanisms and applications of these stimulating materials. Herein, the state-of-the-art of colloidal semiconductor NRs is reviewed, with special emphasis on heavy-metal-free materials. The main growth mechanisms of heavy-metal-free colloidal semiconductor NRs are first elaborated, including anisotropic-controlled growth, oriented attachment, solution-liquid-solid method, and cation exchange. Then, structural engineering and properties of semiconductor NRs are discussed, with a comprehensive overview of core/shell structures, alloying, and doping, as well as semiconductor-metal hybrid nanostructures, followed by highlighted practical applications in terms of photocatalysis, photodetectors, solar cells, and biomedicine. Finally, challenges and future opportunities in this fascinating research area are proposed.

Pronounced Impact of p-Type Carriers and Reduction of Bandgap in Semiconducting ZnTe Thin Films by Cu Doping for Intermediate Buffer Layer in Heterojunction Solar Cells

Stabilized un-doped Zinc Telluride (ZnTe) thin films were grown on glass substrates under vacuum using a closed space sublimation (CSS) technique. A dilute copper nitrate solution (0.1/100 mL) was prepared for copper doping, known as an ion exchange process, in the matrix of the ZnTe thin film. The reproducible polycrystalline cubic structure of undoped and the Cu doped ZnTe thin films with preferred orientation (111) was confirmed by X-rays diffraction (XRD) technique. Lattice parameter analyses verified the expansion of unit cell volume after incorporation of Cu species into ZnTe thin films samples. The micrographs of scanning electron microscopy (SEM) were used to measure the variation in crystal sizes of samples. The energy dispersive X-rays were used to validate the elemental composition of undoped and Cu-doped ZnTe thin films. The bandgap energy 2.24 eV of the ZnTe thin film decreased after doping Cu to 2.20 eV and may be due to the introduction of acceptors states near to valance band. Optical studies showed that refractive index was measured from 2.18 to 3.24, whereas thicknesses varied between 220 nm to 320 nm for un-doped and Cu doped ZnTe thin film, respectively, using the Swanepoel model. The oxidation states of Zn⁺², Te⁺², and Cu⁺¹ through high resolution X-ray photoelectron spectroscopy (XPS) analyses was observed. The resistivity of thin films changed from ~10⁷ Ω·cm or undoped ZnTe to ~1 Ω·cm for Cu-doped ZnTe thin film, whereas p-type carrier concentration increased from 4 × 10⁹ cm⁻² to 1.4 × 10¹¹ cm⁻², respectively. These results predicted that Cu-doped ZnTe thin film can be used as an ideal, efficient, and stable intermediate layer between metallic and absorber back contact for the heterojunction thin film solar cell technology.

Ultrasonic and microwave assisted extraction as rapid and efficient techniques for plant mediated synthesis of quantum dots: green synthesis, characterization of zinc telluride and comparison study of some biological activities

In this study, a simple, rapid, and efficient plant-mediated green approach was presented for the synthesis of stable and ultra-small zinc telluride quantum dots (ZnTe QDs) using the aqueous extract of the Ficus johannis plant.

Tailored Emission Properties of ZnTe/ZnTe:O/ZnO Core-Shell Nanowires Coupled with an Al Plasmonic Bowtie Antenna Array

The ability to manipulate light-matter interaction in semiconducting nanostructures is fascinating for implementing functionalities in advanced optoelectronic devices. Here, we report the tailoring of radiative emissions in a ZnTe/ZnTe:O/ZnO core-shell single nanowire coupled with a one-dimensional aluminum bowtie antenna array. The plasmonic antenna enables changes in the excitation and emission processes, leading to an obvious enhancement of near band edge emission (2.2 eV) and subgap excitonic emission (1.7 eV) bound to intermediate band states in a ZnTe/ZnTe:O/ZnO core-shell nanowire as well as surface-enhanced Raman scattering at room temperature. The increase of emission decay rate in the nanowire/antenna system, probed by time-resolved photoluminescence spectroscopy, yields an observable enhancement of quantum efficiency induced by local surface plasmon resonance. Electromagnetic simulations agree well with the experimental observations, revealing a combined effect of enhanced electric near-field intensity and the improvement of quantum efficiency in the ZnTe/ZnTe:O/ZnO nanowire/antenna system. The capability of tailoring light-matter interaction in low-efficient emitters may provide an alternative platform for designing advanced optoelectronic and sensing devices with precisely controlled response.

Sandwich-interface inspired strategy for controlled formation of nanoparticles

Nanoparticles are functional materials able to offer improved or new synergetic properties. By manipulating the interfacial properties, we demonstrate an innovative sandwich-interface method capable of forming various monodispersed nanostructures including metals, semiconductors, and inorganic and coordinated nanoparticles. By analysing of the reaction mechanism, we show that reaction time, the height of transition and presence of surfactant have the greatest influence on the formation of the products. These advances in the sandwich-interface synthesis significantly extend the scope of interface synthetic methods, facilitating a new level of structural-architectural control which may lead to future developments in the field of crystallography.

Nanocluster-Mediated Synthesis of Diverse ZnTe Nanostructures: from Nanocrystals to 1D Nanobelts

Liquid phase one-pot synthesis of semiconductor nanocrystals, by direct nucleation-growth crystallization, is unsuccessful for synthesis of some kinds of semiconductors. Using ZnTe as an example here, highly disperse ZnTe nanoclusters with diameters of 2-3 nm were first synthesized by a facile solvothermal method. Then the ZnTe nanoclusters were chosen as starting crystallization seeds to mediate the synthesis of flexible semiconductor nanostructures. Three-dimensional (3D) oriented assembly of ZnTe nanoclusters to monodisperse dendrimer-like nanocrystals (DLNCs), and one-dimensional (1D) ZnTe nanobelts with cubic phase, have been achieved successfully. Supported by TEM characterization of time-dependent morphology evolution, the oriented attachment assisted seed growth, based on ZnTe nanoclusters, enabled the 1D flexible ZnTe nanobelts formation, which could reach to ≈10 micrometers length.

Phosphine-Free Synthesis of Metal Chalcogenide Quantum Dots by Directly Dissolving Chalcogen Dioxides in Alkylthiol as the Precursor

Semiconductor quantum dots (QDs) are competitive emitting materials in developing new-generation light-emitting diodes (LEDs) with high color rendering and broad color gamut. However, the use of highly toxic alkylphosphines cannot be fully avoided in the synthesis of metal selenide and telluride QDs because they are requisite reducing agents and solvents for preparing chalcogen precursors. In this work, we demonstrate the phosphine-free preparation of selenium (Se) and tellurium (Te) precursors by directly dissolving chalcogen dioxides in the alkylthiol under the mild condition. The chalcogen dioxides are reduced to elemental chalcogen clusters, while the alkylthiol is oxidized to disulfides. The chalcogen clusters further combine with the disulfides, generating dispersible chalcogen precursors. The resulting chalcogen precursors are suitable for synthesizing various metal chalcogenide QDs, including CdSe, CdTe, Cu₂Te, Ag₂Te, PbTe, HgTe, and so forth. In addition, the precursors are of high reactivity, which permits a shorter QD synthesis process at lower temperature. Owing to the high quantum yield (QYs) and easy tunability of the photoluminescence (PL), the as-synthesized QDs are further employed as down-conversion materials to fabricate monochrome and white LEDs.

One-dimensional silicon nanoshuttles simultaneously featuring fluorescent and magnetic properties

Fluorescent and magnetic one-dimensional silicon nanoshuttles are prepared in situ through a metal ions-assisted microwave synthetic strategy.

Interfacial rheology of polymer/carbon nanotube films co-assembled at the oil/water interface

At appropriate conditions, water-dispersed acid-functionalized single-walled carbon nanotubes (SWCNTs) co-assemble at the oil/water interface with toluene-dissolved amine-terminated polystyrene (PS-NH₂) to form composite thin films displaying pronounced interfacial viscoelasticity. To probe this viscoelasticity, the films were examined under dilatational deformations of pendant drop tensiometry/rheometry, with storage and loss moduli recorded against frequency ω (0.003 < ω < 3 Hz) and time-dependent relaxation modulus recorded against time t (0.2 < t < 2000 s). Without the SWCNTs, PS-NH₂-decorated interfaces have little dilatational stiffness, i.e., low storage modulus, but their stiffness grows as SWCNTs are added, reaching 50-100 mN m^-1 at large ω. Two characteristic relaxation processes are identified in the composite films: a fast process (ω ∼ 0.1-0.2 Hz) attributable to local structural relaxation of confined PS-NH₂ and a slow process (t ∼ 300-2000 s) attributable to component adsorption/desorption (or attachment/detachment). Among the variables that affect positions and strengths of these relaxations are SWCNT and PS-NH₂ bulk concentrations as well as water phase pH. In frequency or timescale ranges intermediate between the two relaxations, the co-assembled films display "soft-glass" behavior, with the storage and loss moduli characterized by nearly equal power-law exponents. The relaxation modulus, better able to probe terminal behavior, eventually decays to zero, revealing that the films are fundamentally fluid-like due to the slow relaxation, and in support of this conclusion, large strain compression-induced film wrinkles disappear at large t.

Alternative motif toward high-quality wurtzite MnSe nanorods via subtle sulfur element doping

The manipulated synthesis of high-quality semiconductor nanocrystals (NCs) is of high significance with respect to the exploration of their properties and their corresponding applications. Nevertheless, the preparation of metastable-phase NCs still remains a great challenge due to their high kinetic barriers and harsh synthetic conditions. Herein, we demonstrated the fabrication of high-quality MnSe nanorods with a metastable wurtzite structure via a subtle sulfur-doping strategy. Based on the UV-vis absorption spectra, manganese polysulfide clusters were formed by mixing oleylamine-sulfur and oleylamine-manganese solutions at room temperature. The existence of manganese polysulfide clusters with polymeric sulfur structures makes the system more reactive, inducing fast wurtzite-phase nucleation. This can overcome the natural kinetic barrier of wurtzite MnSe and lead to subsequent growth of targeted NCs. On the other hand, no sulfur doping would produce MnSe NCs in a thermodynamically favorable rock-salt phase. As expected, different doping contents and sulfur sources also resulted in the formation of high-quality wurtzite MnSe nanorods. This success establishes that a facile strategy can be anticipated to synthesize high-quality metal chalcogenide NCs with a metastable phase, especially wurtzite nanorods, for potential applications from spintronics to solar cells.

Water-soluble, luminescent ZnTe quantum dots: supersaturation-controlled synthesis and self-assembly into nanoballs, nanonecklaces and nanowires

A supersaturation-controlled aqueous synthesis route has been developed for ZnTe quantum dots (QDs) with high monodispersity, size tunability, stability, band-edge luminescence (full-width at half-maximum (FWHM) 10-12 nm) and negligibly small Stokes' shift (2-4 nm). The degree of supersaturation of the initial reaction mixture was varied by increasing the reagent concentration, but keeping the molar ratio Zn(2+) : thioglycolic acid : Te(2-) constant at 1 : 2.5 : 0.5. For a 10× increase in supersaturation, the photoluminescence (PL) peak underwent a 50 nm blue shift from 330 to 280 nm at pH 6. The effect was more pronounced at pH 12, where the PL peak blue-shifted by 100 nm from 327 to 227 nm. Concomitantly, the FWHM was also reduced to a low value of 10 nm, indicating high monodispersity. For a 10× change in supersaturation, the particle size decreased by 63% (from 2.2 to 0.8 nm) at pH 12, whereas it changed by 19% (from 2.1 to 1.7 nm) at pH 6. High-resolution transmission electron microscopy and selected area electron diffraction data further revealed that the QDs synthesized at higher supersaturation had a better crystallinity. These QDs exhibited the unique property of undergoing isotropic and anisotropic self-assembly, which resulted in a blue shift and a red shift in the absorption and PL spectra, respectively. Isotropic assembly into spherical nanoballs (100 nm diameter, 1 nm inter-QD separation) occurred when the QDs were stored at pH 12 for 3 weeks at room temperature. The nanoballs further self-assembled into a 'pearl necklace' arrangement. On the partial removal of the capping agents, the QDs self-organized anisotropically into nanowires (1.3 μm long and 4.6 nm in diameter). The self-assembled nanostructures showed exciton-exciton coupling and excellent PL properties, which may be useful in enhanced optoelectronics, photovoltaics and biochemical sensing.

Construction of Ag-doped Zn–In–S quantum dots toward white LEDs and 3D luminescent patterning

The synthesis of green photoluminescent Ag-doped Zn–In–S quantum dots and their applications in patterning and white LEDs are reported.

Interfacial synthesis of SnSe quantum dots for sensitized solar cells

A new interfacial synthesis of colloidal SnSe quantum dots (QDs) was realized from common precursors at a mild condition. SnSe QD-sensitized solar cells were fabricated to show an improved power conversion efficiency with a high fill factor of 0.71.

New insights into the phosphine-free synthesis of ultrasmall Cu2−xSe nanocrystals at the liquid–liquid interface

A liquid–liquid interfacial strategy to prepare ultra small (<5 nm) colloidal Cu_2−​xSe NCs with blue-fluorescence, noncaustic and environmentally friendly NH₄SCN replaces the long-chain organic ligands for fabrication of NC-sensitized solar.

Green interfacial synthesis of two-dimensional poly(2,5-dimethoxyaniline) nanosheets as a promising electrode for high performance electrochemical capacitors

2D poly(2,5-dimethoxyaniline) nanosheets were designed via a green interfacial strategy, and demonstrated large specific capacitance and striking stability at high rates as a pseudo-capacitive electrode.