Ultrathin Nanocrystals Research Articles

MoS2 Photoelectrodes for Hydrogen Production: Tuning the S-Vacancy Content in Highly Homogeneous Ultrathin Nanocrystals.

Tuning the electrocatalytic properties of MoS2 layers can be achieved through different paths, such as reducing their thickness, creating edges in the MoS2 flakes, and introducing S-vacancies. We combine these three approaches by growing MoS2 electrodes by using a special salt-assisted chemical vapor deposition (CVD) method. This procedure allows the growth of ultrathin MoS2 nanocrystals (1-3 layers thick and a few nanometers wide), as evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS2 layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS2 layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H2 mixtures as a carrier gas. Detailed optical microtransmittance and microreflectance spectroscopies, micro-Raman, and X-ray photoelectron spectroscopy measurements with sub-millimeter spatial resolution show that the obtained samples present an excellent homogeneity over areas in the cm2 range. The electrochemical and photoelectrochemical properties of these MoS2 layers were investigated using electrodes with relatively large areas (0.8 cm2). The prepared MoS2 cathodes show outstanding Faradaic efficiencies as well as long-term stability in acidic solutions. In addition, we demonstrate that there is an optimal number of S-vacancies to improve the electrochemical and photoelectrochemical performances of MoS2.

Physical Vapor Deposition Features of Ultrathin Nanocrystals of Bi2(TexSe1-x)3.

Structural and electronic properties of ultrathin nanocrystals of chalcogenide Bi2(Tex Se1-x)3 were studied. The nanocrystals were formed from the parent compound Bi2Te2Se on as-grown and thermally oxidized Si(100) substrates using Ar-assisted physical vapor deposition, resulting in well-faceted single crystals several quintuple layers thick and a few hundreds nanometers large. The chemical composition and structure of the nanocrystals were analyzed by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, electron backscattering, and X-ray diffraction. The electron transport through nanocrystals connected to superconducting Nb electrodes demonstrated Josephson behavior, with the predominance of the topological channels [Stolyarov et al. Commun. Mater., 2020, 1, 38]. The present paper focuses on the effect of the growth conditions on the morphology, structural, and electronic properties of nanocrystals.

In Situ Surface Restraint-Induced Synthesis of Transition-Metal Nitride Ultrathin Nanocrystals as Ultrasensitive SERS Substrate with Ultrahigh Durability.

It is a major challenge to synthesize crystalline transition-metal nitride (TMN) ultrathin nanocrystals due to their harsh reaction conditions. Herein, we report that highly crystalline tungsten nitride (W2N, WN, W3N4, W2N3) nanocrystals with small size and excellent dispersibility are prepared by a mild and general in situ surface restraint-induced growth method. These ultrafine tungsten nitride nanocrystals are immobilized in ultrathin carbon layers, forming an interesting hybrid nanobelt structure. The hybrid WN/C nanobelts exhibit a strong localized surface plasmon resonance (LSPR) effect and surface-enhanced Raman scattering (SERS) effect, including a lowest detection limit of 1 × 10-12 M and a Raman enhancement factor of 6.5 × 108 comparable to noble metals, which may be one of the best records for non-noble metal SERS substrates. Moreover, they even can maintain the SERS performance in a variety of harsh environments, showing outstanding corrosion resistance, radiation resistance, and oxidation resistance, which is not available on traditional noble metal and semiconductor SERS substrates. A synergistic Raman enhancement mechanism of LSPR and interface charge transfer is found in the carbon-coated tungsten nitride substrate. A microfluidic SERS channel integrating the enrichment and detection of trace substances is constructed with the WN/C nanobelt, which realizes high-throughput dynamic SERS analysis.

Magic “K-insertion and unzipping’’ way to form novel 2D ultrathin nanocrystals

Fabrication of 2D ultrathin nanocrystals with rich electronic and band structures from bulk nonlayered structures remains a great challenge. Chen et al. report a novel K-insertion and unzipping'' approach to efficiently break strong chemical bonds between planes and atoms of bulk phase, yet opening up a whole new avenue to construct 2D ultrathin nanocrystals from bulk non-layered structures for various useful applications.

Unzipping chemical bonds of non-layered bulk structures to form ultrathin nanocrystals

The rich electronic and band structures of monolayered crystals distinguished from their layered bulk counterparts offer versatile physical/chemical properties and applications.1-5 Their fabrications, particularly the top-down "exfoliations", are successful promised by the weak Van der Waals force between monolayers.6-9 Differentially, un-zipping ultra-thin crystals (e.g. with only one layer of crystal plane) from non-layered structures is highly challenging due to the strong chemical bond between planes and atoms. Alterative finely controlled growth of these ultra-thin materials is not really successful. This work demonstrates how a technique can be used to unzip and disintegrate ultra-thin crystal plane (e.g. monolayered nanocrystals and nanosheets) from bulk non-layered structures (ZnO, alpha/belta-MnO2, TiO2, alpha-TiB2), and present how the basic optical properties changed to distinguish from their bulk phases. The work here gives a strong tool kit to various novel 2D non-layered nanomaterials, providing significant contributions to the family of two-dimensional materials, potentially paving the way for various practical applications.

Self-Assembly of Ultrathin Nanocrystals to Multidimensional Superstructures.

The self-assembly of ultrathin nanocrystals (UTNCs) into well-organized multidimensional superstructures is one of the key topics in material chemistry and physics. Highly ordered nanocrystal assemblies also known as superstructures or synthetic structures have remained a focus for researchers over the past few years due to synergy in their properties as compared to their components. Here, we aim to present the recent progress being made in this field with highlights of our research group endeavors in the engineering of self-assembled complex multidimensional superstructures of various inorganic materials, including polyoxometalates. The driving forces for the assembly process and its kinetics along with the potential applications associated with these unique ordered and spatially complex superstructures are also discussed.

Solution-processed near-infrared phototransistor based on ultrathin nanocrystals of octamethyl substituted zinc(II) phthalocyanine

Abstract A drop-casted near-infrared (NIR) organic phototransistor (OPT) based on ultrathin nanocrystals of octamethyl substituted zinc(II) phthalocyanine (ZnMe2Pc) has been developed, as an attempt to realize NIR detecting property of metal phthalocyanine (MPc) in OPT, and a preliminary feasibility exploration of solution processing for MPc-based OPT fabrication. The broad and strong NIR light absorption together with the enhanced hole mobility of these ZnMe2Pc nanocrystals led to the significantly increased performances in NIR detecting and sensing compared with that of the vacuum deposited OPT. The cost-effective processes both in materials preparation and device fabrication are promising in future application.

Fabrication of octamethyl substituted zinc(II) phthalocyanine nanostructure via exfoliation and use for solution-processed field-effect transistor

Abstract Ultrathin nanocrystals with thicknesses of only several monolayers and even a single monolayer were prepared via exfoliation by ultrasound from an octamethyl substituted zinc(II) phthalocyanine (ZnMe2Pc). The drop-casted thin films of these nanocrystals exhibited higher crystallinity than that of the vacuum-deposited film. The drop-casted field-effect transistor based on these nanocrystals showed a significant increase in charge carrier mobility because of enhanced intermolecular π-π stacking in the ultrasonic-treated ZnMe2Pc, and the measured mobility was two orders of magnitude higher than that of the vacuum-deposited transistor.

Silver nanocrystal-decorated polyoxometalate single-walled nanotubes as nanoreactors for desulfurization catalysis at room temperature.

Ultrathin nanocrystals generally provide a remarkable catalytic performance due to their high specific surface area and exposure of certain active sites. However, deactivation caused by growth and gathering limits the catalytic application of ultrathin nanocrystals. Here we report Ag nanocrystal-decorated polyoxometalate (Ag-POM) single-walled nanotubes assembled via a concise, surfactant-free soaking method as a new kind of well-defined core-sheath nanoreactor. The diameter of Ag nanocrystals inside polyoxometalate nanotubes can be controlled via simply adjusting the reactant concentration. Ag-POM provided outstanding oxidative desulfurization (ODS) catalytic performance for aromatic sulfocompounds at room temperature. It was suggested that Ag nanocrystals decorated on the inner surface played a key role in adjusting the electronic distribution and enhancing the catalytic activity. The as-prepared Ag-POM nanotubes are promising candidate catalysts with enhanced performance for practical catalytic applications in the gasoline desulfurization industry.

From cluster assembly to ultrathin nanocrystals and complex nanostructures

Ultrathin nanostructures have attracted much attention in recent years due to their predictable novel properties and various potential applications. The improvement in synthetic skills has led to many successful syntheses of nanostructures including zero-dimensional (0D) nanoclusters, one-dimensional (1D) nanowires, two-dimensional (2D) nanosheets and other higher-level complex nanostructures, where cluster-assembly of primary nanocrystals is a common process. In this review, progress in ultrathin nanocrystals in the last decade and several important factors in the growth mechanisms are covered. By giving examples of cluster assembly from 1D to 3D nanostructures, the utility of cluster assembly in the synthesis of new materials has been demonstrated.

From cluster assembly to ultrathin nanocrystals and complex nanostructures

Ultrathin nanostructures have attracted much attention in recent years due to their predictable novel properties and various potential applications. The improvement in synthetic skills has led to many successful syntheses of nanostructures including zero-dimensional (0D) nanoclusters, one-dimensional (1D) nanowires, two-dimensional (2D) nanosheets and other higher-level complex nanostructures, where cluster-assembly of primary nanocrystals is a common process. In this review, progress in ultrathin nanocrystals in the last decade and several important factors in the growth mechanisms are covered. By giving examples of cluster assembly from 1D to 3D nanostructures, the utility of cluster assembly in the synthesis of new materials has been demonstrated.

Self-Organized Ultrathin Oxide Nanocrystals

Sub-2-nm (down to one-unit cell) uniform oxide nanocrystals and highly ordered superstructures were obtained in one step using oleylamine and oleic acid as capping and structure directing agents. The cooperative nature of the nanocrystal growth and assembly resulted in mesoscopic one-dimensional ribbon-like superstructures made of these ultrathin nanocrystals. The process reported here is general and can be readily extended to the production of many other transition metal (TiO2, ZnO, Nb2O5) and rare earth oxide (Eu2O3, Sm2O3, Er2O3, Y2O3, Tb2O3, and Yb2O3) systems.

Self-Trapped Excitons in Ultrathin PbI2Nanocrystals Embedded in a Polymer

We have found broad band luminescence due to a self-trapped exciton (STE) and disappearance of a free exciton luminescence in ultrathin nanocrystals of PbI 2 embedded in ethylene-methacrylic acid c...