Unprecedented generation of 3D heterostructures by mechanochemical disassembly and re-ordering of incommensurate metal chalcogenides.

Three-dimensional heterostructures are usually created either by assembling two-dimensional building blocks into hierarchical architectures or using stepwise chemical processes that sequentially deposit individual monolayers. Both approaches suffer from a number of issues, including lack of suitable precursors, limited reproducibility, and poor scalability of the preparation protocols. Therefore, development of alternative methods that enable preparation of heterostructured materials is desired. We create heterostructures with incommensurate arrangements of well-defined building blocks using a synthetic approach that comprises mechanical disassembly and simultaneous reordering of layered transition-metal dichalcogenides, MX2, and non-layered monochalcogenides, REX, where M = Ta, Nb, RE = Sm, La, and X = S, Se. We show that the discovered solid-state processes are rooted in stochastic mechanochemical transformations directed by electronic interaction between chemically and structurally dissimilar solids toward atomic-scale ordering, and offer an alternative to conventional heterostructuring. Details of composition-structure-properties relationships in the studied materials are also highlighted.

Unveiling the Photo- and Thermal-Stability of Cesium Lead Halide Perovskite Nanocrystals.

Lead halide perovskites possess unique characteristics that are well-suited for optoelectronic and energy capture devices, however, concerns about their long-term stability remain. Limited stability is often linked to the methylammonium cation, and all-inorganic CsPbX3 (X=Cl, Br, I) perovskite nanocrystals have been reported with improved stability. In this work, the photostability and thermal stability properties of CsPbX3 (X=Cl, Br, I) nanocrystals were investigated by means of electron microscopy, X-ray diffraction, thermogravimetric analysis coupled with FTIR (TGA-FTIR), ensemble and single particle spectral characterization. CsPbBr3 was found to be stable under 1-sun illumination for 16 h in ambient conditions, although single crystal luminescence analysis after illumination using a solar simulator indicates that the luminescence states are changing over time. CsPbBr3 was also stable to heating to 250 °C. Large CsPbI3 crystals (34±5 nm) were shown to be the least stable composition under the same conditions as both XRD reflections and Raman bands diminish under irradiation; and with heating the γ (black) phase reverts to the non-luminescent δ phase. Smaller CsPbI3 nanocrystals (14±2 nm) purified by a different washing strategy exhibited improved photostability with no evidence of crystal growth but were still thermally unstable. Both CsPbCl3 and CsPbBr3 show crystal growth under irradiation or heat, likely with a preferential orientation based on XRD patterns. TGA-FTIR revealed nanocrystal mass loss was only from liberation and subsequent degradation of surface ligands. Encapsulation or other protective strategies should be employed for long-term stability of these materials under conditions of high irradiance or temperature.

Multi-principal element transition metal dichalcogenides via reactive fusion of 3D-heterostructures.

Transition metal dichalcogenides combining multiple principal elements in their structures are synthesized via mechanochemical exfoliation and spontaneous reassembly of binary precursors into 3D-heterostructures that are converted into single-phase layered materials by high-temperature reactive fusion. Physical and chemical events enabling these transformations are summarized in the form of a conceivable reaction mechanism.

Sub-100 nm anisotropic gold nanoparticles as surface-enhanced Raman spectroscopy substrates.

This study describes a reliable preparation of relatively small Ag/Au-based anisotropic nanostructures possessing tunable absorption bands and their use as surface-enhanced Raman spectroscopy (SERS) substrates. These Au nanostructures were prepared via the seed growth process of small Ag-core-Au-shell-type nanoparticles that were obtained by the subsequent reduction of Ag and Au ions by NaBH(4) and L-ascorbic acid at room temperature. The presence of Ag during the transformation process of the Ag-Au core-shell nanoparticles under light irradiation led to the formation of various small anisotropic Au nanoparticles which clearly exhibited different structural and optical properties from those of nanoparticles prepared from typical Ag-Au alloy or bare Ag or Au seeds. As the optimal size of Au-based substrates for SERS applications was reported to be below 100 nm in diameter under a constant concentration, we tested our moderately small anisotropic nanoparticles (∼55 nm in diameter) as a SERS substrate to examine the signal enhancement of 4-nitrobenzenethiol. These nanoparticles exhibited a greatly increased SERS response compared to those of similar sizes of uniform Ag and Au nanoparticles, presumably because of the increased surface area due to the nanoparticles' anisotropic nature (i.e., chemical effect) and partial overlap of their absorption bands with the SERS excitation wavelength (i.e., electromagnetic effect). In addition, these nanoparticles have shown a suitable stability to prevent significant SERS signal fluctuations caused by unpredictable aggregations. Due to our simple synthetic and modification approaches, relatively small Au-based anisotropic nanostructures can be easily designed to serve as attractive SERS templates.

Silver-gold bimetallic nanoparticles and their applications as optical materials.

Recently, nanoscale metallic particles have been studied extensively due to their tunable and strong optical properties that are well beyond those of organic chromophores. As monometallic nanoparticles have shown strong but narrow absorption bands within the ultraviolet and visible wavelengths, the preparation of bimetallic core-shell structures can give rise to strong, wide, and tunable absorption bands across the visible to near infrared areas. The silver-gold bimetallic nanoparticles with core-shell structures can offer unique physical and optical properties inaccessible to monometallic systems. These nanoparticles have been utilized in many areas of research including chemical catalysis, surface-enhanced Raman spectroscopy, and photothermal therapy. This review article is a comprehensive overview of bimetallic nanoparticle systems consisting of gold and silver; it is based on the recent advances in wet-chemical synthetic methodologies, the characterization of size and shape-dependent optical properties, and various optically driven applications including catalysis, signal-enhancing devices, and biomedical purposes.

Thermally tunable catalytic and optical properties of gold-hydrogel nanocomposites.

We have developed a very simple approach for preparing physically embedded gold cores in a temperature-responsive hydrogel polymer nanoparticle under fluorescent light irradiation. The complete encapsulation of the multiple gold core nanoparticles is confirmed by the catalytic reduction of 4-nitrophenol, whose reactivity is significantly retarded above the lower critical solution temperature (LSCT) due to the deswelled polymer structure; its increased hydrophobicity slows the access of hydrophilic reactants to the cores. Since these gold cores are physically embedded in the polymer nanoparticles, further growth of the cores is reliably achieved in situ under light irradiation. Interestingly, the resulting composite nanoparticles exhibit reversible solution color changes as well as absorption bands from the visible to near-IR regions below and above the LSCT.

Controlled synthesis of gold nanoparticles by fluorescent light irradiation.

A novel photochemical synthesis of size-controlled gold nanoparticles was reliably accomplished via both a direct reduction and a seeded-growth method at room temperature under the irradiation of fluorescent light. These methods utilized the intensity of fluorescent light that closely resembles daily sunlight (∼100 mW cm(-2)). This effectively allowed for the formation of gold nanoparticles with tunable sizes simply by controlling the concentration of trisodium citrate and gold chloride. The broad band fluorescent light was found to be an efficient source for inducing the formation of gold nanoparticles at ambient conditions. The size distribution and absorption property of the resulting nanoparticles were thoroughly characterized by scanning/transmission electron microscopy, dynamic light scattering, UV-visible spectroscopy and powder x-ray diffraction. This photochemical synthesis demonstrates, for the first time, the reliable preparation of gold nanoparticles at room temperature upon irradiation with fluorescent light.