Functional Alkali Metal-Based Ternary Chalcogenides: Design, Properties, and Opportunities

The search for novel materials has recently brought research attention to alkali metal-based chalcogenides (ABZ) as a new class of semiconducting inorganic materials. Various theoretical and computational studies have highlighted many compositions of this class as ideal functional materials for application in energy conversion and storage devices. This Perspective discusses the expansive compositional landscape of ABZ compositions that inherently gives a wide spectrum of properties with great potential for application. In the present paper, we examine the technique of synthesizing this particular class of materials and explore their potential for compositional engineering in order to manipulate key functional properties. This study presents the notable findings that have been documented thus far in addition to outlining the potential avenues for implementation and the associated challenges they present. By fulfilling the sustainability requirements of being relativity earth-abundant, environmentally benign, and biocompatible, we anticipate a promising future for alkali metal chalcogenides. Through this Perspective, we aim to inspire continued research on this emerging class of materials, thereby enabling forthcoming breakthroughs in the realms of photovoltaics, thermoelectrics, and energy storage.

Self-assembly of anisotropic nanoparticles into functional superstructures

Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technological applications. In this field, anisotropic NPs with size- and shape-dependent physical properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technological applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the experimental techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.

Dopant Mediated Assembly of Cu2ZnSnS4 Nanorods into Atomically Coupled 2D Sheets in Solution

Assembly of anisotropic nanocrystals into ordered superstructures is an area of intense research interest due to its relevance to bring nanocrystal properties to macroscopic length scales and to impart additional collective properties owing to the superstructure. Numerous routes have been explored to assemble such nanocrystal superstructures ranging from self-directed to external field-directed methods. Most of the approaches require sensitive control of experimental parameters that are largely environmental and require extra processing steps, increasing complexity and limiting reproducibility. Here, we demonstrate a simple approach to assemble colloidal nanorods in situ, wherein dopant incorporation during the particle synthesis results in the formation of preassembled 2D sheets of close-packed ordered arrays of vertically oriented nanorods in solution.

Reversible, Tunable, Electric-Field Driven Assembly of Silver Nanocrystal Superlattices

Nanocrystal superlattices are typically fabricated by either solvent evaporation or destabilization methods that require long time periods to generate highly ordered structures. In this paper, we report for the first time the use of electric fields to reversibly drive nanocrystal assembly into superlattices without changing solvent volume or composition, and show that this method only takes 20 min to produce polyhedral colloidal crystals, which would otherwise need days or weeks. This method offers a way to control the lattice constants and degree of preferential orientation for superlattices and can suppress the uniaxial superlattice contraction associated with solvent evaporation. In situ small-angle X-ray scattering experiments indicated that nanocrystal superlattices were formed while solvated, not during drying.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Facile synthesis of scandium fluoride oriented single-crystalline rods and urchin-like structures by a gas–solution interface technique

ScF₃ rod-like oriented crystals and urchin-like structures were synthesized at the gas–solution interface for the first time.

Two-Dimensional Bipyramid Plasmonic Nanoparticle Liquid Crystalline Superstructure with Four Distinct Orientational Packing Orders

Anisotropic plasmonic nanoparticles have been successfully used as constituent elements for growing ordered nanoparticle arrays. However, orientational control over their spatial ordering remains challenging. Here, we report on a self-assembled two-dimensional (2D) nanoparticle liquid crystalline superstructure (NLCS) from bipyramid gold nanoparticles (BNPs), which showed four distinct orientational packing orders, corresponding to horizontal alignment (H-NLCS), circular arrangement (C-NLCS), slanted alignment (S-NLCS), and vertical alignment (V-NLCS) of constituent particle building elements. These packing orders are characteristic of the unique shape of BNPs because all four packing modes were observed for particles with various sizes. Nevertheless, only H-NLCS and V-NLCS packing orders were observed for the free-standing ordered array nanosheets formed from a drying-mediated self-assembly at the air/water interface of a sessile droplet. This is due to strong surface tension and the absence of particle-substrate interaction. In addition, we found the collective plasmonic coupling properties mainly depend on the packing type, and characteristic coupling peak locations depend on particle sizes. Interestingly, surface-enhanced Raman scattering (SERS) enhancements were heavily dependent on the orientational packing ordering. In particular, V-NLCS showed the highest Raman enhancement factor, which was about 77-fold greater than the H-NLCS and about 19-fold greater than C-NLCS. The results presented here reveal the nature and significance of orientational ordering in controlling plasmonic coupling and SERS enhancements of ordered plasmonic nanoparticle arrays.

A facile phosphine-free colloidal synthesis of Cu2SnS3 and Cu2ZnSnS4 nanorods with a controllable aspect ratio

Cu₂SnS₃ nanorods were synthesized with a controllable aspect ratio via a facile phosphine-free colloidal synthesis, which readily extended to obtain Cu₂ZnSnS₄ nanorods.

Complete study of the composition and shape evolution in the synthesis of Cu2ZnSnS4 (CZTS) semiconductor nanocrystals

This article describes a complete study of the evolution of composition (from binary to quaternary) and shape (0D–1D) during the synthesis of CZTS nanocrystals.