Molybdenum and Tungsten Sulfide Ligands for Versatile Functionalization of All-Inorganic Nanocrystals

3D microprinting of inorganic porous materials by chemical linking-induced solidification of nanocrystals

Three-dimensional (3D) microprinting is considered a next-generation manufacturing process for the production of microscale components; however, the narrow range of suitable materials, which include mainly polymers, is a critical issue that limits the application of this process to functional inorganic materials. Herein, we develop a generalised microscale 3D printing method for the production of purely inorganic nanocrystal-based porous materials. Our process is designed to solidify all-inorganic nanocrystals via immediate dispersibility control and surface linking-induced interconnection in the nonsolvent linker bath and thereby creates multibranched gel networks. The process works with various inorganic materials, including metals, semiconductors, magnets, oxides, and multi-materials, not requiring organic binders or stereolithographic equipment. Filaments with a diameter of sub-10 μm are printed into designed complex 3D microarchitectures, which exhibit full nanocrystal functionality and high specific surface areas as well as hierarchical porous structures. This approach provides the platform technology for designing functional inorganics-based porous materials.

Self-Assembly of Matchstick-Shaped Inorganic Nano-Surfactants with Controlled Surface Amphiphilicity

Molecular and nanoscale amphiphiles have been extensively studied as building blocks for organizing macroscopic matter through specific and local interactions. Among various amphiphiles, inorganic Janus nanoparticles have attracted a lot of attention owing to their ability to impart multifunctionalities, although the programmability to achieve complicated self-assembly remains a challenge. Here, we synthesized matchstick-shaped Janus nano-surfactants that mimic organic surfactant molecules and studied their programmable self-assembly. High amphiphilicity was achieved through the hard-soft acid-base-based ligand-exchange reaction with strong selectivity on the surface of nano-matchsticks consisting of Ag₂S heads and CdS stems. The obtained nano-surfactants spontaneously assembled into diverse ordered structures such as lamellar, curved, wrinkled, cylindrical, and micellar structures depending on the vertical asymmetry and the interfacial tension controlled by their geometry and surface ligands. The correlation between the phase selectivity of suprastructures and the characteristics of nano-surfactants is discussed. This study realized the molecular amphiphile-like programmability of inorganic Janus nanostructures in self-assembly with the precise control on the surface chemistry.

Generalised optical printing of photocurable metal chalcogenides

Optical three-dimensional (3D) printing techniques have attracted tremendous attention owing to their applicability to mask-less additive manufacturing, which enables the cost-effective and straightforward creation of patterned architectures. However, despite their potential use as alternatives to traditional lithography, the printable materials obtained from these methods are strictly limited to photocurable resins, thereby restricting the functionality of the printed objects and their application areas. Herein, we report a generalised direct optical printing technique to obtain functional metal chalcogenides via digital light processing. We developed universally applicable photocurable chalcogenidometallate inks that could be directly used to create 2D patterns or micrometre-thick 2.5D architectures of various sizes and shapes. Our process is applicable to a diverse range of functional metal chalcogenides for compound semiconductors and 2D transition-metal dichalcogenides. We then demonstrated the feasibility of our technique by fabricating and evaluating a micro-scale thermoelectric generator bearing tens of patterned semiconductors. Our approach shows potential for simple and cost-effective architecturing of functional inorganic materials.

Optical 3D printing techniques are low-cost mask-less patterning methods, but their application is limited by the number of printable materials. Here, the authors report a generalized optical method to print 2D or micrometre-thick 2.5D architectures based on metal chalcogenides inks, showing the realization of micro-scale thermoelectric generators.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe2 (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO2. QDs with monofunctional ligands are directly attached on TiO2 through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO2. In contrast, bifunctional ligands bridge QDs and TiO2, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm2, while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm2 (at 0.6 VRHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Chemical Transformations of Colloidal Semiconductor Nanocrystals Advance Their Applications

Recently, colloidal semiconductor nanocrystals (NCs) are finding more and more applications in optoelectronic devices. Their usage, however, is still very far from the great potential already demonstrated in many fields owing to their unique features. While researchers are still struggling to achieve a wider gamut of different semiconductor nanomaterials with more controllable properties, the library of already existing candidates is large enough to harness their potential. Modification of well-studied semiconductor NCs by means of their chemical transformations can greatly advance their practical exploitation. In this Perspective, the main types of chemical transformations represented by ligand and cation exchange reactions and their recent examples are summarized. While ligand exchange is used to adjust the surface of a semiconductor NC, cation exchange allows us to engineer its core composition. Both approaches greatly extend the range of properties of the resulting nanomaterials, advancing their further incorporation into optoelectronic devices.

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

Colloidal nanocrystals (NCs) are intriguing building blocks for assembling various functional thin films and devices. The electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganic ligand (IL)-capped colloidal NCs are especially promising as the performance of related devices can substantially outperform their organic ligand-capped counterparts. This in turn highlights the significance of preparing IL-capped NC dispersions. The replacement of initial bulky and insulating ligands capped on NCs with short and conductive inorganic ones is a critical step in solution-phase ligand exchange for preparing IL-capped NCs. Solution-phase ligand exchange is extremely appealing due to the highly concentrated NC inks with completed ligand exchange and homogeneous ligand coverage on the NC surface. In this review, the state-of-the-art of IL-capped NCs derived from solution-phase inorganic ligand exchange (SPILE) reactions are comprehensively reviewed. First, a general overview of the development and recent advancements of the synthesis of IL-capped colloidal NCs, mechanisms of SPILE, elementary reaction principles, surface chemistry, and advanced characterizations is provided. Second, a series of important factors in the SPILE process are offered, followed by an illustration of how properties of NC dispersions evolve after ILE. Third, surface modifications of perovskite NCs with use of inorganic reagents are overviewed. They are necessary because perovskite NCs cannot withstand polar solvents or undergo SPILE due to their soft ionic nature. Fourth, an overview of the research progresses in utilizing IL-capped NCs for a wide range of applications is presented, including NC synthesis, NC solid and film fabrication techniques, field effect transistors, photodetectors, photovoltaic devices, thermoelectric, and photoelectrocatalytic materials. Finally, the review concludes by outlining the remaining challenges in this field and proposing promising directions to further promote the development of IL-capped NCs in practical application in the future.

Immobilized Precursor Particle Driven Growth of Centimeter-Sized MoTe2 Monolayer

Molybdenum ditelluride (MoTe₂) has attracted ever-growing attention in recent years due to its novel characteristics in spintronics and phase-engineering, and an efficient and convenient method to achieve large-area high-quality film is an essential step toward electronic applications. However, the growth of large-area monolayer MoTe₂ is challenging. Here, for the first time, we achieve the growth of a centimeter-sized monoclinic MoTe₂ monolayer and manifest the mechanism of immobilized precursor particle driven growth. Microscopic characterizations reveal an obvious trend of immobilized precursor particles being consumed by the monolayer and continuing to provide a source for the growth of the monolayer. Time-of-flight secondary ion mass spectrometry verifies the attachment of hydroxide ions on the surface of the MoTe₂ monolayer, thereby realizing the inhibition of crystal growth along the [001] zone axis and the continuous growth of the MoTe₂ monolayer. The first-principles DFT calculations prove the mechanism of immobilized precursor particles and the absorption of hydroxide ions on the MoTe₂ monolayer. The as-grown MoTe₂ monolayer exhibits a surface roughness of 0.19 nm and average conductivity of 1.5 × 10^-5 S/m, which prove the smoothness and uniformity of the MoTe₂ monolayer. Temperature-dependent electrical measurements together with the transfer characteristic curves further demonstrate the typical semimetallic properties of monoclinic MoTe₂. Our research elaborates the microscopic process of immobilized precursor particles to grow large-area MoTe₂ monolayer and provides a new thinking about the growth of many other two-dimensional materials.

Thiometallate precursors for the synthesis of supported Pt and PtNi nanoparticle electrocatalysts: Size-focusing by S capping

Herein, we report for the first time the successful preparation of thiometallate-based precursors for use in a bottom-up synthetic process of supported Pt and PtNi nanoparticle catalyst. This precursor enabled the monodisperse synthesis of supported Pt nanoparticles and the in situ formation of S, which were caught directly in a collection system by the nanoparticle synthetic processes consisting of impregnation and thermal processes. S is proven to act as a capping agent in generating highly stable nanoparticles with the size ranging from 2 nm to 3 nm and further favors the formation of monodispersed particles by solid-state digestive ripening. The proposed synthetic methodology can be applied to high-quality PtNi alloy nanoparticle systems. The current route is readily scalable, and multi-gram quantities can be prepared. The prepared carbon-supported Pt and PtNi nanoparticles were characterized as electrocatalysts for the oxygen reduction reaction and exhibited superior performance and durability to commercial Pt/C.

Controlled Grafting of Colloidal Nanoparticles on Graphene through Tailored Electrostatic Interaction

Nanoparticle/graphene hybrid composites have been of great interest in various disciplines due to their unique synergistic physicochemical properties. In this study, we report a facile and generalized synthesis method for preparing nanoparticle/exfoliated graphene (EG) composites by tailored electrostatic interactions. EG was synthesized by an electrochemical method, which produced selectively oxidized graphene sheets at the edges and grain boundaries. These EG sheets were further conjugated with polyethyleneimine to provide positive charges at the edges. The primary organic ligands of the colloidal nanoparticles were exchanged with Cl^- or MoS₄^2- anions, generating negatively charged colloidal nanoparticles in polar solvents. By simple electrostatic interactions between the EG and nanoparticles in a solution, nanoparticles were controllably assembled at the edges of the EG. Furthermore, the generality of this process was verified for a wide range of nanoparticles, such as semiconductors, metals, and magnets, on the EG. As a model application, designed composites with size-controlled FeCo nanoparticle/EG were utilized as electromagnetic interference countermeasure materials that showed a size-dependent shift of the frequency ranges on the electromagnetic absorption properties. The current generalized process will offer great potential for the large-scale production of well-designed graphene nanocomposites for electronic and energy applications.

MoS2 Liquid Cell Electron Microscopy Through Clean and Fast Polymer-Free MoS2 Transfer

Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS₂ as a functional substrate as well as the membrane window for liquid cell TEM, which is enabled by our facile and polymer-free MoS₂ transfer process. This provides the opportunity to investigate the growth of Pt nanocrystals on MoS₂ substrates, which elucidates the formation mechanisms of such heterostructured 2D materials. We find that Pt nanocrystals formed in MoS₂ liquid cells have a strong tendency to align their crystal lattice with that of MoS₂, suggesting a van der Waals epitaxial relationship. Importantly, we can study its impact on the kinetics of the nanocrystal formation. The development of MoS₂ liquid cells will allow further study of various liquid phenomena on MoS₂, and the polymer-free MoS₂ transfer process will be implemented in a wide range of applications.

Interface control of electronic and optical properties in IV–VI and II–VI core/shell colloidal quantum dots: a review

Core/shell heterostructures provide controlled optical properties, tuneable electronic structure, and chemical stability due to an appropriate interface design.