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

Cation Exchange and Spontaneous Crystal Repair Resulting in Ultrathin, Planar CdS Nanosheets

Cation exchange has become a major postsynthetic tool to obtain nanocrystals with a combination of stoichiometry, size, and shape that is challenging to achieve by direct wet-chemical synthesis. Here, we report on the transformation of highly anisotropic, ultrathin, and planar PbS nanosheets into CdS nanosheets of the same dimensions. We monitor the evolution of the Cd-for-Pb exchange by ex-situ TEM, HAADF-STEM, and EDX. We observe that in the early stages of the exchange the sheets show large in-sheet voids that repair spontaneously upon further exchange and annealing, resulting in ultrathin, planar, and crystalline CdS nanosheets. After cation exchange, the nanosheets show broad sub-band gap luminescence, as often observed in CdS nanocrystals. The photoluminescence excitation spectrum reveals the heavy- and light-hole exciton features, with very strong quantum confinement and large electron-hole Coulomb energy, typical for 2D ultrathin Cd-chalcogenide nanosheets.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

Synthesis of Layered Lead-Free Perovskite Nanocrystals with Precise Size and Shape Control and Their Photocatalytic Activity

Halide perovskites have attracted enormous attention due to their potential applications in optoelectronics and photocatalysis. However, concerns over their instability, toxicity, and unsatisfactory efficiency have necessitated the development of lead-free all-inorganic halide perovskites. A major challenge in designing efficient halide perovskites for practical applications is the lack of effective methods for producing nanocrystals with precise size and shape control. In this work, a layered perovskite, Cs4ZnSb2Cl12 (CZS), is found from calculations to exhibit size- and facet-dependent optoelectronic properties in the nanoscale, and thus, a colloidal method is used to synthesize the CZS nanoparticles with size-tunable morphologies: zero- (nanodots), one- (nanowires and nanorods), two- (nanoplates), and three-dimensional (nanopolyhedra). The growth kinetics of the CZS nanostructures, along with the effects of surface ligands, reaction temperature, and time were investigated. The optoelectronic properties of the nanocrystals varied with size due to quantum confinement effects and with shape due to anisotropy within the crystals and the exposure of specific facets. These properties could be modulated to enhance the visible-light photocatalytic performance for toluene oxidation. In particular, the 9.7 nm CZS nanoplates displayed a toluene to benzaldehyde conversion rate of 1893 μmol g-1 h-1 (95% selectivity), 500 times higher than the bulk synthesized CZS, and comparable with the reported photocatalysts. This study demonstrates the integration of theoretical calculations and synthesis, revealing an approach to the design and fabrication of novel, high-performance colloidal perovskite nanocrystals for optoelectronic and photocatalytic applications.

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.

Unravelling the Interfacial Dynamics of Bandgap Funneling in Bismuth-Based Halide Perovskites

An environmentally friendly mixed-halide perovskite MA₃ Bi₂ Cl_{9- x} I_x with a bandgap funnel structure has been developed. However, the dynamic interfacial interactions of bandgap funneling in MA₃ Bi₂ Cl_{9- x} I_x perovskites in the photoelectrochemical (PEC) system remain ambiguous. In light of this, single- and mixed-halide lead-free bismuth-based hybrid perovskites-MA₃ Bi₂ Cl_{9- y} I_y and MA₃ Bi₂ I₉ (named MBCl-I and MBI)-in the presence and absence of the bandgap funnel structure, respectively, are prepared. Using temperature-dependent transient photoluminescence and electrochemical voltammetric techniques, the photophysical and (photo)electrochemical phenomena of solid-solid and solid-liquid interfaces for MBCl-I and MBI halide perovskites are therefore confirmed. Concerning the mixed-halide hybrid perovskites MBCl-I with a bandgap funnel structure, stronger electronic coupling arising from an enhanced overlap of electronic wavefunctions results in more efficient exciton transport. Besides, MBCl-I's effective diffusion coefficient and electron-transfer rate demonstrate efficient heterogeneous charge transfer at the solid-liquid interface, generating improved photoelectrochemical hydrogen production. Consequently, this combination of photophysical and electrochemical techniques opens up an avenue to explore the intrinsic and interfacial properties of semiconductor materials for elucidating the correlation between material characterization and device performance.

Subsuming the Metal Seed to Transform Binary Metal Chalcogenide Nanocrystals into Multinary Compositions

Direct colloidal synthesis of multinary metal chalcogenide nanocrystals typically develops dynamically from the binary metal chalcogenide nanocrystals with the subsequent incorporation of additional metal cations from solution during the growth process. Metal seeding of binary and multinary chalcogenides is also established, although the seed is solely a catalyst for nanocrystal nucleation and the metal from the seed has never been exploited as active alloying nuclei. Here we form colloidal Cu-Bi-Zn-S nanorods (NRs) from Bi-seeded Cu_2-xS heterostructures. The evolution of these homogeneously alloyed NRs is driven by the dissolution of the Bi-rich seed and recrystallization of the Cu-rich stem into a transitional segment, followed by the incorporation of Zn²⁺ to form the quaternary Cu-Bi-Zn-S composition. The present study also reveals that the variation of Zn concentration in the NRs modulates the aspect ratio and affects the nature of the majority charge carriers. The NRs exhibit promising thermoelectric properties with very low thermal conductivity values of 0.45 and 0.65 W/mK at 775 and 605 K, respectively, for Zn-poor and Zn-rich NRs. This study highlights the potential of metal seed alloying as a direct growth route to achieving homogeneously alloyed NRs compositions that are not possible by conventional direct methods or by postsynthetic transformations.

Inkjet printed patterned bank structure with encapsulated perovskite colour filters for modern display

Inorganic multicolour perovskite nanocrystals (NCs) of CsPbX₃ (X = Cl, Br, I) with high photoluminescence (PL) quantum yield (QY) and saturated colours are considered promising candidates for a high-performance colour conversion layer. However, integration of these materials into industrial applications still faces a significant challenge due to their tendency for aggregation and quenching of the emission during deposition and processing. In this work, we explore a new ink composition with oleylamine (OLA) and hexylphosphonic acid (HPA) ligands in combination with a liquid crystal monomer (LCM) composing a superior solution for an inkjet-printed colour conversion layer. This work provides a simple technique for preparing high-quality perovskite pixels for high-performance displays.

Highly Stable and Efficient Mn2+ Doping Zero-Dimension Cs2ZnxPb1-xCl4 Alloyed Nanorods toward White Electroluminescent Light-Emitting Diodes

Zero-dimensional (0D) crystal structure perovskite NCs have reemerged as promising materials owing to their superior long-term stability; however, their poor conductivity leads to the inferior electrical performances and critically restricts the optoelectronic application of 0D perovskite materials. Herien, the alloyed 0D crystal structure Cs₂Zn_xPb_1-xCl₄ nanorods (NRs) have been synthesized by the modified hot-injection method, which emits bright blue-violet light at 408 nm, and the optimized photoluminescence quantum yield (PLQY) reaches 26%. The Cs₂Zn_0.88Pb_0.12Cl₄ NRs display more excellent air stability and an order of magnitude higher conductivity than CsPbCl₃ nanocube films. In addition, we dope Mn²⁺ ions into the Cs₂Zn_0.88Pb_0.12Cl₄ NRs, which accomplished the optimized PLQY of 40.3% and polarized emission with r = 0.19. The light-emitting diodes (LEDs) based on Mn²⁺ ion doped Cs₂Zn_0.88Pb_0.12Cl₄ NRs exhibit a chromaticity coordinate (CIE) of (0.36, 0.33), an EQE of 0.3%, and a maximum luminance of 98 cd m^-2. This work has enriched ideas for the production of white light perovskite LEDs.

Catalyzed Growth for Atomic-Precision Colloidal Chalcogenide Nanowires and Heterostructures: Progress and Perspective

One-dimensional colloidal semiconductor nanowires are of wide interest in nanoscale electronics and photonics. As compared to the zero-dimensional counterparts, their geometrical anisotropy offers an additional degree of freedom to tailor the electronic and optical properties and enables customized heterostructures with increased complexity. The colloidal synthetic chemistry developed over past decades has fueled the emergence of diverse one-dimensional nanocrystals and heterostructures, whereas the synthetic pursuit for compositionally and structurally defining them at the atomic-level precision remains yet a giant challenge. Catalyzed growth, wherein nanowires grow at the catalyst-nanowire interfaces in a layer-by-layer manner, offers a promising path toward such an ultimate goal. In this Perspective, we will take a close look at how catalyzed growth would enable the on-demand, atomic-precision control of colloidal nanowires and their heterostructures. We then further highlight their potentials for constructing higher-order heteroarchitectures with new and/or enhanced performances. Finally, we conclude with a forward-looking perspective on future challenges.

Accelerated screening of colloidal nanocrystals using artificial neural network-assisted autonomous flow reactor technology

Colloidal semiconductor nanocrystals with tunable optical and electronic properties are opening up exciting opportunities for high-performance optoelectronics, photovoltaics, and bioimaging applications. Identifying the optimal synthesis conditions and screening of synthesis recipes in search of efficient synthesis pathways to obtain nanocrystals with desired optoelectronic properties, however, remains one of the major bottlenecks for accelerated discovery of colloidal nanocrystals. Conventional strategies, often guided by limited understanding of the underlying mechanisms remain expensive in both time and resources, thus significantly impeding the overall discovery process. In response, an autonomous experimentation platform is presented as a viable approach for accelerated synthesis screening and optimization of colloidal nanocrystals. Using a machine-learning-based predictive synthesis approach, integrated with automated flow reactor and inline spectroscopy, indium phosphide nanocrystals are autonomously synthesized. Their polydispersity for different target absorption wavelengths across the visible spectrum is simultaneously optimized during the autonomous experimentation, while utilizing minimal self-driven experiments (less than 50 experiments within 2 days). Starting with no-prior-knowledge of the synthesis, an ensemble neural network is trained through autonomous experiments to accurately predict the reaction outcome across the entire synthesis parameter space. The predicted parameter space map also provides new nucleation-growth kinetic insights to achieve high monodispersity in size of colloidal nanocrystals.

Nanoscale metal-organic frameworks for tumor phototherapy

Metal-Organic Frameworks (MOFs) are constructed from metal ions/cluster nodes and functional organic ligands through coordination bonds. Owing to the advantages of diverse synthetic methods, easy modification after synthesis, large adsorption capacity for heavy metals, and short equilibrium time, considerable attention has recently been paid to MOFs for tumor phototherapy. Through rational tuning of metal ions and ligands, MOFs present abundant properties for various applications. Light-triggered phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT), is an emerging cancer treatment approach. Nanosized MOFs can be applied as phototherapeutic agents to accomplish phototherapy with excellent phototherapeutic efficacy. This review outlines the latest advances in the field of phototherapy with various metal ion-based MOFs.

Light-Mediated Polymerization Induced by Semiconducting Nanomaterials: State-of-the-Art and Future Perspectives

Direct capture of solar energy for chemical transformation via photocatalysis proves to be a cost-effective and energy-saving approach to construct organic compounds. With the recent growth in photosynthesis, photopolymerization has been established as a robust strategy for the production of specialty polymers with complex structures, precise molecular weight, and narrow dispersity. A key challenge in photopolymerization is the scarcity of effective photomediators (photoinitiators, photocatalysts, etc.) that can provide polymerization with high yield and well-defined polymer products. Current efforts on developing photomediators have mainly focused on organic dyes and metal complexes. On the other hand, nanomaterials (NMs), particularly semiconducting nanomaterials (SNMs), are suitable candidates for photochemical reactions due to their unique optical and electrical properties, such as high absorption coefficients, large charge diffusion lengths, and broad absorption spectra. This review provides a comprehensive insight into SNMs' photomediated polymerizations and highlights the roles SNMs play in photopolymerizations, types of polymerizations, applications in producing advanced materials, and the future directions.

Synthesis and Formation Mechanism of Colloidal Janus-Type Cu2-xS/CuInS2 Heteronanorods via Seeded Injection

Colloidal heteronanocrystals allow for the synergistic combination of properties of different materials. For example, spatial separation of the photogenerated electron and hole can be achieved by coupling different semiconductors with suitable band offsets in one single nanocrystal, which is beneficial for improving the efficiency of photocatalysts and photovoltaic devices. From this perspective, axially segmented semiconductor heteronanorods with a type-II band alignment are particularly attractive since they ensure the accessibility of both photogenerated charge carriers. Here, a two-step synthesis route to Cu_2-xS/CuInS₂ Janus-type heteronanorods is presented. The heteronanorods are formed by injection of a solution of preformed Cu_2-xS seed nanocrystals in 1-dodecanethiol into a solution of indium oleate in oleic acid at 240 °C. By varying the reaction time, Janus-type heteronanocrystals with different sizes, shapes, and compositions are obtained. A mechanism for the formation of the heteronanocrystals is proposed. The first step of this mechanism consists of a thiolate-mediated topotactic, partial Cu⁺ for In³⁺ cation exchange that converts one of the facets of the seed nanocrystals into CuInS₂. This is followed by homoepitaxial anisotropic growth of wurtzite CuInS₂. The Cu_2-xS seed nanocrystals also act as sacrificial Cu⁺ sources, and therefore, single composition CuInS₂ nanorods are eventually obtained if the reaction is allowed to proceed to completion. The two-stage seeded growth method developed in this work contributes to the rational synthesis of Cu_2-xS/CuInS₂ heteronanocrystals with targeted architectures by allowing one to exploit the size and faceting of premade Cu_2-xS seed nanocrystals to direct the growth of the CuInS₂ segment.

Nanocrystal Quantum Dots: From Discovery to Modern Development

This review traces nanocrystal quantum dot (QD) research from the early discoveries to the present day and into the future. We describe the extensive body of theoretical and experimental knowledge that comprises the modern science of QDs. Indeed, the spatial confinement of electrons, holes, and excitons in nanocrystals, coupled with the ability of modern chemical synthesis to make complex designed structures, is today enabling multiple applications of QD size-tunable electronic and optical properties.

Morphology effect on zinc oxide quantum photoinitiators for radical polymerization

Semiconductor nanocrystal based photoinitiators, quantum PIs, are a viable alternative to organic photoinitiators demonstrating unique advantages, including a broad and tunable excitation window, limited migration, and more. Aiming towards efficient quantum PIs with tunable properties, a deeper understanding of the relationships between the nanoparticle properties and their efficiency is required. Herein, we studied the morphological effect on ZnO nanocrystals functioning as photoinitiators in both water-based and solvent-free formulations by comparing rod and pyramidal shaped particles of similar volumes and nearly identical surface area. Superior polymerization performances are measured for the nanorods. Photocatalytic characterization including oxygen consumption and reactive oxygen species formation as well as dyes reduction and oxidation, also showed enhanced activities for the nanorods. The different performances were attributed to the anisotropic nanorod morphology which is beneficial for charge separation as well as to the presence of a reactive [0001] facet in the nanorods, which is known to increase the adsorption of molecular oxygen and anionic molecules, thus affecting the catalytic activity. These observations, along with the higher photoinitiation efficiency of the ZnO nanorods, bring them closer to functionality as photoinitiators in numerous photopolymerization applications.

CoP imbedded g-C3N4 heterojunctions for highly efficient photo, electro and photoelectrochemical water splitting

Nanorod-like CoP nanoparticles were fabricated from different precursors of Co(OH)₂ and Co₃O₄ by gas-solid reaction, then further embedded into g-C₃N₄ nanosheets to form intimate heterojunctions via the (011) crystal planes of CoP nanoparticles. The heterojunction hybrid obtained from Co(OH)₂ exhibits superior activity in photo, electro and photoelectrochemical water splitting processes. In photocatalytic water half-splitting for hydrogen evolution reaction, the as-obtained 0.5% CoP-CN achieved a rate at 959.4 μmol·h^-1·g^-1 and 59.1 μmol·h^-1·g^-1 when irradiated by simulated sunlight and visible light respectively, almost 3.1 times and 15.8 times that of pristine g-C₃N₄, For photocatalytic water full-splitting, a stoichiometric evolution of H₂ (14.7 μmol·h^-1·g^-1) and O₂ (7.6 μmol·h^-1·g^-1) was observed on 3%Pt-0.5% CoP-CN composite. The onset potential for electrochemical HER process was drastically reduced after deposition with 0.5% CoP. Meanwhile, a higher photocurrent response and larger anodic photocurrent was detected over 0.5% CoP-CN photoanode during the photoelectrochemical water splitting process, relative to pristine g-C₃N₄ and its analogues. The comprehensive enhancements for catalytic activity of 0.5% CoP-CN could be attributed to its reduced over-potentials, more negative photo-reductive potentials, boosted interfacial charge transfer efficiency, as well as a much higher solar to hydrogen efficiency. The contrastive redox roles of CoP in both photocatalytic water half-splitting and full-splitting processes have been fully explored and revealed. This design on covalent organic framework of highly efficient CoP-based heterojunctions holds great promise for direct water splitting applications in utilizing solar energy.

Nanotechnology for catalysis and solar energy conversion

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

Photoelectrochemical Performance Enhancement of ZnSe Nanorods versus Dots: Combined Experimental and Computational Insights

Size- and shape-tunable colloidal semiconductor nanocrystals are among the most promising materials for photoelectrochemical water splitting. However, in-depth insights into dimension-dependent charge carrier separation and transport for colloidal semiconductor NCs are still lacking in the contemporary literature. Herein, we experimentally compared photoelectrochemical performance of heavy-metal-free ZnSe nanodots and nanorods with the same cubic structure (zinc blende), similar volumes, and similar absorption edge positions and performed density functional theory (DFT) calculations to study the correlation between the dimension and the electronic structures of ZnSe dots and rods. To eliminate the influence of the different deposition amount of NRs and NDs on each phtoanode, we quantified an average photocurrent density contribution of each single ZnSe dot and rod to be 5 × 10^-12 and 9 × 10^-12 μA·cm^-2, respectively, which highlights a significant PEC performance enhancement of 80% for rods versus dots. DFT calculations have shown that the one-dimensional morphology and crystal plane orientation (⟨111⟩) are both major factors for extremely high transition dipole moment density, which facilitate the charge carrier separation and mobility for ZnSe nanocrystals of different dimensions. This work provides useful insights into the mechanism of photoelectrochemical performance enhancement of colloidal nanocrystals and is beneficial for the design of semiconductor materials for optimal photoelectrochemical cells.

Metal Halide Perovskite Nanorods: Shape Matters

Quasi-1D metal halide perovskite nanorods (NRs) are emerging as a type of materials with remarkable optical and electronic properties. Research into this field is rapidly expanding and growing in the past several years, with significant advances in both mechanistic studies of their growth and widespread possible applications. Here, the recent advances in 1D metal halide perovskite nanocrystals (NCs) are reviewed, with a particular emphasis on NRs. At first, the crystal structures of perovskites are elaborated, which is followed by a review of the major synthetic approaches toward perovskite NRs, such as wet-chemical synthesis, substrate-assisted growth, and anion exchange reactions, and discussion of the growth mechanisms associated with each synthetic method. Then, thermal and aqueous stability and the linear polarized luminescence of perovskite NRs are considered, followed by highlighting their applications in solar cells, light-emitting diodes, photodetectors/phototransistors, and lasers. Finally, challenges and future opportunities in this rapidly developing research area are summarized.

Atomically thin heavy-metal-free ZnTe nanoplatelets formed from magic-size nanoclusters

Atomically thin colloidal quasi-two-dimensional (2D) semiconductor nanoplatelets (NPLs) have attracted tremendous attention due to their excellent properties and stimulating applications. Although some advances have been achieved in Cd- and Pb-based semiconductor NPLs, research into heavy-metal-free NPLs has been reported less due to the difficulties in the synthesis and the knowledge gap in the understanding of the growth mechanism. Herein wurtzite ZnTe NPLs with an atomic thickness of about 1.5 nm have been successfully synthesized by using Superhydride (LiEt₃BH) reduced tributylphosphine-Te (TBP-Te) as the tellurium precursor. Mechanistic studies, both experimentally and theoretically, elucidate the transformation from metastable ZnTe MSC-323 magic-size nanoclusters (MSCs) to metastable ZnTe MSC-398, which then forms wurtzite ZnTe NPLs via an oriented attachment mechanism along the [100] and [002] directions of the wurtzite structure. This work not only provides insightful views into the growth mechanism of 2D NPLs but also opens an avenue for their applications in optoelectronics.

Dominant Polar Surfaces of Colloidal II-VI Wurtzite Semiconductor Nanocrystals Enabled by Cation Exchange

Polar surfaces of ionic crystals are of growing technological importance, with implications for the efficiency of photocatalysts, gas sensors, and electronic devices. The creation of ionic nanocrystals with high percentages of polar surfaces is an option for improving their efficiency in the aforementioned applications but is hard to accomplish because they are less thermodynamically stable and prone to vanish during the growth process. Herein, we develop a strategy that is capable of producing polar surface-dominated II-VI semiconductor nanocrystals, including ZnS and CdS, from copper sulfide hexagonal nanoplates through cation exchange reactions. The obtained wurtzite ZnS hexagonal nanoplates have dominant {002} polar surfaces, occupying up to 97.8% of all surfaces. Density functional theory calculations reveal the polar surfaces can be stabilized by a charge transfer of 0.25 eV/formula from the anion-terminated surface to the cation-terminated surface, which also explains the presence of polar surfaces in the initial Cu_1.75S hexagonal nanoplates with cation deficiency prior to cation exchange reactions. Experimental results showed that the HER activity could be boosted by the surface polarization of polar surface-dominated ZnS hexagonal nanoplates. We anticipate this strategy is general and could be used with other systems to prepare nanocrystals with dominant polar surfaces. Furthermore, the availability of colloidal semiconductor nanocrystals with dominant polar surfaces produced through this strategy opens a new avenue for improving their efficiency in catalysis, photocatalysis, gas sensing, and other applications.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Nonepitaxial Gold-Tipped ZnSe Hybrid Nanorods for Efficient Photocatalytic Hydrogen Production

For the first time, colloidal gold (Au)-ZnSe hybrid nanorods (NRs) with controlled size and location of Au domains are synthesized and used for hydrogen production by photocatalytic water splitting. Au tips are found to grow on the apices of ZnSe NRs nonepitaxially to form an interface with no preference of orientation between Au(111) and ZnSe(001). Density functional theory calculations reveal that the Au tips on ZnSe hybrid NRs gain enhanced adsorption of H compared to pristine Au, which favors the hydrogen evolution reaction. Photocatalytic tests reveal that the Au tips on ZnSe NRs effectively enhance the photocatalytic performance in hydrogen generation, in which the single Au-tipped ZnSe hybrid NRs show the highest photocatalytic hydrogen production rate of 437.8 µmol h^-1 g^-1 in comparison with a rate of 51.5 µmol h^-1 g^-1 for pristine ZnSe NRs. An apparent quantum efficiency of 1.3% for hydrogen evolution reaction for single Au-tipped ZnSe hybrid NRs is obtained, showing the potential application of this type of cadmium (Cd)-free metal-semiconductor hybrid nanoparticles (NPs) in solar hydrogen production. This work opens an avenue toward Cd-free hybrid NP-based photocatalysis for clean fuel production.

Adjustable Ternary FeCoNi Nanohybrids for Enhanced Oxygen Evolution Reaction

Water splitting as a greatly desired technology to produce clean renewable energy, but is hampered by the sluggish oxygen evolution reaction. So, the development of highly active and durable water oxidation electrocatalysts is of primarily significance for energy conversion. Here, a facial strategy to synthesize FeCoNi nanohybrids with adjustable morphological structures by using fluorine is introduced. The morphology and electrocatalytic activity of the sample is determined by the innovative introduction of fluorine. Among them, the overpotential at 10 mA cm^-2 of the best sample is approximately 97 mV lower than the commercial RuO₂ toward the oxygen evolution reaction in 1 m KOH. Additionally, the catalysts also have low Tafel slopes and remarkable stability.

Why Do Colloidal Wurtzite Semiconductor Nanoplatelets Have an Atomically Uniform Thickness of Eight Monolayers?

Herein we employed a first-principles method based on density functional theory to investigate the surface energy and growth kinetics of wurtzite nanoplatelets to elucidate why nanoplatelets exhibit a uniform thickness of eight monolayers. We synthesized a series of wurtzite nanoplatelets (ZnSe, ZnS, ZnTe, and CdSe) with an atomically uniform thickness of eight monolayers. As a representative example, the growth mechanism of 1.39 nm thick (eight monolayers) wurtzite ZnSe nanoplatelets was studied to substantiate the proposed growth kinetics. The results show that the growth of the seventh and eighth layers along the [112̅0] direction of 0.99 nm (six monolayers) ZnSe magic-size nanoclusters is accessible, whereas the growth of the ninth layer is unlikely to occur because the formation energy is large. This work not only gives insights into the synthesis of atomically uniform thick wurtzite semiconductor nanoplatelets but also opens up new avenues to their applications in light-emitting diodes, catalysis, detectors, and lasers.

Colloidal quasi-one-dimensional dual semiconductor core/shell nanorod couple heterostructures with blue fluorescence

Herein we report a nanorod couple heterostructure made of dual semiconductors, in which two parallelly aligned ZnSe nanorods are connected by the growth of ZnS on both end and side facets, producing hetero-ZnS (short arms)-ZnSe (long arms)/ZnS shell nanorod couples. As evidenced by electronic structure studies, both experimental and theoretical, such core/shell nanorod couple heterostructures can act as a platform to precisely tailor the quantum confinement of charge carriers between the constituting components within a single nano-object, generating blue fluorescence after the overgrowth of an alloyed ZnCdS layer on the heterostructures. We foresee the mechanistic insights gained and electronic structures revealed in this work would shed light on the rational design of more complex heterostructures with novel functionalities.

Spontaneous shape and phase control of colloidal ZnSe nanocrystals by tailoring Se precursor reactivity

A novel synthetic method of shape and phase control of ZnSe nanocrystals by tailoring Se precursor reactivity is reported.