Colloidal Magnetic Heterostructured Nanocrystals with Asymmetric Topologies: Seeded-Growth Synthetic Routes and Formation Mechanisms

Colloidal inorganic nanocrystals, free-standing crystalline nanostructures generated and processed in solution phase, represent an important class of advanced nanoscale materials owing to the flexibility with which their physical–chemical properties can be controlled through synthetic tailoring of their compositional, structural and geometric features and the versatility with which they can be integrated in technological fields as diverse as optoelectronics, energy storage/ conversion/production, catalysis and biomedicine. In recent years, building upon mechanistic knowledge acquired on the thermodynamic and kinetic processes that underlie nanocrystal evolution in liquid media, synthetic nanochemistry research has made impressive advances, opening new possibilities for the design, creation and mastering of increasingly complex “colloidal molecules”, in which nanocrystal modules of different materials are clustered together via solid-state bonding interfaces into free-standing, easily processable multifunctional nanocomposite systems. This Review will provide a glimpse into this fast-growing research field by illustrating progress achieved in the wet-chemical development of last-generation breeds of all-inorganic heterostructured nanocrystals (HNCs) in asymmetric non-onionlike geometries, inorganic analogues of polyfunctional organic molecules, in which distinct nanoscale crystalline modules are interconnected in hetero-dimer, hetero-oligomer and anisotropic multidomain architectures via epitaxial heterointerfaces of limited extension. The focus will be on modular HNCs entailing at least one magnetic material component combined with semiconductors and/or metals, which hold potential for generating enhanced or unconventional magnetic properties, while offering diversified or even new chemical-physical properties and functional capabilities. The available toolkit of synthetic strategies, all based on the manipulation of seeded-growth techniques, will be described, revisited and critically interpreted within the framework of the currently understood mechanisms of colloidal heteroepitaxy

Colloidal Magnetic Heterostructured Nanocrystals with Asymmetric Topologies: Seeded-Growth Synthetic Routes and Formation Mechanisms

Colloidal inorganic nanocrystals, free-standing crystalline nanostructures generated and processed in solution phase, represent an important class of advanced nanoscale materials owing to the flexibility with which their physical–chemical properties can be controlled through synthetic tailoring of their compositional, structural and geometric features and the versatility with which they can be integrated in technological fields as diverse as optoelectronics, energy storage/ conversion/production, catalysis and biomedicine. In recent years, building upon mechanistic knowledge acquired on the thermodynamic and kinetic processes that underlie nanocrystal evolution in liquid media, synthetic nanochemistry research has made impressive advances, opening new possibilities for the design, creation and mastering of increasingly complex "colloidal molecules", in which nanocrystal modules of different materials are clustered together via solid-state bonding interfaces into free-standing, easily processable multifunctional nanocomposite systems. This Review will provide a glimpse into this fast-growing research field by illustrating progress achieved in the wet-chemical development of last-generation breeds of all-inorganic heterostructured nanocrystals (HNCs) in asymmetric non-onionlike geometries, inorganic analogues of polyfunctional organic molecules, in which distinct nanoscale crystalline modules are interconnected in hetero-dimer, hetero-oligomer and anisotropic multidomain architectures via epitaxial heterointerfaces of limited extension. The focus will be on modular HNCs entailing at least one magnetic material component combined with semiconductors and/or metals, which hold potential for generating enhanced or unconventional magnetic properties, while offering diversified or even new chemical-physical properties and functional capabilities. The available toolkit of synthetic strategies, all based on the manipulation of seeded-growth techniques, will be described, revisited and critically interpreted within the framework of the currently understood mechanisms of colloidal heteroepitaxy

An Insight into Chemistry and Structure of Colloidal 2D-WS2 Nanoflakes: Combined XPS and XRD Study

The surface and structural characterization techniques of three atom-thick bi-dimensional 2D-WS2 colloidal nanocrystals cross the limit of bulk investigation, offering the possibility of simultaneous phase identification, structural-to-morphological evaluation, and surface chemical description. In the present study, we report a rational understanding based on X-ray photoelectron spectroscopy (XPS) and structural inspection of two kinds of dimensionally controllable 2D-WS2 colloidal nanoflakes (NFLs) generated with a surfactant assisted non-hydrolytic route. The qualitative and quantitative determination of 1T’ and 2H phases based on W 4f XPS signal components, together with the presence of two kinds of sulfur ions, S22− and S2−, based on S 2p signal and related to the formation of WS2 and WOxSy in a mixed oxygen-sulfur environment, are carefully reported and discussed for both nanocrystals breeds. The XPS results are used as an input for detailed X-ray Diffraction (XRD) analysis allowing for a clear discrimination of NFLs crystal habit, and an estimation of the exact number of atomic monolayers composing the 2D-WS2 nanocrystalline samples

Photoluminescence Emission Induced by Localized States in Halide Passivated Colloidal Two-Dimensional WS2 Nanoflakes

Engineering physicochemical properties of two-dimensional transition metal dichalcogenide (2D-TMD) materials by surface manipulation is essential for their practical and large-scale application especially for colloidal 2D-TMDs that are plagued by the unintentional formation of structural defects during the synthetic procedure. However, the available methods to manage surface states of 2D-TMDs in solution-phase are still limited hampering the production of high quality colloidal 2D-TMD inks to be straightforwardly assembled into actual devices. Here, we demonstrate an efficient solution-phase strategy to passivate surface defect states of colloidally synthetized WS2 nanoflakes with halide ligands, resulting in the activation of the photoluminescence emission. Photophysical investigation and density functional theory calculations suggest that halide atoms enable the suppression of non-radiative recombination through the elimination deep gap trap states, and introduce localized states in the energy band structure from which excitons efficiently recombine. Halide passivated WS2 nanoflakes importantly preserve colloidal stability and photoluminescence emission after several weeks of storing in ambient atmosphere, corroborating the potential of our developed 2D-TMD inks

In-plane Aligned Colloidal 2D WS2 Nanoflakes for Solution- Processable Thin Films with High Planar Conductivity

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS2 colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS2 nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS2 thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS2 monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencie

Chapter 3 - Magnetically Active Asymmetric Nanoheterostructures Based on Colloidal All-Inorganic Multicomponent Nanocrystals

Colloidal inorganic nanocrystals (NCs) constitute an important class of advanced nanomaterials owing to the flexibility with which their dimensionality-dependent physical–chemical properties can be controlled by engineering their compositional, structural, and geometric features in the synthesis stage and the versatility with which they can be exploited in disparate technological fields, spanning from optoelectronics, energy conversion/production to catalysis, and biomedicine. In recent years, building upon knowledge acquired on the thermodynamic and kinetic processes that underlie NC evolution in liquid media, synthetic nanochemistry research has made tremendous advances, opening new possibilities for designing, creating, and mastering increasingly complex NC-based assemblies, in which sections of different materials are grouped together into free-standing, easily processable multifunctional nanocomposite systems. This chapter will provide an overview of this fast-growing research field by illustrating progress achieved in the wet-chemical development of last-generation breeds of so-called hybrid or heterostructured nanocrystals (HNCs) in asymmetric non-core/shell geometries, in which distinct material modules are interconnected in heterodimer, heterooligomer, and anisotropic multidomain architectures via heteroepitaxial bonding interfaces of limited extension. The focus will be on HNCs that incorporate at least one magnetic material component combined with semiconductors and/or plasmonic metals, which hold potential for generating enhanced, unconventional magnetic behavior, on one side, and diversified or even new properties and capabilities, on the other side. Various synthetic strategies, all based on the manipulation of seeded-growth techniques, will be described and rationally interpreted within the framework of the currently understood mechanisms of colloidal heteroepitaxy

From capacitance-controlled to diffusion-controlled electrochromism in one-dimensional shape-tailored tungsten oxide nanocrystals

The engineering of electrochemically active films based on structurally and geometrically controlled transition-metal oxide nanocrystals holds promise for the development of a new generation of energy-efficient dynamic windows that may enable a spectrally selective control of sunlight transmission over the near-infrared regime. Herein, the different spectro-electrochemical signatures of two sets of engineered nanotextured electrodes made of distinct anisotropic-shaped tungsten oxide building blocks are comparatively investigated. The electrodes were fabricated starting from corresponding one-dimensional colloidal nanocrystals, namely solid and longitudinally carved nanorods, respectively, which featured identical crystal phase and lattice orientation, but exposed two distinct space-filled volume structures with subtly different lattice parameters and nonequivalent types of accessible surfaces. The shape of nanocrystalline building blocks greatly impacted on the fundamental electrochemical charge-storage mechanisms and, hence on the electrochromic response of these electrodes, due to concomitant bulk and surface-structure effects that could not be entirely traced to mere differences in surface-to-volume ratio. Electrodes made of carved nanorods accommodated more than 80% of the total charge through surface-capacitance mechanisms. This unique prerogative was ultimately demonstrated to enable an outstanding spectral selectivity as well as an extremely efficient dynamic modulation of the optical transmittance at near-infrared frequencies (~ 80% in the range 700–1600 nm)

Exploiting the Transformative Features of Metal Halides for the Synthesis of CsPbBr3@SiO2 Core-Shell Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) are an emerging semiconductive material with a great potential for applications in optoelectronic devices such as photodetectors, solar cells, light-emitting diodes, etc.1 Such an interest in LHP NCs is motivated by their easy synthesis combined with tunable and bright photoluminescence (PL) and strong absorption.2 Despite their impressive optoelectronic properties, LHP NCs experience fast degradation when exposed to UV irradiation, high temperature, moisture, acidic or alkaline environments and polar solvents. Silica has emerged as the most promising material for LHP NCs stabilization.3 However, such enhanced stability is achieved with bulk silica which cannot be employed in technologies that require colloidal stability (e.g. inkjet printing). As a consequence, the community is moving to colloidally stable LHP NCs@SiO2 core@shell systems. The complexity of growing silica shells onto preformed LHP NCs arises from NCs degradation under the conditions needed to grow silica, i.e. the acidic or alkaline environments that catalyze growth. Interestingly, Baranov et al. stabilized CsPbBr3 NCs in an acidic environment through the reaction of the non-luminescent C4PbBr6 NCs with poly(maleic anhydride-alt-1-octadecene) (PMAO). In particular, the oleylamine capping ligands react with the polymer promoting the formation of the luminescent CsPbBr3 NCs and acidifying the reaction environment due to maleamic acid formation.4 In our study, we exploited the acidic environment produced by the reaction of maleic anhydride (MANH, the reactive monomer of PMAO) with the oleylamine ligand of Cs4PbBr6 to prepare CsPbBr3@SiO2 in presence of tetraethyl orthosilicate (TEOS). XRD showed the partial conversion of the Cs4PbBr6 into the CsPbBr3 NCs which was confirmed by their green emission. The CsPbBr3@SiO2 were further coated with SiO2 enhancing the stability towards polar solvents and removing the residual Cs4PbBr6 NCs. These results provide interesting insights onto the mechanism of silica shell formation. Namely, Both the acidic environment and the Cs4PbBr6 NCs as starting material are needed to prepare CsPbBr3@SiO2. [1] S. Tiam Tan, X. Li, H. Volkan Demir. Small 2019, 15, 1902079. [2] L. Protesescu, S. Yakunin, M. Bodnarchuk, F. Krieg, R. Caputo, C. Hendon, R. Xi Yang, A. Walsh, M. Kovalenko. Nano Lett. 2015, 15, 3692\u20133696. [3] Q. Zhang, B. Wang, L. Kong, Q. Wan, C. Zhang, Z. Li, X. Cao, M. Liu, L. Li. Nat. Comm. 2020, 11, 31. [4] D. Baranov, G. Caputo, L. Goldoni, Z. Dang, R. Scarfiello, L. De Trizio, A. Portone, F. Fabbri, A. Camposeo, D. Pisignano, L. Manna. Chem. Sci., 2020, 11, 3986-3995

Photochromic Textiles Based upon Aqueous Blends of Oxygen-Deficient WO_3-x and TiO₂ Nanocrystals

With the main objective being to develop photochromic smart textiles, in this paper, we studied the photochromic behavior of WO3-x nanocrystals (NCs) cooperatively interacting with variable amounts of TiO2 NCs. We tested several blends of WO3-x:TiO2 NCs, admixed in different compositions (relative molar ratio of 4:0, 3:1, 2:2, 1:3, 0:4) and electrostatically interfacing because of opposite values of Z-potential, for photo-induced chromogenic textiles. We further monitored the photochromic sensitivity of NC-impregnated textiles after exposure to a few solvents (i.e., methanol, ethanol, and isopropanol) or when over-coated with different polymeric matrices such as natural cellulose or ionic conductive Nafion. The optimization of the compositions of the WO3-x:TiO2 blends embedded in polymeric matrices, allowed the nanostructured photochromic textiles to show rapid and tunable coloration (3-x:TiO2-based photochromic smart textiles

Exploiting the Transformative Features of Metal Halides for the Synthesis of CsPbBr3@SiO2 Core\u2013Shell Nanocrystals

The encapsulation of colloidal lead halide perovskite nanocrystals within silica (SiO2) is one of the strategies to protect them from polar solvents and other external factors. Here, we demonstrate the overcoating of CsPbBr3 perovskite nanocrystals with silica by exploiting the anhydride-induced transformation of Cs4PbBr6 nanocrystals. CsPbBr3@SiO2 core\u2013shell nanocrystals are obtained after (i) a reaction between colloidal Cs4PbBr6 nanocrystals and maleic anhydride in toluene that yields CsPbBr3 nanocrystals and maleamic acid and (ii) a silica-shell growth around CsPbBr3 nanocrystals via hydrolysis of added alkoxysilanes. The reaction between Cs4PbBr6 nanocrystals and maleic anhydride is necessary to promote shell formation from alkoxysilanes, as demonstrated in control experiments. The best samples of as-prepared CsPbBr3@SiO2 nanocrystals consist of 3c10 nm single-crystal CsPbBr3 cores surrounded by 3c5\u20137 nm amorphous silica shell. Despite their core\u2013shell structure, such nanostructures are poor emitters and degrade within minutes of exposure to ethanol. The photoluminescence intensity of the core\u2013shell nanocrystals is improved by the treatment with a solution of PbBr2 and ligands, and their stability in ethanol is extended to several days after applying an additional silica growth step. Overall, the investigated approach outlines a strategy for making colloidal core\u2013shell nanocrystals utilizing the transformative chemistry of metal halides and reveals interesting insights regarding the conditions required for CsPbBr3@SiO2 nanocrystal formation