Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly

Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Inside Front Cover: Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly (Adv. Funct. Mater. 7/2006)

The synthesis of three‐dimensionally ordered, transparent gold‐nanocrystal (NC)/silica superlattice thin films using the self‐assembly (by spin‐coating) of water‐soluble gold nanocrystal micelles and soluble silica is reported by Fan and co‐workers on p. 891. The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Ordered Superparticles with an Enhanced Photoelectric Effect by Sub‐Nanometer Interparticle Distance

As the development in self‐assembly of nanoparticles, a main question is directed to whether the supercrystalline structure can facilitate generation of collective properties, such as coupling between adjacent nanocrystals or delocalization of exciton to achieve band‐like electronic transport in a 3D assembly. The nanocrystal surfaces are generally passivated by insulating organic ligands, which block electronic communication of neighboring building blocks in nanoparticle assemblies. Ligand removal or exchange is an operable strategy for promoting electron transfer, but usually changes the surface states, resulting in performance alteration or uncontrollable aggregation. Here, 3D, supercompact superparticles with well‐defined superlattice domains through a thermally controlled emulsion‐based self‐assembly method is fabricated. The interparticle spacing in the superparticles shrinks to ≈0.3 nm because organic ligands lie prone on the nanoparticle surface, which are sufficient to overcome the electron transfer barrier. The ordered and compressed superstructures promote coupling and electronic energy transfer between CdSSe quantum dots (QDs). Therefore, the acquired QD superparticles exhibit different optical properties and enhanced photoelectric activity compared to individual QDs.     

Intraband Cooling in All‐Inorganic and Hybrid Organic–Inorganic Perovskite Nanocrystals

Intraband relaxation in all‐inorganic cesium lead tribromide (CsPbBr₃) and hybrid organic–inorganic formamidinium lead tribromide (FAPbBr₃) nanocrystals is experimentally investigated for a range of particle sizes, excitation energies, sample temperatures, and excitation fluences. Hot carriers in CsPbBr₃ nanocrystals consistently exhibit slower cooling than FAPbBr₃ nanocrystals in the single electron–hole pair per nanocrystal regime. In both compositions, long‐lived hot carriers (>3 ps) are only observed at excitation densities corresponding to production of multiple electron–hole pairs per nanocrystal—and concomitant Auger recombination. These presented results are distinct from previous reports in bulk hybrid perovskite materials that convey persistent hot carriers at low excitation fluences. Time‐resolved photoluminescence confirms the rapid cooling of carriers in the low‐fluence (single electron–hole pair per nanocrystal) regime. Intraband relaxation processes, as a function of excitation energy, size, and temperature are broadly consistent with other nanocrystalline semiconductor materials.     

Highly Emissive and Color‐Tunable CuInS2‐Based Colloidal Semiconductor Nanocrystals: Off‐Stoichiometry Effects and Improved Electroluminescence Performance

The off‐stoichiometry effects and gram‐scale production of luminescent CuInS₂‐based semiconductor nanocrystals, as well as their application in electroluminescence devices are reported. The crystal structures and optical properties of CuInS₂ nanocrystals can be significantly influenced by controlling their [Cu]/[In] molar ratio. A simple model adapted from the bulk materials is proposed to explain their off‐stoichiometry effects. Highly emissive and color‐tunable CuInS₂‐based NCs are prepared by a combination of [Cu]/[In] molar ratio optimization, ZnS shell coating, and CuInS₂–ZnS alloying. The method is simple, hassle‐free, and easily scalable to fabricate tens of grams of nanocrystal powders with photoluminescence quantum yields up to around 65%. Furthermore, the performance of high‐quality CuInS₂‐based NCs in electroluminescence devices is examined. These devices have lower turn‐on voltages of around 5 V, brighter luminance up to approximately 2100 cd m^?2 and improved injection efficiency of around 0.3 lm W^?1 (at 100 cd m^?2) in comparison to recent reports.     

Shape‐Controlled Synthesis of Polyhedral Nanocrystals and Their Facet‐Dependent Properties

Growth of inorganic polyhedral nanocrystals with excellent morphology control presents significant synthetic challenges, especially when the development of synthetic schemes to make nanocrystals with systematic shape evolution is desired. Nanocrystals with fine size and shape control facilitate formation of their self‐assembled packing structures and offer opportunities for examination of their facet‐dependent physical and chemical properties. In this Feature Article, recent advances in the synthesis of nanocrystals with systematic shape evolution are highlighted. The reaction conditions used to achieve this morphology change offer insights into the growth mechanisms of nanocrystals. A novel class of polyhedral core–shell heterostructures fabricated using structurally well‐defined nanocrystal cores is also presented. Facet‐dependent photocatalytic activity, molecular adsorption, and catalytic and electrical properties of nanocrystals have been examined and are discussed. Nanomaterials with enhanced properties and functionality may be obtained through continuous efforts in the synthesis of nanocrystals with well‐defined structures and investigation of their plane‐selective properties.     

Control of the Oxygen and Cobalt Atoms Diffusion through Co Nanoparticles Differing by Their Crystalline Structure and Size

The size‐dependent Kirkendall effect is studied by using Co nanoparticles. The sizes of Co nanoparticles differing by their crystal structures called nanocrystallinity, namely amorphous, polycrystalline fcc, single crystalline hcp, and single crystalline ε phase, are modulated from 4 to 10 nm. The nanoparticles self‐assembled in 2D superlattices and differing by their nanocrystallinities are subjected to oxygen at 200 °C for 10 min. With single‐domain nanocrystals differing by their crystalline structure (ε and hcp phases), marked changes in the final structures are observed: upon increasing the nanocrystal size, the ε phase favors formation of a hollow structure whereas a transition from single‐domain hollow to multidomain core/shell structures takes place with the hcp phase. With polycrystalline fcc Co nanocrystals, a transition from a hollow to a yolk/shell structure is observed, whereas with amorphous cobalt, solid CoO nanoparticles are produced at the smaller size and are converted to the core/shell structure at the larger one. These differences in size effect are attributed to the change in the control of the inward flow of oxygen atoms and the outward flow of Co atoms with the crystalline structure of cobalt nanoparticles. Such a diffusion process described here on the Kirkendall effect can be studied for other metal nanocrystals.     

Ligand‐Driven Wavelength‐Tunable and Ultra‐Broadband Infrared Luminescence in Single‐Ion‐Doped Transparent Hybrid Materials

Here, tuning of the optical properties of emission centers by tailoring the ligand fields is investigated. Experimentally, it is demonstrated that Ni²⁺ can act as a single emission species in multiple octahedral local environments. Nanocrystal‐embedded hybrid materials are employed as hosts in order to take advantage of their convenience in local environment design for practical applications. Novel composite gain materials with high transparence are successfully made, and show interesting wavelength‐tunable and ultra‐broadband infrared luminescence covering the whole near‐infrared region from 1 100 to 1 800 nm. The infrared luminescence peak positions can be finely tuned from 1 300 to 1 450 and to 1 570 nm, with the largest full width at half maximum being about 400 nm and covering the telecommunication bands at 1 200–1 500 nm. According to the results of characterization, the unusual luminescence, interestingly, originates from Ni²⁺ in nanocrystals and the doping efficiency of Ni²⁺ is surprisingly high. The results demonstrate that the method presented may be an effective way to fabricate multifunctional light sources with various fundamental multifunctional applications from efficient broadband optical amplifiers to bio‐imaging.     

Optical properties of <Emphasis Type="Italic">p–i–n</Emphasis> structures based on amorphous hydrogenated silicon with silicon nanocrystals formed via nanosecond laser annealing

Silicon nanocrystals are formed in the i layers of p–i–n structures based on a-Si:H using pulsed laser annealing. An excimer XeCl laser with a wavelength of 308 nm and a pulse duration of 15 ns is used. The laser fluence is varied from 100 (below the melting threshold) to 250 mJ/cm² (above the threshold). The nanocrystal sizes are estimated by analyzing Raman spectra using the phonon confinement model. The average is from 2.5 to 3.5 nm, depending on the laser-annealing parameters. Current–voltage measurements show that the fabricated p–i–n structures possess diode characteristics. An electroluminescence signal in the infrared (IR) range is detected for the p–i–n structures with Si nanocrystals; the peak position (0.9–1 eV) varies with the laser-annealing parameters. Radiative transitions are presumably related to the nanocrystal–amorphous-matrix interface states. The proposed approach can be used to produce light-emitting diodes on non-refractory substrates.     

Ligand‐Free Synthesis of Aluminum‐Doped Zinc Oxide Nanocrystals and their Use as Optical Spacers in Color‐Tuned Highly Efficient Organic Solar Cells

The color of polymer solar cells using an opaque electrode is given by the reflected light, which depends on the composition and thickness of each layer of the device. Metal‐oxide‐based optical spacers are intensively studied in polymer solar cells aiming to optimize the light absorption. However, the low conductivity of materials such as ZnO and TiO₂ limits the thickness of such optical spacers to tenths of nanometers. A novel synthesis route of cluster‐free Al‐doped ZnO (AZO) nanocrystals (NCs) is presented for solution processing of highly conductive layers without the need of temperature annealing, including thick optical spacers on top of polymer blends. The processing of 80 nm thick optical spacers based on AZO nanocrystal solutions on top of 200 nm thick polymer blend layer is demonstrated leading to improved photocurrent density of 17% compared to solar cells using standard active layers of 90 nm in combination with thin ZnO‐based optical spacers. These AZO NCs also open new opportunities for the processing of high‐efficiency color tuned solar cells. For the first time, it is shown that applying solution‐processed thick optical spacer with polymer blends of different thicknesses can process solar cells of similar efficiency over 7% but of different colors.     

Magnetic and Structural Investigation of ZnSe Semiconductor Nanoparticles Doped With Isolated and Core‐Concentrated Mn2+ Ions

X‐Ray magnetic circular dichroism (XMCD) experiments on diluted magnetic semiconductor nanocrystals (2–7 nm) are reported in order to study their local electronic structure and magnetic properties. ZnSe nanoparticles containing either single manganese ions (Mn²⁺) distributed in the lattice of the entire particle or a MnSe core in the center are prepared using high temperature approaches. The Mn²⁺ concentration is varied between less than one to several tens of manganese ions per nanocrystal. For all samples it is shown that the Mn²⁺ is exclusively present in the bulk of ZnSe nanoparticles with no evidence for oxidation to higher Mn‐oxidation states. The magnetic ions are highly polarized inside the nanocrystals reaching about 80% of the theoretical value of a pure d⁵ state under identical conditions for the case of isolated manganese ions. Nanocrystals with a MnSe core ZnSe shell structure reach <50% of this value. Thus, their polarization is significantly more hindered, which is due to the significantly enhanced Mn–Mn interactions and a more distorted crystalline lattice. In contrast, no coupling between the manganese centers is observed in the nanoparticles doped samples with low concentrations of Mn²⁺, indicating that these ions are isolated in the bulk of the nanoparticles.     

Cu2O Nanocrystal‐Templated Growth of Cu2S Nanocages with Encapsulated Au Nanoparticles and In‐Situ Transmission X‐ray Microscopy Study

Cubic and octahedral Cu₂O nanocrystals and Au–Cu₂O core–shell heterostructures are used as sacrificial templates for the growth of Cu₂S nanocages and Au–Cu₂S core–cage structures. A rapid sulfidation process involving a surface reaction of Cu₂O nanocrystals with Na₂S, followed by etching of the Cu₂O cores with HCl solution for ≈5 sec, results in the fabrication of Cu₂S cages with a wall thickness of 10–20 nm. Transmission electron microscopy characterization reveals the formation of crystalline walls and the presence of ultrasmall pores with sizes of 1 nm or less. Formation of Cu₂O–Cu₂S core–shell structures and their conversion into Cu₂S cages is verified by UV–vis absorption spectroscopy. X‐ray photoelectron spectra further confirm the composition of the cages as Cu₂S. The entire hollowing process via the Kirkendall effect is recorded using in‐situ transmission X‐ray microscopy. After shell formation, continuous ionic diffusion removes the interior Cu₂O. Intermediate structures with remaining central Cu₂O portions and bridging arms to the surrounding cages are observed. The nanocages are also shown to allow molecular transport: anthracene and pyrene penetration into the cages leads to enhanced fluorescence quenching immediately upon adsorption onto the surfaces of the encapsulated gold nanocrystals.     

Microfluidic‐Spinning‐Directed Microreactors Toward Generation of Multiple Nanocrystals Loaded Anisotropic Fluorescent Microfibers

Anisotropic fluorescent hybrid microfibers with distinct optical properties and delicate architectures have aroused special interest because of their potential applications in tissue engineering, drug delivery, sensors, and functional textiles. Microfluidic systems have provided an ideal microreactor platform to produce anisotropic fibers due to their simplified manipulation, high efficiency, flexible controllability, and environmental‐friendly chemical process. Here a novel microfiber reactor based on a microfluidic spinning technique for in situ fabrication of nanocrystals loaded anisotropic fluorescent hybrid microfibers is demonstrated. Multiple nanocrystal reactions are carried out in coaxial flow‐based microdevices with different geometric features, and various nanocrystals loaded microfibers with solid, string‐of‐beads and Janus topographies are obtained. Moreover, the resulted anisotropic fluorescent hybrid microfibers present multiple optical signals. This strategy contributes a facile and environmental‐friendly route to anisotropic fluorescent hybrid microfibers and might open a promising avenue to multiplex optical sensing materials.     

Fabrication and Size‐Selective Bioseparation of Magnetic Silica Nanospheres with Highly Ordered Periodic Mesostructure

In this paper, we report a novel synthesis and selective bioseparation of the composite of Fe₃O₄ magnetic nanocrystals and highly ordered MCM‐41 type periodic mesoporous silica nanospheres. Monodisperse superparamagnetic Fe₃O₄ nanocrystals were synthesized by thermal decomposition of iron stearate in diol in an autoclave at low temperature. The synthesized nanocrystals were encapsulated in mesoporous silica nanospheres through the packing and self‐assembly of composite nanocrystal–surfactant micelles and surfactant/silica complex. Different from previous studies, the produced magnetic silica nanospheres (MSNs) possess not only uniform nanosize (90 ～ 140 nm) but also a highly ordered mesostructure. More importantly, the pore size and the saturation magnetization values can be controlled by using different alkyltrimethylammonium bromide surfactants and changing the amount of Fe₃O₄ magnetic nanocrystals encapsulated, respectively. Binary adsorption and desorption of proteins cytochrome c (cyt c) and bovine serum albumin (BSA) demonstrate that MSNs are an effective and highly selective adsorbent for proteins with different molecular sizes. Small particle size, high surface area, narrow pore size distribution, and straight pores of MSNs are responsible for the high selective adsorption capacity and fast adsorption rates. High magnetization values and superparamagnetic property of MSNs provide a convenient means to remove nanoparticles from solution and make the re‐dispersion in solution quick following the withdrawal of an external magnetic field.     

Laser Interference Lithography for the Nanofabrication of Stimuli‐Responsive Bragg Stacks

Dynamic structural coloration in Tmesisternus isabellae beetle elytra is a unique example of Bragg stack‐based wavelength tuning in response to external stimuli. The underlying principles could guide the design of quantitative optical stimuli‐responsive polymers. Existing nanofabrication techniques to create such materials are costly, time‐consuming, and require high expertise. This study reports a nanofabrication method to produce slanted Bragg stack structures in poly(acrylamide‐co‐poly(ethylene glycol) diacrylate) hydrogel films by combining laser interference lithography and silver halide chemistry in a cost‐effective and rapid process (≈10 min). The Bragg stacks consist of silver bromide nanocrystal multilayers having a lattice spacing of ≈200 nm. Upon broadband light illumination, the Bragg stacks diffract a narrow‐band peak at 520 nm at ≈10° with respect to the normal incidence. The lattice spacing of the hydrogel films can be modulated by external stimuli to shift the Bragg peak for dynamic quantitative measurements. To demonstrate the utility of this method, the Bragg stacks are functionalized with phenylboronic acid molecules. Bragg peak shift analysis allows reversible glucose sensing within a physiological dynamic range (0.0–20.0 mmol L^?1) having a sensitivity of 0.2 mmol L^?1. The developed Bragg stacks may have application in portable, wearable, and implantable real‐time medical diagnostics at point‐of‐care settings.     

In Situ Atomic Force Microscopy of the Reconfiguration of On‐Surface Self‐Assembled DNA‐Nanoparticle Superlattices

The ability to dynamically reconfigure superlattices in response to external stimuli is an intriguing prospect for programmable DNA‐guided nanoparticle (NP) assemblies, which promises the realization of “smart” materials with dynamically adjustable interparticle spacing and real‐time tunable properties. Existing in situ probes of reconfiguration processes have been limited mostly to reciprocal space methods, which can follow larger ordered ensembles but do not provide access to real‐space pathways and dynamics. Here, in situ atomic force microscopy is used to investigate DNA‐linked NP assemblies and their response to external stimuli, specifically the contraction and expansion of on‐surface self‐assembled monolayer superlattices upon reversible DNA condensation induced by ethanol. In situ microscopy allows observation and quantification of key processes in solution, e.g., lattice parameter changes, defects, and monomer displacements in small groups of NPs. The analysis of imaging data uncovers important boundary conditions due to DNA bonding of NP superlattices to a substrate. Tension in the NP–substrate DNA bonds, which can elastically extend, break, and re‐form during contraction/expansion cycles, counteracts the changes in lattice parameter and causes hysteresis in the response of the system. The results provide insight into the behavior of supported DNA‐linked NP superlattices and establish a foundation for designing and probing tunable nanocrystal‐based materials in solution.     

Interfacial Polar‐Bonding‐Induced Multifunctionality of Nano‐Silicon in Mesoporous Silica

The optoelectronic response of a material governs its suitability for a wide range of applications, from photon detection to photovoltaic conversion. To conquer the material limitations and achieve improved optoelectronic responses, nanotechnology has been employed to arrange subunits with specific size‐dependent quantum mechanical properties in a hierarchically organized structure. However, building a functional optoelectronic system from nano‐objects remains a formidable challenge. In this paper, the fabrication of a new artificially engineered optoelectronic material by the preferential growth of silicon nanocrystals on the bottom of the pore‐channels of mesoporous silica is reported. The nanocrystals form highly stable interface structures bonded on one side; these structure show strong electron–phonon coupling and a ferroelectric‐like hysteretic switching property. A new class of multifunctional materials is realized by invoking a concept that employs semiconductor nanocrystals for optical sensing and utilizes interfacial polar layers to facilitate carrier transport and emulate ferroelectric‐like switching.     

Heat‐Transport Mechanisms in Superlattices

The heat transport mechanisms in superlattices are identified from the cross‐plane thermal conductivity Λ of (AlN)_x–(GaN)_y superlattices measured by time‐domain thermoreflectance. For (AlN)_{4.1 nm}–(GaN)_{55 nm} superlattices grown under different conditions, Λ varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest Λ, Λ of (AlN)_{4 nm}–(GaN)_y superlattices decreases monotonically as y decreases, Λ = 6.35 W m⁻¹ K⁻¹ at y = 2.2 nm, 35 times smaller than Λ of bulk GaN. For long‐period superlattices (y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G ≈ 620 MW m⁻² K⁻¹. For y < 40 nm, the apparent value of G increases with decreasing y, reaching G ≈ 2 GW m⁻² K⁻¹ at y < 3 nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)_{4.1 nm}–(GaN)_{4.9 nm} superlattice irradiated by 2.3 MeV Ar ions to a dose of 2 × 10¹⁴ ions cm⁻² is reduced by <35%, suggesting that heat transport in these short‐period superlattices is dominated by long‐wavelength acoustic phonons. Calculations using a Debye‐Callaway model and the assumption of a boundary scattering rate that varies with phonon‐wavelength successfully capture the temperature, period, and ion‐dose dependence of Λ.     

Hierarchical Ordering of Quantum Dots and Liquid with Tunable Super‐Periodicity into High Aspect Ratio Moiré Superlattice Structure

In this work, a new approach for construction of high aspect ratio complex moiré superlattice structure with versatile super‐periodicity is developed using the moiré fringe and secondary sputtering lithography. Wide assortments of high aspect ratio complex superstructures having different features on a 10 nm scaled wall are easily fabricated from simple starting components. More important is the finding of a new microscale phenomenon, consisting in trapping fluids in the centres of the moiré hexagonal fringes, as the consequence of the modulation of local hydrophilicty of the pattern. Using this phenomenon, target materials can be selectively and hierarchically confined within the moiré superlattice. Hierarchical nanoparticles (QDs) ordering with tunable super‐periodicity into selective area of moiré superlattice are successfully demonstrated by just solution‐casting of toluene based QD solution on patterned surfaces. This observation is expected to elucidate the key morphological factors that govern the physics of liquid behavior on a complex patterned substrate. Accordingly, in the near future, this facile approach for complex superlattice structure could be used as optical substrate for imaging applications and open interesting perspectives in the assembly processes and the handling of the nano‐microsized particles.     

Transforming Common III–V and II–VI Semiconductor Compounds into Topological Heterostructures: The Case of CdTe/InSb Superlattices

Currently, known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. It is shown via first‐principle calculations that a heterovalent superlattice made of common semiconductor building blocks can transform its non‐TI components into a topological nanostructure, illustrated by III–V/II–VI superlattice InSb/CdTe. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. The relationship between the interfacial stability and the topological transition is revealed, finding a “window of opportunity” where both conditions can be optimized. Once a critical InSb layer thickness above ≈1.5 nm is reached, both [111] and [100] superlattices have a relative energy of 1.7–9.5 meV Å^–2, higher than that of the atomically exchanged interface and an excitation gap up to ≈150 meV, affording room‐temperature quantum spin Hall effect in semiconductor superlattices. The understanding gained from this study could broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next‐generation superstructured functional materials.