Self‐Assembly in Hopper‐Shaped Crystals

Hopper shape is a special type of crystal morphology. Hopper‐shaped crystals possess unique properties and show promise in many different applications. The understanding of how the building blocks (atoms, ions, and molecules) assemble into hopper‐shaped crystals and how the environmental factors influence the assembly process is critical to the properties and applications of hopper‐shaped crystals. In this review, the important interfacial instability theories that outline the underlying mechanisms for the formation of hopper‐shaped crystals are discussed. Next, the relevant experimental developments based on three categories of synthetic approaches are discussed: the growth through the control of the solute concentration, the temperature gradient, and the capping agent. At the end of the review, the applications, opportunities, and potential challenges of the hopper‐shaped crystals are discussed.     

Alignment of Rod‐Shaped Single‐Photon Emitters Driven by Line Defects in Liquid Crystals

Arrays of liquid crystal defects—linear smectic dislocations—are used to trap semiconductor CdSe/CdS dot‐in‐rods which behave as single‐photon emitters. Measurements of the emission diagram are combined together with measurements of the emitted polarization of the single emitters. It is shown that the dot‐in‐rods are confined parallel to the linear defects to allow for a minimization of the disorder energy associated with the dislocation cores. It is demonstrated that the electric dipoles associated with the dot‐in‐rods, tilted with respect to the rods, remain oriented in the plane including the smectic linear defects and perpendicular to the substrate, most likely due to dipole/dipole interactions between the dipoles of the liquid crystal molecules and those of the dot‐in‐rods. Using smectic dislocations, nanorods can consequently be oriented along a unique direction for a given substrate, independently of the ligands' nature, without any induced aggregation, leading as well to a fixed azimuthal orientation for the dot‐in‐rods' dipoles. These results open the way for the fine control of nanoparticle anisotropic optical properties, in particular, fine control of single‐photon emission polarization.     

Star‐Shaped Oligothiophenes for Solution‐Processible Organic Field‐Effect Transistors

We have designed and successfully synthesized star‐shaped oligothiophenes, which could be used as semiconducting materials for solution‐processible organic field‐effect transistors (FETs). By systematically changing the chemical structure of the star‐shaped oligothiophenes we obtained the structural requirements needed for making working FETs from them. UV‐vis fluorescence measurements showed that a molecule of the star‐shaped compounds under consideration is not a fully conjugated molecule, but it has three independently conjugated oligothienyl‐phenylene blocks. A possible scheme of molecular packing of the star‐shaped oligothiophenes in a lamellar structure was proposed and confirmed by atomic force microscopy (AFM) and X‐ray diffraction (XRD) measurements. Although the star‐shaped semiconductors show a somewhat lower mobility than their linear analogs, they possess better solubility and film‐forming properties, leading to improved spin‐coating processing. The best FETs were made by spin‐coating 1,3,5‐tris(5″‐decyl‐2,2′:5′,2″‐terthien‐5‐yl)benzene from a chloroform solution, which resulted in a mobility of 2 × 10^–4 cm² V ^–1s^–1, a 10² on/off ratio at gate voltages of 0 V and –20 V, and a threshold voltage close to 0 V.     

Small‐Molecule‐Doped Organic Crystals with Long‐Persistent Luminescence

Traditional long‐persistent luminescence (LPL) materials, which are based on inorganic systems containing rare elements and with preparation temperatures of at least 1000 °C, exhibit afterglow times of more than 10 h and can be tuned for different applications. However, the development of this field is hindered due to the large thermal energy consumption and the need for nonrenewable resources. Thus, the development of a “green” design and preparation of LPL materials is of some importance. A doped‐crystalline material based on two metal‐free organic small molecules is easily prepared through ultrasonic crystallization at room temperature. It has a high‐quality, single‐crystalline structure, and visible LPL performance with a duration of more than 6 s upon low‐energy photoexcitation. A green, flexible, and convenient screen‐printing technology for controllable pattern anticounterfeiting is then developed from this purely organic material, which improves the prospects for commercial utilization in the future.     

Multicolor Printing Using Electric‐Field‐Responsive and Photocurable Photonic Crystals

Efficient and large scale printing of photonic crystal patterns with multicolor, multigrayscale, and fine resolution is highly desired due to its application in smart prints, sensors, and photonic devices. Here, an electric‐field‐assisted multicolor printing is reported based on electrically responsive and photocurable colloidal photonic crystal, which is prepared by supersaturation‐induced self‐assembly of SiO₂ particles in the mixture of propylene carbonate (PC) and trimethylolpropane ethoxylate triacrylate (ETPTA). This colloidal crystal suspension, named as E‐ink, has tunable structural color, controllable grayscale, and instantly fixable characteristics at the same time because the SiO₂/ETPTA‐PC photonic crystal has metastable and reversible assembly as well as polymerizable features. Lithographical printing with photomask and maskless pixel printing techniques are developed respectively to efficiently prepare multicolor and high‐resolution photonic patterns using a single‐component E‐ink.     

Programmed High‐Hole‐Mobility Supramolecular Polymers from Disk‐Shaped Molecules

In this paper a simple, casting solution technique for the preparation of two‐dimensional (2D) arrays of very‐high molecular weight (MW) 1D‐Pc supramolecular inorganic polymers is described. The soluble fluoroaluminium tetra‐tert‐butylphthalocyanine (ttbPcAlF) is synthesized and characterized, which can be self‐assembled to form 2D arrays of very‐high‐MW 1D‐Pc supramolecular inorganic polymers. High‐resolution transmission electron microscopy (HRTEM) demonstrates that the 1D‐ttbPcAlF, having a cofacial ring spacing of ～0.36 nm and an interchain distance of ～1.7 nm, self‐assembles into 2D‐nanosheets (～140 nm in length, ～20 nm in width, and equivalent to MW of 3.2 × 10⁵ g mol^?1). The film cast from a 1,2‐dichloroethane (DCE) solution shows a minimum hole‐mobility of ～0.3 cm² V^?1 s^?1 at room temperature by flash‐photolysis time‐resolved microwave conductivity (TRMC) measurements and a fairly high dark dc‐conductivity of ～1 × 10^?3 S cm^?1.     

Bowtie‐Shaped NiCo2O4 Catalysts for Low‐Temperature Methane Combustion

Bowtie‐shaped NiCo₂O₄ nanostructures are prepared using a hydrothermal method. Variation of the synthesis parameters, including reaction time, additives, and calcination temperature, allows an understanding of the origin of the bowtie‐shaped structure to be developed. Methane oxidation experiments performed using temperature‐programed oxidation (TPO) show that the new materials, which do not contain precious metals, have excellent activity for low‐temperature methane combustion, with 100% conversion at ≈410 °C (gas hourly space velocity (GHSV): 90 000 mL (STP) g^?1 h^?1). The structure–activity relationships of the bowtie‐shaped nanostructures are explored.     

Site‐Selective Self‐Assembly of Colloidal Photonic Crystals

A scalable method for site‐selective, directed self‐assembly of colloidal opals on topologically patterned substrates is presented. Here, such substrate contains optical waveguides which couple to the colloidal crystal. The site‐selectivity is achieved by a capillary network, whereas the self‐assembly process is based on controlled solvent evaporation. In the deposition process, a suspension of colloidal microspheres is dispensed on the substrate and driven into the desired crystallization sites by capillary flow. The method has been applied to realize colloidal crystals from monodisperse dielectric spheres with diameters ranging from 290 to 890 nm. The method can be implemented in an industrial wafer‐scale process.     

V‐Shaped Thiophene‐Based Oligomers with Improved Electroluminescence Properties

The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film‐forming properties than their linear counterparts. Spin‐coated films show photoluminescence efficiencies up to 50 %. Light‐emitting diodes with spin‐coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10 000 cd m^–2. Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl‐S,S‐dioxide leads an increase in electron affinity by 0.5–0.7 V, analogous to that of the corresponding linear oligomers.     

Sacrificial‐Layer Atomic Layer Deposition for Fabrication of Non‐Close‐Packed Inverse‐Opal Photonic Crystals

A method is presented for predicting and precisely controlling the structure of photonic crystals fabricated using sacrificial‐layer atomic layer deposition. This technique provides a reliable method for fabrication of high‐quality non‐close‐packed inverse shell opals with large static tunability and precise structural control. By using a sacrificial layer during opal infiltration, the inverse‐opal pore size can be increased with sub‐nanometer resolution and without distorting the lattice to allow for a high degree of dielectric backfilling and increased optical tunability. For a 10 % sacrificial layer, static tunability of 80 % is predicted for the inverse opal. To illustrate this technique, SiO₂ opal templates were infiltrated using atomic layer deposition of ZnS, Al₂O₃, and TiO₂. Experimentally, a static tunability of over 600 nm, or 58 %, was achieved and is well described by both a geometrical model and a numerical‐simulation algorithm. When extended to materials of higher refractive index, this method will allow the facile fabrication of 3D photonic crystals with optimized photonic bandgaps.     

Multiple Structural Coloring of Silk‐Fibroin Photonic Crystals and Humidity‐Responsive Color Sensing

In the biological world, numerous creatures such as butterflies, insects, and birds have exploited photonic structures to produce bicolor reflections with important biofunctions in addition to unique brilliant structural coloration. Although the mimicking of bistructural color reflection is possible, the fabrication involves a process of combined layer deposition techniques, which is complicated and less flexible. Here, a bistructural color mimicking, based on silk fibroin, is reported using a simple and inexpensive self‐assembly method. Silk‐fibroin inverse opals with different spectral positions of bistructural color reflection (i.e., ultraviolet and visible peaks, ultraviolet and near infrared peaks, and visible and near infrared peaks) are obtained by simply controlling their lattice constants. Furthermore, the inline and continuous tuning of the peak positions of bistructural color reflection can be achieved by the humidity‐induced cyclic contraction of silk fibroin. The potential applications of silk‐fibroin photonic structures in eco‐dying and multifunctional silk fabrics are also demonstrated.     

Analysis of Novel Approach to Design of Ultra‐wide Stopband Microstrip Low‐Pass Filter Using Modified U‐Shaped Resonator

A novel microstrip low‐pass filter is presented to achieve an ultra‐wide stopband with 11 harmonic suppression and very sharp skirt characteristics. The filter is composed of a modified U‐shaped resonator (which creates two fully adjustable transmission zeroes), a T‐shaped resonator (which determines a cut‐off frequency), and four radial stubs (which provide a wider stopband). The operating mechanism of the filter is investigated based on a proposed equivalent‐circuit model, and the role of each section of the proposed filter in creating null points is theoretically discussed in detail. The presented filter with 3 dB cut‐off frequency has been fabricated and measured. Results show that a relative stopband bandwidth of 164% (referred to as a 22 dB suppression) is obtained while achieving a high figure‐of‐merit of 15,221.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

Oligothiophene‐Functionalized Truxene: Star‐Shaped Compounds for Organic Field‐Effect Transistors

Organic field‐effect transistors (OFETs) based on oligothiophene‐functionalized truxene derivatives have been fabricated for use as novel star‐shaped organic semiconductors in solution‐processible organic electronics. The electronic and optical properties of compounds 1 – 3 , with increasing numbers of thiophene rings at each of the three branches, have been investigated using scanning electron microscopy (SEM), X‐ray diffraction measurements, and ultraviolet–visible (UV‐vis) and photoluminescence spectroscopies. The results show that with a stepwise increase of the thiophene rings at every branch, a transition from a polycrystalline to an amorphous state is observed. The characteristics of compounds 1 , 2 , and 3 used for OFETs exhibit a significant difference. The mobility depends greatly on the morphology in the solid state, and decreases in going from 1 to 3 . Mobilities up to 1.03 × 10^–3 cm² V^–1 s^–1 and an on/off ratio of about 10³ for compound 1 have been achieved; these are the highest values for star‐shaped organic semiconductors used for OFETs so far. All the results demonstrate that the truxene core of the oligothiophene‐functionalized truxene derivatives not only extends the π‐delocalized system, but also leads to high mobilities for the compounds.     

Yellow‐Colored Electro‐Optic Crystals as Intense Terahertz Wave Sources

This study presents newly developed yellow‐colored organic electro‐optic crystals to provide high terahertz (THz) wave generation efficiency. Compared with currently existing red‐ or orange‐colored electro‐optic crystals, which are used for most benchmark organic THz sources, yellow‐colored crystals have additional superior advantages for THz wave generation, e.g., higher transparency in the visible wavelength range with accompanying different phase‐matching possibilities. The new yellow‐colored crystals consist of a highly nonlinear optical 4‐(4‐hydroxystyryl)‐1‐methylpyridinium (OHP) cation, with a relatively short wavelength of maximal absorption at 390 nm in solution, and various halogen‐substituted benzenesulfonate anions, with strong secondary‐bonding ability. OHP 4‐chlorobenzenesulfonate (OHP‐CBS) crystals exhibit large off‐resonant macroscopic optical nonlinearity and high transparency, with a cut‐off wavelength for solid‐state absorption near 490 nm. OHP‐CBS crystals provide excellent THz wave generation characteristics based on optical rectification. A 0.53 mm thick OHP‐CBS crystal delivers ≈27 times higher optical‐to‐THz conversion efficiency and a much broader spectrum bandwidth compared with the standard 1.0 mm thick ZnTe at 1300 nm pumping. Particularly, compared with a benchmark organic quinolinium crystal with a similar thickness of 0.55 mm, OHP‐CBS crystals exhibit 1.7 times higher optical‐to‐THz conversion efficiency, and show a significantly different THz spectral shape.     

Nanostructured Metallo‐Dielectric Quasi‐Crystals: Towards Photonic‐Plasmonic Resonance Engineering

The first evidence of out‐of‐plane resonances in hybrid metallo‐dielectric quasi‐crystal (QC) nanostructures composed of metal‐backed aperiodically patterned low‐contrast dielectric layers is reported. Via experimental measurements and full‐wave numerical simulations, these resonant phenomena are characterized with specific reference to the Ammann‐Beenker (quasi‐ periodic, octagonal) tiling lattice geometry and the underlying physics is investigated. In particular, it is shown that, by comparison with standard periodic structures, a moderately richer spectrum of resonant modes may be excited, due to the easier achievement of phase‐matching conditions endowed by its denser Bragg spectrum. Such modes are characterized by a distinctive plasmonic or photonic behavior, discriminated by their field distribution and dependence on the metal film thickness. Moreover, the response is accurately predicted via computationally affordable periodic‐approximant‐based numerical modeling. The enhanced capability of QCs to control number, spectral position, and mode distribution of hybrid resonances may be exploited in a variety of possible applications. To assess this aspect, label‐free biosensing is studied via characterization of the surface sensitivity of the proposed structures with respect to local refractive index changes. Moreover, it is also shown that the resonance‐engineering capabilities of QC nanostructures may be effectively exploited in order to enhance the absorption efficiency of thin‐film solar cells.     

Polyphenylene Dendrimer‐Templated In Situ Construction of Inorganic–Organic Hybrid Rice‐Shaped Architectures

A novel dendrimer‐templating method for the synthesis of CuO nanoparticles and the in situ construction of ordered inorganic–organic CuO–G2Td(COOH)₁₆rice‐shaped architectures (RSAs) with analogous monocrystalline structures are reported. The primary CuO nanoparticles are linked by the G2Td(COOH)₁₆ dendrimer. This method provides a way to preserve the original properties of primary CuO nanoparticles in the ordered hybrid nanomaterials by using the 3D rigid polyphenylene dendrimer (G2Td(COOH)₁₆) as a space isolation. The primary CuO nanoparticles with diameter of (6.3 ± 0.4) nm are synthesized via four successive reaction steps starting from the rapid reduction of Cu(NO₃)₂ by using NaBH₄ as reducer and G2Td(COOH)₁₆ as surfactant. The obtained hybrid CuO–G2Td(COOH)₁₆ RSA, formed in the last reaction step, possesses a crystal structure analogous to a monocrystal as observed by transmission electron microscopy(TEM). In particular, the formation process of the RSA is monitored by UV–vis, TEM, and X‐ray diffraction. Small angle X‐ray scattering and Fourier transform infrared spectroscopy are used to investigate the role of the dendrimer in the RSA formation process. The obtained results illuminate that Cu²⁺? COO^? coordination bonds play an indispensable role in bridging and dispersing the primary CuO nanoparticles to induce and maintain the hybrid RSA. More importantly, the RSA is retained through the Cu²⁺? COO^?coordination bonds even with HCl treatment, suggesting that the dendrimers and Cu²⁺ ions may form rice‐shaped polymeric complexes which could template the assembly of CuO nanoparticles towards RSAs. This study highlights the feasibility and flexibility of employing the peculiar dendrimers to in‐situ build up hybrid architectures which could further serve as templates, containers or nanoreactors for the synthesis of other nanomaterials.     

Vertical Orientation of Nanodomains on Versatile Substrates through Self‐Neutralization Induced by Star‐Shaped Block Copolymers

Vertical orientation of lamellar and cylindrical nanodomains of block copolymers on substrates is one of the most promising means for developing nanopatterns of next‐generation microelectronics and storage media. However, parallel orientation of lamellar and cylindrical nanodomains is generally preferred due to different affinity between two block segments in a block copolymer toward the substrate and/or air. Thus, vertical orientation of the nanodomains is only obtained under various pre‐ or post‐treatments such as surface neutralization by random copolymers, solvent annealing, and electric or magnetic field. Here, a novel self‐neutralization concept is introduced by designing molecular architecture of a block copolymer. Star‐shaped 18 arm poly(methyl methacrylate)‐block‐polystyrene copolymers ((PMMA‐b‐PS)₁₈) exhibiting lamellar and PMMA cylindrical nanodomains are synthesized. When a thin film of (PMMA‐b‐PS)₁₈ is spin‐coated on a substrate, vertically aligned lamellar and cylindrical nanodomains are obtained without any pre‐ or post‐treatment, although thermal annealing for a short time (less than 30 min) is required to improve the spatial array of vertically aligned nanodomains. This result is attributed to the star‐shaped molecular architecture that overcomes the difference in the surface affinity between PS and PMMA chains. Moreover, vertical orientations are observed on versatile substrates, for instance, semiconductor (Si, SiO_x), metal (Au), PS or PMMA‐brushed substrate, and a flexible polymer sheet of polyethylene naphthalate.