Electrically Conductive Thin Films Prepared from Layer‐by‐Layer Assembly of Graphite Platelets

Layer‐by‐layer (LBL) assembly of carbon nanoparticles for low electrical contact resistance thin film applications is demonstrated. The nanoparticles consist of irregularly shaped graphite platelets, with acrylamide/ββ‐methacryl‐oxyethyl‐trimethyl‐ammonium copolymer as the cationic binder. Nanoparticle zeta (ζζ) potential and thereby electrostatic interactions are varied by altering the pH of graphite suspension as well as that of the binder suspension. Film thickness as a function of zeta potential, immersion time, and the number of layers deposited is obtained using Monte Carlo simulation of the energy dispersive spectroscopy measurements. Multilayer film surface morphology is visualized via field‐emission scanning electron microscopy and atomic‐force microscopy. Thin film electrical properties are characterized using electrical contact resistance measurements. Graphite nanoparticles are found to self‐assemble onto gold substrates through two distinct yet overlapping mechanisms. The first mechanism is characterized by logarithmic carbon uptake with respect to the number of deposition cycles and slow clustering of nanoparticles on the gold surface. The second mechanism results from more rapid LBL nanoparticle assembly and is characterized by linear weight uptake with respect to the number of deposition cycles and a constant bilayer thickness of 15 to 21 nm. Thin‐film electrical contact resistance is found to be proportional to the thickness after equilibration of the bilayer structure. Measured values range from 1.6 mΩ cm^?2 at 173 nm to 3.5 mΩ cm^?2 at 276 nm. Coating volume resistivity is reduced when electrostatic interactions are enhanced during LBL assembly.     

Liquid‐Crystalline Perylene Diester Polymers with Tunable Charge‐Carrier Mobility

New classes of liquid‐crystalline semiconductor polymers based on perylene diester benzimidazole and perylene diester imide mesogens are reported. Two highly soluble side‐chain polymers, poly(perylene diester benzimidazole acrylate) (PPDB) and poly(perylene diester imide acrylate) (PPDI) are synthesized by nitroxide‐mediated radical polymerization (NMRP). PPDB shows n‐type semiconductor performance with electron mobilities of 3.2 × 10^?4 cm² V^?1 s^?1 obtained in a diode configuration by fitting the space‐charge‐limited currents (SCLC) according to the Mott–Gurney equation. Interestingly, PPDI performs preferentially as a p‐type material with a hole mobility of 1.5 × 10^?4 cm² V^?1 s^?1, which is attributed to the less electron‐deficient perylene core of PPDI compared to PPDB. Optical properties are investigated by UV‐vis and fluorescence spectroscopy. The extended π‐conjugation system due to the benzimidazole unit of PPDB leads to a considerably broader absorption in the visible region compared to PPDI. HOMO and LUMO levels of the polymers are also determined by cyclic voltammetry; the resulting energy band‐gaps are 1.86 eV for PPDB and 2.16 eV for PPDI. Thermal behavior and liquid crystallinity are studied by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction measurements. The results indicate liquid‐crystalline order of the polymers over a broad temperature range. These thermal, electrical, and optical properties make the perylene side‐chain polymers attractive materials for organic photovoltaics.     

Molten‐Salt‐Assisted Self‐Assembly (MASA)‐Synthesis of Mesoporous Metal Titanate‐Titania,Metal Sulfide‐Titania,and Metal Selenide‐Titania Thin Films

New synthetic strategies are needed for the assembly of porous metal titanates and metal chalcogenite‐titania thin films for various energy applications. Here, a new synthetic approach is introduced in which two solvents and two surfactants are used. Both surfactants are necessary to accommodate the desired amount of salt species in the hydrophilic domains of the mesophase. The process is called a molten‐salt‐assisted self‐assembly (MASA) because the salt species are in the molten phase and act as a solvent to assemble the ingredients into a mesostructure and they react with titania to form mesoporous metal titanates during the annealing step. The mesoporous metal titanate (meso‐Zn₂TiO₄ and meso‐CdTiO₃) thin films are reacted under H₂S or H₂Se gas at room temperature to yield high quality transparent mesoporous metal chalcogenides. The H₂Se reaction produces rutile and brookite titania phases together with nanocrystalline metal selenides and H₂S reaction of meso‐CdTiO₃ yields nanocrystalline anatase and CdS in the spatially confined pore walls. Two different metal salts (zinc nitrate hexahydrate and cadmium nitrate tetrahydrate) are tested to demonstrate the generality of the new assembly process. The meso‐TiO₂‐CdSe film shows photoactivity under sunlight.     

Enhanced Hole‐Transporting Behavior of Discotic Liquid‐Crystalline Physical Gels

Discotic liquid‐crystalline (LC) physical gels have been prepared by combining the self‐assembled fibers of a low‐molecular‐weight gelator and semiconducting LC triphenylene derivatives. The hole mobilities of the discotic LC physical gels measured by a time‐of‐flight method become higher than those of LC triphenylenes alone. The introduction of the finely dispersed networks of the gelator in the hexagonal columnar phases may affect the molecular dynamics of the liquid crystals, resulting in the enhancement of hole transporting behavior in the LC gel state.     

Highly Fluorescent Mesostructured Films that consist of Oligo(phenylenevinylene)–Silica Hybrid Frameworks

Highly fluorescent and visible‐light‐responsive mesostructured organosilica films are successfully obtained by acidic sol–gel polycondensation of oligo(phenylenevinylene) (OPV)‐bridged organosilane and tetraethoxysilane precursors in the presence of a template surfactant. The OPV‐bridged organosilane precursors with different lateral alkoxy substituents, hexyloxy and 2‐ethylhexyloxy, and no substituent, are synthesized by Rh‐catalyzed silylation of corresponding aromatic iodides. From the organosilane precursors, three kinds of mesostructured OPV–silica hybrid films are prepared by spin‐casting using evaporation‐induced self‐assembly. UV‐vis absorption and fluorescence behavior of the OPV–silica hybrid films show that the optical properties and intermolecular interactions of the OPV moieties embedded within the organosilica frameworks strongly depend on the lateral alkoxy substituents in the precursors. The hexyloxy and 2‐ethylhexyloxy substituents prevent aggregation of the OPV units in the organosilica frameworks; this result leads to high fluorescence quantum yields of 0.48–0.61 and 0.63–0.66, respectively, while non‐substitution leads to lower fluorescence quantum yields of 0.25–0.34. Fluorescence decay profiles of the organosilica hybrid films also confirm a suppression of the aggregation of OPV moieties by the lateral substituents. These mesostructured organosilica films with significant optical properties in the visible‐light region are promising as a new class of phosphor materials.     

Controlling the Self‐Assembly of Periodic Defect Patterns in Smectic Liquid Crystal Films with Electric Fields

Large‐area periodic defect patterns are produced in smectic A liquid crystals confined between rigid plate electrodes that impose conflicting parallel and normal anchoring conditions, inducing the formation of topological defects. Highly oriented stripe patterns are created in samples thinner than 2 μm due to self‐assembly of linear defect domains with period smaller than 4 μm, whereas hexagonal lattices of focal conic domains appear for thicker samples. The pattern type (1d/2d) and period can be controlled at the nematic–smectic phase transition by applying an electric field, which confines the defect domains to a thin surface layer with thickness comparable to the nematic coherence length. The pattern morphology persists in the smectic phase even after varying the field or switching it off. Bistable, non‐equilibrium patterns are stabilized by topological constraints of the smectic phase that hinder the rearrangement of defects in response to field variations.     

Control of the Properties of Micrometer‐Sized Actuators from Liquid Crystalline Elastomers Prepared in a Microfluidic Setup

In this article new results on the preparation of monodisperse particles from a liquid crystalline elastomer in a microfluidic setup are presnted. For this, droplets from a liquid crystalline monomer are prepared in a microfluidic device and polymerized while they are flowing inside a microtube. The parti­cles obtained by this method possess an internal orientation, which gives them actuating properties. When they are heated into the isotropic phase of the liquid crystalline material they show a reversible change in shape whereby they change their length in one direction by almost 100%. It is shown how the variation of experimental parameters during their synthesis impacts the properties of these micro‐actuators. Influence over their primal shape, the strength of their shape changing properties, their size, and their mechanical properties is demontrated. From the systematic variation of experimental parameters a deep understanding of the complex processes taking place in a flowing droplet of a liquid crystalline material is obtainted. Additionally NMR analysis and swelling experiments on these actuating materials are provided.     

Large Area Self‐Assembly of Nematic Liquid‐Crystal‐Functionalized Gold Nanorods

Fascinating nematic‐ and smectic‐like self‐assembled arrays are observed for gold nanorods partially capped with either laterally or terminally attached nematic liquid crystals upon slow evaporation of an organic solvent on TEM grids. These arrays can be manipulated and reoriented by applying an external magnetic field from quasi‐planar to vertical similar to a Fréedericksz transition of common organic nematic liquid crystals. Birefringence and thin film textures of these self‐assembled gold nanorod arrays observed by polarized optical microscopy are strongly reminiscent of common organic nematic liquid crystal textures between crossed polarizers and, additionally, support the formation of ordered liquid crystal‐like anisotropic superstructures. The ordering within these arrays is also confirmed in bulk samples using small angle X‐ray scattering (SAXS).     

π‐Conjugated Oligothiophene‐Based Polycatenar Liquid Crystals: Self‐Organization and Photoconductive,Luminescent, and Redox Properties

A series of liquid‐crystalline (LC) π‐ ‐conjugated oligothiophenes bearing three or two alkoxy chains at their extremities has been designed and synthesized. These polycatenar oligothiophenes form various LC nanostructures including smectic, columnar, and micellar cubic phases. These properties depend on the number and length of the terminal alkoxy chains. The hole mobilities for the oligothiophenes have been measured. The layered smectic and columnar structures are capable of transporting holes, leading to mobilities of up to 0.01 cm² V^?1 s^?1. The columnar LC assemblies have also been explored to produce linearly polarized light‐emission. Fine red polarized fluorescence is observed from a uniaxially aligned film of the oligothiophenes. The redox properties of the oligothiophenes both in solutions and in films have been examined. The oligothiophenes exhibit electrochromism upon applying an oxidative potential. The present design strategy is useful for fabricating a variety of functional electro‐active molecular assemblies.     

Supramolecular Surface Chemistry: Substrate‐Independent,Phosphate‐Driven Growth of Polyamine‐Based Multifunctional Thin Films

The ability to engineer surfaces at the supramolecular level by controlled integration of specific chemical units through substrate‐independent methodologies represents one of the new paradigms of contemporary materials science. Here, a method is reported to form multifunctional supramolecular coatings through simple dip‐coating of substrates in an aqueous solution of polyamine in the presence of phosphate anions. The chemical richness and versatility of polyamines are exploited as phosphate receptors to form thin functional films on a broad variety of substrates, ranging from metal to carbonaceous surfaces. It is shown that the simple derivatization of pendant amino groups of polyallylamine precursors with different chemical groups can endow films with predefined responsiveness or multiple functions—this translates into one‐pot and one‐step preparation of substrate‐adherent films displaying built‐in functions. It is believed that the flexibility, speed, and versatility with which this method provides such robust functional films make it very attractive for preparing samples of fundamental and technological interest.     

Liquid‐Crystalline Electrolytes for Lithium‐Ion Batteries: Ordered Assemblies of a Mesogen‐Containing Carbonate and a Lithium Salt

Thermotropic liquid‐crystalline (LC) electrolytes for lithium‐ion batteries are developed for the first time. A rod‐like LC molecule having a cyclic carbonate moiety is used to form self‐assembled two‐dimensional ion‐conductive pathways with lithium salts. Electrochemical and thermal stability, and efficient ionic conduction is achieved for the liquid crystal. The mixture of the carbonate derivative and lithium bis(trifluoromethylsulfonyl)imide is successfully applied as an electrolyte in lithium‐ion batteries. Reversible charge–discharge for both positive and negative electrodes is observed for the lithium‐ion batteries composed of the LC electrolyte.     

Supramolecular Assembly of Perylene Bisimide with β‐Cyclodextrin Grafts as a Solid‐State Fluorescence Sensor for Vapor Detection

A nanoscopic supramolecular aggregate is constructed from perylene bisimide‐bridged bis‐(permethyl‐β‐cyclodextrins) 1 via π–π stacking interactions. Its self‐assembly behavior in organic and aqueous solutions is investigated by UV–Vis, fluorescence, and ¹H NMR spectroscopy. Transmission electron microscopy and scanning electron microscopy images show the 1D nanorod aggregation of 1 , which is birefringent under crossed polarizer conditions and strongly fluorescent as depicted in the fluorescence microscopy image. X‐ray powder diffraction measurements indicate that 1 forms a well‐ordered crystalline arrangement with a π–π stacking distance of 4.02 Å. Furthermore, the solid‐state fluorescence sensing is explored by utilizing the poly(vinylidene fluoride) membrane‐embedded 1 , giving that 1 , as a novel vapor detecting material, can probe several kinds of volatile organic compounds and, especially, exhibits high sensitivity to organic amines.     

Spontaneous Outcropping of Self‐Assembled Insulating Nanodots in Solution‐Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 Films

A new mechanism is proposed for the generation of self‐assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self‐assembled Sr–La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) film grown on SrTiO₃ from chemical solutions. TEM analysis reveals that, underneath the La–Sr oxide (LSO) nanodots, the film switches from the compressive out‐of‐plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden–Popper phase Sr₃Mn₄O₇ embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski–Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy.     

Size‐Selective Binding of Sodium and Potassium Ions in Nanoporous Thin Films of Polymerized Liquid Crystals

The development of a nanoporous material from a columnar liquid crystalline complex between a polymerizable benzoic acid derivative and a 1,3,5‐tris(1H‐benzo[d]imidazol‐2‐yl)benzene template molecule is described. The morphology of the liquid crystalline complex is retained upon polymerization and quantitative removal of the template molecule affords a nanoporous material with the same lattice parameters. The nanoporous material selectively binds cations from aqueous solution, with selectivity for sodium and potassium ions over lithium and barium ions, as shown with FT‐IR. Binding is also quantified gravimetrically with a quartz crystal microbalance with dissipation monitoring, a technique that is used for this purpose for the first time here.     

Highly Oriented Nanofibrils of Regioregular Poly(3‐hexylthiophene) Formed via Block Copolymer Self‐Assembly in Liquid Crystals

A novel method making use of block copolymer self‐assembly in nematic liquid crystals (LCs) is described for preparing macroscopically oriented nanofibrils of π‐conjugated semiconducting polymers. Upon cooling, a diblock copolymer composed of regioregular poly(3‐hexylthiophene) (P3HT) and a liquid crystalline polymer (LCP) in a block‐selective LC solvent can self‐assemble into oriented nanofibrils exhibiting highly anisotropic absorption and polarized photoluminescence emission. An unusual feature of the nanofibrils is that P3HT chains are oriented along the fibrils' long axis. This general method makes it possible to use LCs as an anisotropic medium to grow oriented nanofibrils of many semiconducting polymers insoluble in LCs.     

Photoconductive Hybrid Films via Directional Self‐Assembly of C60 on Aligned Carbon Nanotubes

Hybrid nanostructured materials can exhibit different properties than their constituent components, and can enable decoupled engineering of energy conversion and transport functions. Novel means of building hybrid assemblies of crystalline C₆₀ and carbon nanotubes (CNTs) are presented, wherein aligned CNT films direct the crystallization and orientation of C₆₀ rods from solution. In these hybrid films, the C₆₀ rods are oriented parallel to the direction of the CNTs throughout the thickness of the film. High‐resolution imaging shows that the crystals incorporate CNTs during growth, yet grazing‐incidence X‐ray diffraction (GIXD) shows that the crystal structure of the C₆₀ rods is not perturbed by the CNTs. Growth kinetics of the C₆₀ rods are enhanced 8‐fold on CNTs compared to bare Si, emphasizing the importance of the aligned, porous morphology of the CNT films as well as the selective surface interactions between C₆₀ and CNTs. Finally, it is shown how hybrid C₆₀–CNT films can be integrated electrically and employed as UV detectors with a high photoconductive gain and a responsivity of 10⁵ A W^?1 at low biases (± 0.5 V). The finding that CNTs can induce rapid, directional crystallization of molecules from solution may have broader implications to the science and applications of crystal growth, such as for inorganic nanocrystals, proteins, and synthetic polymers.     

Electrotunable Non‐reciprocal Laser Emission from a Liquid‐Crystal Photonic Device

A liquid crystal (LC) photonic device with an anisotropic optical heterojunction structure has been fabricated. The device has a phase‐retarding nematic LC (NLC) layer sandwiched between two polymer cholesteric LC films with right‐handed helices of different pitches. Electrotunable non‐reciprocal light transmittance and unidirectional circularly polarized (CP) lasing emission have been successfully demonstrated for this device structure. Two left CP (LCP) lasing emission peaks are observed at the edges of the overlapping region between the two photonic bands in the structure and are shifted upon the application of a voltage. In contrast, a non‐reciprocal right CP (RCP) lasing emission peak emerges at one of the band edges and diminishes upon the application of a voltage. These phenomena are interpreted based on the selective reflection of RCP light and the reorientation of the NLC molecules by the application of a voltage.     

Electrochemically Triggered Selective Adsorption of Biotemplated Nanoparticles on Self‐Assembled Organometallic Diblock Copolymer Thin Films

The controlled adsorption of the iron‐containing cage protein ferritin at the nanoscale using stimuli‐responsive self‐assembled diblock copolymer thin‐film templates is reported. The diblock copolymer used study consists of a cylinder‐forming polystyrene‐block‐polyferrocenylsilane (PS‐b‐PFS), with PFS as the minor block, and shows reversible redox properties. To prevent any spontaneous protein adsorption on either block, the electrolyte pH is selected to leave the ferritin negatively charged, and the protein concentration and solution ionic strength are carefully tuned. Selective adsorption of ferritin on the PFS domains of the self‐assembled thin films is then triggered in situ by applying a positive potential, simultaneously oxidizing the PFS and attracting the ferritin electrostatically.