Advanced Materials

Organic electroluminescent devices that emit polarized light will be useful as backlights in conventional liquid-crystal displays. The necessary high polarization ratios require, in turn, highly aligned emitters. The various methods of orientation are discussed, together with the materials to which they are applicable. The Figure shows poly(phenylene vinylene), which is used as a luminescent guest molecule in an aligned host matrix.

The integration of organic light-emitting diodes (OLEDs) into full-color flat-panel displays is a current research objective. Metal-free stacked OLEDs (MF-SOLEDs) are the latest development: a red, a blue, and a green subpixel, each of which is transparent to the light emitted by itself and the other subpixels, are stacked on top of each other. A MF-SOLED is described in which an insulating layer is interposed between the middle and top highly transparent subpixels, allowing the three subpixels to be referenced to a common ground and eliminating the differential drive schemes required in previous SOLEDs.

The open–closed interconversion of four photochromes, including their photochemical interconversion kinetics and their use in multicolor systems based on multicomponent mixtures, is presented. The Figure shows a picture prepared with three dithienyl photochromes in closed form on a thin-layer chromatography plate after exposure to UV light for 2 min (left) and subsequently to daylight for 12 h (right).

Two ambient-temperature molecule-based magnets (Curie temperatures T_c of ∼350 K and 372 K) are presented, which are the result of research to identify factors in synthesis that control the magnetic properties of compounds. The goal is to increase T_c. The magnetic properties of the two hexacyanochromate magnets are described (see also the cover of this issue) and the reasons for the differences between them and earlier hexacyanochromate magnets (T_c ∼ 315 K) are discussed. High Curie temperature materials may find applications in magnetic shielding and memory storage devices, for example.

Control over surface texture in thin film production is necessary for preparing smooth films (e.g., for optical surfaces), but also where some roughening of the surface is required (e.g., patterning for liquid-crystalline display devices). These authors have found a way to control nanoscale roughening by adsorbing negatively charged colloidal silica particles onto a positively charged polyelectrolyte preadsorbed onto a silicon wafer. Using this method, model surfaces have been prepared which are then substrates for liquid-crystalline polymers. The properties of these films (surface ordering and film stability) are compared with their smooth counterparts.

Photovoltaic devices remain an important aim for thin films of conjugated polymers. Here is reported the construction of devices with improved photovoltaic performance, which is achieved by blending elongated CdSe nanocrystals (see Figure) with regioregular poly(3-hexylthiophene). Improved transport arising from denser aggregation between the elongated particles is a probable source of the enhanced energy conversion.

Intermolecular interactions caused by substituents, van der Waals interactions, hydrogen bonds, etc. need to be understood if the directed formation of supramolecular structures is to succeed. A study of the influence of intermolecular and substrate–molecule interactions on the epitaxy of monolayers of terminally mono- and disubstituted 3,3-dipentyl-α-quinquethiophenes such as that shown in the Figure is presented.

Small-diameter nanotubes are predicted to exhibit quantum size effects involving an interdependence between the diameter of the nanotubes and their electronic and magnetic properties. These authors ask whether this dependence on diameter might be extended to the mechanical properties of single-wall carbon nanotubes (SWNTs). Results were obtained by inducing a compressive deformation in SWNTs embedded in a polymer matrix by cooling the specimen to 81 K and monitoring the deformation via the Raman D* band frequency shift of the embedded nanotubes.

Inorganic fullerene-like nested nanostructures (IFs) such as the WS₂ IFs shown in the Figure deposited on an a-SiO_x-coated TEM grid are confirmed to confer tribological advantages when added to a lubricant fluid (tetradecane) between two shearing mica surfaces. It is shown that the lower friction is associated with friction-induced material transfer of WS₂ from the IFs to the mica surfaces.

Are nanotubes ideally suited to a straightforward reinforcing role? Results reported here indicate that that may not be the case, but they could find application as a polymer modifier. A successful route is described for the fabrication of large composite films containing carbon nanotubes based on the formation of a stable colloidal intermediate, a route that should be broadly applicable to a range of nanotube materials and polymers. The resulting thermo-mechanical and electrical properties are discussed. While the stiffness of the composites at room temperature is rather low they show promise at high temperatures.

High yields of nanocrystals at ambient temperaturehave been attained for several inorganic salts such as ZnS and CuO. The authors report one-step, solid-state reactions between easily obtained starting materials such as CuCl₂·2H₂O and NaOH to give nanoparticles with narrow size distribution. The process is carried out in air and requires no complex apparatus, reagents, or techniques and thus it shows potential for mass production of nanocrystals. All of the reactions investigated involve hydrated salts, and the role of the water in the reaction mechanism is discussed.

A low-cost method of producing nanostructured titania thick films is required if nanostructured titania is to be used to its full potential in applications in areas ranging from optics via solar energy to gas sensing. It is demonstrated that screen printing forms the basis of such a method, and the effect of annealing on the nanophase and the role of doping to prevent coarse-grain growth at relatively high temperature are discussed. Finally, miniaturized devices with deposited titania layers are shown to act as sensors for CO, their performance depending on the size of the grains.

Rapid micropatterning of polymers on rigid and flexible substrates can be achieved by the method—vacuum-assisted micromolding in capillaries (MIMIC)—introduced here. The Figure shows an oblique-view SEM image of a single hexagonal cell of a pattern produced by vacuum-assisted MIMIC. The use of vacuum reduced the time taken to fill the pattern with UV-curable polyurethane from ∼30 min to ∼15 s.

Colloidal particles with magnetic properties are attractive due to their tunable anisotropic interactions. Here is reported a new class of polystyrene-core magnetite-shell particles produced by the sequential adsorption of nanoparticles and polyelectrolyte, a process that allows the shell thickness and composition to be controlled with nanometer precision. The Figure shows the alignment of the nanocomposite particles in the presence of a magnetic field.

Conjugated polymers such as polypyrrole (PPy) have potential use as artificial muscles or in microsystems such as valves for microfluid handling. One of the most important parameters in such uses is the magnitude of volume change during associated redox processes; however, until now, estimates have varied greatly. Atomic force microscopy is reported here as allowing direct measurement of the in situ thickness change during oxidation and reduction of thin films of PPy doped with dodecylbenzenesulfonate.

Is “self-healing” the source of the stability of Cu(In, Ga)Se₂-based solar modules? The proven remarkable stability and radiation hardness of Cu(In,Ga)Se₂ (CIGS) solar cells stand in apparent contradiction to the fact that CIGS shows both short-range (metastable defect centers) and long-range (significant Cu migration) instabilities. The authors suggest that these instabilities may in fact be a prerequisite for CIGS's stability as they allow a degree of flexibility or “smartness” in accommodating externally imposed changes. Two self-healing cycles are proposed, in which copper species play a particularly important role.