Advanced Materials

Core/multishell-structured semiconductor nanocrystals (NCs) are prepared in multigram amounts through a one-pot process without a core-separation step (see figure and cover). The NCs show a maximum quantum efficiency of 85 % at a controlled wavelength and strong photostability against UV-light bleaching. The NCs are applied as phosphors on UV-light-emitting diodes (390 nm source), which emit light at 620 nm with a 48 % power conversion efficiency.

Various classes of inorganic semiconductors can be grown into high-quality thin films to serve as channels of high-performance thin-film transistors (TFTs) fabricated on flexible plastic substrates, which are referred to as macroelectronics. The image shows an example of an array of TFTs built with GaAs ribbons on a thin polyurethane sheet laminated on the surface of a glass rod with diameter of 7 mm (with electrical characterization). This Review describes progress in this emerging area.

Thick films containing ultrafine multilayer structures of two complex metal oxides are grown with a single, sectored target in conjunction with pulsed laser deposition on stationary or moving templates. This process is demonstrated with the high-temperature superconductors YBa₂Cu₃O_y and EuBa₂Cu₃O_y. Multilayer periodicities (see figure) as low as 5.5 nm (approximately five unit cells) are obtained in thick films (1.84 μm ≈ 335 bilayer pairs) with improved superconducting properties.

Nonradiative fluorescence resonance energy transfer (FRET) between a luminescent quantum dot (QD) donor and a proximal dye brought in close proximity of the QD surface via conjugation with a dye-labeled peptide (or a protein) is shown (see figure). The system is excited with near IR irradiation (well below the absorption band of the QD), via a fast two-photon process, which produces a FRET signal with very low background contribution due to a substantially reduced nonlinear direct excitation of the dye.

Core/multishell-structured semiconductor nanocrystals (NCs) are prepared in multigram amounts through a one-pot process without a core-separation step (see figure and cover). The NCs show a maximum quantum efficiency of 85 % at a controlled wavelength and strong photostability against UV-light bleaching. The NCs are applied as phosphors on UV-light-emitting diodes (390 nm source), which emit light at 620 nm with a 48 % power conversion efficiency.

Highly sensitive detection of DNA hybridization is demonstrated for the functionalized nanotubes (NTs) described here. With a cascaded-energy-transfer architecture produced by depositing, layer-by-layer, three types of Zn_xCd_1–xSe alloy quantum dots (QDs)—that is, graded-bandgap QDs—into ordered porous alumina membranes (see figure), the NTs provide the ability to detect small but significant changes in the signal during biological detection.

Copper-filled carbon nanotubes can electrically be probed and engineered (see figure) inside a high-resolution transmission electron microscope using a piezo-driven stage. Unique switchlike or rheostatlike electrical behavior and electrical-driven copper femtogram mass transport within nanotubes is shown.

Macroscopic to sub-micrometer-scale patterns and objects are generated by successive miniaturization of an original pattern using the volume-shrinking characteristics of hydrogels. A pattern from a stamp is transferred onto a hydrogel block, which is then dried to shrink its size. The shrunk pattern is subsequently transferred to a polymer stamp, which can be used as master for the next cycle. The figure shows the shrinking of the patterns of a CD (left) and a DVD (right).

A direct-writing technique that allows the fabrication of 3D chemically active nanostructures by combining low- pressure plasma polymerization with an electron-beam process is reported. The figure shows functional nanopillars consisting of acrylic acid (carboxylic acid moieties) on a protein-repellent layer of poly(ethylene glycol). The versatility of the structuring process makes it a promising tool for probing biological systems at the nanoscale.

Site-selective deposition of nanoparticles onto surfaces is desirable for the fabrication of nanoscale devices. For nanoparticles with vastly different numbers of DNA chains on their surfaces, multivalent binding of short-sequence motifs and nonspecific adsorption complicate sequence-specific immobilization from mixtures. A new nanoparticle coating method that suppresses salt-induced aggregation and undesirable binding events is reported. Size-selective sorting of gold nanoparticles up to 60 nm diameter onto nanopatterned surfaces is shown (see figure).

Real-time X-ray microtomography is used to study the underlying mechanisms of the yielding and collapse of a closed-cell metallic glass foam (see figure). Intercellular and intracellular deformations upon compressive loading are examined in situ. A shear-stability analysis reveals that glassy foams obey the same universal yield criterion as zero-temperature glasses, suggesting a link between the floppy modes in a glass and the buckling instabilities in a stochastic cellular structure.

By using nanometer-sized RuO₂ to “metalize” tiny pores and even “repair” incomplete electronically conducting (carbon) networks in porous carbon-containing LiFePO₄ (see figure), the kinetics and rate capability of the composite are significantly improved. The key lies in the bonding properties of RuO₂, which enables good contact to both the oxidic storage material as well as the carbon structures used as current collector.

Submerged laser ablation is used to obtain multicomponent, patterned alkanethiol monolayers on template-stripped gold. In a decanethiol (DT) self-assembled monolayer first squares of mercaptoundecanoic acid (MUA) and then lines comprising of hexadecanethiol (HDT) were prepared. The patterned monolayers are imaged by scanning electron microscopy (see figure).

Structured surfaces with 3D pillars with different geometries resembling those found in biological-attachment devices are microfabricated and their adhesion performance is tested (see figure). This work provides the first experimental evidence of the influence of the contact shape on the adhesion of structured surfaces and paves the road to a better understanding of biological-attachment systems and to optimum designs of artificial analogues.

Electron–phonon coupling in a π-conjugated polymer is revealed by single-molecule spectroscopy in combination with statistical pattern recognition techniques. The technique allows to reveal the phonon-side band in the spectra of methyl-substituted ladder-type poly(para-phenylene) (see figure). For this polymer a weak electron–phonon coupling strength is found at low temperatures. The distribution of the phonon frequencies provides strong evidence that the low-energy vibrational modes, which couple to the electronic transitions, stem from vibrations of the host matrix.

A modified sol–gel approach is used for the preparation of stable silica materials based on a surfactant-encapsulated polyoxometalate (see figure, top panel). In this hybrid material, the luminescence properties of the polyoxometalate and the processibility of the sol–gel matrix are synergistically combined (see figure, bottom panel). This method is, in principle, applicable to all varieties of polyoxometalates, and the resulting composites could be used for applications in optics and catalysis.

A two-photon excitation fluorescence resonance energy transfer scheme, in a system consisting of a cationic conjugated polymer, dsDNA, and a DNA intercalator, is reported. The conjugated polymer acts as two-photon excitation light-harvesting complex (see figure). The two-photon excitation fluorescence of the intercalator is found to be enhanced by a factor of over 35 upon addition of conjugated polymers. The obtained results may have profound implications in two-photon imaging and phototherapy.

A theoretical study showing the importance of intra- and intermolecular hydrogen bonding in the formation of self-assembled systems is reported. Arene-based species with appropriate functional groups to favor intermolecular hydrogen bonding are studied. It is found that molecular flexibility can create strong intermolecular hydrogen bonds within oligomers in which molecules are generally separated by large interplanar distances (see figure).

High-performance H₂sensors are becoming increasingly important in detecting and monitoring H₂. By using pyrolytic carbon as a transition layer between a Pd film and an anodic aluminum oxide substrate, a nanoporous Pd film sensor is fabricated (see figure) that has a remarkable sensing performance with the ability to detect both dilute and high concentrations of H₂ gas quickly and at room temperature.

The absolute value of the hole mobility, computed for the case of rubrene without adjustable parameters (see figure), is in excellent agreement with experiments. The diffusion of the hole is limited by thermal fluctuations of the intermolecular coupling. The system parameters are computed using a combination of classical molecular-dynamics simulations and quantum chemical methods. The effect of intramolecular reorganization energy is included in the model.

A framework for large-scale synthesis of a variety of uniform nonspherical particle types (see figure) is introduced. The technique involves controlling the directionality of phase separations in the seeded-polymerization technique by manipulating the crosslinking density gradients of the dimer seed particles, thus allowing the obtainment of novel nonspherical particle shapes and the production of sufficient quantities to characterize their bulk properties.

A novel conjugated neutral surfactant poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH; see figure) has been developed as an efficient electron-injection material for fabricating highly efficient multilayer polymer light-emitting diodes. Compared to the traditional neutral surfactants used, PFN-OH possesses the combined advantages of good conductivity and excellent electron-injection ability from high-work-function metals (e.g., Al, Ag, or Au).

Charge-carrier concentrations of well-aligned ZnO nanorods grown on p⁺⁺-Si substrates are determined by means of ac impedance analysis of devices fabricated directly on as-deposited samples. This approach also demonstrates that the carrier concentration can be controlled by varying the Zn/O source molar ratio (MR) during metal–organic chemical vapor deposition (see figure).

A new surface-patterning method that involves the intact transferring of interfacial non-densely packed hexagonal colloidal crystals onto silicon wafer substrates has been developed. The arrays have long-range order on the square millimeter scale. The laser diffraction pattern of a micropatterned surface is shown in the figure.

Slide-ring topological gels with mobile cross-linking units are a new class of soft materials that exhibit distinct features differing from those of conventional chemically or physically cross-linked gels. Photoresponsive behavior arising from the dynamic nature of the cross-linkers is demonstrated for a slide-ring gel prepared by adding azobenzene units to the mobile α-cyclodextrin units of a poly(ethylene oxide)-based polyrotaxane (see figure).