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We directly track the tubelike motion of individual fluorescently labeled polymer molecules in a concentrated solution of unlabeled polymers. We use a single molecule wide-field fluorescence microscopy technique that is able to determine characteristic properties of the polymer dynamics, such as the confining potential, the tube diameter, and the Rouse time. The use of synthetic polymers allows us to investigate the confined motion of the polymer chains not only as a function of polymer concentration (mesh size) but also versus the persistence length of the matrix polymers. Although the polymers used have a persistence length much smaller than their contour length, our experimental results lead to a dependence of the tube diameter on both the mesh size and the persistence length, which follows the theoretically predicted relation for semiflexible chains. [ABSTRACT FROM AUTHOR]

Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states. Although it is widely assumed that these surface states display a Dirac-type dispersion that is symmetric above and below the Dirac point, this exact equivalence across the Fermi level has yet to be established experimentally. Here, we present a detailed transport study of the 3D topological insulator-strained HgTe that strongly challenges this prevailing viewpoint. First, we establish the existence of exclusively surface-dominated transport via the observation of an ambipolar surface quantum Hall effect and quantum oscillations in the Seebeck and Nernst effect. Second, we show that, whereas the thermopower is diffusion driven for surface electrons, both diffusion and phonon drag contributions are essential for the hole surface carriers. This distinct behavior in the thermoelectric response is explained by a strong deviation from the linear dispersion relation for the surface states, with a much flatter dispersion for holes compared with electrons. These findings show that the metallic surface states in topological insulators can exhibit both strong electron–hole asymmetry and a strong deviation from a linear dispersion but remain topologically protected. [ABSTRACT FROM AUTHOR]

Review: 117 refs. [ABSTRACT FROM AUTHOR]

The puzzling electronic and transport properties of the Ga(AsBi) alloy are investigated for a wide range of Bi-concentrations ( x = 0-10.6%) by means of various experimental techniques in high magnetic fields ( B up to 30 T): magneto-photoluminescence spectroscopy, magneto-far-infrared (FIR) absorption spectroscopy, and Hall effect measurements. Our experimental findings suggest that the strength of hybridization of the continuum states of the valence and conduction bands with the Bi-related electronic levels depends on the Bi-concentration, thus leading to band edges with a localized (for x < 6%) or band-like character (for x > 8%). We report an unusual compositional-dependence of the exciton reduced mass ( µexc), whose value can be larger (for x < 6%) or smaller (for x > 8%) than in GaAs depending on the Bi-concentration. Correspondingly, the free-hole mobility ( µh) decreases with increasing Bi-concentration and eventually tends to increase for x > ∼8%. The incorporation of Bi in GaAs also induces the formation of acceptor levels, which we reveal by FIR absorption spectroscopy and Hall effect measurements. The Bi-induced acceptors are characterized by an exceedingly high value of the effective ground-state g-factor ( geff ∼ 15) and are responsible for the increasing p-type conductivity observed in nominally undoped Ga(AsBi) alloys with increasing Bi-concentration. [ABSTRACT FROM AUTHOR]

The properties of InGaAsN V-groove QWRs are assessed here by polarization-dependent photoluminescence (PL) and micro-magneto-PL. Both the polarization anisotropy of the QWR emission and the strong dependence of the diamagnetic shift on the orientation of the applied magnetic field confirm the 1D nature of the QWR excitons. Further, the possibility of passivating N impurities by H irradiation is used to estimate the N content ( x) in the QWRs by turning off the effects of N incorporation. Both the H-induced blueshift of the QWR emission (70 meV) and the measured value of the electron effective mass are consistent with x ∼1%. Nitrogen is also found to enhance the In intake in the QWR, likely due to the strain reduction resulting from the smaller lattice parameter of the InGaAsN alloy. Such strain reduction is also responsible for the quick decay of the degree of linear polarization (ρ) of the QWR emission with increasing temperature, indicating a small splitting between the QWR valence-band levels. In fully hydrogenated samples, conversely, ρ remains roughly constant up to ∼240 K, suggesting the recovery of a larger energy separation between the QWR hole states upon N passivation. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The properties of single site-controlled InGaAsN quantum wires (QWRs)--both untreated and irradiated with atomic hydrogen--are probed by micro-magneto photolumine scence spectroscopy. The strong anisotropy of the diamagnetic shift measured for different orientations of the applied magnetic field confirms the one-dimensional nature of the QWR carrier wave function. In addition, the strain reduction associated with N incorporation is found to promote a larger indium intake in the QWR, enabling the realization of site-controlled QWRs emitting at long (&sce;1.3 ßtn), technologically relevant wavelengths. [ABSTRACT FROM AUTHOR]

A method is presented to design magnetic molecules in which the exchange interaction between adjacent metal ions is controlled by electron density withdrawal through their bridging ligands. We synthesized a novel Mn4 cluster in which the choice of the bridging carboxylate ligands (acetate, benzoate, or trifluoroacetate) determines the type and strength of the three magnetic exchange couplings (J1, ,J2, and .J3) present between the metal ions. Experimentally measured magnetic moments in high magnetic fields show that, upon electron density withdrawal, the main antiferromagnetic exchange constant J1 decreases from –2.2 K for the [Mn4(OAc)4] cluster to –1.9 K for the [Mn4(H5C6COO)4] cluster and –0.6 K for the [Mn4(F3CCOO)4] cluster, while .12 decreases from -1.1 K to nearly 0 K and .J3 changes to a small ferromagnetic coupling. These experimental results are further supported with density- functional theory calculations based on the obtained crystallographic structures of the [Mn4(OAc)4] and [Mn4(F3CCOO)4] clusters. [ABSTRACT FROM AUTHOR]

Most enzyme studies are carried out in bulk aqueous solution, at the so-called ensemble level, but more recently studies have appeared in which enzyme activity is measured at the level of a single molecule, revealing previously unseen properties. To this end, enzymes have been chemically or physically anchored to a surface, which is often disadvantageous because it may lead to denaturation. In a natural environment, enzymes are present in a confined reaction space, which inspired us to develop a generic method to carry out single-enzyme experiments in the restricted spatial environment of a virus capsid. We report here the incorporation of individual horseradish peroxidase enzymes in the inner cavity of a virus, and describe single-molecule studies on their enzymatic behaviour. These show that the virus capsid is permeable for substrate and product and that this permeability can be altered by changing pH. [ABSTRACT FROM AUTHOR]

Virus particles are probably the most precisely defined nanometre-sized objects that can be formed by protein self-assembly. Although their natural function is the storage and transport of genetic material, they have more recently been applied as scaffolds for mineralization and as containers for the encapsulation of inorganic compounds. The reproductive power of viruses has been used to develop versatile analytical methods, such as phage display, for the selection and identification of (bio)active compounds. To date, the combined use of self-assembly and reproduction has not been used for the construction of catalytic systems. Here we describe a self-assembled system based on a plant virus that has its coat protein genetically modified to provide it with a lipase enzyme. Using single-object and bulk catalytic studies, we prove that the virus-anchored lipase molecules are catalytically active. This anchored biocatalyst, unlike man-made supported catalysts, has the capability to reproduce itself in vivo, generating many independent catalytically active copies. [ABSTRACT FROM AUTHOR]

The use of bottom-up approaches to construct patterned surfaces for technological applications is appealing, but to date is applicable to only relatively small areas (-10 square micrometers). We constructed highly periodic patterns at macroscopic length scales, in the range of square millimeters, by combining self-assembly of disk-like porphyrin dyes with physical dewetting phenomena. The patterns consisted of equidistant 5-nanometer-wide lines spaced 0.5 to 1 micrometers apart, forming single porphyrin stacks containing millions of molecules, and were formed spontaneously upon drop-casting a solution of the molecules onto a mica surface. On glass, thicker lines are formed, which can be used to align liquid crystals in large domains of square millimeter size. [ABSTRACT FROM AUTHOR]

Magnetic birefringence was used for in situ monitoring of the morphological changes in diamagnetic polymersomes during shapetransformation by dialysis. The birefringence was found to be very sensitive to the polymersome morphology, as determined by electron microscopy. The deflation of polymersomes into disks was observed, followed by a bending and partial inflation into stomatocytes. [ABSTRACT FROM AUTHOR]

Examines the transportation of electron-hole complexes in semiconductor quantum wells. Disturbance of negatively charged excitons with the application of voltage between the source and drain; Existence of excitons in best-quality samples; manipulation of light emission under applied electric fields.

We show evidence of the backscattering of quantum Hall edge channels in a narrow graphene Hall bar, induced by the gating effect of the conducting tip of a scanning gate microscope, which we can position with nanometer precision. We show full control over the edge channels and are able, because of the spatial variation of the tip potential, to separate copropagating edge channels in the Hall bar, creating junctions between regions of different charge carrier density, that have not been observed in devices based on top or split gates. The solution of the corresponding quantum scattering problem is presented to substantiate these results, and possible follow-up experiments are discussed. [ABSTRACT FROM AUTHOR]

The article reports that the development of nanofibers is a promising approach to miniaturize optoelectronic devices. Over the recent years, such fibers have been constructed from inorganic materials and carbon nanotubes. Alternatively, self-assembled nanofibers based on pie-conjugated systems provide a versatile means to fabricate fibers with tailor-made functionalities similar to those of plastic electronics. So far, however, their use in devices has been limited to pie-conjugated polymers and doped oligomers. A crucial step toward plastic nanoelectronics is the creation of identical, highly organized fibers, about one-molecule thick, on a solid support.