Arrays of Individual DNA Molecules on Nanopatterned Substrates

Arrays of individual molecules can combine the advantages of microarrays and single-molecule studies. They miniaturize assays to reduce sample and reagent consumption and increase throughput, and additionally uncover static and dynamic heterogeneity usually masked in molecular ensembles. However, realizing single-DNA arrays must tackle the challenge of capturing structurally highly dynamic strands onto defined substrate positions. Here, we create single-molecule arrays by electrostatically adhering single-stranded DNA of gene-like length onto positively charged carbon nanoislands. The nanosites are so small that only one molecule can bind per island. Undesired adsorption of DNA to the surrounding non-target areas is prevented via a surface-passivating film. Of further relevance, the DNA arrays are of tunable dimensions, and fabricated on optically transparent substrates that enable singe-molecule detection with fluorescence microscopy. The arrays are hence compatible with a wide range of bioanalytical, biophysical, and cell biological studies where individual DNA strands are either examined in isolation, or interact with other molecules or cells

Experimental evidence for the formation of stripe phases in Si/SiGe

We observe pronounced transport anisotropies in magneto-transport experiments performed in the two-dimensional electron system of a Si/SiGe heterostructure. They occur when an in-plane field is used to tune two Landau levels with opposite spin to energetic coincidence. The observed anisotropies disappear drastically for temperatures above 1 K. We propose that our experimental findings may be caused by the formation of a unidirectional stripe phase oriented perpendicular to the in-plane field.Comment: 4 pages, 3 figure

Nanoimprinted superlattice metallic photonic crystal as ultraselective solar absorber

A two-dimensional superlattice metallic photonic crystal (PhC) and its fabrication by nanoimprint lithography on tantalum substrates are presented. The superior tailoring capacity of the superlattice PhC geometry is used to achieve spectrally selective solar absorption optimized for high-temperature and high-efficiency solar-energy-conversion applications. The scalable fabrication route by nanoimprint lithography allows for a high-throughput and high-resolution replication of this complex pattern over large areas. Despite the high fill factor, the pattern of polygonal cavities is accurately replicated into a resist that hardens under ultraviolet radiation over an area of 10  mm². In this way, cavities of 905 nm and 340 nm width are achieved with a period of 1 μm. After pattern transfer into tantalum via a deep reactive ion-etching process, the achieved cavities are 2.2 μm deep, separated by 85–95 nm wide ridges with vertical sidewalls. The room-temperature reflectance spectra of the fabricated samples show excellent agreement with simulated results, with a high spectral absorptance approaching blackbody absorption in the range from 300 to 1900 nm and a steep cutoff. The calculated solar absorptivity of this superlattice PhC is 96% and its thermal transfer efficiency is 82.8% at an operating temperature of 1500 K and an irradiance of 1000  kW/m².United States. Army Research Office (W911NF-13-D-0001)United States. Department of Energy (DE-SC0001299

Nocturnal bee pollinators are attracted to guarana flowers by their scents.

Floral scent is an important component of the trait repertoire of flowering plants, which is used to attract and manipulate pollinators. Despite advances during the last decades about the chemicals released by flowers, there is still a large gap in our understanding of chemical communication between flowering plants and their pollinators. We analyzed floral scents of guarana (Paullinia cupana, Sapindaceae), an economically important plant of the Amazon, using chemical analytical approaches, and determined the attractiveness of the scent to its nocturnal bee pollinators using behavioral assays in the field. Pollen loads of attracted bees were also analyzed. Inflorescences of guarana emit strong scents, both during day and at night, with some semi-quantitative differences between day- and night-time scents

Decoherence of electron spin qubits in Si-based quantum computers

Direct phonon spin-lattice relaxation of an electron qubit bound by a donor impurity or quantum dot in SiGe heterostructures is investigated. The aim is to evaluate the importance of decoherence from this mechanism in several important solid-state quantum computer designs operating at low temperatures. We calculate the relaxation rate $1/T_1$ as a function of [100] uniaxial strain, temperature, magnetic field, and silicon/germanium content for Si:P bound electrons. The quantum dot potential is much smoother, leading to smaller splittings of the valley degeneracies. We have estimated these splittings in order to obtain upper bounds for the relaxation rate. In general, we find that the relaxation rate is strongly decreased by uniaxial compressive strain in a SiGe-Si-SiGe quantum well, making this strain an important positive design feature. Ge in high concentrations (particularly over 85%) increases the rate, making Si-rich materials preferable. We conclude that SiGe bound electron qubits must meet certain conditions to minimize decoherence but that spin-phonon relaxation does not rule out the solid-state implementation of error-tolerant quantum computing.Comment: 8 figures. To appear in PRB-July 2002. Revisions include: some references added/corrected, several typos fixed, a few things clarified. Nothing dramati

Double quantum dot with integrated charge sensor based on Ge/Si heterostructure nanowires

Coupled electron spins in semiconductor double quantum dots hold promise as the basis for solid-state qubits. To date, most experiments have used III-V materials, in which coherence is limited by hyperfine interactions. Ge/Si heterostructure nanowires seem ideally suited to overcome this limitation: the predominance of spin-zero nuclei suppresses the hyperfine interaction and chemical synthesis creates a clean and defect-free system with highly controllable properties. Here we present a top gate-defined double quantum dot based on Ge/Si heterostructure nanowires with fully tunable coupling between the dots and to the leads. We also demonstrate a novel approach to charge sensing in a one-dimensional nanostructure by capacitively coupling the double dot to a single dot on an adjacent nanowire. The double quantum dot and integrated charge sensor serve as an essential building block required to form a solid-state spin qubit free of nuclear spin.Comment: Related work at http://marcuslab.harvard.edu and http://cmliris.harvard.ed

Valley splitting in a Si/SiGe quantum point contact

We present the theory and measurement of valley splitting in a quantum point contact (QPC) in a modulation doped Si/SiGe heterostructure. Our measurements are performed on a submicron Schottky-gated device. An effective mass theory is developed for a QPC formed in a quantum well, grown on a miscut substrate. Both theory and experiments include a perpendicular magnetic field. Our results indicate that both QPC and magnetic confinement can enhance the valley splitting by reducing the spatial extent of the electronic wavefunction. Consequently, the valley splitting can be much larger than the spin splitting for small magnetic fields. We also observe different valley splittings for different transverse modes in the QPC, supporting the notion that when steps are present at the quantum well interface, the spatial extent of the wavefunction plays a dominant role in determining the valley splitting.Comment: 23 pages, 14 figure

Size and surface charge of gold nanoparticles determine absorption across intestinal barriers and accumulation in secondary target organs after oral administration

It is of urgent need to identify the exact physico-chemical characteristics which allow maximum uptake and accumulation in secondary target organs of nanoparticulate drug delivery systems after oral ingestion. We administered radiolabelled gold nanoparticles in different sizes (1.4-200 nm) with negative surface charge and 2.8 nm nanoparticles with opposite surface charges by intra-oesophageal instillation into healthy adult female rats. The quantitative amount of the particles in organs, tissues and excrements was measured after 24 h by gamma-spectroscopy. The highest accumulation in secondary organs was mostly found for 1.4 nm particles; the negatively charged particles were accumulated mostly more than positively charged particles. Importantly, 18 nm particles show a higher accumulation in brain and heart compared to other sized particles. No general rule accumulation can be made so far. Therefore, specialized drug delivery systems via the oral route have to be individually designed, depending on the respective target organ