共查询到20条相似文献，搜索用时 15 毫秒
1.
Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence. 相似文献
2.
Metamaterials and transformation optics play substantial roles in various branches of optical science and engineering by providing schemes to tailor electromagnetic fields into desired spatial patterns. We report a theoretical study showing that by designing and manipulating spatially inhomogeneous, nonuniform conductivity patterns across a flake of graphene, one can have this material as a one-atom-thick platform for infrared metamaterials and transformation optical devices. Varying the graphene chemical potential by using static electric field yields a way to tune the graphene conductivity in the terahertz and infrared frequencies. Such degree of freedom provides the prospect of having different "patches" with different conductivities on a single flake of graphene. Numerous photonic functions and metamaterial concepts can be expected to follow from such a platform. 相似文献
3.
The unique band structure of graphene allows reconfigurable electric-field control of carrier type and density, making graphene an ideal candidate for bipolar nanoelectronics. We report the realization of a single-layer graphene p-n junction in which carrier type and density in two adjacent regions are locally controlled by electrostatic gating. Transport measurements in the quantum Hall regime reveal new plateaus of two-terminal conductance across the junction at 1 and 32 times the quantum of conductance, e(2)/h, consistent with recent theory. Beyond enabling investigations in condensed-matter physics, the demonstrated local-gating technique sets the foundation for a future graphene-based bipolar technology. 相似文献
4.
Transparent, conductive carbon nanotube films 总被引：1，自引：0，他引：1
Wu Z Chen Z Du X Logan JM Sippel J Nikolou M Kamaras K Reynolds JR Tanner DB Hebard AF Rinzler AG 《Science (New York, N.Y.)》2004,305(5688):1273-1276
We describe a simple process for the fabrication of ultrathin, transparent, optically homogeneous, electrically conducting films of pure single-walled carbon nanotubes and the transfer of those films to various substrates. For equivalent sheet resistance, the films exhibit optical transmittance comparable to that of commercial indium tin oxide in the visible spectrum, but far superior transmittance in the technologically relevant 2- to 5-micrometer infrared spectral band. These characteristics indicate broad applicability of the films for electrical coupling in photonic devices. In an example application, the films are used to construct an electric field-activated optical modulator, which constitutes an optical analog to the nanotube-based field effect transistor. 相似文献
5.
Zaric S Ostojic GN Kono J Shaver J Moore VC Strano MS Hauge RH Smalley RE Wei X 《Science (New York, N.Y.)》2004,304(5674):1129-1131
We report interband magneto-optical spectra for single-walled carbon nanotubes in high magnetic fields up to 45 tesla, confirming theoretical predictions that the band structure of a single-walled carbon nanotube is dependent on the magnetic flux phi threading the tube. We have observed field-induced optical anisotropy as well as red shifts and splittings of absorption and photoluminescence peaks. The amounts of shifts and splittings depend on the value of phi/phi(0) and are quantitatively consistent with theories based on the Aharonov-Bohm effect. These results represent evidence of the influence of the Aharonov-Bohm phase on the band gap of a solid. 相似文献
6.
Mayorov AS Elias DC Mucha-Kruczynski M Gorbachev RV Tudorovskiy T Zhukov A Morozov SV Katsnelson MI Fal'ko VI Geim AK Novoselov KS 《Science (New York, N.Y.)》2011,333(6044):860-863
The nematic phase transition in electronic liquids, driven by Coulomb interactions, represents a new class of strongly correlated electronic ground states. We studied suspended samples of bilayer graphene, annealed so that it achieves very high quasiparticle mobilities (greater than 10(6) square centimers per volt-second). Bilayer graphene is a truly two-dimensional material with complex chiral electronic spectra, and the high quality of our samples allowed us to observe strong spectrum reconstructions and electron topological transitions that can be attributed to a nematic phase transition and a decrease in rotational symmetry. These results are especially surprising because no interaction effects have been observed so far in bilayer graphene in the absence of an applied magnetic field. 相似文献
7.
We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films on graphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic. 相似文献
8.
Transmission electron microscopy of a hydrated interplanetary dust particle (IDP) indicates that it contains abundant magnesium-iron carbonates, primarily breunnerite and magnesian siderite. This IDP displays a strong absorption band at 6.8 micrometers in its infrared spectrum, similar to that in certain protostellar spectra. The carbonates probably account for the 6.8-micrometer band in the IDP spectrum, suggesting that carbonate also may occur in interstellar dust and be the source of the controversial 6.8-micrometer feature from the protostellar spectra. 相似文献
9.
Miao F Wijeratne S Zhang Y Coskun UC Bao W Lau CN 《Science (New York, N.Y.)》2007,317(5844):1530-1533
As an emergent electronic material and model system for condensed-matter physics, graphene and its electrical transport properties have become a subject of intense focus. By performing low-temperature transport spectroscopy on single-layer and bilayer graphene, we observe ballistic propagation and quantum interference of multiply reflected waves of charges from normal electrodes and multiple Andreev reflections from superconducting electrodes, thereby realizing quantum billiards in which scattering only occurs at the boundaries. In contrast to the conductivity of conventional two-dimensional materials, graphene's conductivity at the Dirac point is geometry-dependent because of conduction via evanescent modes, approaching the theoretical value 4e(2)/pih (where e is the electron charge and h is Planck's constant) only for short and wide devices. These distinctive transport properties have important implications for understanding chaotic quantum systems and implementing nanoelectronic devices, such as ballistic transistors. 相似文献
10.
ClC channels conduct chloride (Cl-) ions across cell membranes and thereby govern the electrical activity of muscle cells and certain neurons, the transport of fluid and electrolytes across epithelia, and the acidification of intracellular vesicles. The structural basis of ClC channel gating was studied. Crystal structures of wild-type and mutant Escherichia coli ClC channels bound to a monoclonal Fab fragment reveal three Cl- binding sites within the 15-angstrom neck of an hourglass-shaped pore. The Cl- binding site nearest the extracellular solution can be occupied either by a Cl- ion or by a glutamate carboxyl group. Mutations of this glutamate residue in Torpedo ray ClC channels alter gating in electrophysiological assays. These findings reveal a form of gating in which the glutamate carboxyl group closes the pore by mimicking a Cl- ion. 相似文献
11.
Li L Checkelsky JG Hor YS Uher C Hebard AF Cava RJ Ong NP 《Science (New York, N.Y.)》2008,321(5888):547-550
The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy. 相似文献
12.
Spectral measurements of the thermal radiation from Jupiter in the band from 16 to 40 micrometers are analyzed under the assumption that pressure-broadened molecular hydrogen transitions are responsible for the bulk of the infrared opacity over most of this spectral interval. Both the vertical pressure-temperature profile and the molecular hydrogen mixing ratio are determined. The derived value ofthe molecular hydrogen mixing ratio, 0.89 +/- 0.11, is consistent with the solar value of 0.86. 相似文献
13.
Gabor NM Song JC Ma Q Nair NL Taychatanapat T Watanabe K Taniguchi T Levitov LS Jarillo-Herrero P 《Science (New York, N.Y.)》2011,334(6056):648-652
We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population, may offer a path to hot carrier-assisted thermoelectric technologies for efficient solar energy harvesting. 相似文献
14.
Shankland TJ 《Science (New York, N.Y.)》1968,161(3836):51-53
Optical absorption and reflectivity measurements on synthetic forsterite show that this silicate has a wide band gap of the order of 8.4 electron volts; thus it resembles other pure insulating oxides such as MgO. For natural olivines, in which divalent cations, mainly Fe(2+), can replace Mg(2+), all the optical absorption bands between the lattice overtones in the infrared and the first excitonic transition at 8.4 electron volts are due to the presence of iron. 相似文献
15.
Ferguson AD Chakraborty R Smith BS Esser L van der Helm D Deisenhofer J 《Science (New York, N.Y.)》2002,295(5560):1715-1719
Siderophore-mediated acquisition systems facilitate iron uptake. We present the crystallographic structure of the integral outer membrane receptor FecA from Escherichia coli with and without ferric citrate at 2.5 and 2.0 angstrom resolution. FecA is composed of three distinct domains: the barrel, plug, and NH2-terminal extension. Binding of ferric citrate triggers a conformational change of the extracellular loops that close the external pocket of FecA. Ligand-induced allosteric transitions are propagated through the outer membrane by the plug domain, signaling the occupancy of the receptor in the periplasm. These data establish the structural basis of gating for receptors dependent on the cytoplasmic membrane protein TonB. By compiling available data for this family of receptors, we propose a mechanism for the energy-dependent transport of siderophores. 相似文献
16.
Steger M Saeedi K Thewalt ML Morton JJ Riemann H Abrosimov NV Becker P Pohl HJ 《Science (New York, N.Y.)》2012,336(6086):1280-1283
A quantum computer requires systems that are isolated from their environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spins in solids promise long coherence lifetimes, but they are difficult to initialize into known states and to detect with high sensitivity. We show how the distinctive optical properties of enriched (28)Si enable the use of hyperfine-resolved optical transitions, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these resolved hyperfine transitions permit rapid nuclear hyperpolarization and electrical spin-readout. We combine these techniques to detect nuclear magnetic resonance from dilute (31)P in the purest available sample of (28)Si, at concentrations inaccessible to conventional measurements, measuring a solid-state coherence time of over 180 seconds. 相似文献
17.
Tsen AW Brown L Levendorf MP Ghahari F Huang PY Havener RW Ruiz-Vargas CS Muller DA Kim P Park J 《Science (New York, N.Y.)》2012,336(6085):1143-1146
Graphene produced by chemical vapor deposition (CVD) is polycrystalline, and scattering of charge carriers at grain boundaries (GBs) could degrade its performance relative to exfoliated, single-crystal graphene. However, the electrical properties of GBs have so far been addressed indirectly without simultaneous knowledge of their locations and structures. We present electrical measurements on individual GBs in CVD graphene first imaged by transmission electron microscopy. Unexpectedly, the electrical conductance improves by one order of magnitude for GBs with better interdomain connectivity. Our study suggests that polycrystalline graphene with good stitching may allow for uniformly high electrical performance rivaling that of exfoliated samples, which we demonstrate using optimized growth conditions and device geometry. 相似文献
18.
果蝇唾腺染色体为多线染色体,其上的每一条横纹带就是一个具有多个相同DNA序列的基因集合体,这个集合体至少相当于一个基因的克隆。本研究利用8.8NHCl打断果蝇唾腺染色体的间带区,使每条横纹带相互分离,实现了天然基因克隆的切割。本方法简单易行,为开发转基因动物和转基因植物提供了新的研究途径。 相似文献
19.
Britnell L Gorbachev RV Jalil R Belle BD Schedin F Mishchenko A Georgiou T Katsnelson MI Eaves L Morozov SV Peres NM Leist J Geim AK Novoselov KS Ponomarenko LA 《Science (New York, N.Y.)》2012,335(6071):947-950
An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration. 相似文献
20.
Colombelli R Srinivasan K Troccoli M Painter O Gmachl CF Tennant DM Sergent AM Sivco DL Cho AY Capasso F 《Science (New York, N.Y.)》2003,302(5649):1374-1377
We combine photonic and electronic band structure engineering to create a surface-emitting quantum cascade microcavity laser. A high-index contrast two-dimensional photonic crystal is used to form a micro-resonator that simultaneously provides feedback for laser action and diffracts light vertically from the surface of the semiconductor surface. A top metallic contact allows electrical current injection and provides vertical optical confinement through a bound surface plasmon wave. The miniaturization and tailorable emission properties of this design are potentially important for sensing applications, while electrical pumping can allow new studies of photonic crystal and surface plasmon structures in nonlinear and near-field optics. 相似文献