No abstract

Scanning tunneling microscopy, spectroscopy, and tight-binding calculations have been used to elucidate the unique structural and electronic properties of single-walled carbon nanotubes (SWNTs). First, the unique relationship between SWNT atomic structure and electronic properties, and the richness of structures observed in both purified and chemically etched nanotube samples are discussed. Second, a more detailed picture of SWNT electronic band structure is developed and compared with experimental tunneling spectroscopy measurements. Third, experimental and theoretical investigations of localized structures, such as bends and ends in nanotubes, are presented. Last, quantum size effects in nanotubes with lengths approaching large molecules are discussed. The implications of these studies and important future directions are considered.

A new class of intracellular nanoprobe, termed fluorescence resonance energy transfer (FRET) nanoflares, was developed to sense mRNA in living cells. It consists of a gold nanoparticle (AuNP), recognition sequences, and flares. Briefly, the AuNP functionalized with recognition sequences hybridized to flares, which are designed as hairpin structures and fluorescently labeled donors and acceptors at two ends, respectively. In the absence of targets, the flares are captured by binding with the recognition sequences, separating of the donor and acceptor, and inducing low FRET efficiency. However, in the presence of targets, the flares are gradually displaced from the recognition sequences by the targets, subsequently forming hairpin structures that bring the donor and acceptor into close proximity and result in high FRET efficiency. Compared to the conventional single-dye nanoflares, the upgraded FRET nanoflares can avoid false positive signals by chemical interferences (such as nuclease and GSH) and thermodynamic fluctuations. Moreover, the signal generation in FRET nanoflares can be easily made with ratiometric measurement, minimizing the effect of system fluctuations.

The electronic density of states of atomically resolved single-walled carbon nanotubes have been investigated using scanning tunneling microscopy. Peaks in the density of states due to the onedimensional nanotube band structure have been characterized and compared with the results of tight-binding calculations. In addition, tunneling spectroscopy measurements recorded along the axis of an atomically-resolved nanotube exhibit new, low-energy peaks in the density of states near the tube end. Calculations suggest that these features arise from the specific arrangement of carbon atoms that close the nanotube end.PACS numbers: 71.20.Tx;61.16.Ch;73.20.At The electronic properties of single-walled carbon nanotubes (SWNTs) are currently the focus of considerable interest [1]. According to theory [2][3][4], SWNTs can exhibit either metallic or semiconducting behavior depending on diameter and helicity. Recent scanning tunneling microscopy (STM) studies of SWNT [5,6] have confirmed this predicted behavior, and have reported peaks in the density of states (DOS), Van Hove singularities (VHS), that are believed to reflect the 1D band structure of the SWNTs. A detailed experimental comparison with theory has not been carried out, although such a comparison is critical for advancing our understaning of these fascinating materials. For example, chiral SWNTs have unit cells that can be significantly larger than the cells of achiral SWNTs of similar diameter, and thus chiral tubes may exhibit a larger number of VHS than achiral ones [7]. Recent theoretical work [8,9] suggested, however, that semiconducting (or metallic) SWNTs of similar diameters will have a similar number of VHS near the Fermi level, independent of chiral angle. In addition, the electronic properties of localized SWNT structures, including end caps, junctions and bends [10][11][12][13], which are essential to proposed device applications, have not been characterized experimentally in atomically resolved structures.In this Letter, we report STM investigations of the electronic structure of atomically resolved SWNTs and compare these results with tight-binding calculations. Significantly, we find that the VHS in the DOS calculated using a straight-forward zone-folding approach agree with the major features observed in our experiments. We have observed new peaks in the local DOS (LDOS) at an end of a metallic SWNT and compared these results to calculations. This analysis suggests that the new peaks can be associated with a specific topology required to cap the SWNT. The implications of these results and important unresolved issues are discussed. Experimental procedures and are described elsewhere in detail [6,14]. In brief, SWNT samples were prepared by laser vaporization [15], purified and then deposited onto a Au (111)/mica substrate. Immediately after deposition, the sample was loaded into a UHV STM that was stabilized at 77 K; all of the experimental data reported in this Letter was recorded at 77 K. Imaging and spectroscopy were measured using etched tungst...

China Scholarship Council (CSC); ACS; US NIH; China NSFC[20805038]; National Basic Research Program of China[2007CB935603, 2010CB732402]; China National Grand Program on Key Infectious Disease[2009ZX10004-312]; Key Project of Natural Science Foundation of China[90606003]; International Science & Technology Cooperation Program of China[2010DFB30300]; Hunan Provincial Natural Science Foundation of China[10JJ7002