Electron scattering rates in metallic single-walled carbon nanotubes are studied using an atomic force microscope as an electrical probe. From the scaling of the resistance of the same nanotube with length in the low and high bias regimes, the mean free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic phonon scattering at low biases and zone boundary/optical phonon scattering at high biases.

Semiconducting single-walled carbon nanotubes are studied in the diffusive transport regime. The peak mobility is found to scale with the square of the nanotube diameter and inversely with temperature. The maximum conductance, corrected for the contacts, is linear in the diameter and inverse temperature. These results are in good agreement with theoretical predictions for acoustic phonon scattering in combination with the unusual band structure of nanotubes. These measurements set the upper bound for the performance of nanotube transistors operating in the diffusive regime.

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a bandgap in a metallic NT and modify the bandgap in a semiconducting NT. Theoretical work predicts that bandgap changes can range between ± 100 meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range. PACS numbers: 62.25.+g, 71.20.Tx, 73.63.Fg, 81.07.De, 85.35.Kt The electronic and mechanical properties of carbon NTs make them interesting for both technological applications and basic science. A NT can be either metallic or semiconducting depending on the orientation between the atomic lattice and the tube axis [1,2]. NTs can accommodate very large mechanical strains [3] and have an extremely high Young's modulus [4]. Both theory and experiment indicate that NTs also have interesting electromechanical properties [5][6][7][8][9][10][11][12]. A pioneering experiment [10] showed that the conductance of a metallic NT could decrease by orders of magnitude when strained by an atomic force microscope (AFM) tip. The authors suggest that a local distortion of the sp 2 bonding where the NT is touched by the AFM tip causes the drop in conductance. In Ref.[12], however, it is argued that the observed drop in conductance is due to a bandgap induced in the NT as it is axially stretched [5,8,11] as illustrated in Fig. 1(a). Evidence for the effect of strain on NT bandgap also comes from recent STM work on semiconducting NTs containing encapsulated metallofullerenes [13]. The authors found a bandgap reduction of 60% at the expected positions of the metallofullerenes and postulated that strain could account for this change.Here we present measurements to demonstrate conclusively that strain modulates the band structure of NTs. We employ an AFM tip to simultaneously vary the NT strain and to electrostatically gate the tube. We find that, under strain, the conductance of the NT can increase or decrease, depending on the tube. By using the tip as a gate, we show that this is related to the increase or decrease in the bandgap of a NT under strain. The magnitude of the effect and its dependence on strain are consistent with theoretical expectations.The samples consist of NTs suspended over a trench and clamped at both ends by electrical contacts [10,[14][15][16][17]. CVD growth is utilized to grow NTs with diameters between 1 and 10 nm at lithographically defined catalyst sites [18] on a Si substrate with a 500nm oxide. Metal contacts (5nm Cr and 50-80nm gold) are made using photolithography, as described previously [19]. An ashing step (400°C for 10 minutes in Ar atmosphere) removes photoresist residue and improves contact resistances. An HF etch (3 minutes in 6:1 BHF, etch rate 80 nm/min) followed by critical point drying is used to suspend the NTs [16]. Device conductances are not changed significantly by the etching/drying p...

Although aberrant protein aggregation has been conclusively linked to dozens of devastating amyloid diseases, scientists remain puzzled about the molecular features that render amyloid fibrils or small oligomers toxic. Here, we report a previously unobserved type of amyloid fibril that tests as cytotoxic: one in which the strands of the contributing β-sheets are out of register. In all amyloid fibrils previously characterized at the molecular level, only inregister β-sheets have been observed, in which each strand makes its full complement of hydrogen bonds with the strands above and below it in the fibril. In out-of-register sheets, strands are sheared relative to one another, leaving dangling hydrogen bonds. Based on this finding, we designed out-of-register β-sheet amyloid mimics, which form both cylindrin-like oligomers and fibrils, and these mimics are cytotoxic. Structural and energetic considerations suggest that out-of-register fibrils can readily convert to toxic cylindrins. We propose that out-of-register β-sheets and their related cylindrins are part of a toxic amyloid pathway, which is distinct from the more energetically favored in-register amyloid pathway.X-ray crystallography | BAMs I n contrast to infectious and metabolic disorders, for which researchers can usually uncover the causative entity and the pathway to disease, amyloid diseases have challenged scientists to identify the etiologic agents and the initial pathological events (1-3). Part of the difficulty is that pathways of protein aggregation are diverse, leading to multiple species differing in size, structure, lifetime, and cytotoxicity (4-8). As the most-studied aggregation species, amyloid fibrils have long been associated with devastating human pathologies, including Alzheimer's disease, type II diabetes, and prion disease (2). However, other proteins form amyloid-like aggregates with normal biological functions (9). Studies by NMR, EPR, X-ray diffraction, and scanning mutagenesis have shown that both deleterious and functional amyloid fibrils are made up of extended β-strands running perpendicular to the fibril axis and hydrogen bonded into β-sheets (10-14). The sheets are normally paired into steric zippers, and most often, the strands run parallel to each other and are strictly in-register (10,11,(15)(16)(17). However, in some cases, the strands are antiparallel (18), and some antiparallel fibrils have been found to be more cytotoxic than parallel counterparts (19). Studies of prion (20), HypF-N (21), and Aβ 1-40 (22) indicate that different aggregate morphologies have different levels of cytotoxicity. Therefore, it is important to investigate the various amyloid fibrils from different aggregation pathways, especially those fibrils related to toxic amyloid pathogenesis.Complicating the picture is the variety of oligomers found apparently on the pathways to fibrils, which are more toxic than the fibrils (2, 23, 24). Amyloid oligomers with distinct structural features exhibit different cytotoxicity. A diversity of structural models...

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Quantum chemical molecular modeling has become a standard tool in organometallic chemistry. In particular, density functional theory calculations are now indispensable for investigating the mechanism of even complex reactions and deliver precise energies of intermediates and transition states. Because software packages have become user-friendly and are widely available, even nonexperts can now produce highquality computer models. In this tutorial, we highlight nontrivial mistakes, misconceptions, and misinterpretations often encountered when producing models of a chemical reaction that can lead to wrong conclusions. The reasons for these errors are conceptually explained in simple terms, and remedies are offered.

This communication describes the first preparation of highly ordered large-pore hexagonal periodic mesoporous organosilicas (PMOs) using triblock copolymer P123 as the template and inorganic salts as the auxiliary agents under strongly acidic conditions. The long-range structural order of the PMOs results from the enhanced self-assembly interaction imparted by inorganic salts.

Single-walled carbon nanotubes (SWNTs) are emerging as an important new class of electronic materials. Both metallic and semiconducting SWNTs have electrical properties that rival or exceed the best metals or semiconductors known. In this article, we review recent transport and scanning probe experiments that investigate the electrical properties of SWNTs.We address the fundamental scattering mechanisms in SWNTs, both in linear response and at high bias.We also discuss the nature and properties of contacts to SWNTs. Finally, we discuss device performance issues and potential applications in electronics and sensing.