We fabricated field-effect transistors based on individual single-and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations-in our case a collapsed tube-can make them operate as field-effect transistors. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00143-0͔Carbon nanotubes ͑NTs͒ are a new form of carbon with unique electrical and mechanical properties.1 They can be considered as the result of folding graphite layers into carbon cylinders and may be composed of a single shell-single wall nanotubes ͑SWNTs͒, or of several shells-multi-wall nanotubes ͑MWNTs͒. Depending on the folding angle and the diameter, nanotubes can be metallic or semiconducting. Simple theory also shows that the band gap of semiconducting NTs decreases with increasing diameter. These predictions have been verified in recent scanning tunneling spectroscopy experiments. 2,3Their interesting electronic structure makes carbon nanotubes ideal candidates for novel molecular devices. Metallic NTs, for example, were utilized as Coulomb islands in single-electron transistors 4,5 and, very recently, Tans and coworkers built a molecular field-effect transistor ͑FET͒ with a semiconducting nanotube. 6In this letter, we report on the fabrication and performance of a SWNT-based FET and explore whether MWNTs can be utilized as the active element of carbon-based FETs. Despite their large diameter, we find that structurally deformed MWNTs may well be employed in NT-FETs. Based on the output and transfer characteristics of our NT devices, we evaluate their carrier density and discuss the transport mechanism.The SWNTs used in our study were produced by laser ablation of graphite doped with cobalt and nickel catalysts. For cleaning, the SWNTs were ultrasonically treated in an H 2 SO 4 /H 2 O 2 solution. MWNTs were produced by an arcdischarge evaporation technique 8 and used without further treatment. The NTs were dispersed by sonication in dichloroethane and then spread on a substrate with predefined electrodes. A schematic cross section of a NT device is shown in Fig. 1. They consist of either an individual SWNT or MWNT bridging two electrodes deposited on a 140 nm thick gate oxide film on a doped Si wafer, which is used as a back gate. The 30 nm thick Au electrodes were defined using electron beam lithography. For imaging, we used an atomic force microscope operating in the noncontact mode. The sourcedrain current I through the NTs was measured at room temperature as a function of the bias voltage V SD and the gate voltage V G . Figure 2͑a͒ shows the output characteristics I -V SD of a device consisting of a single SWNT with a diameter of 1.6 nm for several values of the gate voltage. At V G ϭ0 V, the I -V SD curv...
Renin angiotensin aldosterone system inhibitors/antagonists/blockers (RAASi) are a cornerstone in treatment of patients with cardiovascular diseases especially in those with heart failure (HF) due to their proven effect on surrogate and hard endpoints. Renin angiotensin aldosterone system inhibitors are also the basis in treatment of arterial hypertension, and they are furthermore indicated to reduce events and target organ damage in patients with diabetes and chronic kidney disease, where they have specific indication because of the evidence of benefit. Renin angiotensin aldosterone system inhibitor therapy, however, is associated with an increased risk of hyperkalaemia. Patients with chronic kidney disease and HF are at increased risk of hyperkalaemia and ∼50% of these patients experience two or more yearly recurrences. A substantial proportion of patients receiving RAASi therapy have their therapy down-titrated or more often discontinued even after a single episode of elevated potassium (K+) level. Since RAASi therapy reduces mortality and morbidity in patients with cardiovascular disease steps should, when hyperkalaemia develops, be considered to lower K+ level and enable patients to continue their RAASi therapy. The use of such measures are especially important in those patients with the most to gain from RAASi therapy.
Palladium-catalyzed decarboxylative asymmetric allylic alkylation (DAAA) of allyl enol carbonates as a highly chemo-, regio-and enantio-selective process for the synthesis of ketones bearing either a quaternary or a tertiary α-stereogenic center has been investigated in detail. Chiral ligand L4 was found to be optimal in the DAAA of a broad scope of cyclic and acyclic ketones including simple aliphatic ketones with more than one enolizable proton. The allyl moiety of the carbonates has been extended to a variety of cyclic or acyclic di-substituted allyl groups. Our mechanistic studies reveal that, similar to the direct allylation of lithium enolates, the DAAA reaction proceeds through an "outer sphere" S N 2 type of attack on the π-allylpalladium complex by the enolate. An important difference between the DAAA reaction and the direct allylation of lithium enolates is that in the DAAA reaction, the nucleophile and the electrophile were generated simultaneously. Since the π-allylpalladium cation must serve as the counterion for the enolate, the enolate probably exists as a tight-ion-pair. This largely prevents the common side reactions of enolates associated with the equilibrium between different enolates. The much milder reaction conditions as well as the much broader substrate scope also represent the advantages of the DAAA reaction over the direct allylation of preformed metal enolates.
Carbon nanotubes are novel materials with unique electrical and mechanical properties. Here we present results on their atomic structure and mechanical properties in the adsorbed state, on ways to manipulate individual nanotubes, on their electrical properties and, finally, on the fabrication and characteristics of nanotube-based electron devices. Specifically, Ž . atomic force microscopy AFM and molecular mechanics simulations are used to investigate the effects of van der Waals interactions on the atomic structure of adsorbed nanotubes. Both radial and axial structural deformations are identified and the interaction energy itself is obtained from the observed deformations. The conditions under which the structure of a nanotube will adjust to the topography of the substrate are defined. We show that the strong substrate-nanotube interaction allows the manipulation of both the position and shape of individual nanotubes at inert surfaces using the AFM. AFM manipulation is then utilized to position individual nanotubes on electrical pads so that their electrical characteristics can be evaluated. We demonstrate the operation of a field-effect transistor based on a single semiconducting nanotube and of a single-electron transistor using a nanotube bundle as Coulomb island. Finally, conducting nanotubes are employed as tips for AFM lithography. q
This Highlight covers the current status of relatively unexplored sp 3 -sp 3 cross-coupling reactions with particular focus on natural product and related syntheses.
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