We report on the transport properties of random networks of single-wall carbon nanotubes fabricated into thin-film transistors. At low nanotube densities ͑ϳ1 m Ϫ2 ͒ the networks are electrically continuous and behave like a p-type semiconductor with a field-effect mobility of ϳ10 cm 2 /V s and a transistor on-to-off ratio ϳ10 5 . At higher densities ͑ϳ10 m Ϫ2 ͒ the field-effect mobility can exceed 100 cm 2 /V s; however, in this case the network behaves like a narrow band gap semiconductor with a high off-state current. The fact that useful device properties are achieved without precision assembly of the nanotubes suggests the random carbon nanotube networks may be a viable material for thin-film transistor applications.Perhaps the most intriguing electronic property of single-wall carbon nanotubes ͑SWNTs͒ is the high roomtemperature mobility of semiconducting SWNTs ͑s-SWNTs͒ that is more than an order of magnitude larger than the mobility of crystalline Si. 1,2 This high mobility has prompted researchers to fabricate and study field-effect transistors in which a single s-SWNT serves as a high-mobility transport channel. [1][2][3][4][5][6][7] Recent measurements on such devices yield a transconductance per unit channel width greater than that of state-of-the-art Si transistors. 7 However, because of the limited current-carrying capacity of individual SWNTs, many s-SWNTs aligned side by side in a single device would be required in order to surpass the current drive of a Si device. Such precise positioning of SWNTs is beyond the capability of current growth and assembly technology and presents a major technological hurdle for carbon nanotube-based electronic applications.In contrast, random arrays of SWNTs are easily produced either by direct growth on a catalyzed substrate or by deposition onto an arbitrary substrate from a solution of suspended SWNTs. If the density of SWNTs in such an array is sufficiently high, the nanotubes will interconnect and form continuous electrical paths. Such random arrays of SWNTs have not previously been seriously investigated for use as channels in field-effect transistors.In this letter we explore the transport properties of random networks of SWNTs and find that low density networks ͑ϳ1 m Ϫ2 ͒ behave like a p-type semiconducting thin film with a field-effect mobility ϳ10 cm 2 /V s, approximately an order of magnitude larger than the mobility of materials typically used in commercial thin-film transistors, e.g., amorphous Si. These mobility values and correspondingly good electronic quality of the random SWNT network are due to a combination of the low resistance of inter-SWNT contacts and the high mobility of the individual SWNTs, which together compensate for the extremely low fill factor of the network. These initial transport results are promising and indicate that such random nanotube networks ͑easily produced with no need for precision assembly͒ form an interest-ing electronic material that has potential for use in thin-filmtransistor applications to produce active electronic...

We report the use of carbon nanotubes as a sensor for chemical nerve agents. Thin-film transistors constructed from random networks of single-walled carbon nanotubes were used to detect dimethyl methylphosphonate (DMMP), a simulant for the nerve agent sarin. These sensors are reversible and capable of detecting DMMP at sub-ppb concentration levels, and they are intrinsically selective against interferent signals from hydrocarbon vapors and humidity. We provide additional chemical specificity by the use of filters coated with chemoselective polymer films. These results indicate that the electronic detection of sub-ppb concentrations of nerve agents and potentially other chemical warfare agents is possible with simple-to-fabricate carbon nanotube devices.

Nanometer-sized metal particles (e.g., gold and silver) are certain to be important fundamental building blocks of future nanoscale electronic and optical devices. However, there are numerous challenges and questions which must be addressed before nanoparticle technologies can be implemented successfully. For example, basic capping ligand chemistrysnanoparticle electronic function relationships must be addressed in greater detail. New methods for assembling nanoparticles together into higher-order arrays with more complex electronic functions are also required. This review highlights our recent progress toward characterizing electron transport in gold nanoparticles as a function of capping ligand charge state. These studies have shown that single electron tunneling energies can be manipulated predictably via pH-induced charge changes of surfacebound thiol capping ligands. We also show that rigid phenylacetylene molecules are useful bridges for assembling gold and silver nanoparticles into arrays of two, three, and four particles with psuedo D ∞h , D 3h , and T d symmetries. These nanoparticle "molecules" interact electromagnetically in a manner qualitatively consistent with dipole coupling models.

A method for synthesizing hollow nanoscopic polypyrrole and poly(N-methylpyrrole) capsules is described. The method employs gold nanoparticles as templates for polymer nucleation and growth. Etching the gold leaves a structurally intact hollow polymer capsule with a shell thickness governed by polymerization time (ca. 5 to >100 nm) and a hollow core diameter dictated by the diameter of the template particle (ca. 5−200 nm). Transport rates of gold etchant through the polymer shell to the gold core were found to depend on the oxidation state of the polymer, those rates being a factor of 3 greater for the reduced form of the polymer. We show for the first time that not only is the particle a useful template material but also that it can be employed to deliver guest molecules into the capsule core. For example, ligands attached to the gold surface prior to poly(N-methylpyrrole) formation remained trapped inside the hollow capsule following polymer formation and gold etching.

We report the development of high-mobility carbon-nanotube thin-film transistors fabricated on a polymeric substrate. The active semiconducting channel in the devices is composed of a random two-dimensional network of single-walled carbon nanotubes (SWNTs). The devices exhibit a field-effect mobility of 150cm2∕Vs and a normalized transconductance of 0.5mS∕mm. The ratio of on-current (Ion) to off-current (Ioff) is ∼100 and is limited by metallic SWNTs in the network. With electronic purification of the SWNTs and improved gate capacitance we project that the transconductance can be increased to ∼10–100mS∕mm with a significantly higher value of Ion∕Ioff, thus approaching crystalline semiconductor-like performance on polymeric substrates.

We report the scaling behavior of 1 / f noise in single-walled carbon nanotube devices. In this study we use two-dimensional carbon nanotube networks to explore the geometric scaling of 1 / f noise and find that for devices of a given resistance the noise scales inversely with device size. We have established an empirical formula that describes this behavior over a wide range of device parameters that can be used to assess the noise characteristics of carbon nanotube-based electronic devices and sensors.

We present local conductance measurements of carbon nanotube networks with nanometer scale resolution and show that there are discrete drops in conductance that correspond to junctions of metallic nanotubes and semiconducting nanotubes. The anomalies of these networks compared to thin films are shown, and a new method of discerning between semiconducting and metallic single-wall carbon nanotubes is demonstrated.

Studies of inorganic clusters continue to reveal fundamental information regarding the size, shape and mediumdependent optical and electronic behaviors of nanoscopic materials.[1] Much of this research has involved characterization of the collective properties of disordered and crystalline two-dimensional (2D) and three-dimensional (3D) arrangements of clusters.[2] Optical absorptions and electron hopping in these crystals of clusters have proven to be strongly dependent on the distance and medium between clusters. These observations have generated interest in nanoclusters on several more applied fronts; e.g., gold cluster chemiresistive sensors and deoxyribonucleic acid assay methods have been reported recently.[3]The fundamental and applied advances described above vis-à-vis extended cluster networks prompted our group, [4] and others, [5] to examine the properties of more discrete assemblies of nanoclusters (e.g., dimers, trimers, etc.) so that the effects of local symmetry on collective particle properties could be better assessed. Herein the assembly of phenylacetylene-bridged gold nanoparticle dimers and trimers from solution is reported (Scheme 1). Phenylacetylene oligomers I and II (PA I, II) were chosen as basic linker repeat units because: 1) they are conformationally rigid molecules which could be expected to keep coupled nanoparticles at a fixed distance, an important difference from the DNA-linked systems reported previously; 2) they can be coupled to form a variety of geometries (e.g., linear, bent, trigonal planar, tetrahedral); [6] 3) lengths of up to 16 repeat units (ca. 20 nm) are readily synthesized without significant solubility problems; and 4) they have been discussed as potential wire candidates for molecular electronic devices. Given these advantages, we anticipated that the successful synthesis of PA-bridged gold nanoparticles would allow particle array symmetry±optical property relationships to be established. Indeed, in our initial optical studies reported herein, we have found, in accord with theoretical predictions, that array symmetry does influence optical properties. Gold nanoparticles were synthesized using the citrate-reduction method reported previously.[7] The strategy for formation of gold nanoparticle dimers and trimers involves producing a locally high concentration of the gold sol with respect to the PA linkers. This was accomplished by the rapid addition of an aqueous gold nanoparticle solution (5 mL of a 0.1 mM solution) to a solution of linker (20 mL of 10 mM solution) in a mixture of CH 3 CH 2 OH/water (1:3). Ethanol was necessary to ensure solubility of the linkers, however, the addition of higher concentrations of ethanol resulted in rapid flocculation of the gold nanoparticles. Phenylacetylene-bridged gold nanoparticle dimer formation was followed in time by visible spectroscopy (Fig. 1).Prior to the addition of PA linker I, the wellknown plasmon of~12 nm diameter gold particles was observed at l max = 525 nm. [7] Following addition of nanoparticles to a slight exce...