We have used a number of methods to grow long aligned single-walled carbon nanotubes. Geometries include individual long tubes, dense parallel arrays, and long freely suspended nanotubes. We have fabricated a variety of devices for applications such as multiprobe resistance measurement and high-current field effect transistors. In addition, we have measured conductance of single-walled semiconducting carbon nanotubes in field-effect transistor geometry and investigated the device response to water and alcoholic vapors. We observe significant changes in FET drain current when the device is exposed to various kinds of different solvent. These responses are reversible and reproducible over many cycles of vapor exposure. Our experiments demonstrate that carbon nanotube FETs are sensitive to a wide range of solvent vapors at concentrations in the ppm range.
We have developed chemical-based methods to produce binary assemblies of nanocrystals. The ordered arrays that result are superlattices that mimic the structures of known crystal phases. Applications of this new type of material extends into the realm of optical science and technology. The model is of single component nanocrystals in the 5-20 nm range, which build multicomponent structures of micrometer dimensions. The method presents the opportunity to choose from a variety of inorganic nanocrystals (e.g. semiconducting, magnetic) in order to prepare superlattices with uniquely tunable properties. Transition metal and transition metal oxide nanocrystals are nanometer dimension crystals composed of one or more metals from the d block of the periodic table, and oxygen. The nanocrystals have capping groups which render them discrete, stable, and enable them to be manipulated in a variety of media such as solvents or polymers. The nanocrystals are ideally monodisperse, uniform in composition, crystalline, and can be prepared over a range of sizes from 5-20 nm. The selection of composition for the nanocrystals is based on materials with known interesting properties (optical, electronic or electrical) in the bulk phase. Once fully characterized, the nanocrystals can be considered as components for the assembly of a nanostructured composite material designed to exhibit interesting collective properties with tunable control at the nanoscale.
The relaxation dynamics of Cu2O and Cu1.8S quantum dots (QDs) are compared via time-resolved femtosecond pump probe experiments. It is found that Cu2O shows extremely long-lived excited states on the microsecond time scale and Cu1.8S exhibits much shorter lifetimes in the picosecond time regime. While copper sulfide systems are described in the literature as p-type direct band gap materials, the Cu2O system is direct band gap, however it has a forbidden lowest-energy state. These differences are expressed in the different lifetimes displayed in the time-resolved femtosecond and nanosecond measurements. Moreover, it is confirmed by photoluminescence spectroscopy that reveals that only the Cu1.8S QDs show efficient PL and the Cu2O QDs do not luminescence. In all of the systems, carrier trapping is probably the lifetime limiting process for the conduction band edge depopulation.