Session K6: Carbon Nanotubes

Explaining the Purification of Single-Walled Carbon Nanotubes by 248 nanometer UV Light

It has been experimentally observed that amorphous carbon is removed from as-prepared, bulk, single-walled carbon nanotubes by illumination with 248 nm (5 eV) UV light. The process via which this occurs, though, has not yet been rigorously identified. We use a combination of experiments and modeling to explain how localized surface plasmon pairs can be induced at the surfaces of nanotubes. The 248 nm light is near the resonant frequency of one of these plasmon pairs for small diameter nanotubes, and this causes a large electric field enhancement in the vicinity of the tubes. The enhanced field increases the rate at which sp-2 bonds in the amorphous carbon are excited into a state from which the carbon is more easily oxidized. Electromagnetic catalysis by embedded nanoparticles describe these processes. Classical electromagnetics, in conjunction with density functional theory, is used to quantify the field enhancement and the relationship between laser wavelength and nanotube radius which will result in cleaning. Cleaning is defined as the removal of a-C from SWCNT by optical processes. The absorption spectra in the region of 5 eV, for amorphous carbon, is also described by using density functional theory to study the effects of deformation on related molecules such as ethylene and benzene.   

Using Carbon Nanotubes for Nanometer-Scale Energy Transfer Microscopy

We investigate optical energy transfer between fluorophores and carbon nanotubes (CNTs). CNTs are grown on Si-oxide wafers by chemical vapor deposition (CVD), lifted off substrates by atomic force microscope (AFM) tips via Van der Waals forces, then shortened by electrical pulses. The tip-attached CNTs are scanned over fluorescent CdSe-ZnS quantum dots (QDs) with sub-nm precision while recording the fluorescence rate. A novel photon counting technique enables us to produce 3D maps of the QD-CNT coupling, revealing nanoscale lateral and vertical features. All CNTs tested ($>$50) strongly quenched the QD fluorescence, apparently independent of chirality. In some data, a delay in the recovery of QD fluorescence following CNT-QD contact was observed, suggesting possible charge transfer in this system. In the future, we will perform time-resolved studies to quantify the rate of energy and charge transfer processes and study the possible differences in fluorescence quenching and nanotube-QD energy transfer when comparing single-walled (SW) versus multi-walled (MW) CNTs, attempting to grow substrates consisting primarily of SW or MWCNTs and characterizing the structure of tip-attached CNTs using optical spectroscopy.   

Controlled Placement of Carbon Nanotubes using Massively Parallel Indirect Dielectrophoresis

Dielectrophoresis has been used to place nanotubes, nanorods, nanowires, and other nanostructures between surface patterned metal electrodes. This technique can deposit a varying number of structures between each set of electrodes. We have developed a method to control the number of deposited singled-walled carbon nanotubes by tuning the impedance of capacitively-coupled electrodes through parameters such as electrode geometry and AC driving frequency. Controlled placement of nanotubes at high yield is a prerequisite for the use of carbon nanotube devices in modern integrated circuitry.   

Strength and Mechanical Properties of Carbon Nanotube Templated Materials

The type of material a structure is made of limits its kinds and extent of practical applications. Carbon nanotubes have an unusually high strength-to-weight ratio and thus present an exciting material for use in reinforcing the structural integrity of microstructures. However, despite their desirable properties, carbon nanotubes have proved difficult to incorporate in materials as strengthening elements. Our group has developed a method for patterning and infiltrating, or filling, carbon nanotube forests to make structures. This filling of the space between the carbon nanotubes locks the structure together. Carbon infiltration proceeds by flowing an ethylene/argon mixture across a sample at a temperature of 900 C, thus depositing amorphous carbon and creating a carbon/nanotube composite material. Using cantilever structures, we have begun to measure key mechanical properties of this composite material. We are able to determine the maximum applied force that a carbon-infiltrated microstructure can withstand which allows us to calculate mechanical properties, such as the Young's modulus of the composite material.   

Analysis of Silicon Carbide Coated Carbon Nanotubes

The purpose of this research is to explore the use of silicon carbide coated carbon nanotubes in microelectromechanical systems or MEMS. In our research group at Brigham Young University, we are developing a method of MEMS fabrication through the use of carbon nanotube (abbreviated CNT) ``scaffolds.'' Traditional MEMS fabrication techniques are use chemical etching to create three dimensional structures. Our group is seeking to overcome some of the shortcomings of this method by using patterned vertically aligned CNT's filled with bulk materials to create new MEMS devices. This technique allows the creation of MEMS devices with geometries that cannot be created using standard methods. This research focuses on the use of chemical vapor deposition to fill the CNT arrays with silicon carbide, which is a very durable and robust material that could have a wide range of applications in MEMS. We will report on preliminary results of silicon carbide production as determined by electron microscopy and X-ray spectroscopy.   

Structural Optimization of CNTs for MEMS Devices

MicroElectroMechanical Systems (MEMS) fabricated from a carbon nanotube (CNT) scaffolding are extremely versatile devices. They provide a means to create high aspect-ratio structures out of nearly any material, and minimize the required effort. In order for these great properties to be exploited, however, the nanotube framework must be perfected. Many aspects of CNT synthesis have been extensively studied in the past, yet rarely have they been viewed from the standpoint of using them as a basis for filled MEMS devices. Seldom has such absolute structural perfection and replicative fidelity been required of CNT forests. To achieve these lofty requirements, variables such as catalyst thickness, substrate preparation, and CNT synthesis conditions were carefully varied. Measurements were made on many resultant properties, such as sidewall straightness and fabrication robustness. These findings vastly improve the nanotube framework, opening new avenues for research in CNT MEMS.