The optical properties of periodic and nonperiodic arrays of aligned multiwalled carbon nanotubes are presented.
Experimental analysis indicates a complex optical response that is attributed to both the individual carbon nanotube
scatterers and also to the array ensembles. These studies indicate that by controlling the geometry and spacing of the
arrays, it is possible to create structures that respond very strongly to specific wavelengths or bands of wavelengths.
Structures such as these may form the basis for numerous applications in energy conversion. This would include highly
efficient solar energy conversion as well as sensitive, finely tuned detectors that can respond to predetermined
wavelength bands ranging from the ultraviolet to the infrared region. Experimental, theoretical and modeled results are
discussed as they apply to these applications. Challenges and issues are discussed.
We have studied growth of carbon nanotubes by chemical vapor deposition and zinc oxide nanowires by physical vapor deposition on carbon cloth with iron sulfate or stainless steel as a catalyst, and various combinations of carbon source and carrier gases. The field emission of these nanostructures shows a combined result of field enhancement from carbon nanotubes or Zinc oxide nanowires and carbon cloth. An emission current density of 1 mA/cm<sup>2</sup> was obtained at 0.4 V/μm and 0.7 V/μm for nanotubes and nanowires, respectively. Field enhancement factor of ~4x10<sup>4</sup> has been observed. Moreover, electron transport characteristics and structural studies of carbon nanotubes have been investigated. Microscopic observations of electric wall-by-wall breakdown imply that transport in the nanotubes is not ballistic and that a significant scattering occurs as carriers traverse nanotubes length.
Large-scale, two-dimensional arrays of periodic particles were prepared by nanosphere lithography. We modified the fabrication technique based on a self-assembly of latex particles on water surface in order to improve mask quality and size. Modifications of particles arrangement in an array were also practicable by using double-layered masks and mask transfer method. Such particle arrays were used for catalytic growth of aligned carbon nanotubes and ZnO nanorods with various configurations, length, and diameter. These exhibit interesting phenomena - antenna effects, photonic bandgap behavior, subwavelength lensing, and enhanced field emission. Therefore, they can be used in variety of future optoelectronic devices, such as THz and IR detectors.
Spectroscopic observations are presented for carbon nanotubes grown on silicon and quartz substrates in a hexagonal honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition method. A white light source is used as an incident beam and light reflected from the surface of the carbon nanotubes results in a distinctive signature in the reflected spectrum. A comparison of non-periodic arrays and periodic arrays of
carbon nanotubes show that the reflectance signature is only observed when the carbon nanotubes are oriented in a periodic array. Further observations regarding the light antenna effect observed in nonperiodic arrays are also reported. Theoretical curves show good agreement to experimentally observed phenomena. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicate that these materials may find many uses in the field of optoelectronics.