Single-wall carbon nanotubes (SWNTs) are the most attractive material in the fields of nanoscience and nanotechnology due to their chemical, physical, and electrical properties. Carbon nanotubes (CNTs) are being applied in various kinds of nano devices such as transistors, electrodes, sensors, and filters because of the semiconducting, metallic, optical, and structural properties. To date, CNTs are mainly synthesized, grown, and dispersed on a two-dimensional substrate. However, it is not easy to fabricate and manipulate CNTs on a particularly structured substrate for various applications. In the case of CNTs on planar substrates, it is hard to get high conductivity and sensitivity because of physical disconnection and low surface areas due to randomly oriented two-dimensional structure.

(a) SEM image of SWNT-3DNs coated with Al2O3 without UV-O3 pretreatment; and (b) SEM image of SWNT-3DNs coated with Al2O3 with UV-O3 pretreatment. It exhibits continuous Al2O3 ALD coating. The measured thickness of deposited Al2O3 was 10 nm. A continuous and uniform coating of metal oxide on SWNTs cannot be achieved without UV-O3 pretreatment.
Three-dimensionally networked structured CNTs (3DNs) with enlarging surface areas on a pre-patterned substrate are ideal for device and sensor applications, but they need enhanced conductivity and sensitivity. The 3D networkstructured CNTs can be used for mechanical filtration of submicron components by controlling the size of network mesh while enhancing mechanical strength.

Synthesis of SWNT-3DNs was performed using PE-CVD equipment, and functionalization of the surface of SWNTs was also performed. A coaxial coating technique was introduced for improving the physical hardness and surface functionality of SWNT-3DNs. The SWNT-3DNs are easily bundled and collapsed during the wetting and drying process because the capillary forces of the solution drew the suspended SWNT channels closer together as the solution dried and evaporated.

Atomic layer deposition (ALD) appears to be one of the most versatile techniques for the well-controlled deposition of thin films on complex-shaped supports. ALD permits a precise control of the thickness of the deposited films at the subnanometer level while preserving their high homogeneity and conformality independent of the complexity of the substrate. However, the coating of CNTs with metal oxides of a well-defined and controllable thickness was not yet achieved. CNTs can be homogeneously coated on the outer and inner surfaces with a nanometric thick film of aluminum oxide to prevent collapsing of SWNT networks.

The Al2O3 thin films were deposited onto the SWNT-3DNs substrates using [Al(CH3)3] and H2O as ALD precursors. The Argon served as both a carrier and a purging gas. The trimethyl aluminum (TMA) and water were evaporated at 20 ºC. The cycle consisted of 1 s exposure to TMA, 5 s Ar purge, 1 s exposure to water, and 5 s Ar purge. The total flow rate of the Ar was 50 sccm. The Al2O3 thin films were grown at temperatures of 150 to 200 ºC under a pressure of 300 mTorr.

The Al2O3 coating is uniform along the whole surface of the CNTs and shows approximately the same thickness of Al2O3 on the outer surface. Metal-oxidecoated SWNT-3DNs endured the capillary force, because the rigidity of the metal oxide layer was increased from the untreated carbon nanotubes, so the SWNT-3DNs could stand without collapsing in solvent flow.

The functionalization of SWNTs with a UV-O3 pretreatment reduces a probability of any nucleation inhibition. These results show that a nitro functional group specifically facilitates the reaction with the gas-phase ALD precursor molecules. This type of reaction is common in organometallic chemistry, and illustrates the significance of an appropriate ligand selection for functionalizing SWNTs for ALD coating purposes. In order to achieve continuous and uniform coating, the SWNT-3DNs should be pretreated by UV-O3.

Unlike a thermal CVD system, a plasma- enhanced CVD (PECVD) system uses plasma to decompose a hydrocarbon gas such as C2H2, CH4, and C2H4. Thus, the synthesis of CNTs at low temperature is possible compared to the conventional thermal CVD method. But CNTs synthesized by PECVD have many defects and are short in size compared to those prepared by a conventional CVD system due to a chemical etching effect of the plasma ions. In order to fabricate SWNT-3DNs using PECVD, the plasma etching effect should be decreased or avoided by certain treatments, because one or two graphene sheets of CNTs are easily broken by accelerated ions during the PECVD process.

The structure of CNTs synthesized by the PECVD process was different from those prepared by the thermal CVD process. For the best network structure, the synthesis conditions are needed to be optimized. The number of CNT walls depends on the catalyst particle size. The size of catalyst particles is related to substrate temperature, annealing time, and pretreatment plasma power during the PECVD process. Both annealing and pretreatment steps were conducted simultaneously to control the catalyst size effectively, unlike a conventional CVD process. The size of catalyst particles decreases by plasma etching effect. As a result, the CNT network was formed as prepared in the thermal CVD system.

This work was done by Haiwon Lee of Hanyang University for the Air Force Asian Office of Aerospace Research and Development. AFRL-0198