Tech Briefs

Applications of 3D network-structured carbon nanotubes include solar cells, functional filters, and sensors in microfluidic devices.

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.