Controlling nanotube orientation in 3D architectures
(Nanowerk Spotlight) Since their discovery in 1991, carbon nanotubes (CNTs) have generated much excitement in a variety of fields, due to their ultrahigh aspect ratios, which can lead to extraordinary electronic and photonic devices if a macroscopic ensemble of highly aligned CNTs can be made. Generally, variant CNT microstructures determine their properties, for example, highly graphitized CNTs exhibit excellent mechanical and electrical properties; while CNTs with defects and poor crystallinity are beneficial for research on field emission property and hydrogen storage capacity (see for instance: "Nanotube defects equal better energy and storage systems").
Therefore, it is of vital importance to control the CNT microstructures effectively for desired applications.
There have been reports on successful formation of simple structures – fibers and films – in which CNTs are aligned. However, presently, no method exists for controlling the orientation direction of CNTs into more complicated, desired 3D architectures, which would widely expand the application range of CNTs.
A new technique, developed by a group of Japanese scientists, can solve a problem of three-dimensional orientation control of CNTs in microscopic scale. The team developed a technique based on two-photon polymerization (TPP) lithography to fabricate arbitrary 3D structures in which aligned single-walled CNTs (SWCNTs) are embedded.
3D fabrication of SWCNT/polymer composites by TPP lithography. a) Schematic showing 3D microfabrication of SWCNT/polymer composites based on TPP lithography. Femtosecond pulsed laser beam (780 nm) is focused on SWCNT-dispersed photo-resin through an oil-immersion objective lens (100x, NA 1.4). The focus spot is three dimensionally moved relative to the stage, and the composites are created following the trajectory of the focus spot. b,c) SEM images of a 375 nm wide, 10 µm long nanowire, suspended between two microboxes, ca. 7 µm above the substrate. (b) and (c) are top and perspective views, respectively. d) SEM image of a cross-section of the nanowire, showing that SWCNTs are embedded inside. (Reprinted by permission of Wiley-VCH Verlag) (click on image to enlarge)
"This work represents a significant breakthrough laser technique in the fabrication of arbitrary three-dimensional architectures consisting of aligned single-wall carbon nanotubes," Satoru Shoji, an associate professor at The University of Electro-Communications in Tokyo, tells Nanowerk. "We achieved not only the fabrication of 3D structures of CNT/polymer composites, but also the control of the direction of the alignment of CNTs in the structures."
This work opens up possibilities of making devices and systems consisting of macroscopic ensembles of aligned SWCNTs whose extremely anisotropic properties approach those of individual SWCNTs.
To make their SWCNT/polymer composites, the researchers loaded a 0.01 wt% solution of SWCNTs into an UV-curable monomer. Then they carried out TPP lithography on the SWCNT-dispersed photo-resin, and SWCNTs were simultaneously fixed in tiny polymer structures. After the structures were created, unsolidified resin was rinsed way using acetone, and the structures were then dried.
Alignment manipulation of SWCNTs in a single microstructure. a,b) SEM images of a single microstructure that is composed of two sections. The left section is made up of an array of nanowires along x-axis, while the right section is made up of an array of nanowires along y-axis. The coordinate is defined in (c). (a) and (b) are top and perspective views, respectively. c,d) Schematic images showing that the structure is made up of arrays of nanowires in two different orientations. The inset defines the coordinate, and θ is defined as the angle between the polarization of the incident laser beam and the x-axis. (a) and (b) are top and perspective views, respectively. e) Polar-diagram showing the G-band intensity as a function of the angle θ representing that SWCNTs are oriented along the scanning direction. The square and diamond symbols represent the angular dependence in the x-scanning area and y-scanning area, respectively. The excitation wavelength, laser power, and exposure time for the Raman measurements were 785 nm, 3.3 mW, and 7 s, respectively. (Reprinted by permission of Wiley-VCH Verlag) (click on image to enlarge)
Using this technique, 3D structures as well as suspended nanowires – in all of which SWCNTs were embedded – were fabricated with feature resolution far beyond the diffraction limit of light. Using polarized Raman microspectroscopy, the team then investigated and confirmed the orientation direction and degree of the alignment of SWCNTs in the obtained structures.
Shoji points out that this method would provide one of the key technique of building 3D structural devices based on aligned CNTs including photodetectors, polarizers, actuators, and metamaterials: "Our technique provides a versatile and convenient way of controlling the orientation of CNTs in any desired directions into arbitrary 3D nanostructures, which should be of great and immediate interest to researchers working in diverse fields of nanoscience and nanotechnology – including nanoelectronics, nanomaterial synthesis, nanostructures, nanocomposites, and nanolithography."