Visualization and manipulation of carbon nanotubes under an optical microscope
(Nanowerk Spotlight) Direct visualization and manipulation of individual carbon nanotubes (CNTs) in ambient conditions is of great significance for their characterizations and applications. However, the direct visualization, location, and manipulation of individual CNTs is extremely difficult due to their nanoscale diameters. The observation of individual CNTs usually requires electron microscopes under high vacuum. Advanced analytical equipment, such as scanning electron microscope (SEM), transmission electron microscope (TEM), and atomic force microscope (AFM), etc., have been employed to characterize the location, morphology, and structure of CNTs.
However, all the above mentioned techniques cannot guarantee the efficient location and manipulation of ultralong CNTs, due to their characteristics such as limited accessibility, requiring high vacuum, narrow field of view, and small operating space.
A research group in China, led by Prof. Fei Wei in the Department of Chemical Engineering of Tsinghua University, have proposed a facile way to realize optical visualization of individual carbon nanotubes and, based on that, macroscale manipulation of individual carbon nanotubes that could be carried out under an optical microscope.
"Optical microscopes are routine facilities in many laboratories," says Yingying Zhang, an Associate Professor at Tsinghua University's Center for Nano and Micro Electronics. "Combined with the open operation space of optical microscopes, if individual CNTs can be directly observed with an optical microscope, the in situ observation and manipulation of ultralong CNTs will become much easier."
Compared with other methods proposed before, the optical visualization method proposed here has various advantages. If individual CNTs can be directly observed with an optical microscope, the in situ observation and manipulation of ultralong CNTs will become much easier. Since this process is very easy to perform without any special facilities, this will allow many more laboratories to conduct work in this field.
Deposition of TiO2 nanoparticles on ultralong CNTs. (a–d) Schematic illustration for depositing TiO2 nanoparticles on suspended CNTs. (e) TEM images of CNTs with small TiO2 nanoparticles prepared with contact time of 0.1–0.3 s. Scale bar, 200 nm. (f) TEM image of CNT with TiO2 nanoparticles with various sizes prepared with contact time of 7.0 s. Scale bar, 500 nm. Inset: high-resolution TEM image of the CNT shown in panel f. Scale bar, 5 nm. (g,h) SEM images of suspended CNTs decorated with TiO2 nanoparticles prepared with contact time of 5.0 s. Scale bar, 10 mm (in g); 2 mm (in h). (Reprinted with permission from Nature Publishing Group)
"The strong scattering of TiO2 nanoparticles to visible light enables the CNTs to be easily observed under optical microscopes even at low magnification, which allows the direct manipulation of individual CNTs, even ultralong ones with length up to centimeters, under optical microscopes,"Zhang explains to Nanowerk. "Based on that, we were able to demonstrate various manipulations of individual ultralong CNTs under an optical microscope."
This method does not need any special facilities. The deposited TiO2 nanoparticles do not change the pristine properties of carbon nanotubes.
Optical images of suspended CNTs. (a,c) Bright field modes. The red arrow in panel a shows a broken CNT falling on the substrate. Scale bar, 100 mm (in a); 20 mm (in c). (b) Dark field mode. Scale bar, 100 mm. (d,e) Optical images with low magnification (50). Scale bar, 1 mm. The light source in panel d is green laser, whereas the light source in panel e is visible light. (f) High-resolution optical image of a suspended CNT decorated with TiO2 nanoparticles. The small TiO2 nanoparticles indicated by the red arrows have diameters o200 nm. Scale bar, 1 mm. (Reprinted with permission from Nature Publishing Group)
Zhang notes that the main problem during this work was the choice of a suitable mediator to realize effective identification of individual carbon nanotubes.
"After a series of trials, we finally found that TiCl4 is a very good candidate to realize this goal. As we know, TiCl4 is a highly volatile metal halide, which forms a misty fog of TiO2 and HCl upon contact with humid air. When TiCl4 vapor gets into contact with suspended CNTs, TiO2 nanoparticles nucleate on the CNTs. By controlling the contact time of TiCl4 and carbon nanotube and the air humidity, we can effectively control the size and number distribution of TiO2 nanotubes on carbon nanotubes."
With this optical visualization method, the researchers developed various macroscale manipulation methods for individual CNTs, which greatly facilitates their characterization and study (in a recent Nanowerk Spotlight we reported on the group's work with ultralong CNTs: "Researchers grow half-meter long carbon nanotubes").
One issue that remains to be dealt with is the removal of the TiO2 nanoparticles from the carbon nanotubes. Although the team developed a method – by acid- or base-washing – this process may result in defects to the structure of the carbon nanotubes. They still need to come up with a technique that doesn't influence on the structures or properties of the CNTs.