1. ? Templated Growth of Carbon Nanotube Arrays
Using Anodic Porous Alumina with and without Catalyst
Niranjan Patra1,2, Marco Salerno2, Vladimir A. Basiuk3, Alberto Diaspro2
1University of Genova, viale Causa 13, I‑16145 Genova, Italy
2Italian Institute of Technology, via Morego 30, I‑16163 Genova, Italy
3Instituto de Ciencias Nucleares, UNAM, Circuito Exterior, C.U. Mexico
Background
Thanks to their exceptional electronic and mechanical properties, together with their nanoscale diameter and hollowness, carbon nanotubes (CNTs) are one of the candidate building blocks for molecular electronics. Future integration with conventional microelectronics requires that CNTs are grown in ordered arrays or located at defined positions, such as pads of predeposited catalyst or partially exposed nanoscale template areas. Anodic porous alumina (APA) has already demonstrated to help in fabricating nanometer-sized structures (e.g. polymer pillars by replica molding, magnetic metal nanowires by in-situ growth). Therefore, APA appears as a good candidate for controlled templated growth of long-range spatially arranged CNTs.
Methods
APA was potentiostatically deposited on previously electropolished aluminum, at 40 V in a one step anodization process in oxalic acid at 7oC. The resulting pore diameter was typically 50 to 70 nm.
Multi-walled CNTs were grown on a solid substrate from previously drop casted Ni or binary Ni-Co catalyst particles, by chemical vapor deposition at 650oC for 60 min, using Ar bubbling through ethanol as the carbon source. The resulting CNT diameter was typically 30 to 50 nm.
Preliminary CNTs test growth was carried out on flat silicon wafer substrates. Tapping mode AFM topography images acquired in air for both nanomaterials are presented hereby.
Catalyst eventually deposited on the pore bottom is expected to give rise to CNTs growning aligned along the pores and vertically standing outside the APA
CNTs grown ontop of the APA may lay or stand, grow at pore mouth or at wall crossing points, be oriented randomly or according to a given (unknown) pattern
APA-directed synthesis of CNTs
Using APA as a CNT template can take advantage of either the cylindrical pore vessels (3D in-depth structure) or the top pattern only (2D structure). In case of APA substrate, catalyst nanoparticles, when used, were deposited on the samples by simple impregnation with an ethanol solution containing the nanoparticles (5 min dipping at RT).
CNTs formed with Ni and Ni-Co catalyst
Most CNTs were grown ontop of the APA and only few of them appeared along the inner pore walls.
Raman spectroscopy showed that the Ni made CNT sample had much higher amorphous carbon content as compared to Ni-Co made sample, irrespective of the APA configuration. Obviously the quality of CNTs depends on catalyst type and particle size. ?
CNTs formed without catalyst
CNTs growth was also tried without the use of catalyst particles. It is expected that the regular nanoscale surface texture of APA plays a catalytic role in decomposition of the carbon containing gas, replacing the seed metal particles.
Unexpectedly, higher quality CNTs were obtained, as shown by the G band being approximately as intense as the D band in the Raman spectra.
Furthermore, CNTs were straight and pinned at the top into upside-down bunches, shaped like the downward oriented end bundle of a cut rope. This provides with an interesting surface morphology on the mesoscale (bunch pinning points period 30 pore cell diameter). SEM of material cross-section after fracture showed no appearance of CNTs along the inner pore walls, stressing the role of the APA surface pattern alone. The exact origin of the ‘bunch’ structure is not understood and is currently under investigation.
Conclusions
Carbon nanotubes were obtained by pyrolysis of Ar-bubbled ethanol at 650 °C on APA templates, with and without metal catalyst particles.
In both cases CNTs showed preferential growth on the top APA surface.
Catalyst grown CNTs presented highly dense yet completely disordered structure, with highly curved CNTs and high content of amorphous carbon.
CNTs grown without catalyst were high purity, and presented a ‘bunch’ arrangement that was never obtained previously to our knowledge.
The 3D CNT ‘bunch’ morphology can be of interest for several applications:
gas sensors, with the CNTs bunches working as independent and possibly individually addressable micro-nano electrodes;
integration in biomedical applications such as biosensors or active implants;
scaffolding for living cell adhesion and growth, thanks to the biologically inertness of APA;
as a material in layered radar absorbing devices to disguise a vehicle from radar detection, or in microwave shielding devices.
Further studies will investigate the effect of APA structural parameters (pore diameter, cell diameter, and regular hexagonal arrangement in two-step anodized APA) on the resulting parameters of the CNTs bunches (pinning point spacing, vertical corner angle, CNTs length).
References
Michaël Daenen et al., “The wondrous world of Carbon Nanotubes”, Eindhoven Technical University, CNT technologies review commited by Philips, free download (2003).
YeoHeung Yun et al., “Nanotube electrodes and biosensors”, Nanotoday 2, 30 (2007),.
Shoso Shingubara, “Fabrication of nanomaterials using porous alumina templates”, Journal of Nanoparticle Research 5, 17 (2003).
Y.C. Sui et al.,“Structure, Thermal Stability and Deformation of Multibranched CNTS Synthesized by CVD in AAO Template”, Journal of Physical Chemistry B 105, 1523 (2001).
Yu Guojun et al., “Synthesis of carbon nanotube arrays using ethanol in porous anodic aluminum oxide template”, Chinese Science Bulletin 50, 1097 (2005 ).
Jaya Sarkar et al., “Nanowires: properties, applications and synthesis via porous anodic aluminium oxide template”, Bulletin of Material Science 30, 271 (2007).