Dipartimento di Fisica
JOURNAL CLUB
Francesco DE NICOLA
Multi-Functional Carbon Nanotube Films
1Dipartimento di Fisica, Università di Roma Tor Vergata, Italy
Carbon nanotubes are hollow cylindrical nanostructures composed of one graphitic sheet rolled-up (single-walled carbon nanotubes) or more coaxially symmetric (multi-walled carbon nanotubes). Furthermore, due to their high aspect ratio (~105) they can be considered as one-dimensional solids. This exceptional feature confers to carbon nanotubes unique mechanical, thermodynamical, and optoelectronic properties [1]. Particularly, they may exhibit semiconducting or metallic behavior depending on their chirality, i.e. the wrapping direction along the tube axis. Interestingly, also metallic carbon nanotubes are optically active.
Recently, we are realizing single-walled carbon nanotube films prepared by simple, rapid, and inexpensive filtration process from a well-dispersed suspension of highly pure carbon nanotube powder, and then dry transfer printing deposited on a substrate. This recently introduced deposition method allows to easily placing CNT films on several substrates (i.e. glass, silicon, plastics, and metals) at room temperature. The investigation of carbon nanotube films with scanning electron microscopy reveals that, as-realized coatings consist of a multi-fractal random network of carbon nanotubes. In particular, the film physical properties can be tailored in a controlled fashion by varying their thickness ranging from 10-1700 nm. Furthermore, carbon nanotube films are chemically stable and may be semi-transparent (>70%), highly conductive (density of carriers: 1022 electron/cm3) with low sheet resistance (<50 k/sq), and also may have amphiphobic properties with contact angle values from 42° up to 109°. Moreover, owing to the one-dimensional electronic structure of single-walled carbon nanotubes, films made by different chirality introduce a new class of materials highly conductive and optically active broadband in the ultraviolet, visible, and near-infrared ranges. Therefore, single-walled carbon nanotube films could pave the way to a large field of applications for a new generation of optoelectronic devices.
We are also fabricating carbon nanotube/silicon solar cells with photo-conversion efficiency up to 11%. In our CNT/Si cells [2], the top emitter layer is made by a semi-transparent carbon nanotube thin (~50 nm) film deposited on a n-doped silicon wafer. In particular, the carbon nanotube high absorption leads to a substantially cell thickness and weight reduction allowing thin film of crystalline, poly-crystalline, amorphous, organic, or low-dimensional materials to be used. The fabrication of such hetero-junctions is fast, simple, low-cost, lightweight, and made at low-temperature (<1000°C). In this scenario, it is crucial to take into account several parameters in order not only to improve the power conversion efficiency of carbon nanotube/silicon solar cells, but also to enhance the cell spectral response. Actually, the external quantum efficiency of carbon nanotube/silicon devices extends the conventional silicon cell working range in the ultraviolet and near-infrared regions. In addition, we have realized for the first time carbon nanotube/amorphous silicon hybrid solar cells [4] obtaining encouraging results.
More generally, carbon nanotube/silicon hetero-structures are promising devices with photo-conversion efficiency up to 15% [3] comparable to a conventional silicon p-n junction solar cell.
Recently, wettability of carbon nanotubes is a phenomenon of great theoretical and practical interest [5] with several technological applications, for instance to fabricate self-cleaning, anti-fouling, anti-sticking, and anti-fogging surfaces but also for improving self-assembling and adhesion. Therefore, hydrophobic carbon nanotube films with a hierarchical micro-/nano-structured surface roughness have been addressed [7]. Furthermore, in order to explain wetting properties of such composite surfaces, we have suggested the two-dimensional extension of the Wenzel-Cassie-Baxter theoretical model [6]. Moreover, we have demonstrated that by varying the thickness of carbon nanotube films, it is possible control their wetting properties owing to capillary phenomena.
[1]R. Saito et al., Physical Properties of Carbon Nanotubes, Imperial College Press (1998)
[2]S. Del Gobbo, P. Castrucci , F. De Nicola et al., J. Materials Chemistry C (2013). M. A. El Khakani, P. Castrucci, M. De Crescenzi et al., Appl. Phys. Lett. 95, 083114 (2009); V. Le Borgne, P. Castrucci, M. De Crescenzi et al., Appl. Phys. Lett. 97, 193105 (2010); P. Castrucci, M. De Crescenzi et al., Nanotechnology 22, 115701(2011); P. Castrucci et al., Appl. Phys. Lett. 89, 253107 (2006); V. Le Borgne, L. A. Gautier, P. Castrucci, S. Del Gobbo, M. De Crescenzi and M. A. El Khakani, Nanotechnology 23, 215206 (2012).
[3]S. Del Gobbo, P. Castrucci, M. De Crescenzi et al., Appl. Phys. Lett. 98,183113 (2011).
[4]J. Wei et al., Nano Lett. 7, 2317 (2007); Z. Li et al., ACS Nano 3, 1407 (2009); Y. Jia et al., Advanced Materials 20, 4594 (2008), Y. Jia et al., Appl. Phys. Lett. 98, 133115 (2011), Y. Jia et al. Nano Lett. 11, 1901 (2011), E. Shi et al., Scientific Reports 2, 884 (2012).
[5]I. Y.Y. Bu, S. P. Oei, Applied Surface Science 256, 6699 (2010); Z. J. Han et al., Applied Physics Letters 94, 223106 (2009); Y. Li et al. Langmuir 23, 2169 (2007); S.-G. Cho and K.-C. Ko, Thin Solid Films 518, 6619 (2010).
[6]R. N. Wenzel, Industrial and Engineering Chemistry 28, 8 (1936); A. B. D. Cassie and S. Baxter, Transactions of the Faraday Society 40, 546 (1944).
[7]F. De Nicola, P. Castrucci, M. Scarselli, and M. De Crescenzi, Hydrophobic Multi-Fractal Single-Walled Carbon Nanotube Coatings (to be published)
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