Physica E 44 (2012) 1024–1027
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Physica E journal homepage: www.elsevier.com/locate/physe
Oriented carbon nanostructures grown by hot-filament plasma-enhanced CVD from self-assembled Co-based catalyst on Si substrates Claudiu Teodor Fleaca a,n, Ion Morjan a, Alexandrescu Rodica a, Florian Dumitrache a, Iuliana Soare a, Lavinia Gavrila-Florescu a, Ion Sandu a, Elena Dutu a, Franc- ois Le Normand b, Jacques Faerber b a b
Laser Department, NILPRP, 409 Atomistilor Street, P.O. Box-MG-36, Bucharest, Romania Surfaces and Interfaces Group, IPCMS, UMR 7504 CNRS, 23 rue du Loess, 67034 Strasbourg Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2010 Accepted 16 November 2010 Available online 26 November 2010
We report the synthesis of coral- and caterpillar-like carbon nanostructures assemblies starting from cobalt nitrate ethanol solutions deposited by drop-casting onto blank or carbon nanoparticles film covered Si(1 0 0) substrates. The seeded films were pre-treated with glow discharge hydrogen plasma aided by hot-filaments at 550 1C followed by introduction of acetylene at 700 1C. The resultant carbon nanostructure assemblies contain a high density of aligned carbon nanotubes/nanofibers (CNTs/CNFs). The influence of the forces that act during liquid-mediated self-assembly of Co catalyst precursor is discussed. & 2010 Elsevier B.V. All rights reserved.
1. Introduction Oriented carbon nanostructures (nanofibers and nanotubes) have attracted in the last decade increased interest for their utilization in modern devices such as microfabricated field emission sources [1], biosensors [2], pressure sensors [3], actuatable membranes [4], gene delivery arrays [5], microwave amplifiers [6] and nanoelectromechanical switches [7]. The performances of the above mentioned devices strongly depend on the orientation and packing degree, surface dispersion and positional control on solid substrates of CNTs/CNFs [8]. These characteristics themselves depend on the synthesis method and on the catalyst preparation. It is well known that chemical vapor deposition (CVD), as a basic method, can be used to grow CNTs/CNFs. While the thermally activated CVD methods always yield spaghetti-like films or compact assemblies made of wavy individual nanotubes, the plasma activated CVD ones generate individual, self-sustaining and vertical carbon nanostructures [9]. The positional control of the carbon nanostructures is usually realized through the substrate’s patterning by photolithography [8,10] electron-beam lithography [10,11] or focused ion beam [12] followed by catalyst deposition from metal vapor-phase species (by magnetron sputtering [10] or vacuum evaporation [11]) or through liquid-based routes, in which the patterning step is absent and the catalyst (precursor) suspension/solution is deposited onto the substrate: pulsed electrodeposition [13], drop-casting [14], self-assembly in immersion [15] or spin-coating [16].
n
Corresponding author. Tel.: + 4021 4574489; fax: + 4021 4574243. E-mail address: claudiufl
[email protected] (C.T. Fleaca).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.11.017
Exotic assemblies of CNTs/CNFs such as rows of CNTs perpendicularly oriented (but not suspended) on dithiol treated Au/Si substrate can be observed in the Ni-based nanoparticles catalyzed deposit obtained by PECVD in the presence of C2H2 and NH3 [15]; partially suspended carpets of Ni catalyzed vertical CNFs on SiO2/Ti layers were also fabricated by PECVD in combination with oxygen plasma etching of sacrificial cross-linked poly(methylmethacrylate), in order to demonstrate their integration on micromechanical structures [17]. In this work we used drop-casting deposition of the catalyst precursor onto bare Si or carbon nanoparticles pre-deposited film, followed by hot-filament assisted direct current plasma enhanced (HF d.c. PE) treatment and CVD for the growth of exotic assemblies of carbon CNTs/CNFs.
2. Experimental Carbon nanoparticles synthesized by the laser pyrolysis technique from C2H4 and C6H6 vapors in the presence of N2O [18], Si(1 0 0) substrates p-doped at 1–3 O cm (7 5 0.5 mm3), cobalt nitrate hexahydrate (99.999% trace metal basis, Sigma Aldrich), acetone (analytical grade, Carlo Erba), ethanol ( Z99.8%, Carl Roth), acetylene and hydrogen (both 99.9999%, Air Liquide) were used as raw or auxiliary materials for these experiments. The Si substrates were successively cleaned in acetone and ethanol by ultrasonication and then dried. For the first experiment, one droplet (0.1 cm3) of ethanolic solution of cobalt nitrate hexahydrate (0.3 mg/cm3) was drop-casted on Si and dried using a lamp with an incandescent bulb (50 W) positioned 5 mm above the substrate. For the second experiment, a dilute suspension
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(0.04 mg/cm3) was prepared by dispersing carbon nanopowder in acetone under ultrasonication (20 min) in a bath. Unlike ethanol, the acetone dispersant allows better dispersion and more homogeneous deposition of the carbon nanoparticles on the Si surface, due to the lower viscosity and higher volatility. From this suspension, 20 droplets were successively dropped on the Si(1 0 0) substrate and evaporated (using the same lamp’s heat). Finally, 10 droplets of the initial solution of Co salt were also dropped on the pre-deposited (with carbon nanoparticles) Si wafer and dried in the same way. The seeded Si wafers were introduced into a reactor described in Ref. [19] and treated first with H2 (at 550 1C) and then with a H2–C2H2 mixture (at 700 1C) in a hot-filaments assisted d.c. plasma environment [20]. The carbon nanostructures were investigated by Scanning Electron Microscopy (SEM) using a XL30S-FEG Philips microscope (on 451 tilted substrates) and Raman Spectroscopy using a Renishaw Ramascope 1000 micro-spectrometer equipped with an Ar ion laser (l ¼488 nm).
3. Results and discussion
Fig. 2. SEM image of a typical island comprising the deposit grown from Co nitrate solution on Si(1 0 0) substrate.
The deposit resulting from the catalyst precursor deposition on bare Si wafer followed by HF d.c. PE reduction/etching in H2 and HF d.c. PE CVD in H2 and C2H2 can be seen in the SEM image in Fig. 1; the Si surface is rather uniformly patterned with a few microns size round islands or rings containing the catalyzed carbon nanostructures, each of them surrounded by a halo. In Fig. 2, the SEM image reveals the detailed structure of a typical island: short and stubby vertically semi-oriented CNTs/CNFs that form three-dimensional coral-like aggregates at the center and around the perimeter. Other islands are fully occupied by these aggregates. Inside the rest of the islands, the nanostructures appear to have grown perpendicularly and directly on the Si surface. They also seem thinner than those at the center. Each halo is composed of the thinnest vertically oriented nanotubes (30–50 nm in diameter) and they begin to organize in radial rows that surround the islands (Fig. 3). This unusual morphology originates in the complex process occurring along the drying of the catalyst precursor solution. During fast evaporation, the concentration of the ethanolic solution of Co nitrate keeps increasing until saturation is reached. Consequently, nano- and micro-crystals appear and interact with the liquid and solid surface Fig. 3. SEM image of the nanotube carpet grown between the islands from Co nitrate solution deposited on Si(1 0 0).
Fig. 1. Low magnification SEM image of the carbon assemblies grown from Co nitrate solution on Si(1 0 0) surface; the Raman spectrum from one island is presented in the inset.
during the drying process, undergoing a self-assembly process. We assume that bigger crystals aggregate mainly as circular- or ringshaped spots, while the smaller ones are left outside the spots, many of them arranged radially towards the spot’s boundaries. The process of rings formation from suspensions under specific drying conditions was attributed to the ‘‘coffee-ring’’ effect due to the capillary flow [21] or to the surface tension gradients that induce Benard–Marangoni instabilities in the liquid film [22], combined with lateral capillary forces of immersion [23]. After drying and before the introduction of acetylene, the as-deposited Co salt undergoes a transformation sequence during the heating and hot-filament assisted plasma treatment in hydrogen atmosphere. Above 50 1C and under the low pressure conditions, the crystalline Co nitrate solvate quickly loses the solvation molecules (water, ethanol). Then, under hydrogen treatment at a higher temperature the anhydrous Co nitrate is decomposed to Co2O3 and is successively reduced to Co3O4, CoO and finally to metallic Co [24]. During the growth stage at 700 1C in the H2–C2H2 mixture activated by plasma and hot-filaments and under the orienting electric field
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influence, thin nanotubes emerge vertically on Si surface from the few tens of nanometer length catalyst particles. At the same time, from the bigger particles (originating from bigger Co precursor salt crystals and/or by coalescence of smaller aggregated particles at high temperature) short, distorted, stubby and less oriented nanofibers (mainly between 100 and 200 nm diameter, discernible also in the right down corner of the Fig. 3) grow in the form of the corallike structures. The Raman spectrum of an island presented as an inset in Fig. 1 reveals the presence of sharp G and D bands (at 1585 and 1358 cm 1, respectively), which is typical for multi-walled CNTs. The emergence of the shoulder at 1318 cm 1 suggests the presence of carbon nanofibers with a stacked-cone structure [25]. The second experiment involves the deposition of Co nitrate from solution on a carbon nanoparticles pre-deposited film and the same hot-filaments assisted plasma treatments as in the first one. To our knowledge, the obtained morphology (see Figs. 4 and 5) was not reported before. One can observe a few microns sized caterpillar-like structures, many of them semi-suspended on the surface. The high resolution SEM images in Figs. 5 and 6 reveal vertically grown, short and crowded carbon nanotubes. The rest of the flat surface seems to be covered by small groups of nanotubes. Also, from Fig. 3, the tip growth mechanism of the nanotubes can be deduced from the presence of catalytic particles at their tips. The morphology of the nanotubes, which appear to be segmented, suggests a non-uniform growth, while the diminishing diameter towards the tip can be an effect of the hydrogen etching in the hot-filament activated plasma environment during their growth. The presence of structural defects is also reflected in their intense Raman spectrum D band (see the grey curve shown in the inset of Fig. 4), at 1358 cm 1. This Raman spectrum is very similar to that of carbon nanostructures obtained from the first experiment (in the absence of carbon pre-deposited film) and is clearly different from that arising from raw carbon nanoparticles used as substrate, presented for comparison as the upper black curve (the same inset). The D band of the Raman spectrum of the carbon nanopowder is much broader (full width at half maximum FWHM¼168 cm 1) than the corresponding band from the as-grown nanotubes (FWHM¼ 51 cm 1) and enveloped with the G band due to the presence of the additional D3 band. This hidden band, situated at 1518 cm 1, between the D (at 1349 cm 1) and the G bands (at 1588 cm 1), seems to be an indication of the presence of amorphous carbon (organic molecules, fragments or functional groups) [26].
Fig. 5. SEM image of semi-suspended carbon structures grown from Co nitrate solution on carbon nanoparticle layer pre-deposited Si surface.
Fig. 6. High magnification SEM image of a suspended structure from Co nitrate solution on carbon nanoparticle layer pre-deposited Si surface.
Fig. 4. Low magnification SEM image of carbon assemblies grown from Co nitrate solution on carbon nanoparticle layer pre-deposited Si surface; the superposed Raman spectra of the raw carbon nanoparticles (up) and of a nanostructure present on this image (down).
The difference between the two resultant deposits can be attributed to the carbon nanoparticle pre-deposited layer and also to the multi-droplets successive deposition/drying technique used in the second experiment. The drying of the ethanolic Co solution salt is different on the covered carbon nanoparticles than on the bare Si wafer, due to the much higher specific surface, which leads to an enhanced role of the lateral capillary forces in the final stage of solvent evaporation [26]. The deposited nanocarbon aggregates play in this case the same role as played by the submicron patterns made on the resist layer on SiO2/Si substrates [27]. Moreover, even if the carbon nanoparticles layer appears to be macroscopically homogeneous, at the micron and the submicron levels the situation is different due to the existence of disordered aggregates arranged as networks [25]. Also, the thickness of this layer can fluctuate due to the multi-droplet deposition, which could superpose the aggregates. The surface functionalization of the carbon nanoparticles should be considered also from the point of view of the interaction with the cobalt ions in the solution. Since they were obtained in the presence of controlled number of reactive oxygen atoms (from N2O decomposition) by the laser pyrolysis technique, these
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nanoparticles have a fullerene-like structure [18] and a surface reach in oxygenated functional groups [28], such as COOH. This surface is able to promote cobalt adsorption even in the smallest pores, enhancing the carbon–cobalt interaction in high temperature processes that follow. The resultant carbon–cobalt nitrate composite is exposed to a complex process of etching and reduction in the presence of the reactive hydrogen atoms generated by the hot-filaments in plasma at low pressure. We suppose that a part of the formed metallic cobalt coalesces and acts as a shield, protecting some carbon structures from complete etching. This behavior can explain the formation of the semi-suspended a few microns in size long caterpillar-like structures. After the introduction of C2H2 in the H2 plasma at higher temperature, the oriented carbon nanotubes/ nanofibers start to grow from the gas phase carbon-based species, catalyzed by the Co nanoparticles. Also, a Co catalyzed solid-state conversion of the non-etched carbon aggregates into CNTs/CNTs could be possible, taking into account the behavior the Fe–C nanocomposites heated in vacuum at 800 1C [29]. The smaller assemblies of oriented carbon nanostructures on the flat substrate (as shown in Figs. 4 and 5) could originate from the cobalt nitrate nanocrystals that were deposited between the carbon nanoparticle aggregates. In the same SEM images, an amorphous-like layer is visible, which covers the flat surface and the smallest structures. This could be a consequence of the prevalence of C2H2 decomposition during hydrogen etching in regions without a catalyst.
4. Conclusions The liquid-mediated self-assembly of Co catalyst precursor on Si substrates in the absence or the presence of a carbon nanoparticle layer yielded different complex assemblies of vertically oriented carbon nanotubes/nanofibers during the hot-filaments assisted d.c. plasma etching and growth processes. A better understanding of the forces involved in the self-assembly processes may lead to an improvement in the control of the packing, orientation and position of CNTs/CNFs onto the substrate. These carbon-based assemblies may have potential applications in the fabrication of microdevices which would require simultaneously the vertical and the horizontal positioning of the individual carbon nanostructures.
Acknowledgements We would like to acknowledge the support for this research from the Romanian Ministry of Education, Research, Youth and Sport through the ID Project no. 349/2007.
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