Structural properties of the samples

thereby maintaining the 2D ordered structure. As a consequence, the deposition process leads to the formation of a layer with well-defined micro-nanotopography. The created layer possesses sufficient depth to wrap the entire surface around the spheres. In addition, only few carbon nanotubes provide bridge connections between neighboring spheres. The presence of these connections indicates that carbon nanotube deposition takes place only on the upper surface of the particles, without infiltration within the interstices. Moreover, the individual carbon nanotubes are interpenetrated and homogeneously dispersed within the polylelectrolyte, without any sign of phase segregation. As shown in Fig. 5.16, MWNTs connections increase with the growth of MWNT/PE multilayers. After deposition of ten carbon nanotube layers, the gaps between the spheres are fully covered (Fig. 5.16 C).

To obtain free-standing MWNT-based matrices, the films were peeled off from the substrate through chemical delamination (see section 4.1.2.2). The thus-obtained black compact films could be cut into pieces of a desired size. Fig. 5.17 shows a CNT-based film on a polystyrene mask covering the silicon substrate (A) and a free-standing LBL film after delamination from the substrate (B). The thickness of the free-standing film could be determined from SEM images and was found to be around 2 µm (deposition cycles n = 30).

Figure 5.18 shows a scanning electron micrograph of the morphology of the film before and after chemical delamination. THF etching completely removes the latex particles. However, it leaves a thin polyelectrolyte membrane. This polymer residuum was created due to the infiltration of the polyelectrolytes into the gaps between the microspheres during the LbL composite fabrication.

Figure 5.16: SEM images of the LBL-grown MWNT-PEI/PSS composites deposited on polysty-rene particles after various numbers of deposition cycles: one bi-layer (A), four bi-layers (B), ten bi-layers (C) of MWNT-PEI/PSS.

A B C

Figure 5.17: Digital camera pictures of a CNT-based film created on a polystyrene mask (A) and its free-standing form obtained after chemical delamination (B).

A B

R^ESULTSAND D^ISCUSSION 49

Despite THF treatment, it was not possible to remove this polymer membrane. Therefore, to dislodge residual material from the surface, reactive ion etching (RIE) was employed. During RIE, the ions of the plasma react with the surface atoms forming compounds or molecules, which subsequently leave the surface thermally or as a result of the ion bombardment. The etch rates were determined by controlling the structural changes of the film after 40 s of etching. SEM investigations show the morphology of the film during various stages of O₂ and O₂ + Ar etching.

For 25 % O₂ + 75 % Ar, the initially porous membrane disappeared leaving a polystyrene residuum on the edge of the cavities (Fig. 5.19 A).

Progressively decreasing the argon flow rate and thereby, increasing the oxygen ion concentration, leads to efficient polymer etching. As shown in Fig. 5.19 D, optimal etch conditions have been found for O₂ flow rates equaling 20 sccm. Moreover, the same image reveals that the RIE process induces morphological changes affecting the surface. The originally smooth surfaces became rough with the carbon nanotubes randomly sticking out. A detailed examination of these exposed MWNTs indicates that the oxygen plasma not only removes the polymer residuum but also the superficial polyelectrolyte layer. Figure 5.19 D also indicates that carbon nanotubes can withstand this etching process, which leaves them practically undamaged [1,3]. As has previously been reported, plasma etching results in the formation of highly polar surfaces of the carbon nanotubes [4] and does not affect the mechanical properties of the MWNT-based matrices [5].

Thus, we can assume that oxygen etching under these experimental conditions results in no detectable change in the mechanical properties of the MWNT-based matrices.

Figure 5.18: SEM micrograph of a MWNT-based film (A) before and (B) after chemical dela-mination of the latex. Scale bar: 1 mm.

A B

Figure 5.20 shows the final complete network architecture consisting of successive layers of cross-linked carbon nanotubes that self-assemble into ordered structures. The film, as a free-standing matrix, is characterized by controlled geometry, surface topography, and chemical composition [5].

As mentioned before, the NSL technique combined with a LbL assembly process was employed to produce a model system with both the exceptional nanotopography and nanochemistry to explore the cellular response to carbon nanotube-based materials. The methods presented here offer many advantages. In a simple manner, carbon nanotubes can be assembled into monolayers

Figure 5.19: SEM images after following RIE processes: (A) 25 % O₂ + 75 % Ar, (B) 50 % O₂ + 50 % Ar, (C) 75 % O₂ + 25 % Ar, (D) 100 % O₂. Scale bar: 1 µm.

A B

C D

Figure 5.20: SEM image of the final free-standing matrix made up of carbon nanotubes ar-ranged in a regular network of micro-cavities. Scale bar in A: 10 µm.

A B

R^ESULTSAND D^ISCUSSION 51

with a cavity-like structure. Since the cavity dimension is related to the polystyrene microspheres, their diameter can easily be changed by formation of the polystyrene mask from particles of various sizes. Additionally, the methods used guarantee considerable chemical stability of the self-assembled monolayers and allow for high reproducibility in manufacturing.

5.1.2 VACNT matrices

Vertically-aligned carbon nanotubes were obtained using a plasma enhanced chemical vapor deposition process (PECVD), described in detail in section 4.1.4.

A typical image of catalytically-grown carbon nanotubes is presented in Fig. 5.21. The produced nanotube arrays exhibit perfect vertical alignment and a very good separation between the individual CNTs. The diameters and lengths of randomly distributed nanotubes is around 50 nm and 10 µm, respectively.

Substrates with periodically-aligned nanotubes show that the position of the nanotubes corresponds to the honeycomb pattern of the nickel catalysts. On most of the periodic islands, only single nanotubes were grown. However, as illustrated in the enlarged SEM image (Fig. 5.22, right), double growth also took place. This defect may arise from improper PECVD

Figure 5.21: SEM image of a “carpet” composed of aligned nanotubes grown perpendicular to the silicon substrate. Scale bar: 1 µm.

Figure 5.22: SEM images showing an array of vertically-aligned carbon nan-otubes with periodic distribution on a sapphire substrate. Scale bar: 5 µm (left) and 100 nm (right).

parameters as well as from improper preparation of the nickel structure. Probably, the final step in the substrate preparation, i.e. the annealing phase, influences the creation of additional smaller catalytic centers surrounding the main growth core, which subsequently contributes to the growth of shorter and thinner carbon nanotubes. This imperfection can be eliminated by replacing the quasi-triangular catalyst islands by round catalyst dots [7]. Periodically-aligned CNTs are 50 nm in diameter and ~ 5 µm in length.