**3. Complex catalyst-free arrays by mechanical writing**

Plasma-based techniques yet being capable of producing high-quality nanotube arrays, still require metal catalyst to initialize the control the nanotube growth process. However, there is a strong demand in metal-free CNTs, i.e. the CNTs not containing a catalyst metal which is usually incorporated in the nanotube structure (from the nanotube top or bottom, depending on the process used). Removal of metal catalyst from CNTs implies a complex post-processing [20] which results in significant disadvantages, such as essential change in electronic properties or degradation of the nanotube ordering or orientation (in particular, post-processing deteri‐ orates the vertical orientation of the nanotubes), damages the substrate structure in high temperature annealing process, etc. Thus, removal of the metallic catalyst by after-growth postprocessing is feasible only for limited small-scale experimental production [21]. Hence, the development of the catalyst-free methods for growing arrays of high quality, dense vertically aligned nanotubes is a pressing demand now. The metal-free nucleation and growth of carbon nanotubes is possible, yet with the use of other catalytic material, and with a low quality outcome. For example, the nucleation and growth on semiconductor nanoparticles in CVD process was recently reported [22,23,24]. In these works, the nanotubes were catalyzed and grown without metal catalyst, but those nanotubes are not vertically aligned but highly tangled, tousled, and the surface density is quite low. Therefore, obtaining high quality arrays of CNTs on a catalyst-free silicon substrate still remains elusive.

such morphology, as compared to individual nanotubes, are scalability, stability, reproduci‐ bility, and low cost of the CNT-based devices. They are therefore widely used as thin film transistors, transparent conductive coatings, solar cells, gas and biosensors. The electrical resistivity in entangled SWCNTs is determined by the nanotube-nanotube junctions in the network, and the nanotube-metal junctions at the electrodes (so-called Schottky barrier). The intrinsic resistance of the nanotubes usually plays a minor role if the array density is not far away from the percolation threshold [15]. In addition, it is generally perceived that for the CNT-based device to deliver outstanding performance, chirality-selected growth of CNT is a pre-requisite. However, for entangled SWCNTs, this stringent requirement may be avoided if

There are many parameters of the CVD process that should be controlled to grow entangled CNTs with some special patterns. For example, the length of the nanotubes could be deter‐ mined by the exposure time of the carbon feedstocks. Recently, we have demonstrated that the density of entangled SWCNTs, which is a critical factor in device performance, could be controlled over 3-order-of-magnitude in acetylene-modulated CVD processes (Figure 6a) [2]. In addition, we also obtained a special 'knotted' morphology of the CNT network by using porous silica as the catalyst-supporting layer (Figure 6b) [19]. In contrast to this morphology,

) [16,17,18].

the density is within a certain range (usually 1–3 nanotubes/µm2

24 Syntheses and Applications of Carbon Nanotubes and Their Composites

a much lower density of nanotubes was observed on flat silica surface.

**Figure 6.** Representative arrays of entangled carbon nanotubes [2,19].

**3. Complex catalyst-free arrays by mechanical writing**

Plasma-based techniques yet being capable of producing high-quality nanotube arrays, still require metal catalyst to initialize the control the nanotube growth process. However, there is a strong demand in metal-free CNTs, i.e. the CNTs not containing a catalyst metal which is usually incorporated in the nanotube structure (from the nanotube top or bottom, depending on the process used). Removal of metal catalyst from CNTs implies a complex post-processing [20] which results in significant disadvantages, such as essential change in electronic properties

**Figure 7.** a) Three typical process configurations: localized plasma, remote plasma, gas environment; (b) nanotubes growth on Si substrate contacting with plasma: dense nanotubes as-grown on a doted spot; (c) photo of the plasma above substrate and (d) photo of the microwave reactor; (e) complete experiment matrix, which indicates the sub‐ strate condition (for scratched or non-scratched surface), and the process environment condition; remote gas/plasma and contacting gas/plasma. Among all possible 6 variants tested, only localized plasma process have produced nano‐ tubes on substrate [25].

strate was varied to study in detail the plasma effect on CNT growth process (Figure 1e); namely, the process conducting with plasma contacting the wafer surface was effective for nucleation and growth of CNTs. More details on making the mechanical pattern are

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The scanning electron microscopy (SEM) investigations (Figure 9) show that a fast growth of a high-density, highly aligned CNTs are produced exactly replicating the pattern configuration (a complex pattern configuration which consists of a linear notch and spot applied directly on the notch have been achieved). The SEM images clearly show that the complex pattern of CNTs was perfectly replicated by nanotubes and the longest nanotubes reach ~140 µm in length. The growth sites are very densely occupied, and the rest wafer surface is absolutely free of nanotubes. Notably, these very dense arrays were formed in a very fast process, such unusual growth rates (up to ~48 µm/minute) were not reported previously in the absence of metal

catalyst. Figure 9a shows the high-magnification SEM image of the nanotube array.

**Figure 9.** a) High-magnification SEM image of the vertically-oriented CNTs; (b) a high-resolution TEM image showing the planes in CNTs, with the inter-planer distance of ~0.34 nm; (c) the electron diffraction pattern of CNTs; (d) lowresolution TEM image showing the nanotube diameter of about 10 nm; (e) micro-Raman spectrum of the carbon

shown in Figure 8.

nanotubes [25].

**Figure 8.** a,b) Top-view of CNTs on doted spots (SEM images); (c) microwave reactor; (d, e) tilted SEM image of CNT arrays showing a high number density of CNTs. Insets illustrate the process of making pattern and TEM image of the carbon nanotube [25].

Here we describe a novel plasma-based catalyst-free growth technique that is capable of producing very dense, strongly aligned arrays of extremely long (up to several hundred µm) CNTs on Si wafer surface in very fast process (with growth rate achieving 50 µm/sec), with experimentally proven possibility to arrange the nanotubes into complex arrays of various shapes such as separate nests and linear strands.

The six different experimental variations were used, with respect to the plasma/gas environ‐ ments and plasma location relative to the substrate, as shown in Figure 7. We did not observe the nanotube nucleation in gas environment, on both smooth and patterned surfaces; we also did not observe the nucleation on both smooth and patterned surfaces with the remotely located plasma, and only the process conducted in plasma contacting the patterned surface resulted in the nucleation and growth of CNTs. The process starts by applying a special notch pattern (NP) on the prepared Si(100) wafers.

Then, the substrates with a specific NP (we used a linear NP consisting of parallel notch‐ es, and spot pattern of small pits) was treated in a chemical vapor deposition (CVD) reac‐ tor (Figure 8) where a microwave discharge was ignited in gas mixture of CH4 and N2, at pressure of 13 Torr and power density of 1.28 W/cm3 , typically for 3 min. The substrates were heated up to ~800 °C only by the plasma. The plasma localization relative to the sub‐ strate was varied to study in detail the plasma effect on CNT growth process (Figure 1e); namely, the process conducting with plasma contacting the wafer surface was effective for nucleation and growth of CNTs. More details on making the mechanical pattern are shown in Figure 8.

The scanning electron microscopy (SEM) investigations (Figure 9) show that a fast growth of a high-density, highly aligned CNTs are produced exactly replicating the pattern configuration (a complex pattern configuration which consists of a linear notch and spot applied directly on the notch have been achieved). The SEM images clearly show that the complex pattern of CNTs was perfectly replicated by nanotubes and the longest nanotubes reach ~140 µm in length. The growth sites are very densely occupied, and the rest wafer surface is absolutely free of nanotubes. Notably, these very dense arrays were formed in a very fast process, such unusual growth rates (up to ~48 µm/minute) were not reported previously in the absence of metal catalyst. Figure 9a shows the high-magnification SEM image of the nanotube array.

**Figure 8.** a,b) Top-view of CNTs on doted spots (SEM images); (c) microwave reactor; (d, e) tilted SEM image of CNT arrays showing a high number density of CNTs. Insets illustrate the process of making pattern and TEM image of the

Here we describe a novel plasma-based catalyst-free growth technique that is capable of producing very dense, strongly aligned arrays of extremely long (up to several hundred µm) CNTs on Si wafer surface in very fast process (with growth rate achieving 50 µm/sec), with experimentally proven possibility to arrange the nanotubes into complex arrays of various

The six different experimental variations were used, with respect to the plasma/gas environ‐ ments and plasma location relative to the substrate, as shown in Figure 7. We did not observe the nanotube nucleation in gas environment, on both smooth and patterned surfaces; we also did not observe the nucleation on both smooth and patterned surfaces with the remotely located plasma, and only the process conducted in plasma contacting the patterned surface resulted in the nucleation and growth of CNTs. The process starts by applying a special notch

Then, the substrates with a specific NP (we used a linear NP consisting of parallel notch‐ es, and spot pattern of small pits) was treated in a chemical vapor deposition (CVD) reac‐ tor (Figure 8) where a microwave discharge was ignited in gas mixture of CH4 and N2, at

were heated up to ~800 °C only by the plasma. The plasma localization relative to the sub‐

, typically for 3 min. The substrates

carbon nanotube [25].

shapes such as separate nests and linear strands.

26 Syntheses and Applications of Carbon Nanotubes and Their Composites

pattern (NP) on the prepared Si(100) wafers.

pressure of 13 Torr and power density of 1.28 W/cm3

**Figure 9.** a) High-magnification SEM image of the vertically-oriented CNTs; (b) a high-resolution TEM image showing the planes in CNTs, with the inter-planer distance of ~0.34 nm; (c) the electron diffraction pattern of CNTs; (d) lowresolution TEM image showing the nanotube diameter of about 10 nm; (e) micro-Raman spectrum of the carbon nanotubes [25].

Further characterization of the nanotube structure was done with a high-resolution transmis‐ sion electron microscopy (TEM) and Raman techniques (Figure 9). The TEM images (inset in Figure 8, Figures 9b and 9d) clearly show the absence of catalyst particle at the closed end tip of the CNTs, this reveals that the nanotubes were following in a "base-growth" mode [25]. As follows from TEM images, the diameters of the nanotubes are in the range of 10-80 nm, with up to 25 walls. Figure S13c shows the electron diffraction pattern of multi-wall nanotubes. Raman spectrum of as-grown nanotubes obtained at a room temperature (Figure 9e) shows a Raman broad-band peak at 1585 cm-1, which is the characteristic of in-plane C-C stretching E2g mode of the hexagonal sheet. The appearance of a broad-band peak at 1355 cm-1 indicates the disordered graphitic nature of the nanotubes.

Further, the heated silicon nano-hillock starts reshaping [27] by forming multiple step-like features due to thermal re-arrangement of silicon (to minimize the surface energy), and partially due to the possible carbon solution and saturation in the upper overheated Si layer. Then, carbon atoms incorporate into the steps and form closed chains. Simultaneously, the steps become well-shaped and thus carbon atoms assist the nanotube nucleation along the multiple steps. Later, multi-walled nanotubes start growing. Eventually, when the nanotube

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Thus, in this process the carbon catalization proceeds by the minimization of surface energy at the nano-hillock steps [28], since the adatom adsorbed in the step can be considered as 'partially dissolved'. As a result, this process leads to the formation of very dense array of very long multiwall nanotubes on the mechanically patterned areas. Thus, the proposed growth mechanism explains all the observed features; it is noteworthy that just the effect of plasma on the patterned surface explains several important characteristics due to plasma-related heating and high rate of material delivery. As a result, the catalyst-free, very dense arrays of long (up to 150 µm) vertically oriented multiwall carbon nanotubes were grown on the mechanically patterned silicon wafers in a low-temperature microwave discharge. These experiments have

reaches 100-150 µm in length, the tip of carbon nanotube closes.

demonstrated an extremely high (up to 48 µm/min) growth rate.

**4. Three-dimensional CNT arrays by post-processing with liquids**

factor of the resultant structure fabricated by immersing the CNT array into liquid.

transforming applications requiring very high levels of the light absorbance.

The above-described method can be used to produce 'planar', drawing-like arrays of the verti‐ cally-aligned carbon nanotubes on silicon surfaces. When a need in a complex three-dimen‐ sional array arises, post-processing of the uniform array (array-precursor) can be used. Among others, the post-treatment with a liquid is the most cheap and convenient [29-38]. Nevertheless, this technique still lacks controllability. In this section we show several possible ways of en‐ hancing controllability of the fabrication of three-dimensional structures of the verticallyaligned carbon nanotube arrays. Specifically, we show that the array structure can be a key

Figure 11a is an SEM image of the cross-section of the array of vertically-aligned CNTs grown using the CVD technique. This array exhibits super hydrophobic properties and thus, it cannot be wetted by water. After immersing into water, only weakly-collapsed irregular structure was produced (Figure 11b). In contrast, this array does not show super-hydrofobicity to acetone, and thus, highly-regular completely collapsed pattern was produced by immersing this array into acetone (Figures 11c, 11d). As one can see in this figure, this pattern exhibits very high surface area of the 'sponge', produced by carbon nanotubes (and hence, the walls of this sponge can be highly-conductive or semi-conductive). Such structures could be very useful for the fabrication of gas and bio-sensors, gas storage devices, as well as energy-

It is apparent that the control over the resultant structure of such patterns is a key issue for the above applications. Using different growth conditions, we have grown a similar CNT array with denser structure (see Figure 12a), which does not exhibit super-hydrophobic properties.

Thus, the nanotubes in our experiments were grown on the features mechanically written on the surface of Si wafer, and no nanotubes were formed on the intact silicon. To explain this, we propose a mechanism based on the key role of nano-elements on Si surface, so-called 'nanohillocks'. These hillocks are formed on the surface when writing pattern, they establish a strong covalent bond to the Si surface at a temperature of ~800 °C during the process of CNT nucleation, and thus remain on Si surface, and hence at the bottom of nanotube during the whole growth process. Indeed, the solubility of carbon in Si is very low (10-3 %) [26] as compared to the conventional metallic catalyst such as Fe, Co, Ni etc., and thus the extremely high (up to 1 µm/s) growth rate observed in these experiments indicates that the nanotubes were grown via a surface diffusion, without involving very slow bulk-Si diffusion. Thus, a vapor-liquid-solid (VLS) mechanism was not involved, and the plasma played a key role in this process. We propose the following mechanism, so-called *reshaping-enhanced surface catalyzed* (RESC)growth. During the first stage, the tip region of a Si nano-hillock was heated up by plasma due to increased current density to the nano-sized tip (Figure 10).

**Figure 10.** Scheme of the proposed mechanism of carbon nanotube nucleation and growth on silicon nano-hillocks in the plasma environment. (a) Si nano-hillock (with the shape 'as-produced' by mechanical patterning) is locally (mainly at the top) heated by the plasma; (b) heated Si nano-hillock starts reshaping – multiple step-like features are formed due to thermal re-arrangement and carbon saturation of the upper (overheated) Si layer; single carbon atoms incor‐ porate into the steps; (c) reshaping continues, the steps become well-shaped, carbon atoms form chains (nanotube nuclei) along the multiple steps; (d) carbon chains close, nanotube start growing; (e) nanotube grow and close; (f) supposed reshaping of the silicon nano-hillock during plasma heating and nanotube nucleation [25].

Further, the heated silicon nano-hillock starts reshaping [27] by forming multiple step-like features due to thermal re-arrangement of silicon (to minimize the surface energy), and partially due to the possible carbon solution and saturation in the upper overheated Si layer. Then, carbon atoms incorporate into the steps and form closed chains. Simultaneously, the steps become well-shaped and thus carbon atoms assist the nanotube nucleation along the multiple steps. Later, multi-walled nanotubes start growing. Eventually, when the nanotube reaches 100-150 µm in length, the tip of carbon nanotube closes.

Further characterization of the nanotube structure was done with a high-resolution transmis‐ sion electron microscopy (TEM) and Raman techniques (Figure 9). The TEM images (inset in Figure 8, Figures 9b and 9d) clearly show the absence of catalyst particle at the closed end tip of the CNTs, this reveals that the nanotubes were following in a "base-growth" mode [25]. As follows from TEM images, the diameters of the nanotubes are in the range of 10-80 nm, with up to 25 walls. Figure S13c shows the electron diffraction pattern of multi-wall nanotubes. Raman spectrum of as-grown nanotubes obtained at a room temperature (Figure 9e) shows a Raman broad-band peak at 1585 cm-1, which is the characteristic of in-plane C-C stretching E2g mode of the hexagonal sheet. The appearance of a broad-band peak at 1355 cm-1 indicates the

Thus, the nanotubes in our experiments were grown on the features mechanically written on the surface of Si wafer, and no nanotubes were formed on the intact silicon. To explain this, we propose a mechanism based on the key role of nano-elements on Si surface, so-called 'nanohillocks'. These hillocks are formed on the surface when writing pattern, they establish a strong covalent bond to the Si surface at a temperature of ~800 °C during the process of CNT nucleation, and thus remain on Si surface, and hence at the bottom of nanotube during the whole growth process. Indeed, the solubility of carbon in Si is very low (10-3 %) [26] as compared to the conventional metallic catalyst such as Fe, Co, Ni etc., and thus the extremely high (up to 1 µm/s) growth rate observed in these experiments indicates that the nanotubes were grown via a surface diffusion, without involving very slow bulk-Si diffusion. Thus, a vapor-liquid-solid (VLS) mechanism was not involved, and the plasma played a key role in this process. We propose the following mechanism, so-called *reshaping-enhanced surface catalyzed* (RESC)growth. During the first stage, the tip region of a Si nano-hillock was heated

up by plasma due to increased current density to the nano-sized tip (Figure 10).

**Figure 10.** Scheme of the proposed mechanism of carbon nanotube nucleation and growth on silicon nano-hillocks in the plasma environment. (a) Si nano-hillock (with the shape 'as-produced' by mechanical patterning) is locally (mainly at the top) heated by the plasma; (b) heated Si nano-hillock starts reshaping – multiple step-like features are formed due to thermal re-arrangement and carbon saturation of the upper (overheated) Si layer; single carbon atoms incor‐ porate into the steps; (c) reshaping continues, the steps become well-shaped, carbon atoms form chains (nanotube nuclei) along the multiple steps; (d) carbon chains close, nanotube start growing; (e) nanotube grow and close; (f)

supposed reshaping of the silicon nano-hillock during plasma heating and nanotube nucleation [25].

disordered graphitic nature of the nanotubes.

28 Syntheses and Applications of Carbon Nanotubes and Their Composites

Thus, in this process the carbon catalization proceeds by the minimization of surface energy at the nano-hillock steps [28], since the adatom adsorbed in the step can be considered as 'partially dissolved'. As a result, this process leads to the formation of very dense array of very long multiwall nanotubes on the mechanically patterned areas. Thus, the proposed growth mechanism explains all the observed features; it is noteworthy that just the effect of plasma on the patterned surface explains several important characteristics due to plasma-related heating and high rate of material delivery. As a result, the catalyst-free, very dense arrays of long (up to 150 µm) vertically oriented multiwall carbon nanotubes were grown on the mechanically patterned silicon wafers in a low-temperature microwave discharge. These experiments have demonstrated an extremely high (up to 48 µm/min) growth rate.
