In this study we examine catalyst preparation and chemical vapor deposition (CVD) parameters related to synthesis of single-walled carbon nanotubes (SWNTs) by alcohol catalytic CVD. We show that modifying the catalyst recipe considerably changes the average SWNT diameter, and vertically aligned arrays with an average diameter of 1.5 nm were obtained. The height of vertically aligned SWNT arrays can be significantly enhanced by surface modification of the substrate prior to dip-coating, although this produces SWNTs with larger diameters at the root of the array. We demonstrate patterned growth by combining this method with suppression of SWNT synthesis by formation of a hydrophobic surface. We also consider the effects of ethanol flow rate and thermal decomposition on the chemical environment at the substrate. The growth process is considerably altered by the extent of ethanol decomposition, with sudden termination of the growth occurring in the extreme low-flow (complete decomposition) case.

Interfaces dominate the thermal resistances in aligned carbon nanotube arrays. This work uses nanosecond thermoreflectance thermometry to separate interface and volume resistances for 10 microm thick aligned SWNT films coated with Al, Ti, Pd, Pt, and Ni. We interpret the data by defining the nanotube-metal engagement factor, which governs the interface resistance and is extracted using the measured film heat capacity. The metal-SWNT and SWNT-substrate resistances range between 3.8 and 9.2 mm(2)K/W and 33-46 mm(2)K/W, respectively. The temperature dependency of the heat capacity data, measured between 125 and 300 K, is in good agreement with theoretical predictions. The temperature dependence demonstrated by the metal-SWNT interface resistance data suggests inelastic phonon transmission.

We report a microfabricated gas chromatography (GC) column that uses a thin layer of high‐quality single‐walled carbon nanotubes (SWNTs) as a stationary phase. This 1.0‐m‐long, 160‐μm‐wide, 250‐μm‐deep column has the highest separation efficiency reported to date for microfabricated columns having an SWNT stationary phase. Separation efficiency was evaluated with a Golay plot, in which the minimum of the height equivalent to a theoretical plate was 0.062 cm. The microfabricated column was able to separate n‐alkanes having high boiling points under temperature‐programmed conditions. Use of SWNTs as a stationary phase will be potentially useful for high‐performance micro‐GC.

Addition of only 1% acetylene into ethanol was found to enhance the growth rate of single-walled carbon nanotubes (SWNTs) by up to 10-fold. This accelerated growth, however, only occurred in the presence of ethanol, whereas pure acetylene at the same partial pressure resulted in negligible growth and quickly deactivated the catalyst. The dormant catalyst could be revived by reintroduction of ethanol, indicating that catalyst deactivation is divided into reversible and irreversible stages. Since the thermal decomposition of ethanol also yields some amount of acetylene, the possible contribution to the formation of SWNTs from these decomposed gases is also discussed.

We review our recent studies on the synthesis of vertically aligned single-walled carbon nanotube (VA-SWNT) films by chemical vapor deposition (CVD) of alcohol. In particular, we look at how the ethanol pressure affects the growth process in real-time using an in situ optical absorbance measurement [1]. We find there is an optimum ethanol pressure that increases with CVD temperature [2], below which the growth reaction is governed by a first-order process (Fig. 1). The growth rate of the film is also found to be sensitive to changes in the CVD environment, and can change on short time scales. We also report preliminary results from polarized Raman spectroscopic studies that show anomalous anisotropic behavior in the radial breathing mode. A possible connection between this anisotropy and the small-bundle structure of the VA-SWNT films [3] is discussed. 0 1 2 3 0 50 100 150

We have been attempting to use vertically aligned single-walled carbon nanotube (VA-SWNT) films [1] synthesized by ACCVD method [2] for counter electrodes (CEs) of dye-sensitized solar cells (DSCs). Previously [3], we developed a solar cell in which VA-SWNT films were used for a CE, in place of conventional sputtered Pt on fluorine-doped tin oxide (FTO) layers, by transferring a VA-SWNT film on a FTO-coated glass substrate using our hot-water detachment technique [4]. However, I-V curves of the fabricated solar cells showed smaller fill factor than our reference solar cell in which sputtered Pt was used for CE. Based on fitting analyses on these I-V characteristics using the one-diode equivalent circuit model [5], we attributed the small fill factor in the developed cell to contact resistance at the interface between the SWNT film and the FTO layer. In the present study, in order to reduce the contact resistance, we adopted Si substrates deposited with Au/Cr metallic bilayer (Fig. 1), instead of the FTO-coated glass substrates. Figure 2 shows an improvement of the fill factor achieved by this change. We will discuss the reason of this improvement as well as the possibility of replacing conventional Pt/FTO CEs with VA-SWNT films in DSCs.

Arrays of vertically aligned single-walled carbon nanotubes (VASWNTs) were coated with thin films of Ti, Pd, Au, and Al by evaporative deposition. Scanning electron microcopy showed the Ti and Pd coatings were continuous or quasi-continuous, whereas Au and Al agglomerated into discrete deposits on SWNT bundles. The mechanism of metal film formation on VASWNT arrays was studied by observing the film at various stages of the deposition process. Uniformity of the deposition was found to be strongly dependent on the metal species and the deposition conditions, such as substrate temperature, deposition rate, and deposition thickness. The optimization of the deposition conditions was demonstrated for Pd. The results suggest that the deposition efficiency for a smooth coating layer is determined by the balance of two processes with significantly different speeds: the coating along an SWNT bundle and coating of an interbundle, which depend on the metal type and the deposition conditions. These findings may be useful regarding both fundamental and practical aspects of VASWNT applications in thermal and electronic devices.

The unique physical properties of single-walled carbon nanotubes (SWNTs) have generated considerable interested in various scientific fields, as well as high expectations for novel applications. However, few applications have thus far been demonstrated due to difficulties in controlling SWNT morphology, electrical character, etc. We present some of our recent activities related to applications of SWNTs, particularly patterned synthesis and the use of SWNTs as the counter-electrode in a dye-sensitized solar cell (DSSC). The SWNTs used in this study were synthesized by the alcohol catalytic chemical vapor deposition (ACCVD) method [1], in which SWNTs are formed via the reaction between ethanol and metallic catalyst nanoparticles. The catalyst is deposited on quartz or silicon substrates by a liquid-based dip-coat method [2]. By treating the substrate surface prior to the dip-coating process (Fig. 1), we show we are able to predetermine the regions where catalyst will be deposited (hydrophilic regions) and the areas that will remain catalyst-free. This surface treatment consists of forming a self-assembled monolayer (SAM) on the surface, then selectively removing portions of the SAM either by UV light or electron beam exposure [3]. A vertically aligned SWNT array [4] was also employed as the counter-electrode in a DSSC by transferring the array onto a fluorinated tin oxide (FTO) substrate [5]. The performance was found to be comparable to industry-standard platinum thin films, but had slightly lower efficiency. This was attributed primarily to high resistance at the SWNT-substrate interface. Depositing a gold thin film onto a silicon substrate produced a flatter surface, and resulted in an increase in performance due to the better contact between the substrate and the SWNT array.

An in situ optical absorbance technique was used to monitor the growth of vertically aligned single-walled carbon nanotubes (VA-SWNTs) at various temperatures and pressures. The effects of the growth temperature and ethanol pressure on the initial growth rate and catalyst lifetime were investigated. It was found that the ideal pressure for VA-SWNT synthesis changes with the growth temperature, shifting toward higher pressure as the growth temperature increases. It was also found that the growth reaction is first-order below this ideal pressure. Additionally, the internal structure of the VA-SWNT film was observed at different depths into the film by transmission electron microscopy. The absence of large bundles was confirmed, and little change in the structure was observed to a depth of approximately 1 microm.

We have performed a systematic investigation of the influence of growth parameters on the synthesis of vertically aligned single-walled carbon nanotubes (VA-SWNTs) by the alcohol catalytic chemical vapor deposition (ACCVD) method. The growth process of the VA-SWNTs was monitored using an in situ optical absorbance technique and the effects of CVD temperature and ethanol pressure on the initial growth rate and the catalyst lifetime were investigated. We found that for a given CVD temperature, there is an optimum pressure at which VA-SWNT film growth is maximized, and this pressure increases with temperature. Below this optimum pressure, the growth reaction is first-order, with the arrival of ethanol to the catalyst being the rate-limiting step. The activation energy of the growth reaction was determined to be approximately 1.5 eV. The root-growth mechanism of VA-SWNTs synthesized by the alcohol CVD method was also confirmed by a two-stage growth process. Following a short growth period using normal ethanol, 13C -labeled ethanol was introduced to continue the growth. The location of the 13C was determined from resonance Raman spectra, confirming the root-growth mechanism.

We present the successful synthesis of aligned 13C labeled single-walled carbon nanotube (SWNT) arrays from alcohol by a modified no-flow chemical vapor deposition (CVD) method that makes efficient growth possible using a small amount of carbon source. The synthesis of high-quality SWNTs by this alternative method was confirmed by resonance Raman spectroscopy, which also showed that the quality of the grown SWNTs is uniform in growth direction. The synthesis of 13C labeled SWNTs provides solid evidence for the root growth mechanism in alcohol catalytic CVD, which agrees well with the transmission electron microscopy (TEM) observations.

An optical absorbance technique was used to study the burning temperature and burning mechanism of vertically aligned single-walled carbon nanotube (VA-SWNT) films. The use of this simple optical method is shown to be consistent with the standard thermogravimetric analysis (TGA) method, but it can be applied to a very small amount of SWNTs. Experimental results indicate that burning of the VA-SWNTs is not localized, but occurs throughout the film. Furthermore, thick films have a slightly higher burning temperature than thin films synthesized under the same conditions. This is believed to be due to a higher bundle density and more uniform distribution of SWNTs within thicker films.

Towards the application of SWNTs, clarification of their growth mechanism and control of their diameter, alignments and chirality is important. For these purposes, we firstly investigated the growth process of vertically aligned SWNTs (VA-SWNTs). We have monitored the growth of VA-SWNT films using optical absorbance technique [1] and obtained the growth curves of these films. Variation of growth profiles was observed when the flow rate of ethanol during the ACCVD [2] was controlled precisely (Fig. 1). Furthermore, careful consideration was given for these outcomes by comparing with the results of CHEMKIN simulations and FT-IR gas analysis. Secondly, as an example of the applications, we attempted to use VA-SWNT films as counter electrodes (CEs) for dye-sensitized solar cells (DSCs). Although platinum has been popularly adopted as a catalyst on CEs, it is rare and expensive. Hence, there is a strong demand for new materials for CEs. By detachment technique of SWNT films from substrates [3], the film was transferred on a transparent conductive oxide (TCO) substrate to form a CE. Figure 2 shows the I-V characteristics of DSCs using Pt and SWNT films. Even though the fill factor of the cell with SWNT was smaller than that with Pt, the similar short circuit current (Isc) and open ciruit voltage (Voc) was obtained. Smaller fill factor may be caused by the contact resistance between the SWNT film and TCO. Although this problem needs to be solved, current result indicates SWNT films can be useful as a material of CEs.

Addition of only 1% of acetylene into ethanol was found to enhance the growth rate of singlewalled carbon nanotubes (SWNTs) by up to ten times. Since acetylene is a byproduct of the thermal decomposition of ethanol, this suggests an alternative fast reaction pathway to the formation of SWNTs from ethanol via byproducts of decomposition. This accelerated growth, however, only occurred in the presence of ethanol, whereas pure acetylene at the same partial pressure resulted in negligible growth and quickly deactivated the catalyst. The dormant catalyst could be revived by reintroduction of ethanol, indicating that catalyst deactivation is divided into reversible and irreversible stages.