ferrocene-ethanol-mist cvd grown swcnt films as ... · ferrocene-ethanol-mist cvd grown swcnt films...

Procedia Engineering 93 ( 2014 ) 49 – 58

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1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and/or peer-review under responsibility of the scientific committee of Symposium [Symposium G - Carbon Nanotubes and Graphene: Synthesis, Functionalisation and Applications] – ICMATdoi: 10.1016/j.proeng.2013.11.022

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7th International conference on materials for Advanced technologies, ICMAT 2013

Ferrocene-ethanol-mist CVD grown SWCNT films as

transparent electrodes Kittipong Jantharamatsakarna, Sutichai Chaisitsaka,*

aDepartment of Electronics Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok and 10520, Thailand

Abstract

The floating catalyst CVD using ferrocene–ethanol mist was successfully used to deposit single-walled carbon nanotube (SWCNT) films from the gaseous phase onto a Si substrate and a membrane filter. The home-built vertical mist-CVD system was used without the use of H2 or CO gases. The tiny mist was generated using a high-frequency ultrasonic vibration. The effects of various parameters including furnace temperature, ferrocene/ethanol ratio, the deposition position in the reactor, flow rate of carrier gas, and deposition time on the SWCNTs formation were investigated using SEM, TEM and Raman spectroscopy. The furnace temperature and the flow rate of carrier gas were found to determine the diameter and crystallinity of nanotubes. The ferrocene concentration in ethanol influenced the diameter distribution of nanotubes and the amount of impurity particles in the materials. The collected SWCNT films on a filter were directly transferred to a laminate sheet by using a hot press laminator. The properties of SWCNTs were investigated by UV-Vis spectrophotometer and four-point probe measurement. The high sheet resistance was observed for the as-transferred films. However, the initial results show that a sheet resistance of 10 kΩ/cm2 and an 80% transmittance at the wavelength of 550 nm after nitric acid treatment. © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium [Carbon Nanotubes and Graphene: Synthesis, Functionalisation and Applications] – ICMAT. . Keywords: Single-walled carbon nanotube; Ethanol; Ferrocene; Mist-CVD; Conductive and transparent film.

* Corresponding author. Tel.: +66-2-329-8344; fax: +66-2-329-8346. E-mail address: [email protected]

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and/or peer-review under responsibility of the scientific committee of Symposium [Symposium G - Carbon Nanotubes and Graphene: Synthesis, Functionalisation and Applications] – ICMAT

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50 Kittipong Jantharamatsakarn and Sutichai Chaisitsak / Procedia Engineering 93 ( 2014 ) 49 – 58

1. Introduction

Transparent and conductive film have been utilized as components of nano-devices, such as thin displays [1], touch sensors [1], organic photovoltatics [2], etc. Currently used transparent metal oxides such as indium-tin oxide (ITO) has several drawbacks, including brittle, expensive and harmful to the environment [3]. Single-walled carbon nanotube (SWCNT) was one of the materials which attract much attention to replace ITO, due to their unique mechanical, electrical, and optical properties [4-7]. The synthesis of CNT has several methods, The chemical vapor deposition (CVD) method is the most appropriate for industrial scale production of CNT, owing to low cost and low deposition temperature [8-9].

Recently, Maruyama et al. has reported the synthesis of SWCNTs from liquid alcohols (methyl, ethyl) and Fe–Co catalysts using a thermal CVD [10-11]. These resulted in a high yield of the SWCNT owing to the role of the decomposed OH radicals. The reaction of OH radical with solid carbon reduces the formation of soot and hence restricts the generation of amorphous carbon in the SWCNT product [10]. In addition, S. Chaisitsak et al. has reported the floating catalyst CVD using ferrocene–ethanol mist [8]. These results were successfully synthesized SWCNTs on a silicon substrate at low substrate temperatures and may be suitable for the fabrication of CNT-based sensors [12]. However, SWCNTs on the silicon substrate had low quantity and could not easily transferred to several substrates.

In this work, we have developed a low-coat CVD for synthesis of SWCNTs. The SWCNTs were collected directly from the gas phase by a home-built vertical CVD using ferrocene-ethanol mist. This method, the mist of ferrocene-ethanol solution was generated by atomizer. Ethanol and ferrocene were used as a carbon source and a catalyst precursor, respectively. The effects of various growth parameters (furnace temperature , ferrocene concentration in ethanol solution , the deposition position in the reactor, flow rate of carrier gas and deposition time) on the formation of SWCNTs were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The optimal condition of SWCNT film were easily transferred to Polyethylene Terephthalate (PET) substrate by a hot press laminator and were used as transparent and conductive SWCNT film. The electrical and optical properties of SWCNTs were investigated by UV-Vis spectrophotometer and four-point probe measurement, respectively. Sheet resistance of a thin film was improved by nitric acid treatment.

2. Experimental

Single-walled carbon nanotubes (SWCNTs) were synthesized by a home-built vertical CVD system (Fig. 1(a)) using the tiny mist of liquid hydrocarbon [8]. Our home-built CVD apparatus consisted of a furnace (500 mm in length), a quartz tube (55mm in outer diameter and 1200 mm in length), a mass flow controllers (MFC), an atomizer ( 1.7 MHz, 20–30 W), Ar carrier gas (purity 99.99%) and a precursor solution, as shown in Fig. 1(a). Ethanol (C2H5OH; purity 99.8%) and ferrocene (Fe(C5H5)2; purity 98%) were used as a carbon source and a catalytic source, respectively. A precursor was prepared by dissolving ferrocene powder in ethanol solution; the solution was ultrasonically stirred for 30 minutes, then transferred into a container attached to the atomizer. After connecting all equipment (Fig. 1(a)), argon was fed into the system to eliminate oxygen from the system. Then, the furnace was heated to the setting temperature in ~30 min. After furnace temperature was stable, the atomizer was turned on, creating a small droplet of ferrocene–ethanol in the reactor. SWCNTs were collected directly from gas phase on silicon substrate and MCE membrane filter (47mm in diameter and pore size 0.45

m). After deposition time completed, the atomizer was turned off and the reactor cooled down to room temperature under argon atmosphere. It should be noted that the resulting CNTs grew on both substrates as a black mat-like film that could be easily scraped off.


Fig. 1 (a) Schematic diagram of the vertical CVD system used for growing SWCNTs, (b) Schematic diagram of a hot press laminator production of CNTs films grown on a PET and nitric acid treatment process.

The characterization of Thin-film CNT was performed by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Raman spectroscopic measurements were also carried out at room temperature using Ar laser (514 nm).

The thin-film SWCNT was directly transferred from a low adhesion filter to a laminate sheet by using a hot press laminator (Fig 1 (b)), and then the upper PET was peeled off. A transparency of SWCNT film was varied via the deposition time. To decrease the sheet resistance of SWCNT film, the thin-film was treated with nitric acid (69%) for 1-300 min and was washed with deionized water for 15 min. After the chemical treatment, the transparent film was baked in air at 100 °C for 1 hour. The electrical sheet resistance and the optical transmittance of SWCNT film were investigated by four-point probe measurement and UV-Vis spectrophotometer, respectively.

3. Results and discussion

3.1. Effect of the furnace temperature

To investigate the effect of temperature on the formation of CNTs, the furnace temperature was varied in the range of 800-1050°C, while the argon flow rate, ferrocene/ethanol ratio and deposition time were kept constant at 2.5 L/min, 1 wt.% and 30 min, respectively. The web-like CNTs were collected on Si substrate below the furnace (z = 5 cm)

Figure 2(a) shows the SEM images of CNTs grown at different temperatures. These SEM images show a significant temperature-dependent morphological change. At the temperature 850°C and below, very short-length and low density CNTs were found. On the other hand, when the temperature was increased to 900°C and above, the long-length and high density CNTs could be obtained, as shown in Fig.2 (a-2)-(a-4). Although further increase in furnace temperature to 1050°C could enhance the amount of long tubes in the films, the

(a) (b)


amount of large catalyst particles which identified as iron/iron oxide [8] also increased. This could be attributed to the excessively high supply of both carbon and catalysts under a high temperature.

Fig. 2 (a) SEM images of CNT films deposited at different temperatures, (b and c) Raman spectra in low-frequency region and high-frequency region of CNT films, respectively. Inset in (c) shows the corresponding IG/ID values. Growth parameters; Ar flow rate: 2.5 L/min, ferrocene/ethanol ratio: 1.0 wt.%, position of Si substrate: z = 5 cm, 30 min.

Figure 2(b) and (c) show the corresponding Raman spectra in radial breathing mode (RBM) frequency region and high-frequency region, respectively. The spectra in RBM (100-350 cm-1) are very sensitive to the diameter of SWCNTs [13]. The intense G-band at 1530–1590 cm−1 is assigned to the tangential mode of the highly ordered graphite, while the broad D-band at 1330 cm-1 is generally associated with disordered or nanocrystalline carbons [13]. As shown in Fig. 2(b), the SWCNTs synthesized above 850°C show RBM peaks between 170 cm-1 and 270 cm-1. The corresponding diameters of SWCNTs were in the range of 0.9 –1.4 nm, using the empirical relation

RBMtd 248

(1)

where dt and ωRBM are the tube diameter (nm) and the Raman shift (cm− 1), respectively [13]. A clearly observed multiple-splitting of the G-band in Fig. 2(c) also provides a reliable indication of a presence of SWCNTs within the samples [13]. Moreover, it was found that the RBM peak in low frequency region shifted toward lower frequencies with increasing the furnace temperature, indicating that the diameter of SWCNTs tended to become larger. It has been previously observed that an increase in temperature results in an increase in catalyst particles size due to the higher agglomeration rate of catalyst particles, thereby forming larger diameter nanotubes [14-15].

Inset in Fig. 2(c) shows the relative area intensities of the G band to D band (i.e. IG/ID), which used as an estimate of the quality of the SWCNTs sample. The result shows that the IG/ID value increased with increasing the temperature, indicating an increase in crystallinity of the films. This crystallinity enhancement could be possibly due to the etching effect of OH radical decomposed from the ethanol under high temperatures [10, 16].

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In this experiment, the SWCNTs synthesized at 1050°C exhibited the highest IG/ID value of about 9.2. However, the optimal temperature for this study was 950°C, since the CNTs synthesized at this temperature show the lowest amount of impurity particles in the material (Fig. 2(a)).

3.2. Effect of the ferrocene concentration

To investigate the effect of ferrocene concentration on the formation of CNTs, the ferrocene concentration was varied in the range of 0.1-2 wt.%, while the argon flow rate, the furnace temperature and deposition time were kept constant at 2.5 L/min, 950°C and 30min, respectively. The web-like CNTs were collected on Si substrate below the furnace (z = 5 cm)

Fig. 3 (a) SEM images of CNT films deposited at different ferrocene/ethanol ratio, (b and c) Raman spectra in low-frequency region and high-frequency region of CNT films, respectively. Inset in (c) shows the corresponding IG/ID values. Growth parameters; temperature: 950°C, Ar flow rate: 2.5 L/min, position of Si substrate: z = 5 cm, 30 min.

Figure 3(a) shows the SEM images of CNTs grown at different ferrocene/ethanol ratio. The SEM image shows the density of CNTs. For the synthesis under low ferrocene/ethanol ratio, the low density of CNTs and small number of catalyst particles were found, as shown in Fig. 3(a-1)-(a-2). The density of CNTs tended to increase with increased ferrocene/ethanol ratio. It was due to high concentration of solution resulted in high catalyst particles , and subsequently, more CNT grow [8]. However, a too high concentration would result a large number of catalyst particle.

The multiple RBM peaks (Fig. 3(b)) were observed under condition 0.5-2 wt.% of ferrocene/ethanol ratio, indicating that various diameter SWCNTs. On the order hand, The RBM of the film synthesized at low ferrocene/ethanol ratio (0.1-0.25 wt.%), the RBM shifted toward higher frequencies, indicating that narrow diameter of SWCNTs [8].

Singh et al. [17] and BAI Xiaodong et al. [18] reported similar results. Increasing of ferrocene concentration resulted in larger CNTs. They believe that the aggregation of catalyst atoms increases as an increasing of ferrocene concentration, resulting in the formations of large particles and large CNTs.

High-crystallinity SWCNTs were obtained at ferrocene/ethanol ratio in the range of 0.25-2 wt.%. Inset in figure 3(c) indicated that the IG/ID value of films slightly decrease at lower this range. This could be attributed

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to the excessively high supply of carbon. The high carbons will dissolve continually into the melted catalysts and then make the catalyst inactive [8].

3.3. Effect of the deposition position in the reactor

Figure 4 shows Raman spectra of CNTs which were deposited at different positions in the reactor. CNTs were collected at bottom position (z=5, with Si substrate) and bottom flange position (z=40, with filter), while the argon flow rate, the furnace temperature, ferrocene/ethanol ratio and deposition time were kept constant at 2.5 L/min, 950°C, 1 wt.% and 30 min, respectively.

Fig. 4 Raman spectra of CNT films deposited at bottom (z=5, with Si substrate) and bottom flange (z=40, filter) positions, (a and b) Raman spectra in low-frequency region and high-frequency region of CNT films, respectively. Inset in (b) shows the corresponding IG/ID values. Growth parameters; furnace temperature: 950°C, ferrocene/ethanol ratio: 1.0 wt.%, Ar flow rate: 2.5 L/min, 30 min.

For the synthesis at position (z=5), The IG/ID ratio and RBM peak were similar to the position (z=40), implying that the diameter and crystallinity of SWCNTs were similar at both positions. It could be possible due to ferrocene and ethanol more decomposed in the hot region of furnace [8]. On the other hand, the ferrocene and ethanol less decomposed in warm region. Therefore, the position at outside the reactor slightly affect the characteristic of SWCNT.

3.4. Effect of the carrier gas flow rate

In order to investigate the effect of carrier gas flow rate on the formation of CNTs, the argon flow rate was set in the range of 0.3-2.5 L/min, while the furnace temperature, ferrocene/ethanol ratio and deposition time were kept constant at 950°C, 1 wt.% and 30min, respectively. CNTs were collected from Si substrate below the furnace (z = 5 cm).

Figure 5(a) shows SEM images of the CNTs synthesized at the different Ar flow rate; (a-1) 0.3 L/min, (a-2) 0.6 L/min, (a-3) 1.0 L/min and (a-4) 2.5 L/min. As shown in the figure, the flow rate of Ar affected the length of CNTs. For the synthesis under a low flow rate (less than 0.6 L/min), only large number of particle impurities were found, as shown in Fig. 5(a-1). This could be due to the excessively long residence time in the reactor

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which provides a greater opportunity for catalyst particles to aggregate forming lager particles [19]. For the synthesis under a higher low flow rate, however, the long-length and high density CNTs formed and tended to increase with increasing the flow rate (Fig. 5(a-2)-(a-4)). This could be attributed to the increased feed of carbon to the metal particles, due to the optimum residence time of the catalyst [20]. For the synthesis under a medium and high flow rate (0.6-2.5 L/min), the IG/ID value of films did not change with the flow rate, indicating no significant change in crystallinity (Fig. 5(c)). However, it was found from the RBM peaks in Fig. 5(b) that the RBM peak shifted toward lower frequencies with decreasing the flow rate, implying that the diameter of SWCNTs tended to become larger. This could be assumed that the lower flow rates lead to the longer residence time which provides a greater opportunity for Fe catalyst particles to collide and to aggregate forming larger particles [19]. As a consequence, the diameter of CNTs becomes larger. On the other hand, the RBM peaks did not appear when grown at a 0.3 L/min flow rate. This result was consistent with the SEM results described earlier. However, the optimal Ar flow rate for this study 0.6 L/min, since the CNTs synthesized at this flow rate show the long-length SWCNTs and the lowest amount of impurity particles in the material (Fig. 5(a)).

Fig. 5 (a) SEM images of CNT films deposited at different Ar flow rate, (b and c) Raman spectra in low-frequency region and high-frequency region of CNT films, respectively. Inset in (c) shows the corresponding IG/ID values. Growth parameters; furnace temperature: 950°C, ferrocene/ethanol ratio: 1.0 wt.%, position of Si substrate: z = 5 cm, 30 min.

3.5. Effect of deposition time

Although, the synthesis of SWCNTs under this condition (Ar flow rate: 0.6 L/min, temperature: 950°C and ferrocene/ethanol: 1 wt.%) was the optimal condition. However, the crystallinity of SWCNT films did not high. According to earlier result, the low ferrocene/ethanol ratio would result the low density of SWCNTs. However, the low ferrocene/ethanol ratio would result a small number of catalyst particles. We therefore hypothesized that the lower concentration (0.25 wt.%) may be synthesized the higher crystallinity of SWCNTs than 1 wt.% of concentration. Moreover, filter substrate was found that it could be transferred to several substrates. However, the filter could not resist at high temperature. Therefore, SWCNT need to be collected at bottom flange position.

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Fig. 6 Raman spectra of CNT films deposited at different deposition time, (a and b) Raman spectra in low-frequency region and high-frequency region of CNT films, respectively. Inset in (b) shows the corresponding IG/ID values, (C) TEM image of a bundle and an individual of SWCNTs with catalyst particles. Growth parameters; furnace temperature: 950°C, ferrocene/ethanol ratio: 1.0 wt.%, position of filter: z = 40 cm, 30 min.

Figure 6 shows the Raman spectra of CNT films which deposited between 5 min and 30 min, while the argon flow rate, the furnace temperature and ferrocene/ethanol ratio were kept constant at 0.6 L/min, 950°C and 0.25 wt.%, respectively. The web-like CNTs were collected directly on filter below the furnace (z = 40 cm)

Figure 6(a) and (b) shows the RBM peaks and IG/ID ratio which did not change with the deposition time, indicating that no change in the diameter of SWCNT and no change the crystallinity of SWCNT films, respectively. In this experiment, SWCNTs were successfully synthesized with the highest IG/ID value of about 18.9. This results could be possible due to the conditions: argon flow rate, the furnace temperature and ferrocene concentration were kept constant. Thus, the residence time and aggregation of metal were kept constant. However, for fabrication of transparent and conductive films, the deposition time had influence on transmittance and sheet resistance value. Moreover, it was found that the SWCNTs deposited by ferrocene – ethanol method tend to grow in a bundle and an individual tube, as demonstrated in Fig. 6(c).

3.6. Acid treatments of conductive films

The transmittance and sheet resistance of films were controlled by the web-like SWCNTs deposition time. After web-like SWCNTs were collected downstream of the reactor by filtering, the SWCNTs were transferred from this filter to PET substrate by a hot press laminator. Figure 7(a) indicate that the transmittance and sheet resistance of films decreased with increasing in deposition time of SWCNTs. Sheet resistance (Rs) of the sample short deposition time (5min) was 50 kΩ/cm2 at T = 83 %. The sample long deposition time (30min) was 8 kΩ/cm2 at T = 36 %, respectively.

To decrease the Rs of the web-like SWCNTs, we improved a postdeposition treatment. SWCNT film was dipped into 69% of HNO3 concentration for 1 - 300 min and was washed in deionized water for 15 min to decrease inter bundle junction resistance [1]. After the chemical treatment, the transparent film was baked in air at 100 °C for 1 hour to eliminate the water.

Figure 7 (b) shows sheet resistance decrease after contact with HNO3 more than 1 min. It was due to the structure of HNO3 that contains oxygen atoms. It is know that oxygen can be attacked the end and structure

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defect in the sidewalls of SWCNT [21-23]. Such a phenomenon resulted in decreasing inter bundle junction resistance. However, for a long time treatment, transparence and sheet resistance increased slightly. It could be possible due to the oxidation at defect sidewalls may cut the tube into pieces [21]. This treatment, the sheet resistance was decreased to 10 kΩ/cm2 and the transparence was improved to 80 % , which shown in Fig.7 (b) and (c).

Fig. 7 (a) Optical transmittance (at 550 nm) and sheet resistance of the SWCNT-PET thin film, as a function of SWCNTs deposition time. (b) Optical transmittance and sheet resistance of the SWCNT-PET thin film (at deposition time: 5min.) as a function of time in nitric treatment (1-300 min.). (c) Transmittance spectra of SWCNTs film.

4. Conclusion

In this paper, we have presented a low-coat CVD for production of SWCNTs by using a vertical CVD apparatus. This method was performed at a normal pressure (1atm.) using the mist of ferrocene-ethanol. The effects of the furnace temperature (800-1050°C) and total flow rate of argon gas (0.3-2.5 L/min) on the formation of SWCNTs were investigated using high-resolution scanning electron microscopy (SEM) and Raman spectroscopy. According to the result, the effect of furnace temperature and the flow rate of carrier gas had an impact on diameter and crystallinity of the obtained CNTs. By varying the furnace temperature, the formation of SWCNTs could be observed when the temperature was higher than 850°C. Increasing temperature resulted in both large diameter and high crystallinity of SWCNTs. For the flow rate in the range of 0.6-2.5 L/min, there was no significant relation between the flow rate of carrier gas and the crystallinity of CNTs. However, it was found that the diameter of CNTs tended to decrease with increasing the flow rate. Moreover, the optimal flow rate (0.6 L/min) could synthesize the long-length CNTs. The ferrocene concentration in ethanol influenced the density and the crystallinity of CNTs. There was no significant relation between the deposition time and the diameter of CNTs. Nevertheless, we have demonstrated a simple transfer technique of preparing transparent and conductive SWCNT films on PET substrate. We developed the sheet resistance of films by Nitric acid treatment. This treatment resulted in a low sheet resistance (10 k /cm2,80%T).

Acknowledgements

The authors are grateful to the Western Digital (Thailand) for the Raman measurements. The work has been supported, in part, by the King Mongkut's Institute of Technology Ladkrabang Research Fund.

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ferrocene-ethanol-mist cvd grown swcnt films as ... · ferrocene-ethanol-mist cvd grown swcnt films...

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