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Synthesis of vertically aligned carbon nanoflakes by hot-wire chemical vapor deposition: Influence of process pressure and different substrates
Thin Solid Films 678 (2019) 26–31
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis of vertically aligned carbon nanoflakes by hot-wire chemical vapor deposition: Influence of process pressure and different substrates Mukesh Singha,1, Himanshu S. Jhaa,2, Pratima Agarwala,b, a b
T
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Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India Centre for Energy, Indian Institute of Technology Guwahati, Guwahati 781039, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon nanoflakes Hot-wire chemical vapor deposition Low substrate temperature Glass
The vertically aligned carbon nanoflakes thin films were synthesized on Si (100) and alkali free borosilicate glass substrates at relatively low substrate temperature of 400 °C, without any catalyst or surface pre-treatment, using hot-wire chemical vapor deposition (HWCVD) technique. Raman Spectroscopy studies reveal that the crystallinity of the films increases with increase in process pressure. Our studies show that HWCVD is a versatile technique for the synthesis of these films at low temperature compared to plasma enhanced CVD on both semiconducting Si as well as insulating glass substrates.
1. Introduction Vertically aligned carbon nanoflakes (CNFs) is one of the remarkable allotropes like carbon nanofibers and carbon nanotubes (CNT) in the carbon family due to its open edge interconnects and high aspect ratio [1]. In recent years, CNFs has attracted considerable attention of researcher and technologist in the highly emerging field of vacuum electronics, particularly field emission cathode for the transparent electronic window [1]. It has a wall structure, which consists of vertically aligned multilayer graphene, and can freely stand vertically on the substrates [2]. It has a large surface area (300–700 m2/g) [3] and large surface to volume ratio. Furthermore, since CNFs essentially consisting of multi-layer graphene sheets, it is expected to have high carrier mobility and high sustainable current density [4,5] as well as having the capacity of easy functionalization [6]. These unique inherent properties of CNFs can be accounted as a significant feature for the application in the field of bio-sensors [7], gas sensors [8], energy storage device [9,10], and field emission display [1]. Carbon nanoflakes were discovered for the first time in year 2002 during the growth of CNT using microwave plasma enhanced chemical vapor deposition (PECVD) on catalytic Ni substrate [11]. Now a days, several methods have been developed for the preparation of CNFs such as PECVD [4,11–16], sputtering [17], arc-discharge [18], and hot-wire chemical vapor deposition (HWCVD) [19,20]. Among these, PECVD has shown the potential for the uniform growth of vertically aligned carbon
nanoflakes with different source gases. In recent years, several studies have been done on the growth of CNFs with/without catalysts using different source gases like CH4 [4,12,15,21], C2H2 [14], C2F6 [13,22] using plasma assisted CVD method. Kim et al. [21] have examined the microstructural properties of CNFs at different substrate temperature of 700–950 °C using microwave PECVD. Wang et al. [15] have studied the crystallinity of the CNFs films with different methane gas concentration varying from 10% to 100% using rf-PECVD. Usually, PECVD is a very good technique for the homogeneity in the deposition of thin films, whereas it can increase the disorder due to the continuous striking of ion-bombardment from high density plasma [23,24]. Therefore, an alternative way to reduce the defects is to use HWCVD, where source gases are decomposed at high temperature and films are deposited on the desired substrates. It has the advantage of large decomposition rates and low gas flow rate as well as important for the mass production [20,25]. HWCVD is very popular technique for the growth of diamond like carbon [26,27], Si [28], SiC [29]. However, this method is not much explored for the synthesis of CNFs. Only, a few reports are available on the synthesis of CNFs using HWCVD method by varying substrate temperature [19,20], source gas concentration [30] and filament temperature [19]. Whereas, the effect of process pressure on the structural properties of CNFs has been rarely studied. Also, the comparative study of the growth of CNFs thin films have been made extensively on different substrates like metallic (stainless steel, Ni, Cu), Si and glass
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Corresponding author at: Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India. E-mail address: [email protected] (P. Agarwal). 1 Present Address: Department of Physics, National Taiwan University, Taipei 10617, Taiwan. 2 Present address: Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu-501-1193, Japan. https://doi.org/10.1016/j.tsf.2019.03.033 Received 26 October 2018; Received in revised form 27 March 2019; Accepted 27 March 2019 Available online 28 March 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. FESEM images of CNFs deposited on Si substrate at different process pressure of (a) 100 Pa, (b) 300 Pa and (c) 500 Pa at a substrate temperature of 600 °C.
Fig. 2. AFM images of CNFs on Si substrate at different process pressure of (a) 100 Pa, (b) 300 Pa and (c) 500 Pa at a substrate temperature of 600 °C.
temperature of 600 °C using HWCVD. We have also reported the comparative study of the growth of CNFs on Si and borosilicate glass substrates simultaneously at a low substrate temperature of 400 °C. The microstructural properties in term of the crystallinity of the films were studied with different deposition parameters.
substrates using PECVD [16,31–33]. In case of HWCVD method, comparative study has been made mostly on metallic (stainless steel/Ni) and semiconductor (c-Si) substrates [20,30,34], whereas the growth on c-Si and amorphous substrates is not explored simultaneously. In this paper, we have reported the synthesis of CNFs thin films on Si substrate with different process pressure of 100–500 Pa at a substrate 27
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Fig. 3. (a) Raman spectra of CNFs on Si substrate at different process pressure of 100, 300 and 500 Pa at substrate temperature of 600 °C and (b) enlarged view (in the range of 1100–1800 cm−1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Deconvoluted Raman spectra of CNFs on Si substrate at different process pressure of (a) 100 Pa, (b) 300 Pa and (c) 500 Pa at substrate temperature of 600 °C.
2. Experimental details
was used as a source gas and H2 is used for the dilution. No surface pretreatment or catalyst were used to grow the film. The substrate temperature was kept fixed at 600 °C for the films deposited at a different process pressure of 100, 300 and 500 Pa, whereas, in case of comparative study of different substrates, films were grown at 400 °C on glass and Si substrates simultaneously at a process pressure of 300 Pa. Other deposition parameter such as filament temperature was kept at 2100 ± 50 °C for all the process. Temperature of the filament was
Carbon nanoflakes thin films were deposited on c-Si (100) at a different process pressure of 100, 300 and 500 Pa using HWCVD method. Thin films were also deposited at two different substrates namely c-Si (100) and alkali-free borosilicate glass (Corning 1737 ®, henceforth referred as glass) substrates at a process pressure of 300 Pa. CH4 and H2 are used as precursors at a flow rate ratio of 3:1, where CH4 28
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respectively. No significant change is observed in the width of the flakes with the variation of process pressure, and it is in the range of 8–12 nm. This indicates that the overall size (i.e length and width) of the flakes increases with increasing process pressure. Different nucleation models in CVD method like Stranski – Krastanov model [36], gas phase nucleation [37], etc. are used to explain the growth mechanism for the formation of thin films. Stranski – Krastanov model is widely used in the view of directional growth [20], whereas, gas phase nucleation is more applicable for the growth of continuous thin films like SiC [38] and nano-crystalline diamond thin films [39,40]. According to the S-K model, after the formation of nucleation sites, those standing vertically on the substrates with time, continue to grow faster in upward direction due to shadow effect and therefore activated carbon species arrives only at the top of the wall. This shows that by increasing process pressure, the concentration of activated carbon species is increased. As a result, shadow effect is dominant at high process pressure and therefore, density of the flakes is decreased with increase in process pressure. Fig. 2 shows the 3D view of AFM images of CNFs on Si substrate at different process pressure of 100, 300 and 500 Pa at a substrate temperature of 600 °C. AFM images confirm the uniform growth of CNFs, aligned vertically on the substrates. AFM image also shows that with increase of process pressure, the height of the wall is increased. It further confirms about the dominating shadow effect at higher process pressure. With the increase in shadow effect, the activated carbon species grow faster in upward direction. And therefore, height of the wall is increased with increase in process pressure. Fig. 3(a) shows the Raman spectra of CNFs on Si substrate at different process pressure of 100, 300 and 500 Pa at fixed substrate temperature of 600 °C. It is an important characterization technique for the carbon-based materials to determine the crystallinity of the films. Raman spectra of all the samples were recorded with an excitation wavelength of 514.53 nm. The prominent characteristic peaks are observed at approximately 1350 cm−1 (D band), 1596 cm−1 (G band), 2700 cm−1 (2D band) and 3250 cm−1 (2D' band) [2,4,14,23]. G band is the in-plane vibration of sp2 bonded carbon, whereas D band represent defect induced Raman features due to finite crystallite size [2,23]. For the comparative study, Raman spectra are shown in the range of 1100–1800 cm−1, in Fig. 3(b). Fig. 3(b) indicates that the peak intensity ratio ID/IG is increasing with increasing process pressure. In order to find the different bonding modes of carbon in the film, Raman spectra are further deconvoluted, in the range of 1000–1800 cm−1, into five Lorentzian peaks including D and G band, as shown in Fig. 4. Two broad peaks with small intensity at ~ 1290 cm−1 and 1560 cm−1 could be due to the amorphous phase/sp3 grains in the graphitic phase [41]. In addition, a weak D' band is also observed at ~ 1620 cm−1 which appears in graphite like carbon such as nanocrystalline graphite with low disorder [2]. Full width at half maxima (FWHM) of D and G band, peak intensity ratio ID/IG and ID'/IG, from the
Table 1 FWHM of D and G band, peak intensity ratio ID/IG and ID'/IG for the films deposited at different parameters.
Si_600°C_100 Pa Si_600°C_300 Pa Si_600°C_500 Pa Borosilicate glass_400°C Si_400°C
FWHM (D band, cm−1)
FWHM (G band, cm−1)
ID/IG
ID'/IG
69.9 63.2 52.2 59.4 51.6
38.9 37.6 40.8 44.2 38.2
1.69 1.84 2.42 1.47 2.11
0.65 0.66 0.59 0.55 0.68
measured using pyrometer (model: IRCON, MR-0R05-24C) by focusing at the centre of tungsten filament. The experimental setup used in this study has been described in details in our previous work [35]. In brief, HWCVD system consist of two cylindrical chambers made of stainless steel (SS 304 grade) separated by a gate valve. One of them is process chamber and other is load-lock chamber. A high temperature water cooled substrate heater (~ 900 °C) is attached with the system. Substrate temperature was measured by a thermocouple, which is inbuilt into the heater assembly. Process chamber is designed in such a way that heater assembly can move up and down to adjust the separation between filament and substrate. The separation between filament and substrate were fixed at 1.5 cm in this study. The as grown vertically aligned carbon nanoflakes were characterized by field emission scanning electron microscopy (FESEM, model: ƩIGMA ZEISS) (operated at 2 kV) and atomic force microscopy (AFM, model: Agilent, 5500 series). Films were also characterized by micro Raman spectroscopy (model: HORIBA JOBIN YVON LabRAM HR800) with an excitation wavelength of 514.5 nm. The CNFs films deposited on carbon coated Cu mesh were characterized using Transmission electron microscopy (TEM).
3. Results and discussion 3.1. Influence of process pressure on Si substrate Fig. 1 shows the FESEM image of vertically aligned CNFs thin films on Si substrate deposited at different process pressure of 100, 300 and 500 Pa at a fixed substrate temperature of 600 °C. CH4/H2 source gases were used in the ratio of 3:1 and deposition time was 20 min. The alignment of CNFs in vertical direction is confirmed from three-dimensional AFM analysis, as discussed later (Fig. 2). From FESEM image, it is clear that with increasing process pressure, lateral size (i.e. length) of the flakes increases. On the other hand, the density of the flakes decreases with increase in process pressure from 100 to 500 Pa. The lateral size of the flakes is in the range of 20–30, 25–35 and 55–90 nm for the films deposited at process pressure of 100, 300 and 500 Pa
Fig. 5. (a) TEM image and (b) SAED pattern of CNFs deposited at process pressure of 300 Pa and substrate temperature of 400 °C. 29
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Fig. 6. FESEM image of CNFs deposited on (a) glass and (b) Si substrates at substrate temperature of 400 °C.
increased with increasing process pressure, which corresponds to increase in density of edges. As a result, ID/IG is increased with increasing process pressure. Fig. 5 Shows the TEM image and selected area electron diffraction (SAED) pattern of CNFs deposited on Cu mesh TEM grids at process pressure of 300 Pa and substrate temperature of 400 °C. TEM image further confirms the growth of high-density flakes connected with each other [33,42]. SAED pattern shows the high crystallinity of the flakes. From the SAED pattern, the interplanar spacing is calculated using “Digital Gatan” software and compared with the JCPDS data. We have observed that the measured interplanar spacing is in agreement with the standard value of 0.335 nm, as reported in literature [42]. The centre ring with an interplanar spacing of 0.346 nm corresponds to (002) pane of graphite. As we move further from the centre ring, diffraction rings corresponding to (002)/(100), (100), (102), (103)/(004), and (110) panes of graphitic carbon were observed. Fig. 7. Raman spectra of CNFs on glass and Si substrates at substrate temperature of 400 °C and process pressure of 300 Pa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Influence of different substrates (glass/c-Si) To study the substrate effect on the microstructural properties of CNFs, films were deposited simultaneously on two different substrates, glass and c-Si, at process pressure of 300 Pa. Due to low temperature sustainability of glass, films were deposited at relatively low substrate temperature of 400 °C. Also, since we have lowered the substrate temperature and due to high decomposition rate of radicals in HWCVD, CH4 concentration was also reduced to 1:1. Fig. 6 shows the FESEM image of CNFs thin films on glass and Si substrates at a substrate temperature of 400 °C. The lateral size of the flakes is in the range of 20–30 nm and 40–60 nm for the films deposited on glass and Si substrates respectively. The comparison of CNFs thin films on Si and glass substrates indicate that films deposited on Si substrate have a higher
deconvoluted spectra, for all the films are listed in Table 1. The FWHM of D band is decreased with increasing process pressure from 100 to 500 Pa, however, no significant change is observed in the FWHM of G band. The decrease in FWHM of D band and peak intensity ratio ID'/IG indicate that films possess more crystallinity at high process pressure. The peak intensity ratio ID/IG is increased with increasing process pressure. The increase in peak intensity ratio ID/IG represents more defects i.e. increase in number of edges (benzene rings separated by defects). As we can see from AFM images (Fig. 2), the height of wall is
Fig. 8. Deconvoluted Raman spectra of CNFs (a) on glass (b) on Si substrates at substrate temperature of 400 °C and process pressure of 300 Pa. 30
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growth rate (in terms of higher lateral size of the flakes) in comparison to glass substrate. This is probably because of the difference in thermal conductivity of Si and glass substrates. The higher thermal conductivity of Si substrate can make the nucleation growth faster than glass substrate and therefore higher lateral size of the flakes on Si substrate [31]. Fig. 7 shows the Raman spectra CNFs thin film on glass and Si substrates at a substrate temperature of 400 °C and a fixed process pressure of 300 Pa. The Raman spectra for the films on different substrates clearly show the decrease in FWHM of D and G band for the films deposited on Si substrate. This indicates that films on Si substrate have more crystallinity than on glass. Furthermore, to identify the different Raman active modes, Raman spectra were deconvoluted into five different Lorentzian peaks, as shown in Fig. 8. All the deconvoluted parameters for the films deposited on glass and Si substrates are also listed in Table 1. Apart from the higher crystallinity of the film on Si substrate, the peak intensity ratio ID/IG is higher for the films on Si substrate, indicating more defects. However, higher ID/IG could also be due to crystallite size [2]. Since, the empirical formula ID C (λ ) = L , where C (514.5 nm) = 44 Å by Tuinstra-Kornig [2,43], reIG a presents that the crystallite size is inversely proportional of ID/IG. The higher crystallinity of the films may possess less crystallite size and therefore ID/IG is higher for the films deposited on Si substrate. The crystallinity of the films can also be corelated with the size of the flakes. The lateral size of the flakes is in the range of 40–60 nm for the films deposited on Si substrate, which is higher in comparison to glass. This indicates that, crystallinity during deposition can support the growth of CNFs in well aligned direction.
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4. Summary High crystallinity CNFs thin films were deposited on c-Si and borosilicate glass substrates using HWCVD method. The effect of process pressure was studied on the microstructural properties of CNFs thin film. With the increase in process pressure, improvement in the crystallinity of the film was observed. Our study shows that, CNFs thin film can be deposited at low temperature on glass substrate by HWCVD and can be used as a substrate for making transparent electronic window. Acknowledgements Authors would like to acknowledge CIF, IIT Guwahati for using FESEM, AFM, TEM and Raman facilities. References [1] A. Malesevic, R. Kemps, A. Vanhulsel, M.P. Chowdhury, A. Volodin, C.V. Haesendonck, Field emission from vertically aligned few-layer graphene, J. Appl. Phys. 104 (2008) 084301. [2] S. Kurita, A. Yoshimura, H. Kawamoto, T. Uchida, K. Kojima, M. Tachibana, P. MolinaMorales, H. Nakai, Raman spectra of carbon nanowalls grown by plasma-enhanced chemical vapor deposition, J. Appl. Phys. 97 (2005) 104320. [3] T.-C. Hung, C.-F. Chen, C.-C. Chen, W.-T. Whang, Catalyst-free, low-temperature growth of high-surface area carbon nanoflakes on carbon cloth, Appl. Surf. Sci. 255 (2009) 3676–3681. [4] H.J. Cho, H. Kondo, K. Ishikawa, M. Sekine, M. Hiramatsu, M. Hori, Density control of carbon nanowalls grown by CH4/H2 plasma and their electrical properties, Carbon 68 (2014) 380–388. [5] W. Takeuchi, M. Ura, M. Hiramatsu, Y. Tokuda, H. Kano, M. Hori, Electrical conduction control of carbon nanowalls, Appl. Phys. Lett. 92 (2008) 213103. [6] S. Elena Claudia, S. Ana-Maria, V. Sorin, L. Catalin, M. Lucia, A. Amine, D. Gheorghe, Plasma functionalization of carbon nanowalls and its effect on attachment of fibroblastlike cells, J. Phys. D. Appl. Phys. 47 (2014) 265203. [7] C. Qianwei, S. Tai, S. Xuefen, R. Qincui, Y. Chongsheng, Y. Jun, F. Hua, Y. Leyong, W. Dapeng, Flexible electrochemical biosensors based on graphene nanowalls for the realtime measurement of lactate, Nanotechnology 28 (2017) 315501. [8] K. Yu, Z. Bo, G. Lu, S. Mao, S. Cui, Y. Zhu, X. Chen, R.S. Ruoff, J. Chen, Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication, Nanoscale Res. Lett. 6 (2011) 202. [9] D.J. Cott, M. Verheijen, O. Richard, I. Radu, S.D. Gendt, S.v. Elshocht, P.M. Vereecken, Synthesis of large area carbon nanosheets for energy storage applications, Carbon 58 (2013) 59–65.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis of vertically aligned carbon nanoflakes by hot-wire chemical vapor deposition: Influence of process pressure and different substrates Mukesh Singha,1, Himanshu S. Jhaa,2, Pratima Agarwala,b, a b
T
⁎
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India Centre for Energy, Indian Institute of Technology Guwahati, Guwahati 781039, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon nanoflakes Hot-wire chemical vapor deposition Low substrate temperature Glass
The vertically aligned carbon nanoflakes thin films were synthesized on Si (100) and alkali free borosilicate glass substrates at relatively low substrate temperature of 400 °C, without any catalyst or surface pre-treatment, using hot-wire chemical vapor deposition (HWCVD) technique. Raman Spectroscopy studies reveal that the crystallinity of the films increases with increase in process pressure. Our studies show that HWCVD is a versatile technique for the synthesis of these films at low temperature compared to plasma enhanced CVD on both semiconducting Si as well as insulating glass substrates.
1. Introduction Vertically aligned carbon nanoflakes (CNFs) is one of the remarkable allotropes like carbon nanofibers and carbon nanotubes (CNT) in the carbon family due to its open edge interconnects and high aspect ratio [1]. In recent years, CNFs has attracted considerable attention of researcher and technologist in the highly emerging field of vacuum electronics, particularly field emission cathode for the transparent electronic window [1]. It has a wall structure, which consists of vertically aligned multilayer graphene, and can freely stand vertically on the substrates [2]. It has a large surface area (300–700 m2/g) [3] and large surface to volume ratio. Furthermore, since CNFs essentially consisting of multi-layer graphene sheets, it is expected to have high carrier mobility and high sustainable current density [4,5] as well as having the capacity of easy functionalization [6]. These unique inherent properties of CNFs can be accounted as a significant feature for the application in the field of bio-sensors [7], gas sensors [8], energy storage device [9,10], and field emission display [1]. Carbon nanoflakes were discovered for the first time in year 2002 during the growth of CNT using microwave plasma enhanced chemical vapor deposition (PECVD) on catalytic Ni substrate [11]. Now a days, several methods have been developed for the preparation of CNFs such as PECVD [4,11–16], sputtering [17], arc-discharge [18], and hot-wire chemical vapor deposition (HWCVD) [19,20]. Among these, PECVD has shown the potential for the uniform growth of vertically aligned carbon
nanoflakes with different source gases. In recent years, several studies have been done on the growth of CNFs with/without catalysts using different source gases like CH4 [4,12,15,21], C2H2 [14], C2F6 [13,22] using plasma assisted CVD method. Kim et al. [21] have examined the microstructural properties of CNFs at different substrate temperature of 700–950 °C using microwave PECVD. Wang et al. [15] have studied the crystallinity of the CNFs films with different methane gas concentration varying from 10% to 100% using rf-PECVD. Usually, PECVD is a very good technique for the homogeneity in the deposition of thin films, whereas it can increase the disorder due to the continuous striking of ion-bombardment from high density plasma [23,24]. Therefore, an alternative way to reduce the defects is to use HWCVD, where source gases are decomposed at high temperature and films are deposited on the desired substrates. It has the advantage of large decomposition rates and low gas flow rate as well as important for the mass production [20,25]. HWCVD is very popular technique for the growth of diamond like carbon [26,27], Si [28], SiC [29]. However, this method is not much explored for the synthesis of CNFs. Only, a few reports are available on the synthesis of CNFs using HWCVD method by varying substrate temperature [19,20], source gas concentration [30] and filament temperature [19]. Whereas, the effect of process pressure on the structural properties of CNFs has been rarely studied. Also, the comparative study of the growth of CNFs thin films have been made extensively on different substrates like metallic (stainless steel, Ni, Cu), Si and glass
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Corresponding author at: Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India. E-mail address: [email protected] (P. Agarwal). 1 Present Address: Department of Physics, National Taiwan University, Taipei 10617, Taiwan. 2 Present address: Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu-501-1193, Japan. https://doi.org/10.1016/j.tsf.2019.03.033 Received 26 October 2018; Received in revised form 27 March 2019; Accepted 27 March 2019 Available online 28 March 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. FESEM images of CNFs deposited on Si substrate at different process pressure of (a) 100 Pa, (b) 300 Pa and (c) 500 Pa at a substrate temperature of 600 °C.
Fig. 2. AFM images of CNFs on Si substrate at different process pressure of (a) 100 Pa, (b) 300 Pa and (c) 500 Pa at a substrate temperature of 600 °C.
temperature of 600 °C using HWCVD. We have also reported the comparative study of the growth of CNFs on Si and borosilicate glass substrates simultaneously at a low substrate temperature of 400 °C. The microstructural properties in term of the crystallinity of the films were studied with different deposition parameters.
substrates using PECVD [16,31–33]. In case of HWCVD method, comparative study has been made mostly on metallic (stainless steel/Ni) and semiconductor (c-Si) substrates [20,30,34], whereas the growth on c-Si and amorphous substrates is not explored simultaneously. In this paper, we have reported the synthesis of CNFs thin films on Si substrate with different process pressure of 100–500 Pa at a substrate 27
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Fig. 3. (a) Raman spectra of CNFs on Si substrate at different process pressure of 100, 300 and 500 Pa at substrate temperature of 600 °C and (b) enlarged view (in the range of 1100–1800 cm−1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Deconvoluted Raman spectra of CNFs on Si substrate at different process pressure of (a) 100 Pa, (b) 300 Pa and (c) 500 Pa at substrate temperature of 600 °C.
2. Experimental details
was used as a source gas and H2 is used for the dilution. No surface pretreatment or catalyst were used to grow the film. The substrate temperature was kept fixed at 600 °C for the films deposited at a different process pressure of 100, 300 and 500 Pa, whereas, in case of comparative study of different substrates, films were grown at 400 °C on glass and Si substrates simultaneously at a process pressure of 300 Pa. Other deposition parameter such as filament temperature was kept at 2100 ± 50 °C for all the process. Temperature of the filament was
Carbon nanoflakes thin films were deposited on c-Si (100) at a different process pressure of 100, 300 and 500 Pa using HWCVD method. Thin films were also deposited at two different substrates namely c-Si (100) and alkali-free borosilicate glass (Corning 1737 ®, henceforth referred as glass) substrates at a process pressure of 300 Pa. CH4 and H2 are used as precursors at a flow rate ratio of 3:1, where CH4 28
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respectively. No significant change is observed in the width of the flakes with the variation of process pressure, and it is in the range of 8–12 nm. This indicates that the overall size (i.e length and width) of the flakes increases with increasing process pressure. Different nucleation models in CVD method like Stranski – Krastanov model [36], gas phase nucleation [37], etc. are used to explain the growth mechanism for the formation of thin films. Stranski – Krastanov model is widely used in the view of directional growth [20], whereas, gas phase nucleation is more applicable for the growth of continuous thin films like SiC [38] and nano-crystalline diamond thin films [39,40]. According to the S-K model, after the formation of nucleation sites, those standing vertically on the substrates with time, continue to grow faster in upward direction due to shadow effect and therefore activated carbon species arrives only at the top of the wall. This shows that by increasing process pressure, the concentration of activated carbon species is increased. As a result, shadow effect is dominant at high process pressure and therefore, density of the flakes is decreased with increase in process pressure. Fig. 2 shows the 3D view of AFM images of CNFs on Si substrate at different process pressure of 100, 300 and 500 Pa at a substrate temperature of 600 °C. AFM images confirm the uniform growth of CNFs, aligned vertically on the substrates. AFM image also shows that with increase of process pressure, the height of the wall is increased. It further confirms about the dominating shadow effect at higher process pressure. With the increase in shadow effect, the activated carbon species grow faster in upward direction. And therefore, height of the wall is increased with increase in process pressure. Fig. 3(a) shows the Raman spectra of CNFs on Si substrate at different process pressure of 100, 300 and 500 Pa at fixed substrate temperature of 600 °C. It is an important characterization technique for the carbon-based materials to determine the crystallinity of the films. Raman spectra of all the samples were recorded with an excitation wavelength of 514.53 nm. The prominent characteristic peaks are observed at approximately 1350 cm−1 (D band), 1596 cm−1 (G band), 2700 cm−1 (2D band) and 3250 cm−1 (2D' band) [2,4,14,23]. G band is the in-plane vibration of sp2 bonded carbon, whereas D band represent defect induced Raman features due to finite crystallite size [2,23]. For the comparative study, Raman spectra are shown in the range of 1100–1800 cm−1, in Fig. 3(b). Fig. 3(b) indicates that the peak intensity ratio ID/IG is increasing with increasing process pressure. In order to find the different bonding modes of carbon in the film, Raman spectra are further deconvoluted, in the range of 1000–1800 cm−1, into five Lorentzian peaks including D and G band, as shown in Fig. 4. Two broad peaks with small intensity at ~ 1290 cm−1 and 1560 cm−1 could be due to the amorphous phase/sp3 grains in the graphitic phase [41]. In addition, a weak D' band is also observed at ~ 1620 cm−1 which appears in graphite like carbon such as nanocrystalline graphite with low disorder [2]. Full width at half maxima (FWHM) of D and G band, peak intensity ratio ID/IG and ID'/IG, from the
Table 1 FWHM of D and G band, peak intensity ratio ID/IG and ID'/IG for the films deposited at different parameters.
Si_600°C_100 Pa Si_600°C_300 Pa Si_600°C_500 Pa Borosilicate glass_400°C Si_400°C
FWHM (D band, cm−1)
FWHM (G band, cm−1)
ID/IG
ID'/IG
69.9 63.2 52.2 59.4 51.6
38.9 37.6 40.8 44.2 38.2
1.69 1.84 2.42 1.47 2.11
0.65 0.66 0.59 0.55 0.68
measured using pyrometer (model: IRCON, MR-0R05-24C) by focusing at the centre of tungsten filament. The experimental setup used in this study has been described in details in our previous work [35]. In brief, HWCVD system consist of two cylindrical chambers made of stainless steel (SS 304 grade) separated by a gate valve. One of them is process chamber and other is load-lock chamber. A high temperature water cooled substrate heater (~ 900 °C) is attached with the system. Substrate temperature was measured by a thermocouple, which is inbuilt into the heater assembly. Process chamber is designed in such a way that heater assembly can move up and down to adjust the separation between filament and substrate. The separation between filament and substrate were fixed at 1.5 cm in this study. The as grown vertically aligned carbon nanoflakes were characterized by field emission scanning electron microscopy (FESEM, model: ƩIGMA ZEISS) (operated at 2 kV) and atomic force microscopy (AFM, model: Agilent, 5500 series). Films were also characterized by micro Raman spectroscopy (model: HORIBA JOBIN YVON LabRAM HR800) with an excitation wavelength of 514.5 nm. The CNFs films deposited on carbon coated Cu mesh were characterized using Transmission electron microscopy (TEM).
3. Results and discussion 3.1. Influence of process pressure on Si substrate Fig. 1 shows the FESEM image of vertically aligned CNFs thin films on Si substrate deposited at different process pressure of 100, 300 and 500 Pa at a fixed substrate temperature of 600 °C. CH4/H2 source gases were used in the ratio of 3:1 and deposition time was 20 min. The alignment of CNFs in vertical direction is confirmed from three-dimensional AFM analysis, as discussed later (Fig. 2). From FESEM image, it is clear that with increasing process pressure, lateral size (i.e. length) of the flakes increases. On the other hand, the density of the flakes decreases with increase in process pressure from 100 to 500 Pa. The lateral size of the flakes is in the range of 20–30, 25–35 and 55–90 nm for the films deposited at process pressure of 100, 300 and 500 Pa
Fig. 5. (a) TEM image and (b) SAED pattern of CNFs deposited at process pressure of 300 Pa and substrate temperature of 400 °C. 29
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Fig. 6. FESEM image of CNFs deposited on (a) glass and (b) Si substrates at substrate temperature of 400 °C.
increased with increasing process pressure, which corresponds to increase in density of edges. As a result, ID/IG is increased with increasing process pressure. Fig. 5 Shows the TEM image and selected area electron diffraction (SAED) pattern of CNFs deposited on Cu mesh TEM grids at process pressure of 300 Pa and substrate temperature of 400 °C. TEM image further confirms the growth of high-density flakes connected with each other [33,42]. SAED pattern shows the high crystallinity of the flakes. From the SAED pattern, the interplanar spacing is calculated using “Digital Gatan” software and compared with the JCPDS data. We have observed that the measured interplanar spacing is in agreement with the standard value of 0.335 nm, as reported in literature [42]. The centre ring with an interplanar spacing of 0.346 nm corresponds to (002) pane of graphite. As we move further from the centre ring, diffraction rings corresponding to (002)/(100), (100), (102), (103)/(004), and (110) panes of graphitic carbon were observed. Fig. 7. Raman spectra of CNFs on glass and Si substrates at substrate temperature of 400 °C and process pressure of 300 Pa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Influence of different substrates (glass/c-Si) To study the substrate effect on the microstructural properties of CNFs, films were deposited simultaneously on two different substrates, glass and c-Si, at process pressure of 300 Pa. Due to low temperature sustainability of glass, films were deposited at relatively low substrate temperature of 400 °C. Also, since we have lowered the substrate temperature and due to high decomposition rate of radicals in HWCVD, CH4 concentration was also reduced to 1:1. Fig. 6 shows the FESEM image of CNFs thin films on glass and Si substrates at a substrate temperature of 400 °C. The lateral size of the flakes is in the range of 20–30 nm and 40–60 nm for the films deposited on glass and Si substrates respectively. The comparison of CNFs thin films on Si and glass substrates indicate that films deposited on Si substrate have a higher
deconvoluted spectra, for all the films are listed in Table 1. The FWHM of D band is decreased with increasing process pressure from 100 to 500 Pa, however, no significant change is observed in the FWHM of G band. The decrease in FWHM of D band and peak intensity ratio ID'/IG indicate that films possess more crystallinity at high process pressure. The peak intensity ratio ID/IG is increased with increasing process pressure. The increase in peak intensity ratio ID/IG represents more defects i.e. increase in number of edges (benzene rings separated by defects). As we can see from AFM images (Fig. 2), the height of wall is
Fig. 8. Deconvoluted Raman spectra of CNFs (a) on glass (b) on Si substrates at substrate temperature of 400 °C and process pressure of 300 Pa. 30
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growth rate (in terms of higher lateral size of the flakes) in comparison to glass substrate. This is probably because of the difference in thermal conductivity of Si and glass substrates. The higher thermal conductivity of Si substrate can make the nucleation growth faster than glass substrate and therefore higher lateral size of the flakes on Si substrate [31]. Fig. 7 shows the Raman spectra CNFs thin film on glass and Si substrates at a substrate temperature of 400 °C and a fixed process pressure of 300 Pa. The Raman spectra for the films on different substrates clearly show the decrease in FWHM of D and G band for the films deposited on Si substrate. This indicates that films on Si substrate have more crystallinity than on glass. Furthermore, to identify the different Raman active modes, Raman spectra were deconvoluted into five different Lorentzian peaks, as shown in Fig. 8. All the deconvoluted parameters for the films deposited on glass and Si substrates are also listed in Table 1. Apart from the higher crystallinity of the film on Si substrate, the peak intensity ratio ID/IG is higher for the films on Si substrate, indicating more defects. However, higher ID/IG could also be due to crystallite size [2]. Since, the empirical formula ID C (λ ) = L , where C (514.5 nm) = 44 Å by Tuinstra-Kornig [2,43], reIG a presents that the crystallite size is inversely proportional of ID/IG. The higher crystallinity of the films may possess less crystallite size and therefore ID/IG is higher for the films deposited on Si substrate. The crystallinity of the films can also be corelated with the size of the flakes. The lateral size of the flakes is in the range of 40–60 nm for the films deposited on Si substrate, which is higher in comparison to glass. This indicates that, crystallinity during deposition can support the growth of CNFs in well aligned direction.
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4. Summary High crystallinity CNFs thin films were deposited on c-Si and borosilicate glass substrates using HWCVD method. The effect of process pressure was studied on the microstructural properties of CNFs thin film. With the increase in process pressure, improvement in the crystallinity of the film was observed. Our study shows that, CNFs thin film can be deposited at low temperature on glass substrate by HWCVD and can be used as a substrate for making transparent electronic window. Acknowledgements Authors would like to acknowledge CIF, IIT Guwahati for using FESEM, AFM, TEM and Raman facilities. References [1] A. Malesevic, R. Kemps, A. Vanhulsel, M.P. Chowdhury, A. Volodin, C.V. Haesendonck, Field emission from vertically aligned few-layer graphene, J. Appl. Phys. 104 (2008) 084301. [2] S. Kurita, A. Yoshimura, H. Kawamoto, T. Uchida, K. Kojima, M. Tachibana, P. MolinaMorales, H. Nakai, Raman spectra of carbon nanowalls grown by plasma-enhanced chemical vapor deposition, J. Appl. Phys. 97 (2005) 104320. [3] T.-C. Hung, C.-F. Chen, C.-C. Chen, W.-T. Whang, Catalyst-free, low-temperature growth of high-surface area carbon nanoflakes on carbon cloth, Appl. Surf. Sci. 255 (2009) 3676–3681. [4] H.J. Cho, H. Kondo, K. Ishikawa, M. Sekine, M. Hiramatsu, M. Hori, Density control of carbon nanowalls grown by CH4/H2 plasma and their electrical properties, Carbon 68 (2014) 380–388. [5] W. Takeuchi, M. Ura, M. Hiramatsu, Y. Tokuda, H. Kano, M. Hori, Electrical conduction control of carbon nanowalls, Appl. Phys. Lett. 92 (2008) 213103. [6] S. Elena Claudia, S. Ana-Maria, V. Sorin, L. Catalin, M. Lucia, A. Amine, D. Gheorghe, Plasma functionalization of carbon nanowalls and its effect on attachment of fibroblastlike cells, J. Phys. D. Appl. Phys. 47 (2014) 265203. [7] C. Qianwei, S. Tai, S. Xuefen, R. Qincui, Y. Chongsheng, Y. Jun, F. Hua, Y. Leyong, W. Dapeng, Flexible electrochemical biosensors based on graphene nanowalls for the realtime measurement of lactate, Nanotechnology 28 (2017) 315501. [8] K. Yu, Z. Bo, G. Lu, S. Mao, S. Cui, Y. Zhu, X. Chen, R.S. Ruoff, J. Chen, Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication, Nanoscale Res. Lett. 6 (2011) 202. [9] D.J. Cott, M. Verheijen, O. Richard, I. Radu, S.D. Gendt, S.v. Elshocht, P.M. Vereecken, Synthesis of large area carbon nanosheets for energy storage applications, Carbon 58 (2013) 59–65.
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