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Carbon fractals grown from carbon nanotips by plasma-enhanced hot filament chemical vapor deposition
Applied Surface Science 258 (2011) 1677–1681
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Carbon fractals grown from carbon nanotips by plasma-enhanced hot filament chemical vapor deposition B.B. Wang a,∗ , G.B. Dong b , X.Z. Xu c a b c
College of Chemistry and Chemical Engineering, Chongqing University of Technology, 69 Hongguang Rd., Lijiatuo, Banan District, Chongqing 400054, PR China School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, PR China College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 12 July 2011 Received in revised form 10 September 2011 Accepted 27 September 2011 Available online 18 October 2011 Keywords: Carbon nanotips Carbon fractals Ion bombardment
a b s t r a c t Carbon nanomaterials with different structures were prepared in a custom-designed plasma-enhanced hot filament chemical vapor deposition system using methane, hydrogen and nitrogen. They were investigated by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The SEM images show that the smooth carbon nanotips are formed under a high bias current and the carbon fractals can grow from the tips of the carbon nanotips under a low bias current. The results of micro-Raman spectroscopy indicate that the graphitization of the carbon nanomaterials was improved by ion bombardment. Combined the ion bombardment, electric field enhancement and electron emission mechanisms, the formation model of the carbon fractals was suggested. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The carbon-related nanomaterials such as carbon nanotubes, nanofibers, nanowires, nanotips and nanodiamond have been extensively studied due to their unique structures and applications [1–5]. In particular, the carbon nanotips (CNTPs) have attracted much attention in recent years because of their excellent mechanical and electronic properties and the potential applications in the field of micro-electron devices such as scanning microscopy probes, field emission sources and optoelectronic devices [4,6–9]. To date, the CNTPs have successfully been prepared by different methods such as plasma-enhanced chemical vapor deposition (PECVD) [10], plasma-enhanced hot filament chemical vapor deposition (PEHFCVD) [11], ion etching [12] and electron-beam-induced deposition (EBID) [8,13]. It is interesting that the fractal-like CNTPs were grown by EBID [8], which the phenomena were observed in the growth of carbon nanotubes [14–16]. Like the fractals of carbon nanotubes [15], the formation of fractals of CNTPs will result in the change in the properties and expansion of the applications in the field of micro-electron devices. Reactive plasma is extensively used to prepare the carbonbased nanomaterials [1,4,5,9–11,17–20]. In this work, the carbon nanomaterials were prepared by reactive plasma produced in a conventional PEHFCVD system, in which methane, nitrogen and
∗ Corresponding author. Tel.: +86 23 6256 3221; fax: +86 23 6256 3221. E-mail address: [email protected] (B.B. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.124
hydrogen were used as the reaction gases. It is interesting to find that the carbon fractals were grown from the CNTPs like the fractals grown by EBID [8]. According to the ion bombardment, field enhancement and electron emission mechanisms, the formation of the fractals were analyzed and a formation model was suggested. We think that our results are significance for further study of the properties and applications of carbon nanomaterials. 2. Experimental The carbon nanomaterials were synthesized by PEHFCVD, the reaction chamber was similar to the system described in Ref. [19]. Briefly, the reactor has three coiled hot tungsten (heated to about 1600–1800 ◦ C) filaments for precursor gases heating and pre-ionization. Moreover, a variable DC bias was applied to the deposition substrate to draw the ion flux towards the specimens and sustain the plasma discharge by enhancing the ionization of the feedstock gas. The substrate was a silicon wafer with carbon film deposited by sputtering. The substrate temperature was measured by a thermocouple. The distance from the substrate to the filaments was about 8 mm. The reaction gas was a mixture of methane, nitrogen and hydrogen. The working pressure was 2 × 103 Pa. A negative bias relative to the filaments was applied to silicon substrate through the molybdenum holder to produce the plasma. Before preparation of the carbon nanomaterials, a thin carbon film was pre-deposited on the (1 0 0) single crystal silicon substrate using a magnetron sputtering deposition system. The carbon film acts as a seed layer for the growth of the carbon
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nanomaterials, which was carefully described in our previous work [19]. During deposition of the carbon film, the vacuum chamber was pre-evacuated to a base pressure lower than 2 × 10−3 Pa through the use of a combination of rotary and turbo-molecular pumps. Then, a high-purity (99.999%) Ar gas with a flow rate of 30 sccm (sccm denotes cubic centimeters per minute at standard temperature and pressure) was let into the chamber and the total working gas pressure was maintained at 0.5 Pa. Later, the radio-frequency power setting at 100 W was turned on to deposit the carbon film by sputtering carbon target for 30 min. After the silicon wafer deposited with the carbon film was loaded into the PEHFCVD chamber, the chamber was evacuated to a pressure lower than 2 Pa, and then a mixture of methane, nitrogen and hydrogen gases was let into the chamber. After the total working gas pressure was maintained at 2 × 103 Pa, the filament was rapidly heated and the substrate surface was heated by the hot filaments. Later, a bias power with a setting value was switched on to produce plasma and deposit the carbon nanomaterials. The detailed experimental parameters are listed in Table 1. In these parameters, we changed the bias current to grow carbon nanomaterials. Due to the constant current power supply, the bias changes with the bias current. The surface morphology of the nanomaterials was observed using a Hitachi S-4800 field emission scanning electron microscope (FESEM). The chemical composition was investigated by a Renishaw micro-Raman spectroscopy, in which an excitation sources was an Ar+ laser with a wavelength of 514 nm.
3. Results Fig. 1(a) and (b) are the FESEM image and Raman spectrum of the carbon film, respectively. Fig. 1(a) indicates that the thickness of the carbon film is about 60 nm. The Raman spectrum shows two main scattering peaks centered about 1394 and 1578 cm−1 , which are the D and G peaks of the graphitic carbon nanomaterials [21,22]. The intensity ratio of the D to G peak is about 0.66, which indicates that the carbon film is amorphous film [23]. Fig. 2(a)–(d) shows the FESEM images of the specimens grown under the conditions (1)–(4). According to Fig. 2, the geometrical characteristics of the CNTPs can be obtained and they are summarized in Table 2. Compared the data in Table 2 with the thickness of the carbon film, it is obvious that the CNTP formation is a result of growth process. From the data in Table 2, one can know that the growth of the CNTPs is enhanced with the increase in the bias current. As shown in Fig. 2, the fractals are formed on the tips of the CNTPs, but they disappear when the bias current was 180 mA. Fig. 3 reveals the Raman spectra recorded from the specimens synthesized under the conditions (1)–(4). As one can see in Fig. 3, there are two main D and G peaks located about 1355 and 1597 cm−1 , respectively. The D peak centered about 1355 cm−1 is associated with the presence of disordered graphite, while the G peak located about 1597 cm−1 indicates the formation of the nanocrystalline graphite or sp2 hybridized carbon clusters [23]. From Fig. 3, the intensity ratios of G to D peak and the full widths at half maximum (FWHMs) of the D and G peaks can be obtained and they are summarized in Table 2. The data of the ratios indicate that the graphitization of the CNTPs is improved with the increase in the bias currents. From Table 2, the FWHMs of the D and G peaks are different for different specimens, which are related to the change in their structures. For the carbon materials with the low disorder, there are a narrow G peak and a wide D peak [23], which means that the FWHMs of the D and G peaks become wide and narrow with the increasing of degree of graphitization of carbon materials, respectively. Because the graphitization of the CNTPs is improved with the increase in the bias currents, Table 2 shows that the FWHM values
Fig. 1. The FESEM image (a) and Raman spectrum (b) of the carbon film.
of the D and G peaks become large and small with the increase in the bias currents, respectively. As show in Fig. 3, the positions of D and G peaks basically keep the same positions for different specimens, which illustrates that there are not great change in their structures. If the structures are greatly changed, the positions of D and G peaks will have a large shift, respectively. For example, the G peaks of sp2 cluster and graphite are located about 1600 and 1580 cm−1 , respectively [23]. 4. Discussion As shown in Fig. 2, the fractals are grown from the CNTPs. The formation of the CNTPs was studied in previous work [11,18,19,24], here the formation of the fractals was emphatically analyzed. From Fig. 2, one can see that the fractals are formed on the tips which are irregular, thus we believe that the formation of the fractals is related to the ion bombardment and the enhancement of the electric field near the tips of the CNTPs. Due to the production of plasma, a cathode sheath is formed near the substrate and there are a number of carbonaceous and nitrogenous ions to form in the sheath, which they are C2 H3 + , C2 H2 + , C2 H+ CH3 + , CH2 + , CH+ , C+ , NH3 + , NH2 + , NH+ , N+ , etc. [18]. Among these ions, the nitrogenous ions play a role of etching the carbon materials. In DC plasma, the bias mainly falls on the sheath so that a strong electrical field is built up near the substrate surface [25]. After the ions in the sheath can obtain high energy from the electric field, some nitrogenous ions with high energy bombard the CNTPs to remove the tips from the CNTPs [26]. As a result, the edges of the CNTPs are formed, which
B.B. Wang et al. / Applied Surface Science 258 (2011) 1677–1681
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Table 1 Main experimental parameters including the gas fluxes, the substrate temperature Ts , the bias current Ib , the corresponding bias (U) and the growth time t. No.
CH4 (sccm)
H2 (sccm)
N2 (sccm)
Ts (◦ C)
Ib (mA)
U (V)
t (min)
1 2 3 4
20 20 20 20
70 70 70 70
10 10 10 10
∼800 ∼800 ∼800 ∼800
120 140 160 180
∼800 880–890 960–970 1040–1060
20 20 20 20
Table 2 The bottom width (BW), height (H), top diameter (D) of the typical CNTPs, the intensity ratio of G to D peak (IG /ID ), FWHM(D) and FWHM(G) of D and G peaks. Sample
BW (nm)
H (nm)
D (nm)
IG /ID
FWHM(D) (cm−1 )
FWHM(G) (cm−1 )
Fig. 2(a) Fig. 2(b) Fig. 2(c) Fig. 2(d)
42–75 29–63 92–111 42–79
183–267 158–333 275–458 243–525
∼15 ∼15 ∼15 ∼15
0.97 0.99 1.04 3.84
77.1 79.6 97.8 113.5
64.6 61.3 58.3 21.3
is confirmed by the CNTP marked the “A” letter in Fig. 2(c). Simultaneously, the local electric field near the edges of the CNTPs is greatly enhanced like the carbon nanotubes which their tops are opened [27]. Our experimental conditions approach to that in Ref. [11], thus the sheath thickness should be about 2 mm. From the data in Table 2, the maximum height of the CNTPs is about 530 nm which is much smaller than 2 mm, thus the electric field near the edges should be larger than 0.4 V/m. The result of Jang et al. indicates that electrons can emit form CNTPs under a field of 0.1 V/m [28],
that is to say, electrons can emit from the edges of the CNTPs during the preparation of the CNTPs. The emission of electrons results in further ionization of the ions near the edges of the CNTPs in following ways, Cx Hy + + e → Cx Hy−1 + + H+
(1)
NHy + + e → NHy−1 + + H+
(2)
Fig. 2. The FESEM images of CNTPs grown under different bias currents (a) 120 mA, (b) 140 mA, (c) 160 mA, and (d) 180 mA.
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1200
of the bias results in the improvement of electron emission so that the electron density near the tips is enhanced. According to Eqs. (1) and (2), the hydrogen ion density near the tip is enhanced. In this case, the speed of electrons can be reduced due to frequent collision of electrons with ions so that electrons can react to hydrogen ions, thus there is a great deal of atomic hydrogen [H] to form in following reaction
1597
1000
Intensity (arb.u)
1355 800
(d)
600
(c)
H+ + e → [H]
400 (b) 200 0 1000
(a) 1250
1500
1750
2000
2250
2500
Raman shift (cm-1 ) Fig. 3. The Raman spectra of CNTPs synthesized under different bias currents (a) 120 mA, (b) 140 mA, (c) 160 mA, and (d) 180 mA.
where x = 1, 2 and y = 0, 1, 2, 3, 4. Eq. (1) indicates that the masses of the carbonaceous ions are reduced by the ionization, thus the carbonaceous ions are accelerated along the field lines [13] and the carbon concentration near the edges is enhanced. Because there are step defects near the edges of the CNTPs which have high surface energy, the carbon atoms nucleate on the defects due to high carbon concentration [29]. The strong local field near the substrate results in the fast movement of the carbonaceous ions toward the step defects to form the fractals. According to the above analyses, the process of the fractal formation can be described in Fig. 4. As shown in Fig. 4(b), the tip is removed from the CNTP and electrons emit from the edges formed by the damaged tip. Due to electron emission, the carbon concentration near the edges is enhanced and the carbon atoms nucleate on the step defect shown in Fig. 4(c). The acceleration movement of the carbonaceous ions alone the field lines results in the formation of the fractal shown in Fig. 4(d). From Fig. 2(d), one can see that the carbon fractals disappear, which it maybe result from the strong bombardment of the nitrogenous ions and erosion of atomic hydrogen. According to the data in Table 1, the bias is the highest in the condition (4), thus the energy of the nitrogenous ions is enhanced by formula,
E = eU
2
(1 − 2/l) exp(l/) +1−2 l 1 − exp(l/)
(3)
where is the mean free path of the ion and l is the thickness of the sheath [30]. Due to the increase of sputtering yield by the high energetic ions [31], a segment of the fractal is easily etched due to its divagation from the field lines [29]. Simultaneously, the increase Ion beam
Ion beam
Ion beam
Ion beam
(4)
It is known that [H] can strongly erode amorphous carbon. Thus, the strong sputtering of the nitrogen ions together with the erosion of atomic hydrogen result in disappearance of the fractals. Compared the fractals with them prepared by EBID in Ref. [8], they are amorphous carbon, but there is a great difference in their morphologies. For the fractals formed by EBID in Ref. [8], the whole morphology is like a tree which has many long branches. However, our specimens have many short branches located near the tips of the CNTPs. It is possible that the difference results from the different substrates and growth conditions. In Ref. [8], the substrate was the 2 mm2 porous silicon attached to copper TEM grid and the ionization of hydrocarbon molecules depended on the incident electron beam produced under the voltage of 100 kV in TEM. Due to the porosity of the substrate with a small area and the special structure of TEM, the local field and a high density of hydrocarbon ions can form near the porous silicon at beginning of deposition. In this case, the hydrocarbon ions can fast deposit along the field lines to form many long branches. For our specimens, the substrate was silicon with amorphous carbon film of about 150 mm2 . From the above analysis, the local field and the enhancement of hydrocarbon ions formed near the CNTPs after the CNTPs formed. Thus, our specimens have many short branches located near the tips of the CNTPs. 5. Conclusion In summary, the carbon fractals and CNTPs were prepared in a custom-designed PEHFCVD system depending on ion bombardment. The carbon fractals can grow from the CNTPs under the low bias current while the smooth CNTPs can be formed under the high bias current. The formation of the carbon fractals results from the tip damage of the CNTPs and the enhancement of the carbon concentration near the tips of the CNTPs due to electron emission. The formation of the smooth CNTPs in the case of the high bias current is due to the strong sputtering of the nitrogen ions together with the erosion of atomic hydrogen. Our results indicate that the carbon fractals can be prepared by a conventional PEHFCVD. Future work will focus on how to promote the growth of the fractals so that carbon nanowires can be naturally connected with CNTPs. Once the natural joint of carbon nanowires with CNTPs is realized, they will have expansive applications in the fields of nanodevices. Acknowledgements
Si (a)
Electron
Si
Si
Si
This work is partially supported by Chongqing Natural Science Foundation of China (CSTC, 2009BA4027) and partially supported by Scientific Research Foundation of Chongqing University of Technology of China.
(b)
(c)
(d)
References
Carbon atom
Electric field line
Fig. 4. The schematic diagram of the formation of a carbon fractal (a) CNTPs, (b) the formation of the step defect and electron emission, (c) nucleation of carbon atoms nucleate on the step defect, and (d) the formation of the fractal.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Carbon fractals grown from carbon nanotips by plasma-enhanced hot filament chemical vapor deposition B.B. Wang a,∗ , G.B. Dong b , X.Z. Xu c a b c
College of Chemistry and Chemical Engineering, Chongqing University of Technology, 69 Hongguang Rd., Lijiatuo, Banan District, Chongqing 400054, PR China School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, PR China College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
a r t i c l e
i n f o
Article history: Received 12 July 2011 Received in revised form 10 September 2011 Accepted 27 September 2011 Available online 18 October 2011 Keywords: Carbon nanotips Carbon fractals Ion bombardment
a b s t r a c t Carbon nanomaterials with different structures were prepared in a custom-designed plasma-enhanced hot filament chemical vapor deposition system using methane, hydrogen and nitrogen. They were investigated by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The SEM images show that the smooth carbon nanotips are formed under a high bias current and the carbon fractals can grow from the tips of the carbon nanotips under a low bias current. The results of micro-Raman spectroscopy indicate that the graphitization of the carbon nanomaterials was improved by ion bombardment. Combined the ion bombardment, electric field enhancement and electron emission mechanisms, the formation model of the carbon fractals was suggested. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The carbon-related nanomaterials such as carbon nanotubes, nanofibers, nanowires, nanotips and nanodiamond have been extensively studied due to their unique structures and applications [1–5]. In particular, the carbon nanotips (CNTPs) have attracted much attention in recent years because of their excellent mechanical and electronic properties and the potential applications in the field of micro-electron devices such as scanning microscopy probes, field emission sources and optoelectronic devices [4,6–9]. To date, the CNTPs have successfully been prepared by different methods such as plasma-enhanced chemical vapor deposition (PECVD) [10], plasma-enhanced hot filament chemical vapor deposition (PEHFCVD) [11], ion etching [12] and electron-beam-induced deposition (EBID) [8,13]. It is interesting that the fractal-like CNTPs were grown by EBID [8], which the phenomena were observed in the growth of carbon nanotubes [14–16]. Like the fractals of carbon nanotubes [15], the formation of fractals of CNTPs will result in the change in the properties and expansion of the applications in the field of micro-electron devices. Reactive plasma is extensively used to prepare the carbonbased nanomaterials [1,4,5,9–11,17–20]. In this work, the carbon nanomaterials were prepared by reactive plasma produced in a conventional PEHFCVD system, in which methane, nitrogen and
∗ Corresponding author. Tel.: +86 23 6256 3221; fax: +86 23 6256 3221. E-mail address: [email protected] (B.B. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.124
hydrogen were used as the reaction gases. It is interesting to find that the carbon fractals were grown from the CNTPs like the fractals grown by EBID [8]. According to the ion bombardment, field enhancement and electron emission mechanisms, the formation of the fractals were analyzed and a formation model was suggested. We think that our results are significance for further study of the properties and applications of carbon nanomaterials. 2. Experimental The carbon nanomaterials were synthesized by PEHFCVD, the reaction chamber was similar to the system described in Ref. [19]. Briefly, the reactor has three coiled hot tungsten (heated to about 1600–1800 ◦ C) filaments for precursor gases heating and pre-ionization. Moreover, a variable DC bias was applied to the deposition substrate to draw the ion flux towards the specimens and sustain the plasma discharge by enhancing the ionization of the feedstock gas. The substrate was a silicon wafer with carbon film deposited by sputtering. The substrate temperature was measured by a thermocouple. The distance from the substrate to the filaments was about 8 mm. The reaction gas was a mixture of methane, nitrogen and hydrogen. The working pressure was 2 × 103 Pa. A negative bias relative to the filaments was applied to silicon substrate through the molybdenum holder to produce the plasma. Before preparation of the carbon nanomaterials, a thin carbon film was pre-deposited on the (1 0 0) single crystal silicon substrate using a magnetron sputtering deposition system. The carbon film acts as a seed layer for the growth of the carbon
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nanomaterials, which was carefully described in our previous work [19]. During deposition of the carbon film, the vacuum chamber was pre-evacuated to a base pressure lower than 2 × 10−3 Pa through the use of a combination of rotary and turbo-molecular pumps. Then, a high-purity (99.999%) Ar gas with a flow rate of 30 sccm (sccm denotes cubic centimeters per minute at standard temperature and pressure) was let into the chamber and the total working gas pressure was maintained at 0.5 Pa. Later, the radio-frequency power setting at 100 W was turned on to deposit the carbon film by sputtering carbon target for 30 min. After the silicon wafer deposited with the carbon film was loaded into the PEHFCVD chamber, the chamber was evacuated to a pressure lower than 2 Pa, and then a mixture of methane, nitrogen and hydrogen gases was let into the chamber. After the total working gas pressure was maintained at 2 × 103 Pa, the filament was rapidly heated and the substrate surface was heated by the hot filaments. Later, a bias power with a setting value was switched on to produce plasma and deposit the carbon nanomaterials. The detailed experimental parameters are listed in Table 1. In these parameters, we changed the bias current to grow carbon nanomaterials. Due to the constant current power supply, the bias changes with the bias current. The surface morphology of the nanomaterials was observed using a Hitachi S-4800 field emission scanning electron microscope (FESEM). The chemical composition was investigated by a Renishaw micro-Raman spectroscopy, in which an excitation sources was an Ar+ laser with a wavelength of 514 nm.
3. Results Fig. 1(a) and (b) are the FESEM image and Raman spectrum of the carbon film, respectively. Fig. 1(a) indicates that the thickness of the carbon film is about 60 nm. The Raman spectrum shows two main scattering peaks centered about 1394 and 1578 cm−1 , which are the D and G peaks of the graphitic carbon nanomaterials [21,22]. The intensity ratio of the D to G peak is about 0.66, which indicates that the carbon film is amorphous film [23]. Fig. 2(a)–(d) shows the FESEM images of the specimens grown under the conditions (1)–(4). According to Fig. 2, the geometrical characteristics of the CNTPs can be obtained and they are summarized in Table 2. Compared the data in Table 2 with the thickness of the carbon film, it is obvious that the CNTP formation is a result of growth process. From the data in Table 2, one can know that the growth of the CNTPs is enhanced with the increase in the bias current. As shown in Fig. 2, the fractals are formed on the tips of the CNTPs, but they disappear when the bias current was 180 mA. Fig. 3 reveals the Raman spectra recorded from the specimens synthesized under the conditions (1)–(4). As one can see in Fig. 3, there are two main D and G peaks located about 1355 and 1597 cm−1 , respectively. The D peak centered about 1355 cm−1 is associated with the presence of disordered graphite, while the G peak located about 1597 cm−1 indicates the formation of the nanocrystalline graphite or sp2 hybridized carbon clusters [23]. From Fig. 3, the intensity ratios of G to D peak and the full widths at half maximum (FWHMs) of the D and G peaks can be obtained and they are summarized in Table 2. The data of the ratios indicate that the graphitization of the CNTPs is improved with the increase in the bias currents. From Table 2, the FWHMs of the D and G peaks are different for different specimens, which are related to the change in their structures. For the carbon materials with the low disorder, there are a narrow G peak and a wide D peak [23], which means that the FWHMs of the D and G peaks become wide and narrow with the increasing of degree of graphitization of carbon materials, respectively. Because the graphitization of the CNTPs is improved with the increase in the bias currents, Table 2 shows that the FWHM values
Fig. 1. The FESEM image (a) and Raman spectrum (b) of the carbon film.
of the D and G peaks become large and small with the increase in the bias currents, respectively. As show in Fig. 3, the positions of D and G peaks basically keep the same positions for different specimens, which illustrates that there are not great change in their structures. If the structures are greatly changed, the positions of D and G peaks will have a large shift, respectively. For example, the G peaks of sp2 cluster and graphite are located about 1600 and 1580 cm−1 , respectively [23]. 4. Discussion As shown in Fig. 2, the fractals are grown from the CNTPs. The formation of the CNTPs was studied in previous work [11,18,19,24], here the formation of the fractals was emphatically analyzed. From Fig. 2, one can see that the fractals are formed on the tips which are irregular, thus we believe that the formation of the fractals is related to the ion bombardment and the enhancement of the electric field near the tips of the CNTPs. Due to the production of plasma, a cathode sheath is formed near the substrate and there are a number of carbonaceous and nitrogenous ions to form in the sheath, which they are C2 H3 + , C2 H2 + , C2 H+ CH3 + , CH2 + , CH+ , C+ , NH3 + , NH2 + , NH+ , N+ , etc. [18]. Among these ions, the nitrogenous ions play a role of etching the carbon materials. In DC plasma, the bias mainly falls on the sheath so that a strong electrical field is built up near the substrate surface [25]. After the ions in the sheath can obtain high energy from the electric field, some nitrogenous ions with high energy bombard the CNTPs to remove the tips from the CNTPs [26]. As a result, the edges of the CNTPs are formed, which
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Table 1 Main experimental parameters including the gas fluxes, the substrate temperature Ts , the bias current Ib , the corresponding bias (U) and the growth time t. No.
CH4 (sccm)
H2 (sccm)
N2 (sccm)
Ts (◦ C)
Ib (mA)
U (V)
t (min)
1 2 3 4
20 20 20 20
70 70 70 70
10 10 10 10
∼800 ∼800 ∼800 ∼800
120 140 160 180
∼800 880–890 960–970 1040–1060
20 20 20 20
Table 2 The bottom width (BW), height (H), top diameter (D) of the typical CNTPs, the intensity ratio of G to D peak (IG /ID ), FWHM(D) and FWHM(G) of D and G peaks. Sample
BW (nm)
H (nm)
D (nm)
IG /ID
FWHM(D) (cm−1 )
FWHM(G) (cm−1 )
Fig. 2(a) Fig. 2(b) Fig. 2(c) Fig. 2(d)
42–75 29–63 92–111 42–79
183–267 158–333 275–458 243–525
∼15 ∼15 ∼15 ∼15
0.97 0.99 1.04 3.84
77.1 79.6 97.8 113.5
64.6 61.3 58.3 21.3
is confirmed by the CNTP marked the “A” letter in Fig. 2(c). Simultaneously, the local electric field near the edges of the CNTPs is greatly enhanced like the carbon nanotubes which their tops are opened [27]. Our experimental conditions approach to that in Ref. [11], thus the sheath thickness should be about 2 mm. From the data in Table 2, the maximum height of the CNTPs is about 530 nm which is much smaller than 2 mm, thus the electric field near the edges should be larger than 0.4 V/m. The result of Jang et al. indicates that electrons can emit form CNTPs under a field of 0.1 V/m [28],
that is to say, electrons can emit from the edges of the CNTPs during the preparation of the CNTPs. The emission of electrons results in further ionization of the ions near the edges of the CNTPs in following ways, Cx Hy + + e → Cx Hy−1 + + H+
(1)
NHy + + e → NHy−1 + + H+
(2)
Fig. 2. The FESEM images of CNTPs grown under different bias currents (a) 120 mA, (b) 140 mA, (c) 160 mA, and (d) 180 mA.
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1200
of the bias results in the improvement of electron emission so that the electron density near the tips is enhanced. According to Eqs. (1) and (2), the hydrogen ion density near the tip is enhanced. In this case, the speed of electrons can be reduced due to frequent collision of electrons with ions so that electrons can react to hydrogen ions, thus there is a great deal of atomic hydrogen [H] to form in following reaction
1597
1000
Intensity (arb.u)
1355 800
(d)
600
(c)
H+ + e → [H]
400 (b) 200 0 1000
(a) 1250
1500
1750
2000
2250
2500
Raman shift (cm-1 ) Fig. 3. The Raman spectra of CNTPs synthesized under different bias currents (a) 120 mA, (b) 140 mA, (c) 160 mA, and (d) 180 mA.
where x = 1, 2 and y = 0, 1, 2, 3, 4. Eq. (1) indicates that the masses of the carbonaceous ions are reduced by the ionization, thus the carbonaceous ions are accelerated along the field lines [13] and the carbon concentration near the edges is enhanced. Because there are step defects near the edges of the CNTPs which have high surface energy, the carbon atoms nucleate on the defects due to high carbon concentration [29]. The strong local field near the substrate results in the fast movement of the carbonaceous ions toward the step defects to form the fractals. According to the above analyses, the process of the fractal formation can be described in Fig. 4. As shown in Fig. 4(b), the tip is removed from the CNTP and electrons emit from the edges formed by the damaged tip. Due to electron emission, the carbon concentration near the edges is enhanced and the carbon atoms nucleate on the step defect shown in Fig. 4(c). The acceleration movement of the carbonaceous ions alone the field lines results in the formation of the fractal shown in Fig. 4(d). From Fig. 2(d), one can see that the carbon fractals disappear, which it maybe result from the strong bombardment of the nitrogenous ions and erosion of atomic hydrogen. According to the data in Table 1, the bias is the highest in the condition (4), thus the energy of the nitrogenous ions is enhanced by formula,
E = eU
2
(1 − 2/l) exp(l/) +1−2 l 1 − exp(l/)
(3)
where is the mean free path of the ion and l is the thickness of the sheath [30]. Due to the increase of sputtering yield by the high energetic ions [31], a segment of the fractal is easily etched due to its divagation from the field lines [29]. Simultaneously, the increase Ion beam
Ion beam
Ion beam
Ion beam
(4)
It is known that [H] can strongly erode amorphous carbon. Thus, the strong sputtering of the nitrogen ions together with the erosion of atomic hydrogen result in disappearance of the fractals. Compared the fractals with them prepared by EBID in Ref. [8], they are amorphous carbon, but there is a great difference in their morphologies. For the fractals formed by EBID in Ref. [8], the whole morphology is like a tree which has many long branches. However, our specimens have many short branches located near the tips of the CNTPs. It is possible that the difference results from the different substrates and growth conditions. In Ref. [8], the substrate was the 2 mm2 porous silicon attached to copper TEM grid and the ionization of hydrocarbon molecules depended on the incident electron beam produced under the voltage of 100 kV in TEM. Due to the porosity of the substrate with a small area and the special structure of TEM, the local field and a high density of hydrocarbon ions can form near the porous silicon at beginning of deposition. In this case, the hydrocarbon ions can fast deposit along the field lines to form many long branches. For our specimens, the substrate was silicon with amorphous carbon film of about 150 mm2 . From the above analysis, the local field and the enhancement of hydrocarbon ions formed near the CNTPs after the CNTPs formed. Thus, our specimens have many short branches located near the tips of the CNTPs. 5. Conclusion In summary, the carbon fractals and CNTPs were prepared in a custom-designed PEHFCVD system depending on ion bombardment. The carbon fractals can grow from the CNTPs under the low bias current while the smooth CNTPs can be formed under the high bias current. The formation of the carbon fractals results from the tip damage of the CNTPs and the enhancement of the carbon concentration near the tips of the CNTPs due to electron emission. The formation of the smooth CNTPs in the case of the high bias current is due to the strong sputtering of the nitrogen ions together with the erosion of atomic hydrogen. Our results indicate that the carbon fractals can be prepared by a conventional PEHFCVD. Future work will focus on how to promote the growth of the fractals so that carbon nanowires can be naturally connected with CNTPs. Once the natural joint of carbon nanowires with CNTPs is realized, they will have expansive applications in the fields of nanodevices. Acknowledgements
Si (a)
Electron
Si
Si
Si
This work is partially supported by Chongqing Natural Science Foundation of China (CSTC, 2009BA4027) and partially supported by Scientific Research Foundation of Chongqing University of Technology of China.
(b)
(c)
(d)
References
Carbon atom
Electric field line
Fig. 4. The schematic diagram of the formation of a carbon fractal (a) CNTPs, (b) the formation of the step defect and electron emission, (c) nucleation of carbon atoms nucleate on the step defect, and (d) the formation of the fractal.
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