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Plasma effects in aligned carbon nanoflake growth by plasma-enhanced hot filament chemical vapor deposition
Accepted Manuscript Title: Plasma effects in aligned carbon nanoflake growth by plasma-enhanced hot filament chemical vapor deposition Author: B.B. Wang K. Zheng Q.J. Cheng K. Ostrikov PII: DOI: Reference:
S0169-4332(14)02545-8 http://dx.doi.org/doi:10.1016/j.apsusc.2014.11.072 APSUSC 29117
To appear in:
APSUSC
Received date: Accepted date:
30-9-2014 11-11-2014
Please cite this article as: B.B. Wang, K. Zheng, Q.J. Cheng, K. Ostrikov, Plasma effects in aligned carbon nanoflake growth by plasma-enhanced hot filament chemical vapor deposition, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.11.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
►Plasma-specific effects in the growth of carbon nanoflakes (CNFs) are studied.
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►Electic field in the plasma sheath promotes separation of CNFs from the substrate.
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►The orentention of GNFs is related to the combined electic force and growth
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effects.
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►The high growth grates of aligned GNFs are plasma-related.
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Plasma effects in aligned carbon nanoflake growth by plasma-enhanced hot filament
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chemical vapor deposition
College of Chemistry and Chemical Engineering, Chongqing University of
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1
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B. B. Wang1, K. Zheng2, Q. J. Cheng3,*, K. Ostrikov4,5,6
Technology, 69 Hongguang Rd, Lijiatuo, Banan District, Chongqing 400054, PR
2
an
China.
Institute of Microstructure and Properties of Advanced Materials, Beijing University
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of Technology, Beijing 100124, P. R. China 3
School of Energy Research, Xiamen University, Xiamen 361005, PR China
4
Plasma
Australia
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Center
(PNCA),
Manufacturing
Flagship,
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Nanoscience
Commonwealth Scientific and Industrial Research Organization, P. O. Box 218, NSW 2070, Australia
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Lindfield 5
Institute for Future Environments and School of Chemistry, Physics and Mechanical
Engineering, Queensland University of Technology, Brisbane QLD 4000, Australia 6
Plasma Nanoscience, School of Physics, The University of Sydney, Sydney NSW 2006,
Australia.
*Corresponding author: Assoc. Prof. Q. J. Cheng, Tel.: +86 592 5952797; fax: +86 592 2188053. E-mail address: [email protected] (Q.J. Cheng).
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ABSTRACT Carbon nanofilms are directly grown on silicon substrates by plasma-enhanced hot filament chemical vapor deposition in methane environment. It is shown that the
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nanofilms are composed of aligned carbon nanoflakes by extensive investigation of
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experimental results of field emission scanning electron microscopy, micro-Raman
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spectroscopy and transmission electron microscopy. In comparison with the graphene-like films grown without plasmas, the carbon nanoflakes grow in an
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alignment mode and the growth rate of the films is increased. The effects of the plasma on the growth of the carbon nanofilms are studied. The plasma plays three
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main effects of (1) promoting the separation of the carbon nanoflakes from the silicon substrate, (2) accelerating the motion of hydrocarbon radicals, and (3) enhancing the
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deposition of hydrocarbon ions onto the substrate surface. Due to these plasma-specific effects, the carbon nanofilms can be formed from the aligned carbon
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nanoflakes with a high rate. These results advance our knowledge on the synthesis, properties and applications of graphene-based materials.
Keywords: Carbon nanoflakes; Chemical vapor deposition; Plasma; Polarization
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1. Introduction Since graphene was discovered [1], graphene-based materials, which include graphene flakes, reduced graphene oxide, graphene quantum dots, etc., have been
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extensively studied due to their unique structures and potential applications [2-5].
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Because of the gapless property of perfect graphene, it requires further treatment to
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produce a bandgap for its applications in the areas of electronics and optoeletronics [6]. For example, the bandgap of graphene can be induced to form by oxygen and
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hydrogen plasma treatment [6,7]. It is interesting to find that the graphene nanoribbons can produce the bandgap [8,9], which motivates extensive studies of
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graphene nanoflake-like carbon nanomaterials.
Graphene nanoflakes have been successfully synthesized by chemical vapor
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deposition with and without using catalysts [10-15], however the growth mechanism has not been well understood. For example, Yu et al. suggested that the synthesis of
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graphene is a surface precipitation process with a controlled cooling [12], while Li et
al. proposed that the formation of graphene is a surface-catalyzed process rather than a surface precipitation process [13], and Seo et al. suggested that the formation of
graphene sheets starts from the carbon nanorods [14]. Graphene-like nanoflakes have also been produced in hot filament chemical
vapor deposition, where a complex process has been involved including catalysis, diffusion, assembly of carbon atoms, and several other microscopic effects [15,16]. These results indicate that the growth mechanism of graphene flakes still remains an open question. Since the properties and applications of graphene-based materials are 4
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highly related to the structure and stacking-assembly order of graphene sheets [14], the formation of graphene flakes requires further study to control the structure and growth of graphene-based materials.
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In the previous works [15,16], graphene-like nanoflakes were synthesized in hot
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filament chemical vapor deposition (HFCVD), where methane was used as reactive
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precursor gas. Plasma is extensively used to fabricate various nanomaterials such as carbon nanotubes [17], metal oxide nanowires and nanobelts [18], silicon nanocones
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[19], and organic nanofilms [20], etc. This is why in this work the plasma is generated in the HFCVD system to enhance the growth of graphene flake-like carbon
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nanomaterials. As a result, the carbon films composed of carbon nanoflakes were formed, which are different from the previously reported results [14]. Namely, the
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carbon nanofilms are composed of different aligned carbon nanoflakes rather than the vertical graphene sheets. Here, the carbon nanofilms composed of carbon nanoflakes
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are named graphene-like films (GLFs). Extensive characterization results suggest that the formation of carbon
nanoflakes is a complex process which is rather different from the previously reported processes [12,13]. In comparison with the graphene-like nanoflakes grown in HFCVD [21], the plasma is responsible for the aligned growth of the carbon nanoflakes. Simultaneously, it improves the growth rate of GLFs. Since the GLFs are composed of aligned carbon nanoflakes, more surfaces and edges of aligned carbon nanoflakes can be utilized in applications in the areas of fuel cells, bio-sensing, catalysis and energy storage devices, which may be superior to the applications based on 5
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substrate-bound flat graphene films [22,23]. However, the synthesis of vertical graphene-based structures is quite challenging [22]; this is the reason why the effects of the plasma on the growth of GLFs are
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studied in this paper. Although the growth of vertical carbon nanoflakes has recently
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attracted strong interest and a few growth models were suggested [23, 24], a few
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issues remain and require further studies. For example, how do the carbon nanoflakes bend upward? What determines the orientation of the carbon nanoflakes? Why is the
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growth rate of the carbon nanoflakes parallel to the electric field produced by plasma higher than that perpendicular to the electric field produced by plasma? This work
2.1. Synthesis of GLFs
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2. Experiments
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further clarifies these issues.
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The GLFs were synthesized by plasma-enhanced hot filament chemical vapor
deposition (PEHFCVD), which is described in Ref.(21). Briefly, the PEHFCVD system is composed of the CVD chamber, heating and bias systems. The heating and bias systems constructed with three tungsten filaments and a DC power supply are situated in the CVD chamber for the decomposition of reactive gases, heating of substrate and production of plasma, respectively. Due to the high temperature of filaments (~ 2000 ºC), the substrate is easily heated by the filaments to the growth temperature (>900 ºC) within a short time because the distance between the filaments and substrate is about 8 mm. The anode and cathode of DC power supply were 6
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connected to the filaments and substrate through a molybdenum holder, respectively. Before the growth of GLFs, the silicon wafers were pretreated to clean their surfaces. Firstly, the silicon wafers were ultrasonically cleaned for 15 min in
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methylbenzene, acetone and alcohol liquid, respectively, and then they were boiled for
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15 min at 75 ºC in a solution mixed with aqueous ammonia, oxydol and deionized
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water. Finally, the silicon wafers were cleaned for many times with deionized water. After the clean silicon substrate was placed in the CVD chamber, the chamber
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was evacuated till the basic pressure was lower than 2 Pa, and then 20 sccm nitrogen and 80 sccm hydrogen were introduced into the chamber. Consequently, the pressure
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was increased. Through adjusting the valve between the chamber and vacuum pump, the pressure was stabilized at about 2×103 Pa. In this case, the filaments were heated
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to about 1800 ºC. Once the silicon substrate was heated by the filaments to ~ 850 ºC, the DC power supply was turned on to produce the plasma. The surface of silicon
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substrate was further treated for 5 min in the H2-N2 plasma under a bias current of 160
mA.
After the silicon surface was treated in the H2-N2 plasma, 50 sccm methane was
inlet into the chamber, and then the pressure was adjusted to about 2×103 Pa. To fully
decompose methane, the filaments were heated to about 2000 ºC through setting the current of filaments. As a result, the silicon substrate was heated to ~ 930 ºC. In this case, the DC power supply was turned on again and the bias current was set to produce the GLFs. In this work, three specimens were prepared and the growth parameters are shown in Table I. 7
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2.2. Characterization of GLFs The structure and composition of GLFs were studied using a Hitachi S-4800 field emission scanning electron microscope (FESEM) operated at 15 kV, a JEOL
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2010F transmission electron microscope (TEM) operated at 200KV and a HR 800
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Raman spectroscope using the 532 nm line of semiconductor laser as the excitation
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source. To study the structural change of graphene-like flakes, the specimen A was
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also measured at different temperatures in the Raman spectroscope.
3. Results
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Figure 1 shows the FESEM images of specimens A-C. From Fig.1, one can see that the films are composed of the aligned nanoflakes. In comparison with the films
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grown without the plasma [21], the growth rate of GLFs is enhanced by the plasma because the thickness of GLF grown for 5 min in Ref.(21) is about 128 nm and the
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thickness of GLF shown in Fig.1(a) (grown for 3 min) is about 230 nm. In addition, Fig.1(d)~(f) indicate that the nanoflakes are grown in an alignment mode. Figure 2 shows the Raman spectra of specimens A-C. As shown in Fig.2, every
spectrum exhibits the D, G, 2D and D+G Raman peaks of grahene sheets at about 1352, 1592, 2703 and 2949 cm-1 [7,14], which indicates that the films are composed
of graphene-like nanoflakes. To further confirm the structure of GLFs, the specimen B was studied by TEM after the carbon nanoflakes were scraped from silicon surface and the TEM results are shown in Fig.3. From Fig.3(a) one can clearly see the carbon nanoflakes, which 8
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further indicate that the carbon films are composed of carbon nanoflakes. As shown in Fig.3(b), the carbon nanoflake are the hybrid of crystalline and amorphous structures because the right side of Fig.3(b) show the crystalline and amorphous structures. Due
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to the aggregation of carbon nanoflakes, the structure of carbon nanoflakes is not
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clearly shown in left side of Fig.3(b).
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In the growth process of carbon nanoflakes, the structure of carbon nanoflakes continuously changes. To illuminate the structural change, the specimen A was studied
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in micro-Raman system at different temperatures and the results are shown in Fig.4. According to Fig.4, we obtain the positions of Raman peaks and they are summarized
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in Table II. From the data in Table II, one can obviously see that the G and 2D peaks
4. Discussion
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shift towards low frequency with the increase of temperature.
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Figure 1 indicates that the carbon nanoflakes are grown in an alignment mode,
which is related to the plasma effects. For the growth of vertical carbon nanoflakes, Zhao et al. suggested that it is related to the nucleation of graphene and correlates with the supply rate of carbon adatoms [23], but the relation of the growth of vertical graphenes with the plasma was not discussed in detail. In Ref.(24), Zhu et al.
suggested that the growth of vertical carbon nanoflakes is affected by the strain force at the grain boundaries and that the orientation of carbon nanoflakes was determined by the polarization of graphite layers associated with the electric field in the plasma sheath. However, how the force is formed is not explained so that the effect of the 9
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force on the bending of carbon nanoflakes is not well understood. Indeed, the carbon nanoflakes are polarized in the electric field, but the effects of polarization are not discussed. In fact, the bending and orientation of the carbon nanoflakes are
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determined by several forces related to the plasma. In this section, we study the
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effects of the plasma on the growth of GLFs.
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4.1. Effects of plasma on silicon surface
Prior to the growth of GLFs, the pretreatment of silicon substrate can lead to the
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change of silicon surface. In the H2-N2 plasma, hydrogen and nitrogen are ionized into hydrogen and nitrogenous ions. Simultaneously, a strong electric field is formed near
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the silicon surface [25]. Under the electric field, the ions bombard the surface of silicon substrate. As a result, the residual oxygen on silicon surface is removed by
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hydrogen ions through the following reactions, (1)
2H + O → H2O.
(2)
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H+ + e → H,
Eq.(1) represents ion neutralization on the substrate [26]. Due to the large mass of nitrogen ions relative to hydrogen ions, the bombardment of nitrogen ions results in the production of microscopic pits (micro-pits) on the silicon surface. These micro-pits will become the nucleation spots for precipitated carbon material, which are confirmed by the results of Ref.(27). 4.2. Formation of vertical carbon nanoflakes under plasma conditions After the pretreatment of the silicon substrate is completed, methane is let into the CVD chamber to replace hydrogen and nitrogen. Consequently, methane is 10
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ionized into various hydrocarbon ions by the plasma [28,29]. When these hydrocarbon ions deposit on the silicon surface, they nucleate on the micro-pits to form carbon islands [27]. Depending on the carbon island size and morphology, the
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carbon nanoflakes form on silicon surface [15,16].
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In comparison with Fig.2, Fig.4 shows a shift of G and 2D peaks towards lower
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wavenumbers with the increase of temperature, which indicates that stress is generated within carbon nanoflakes [30]. According to the results of Ref.(31), the
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stress is the tensile stress (FT) parallel to the carbon nanoflakes at high temperature, which results from the shrinking of carbon nanoflakes due to the negative expansion
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coefficient of graphene and the basal plane of graphite at high temperature [32-34]. Since one edge of a carbon nanoflake is anchored on the carbon island [16], the free
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edge of the carbon nanoflake bends upwards under the tensile stress [32]. During the growth, the hydrocarbon ions deposit on the surfaces of carbon
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nanoflakes to become the hydrocarbon radicals because the silicon substrate is connected with the cathode of the bias power supply [26]. However, the interaction probability of these hydrocarbon radicals is low at our growth temperature because there is no chemical reaction to occur on the surface of the highly-oriented pyrolytic graphite at above 600 ºC [35]. Thus, the hydrocarbon radicals rapidly move along the surfaces of carbon nanoflakes and reach the free edges of carbon nanoflakes in the absence of high flux of hydrogen [24]. Due to the difference in the electro-negativtivity of carbon and hydrogen elements, the C-H bonds are of the polar bond type, which are polarized by the 11
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electric field formed by the plasma. As a result, the free edges of carbon nanoflakes are negatively charged and a repelling force (FR) parallel to the substrate is formed between two carbon nanoflakes. The repelling force prevents both carbon nanoflakes
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from contact and further promotes the upward bending of the carbon nanoflakes.
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Simultaneously, an electric force (FE) is applied to the free edges of carbon
E
2U c x 1 , d d
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nanoflakes. For the field formed by the plasma, it can be expressed by
(3)
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where U c is the voltage applied to the substrate, d is the thickness of cathode
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sheath and x is the distance from the substrate surface [36]. Eq.(3) indicates that the field is strong near the substrate surface, which implies that a strong electric force is
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applied to the carbon nanoflakes.
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With the growth of carbon nanoflakes, the direction of tensile stress tends to the
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direction of electric force. Once they have the same direction, both the tensile stress and the electric force result in the separation of carbon nanoflakes from the silicon surface. Under the repelling force, it is difficult for two carbon nanoflakes to come into contact. Eventually, the vertical carbon nanoflakes are formed. To be clear, the formation of vertical carbon nanoflakes under plasma conditions
can be described by Fig.5. As shown in Fig.5(a), the carbon nanoflakes form on silicon surface depending on the carbon islands. Fig.5(b) shows that the free edge of the carbon nanoflake bends upwards under the tensile stress and the hydrocarbon radicals move along the surfaces of carbon nanoflakes. In Fig.5(c), the repelling and electric forces are applied to the carbon nanoflakes after the free edges of carbon 12
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nanoflakes are negatively charged. Under the repelling force, the carbon nanofalkes further bend upward. With the growth of carbon nanoflakes, the direction of tensile stress tends to the direction of electric force. After the tensile stress has the same
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4.3. Orientation of carbon nanoflakes under plasma conditions
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carbon nanoflakes vertical growth, which is described by Fig.5(d).
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direction as the electric force, both the tensile stress and electric force make the
From Fig.1, we obtain the thickness of carbon nanoflakes and it is more than 4
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nm, which indicates that a carbon nanoflake is composed of multilayer graphene-like nanosheets. Thus, the carbon nanoflakes expand along their normal direction during
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the growth because the expansion coefficient of graphite in the direction of c-axis is
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the carbon nanoflakes.
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positive [37]. As a result, a compressive stress is generated in the normal direction of
However, the high aspect ratio of thin carbon nanoflakes leads to the electric
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field focusing near the free end of the carbon nanoflakes. For the DC plasma, the thickness of the cathode sheath is about several millimeters [38]. From Fig.1(d), the height of a vertical carbon nanoflake is about 48 nm. To facilitate analysis, the thickness of cathode sheath is assumed to be 1 mm. According to Eq.(3), the field on the free edge of vertical carbon nanoflakes is about 1.999 U c / d . Due to the vertical
carbon nanoflakes, the field enhancement factor
h/l
(4)
is about 11, where h and l are the height and thickness of vertical carbon nanoflakes [39]. Consequently, the actual electric field on the free end of the vertical 13
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carbon nanoflakes is about 22 U c / d . Thus, the electric field near the free edge of the carbon nanoflakes can be enhanced by the growth of the carbon nanoflakes. Because of the change of the height and thickness of the carbon nanoflakes with the growth
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time, the electric force changes during the growth of the carbon nanoflakes. In
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addition, the distance between two carbon nanoflakes and the charge on the edges of
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carbon nanoflakes are different, which results in the change of repelling forces acted on every carbon nanoflake.
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Because of different bending torques for the stress, repelling and electric forces, the effects of these torques on the carbon nanoflakes are quite different. When the net
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torque is zero, the carbon nanoflake can grow vertically. Otherwise, the carbon nanoflakes are tilted with respect to the substrate surface. Since the forces vary
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dynamically, the system is continuously driven out of equilibrium. As a result, the carbon nanoflakes grow in an alignment mode shown in Fig.1. We emphasize that the
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orientation of the carbon nanoflakes is determined by several forces. For the graphene-like nanoflakes grown by HFCVD [21], a low inclination angle with respect to the silicon surface was reported, while Fig.1(d) shows that the carbon nanoflakes are basically perpendicular to the silicon surface. Thus we conclude that plasma-specific effects significantly contribute to the aligned growth of the carbon nanoflakes. 4.4. Plasma effects on the growth rates Due to the formation of aligned carbon nanoflakes, the hydrocarbon radicals
mainly move towards the free edges of carbon nanoflakes because there is a low 14
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reaction probability of them on the surface of carbon nanoflakes at high growth temperature [35]. After the hydrocarbon ions deposit on the substrate surface to become hydrocarbon radicals, they are polarized by the electric field into the polar
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radicals such as Cδ--Hδ+ due to the difference in the electro-negativity of carbon and
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hydrogen elements [40]. Under the electric field, the radicals rapidly move towards
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the free edges of carbon nanoflakes. After the radicals reach the free edges of the carbon nanoflakes, they bond the free edges due to the dangling bonds of free edges.
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As a result, the area of carbon nanoflakes is increased. Because there are some defects on the surfaces of the carbon nanoflakes, the radicals can bond with the
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defects if they cannot overcome the potential barrier formed by the defects under the electric field. In this case, some radicals can bond with other radicals to increase the
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thickness of the carbon nanoflakes. However, the high growth temperature can result in the re-evaporation of the deposited radicals to slow down the increase of the carbon
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nanoflakes in thickness. This is the reason that the growth rate of the carbon nanoflakes along the electric field is the highest. When the two aligned carbon nanoflakes touch, the free edges of them combine
together to form defects on the joint. Depending on the defects, new carbon nanoflakes may form and then develop in the alignment mode. Finally, the carbon nanoflakes form the thin GLFs. In section III, the results indicate that the growth rates of GLFs are increased by the plasma, which is related to the motion of hydrocarbon ions in the plasma. After methane is ionized into the hydrocarbon ions such as CH3+ and C2H2+ [27,28], they 15
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move towards the substrate under the electric field and temperature difference between the filaments and the substrate. When the GLFs grow in the HFCVD system, methane is predominantly decomposed into hydrocarbon radicals, e.g. (5)
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CH4 → CH3 + H.
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The motion of hydrocarbon radicals is also affected by the temperature difference
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between the filaments and the substrate. In other words, the motion of hydrocarbon ions is accelerated in the plasma, which results in the fast formation of carbon
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nanoflakes. Consequently, the growth of GLFs is enhanced by the plasma.
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5. Conclusion
In summary, GLFs were directly synthesized on silicon substrates by PEHFCVD
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in reactive methane environment. The structural and compositional properties of GLFs were studied using FESEM/TEM and micro-Raman spectroscopy, respectively.
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The characterization results help elucidating the effects of the plasma on the growth of GLFs. The plasma promotes the separation of carbon nanoflakes from the silicon substrate and improves the aligned growth of the carbon nanoflakes. Moreover, the hydrocarbon ions can move towards the substrate faster under the plasma conditions, which results in the high growth rates of GLFs. The achievements obtained here can enrich our knowledge of synthesizing graphene-based materials and contribute to the development of new applications of graphene-based nanomaterials.
Acknowledgements 16
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This work was partially supported by the Fundamental Research Funds for the Central Universities, Program for New Century Excellent Talents in Fujian Province University (NCETFJ), Scientific Research Foundation for the Returned Overseas
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Chinese Scholars, and CSIRO’s OCE Science Leadership Scheme and the Australian
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Research Council.
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[23] J. Zhao, M. Shaygan, J. Eckert, M. Meyyappan, Mark H. Rümmeli, A growth
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mechanism for free-standing vertical graphene, Nano Lett. 14 (2014) 3064-3071. [24] M. Zhu, J. Wang, B. C. Holloway, R. A. Outlaw, X. Zhao, K. Hou, V.
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Shutthanandan, D. M. Manos, A mechanism for carbon nanosheet formation,
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Carbon 45 (2007) 2229–2234.
[25] Y. P. Raizer, Gas Discharge Physics (Springer-Verlage, Berlin, 1991).
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[26] Y. Itikawa, Molecular Processes in Plasmas: Collisions of charged Particles with
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Molecules (Springer, Berlin, 2007).
[27] B. B. Wang, W. L. Wang, K. J. Liao, Micro-defects produced on a substrate by a
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Films 401 (2001) 77-83.
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glow discharge and the role of such defects on diamond nucleation, Thin Solid
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[28] I. Denysenko, K. Ostrikov, U. Cvelbar, M. Mozetic, N. A. Azarenkov, Carbon nanofiber growth in plasma-enhanced chemical vapor deposition, J. Appl. Phys. 104 (2008) 073301-9.
[29] Sz. Kátai, Z. Tass, Gy. Hárs, P. Deák, Measurement of ion energy distributions in the bias enhanced nucleation of chemical vapor deposited diamond, J. Appl. Phys. 86 (1999) 5549-5555.
[30] H. Zhou, C. Qiu, F. Yu, H. Yang, M. Chen, L. Hu, Y. Guo, L. Sun, Raman scattering of monolayer graphene: the temperature and oxygen doping effects, J. Phys. D: Appl. Phys. 44 (2011) 185404-6. [31] E. del Corro, M. Taravillo, V. G. Baonza, Nonlinear strain effects in 21
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double-resonance Raman bands of graphite, graphene, and related materials, Phys. Rev. B 85 (2012) 033407-5. [32] M. Ohring, The Materials Science of Thin Films (Academic Press, Boston,
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1992).
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[33] N. Mounet and N. Marzari, First-principles determination of the structural,
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vibrational and thermodynamic properties of diamond, graphite, and derivatives, Phys. Rev. B 71(2005) 205214-14.
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[34] Y. Su, H Wei, R. Gao, Z. Yang, J. Zhang, Z. Zhong, Y. Zhang, Exceptional
Carbon 50 (2012) 2804-2809.
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negative thermal expansion and viscoelastic properties of graphene oxide paper,
[35] J. Roithová, J. Žabka, Z. Dolejšek, Z. Herman, Collisions of slow polyatomic
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Chem. B106 (2002) 8293-8301.
[36] S. Maniv, W. D. Westwood, and P. J. Scanlon, Calculation of the current-voltage-pressure characteristics of dc diode sputtering discharges, J. Appl. Phys. 53 (1982) 856-860.
[37] P. H. Tan, Y. M. Deng, Q. Zhao, and W. C. Cheng, The intrinsic temperature effect of the Raman spectra of graphite, Appl. Phys. Lett. 74 (1999) 1818-1820. [38] B. B. Wang, B. Zhang, Effects of carbon film roughness on growth of carbon nanotip arrays by plasma-enhanced hot filament chemical vapor deposition, Carbon 44 (2006) 1949-1953. 22
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[39] R. G.Forbes, C.J. Edgcombe, U.Valdrè, Some comments on models for field enhancement, Ultramicroscopy 95 (2003) 57-65. [40] C. Y. Zhi, X. D. Bai, E. G. Wang, Enhanced field emission from carbon
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an
us
cr
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nanotubes by hydrogen plasma treatment, Appl. Phys. Lett. 81 (2002) 1690-1692.
23
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Table captions Table I Growth parameters: bias current Ib, bias Ub and growth time t.
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Table II Positions of Raman peaks measured at different temperatures.
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Figure captions
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Figure 1 Low- and high-resolution FESEM images of specimens A(a, d), B(b, e) and
M
C(c, f).
te
d
Figure 2 Raman spectra of specimens A (1), B(2) and C(3).
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Figure 3 (a) Low-resolution and (b) high-resolution TEM images of specimen B.
Figure 4 Raman spectra of specimen A measured at different temperatures.
Figure 5 Schematics of formation of vertical carbon nanoflakes under plasma conditions
24
Page 24 of 30
Table I Growth parameters: bias current Ib, bias Ub and growth time t.
Ub(V) ~1000 ~1000 ~1080
ip t
t(min) 3 5 5
cr
Ib(mA) 160 160 180
an
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Specimen A B C
d
G(cm-1) 1593 1593 1588 1581
te
D(cm-1) 1355 1353 1350 1352
2D(cm-1) 2704 2703 2701 2696
D+G(cm-1) 2944 2952 2930 2930
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Peak -30ºC 100ºC 300ºC 450ºC
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Table II Positions of Raman peaks measured at different temperatures.
25
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ip t cr us an M d te Ac ce p Figure 1 Low- and high-resolution FESEM images of specimens A(a, d), B(b, e) and C(c, f).
26
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ip t cr us an M d te
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Figure 2 Raman spectra of specimens A (1), B(2) and C(3).
27
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Figure 3 (a) Low-resolution and (b) high-resolution TEM images of specimen B.
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ip t cr us an M d te Ac ce p
Figure 4 Raman spectra of specimen A measured at different temperatures.
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Figure 5 Schematics of formation of vertical carbon nanoflakes under plasma conditions
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S0169-4332(14)02545-8 http://dx.doi.org/doi:10.1016/j.apsusc.2014.11.072 APSUSC 29117
To appear in:
APSUSC
Received date: Accepted date:
30-9-2014 11-11-2014
Please cite this article as: B.B. Wang, K. Zheng, Q.J. Cheng, K. Ostrikov, Plasma effects in aligned carbon nanoflake growth by plasma-enhanced hot filament chemical vapor deposition, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.11.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
►Plasma-specific effects in the growth of carbon nanoflakes (CNFs) are studied.
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►Electic field in the plasma sheath promotes separation of CNFs from the substrate.
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►The orentention of GNFs is related to the combined electic force and growth
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effects.
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►The high growth grates of aligned GNFs are plasma-related.
1
Page 1 of 30
Plasma effects in aligned carbon nanoflake growth by plasma-enhanced hot filament
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chemical vapor deposition
College of Chemistry and Chemical Engineering, Chongqing University of
us
1
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B. B. Wang1, K. Zheng2, Q. J. Cheng3,*, K. Ostrikov4,5,6
Technology, 69 Hongguang Rd, Lijiatuo, Banan District, Chongqing 400054, PR
2
an
China.
Institute of Microstructure and Properties of Advanced Materials, Beijing University
M
of Technology, Beijing 100124, P. R. China 3
School of Energy Research, Xiamen University, Xiamen 361005, PR China
4
Plasma
Australia
d
Center
(PNCA),
Manufacturing
Flagship,
te
Nanoscience
Commonwealth Scientific and Industrial Research Organization, P. O. Box 218, NSW 2070, Australia
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Lindfield 5
Institute for Future Environments and School of Chemistry, Physics and Mechanical
Engineering, Queensland University of Technology, Brisbane QLD 4000, Australia 6
Plasma Nanoscience, School of Physics, The University of Sydney, Sydney NSW 2006,
Australia.
*Corresponding author: Assoc. Prof. Q. J. Cheng, Tel.: +86 592 5952797; fax: +86 592 2188053. E-mail address: [email protected] (Q.J. Cheng).
2
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ABSTRACT Carbon nanofilms are directly grown on silicon substrates by plasma-enhanced hot filament chemical vapor deposition in methane environment. It is shown that the
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nanofilms are composed of aligned carbon nanoflakes by extensive investigation of
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experimental results of field emission scanning electron microscopy, micro-Raman
us
spectroscopy and transmission electron microscopy. In comparison with the graphene-like films grown without plasmas, the carbon nanoflakes grow in an
an
alignment mode and the growth rate of the films is increased. The effects of the plasma on the growth of the carbon nanofilms are studied. The plasma plays three
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main effects of (1) promoting the separation of the carbon nanoflakes from the silicon substrate, (2) accelerating the motion of hydrocarbon radicals, and (3) enhancing the
te
d
deposition of hydrocarbon ions onto the substrate surface. Due to these plasma-specific effects, the carbon nanofilms can be formed from the aligned carbon
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nanoflakes with a high rate. These results advance our knowledge on the synthesis, properties and applications of graphene-based materials.
Keywords: Carbon nanoflakes; Chemical vapor deposition; Plasma; Polarization
3
Page 3 of 30
1. Introduction Since graphene was discovered [1], graphene-based materials, which include graphene flakes, reduced graphene oxide, graphene quantum dots, etc., have been
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extensively studied due to their unique structures and potential applications [2-5].
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Because of the gapless property of perfect graphene, it requires further treatment to
us
produce a bandgap for its applications in the areas of electronics and optoeletronics [6]. For example, the bandgap of graphene can be induced to form by oxygen and
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hydrogen plasma treatment [6,7]. It is interesting to find that the graphene nanoribbons can produce the bandgap [8,9], which motivates extensive studies of
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graphene nanoflake-like carbon nanomaterials.
Graphene nanoflakes have been successfully synthesized by chemical vapor
te
d
deposition with and without using catalysts [10-15], however the growth mechanism has not been well understood. For example, Yu et al. suggested that the synthesis of
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graphene is a surface precipitation process with a controlled cooling [12], while Li et
al. proposed that the formation of graphene is a surface-catalyzed process rather than a surface precipitation process [13], and Seo et al. suggested that the formation of
graphene sheets starts from the carbon nanorods [14]. Graphene-like nanoflakes have also been produced in hot filament chemical
vapor deposition, where a complex process has been involved including catalysis, diffusion, assembly of carbon atoms, and several other microscopic effects [15,16]. These results indicate that the growth mechanism of graphene flakes still remains an open question. Since the properties and applications of graphene-based materials are 4
Page 4 of 30
highly related to the structure and stacking-assembly order of graphene sheets [14], the formation of graphene flakes requires further study to control the structure and growth of graphene-based materials.
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In the previous works [15,16], graphene-like nanoflakes were synthesized in hot
cr
filament chemical vapor deposition (HFCVD), where methane was used as reactive
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precursor gas. Plasma is extensively used to fabricate various nanomaterials such as carbon nanotubes [17], metal oxide nanowires and nanobelts [18], silicon nanocones
an
[19], and organic nanofilms [20], etc. This is why in this work the plasma is generated in the HFCVD system to enhance the growth of graphene flake-like carbon
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nanomaterials. As a result, the carbon films composed of carbon nanoflakes were formed, which are different from the previously reported results [14]. Namely, the
te
d
carbon nanofilms are composed of different aligned carbon nanoflakes rather than the vertical graphene sheets. Here, the carbon nanofilms composed of carbon nanoflakes
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are named graphene-like films (GLFs). Extensive characterization results suggest that the formation of carbon
nanoflakes is a complex process which is rather different from the previously reported processes [12,13]. In comparison with the graphene-like nanoflakes grown in HFCVD [21], the plasma is responsible for the aligned growth of the carbon nanoflakes. Simultaneously, it improves the growth rate of GLFs. Since the GLFs are composed of aligned carbon nanoflakes, more surfaces and edges of aligned carbon nanoflakes can be utilized in applications in the areas of fuel cells, bio-sensing, catalysis and energy storage devices, which may be superior to the applications based on 5
Page 5 of 30
substrate-bound flat graphene films [22,23]. However, the synthesis of vertical graphene-based structures is quite challenging [22]; this is the reason why the effects of the plasma on the growth of GLFs are
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studied in this paper. Although the growth of vertical carbon nanoflakes has recently
cr
attracted strong interest and a few growth models were suggested [23, 24], a few
us
issues remain and require further studies. For example, how do the carbon nanoflakes bend upward? What determines the orientation of the carbon nanoflakes? Why is the
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growth rate of the carbon nanoflakes parallel to the electric field produced by plasma higher than that perpendicular to the electric field produced by plasma? This work
2.1. Synthesis of GLFs
d te
2. Experiments
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further clarifies these issues.
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The GLFs were synthesized by plasma-enhanced hot filament chemical vapor
deposition (PEHFCVD), which is described in Ref.(21). Briefly, the PEHFCVD system is composed of the CVD chamber, heating and bias systems. The heating and bias systems constructed with three tungsten filaments and a DC power supply are situated in the CVD chamber for the decomposition of reactive gases, heating of substrate and production of plasma, respectively. Due to the high temperature of filaments (~ 2000 ºC), the substrate is easily heated by the filaments to the growth temperature (>900 ºC) within a short time because the distance between the filaments and substrate is about 8 mm. The anode and cathode of DC power supply were 6
Page 6 of 30
connected to the filaments and substrate through a molybdenum holder, respectively. Before the growth of GLFs, the silicon wafers were pretreated to clean their surfaces. Firstly, the silicon wafers were ultrasonically cleaned for 15 min in
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methylbenzene, acetone and alcohol liquid, respectively, and then they were boiled for
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15 min at 75 ºC in a solution mixed with aqueous ammonia, oxydol and deionized
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water. Finally, the silicon wafers were cleaned for many times with deionized water. After the clean silicon substrate was placed in the CVD chamber, the chamber
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was evacuated till the basic pressure was lower than 2 Pa, and then 20 sccm nitrogen and 80 sccm hydrogen were introduced into the chamber. Consequently, the pressure
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was increased. Through adjusting the valve between the chamber and vacuum pump, the pressure was stabilized at about 2×103 Pa. In this case, the filaments were heated
te
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to about 1800 ºC. Once the silicon substrate was heated by the filaments to ~ 850 ºC, the DC power supply was turned on to produce the plasma. The surface of silicon
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substrate was further treated for 5 min in the H2-N2 plasma under a bias current of 160
mA.
After the silicon surface was treated in the H2-N2 plasma, 50 sccm methane was
inlet into the chamber, and then the pressure was adjusted to about 2×103 Pa. To fully
decompose methane, the filaments were heated to about 2000 ºC through setting the current of filaments. As a result, the silicon substrate was heated to ~ 930 ºC. In this case, the DC power supply was turned on again and the bias current was set to produce the GLFs. In this work, three specimens were prepared and the growth parameters are shown in Table I. 7
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2.2. Characterization of GLFs The structure and composition of GLFs were studied using a Hitachi S-4800 field emission scanning electron microscope (FESEM) operated at 15 kV, a JEOL
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2010F transmission electron microscope (TEM) operated at 200KV and a HR 800
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Raman spectroscope using the 532 nm line of semiconductor laser as the excitation
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source. To study the structural change of graphene-like flakes, the specimen A was
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also measured at different temperatures in the Raman spectroscope.
3. Results
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Figure 1 shows the FESEM images of specimens A-C. From Fig.1, one can see that the films are composed of the aligned nanoflakes. In comparison with the films
te
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grown without the plasma [21], the growth rate of GLFs is enhanced by the plasma because the thickness of GLF grown for 5 min in Ref.(21) is about 128 nm and the
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thickness of GLF shown in Fig.1(a) (grown for 3 min) is about 230 nm. In addition, Fig.1(d)~(f) indicate that the nanoflakes are grown in an alignment mode. Figure 2 shows the Raman spectra of specimens A-C. As shown in Fig.2, every
spectrum exhibits the D, G, 2D and D+G Raman peaks of grahene sheets at about 1352, 1592, 2703 and 2949 cm-1 [7,14], which indicates that the films are composed
of graphene-like nanoflakes. To further confirm the structure of GLFs, the specimen B was studied by TEM after the carbon nanoflakes were scraped from silicon surface and the TEM results are shown in Fig.3. From Fig.3(a) one can clearly see the carbon nanoflakes, which 8
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further indicate that the carbon films are composed of carbon nanoflakes. As shown in Fig.3(b), the carbon nanoflake are the hybrid of crystalline and amorphous structures because the right side of Fig.3(b) show the crystalline and amorphous structures. Due
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to the aggregation of carbon nanoflakes, the structure of carbon nanoflakes is not
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clearly shown in left side of Fig.3(b).
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In the growth process of carbon nanoflakes, the structure of carbon nanoflakes continuously changes. To illuminate the structural change, the specimen A was studied
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in micro-Raman system at different temperatures and the results are shown in Fig.4. According to Fig.4, we obtain the positions of Raman peaks and they are summarized
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in Table II. From the data in Table II, one can obviously see that the G and 2D peaks
4. Discussion
te
d
shift towards low frequency with the increase of temperature.
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Figure 1 indicates that the carbon nanoflakes are grown in an alignment mode,
which is related to the plasma effects. For the growth of vertical carbon nanoflakes, Zhao et al. suggested that it is related to the nucleation of graphene and correlates with the supply rate of carbon adatoms [23], but the relation of the growth of vertical graphenes with the plasma was not discussed in detail. In Ref.(24), Zhu et al.
suggested that the growth of vertical carbon nanoflakes is affected by the strain force at the grain boundaries and that the orientation of carbon nanoflakes was determined by the polarization of graphite layers associated with the electric field in the plasma sheath. However, how the force is formed is not explained so that the effect of the 9
Page 9 of 30
force on the bending of carbon nanoflakes is not well understood. Indeed, the carbon nanoflakes are polarized in the electric field, but the effects of polarization are not discussed. In fact, the bending and orientation of the carbon nanoflakes are
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determined by several forces related to the plasma. In this section, we study the
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effects of the plasma on the growth of GLFs.
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4.1. Effects of plasma on silicon surface
Prior to the growth of GLFs, the pretreatment of silicon substrate can lead to the
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change of silicon surface. In the H2-N2 plasma, hydrogen and nitrogen are ionized into hydrogen and nitrogenous ions. Simultaneously, a strong electric field is formed near
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the silicon surface [25]. Under the electric field, the ions bombard the surface of silicon substrate. As a result, the residual oxygen on silicon surface is removed by
d
hydrogen ions through the following reactions, (1)
2H + O → H2O.
(2)
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te
H+ + e → H,
Eq.(1) represents ion neutralization on the substrate [26]. Due to the large mass of nitrogen ions relative to hydrogen ions, the bombardment of nitrogen ions results in the production of microscopic pits (micro-pits) on the silicon surface. These micro-pits will become the nucleation spots for precipitated carbon material, which are confirmed by the results of Ref.(27). 4.2. Formation of vertical carbon nanoflakes under plasma conditions After the pretreatment of the silicon substrate is completed, methane is let into the CVD chamber to replace hydrogen and nitrogen. Consequently, methane is 10
Page 10 of 30
ionized into various hydrocarbon ions by the plasma [28,29]. When these hydrocarbon ions deposit on the silicon surface, they nucleate on the micro-pits to form carbon islands [27]. Depending on the carbon island size and morphology, the
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carbon nanoflakes form on silicon surface [15,16].
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In comparison with Fig.2, Fig.4 shows a shift of G and 2D peaks towards lower
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wavenumbers with the increase of temperature, which indicates that stress is generated within carbon nanoflakes [30]. According to the results of Ref.(31), the
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stress is the tensile stress (FT) parallel to the carbon nanoflakes at high temperature, which results from the shrinking of carbon nanoflakes due to the negative expansion
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coefficient of graphene and the basal plane of graphite at high temperature [32-34]. Since one edge of a carbon nanoflake is anchored on the carbon island [16], the free
te
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edge of the carbon nanoflake bends upwards under the tensile stress [32]. During the growth, the hydrocarbon ions deposit on the surfaces of carbon
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nanoflakes to become the hydrocarbon radicals because the silicon substrate is connected with the cathode of the bias power supply [26]. However, the interaction probability of these hydrocarbon radicals is low at our growth temperature because there is no chemical reaction to occur on the surface of the highly-oriented pyrolytic graphite at above 600 ºC [35]. Thus, the hydrocarbon radicals rapidly move along the surfaces of carbon nanoflakes and reach the free edges of carbon nanoflakes in the absence of high flux of hydrogen [24]. Due to the difference in the electro-negativtivity of carbon and hydrogen elements, the C-H bonds are of the polar bond type, which are polarized by the 11
Page 11 of 30
electric field formed by the plasma. As a result, the free edges of carbon nanoflakes are negatively charged and a repelling force (FR) parallel to the substrate is formed between two carbon nanoflakes. The repelling force prevents both carbon nanoflakes
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from contact and further promotes the upward bending of the carbon nanoflakes.
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Simultaneously, an electric force (FE) is applied to the free edges of carbon
E
2U c x 1 , d d
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nanoflakes. For the field formed by the plasma, it can be expressed by
(3)
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where U c is the voltage applied to the substrate, d is the thickness of cathode
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sheath and x is the distance from the substrate surface [36]. Eq.(3) indicates that the field is strong near the substrate surface, which implies that a strong electric force is
d
applied to the carbon nanoflakes.
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With the growth of carbon nanoflakes, the direction of tensile stress tends to the
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direction of electric force. Once they have the same direction, both the tensile stress and the electric force result in the separation of carbon nanoflakes from the silicon surface. Under the repelling force, it is difficult for two carbon nanoflakes to come into contact. Eventually, the vertical carbon nanoflakes are formed. To be clear, the formation of vertical carbon nanoflakes under plasma conditions
can be described by Fig.5. As shown in Fig.5(a), the carbon nanoflakes form on silicon surface depending on the carbon islands. Fig.5(b) shows that the free edge of the carbon nanoflake bends upwards under the tensile stress and the hydrocarbon radicals move along the surfaces of carbon nanoflakes. In Fig.5(c), the repelling and electric forces are applied to the carbon nanoflakes after the free edges of carbon 12
Page 12 of 30
nanoflakes are negatively charged. Under the repelling force, the carbon nanofalkes further bend upward. With the growth of carbon nanoflakes, the direction of tensile stress tends to the direction of electric force. After the tensile stress has the same
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4.3. Orientation of carbon nanoflakes under plasma conditions
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carbon nanoflakes vertical growth, which is described by Fig.5(d).
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direction as the electric force, both the tensile stress and electric force make the
From Fig.1, we obtain the thickness of carbon nanoflakes and it is more than 4
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nm, which indicates that a carbon nanoflake is composed of multilayer graphene-like nanosheets. Thus, the carbon nanoflakes expand along their normal direction during
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the growth because the expansion coefficient of graphite in the direction of c-axis is
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the carbon nanoflakes.
d
positive [37]. As a result, a compressive stress is generated in the normal direction of
However, the high aspect ratio of thin carbon nanoflakes leads to the electric
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field focusing near the free end of the carbon nanoflakes. For the DC plasma, the thickness of the cathode sheath is about several millimeters [38]. From Fig.1(d), the height of a vertical carbon nanoflake is about 48 nm. To facilitate analysis, the thickness of cathode sheath is assumed to be 1 mm. According to Eq.(3), the field on the free edge of vertical carbon nanoflakes is about 1.999 U c / d . Due to the vertical
carbon nanoflakes, the field enhancement factor
h/l
(4)
is about 11, where h and l are the height and thickness of vertical carbon nanoflakes [39]. Consequently, the actual electric field on the free end of the vertical 13
Page 13 of 30
carbon nanoflakes is about 22 U c / d . Thus, the electric field near the free edge of the carbon nanoflakes can be enhanced by the growth of the carbon nanoflakes. Because of the change of the height and thickness of the carbon nanoflakes with the growth
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time, the electric force changes during the growth of the carbon nanoflakes. In
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addition, the distance between two carbon nanoflakes and the charge on the edges of
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carbon nanoflakes are different, which results in the change of repelling forces acted on every carbon nanoflake.
an
Because of different bending torques for the stress, repelling and electric forces, the effects of these torques on the carbon nanoflakes are quite different. When the net
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torque is zero, the carbon nanoflake can grow vertically. Otherwise, the carbon nanoflakes are tilted with respect to the substrate surface. Since the forces vary
te
d
dynamically, the system is continuously driven out of equilibrium. As a result, the carbon nanoflakes grow in an alignment mode shown in Fig.1. We emphasize that the
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orientation of the carbon nanoflakes is determined by several forces. For the graphene-like nanoflakes grown by HFCVD [21], a low inclination angle with respect to the silicon surface was reported, while Fig.1(d) shows that the carbon nanoflakes are basically perpendicular to the silicon surface. Thus we conclude that plasma-specific effects significantly contribute to the aligned growth of the carbon nanoflakes. 4.4. Plasma effects on the growth rates Due to the formation of aligned carbon nanoflakes, the hydrocarbon radicals
mainly move towards the free edges of carbon nanoflakes because there is a low 14
Page 14 of 30
reaction probability of them on the surface of carbon nanoflakes at high growth temperature [35]. After the hydrocarbon ions deposit on the substrate surface to become hydrocarbon radicals, they are polarized by the electric field into the polar
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radicals such as Cδ--Hδ+ due to the difference in the electro-negativity of carbon and
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hydrogen elements [40]. Under the electric field, the radicals rapidly move towards
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the free edges of carbon nanoflakes. After the radicals reach the free edges of the carbon nanoflakes, they bond the free edges due to the dangling bonds of free edges.
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As a result, the area of carbon nanoflakes is increased. Because there are some defects on the surfaces of the carbon nanoflakes, the radicals can bond with the
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defects if they cannot overcome the potential barrier formed by the defects under the electric field. In this case, some radicals can bond with other radicals to increase the
te
d
thickness of the carbon nanoflakes. However, the high growth temperature can result in the re-evaporation of the deposited radicals to slow down the increase of the carbon
Ac ce p
nanoflakes in thickness. This is the reason that the growth rate of the carbon nanoflakes along the electric field is the highest. When the two aligned carbon nanoflakes touch, the free edges of them combine
together to form defects on the joint. Depending on the defects, new carbon nanoflakes may form and then develop in the alignment mode. Finally, the carbon nanoflakes form the thin GLFs. In section III, the results indicate that the growth rates of GLFs are increased by the plasma, which is related to the motion of hydrocarbon ions in the plasma. After methane is ionized into the hydrocarbon ions such as CH3+ and C2H2+ [27,28], they 15
Page 15 of 30
move towards the substrate under the electric field and temperature difference between the filaments and the substrate. When the GLFs grow in the HFCVD system, methane is predominantly decomposed into hydrocarbon radicals, e.g. (5)
ip t
CH4 → CH3 + H.
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The motion of hydrocarbon radicals is also affected by the temperature difference
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between the filaments and the substrate. In other words, the motion of hydrocarbon ions is accelerated in the plasma, which results in the fast formation of carbon
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nanoflakes. Consequently, the growth of GLFs is enhanced by the plasma.
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5. Conclusion
In summary, GLFs were directly synthesized on silicon substrates by PEHFCVD
te
d
in reactive methane environment. The structural and compositional properties of GLFs were studied using FESEM/TEM and micro-Raman spectroscopy, respectively.
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The characterization results help elucidating the effects of the plasma on the growth of GLFs. The plasma promotes the separation of carbon nanoflakes from the silicon substrate and improves the aligned growth of the carbon nanoflakes. Moreover, the hydrocarbon ions can move towards the substrate faster under the plasma conditions, which results in the high growth rates of GLFs. The achievements obtained here can enrich our knowledge of synthesizing graphene-based materials and contribute to the development of new applications of graphene-based nanomaterials.
Acknowledgements 16
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This work was partially supported by the Fundamental Research Funds for the Central Universities, Program for New Century Excellent Talents in Fujian Province University (NCETFJ), Scientific Research Foundation for the Returned Overseas
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Chinese Scholars, and CSIRO’s OCE Science Leadership Scheme and the Australian
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Research Council.
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Table captions Table I Growth parameters: bias current Ib, bias Ub and growth time t.
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Table II Positions of Raman peaks measured at different temperatures.
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Figure captions
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Figure 1 Low- and high-resolution FESEM images of specimens A(a, d), B(b, e) and
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C(c, f).
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Figure 2 Raman spectra of specimens A (1), B(2) and C(3).
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Figure 3 (a) Low-resolution and (b) high-resolution TEM images of specimen B.
Figure 4 Raman spectra of specimen A measured at different temperatures.
Figure 5 Schematics of formation of vertical carbon nanoflakes under plasma conditions
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Table I Growth parameters: bias current Ib, bias Ub and growth time t.
Ub(V) ~1000 ~1000 ~1080
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t(min) 3 5 5
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Ib(mA) 160 160 180
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Specimen A B C
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G(cm-1) 1593 1593 1588 1581
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D(cm-1) 1355 1353 1350 1352
2D(cm-1) 2704 2703 2701 2696
D+G(cm-1) 2944 2952 2930 2930
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Peak -30ºC 100ºC 300ºC 450ºC
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Table II Positions of Raman peaks measured at different temperatures.
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ip t cr us an M d te Ac ce p Figure 1 Low- and high-resolution FESEM images of specimens A(a, d), B(b, e) and C(c, f).
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Figure 2 Raman spectra of specimens A (1), B(2) and C(3).
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Figure 3 (a) Low-resolution and (b) high-resolution TEM images of specimen B.
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Figure 4 Raman spectra of specimen A measured at different temperatures.
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Figure 5 Schematics of formation of vertical carbon nanoflakes under plasma conditions
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