Patent Application
20130302592
This application claims priority to Korean Patent Application No. 10-2012-0049140, filed on May 9, 2012, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference.
1. Field
Embodiments relate to a method for growing carbon nanoflakes and a carbon nanoflake structure formed thereby. More particularly, the embodiments relate to a method for growing carbon nanoflakes, including inducing partial etching of graphene layers of carbon nanotubes through an adequate composition of precursor gases, CH4, H2 and Ar, while allowing carbon nanoflakes to grow at the etched site in a plane-like shape, as well as to a carbon nanoflake structure formed by the same method.
2. Description of the Related Art
Carbon nanomaterials have potential applicability in field emission devices, electronic devices, optoelectronic devices, gas and energy storage devices, or the like. Particularly, carbon nanoflakes (CNFs) and carbon nanowalls (CNWs) are carbon nanomaterials having a two-dimensional structure, and have excellent physical and chemical properties, such as a high specific surface area and high hydrophobicity. Thus, they are applicable to large-area field emission sources, gas sensors, high-capacity capacitors, or the like.
Carbon nanoflakes may be synthesized through various methods. Since carbon nanoflakes have been synthesized through an evaporation process using direct current arc discharge (Ando Y., Zhao X., Ohkohchi M., Production of petal-like graphite sheets by hydrogen arc discharge, Carbon, 1997: 35(1): 153-8), attempts have been made to synthesize carbon nanoflakes by using plasma assisted chemical vapor deposition (PACVD), to which DC plasma, helicon plasma or microwave plasma is applied individually, and hot filament CVD (HFCVD). In addition, various types of catalysts, growing conditions and substrates have been applied as conditions for synthesis independently from deposition methods. Nevertheless, growth mechanisms of carbon nanoflakes still have not been clearly understood.
An aspect of the present disclosure is directed to providing a method for growing carbon nanoflakes, including inducing partial etching of graphene layers of carbon nanotubes through an adequate composition of precursor gases, CH4, H2 and Ar, while allowing carbon nanoflakes to grow at the etched site in a plane-like shape, as well as to a carbon nanoflake structure formed by the same method.
According to an embodiment, a method for growing carbon nanoflakes includes: providing a silicon substrate having carbon nanotubes; and growing carbon nanoflakes on the carbon nanotubes through a chemical vapor deposition process using a mixed gas of CH4, H2 and Ar as a precursor. During the chemical vapor deposition process, the mixed gas of CH4, H2 and Ar may be in an atmosphere with excess Ar, graphene layers forming the carbon nanotubes may be etched partially under the atmosphere with excess Ar, and graphene layers of carbon nanoflakes may be grown at the etched site.
The mixed gas of CH4, H2 and Ar may have a composition of CH4:H2:Ar=1:4-15:84-95. In addition, the carbon nanotubes may be multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs).
The operation of providing a silicon substrate having carbon nanotubes may include: preparing a methanol solution in which carbon nanotubes are dispersed; casting the methanol solution in which carbon nanotubes are dispersed onto a silicon substrate; and drying the substrate to evaporate methanol.
According to an embodiment, a carbon nanoflake structure includes carbon nanotubes provided on a silicon substrate, and carbon nanoflakes grown on the carbon nanotubes, wherein the carbon nanoflakes are grown through a chemical vapor deposition process using a mixed gas of CH4, H2 and Ar in an atmosphere with excess Ar as a precursor. During the chemical vapor deposition process, graphene layers forming the carbon nanotubes may be etched partially under an atmosphere with excess Ar, and graphene layers of carbon nanoflakes may be grown at the etched site. The mixed gas of CH4, H2 and Ar may have a composition of CH4:H2:Ar=1:4-15:84-95.
The above and other aspects, features and advantages of the disclosed exemplary embodiments will be more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
Exemplary embodiments now will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown
According to an embodiment, carbon nanoflakes (CNFs) may be formed on carbon nanotubes (CNTs). The carbon nanoflakes are formed through a chemical vapor deposition process, and a mixed gas of CH4, H2 and Ar is used as a precursor gas.
The mixed gas of CH4, H2 and Ar serves to carry out partial etching and removal of graphene layers forming carbon nanotubes, and functions as a carbon source for the carbon nanoflakes to be grown on the site from which the graphene layers are etched out.
It is required for carbon nanotubes to be etched adequately to allow growth of carbon nanoflakes. As used herein, the expression ‘etched adequately’ means that graphene layers forming carbon nanotubes are etched partially to such a degree that the graphene layers retain dangling bonds. The dangling bonds of the graphene layers serve as growth nuclei for carbon nanoflakes.
To perform partial etching of the graphene layers of carbon nanotubes, it is required to control the composition of a mixed gas of CH4, H2 and Ar. When the mixed gas of CH4, H2 and Ar is in an atmosphere with excess H2, carbon nanotubes may be etched excessively due to H2, thereby making it difficult to grow carbon nanoflakes. On the other hand, when the mixed gas is in an atmosphere with excess Ar, excessive etching of carbon nanotubes is inhibited. In other words, it is possible to induce partial etching of carbon nanotubes so that carbon nanoflakes may be grown.
To allow growth of carbon nanoflakes, the mixed gas of CH4, H2 and Ar may have a composition of CH4:H2:Ar=1:4-15:84-95. When H2 is present in an amount greater than 15 vol %, carbon nanotubes may be etched excessively. On the other hand, when Ar is present in an amount greater than 95 vol %, carbon atom sources become insufficient, thereby making it difficult to grow carbon nanoflakes.
Carbon nanotubes are dispersed and fixed on a silicon substrate. Particular examples of carbon nanotubes that may be used herein include both multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). MWCNTs have a plurality of graphene layers wound into a cylindrical shape, while SWCNTs have a single graphene layer wound into a cylindrical shape.
While MWCNTs and SWCNTs have a cylindrical shape, carbon nanoflakes formed thereon have a plane-like shape with no curved surface. This is because internal stress applied to carbon nanotubes is released due to the partial etching of carbon nanotubes. The internal stress applied to the inside of carbon nanotubes so that they have a cylindrical shape is released by the partial etching of graphene layers, and then graphene layers of carbon nanoflakes are grown on the etched site from which the internal stress is released. In this manner, carbon nanoflakes grown in a plane-like shape.
According to an embodiment, since the carbon nanoflakes grow on such partially etched graphene layers of carbon nanotubes, it is not possible to grow carbon nanoflakes on nanocrystalline diamond or mesoporous carbon having no graphene layer structure.
Meanwhile, when growing carbon nanoflakes according to an embodiment, no additional catalyst is required for the growth of carbon nanoflakes and no additional plasma application is required for stimulating reaction. According to an embodiment, carbon nanoflakes may be grown on carbon nanotubes through hot filament CVD (HFCVD).
The examples and experiments will now be described together with the results of experiments to illustrate the method for growing carbon nanoflakes disclosed herein.
MWCNTs having a purity of 95 wt % or more and available from Carbon Nano-material Technology Co., Ltd. are dispersed in methanol and treated in an ultrasonic bath for 30 minutes. Then, the methanol solution containing the MWCNTs dispersed therein is applied by drop-casting to a p-type silicon substrate grown in the direction of (100) and having a size of 1×1 inch2, followed by drying at room temperature for 12 hours.
Then, the substrate is mounted to the substrate holder of a hot filament CVD (HFCVD) system. The substrate holder is provided on a water-cooling block. A carbonized tungsten filament with a diameter of 0.3 mm is provided on the top of the substrate holder, and the substrate is spaced apart from the tungsten filament by about 10 mm. The reaction chamber maintains a vacuum state of ˜10−3 Torr before deposition. As a mixed gas of CH4, H2 and Ar is introduced to the chamber, the pressure in the chamber is increased. When the chamber reaches an internal pressure of 7.5 Torr, the current applied to the tungsten filament is increased from 0 to a reaction condition of 8.5 A. The time required to increase the current to 8.5 A is 4 minutes.
While the chamber is maintained continuously at an internal pressure of 70.5 Torr, deposition is carried out for 2 hours. During deposition, the tungsten filament is measured to have a temperature of 2400° C. After measuring the deposition temperature with a thermocouple provided on the substrate holder, it is observed that the deposition temperature is 840° C.
When carrying out a deposition process, the mixed gas of CH4, H2 and Ar is set to a total feed flux of 100 sccm (standard cubic centimeter per minute). While the flux of CH4 is fixed at 1 sccm, the flux of H2 and that of Ar are varied. In other words, the flux of CH4/H2/Ar is varied within a range of 1/84/15 to 1/15/84.
To investigate the growth mechanism of carbon nanoflakes, silicon substrates, on which nanocrystalline diamond (diameter 5 nm), mesoporous carbon (available from Sigma Aldrich Co.) or SWCNTs (available from Carbon Nano-material Technology, Co. Ltd.) are dispersed individually, are subjected to the same processing conditions as the substrate on which MWCNTs are dispersed to carry out deposition.
Under an atmosphere with excess hydrogen atoms, carbon (SP2) is etched with ease. Similarly, even under a low content of Ar gas, carbon (SP2) is etched. Only under an atmosphere with excess Ar, carbon nanoflakes are formed sufficiently. This suggests that such an atmosphere with excess Ar inhibits carbon (SP2) of MWCNTs from being etched, while facilitating nucleation of carbon nanoflakes.
The mixed gas composition (CH4/H2/Ar=1/15/84) that allows formation of carbon nanoflakes on a silicon substrate on which MWCNTs are dispersed is also applied to silicon substrates on which nanocrystalline diamond, mesoporous carbon and SWCNTs are dispersed individually. The same HFCVD process as described above is also applied.
As can be seen from the above results, carbon nanoflakes are grown on a substrate on which MWCNTs or SWCNTs are dispersed. Based on this, it is believed that the growth mechanism of carbon nanoflakes is related closely with CNT structures. Meanwhile, SWCNTs or SWCNTs have SP2 carbon atoms aligned in a honeycomb-like form.
To investigate the growth mechanism of carbon nanoflakes, MWCNTs are observed after a ramp stage is completed. As used herein, the term ‘ramp stage’ refers to the initial stage of growth from the time at which point electric current is applied to a tungsten filament to the time at which point a target current is applied. According to some embodiments, the term ‘ramp stage’ refers to a 4-minute stage during which a current of 0 to 8.5 A is applied.
The MWCNTs etched partially after the ramp stage, like in portion (b) of
Such partial etching of MWCNTs results from hydrogen atoms, and a dangling bond is formed at the etched site. The dangling bond functions as a growth nucleus for carbon nanoflakes, and carbon bonding and growth are performed at the dangling bond. In other words, carbon nanoflakes are grown at each etched site of MWCNT graphene layers with a direction of growth parallel to the MWCNT graphene layers.
Portion (e) of
The fact that growth of carbon nanoflakes is allowed not only on MWCNTs but also on SWCNTs is one of the most important findings. Carbon nanotubes have graphene layers wound into a cylindrical shape, and thus are subjected to internal stress. As mentioned above, the partial etching of CNTs breaks a connected structure of graphene layers to release internal stress, which, in turn, allows the carbon nanoflakes grown at the etched site of CNT graphene layers to grow in a plane-like shape having no curved surface. On the contrary, it is a matter of course that such a CNT-based carbon nanoflake growth mechanism cannot be applied to nanocrystalline diamond and mesoporous carbon having no graphene structure.
The method for growing carbon nanoflakes and the carbon nanoflake structure obtained thereby provide the following effects.
The method includes inducing partial etching of carbon nanotubes under an atmosphere with excess Ar, and thus it is possible to grow carbon nanoflakes easily with no need for application of an additional catalyst or plasma.
While the exemplary embodiments have been shown and described, it will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the spirit and scope of the present disclosure as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2012-0049140 | May 2012 | KR | national |
