14.1 Carbon Nanostructures: Nanotubes and Nanoonions
513
curvature can be ensured with the incorporations of carbon pentagons. Therefore,
closed SWCNTs can also be derived in another imaginary experiment as cutting a
fullerene molecule into halves and packing hexagonal carbon atom units between
these hemispherical caps.
Carbon nanoonions are multiwalled carbon nanostructured whose inner core is a
fullerene-like nearly spherical molecule. Packing further carbon layers onto the core
by sustaining the curvature with the required 0.34 nm spacing results in nanoonions.
To obtain such objects, the proportion of the five-member rings should be decreased
as the diameter gets larger.
Although the identification of various carbon nanostructures can be carried out on
the basis of their physical (e.g., optical absorption) properties, the direct visualization
of their structure is often decisive. This can be performed by TEM. A representative collection of TEM images of various carbon nanostructures obtained with arc
discharge synthesis is shown in Fig. 14.3.
14.2 General Features of the Discharge Devices
and Processes
The review of Aora and Sharma [12] yields a very comprehensive list of former
publications on arc discharge-produced carbon nanostructures with detailed experimental conditions. As it can be seen from this collection, the common features of the
synthesis devices and conditions do not provide a full transparency for this field, and
minor differences in the experiments make the available information rather puzzled.
Below the major routes of the synthesis methods are summarized only.
At high plasma current densities when the cathode is heated up by the impinging
ions and hence, the electrons are thermally emitted from the cathode, we can speak
about an arc discharge. Arc discharges are typically driven with d.c. excitation, even
though a.c. arc plasmas can also be produced with frequencies not larger than a few
tens of kHz. At 1 MHz and above, the electrons become the major carriers of the
current in the plasma, and the motion of the ions can no longer follow the oscillation
of the electric field (radiofrequency /RF/ plasma). Above the GHz frequency level
(microwave /MW/ plasma), the heating mechanism depends on the density of the
plasma, ranging from the surface-wave mode at high plasma density (and small
penetration depth limited by the so-called skin effect) to resonator mode at small
plasma density (and penetration depths much larger than the plasma dimension).
Most RF and MW plasma sources are operated at specific frequencies assigned for
industrial use in order to avoid interference with radio communication. Hence, the
frequency used for RF and MW plasmas cannot be optimized for the specific goal
of the research. The majority of the works to be cited below refers to arc discharges,
and hence, this will be discussed below in detail.
Arc discharge devices work with electrodes positioned in close vicinity of each
other. The electrodes can be momentarily contacted for the ignition of the discharge,
513
curvature can be ensured with the incorporations of carbon pentagons. Therefore,
closed SWCNTs can also be derived in another imaginary experiment as cutting a
fullerene molecule into halves and packing hexagonal carbon atom units between
these hemispherical caps.
Carbon nanoonions are multiwalled carbon nanostructured whose inner core is a
fullerene-like nearly spherical molecule. Packing further carbon layers onto the core
by sustaining the curvature with the required 0.34 nm spacing results in nanoonions.
To obtain such objects, the proportion of the five-member rings should be decreased
as the diameter gets larger.
Although the identification of various carbon nanostructures can be carried out on
the basis of their physical (e.g., optical absorption) properties, the direct visualization
of their structure is often decisive. This can be performed by TEM. A representative collection of TEM images of various carbon nanostructures obtained with arc
discharge synthesis is shown in Fig. 14.3.
14.2 General Features of the Discharge Devices
and Processes
The review of Aora and Sharma [12] yields a very comprehensive list of former
publications on arc discharge-produced carbon nanostructures with detailed experimental conditions. As it can be seen from this collection, the common features of the
synthesis devices and conditions do not provide a full transparency for this field, and
minor differences in the experiments make the available information rather puzzled.
Below the major routes of the synthesis methods are summarized only.
At high plasma current densities when the cathode is heated up by the impinging
ions and hence, the electrons are thermally emitted from the cathode, we can speak
about an arc discharge. Arc discharges are typically driven with d.c. excitation, even
though a.c. arc plasmas can also be produced with frequencies not larger than a few
tens of kHz. At 1 MHz and above, the electrons become the major carriers of the
current in the plasma, and the motion of the ions can no longer follow the oscillation
of the electric field (radiofrequency /RF/ plasma). Above the GHz frequency level
(microwave /MW/ plasma), the heating mechanism depends on the density of the
plasma, ranging from the surface-wave mode at high plasma density (and small
penetration depth limited by the so-called skin effect) to resonator mode at small
plasma density (and penetration depths much larger than the plasma dimension).
Most RF and MW plasma sources are operated at specific frequencies assigned for
industrial use in order to avoid interference with radio communication. Hence, the
frequency used for RF and MW plasmas cannot be optimized for the specific goal
of the research. The majority of the works to be cited below refers to arc discharges,
and hence, this will be discussed below in detail.
Arc discharge devices work with electrodes positioned in close vicinity of each
other. The electrodes can be momentarily contacted for the ignition of the discharge,