**2. The interaction between CNTs and ion beams**

### **2.1 The modification of CNTs by ion beams**

When the energy of ions is very low, the penetration depth of ion beam is only several nanometers, and the damage produced by ion beam occurs on the surface of CNTs. The achieved energy of carbon atom in the CNTs by the collision is small. Therefore, the interaction between ion beam and the CNTs in this energy range should produce the lattice defects. These defects can be used to adjust the mechanical, electronic, and optoelectronic properties. The defects can also be used as the sources to add some new functional group, functional structure, and nanoparticles. The CNTs can be modified by non-covalently attached or covalently attached means through some chemical technology, chemical vapor deposition (CVD), etc. [9–12]. The structure by the noncovalently modification is unsteady. The hybrid material by the covalently attached means has a very strong force between the CNTs and the functional group, function structure, or nanoparticles. In general, the CNTs can be modified by covalently attached means under the irradiation of ion beams. The connection between CNTs and the functional group, function structure, or nanoparticles depends on the formation of the new covalent bond around the defects induced by the ion beams. The hybrid material is very steady. The modification of CNTs should be some new complex properties by the introduction of some new structure and can be used as new composite material.

In our previous work [13, 14], the interaction between CNTs and hydrocarbon ion beams having energy in the range 80–200 eV with substrate temperature from room temperature to 700°C has been studied. The ion beams are produced by Kaufman ion source. The CNTs are dispersed on the silicon wafer as the sample. **Figure 1** represents SEM images of CNTs being irradiated by hydrocarbon ions with 80 eV and different percentages of hydrogen in gas phase.

After CNTs are being irradiated by hydrocarbon ions, surface of CNTs becomes rough, and diameter is increased. As the ratio of H2:CH4 will be increased in gas phase, diameters of CNTs will decrease such as 60–70, 40–50, 8–40 nm to forming fragments and clusters. In the process hydrocarbon ion irradiation, there are two competing effects.

The first effect is deposition of carbon and carbon-based ions, and the second is etching of hydrogen ions.

**55**

**Figure 1.**

*(a) 0; (b) 10:1; (c) 20:1; and (d) 30:1 [13].*

encircled as "E" and "F."

*Reaction between Energy Particle Ion Beam with Carbon Nanotube*

The deposition rate is high due to dominance of deposition effect in case if hydrogen content is less in gas phase. If the hydrogen content is intermediate in the gas phase, then deposition rate will be time-consuming which results in depositing a thin layer. At high content of hydrogen ions, the deposition rate of carbon-based ions is entirely suppressed due to dominance of etching effect. The bare exposure of CNTs to hydrogen ions initiated the etching of CNTs which leads to etch these CNTs in parts and finally results in formation of pieces of carbon clusters. **Figure 2** represents TEM images of CNTs being exposed to 80 eV hydrocarbon ions (T = 700°C). The surface of CNTs is rough and inner hollow structure is undamaged. The selected area electron diffraction pattern (SAED) displays reflections (002 and 004) which correspond to intergraphene. **Figure 2b, c** represents the high-

*SEM images of CNTs being irradiated by hydrocarbon ions of energy 80 eV and varying ratios of H2:CH4, i.e.,* 

resolution transmission electron microscope (HRTEM) images of CNTs after treating with hydrocarbon ions having ratio H2: CH2 = 5:1 at 700°C for 30 and 90 min, respectively. Carbon nanoparticles of graphene stacks in size of ~5 nm are formed on the surface of CNTs after the treatment of 30 min, and the whole surface is covered with a coating of graphene stack carbon nanoparticles (size ~15–20 nm). HRTEM image of CNTs after being treated with hydrocarbon ions having ratio

The whole surface of CNT is covered with a coating of carbon nanoparticles of graphene stacks (size ~5 nm) and highlighted in the region encircled as "D." The existence of a large number of defects can obviously be seen in the regions that are

High temperature is believed to be the most important factor in order to form the carbon nanoparticles of graphene stacks. Usually, carbon nanomaterials that grow on low temperature have amorphous structures. Besides, at high temperature

carbon because of a sharp drop

H2:CH4 = 10:1 for 90 min at 700°C is shown in **Figure 2d**.

carbon clusters are oriented in the film due to SP2

*DOI: http://dx.doi.org/10.5772/intechopen.85529*

*Reaction between Energy Particle Ion Beam with Carbon Nanotube DOI: http://dx.doi.org/10.5772/intechopen.85529*

### **Figure 1.**

*Ion Beam Techniques and Applications*

In general, if the energy is very low (such as 100 eV), the cascade collision effect does not occur. The interaction between ion beam and CNTs is producing the defect in the graphite lattice, and the structure of CNTs is still graphite shell. In this range of ion beam energy, the number of defect on the CNT surface can be controlled precisely by adjusting the ion energy and ion doses, and the corresponding properties of CNTs can be tuned. These defects can be used as the source to add some new functional group, functional material, and nanoparticles. Even, the C atoms around the defects can be transferred into the carbon onion structure or diamond by the H ion beam. When the energy is high (such as 30 keV), the cascade collision effect occurs, and it can produce a large number of defects by the implantation of ion beam. In this ion beam energy range, the graphite layer structure of CNTs should be damaged under the irradiation of ion beam. The rearrangement of carbon atoms should happen. The amorphous carbon nanostructure, carbon onion structure, or diamond structure can be formed. The defects can also be used to link the CNTs, and the welding of CNTs can be realized. Therefore, the modification of CNTs, transformation of CNTs, welding of CNTs, fabrication of carbon nanowire networks, etc. have been interesting under the interaction between the ion beam and CNTs.

When the energy of ions is very low, the penetration depth of ion beam is only several nanometers, and the damage produced by ion beam occurs on the surface of CNTs. The achieved energy of carbon atom in the CNTs by the collision is small. Therefore, the interaction between ion beam and the CNTs in this energy range should produce the lattice defects. These defects can be used to adjust the mechanical, electronic, and optoelectronic properties. The defects can also be used as the sources to add some new functional group, functional structure, and nanoparticles. The CNTs can be modified by non-covalently attached or covalently attached means through some chemical technology, chemical vapor deposition (CVD), etc. [9–12]. The structure by the noncovalently modification is unsteady. The hybrid material by the covalently attached means has a very strong force between the CNTs and the functional group, function structure, or nanoparticles. In general, the CNTs can be modified by covalently attached means under the irradiation of ion beams. The connection between CNTs and the functional group, function structure, or nanoparticles depends on the formation of the new covalent bond around the defects induced by the ion beams. The hybrid material is very steady. The modification of CNTs should be some new complex properties by the introduction of some

In our previous work [13, 14], the interaction between CNTs and hydrocarbon ion beams having energy in the range 80–200 eV with substrate temperature from room temperature to 700°C has been studied. The ion beams are produced by Kaufman ion source. The CNTs are dispersed on the silicon wafer as the sample. **Figure 1** represents SEM images of CNTs being irradiated by hydrocarbon ions with

After CNTs are being irradiated by hydrocarbon ions, surface of CNTs becomes rough, and diameter is increased. As the ratio of H2:CH4 will be increased in gas phase, diameters of CNTs will decrease such as 60–70, 40–50, 8–40 nm to forming fragments and clusters. In the process hydrocarbon ion irradiation, there are two competing effects. The first effect is deposition of carbon and carbon-based ions, and the second is

**2. The interaction between CNTs and ion beams**

new structure and can be used as new composite material.

80 eV and different percentages of hydrogen in gas phase.

**2.1 The modification of CNTs by ion beams**

**54**

etching of hydrogen ions.

*SEM images of CNTs being irradiated by hydrocarbon ions of energy 80 eV and varying ratios of H2:CH4, i.e., (a) 0; (b) 10:1; (c) 20:1; and (d) 30:1 [13].*

The deposition rate is high due to dominance of deposition effect in case if hydrogen content is less in gas phase. If the hydrogen content is intermediate in the gas phase, then deposition rate will be time-consuming which results in depositing a thin layer. At high content of hydrogen ions, the deposition rate of carbon-based ions is entirely suppressed due to dominance of etching effect. The bare exposure of CNTs to hydrogen ions initiated the etching of CNTs which leads to etch these CNTs in parts and finally results in formation of pieces of carbon clusters. **Figure 2** represents TEM images of CNTs being exposed to 80 eV hydrocarbon ions (T = 700°C).

The surface of CNTs is rough and inner hollow structure is undamaged. The selected area electron diffraction pattern (SAED) displays reflections (002 and 004) which correspond to intergraphene. **Figure 2b, c** represents the highresolution transmission electron microscope (HRTEM) images of CNTs after treating with hydrocarbon ions having ratio H2: CH2 = 5:1 at 700°C for 30 and 90 min, respectively. Carbon nanoparticles of graphene stacks in size of ~5 nm are formed on the surface of CNTs after the treatment of 30 min, and the whole surface is covered with a coating of graphene stack carbon nanoparticles (size ~15–20 nm). HRTEM image of CNTs after being treated with hydrocarbon ions having ratio H2:CH4 = 10:1 for 90 min at 700°C is shown in **Figure 2d**.

The whole surface of CNT is covered with a coating of carbon nanoparticles of graphene stacks (size ~5 nm) and highlighted in the region encircled as "D." The existence of a large number of defects can obviously be seen in the regions that are encircled as "E" and "F."

High temperature is believed to be the most important factor in order to form the carbon nanoparticles of graphene stacks. Usually, carbon nanomaterials that grow on low temperature have amorphous structures. Besides, at high temperature carbon clusters are oriented in the film due to SP2 carbon because of a sharp drop

### **Figure 2.** *TEM images of CNTs being irradiated by hydrocarbon ions [13].*

in SP3 fraction of carbon with increasing substrate temperature. Usually, all carbon materials that grow at low temperature have a common characteristic of amorphous structure. Turbostratic stacked graphenes are formed due to structural modifications which occurred at high annealing temperature.

Structural defects such as vacancies and interstitials may be generated due to collision cascade effect on walls of tubes due to the bombardment of hydrocarbon ions. The dominant defects are single vacancies. Thereafter, a pentagon ring is formed by these single vacancies which are escorted by movement of dangling bond atoms away from the shell by the distance 0.5–0.7 Å. Saturation in defects by hydrocarbon bonds will occur due to the effect of active hydrogen ions. Subsequently, protruding atoms induce a stress in the crystal lattice which will put neighboring bonds at risk of additional hydrogenation. In the meantime, active hydrocarbon atoms replace hydrogen atoms by process of abstraction and adsorption of hydrogen and lead to deposit the carbon. The deposited carbon will make carbon nanoparticles of graphene stacks extended from the surface under the influence of hightemperature annealing.

Penetration depth for 80 eV hydrocarbon ion CNTs is ~1 nm; therefore, ion irradiation induces the damage in the topmost shell, and graphene will be formed up to a depth of few "nanometers" which will protect CNTs from further damage.

**57**

**Figure 3.**

*Reaction between Energy Particle Ion Beam with Carbon Nanotube*

nanoparticles of turbostratic stacked graphenes with increase in deposition time. As the hydrogen content in gas phase will be increased, the deposition effect is subdued, and carbon atoms of C–H bonds are supposed to be etched by hydrogen ions prior to assembly with active hydrocarbon ions which lead to appear as the amorphous carbon. Hence, structural quality and size of carbon nanoparticles of turbostratic

When the CNTs are dispersed on Cu network as the sample and substrate temperature is about 900°C, a new complex material of CNTs decorated by graphitic shellencapsulated Cu nanoparticles is achieved [15]. After treating with hydrocarbon ions, internal vacant CNT structure remains safe, whereas surface becomes extremely rough. HRTEM image of **Figure 3** shows the products that are composed of Cu-C core-shell structure nanoparticles with core size 1–2 nm (size of shell = 5–10 nm). The core nanoparticles are firmly encased in carbon shells without having any space in between shell and core. The shells are composed of 8–10 layers and have equal

When the hold substrate of CNTs is copper, the substrate is heated to

chemistry, catalysis, field emission, composite mechanical materials, etc.

When the energy of ions is high, the carbon atom can gain enough energy to escape from the graphite lattice of CNTs in the collision process and can be a free atom. The free carbon atoms should collide with the other carbon atoms in the

**2.2 The phase transformation occurred by ion beam irradiation**

*The TEM image of CNTs decorated by graphitic shell-encapsulated Cu nanoparticles [15].*

900°C. Comparing with the hold substrate of silicon, some new process is produced: initially, several Cu atoms can gain adequate energy and fled from surface of substrate at elevated temperature and turned into Cu species. Thereafter, carbon and Cu alloys were formed due to the interaction of Cu and carbon species introduced by means of low-energy hydrocarbon ions. Afterward, with the period of hydrocarbon ions, dissolution of carbon in Cu nanoparticles approaches saturation, and a precipitation of pure carbon around the nanoparticles would commence in shape of graphite; consequently, the graphitic shell-encapsulated Cu nanoparticles are formed. To sum up, only defects can be created on the surface of the CNTs, and the cascade collision effect does not happen during the irradiation of low-energy ion beam. The new functional groups or new nanostructures are connected on the carbon atoms by covalent bond through utilizing the activity of the defects and form new complexes. The CNTs after modification have some new properties. These new composite materials can be used as new chemical materials, electrical materials, mechanical materials, etc. and can be applied in the fields of medicine, functional

*DOI: http://dx.doi.org/10.5772/intechopen.85529*

stacked graphenes are lessened.

thicknesses.

Initially, the distribution of nanoparticles over the surface of CNTs is random. Later, the surface of CNTs will be entirely covered by a coating of carbon

### *Reaction between Energy Particle Ion Beam with Carbon Nanotube DOI: http://dx.doi.org/10.5772/intechopen.85529*

*Ion Beam Techniques and Applications*

**56**

temperature annealing.

in SP3

**Figure 2.**

 fraction of carbon with increasing substrate temperature. Usually, all carbon materials that grow at low temperature have a common characteristic of amorphous structure. Turbostratic stacked graphenes are formed due to structural modifica-

Structural defects such as vacancies and interstitials may be generated due to collision cascade effect on walls of tubes due to the bombardment of hydrocarbon ions. The dominant defects are single vacancies. Thereafter, a pentagon ring is formed by these single vacancies which are escorted by movement of dangling bond atoms away from the shell by the distance 0.5–0.7 Å. Saturation in defects by hydrocarbon bonds will occur due to the effect of active hydrogen ions. Subsequently, protruding atoms induce a stress in the crystal lattice which will put neighboring bonds at risk of additional hydrogenation. In the meantime, active hydrocarbon atoms replace hydrogen atoms by process of abstraction and adsorption of hydrogen and lead to deposit the carbon. The deposited carbon will make carbon nanoparticles of graphene stacks extended from the surface under the influence of high-

Penetration depth for 80 eV hydrocarbon ion CNTs is ~1 nm; therefore, ion irradiation induces the damage in the topmost shell, and graphene will be formed up to a depth of few "nanometers" which will protect CNTs from further damage. Initially, the distribution of nanoparticles over the surface of CNTs is random. Later, the surface of CNTs will be entirely covered by a coating of carbon

tions which occurred at high annealing temperature.

*TEM images of CNTs being irradiated by hydrocarbon ions [13].*

nanoparticles of turbostratic stacked graphenes with increase in deposition time. As the hydrogen content in gas phase will be increased, the deposition effect is subdued, and carbon atoms of C–H bonds are supposed to be etched by hydrogen ions prior to assembly with active hydrocarbon ions which lead to appear as the amorphous carbon.

Hence, structural quality and size of carbon nanoparticles of turbostratic stacked graphenes are lessened.

When the CNTs are dispersed on Cu network as the sample and substrate temperature is about 900°C, a new complex material of CNTs decorated by graphitic shellencapsulated Cu nanoparticles is achieved [15]. After treating with hydrocarbon ions, internal vacant CNT structure remains safe, whereas surface becomes extremely rough.

HRTEM image of **Figure 3** shows the products that are composed of Cu-C core-shell structure nanoparticles with core size 1–2 nm (size of shell = 5–10 nm). The core nanoparticles are firmly encased in carbon shells without having any space in between shell and core. The shells are composed of 8–10 layers and have equal thicknesses.

When the hold substrate of CNTs is copper, the substrate is heated to 900°C. Comparing with the hold substrate of silicon, some new process is produced: initially, several Cu atoms can gain adequate energy and fled from surface of substrate at elevated temperature and turned into Cu species. Thereafter, carbon and Cu alloys were formed due to the interaction of Cu and carbon species introduced by means of low-energy hydrocarbon ions. Afterward, with the period of hydrocarbon ions, dissolution of carbon in Cu nanoparticles approaches saturation, and a precipitation of pure carbon around the nanoparticles would commence in shape of graphite; consequently, the graphitic shell-encapsulated Cu nanoparticles are formed.

To sum up, only defects can be created on the surface of the CNTs, and the cascade collision effect does not happen during the irradiation of low-energy ion beam. The new functional groups or new nanostructures are connected on the carbon atoms by covalent bond through utilizing the activity of the defects and form new complexes. The CNTs after modification have some new properties. These new composite materials can be used as new chemical materials, electrical materials, mechanical materials, etc. and can be applied in the fields of medicine, functional chemistry, catalysis, field emission, composite mechanical materials, etc.

## **2.2 The phase transformation occurred by ion beam irradiation**

When the energy of ions is high, the carbon atom can gain enough energy to escape from the graphite lattice of CNTs in the collision process and can be a free atom. The free carbon atoms should collide with the other carbon atoms in the

**Figure 3.** *The TEM image of CNTs decorated by graphitic shell-encapsulated Cu nanoparticles [15].*

lattice to decrease its energy and stop inside the CNTs until its energy is zero. The cascade collision effect happens, and a large number of lattice defects should be produced. In the cascade collision process, the order of the crystal lattice will be destroyed, and even the hollow tube structure may be destroyed [16]. The CNTs can transform into the other structure, such as amorphous structure. In the disappearance process of the tube structure, the carbon nanowire with some special morphology structure may be formed such as case, pillbox-like, etc. If the substrate temperature is high, the disordered carbon atom should be rearranged and can transform into the carbon onions or diamond-like carbon structure or even diamond structure. Structural evolution of CNTs irradiated by in situ electron beam carried out by Banhart et al. designated that basal planes of CNTs were found to be cracked, crooked, and tilted after irradiation [17, 18]. In another report, structure of CNTs had been modified to carbon onion or diamonds [19]. Wei et al. irradiated CNTs by beam of 30 or 50 keV Ga ions at a range of fluencies [20]. In their report, at the low beam fluence ~1013 ions/cm2 , they found alteration of CNTs from wellordered pillbox-like nano-compartments of unfixed lengths to amorphous rods without changing the tubelike shapes of CNTs. At high beam fluence ~1015 ions/cm2 , CNTs were modified in form of homogenous amorphous rods.

Kim et al. found alterations in morphologies of CNTs after exposure to beams of 4 MeV Cl ions [21]. In their report, morphologies of CNTs had been altered from CNTs to nano-compartments having bamboo-like structures inside the tubes at a fluence ~3 × 1016 ions/cm2 . They reported the reason for formation of nanocompartments with bamboo-like structure by folding the inner walls.

In our previous work [22], the interaction of CNTs and a 40 keV beam of Si ions at fluencies ~1 × 1015 and ~1 × 1017 ions/cm2 in a 100 keV electromagnetic isotope separator is investigated. HRTEM representations of CNTs exposed to a beam of Si ions having fluencies ranging from 5 × 1015 to 1 × 1017 ions/cm2 are shown in **Figure 4**.

**59**

**Figure 5.**

*Reaction between Energy Particle Ion Beam with Carbon Nanotube*

unfilled case to solid structure amorphous carbon nanowire.

With beam fluencies of ions ranging from ~5 × 1015 to ~1 × 1017 ions/cm2

transformed into CNTs with some disorder in graphite structures and integral hollow tubular shape, semisolid carbon nanowire having amorphous structure and unfilled tubular shape, semisolid amorphous carbon nanowire with intermittent

The procedure for structural modifications of CNTs and fabrication of amorphous carbon nanowires is shown in the model diagram of **Figure 5**. During the ion beam bombardment of CNTs by Si ions, an energetic ion transfers its energy to the atoms of the topmost shell, and several recoils of primary carbon atom and vacancies will be formed in this way. These recoil atoms will produce more recoils by colliding with carbon atoms in other shells of CNTs. Structure of nanotubes becomes highly unstable due to the presence of large number of defects, i.e., vacancies and interstitials, and carbon atoms in graphitic structures around vacancies will

Consequently, well-ordered graphite sheets of CNTs are divided into local ordered graphite. Production of defects is gradually increased with increasing beam fluence. The tube walls of CNTs with local ordered graphite will then be transformed into completely disordered phase having hollow structure due to continuous

Amorphous carbon nanowires are formed in the first period. Secondly, at higher irradiation doses, the semisolid amorphous carbon nanowires converted entirely into solid amorphous carbon nanowire. The nanotubes alter into multishelled pillbox-like nanocompartments prior to breakup in form of homogenous amorphous rods in later case. The procedure for makeup of solid amorphous carbon nanowire at an intermedial stage is displayed in the structure. In addition, CNTs used in this experiment were dispersed randomly on substrate and overlapped. Low beam fluence is received by a shielded part of the tube in comparison with unshielded part as beam of energetic ions assails tubes. Consequently, disordered graphite, semisolid structure, and solid structure are found concurrently on nano-

In our previous work [23], the interaction of CNTs and 1.2 keV Ar ion has been investigated. **Figure 6** shows the SEM images of CNTs irradiated by 1.2 keV Ar ions for 15–60 min. After 15 min sputtering, the tube shapes of CNTs are almost intact, and the diameters of CNTs are 4–35 nm, and only a few CNTs are broken into several parts along the tube axis; the inset TEM images of typical CNTs shows that the CNTs are transformed into amorphous carbon nanowires, consistent with the results of 40 keV Si ion irradiation. After 30 min irradiation, the CNTs are separated into some particles with the size from 20–30 to 300–400 nm along the tube

axis, and the surfaces of particles are smooth with no conical protrusion.

*The process of amorphous carbon nanowire formation by 40 keV Si ion beam irradiation [22].*

, CNTs

*DOI: http://dx.doi.org/10.5772/intechopen.85529*

reorganize to lessen the surface free energy.

tubes irradiated by the almost the same beam fluence.

production and collection of defects.

*Ion Beam Techniques and Applications*

at the low beam fluence ~1013 ions/cm2

ions at fluencies ~1 × 1015 and ~1 × 1017 ions/cm2

at a fluence ~3 × 1016 ions/cm2

shown in **Figure 4**.

lattice to decrease its energy and stop inside the CNTs until its energy is zero. The cascade collision effect happens, and a large number of lattice defects should be produced. In the cascade collision process, the order of the crystal lattice will be destroyed, and even the hollow tube structure may be destroyed [16]. The CNTs can transform into the other structure, such as amorphous structure. In the disappearance process of the tube structure, the carbon nanowire with some special morphology structure may be formed such as case, pillbox-like, etc. If the substrate temperature is high, the disordered carbon atom should be rearranged and can transform into the carbon onions or diamond-like carbon structure or even diamond structure. Structural evolution of CNTs irradiated by in situ electron beam carried out by Banhart et al. designated that basal planes of CNTs were found to be cracked, crooked, and tilted after irradiation [17, 18]. In another report, structure of CNTs had been modified to carbon onion or diamonds [19]. Wei et al. irradiated CNTs by beam of 30 or 50 keV Ga ions at a range of fluencies [20]. In their report,

ordered pillbox-like nano-compartments of unfixed lengths to amorphous rods without changing the tubelike shapes of CNTs. At high beam fluence ~1015 ions/cm2

Kim et al. found alterations in morphologies of CNTs after exposure to beams of 4 MeV Cl ions [21]. In their report, morphologies of CNTs had been altered from CNTs to nano-compartments having bamboo-like structures inside the tubes

In our previous work [22], the interaction of CNTs and a 40 keV beam of Si

isotope separator is investigated. HRTEM representations of CNTs exposed to a beam of Si ions having fluencies ranging from 5 × 1015 to 1 × 1017 ions/cm2

*TEM images of CNTs by irradiation of 40 keV Si ion beam at fluence of 1 × 1015–1 × 1017 ions/cm2*

CNTs were modified in form of homogenous amorphous rods.

compartments with bamboo-like structure by folding the inner walls.

, they found alteration of CNTs from well-

in a 100 keV electromagnetic

. They reported the reason for formation of nano-

,

are

 *[22].*

**58**

**Figure 4.**

With beam fluencies of ions ranging from ~5 × 1015 to ~1 × 1017 ions/cm2 , CNTs transformed into CNTs with some disorder in graphite structures and integral hollow tubular shape, semisolid carbon nanowire having amorphous structure and unfilled tubular shape, semisolid amorphous carbon nanowire with intermittent unfilled case to solid structure amorphous carbon nanowire.

The procedure for structural modifications of CNTs and fabrication of amorphous carbon nanowires is shown in the model diagram of **Figure 5**. During the ion beam bombardment of CNTs by Si ions, an energetic ion transfers its energy to the atoms of the topmost shell, and several recoils of primary carbon atom and vacancies will be formed in this way. These recoil atoms will produce more recoils by colliding with carbon atoms in other shells of CNTs. Structure of nanotubes becomes highly unstable due to the presence of large number of defects, i.e., vacancies and interstitials, and carbon atoms in graphitic structures around vacancies will reorganize to lessen the surface free energy.

Consequently, well-ordered graphite sheets of CNTs are divided into local ordered graphite. Production of defects is gradually increased with increasing beam fluence. The tube walls of CNTs with local ordered graphite will then be transformed into completely disordered phase having hollow structure due to continuous production and collection of defects.

Amorphous carbon nanowires are formed in the first period. Secondly, at higher irradiation doses, the semisolid amorphous carbon nanowires converted entirely into solid amorphous carbon nanowire. The nanotubes alter into multishelled pillbox-like nanocompartments prior to breakup in form of homogenous amorphous rods in later case. The procedure for makeup of solid amorphous carbon nanowire at an intermedial stage is displayed in the structure. In addition, CNTs used in this experiment were dispersed randomly on substrate and overlapped.

Low beam fluence is received by a shielded part of the tube in comparison with unshielded part as beam of energetic ions assails tubes. Consequently, disordered graphite, semisolid structure, and solid structure are found concurrently on nanotubes irradiated by the almost the same beam fluence.

In our previous work [23], the interaction of CNTs and 1.2 keV Ar ion has been investigated. **Figure 6** shows the SEM images of CNTs irradiated by 1.2 keV Ar ions for 15–60 min. After 15 min sputtering, the tube shapes of CNTs are almost intact, and the diameters of CNTs are 4–35 nm, and only a few CNTs are broken into several parts along the tube axis; the inset TEM images of typical CNTs shows that the CNTs are transformed into amorphous carbon nanowires, consistent with the results of 40 keV Si ion irradiation. After 30 min irradiation, the CNTs are separated into some particles with the size from 20–30 to 300–400 nm along the tube axis, and the surfaces of particles are smooth with no conical protrusion.

**Figure 5.**

*The process of amorphous carbon nanowire formation by 40 keV Si ion beam irradiation [22].*

After 45 min sputtering, the all-tube morphology of CNTs on the top layer of CNTs stacks is broken, and the tube morphology of CNTs at the bottom of CNTs stacks is almost intact; some protrusions can be observed on the coarse aggregated nanoparticle surface. With 60-min sputtering, all CNTs are broken, and some nanofibers can be observed; the lengths of nanofibers are ranged from several ten nanometers to several micrometers. The high-resolution SEM images of typical nanofiber show that the nanofibers grow on the tip of protrusion. The formation of carbon nanofibers by Ar+ sputtering CNTs is speculated.

Initially, structures of CNTs are modified due to the presence of large number of defects, i.e., vacancies and interstitials between and on tube walls due to collision cascade effect that gives rise to degree of disorder in the structure, and consequently, CNTs might be scrunched up in form of amorphous nanowires.

Carbon atoms are sputtered quickly on some regions because of difference in sputtering yields which depends on curvature of the amorphous nanowire surface and lead to break CNTs, and some particles will be deposited along the axis of tubes. Afterward, flanges are formed due to competition between smoothening process and roughening process. At last, the migration of mass redeposition atom toward the tip leads to the growth of carbon nanofibers on the protrusion.
