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Modification on graphite due to helium ion irradiation
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Physics Letters A ••• (••••) •••–•••
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Modification on graphite due to helium ion irradiation N.J. Dutta, S.R. Mohanty ∗ , N. Buzarbaruah Centre of Plasma Physics-Institute for Plasma Research, Sonapur, Kamrup 782402, India
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Article history: Received 23 December 2015 Received in revised form 5 April 2016 Accepted 24 May 2016 Available online xxxx Communicated by F. Porcelli Keywords: Plasma focus Helium ion Graphite Atomic force microscopy Transmission electron microscopy X-ray diffraction
a b s t r a c t This paper studies the influence of helium ion irradiation on morphological and structural properties of graphite samples. The helium ions emanated from a plasma focus device have been used to irradiate graphite samples by varying the number of ion pulses. The effect of radiation induced changes in morphology and structure are examined by using optical microscopy, atomic force microscopy, transmission electron microscopy along with selected area electron diffraction and x-ray diffraction. A distinct change in the surface topography is marked in the case of the ion irradiated samples when viewed under the optical microscope. The micrographs of the ion irradiated samples confirm mostly rounded and sparely elongated type of structures arising due to intense melting and local ablation accompanied with ejection of graphite melts that depends upon the ion fluence. The atomic force microscopy images also reveal the formation of globules having sizes ∼50–200 nm which are the agglomeration of small individual clusters. Transmission electron micrographs of the ion irradiated samples furnish that the diameter of these individual small clusters are ∼10.4 nm. Moreover, selected area electron diffraction patterns corroborate that the ion irradiated sample retains its crystalline nature, even after exposure to larger helium ion pulses. It is noticed from the x-ray diffraction patterns that some new phases are developed in the case of ion irradiated sample. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Ion irradiation is now considered a widely accepted technique for the synthesis, modification and characterization of materials [1,2]. This technique has drawn much attention because of its imperative advantages [3,4] over the conventional surface processes like chemical vapor deposition and physical vapor deposition. Being a thermal process it can be easily used to tailor the properties of all types of materials whereas the conventional processes can only manipulate some of them [3]. The non-equilibrium behavior of ion irradiation permits to have a control in nanostructure formation by playing with the parameters like temperature during ion-material interaction and pressure during phase formation simultaneously at the time of experimentation. Another important feature of ion irradiation is its flexibility in selecting the area of exposure either by focusing an ion beam or by using a suitable mask [4]. In the past, there have been a number of efforts that dealt with the investigation of ion-graphite interaction keeping in mind their applications either in the first wall of fusion device [5,6] or in the semiconductor industries [7,8]. In contrast to such past efforts, the present trend of research on the ion-graphite interactions
*
Corresponding author. E-mail address: [email protected] (S.R. Mohanty).
http://dx.doi.org/10.1016/j.physleta.2016.05.044 0375-9601/© 2016 Elsevier B.V. All rights reserved.
is mainly focussed on the synthesis of various carbon nanostructures [9,10] which may have important applications in the next generation technology [11,12]. It is noteworthy to mention here that the early studies on the ion-graphite interactions were carried out mostly by employing continuous ion beam sources. Recently, the pulsed ion beams from plasma focus (PF) devices are being used for the study of material modification, thin film deposition, coating, nanofabrication and new material synthesis [13–18]. This device is a very effective tool for ion irradiation on the materials since it can provide the ion energy in the range of a few tens of keV to MeV [19] within a few hundred of nanoseconds. Normally the ion irradiation induced changes on the material are supposed to be due to either nuclear or electronic loss. The lower energy ions mainly lose their energy on the material by the process of nuclear collisions whereas higher energy ions dissipate their energy by electronic collisions. Hence, the consequence of both the collision processes can be easily studied by bombarding ions from the PF device on material. Another obvious advantage of this ion source is that it is possible not only to examine the final output in a multi-PF shots exposure experiment but also the effect of individual ion pulse impact on the target material in a multi-PF shots exposure experiment. Hence, one can have an optimum operational mode in which the ion beam has the control both over time as well as on the fluence (the energy) inputs to the material. The development and optimization of PF device as an ion source
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Table 1 Concentrations of impurities in graphite sample.
Fig. 1. Schematic of the experimental setup.
for the synthesis of a new material is still one of the most active field of research. The PF device is a simple plasma accelerator that makes use of the self generated magnetic field to propel the plasma towards the open end of the coaxial cylindrical electrodes and subsequently focusing them to form a very high density (1025 –1026 m−3 ) and high temperature (>1 keV) plasma column. The plasma column lasts only for a few hundred of nanoseconds because of the instabilities. The device is renowned as a multi radiation source of various charged particle beams and electromagnetic (EM) radiations and, therefore, it finds applications in microlithography [20], EUV lithography [21], nanofabrication [22], thin film deposition [23,28] and nuclear materials testing [24,25]. Realizing the aforesaid advantages of the PF device, the helium ions emerging out of it are used to irradiate the graphite samples so as to examine the irradiation induced changes on them. The reference and ion irradiated samples are characterized by employing various characterization instruments, namely optical microscopy (OM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) to analyze the surface morphologies and x-ray diffraction (XRD) to investigate the structural changes that have taken place on the samples due to the helium ion irradiation. 2. Experimental details The schematic of the experimental setup used as a helium ion source to irradiate graphite samples is shown in Fig. 1. The device is basically a Mather type [26] that mainly consists of a high energy capacitor (7.1 μF, 25 kV), a high voltage charger, a coaxial and concentric electrode assembly enclosed inside a stainless steel vacuum chamber. The electrode assembly is the combination of a central anode and twelve cathode rods encircling the anode in squirrel cage fashion. The chamber is first evacuated with the help of a diffusion pump that is backed by a rotary pump down to a pressure of ∼10−5 Torr and then is filled with helium gas at an optimum working pressure of 0.5 Torr. After that, the capacitor is charged to its maximum value and followed by discharge of capacitor across the electrode system through a low inductance transmission line. When the capacitor is discharged, the current sheaths initially develop across the insulator sleeves due to the breakdown of gas medium and these sheaths are axially acceler-
Impurity
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B
Ca
Cu
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Si
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1
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ated towards the open end of the anode by the axial component of self-generated J × B force. Subsequently, the sheaths undergo a radial pinching resulting in the formation of a hot and dense plasma column, which lasts only for a few hundred of nanoseconds due to the instabilities. These instabilities enhance the induced electric field locally which in association with the magnetic field breaks down the plasma column [26] and accelerates the high energy ions towards the top of the chamber. These ions were used to irradiate the graphite samples that were kept at a distance of 0.06 m from the anode top with the help of a movable sample holder as shown in Fig. 1. The ion fluence and flux at the location of the sample for a single PF shot (that is, one single pulse of helium ion) as measured by the ion collector are found to be 1018 m−2 and 1025 m−2 s−1 respectively which are similar to the results obtained earlier [27]. The samples were kept perpendicular to the ion trajectories at room temperature. The experimental conditions were maintained similar to the experiment reported in [24] and it was noted that the instantaneous bulk temperature of the ion irradiated samples increase by about 70 to 100 ◦ C for a single PF shot irradiation as measured by a thermocouple system. The graphite rods having a purity of 99.99% were procured from Good-fellow U.K. The chemical composition of the material is tabulated in Table 1. These graphite rods were cut into disc shape having diameter 0.01 m and thickness 0.002 m using a precision cutter, followed by mechanical polishing of the sample surface with the help of a set of abrasive paper arranging with decreasing order of their particle size. After polishing, the samples were cleaned and then mounted inside the chamber as mentioned earlier and exposed to numbers of PF shots namely 5, 10 and 20 shots etc. The exposed and reference samples were investigated by OLYMPUS BX 51M optical microscopy for surface topography. The surface morphology was also observed at higher resolution by employing NTEGRA PRIMA (NT-MDT) atomic force microscopy (AFM), Russia, with a silicon cantilever operating in semi-contact mode. TEMTECNAI G2 FEI, Netherlands, operated at 200 kV was used to capture the TEM images along with selected area electron diffraction (SAED) pattern images. The TEM sample was prepared by scraping fine powder from the graphite surface and dispersing the resulting powder in isopropanol solution by sonication. A drop of the solution was then allowed to evaporate on a copper mesh grid under the IR lamp. The crystallographic phases of the samples were studied using a Bruker-D8 diffract meter having Cu K alpha source (wavelength 1.54 Å) at 3 degree grazing angle incidence with step size of 0.019917. 3. Results and discussion As mentioned earlier, the graphite samples were irradiated to different fluxes of helium ions by simply varying the number of PF shots (5, 10, and 20 shots). After ion irradiation, the samples were preliminarily inspected visually and it was noticed that the shining of the polished sample was disappeared and the surface become white ash in color. For the better view of the surface morphology, both the reference and ion irradiated samples were studied by employing the optical microscope by taking numerous images of the surface of reference as well as ion irradiated samples. A typical optical micrograph of reference sample shown in Fig. 2 illustrates a reasonably smooth surface without any major defects and cracks. Only a few small voids and some hairline marks are noticed at
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Fig. 2. Optical micrographs of the reference graphite sample at different magnifications.
certain locations that may be arising during the mechanical polishing of the samples. On the contrary, a distinct morphological change is visible in the ion irradiated samples clearly indicating some surface deformation. At lower ion doses (5 numbers of PF shots), the surface of the ion irradiated sample shown in Fig. 3(a) exhibits randomly distributed rounded island like grains whereas in the case of 10 numbers of PF shots the rounded grains become more prominent (as shown in Fig. 3(b)) and are appeared throughout the surface. On further increase in helium ion pulses up to 20 numbers of PF shots, the micrograph (as shown in Fig. 3(c)) indicates mostly rounded grains along with a few elongated structures. It can be speculated from these micrographs (shown in Fig. 3) that the size of the rounded structures is dependent upon the numbers of helium ion pulses imposed on the samples. At 20 PF shots, the size of the grains is bigger than that observed in the case of 5 and 10 PF shots. Moreover, the elongated type structures are resulted on the surface of sample due to intense melting and local ablation accompanied with ejection of graphite melts [28]. Since the samples irradiated to 20 numbers of PF shots show a well developed structures that is why we have further done some other advanced characterization of those to acquire information on formation of the rounded structures. AFM analyses have been performed in order to investigate the possible transformation of the surface topography of the graphite sample due to the ion irradiation in higher resolution and thereby bring further clarity to the optical micrograph results. Fig. 4 shows the typical AFM images for both the reference and 20 shots helium ion irradiated samples, and it depicts that the reference sample, as compared to ion irradiated sample, is relatively smooth with lesser structural features. Similar observations were earlier made by using scanning electron microscopy [29] where authors have discussed about the underlying physical processes responsible for the formation of the observed structures. A close observation to the micrographs of ion irradiated samples suggests the association of some small individual clusters. In our case the diameters of the globules were 50–200 nm and were alike in shape with rounded structures as shown in the optical micrographs in Fig. 3. As men-
Fig. 3. Optical micrographs of helium ion irradiated graphite samples: (a) 5 PF shots, (b) 10 PF shots and (c) 20 PF shots.
tioned earlier, the ions lose their energy during the interaction with the graphite target mainly either through nuclear collisions between the incident ions and the target atoms or electronic collisions in between incident ions and electrons of the target. If an ion is irradiated with an energy which is sufficient to overcome the threshold energy of the target material, then the target atom is displaced from its original lattice position. Considering the displacement energy for carbon atom in graphite as 28 eV [30], it is estimated that the helium ion of energy 37.33 eV is enough to eject one carbon atom from the graphite crystal. Since in our case, the ion energy is much more than above mentioned energy value, so we can expect enormous number of carbon atoms on the surface of the graphite not only by the process of primary knockon but also by secondary, tertiary or even higher order atomic knock-on. On the other hand, the higher part of ion energy (>500 keV) contributes to the electronic loss which gives rise to the localized heating sample surface [31]. The displaced carbon atoms are coagulated with each other into small clusters because of their high structural fluidity [32] under the ion irradiation. Further the subsequent irradiation of the helium ion pulses causes more energy being transferred to the sample surface which provides more mobility to the clusters and the coalescence between the clusters occurs resulting in a larger size structure. These agglomerations are appearing mostly as globules since the successive ion pulses owing to the surface energy minimization [29] of the small clusters
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Fig. 4. AFM micrographs of graphite samples: (a) reference and (b) 20 shots helium ion irradiated.
and surface tension of materials forced them to form a spherical shape. Thus, it can be expected that the formation of the structures solely depends on the nucleation of the carbon atoms and the heat load due to ion irradiation. Further the formation of elongated structures that are also seen along with the globules may be due to nucleation of more than one globule when subsequent PF shots are imposed. We have measured the rms roughness of both the reference and ion irradiated samples over a distance of 1 μm and they are estimated to be 18 and 68 nm, respectively. The increase of the surface roughness in the case of the ion irradiated sample might be due to the formation of agglomerated structures because of the ejected carbon atoms unification as discussed earlier. TEM characterization of both the reference as well as 20 shots helium ion irradiated samples was performed to obtain the insights on the microstructural evolution as a result of ion irradiation. Figs. 5(a) and (b) display the typical micrographs of the reference and ion irradiated samples, respectively. As expected the image of the reference sample shows a two dimensional layered type structure whereas the micrograph of the ion irradiated sample illustrates randomly distributed rounded structures. The diameter of the rounded structures varies in the range of 2–22 nm and the mean diameter of these structures is found to be 10.4 ± 2.706 nm as shown in Fig. 6. As mentioned earlier the mechanism of the development of these rounded structures can be attributed to the aggregations of carbon atoms that are dislodged from the crystallite sites and arranged in some clusters. It is noteworthy to mention that these structures are uniformly dispersed on the graphene sheet and a very few of them are black in color. These darker
Fig. 5. TEM pictures of the graphite samples: (a) reference and (b) 20 shots helium ion irradiated.
Fig. 6. Histogram showing size distributions of rounded structure.
rounded features are tungsten carbide impurities which may arise due to the use of tungsten as anode of the PF device. Figs. 7(a) and (b) show the SAED pattern of reference and 20 shots helium ion irradiated samples, respectively. The bright points arranged in the diffraction rings as shown in the Fig. 7(a) confirm that the reference sample is polycrystalline in nature. On the other hand, the diffraction pattern of the ion irradiated sample reveals that some of the bright points are relocated in new rings thus ensuring the structural rearrangements. The d (interlayer distance) values obtained from the diffraction rings of reference sample are estimated
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Fig. 7. SAED patterns of the graphite samples: (a) reference and (b) 20 shots helium ion irradiated.
intense peak (002) got enhanced to 10.3 nm from the corresponding reference sample value of 9.8 nm when calculated by using the Scherrer’s formula,
D=
0.94.λ β2θ . cos θ
(1)
where D is the grain size, λ is wavelength of X-ray, β2θ is FWHM of the plane, and θ is Bragg’s angle. The increase in the crystallite size may be due to the successive bombardment of ion pulses on material surface that allows more mobility to the nanostructures such that larger size structures can be formed. 4. Conclusion
Fig. 8. XRD patterns of the reference and 20 shots helium ion irradiated graphite samples.
from SAED pattern and found to be 3.352, 2.108, 2.024, 1.668, 1.5, 1.226 and 1.15 angstrom. These values are approximately identical to d002 , d100 , d101 , d004 , d103 , d110 and d112 planes of graphite. On the other hand, in the case of exposed sample the d values of diffraction rings are 3.39, 2.58, 2.341, 2.25, 2.126, 2.03, 1.68, 1.537, 1.464, 1.32, 1.21 and 1.136 angstrom. Fig. 8 presents the measured XRD pattern from 2 theta value 5 to 90 degrees at grazing incidence angle for both the reference and 20 shots ion irradiated samples. The diffraction planes (002), (100), (101), (004), (110) and (112) are observed in the both reference and ion irradiated samples. However, some new planes have been observed in the ion irradiated sample at 2 theta values 34.78, 38.23, 39.95, 62.47 and 70.46 degrees which are assigned to be the peaks of tungsten carbide (W2 C). The formation of these new planes is due to the presence of tungsten anode material which combines with the sample material to form tungsten carbide during the ion irradiation process. The magnified view of the most intense peak of graphite (i.e., (002) peak) undergoes a shift towards the lower Bragg angles thus suggesting the built up of expansive stress in the ion irradiated sample [33]. We have calculated the interlayer spacing for the reference as well as ion irradiated samples for (002) diffraction peak and it is found to be 0.3354 and 0.3393 nm, respectively. The marginal increase in interlayer spacing in the irradiated sample is either due to the displacement of carbon atoms to the interstitial positions or on account of inclusion of helium atoms at the interlayer positions [34]. It is also observed that the crystallite size of the ion irradiated sample for the most
We have successfully performed the helium ion irradiation onto the graphite samples using an indigenous pulsed ion source. The variation in surface morphologies in different numbers of ion pulses (PF shots) has been studied by employing the optical microscope. It is inferred from the optical micrograph results that the surface deformation depends upon the number of ion pulses. The AFM study also indicates the formation of globule structures that may be due to the unification of small clusters. It is also observed that surface roughness increases with ion irradiation because of formation of these structures on the surface of the samples. Moreover, the TEM micrographs of the ion irradiated graphite exhibit rounded structure having mean diameter ∼10.4 nm. Comparing the SAED pattern of the reference and ion irradiated samples, it is noted that the crystalline nature of the ion irradiated sample is retained and there is emergence of some new planes. The XRD results also corroborate the SAED pattern confirming the appearance of new planes that may arise due to chemical reaction of graphite with tungsten impurity material. The accumulation of expansive stress is also confirmed in the ion irradiated samples. However, further studies are needed to establish the basic mechanism lying behind the formation of these carbon nanostructures in more details by employing additional advanced characterization tools. Nevertheless, our study contributes towards developing improved understanding of the ion-graphitic material interactions for the formation of carbon nanostructures. Acknowledgements Authors would like to acknowledge the Director, Institute for Plasma Research, Gandhinagar, India and Centre Director, Centre of Plasma Physics-Institute for Plasma Research, Assam, India, for the financial support to carry out the present work. We are also thankful to Dr. S. Kundu, Dr. N. Adhikary of IAAST, Guwahati, India and Dr. B.B. Nayak of IMMT Bhubneswar, India for supporting our
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Modification on graphite due to helium ion irradiation N.J. Dutta, S.R. Mohanty ∗ , N. Buzarbaruah Centre of Plasma Physics-Institute for Plasma Research, Sonapur, Kamrup 782402, India
a r t i c l e
i n f o
Article history: Received 23 December 2015 Received in revised form 5 April 2016 Accepted 24 May 2016 Available online xxxx Communicated by F. Porcelli Keywords: Plasma focus Helium ion Graphite Atomic force microscopy Transmission electron microscopy X-ray diffraction
a b s t r a c t This paper studies the influence of helium ion irradiation on morphological and structural properties of graphite samples. The helium ions emanated from a plasma focus device have been used to irradiate graphite samples by varying the number of ion pulses. The effect of radiation induced changes in morphology and structure are examined by using optical microscopy, atomic force microscopy, transmission electron microscopy along with selected area electron diffraction and x-ray diffraction. A distinct change in the surface topography is marked in the case of the ion irradiated samples when viewed under the optical microscope. The micrographs of the ion irradiated samples confirm mostly rounded and sparely elongated type of structures arising due to intense melting and local ablation accompanied with ejection of graphite melts that depends upon the ion fluence. The atomic force microscopy images also reveal the formation of globules having sizes ∼50–200 nm which are the agglomeration of small individual clusters. Transmission electron micrographs of the ion irradiated samples furnish that the diameter of these individual small clusters are ∼10.4 nm. Moreover, selected area electron diffraction patterns corroborate that the ion irradiated sample retains its crystalline nature, even after exposure to larger helium ion pulses. It is noticed from the x-ray diffraction patterns that some new phases are developed in the case of ion irradiated sample. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Ion irradiation is now considered a widely accepted technique for the synthesis, modification and characterization of materials [1,2]. This technique has drawn much attention because of its imperative advantages [3,4] over the conventional surface processes like chemical vapor deposition and physical vapor deposition. Being a thermal process it can be easily used to tailor the properties of all types of materials whereas the conventional processes can only manipulate some of them [3]. The non-equilibrium behavior of ion irradiation permits to have a control in nanostructure formation by playing with the parameters like temperature during ion-material interaction and pressure during phase formation simultaneously at the time of experimentation. Another important feature of ion irradiation is its flexibility in selecting the area of exposure either by focusing an ion beam or by using a suitable mask [4]. In the past, there have been a number of efforts that dealt with the investigation of ion-graphite interaction keeping in mind their applications either in the first wall of fusion device [5,6] or in the semiconductor industries [7,8]. In contrast to such past efforts, the present trend of research on the ion-graphite interactions
*
Corresponding author. E-mail address: [email protected] (S.R. Mohanty).
http://dx.doi.org/10.1016/j.physleta.2016.05.044 0375-9601/© 2016 Elsevier B.V. All rights reserved.
is mainly focussed on the synthesis of various carbon nanostructures [9,10] which may have important applications in the next generation technology [11,12]. It is noteworthy to mention here that the early studies on the ion-graphite interactions were carried out mostly by employing continuous ion beam sources. Recently, the pulsed ion beams from plasma focus (PF) devices are being used for the study of material modification, thin film deposition, coating, nanofabrication and new material synthesis [13–18]. This device is a very effective tool for ion irradiation on the materials since it can provide the ion energy in the range of a few tens of keV to MeV [19] within a few hundred of nanoseconds. Normally the ion irradiation induced changes on the material are supposed to be due to either nuclear or electronic loss. The lower energy ions mainly lose their energy on the material by the process of nuclear collisions whereas higher energy ions dissipate their energy by electronic collisions. Hence, the consequence of both the collision processes can be easily studied by bombarding ions from the PF device on material. Another obvious advantage of this ion source is that it is possible not only to examine the final output in a multi-PF shots exposure experiment but also the effect of individual ion pulse impact on the target material in a multi-PF shots exposure experiment. Hence, one can have an optimum operational mode in which the ion beam has the control both over time as well as on the fluence (the energy) inputs to the material. The development and optimization of PF device as an ion source
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Table 1 Concentrations of impurities in graphite sample.
Fig. 1. Schematic of the experimental setup.
for the synthesis of a new material is still one of the most active field of research. The PF device is a simple plasma accelerator that makes use of the self generated magnetic field to propel the plasma towards the open end of the coaxial cylindrical electrodes and subsequently focusing them to form a very high density (1025 –1026 m−3 ) and high temperature (>1 keV) plasma column. The plasma column lasts only for a few hundred of nanoseconds because of the instabilities. The device is renowned as a multi radiation source of various charged particle beams and electromagnetic (EM) radiations and, therefore, it finds applications in microlithography [20], EUV lithography [21], nanofabrication [22], thin film deposition [23,28] and nuclear materials testing [24,25]. Realizing the aforesaid advantages of the PF device, the helium ions emerging out of it are used to irradiate the graphite samples so as to examine the irradiation induced changes on them. The reference and ion irradiated samples are characterized by employing various characterization instruments, namely optical microscopy (OM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) to analyze the surface morphologies and x-ray diffraction (XRD) to investigate the structural changes that have taken place on the samples due to the helium ion irradiation. 2. Experimental details The schematic of the experimental setup used as a helium ion source to irradiate graphite samples is shown in Fig. 1. The device is basically a Mather type [26] that mainly consists of a high energy capacitor (7.1 μF, 25 kV), a high voltage charger, a coaxial and concentric electrode assembly enclosed inside a stainless steel vacuum chamber. The electrode assembly is the combination of a central anode and twelve cathode rods encircling the anode in squirrel cage fashion. The chamber is first evacuated with the help of a diffusion pump that is backed by a rotary pump down to a pressure of ∼10−5 Torr and then is filled with helium gas at an optimum working pressure of 0.5 Torr. After that, the capacitor is charged to its maximum value and followed by discharge of capacitor across the electrode system through a low inductance transmission line. When the capacitor is discharged, the current sheaths initially develop across the insulator sleeves due to the breakdown of gas medium and these sheaths are axially acceler-
Impurity
Al
B
Ca
Cu
Fe
Mg
Na
Si
Content (p.p.m.)
0.2
1
0.05
0.01
0.2
0.05
0.02
1
ated towards the open end of the anode by the axial component of self-generated J × B force. Subsequently, the sheaths undergo a radial pinching resulting in the formation of a hot and dense plasma column, which lasts only for a few hundred of nanoseconds due to the instabilities. These instabilities enhance the induced electric field locally which in association with the magnetic field breaks down the plasma column [26] and accelerates the high energy ions towards the top of the chamber. These ions were used to irradiate the graphite samples that were kept at a distance of 0.06 m from the anode top with the help of a movable sample holder as shown in Fig. 1. The ion fluence and flux at the location of the sample for a single PF shot (that is, one single pulse of helium ion) as measured by the ion collector are found to be 1018 m−2 and 1025 m−2 s−1 respectively which are similar to the results obtained earlier [27]. The samples were kept perpendicular to the ion trajectories at room temperature. The experimental conditions were maintained similar to the experiment reported in [24] and it was noted that the instantaneous bulk temperature of the ion irradiated samples increase by about 70 to 100 ◦ C for a single PF shot irradiation as measured by a thermocouple system. The graphite rods having a purity of 99.99% were procured from Good-fellow U.K. The chemical composition of the material is tabulated in Table 1. These graphite rods were cut into disc shape having diameter 0.01 m and thickness 0.002 m using a precision cutter, followed by mechanical polishing of the sample surface with the help of a set of abrasive paper arranging with decreasing order of their particle size. After polishing, the samples were cleaned and then mounted inside the chamber as mentioned earlier and exposed to numbers of PF shots namely 5, 10 and 20 shots etc. The exposed and reference samples were investigated by OLYMPUS BX 51M optical microscopy for surface topography. The surface morphology was also observed at higher resolution by employing NTEGRA PRIMA (NT-MDT) atomic force microscopy (AFM), Russia, with a silicon cantilever operating in semi-contact mode. TEMTECNAI G2 FEI, Netherlands, operated at 200 kV was used to capture the TEM images along with selected area electron diffraction (SAED) pattern images. The TEM sample was prepared by scraping fine powder from the graphite surface and dispersing the resulting powder in isopropanol solution by sonication. A drop of the solution was then allowed to evaporate on a copper mesh grid under the IR lamp. The crystallographic phases of the samples were studied using a Bruker-D8 diffract meter having Cu K alpha source (wavelength 1.54 Å) at 3 degree grazing angle incidence with step size of 0.019917. 3. Results and discussion As mentioned earlier, the graphite samples were irradiated to different fluxes of helium ions by simply varying the number of PF shots (5, 10, and 20 shots). After ion irradiation, the samples were preliminarily inspected visually and it was noticed that the shining of the polished sample was disappeared and the surface become white ash in color. For the better view of the surface morphology, both the reference and ion irradiated samples were studied by employing the optical microscope by taking numerous images of the surface of reference as well as ion irradiated samples. A typical optical micrograph of reference sample shown in Fig. 2 illustrates a reasonably smooth surface without any major defects and cracks. Only a few small voids and some hairline marks are noticed at
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Fig. 2. Optical micrographs of the reference graphite sample at different magnifications.
certain locations that may be arising during the mechanical polishing of the samples. On the contrary, a distinct morphological change is visible in the ion irradiated samples clearly indicating some surface deformation. At lower ion doses (5 numbers of PF shots), the surface of the ion irradiated sample shown in Fig. 3(a) exhibits randomly distributed rounded island like grains whereas in the case of 10 numbers of PF shots the rounded grains become more prominent (as shown in Fig. 3(b)) and are appeared throughout the surface. On further increase in helium ion pulses up to 20 numbers of PF shots, the micrograph (as shown in Fig. 3(c)) indicates mostly rounded grains along with a few elongated structures. It can be speculated from these micrographs (shown in Fig. 3) that the size of the rounded structures is dependent upon the numbers of helium ion pulses imposed on the samples. At 20 PF shots, the size of the grains is bigger than that observed in the case of 5 and 10 PF shots. Moreover, the elongated type structures are resulted on the surface of sample due to intense melting and local ablation accompanied with ejection of graphite melts [28]. Since the samples irradiated to 20 numbers of PF shots show a well developed structures that is why we have further done some other advanced characterization of those to acquire information on formation of the rounded structures. AFM analyses have been performed in order to investigate the possible transformation of the surface topography of the graphite sample due to the ion irradiation in higher resolution and thereby bring further clarity to the optical micrograph results. Fig. 4 shows the typical AFM images for both the reference and 20 shots helium ion irradiated samples, and it depicts that the reference sample, as compared to ion irradiated sample, is relatively smooth with lesser structural features. Similar observations were earlier made by using scanning electron microscopy [29] where authors have discussed about the underlying physical processes responsible for the formation of the observed structures. A close observation to the micrographs of ion irradiated samples suggests the association of some small individual clusters. In our case the diameters of the globules were 50–200 nm and were alike in shape with rounded structures as shown in the optical micrographs in Fig. 3. As men-
Fig. 3. Optical micrographs of helium ion irradiated graphite samples: (a) 5 PF shots, (b) 10 PF shots and (c) 20 PF shots.
tioned earlier, the ions lose their energy during the interaction with the graphite target mainly either through nuclear collisions between the incident ions and the target atoms or electronic collisions in between incident ions and electrons of the target. If an ion is irradiated with an energy which is sufficient to overcome the threshold energy of the target material, then the target atom is displaced from its original lattice position. Considering the displacement energy for carbon atom in graphite as 28 eV [30], it is estimated that the helium ion of energy 37.33 eV is enough to eject one carbon atom from the graphite crystal. Since in our case, the ion energy is much more than above mentioned energy value, so we can expect enormous number of carbon atoms on the surface of the graphite not only by the process of primary knockon but also by secondary, tertiary or even higher order atomic knock-on. On the other hand, the higher part of ion energy (>500 keV) contributes to the electronic loss which gives rise to the localized heating sample surface [31]. The displaced carbon atoms are coagulated with each other into small clusters because of their high structural fluidity [32] under the ion irradiation. Further the subsequent irradiation of the helium ion pulses causes more energy being transferred to the sample surface which provides more mobility to the clusters and the coalescence between the clusters occurs resulting in a larger size structure. These agglomerations are appearing mostly as globules since the successive ion pulses owing to the surface energy minimization [29] of the small clusters
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Fig. 4. AFM micrographs of graphite samples: (a) reference and (b) 20 shots helium ion irradiated.
and surface tension of materials forced them to form a spherical shape. Thus, it can be expected that the formation of the structures solely depends on the nucleation of the carbon atoms and the heat load due to ion irradiation. Further the formation of elongated structures that are also seen along with the globules may be due to nucleation of more than one globule when subsequent PF shots are imposed. We have measured the rms roughness of both the reference and ion irradiated samples over a distance of 1 μm and they are estimated to be 18 and 68 nm, respectively. The increase of the surface roughness in the case of the ion irradiated sample might be due to the formation of agglomerated structures because of the ejected carbon atoms unification as discussed earlier. TEM characterization of both the reference as well as 20 shots helium ion irradiated samples was performed to obtain the insights on the microstructural evolution as a result of ion irradiation. Figs. 5(a) and (b) display the typical micrographs of the reference and ion irradiated samples, respectively. As expected the image of the reference sample shows a two dimensional layered type structure whereas the micrograph of the ion irradiated sample illustrates randomly distributed rounded structures. The diameter of the rounded structures varies in the range of 2–22 nm and the mean diameter of these structures is found to be 10.4 ± 2.706 nm as shown in Fig. 6. As mentioned earlier the mechanism of the development of these rounded structures can be attributed to the aggregations of carbon atoms that are dislodged from the crystallite sites and arranged in some clusters. It is noteworthy to mention that these structures are uniformly dispersed on the graphene sheet and a very few of them are black in color. These darker
Fig. 5. TEM pictures of the graphite samples: (a) reference and (b) 20 shots helium ion irradiated.
Fig. 6. Histogram showing size distributions of rounded structure.
rounded features are tungsten carbide impurities which may arise due to the use of tungsten as anode of the PF device. Figs. 7(a) and (b) show the SAED pattern of reference and 20 shots helium ion irradiated samples, respectively. The bright points arranged in the diffraction rings as shown in the Fig. 7(a) confirm that the reference sample is polycrystalline in nature. On the other hand, the diffraction pattern of the ion irradiated sample reveals that some of the bright points are relocated in new rings thus ensuring the structural rearrangements. The d (interlayer distance) values obtained from the diffraction rings of reference sample are estimated
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Fig. 7. SAED patterns of the graphite samples: (a) reference and (b) 20 shots helium ion irradiated.
intense peak (002) got enhanced to 10.3 nm from the corresponding reference sample value of 9.8 nm when calculated by using the Scherrer’s formula,
D=
0.94.λ β2θ . cos θ
(1)
where D is the grain size, λ is wavelength of X-ray, β2θ is FWHM of the plane, and θ is Bragg’s angle. The increase in the crystallite size may be due to the successive bombardment of ion pulses on material surface that allows more mobility to the nanostructures such that larger size structures can be formed. 4. Conclusion
Fig. 8. XRD patterns of the reference and 20 shots helium ion irradiated graphite samples.
from SAED pattern and found to be 3.352, 2.108, 2.024, 1.668, 1.5, 1.226 and 1.15 angstrom. These values are approximately identical to d002 , d100 , d101 , d004 , d103 , d110 and d112 planes of graphite. On the other hand, in the case of exposed sample the d values of diffraction rings are 3.39, 2.58, 2.341, 2.25, 2.126, 2.03, 1.68, 1.537, 1.464, 1.32, 1.21 and 1.136 angstrom. Fig. 8 presents the measured XRD pattern from 2 theta value 5 to 90 degrees at grazing incidence angle for both the reference and 20 shots ion irradiated samples. The diffraction planes (002), (100), (101), (004), (110) and (112) are observed in the both reference and ion irradiated samples. However, some new planes have been observed in the ion irradiated sample at 2 theta values 34.78, 38.23, 39.95, 62.47 and 70.46 degrees which are assigned to be the peaks of tungsten carbide (W2 C). The formation of these new planes is due to the presence of tungsten anode material which combines with the sample material to form tungsten carbide during the ion irradiation process. The magnified view of the most intense peak of graphite (i.e., (002) peak) undergoes a shift towards the lower Bragg angles thus suggesting the built up of expansive stress in the ion irradiated sample [33]. We have calculated the interlayer spacing for the reference as well as ion irradiated samples for (002) diffraction peak and it is found to be 0.3354 and 0.3393 nm, respectively. The marginal increase in interlayer spacing in the irradiated sample is either due to the displacement of carbon atoms to the interstitial positions or on account of inclusion of helium atoms at the interlayer positions [34]. It is also observed that the crystallite size of the ion irradiated sample for the most
We have successfully performed the helium ion irradiation onto the graphite samples using an indigenous pulsed ion source. The variation in surface morphologies in different numbers of ion pulses (PF shots) has been studied by employing the optical microscope. It is inferred from the optical micrograph results that the surface deformation depends upon the number of ion pulses. The AFM study also indicates the formation of globule structures that may be due to the unification of small clusters. It is also observed that surface roughness increases with ion irradiation because of formation of these structures on the surface of the samples. Moreover, the TEM micrographs of the ion irradiated graphite exhibit rounded structure having mean diameter ∼10.4 nm. Comparing the SAED pattern of the reference and ion irradiated samples, it is noted that the crystalline nature of the ion irradiated sample is retained and there is emergence of some new planes. The XRD results also corroborate the SAED pattern confirming the appearance of new planes that may arise due to chemical reaction of graphite with tungsten impurity material. The accumulation of expansive stress is also confirmed in the ion irradiated samples. However, further studies are needed to establish the basic mechanism lying behind the formation of these carbon nanostructures in more details by employing additional advanced characterization tools. Nevertheless, our study contributes towards developing improved understanding of the ion-graphitic material interactions for the formation of carbon nanostructures. Acknowledgements Authors would like to acknowledge the Director, Institute for Plasma Research, Gandhinagar, India and Centre Director, Centre of Plasma Physics-Institute for Plasma Research, Assam, India, for the financial support to carry out the present work. We are also thankful to Dr. S. Kundu, Dr. N. Adhikary of IAAST, Guwahati, India and Dr. B.B. Nayak of IMMT Bhubneswar, India for supporting our
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