Evidence of diamond-like carbon phase formation due to 80 keV Xe⁺ ion impact on pencil-lead graphitic systems with oblique angle incidence

Initially employed to track particles, latter ion irradiation embarked as a versatile technique for material modification in a much controlled manner [1,2]. It has been established that a projectile can deliver its energy into the target material via two important energy loss mechanisms: electronic $(S_{\mathrm{e}})$ and nuclear $(S_{\mathrm{n}})$ losses [2,3]. In the low-energy regime, the collisions between the ions with the material medium are mostly elastic in nature and consequently, $S_{\mathrm{n}}$ becomes more effective (∼keV/nucleon) as compared to $S_{\mathrm{e}}$. Conversely, $S_{\mathrm{e}}$ arises as a result of frequent inelastic collision of projectile ions with the atoms of the target material and accounts for the high-energy feature (several MeV /nucleon) [2–4]. In contrast to the MeV ions, which are beneficial for creating columnar defects and phase transformation being desired for making etched-membranes with uniformly distributed holes and also in ion track technology, keV beams were shown to be useful for nano-structuring, implantation, surface modification and point defect engineering. This occurs because of the progressive development of nucleating sites and enhanced surface reactivity during the profound elastic collisional events. As an important outcome, in this regime, angle-dependent keV-scale ion irradiation could lead to ripple formation on the target surface, which is normally observable for an incident angle, $\theta_{\mathrm{i}}> 45^{\circ}$ [5,6].

Graphite finds a special place not only as a household material but also in commercial products. Moreover, it is an excellent candidate and is being widely used in nuclear technology across the globe, owing to its low cost and environment-friendly response [7]. It is a layered system with its in-plane atoms held in a single layer via strong covalent bonding. The existence of weak van der Waals forces between every pair of stacked layers makes graphite effortlessly exfoliable, which helps in writing via physical abrasion and scratch. Graphite is anisotropic and a good conductor of electricity and heat due to the vast electron delocalization and because of aromaticity, within the carbon layers [8]. However, normal to the direction of graphite planes, it is slightly less conductive compared to its in-plane direction due to the weak van der Waals forces between the subsequent layers [8]. As far as versatility of graphite is concerned, its usefulness as a moderator in a nuclear plant to an ordinary electrode in electrolysis is already known. Revisiting its single layer has led to the discovery of "graphene"—the thinnest as well as strongest atomic layer ever known till date [9]. These layers can be rolled in different directions and could give rise to what is termed as carbon nanotubes (CNTs). One can have single and multi-wall CNTs depending on the number of outer coverings [10].

As an old allotrope of carbon, sparkling diamond is believed to have formed out of accumulated carbon kept in nature under high temperature and pressure conditions for a prolonged duration of time. Essentially, the diamond structure is created as a result of interpenetrated face-centred cells along the body diagonal by one-fourth of the side edge length. In the past, development of diamond-like carbon (DLC) was realized with the use of nanopulse plasma chemical vapour deposition in ambient laboratory conditions [11]. Defect creation, amorphization and growth of the DLC phase have also been reported for the graphitic systems as a consequence of ion or electron irradiation [12,13]. Conversely, angle-dependent energetic ion irradiation on the graphitic system is not discussed in the existing literature.

Herein, we highlight both surface patterning as well as evolution of DLC phase in the graphitic systems subjected to angle-dependent low-energy Xe⁺ ion impact.

Sample preparation and irradiation

Graphite was collected from a high-quality drawing pencil lead (10B, Kokuyo Camlin Ltd., diameter ${\sim}5\ \text{mm}$). After removing the wood cover from the pencil, it was cut into small pellet-like pieces with the help of a sharp knife, followed by appropriate mechanical polishing to remove away the undesired dirt and debris from the cut surfaces.

Table 1:.  Physical parameters estimated for pencil-graphitic samples subjected to 80 keV Xe⁺ ion irradiation ($S_{\mathrm{n}}=285.6\ \text{eV/}\AA$, $S_{\mathrm{e}}=57.9\ \text{eV/}\AA$, fluence${}=1\times 10^{14}\ \text{ions/cm}^{2}$). SU: un-irradiated; SN: normal irradiation; S50 and S70: oblique angle irradiation at an incidence angle of 50° and 70°, respectively.

Sample code	Average crystallite size (nm)	RMS Roughness (nm)	Ripple amplitude (nm) (peak-to-peak)	FWHM of Raman active G-band (cm−1)	Phonon lifetime (ps)
SU	$171\pm4.2$	$12.5\pm0.8$	–	$7.7\pm0.4$	$0.69\pm0.04$
SN	$133\pm4.1$	$29.4\pm0.7$	–	$12.6\pm0.3$	$0.42\pm0.01$
S50	$131\pm4.2$	$20.6\pm0.6$	$33\pm4.2$	$15.2\pm0.6$	$0.35\pm0.02$
S70	$104\pm4.7$	$54\pm3.9$	$77\pm6.2$	$17.7\pm0.6$	$0.30\pm0.01$

As for irradiation, ¹²⁹Xe⁺ ions were selected because of their heavy mass and inert nature. Whereas the heavy mass enables them to strike the target with high momentum, the inertness helps avoiding chemical bonding as a consequence of irradiation. Apparently, this offers a clean environment to study the irradiation effect without changing the surface chemistry at large. The graphitic specimens were subjected to 80 keV ¹²⁹Xe⁺ irradiation with an oblique angle incidence θ_i = 0°–70° and considering a fixed ion fluence $(1\times 10^{14}\ \text{ions/cm}^{2})$. Naturally, the normal incidence corresponds to $\theta_{\mathrm{i}} =0^{\circ}$. The samples were irradiated in a high-vacuum chamber $({\sim}10^{\mathrm{-6}}\ \text{Torr})$ and with a typical beam current of $2\ \mu \text{A}$, available in the Low Energy Ion Beam Facility (LEIBF) of the Inter University Accelerator Centre (IUAC), New Delhi. The pristine, and irradiated ($\theta_{\mathrm{i}} =0^{\circ}$, 50°, and 70°) graphitic specimens are labelled as SU, SN, S50 and S70, respectively.

Characterization techniques

Structural investigation of the graphitic specimens, before and after 80 keV Xe⁺ ion irradiation, was performed by employing the JANA-2006^© software on the acquired XRD data [14]. Figure 1(A)((a)–(d)) shows the Bragg diffraction peaks located at, $2\theta =26.4^{\circ}$, 44.3° and 54.4° which correspond to the respective crystallographic planes (002), (101) and (004) of the hexagonal phase (JCPDS file No. 751621) bearing a space group P63/mc. Apparently, the most preferred orientation of the crystallites is along the [002] direction of the unit cell. Using the single line fitting relevant to the present case, the average crystallite size of the system is dropped from a value of $171 \pm 4.2\ \text{nm}$ to $104\pm 4.7\ \text{nm}$ (table 1). Moreover, the relative intensities between the (002) to (004) peaks tend to decline with irradiation (fig. 1(B)), thereby suggesting the loss of multiple facets from the specimen due to oblique angle ion impact. The behaviour may not necessarily be due to complete vanishing of the preferred planes but might be linked to localized compression or/and dislodgement at oblique angle incidence.

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The surface morphology of the pristine and irradiated graphitic systems has been characterized through the AFM technique operating in non-contact mode, and further analyzed with the help of WSxM software^© [15]. As depicted in fig. 2(a), (b), un-irradiated pencil-lead graphite (SU) shows a fairly smooth surface topography, whereas the specimen (SN) under normal ion incidence resulted in the creation of microscopic defects and craters. Often, the crater dimension is of several tens of nanometers in depth and noticeably of uneven shape when viewed from the top (fig. 2(b)). Conversely, at an oblique angle of incidence, wavy surface patterns that resemble ripples in the sand of a river-bed can be noticed (fig. 2(c), (d)). The ripple amplitude tends to increase from $33\pm 4.2\ \text{nm}$ to $77\pm 6.2\ \text{nm}$ when the angle of incidence, $\theta_{\mathrm{i}}$, was altered from 50° (S50) to 70° (S70). The root-mean-square (RMS) roughness of the irradiated system was seen to be enhanced owing to either creation of defects or nanoscale ripples within micron scale ones, and offering a maximal RMS value of $54\pm 3.9\ \text{nm}$, obtained for $\theta_{\mathrm{i}}=70^{\circ}$ (table 1). For the sake of clarity, the 3D AFM images of un-irradiated and irradiated with normal and oblique incidences are also presented (fig. 3(a)–(d)). Note how a polished surface gets modified with normal and oblique incidence of the ion impact. A prominent wavy patterning can be visualized in the latter case.

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Raman spectra of the studied specimens are shown in fig. 4(A)((a)–(d)). The pristine graphite exhibits a sharp Raman active mode of lattice vibration located at ${\sim}1580\ \text{cm}^{-1}$ (G-band), whereas the irradiated graphitic systems gave two additional peaks which appeared at ${\sim}1330\ \text{cm}^{-1}$ (DLC) and $1360\ \text{cm}^{-1}$ (Defective). Earlier, the D^*-peak was identified as the characteristic feature of the DLC phase, along with the origin of the D-peak assigned to the presence of defective graphite [16,17]. We further observed that, though the D-band is evident at a lower incidence angle, it is the DLC phase which is more prominent for the specimen irradiated at a higher $\theta_{\mathrm{i}}$ value. Apparently, the development of the sp³ bonded DLC phase exists through initiation of localized disorder in the sp² bonded graphitic layers. Not surprisingly, the broadened G-band with an increasing $\theta_{\mathrm{i}}$ value, predicts partial loss of (002) planes due to oblique ion impact. The signatures of DLC and defective graphite peaks with respect to the G-band are depicted in the histograms, as shown in fig. 4(B). Previously, the formation of nano-diamond and DLC phase has been realized by employing either nanopulse plasma or ultrashort laser pulses [11,18]. High-yield cubic diamond was also produced by employing 3 MeV Ne³⁺ ion irradiation on carbon onions [19]. Moreover, it was established that the ratio of intensity of the D-to-G peak $(I_{D}/I_{G})$ depends largely on the micro-crystallite size and in-plane phonon correlation length [17,20]. Our XRD analysis also confirms the reduction of average crystallite size with increasing angle of incidence, $\theta_{\mathrm{i}}$. As evidenced from the characteristic broadening of the G-band in the Raman spectra (table 1), we anticipate a reduction in the phonon correlation length, as well as in the phonon lifetime $(\tau_{ph})$. $\tau_{ph}$ can be calculated by using the line width of the G-band and using the energy-time uncertainty relation [21–23]: $\frac{\Delta E}{\hslash}=\frac{1}{\tau_{\mathrm{ph}}} = 2\pi c\Gamma$, where $\Delta E$ is the uncertainty in energy, ℏ is the reduced Planck's constant, c is the velocity of light and Γ is the full width at half-maxima (FWHM) of the G peak, in each case.

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Whereas normal incidence offered a slightly lowered value of $\tau_{\mathrm{ph}}$, with oblique incidence it is drastically lowered, i.e., from a value of $0.69\pm 0.04\ \text{ps}$ for the pristine graphite (SU) to a value of $0.30\pm 0.01\ \text{ps}$, considering irradiation at $\theta_{\mathrm{i}} =70^{\circ}$ (S70). A reduction in $\tau_{\mathrm{ph}}$ is attributed to the development of surface ripples which essentially depict periodic surface patterning/roughening created as a result of the virtual loss of hexagonally bonded C-atoms at regular intervals. The oblique incidence was believed to have induced spatially varied energy deposition as a result of which atomic dislodgements and localized deformation would occur. The ensuing DLC phase is a consequence of regular, yet instantaneous removal of C-atoms that give rise to rippled surface structures along with accumulation of defective, sp² bonded graphite.

To conclude, 10B pencil-lead graphitic systems have been irradiated with 80 keV ¹²⁹Xe⁺ ions considering both normal and oblique angle incidences. Both irradiated and un-irradiated graphitic systems exhibited close-packed hexagonal phase, while the average crystallite size showed a declining trend after irradiation. The surface morphology analysis confirmed the formation of surface ripples when subjected to non-zero angle of incidence. Moreover, apart from characteristic G-band, the Raman active DLC peak has been witnessed only for oblique angle irradiation cases. The vanishing and shrinking of (002) planes may lead to surface ripples, whereas nucleation of defects was believed to be primarily responsible for the development of DLC. The co-existence of surface ripples and DLC phase formation can be of value in miniaturized technological assets, especially when superior mechanical and surface transport properties are desired.

We acknowledge IUAC, New Delhi for the financial support (UFR- 62312/2017) and for extending the LEIBF (BTR No. 61406/2017). The authors extend their sincere thanks to Dr. D. Kanjilal, Mr. S. Hazarika and Mr. Kedar Mal for their advice and support. The authors also acknowledge IASST, Guwahati for extending AFM facility, and SAIC, Tezpur University for XRD and Raman measurements.