Graphitization of diamond by laser-accelerated proton beams

Accepted Manuscript Graphitization of diamond by laser-accelerated proton beams M. Barberio, S. Vallières, M. Scisciò, G. Kolhatkar, A. Ruediger, P. Antici PII:

S0008-6223(18)30587-6

DOI:

10.1016/j.carbon.2018.06.031

Reference:

CARBON 13236

To appear in:

Carbon

Received Date: 5 February 2018 Revised Date:

12 June 2018

Accepted Date: 13 June 2018

Please cite this article as: M. Barberio, S. Vallières, M. Scisciò, G. Kolhatkar, A. Ruediger, P. Antici, Graphitization of diamond by laser-accelerated proton beams, Carbon (2018), doi: 10.1016/ j.carbon.2018.06.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Focused laser pulse

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Au 10 µm (proton source) Protons

Diamond

40-180 µm

Proton virtual point source

q 2.5 cm

Half-angle

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Graphitization of Diamond by Laser-Accelerated Proton Beams M. Barberio1, S. Vallières1, M. Scisciò1, G. Kolhatkar 1, A. Ruediger1, P. Antici1* INRS-EMT, 1650, boulevard Lionel-Boulet, J3X 1S2 Varennes (Québec), Canada

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Abstract

In this work, we investigate the interaction between fast energetic proton beams generated by laser-plasma based accelerators with carbon lattices of graphite and diamond.

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Theoretical and experimental results indicate that these proton beams cause both, a low sputtering of carbon atoms from lattices (estimated to be in the order of 3 x 10-8 displacements per atom for each single shot), and a heating (inside the target and within a depth of about 10

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microns below the surface) at a temperature close to the sp3/sp2 transition. The defects generated inside the lattices by the atom displacements cause the random formation of amorphous carbon islands, while the higher temperature causes the start of diamond partial graphitization. All the results suggest the impossibility to use diamond as detector in laser-driven accelerators when placed close to the source, while they confirm that proton beams can be used as alternative to

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classical laser or ion beam methods to graphitize diamond in specific optoelectronic applications.

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*Corresponding author. E-mail: [email protected]

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1. Introduction

The physical properties of carbon allotropes, diamond and graphite, and of their nanostructured materials (carbon nanotubes, graphene, and fullerenes) are used in many

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applications, which range from optoelectronics, to nuclear physics and medical science1,2,3. Their high thermal conductivity, high carrier mobility and chemical stability suggest their use in electronics, while the high resistance to radiation and the elevated melting point promote their application in nuclear and medical physics4, 5. In particular diamond devices are considered to be

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well-performing sensors for detection, beam monitoring, and flight time measurements in radiation physics6. In this application framework, the realization of electrical contacts with good

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adhesion, stability, radiation hardness and Ohmic behavior on the diamond surface is strongly required, and many research groups are working on the possibility to realize graphite electrodes directly in the diamond bulk7.

The process of diamond graphitization is strongly related to the chemical bonds of the two carbon allotropes: Graphite is characterized by sp2-hybridized bonds, while diamond exhibits sp3 hybridization. The complete or local transformation from the diamond allotrope to

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graphite is only possible by breaking the sp3 bonding in the diamond. The process requires that an input energy be provided by a laser or by particle irradiation, breaking several sp3 bonds and forming a state of amorphous graphite-like carbon. A subsequent annealing phase at a temperature of about 2100 °C in air (1700 °C at low pressure) transforms the amorphous carbon

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in graphite thus reconstructing the sp2 bonds4. Many groups are currently working on the realization of controlled graphitized regions on artificial or natural diamonds. For this purpose they propose the use of a high power laser8, or the irradiation with energetic particle beams9 or

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with a cold plasma jet10.

In the last decade, intense research has been conducted with regard to new accelerator

technologies and in particular on the topic of laser-accelerated proton beams, produced during the interaction of a solid target with a high-intensity (I>1018 W/cm2), short pulse (<1 ps) laser, since these beams have some characteristics which, combined, can be superior to present conventional radio-frequency accelerators11, 12. Today, routinely obtained laser-generated protons exhibit about 1013 particles per shot, are ps duration at the source, have an energy in the tens of MeV13 with very good laminarity14. While strong effort is put into laser-driven particles in

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different applications such as in astrophysics15, injectors for large-scale accelerators

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, bright ultra-short neutron source17,

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, as

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, or medicine , material science applications are still

in a very embryonic state with some interesting pioneering work presented recently23, 24, 25, 26. Conversely, laser-driven particles offer many opportunities in this field27,

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, in particular when

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benefitting from their high particle flux and their short bunch duration.

In this paper, we demonstrate that laser-accelerated protons allow inducing graphitization states on natural diamonds and can be used as an alternative to classical laser methods. Comparing the effects of the irradiation with laser-accelerated protons and with classical protons

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on different diamonds, our results clearly indicate that partial (incomplete) graphitization using laser-generated protons is much more effective than what is obtained with classical irradiation

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from conventional accelerators. In addition to this, our partial graphitization process is much faster and more efficient, and allows achieving a comparable incomplete graphitization depth than what is obtained using conventional laser methods. To confirm our claim, we compare the effects of our laser-generated proton irradiation on both, diamonds and on a graphite surface, in order to better understand the effect of the fast proton irradiation on different lattice structures.

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2. Experiments and Methods

The experiment was performed using the TITAN laser of the Jupiter Laser Facility (JLF) located at the Lawrence Livermore National Laboratory (LLNL), Livermore (USA). The experimental set-up is shown in Figure 1.A. A laser with pulse energy E~220 J, pulse duration

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τ=700 fs, wavelength λ=1.053 µm, and beam diameter of about 25 cm is focused down by an f/3 off-axis parabola to an about 9 µm focal spot diameter (FWHM), producing an intensity I~1020

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W/cm2. The laser was hitting with normal incidence onto a 10 µm thick gold foil (gold purity 99.9 %, commercially available from Goodfellow) in order to accelerate protons in the laserforward direction using the Target-Normal Sheath Acceleration (TNSA)29 mechanism. In this acceleration process, the focused laser pulse generates at the front surface, resulting from the ponderomotive force, energetic (“hot”) electrons with a mean energy of a few MeV that travel through the target. While some electrons escape the target at its rear surface, most electrons are retained by the positively charged bulk of the target and form a dense electron sheath at the rear target surface over a distance comparable to the Debye length (in this case about 1 µm). This creates a charge separation electric field in the order of TV/m that accelerates residual water

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contaminants (mainly hydrogen) located on the back of the initially unperturbed target surface (the acceleration occurs in a timeframe shorter than the typical relaxation time of the bulk of the target). The ion beam is therefore accelerated normally from the almost rigid rear surface of the irradiated target, generating a diverging (within a cone of about 25° half-angle), short and intense

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proton source. The TITAN laser is able to shoot one laser pulse every 30 minutes, waiting time needed to cool down the optical amplifiers; however, protons can be achieved also on much higher repetition rate lasers that are now commercially available30.

In our setup, the laser-accelerated protons stemming from the gold foil were impinging,

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under vacuum (10-6 mbar), onto the pristine secondary targets, one being graphite, size 2×2 cm2 with a thickness of 250 µm and, separately, another one being a natural diamond (class Ia

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diamond, size of 0.23 cts, clarity Vs-2, color I). A 5 µm aluminum foil was put in front of the secondary targets in order to protect them from the residuals produced during the TNSA acceleration mechanism (e.g. gamma radiation, heavy ions, etc). The foil was blocking protons with energies below about 470 keV, which produces a negligible effect on the proton irradiation since these particles are at the wings of the spectrum with little contribution (<3%). The targets were placed on axis at a distance of 2.5 cm from the proton source (Figure

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1.A). This distance between the proton source and the targets was chosen in order to generate a temperature lower than the melting point on the secondary target surface, as previously calculated using an Energy Deposition Code (see later). Since the intense proton flux can generate very high temperatures on the target, we needed to ensure that, due to the ballistic spray

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of the laser-driven protons (see Figure 1.B), the flux was sufficiently low not to melt the target31. The proton beam covered the entire surface of the irradiated targets and the beam center

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corresponded exactly to the target center, allowing us to obtain a symmetric conical distribution of the proton beam distribution on the target surface. As diagnostics for the proton beam, we used two calibrated Thomson parabolas (TPs)

located at 0° (TP 0°) and 9° (TP 9°) with respect to the main laser pulse axis to measure the forward generated proton spectrum (Figure 1.A). The TPs were placed respectively at a distance of 690 and 565 mm from the proton source (distance from the gold foil to the entrance slit of the TPs). Proton spectra measured by the TPs were read out in an absolute manner32, 33 using Image Plates (BAS-TR 2025 from Fuji Photo Film Co. Ltd) that were analyzed using a FUJIFILM

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FLA-7000 reader. A typical proton spectrum obtained during the experiment is shown in Figure 1C. The interaction between the laser-generated proton beam and the diamond was modeled using a two-dimensional Energy Deposition Code to simulate the energy deposition phase and

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estimate the temperature reached by the proton heating. The code takes our experimentally measured beam parameters presented in Figure 1.C-E as input, typical for laser-generated proton sources34, which includes the number of protons, the cone beam half-angle from which the protons stem out from the source, and the change of the proton source diameter with energy (this

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is computed by changing the distance of a so-called virtual source point (see Figure 1.A)). The code considers a Gaussian transverse fluence profile and the energy deposition is calculated for our specific cone beam geometry using the proton stopping power tables available from the

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NIST-PSTAR35 database. All these beam characteristics (spectrum, half-angle and source diameter) were obtained experimentally using Thomson parabolas (TP) as proton spectrometers and radiochromic films (RCF) for determining the half-angle and virtual source position for each proton energy. Several simulations were run in order to find a distance between the proton source and the secondary target for catalyzing the above-described process, i.e. that the maximum

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temperature in the secondary target was between the melting point and the sp3/sp2 transition. Morphological and chemical analysis on the pristine and irradiated surfaces was conducted using SEM microscopy and EDX analysis under SEM condition. Crystallinity and allotropic characteristics of the surfaces were investigated by Raman spectroscopy, using a

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Raman microscope in confocal backscattering geometry36. The Raman characterizations were executed at room temperature using a Horiba system with a customized Olympus BX-41

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confocal microscope, and a 0.25 NA 10× objective. A linearly polarized blue solid state Cobolt 04-01 laser diode-pumped operating at 473 nm was employed, as well as a back illuminated deep depletion Synapse CCD from Horiba Scientific. The laser used during the Raman characterizations had an ellipsoid focal spot with dimensions in the two axes of 0.650 x 2.5 µm, the total power illuminating the target was 1 mW. In the figures related to the Raman spectroscopy we show the “as taken” spectra without using the correction for the sensitivity factors, nevertheless, in all our discussion we take into consideration that the difference between the Raman scattering cross section for graphite is ~50 times larger than that for diamond phase37, 38

. The XPS data are fitted assuming a Gaussian distribution. The analysis of the material bulk

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was conducted using Auger Electron Spectroscopy (AES) using a JEOL Jamp30 spectrometer equipped with a LaB6 filament (current of 500 nAs and acceleration voltage of 10 kV). The electron detector was working in the range between 0 and 3 keV. The Auger spectra were

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displayed in differential mode.

3. Results and discussion

Preliminary simulations using our beam parameters (see Figure 2) indicate a source-totarget distance of 2.5 cm as optimized distance for the required temperature. We can see in

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Figure 2.A that the temperature near the surface reaches 2100 °C, which is high enough to enable a phase transition in this low pressure environment (10-6 mbar) without reaching the carbon

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melting point (which is about 4000 °C). We can also note that most of the proton energy is deposited within the first 10 microns of depth (see Figure 2.B) and then decreases very rapidly with increasing depth in the sample. This is due to a high amount of low energy protons in the spectrum (<2 MeV), which have a very high stopping power for diamond in this region (Bragg peak). Concerning the radial temperature profile, we can note that the transition is enabled up to a radius of around 6 mm (see Figure 2.C), which is larger than the actual sample’s width of 5

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mm (i.e. radius of 2.5 mm), in line with the fully graphitized surface observed on the diamond sample. The cooling phase for reaching temperatures lower than the transition temperature is estimated to last tens of nanoseconds.

The obtained thermodynamic conditions indicate a sputtering process for both allotropes, which

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includes the removal of some atoms from the lattice and the consequent formation of lattice defects, and the aggregation of removed atoms in amorphous carbon fragments. Moreover, the

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higher temperature, exceeding 2100°C on the diamond surface, can cause the transition from the sp3 to sp2 hybridization in the diamond. The long cooling phase, the temperature being lower than the melting point, and the high penetration depth of our protons in the diamond bulk, there three conditions provoke a partial graphitization of the material bulk surface, without formation of nanostructures (fullerenes, graphene flakes, carbon nanotubes), which would require shorter synthesis times and higher temperatures39. We use the term partial (incomplete graphitization) due to the probable presence of sp3 hybridized carbon atoms (nanodiamond phase, diamond nanoclusters) on the nanoscale.

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These theoretical considerations are confirmed by our experimental data. SEM images acquired on the graphite sample (see supplementary materials, Figure S1) indicate that the typical smooth surface of the pristine graphite becomes very rough after our proton irradiation. Similarly to the graphite sample, also the flat surface of the pristine natural diamond (Figure 3.A)

formation of carbon aggregates (details in Figure 3.C-D).

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shows (Figure 3.B) a very high roughness and a high damage after the irradiation, with the

Chemical analysis, obtained by EDX spectroscopy under SEM conditions (see supplementary materials Figure S2), indicates the presence on both surfaces of Carbon, Oxygen

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(2%), and Gold (about 3%). The small presence of oxygen can be attributed to a simple adsorption of atmospheric oxygen, while the presence of a small weight percent of Gold (~ 3%)

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indicates the implantation of energetic gold atoms present in the proton beam as residuals of the TNSA mechanism. Auger spectra (see Figure 4.A) confirm this chemical composition, while the structure of the KLL line in both displayed samples (black curve for the irradiated diamond and red curve for the non–irradiated graphite sample) indicate that on both surfaces the main allotropic species is graphite. In both spectra the D parameter (the distance between the positive maximum and negative minimum of the KLL line derivative, which identifies the carbon

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allotropes) is of about 22 eV, indicative of graphite, while the expected value for diamond would be about 13 eV 40. The chemical shift of about 5 eV in the KLL position for graphite, as visible in Figure 4.A, is caused by the charging of the sample during the AES measurements. These results clearly indicate that in the entire region investigated by the EDX and AES

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spectroscopies (up to about 5 µm under the sample surfaces) the interaction between the carbon lattice and the high energetic protons causes the destruction of the crystalline lattice and the

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transition from the allotropic configuration of diamond to that of graphite, with the formation of a small amount of amorphous carbon. Raman spectra shown in Figure 4.B-C confirm these conclusions displaying strong modifications after the proton irradiation for the diamond sample. As visible in Figure 4.B, the graphite Raman spectrum before and after irradiation displays a very similar structure, with the presence of D, G, and 2D lines at the positions expected for graphite (1369 cm-1 for the D structure, 1586 cm-1 for the G structure, and 2737 cm-1 for the 2D structure)

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. The G and D-bands are attributed to the sp2 hybridized carbon and its related

defects, while the 2D band is the second order of the D-band42. The spectrum of the pristine diamond surface (Figure 4.C) shows the mere presence of a strong band around ~1340 cm-1,

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representative of the sp3 phase for the diamond allotropic species. We would like to point out that the diamond spectrum ranges from phonon frequencies in the interval of 1333 to 1340 cm-1 depending on the order in the material: the higher the frequencies, the more disordered is the diamond material43; the presence of disorder in our natural diamond is further confirmed by the

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full width at half maximum of the sp3, in our case with a value of 4 cm-1, higher than that of a perfect diamond (~2 cm-1). Nevertheless, the band around ~1340 cm-1 is therefore still representative for the conventional ~1332 cm-1, and, given the low power of the impinging laser during the Raman analysis and the high thermal conductivity of ~2200 W/(mK) for diamond, not

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to overheating of the diamond. After proton irradiation, this band intensity is strongly reduced (see red curve in Figure 4.C, note that the red curve follows the red scale on the right, while the

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black curve follows the left scale on the left) while the D, G and 2D bands appear on the diamond spectra (see still Figure 4.C) confirming the partial graphitization of the sample surface and the transition of several bonds on the diamond from the sp3 to the sp2 configuration (Raman spectra to verify that the material in the SEM images cannot be attributed to the formation of a grapheme layer are shown in the supplementary materials, Figure S3). Furthermore, we can see that the G band consists of two neighboring bands, located at ~1590 cm-1 and ~1620 cm-1. While

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the first peak is characteristic of the G-band, the second peak is often found in graphite and attributed to defects 44, 45. Indeed both conditions, i.e. the relatively fast cooling phase (estimated to be in the order of tens of ns), and the fact that the temperature is lower than the melting point, prevent the formation of graphene layers.

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All our experimental results can be interpreted in terms of progressive partial graphitization caused by the fast (tens of ns) interaction between the energetic protons and the

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carbon lattices. This interaction provokes, at first, sputtering of atoms from the surface. We can evaluate the sputtering yield defining the displacements per atom (or dpa) as the number of times that an atom is displaced for a given proton fluence:

,where

is the proton beam fluence and

= is the cross section of the process, i.e. the probability

that the incident beam interacts with a matrix atom and produce a knock-on collision that ejects the atom from its initial position. The fluence

was evaluated using the proton beam spectrum

that is irradiating the front surface of the target, such as obtained during the shots, as function of proton energy. Considering the following formula, where N(E) is the measured proton spectrum

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(see Figure 1), A the surface onto which the proton beam impinges (in our case about 1 cm2 ), we obtain for the first surface layer using the TITAN laser: =

( )

≈ 3.2 × 10

obtain a

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Estimating the interaction cross-section σ for our materials to be in the order of 10-21 cm2 46, we ~3×10-8 dpa for one single shot on the TITAN laser. This high amount of atom

displacements well justifies the presence of defects in the graphite and diamond lattice, which are visible in the SEM images in Figure 3, and produces the formation of amorphous carbon

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islands on both surfaces. Moreover, the surface heating (reaching a maximum temperature of about 2500 °C) causes the start of the transition from the sp3 to the sp2 configuration. The transition starts with the proton irradiation and lasts as long as the temperature exceeds 2100 °C,

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which is estimated to be in the order of tens of nanoseconds. The presence of a residual diamond phase (a conservative estimation, based on the net peak height and the width of the noise band provides an upper concentration threshold of 50%) can be attributed to different phenomena: the formation of small nanodiamond areas temperatures 48.

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, or to the stress of graphite generated by the high

The progressive partial graphitization of diamonds when irradiated by an intense flux of

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laser-generated protons, makes it difficult to use diamonds as detector for laser-driven accelerators. Proton beams generated in classical accelerators show a fluence in the same order of magnitude as our proton beam and energies of the same order or greater6. However, classical

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proton beams are typically monoenergetic, which reduces the volume subjected to sputtering and limits a higher heating only to some layers close to the Bragg peak region relative to the incident proton energy. The limited interaction region in conventional accelerators prevents the sp3/sp2

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transition and limits the formation of lattice defects and amorphous carbon. Similarly to conventional laser beams9, our laser-driven proton beam heats a large region under the diamond surface, causing the partial graphitization process and suggesting the use of laser driven proton beams as valid alternative to laser devices to trigger the graphization process. In advantage to laser-technologies, the graphization process occurs on much quicker timescales (in one single laser-shot) and can be performed on larger areas (provided the proton flux is sufficient).

4. Conclusions

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In conclusion, we investigated the effects of the interaction between fast energetic proton beams generated by laser-plasma based accelerators with the carbon lattices of graphite and diamond. Theoretical and experimental results indicate that proton irradiation causes (1) a low sputtering of carbon atoms from lattices (estimated to be in the order of 3×10-8 displacements per

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atom for each single shot), and (2) a heating (inside the target and within a depth of about 10 µm below the surface) at a temperature close to the sp3/sp2 transition. The higher temperature initiates the partial diamond graphitization (as evidenced by Raman and Auger spectroscopies). All the results suggest a non-favorable use of diamond as detector in laser-driven accelerators

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when placed close to the source, while they confirm that proton beams can be used as an alternative to classical laser or ion beam methods to graphitize diamond in specific electronic

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applications.

Acknowledgements

We thank the JLF-TITAN laser staff, S. Veltri and A. Morabito for their precious support. We also thank the INF-LMN laboratory of INRS-EMT and C. Chabanier for the crystallographic analysis and useful suggestions, the CNIS center of the University “Sapienza” (Rome, Italy) and

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F. Mura for the material sample analysis, and the Centre de Caractérisation Microscopique des Matériaux (CM)² from the École Polytechnique de Montréal and P. Plamondon for the AES. This work is funded by FRQNT (Team Grant 2016-PR-189974), NSERC Discovery Grant (Grant No. 435416), and ComputeCanada (Job: pve-323-ac, P. Antici). The use of the Jupiter

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Laser Facility was supported by the U.S. Department of Energy, Lawrence Livermore National

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Laboratory, under Contract No. DE-AC52-07NA27344.

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Figures

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Au 10 µm (proton source) Protons

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Diamond

φ (r, θ , E)

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Focused laser pulse

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(D) Half-angle [°]

dN/dE [protons/MeV]

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Local proton orientation

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2.5 cm

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Energy [MeV]

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Energy [MeV]

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Figure 1: Experimental setup and proton beam characteristics. (A) Experimental setup on the TITAN laser. (B) Proton beam cross section in the transverse plane showing a Gaussian proton fluence distribution. (C) Proton spectrum acquired with Thomson parabolas on the central axis normal to the beam. (D) Proton divergent cone half-angle θ dependence with energy. (E) Proton virtual source position with respect to real proton source (Au target). Dimensions are not to scale.

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Temperature (°C) Phase transition : r = 6.4 mm

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(A)

Temperature (°C)

Depth (µm)

Temperature (°C)

Phase transition : z = 8.6 µm

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Radial (mm) Radial distance distance (cm)

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Figure 2: (A) Temperature map of the graphite sample when irradiated by laser-generated protons at a distance of 2.5 cm; (B) in-depth lineout of the temperature map, at beam center (r=0), showing the depth where the temperature reaches the phase transition; (C) radial lineout of the temperature map, showing the radial distance where the temperature reaches the phase transition.

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(C)

(B)

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(A)

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Figure 3: SEM images of pristine (A) and irradiated diamond (B); (C-D) Details of an irradiated region. All images are obtained by a single shot measurement, i.e. by irradiation of the sample with a single proton burst.

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Figure 4: (A) AES survey spectra for irradiated diamond (black curve) and non -irradiated graphite (red curve); (inset) zoom of the AES spectra shown in (A) for the region of the KLL band. (B) Raman spectra of pristine (black curve) and irradiated graphite in three different regions (named “A-C”) covered by the laser-accelerated proton beam (blue, green, and red curves); spectra are normalized to the G band before irradiation. (C) Raman survey spectra of pristine diamond (black curve, left axis) and irradiated diamond (red curve, right axis).

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