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Scientific Reports Volume 5, Article Number: 11812 (2015) Cite this article

The rapid change in environmental energy caused by the ultrashort laser pulse causes the phase transition of the carbon allotrope, so it is expected to reveal a new carbon phase. Here, by exposing polycrystalline graphite to 25 fs laser pulses at an energy density of 4 J/cm2 in a standard air atmosphere, we demonstrated the synthesis of translucent micron-scale structures with diamond-like and onion-like carbon phases. The texture domain of the diamond phase was also identified. Regarding the different synthetic carbon forms, pulse superposition and singularity of the thermodynamic process, we determine the synthesis mechanism through laser-induced energy evolution of subsequent products to obtain diamond-like phases.

In recent years, carbon allotropes, such as nanodiamonds, have shown promising new applications in many fields due to their physical, chemical and surface properties. Their high electron mobility, field electron emission and magnetism1 make them important players in carbon-based electronics2,3. Their tribological and mechanical properties1,4,5 produce harder coatings1,5 that are biocompatible and can provide improved bioprosthetic joints 6 while reducing wear. This biocompatibility combined with its biosensing, optical and nanodetection capabilities7,8 provides drug delivery and cell labeling capabilities7. This wide range of new applications has facilitated the active exploration of new and more effective synthesis and production methods of carbon allotropes.

In this pursuit, it has been shown that dynamic compression is a method of directing carbon allotropes to phase changes through rapid energy changes. For example, the transformation from graphite to diamond through impact compression experiments occurs through a rapid martensite mechanism; in a device designed to quench impact compressed samples at a rapid cooling rate, n diamond phase 10 has been produced, and the impact impact meteorite Nanodiamond 11 has been formed in the area. These observations reveal the importance of carbon phase transitions in non-equilibrium states, spurring the use of ultrashort pulses to conduct research on faster time scales, which trigger shock waves that carry extreme temperatures and pressures in matter12,13. Experiments using ultrashort laser pulses with high peak power and a duration of about 100 femtoseconds to irradiate highly oriented pyrolytic graphite (HOPG), usually under vacuum 14, 15, 16 or at the graphite/liquid interface 17, produce sp3 on the graphite. Boundary lattices 14, 18 and provide preliminary evidence of diamond formation on the substrate 14, 15, 16, 17, 19, 20. The ultra-short laser shot brings hope for the synthesis of the known 21 and the theoretically still predicted 22,23 sp3 carbon structure. Here, we use a medium-energy ultrashort laser pulse to generate shock waves and induce sp2 carbon polycrystalline graphite precursor to form a diamond-like phase.

In the current work, laser irradiation of graphite in the air significantly changes the surface and produces a micron-level translucent structure, in which diamond-like crystallites are found to coexist with onion-like phases and quasi-amorphous nano-scale graphite. These materials are recovered after the laser irradiation process and analyzed by Raman microscopy, scanning electron microscope (SEM) and high resolution electron microscope (HREM). Considering the ultrafast optical excitation, ablation mechanism, and unique laser-synthesized carbon structure, we propose an indirect mechanism in which the subsequent laser-induced transformation leads to the synthesis of the diamond-like phase.

The Raman spectroscopy and SEM micrographs shown in Figure 1 pass the stage of evaluating the precursors (Figures 1a, b), that is, the intermediate stage found at the irradiated site, showing the induction of ultrashort laser pulses in the polycrystalline graphite sample. Modified evolution. Surface (Figure 1c, d), to the final diamond-like structure (Figure 1e, f).

The Raman spectrum is shown on the left, and the corresponding SEM image is shown on the right.

(a) and (b) refer to polycrystalline graphite (precursor). (c) and (d) correspond to the laser-modified surface. In (c), the thicker red curve is the measured Raman spectrum, and the gray curve is the component band obtained from the Voigt function fitting. (e) and (f) show the measured values ​​of the laser-created structure (the corresponding optical image is shown in Figure S1 of the supplementary information, which also shows the Raman spectra of the other two laser-created particles). A clear morphological difference was noted between the precursor and the laser modified material. The virgin graphite contains disordered and buckled flakes (b). In (d), the material has a micron-sized spherical structure composed of nano-sized structures. The stacked layers produced by the dynamic compression of the carbon by the shock wave produce the structure shown in (e).

In Figure 1a, the original graphite exhibits its characteristic Raman patterns at 1343 and 1615 cm-1 (correlated to structural defect bands D1 and D2, respectively) and 1579 cm-1, which are derived from the stretching formation of sp2 bound carbon atoms. The hexagonal structure (G-band)24. After irradiation, a significant change was observed in the Raman spectrum of the laser-modified surface, as shown in Figure 1c, relative to the precursor (Figure 1a). Although the D1 band appears at 1340 cm-1, the displacement of the G band to 1595 cm-1 (Figure 1c) confirms the laser-induced pressure, because HOPG at room temperature demonstrates a similar shift to higher wavenumbers. In addition, Raman resonances appeared at 1087, 1245, 1425, and 1552 cm-1, which did not appear in the precursor (Figure 1a). Similar spectroscopic experiments have been reported in which carbon black was quenched in high-pressure and high-temperature environments26 (15 GPa and 1700 °C for 15 minutes) and impact meteorites27. In the former work, the new vibrations were declared as unexplainable Raman modes, while in the latter work, they were attributed to the new carbon phase. Taking into account the previous work 26, 27, 28 and our experimental observations, we believe that these vibrations are evidence of the sp3 carbon phase generated by ultrafast laser excitation, because the theoretical model 29 has predicted that the sp3 lattice has a fairly specific Raman resonance.

The Raman spectrum of the laser-created structure transferred to the Cu grating (Figure 1e) shows a very distinct G band at 1580 cm-1. Although this band almost dominates the spectrum, there are also less intense vibrational resonances similar to laser-modified surfaces. The advantage of the G-band is very similar to the Raman spectrum of HOPG26. Therefore, it indicates that the laser shock wave induces the disordered arrangement of the original graphite to be reconstructed into a more ordered graphite phase. However, the selected diffraction area (SAD) analysis results of the crystallites in the multiple regions of the laser-created structure (Figure 1f) are quite different from graphite, as shown in Figure 2a (laser-created particles) and 2b (polycrystalline graphite precursor) . By evaluating the relevant diffraction peaks plotted as a function of the reciprocal of the d-spacing (Figure 2d), the laser-induced changes are confirmed by the apparent absence of the 0.338 nm interlayer distance (characteristic of the graphite phase). Structure. In addition, the electron diffraction pattern of this laser-synthesized carbon form reasonably matches the electron diffraction pattern of the zinc mixed diamond phase 10,30. Therefore, we believe that the laser excitation synthesizes the diamond-like phase. Figure 2c shows a high-resolution image of a small area corresponding to the diffraction pattern area shown in Figure 2a. The entire image area (16 × 16 nm2) of Figure 2c shows a plane (diamond feature plane distance) with a distance of 0.205 nm and an amorphous background, which is also noticed in Figure 2a.

High-resolution electron diffraction analysis of the diamond-like carbon phase.

(a) Laser-created structure and (b) SAD of its graphite precursor; (c) HREM micrograph of the laser-generated structure, the characteristic 0.205 nm d spacing of the diamond phase is evidenced by the zoom of the small area shown in the bottom inset; The above illustration shows the Fourier transform of the entire image; (d) shows the corresponding electron diffraction peaks as a function of the reciprocal of the d-spacing of the polycrystalline graphite (PG, gray spectrum) and the diamond-like phase (blue spectrum).

The dark field image shown in Figure 3b was obtained using the objective lens aperture to select the diffraction spots corresponding to the two stronger peaks of the diamond-like phase electron diffraction pattern (Figure 3a). The texture micro-domains of the latter phase can be observed (Figure 3b), with a typical size of about 50 × 25 nm2. Near them, onion-like structures were also encountered (Figure 3c, d).

The texture domains of diamond-like phase and onion-like carbon coexist in the particles generated by the laser.

(A) SAD from the diamond-like phase of the small area on the laser created structure area shown in (b). The highlighted red dot in frame (a) represents the selected diffraction peak used to construct the pattern domain shown in (b). An onion-like phase was also found in the structure created by the laser, as shown in Figures (c) and (d).

Here, we propose that the mechanism of the transition from the graphite phase to the diamond-like phase follows an indirect approach, which is based on the morphology of the starting material, the specific thermodynamic events determined by the flux of overlapping ultrashort laser pulses and the formation of natural catalysts. , Such as onion-like structure 31 and laser-driven nano-scale graphite 26. Initially, the high density of free electrons accumulated at the boundary of the precursor defect graphite flakes is conducive to the absorption of the ultrashort pulse energy, thereby generating a super excited state and finally ablating 32. Due to the ultra-fast excitation above the high-flux ablation threshold and subsequent explosive ablation, the non-thermal shock wave first propagates into the material after the electrons relax, and then heating and thermal equilibrium occur. This thermodynamic process, which occurs with each laser shot, produces cumulative incremental lattice distortions14,33, leading to more ordered carbon forms34. Taking into account the exposure time of pulse superimposition, these latter structures appear as transients, and their energy barrier to phase change decreases, while the crystallinity gradually increases with each shock wave shot. Therefore, the formation of the diamond-like phase is assisted by the intermediate carbon structures that assume the role of nucleation sites, and these carbon structures are more prone to phase transitions35.

In conclusion, as far as we know, we have demonstrated for the first time that the diamond-like phase synthesized by blasting polycrystalline graphite with spatially overlapping 25 fs ultrashort laser pulses in an air atmosphere relaxes the experimental conditions for manufacturing nanodiamonds. Through Raman microspectroscopy, SEM and HREM research, it is found that the diamond-like phase and the onion-like carbon structure coexist in the micron-sized translucent laser particles. Based on the results of previous literature work and our observations, we propose a mechanism for the formation of diamond-like phases from graphite precursors. The deduced synthesis pathway correlates excitation, temperature and pressure kinetics with phenomenological observations to generate relevant insights into the phase transition of carbon allotropes under ultra-fast extreme conditions.

Samples were prepared by cutting 99.99% pure polycrystalline graphite rods (Alpha Aesar) into rectangular pieces (15 × 7 × 4 mm3), and the larger surface was manually polished with fine sandpaper, with a roughness of up to 10 μm (RMS). Raman microscopy (Xplora Plus microscope from Horiba, laser excitation wavelength is 532 nm), scanning electron microscope (SEM, FEI Inspect 50F) and high resolution electron microscope (HREM, JEOL JEM 2100F) are used to characterize precursors Laser irradiation of samples and subsequent products. In order to complete the ablation experiment, the sample was irradiated with ultrashort pulses from the Ti:Sapphire magnified laser system (Femtopower Compact Pro HR/HP from Femtolasers). The system generates 25 fs pulses in a 4 kHz pulse train with a center of 780 nm and an energy of 558 μJ. The beam is focused in the air by a 75 mm focal length achromatic doublet lens, the sample is placed 1 mm in front of the focal plane, and the laser beam is irradiated on its polished surface. In order to create a suitable surface for the analysis, the sample was moved transversely to the beam at a speed of 10 mm/s, in such a way that a line was etched across its width; then, 400 parallel lines were etched, displaced by 10 μm, and formed 7 × 4 mm2 irradiation area. On the sample surface, the calculated beam spot size (radius) is 67 μm, and each pulse produces a fluence of 3.96 J/cm2, which corresponds to a peak intensity of 1.58 × 1014 W/cm2. The fluence for this application is three times higher than the 1.3 J/cm2 single shot high fluence ablation threshold we measured for this material. In addition, we estimate that the pulse superposition of each point is close to 300 pulses, and the high-throughput ablation threshold is reduced to 0.8 J/cm2,36 at this time. In contrast, it is known that ultra-short laser pulses with a flow rate as low as 60 mJ/cm2 can cause the interplanar spacing of graphite to change, which is the beginning of the laser ablation process. After irradiation, the sample is annealed in an ambient atmosphere (5 hours at 450°C) to eliminate debris 37. After visual inspection of the ablated surface through the 10x magnification objective of a Raman microscope, the white illumination highlights the translucent micron-scale structures with different shapes, ranging in size from 10 μm to 50 μm (Supplementary Information Figures S1 to S3) And obvious photoluminescence (Figure S2 for supplementary information). The electrostatic attraction between these structures may be separated from the substrate due to the ablation process and the tip of the atomic force microscope, which allows them to be transferred to the Cu grating for HREM analysis.

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The work of LNLS and LNNano is supported by the Brazilian Ministry of Science and Technology. We thank the Brazilian funding agencies CNPq, CAPES and FAPESP for their financial support. We thank the Multi-User Laboratory (LAMULT) of the University of Campinas for the micro-Raman spectroscopy measurement.

Laboratório Nacional de Luz Síncrotron (LNLS), Campinas, 13083-970, Sao Paulo, Brazil

Francisco CB Maia, Raul O. Freitas and Narcizo M. Souza-Neto

Instituto de Pesquisas Energéticas e Nucleares (IPEN-CNEN/SP), 05508-000, São Paulo, Brazil

Ricardo E. Samad & Nilson D. Vieira Junior

National Nanotechnology Laboratory (LNNano), Campinas, 13083-970, Sao Paulo, Brazil

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RES and NDV Jr. conducted ablation experiments. The FCBM performed Raman measurements. ROF uses an atomic force microscope to guide the experiment. JB completed electron micrographs and diffraction experiments. NMS-N. Propose experiments and coordinate work from start to finish. All authors participated in the discussion and made effective contributions to the conclusions.

The author declares that there are no competing economic interests.

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Maia, F., Samad, R., Bettini, J. etc. Synthesize diamond-like carbon phase from graphite by ultrafast laser driven dynamic compression. Scientific Report 5, 11812 (2015). https://doi.org/10.1038/srep11812

DOI: https://doi.org/10.1038/srep11812

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