Open Submitted: 27 June 2017 Accepted: 07 August 2017 Published Online: 21 August 2017
Appl. Phys. Lett. 111, 081103 (2017); https://doi.org/10.1063/1.4993118
more...View Affiliations
View Contributors
  • Patrick S. Salter
  • Martin J. Booth
  • Arnaud Courvoisier
  • David A. J. Moran
  • Donald A. MacLaren

Laser writing with ultrashort pulses provides a potential route for the manufacture of three-dimensional wires, waveguides, and defects within diamond. We present a transmission electron microscopy study of the intrinsic structure of the laser modifications and reveal a complex distribution of defects. Electron energy loss spectroscopy indicates that the majority of the irradiated region remains as sp3 bonded diamond. Electrically conductive paths are attributed to the formation of multiple nano-scale, sp2-bonded graphitic wires and a network of strain-relieving micro-cracks.
Diamond has long been exploited for its extreme mechanical properties, but advances in the synthesis of large single crystals also make it attractive for a variety of new technologies, including harsh environment applications,11. W. Adam, C. Bauer, E. Berdermann, P. Bergonzo, F. Bogani, E. Borchi, A. Brambilla, M. Bruzzi, C. Colledani, J. Conway et al., Nucl. Instrum. Methods Phys. Res., Sec. A 434, 131 (1999). https://doi.org/10.1016/S0168-9002(99)00447-7 high frequency electronics,22. S. A. O. Russell, S. Sharabi, A. Tallaire, and D. A. J. Moran, IEEE Electron Device Lett. 33, 1471 (2012). https://doi.org/10.1109/LED.2012.2210020 biosensors,33. F. Picollo, S. Gosso, E. Vittone, A. Pasquarelli, E. Carbone, P. Olivero, and V. Carabelli, Adv. Mater. 25, 4696 (2013). https://doi.org/10.1002/adma.201300710 and photonics.44. I. Aharonovich, A. D. Greentree, and S. Prawer, Nat. Photonics 5, 397 (2011). https://doi.org/10.1038/nphoton.2011.54 In particular, the recent use of nitrogen vacancy (NV) centers for both quantum processing55. H. Bernien, B. Hensen, W. Pfaff, G. Koolstra, M. Blok, L. Robledo, T. Taminiau, M. Markham, D. Twitchen, L. Childress, and R. Hanson, Nature 497, 86 (2013). https://doi.org/10.1038/nature12016 and as extremely sensitive magnetic field and temperature sensors6–96. G. Balasubramanian, I. Chan, R. Kolesov, M. Al-Hmoud, J. Tisler, C. Shin, C. Kim, A. Wojcik, P. R. Hemmer, A. Krueger, T. Hanke, A. Leitenstorfer, R. Bratschitsch, F. Jelezko, and J. Wrachtrup, Nature 455, 648 (2008). https://doi.org/10.1038/nature072787. J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, P. Cappellaro, L. Jiang, M. V. G. Dutt, E. Togan, A. S. Zibrov, A. Yacoby, R. L. Walsworth, and M. D. Lukin, Nature 455, 644 (2008). https://doi.org/10.1038/nature072798. P. Neumann, I. Jakobi, F. Dolde, C. Burk, R. Reuter, G. Waldherr, J. Honert, T. Wolf, A. Brunner, J. H. Shim, D. Suter, H. Sumiya, J. Isoya, and J. Wrachtrup, Nano Lett. 13, 2378 (2013). https://doi.org/10.1021/nl401216y9. H. Clevenson, M. E. Trusheim, C. Teale, T. Schröder, D. Braje, and D. Englund, Nat. Phys. 11, 393 (2015). https://doi.org/10.1038/nphys3291 has stimulated intense research.
Direct laser writing (DLW) with ultrashort pulses is a versatile, emerging platform for diamond functionalisation that could benefit all of these applications. Sub-micrometre scale features can be processed over large volumes in manageable timescales, and there are new prospects for three dimensional device architectures fabricated directly within the bulk material. Aside from surface modification,10–1210. M. Shinoda, R. R. Gattass, and E. Mazur, J. Appl. Phys. 105, 053102 (2009). https://doi.org/10.1063/1.307951211. A. Lehmann, C. Bradac, and R. P. Mildren, Nat. Commun. 5, 3341 (2014). https://doi.org/10.1038/ncomms434112. M. S. Komlenok, V. V. Kononenko, V. G. Ralchenko, S. M. Pimenov, and V. I. Konov, Phys. Procedia 12, 37 (2011). https://doi.org/10.1016/j.phpro.2011.03.103 there are two key regimes for sub-surface processing: (i) at very low pulse energy, the highly non-linear interaction generates an ensemble of vacancies at the laser focus1313. Y. C. Chen, P. S. Salter, S. Knauer, L. Weng, A. C. Frangeskou, C. J. Stephen, S. N. Ishmael, P. R. Dolan, S. Johnson, B. L. Green, G. W. Morley, M. E. Newton, J. G. Rarirty, M. J. Booth, and J. M. Smith, Nat. Photonics 11, 77 (2017). https://doi.org/10.1038/nphoton.2016.234 while (ii) at higher pulse energies, there is break-down of the diamond lattice leaving a conductive graphitic phase.1414. T. V. Kononenko, M. Meier, M. S. Komlenok, S. M. Pimenov, V. Romano, V. P. Pashinin, and V. I. Konov, Appl. Phys. A 90, 645 (2008). https://doi.org/10.1007/s00339-007-4350-9 Regime (i) is an important precursor for the formation of coherent NV centers for quantum applications.13,1513. Y. C. Chen, P. S. Salter, S. Knauer, L. Weng, A. C. Frangeskou, C. J. Stephen, S. N. Ishmael, P. R. Dolan, S. Johnson, B. L. Green, G. W. Morley, M. E. Newton, J. G. Rarirty, M. J. Booth, and J. M. Smith, Nat. Photonics 11, 77 (2017). https://doi.org/10.1038/nphoton.2016.23415. J. P. Hadden, V. Bharadwaj, B. Sotillo, S. Rampini, R. Osellame, T. T. Fernandez, A. Chiappini, C. Armellini, M. Ferrari, R. Ramponi, P. E. Barclay, and S. M. Eaton, preprint arXiv:1701.05885v6 (2017). Regime (ii) enables the creation of embedded electrodes, particularly for use in 3D radiation detectors offering a superior radiation tolerance and a faster response than their planar counterparts.16–1816. B. Caylar, M. Pomorski, and P. Bergonzo, Appl. Phys. Lett. 103, 043504 (2013). https://doi.org/10.1063/1.481632817. A. Oh, B. Caylar, M. Pomorski, and T. Wengler, Diamond Relat. Mater. 38, 9 (2013). https://doi.org/10.1016/j.diamond.2013.06.00318. S. Lagomarsino, M. Bellini, C. Corsi, F. Gorelli, G. Parrini, M. Santoro, and S. Sciortino, Appl. Phys. Lett. 103, 233507 (2013). https://doi.org/10.1063/1.4839555 Furthermore, Regime (ii) modifications provide a potential route for all-carbon metamaterials,1919. M. Shimizu, Y. Shimotsuma, M. Sakakura, T. Yuasa, H. Homma, Y. Minowa, K. Tanaka, K. Miura, and K. Hirao, Opt. Express 17, 46 (2009). https://doi.org/10.1364/OE.17.000046 solar cells,2020. M. Girolami, L. Criante, F. Di Fonzo, S. Lo Turco, A. Mezzetti, A. Notargiacomo, M. Pea, A. Bellucci, P. Calvani, V. Valentini, and D. M. Trucchi, Carbon 111, 48 (2017). https://doi.org/10.1016/j.carbon.2016.09.061 photonic crystals,2121. T. V. Kononenko, P. N. Dyachenko, and V. I. Konov, Opt. Lett. 39, 6962 (2014). https://doi.org/10.1364/OL.39.006962 and waveguides.22,2322. A. Courvoisier, M. J. Booth, and P. S. Salter, Appl. Phys. Lett. 109, 031109 (2016). https://doi.org/10.1063/1.495926723. B. Sotillo, V. Bharadwaj, J. P. Hadden, M. Sakakura, A. Chiappini, T. T. Fernandez, S. Longhi, O. Jedrkiewicz, Y. Shimotsuma, L. Criante, R. Osellame, G. Galzerano, M. Ferrari, K. Miura, R. Ramponi, P. E. Barclay, and S. M. Eaton, Sci. Rep. 6, 35566 (2016). https://doi.org/10.1038/srep35566 For successful development of DLW in these applications and other diamond based technologies, it is of vital importance to carry out detailed structural analysis of the subsurface laser induced changes to the diamond.
When imaged in an optical microscope, the structural modifications in Regime (ii) appear uniform and strongly absorbing, suggesting a bulk conversion of diamond into graphite. However, previous Raman studies revealed only the partial formation of sp2 bonded graphite within the laser irradiated zones.14,19,23,2414. T. V. Kononenko, M. Meier, M. S. Komlenok, S. M. Pimenov, V. Romano, V. P. Pashinin, and V. I. Konov, Appl. Phys. A 90, 645 (2008). https://doi.org/10.1007/s00339-007-4350-919. M. Shimizu, Y. Shimotsuma, M. Sakakura, T. Yuasa, H. Homma, Y. Minowa, K. Tanaka, K. Miura, and K. Hirao, Opt. Express 17, 46 (2009). https://doi.org/10.1364/OE.17.00004623. B. Sotillo, V. Bharadwaj, J. P. Hadden, M. Sakakura, A. Chiappini, T. T. Fernandez, S. Longhi, O. Jedrkiewicz, Y. Shimotsuma, L. Criante, R. Osellame, G. Galzerano, M. Ferrari, K. Miura, R. Ramponi, P. E. Barclay, and S. M. Eaton, Sci. Rep. 6, 35566 (2016). https://doi.org/10.1038/srep3556624. P. Salter and M. J. Booth, Proc. SPIE 8974, 89740T (2014). https://doi.org/10.1117/12.2040384 When using such modifications to create wires, the resultant resistivity varies from 0.02 to 2 Ω cm,18–20,25,2618. S. Lagomarsino, M. Bellini, C. Corsi, F. Gorelli, G. Parrini, M. Santoro, and S. Sciortino, Appl. Phys. Lett. 103, 233507 (2013). https://doi.org/10.1063/1.483955519. M. Shimizu, Y. Shimotsuma, M. Sakakura, T. Yuasa, H. Homma, Y. Minowa, K. Tanaka, K. Miura, and K. Hirao, Opt. Express 17, 46 (2009). https://doi.org/10.1364/OE.17.00004620. M. Girolami, L. Criante, F. Di Fonzo, S. Lo Turco, A. Mezzetti, A. Notargiacomo, M. Pea, A. Bellucci, P. Calvani, V. Valentini, and D. M. Trucchi, Carbon 111, 48 (2017). https://doi.org/10.1016/j.carbon.2016.09.06125. B. Sun, P. S. Salter, and M. J. Booth, Appl. Phys. Lett 105, 231105 (2014). https://doi.org/10.1063/1.490299826. S. Lagomarsino, M. Bellini, C. Corsi, S. Fanetti, F. Gorelli, I. Liontos, G. Parrini, M. Santoro, and S. Sciortino, Diamond Relat. Mater. 43, 23 (2014). https://doi.org/10.1016/j.diamond.2014.01.002 with values at least an order of magnitude higher than that for polycrystalline graphite.2727. J. D. Cutnell and K. W. Johnson, Physics ( Wiley, New York, 2004). Similarly, when writing optical waveguides using the stress field generated by modifications in Regime (ii), the propagation losses are significantly lower than those expected from a complete conversion of the irradiated zones to graphite.2323. B. Sotillo, V. Bharadwaj, J. P. Hadden, M. Sakakura, A. Chiappini, T. T. Fernandez, S. Longhi, O. Jedrkiewicz, Y. Shimotsuma, L. Criante, R. Osellame, G. Galzerano, M. Ferrari, K. Miura, R. Ramponi, P. E. Barclay, and S. M. Eaton, Sci. Rep. 6, 35566 (2016). https://doi.org/10.1038/srep35566 These observations indicate that the wires' internal structure is more complex than suggested by optical imaging. As illustration, a recent scanning electron microscopy (SEM) study of subsurface DLW structures exposed by mechanical polishing revealed a main fracture containing graphenic carbon and smaller conductive nanocracks propagating from it.2828. K. K. Ashikkalieva, T. V. T. V. Kononenko, E. A. Obraztsova, E. V. Zavedeev, A. A. Khomich, E. E. Ashkinazi, and V. I. Konov, Carbon 102, 383 (2016). https://doi.org/10.1016/j.carbon.2016.02.044 Here, we use transmission electron microscopy (TEM) to provide depth resolution and better spatial resolution of these sub-surface structures and employ electron energy loss spectroscopy (EELS) to assess composition. The results provide a vital perspective on the nature of DLW subsurface modifications in diamond, advancing our understanding of both processing regimes.
The diamond was a single crystal CVD sample (Element 6 optical grade, 3 mm square) cut with {100} edges. A regeneratively amplified titanium sapphire laser producing a 1 kHz train of 250 fs, 25 nJ pulses at a wavelength of 790 nm was focused 4 μm beneath the surface of the diamond using an oil immersion objective lens with a numerical aperture of 1.4. A liquid crystal spatial light modulator was used to correct for focal distortion arising from refraction at the diamond surface, but we note that the aberration is relatively small at such a shallow depth. The diamond was translated through the laser focus at 10 μm/s to produce subsurface tracks parallel to the diamond's top surface, as illustrated in Fig. 1(a); further details can be found elsewhere.2525. B. Sun, P. S. Salter, and M. J. Booth, Appl. Phys. Lett 105, 231105 (2014). https://doi.org/10.1063/1.4902998 Wires were thus fabricated, and the sample was annealed at 900 °C in a nitrogen atmosphere for one hour.
A subsurface group of four wires can be seen in the optical transmission microscopy image in Fig. 1(b), along with laser-written surface registration marks. The wires appear optically to have a width around 2 μm, with edge roughness on a 100 nm length-scale. The resistivity was measured for the wires as 1.6 Ω cm, which is toward the top of the range reported in the literature. The relatively high resistivity can be understood by noting the difficulties in writing buried graphitic wires both parallel and close to the top diamond surface. The pulse energy has to be low to avoid any surface damage and a high degree of axial confinement is necessary for the subsequent processing, precluding the use of an axial multi-scan fabrication technique to improve the wire conductivity.20,2520. M. Girolami, L. Criante, F. Di Fonzo, S. Lo Turco, A. Mezzetti, A. Notargiacomo, M. Pea, A. Bellucci, P. Calvani, V. Valentini, and D. M. Trucchi, Carbon 111, 48 (2017). https://doi.org/10.1016/j.carbon.2016.09.06125. B. Sun, P. S. Salter, and M. J. Booth, Appl. Phys. Lett 105, 231105 (2014). https://doi.org/10.1063/1.4902998
Cross-sections of wires were extracted by using standard “lift-out” protocols (e.g., see Ref. 2929. D. Tomus and H. Ng, Micron 44, 115 (2013). https://doi.org/10.1016/j.micron.2012.05.006) on an FEI Nova DualBeam Focused Ion Beam (FIB) microscope. A protective Pt cap was deposited prior to ion milling, and a typically (10 μm)2 cross-section through the wire was removed and ion-polished to <150 nm thickness (i.e., thicker than usually desired for high resolution TEM because the final low-energy FIB polishing step was less effective than for softer materials). TEM and scanning TEM (STEM) were performed on a JEOL ARM cFEG instrument operating at 200 kV, using a Gatan Quantum spectrometer for electron energy loss spectroscopy (EELS). The EELS data were acquired using the “spectrum imaging” methodology3030. J. Hunt and D. Williams, Ultramicroscopy 38, 47 (1991). https://doi.org/10.1016/0304-3991(91)90108-I and processed using standard routines for background subtraction and deconvolution within Gatan's Digital Micrograph software.
The extent of sub-surface modifications is revealed by TEM in Fig. 2, which plots both (a) TEM and (b) bright-field STEM images of an annealed wire cross-section. Contrast in the TEM image is dominated by interference effects due to variations in the thickness and angle of the sample, producing radial black and white lines from strain within the modified region. Fine details within the annotated ellipse are attributed to DLW structural modifications. Interference effects are largely absent from the STEM image, which picks out more clearly disruptions of the diamond lattice as dark features. Visible damage is constrained within a region that is 1.9 μm wide, lying between 0.5 μm and 5.0 μm beneath the diamond surface. Although the elongated shape of the damage region matches that of the laser intensity profile, it is interesting that its location is not clearly centred on the laser focus, which is illustrated to scale in Fig. 2(c) and centred at a depth of 4 μm.
The most striking observation of Fig. 2(b) is that there is no single homogenous region of modified diamond, rather the wires comprise a disperse cluster of smaller structures. Their irregularity is consistent with the slightly ragged wire edges observed optically. The majority of them are individually less than a few hundred nanometres in size, so below the optical diffraction limit, but sufficiently closely spaced that the wire appears as a single continuous feature when imaged optically. We attribute the optical contrast [see Fig. 1(b)] to a combination of absorption, scattering from defects, and refractive index changes arising from strain fields that propagate out from the defects. This demonstrates that standard optical methods lack the resolution to accurately characterize DLW wires in diamond.
The bonding configuration and chemistry of the defects are revealed by EELS, which is summarised for an annealed wire in Fig. 3. A 3 × 0.7 μm2 region straddling the structural features was used for spectrum imaging, where both low-loss and core-loss EELS spectra were acquired on a pixel-by-pixel manner. Note that Fig. 3(a) is a dark field image and so has reversed contrast compared to Fig. 2(b). Figure 3(b), on the other hand, is a high angle annular dark field (HAADF) image with contrast that derives principally from mass or thickness variations, with denser regions appearing brightest. Together, the two images reveal the structural modifications as a combination of (i) discrete oval patches that appear dark in HAADF and so are less dense than the surrounding matrix and (ii) sharp diagonal lines lying at 45° to the [100] direction.
Typical EELS low-loss and carbon K-edge spectra are shown in Figs. 3(c) and 3(d), respectively, and were acquired from the areas indicated in Fig. 3(a). Area 1 is located within one of the oval patches; area 2 lies within a nearby bright feature; and area 3 represents the unmodified diamond. Both the core-loss and the low-loss spectra reveal area 1 to differ chemically and electronically from the surrounding material.
The diamond low-loss spectrum (area 3) is dominated by an asymmetric peak with a maximum at 34.4 eV that agrees with previous studies of the diamond bulk plasmon (≈35 eV) with an asymmetry caused by a surface excitation around 25 eV.31–3331. L. Garvie, P. Buseck, and A. Craven, Can. Mineral. 33, 1157 (1995), available at http://www.canmin.org/content/33/6/1157.full.pdf+html.32. K. Kushita, K. Hojou, S. Furuno, and H. Otsu, J. Nucl. Mater. 191, 346 (1992). https://doi.org/10.1016/S0022-3115(09)80063-933. U. Bangert, R. Barnes, L. Hounsome, R. Jones, A. Blumenau, P. Briddon, M. Shaw, and S. Oberg, Philos. Mag. 86, 4757 (2006). https://doi.org/10.1080/14786430600776348 Area 2's spectrum is similar while the plasmon in area 1 shifts downwards in energy, an effect that has previously been attributed to damage3232. K. Kushita, K. Hojou, S. Furuno, and H. Otsu, J. Nucl. Mater. 191, 346 (1992). https://doi.org/10.1016/S0022-3115(09)80063-9 and reflects a reduction in valence electron density.3434. H. Daniels, R. Brydson, B. Rand, and A. Brown, Philos. Mag. 87, 4073 (2007). https://doi.org/10.1080/14786430701394041 Amorphous carbon, carbon onions, fullerenes, and graphite are all known to have bulk plasmons at lower energies (around 24, 24.5, 26, and 27 eV, respectively3535. Z.-L. Zhang, R. Brydson, Z. Aslam, S. Reddy, A. Brown, A. Westwood, and B. Rand, Carbon 49, 5049 (2011). https://doi.org/10.1016/j.carbon.2011.07.023) and it would be difficult to decompose the spectrum here into distinct components. The sharp peak at 7 eV, however, is more distinctive. It is at too high an energy for purely amorphous carbon3636. K. Mkhoyan, A. W. Contryman, J. Silcox, D. Stewart, G. Eda, C. Mattevi, S. Miller, and M. Chhowalla, Nano Lett. 9, 1058 (2009). https://doi.org/10.1021/nl8034256 but has previously been attributed to the π* plasmon excitation of graphite36,3736. K. Mkhoyan, A. W. Contryman, J. Silcox, D. Stewart, G. Eda, C. Mattevi, S. Miller, and M. Chhowalla, Nano Lett. 9, 1058 (2009). https://doi.org/10.1021/nl803425637. M. Gass, U. Bangert, A. Bleloch, P. Wang, R. Nair, and A. Geim, Nat. Nanotechnol. 3, 676 (2008). https://doi.org/10.1038/nnano.2008.280 and has also been observed from “brown diamond,” which is known to contain intrinsic defects of sp2 character.3333. U. Bangert, R. Barnes, L. Hounsome, R. Jones, A. Blumenau, P. Briddon, M. Shaw, and S. Oberg, Philos. Mag. 86, 4757 (2006). https://doi.org/10.1080/14786430600776348
Turning to the spectra in Fig. 3(d), previous K-edge studies have shown clear spectral differences between carbonaceous materials.3131. L. Garvie, P. Buseck, and A. Craven, Can. Mineral. 33, 1157 (1995), available at http://www.canmin.org/content/33/6/1157.full.pdf+html. Pure diamond typically shows a sharp exciton around 289 eV, followed by multiple overlapping σ* excitations at higher energies,3838. F. Coffman, R. Cao, P. Pianetta, S. Kappor, M. Kelly, and L. Terminello, Appl. Phys. Lett. 69, 568 (1996). https://doi.org/10.1063/1.117789 as observed from areas 2 and 3. The pronounced dip in intensity 302.5 eV is caused by a bandgap in the unoccupied density of states (and is therefore a good indication of insulating or semiconducting character)3838. F. Coffman, R. Cao, P. Pianetta, S. Kappor, M. Kelly, and L. Terminello, Appl. Phys. Lett. 69, 568 (1996). https://doi.org/10.1063/1.117789 whilst a subsequent peak at 327 eV is caused by multiple scattering events and is a good indication of crystallographic order.3939. J. Diaz, O. Monteiro, and Z. Hussain, Phys. Rev. B. 76, 094201 (2007). https://doi.org/10.1103/PhysRevB.76.094201 Similar to previous studies,4040. S. Porro, G. De Temmerman, D. A. MacLaren, S. Lisgo, D. L. Rudakov, J. Westerhout, M. Wiora, P. John, I. Villalpando, and J. I. B. Wilson, Diamond Relat. Mater. 19, 818 (2010). https://doi.org/10.1016/j.diamond.2010.01.051 area 1 exhibits an additional weak “pre-edge” peak at 285.5 eV that is attributed to excitation to a π* state and is therefore indicative of the formation of unsaturated, sp2 bonded carbon. The spectrum from area 1, however, retains the other distinctive diamond features (notably the dip and peak at 302.5 eV and 327 eV, respectively) and lacks either the broad, featureless profile above 289 eV that is typically seen in amorphous or defective carbon, or the sharp exciton and σ* features that are shifted above 291 eV for graphite.3131. L. Garvie, P. Buseck, and A. Craven, Can. Mineral. 33, 1157 (1995), available at http://www.canmin.org/content/33/6/1157.full.pdf+html. Both low-loss and K-edge spectra from area 1 are therefore consistent with the coexistence of sp3-bonded, diamond-like material and <20% of sp2 bonded material. The weakened band-gap dip and the emergence of peaks at both 7 eV and 285.5 eV are all consistent with the existence of electrically conductive carbon phases and therefore underpin the formation of conductive wires by DLW. Area 2, which lies 200 nm away from a conductive region, shows no evidence of sp2 carbon, illustrating the extremely compact nature of the conductive regions.
The spatial distribution of the mixed bonding regions is shown in Fig. 3(e), which maps the strength of the “pre-edge” π* feature and shows a clear correlation with the dark oval features observed by HAADF. Together, the data indicate that conversion of diamond to graphitic carbon occurs in a series of discrete oval patches that lie along the surface normal direction and are 200 nm × 80 nm in size. The diagonal features from Fig. 2(b) (oriented at ≈45° to the surface normal) are barely visible in Fig. 3(e) but do still produce a weak pre-edge component, suggesting that they too may be electrically conductive. These features lie preferentially along the 110 directions and are therefore likely contained in {111} cleavage planes. Each set is accompanied by an sp2 patch, including those closest to the diamond surface and furthest from the laser focus. This strongly suggests that these diagonal features are strain-relieving dislocations that form as a consequence of the volume increase on transformation of the diamond. Also of note is an apparent periodicity in the formation of the sp2 clusters, such as the sequence of oval patches in the upper middle of Fig. 3(e). All wires analyzed displayed a similar ordering with an approximate periodicity of 110 nm along the direction of propagation for the laser, suggesting that their formation is not random. Indeed, it is similar to the nanograting structures formed by DLW inside glass with a periodicity dependent on the pulse energy, which are accredited to a coupling between the electric field of the fabrication laser and excited electron plasma at the focus.4141. Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, Phys. Rev. Lett. 91, 247405 (2003). https://doi.org/10.1103/PhysRevLett.91.247405
Using the EELS analysis presented in Fig. 3 (e), we estimate that only ≈4% of the wire cross-sectional area contains conductive sp2 bonded carbon. Indeed, even if it is assumed that all the regions identified as sp2 are continuous along the wire and can hence contribute to any DC conductivity, values for the intrinsic structural resistivity are expected to be over an order of magnitude lower than those previously reported for laser written wires.18–20,25,2618. S. Lagomarsino, M. Bellini, C. Corsi, F. Gorelli, G. Parrini, M. Santoro, and S. Sciortino, Appl. Phys. Lett. 103, 233507 (2013). https://doi.org/10.1063/1.483955519. M. Shimizu, Y. Shimotsuma, M. Sakakura, T. Yuasa, H. Homma, Y. Minowa, K. Tanaka, K. Miura, and K. Hirao, Opt. Express 17, 46 (2009). https://doi.org/10.1364/OE.17.00004620. M. Girolami, L. Criante, F. Di Fonzo, S. Lo Turco, A. Mezzetti, A. Notargiacomo, M. Pea, A. Bellucci, P. Calvani, V. Valentini, and D. M. Trucchi, Carbon 111, 48 (2017). https://doi.org/10.1016/j.carbon.2016.09.06125. B. Sun, P. S. Salter, and M. J. Booth, Appl. Phys. Lett 105, 231105 (2014). https://doi.org/10.1063/1.490299826. S. Lagomarsino, M. Bellini, C. Corsi, S. Fanetti, F. Gorelli, I. Liontos, G. Parrini, M. Santoro, and S. Sciortino, Diamond Relat. Mater. 43, 23 (2014). https://doi.org/10.1016/j.diamond.2014.01.002 The relatively low sp2 content of even the patches in Fig. 3(e) is surprising, and at no point, purely graphenic EELS spectra are recorded.
The work here clarifies a recent SEM analysis of subsurface DLW modifications in diamond,2828. K. K. Ashikkalieva, T. V. T. V. Kononenko, E. A. Obraztsova, E. V. Zavedeev, A. A. Khomich, E. E. Ashkinazi, and V. I. Konov, Carbon 102, 383 (2016). https://doi.org/10.1016/j.carbon.2016.02.044 which also found only a small volume fraction of sp2 bonded carbon in the laser irradiated zones. Here, the multiplicity of conductive regions and their spatial locations are revealed in detail. TEM images indicate the formation of discrete, 100 nm elliptical nanowires that are partially converted into sp2 carbon. Stress-relieving dislocations radiate out from each region along {111} planes and also contain trace quantities of sp2 carbon. It is anticipated that both these features contribute to electrical conductivity.
In conclusion, the internal structure is revealed for laser written graphitic wires buried inside the bulk of diamond. While viewed in an optical microscope, the wires appear to show a bulk change from diamond to graphite, but the higher resolution of TEM shows that the structural modification is relatively sparse, comprising micro-cracks and nano-clusters of sp2 bonded carbon. This disparity is particularly important when using the measured wire resistivity as a proxy measurement of wire composition. Indeed, the small effective wire cross-sections discovered here indicate the presence of a remarkably conductive carbonaceous phase.
The authors gratefully acknowledge the Leverhulme Trust (RPG-2013-044) and the UK Engineering and Physical Sciences Research Council (EP/K034480/1, EP/I00419X/1) for financial support. We would additionally like to thank Philip Martineau and David Fisher for enlightening discussions.
  1. 1. W. Adam, C. Bauer, E. Berdermann, P. Bergonzo, F. Bogani, E. Borchi, A. Brambilla, M. Bruzzi, C. Colledani, J. Conway et al., Nucl. Instrum. Methods Phys. Res., Sec. A 434, 131 (1999). https://doi.org/10.1016/S0168-9002(99)00447-7, Google ScholarCrossref
  2. 2. S. A. O. Russell, S. Sharabi, A. Tallaire, and D. A. J. Moran, IEEE Electron Device Lett. 33, 1471 (2012). https://doi.org/10.1109/LED.2012.2210020, Google ScholarCrossref
  3. 3. F. Picollo, S. Gosso, E. Vittone, A. Pasquarelli, E. Carbone, P. Olivero, and V. Carabelli, Adv. Mater. 25, 4696 (2013). https://doi.org/10.1002/adma.201300710, Google ScholarCrossref
  4. 4. I. Aharonovich, A. D. Greentree, and S. Prawer, Nat. Photonics 5, 397 (2011). https://doi.org/10.1038/nphoton.2011.54, Google ScholarCrossref
  5. 5. H. Bernien, B. Hensen, W. Pfaff, G. Koolstra, M. Blok, L. Robledo, T. Taminiau, M. Markham, D. Twitchen, L. Childress, and R. Hanson, Nature 497, 86 (2013). https://doi.org/10.1038/nature12016, Google ScholarCrossref, ISI
  6. 6. G. Balasubramanian, I. Chan, R. Kolesov, M. Al-Hmoud, J. Tisler, C. Shin, C. Kim, A. Wojcik, P. R. Hemmer, A. Krueger, T. Hanke, A. Leitenstorfer, R. Bratschitsch, F. Jelezko, and J. Wrachtrup, Nature 455, 648 (2008). https://doi.org/10.1038/nature07278, Google ScholarCrossref, ISI
  7. 7. J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, P. Cappellaro, L. Jiang, M. V. G. Dutt, E. Togan, A. S. Zibrov, A. Yacoby, R. L. Walsworth, and M. D. Lukin, Nature 455, 644 (2008). https://doi.org/10.1038/nature07279, Google ScholarCrossref, ISI
  8. 8. P. Neumann, I. Jakobi, F. Dolde, C. Burk, R. Reuter, G. Waldherr, J. Honert, T. Wolf, A. Brunner, J. H. Shim, D. Suter, H. Sumiya, J. Isoya, and J. Wrachtrup, Nano Lett. 13, 2378 (2013). https://doi.org/10.1021/nl401216y, Google ScholarCrossref
  9. 9. H. Clevenson, M. E. Trusheim, C. Teale, T. Schröder, D. Braje, and D. Englund, Nat. Phys. 11, 393 (2015). https://doi.org/10.1038/nphys3291, Google ScholarCrossref, ISI
  10. 10. M. Shinoda, R. R. Gattass, and E. Mazur, J. Appl. Phys. 105, 053102 (2009). https://doi.org/10.1063/1.3079512, Google ScholarScitation, ISI
  11. 11. A. Lehmann, C. Bradac, and R. P. Mildren, Nat. Commun. 5, 3341 (2014). https://doi.org/10.1038/ncomms4341, Google ScholarCrossref
  12. 12. M. S. Komlenok, V. V. Kononenko, V. G. Ralchenko, S. M. Pimenov, and V. I. Konov, Phys. Procedia 12, 37 (2011). https://doi.org/10.1016/j.phpro.2011.03.103, Google ScholarCrossref
  13. 13. Y. C. Chen, P. S. Salter, S. Knauer, L. Weng, A. C. Frangeskou, C. J. Stephen, S. N. Ishmael, P. R. Dolan, S. Johnson, B. L. Green, G. W. Morley, M. E. Newton, J. G. Rarirty, M. J. Booth, and J. M. Smith, Nat. Photonics 11, 77 (2017). https://doi.org/10.1038/nphoton.2016.234, Google ScholarCrossref, ISI
  14. 14. T. V. Kononenko, M. Meier, M. S. Komlenok, S. M. Pimenov, V. Romano, V. P. Pashinin, and V. I. Konov, Appl. Phys. A 90, 645 (2008). https://doi.org/10.1007/s00339-007-4350-9, Google ScholarCrossref
  15. 15. J. P. Hadden, V. Bharadwaj, B. Sotillo, S. Rampini, R. Osellame, T. T. Fernandez, A. Chiappini, C. Armellini, M. Ferrari, R. Ramponi, P. E. Barclay, and S. M. Eaton, preprint arXiv:1701.05885v6 (2017). Google Scholar
  16. 16. B. Caylar, M. Pomorski, and P. Bergonzo, Appl. Phys. Lett. 103, 043504 (2013). https://doi.org/10.1063/1.4816328, Google ScholarScitation
  17. 17. A. Oh, B. Caylar, M. Pomorski, and T. Wengler, Diamond Relat. Mater. 38, 9 (2013). https://doi.org/10.1016/j.diamond.2013.06.003, Google ScholarCrossref
  18. 18. S. Lagomarsino, M. Bellini, C. Corsi, F. Gorelli, G. Parrini, M. Santoro, and S. Sciortino, Appl. Phys. Lett. 103, 233507 (2013). https://doi.org/10.1063/1.4839555, Google ScholarScitation, ISI
  19. 19. M. Shimizu, Y. Shimotsuma, M. Sakakura, T. Yuasa, H. Homma, Y. Minowa, K. Tanaka, K. Miura, and K. Hirao, Opt. Express 17, 46 (2009). https://doi.org/10.1364/OE.17.000046, Google ScholarCrossref
  20. 20. M. Girolami, L. Criante, F. Di Fonzo, S. Lo Turco, A. Mezzetti, A. Notargiacomo, M. Pea, A. Bellucci, P. Calvani, V. Valentini, and D. M. Trucchi, Carbon 111, 48 (2017). https://doi.org/10.1016/j.carbon.2016.09.061, Google ScholarCrossref
  21. 21. T. V. Kononenko, P. N. Dyachenko, and V. I. Konov, Opt. Lett. 39, 6962 (2014). https://doi.org/10.1364/OL.39.006962, Google ScholarCrossref
  22. 22. A. Courvoisier, M. J. Booth, and P. S. Salter, Appl. Phys. Lett. 109, 031109 (2016). https://doi.org/10.1063/1.4959267, Google ScholarScitation, ISI
  23. 23. B. Sotillo, V. Bharadwaj, J. P. Hadden, M. Sakakura, A. Chiappini, T. T. Fernandez, S. Longhi, O. Jedrkiewicz, Y. Shimotsuma, L. Criante, R. Osellame, G. Galzerano, M. Ferrari, K. Miura, R. Ramponi, P. E. Barclay, and S. M. Eaton, Sci. Rep. 6, 35566 (2016). https://doi.org/10.1038/srep35566, Google ScholarCrossref
  24. 24. P. Salter and M. J. Booth, Proc. SPIE 8974, 89740T (2014). https://doi.org/10.1117/12.2040384, Google ScholarCrossref
  25. 25. B. Sun, P. S. Salter, and M. J. Booth, Appl. Phys. Lett 105, 231105 (2014). https://doi.org/10.1063/1.4902998, Google ScholarScitation, ISI
  26. 26. S. Lagomarsino, M. Bellini, C. Corsi, S. Fanetti, F. Gorelli, I. Liontos, G. Parrini, M. Santoro, and S. Sciortino, Diamond Relat. Mater. 43, 23 (2014). https://doi.org/10.1016/j.diamond.2014.01.002, Google ScholarCrossref
  27. 27. J. D. Cutnell and K. W. Johnson, Physics ( Wiley, New York, 2004). Google Scholar
  28. 28. K. K. Ashikkalieva, T. V. T. V. Kononenko, E. A. Obraztsova, E. V. Zavedeev, A. A. Khomich, E. E. Ashkinazi, and V. I. Konov, Carbon 102, 383 (2016). https://doi.org/10.1016/j.carbon.2016.02.044, Google ScholarCrossref
  29. 29. D. Tomus and H. Ng, Micron 44, 115 (2013). https://doi.org/10.1016/j.micron.2012.05.006, Google ScholarCrossref
  30. 30. J. Hunt and D. Williams, Ultramicroscopy 38, 47 (1991). https://doi.org/10.1016/0304-3991(91)90108-I, Google ScholarCrossref
  31. 31. L. Garvie, P. Buseck, and A. Craven, Can. Mineral. 33, 1157 (1995), available at http://www.canmin.org/content/33/6/1157.full.pdf+html. Google Scholar
  32. 32. K. Kushita, K. Hojou, S. Furuno, and H. Otsu, J. Nucl. Mater. 191, 346 (1992). https://doi.org/10.1016/S0022-3115(09)80063-9, Google ScholarCrossref
  33. 33. U. Bangert, R. Barnes, L. Hounsome, R. Jones, A. Blumenau, P. Briddon, M. Shaw, and S. Oberg, Philos. Mag. 86, 4757 (2006). https://doi.org/10.1080/14786430600776348, Google ScholarCrossref
  34. 34. H. Daniels, R. Brydson, B. Rand, and A. Brown, Philos. Mag. 87, 4073 (2007). https://doi.org/10.1080/14786430701394041, Google ScholarCrossref
  35. 35. Z.-L. Zhang, R. Brydson, Z. Aslam, S. Reddy, A. Brown, A. Westwood, and B. Rand, Carbon 49, 5049 (2011). https://doi.org/10.1016/j.carbon.2011.07.023, Google ScholarCrossref
  36. 36. K. Mkhoyan, A. W. Contryman, J. Silcox, D. Stewart, G. Eda, C. Mattevi, S. Miller, and M. Chhowalla, Nano Lett. 9, 1058 (2009). https://doi.org/10.1021/nl8034256, Google ScholarCrossref
  37. 37. M. Gass, U. Bangert, A. Bleloch, P. Wang, R. Nair, and A. Geim, Nat. Nanotechnol. 3, 676 (2008). https://doi.org/10.1038/nnano.2008.280, Google ScholarCrossref, ISI
  38. 38. F. Coffman, R. Cao, P. Pianetta, S. Kappor, M. Kelly, and L. Terminello, Appl. Phys. Lett. 69, 568 (1996). https://doi.org/10.1063/1.117789, Google ScholarScitation
  39. 39. J. Diaz, O. Monteiro, and Z. Hussain, Phys. Rev. B. 76, 094201 (2007). https://doi.org/10.1103/PhysRevB.76.094201, Google ScholarCrossref
  40. 40. S. Porro, G. De Temmerman, D. A. MacLaren, S. Lisgo, D. L. Rudakov, J. Westerhout, M. Wiora, P. John, I. Villalpando, and J. I. B. Wilson, Diamond Relat. Mater. 19, 818 (2010). https://doi.org/10.1016/j.diamond.2010.01.051, Google ScholarCrossref
  41. 41. Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, Phys. Rev. Lett. 91, 247405 (2003). https://doi.org/10.1103/PhysRevLett.91.247405, Google ScholarCrossref, ISI
  1. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).