Optically-Induced Structural Change in Graphite

1. YOSHIDA Lab. Naoki HOSOYA [1]Ramani K. Raman, YoshieMurooka, Chong-Yu Ruan, Teng Yang, Savas Berber, and David Tománek, Phys. Rev. Lett. 101 077401 (2008). Optically-Induced Structural Change in Graphite

2. Contents • Introduction ・Structural Change ・Graphite-Diamond Transition ・Previous Research of Optical Irradiation to Graphite ・This Letter • Main Issue ・Ultrafast Electron Crystallography (UEC) ・Equilibrium ・Near-Equilibrium ・Far-from-Equilibrium ・Further Elucidation ・Calculations • Summary

3. Structural Change (SC) • Temperature-induced SC • Pressure-induced SC • Electric / Magnetic field-induced SC • Optically-induced SC Ultrafast and efficient ・Development of materials for optical memory ・Material Design without changing chemical composition ・Building new concept for material science

4. Graphite-Diamond Transition • Graphite-Diamond transition by temperature or/and pressure. • This can be induced by optical irradiation to Graphite. [2] T. Meguro et al., Appl Phys. Lett. 79, 3866 (2001) [3] H. Nakayama and H. Katayama-Yoshida, J. Phys. Condens. Matter 15, R1077 (2003) Diamond Graphite

5. Previous Researches ofOptical Irradiation to Graphite • Photo-induced melting [2] S. Ashitkovet al., JETP Lett. 75, 87 (2002). [3] D.H. Reitze, H. Ahn, and M.C. Downer, Phys. Rev. B 45, 2677 (1992). • Generation of coherent phonon [4] T. Mishima, K. Nitta, and Y. Masumoto, Phys. Rev. B 62, 2908 (2000). [5] K. Ishioka, M. Hase, M. Kitajima, and K. Ushida, Appl. Phys. Lett. 78, 3965 (2001). • Auger decay process [6] H. Nakayama and H. Katayama-Yoshida, J. Phys. Condens. Matter 15, R1077 (2003) By observing changes in the electronic properties (Indirect observation of atomic motion)

6. This Letter First direct determination of by for Optically induced structural change in graphite Ultrafast electron crystallography (UEC) and ab initio DFT calculation Graphite-diamond transition

7. Ultrafast Electron Crystallography (UEC) Sample : Highly oriented pyrolytic graphite (HOPG) Pump : A mode-locked Ti-Sapphire laser pulse Probe : A photo generated electron beam Feature of using electron beam • Short wavelength (λe = 0.069 Å) • Large scattering cross section • Femtosecond temporal resolution Direct observation of atomic motion of carbon The layered structurte of graphite

8. Equilibrium regime (not excited) Left : Diffraction pattern of graphite Right : Layer density distribution function (LDF), obtained via Fourier transform of the diffraction pattern. • The peaks are good agreement with the structure of bulk graphite • The decay of LDF peaks suggest a probing depth of ≈ 1 nm.

9. Near-equilibrium (weakly-excited) Dropping the intensity of all 3 maxima Out-of-plane displacement of the atoms. 8ps Recent Report : ・Generation of coherent phonons with E2g symmetry.[4][5] ・Phonon relaxation times is 7 ps.[7] [7] T. Kampfrathet al., Phys. Rev. Lett. 95, 187403 (2005) Direct measure of the phonon-phonon interaction.

10. Far-from-equilibrium (strongly-excited) • Lattice vibration : • linearly increasing (Near-equilibrium) • saturation (Far-from-equilibrium) Why saturate ? − Metastable structure. Expansion of interlayer distance (6% at F = 40 mJ/cm2) Why expansion ? • − Effect of rise surface potential.

11. Effect of surface potential Maximum of surface potential Vs ≈ 12 V Contraction of interlayer distance ≈ 6%. The potential rise Vs yields an internal field of E ≈ 1.2 V/Å (probe depth ≈ 1nm), which causes Coulomb stress.

12. Further elucidation of the structural change The time evolution of the LDF curves at F = 77 mJ/cm2. New peak at R ≈ 1.9 Å appears. Diamond peak R ≈ 1.99 Å The transient sp3-like structure emerged.

13. Calculation technique ab initio DFT calculationin the LDA • Slab model in hexagonal graphite . • The ABINIT code. • 64 Ry energy cutoff. • Troullier-Martins pseudopotential (norm conseriving). • Ceperley-Alder form of the exchange-correlation functional. • Brillouin zone of the 4 atoms bulk unit cell. • 24×24×12k-point. Density of states (solid line) • Fermi-Dirac distribution at 0 K (dashed line) • at kBT = 1.0 eV (dotted line) Total charge density ρ(r) at 0 K

14. Effect of temperature Calculation of the effect of temperature instead of electron excitation kBTe = 1.0 eV(≈ 10000K) Δρ (r) = ρ(r; kBTe) − ρ(r; 0) Increase of the population of C2pzorbitals. → Increase of layer attraction Decrease of the population of in-layer bonding-states. → Expansion of in-layer Global structure optimization calculation result Contraction of interlayer distance by 1.5%

15. Effect of Coulomb stress Charge separation by the laser pulse induces the Coulomb stress. To take the effect into account, the charge distribution is created followingbelow scheme. Contraction of interlayer distance by 2-3 % Combined with the result of previous page (1.5% contraction), the contraction of the interlayer distance by ≈ 5% can be explained. • ＋ • 1.2 V/Å • －

16. Summary • The first direct determination of structural changes induced in graphiteby a femtosecond laser pulse using UEC. • Graphite is driven into a transient state with sp3-like character. • The main forces of this structural change are the modified force field in the excited state andthe Coulomb stress. Issue : More precise theoretical analysis (e.g. Molecular Dynamics using time-dependent DFT)