Optically-Induced Structural Change

in Graphite

YOSHIDA Lab.

Naoki HOSOYA

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[1]Ramani K. Raman, Yoshie Murooka, Chong-Yu Ruan,

Teng Yang, Savas Berber, and David Tománek,

Phys. Rev. Lett. 101 077401 (2008).

Contents

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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

Structural Change (SC)

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Temperature-induced SC Pressure-induced SC Electric / Magnetic field-induced SC

Optically-induced SCUltrafast and efficient

・ Development of materials for optical memory

・Material Design without changing chemical composition

・ Building new concept for material science

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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)

Graphite Diamond

Previous Researches ofOptical Irradiation to Graphite

Photo-induced melting[2] S. Ashitkov et 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)

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This Letter

First direct determination

of

by

for

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Optically induced structural change in graphite

Ultrafast electron crystallography (UEC) and ab initio DFT calculation

Graphite-diamond transition

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

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Ultrafast Electron Crystallography (UEC)

The layered structurte of graphite

Left : Diffraction pattern of graphite

Right : Layer density distribution function (LDF), obtained via Fourier transform of the diffraction pattern.

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Equilibrium regime (not excited)

The peaks are good agreement with the structure of bulk graphite

The decay of LDF peaks suggest a probing depth of ≈ 1 nm.

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Near-equilibrium (weakly-excited)

Dropping the intensity of all 3 maxima

[7] T. Kampfrath et al., Phys. Rev. Lett. 95, 187403 (2005)

Recent Report :・ Generation of coherent phonons with E2g symmetry.[4][5]

・ Phonon relaxation times is 7 ps.[7]

Direct measure of the phonon-phonon interaction.

Out-of-plane displacement of the atoms.

8ps

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Far-from-equilibrium (strongly-excited)

Why saturate ?

− Metastable structure.

Expansion of interlayer distance

(6% at F = 40 mJ/cm2)

Why expansion ?

− Effect of rise surface potential.

Lattice vibration :

linearly increasing (Near-equilibrium)

saturation (Far-from-equilibrium)

Maximum of surface potential Vs ≈ 12 VContraction 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.

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Effect of surface potential

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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.

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Calculation technique

ab initio DFT calculation in 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. 24×24×12 k-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

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 C2pz orbitals.

→ 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%

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Effect of temperature

Charge separation by the laser pulse induces the Coulomb stress.

To take the effect into account,

the charge distribution is created following below 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.

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Effect of Coulomb stress

1.2 V/Å

－

＋

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)

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