Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography

Abstract

By means of time-resolved electron crystallography, we report direct observation of the structural dynamics of graphite, providing new insights into the processes involving coherent lattice motions and ultrafast graphene ablation. When graphite is excited by an ultrashort laser pulse, the excited carriers reach their equilibrium in less then one picosecond by transferring heat to a subset of strongly coupled optical phonons. The time-resolved diffraction data show that on such a time scale the crystal undergoes a contraction whose velocity depends on the excitation fluence. The contraction is followed by a large expansion which, at sufficiently high fluence, leads to the ablation of entire graphene layers, as recently predicted theoretically. DOI: 10.1103/PhysRevLett.100.035501 PACS numbers: 61.82.ÿd, 07.78.+s, 63.20.Kÿ, 78.47.ÿp The unique semimetal physical properties of graphite and its chemical inertness make possible numerous applications [1][2][3] including the use in nuclear reactors [4,5]. When subjected to a shock, the structure distorts, and the ablation to form graphene [6] may result from its instability. It is of fundamental importance to visualize the associated dynamics with atomic-scale spatial and temporal resolutions. Here, by means of ultrafast electron crystallography, we report direct observation of the structural dynamics of graphite following an impulsive near infrared excitation. The diffraction shows that the structure of graphite first contracts perpendicular to the layer planes on the time scale of 0.5 to 3 ps, depending on fluence, and then nonthermally expands in 7 ps. The excitation fluence dependence of both processes indicates a distinct behavior. The highly anisotropic transfer of carrier energy to a subset of phonons of the quasi-2D-structure alters the forces between layers which control the time scale and amplitude of structural change. These atomic motions on different time scales for compression, expansion and restructuring determine the degree of the instability, which at sufficiently high fluences reaches that of graphene ablation [7,8], as predicted theoretically [9].Previous optical studies have focused on the carrier dynamics induced by both high [10,11] and low fluence [12] optical excitation, revealing features of the unique phonon excitations in quasi-two-dimensional graphite and the coherent modes involved. In order to resolve details of the structure, diffraction is our method of choice. In ultrafast electron crystallography, the probing wavelength is 0.07 Å at 30 keV electron acceleration energy, and we use femtosecond (fs) near infrared pulses (800 nm) for the optical excitation of the system of graphite. The overall resolution is determined by group velocity mismatch [13], but as shown here, by tilting the optical wave front, see the inset in Fig. 4, we are able to resolve subpicosecond coherent dynamics. Highly oriented and single crystal graphite were studied, and indeed the static diffraction patterns of both display the characteristic structural features...

Ordered water structure at hydrophobic graphite interfaces observed by 4D, ultrafast electron crystallography

“…The lattice of graphite has been shown to undergo an interlayer contraction followed by a large expansion with a time constant of Ϸ7 ps (22). The ''old-structure'' disappearance time melting for ice given here matches well with the convolution of the 7-ps time for graphite with the instrumental response, indicating that the breakage of the original assembly conformation is due to the vibrational coupling of ice with underlying graphite.…”

“…23 for ice on CO/Pt (111) but the picture is valid. After the ultrafast heating initiated by an infrared light pulse, the substrate undergoes lattice contraction followed by expansion (22). The transfer of vibrational energy to interfacial ice assembly leads to (partial) structural randomization in the first 10 ps.…”

“…The optical excitation fluence used was up to 39 mJ/cm 2 at the peak. We did not employ the scheme of optical-pulse tilting (21,22,37,38), which enables femtosecond resolution, because in the case discussed here the doser system for water deposition is connected on top of the chamber and the temporal resolution (capable of detecting a 2-ps change; 7 ps in total) was sufficient for the dynamics. The ice-substrate composite was fully recovered in Ͻ1 ms without noticeable water sublimation, as evidenced by the reproducibility of the diffraction pattern at negative times and for our experimental repetition rate of 1 kHz.…”

Reduced Charge Transfer Exciton Recombination in Organic Semiconductor Heterojunctions by Molecular Doping

“…A gread deal of information has been obtained by a set of experiments based on ultrafast electron scattering techniques [92][93][94][95], revealing the tendency of the c-axis of graphite to be destabilized by the photoinduced coherent motion of ions. Recenctly, the transition of graphite to diamond was observed by very intense femtosecond laser pulses [96].…”

“…A subsequent expansion and oscillation of the graphite lattice after the inital compression is expected [92] and should be observable in the transients [98]. We analyzed the transient dynamics at a probe photon energy of 0.34 eV and a carrier density vibrational energy is about 43 cm −1 [110].…”