Yoshiyuki Miyamoto (Leader), Dynamic Process Simulation Group, the Nanosystem Research Institute (Director: Kiyoshi Yase) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomakuchi), has performed the first-principles simulations to show that an ultra-short laser pulse can induce electron excitation which activates synchronized rotational motion of two acetylene molecules encapsulated in a semiconducting carbon nanotube using the Earth Simulator of Japan Agency for Marine-Earth Science and Technology. The motion cannot be realized when acetylene molecules are in a gas phase. The simulation also showed that the excitation can be preserved with a reactive configuration of the acetylene molecules.

This work is expected to contribute to new chemical synthesis achieved by combining a femtosecond laser and carbon nanotubes.

The details of this research were published online in a US scientific journal, Proceedings of the National Academy of Sciences USA, on May 21, 2012.

Two acetylene molecules encapsulated in a semiconducting carbon nanotube (top) and synchronized rotational motion of the molecules in the carbon nanotube after laser irradiation (bottom) (The display of nanotube structures during the molecular rotation was omitted.)

Experimental works proved that a carbon nanotube (CNT) can encapsulate metal atoms and organic molecules. It is expected that CNT behaves as a test tube useful for new material synthesis by photo-chemical reactions of encapsulated molecules.

It has been known that an ultra-short laser pulse causes electronic excitation of molecules that non-thermally changes molecular structures. Furthermore, the higher efficiency of laser irradiation is expected when the laser irradiate the molecules encapsulated in CNT than the efficiency when it irradiate the molecules in a gas phase. However, it is difficult to experimentally examine how an optical field is modulated by CNT and what kind of dynamics occurs in a narrow internal space of CNT. Current study examined the effect of an ultra-short laser pulse on acetylene molecules encapsulated in CNT by performing the first-principles calculations, which can treat dynamics of electrons and nuclei simultaneously. The acetylene encapsulation was attracting attention from a viewpoint of molecular storage as well as possibility of polymerization of acetylene.

AIST is aiming at the development of innovative technology of CNT that contributes to industry and at accumulating necessary basic knowledge. For these purposes, AIST is conducting research based on the first-principles simulations to predict CNT properties.

This research has been done under an Earth Simulator collaboration project in 2011, "Large-scale Simulation on the Properties of Carbon-nanotube" (Leader; Syogo Tejima, Research Organization for Information Science and Technology), that used the Earth Simulator of Japan Agency for Marine-Earth Science and Technology. This research was done in collaboration with Prof. Hong Zhang (Sichuan University, China) and Prof. Angel Rubio (University of País Vasco, Spain). All numerical simulations using the Earth Simulator were done by AIST.

In this research, the dynamics of molecules encapsulated in CNT after the electron excitation induced by an ultra-short laser pulse was simulated by numerical calculations. The pulse was polarized perpendicular to CNT axis and has a pulse shape shown in Fig. 1.

Figure 1 : Pulse shape and polarization of the electric field of the laser irradiating CNT encapsulating acetylene

The (14,0) CNT used for the calculation has electronic structure of semiconductor, so the optical field can penetrate to the inner cavity of the CNT without shielding. (Meantime, in metallic CNT, the optical field is shielded by fast-moving electrons.) The diameter is just suitable to encapsulate an acetylene dimer. The dynamics of this dimer induced by a laser pulse (wavelength : 800 nm, full width at half maximum : 2 fs) was studied. This pulse shape has a specific property having an electric field with particular direction for a longer time, unlikely to ordinary sinusoidal shapes directing the electric field in two alternating directions.

The dynamics of gas-phase acetylene induced by the same pulse shown in Fig. 1 was investigated for comparison, and hydrogen atoms at the end of the molecule immediately detached after the application of the pulse. Assuming the same configuration as the dimer in (14,0) CNT, the dynamics of a hypothetical acetylene dimer in gas phase induced by the same laser pulse was also examined. Unlike the case of the single molecule, it was found that C-H bonds showed swing motion and hydrogen atoms detached. Furthermore, when the dimer is in the (14,0) CNT, the numerical simulation interestingly showed that the same laser pulse can induce the swing motions in opposite directions, and that the detachment of hydrogen atoms was suppressed. The result suggests modulation of the laser field by a response of the CNT wall and indirectly suggests that the field is non-uniform in space. Figure 2 shows comparative results of the dynamics of acetylene dimers from the direction parallel to CNT axis, showing the isolated molecule, the isolated dimer, and the dimer encapsulated in CNT.

Figure 2 : Comparison of dynamics of the isolated acetylene molecule (top), the acetylene dimer (middle), and the acetylene dimer encapsulated in CNT, induced by the laser pulse.

The longer time simulation was also made to trace the rotational motion of the acetylene dimer in CNT. Figure 3 shows comparison of the results induced by laser pulses with same shape but different intensities. In both cases, the polar angles of the carbon-carbon axes (C-C axes), as denoted by gray semi-circles in Fig. 3, of acetylene molecules showed synchronized rotation. Throughout the simulation time, the C-C axes which have triple bonds rotate in synchronized manner keeping laser-induced electronically excited states for a long time suggesting the high reactivity of these molecules, while the swing and stretching motions of C-H have no correlation. Because of retention of the electronically excited state, anti-bonding nature in the C-H bond was found. One hydrogen atom in one of the two acetylene molecules detached when maximum intensity of laser field is 12 V/Å. In this case, the simulation also showed induction of defects in the CNT.

Figure 3 : Laser induced dynamics of the acetylene dimer in semiconducting (14,0) CNT Small circles indicate hydrogen atoms. (Structure of CNT is not shown.)

The researchers will continue to perform numerical simulation to explore conditions that maximize the efficiency of chemical reactions of molecules encapsulated in CNT to accelerate frontier research on structure and synthesis of new low-dimensional materials. They will also explore the same effect in other porous materials besides CNT. Chirality dependence of the damage to CNT, a test tube, caused by pulse laser irradiation will also be investigated.