Simulation of pulse-induced nonthermal dynamics of molecules encapsulated in carbon nanotubes
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Density functional theory, which can be used for simulating the properties of materials in the electronic ground state, was extended to time-dependent density functional theory (TDDFT). This extension enabled us to simulate nonthermal (nonequilibrium) dynamics under electronic excitation as well as to analyze and predict phenomena observed in experiments using femtosecond lasers. In this invited paper, a numerical simulation based on TDDFT for laser-induced dynamics of molecules encapsulated in carbon nanotubes (CNTs) is presented. Fast motion of molecules can be induced by short strong laser pulses that cause electronic excitation. The role of CNTs is not simply trapping the molecules but also modulating the electric field of the laser pulse. This knowledge of microscopic-scale processes will be useful when using CNTs as nanoscale test tubes in future photochemistry experiments in confined spaces for the synthesis of exotic materials. The numerical scheme and detailed results of the simulation are also presented here.
I. INTRODUCTION 1
Density functional theory (DFT) opened the door for computational material design through the solution of the Kohn–Sham equation2 of electron wave functions in actual materials. The properties of several phases of materials can be determined from DFT calculations without any empirical parameters. The total energy and force formalism3 within DFT is a standard tool for examining the stability and thermal properties of condensed matter in the electronic ground state. On the other hand, experimental developments using femtosecond lasers for monitoring ultrafast dynamics,4 controlling chemical reactions,5 and fabricating materials6 have necessitated the elucidation of atomic-scale dynamics under electronic excitation. One of the powerful tools for simulating dynamics under electronic excitation is the application of time-dependent density functional theory (TDDFT)7 to molecular dynamics (MD) within an Ehrenfest approximation.8 In this invited paper, recent computational approaches for simulating laser-induced dynamics of molecules encapsulated in carbon nanotubes (CNTs) are discussed. Since the chiral structure of CNTs9,10 and their capillary effect11 were first identified, the nature of the nanoscale hollow space inside CNTs has attracted considerable attention. CNTs have been capable of encapsulating not only metallic atoms but also fullerene,12 metallofullerene,13 water,14 and coronene molecules.15 The next point of interest was then whether a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2012.404 558
J. Mater. Res., Vol. 28, No. 4, Feb 28, 2013
http://journals.cambridge.org
Downloaded: 02 Jul 2014
these encapsulated molecules could interact with light irradiated from outside the CNT, and a method to measure the optical absorption of photoharvesting molecules encapsulated in CNTs became available in the mid 2000s.16 These experimental developments have motivated us to study th
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