Structural Control of Layered Materials by Infrared Laser Light

The possibility of controlling the interlayer distance of materials like graphene or hexagonal boron nitride could provide a new route for nanospace chemistry

April 01, 2015

In a recent publication in Physical Review Letters, a group of researchers including Angel Rubio, Director of the Theory Department of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Yoshiyuki Miyamoto and Takehide Miyazaki of the National Institute of Advanced Industrial Science and Technology AIST in Japan, and Hong Zhang of Sichuan University in China, theoretically demonstrated the possibility of reducing the interlayer distance of layered materials by infrared laser irradiation. Using ab initio simulations, they found that in hexagonal boron nitride (h-BN) the interlayer distance could be reduced by up to 11.3% through Coulomb interactions between the laser-excited layers. The control of the interlayer distance could be applied to chemical reactions occurring at the interlayer space and thereby contribute to the exploration of new materials based on stacking 2D atomic layers.

Schematic illustration of the interlayer compression in hexagonal boron nitride by the lattice vibration induced by an infrared laser

Hexagonal boron nitride (h-BN) is a compound that consists of alternatingly positioned boron (B) and nitrogen (N) atoms in a honeycomb lattice in each stacked layer. The origin of the interlayer cohesion are weak interactions like the van der Waals force. The exposure of h-BN to an infrared laser of 1.4 µm wavelength triggers a vibration in the h-BN lattice with boron and nitrogen atoms moving in opposite directions. Since boron and nitrogen atoms have positive and negative effective charges, respectively, such a vibration with large amplitude can enhance the attractive dipole-dipole Coulomb interaction between the atomic layers. To achieve this prediction, extensive and highly accurate ab initio simulations were applied for electron dynamics under laser illumination by solving the time-dependent Schrödinger equation of electrons and Newton's equation of ion motion simultaneously.

Schematic illustration of the lattice vibration induced by the infrared laser and subsequent polarization of the individual hexagonal boron nitride sheets

It was found that the Coulomb force originating from the dynamic dipoles shortens the interlayer distance of h-BN by up to 11.3% of the original separation. Furthermore, for very intense infrared laser illumination, electronic excitation suppresses the interlayer contraction of h-BN. Thus, a laser power tuned to around 1012 W/cm2 is essential for the interlayer contraction. This intensity can be realized with a commercial laser package by focusing the laser beam to a diameter of micrometer-scale.

In the future, the researchers will collaborate in experiments to confirm this theoretical prediction, and apply the infrared-laser-induced interlayer contraction to the formation of new materials using novel chemical reactions of substances being intercalated between the sheets of layered materials. Furthermore, they will extend the application of infrared lasers from conventional heating to a new frontier of chemical reactions triggered by laser-induced lattice vibrations.

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