Yoshiyuki Miyamoto (Leader) of Nano Carbon Materials Simulation Group and Takehide Miyazaki (Leader) of Nonequilibrium Materials Simulation Group, the Nanosystem Research Institute (Director: Tomohiko Yamaguchi) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), in collaboration with Hong Zhang (Professor) of Sichuan University (China) and Angel Rubio (Professor) of The Max Planck Institute for Structure and Dynamics of Matters (Germany), theoretically present feasibility of reducing inter-layer distance of a layered material, hexagonal boron nitride (hBN), by illuminating an infrared (IR) laser according to the results of first-principles simulations.

This proposal is based on the simulated results that showed large amplitude of an alternating lattice vibration of boron (B) and nitrogen (N) atoms by IR-laser illumination. Since B and N atoms have positive and negative effective charges, respectively, such a vibration with large amplitude can induce attractive Coulomb interaction between atomic layers. The interaction compresses the inter-layer distance and the distance is reduced by more than 10 % of the original distance. These simulated results suggested application to chemical reactions occurring at the inter-layer space with controlled distance etc., thereby contributing to new-material exploration.

The details of the present research will be published online in Physical Review Letters published by American Physical Society on March 19, 2015 (US EST).

Schematic of inter-layer compression of hBN by the lattice vibration induced by infrared laser illumination

Recently, low-dimensional materials has attracted much attention. Especially, layered materials with single-atomic thickness like graphene have potential applications to low-power transistors, highly efficient light-electric convertors, sensors and so on, based on their unique electronic properties such as high carrier mobility and light absorption insensitive to wavelengths and their ability to host substances between the layers. Their electronic properties depending on the inter-layer distance have been studied. However, there has been no technologies to control the distance. IR lasers, available with commercialized package, can disintegrate the layered materials by exfoliating single layer or ablation of a few layers. In contrast, the present study proposed an unprecedented technique to strengthen the inter-layer interaction.

AIST conducts research in graphene and other low-dimensional materials, aiming at higher performance and replacing conventional materials. To accelerated experimental research, AIST also developed simulation techniques with high accuracy and predictability that can guide synthesis and modifications of layered materials by laser. Such simulation technique are possessed by few research institutes in the world.

Part of the present work was supported by MEXT Grant-in-Aid for Scientific Research on Innovative Area, “Science of Atomic Layers,” (FY2013 - FY2017).

The present work theoretically proposed enhancement of dipole-dipole inter-layer attraction of hBN, a layered material, due to increase of the amplitude of a lattice vibration triggered by an IR laser. To predict this, highly accurate first-principles simulations were applied for electron dynamics under laser illumination by solving the time-dependent Schrödinger equation of electrons and the Newton's equation of ion motion simultaneously.

hBN is a compound that consists of alternatively positioned boron (B) and nitrogen (N) atoms in a honeycomb lattice in each stacked layer (Fig. 1).

Figure 1: Honeycomb lattice of hexagonal boron nitride (hBN) sheet consisting of boron and nitrogen atoms

The origin of inter-layer cohesion was a weak interaction, so called van der Waals force, which was found to be enhanced by electronic excitation in noble gas by ultra-violet laser illumination (AIST press release on May 19, 2014). The present work enhances the van der Waals force by induction of a lattice vibration instead of the electronic excitation.

When an IR laser with tuned wavelength of 1.4 µm, the lattice vibration of B and N atoms in opposite directions can be induced as shown by green arrows in Fig. 2. Since B and N atoms possess positive and negative effective charges, respectively, such a vibration with large amplitude can induce dynamic dipoles on hBN sheets in parallel that can induce attractive force between the sheets.

Figure 2: Schematic of lattice vibration induced by the IR laser and subsequent polarization of each hBN sheet

By performing the first-principles simulation, it was found that the Coulomb force originated from dynamic dipoles shortens the inter-layer distance of hBN by up to 11.3 % of the original distance. Past experimental report showed 6 % inter-layer contraction in graphite by illuminating a pulse-compressed laser with wavelength of 800 nm, while the contraction of hBN in the present study is beyond the past value. Moreover, with intense IR laser illumination, simulation tells electronic excitation suppresses the inter-layer contraction of hBN. Thus, it is learned from the simulations that tuning the laser power around 1x10¹² W/cm² is essential for the inter-layer contraction. This intensity can be realized with a commercial laser package by focusing the laser bean in diameter of micrometer-scale.

In future, the researchers will conduct experimental research to confirm this theoretical prediction, and apply the IR-laser induced inter-layer contraction to formation of new materials using novel chemical reactions of substances being intercalated between the sheets of layered materials. Furthermore, they will extend application of IR lasers from conventional heating to new frontier of chemical reactions triggered by laser-induced lattice vibrations.