Materials Design through Computics*: Nanowires and Nanotubes

Atsushi Oshiyama

The University of Tokyo, Graduate School of Engineering, Applied Physics, Tokyo, Japan

I report our recent efforts to expand applicability of density-functional-theory (DFT) based calculations to clarify static and dynamical properties of materials. One of the efforts is to develop a new scheme which allows us to perform atomic and electronic structure calculations for unprecedentedly large-scale targets. Such targets include nano-scale electron devices which emerge as boosters in post-scaling semiconductor technology [1]. I explain our RSDFT (Real Space Density Functional Theory) scheme which is most suitable for next-generation massively parallel multi-core computer architectures since it is free from the communication burden caused by Fast Fourier Transform. We have introduced several new algorithms to accelerate computations, and have performed DFT calculations for Si nanodots and nanowires consisting of a few tens of thousands atoms [2]. Calculated energy-band structures are sensitive to nanomorphology of wire cross sections, thus providing useful information for fabrication of nanowire devices [3].

Carbon-nanotubes (CNTs) are also good candidates for the next-generation devices. However, the controlled alignment of CNTs on substrate surfaces is imperative to utilize fascinating properties of CNTs in semiconductor devices. CVD growth on sapphires surfaces provides promising stages for such alignment. We show that formation of covalent and ionic bonds between the CNT and the sapphire surface is the reason for such alignment [4].

Improvement of the local or the semilocal approximations in DFT is another important issue. We have implemented several hybrid functionals for the exchange-correlation energy and found that the calculated structural constants and the band gaps for various semiconductors and insulators are substantially improved by using some hybrid functionals [5].

* Computics is the word used in a research project, ``Materials Design through Computics: Complex Correlation and Non-equilibrium Dynamics”, supported by MEXT Japan as Scientific Research on Innovative Areas. http://computics-material.jp/

[1] International Technology Roadmap for Semiconductiors, http://www.itrs.net/
[2] J.-I. Iwata, D. Takahashi, A. Oshiyama, B. Boku, K. Shiraishi, S. Okada & K. Yabana, J. Comp. Phys. 229, 2339 (2010).
[3] S. Kyogoku, J.-I. Iwata & A. Oshiyama, Proc. 11th IEEE International Conference on Nanotechnlogy (Portland, USA, August 2011) pp1322.
[4] S. Jeong & A. Oshiyama, Phys. Rev. Lett. 107, 065501 (2011).
[5] Y. Matsushita, K. Nakamura and A. Oshiyama, Phys. Rev. B 84, 075205 (2011).

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