2D chiral growth of 6,13-pentacenequinone thin films on Si(111)
A. Al-Mahboob, J. T. Sadowski, T. Nishihara, Y. Fujikawa and T. Sakurai
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku,
Sendai 980-8577, Japan
Recent progress in the application of organic materials to the microelectronic devices
signifies the big advantages of the organic electronics, such as low cost fabrication,
biodegradability and flexibility. Among currently investigated organics, the pentacene(C22H14, Pn) is studied widely, because of its high field-effect mobility. Pn, however, oxidizes
in the presence of oxygen forming 6,13-pentacenequinone (C22H12O2, PnQ) and showsincreased saturation current in field-effect transistor (FET) in oxygen atmosphere [1]. The
decrease in mobility and on/off ratio in Pn-based FETs induced by a long exposure to air has
also been reported [2]. Recently, the n-type electronic transport and intense yellow
luminescence in PnQ thin film has been demonstrated [3].
Organic materials are usually highly anisotropic, whether in their molecular or crystalstructures, or in the bonding energies. This leads to complicated growth mechanisms,
resulting sometimes in the lowering of the crystal symmetry of the film. However, since nodirect observation of initial stage of the spontaneous, chiral evolution has been reported yet,there is no well understood mechanism for such evolution, either at mesoscopic level or evenat macro level.
In this work the thin film growth of 6,13-pentacenequinone on Si(111) at room
temperature (RT) was studied by low energy electron microscopy (LEEM) and ab initio
density functional theory (DFT) calculations [4]. At the initial stage of film growth, noncrystalline islands or clusters (type-I) nucleate on top of a disordered wetting layer. This isfollowed by nucleation of crystalline islands (type-II). An anisotropy in the growth of type-II
islands has been observed and related to: (1) asymmetric mass incorporation and (2) masstransport from non-crystalline islands via a large diffusion length. The observed left-handedor right-handed chirality is related to the orientation of the c axis and associated with theasymmetric mass incorporation. The pronounced, ring-like shapes of the islands originate
from the gradual rotation of crystallites associated with asymmetric arrangement ofdislocations.
From the observation of the growth and subsequent relaxation of the crystalline PnQ
islands, with the support of DFT calculations, we conclude that kinetically stiff direction(here one of the diagonals of the in-plane unit cell) differs from the energetic one. Thisfeature together with the asymmetric mass incorporation associated with molecular tilt andbond anisotropy can determine the chirality (enantiomorphic evolution) of organic growth,and can help to understand the principles of chiral bio-crystallization.
[1] R. Ye, M. Baba, K. Suzuki, Y. Ohishi and K. Mori, Thin Solid Films 464-465, 437 (2004).
[2] Y. Qiu, Y. Hu, G. Dong, L. Wang, J. Xie and Y. Ma, Appl. Phys. Lett. 83, 1644 (2003).
[3] D.K. Hwang, K. Kim, J.H. Kim, D.-Y. Jung, E. Kim and S. Im, Appl. Surf. Sci. 244, 615 (2005).
[4] A. Al-Mahboob, J. T. Sadowski, T. Nishihara, Y. Fujikawa and T. Sakurai, Phys. Rev. B, pending.