© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 9, FEBRUARY 27, 1997
LETTERS Formation of Ordered, Anisotropic Organic Monolayers on the Si(001) Surface Robert J. Hamers,* Jennifer S. Hovis, Seung Lee, Hongbing Liu, and Jun Shan Department of Chemistry, UniVersity of WisconsinsMadison, 1101 UniVersity AVenue, Madison, Wisconsin 53706 ReceiVed: August 30, 1996; In Final Form: December 19, 1996X
It is shown that the SidSi dimers of the reconstructed Si(001) surface can react with the π bonds of unsaturated organic molecules to produce well-defined organic films with novel physical properties. Scanning tunneling microscopy (STM) studies show that the resulting layers are ordered both translationally and rotationally, with the SidSi dimers acting as a template for extending the translational and rotational order from the silicon substrate to the organic film. STM images and infrared spectroscopy experiments show that by using vicinal Si(001) surface having primarily double-height steps, the rotational order of the molecules can be preserved over macroscopic length scales, leading to measurable anisotropy in optical properties. It is proposed that this chemistry may provide a general method for formation of controlled organic films on Si(001) surfaces.
Introduction The (001) surface of silicon is very important technologically as the starting point for most microelectronic devices. While microelectronics processing typically involves reactions of silicon surfaces with inorganic compounds, there is increasing interest in developing methods for coupling existing microelectrics technology with organic-based structures for applications such as nonlinear optics, thin-film displays, lithography, and molecular electronics. The Si(001) surface is characterized by a surface reconstruction in which adjacent atoms pair together, forming dimers. Since each silicon atom of the bulk-terminated surface has two dangling bonds, the hybridization of two atoms to form a dimer can be described as formation of a strong σ bond and a weak π bond. Indeed, scanning tunneling microscope images of Si(001) reveal filled and empty electronic states having the symmetry properties expected for π-type electronic states as the highest-occupied and lowest-unoccupied electronic states.1 The π bonding of Si(001) suggests that the reactions of this * Author to whom correspondence should be addressed. Phone: 608262-6371. FAX: 608-262-0453. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, February 1, 1997.
S1089-5647(96)02677-6 CCC: $14.00
surface should in some ways be analogous to those of doublebonded compounds of carbon and of silicon.2 Despite the great technological importance of Si(001), there has been comparatively little work attempting to fabricate welldefined organic layers. Chidsey and co-workers3 successfully formed layers of densely-packed alkyl monolayers on hydrogenterminated Si(111) and Si(001) surfaces through a free-radical mechanism. It has been observed previously that ethylene and acetylene are capable of chemisorbing on Si(001) in a so-called “di-σ” configuration, in which the double bonds of the Si dimers and the double bonds of the adsorbed organic species are broken, forming two new Si-C σ bonds.4-7 In this paper, we show that it is possible to use reactions between the oriented dimers present on the Si(001) surface and the double bonds present in unsaturated organic molecules to fabricate well-defined organic layers on Si(001) in which the molecules are ordered translationally as well as rotationally. Using cyclopentene as a prototypical example, we show specifically that (1) such reactions can be used to produce a well-defined interface between silicon and organic layers, (2) this layer contains molecules aligned along specific crystallographic direction with minimal dissociation, (3) the direc© 1997 American Chemical Society
1490 J. Phys. Chem. B, Vol. 101, No. 9, 1997
Letters
tionality of molecular orientation can be maintained over macroscopic distances using miscut (“vicinal”) Si(001) substrates, and (4) the resulting molecular orientation leads to measurable anisotropy in optical properties measured on centimeter length scales. We also show that addition reactions can be extended to other molecules, suggesting that the interaction of unsaturated organic molecules with Si(001) should provide a general method for fabrication of organic layers with controlled chemical functionality. Experimental Section All experiments were performed in an ultrahigh vacuum (UHV) system achieving a base pressure of
