Molecular structures on crystalline metallic surfaces – From STM images to molecular electronics

doi:10.1016/j.mee.2005.07.087

Microelectronic Engineering

Volume 82, Issues 3–4, December 2005, Pages 207-214

https://doi.org/10.1016/j.mee.2005.07.087 Get rights and content

Abstract

We present results from scanning tunneling microscopy obtained for organic molecules – coronene and different phthalocyanine derivatives – adsorbed on crystalline metallic substrates under UHV conditions. Molecular structures resolved till to submolecular details are observed. The principles governing regular self-assembled adsorbate structures are discussed. There are accessible close-packed ultra-thin molecular films as well as open network structures suited as templates for further processing. Switchable single-molecular configurations as well as potential storage and nano-engine devices placed on a regular self-assembled molecular pattern are demonstrated.

Introduction

The huge and ever increasing variety of organic species continuously invented by modern organic chemistry is far away from a detailed understanding of these materials and their interfaces to other inorganic materials. On the other hand, such type of basic knowledge is prerequisite for any systematic applications.

Scanning tunneling microscopy (STM) [1] combined with the standard UHV techniques and optionally low-temperatures is a versatile tool to study adsorbed organic species on well-defined crystalline substrates as has been first demonstrated in [2]. By direct imaging of molecular arrangements on such substrates, systematic investigations can be carried out which give impressive insights into the nature of the interactions at the adsorbate–substrate interface.

We have chosen medium-sized organic molecules for our study. These molecules have dimensions larger than the lattice constant of the substrate but not as large that the substrate periodicity becomes almost negligible. Coronene and metal–phthalocyanines (Me–Pc) as well as metal–naphthalocyanines (Me–Nc) adsorbed on either highly-oriented pyrolytic graphite (HOPG) (which is a layered material with an almost inert (0 0 0 1) surface) or on a densely packed (1 1 1) silver single crystal surface will be used in this work. Both types of substrates show a metallic surface conductivity. Monolayers of these adsorbates form close-packed adsorbate structures on these surfaces. In contrast, trimesic acid (TMA) is another organic substance which is reactive due to carboxylic end-groups and therefore able to self-organize in open structures by formation of H-bonds. Fig. 1 shows the different adsorbate molecules involved in this study. Whereas coronene is sixfold-symmetric and planar and nothing else than a flake of graphite with a saturation of the outermost bonds by additional H atoms, Pd–Pc is a fourfold-symmetric and flat molecule. Sn–Pc has a shuttle-cock shape due to the fact that the central metal atom is too big to fit completely inside the central cavity of the Pc stage. TMA is a threefold-symmetric molecule which opens the opportunity for a self-assembling via H-bonds to identical molecules into a regular network.

In this paper, we report investigations on different types of adsorption from a comparative point of view. We start by a short description of the experimental details. Then we describe the results of STM imaging experiments using the qualitatively different adsorbates on HOPG(0 0 0 1) and coronene and Sn–Pc on Ag(1 1 1) as well. We finish by conclusions and an outlook.

Section snippets

Experimental

HOPG(0 0 0 1) and Ag(1 1 1) have been chosen as crystalline substrates. The former one is a layered material whose electronic surface structure is dominated by π-electron systems within the (0 0 0 1) crystal plane and this surface is expected to interact only weakly via van der Waals forces with the adsorbed molecules. In contrast, the silver surface is dominated by free-electron like surface states which allow also some electrostatic interaction and charge transfer. Both crystallographic surfaces

Single-molecule switch

When the basic ideas of molecular electronics have been brought forward by Forrest L. Carter in the 80s [10], most of them have been just visions and speculations in that time. Nowadays there are some first steps already done and organic molecules on crystalline substrates are one of the prospective directions for further development.

The two possible adsorption states observed for non-planar Pcs offer the opportunity for a single-molecular switch supposed that this state can be changed in a

Conclusion and outlook

In this paper, we have demonstrated that there is a huge variety of self-assembling monolayer structures made of medium-sized organic molecules. Whereas chemically quite inert molecules adsorb as more or less densely packed monolayers there are reactive molecules as TMA which can assemble as quite open network patterns. The latter can be useful templates for the construction of more complex molecular architectures.

Acknowledgments

The authors acknowledge financial support by Deutsche Forschungsgemeinschaft (DFG) and Deutscher Akademischer Austauschdienst (DAAD).

References (13)

J.K. Gimzewski et al.
Surf. Sci.
(1987)
M. Lackinger et al.
Surf. Sci.
(2002)
G. Binnig et al.
Rev. Mod. Phys.
(1987)
M. Lackinger et al.
Anal. Bioanal. Chem.
(2002)
M. Lackinger et al.
J. Phys. Chem. B
(2002)
T.G. Gopakumar et al.
J. Phys. Chem. B
(2004)

There are more references available in the full text version of this article.

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