The scanning tunneling microscope (STM) enables us to precisely image and manipulate individual atoms and molecules. Of particular interest are STM studies of organic molecules on metal surfaces. The electrical and chemical properties of a metal surface can be significantly affected by applying a mono-layer of organic molecules on it. These types of nano interfaces have a huge potential as building blocks for novel nano devices. Examples of applications are; molecular-electronics with superior characteristics compared to conventional silicon based devices, protecting layers with enhanced chemical resistances and/or improved lubrication and biocompatible coatings.

A certain group of thiol-based organic molecules have been shown to be extraordinary versatile when applied onto gold surfaces and other similar coinage metals. Thiol-organo molecules have the abililty to self assemble onto gold to form layers with strict periodic patterns.

Such self-assembled monolayers are easy to prepare and have been shown to improve on the characteristics of the underlying surface such as improved corrosion inhibition, better biocompatibility and more wear resistant lubricants. Understanding how these layers are formed step-by-step in atomistic detail is necessary in order to be able to design and tailor make novel materials with superior properties compared to conventional materials.

Our research projects concerns various types of thiolphenol (TP) molecules on the copper surface. The TP molecule is simply a benzene ring with one of the hydrogen atoms replaced by a sulphur atom. The sulphur atom is the part that will anchor the molecule to the metal surface. By substituting the remaning hydrogen atoms on the TP molecule with the halogen atoms Flourine, Chlorine and Bromine according to the picture, it is possible to study in a systematic way how small changes of a molecule influences the self-assembling properities.

We study these molecules on copper because copper share many common features with gold and it also have the advantage that the current semi-conductor industry already is using copper in mass production of, for example, computer processors.

When the molecules are deposited onto a clean Cu(111) surface in ultra high vacuum, it is observed that they do form periodic arrays even at a temperature as low as 80 K. The following images show the patterns formed by Br-TP and F-TP respectively. Notice how the F-TP pattern looks much more complicated. It requires 7 molecules to form a unit cell which is repeated periodically, whereas the Br-TP pattern is a simple rectangular array with an one-molecule unit cell. The difference in the observed patterns can be attributed to the change in size and electronegativity of the halogen atoms Br and F.

How about the plain TP molecule without any halogen substitutions? This molecule do not form ordered patterns on Cu(111). Substituting all the remaning hydrogens on the TP molecule with fluorine atoms again leads to the observation that no ordered patterns are formed. These experimental observations point to the perimeter atoms on the benzene ring as significant modifiers for the self-assembled monolayers. Understanding exactly the effects of each substitution will help towards the goal of synthesising monolayers with arbitrary desired patterns.

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