Theoretical characterisation of assemblies of flat organic molecules on Au(111) surface

by Dr Manuela Mura

Centre for Materials Science Research Colloquium

Wednesday 20th of February 2013 at 13:00

Foster Building Lecture Theatre 3

Abstract: The phenomenon of self-assembly of atomic and molecular superstructures on crystal surfaces is attracting an increasing interest in nanotechnology. Self-organised nano-templates, where the self-assembled monolayer traps other molecules with selected functional properties, can be used to build large nanoscale structures. Self-assembled superstructures can form chiral domains ranging from 1D chains to 2D monolayers.

Herringbone phase of PTCDI molecules

Herringbone phase of PTCDI molecules

There have been many scanning tunneling microscopy (STM) studies of self-assembly of melamine, perylene tetra-carboxylic di-imide (PTCDI) or perylene tetra-carboxylic di-anhydride (PTCDA) molecules on the Au(111) and Ag/Si(111) surfaces. STM images of these networks do not reveal the exact details of the intermolecular bonding and the network growth. Therefore theory can help to determine the exact atomic structure of these networks.


Brick-wall PTCDI monolayers observed with STM on the Au(111) surface

We present a theoretical study of self-assembly of molecular networks formed by organic molecules. We propose a systematic approach to build molecular superstructures based on the notion of binding sites. First, we identify all possible sites for hydrogen bonding between molecules. Then we form molecular pairs and larger structures using all possible combinations of these binding sites. In this way, we construct all possible dimers, chains and 2D monolayers dimers and chains of organic molecules. The energies of these structures are calculated using the density-functional theory implemented in SIESTA code. The strength of hydrogen bonding in various molecular arrangements is analysed. The theoretically predicted monolayer structures are in very good agreement with the results of STM measurements. We also investigate the nature of interaction of molecules and superstructures with the gold substrate and analyse the alignment of networks with the Au(111) surface.

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