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Winner of the IUPAC Prize
for Young Chemists - 2001

 

 

Paolo SamorÝ wins one of the five IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics"

Current address (at the time of application)

Humboldt University Berlin
Department of Physics
Invalidenstrasse 110
D-10115 Berlin

Tel: +49-30-20937856
Fax. +49-30-20937632
E-mail: samori@physik.hu-berlin.de
<http://pmm08.physik.hu-berlin.de/samori/paolo1.htm>

Academic degrees

  • Ph.D. in Chemistry, Humboldt University Berlin; October 24, 2000 - mark "Summa cum laude".
  • "Laurea" in Industrial Chemistry, UniversitÓ di Bologna; November 10, 1995 - mark: 110/110.
  • "MaturitÓ" in Science, Liceo Copernico, Bologna; July 1989.

Ph.D. Thesis

Title Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics
Adviser Prof. Dr. JŘrgen P. Rabe, Department of Physics, Humboldt University Berlin
Thesis Committee Prof. Dr. Manfred Meisel (chairman), Department of Chemistry, Humboldt University Berlin; Prof. Dr. Werner Abraham, Department of Chemistry, Humboldt University Berlin; Prof. Dr. Frans C. De Schryver, Department of Chemistry, Katholieke Universiteit Leuven (Belgium); Prof. Dr. Michael Linscheid, Department of Chemistry, Humboldt University Berlin; Prof. Dr. Klaus Rademann, Department of Chemistry, Humboldt University Berlin.

Essay

In the last two decades there has been a growing interest towards the nanoworld. The scientific community was curious to cast new light on the structure of organic, inorganic and biological materials, probing their chemical and physical properties on a molecular scale and comparing the properties of a single molecule with those of an ensemble or Avogadro number of molecules. Manipulating single molecules at room temperature, stimulating and visualizing in-situ chemical reactions at surfaces are just few examples of how the scientific community was able to approach to the nanoworld. In 1982 the Scanning Tunneling Microscope (STM) was invented. This technique enables to generate real-space images of surfaces with a nanometer scale resolution. This discovery represented also a big improvement for the development of miniaturised electronic devices. Even greater importance had the invention of the Atomic Force Microscope, also called Scanning Force Microscope (SFM), that allowed to extend investigations to insulating materials, like polymers and biomolecules.

On the other hand, the report in 1977 about the high electrical conductivity of a p-conjugated molecule, namely trans-poly(acetylene), that can be achieved upon p and n-doping opened new avenues of exploration for chemistry, physics and technology.

Both these two discoveries were recently awarded with the Nobel prize. Based on these two developments, the aim of this thesis was to grow highly ordered molecular nanostructures of conjugated (macro)molecules with well defined chemical functionalities and physical properties that arise from both the molecules themselves and their order at a supramolecular level. These architectures can be useful for building molecular based electronic devices, in particular molecular nanowires. Scanning Probe Microscopies were essential for this project because they allowed to investigate with a sub-molecular resolution the structures and the dynamics of (macro)molecular architectures self-assembled at surfaces.

Chemisorption and physisorption were used alternatively to design (macro)molecular nanostructures from p-conjugated systems. The accomplishment of these two different approaches were performed in the frame of extensive collaborations with of Professors Jean - Marie Lehn and Klaus MŘllen and their co-workers who were responsible for the synthesis of the molecular systems.

In the first case, Self-Assembled Monolayers of thiol-end functionalized alkanes and alkenes were grown both on Au and on Ag surfaces. The role of the substrate in the self-assembly was investigated; for this purpose a novel ultra flat Au surface (Template Stripped Gold) was developed. Different thicknesses of the organic adlayer (length of the alkyl chain) and compositions of the adsorbate (saturated or unsaturated chain) have shown distinct electric properties of the molecular adlayer.

In the latter case, using intramolecular, intermolecular and interfacial forces highly ordered 2D and 3D polymolecular micro- and nano-scopic architectures from rod- and disc-like conjugated molecules were produced. STM investigations at the interface between an almost saturated solution and a graphite (HOPG) substrate allowed to characterize both the structure and the dynamics of the molecular adsorbates. Phenyleneethynylene trimers pack in an oriented 2D polycrystalline structure. The dynamics of the single nanorods on a several minutes time scale was recorded. This Ostwald ripening phenomenon is driven by a minimization of the line energies. Such a high resolution imaging enabled us to gain insight into the kinetics of this process and to draw conclusions on thermodynamic and kinetic contributions to the total energy governing this grain coarsening. The corresponding polydisperse system (PPE) is the first polymer which was visualised with a sub-molecular resolution at the solid-liquid interface. The macromolecules arrange into a nematic-like molecular order on HOPG. Single rods are oriented along preferential directions according to the symmetry of the substrate. The true molecular lengths for several hundreds of physisorbed molecules were determined from STM images. The key result was a narrow macromolecular fractionation occurring at the solid-liquid interface: molecules belonging to the two tails of the molecular weight distribution, namely shorter and longer rods, do not favourably pack on HOPG. This result, which was confirmed by Montecarlo calculations, could be interpreted in terms of the thermodynamics of the physisorption at the solid-liquid interface and in particular with respect to the enthalpic and entropic contributions to the total free energy governing this process.

In addition, dried macromolecular films of PPE were self-assembled by solution casting on insulating substrates under various boundary conditions and studied with Tapping Mode - SFM (TM-SFM). It was possible to understand and drive the growth of these architectures towards epitaxially oriented micrometer long nanoribbons. These nanostructures are typically two monolayers thick with their alkyl side-chains oriented perpendicular to the substrate. The distribution of ribbon widths is in good agreement with the molecular weight distribution according to the Schulz-Zimm distribution. This indicates that SFM offers a valuable alternative route to determine molecular weight distributions for rod-like polymers. Moreover, these nanoribbons are architectures from p-conjugated molecules which upon thiol functionalization at their edges are ready to bridge Au nanoelectrodes in a molecular nanowire device (Figure 1).

> enlarge view

Figure 1: Cartoon of the approach that can be undertaken for developing a hybrid metallic-molecular nanowire: This strategy allows to measure the electric properties of the molecular nanowire.

Results on the electric properties of PPE aggregates between the two Au nanoelectrodes are also presented. Moreover the electronic structures of thin films of pristine and n-doped phenyleneethynylenes were investigated with photoelectron spectroscopies corroborated by theoretical calculations.

Furthermore highly ordered layers of synthetic disc-like "nano-graphene" molecules, namely hexakis-dodecyl-hexabenzocoronenes (HBC)s, were grown from solutions. TM-SFM and angle-resolved photoemission measurements revealed that HBCs can self-assemble on conductive HOPG substrates into monolayers with the p-conjugated core lying preferentially parallel to the basal plane of HOPG. By tuning the rate of the self-assembly, at very slow deposition speeds, it was possible to produce layers aligned preferentially along the crystallographic axes of HOPG. This suggests that the growth of HBC is a kinetically governed phenomenon which on the crystalline support in equilibrium allows a hetero-epitaxial type of growth.

In summary, making use of intra-molecular, inter-molecular and interfacial forces it is possible to grow highly ordered conjugated (macro)molecular nanostructures, which are candidates to be interfaced to metallic nanojunctions to probe the conductivity of this molecular ensemble.

Full-text PDF files can be downloaded from
<http://pmm08.physik.hu-berlin.de/publikat/samorithesis.pdf>


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