Photo-Induced On-Surface Synthesis to Elaborate Highly-Ordered Covalent Structures on Insulating Substrates – Light4Net

Light4Net is a collaborative project which enrolls two major French academic research laboratories (IM2NP and FEMTO-ST), each labelled “research centre of excellence” by the French government. The partners have already proven a fruitful and close collaboration in the past. Light4Net presents an entirely novel interdisciplinary research in the field of Chemistry and Physics and it groups complementary expertise and equipment. It aims to develop a general procedure to create artificial pi-conjugated organic nanostructures on insulating substrates.

CONTEXT
Ordered arrays of nanoscale objects supported on a surface are of growing interest for many technological applications. Nevertheless, the fabrication of extended regular structures is quite challenging and mainly reported for self-assembled supramolecular structures on metal surfaces.
Most of the technological applications in molecular Nanotechnology require however
- the existence of electronic bands with high conduction properties,
- the decoupling of the electronic states from those of the substrate, and
- the mechanical and thermal stability of the molecular layer.
These criteria can only be fulfilled by pi-conjugated nanostructures on insulating surfaces.
The literature on covalently bond molecular layers is mainly restricted to metal surfaces on which the polymerization is assisted by the catalytic effect of the substrate. For insulating surfaces, new tools must be developed to induce the reaction and to guide the polymerization to form highly ordered structures. Based on preliminary work, in Light4Net we propose UV-illumination to induce the radical polymerization on alkali-halide substrates. Specially synthesized precursors with well-chosen anchoring groups will guide the polymerization on these polar substrates to fabricate extended and defect-free covalent structures
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SCIENTIFIC CHALLENGE / NOVELTY
The following novel and challenging issues will be addressed by Light4Net:
(i) UV-induced radical polymerization on insulating bulk and thin-film substrates,
(ii) Fabrication of novel, defect-free, and covalent 1D (length > 10 µm) and 2D (area > 1x1 µm2) polymers,
(iii) Low-Temperature nc-AFM imaging and spectroscopy with sub-molecular resolution of monomers, small oligomers, and extended covalent structures adsorbed on insulators by means of functionalized tips.

WORKPLAN
Three work packages (WP) are planned:
WP1: Molecular Synthesis. Two families of molecules will be synthesized which contain, as common property, functional groups providing photo-generation of organic radicals as initiators to provide the formation of defect-free extended polymers on insulating surfaces.
WP2: On-Surface Polymerization: Highly-ordered covalent 1D and 2D structures will be fabricated on insulating substrates by optimizing the sample preparation and by controlling the growth kinetics under UHV conditions.
WP3: UV-induced activation of monomers and the exact configuration between individual molecules when they polymerize will be investigated by means of Low-Temperature Noncontact Atomic Force Microscopy using functionalized tips.

MAIN OUTPUT
There are three main breakthroughs:
(1) Synthesis of functional molecules which serve as building blocks for radical polymerization to give extended polymers; these precursors will incorporate both UV-dissociable bonds and specific anchoring groups.
(2) Demonstration of how the growth kinetics of UV-induced polymerization can be controlled on insulating substrates.
(3) Characterize the geometry and the properties of molecules (neutral and radical ones) before and after UV-induced covalent bond formation by means of high-resolution Scanning Probe Microscopy (SPM).