Laurent Nony

Cu-TBPP and PTCDA molecules on insulating surfaces studied by ultra-high-vacuum non-contact AFM

The adsorption of two kinds of porphyrin (Cu-TBPP) and perylene (PTCDA) derived organic molecules deposited on KBr and Al2O3 surfaces has been studied by non-contact force microscopy in ultra-high vacuum, our goal being the assembly of ordered molecular arrangements on insulating surfaces at room temperature. On a Cu(100) surface, well ordered islands of Cu-TBPP molecules were successfully imaged. On KBr and Al2O3 surfaces, it was found that the same molecules aggregate in small clusters at step edges, rather than forming ordered monolayers. First measurements with PTCDA on KBr show that nanometre-scale rectangular pits in the surface can act as traps to confine small molecular assemblies.

Dipole-driven self-organization of zwitterionic molecules on alkali halide surfaces.

Summary We investigated the adsorption of 4-methoxy-4′-(3-sulfonatopropyl)stilbazolium (MSPS) on different ionic (001) crystal surfaces by means of noncontact atomic force microscopy. MSPS is a zwitterionic molecule with a strong electric dipole moment. When deposited onto the substrates at room temperature, MSPS diffuses to step edges and defect sites and forms disordered assemblies of molecules. Subsequent annealing induces two different processes: First, at high coverage, the molecules assemble into a well-organized quadratic lattice, which is perfectly aligned with the <110> directions of the substrate surface (i.e., rows of equal charges) and which produces a Moiré pattern due to coincidences with the substrate lattice constant. Second, at low coverage, we observe step edges decorated with MSPS molecules that run along the <110> direction. These polar steps most probably minimize the surface energy as they counterbalance the molecular dipole by presenting oppositely charged ions on the rearranged step edge.

Force microscopy on insulators: imaging of organic molecules

So far, most of the high resolution scanning probe microscopy studies of organic molecules were restricted to metallic substrates. Insulating substrates are mandatory when the molecules need to be electrically decoupled in a electronic circuit. In such a case, atomic force microscopy is required. In this paper we will discuss our recent studies on different organic molecules deposited on KBr surfaces in ultra-high vacuum, and then imaged by AFM at room temperature. The distance between tip and surface was controlled either by the frequency-shift of the cantilever resonance or by the excitation signal required to keep the oscillation amplitude constant. Advantages and drawbacks of both techniques are discussed. The high mobility of the molecules, due to their weak interaction with the substrate, hinders the formation of regular self assembled structures. To overcome this problem we created artificial structures on the surface by annealing and by electron irradiation, which made possible the growth of the molecules onto step edges and their confinement into rectangular pits.

Distance dependence of force and dissipation in non-contact atomic force microscopy on Cu(100) and Al(111)

The dynamic characteristics of a tip oscillating in the nc-AFM mode in close vicinity to a Cu(100)-surface are investigated by means of phase variation experiments in the constant amplitude mode. The change of the quality factor upon approaching the surface deduced from both frequency shift and excitation versus phase curves yield to consistent values. The optimum phase is found to be independent of distance. The dependence of the quality factor on distance is related to true damping, because artefacts related to phase misadjustment can be excluded. The experimental results, as well as on-resonance measurements at different bias voltages on an Al(111) surface, are compared to Joule dissipation and to a model of dissipation in which long-range forces lead to viscoelastic deformations.

The Cu(1 0 0)-c(2 2) N structure studied by combined nc-AFM/STM

The Cu(1 0 0)-c(2 x 2) N reconstructed surface has been studied by combined non-contact force and tunneling microscopy. Frequent tip changes produced all kinds of contrast in the interaction between the tip and differently reconstructed areas on the surface. Atomic resolution has been obtained using the tunneling current as feedback

Investigation of Organic Supramolecules by Scanning Probe Microscopy in Ultra-High Vacuum

In conclusion, we have shown how scanning probe microscopy has been applied to investigate, isolate and manipulate different organic molecules on metal, semiconductor and insulating surfaces. The same chemical compounds give rise to different structures, depending on several factors, such as the adsorbate coverage, the substrate orientation or the temperature. Self-assembled patterns formed on the step edges of insulating surfaces — or raised from metal substrates by specifically designed molecular “legs” — can be used as molecular wires. The different conformations assumed by single molecules can be viewed as different logical states of a digital circuit. Scanning probe microscopy might become one of the essential ingredients to design future molecular electronics devices. Well-defined experiments to investigate self-assembly characteristics and to measure the mechanical and electrical properties of the individual molecules will be performed. Internal degrees of freedoms of the molecules and mechanical instabilities are rather complex phenomena, which will need both experimental and computational efforts to achieve the required degree of control of these novel molecular electronics devices.