Controlling molecular organization for the realization of sub-wavelength light sources

The possibility to use the tip of a scanning tunneling microscope (STM) for the realization of a highly localized molecular light source is discussed. Engineering rules for the control of single molecule luminescence at metallic interfaces are first presented: we report controlled patterning of graphite substrates with well-defined 3D molecular nanostructures using the steered arrangement of multilayered [2.2] paracyclophane-based building blocks. Their in-plane self-assembly is shown to allow exact positioning of vertical structural elements with precisely defined nanoscale spacing both in the substrate plane and in a direction perpendicular to it. In a second part, we demonstrate that electric field induced second harmonic generation (SHG) processes can also be taken into profit to get localized light emission through controlled local electric field application using the STM tip as an electrode. Efficient SHG emission at the tip can be observed when the tip is immersed in a solution of highly nonlinear molecules: the electric field present at the tip apex is shown to lead to local noncentrosymmetry following molecular orientation.