Imaging the Photo-Excited Electronic States at the Molecular Scale – PESoS

Since the late twentieth century, atomic force microscopy in non-contact mode (nc-AFM) has proven its strength for the study of molecular self-assemblies and single molecules in ultimate picometer scale resolution. Ever-increasing advances in combining nc-AFM with its electrostatic mode led to the visualization of charge states with intramolecular resolution already ten years ago. Despite these tremendous progresses, photo-induced excitons or photo-generated carriers have never been mapped by nc-AFM/KPFM at the molecular scale, and even less with intra-molecular resolution.
PESoS is a joint project between two laboratories: UMR5819-SyMMES CEA-CNRS-UGA, Grenoble and UMR CNRS 7334 IM2NP, Marseille. Our goal is to map the spatial distribution of excitons and photo-generated charge carriers in molecular self-assemblies, down to the single molecular layer and ultimately with intra-molecular resolution. For that purpose, we will implement a novel electrostatic mode of the non-contact atomic force microscope (nc-AFM) in ultra-high vacuum (UHV), which allows mapping the Fourier spectral components (amplitude/phase) of any time-periodic surface photovoltage (SPV) signal generated under pulsed/modulated illumination. This new dual-heterodyne KPFM (DHe-KPFM), very recently invented by SyMMES team, features an enhanced sensitivity compared to conventional surface potential imaging by Kelvin Probe Force Microscopy (KPFM), and yields access to the SPV dynamics through the frequency domain analysis with 10 ns resolution. It will be applied to map the photo-excited charge states in two kinds of model molecular self-assemblies. To benchmark our ability to map the surface photovoltage at the molecular level, ultra-thin type II heterojunctions (from a few monolayers down to the monolayer) will be tailored by sequential and/or co-evaporation of donor (D) and acceptor (A) reference molecules, selected for their ability to dissociate the excitons into charge transfer states (and subsequently into delocalized photocarriers). The second category of molecular assemblies will be tailored from molecules based on covalently bonded donor and acceptor sub-units. These molecules will be selected to achieve an excitonic effect resulting from an intermolecular dipolar coupling under illumination. Last, to achieve a sub-molecular resolution, the outputs of room temperature DHe-KPFM will be combined with spectroscopic SPV imaging (contact potential shift map recalculated from a 4D matrix of frequency shift-bias curves) experiments performed with an active atomic tracking in an ultra-highly stable cryogenic setup. Can D-A heterojunctions still act as efficient interfaces for exciton dissociation into charge transfer states and delocalized carriers in the limit of molecular monolayers? Is it possible to detect a shift in the electrostatic potential that would result from the molecular polarization itself (i.e. the exciton generation) under illumination? Ultimately, is it possible to achieve an intra-molecular imaging of photo-excited states? These are the key questions we will answer with PESoS.
Our teams are very well placed to achieve a significant breakthrough in the field; with PESoS we combine a unique expertise in advanced nc-AFM/KPFM techniques applied to photoactive organic materials (SyMMES) and in high-resolution nc-AFM investigations of molecular assemblies on reference surfaces (IM2NP).