Estimation of π-π Electronic Couplings from Current Measurements.

The π-π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π-π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π-π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.

On the mechanical and electronic properties of thiolated gold nanocrystals.

We present a quantitative exploration, combining experiment and simulation, of the mechanical and electronic properties, as well as the modifications induced by an alkylthiolated coating, at the single nanoparticle (NP) level. We determined the response of the NPs to external pressure in a controlled manner using an atomic force microscope tip. We found a strong reduction in their Young's modulus, as compared to bulk gold, and a significant influence of strain on the electronic properties of the alkylthiolated NPs. Electron transport measurements of tiny molecular junctions (NP/alkylthiol/CAFM tip) show that the effective tunnelling barrier through the adsorbed monolayer strongly decreases by increasing the applied load, which translates in a remarkable and unprecedented increase in the tunnel current. These observations are successfully explained using simulations based on the finite element analysis (FEA) and first-principles calculations that permit one to consider the coupling between the mechanical response of the system and the electric dipole variations at the interface.

Application of nano-EBIC to the characterization of GaAs and InP homojunctions.

High resolution electron-beam-induced current (EBIC) analyses were carried out on InP and GaAs substrates with a home-made atomic force microscope (AFM) combined with a scanning electron microscope (SEM). With this scanning nano-EBIC microscope, a sample can be conventionally imaged by SEM, its local topography investigated by AFM and nano-EBIC image simultaneously obtained. In this study, we report the utilization of nano-EBIC microscopy for imaging and characterizing GaAs and InP homojunctions. I-V characteristic measurements allow understanding of the electrical behavior of the AFM tip-sample contact. The electron probe intensity must be larger than about 100 pA to be able to generate an induced current because of the surface states which act as non-radiative recombination centers. The minority carrier diffusion length of InP and GaAs is measured and compared for different electron probe currents and it is shown that the measurements are not perturbed by photon recycling, i.e. the self-absorption of photons that gives rise to an extra generation of electron-hole pairs.

Influence of electron irradiation on the electronic transport mechanisms during the conductive AFM imaging of InAs/GaAs quantum dots capped with a thin GaAs layer.

We have used conductive atomic force microscopy (C-AFM) to study the electronic transport mechanisms through InAs quantum dots (QDs) grown by molecular beam epitaxy on an n-type GaAs(001) substrate and covered with a 5 nm thick GaAs cap layer. The study is performed with a conductive atomic force microscope working inside a scanning electron microscope. Electric images can be obtained only if the sample is preliminarily irradiated with an electron probe current sufficiently high to generate strong electron beam induced current. In these conditions holes are trapped in QDs and surface states, so allowing the release of the Fermi level pinning and thus conduction through the sample. The electronic transport mechanism depends on the type of AFM probe used; it is explained for a metal (Co/Cr) coated probe and p-doped diamond coated probe with the aid of energy band diagrams. The writing (charge trapping) and erasing (untrapping) phenomena is conditioned by the magnitude of the electron probe current. A strong memory effect is evidenced for the sample studied.

High-resolution scanning near-field EBIC microscopy: application to the characterisation of a shallow ion implanted p+-n silicon junction.

High-resolution electron beam induced current (EBIC) analyses were carried out on a shallow ion implanted p+-n silicon junction in a scanning electron microscope (SEM) and a scanning probe microscope (SPM) hybrid system. With this scanning near-field EBIC microscope, a sample can be conventionally imaged by SEM, its local topography investigated by SPM and high-resolution EBIC image simultaneously obtained. It is shown that the EBIC imaging capabilities of this combined instrument allows the study of p-n junctions with a resolution of about 20 nm.