Kacem Smaali

Conductance Statistics from a Large Array of Sub-10 nm Molecular Junctions

Devices made of few molecules constitute the miniaturization limit that both inorganic and organic-based electronics aspire to reach. However, integration of millions of molecular junctions with less than 100 molecules each has been a long technological challenge requiring well controlled nanometric electrodes. Here we report molecular junctions fabricated on a large array of sub-10 nm single crystal Au nanodots electrodes, a new approach that allows us to measure the conductance of up to a million of junctions in a single conducting atomic force microscope (C-AFM) image. We observe two peaks of conductance for alkylthiol molecules. Tunneling decay constant (β) for alkanethiols, is in the same range as previous studies. Energy position of molecular orbitals, obtained by transient voltage spectroscopy, varies from peak to peak, in correlation with conductance values.

Oligothiophene-derivatized azobenzene as immobilized photoswitchable conjugated systems

Immobilization of an azobenzene-bithiophene compound on a gold surface leads to self-assembled monolayers with photoswitchable electrical properties.

Large Array of Sub‐10‐nm Single‐Grain Au Nanodots for use in Nanotechnology

A uniform array of single-grain Au nanodots, as small as 5-8 nm, can be formed on silicon using e-beam lithography. The as-fabricated nanodots are amorphous, and thermal annealing converts them to pure Au single crystals covered with a thin SiO(2) layer. These findings are based on physical measurements, such as atomic force microscopy (AFM), atomic-resolution scanning transmission electron microscopy, and chemical techniques using energy dispersive X-ray spectroscopy. A self-assembled organic monolayer is grafted on the nanodots and characterized chemically with nanometric lateral resolution. The extended uniform array of nanodots is used as a new test-bed for molecular electronic devices.

Initial stage of the overgrowth of InP on InAs∕InP(001) quantum dots: Formation of InP terraces driven by preferential nucleation on quantum dot edges

This letter reports on the growth mechanism of the InP cap layer over InAs∕InP quantum dots (QDs) fabricated by metal organic vapor phase epitaxy (MOVPE). QD edges are shown to act as preferential nucleation sites for the InP cap layer, leading to the formation of InP domains around the nanostructures. As∕P exchange reactions are at the origin of the planarization of the top of the QDs under P-rich ambient, thus leading to a final QD height equal to the local thickness of the InP cap layer. The possibility to use As∕P exchange reactions to homogenize the height distribution of MOVPE grown InAs∕InP QDs is discussed on the basis of these observations.

Imaging the electric properties of InAs∕InP(001) quantum dots capped with a thin InP layer by conductive atomic force microscopy: Evidence of memory effect

Conductive atomic force microscopy has been used to study the topography and the electric properties of InAs quantum dots (QDs) grown by metal organic vapor phase epitaxy on a n-type InP(001) substrate and covered with a 5nm thick InP cap layer. Images reveal that the cap layer has not entirely covered the surface, but has formed rounded terracelike structures surrounding the QDs. A high current is detected on the QDs, about ten times less on the terraces, and not detectable on the wetting layer. Charges can be trapped inside the QDs and the surrounding terraces in forward bias conditions with a temporary memory effect and discharged in reverse bias.

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.

Local electronic transport through InAs/InP(0 0 1) quantum dots capped with a thin InP layer studied by an AFM conductive probe

An AFM combined with a SEM has been used to study the topography and the local electronic transport through InAs QDs grown by metalorganic vapour phase epitaxy (MOVPE) on an n-type InP(0 0 1) substrate and covered by a 5 nm thick InP cap-layer. Images reveal that elliptic terrace-like structures have been formed around the QDs and that the height of the QDs has been decreased to that of the cap-layer. The electric current is very high on the dots, about ten times less on the terraces, and not detectable on the wetting layer. Mechanisms of electronic transport through the sample are discussed, based on current–voltage characteristics and energy band diagrams. The detection of the electron beam induced current (EBIC) with the conductive probe shows that the minority carrier diffusion length, the holes in our case, is about two times larger than that of the reference sample containing no QDs. Mechanisms of charge trapping inside the QDs and the surrounding terraces in forward bias conditions are also discussed. A temporary memory effect is evidenced.