Pascal Chrétien

Conducting probe atomic force microscopy applied to organic conducting blends

Atomic force microscopy (AFM) is used in contact mode with a conducting tip to probe the conducting network of the conductive polymer polyaniline blended in an insulating polymer matrix. The high resistance contrast and sharp boundaries between conductive and insulating phases is observed down to scales in the 10 nm range. The very low scale electric dispersion corresponds to the morphologic phase segregation known from conventional AFM or transmission electron microscopy measurements, which is responsible for the ultralow electrical percolation threshold previously demonstrated in this system.

From single III-nitride nanowires to piezoelectric generators: New route for powering nomad electronics

Ambient energy harvesting using piezoelectric nanomaterials is today considered as a promising way to supply microelectronic devices. Since the first demonstration of electrical energy generation from piezoelectric semiconductor nanowires in 2006, the piezoelectric response of 1D-nanostructures and the development of nanowire-based piezogenerators have become a hot topic in nanoscience. After several years of intense research on ZnO nanowires, III-nitride nanomaterials have started to be explored thanks to their high piezoelectric coefficients and their strong piezo-generation response. This review describes the present status of the field of piezoelectric energy generation with nitride nanowires. After presenting the main motivations and a general overview of the domain, a short description of the main properties of III-nitride nanomaterials is given. Then we review the piezoelectric responses of III-N nanowires and the specificities of the piezo-generation mechanism in these nanostructures. Finally, the design and performance of the macroscopic piezogenerators based on nitride nanowire arrays are described, showing the promise of III-nitride nanowires for ultra-compact and efficient piezoelectric generators.

Impact of the GaN nanowire polarity on energy harvesting

We investigate the piezoelectric generation properties of GaN nanowires (NWs) by atomic force microscopy equipped with a Resiscope module for electrical measurements. By correlating the topography profile of the NWs with the recorded voltage peaks generated by these nanostructures in response to their deformation, we demonstrate the influence of their polarity on the rectifying behavior of the Schottky diode formed between the NWs and the electrode of measurement. These results establish that the piezo-generation mechanism crucially depends on the structural characteristics of the NWs.

Copper sample analyzed with an n-doped silicon tip using conducting probe atomic force microscopy

A study of the electrical properties of the nanocontact between a conducting tip made of n-doped silicon, and the surface of a cleaned copper sample has been carried out. Current–voltage (I–V) curves have been performed in order to clarify the electrical nature of the nanocontact. Apart from the breakdown phenomenon and the noticeable laser diode effect on the I–V curves, the nanocontact can be modeled by a Schottky diode, in series with a resistance of a few kΩ, which probably reflects the resistance of the semiconducting probe. This fact is partially confirmed by a simple calculation of the resistance of the tip/cantilever system using a rough model. Topographical and electrical images have also been recorded for a given tip/sample bias. For a positive sample–tip voltage, the copper surface appears uniformly conducting over 20×20 μm2, and remains stable during the whole period of study (three weeks). This study thus allows one to have a better understanding of the effects which must be kept in mind when...

Specific methodology for capacitance imaging by atomic force microscopy: A breakthrough towards an elimination of parasitic effects

On the basis of a home-made nanoscale impedance measurement device associated with a commercial atomic force microscope, a specific operating process is proposed in order to improve absolute (in sense of “nonrelative”) capacitance imaging by drastically reducing the parasitic effects due to stray capacitance, surface topography, and sample tilt. The method, combining a two-pass image acquisition with the exploitation of approach curves, has been validated on sets of calibration samples consisting in square parallel plate capacitors for which theoretical capacitance values were numerically calculated.

First observations of YBaCuO thin films by atomic force microscopy with conducting tips

YBaCuO thin film surfaces have been studied with an original laboratory-made attachment associated with a commercial atomic force microscope. Using a doped silicon probe coated with doped diamond, we have obtained simultaneously topographical and local contact resistance surface images within a given area of the sample. YBaCuO films on various types of substrates were observed: polycrystalline yttria-doped zirconia (PYSZ), and MgO or SrTiO3 single crystals. For YBaCuO films grown on PYSZ, the electrical image has clearly revealed the presence of electrical disconnection zones between grains, which correspond to grain boundary areas observed on the topographical image. The presence of such defects can explain the modest critical current density (Jc approximately equals 3 X 104 A/cm2 at 77 K) measured on these granular films. On the opposite, for films grown on single-crystal substrates, the electrically connected areas between grains are visible on the electrical images, that can be correlated to better electrical transport properties of the films. Moreover, island-shaped grains exhibiting terraces of one unit cell vertical height could be seen, on close inspection of films grown on PYSZ and MgO substrates. For YBaCuO elaborated on SrTiO3 substrates, the observed grain structure rather exhibited a spiral shape.

High Piezoelectric Conversion Properties of Axial InGaN/GaN Nanowires

We demonstrate for the first time the efficient mechanical-electrical conversion properties of InGaN/GaN nanowires (NWs). Using an atomic force microscope equipped with a modified Resiscope module, we analyse the piezoelectric energy generation of GaN NWs and demonstrate an important enhancement when integrating in their volume a thick In-rich InGaN insertion. The piezoelectric response of InGaN/GaN NWs can be tuned as a function of the InGaN insertion thickness and position in the NW volume. The energy harvesting is favoured by the presence of a PtSi/GaN Schottky diode which allows to efficiently collect the piezo-charges generated by InGaN/GaN NWs. Average output voltages up to 330 ± 70 mV and a maximum value of 470 mV per NW has been measured for nanostructures integrating 70 nm-thick InGaN insertion capped with a thin GaN top layer. This latter value establishes an increase of about 35% of the piezo-conversion capacity in comparison with binary p-doped GaN NWs. Based on the measured output signals, we estimate that one layer of dense InGaN/GaN-based NW can generate a maximum output power density of about 3.3 W/cm2. These results settle the new state-of-the-art for piezo-generation from GaN-based NWs and offer a promising perspective for extending the performances of the piezoelectric sources.

Nitride Nanowires: From Rigid to Flexible Piezo-generators

Here we employ self-assembled Mg-doped GaN nanowires(NWs) grown by plasma assisted molecular beam epitaxy on Si(111) substrates to fabricate piezogenerators. We will first discuss the fabrication and testing of rigid nanowire-based generators and then a flexible generator prototype will be shown.