Louis Giraudet

Synthesis and characterization of 1,7-disubstituted and 1,6,7,12-tetrasubstituted perylenetetracarboxy-3,4:9,10-diimide derivatives

A variety of perylenetetracarboxy-3,4:9,10-diimide derivatives have been synthesized. Particular attention was paid to substituents in positions 1, 6, 7 or 12. The energy differences between the frontier orbitals have been determined using optical spectroscopy (UV and fluorescence). The energy of the lowest unoccupied orbitals (LUMOs) were obtained by cyclic voltammetry. From both studies, the energies of the highest occupied orbitals (HOMOs) were also been calculated. A Hammett-type relationship was observed for the reduction potentials (Ered11/2) when correlated with the σ−ortho parameter. The energies of the frontier orbitals define the domains of application of these compounds. They significantly depend on the substitution in positions 1, 6, 7, or 12.

Characterizations of Ohmic and Schottky-behaving contacts of a single ZnO nanowire

Current-voltage and Kelvin probe force microscopy (KPFM) measurements were performed on single ZnO nanowires. Measurements are shown to be strongly correlated with the contact behavior, either Ohmic or diode-like. The ZnO nanowires were obtained by metallo-organic chemical vapor deposition (MOCVD) and contacted using electronic-beam lithography. Depending on the contact geometry, good quality Ohmic contacts (linear I-V behavior) or non-linear (diode-like) contacts were obtained. Current-voltage and KPFM measurements on both types of contacted ZnO nanowires were performed in order to investigate their behavior. A clear correlation could be established between the I-V curve, the electrical potential profile along the device and the nanowire geometry. Some arguments supporting this behavior are given based on technological issues and on depletion region extension. This work will help to better understand the electrical behavior of Ohmic contacts on single ZnO nanowires, for future applications in nanoscale field-effect transistors and nano-photodetectors.

Tailoring the microstructure and charge transport in conjugated polymers by alkyl side-chain engineering

Charge transport in conjugated polymers is critical to most optoelectronic devices and depends strongly on the polymer structure and conformation in the solid state. Understanding the correlations between charge carrier mobility, energy disorder and molecular assembly is therefore essential to improve device performances. Alkyl side-chains contribute to intermolecular interactions and are key to controlling the polymer microstructure and electronic properties. Investigating a set of polymers with common conjugated units but different side-chain functionalization provides new insights into the complex structure–transport relationship. Here, field-effect transistors and space-charge-limited current devices are used together with in situ grazing-incidence wide-angle X-ray scattering to study charge transport and morphology in a series of donor–acceptor copolymers. Probing hole mobility as a function of carrier density and orientation permits us to assess energy disorder and hopping rate anisotropy, while X-ray diffraction allows us to link transport properties to the polymer microstructure. We show that branched side-chains enhance structural and energy disorder and lead to isotropic transport, whereas linear chains induce either a common lamellar structure or a more exceptional pseudo-hexagonal columnar phase with a helicoidal polymer conformation. The latter enhances out-of-plane mobility but increases energy disorder possibly due to larger interring torsion angles.

Design and optimization of a 1.3/1.55-/spl mu/m wavelength selective p-i-n photodiode based on multimode diluted waveguide

This letter demonstrates a new solution to achieve low-cost, highly integrated, and reliable InP-based p-i-n photodiodes for wavelength-division-multiplexing optical networks, absorbing 1.55-/spl mu/m wavelength and transparent to 1.3 /spl mu/m. It is based on a multimode diluted waveguide (MDW) combined with an evanescently coupled photodiode. The overall structure is optimized using a genetic algorithm linked to a beam propagation method software. The computed responsivity of the MDW photodiode is 0.86 A/W at 1.55 /spl mu/m, and the 1.3/1.55-/spl mu/m optical crosstalk is better than -20 dB (-40-dB electrical). The proposed device is designed for hybridization on silicon platform for low-cost modules, with potential application up to 10 Gb/s.

High voltage surface potential measurements in ambient conditions: Application to organic thin-film transistor injection and transport characterization

A scanning surface potential measurement technique suited for thin-film devices operating under high voltages is reported. A commercial atomic force microscope has been customized to enable a feedback-controlled and secure surface potential measurement based on phase-shift detection under ambient conditions. Measurements of the local potential profile along the channel of bottom-gate organic thin-film transistors (TFTs) are shown to be useful to disentangle the contributions from the channel and contacts to the device performance. Intrinsic contact current-voltage characteristics have been measured on bottom-gate, top-contact (staggered) TFTs based on the small-molecule semiconductor dinaphtho[2,3-b:2′,3-f]thieno[3,2-b]thiophene (DNTT) and on bottom-gate, bottom-contact (coplanar) TFTs based on the semiconducting polymer polytriarylamine (PTAA). Injection has been found to be linear in the staggered DNTT TFTs and nonlinear in the coplanar PTAA TFTs. In both types of TFT, the injection efficiency has been ...

Nanoscale optical and electrical characterizations of ZnO nanostructures by near-field microscopy

The interest in the recent years for nanostructure studies has led to the development of a wide palette of characterization techniques such as the electrical modes in scanning probe microscopy (STM, EFM, KPFM...). Optical characterization at nanoscale remains nevertheless a challenge especially for wide gap semiconductors where high energy is required. In this presentation, we will present our work focusing in the development and the improvement of near-field microscopy techniques to investigate nanoscale properties of ZnO nanostructures and related semiconducting objects. For the optical characterization, cathodoluminescence (CL) studies present many advantages over the classical photoluminescence experiments for ZnO analysis. This contribution presents the development of a scanning near-field cathodoluminescence microscope where a bimorph piezoelectric cantilever is simultaneously used for both actuation and oscillation amplitude detection. Operated inside a scanning electron microscope (SEM) it offers the possibility of performing simultaneous topography and cathodoluminescence charting of the sample surface additionally to the SEM imaging with a resolution in the order of several tenths of nanometers. Different measurements of ZnO nanostructures and related objects will be presented to show the potentiality of our optical characterization setup. Complementary STEM-CL measurements at higher beam energy were performed on the ZnO nanowires confirming the good quality of the investigated nanostructures. As for the electrical characterization, we will focus on the local surface potential mapping of ZnO nanowires used for photoconduction using Kelvin Probe Force Microscopy. While ZnO nanowire photoconduction gains as high as 1010 in the UV region were reported, several issues come into play when it comes to making a precise measurement of a single nanowire. An important issue is the good quality of the injecting contacts on the nanowire and the reproducibility of its characteristics which can be made using KPFM.

Development of an improved Kelvin probe force microscope for accurate local potential measurements on biased electronic devices

An improved setup for accurate near‐field surface potential measurements and characterisation of biased electronic devices using the Kelvin Probe method has been developed. Using an external voltage source synchronised with the raster‐scan of the KPFM‐AM, this setup allows to avoid potential measurement errors of the conventional Kelvin Probe Force Microscopy in the case of in situ measurements on biased electronic devices. This improved KPFM‐AM setup has been tested on silicon‐based devices and organic semiconductor‐based devices such as organic field effect transistors (OFETs), showing differences up to 25% compared to the standard KPFM‐AM lift‐mode measurement method.