Localization and delocalization of charges injected in DNA

The electrical properties of DNA molecules are investigated by charge injection and electric force microscopy experiments. Prior to injection, DNA molecules exhibit a weak positively charged state. We probe the electrical behaviour of DNA by measuring the localized or delocalized character of the DNA charge states upon injection of excess charges. We show that injected charges do not delocalize for overstretched DNA prepared by a receding meniscus technique, while the adjunction of spermidine during the deposition leads to relaxed DNA molecules exhibiting a charge delocalization over microns. The interplay between charge localization/delocalization and deposition techniques may explain that transport behaviors ranging from insulating to conductive have been reported for DNA deposited on surfaces.Comment: To be published in Appl. Phys. Let

Voltage transient analysis as a generic tool for solar junction characterization

International audienc

Doped colloidal InAs nanocrystals in the single ionized dopant limit

We investigate the electronic properties of individual n-type (Cu) doped and p-type (Ag) doped InAs colloidal nanocrystals (NCs) in the 2–8 nm size range from their charge transfers toward a highly oriented pyrolytic graphite (HOPG) substrate, using ultrahigh vacuum Kelvin probe force microscopy (KPFM) with elementary charge sensitivity at 300 K. The NC active dopant concentration is measured as ND = 8 × 1020 cm–3 and NA > 5 × 1020 cm–3 for n- and p-type doping, respectively. The electrostatic equilibrium between the NC and the HOPG reference substrate is investigated and reveals an enhancement of the Fermi-level mismatch between the NCs and the HOPG substrate at reduced NC sizes, both for n- and p-type doping. It also shows, for n-type doped NCs with smallest sizes (∼2 nm), the existence of a full depletion regime, in which smallest NCs contain single ionized dopants. Results are compared with self-consistent tight-binding calculations of the electronic structure of InAs NCs, including hydrogenoid impurities and the presence of a host substrate, in the case of n-type doped NCs. The observed enhancement of the NC–HOPG Fermi-level mismatch can be understood by considering a size-dependent electrostatic contribution attributed to dipolar effects at the NC–ligand interface. The estimated surface dipole density equals a few Debye/nm2 and is increased at smallest NC sizes, which follows the enhancement of ligand densities at small NC sizes previously reported for metallic or semiconducting NCs. The results put forward the role played by the NC–ligand interface electrostatics for electronic applications

Étude par C-AFM de substrats magnétiques fonctionnalisés par des SAMs photo-commutables

International audienceLa spintronique moléculaire utilise des systèmes contenant des molécules et exploite le transport d'électrons polarisé en spin (phénomènes de magnétorésistance). Une approche prometteuse à l'intégration des molécules est la formation en solution de monocouches auto-assemblées, (Self-Assembled Monolayers, SAMs) sur un substrat ferromagnétique (FM) servant d'électrode dans des jonctions FM/SAM/FM. Jusqu'à présent, les SAMs intégrées dans de tels dispositifs jouent le rôle de simple barrière tunnel [1]. Nous avons étudié ici des molécules photo-commutables offrant deux états de conductance électrique par éclairement associés à un changement de conformation afin d'explorer les relations entre conformation moléculaire, couplages molécule/électrodes et transport polarisé en spin, comme proposé théoriquement [2]. Nous avons greffé des SAMs de dérivé de diaryléthène (DDA) (Figure 1, [A]) sur manganite de lanthane dopé au strontium (LSMO), en collaboration avec CNRS/Thales, et de dérivé d'azobenzène (AzBT) (Figure 1, [B]) sur cobalt. Dans les deux cas, la molécule possède une transition réversible entre deux isomères par éclairement UV ou visible. La caractérisation électrique a été réalisée par Conducting Atomic Force Microscope (C-AFM), à l'air et sous atmosphère d'azote au moyen d'un Icon AFM (Bruker) pour LSMO-DDA et sous ultra-vide avec un VT SPM (Scienta Omicron) pour Co-AzBT afin de prévenir l'oxydation de l'électrode. Ces travaux ont montré, pour LSMO, une commutation de résistance du substrat en l'absence de SAM, d'un ratio entre 2 et 1200 (Figure 1, [C]) due à une modification de stoechiométrie du LSMO [3, 4]. Dans le cas de Co-AzBT, un effet similaire est observé d'un ratio 50 pour les substrats non-fonctionnalisés (Figure 1, [D]). Ces effets sont fortement réduits lorsque la surface est fonctionnalisée par une SAM (Figure 1, [C, D]). Les variations de résistance en fonction de l'éclairement caractérisées par spectroscopie I-V sont de l'ordre de 2,5 à ce stade de nos travaux pour DDA et AzBT. Références [1] Tatay, S.; Galbiati, M.; Delprat, S.; Barraud, C.; Bouzehouane, K.; Collin, S.; Deranlot, C.; Jacquet, E.; Seneor, P.; Mattana, R.; Petroff, F. Kim, Y.; Eliseev, E.; Morozovska, A.; Jesse, S.; Biegalski, M.; Mitchell, J.; Zheng, H.; Aarts, J.; Hwang, I.; Oh, S.; Choi, J.; Choi, T.; Park, B.; Kalinin, S.; Maksymovych, P. Nanotechnology 2014, 25, 475302 [4] Moreno, C.; Munuera, C.; Valencia, S.; Kronast, F.; Obradors, X.; Ocal,C. Nano Lett. 2010, 10, 382

Étude par C-AFM de substrats magnétiques fonctionnalisés par des SAMs photo-commutables

National audienc

Coupling of ToF-SIMS and AFM for the identification of the chemical species involved in the nucleation process of soot particles in flame combustion

International audienceSoot formation in combustion is a major research problem because of the negative impact of the carbonaceous particles in the exhausts on human health and the environment. However, state of the art knowledge on the chemical species and reaction pathways leading to the formation of soot particles is not yet complete. In particular, many questions are still open on the transformation of gaseous molecular precursors into condensed phase soot particles (soot nucleation). One of the hypotheses proposed in the literature, the nucleation driven by the dimerization of small polycyclic aromatic hydrocarbons (PAHs), has been recently used in models to successfully reproduce the soot volume fraction profiles measured in laboratory flames. However, experimental evidences on the existence of dimers in the flame environment are still lacking. In this work, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and high-resolution Atomic Force Microscopy (AFM) are coupled with the aim of characterizing the chemistry of the building blocks involved in the formation of nascent soot particles

Operation of near-field scanning millimeter-wave microscopy up to 67 GHz under scanning electron microscopy vision

International audienceA near-field scanning millimeter-wave microscope is developed with broadband capabilities up to 67 GHz. The instrumentation has been designed to operate inside a scanning electron microscope for environment control and water meniscus elimination. Scanning electron microscopy imaging of the tip / sample interaction gives the unique possibility to limit the scan area and preserve the integrity of the probe tip. In addition, hybrid imaging considering simultaneously atomic, microwave and electron microscopy tools is beneficial for further modeling to address the quantitative characterization of nanomaterials. Experimental data are exemplary shown to demonstrate the viability of the solution proposed

Near-Field Scanning Millimeter-Wave Microscope Operating Inside a Scanning Electron Microscope: Towards Quantitative Electrical Nanocharacterization

The main objectives of this work are the development of fundamental extensions to existing scanning microwave microscopy (SMM) technology to achieve quantitative complex impedance measurements at the nanoscale. We developed a SMM operating up to 67 GHz inside a scanning electron microscope, providing unique advantages to tackle issues commonly found in open-air SMMs. Operating in the millimeter-wave frequency range induces high collimation of the evanescent electrical fields in the vicinity of the probe apex, resulting in high spatial resolution and enhanced sensitivity. Operating in a vacuum allows for eliminating the water meniscus on the tip apex, which remains a critical issue to address modeling and quantitative analysis at the nanoscale. In addition, a microstrip probing structure was developed to ensure a transverse electromagnetic mode as close as possible to the tip apex, drastically reducing radiation effects and parasitic apex-to-ground capacitances with available SMM probes. As a demonstration, we describe a standard operating procedure for instrumentation configuration, measurements and data analysis. Measurement performance is exemplarily shown on a staircase microcapacitor sample at 30 GHz