Transport and Phototransport in ITO Nanocrystals with Short to Long-Wave Infrared Absorption

Nanocrystals are often described as an interesting strategy for the design of low-cost optoelectronic devices especially in the infrared range. However the driving materials reaching infrared absorption are generally heavy metalcontaining (Pb and Hg) with a high toxicity. An alternative strategy to achieve infrared transition is the use of doped semiconductors presenting intraband or plasmonic transition in the short, mid and long-wave infrared. This strategy may offer more flexibility regarding the range of possible candidate materials. In particular, significant progresses have been achieved for the synthesis of doped oxides and for the control of their doping magnitude. Among them, tin doped indium oxide (ITO) is the one providing the broadest spectral tunability. Here we test the potential of such ITO nanoparticles for photoconduction in the infrared. We demonstrate that In2O3 nanoparticles presents an intraband absorption in the mid infrared range which is transformed into a plasmonic feature as doping is introduced. We have determined the cross section associated with the plasmonic transition to be in the 1-3x10-13 cm2 range. We have observed that the nanocrystals can be made conductive and photoconductive due to a ligand exchange using a short carboxylic acid, leading to a dark conduction with n-type character. We bring further evidence that the observed photoresponse in the infrared is the result of a bolometric effect

Adsorption de molécules chirales sur catalyseurs hétérogènes supportés sur oxyde : une approche modèle

Enantioselective heterogeneous catalysis is a method of choice for the synthesis of enantiopure chiral products. One current approach involves the modification of a metal surface by a chiral modifier. Despite its great potential, only a small number of successful systems have been developed so far. Most of fundamental works have been devoted to model systems based on single crystal metal surfaces while the role of the oxide support in supported metal catalysts have usually been overlooked. To date, fundamental questions remain on the role of the oxide support on the chiral induction. A rational design of the catalyst requires therefore a molecular scale description of the interactions between the oxide support, the metal nanoparticles and the chiral modifier. In this context, this study aims at understanding the interactions between these three partners through a surface science approach. To mimic the catalytic system, rutile TiO2(110) single crystals, Tartaric Acid molecules (TA) and Ni nanoparticles have been selected. The chemical nature of TA is explored on TiO2(110) by X-ray and Ultraviolet Photoemission Spectroscopy (XPS/UPS) and High Resolution Electron Energy Loss Spectroscopy. Scanning Tunnelling Microscopy (STM) and Low-Energy Electron Diffraction are employed to study the TA layer structure and anchoring points. The molecular decomposition behaviour is studied by Thermal Programmed Desorption (TPD). In parallel, XPS, STM and Surface Differential Reflectivity Spectroscopy are used to probe the growth of Ni NPs on TiO2 at increasing Ni coverage. Finally, perspectives on the TA/Ni/TiO2 system are put forward mainly by XPS and TPD.La catalyse asymétrique hétérogène est une méthode de choix pour la synthèse de composés chiraux énantiopurs. Une approche courante implique la modification d'une surface métallique par un inducteur chiral. Malgré son potentiel, seul un petit nombre de systèmes ont été mis au point jusqu'à présent avec succès. De plus, si l’interaction de cette molécule asymétrique avec des surfaces métalliques monocristallines est maintenant bien comprise, le rôle du support oxyde dans des catalyseurs à base de nanoparticules métalliques supportées demeure encore peu étudié. La conception raisonnée du catalyseur repose sur la maitrise des interactions à l’échelle moléculaire entre l’oxyde, les nanoparticules métalliques et l’inducteur chiral. Dans ce contexte, cette étude vise à comprendre les interactions entre ces trois partenaires grâce à une approche de type science des surfaces. Pour représenter le système catalytique, des monocristaux de rutile TiO2(110), l'acide tartrique (AT) et des nanoparticules de nickel ont été sélectionnés. La nature chimique de l’AT sur TiO2(110) est étudiée par Photoémission X (XPS) et UV et Spectroscopie de Perte d'Énergie d'Électrons Lents à Haute Résolution. La structure de la couche moléculaire et ses points d’ancrage sont étudiés par Microscopie à Effet Tunnel (STM) et Diffraction d’Électrons Lents. Le comportement de décomposition de l’AT est obtenu par désorption thermique (TPD). Les techniques XPS, STM et la Réflectivité Différentielle de Surface sont utilisées pour sonder la croissance du Ni sur le TiO2 lorsque la couverture en Ni augmente. Enfin, des perspectives sur le système AT / Ni / TiO2 sont proposées principalement par XPS et TPD

Chemical nature and thermal decomposition behavior of tartaric acid multilayers on rutile TiO 2 (110)

International audienceR,R-tartaric acid (RR-TA) thermal stability and decomposition on the rutile TiO2(110) surface was investigated by temperature programmed desorption. The authors show that a majority of RR-TA molecules are desorbed intact from multilayers at around 340 K, while they decompose from the first chemisorbed layer between 460 and 480 K. Complementary information on the chemical nature of RR-TA in the multilayer regime was gained by x-ray photoelectron spectroscopy, which shows that biacid molecules form the multilayer while they are monotartrate at the interface

Photoemission Fingerprints for Structural Identification of Titanium Dioxide Surfaces

The wealth of properties of titanium dioxide relies on its various polymorphs and on their mixtures coupled with a sensitivity to crystallographic orientations. It is therefore pivotal to set out methods that allow surface structural identification. We demonstrate herein the ability of photoemission spectroscopy to provide Ti LMV (V = valence) Auger templates to quantitatively analyze TiO₂ polymorphs. The Ti LMV decay reflects Ti 4sp-O 2p hybridizations that are intrinsic properties of TiO₂ phases and orientations. Ti LMV templates collected on rutile (110), anatase (101), and (100) single crystals allow for the quantitative analysis of mixed nanosized powders, which bridges the gap between surfaces of reference and complex materials. As a test bed, the anatase/rutile P25 is studied both as received and during the anatase-to-rutile transformation upon annealing. The agreement with X-ray diffraction measurements proves the reliability of the Auger analysis and highlights its ability to detect surface orientations