La division Spectroscopies d'Electrons de la Société Française du Vide vous invite à participer à son 1er Webinaire le mercredi 13 avril 2022 à 16h pour une durée de 45 mn, sur le thème :

"Photoemission for unusual analysis: nanoscale potential mapping of biased stacks and electronic structure establishment in ionic liquids"
Présenté par 
Dr Sylvie Rangan (Rutgers University, USA)

Dr Sylvie Rangan is currently an assistant research professor in the physics department of Rutgers University, the state university of New Jersey. She obtained her PhD in 2005 from the Laboratoire Chimie-Physique Matiere Rayonnment of the Pierre and Marie Curie University, under the direction of Francois Rochet, studying the reactivity of nitriles on Si surfaces using synchrotron-based spectroscopies and scanning probe techniques.

She then moved to the physics department of Rutgers University, initially working as a postdoc on understanding energy bands alignment at high-k dielectric interfaces, using direct and inverse photoemission.

This work has expanded to organics/oxides interfaces, for which the energy alignment between organics molecular levels and oxides band edges is studied. Recently, she has contributed to the development of strategies based on molecular dipoles, to tune energy alignment and alter electron transfer properties across such interfaces. She is also developing an on-surface chemical toolbox in order to grow 2D organic materials, using on-surface self-assembly processes.

Résumé :

In this seminar, two studies using photoemission in unusual settings will be presented. The first study is dedicated to a method using small spot X-ray photoemission performed on a biased graphene/SiO2/Si structure in order to experimentally determine the potential profile across the system, including discontinuities at the interfaces. Measuring and understanding electric fields in multilayered materials at the nanoscale remains a challenging problem impeding the development of novel devices. At this scale, it is far from obvious that materials can be accurately described by their intrinsic bulk properties, and considerations of the interfaces between layered materials become unavoidable for a complete description of the system’s electronic properties. Here, a general approach to the direct measurement of nanoscale internal fields is proposed, where core levels provide a measure of the local potential and are used to reconstruct the potential profile as a function of the depth through the stack. It is found that each interface plays a critical role in establishing the potential across the dielectric, and the origin of the potential discontinuities at each interface is discussed. The second study is devoted to the electronic structure establishment in a series of cyano-ionic liquids (ILs). The valence band spectra of three cyano-ionic liquids based on 1-ethyl-3-methylimidazolium (Im2,1+) paired with thiocyanate (SCN−), dicyanamide (N(CN)2−), and tricyanomethanide (C(CN)3−) are measured using ultraviolet and x-ray photoemission spectroscopy. Experimental spectra are compared to their corresponding density of states, weighted by photoemission cross sections, calculated for clusters of ions pairs of increasing size. Thus, this study bridges single ion approaches to 3D periodical DFT studies and enables the exploration of the different aspects of electronic structure establishment in ILs. Even for a relatively small cluster size, the relative energy of cation and anions states shifts by an amount that corresponds closely to that expected from the Madelung energy of a bulk IL, and the photoemission cross section-weighted DOS spectra are in good agreement with the measured valence bands. Trends in the relative energy and ionic character of the frontier orbitals across this series of cyano-ILs are discussed.