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Dipartimento di Fisica - Politecnico di Milano

Study of the structural properties of molecular thin films using X-ray adsorption spectroscopy

The proposed thesis activity relates to the characterization of molecular thin films by looking at the fine structure of X-ray adsorption spectra close to the adsorption edges (NEXAFS - Near Edge X-ray Adsorption Spectroscopy) with linearly polarized light. The NEXAFS technique allows to determine the orientation in space of molecular bonds. When applied to complex molecules such as porphyrins or phthalocyanines, containing aromatic units such as pyrrole or benzene rings, it allows to estimate their tilt with respect to the supporting substrate, and consequently to determine the molecular orientation and possible distortions occurring as a result of the adsorption process. Such molecules, characterized by the presence of a metal ion determining their optical and electronic properties, are also present in nature in numerous colored compounds (e.g. chlorophyll, hemoglobin) and find application in devices such as solar cells and gas sensors. With the aim of realizing organic-inorganic hybrid electronic devices with those molecules, the research is focusing on the study of assembled systems characterized by a high degree of structural and electronic order, such as thin and ultra-thin films.

The research, of a fundamental nature, will initially envisage the optimization of preparation procedures for the crystalline substrate and for the growth of molecular films. The growth will take place in a controlled high vacuum environment and exploit organic molecular beam epitaxy (OMBE) in order to allow for the growth of ordered films of variable thickness (from single molecular layers to multilayers). The crystalline structure of the molecular films will be determined using low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) techniques. NEXAFS is characterized by an excellent chemical selectivity and is generally sensitive to the valence electronic structure. Therefore, it will be additionally used to provide complementary information to what is obtained by means of X-ray and UV photoemission spectroscopy techniques (XPS and UPS, respectively), as well as by inverse photoemission spectroscopy (IPES). For further information, applicants are strongly suggested to read the enclosed bibliography, which presents two studies carried out by the proponents of this activity in recent years.

NEXAFS requires a tunable X-ray source, available at large Italian or foreign synchrotron facilities for time periods generally limited to a few days. Most of the thesis, preparatory for the NEXAFS experiment, will take place instead at the "VESI" laboratory of the physics department. Familiarity with the basics of surface physics (provided by the master's degree courses in physical engineering - nanotechnologies and physical technologies branch) is desirable. The thesis requires a full-time activity in the lab (Monday to Friday) and will last for a minimum of six months.

 

Bibliography:

A. Calloni, M.S. Jagadeesh, G. Bussetti, G. Fratesi, S. Achilli, A. Picone, A. Lodesani, A. Brambilla, C. Goletti, F. Ciccacci, L. Duo, M. Finazzi, A. Goldoni, A. Verdini and L. Floreano “Cobalt atoms drive the anchoring of Co-TPP molecules to the oxygen-passivated Fe(001) surface” Appl. Surf. Sci. 505 (2020) 144213 (DOI: 10.1016/j.apsusc.2019.144213)

A. Picone, D. Giannotti, A. Brambilla, G. Bussetti, A. Calloni, R. Yivlialin, M. Finazzi, L. Duò, F. Ciccacci, A. Goldoni, A. Verdini and L. Floreano  “Local structure and morphological evolution of ZnTPP molecules grown on Fe(001)-p(1 × 1)O studied by STM and NEXAFS” Appl. Surf. Sci. 435 (2018) 841–847 (DOI: 10.1016/j.apsusc.2017.11.128)



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