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Dipartimento di Fisica - Politecnico di Milano

Study of the electronic properties of molecular thin films using electronic spectroscopy techniques

The thesis relates to the growth of thin films in a well-controlled environment (high vacuum regime) by means of organic molecular beam epitaxy (OMBE). This technique exploits very low growth rates allowing, if the right conditions of interaction with the substrate are met, for the growth of ordered molecular layers (variable thicknesses from fractions of a single layer to multilayers). Porphyrin or phthalocyanine molecules, i.e. aromatic molecules featuring a macrocycle where it is possible to place a metal ion, thus modifying their optical and electronic properties, will be preferentially used. These molecules, also present in nature in numerous colored compounds (e.g. chlorophyll, hemoglobin), are used in the field of organic-inorganic hybrid electronics for the construction of solar cells (e.g. in dye sensitized solar cells) and gas sensors. With the aim of creating electronic devices based on these molecules, research in this sector has focused on the study of systems, such as ultra-thin films, which have a high crystallographic order and electronic properties controlled on a nanometric scale. By appropriately choosing the nature of the central ion, these molecules can also assume a characteristic magnetic behavior and, in principle, be used in spintronic devices such as "bits" for storing information.

The proposed research, of a fundamental nature, will initially focus on the optimization of growth procedures, including the choice and preparation of an adequate substrate, which should preserve the chemical structure of the molecules and promote their ordered arrangement (a procedure that might involve the chemical passivation of the surface). Subsequently, the presence of a possible supra-molecular order and the morphology of the molecular layer (tilt of the macrocycle with respect to the surface, growth of a "wetting" layer vs. formation of molecular clusters) will be studied. Finally, we will proceed with the determination of the electronic structure using UV (filled states) and inverse (empty states) photoemission techniques, with the aim of determining the energy position of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and their alignment with the substrate bandstructure. If magnetic molecules are used, in combination with a ferromagnetic substrate, the electronic structure investigation will make use of spin-resolved spectroscopy. 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.

The instrumentation necessary to complete the thesis work is present 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:

G. Bussetti, A. Calloni, M. Celeri, R. Yivlialin, M. Finazzi, F. Bottegoni, L. Duò and F. Ciccacci, “Structure and electronic properties of Zn-tetra-phenyl-porphyrin single- and multi-layers films grown on Fe(001)-p(1 × 1)O” Appl. Surf. Sci. 390 (2016) 856–862 (DOI: 10.1016/j.apsusc.2016.08.137)

M. S. Jagadeesh, A. Calloni, A. Brambilla, A. Picone, A. Lodesani, L. Duò, F. Ciccacci, M. Finazzi and G. Bussetti “Room temperature magnetism of ordered porphyrin layers on Fe” Appl. Phys. Lett. 115 (2019) 082404 (DOI: 10.1063/1.5109750)



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