Interaction of ultra-thin CoTPP films on Fe(001) with oxygen: Interplay between chemistry, order, and magnetism

Abstract

This work focuses on the stability against the oxidation of the chemical, structural, and magnetic properties of the system consisting of a layer of Co tetra-phenyl porphyrins grown on the top of Fe(001) – p(1 × 1)O. Such a system is characterized by a very high degree of structural order and the existence of magnetic coupling between the molecules and the substrate, even at room temperature, as we recently reported [Jagadeesh et al., Appl. Phys. Lett. 115, 082404 (2019)]. We highlight, by using x-ray photoemission spectroscopy, the effect of porphyrins in screening the substrate from oxidation. The coupling between the magnetic response of the system and the order of the molecular layer is investigated by means of spin-resolved UV photoemission spectroscopy and low-energy electron diffraction, respectively. As a result, a link is eventually found between this response and the chemical and structural stability of the interface.

An In-Depth Assessment of the Electronic and Magnetic Properties of a Highly Ordered Hybrid Interface: The Case of Nickel Tetra-Phenyl-Porphyrins on Fe(001)–p(1 × 1)O

“…The molecular arrangement is compatible with a (5 × 5) commensurate pattern, as already reported for ZnTPP [32]. The LEED pattern suggests the presence of two domains, rotated by 37° (R 37), similarly to the pattern assumed by CoTPP on Fe-p(1 × 1)O [11,26]. This symmetry is mostly evident at 1 ML coverage.…”

“…Furthermore, Figure 4 c indicates no variations in the position of the Ni 2 p peak and this denotes that the oxidation state of this ion is not modified. These results convey the information that molecular oxygen, at this exposure, does not stick to the molecules and cannot penetrate the molecular layer to further oxidize the substrate underneath, thus suggesting a protecting effect of the organic layer [ 11 ].…”

“…The preparation and the analysis of the sample were performed in situ, in ultra-high vacuum (UHV) conditions with pressure in the high 10 −11 Torr regime [ 34 ]. The surface of a thick Fe film (thickness ≈ 500 nm) grown on a MgO(100) substrate was cleaned by a series of cycles of Ar + sputtering followed by low-temperature (480 °C) annealing [ 11 ]. To obtain a clean Fe– p (1 × 1)O termination, the sample was exposed to 30 L of oxygen (with 1 L = 10 −6 Torr·s) and then to a higher temperature annealing (700 °C) to remove the excess of oxygen.…”

“…IPES spectra were measured in the isochromatic mode by varying the energy of the electrons impinging on the sample and selecting the outgoing photons with a band pass filter centered at 9.6 eV. The total full width at half-maximum (FWHM) resolution of the apparatus is 0.9 eV for XPS, 0.05 eV for UPS, 0.15 eV for SP-UPS [ 11 ], 0.6 eV for IPES [ 38 , 39 ].…”

“…A typical goal in organic electronics consists in preserving some of the features of the free standing molecules, such as the energetic positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [ 9 , 10 ], when the molecules are deposited onto a substrate. Furthermore, in view of the potential application of these molecules in a device, an ordered, directly accessible layer of molecules, with a certain resistance against gaseous contaminants [ 11 ], would be equally important. A further essential feature that molecules grown on top of crystalline substrates must possess for spintronic applications is that they should carry a magnetic moment [ 12 , 13 ].…”