BIGNSPIN · Bismuth and Graphene Nanostructures for Spintronics

Bismuth and graphene nanostructures for spintronics

This project was conceived with two key objectives: - study of the correlation of electronic transport properties (including magnetoresistance, Hall effect and near Fermi level spectroscopy acquired by scanning tunnelling microscope (STM)), in epitaxial bismuth (Bi) thin films supported on silicon (Si)(111) 7x7 reconstruction; - idem for graphene films. The overall objective of this proposal was the identification of suitable growth recipes in which the relevant properties for applications in spintronics become optimised for a subsequent device patterning. Those are: spin-coherence length, electron mean free path, electron effective mass (massless for ideal Dirac electrons) and spin-orbit splitting of surface electrons. The project development has pursued the above described objectives within the practical constrains raised during the reporting period. We would like to highlight that most constrains are positive towards the career integration of the researcher and the scientific throughput of the project. The main project deviation aroused from the early awarding of a Ramón y Cajal research position to the fellow researcher. The position brought up new responsibilities, mainly the purchase, set-up and scientific direction of a scanning probe method (SPM) facility worth EUR 1.2 million. This equipment has been committed to the scientific objectives of this project, which have been partially fulfilled in the line of production of high quality epitaxial Bi and graphene. Below, we provide a chronological enumeration of the tasks completed in the frame of this project: First period (1 April 2010 to 30 September 2011): - set-up of a pulsed magnetic field facility for magnetoresistance and Hall effect measurements (31 Tesla, temperature range 2-300 K). - partial set-up of an ultra-high vacuum (UHV) multichamber system with two STMs (temperature range 1.1 - 1 200 K, 3 Tesla vertical field) and in situ surface cleaning and epitaxial growth facilities; - growth of epitaxial magnesium oxide (MgO) monolayers. The characterisation was performed by simultaneous STM and non-contact atomic force microscopy (AFM) in UHV condition; - implementation of atomic manipulation techniques. Last period (1 October 2011 to 1 April 2013) - growth of graphene islands on nickel (Ni)(111) - engineering of atomic scale structures to produce designated oscillatory patterns of the local density of states (LDOS) on a metal substrate; - growth of graphene monolayer on Ni(111); - growth of Bi thin films on Si(111); - implementation of spin-polarised STM. The completion of these tasks provides, in the first place, a broad scenario of samples to investigate by STM the intrinsic electronic structure of Bi and graphene surface electrons, as well as adequate quality films for a subsequent patterning in collaboration with other groups at the host institutions, where macroscopic transport properties can be addressed. In the second place, the optimisation of the sample preparation required a deep understanding of the chemical-physical processes governing the growth mechanisms, of great interest for the scientific community. Focusing on the specific achievements, we would like to highlight the following results: - The SPM platform at the host institution is running at full strength and providing regular scientific support to multiple research projects, both internal and of collaborative character with other European institutions (University College London, Intitut Catalá de Nanotecnologia, CEMES-Toulouse, University of Groningen). - A novel approach for the growth of ultrathin MgO layers has been demonstrated: direct thermal evaporation of MgO powder over Ag(111). A combination of STM and non-contact AFM imaging under UHV conditions was applied in order to find the MgO monolayer. - Successful growth of epitaxial Bi over the 7x7 Si(111) reconstructed surface. - Identification of the dynamics of nucleation of graphene islands over Ni(111). Yield and shape selection is possible thanks to thermal cycles and controlled Carbon dose over the clean Ni surface. - Identification of the intermediate chemical reactions and nanoscale structural details for the growth of a wetting graphene monolayer on Ni(111). In the presence of propene as carbon precursor, we have found that the formation of Ni2C at high temperatures occurs first, then C is diluted into the surface after stopping the propene flux, and finally carbon gets back to the surface this time in the form of grapheme. - Atomic manipulation procedures have been developed successfully and established as a standard technique in the group, including lateral manipulation, vertical manipulation, and electric field induced manipulation. In the last period we finalised the instrumentation necessary for spin-polarised STM. Potential impact and expected final results: - Training / Doctor of Philosophy (PhD): Three PhD students have been contributing to this project in the various aforementioned tasks. In all three cases, their work has proved relevance to be included in their thesis dissertation. - Relevance of the MgO monolayer: we would like to underline this part of the work which has been performed by Maria Moro, the PhD student partially funded directly by this project. Decoupling electronically single atoms or molecules from the surface state of supporting substrates like Bi or graphene provides a way to probe the interactions with the substrate and to tune them towards practical applications of the atoms / molecules functionalities. Insulting adlayers in between as MgO are perfect for such purpose. Furthermore, MgO entails a specially interesting technological impact, because its production in epitaxial form has been scaled to mass production in the field of magnetic tunnel junctions. Thus, we are using the knowledge gathered during this project in combining MgO over magnetic substrates, Bi and graphene to tune the properties of individual adsorbates over them. This has not been achieved to date and has strategic interest to our lab and to the STM community working in the bistability of atoms and molecules. - Publications: we are in the process of manuscript preparation to publish the data obtained in the graphene on Ni(111) system, additional to the already published investigation on the formation of graphene islands M. Olle et al., Nano Lett. 12, 4431 (2012)). - Atom manipulation: as was mentioned in paragraph B.4.3 of the project (annex 1), atomic manipulation has been successfully implemented in the low temperature STM. Now several members of the SPM group are able to construct atomic scale structures using this technique, and the funding from this project has contributed remarkably to this goal. As an example, we built the word 'ARAGÓN' using 69 cobalt (Co) atoms and 1 silver (Ag) atom, aiming to attract the attention of the media and consequently of promising young scientists which are at the stage of deciding their future. The work was reported in the main regional newspapers and broadcasted on the national radio. After that, we have applied the technique to scientific purposes such as construction of quantum corrals, chemical identification of atomic species, assembly of nanoclusters with catalytic properties and many others to come. - Spin-polarised STM is currently in progress.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз