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Project

Tuning graphene into a spin active material: Exploiting its size-specific interaction with metal clusters

The success of semiconductor industry relies on the continuous improvement of the performances of integrated circuits (IC). So far, this has been achieved by reducing the size of the basic building block of these circuits, the metal-oxide-semiconductor field effect transistor (MOSFET). The downscaling of MOSFET’s has been realized by introducing new materials, like high-k dielectric/metal gate stacks and high-mobility semiconductors (Ge, III-V compounds). Since this down-scaling is approaching its limits, the continuous increase in performance of integrated circuits can only be preserved through the implementation of radically new approaches.  Spintronics, in which one manipulates the spin of electrons next to their charge, could be the required addition to overcome the challenges conventional electronics face today. During the last decades, the scientific community successfully demonstrated injection, transport and manipulation of spins in Si, metals, graphene and more. Despite these significant achievements, a spin-based alternative for logic elements, such as the MOSFET, did not yet reach the level of performance to compete with the charge based alternatives. 

More than 10 years ago, the discovery of graphene's properties ignited the research on two-dimensional (2D) materials, enabling new and exciting opportunities in both fundamental and applied sciences. The 2D material is a zero-gap semiconductor: its valence and conduction band touch in the so-called Dirac point, around which its band energy dispersion is linear. The ultimate confinement in one dimension enables gate control of graphene's transport properties. Using a device with a back-gate structure allows to tune the type (electrons/holes) and number of charge carriers in a continuous fashion, i.e. the (ambipolar) electric field effect. Moreover, graphene exhibits a number of extraordinary properties, including excellent thermal and electrical conductance, extreme flexibility, high mechanical strength, and pronounced quantum-mechanical phenomena such as the quantized Hall effect. Furthermore, graphene has proven to be an excellent starting material for spintronics. Intrinsic graphene has low spin-orbit and hyperfine interactions, resulting in few spin relaxation mechanisms and hence it exhibits a long spin diffusion length, making it a suitable spin transport channel. However, this implies that pristine graphene is predominantly a passive spintronic element: it offers very limited active spin manipulation. 

In the semiconductor industry, doping plays a crucial role: it tailors the semiconducting material to have the desired properties. In case of graphene, the doping effects go beyond the addition or removal of an electron from the conduction band. For example, adatoms and nanoparticles adsorbed on graphene can (locally) tune the spin-orbit coupling. Enhanced spin-orbit coupling is expected to lead to an enlarged and/or tunable spin Hall effect, robust quantum spin Hall states, alter the spin lifetime anisotropy, and spin-splitting in the graphene density of states. As such, graphene decorated with adparticles is expected to activate graphene’s spintronic potential. Due to the extreme sensitivity of graphene devices, one desires a high level of control in adsorbing adparticles on graphene. Such control is offered by state-of-the-art cluster fabrication and deposition techniques, which allows to select the size and composition of metal clusters with atomic resolution. Using these techniques, gas phase clusters showcased a distinct atom-by-atom size dependence, dominated by quantum confinement effects, in the electronic and structural properties. This leads to unique physico-chemical properties, such as magnetism made of atoms that form non-magnetic bulk metals, catalytic activity of gold clusters opposed to their inert bulk phase, and metal-dielectric transitions that occur when adding a single atom. Clusters can be regarded as extensions to the periodic table of elements in the third dimension, or, in the framework of this project, as superdopants.
 
In this thesis, it is demonstrated for the first time, to the best of our knowledge, an experimental study of spin transport in size-selected cluster-decorated graphene devices. Graphene spin valves were fabricated and characterized in-situ using measuring facilities built in the context of this thesis. These spin valves are decorated with Au3 and Au6 clusters, which are created in a DC magnetron sputtering source, size selected, and subsequently deposited on the spin valves. As the density of deposited clusters on graphene is incremented, the spin transport parameters of the graphene channel are carefully monitored using Hanle spin precession measurements. It is found that both gold cluster sizes scatter spins via the Elliot-Yafet mechanism. The induced spin-orbit coupling strength is a few meV for both clusters, with the value for Au3 being roughly twice as large as that of Au6. A gradual increase of the deposited cluster density (up to 1e14 clusters/cm^2) decreases the spin and momentum lifetime of the graphene channel, with Au6 clusters affecting both spin and momentum lifetime more strongly than the Au3 clusters. Density functional theory calculations provide insights into the spin relaxation mechanism. The dependence of graphene's electronic and spintronic properties on the exact cluster size indicates the importance of the microscopic details for graphene functionalisation towards spintronic applications.

In a similar fashion, Ni4 clusters are decorated on a graphene spin valve. Fueled by the debate in literature, this study focusses on the spin scattering mechanisms in graphene by evaluating the Elliot-Yafet and the D'yakonov-Perel spin scattering mechanisms in the cluster decorated sample. 

This thesis aids the understanding of spin scattering in graphene by the controlled modification of the graphene flake by cluster deposition. It provides experimental data to the discussion on the role of the D'yankonov-Perel mechanism and the Elliot-Yafet mechanisms causing spin relaxation in graphene. This thesis is expected to guide future experiments in the search for phenomena useful for spintronic applications such as the (inverse) spin Hall effect or a strong gate dependence of the spin current, enabling the Datta-Das spin transistor.

Date:1 Oct 2017 →  20 Oct 2021
Keywords:graphene, spintronics, clusters
Disciplines:Condensed matter physics and nanophysics
Project type:PhD project