A tunable two-impurity Kondo system in an atomic point contact

Two magnetic atoms, one attached to the tip of a scanning tunnelling microscope and one adsorbed on a metal surface, each constituting a Kondo system, have been proposed as one of the simplest conceivable systems potentially exhibiting quantum critical behaviour. We have succeeded in implementing this concept experimentally for cobalt dimers clamped between a scanning tunnelling microscope tip and a gold surface. Control of the tip-sample distance with subpicometre resolution enables us to tune the interaction between the two cobalt atoms with unprecedented precision. Electronic transport measurements on this two-impurity Kondo system reveal a rich physical scenario, which is governed by a crossover from local Kondo screening to non-local singlet formation due to antiferromagnetic coupling as a function of separation of the cobalt atoms.

Two-dimensional supramolecular architectures and metallic nanostructures on pre-structured surfaces studied by scanning tunneling microscopy

Régis Decker

In this thesis, we present Scanning Tunneling Microscopy investigations of two-dimensional supramolecular architectures and metallic nanostructures on pre-structured surfaces. We studied different self-assembled supramolecular architectures formed by linear dicarbonitrile-polyphenyl organic molecules (NC-Phn-CN, n = 3,4,5,6) on the Ag(111) surface. The molecules differ only in their size, which is determined by the number n of phenyl rings incorporated in the polyphenyl chain. The NC-Ph3-CN self-assemble in a chevron pattern; the NC-Ph4-CN form a chiral open network of rhombic cavities; the NC-Ph5-CN self-assemble in a chiral Kagomé lattice; the NC-Ph6-CN self-assembly is characterized by the formation of several different and coexisting architectures, which are chiral open networks of rhombic cavities in addition to a chiral Kagomé lattice. For all NC-Phn-CN molecules, the coordination with cobalt atoms leads to the formation of metal-organic honeycomb nanomeshes. Thus, by changing the length of the linear dicarbonitrile-polyphenyl molecules, we can fabricate metal-organic honeycomb networks with a tunable cavity size. These open networks have been used as templates to host additional molecules. STM was used to study tip-induced motions of these additional molecules in confined environment. The metal-organic networks have been also exploited to steer the formation of cobalt and iron nanostructures. At low temperature (120 K) and low coverage rate, metal nanostructures form preferentially on top of the organic ligands while they decorate the cobalt coordination nodes at 190 K or at higher coverages. Furthermore, the formation of iron nanostructures at the Al2O3/Ni3Al(111) oxide surface is shown. Iron nucleate at specific sites of the Al2O3/Ni3Al(111) network structure and form three-dimensional nanostructures for deposition temperatures of 300 K and two-dimensional for deposition temperatures of 150 K. It is also shown that two-dimensional nanostructures are stable upon annealing up to 350 K, which is of great interest for further studies concerning catalysis. Finally, first attempts for the ordered growth of gold nanostructures on graphene on the Ru(0001) surface are presented. It is shown that the shape of two-dimensional gold nanostructures is influenced by the underlying moiré structure of the graphene/Ru(0001) surface.

EPFL2008

Magnetism of Single Adatoms and Small Adsorbed Clusters Investigated by Means of Low-Temperature STM

Quentin Dubout

This thesis works reports on the magnetic properties of single transition metal atoms and hydrogenated compounds adsorbed on crystal surfaces. It is an experimental study, based on scanning tunneling microscopy (STM), initiated by the project of demonstrating the existence of magnetic remanence for single adsorbed atoms (adatoms). All the measurements have been performed with a scanning tunneling microscope operating at 0.4 K and equipped with 8.5 T superconducting coils. The investigated systems are cobalt atoms, pure and hydrogenated (cobalt hydrides), adsorbed on Pt(111) and graphene on Pt(111) surfaces. In the first case, we have immediately been confronted to the presence of cobalt hydrides on the surface. Three types of hydrides (CoH, CoH2, and CoH3) have been identified, quite different from the clean adatoms, and readily distinguished by their tunnel spectrum (STS). Twoofthesehydridesdisplaylow-energyvibrationalmodes(1−30meV),identifiedbytheir absence of response to a magnetic field, the effect of an isotopic substitution, and by comparison with known systems. The three hydrides can be deprotonated applying a sufficient voltage between tip and sample. CoH2 on Pt(111) presents a Kondo resonance, due to a half-integer spin (S = 3/2) and a peculiar magnetic anisotropy. This system corresponds to the first demonstration of the appearance of the Kondo effect upon hydrogen adsorption. The results of several attempts to observe remanence for single cobalt adatoms on Pt(111) with spin-polarized scanning tunneling microscopy (SP-STM) are also presented and discussed. The second system differs from the first one in the presence of a graphene layer on the Pt surface. The first experimental determination of the magnetic moment and magnetic anisotropy of adatoms on graphene is reported in this thesis. A magnetic moment of (2.2 ± 0.4) μB was measured by spin-excitation spectroscopy (SES), as well as a hard magnetization axis with an anisotropy energy of 8.1 ± 0.4 meV. This value, comparable to the record value of single cobalt atoms on Pt(111), has been mainly attributed to the strong hybridization between cobalt and graphene. Ab initio calculations confirmed our observations. Graphene thus represents an extremely promising substrate for future applications in nanoscale magnetism and spintronics. This system also exhibits three different cobalt hydrides in addition to the clean atoms. Their respective chemical composition, CoH, CoH2, and CoH3, was determined by direct comparison between high resolution STM images and simulated images obtained from ab initio calculated atomic structures. A controlled deprotonation method is presented. CoH3 is the only hydride to display spin excitations. Its magnetic moment is comparable to that of clean cobalt, it presents a hard magnetization axis as well, however, its magnetic anisotropy energy is sensibly lower, 1.7 ± 0.05 meV. The other two hydrides display vibrational excitations only. The effect of hydrogen on the magnetic properties of metal adatoms is therefore very strong in this system as well, and must be taken into account in future studies, hydrogen being a very common contaminant in ultra-high vacuum set-ups. Finally, a detailed study of the structure graphene mono- and bilayers on Ru(0001) was realized. Two different stackings, AB and AA, have been identified for the bilayers, the last one not observed to date for this system. The moiré structure of monolayer graphene and of this last type of bilayer has been determined: (11.57×11.57)R4.3◦ and (10.54×10.54)R4.7◦, respectively. Moreover, C-C bond distortions of more than 10%, observed in the STM images, have been studied in detail. A quantitative comparison with ab initio calculations allowed to identify their origin as the tilt of the graphene π-orbitals. These distortions are therefore purely artificial. This is an important result for future scanning tunneling microscopy studies of graphene.

EPFL2013

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

Submonolayer epitaxial growth is obtained by the deposition of less than a complete layer of atoms on a single crystal surface. It is of fundamental interest for industrial applications (e.g. in the semiconductor industry) as well as from the point of view of basic research. For example, it is known that nanometer-sized atomic structures (nanostructures) exhibit remarkable physical and chemical properties which can differ greatly from those of bulk matter. In order to investigate these properties, it is often necessary to create large quantities of well defined and possibly spatially ordered nanostructures. One way to achieve this result is self-organized growth where one deposits atoms on a clean crystalline surface and lets the growth process evolve freely. Here, nanostructures result from the atomic diffusion and aggregation processes taking place at the surface. Understanding the exact nature of these processes is of ongoing interest in the field of nanostructure growth. In this thesis we report about submonolayer growth experiments of the ferromagnetic transition metal cobalt. Cobalt was chosen because it exhibits remarkable magnetic properties. These experiments were performed in an ultra-high vacuum chamber and molecular beam epitaxy was used to grow the nanostructures. Observations of the surface were made using a variable-temperature scanning tunneling microscope (STM). In order to get a better understanding of the atomic processes happening at the surface, we developed two adapted computational simulation methods. Kinetic Monte-Carlo (KMC) simulations were used to get an atomistic picture of the surface while mean field rate equations were integrated numerically to yield cluster densities. We study the growth of cobalt on three different surfaces. By depositing Co on a clean Pt(111) crystal surface, we observe that the platinum surface reconstructs by forming star-shaped partial dislocations for sample temperatures above 180 K (-93°C). We also observe that island densities deviate from predictions of all known models towards higher values for these same temperatures. By simulation we are able to show that insertion of Co into the topmost platinum layer creates a repulsive network of dislocations. We show that these dislocations act as diffusion inhibiting barriers and thus influence the island density by constraining the free movement of atoms at the surface. We also show that the Co dimer and trimer diffuse on Pt(111) before dissociating and are able to extract corresponding activation barriers. We also study the deposition of Co on a Ru(0001) surface which shows a more conventional temperature dependence. By comparing simulation and experiment we extract all relevant diffusion activation barriers and show that at high temperatures the Co dimer and trimer dissociate. Finally, we investigate the growth of Co on the hexagonal boron nitride superlattice which forms on a Rh(111) surface. On this surface Co clusters form three-dimensional hemispherical dots. We show how the substrate corrugation influences diffusion of Co atoms and that clusters up to the size of the pentamer can diffuse. By simulation, we also extract the relevant migrations and desorption barriers.

EPFL2008

Two-dimensional supramolecular architectures and metallic nanostructures on pre-structured surfaces studied by scanning tunneling microscopy

Régis Decker

EPFL2008

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

EPFL2008

Magnetism of Single Adatoms and Small Adsorbed Clusters Investigated by Means of Low-Temperature STM

Quentin Dubout

EPFL2013