dc.description.abstract
Topological Insulators (TIs) have become one of the wonder materials of condensed
matter physics over the last decade due to their novel properties, possessing
an insulating bulk in coexistence with metallic boundaries. They present
an inverted band gap consequence of strong spin orbit coupling, which gives rise
to the conductive boundary states with linear dispersion, characteristic of Dirac
fermions, and helical spin polarization. Numerous materials have been predicted
and observed to have TI signatures, holding great perspective for the realization
of novel applications in spintronics, quantum computing and metrology.
The experimental realization of three-dimensional TIs with the the Fermi
Level located well in the bulk band gap is a challenging task because of their
relatively small gap of hundreds of meV, and their high sensibility to crystal
defects and impurities. These can induce electron doping that activates bulk
conduction channels, thus burying the contribution of the surface states to the
transport. Molecular Beam Epitaxy (MBE) has been reported to be the most
suitable growth method to overcome this hindrance, due to its capability to grow
single crystals with fine control over the crystal defects and impurity level. The
first part of this thesis deals with the growth of high-quality TIs that maintain
their pristine insulating bulk behaviour. By using MBE, we studied the impact of
different substrates and growth parameters to the synthesis of Bismuth Telluride
(Bi2Te3) thin films, and the growth of the ternary compound Bismuth-Antimony
Telluride.
We were able to grow insulating Bi2Te3 thin films with complete suppression
of the \twin" domains, mirror-symmetric domains that contribute to the
self-doping of the crystal. By a combination of the initial interaction with the
lattice-matched Barium Fluoride substrate and the high working temperatures,
the growth of Bi2Te3 single-crystalline films is achieved already from the first
layer. More importantly, the films present low-doping level with the the Fermi
Level kept in the bulk band gap. The correlation between the lack of \twin" domains
(measured by Re
ection High-Energy Electron Diffraction, X-ray Diffraction
and Atomic Force Microscopy) and the low-doping level measured by Angle-
Resolved Photoemission Spectroscopy (ARPES), indicates the relation between
the crystal quality and the capability to preserve the bulk insulating character.
This result contrasts to other TIs grown on more conventional substrates,
typically presenting large lattice misfits that lead to the formation of an initial
polycrystalilne or amorphous seed layer. In parallel, we explored a complemeniii
tary approach to the growth of insulating Bi2Te3, based on the addition of Sb at
the expense of Te atoms. A sequence of Bismuth-Antimony Telluride films with
different x content were measured by X-ray Photoemission Spectroscopy (XPS)
and ARPES, showing that the the Fermi Level can be gradually brought to the
bulk valence band. The realization of such TIs, with a controlled level of the the
Fermi Level position is of special interest for counteracting the n-doping effects
typically induced by the addition of magnetic materials.
The second and more extended part of this thesis is devoted to the study
of interfaces formed by magnetic Metal-organic molecules deposited on the TI
thin films. Interfacing TI surfaces with magnetic materials can give rise to novel
magnetoelectronic phenomena, involving the manipulation of spin-torques (Inverse
Edelstein Effect), or the realization of spin polarized edge states (Quantum
Anomalous Hall Effect). The realization of such spin-related effects rely on the
capability to control the interfacial magnetic and electronic interactions. The
use of organic molecules to cage magnetic ions has been proved to be a versatile
approach to engineer inter-ions and ions-surface interactions, due to the
exible
design that molecules offer and to their ability to form structurally perfect selfassembled
structures. Moreover, they can also act as building blocks for covalent
or coordination structures via on-surface reactions.
As a first approach to tune the interfacial properties with Metal-organic molecules,
we showed how the ligand chemistry allows a progressive control over the
magnetic interactions between a hosted Co ion and a prototypical Au surface.
The spin states and magnetic moments are comprehensively studied thanks to the
complementary use of local spectroscopic Scanning Tunneling Spectroscopy and
non-local magnetic sensitive X-ray Magnetic Circular Dichroism (XMCD) techniques,
which are supported theoretically by Density Functional Theory (DFT).
We were able to continuously cover the range of magnetic Co ion-substrate interactions,
from a strong interacting scenario where the magnetic moment is
quenched, to a gradual decrease of the interaction revealed by a lower Kondo
screening of the spin. In addition, by changing the Au surface for a TI surface,
the interfacial interactions reach the weakest limit in which the molecular
magnetic structure is completely decoupled from the substrate electrons.
Thereafter, we explored the electronic and magnetic interactions between the
Topological Surface State of the Bi2Te3 thin film and Co ions caged in two different
planar molecules such as Cobalt - Tetrakis (4-Promophenyl) Porphyrin
(CoTBrPP) and Cobalt - Phthalocyanine (CoPc). We found a Metal-organic /
TI interface with unperturbed electronic and magnetic properties. This is assessed
by a coverage dependent ARPES study in which the Topological Surface
State persists upon the deposition of one (CoTBrPP or CoPc) molecular layer.
On the other hand, XMCD and Scanning Tunneling Spectroscopy measurements
reveal the preservation of the pristine CoTBrPP magnetic moment and electronic
structure respectively. Furthermore, a comprehensive Scanning Tunneling Microscopy
(STM) and DFT study of the CoTBrPP adsorption geometry describes
weak molecule-surface interactions, and corroborates the electronic decoupling of
the Metal-organic layer from the TI surface. In an analogue study with CoPc we
find slightly stronger interactions yet within the non-perturbative regime, that
suggesting ligand chemistry can be used to tune magnetic interactions without
affecting the overall properties of each component of the heterostructure.
Subsequently, the Br-functionalized CoTBrPP on Bi2Te3 system was used to
induce on-surface synthesis of Metal-organic coordination networks on TI. These
more entangled structures are of great interest as a framework in which magnetic
ions could arrange in ordered and mechanically stable arrays. Two different
coordination phases are selectively created after CoTBrPP dehalogenation upon
thermal activation. We track the chemical reaction by XPS, and investigate the
morphological and electronic properties of the final products by combining Scanning
Tunneling Spectroscopy (STS) and DFT calculations. We conclude that the
resulting structures consists of CoTPP coordinated with Te atoms incorporated
from the substrate, and thanks to the supporting DFT calculations, we are able to
explain the presence of linear chains and irregular coordinated networks. In parallel,
the presence of unperturbed Topological Surface State upon the formation
of the Metal-organic structures is confirmed by a coverage-dependent ARPES
study.
Overall, the first part of the thesis constitutes an extensive study of MBE
grown of Bi2Te3 thin films, in which different substrates and growth conditions
are discussed. Furthermore, the results provide a route for the enhancement of
the crystal quality of simple diatomic TIs, crucial for the preservation of their
bulk insulating behaviour. The results presented in the second part conceive the
capabilities of organic molecules to tune magnetic interactions between Co atoms
and Bi2Te3 films, and pave the way for the on-TI surface synthesis of magnetic
supramolecular structures.
