126 research outputs found
On-surface Assembly of Au-Dicyanoanthracene Coordination Structures on Au(111)
On-surface metal-organic coordination provides a promising way for
synthesizing different two-dimensional lattice structures that have been
predicted to possess exotic electronic properties. Using scanning tunneling
microscopy (STM) and spectroscopy (STS), we studied the supramolecular
self-assembly of 9,10-dicyanoanthracene (DCA) molecules on the Au(111) surface.
Close-packed islands of DCA molecules and Au-DCA metal-organic coordination
structures coexist on the Au(111) surface. Ordered DCAAu
metal-organic networks have a structure combining a honeycomb lattice of Au
atoms with a kagome lattice of DCA molecules. Low-temperature STS experiments
demonstrate the presence of a delocalized electronic state containing
contributions from both the gold atom states and the lowest unoccupied
molecular orbital of the DCA molecules. These findings are important for the
future search of topological phases in metal-organic networks combining
honeycomb and kagome lattices with strong spin-orbit coupling in heavy metal
atoms
Two-dimensional band structure in honeycomb metal-organic frameworks
Metal-organic frameworks (MOFs) are an important class of materials that
present intriguing opportunities in the fields of sensing, gas storage,
catalysis, and optoelectronics. Very recently, two-dimensional (2D) MOFs have
been proposed as a flexible material platform for realizing exotic quantum
phases including topological and anomalous quantum Hall insulators.
Experimentally, direct synthesis of 2D MOFs has been essentially confined to
metal substrates, where the interaction with the substrate masks the intrinsic
electronic properties of the MOF. Here, we demonstrate synthesis of 2D
honeycomb metal-organic frameworks on a weakly interacting epitaxial graphene
substrate. Using low-temperature scanning tunneling microscopy (STM) and atomic
force microscopy (AFM) complemented by density-functional theory (DFT)
calculations, we show the formation of 2D band structure in the MOF decoupled
from the substrate. These results open the experimental path towards MOF-based
designer quantum materials with complex, engineered electronic structures
Electrochemistry at electrified soft interfaces : novel approaches to old problems
This thesis addresses some known difficulties in the field of liquid | liquid electrochemistry by presenting novel means of controlling mass transfer, monolayermodification of the interface, and probing interfacial reactivity.
A novel rectangular channel flow electrochemical cell suitable for studying charge transfer at liquid | liquid interfaces is presented. The organic phase is immobilised using a gelling agent, while the aqueous phase flows past the interface. This creates an asymmetric diffusion regime, providing diagnostic criteria to determine, for example, the direction of the ion transfer.
One of enduring problems concerning phospholipid adsorption at liquid | liquid interfaces has been the inability to determine and control the exact nature of the adsorbed monomolecular layer. This difficulty is addressed by a combination of the Langmuir-Blodgett technique and the use of an electrochemical cell as a substrate. It is shown that reproducible layers of known surface pressure can be deposited at the interface and that the deposition surface pressure has a great influence on the behaviour of the layer.
The latter part of this thesis concerns the study of reactivity at liquid | liquid interfaces. To this end, the potential of ring-disk ultramicroelectrodes as probes for scanning electrochemical microscopy is investigated both theoretically and experimentally. In particular, the disk-generation/ring-collection mode of operation is considered. The interaction of two species with the substrate under investigation can be followed simultaneously from a single tip current-distance measurement to the substrate. This method is then applied to investigate the partitioning of iodine across a liquid-liquid interface.
A facile method to determine the lipophilicity of potentially unstable charged products of electron transfer reactions is reported. This is achieved by local electrolysis at a Pt coated micropipette and subsequent transfer of the electrogenerated ions across a polarisable liquid | liquid interface supported at the tip of the micropipette. The formal potential of ion transfer can then be used to give a measure of its relative lipophilicity.reviewe
Benchmarking van der Waals-treated DFT: The case of hexagonal boron nitride and graphene on Ir(111)
There is enormous recent interest in weak, van der Waals-type (vdW)
interactions due to their fundamental relevance for two-dimensional materials
and the so-called vdW heterostructures. Tackling this problem using computer
simulation is very challenging due to the non-trivial, non-local nature of
these interactions. We benchmark different treatments of London dispersion
forces within the density functional theory (DFT) framework on hexagonal boron
nitride or graphene monolayers on Ir(111) by comparing the calculated
geometries to a comprehensive set of experimental data. The geometry of these
systems crucially depends on the interplay between vdW interactions and wave
function hybridisation, making them excellent test cases for vdW-treated DFT.
Our results show strong variations in the calculated atomic geometry. While
some of the approximations reproduce the experimental structure, this is rather
based on \textit{a posteriori} comparison with the ``target results''. General
predictive power in vdW-treated DFT is not achieved yet and might require new
approaches.Comment: More or less published version, with Supporting Materia
Topographic and electronic contrast of the graphene moir\'e on Ir(111) probed by scanning tunneling microscopy and non-contact atomic force microscopy
Epitaxial graphene grown on transition metal surfaces typically exhibits a
moir\'e pattern due to the lattice mismatch between graphene and the underlying
metal surface. We use both scanning tunneling microscopy (STM) and atomic force
microscopy (AFM) experiments to probe the electronic and topographic contrast
of the graphene moir\'e on the Ir(111) surface. While STM topography is
influenced by the local density of states close to the Fermi energy and the
local tunneling barrier height, AFM is capable of yielding the 'true' surface
topography once the background force arising from the van der Waals (vdW)
interaction between the tip and the substrate is taken into account. We observe
a moir\'e corrugation of 3510 pm, where the graphene-Ir(111) distance is
the smallest in the areas where the graphene honeycomb is atop the underlying
iridium atoms and larger on the fcc or hcp threefold hollow sites.Comment: revised versio
Real-space imaging of dispersive triplon excitations in engineered quantum magnets
Quantum magnets provide a powerful platform to explore complex quantum
many-body phenomena. One example is triplon excitations, exotic many-body modes
emerging from deconfined singlet-triplet transitions with no single particle
analog. Triplons are challenging to observe in conventional materials, as the
energy scales of singlet-triplet transitions are associated with Hund's energy
and are dramatically larger than the typical bandwidth of spin fluctuations. We
engineer a minimal quantum magnet from organic molecules and demonstrate the
emergence of dispersive triplon modes in one- and two-dimensional assemblies
probed with scanning tunneling microscopy and spectroscopy. We show the
variable bandwidth of triplon excitations in these two different geometries.
Our results provide the first demonstration of dispersive triplon excitations
from a real-space measurement, suggesting their potential engineering to
realize exotic many-body phenomena in quantum magnets without breaking
time-reversal symmetry
Quantum confined electronic states in atomically well-defined graphene nanostructures
Despite the enormous interest in the properties of graphene and the potential
of graphene nanostructures in electronic applications, the study of quantum
confined states in atomically well-defined graphene nanostructures remains an
experimental challenge. Here, we study graphene quantum dots (GQDs) with
well-defined edges in the zigzag direction, grown by chemical vapor deposition
(CVD) on an iridium(111) substrate, by low-temperature scanning tunneling
microscopy (STM) and spectroscopy (STS). We measure the atomic structure and
local density of states (LDOS) of individual GQDs as a function of their size
and shape in the range from a couple of nanometers up to ca. 20 nm. The results
can be quantitatively modeled by a relativistic wave equation and atomistic
tight-binding calculations. The observed states are analogous to the solutions
of the text book "particle-in-a-box" problem applied to relativistic massless
fermions.Comment: accepted for publication in Phys. Rev. Let
Tuneable topological domain wall states in engineered atomic chains
Topological modes in one- and two-dimensional systems have been proposed for numerous applications utilizing their exotic electronic responses. The 1D, zero-energy, topologically protected end modes can be realized in structures implementing the Su-Schrieffer-Heeger (SSH) model. While the edge modes in the SSH model are at exactly the mid-gap energy, other paradigmatic 1D models such as trimer and coupled dimer chains have non-zero energy boundary states. However, these structures have not been realized in an atomically tuneable system that would allow explicit control of the edge modes. Here, we demonstrate atomically controlled trimer and coupled dimer chains realized using chlorine vacancies in the c(2 x 2) adsorption layer on Cu(100). This system allows wide tuneability of the domain wall modes that we experimentally demonstrate using low-temperature scanning tunneling microscopy (STM).Peer reviewe
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