1,423 research outputs found
On‐surface synthesis and atomic scale characterization of unprotected indenofluorene polymers
Polycyclic hydrocarbons with nonzero radical character have attracted enormous interest as potential active media for organic electronics and spintronics. In this context, indenofluorenes are an intriguing class of formally antiaromatic, biradical materials with a radical character that depends on the connectivity of their six- and five-membered rings. Synthesis of indenofluorene polymers and related compounds, first achieved in the early ‘90s with the production of ladder-type chains, represents a major step toward incorporation of these systems into devices. However, solution-based synthetic protocols require bulky protecting groups to stabilize the most reactive sites and, at the same time, to improve solubility and processability of such compounds. The preparation of various pristine – that is, unprotected-indenofluorene polymers has recently become possible via the on-surface synthesis approach, where the resulting nanostructures are supported and efficiently stabilized by the underlying substrate in ultrahigh vacuum conditions. Here, an overview of these recent works is given, with a focus on synthetic challenges, structural details and electronic properties
Oxidation of Mg(0001): A combined Experimental and Theoretical Study
The oxidation of magnesium proceeds in several stages, beginning with the oxygen dissociation
process and ending with the formation of magnesium oxides. Our focus is on the intermediate
oxidation state at the Mg(0001) surface, whose geometrical structure is an unsettled problem. By
combining the results of high-accuracy electronic structure calculations with angle-scanned x-ray
photoelectron diffraction measurements we are able to unambiguously determine the structure of
the Mg(0001) surface upon oxidation. In contrast to previous studies of Mg(0001) oxidation and
unlike the case of aluminum oxidation we find a rather unanticipated surface oxide structure,
consisting of two mixed oxygen-magnesium layers on top of an almost undisturbed Mg(0001)
surface. This unusual surface oxide structure is locally formed already at very low coverages of
0.1 monolayer and grows laterally with increasing oxygen coverage up to 2 monolayers
Charge-carrier dynamics in single-wall carbon nanotube bundles: A time-domain study
We present a real-time investigation of ultrafast carrier dynamics in
single-wall carbon nanotube bundles using femtosecond time-resolved
photoelectron spectroscopy. The experiments allow to study the processes
governing the subpicosecond and the picosecond dynamics of non-equilibrium
charge-carriers. On the subpicoseond timescale the dynamics are dominated by
ultrafast electron-electron scattering processes which lead to internal
thermalization of the laser excited electron gas. We find that quasiparticle
lifetimes decrease strongly as a function of their energy up to 2.38 eV above
the Fermi-level - the highest energy studied experimentally. The subsequent
cooling of the laser heated electron gas down to the lattice temperature by
electron-phonon interaction occurs on the picosecond time-scale and allows to
determine the electron-phonon mass enhancement parameter lambda. The latter is
found to be over an order of magnitude smaller if compared, for example, with
that of a good conductor such as copper.Comment: 17 pages, 19 igure
Giant edge state splitting at atomically precise zigzag edges
Zigzag edges of graphene nanostructures host localized electronic states that
are predicted to be spin-polarized. However, these edge states are highly
susceptible to edge roughness and interaction with a supporting substrate,
complicating the study of their intrinsic electronic and magnetic structure.
Here, we focus on atomically precise graphene nanoribbons whose two short
zigzag edges host exactly one localized electron each. Using the tip of a
scanning tunneling microscope, the graphene nanoribbons are transferred from
the metallic growth substrate onto insulating islands of NaCl in order to
decouple their electronic structure from the metal. The absence of charge
transfer and hybridization with the substrate is confirmed by scanning
tunneling spectroscopy (STS), which reveals a pair of occupied / unoccupied
edge states. Their large energy splitting of 1.9 eV is in accordance with ab
initio many-body perturbation theory calculations and reflects the dominant
role of electron-electron interactions in these localized states.Comment: 14 pages, 4 figure
Electronic Band Dispersion of Graphene Nanoribbons via Fourier-Transformed Scanning Tunneling Spectroscopy
Atomically precise armchair graphene nanoribbons of width (7-AGNRs) are
investigated by scanning tunneling spectroscopy (STS) on Au(111). The analysis
of energy-dependent standing wave patterns of finite length ribbons allows, by
Fourier transformation, the direct extraction of the dispersion relation of
frontier electronic states. Aided by density functional theory calculations, we
assign the states to the valence band, the conduction band and the next empty
band of 7-AGNRs, determine effective masses of , and , respectively, and a band gap of eV.Comment: 20 pages, 7 figure
Coupled spin states in armchair graphene nanoribbons with asymmetric zigzag edge extensions
Carbon-based magnetic structures promise significantly longer coherence times
than traditional magnetic materials, which is of fundamental importance for
spintronic applications. An elegant way of achieving carbon-based magnetic
moments is the design of graphene nanostructures with an imbalanced occupation
of the two sublattices forming the carbon honeycomb lattice. According to
Lieb's theorem, this induces local magnetic moments that are proportional to
the sublattice imbalance. Exact positioning of sublattice imbalanced
nanostructures in graphene nanomaterials hence offers a route to control
interactions between induced local magnetic moments and to obtain graphene
nanomaterials with magnetically non-trivial ground states. Here, we show that
such sublattice imbalanced nanostructures can be incorporated along a large
band gap armchair graphene nanoribbon on the basis of asymmetric zigzag edge
extensions, which is achieved by incorporating specifically designed precursor
monomers during the bottom-up fabrication of the graphene nanoribbons. Scanning
tunneling spectroscopy of an isolated and electronically decoupled zigzag edge
extension reveals Hubbard-split states in accordance with theoretical
predictions. Investigation of pairs of such zigzag edge extensions reveals
ferromagnetic, antiferromagnetic or quenching of the magnetic interactions
depending on the relative alignment of the asymmetric edge extensions.
Moreover, a ferromagnetic spin chain is demonstrated for a periodic pattern of
zigzag edge extensions along the nanoribbon axis. This work opens a route
towards the design and fabrication of graphene nanoribbon-based spin chains
with complex magnetic ground states
On-Surface Hydrogen-Induced Covalent Coupling of Polycyclic Aromatic Hydrocarbons via a Superhydrogenated Intermediate
The activation and subsequent covalent coupling of polycyclic aromatic
hydrocarbons (PAHs) are of great interest in fields like chemistry, energy,
biology, or health, among others. However, this is not a trivial process. So
far, it is based on the use of catalysts that drive and increase the efficiency
of the reaction. Here, we report on an unprecedented method in which the
dehydrogenation and covalent coupling is thermally activated in the presence of
atomic hydrogen and a surface. This mechanism, which requires of the
superhydrogenation of the PAHs, has been characterized by high-resolution
scanning tunnelling microscopy (STM) and rationalized by density functional
theory (DFT) calculations. This work opens a door toward the formation of
covalent, PAH-based, macromolecular nanostructures on low-reactive surfaces,
thus facilitating its applicability.Comment: This manuscript version is made available under the CC-BY-NC-ND 4.0
licens
Structure-dependent electrical properties of graphene nanoribbon devices with graphene electrodes
Graphene nanoribbons (GNRs) are a novel and intriguing class of materials in
the field of nanoelectronics, since their properties, solely defined by their
width and edge type, are controllable with high precision directly from
synthesis. Here we study the correlation between the GNR structure and the
corresponding device electrical properties. We investigated a series of field
effect devices consisting of a film of armchair GNRs with different structures
(namely width and/or length) as the transistor channel, contacted with narrowly
spaced graphene sheets as the source-drain electrodes. By analyzing several
tens of junctions for each individual GNR type, we observe that the values of
the output current display a width-dependent behavior, indicating electronic
bandgaps in good agreement with the predicted theoretical values. These results
provide insights into the link between the ribbon structure and the device
properties, which are fundamental for the development of GNR-based electronics.Comment: Published in Carbon (2019
- …