109 research outputs found
Molekulare Systeme im Wechselspiel von Struktur und Ladung: Optische in situ Spektroskopie an organischen Dünnschichten
Die optische in situ-Charakterisierung des Aufwachsens organischer Molekülschichten auf isolierenden und metallischen Substraten (Kaliumchlorid, -bromid und Glimmer, beziehungsweise Gold) ist Gegenstand dieser Arbeit. Am Beispiel der Substanzen Perylen-3,4,9,10- tetrakarbonsäuredianhydrid (PTCDA) und Titanylphthalozyanin (TiOPc), die mittels Molekularstrahlepitaxie abgeschieden werden, wird der Einfluss der Anordnung der Moleküle und der gegebenenfalls hinzugefügten Dotierung auf die messbaren Eigenschaften untersucht. Wie wichtig dabei die Feinabstimmung zwischen den Gitterkonstanten des Substrats und der Ausdehnung der Moleküle ist, äußert sich in schmalen Absorptions- und Emissionsbanden im Falle kommensurablenWachstums von PTCDA auf Kaliumchlorid. Diese Anordnung und ihre Metastabilität werden mit begleitenden Rasterkraftmikroskopie-Untersuchungen und Kraftfeldrechnungen nachgewiesen. Ausgehend von Monolagen neutraler Moleküle kann durch die schrittweise Dotierung mit Kalium die spektrale Entwicklung der Absorption von PTCDA-Anionen verfolgt und einzelnen Ladungsstufen – von Mono- bis Tetraanionen – zugeordnet werden. Durch vorherige Bestrahlung mit Elektronen konnte Glimmer so modifiziert werden, dass auch die spektrale Signatur von PTCDA-Monokationen aufgeklärt werden konnte. In Bestätigung früherer Rastertunnelmikoskopie-Ergebnisse zur Ausbildung von TiOPc- Kristallphasen erfolgt das Wachstum von Phase II auf einer Benetzungsschicht, die Phase I-Anordnung aufweist. Die Schichtdickenabhängigkeit der optischen Eigenschaften wird mit einer Genauigkeit im (Sub-)Monolagenbereich bestimmt und der Verlauf von Oszillatorstärke und Emissionslöschung durch die Orientierung der TiOPc-Moleküle zueinander beziehungsweise ihren Abstand zum Substrat erklärt.The aim of this work is the in situ monitoring of the growth of molecular thin films on either insulating (potassium chloride, -bromide and mica) or gold substrates by optical spectroscopy. The influence of the molecular arrangement and an optionally added doping on the properties is studied on perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) and titanyl phthalocyanine (TiOPc), deposited by molecular beam epitaxy. The impact of perfect matching between the substrate’s lattice constants and the dimensions of the molecules, appears in narrow absorption and emission bands in case of commensurate growth of PTCDA on potassium chloride. This arrangement and its metastability are proven by accompanying atomic force microscopy and advanced potential energy calculations. Once potassium can be added stepwise to monolayers of neutral PTCDA molecules, the spectral development towards PTCDA anions can be followed and assigned to the reached levels of charging. The crystal growth of TiOPc molecules in phase II takes place on a wetting layer with phase I arrangement, proving earlier results by scanning tunneling microscopy. Measuring the thickness-dependent optical properties with (sub-)monolayer resolution allows a deeper understanding of the dependences of both, the oscillator strength and the efficiency of luminescence quenching, on the molecules’ orientations with respect to each other and on their distance to the substrate
Binary Atomic Silicon Logic
It has long been anticipated that the ultimate in miniature circuitry will be
crafted of single atoms. Despite many advances made in scanned probe microscopy
studies of molecules and atoms on surfaces, challenges with patterning and
limited thermal stability have remained. Here we make progress toward those
challenges and demonstrate rudimentary circuit elements through the patterning
of dangling bonds on a hydrogen terminated silicon surface. Dangling bonds
sequester electrons both spatially and energetically in the bulk band gap,
circumventing short circuiting by the substrate. We deploy paired dangling
bonds occupied by one movable electron to form a binary electronic building
block. Inspired by earlier quantum dot-based approaches, binary information is
encoded in the electron position allowing demonstration of a binary wire and an
OR gate
Site-selective adsorption of phthalocyanine on h-BN/Rh(111) nanomesh
STM and DFT study of site selectivity of h-BN/Rh(111) (nanomesh) for the adsorption phthalocyanine, showing impressive agreement between experiment and theory.</p
Graphene nanoribbons with mixed cove-cape-zigzag edge structure
A recently developed bottom-up synthesis strategy enables the fabrication of graphene nanoribbons with well-defined width and non-trivial edge structures from dedicated molecular precursors. Here we discuss the synthesis and properties of zigzag nanoribbons (ZGNRs) modified with periodic cove-cape-cove units along their edges. Contrary to pristine ZGNRs, which show antiferromagnetic correlation of their edge states, the edge-modified ZGNRs exhibit a finite single particle band gap without localized edge states. We report the on-surface synthesis of such edge-modified ZGNRs and discuss tunneling conductance dI/dV spectra and dI/dV spatial maps that reveal a noticeable localization of electronic states at the cape units and the opening of a band gap without presence of edge states of magnetic origin. A thorough ab initio investigation of the electronic structure identifies the conditions under which antiferromagnetically coupled, edge-localized states reappear in the electronic structure. Further modifications of the ribbon structure are proposed that lead to an enhancement of such features, which could find application in nanoelectronics and spintronics
On-surface synthesis of graphene nanoribbons with zigzag edge topology
Graphene-based nanostructures exhibit a vast range of exciting electronic
properties that are absent in extended graphene. For example, quantum
confinement in carbon nanotubes and armchair graphene nanoribbons (AGNRs) leads
to the opening of substantial electronic band gaps that are directly linked to
their structural boundary conditions. Even more intriguing are nanostructures
with zigzag edges, which are expected to host spin-polarized electronic edge
states and can thus serve as key elements for graphene-based spintronics. The
most prominent example is zigzag graphene nanoribbons (ZGNRs) for which the
edge states are predicted to couple ferromagnetically along the edge and
antiferromagnetically between them. So far, a direct observation of the
spin-polarized edge states for specifically designed and controlled zigzag edge
topologies has not been achieved. This is mainly due to the limited precision
of current top-down approaches, which results in poorly defined edge
structures. Bottom-up fabrication approaches, on the other hand, were so far
only successfully applied to the growth of AGNRs and related structures. Here,
we describe the successful bottom-up synthesis of ZGNRs, which are fabricated
by the surface-assisted colligation and cyclodehydrogenation of specifically
designed precursor monomers including carbon groups that yield atomically
precise zigzag edges. Using scanning tunnelling spectroscopy we prove the
existence of edge-localized states with large energy splittings. We expect that
the availability of ZGNRs will finally allow the characterization of their
predicted spin-related properties such as spin confinement and filtering, and
ultimately add the spin degree of freedom to graphene-based circuitry.Comment: 15 pages, 4 figure
Second asymptomatic carotid surgery trial (ACST-2): a randomised comparison of carotid artery stenting versus carotid endarterectomy
Background: Among asymptomatic patients with severe carotid artery stenosis but no recent stroke or transient cerebral ischaemia, either carotid artery stenting (CAS) or carotid endarterectomy (CEA) can restore patency and reduce long-term stroke risks. However, from recent national registry data, each option causes about 1% procedural risk of disabling stroke or death. Comparison of their long-term protective effects requires large-scale randomised evidence. Methods: ACST-2 is an international multicentre randomised trial of CAS versus CEA among asymptomatic patients with severe stenosis thought to require intervention, interpreted with all other relevant trials. Patients were eligible if they had severe unilateral or bilateral carotid artery stenosis and both doctor and patient agreed that a carotid procedure should be undertaken, but they were substantially uncertain which one to choose. Patients were randomly allocated to CAS or CEA and followed up at 1 month and then annually, for a mean 5 years. Procedural events were those within 30 days of the intervention. Intention-to-treat analyses are provided. Analyses including procedural hazards use tabular methods. Analyses and meta-analyses of non-procedural strokes use Kaplan-Meier and log-rank methods. The trial is registered with the ISRCTN registry, ISRCTN21144362. Findings: Between Jan 15, 2008, and Dec 31, 2020, 3625 patients in 130 centres were randomly allocated, 1811 to CAS and 1814 to CEA, with good compliance, good medical therapy and a mean 5 years of follow-up. Overall, 1% had disabling stroke or death procedurally (15 allocated to CAS and 18 to CEA) and 2% had non-disabling procedural stroke (48 allocated to CAS and 29 to CEA). Kaplan-Meier estimates of 5-year non-procedural stroke were 2·5% in each group for fatal or disabling stroke, and 5·3% with CAS versus 4·5% with CEA for any stroke (rate ratio [RR] 1·16, 95% CI 0·86–1·57; p=0·33). Combining RRs for any non-procedural stroke in all CAS versus CEA trials, the RR was similar in symptomatic and asymptomatic patients (overall RR 1·11, 95% CI 0·91–1·32; p=0·21). Interpretation: Serious complications are similarly uncommon after competent CAS and CEA, and the long-term effects of these two carotid artery procedures on fatal or disabling stroke are comparable. Funding: UK Medical Research Council and Health Technology Assessment Programme
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