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Fixing the Energy Scale in Scanning Tunneling Microscopy on Semiconductor Surfaces
Full text linkIn scanning tunneling experiments on semiconductor surfaces, the energy scale within the tunneling junction is usually unknown due to tip-induced band bending. Here, we experimentally recover the zero point of the energy scale by combining scanning tunneling microscopy with Kelvin probe force spectroscopy. With this technique, we revisit shallow acceptors buried in GaAs. Enhanced acceptor-related conductance is observed in negative, zero, and positive band-bending regimes. An Anderson-Hubbard model is used to rationalize our findings, capturing the crossover between the acceptor state being part of an impurity band for zero band bending and the acceptor state being split off and localized for strong negative or positive band bending, respectively
Charge-state lifetimes of single molecules on few monolayers of NaCl
No full textAbstract In molecular tunnel junctions, where the molecule is decoupled from the electrodes by few-monolayers-thin insulating layers, resonant charge transport takes place by sequential charge transfer to and from the molecule which implies transient charging of the molecule. The corresponding charge state transitions, which involve tunneling through the insulating decoupling layers, are crucial for understanding electrically driven processes such as electroluminescence or photocurrent generation in such a geometry. Here, we use scanning tunneling microscopy to investigate the decharging of single ZnPc and H2Pc molecules through NaCl films of 3 to 5 monolayers thickness on Cu(111) and Au(111). To this end, we approach the tip to the molecule at resonant tunnel conditions up to a regime where charge transport is limited by tunneling through the NaCl film. The resulting saturation of the tunnel current is a direct measure of the lifetimes of the anionic and cationic states, i.e., the molecule’s charge-state lifetime, and thus provides a means to study charge dynamics and, thereby, exciton dynamics. Comparison of anion and cation lifetimes on different substrates reveals the critical role of the level alignment with the insulator’s conduction and valence band, and the metal-insulator interface state
Implementing Functionality in Molecular Self-Assembled Monolayers
No full textThe planar heterocyclic
molecules 1,6,7,12-tetraazaperylene on
a Ag(111) metal substrate show different charging characteristics
depending on their local environment: next to vacancies in self-assembled
islands, molecules can be charged by local electric fields, whereas
their charge state is fixed otherwise. This enables the activation
of selected molecules inside islands by vacancy creation from scanning-probe-based
manipulation. This concept allows for combining the precise mutual
atomic-scale alignment of molecules by self-assembly, on one hand,
and the implementation of specific functionality into otherwise homogeneous
monolayers, on the other. Activated molecules in the direct neighborhood
influence each other in their charging characteristics, suggesting
their use as molecular quantum cellular automata. Surprisingly, only
very few interacting molecules exhibit a rich spectroscopic signature,
which offers the prospect of implementing complex functionality in
such structures in the future
Imaging on-surface hierarchical assembly of chiral supramolecular networks
Full text linkThe bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,100-dibromo-9,90-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling
scanning tunneling microscopy and spectroscopy of adsorbates on metal and insulator surfaces
No full textTitel 1
Inhaltsverzeichnis 9
1 Einleitung 11
2 Grundlagen 13
2.1 Das Rastertunnelmikroskop 13
2.2 Zur Theorie der Rastertunnelmikroskopie 14
2.3 Spektroskopie mit dem Rastertunnelmikroskop 16
2.4 Oberflächenzustände der Elektronen im NFE-Modell 18
3 Experimentelles 25
3.1 Die verwendete Meßapparatur 25
3.2 Eigenschaften der verwendeten Materialien 26
3.3 Probenpräparation 27
4 Entwicklung einer digitalen DFM-Steuerung 29
4.1 Der prinzipielle Aufbau 30
4.2 Realisierung 31
4.3 Testmessungen 33
5 Substratvermittelte langreichweitige Wechselwirkung 35
5.1 Diffusion einzelner Cu-Adatome auf Cu(111) 36
5.2 Bestimmung des Paarpotentials zwischen Cu-Adatomen 43
5.3 Lokale Zustandsdichte für unterschiedliche Adatomabstände 48
5.4 Zur Theorie dieser Wechselwirkung 53
5.5 Kinetische Monte-Carlo-Simulationen 56
5.6 Das reine Paarpotential 59
5.7 Die Wechselwirkung zwischen Monomeren und Adatom-Ketten 59
5.8 Platzbestimmung von Cu-Monomeren und -Dimeren 65
5.9 Diffusion in künstlichen Nanostrukturen 70
6 Grenzflächenzustände von NaCl(100) auf Cu(111) 71
6.1 Wachstum von NaCl(100) auf Cu(111) 71
6.2 I(z)-Spektroskopie 77
6.3 Elektronische Grenzflächenzustände 79
6.4 Zur Theorie des Grenzflächenzustandsbandes 83
6.5 Aufspaltung des Grenzflächenzustandsbandes 89
6.6 Brechung an NaCl-Stufenkanten 94
6.7 Cl-Fehlstellen und gebundene Zustände 95
7 Einzelne Metallatome auf NaCl(100)/Cu(111) 99
7.1 Abbildung von Gold-Monomeren und -Dimeren 99
7.2 Platzbestimmung einzelner Gold-Adatome 102
7.3 Diffusion von Au/NaCl(100) 103
7.4 Manipulation einzelner Gold-Adatome 105
7.5 Theoretisches Modell zur Manipulation 109
7.6 Desorption von Gold-Adatomen 112
7.7 Gezielte Erzeugung einzelner Cl-Fehlstellen 112
7.8 Messungen an einzelnen Ag- und Cu-Adatomen 114
7.9 Weiterführende Messungen 123
8 Zusammenfassung 125
9 Ausblick 129
Literaturverzeichnis 131
Curriculum Vitae 139
Veröffentlichungen 141
Danksagung 143In dieser Arbeit werden Untersuchungen an Adsorbaten mittels Tieftemperatur-
Rastertunnelmikroskopie und -spektroskopie auf Metall- und Isolatoroberflächen
präsentiert.
Am System Cu/Cu(111) wurde erstmals eine oszillatorische, langreichweitige
Wechselwirkung experimentell nachgewiesen, die durch ein
Oberflächenzustandsband vermittelt wird. Einzelne Cu-Adatome haben eine kleine
Diffusionsbarriere von (37 ± 5)meV und sind auf hcp-Gitterplätzen adsorbiert.
In Übereinstimmung mit theoretischen Erwartungen oszilliert das Paarpotential
zwischen zwei Adatomen mit der halben Fermiwellenlänge und die Einhüllende
fällt wie 1/d² mit dem Abstand d ab. Die Potentialdifferenz zwischen dem
ersten Potentialminimum und -maximum beträgt etwa 1meV. Im Falle eines
Potentialminimums/-maximums ist die lokale Zustandsdichte am Ferminiveau
zwischen den Adatomen reduziert/erhöht. Die Diffusion von Adatomen in
künstlich erzeugten Nanostrukturen kann auf einzelne eindimensionale Kanäle
eingeschränkt werden. In einem Dimer können sich die Kupferatome bei 8K sowohl
auf fcc- als auch auf hcp-Gitterplätzen aufhalten, während in längeren Ketten
ausschließlich eine Adsorption auf fcc-Gitterplätzen beobachtet werden konnte.
NaCl(100)/Cu(111) zeigt ein Interfacezustandsband mit einer gegenüber der
reinen Cu(111)-Oberfläche zu höherer Energie verschobenen Dispersion. Es wird
ein einfaches Modell aufgestellt, das ein grundlegendes Verständnis der
beobachteten Dispersion schafft. Dieses Interfacezustandsband zeigt eine
eindimensionale Bandaufspaltung, die sich in Rastertunnelmikroskopie-Bildern
als stehende Wellen bemerkbar macht. Moiré-Muster, die durch das
inkommensurable Wachstum zustande kommen, bewirken über eine Modulation des
Potentials diese Bandaufspaltung. An Chlor-Fehlstellen findet man einen
gebundenen Zustand. An NaCl-Inselkanten kann darüber hinaus Brechung von
Elektronen im Grenzflächenzustandsband beobachtet werden.
Einzelne Silber-, Kupfer- und Gold-Adatome auf NaCl(100) werden mit dem
Rastertunnelmikroskop als Erhebungen abgebildet. Der für diese Adatome
gefundene Adsorptionsplatz auf einem Chlor-Ion falsifiziert alle
diesbezüglichen bisherigen theoretischen Untersuchungen. Gold hat eine
Diffusionsbarriere von (147 ± 15)meV auf NaCl(100). Atomare Manipulation
einzelner Gold-Adatome durch einen Spannungspuls kann diese gezielt in einen
veränderten, stabilen Zustand versetzen. Der Adsorptionsplatz ändert sich
dabei nicht. Die Adatome können durch einen weiteren, geeigneten Spannungspuls
in ihren ursprünglichen Zustand zurückversetzt werden.
Für den geplanten Aufbau eines kombinierten Tieftemperatur-Rastertunnel- und
Rasterkraftmikroskopes wurde eine digitale Steuerung entwickelt.In this thesis investigations of adsorbates on metal and insulator surfaces
are presented using low temperature scanning tunneling microscopy and
spectroscopy.
With the study of Cu/Cu(111) a long-ranged oscillatory interaction could be
identified for the first time that is mediated by a surface state band. Single
copper adatoms have a low diffusion barrier of (37 ± 5)meV and occupy only hcp
adsorption sites. In agreement with theoretical predictions the pair potential
oscillates with half the Fermi wavelength and the envelope falls off as 1/d
with distance d. The difference between the first potential minimum and
maximum is about 1meV. In the case of a potential minimum/maximum the local
density of states at the Fermi level directly between the adatoms is
reduced/increased. The diffusion of single adatoms in artificial
nanostructures can be forced into discrete diffusion channels. The copper
adatoms within a dimer can occupy either fcc or hcp adsorption sites at a
temperature of 8K. In long chains of adatoms adsorption could be observed on
fcc sites only.
NaCl(100)/Cu(111) shows an interface state band. The dispersion is shifted
towards higher energy compared to the surface state band of the clean Cu(111)
surface. A simple model is proposed that gives a qualitative understanding of
the observed features. The interface state band shows a one dimensional band
gap that appears in the images as standing waves. Moiré patterns arise from
the incommensurate growth. These patterns cause a modulation of the potential
and thereby split up the interface state band. At chlorine vacancies a bound
state can be found. At NaCl island edges the refraction of electrons in the
interface state band can be observed.
Single silver, copper and gold adatoms on NaCl(100) appear as protrusions in
the scanning tunneling microscopy images. For these adatoms the adsorption
site is found to be on top of the chlorine ions. This falsifies all previous
theoretical studies in this regard. Gold adatoms have a diffusion barrier of
(147 ± 15)meV on NaCl(100). Atomic manipulation can be performed applying a
voltage puls that switches single gold adatoms into a different stable state.
This manipulation does not change the adsorption site. The adatoms can be
switched back to the primary state by an additional suitable voltage pulse.
A digital control unit was designed, which will be used in a future combined
low temperature scanning tunneling and atomic force microscope
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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