5,277 research outputs found
Pentacene islands grown on ultra-thin SiO2
Ultra-thin oxide (UTO) films were grown on Si(111) in ultrahigh vacuum at
room temperature and characterized by scanning tunneling microscopy. The
ultra-thin oxide films were then used as substrates for room temperature growth
of pentacene. The apparent height of the first layer is 1.57 +/- 0.05 nm,
indicating standing up pentacene grains in the thin-film phase were formed.
Pentacene is molecularly resolved in the second and subsequent molecular
layers. The measured in-plane unit cell for the pentacene (001) plane (ab
plane) is a=0.76+/-0.01 nm, b=0.59+/-0.01 nm, and gamma=87.5+/-0.4 degrees. The
films are unperturbed by the UTO's short-range spatial variation in tunneling
probability, and reduce its corresponding effective roughness and correlation
exponent with increasing thickness. The pentacene surface morphology follows
that of the UTO substrate, preserving step structure, the long range surface
rms roughness of ~0.1 nm, and the structural correlation exponent of ~1.Comment: 15 pages, 4 figure
Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is
obtained by sublimation of Si from the substrate. The graphene film is
separated from the bulk by a carbon-rich interface layer (hereafter called the
buffer layer) which in part covalently binds to the substrate. Its structural
and electronic properties are currently under debate. In the present work we
report scanning tunneling microscopy (STM) studies of the buffer layer and of
quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling
the buffer layer from the SiC(0001) substrate by means of hydrogen
intercalation. Atomic resolution STM images of the buffer layer reveal that,
within the periodic structural corrugation of this interfacial layer, the
arrangement of atoms is topologically identical to that of graphene. After
hydrogen intercalation, we show that the resulting QFMLG is relieved from the
periodic corrugation and presents no detectable defect sites
STM-induced surface aggregates on metals and oxidized silicon
We have observed an aggregation of carbon or carbon derivatives on platinum
and natively oxidized silicon surfaces during STM measurements in ultra-high
vacuum on solvent-cleaned samples previously structured by e-beam lithography.
We have imaged the aggregated layer with scanning tunneling microscopy (STM) as
well as scanning electron microscopy (SEM). The amount of the aggregated
material increases with the number of STM scans and with the tunneling voltage.
Film thicknesses of up to 10 nm with five successive STM measurements have been
obtained
Exploring the transferability of large supramolecular assemblies to the vacuum-solid interface
We present an interplay of high-resolution scanning tunneling microscopy imaging and the corresponding theoretical calculations based on elastic scattering quantum chemistry techniques of the adsorption of a gold-functionalized rosette assembly and its building blocks on a Au(111) surface with the goal of exploring how to fabricate functional 3-D molecular nanostructures on surfaces. The supramolecular rosette assembly stabilized by multiple hydrogen bonds has been sublimed onto the Au(111) surface under ultra-high vacuum conditions; the resulting surface nanostructures are distinctly different from those formed by the individual molecular building blocks of the rosette assembly, suggesting that the assembly itself can be transferred intact to the surface by in situ thermal sublimation. This unanticipated result will open up new perspectives for growth of complex 3-D supramolecular nanostructures at the vacuum-solid interface
Photon super-bunching from a generic tunnel junction
Generating correlated photon pairs at the nanoscale is a prerequisite to
creating highly integrated optoelectronic circuits that perform quantum
computing tasks based on heralded single-photons. Here we demonstrate
fulfilling this requirement with a generic tip-surface metal junction. When the
junction is luminescing under DC bias, inelastic tunneling events of single
electrons produce a photon stream in the visible spectrum whose super-bunching
index is 17 when measured with a 53 picosecond instrumental resolution limit.
These photon bunches contain true photon pairs of plasmonic origin, distinct
from accidental photon coincidences. The effect is electrically rather than
optically driven - completely absent are pulsed lasers, down-conversions, and
four-wave mixing schemes. This discovery has immediate and profound
implications for quantum optics and cryptography, notwithstanding its
fundamental importance to basic science and its ushering in of heralded photon
experiments on the nanometer scale
Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory
Chalcogen vacancies are considered to be the most abundant point defects in
two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and
predicted to result in deep in-gap states (IGS). As a result, important
features in the optical response of 2D-TMDs have typically been attributed to
chalcogen vacancies, with indirect support from Transmission Electron
Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM
imaging measurements do not provide direct access to the electronic structure
of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a
direct probe of local electronic structure, the interpretation of the chemical
nature of atomically-resolved STM images of point defects in 2D-TMDs can be
ambiguous. As a result, the assignment of point defects as vacancies or
substitutional atoms of different kinds in 2D-TMDs, and their influence on
their electronic properties, has been inconsistent and lacks consensus. Here,
we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS,
and state-of-the-art ab initio density functional theory (DFT) and GW
calculations to determine both the structure and electronic properties of the
most abundant individual chalcogen-site defects common to 2D-TMDs.
Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our
results and analysis strongly suggest that the common chalcogen defects in our
2D-TMDs, prepared and measured in standard environments, are substitutional
oxygen rather than vacancies
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