58 research outputs found
Nucleation and growth of thin films of rod--like conjugated molecules
Thin films formed from small molecules rapidly gain importance in different
technological fields. To explain their growth, methods developed for
zero--dimensional atoms as the film forming particles are applied. However, in
organic thin film growth the dimensionality of the building blocks comes into
play. Using the special case of the model molecule para--Sexiphenyl, we will
emphasize the challenges that arise from the anisotropic and one--dimensional
nature of building blocks. Differences or common features with other rodlike
molecules will be discussed. The typical morphologies encountered for this
group of molecules and the relevant growth modes will be investigated. Special
attention is given to the transition between flat lying and upright orientation
of the building blocks during nucleation. We will further discuss methods to
control the molecular orientation and describe the involved diffusion processes
qualitatively and quantitatively.Comment: as submitted to JPCM (revised version) fixed figures and a few lines
of tex
Helium Ion Microscopy
Helium Ion Microcopy (HIM) based on Gas Field Ion Sources (GFIS) represents a
new ultra high resolution microscopy and nano-fabrication technique. It is an
enabling technology that not only provides imagery of conducting as well as
uncoated insulating nano-structures but also allows to create these features.
The latter can be achieved using resists or material removal due to sputtering.
The close to free-form sculpting of structures over several length scales has
been made possible by the extension of the method to other gases such as Neon.
A brief introduction of the underlying physics as well as a broad review of the
applicability of the method is presented in this review.Comment: Revised versio
The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM
The growth of para-sexiphenyl (6P) thin films as a function of substrate
temperature on Ir{111} supported graphene flakes has been studied in real-time
with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron
Diffraction (\mu LEED) has been used to determine the structure of the
different 6P features formed on the surface. We observe the nucleation and
growth of a wetting layer consisting of lying molecules in the initial stages
of growth. Graphene defects -- wrinkles -- are found to be preferential sites
for the nucleation of the wetting layer and of the 6P needles that grow on top
of the wetting layer in the later stages of deposition. The molecular structure
of the wetting layer and needles is found to be similar. As a result, only a
limited number of growth directions are observed for the needles. In contrast,
on the bare Ir{111} surface 6P molecules assume an upright orientation. The
formation of ramified islands is observed on the bare Ir{111} surface at 320 K
and 352 K, whereas at 405 K the formation of a continuous layer of upright
standing molecules growing in a step flow like manner is observed.Comment: 9 pages, 7 figures, Revised Version as accepted for publication in
Surface Scienc
Visualization of steps and surface reconstructions in Helium Ion Microscopy with atomic precision
Helium Ion Microscopy is known for its surface sensitivity and high lateral
resolution. Here, we present results of a Helium Ion Microscopy based
investigation of a surface confined alloy of Ag on Pt(111). Based on a change
of the work function of 25\,meV across the atomically flat terraces we can
distinguish Pt rich from Pt poor areas and visualize the single atomic layer
high steps between the terraces. Furthermore, dechanneling contrast has been
utilized to measure the periodicity of the hcp/fcc pattern formed in the 2--3
layers thick Ag/Pt alloy film. A periodicity of 6.65\,nm along the
surface direction has been measured. In terms of
crystallography a hcp domain is obtained through a lateral displacement of a
part of the outermost layer by of a nearest neighbour spacing
along . This periodicity is measured with atomic
precision: coincidence between the Ag and the Pt lattices is observed for 23 Ag
atoms on 24 Pt atoms. The findings are perfectly in line with results obtained
with Low Energy Electron Microscopy and Phase Contrast Atomic Force Microscopy.Comment: 15 pages, 7 figure
To see or not to see: Imaging surfactant coated nano--particles using HIM and SEM
Nano--particles are of great interest in fundamental and applied research.
However, their accurate visualization is often difficult and the interpretation
of the obtained images can be complicated. We present a comparative scanning
electron microscopy and helium ion microscopy study of
cetyltrimethylammonium--bromide (CTAB) coated gold nano--rods. Using both
methods we show how the gold core as well as the surrounding thin CTAB shell
can selectively be visualized. This allows for a quantitative determination of
the dimensions of the gold core or the CTAB shell. The obtained CTAB shell
thickness of 1.0 nm--1.5 nm is in excellent agreement with earlier results
using more demanding and reciprocal space techniques.Comment: revised versio
Hierarchy of adhesion forces in patterns of photoreactive surface layers
Precise control of surface properties including electrical characteristics,
wettability, and friction is a prerequisite for manufacturing modern organic
electronic devices. The successful combination of bottom up approaches for
aligning and orienting the molecules and top down techniques to structure the
substrate on the nano and micrometer scale allows the cost efficient
fabrication and integration of future organic light emitting diodes and organic
thin film transistors. One possibility for the top down patterning of a surface
is to utilize different surface free energies or wetting properties of a
functional group. Here, we used friction force microscopy (FFM) to reveal
chemical patterns inscribed by a photolithographic process into a
photosensitive surface layer. FFM allowed the simultaneous visualization of at
least three different chemical surface terminations. The underlying mechanism
is related to changes in the chemical interaction between probe and film
surface.Comment: 11 pages, 5 figures The following article has been submitted to
Journal of Chemical Physics. After it is published, it will be found at
http://jcp.aip.org
Smooth Growth of Organic Semiconductor Films on Graphene for High-Efficiency Electronics
High-quality thin films of conjugated molecules with smooth interfaces are important to assist the advent of organic electronics. Here, we report on the layer-by-layer growth of the organic semiconductor molecule p-sexiphenyl (6P) on the transparent electrode material graphene. Low energy electron microscopy and micro low energy electron diffraction reveal the morphological and structural evolution of the thin film. The layer-by-layer growth of 6P on graphene proceeds by subsequent adding of {(111)} layers
Quantitative nanoscale imaging using transmission He ion channelling contrast: Proof-of-concept and application to study isolated crystalline defects
A newly developed microscope prototype, namely npSCOPE, consisting of a Gas Field Ion Source (GFIS) column and a position sensitive Delay-line Detector (DLD)
was used to perform Scanning Transmission Ion Microscopy (STIM) using keV He+ ions. One experiment used 25 keV ions and a second experiment used 30 keV ions.
STIM imaging of a 50 nm thick free-standing gold membrane exhibited excellent contrast due to ion channelling and revealed rich microstructural features including
isolated nanoscale twin bands which matched well with the contrast in the conventional ion-induced Secondary Electron (SE) imaging mode. Transmission Kikuchi
Diffraction (TKD) and Backscattered Electron (BSE) imaging were performed on the same areas to correlate and confirm the microstructural features observed in
STIM. Monte Carlo simulations of the ion and electron trajectories were performed with parameters similar to the experimental conditions to derive insights related
to beam broadening and its effect in the degradation of transmission image resolution. For the experimental conditions used, STIM imaging showed a lateral resolution close to30 nm. Dark twin bands in bright grains as well as bright twin bands in dark grains were observed in STIM. Some of the twin bands were invisible in
STIM. For the specific experimental conditions used, the ion transmission efficiency across a particular twin band was found to decrease by a factor of 2.8. Surprisingly, some grains showed contrast reversal when the Field of View (FOV) was changed indicating the sensitivity of the channelling contrast to even small
changes in illumination conditions. These observations are discussed using ion channelling conditions and crystallographic orientations of the grains and twin bands.
This study demonstrates for the first time the potential of STIM imaging using keV He+ ions to quantitatively investigate channelling in nanoscale structures including
isolated crystalline defects
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