7 research outputs found
Programmable Sub-nanometer Sculpting of Graphene with Electron Beams
Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without <i>C</i><sub><i>s</i></sub> correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains
Programmable Sub-nanometer Sculpting of Graphene with Electron Beams
Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without <i>C</i><sub><i>s</i></sub> correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains
Programmable Sub-nanometer Sculpting of Graphene with Electron Beams
Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without <i>C</i><sub><i>s</i></sub> correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains
Lattice Expansion in Seamless Bilayer Graphene Constrictions at High Bias
Our understanding of sp<sup>2</sup> carbon nanostructures
is still
emerging and is important for the development of high performance
all carbon devices. For example, in terms of the structural behavior
of graphene or bilayer graphene at high bias, little to nothing is
known. To this end, we investigated bilayer graphene constrictions
with closed edges (seamless) at high bias using <i>in situ</i> atomic resolution transmission electron microscopy. We directly
observe a highly localized anomalously large lattice expansion inside
the constriction. Both the current density and lattice expansion increase
as the bilayer graphene constriction narrows. As the constriction
width decreases below 10 nm, shortly before failure, the current density
rises to 4 × 10<sup>9</sup> A cm<sup>–2</sup> and the
constriction exhibits a lattice expansion with a uniaxial component
showing an expansion approaching 5% and an isotropic component showing
an expansion exceeding 1%. The origin of the lattice expansion is
hard to fully ascribe to thermal expansion. Impact ionization is a
process in which charge carriers transfer from bonding states to antibonding
states, thus weakening bonds. The altered character of C–C
bonds by impact ionization could explain the anomalously large lattice
expansion we observe in seamless bilayer graphene constrictions. Moreover,
impact ionization might also contribute to the observed anisotropy
in the lattice expansion, although strain is probably the predominant
factor
γ‑Iron Phase Stabilized at Room Temperature by Thermally Processed Graphene Oxide
Stabilizing nanoparticles on surfaces,
such as graphene, is a growing
field of research. Thereby, iron particle stabilization on carbon
materials is attractive and finds applications in charge-storage devices,
catalysis, and others. In this work, we describe the discovery of
iron nanoparticles with the face-centered cubic structure that was
postulated not to exist at ambient conditions. In bulk, the γ-iron
phase is formed only above 917 °C, and transforms back to the
thermodynamically favored α-phase upon cooling. Here, with X-ray
diffraction and Mössbauer spectroscopy we unambiguously demonstrate
the unexpected room-temperature stability of the γ-phase of
iron in the form of the austenitic nanoparticles with low carbon content
from 0.60% through 0.93%. The nanoparticles have controllable diameter
range from 30 nm through 200 nm. They are stabilized by a layer of
Fe/C solid solution on the surface, serving as the buffer controlling
carbon content in the core, and by a few-layer graphene as an outermost
shell
Confined Crystals of the Smallest Phase-Change Material
The demand for high-density memory
in tandem with limitations imposed
by the minimum feature size of current storage devices has created
a need for new materials that can store information in smaller volumes
than currently possible. Successfully employed in commercial optical
data storage products, phase-change materials, that can reversibly
and rapidly change from an amorphous phase to a crystalline phase
when subject to heating or cooling have been identified for the development
of the next generation electronic memories. There are limitations
to the miniaturization of these devices due to current synthesis and
theoretical considerations that place a lower limit of 2 nm on the
minimum bit size, below which the material does not transform in the
structural phase. We show here that by using carbon nanotubes of less
than 2 nm diameter as templates phase-change nanowires confined to
their smallest conceivable scale are obtained. Contrary to previous
experimental evidence and theoretical expectations, the nanowires
are found to crystallize at this scale and display amorphous-to-crystalline
phase changes, fulfilling an important prerequisite of a memory element.
We show evidence for the smallest phase-change material, extending
thus the size limit to explore phase-change memory devices at extreme
scales
Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB<sub>2</sub> Nanoparticles
Recent
years have seen an increasing research effort focused on nanoscaling
of metal borides, a class of compounds characterized by a variety
of crystal structures and bonding interactions. Despite being subject
to an increasing number of studies in the application field, comprehensive
studies of the size-dependent structural changes of metal borides
are limited. In this work, size-dependent microstructural analysis
of the VB<sub>2</sub> nanocrystals prepared by means of a size-controlled
colloidal solution synthesis is carried out using X-ray powder diffraction.
The contributions of crystallite size and strain to X-ray line broadening
is separated by introducing a modified Williamson–Hall method
taking into account different reflection profile shapes. For average
crystallite sizes smaller than ca. 20 nm, a remarkable increase of
lattice strain is observed together with a significant contraction
of the hexagonal lattice decreasing primarily the cell parameter <i>c</i>. Exemplary density-functional theory calculations support
this trend. The size-dependent lattice contraction of VB<sub>2</sub> nanoparticles is associated with the decrease of the interatomic
boron distances along the <i>c</i>-axis. The larger fraction
of constituent atoms at the surface is formed by boron atoms. Accordingly,
lattice contraction is considered to be a surface effect. The anisotropy
of the size-dependent lattice contraction in VB<sub>2</sub> nanocrystals
is in line with the higher compressibility of its macroscopic bulk
structure along the <i>c</i>-axis revealed by theoretical
calculations of the respective elastic properties. Transmission electron
microscopy indicates that the VB<sub>2</sub> nanocrystals are embedded
in an amorphous matrix. X-ray photoelectron spectroscopy analysis
reveals that this matrix is mainly composed of boric acid, boron oxides,
and vanadium oxides. VB<sub>2</sub> nanocrystals coated with these
oxygen containing amorphous species are stable up to 789 °C as
evidenced by thermal analysis and temperature dependent X-ray diffraction
measurements carried out under Ar atmosphere. Electrokinetic measurement
indicates that the aqueous suspension of VB<sub>2</sub> nanoparticles
with hydroxyl groups on the surface region has a good stability at
neutral and basic pH arising from electrostatic stabilizatio