8 research outputs found
Probing from Both Sides: Reshaping the Graphene Landscape via Face-to-Face Dual-Probe Microscopy
In two-dimensional samples, all atoms
are at the surface and thereby
exposed for probing and manipulation by physical or chemical means
from both sides. Here, we show that we can access the same point on
both surfaces of a few-layer graphene membrane simultaneously, using
a dual-probe scanning tunneling microscopy (STM) setup. At the closest
point, the two probes are separated only by the thickness of the graphene
membrane. This allows us for the first time to directly measure the
deformations induced by one STM probe on a free-standing membrane
with an independent second probe. We reveal different regimes of stability
of few-layer graphene and show how the STM probes can be used as tools
to shape the membrane in a controlled manner. Our work opens new avenues
for the study of mechanical and electronic properties of two-dimensional
materials
Atomic Structure of Intrinsic and Electron-Irradiation-Induced Defects in MoTe<sub>2</sub>
Studying
the atomic structure of intrinsic defects in two-dimensional
transition-metal dichalcogenides is difficult since they damage quickly
under the intense electron irradiation in transmission electron microscopy
(TEM). However, this can also lead to insights into the creation of
defects and their atom-scale dynamics. We first show that MoTe<sub>2</sub> monolayers without protection indeed quickly degrade during
scanning TEM (STEM) imaging, and discuss the observed atomic-level
dynamics, including a transformation from the 1H phase into 1T′,
3-fold rotationally symmetric defects, and the migration of line defects
between two 1H grains with a 60° misorientation. We then analyze
the atomic structure of MoTe<sub>2</sub> encapsulated between two
graphene sheets to mitigate damage, finding the as-prepared material
to contain an unexpectedly large concentration of defects. These include
similar point defects (or quantum dots, QDs) as those created in the
nonencapsulated material and two different types of line defects (or
quantum wires, QWs) that can be transformed from one to the other
under electron irradiation. Our density functional theory simulations
indicate that the QDs and QWs embedded in MoTe<sub>2</sub> introduce
new midgap states into the semiconducting material and may thus be
used to control its electronic and optical properties. Finally, the
edge of the encapsulated material appears amorphous, possibly due
to the pressure caused by the encapsulation
Atomic Structure of Intrinsic and Electron-Irradiation-Induced Defects in MoTe<sub>2</sub>
Studying
the atomic structure of intrinsic defects in two-dimensional
transition-metal dichalcogenides is difficult since they damage quickly
under the intense electron irradiation in transmission electron microscopy
(TEM). However, this can also lead to insights into the creation of
defects and their atom-scale dynamics. We first show that MoTe<sub>2</sub> monolayers without protection indeed quickly degrade during
scanning TEM (STEM) imaging, and discuss the observed atomic-level
dynamics, including a transformation from the 1H phase into 1T′,
3-fold rotationally symmetric defects, and the migration of line defects
between two 1H grains with a 60° misorientation. We then analyze
the atomic structure of MoTe<sub>2</sub> encapsulated between two
graphene sheets to mitigate damage, finding the as-prepared material
to contain an unexpectedly large concentration of defects. These include
similar point defects (or quantum dots, QDs) as those created in the
nonencapsulated material and two different types of line defects (or
quantum wires, QWs) that can be transformed from one to the other
under electron irradiation. Our density functional theory simulations
indicate that the QDs and QWs embedded in MoTe<sub>2</sub> introduce
new midgap states into the semiconducting material and may thus be
used to control its electronic and optical properties. Finally, the
edge of the encapsulated material appears amorphous, possibly due
to the pressure caused by the encapsulation
Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films
We employ atomically
resolved and element-specific scanning transmission
electron microscopy (STEM) to visualize <i>in situ</i> and
at the atomic scale the crystallization and restructuring processes
of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films.
To this end, we deposit a model heterostructure of thin amorphous
MoS<sub>2</sub> films onto freestanding graphene membranes used as
high-resolution STEM supports. Notably, during STEM imaging the energy
input from the scanning electron beam leads to beam-induced crystallization
and restructuring of the amorphous MoS<sub>2</sub> into crystalline
MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature
MoS<sub>2</sub> synthesis and processing conditions. We thereby directly
observe nucleation, growth, crystallization, and restructuring events
in the evolving MoS<sub>2</sub> films <i>in situ</i> and
at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve
in parallel and that these can dynamically transform into each other.
We further highlight transitions from in-plane to out-of-plane crystallization
of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species,
and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that
underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces
a general approach to <i>in situ</i> visualize at the atomic
scale the growth and restructuring mechanisms of 2D transition-metal
dichalcogenides and other 2D materials
Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films
We employ atomically
resolved and element-specific scanning transmission
electron microscopy (STEM) to visualize <i>in situ</i> and
at the atomic scale the crystallization and restructuring processes
of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films.
To this end, we deposit a model heterostructure of thin amorphous
MoS<sub>2</sub> films onto freestanding graphene membranes used as
high-resolution STEM supports. Notably, during STEM imaging the energy
input from the scanning electron beam leads to beam-induced crystallization
and restructuring of the amorphous MoS<sub>2</sub> into crystalline
MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature
MoS<sub>2</sub> synthesis and processing conditions. We thereby directly
observe nucleation, growth, crystallization, and restructuring events
in the evolving MoS<sub>2</sub> films <i>in situ</i> and
at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve
in parallel and that these can dynamically transform into each other.
We further highlight transitions from in-plane to out-of-plane crystallization
of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species,
and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that
underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces
a general approach to <i>in situ</i> visualize at the atomic
scale the growth and restructuring mechanisms of 2D transition-metal
dichalcogenides and other 2D materials
Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films
We employ atomically
resolved and element-specific scanning transmission
electron microscopy (STEM) to visualize <i>in situ</i> and
at the atomic scale the crystallization and restructuring processes
of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films.
To this end, we deposit a model heterostructure of thin amorphous
MoS<sub>2</sub> films onto freestanding graphene membranes used as
high-resolution STEM supports. Notably, during STEM imaging the energy
input from the scanning electron beam leads to beam-induced crystallization
and restructuring of the amorphous MoS<sub>2</sub> into crystalline
MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature
MoS<sub>2</sub> synthesis and processing conditions. We thereby directly
observe nucleation, growth, crystallization, and restructuring events
in the evolving MoS<sub>2</sub> films <i>in situ</i> and
at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve
in parallel and that these can dynamically transform into each other.
We further highlight transitions from in-plane to out-of-plane crystallization
of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species,
and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that
underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces
a general approach to <i>in situ</i> visualize at the atomic
scale the growth and restructuring mechanisms of 2D transition-metal
dichalcogenides and other 2D materials
Chemical Oxidation of Graphite: Evolution of the Structure and Properties
Graphene
oxide is a complex material whose synthesis is still incompletely
understood. To study the time evolution of structural and chemical
properties of oxidized graphite, samples at different temporal stages
of oxidation were selected and characterized through a number of techniques:
X-ray photoelectron spectroscopy for the content and bonding of oxygen,
X-ray diffraction for the level of intercalation, Raman spectroscopy
for the detection of structural changes, electrical resistivity measurements
for probing charge localization on the macroscopic scale, and scanning
transmission electron microscopy for the atomic structure of the graphene
oxide flakes. We found a nonlinear behavior of oxygen uptake with
time where two concentration plateaus were identified: Uptake reached
20 at % in the first 15 min, and after 1 h a second uptake started,
reaching a highest oxygen concentration of >30 at % after 2 h of
oxidation.
At the same time, the interlayer distance expanded to more than twice
the value of graphite and the electrical resistivity increased by
seven orders of magnitude. After 4 days of chemical processing, the
expanded structure of graphite oxide became unstable and spontaneously
exfoliated; more than 2 weeks resulted in a significant decrease in
the oxygen content accompanied by reaggregation of the GO sheets.
These correlated measurements allow us to offer a comprehensive view
into the complex oxidation process
High-Performance Hybrid Electronic Devices from Layered PtSe<sub>2</sub> Films Grown at Low Temperature
Layered
two-dimensional (2D) materials display great potential
for a range of applications, particularly in electronics. We report
the large-scale synthesis of thin films of platinum diselenide (PtSe<sub>2</sub>), a thus far scarcely investigated transition metal dichalcogenide.
Importantly, the synthesis by thermally assisted conversion is performed
at 400 °C, representing a breakthrough for the direct integration
of this material with silicon (Si) technology. Besides the thorough
characterization of this 2D material, we demonstrate its promise for
applications in high-performance gas sensing with extremely short
response and recovery times observed due to the 2D nature of the films.
Furthermore, we realized vertically stacked heterostructures of PtSe<sub>2</sub> on Si which act as both photodiodes and photovoltaic cells.
Thus, this study establishes PtSe<sub>2</sub> as a potential candidate
for next-generation sensors and (opto-)electronic devices, using fabrication
protocols compatible with established Si technologies