14 research outputs found
Ultrathin 2D Photodetectors Utilizing Chemical Vapor Deposition Grown WS<sub>2</sub> With Graphene Electrodes
In
this report, graphene (Gr) is used as a 2D electrode and monolayer
WS<sub>2</sub> as the active semiconductor in ultrathin photodetector
devices. All of the 2D materials are grown by chemical vapor deposition
(CVD) and thus pose as a viable route to scalability. The monolayer
thickness of both electrode and semiconductor gives these photodetectors
∼2 nm thickness. We show that graphene is different to conventional
metal (Au) electrodes due to the finite density of states from the
Dirac cones of the valence and conduction bands, which enables the
photoresponsivity to be modulated by electrostatic gating and light
input control. We demonstrate lateral Gr–WS<sub>2</sub>–Gr
photodetectors with photoresponsivities reaching 3.5 A/W under illumination
power densities of 2.5 × 10<sup>7</sup> mW/cm<sup>2</sup>. The
performance of monolayer WS<sub>2</sub> is compared to bilayer WS<sub>2</sub> in photodetectors and we show that increased photoresponsivity
is achieved in the thicker bilayer WS<sub>2</sub> crystals due to
increased optical absorption. This approach of incorporating graphene
electrodes in lateral TMD based devices provides insights on the contact
engineering in 2D optoelectronics, which is crucial for the development
of high performing ultrathin photodetector arrays for versatile applications
Separation of Hydrogen Gas from Coal Gas by Graphene Nanopores
We designed a series of porous graphene
as the separation membrane
for hydrogen gas in coal gas. The permeation process of different
gas molecules (H<sub>2</sub>, CO, CH<sub>4</sub>, and H<sub>2</sub>S) in porous graphene was evaluated under the atmospheric pressure
and high pressure conditions. Our results indicate the hydrogen permeability
and selectivity could be tuned by the size and the shape of the porous
graphene. For graphene with bigger pores, the selectivity for hydrogen
gas could decrease. In the porous graphene with same pore area, the
hydrogen gas selectivity could be affected by the shape of the pore.
The potential of mean force (PMF) of different gases to pass through
a good separation candidate was calculated. The order of PMF for different
gases to pass through the good separation candidate is H<sub>2</sub> < CO < CH<sub>4</sub> ≈ H<sub>2</sub>S, which is also
confirmed by the first-principle density function theory (DFT) calculation
<i>In Situ</i> Atomic Level Dynamics of Heterogeneous Nucleation and Growth of Graphene from Inorganic Nanoparticle Seeds
An <i>in situ</i> heating holder inside an aberration-corrected
transmission electron microscope (AC-TEM) is used to investigate the
real-time atomic level dynamics associated with heterogeneous nucleation
and growth of graphene from Au nanoparticle seeds. Heating monolayer
graphene to an elevated temperature of 800 °C removes the majority
of amorphous carbon adsorbates and leaves a clean surface. The aggregation
of Au impurity atoms into nanoparticle clusters that are bound to
the surface of monolayer graphene causes nucleation of secondary graphene
layers from carbon feedstock present within the microscope chamber.
This enables the <i>in situ</i> study of heterogeneous nucleation
and growth of graphene at the atomic level. We show that the growth
mechanism consists of alternating C cluster attachment and indentation
filling to maintain a uniform growth front of lowest energy. Back-folding
of the graphene growth front is observed, followed by a process that
involves flipping back and attaching to the surrounding region. We
show how the highly polycrystalline graphene seed evolves with time
into a higher order crystalline structure using a combination of AC-TEM
and tight-binding molecular dynamics (TBMD) simulations. This helps
understand the detailed lowest-energy step-by-step pathways associated
with grain boundaries (GB) migration and crystallization processes.
We find the motion of the GB is discontinuous and mediated by both
bond rotation and atom evaporation, supported by density functional
theory calculations and TBMD. These results provide insights into
the formation of crystalline seed domains that are generated during
bottom-up graphene synthesis
Europa und das Meer. Deutsches Historisches Museum, Berlin 13 June 2018 – 06 January 2019
The atomic structure of subnanometer pores in graphene, of interest due to graphene’s potential as a desalination and gas filtration membrane, is demonstrated by atomic resolution aberration corrected transmission electron microscopy. High temperatures of 500 °C and over are used to prevent self-healing of the pores, permitting the successful imaging of open pore geometries consisting of between −4 to −13 atoms, all exhibiting subnanometer diameters. Picometer resolution bond length measurements are used to confirm reconstruction of five-membered ring projections that often decorate the pore perimeter, knowledge which is used to explore the viability of completely self-passivated subnanometer pore structures; bonding configurations where the pore would not require external passivation by, for example, hydrogen to be chemically inert
<i>In Situ</i> Atomic Level Dynamics of Heterogeneous Nucleation and Growth of Graphene from Inorganic Nanoparticle Seeds
An <i>in situ</i> heating holder inside an aberration-corrected
transmission electron microscope (AC-TEM) is used to investigate the
real-time atomic level dynamics associated with heterogeneous nucleation
and growth of graphene from Au nanoparticle seeds. Heating monolayer
graphene to an elevated temperature of 800 °C removes the majority
of amorphous carbon adsorbates and leaves a clean surface. The aggregation
of Au impurity atoms into nanoparticle clusters that are bound to
the surface of monolayer graphene causes nucleation of secondary graphene
layers from carbon feedstock present within the microscope chamber.
This enables the <i>in situ</i> study of heterogeneous nucleation
and growth of graphene at the atomic level. We show that the growth
mechanism consists of alternating C cluster attachment and indentation
filling to maintain a uniform growth front of lowest energy. Back-folding
of the graphene growth front is observed, followed by a process that
involves flipping back and attaching to the surrounding region. We
show how the highly polycrystalline graphene seed evolves with time
into a higher order crystalline structure using a combination of AC-TEM
and tight-binding molecular dynamics (TBMD) simulations. This helps
understand the detailed lowest-energy step-by-step pathways associated
with grain boundaries (GB) migration and crystallization processes.
We find the motion of the GB is discontinuous and mediated by both
bond rotation and atom evaporation, supported by density functional
theory calculations and TBMD. These results provide insights into
the formation of crystalline seed domains that are generated during
bottom-up graphene synthesis
<i>In Situ</i> Atomic Level Dynamics of Heterogeneous Nucleation and Growth of Graphene from Inorganic Nanoparticle Seeds
An <i>in situ</i> heating holder inside an aberration-corrected
transmission electron microscope (AC-TEM) is used to investigate the
real-time atomic level dynamics associated with heterogeneous nucleation
and growth of graphene from Au nanoparticle seeds. Heating monolayer
graphene to an elevated temperature of 800 °C removes the majority
of amorphous carbon adsorbates and leaves a clean surface. The aggregation
of Au impurity atoms into nanoparticle clusters that are bound to
the surface of monolayer graphene causes nucleation of secondary graphene
layers from carbon feedstock present within the microscope chamber.
This enables the <i>in situ</i> study of heterogeneous nucleation
and growth of graphene at the atomic level. We show that the growth
mechanism consists of alternating C cluster attachment and indentation
filling to maintain a uniform growth front of lowest energy. Back-folding
of the graphene growth front is observed, followed by a process that
involves flipping back and attaching to the surrounding region. We
show how the highly polycrystalline graphene seed evolves with time
into a higher order crystalline structure using a combination of AC-TEM
and tight-binding molecular dynamics (TBMD) simulations. This helps
understand the detailed lowest-energy step-by-step pathways associated
with grain boundaries (GB) migration and crystallization processes.
We find the motion of the GB is discontinuous and mediated by both
bond rotation and atom evaporation, supported by density functional
theory calculations and TBMD. These results provide insights into
the formation of crystalline seed domains that are generated during
bottom-up graphene synthesis
Elongated Silicon–Carbon Bonds at Graphene Edges
We study the bond lengths of silicon
(Si) atoms attached to both
armchair and zigzag edges using aberration corrected transmission
electron microscopy with monochromation of the electron beam. An <i>in situ</i> heating holder is used to perform imaging of samples
at 800 °C in order to reduce chemical etching effects that cause
rapid structure changes of graphene edges at room temperature under
the electron beam. We provide detailed bond length measurements for
Si atoms both attached to edges and also as near edge substitutional
dopants. Edge reconstruction is also involved with the addition of
Si dopants. Si atoms bonded to the edge of graphene are compared to
substitutional dopants in the bulk lattice and reveal reduced out-of-plane
distortion and bond elongation. An extended linear array of Si atoms
at the edge is found to be energy-favorable due to inter-Si interactions.
These results provide detailed structural information about the Si–C
bonds in graphene, which may have importance in future catalytic and
electronic applications
Atomically Flat Zigzag Edges in Monolayer MoS<sub>2</sub> by Thermal Annealing
The
edges of 2D materials show novel electronic, magnetic, and
optical properties, especially when reduced to nanoribbon widths.
Therefore, methods to create atomically flat edges in 2D materials
are essential for future exploitation. Atomically flat edges in 2D
materials are found after brittle fracture or when electrically biasing,
but a simple scalable approach for creating atomically flat periodic
edges in monolayer 2D transition metal dichalcogenides has yet to
be realized. Here, we show how heating monolayer MoS<sub>2</sub> to
800 °C in vacuum produces atomically flat Mo terminated zigzag
edges in nanoribbons. We study this at the atomic level using an ultrastable
in situ heating holder in an aberration-corrected transmission electron
microscope and discriminating Mo from S at the edge, revealing unique
Mo terminations for all zigzag orientations that remain stable and
atomically flat when cooling back to room temperature. Highly faceted
MoS<sub>2</sub> nanoribbon constrictions are produced with Mo rich
edge structures that have theoretically predicted spin separated transport
channels, which are promising for spin logic applications
Atomically Flat Zigzag Edges in Monolayer MoS<sub>2</sub> by Thermal Annealing
The
edges of 2D materials show novel electronic, magnetic, and
optical properties, especially when reduced to nanoribbon widths.
Therefore, methods to create atomically flat edges in 2D materials
are essential for future exploitation. Atomically flat edges in 2D
materials are found after brittle fracture or when electrically biasing,
but a simple scalable approach for creating atomically flat periodic
edges in monolayer 2D transition metal dichalcogenides has yet to
be realized. Here, we show how heating monolayer MoS<sub>2</sub> to
800 °C in vacuum produces atomically flat Mo terminated zigzag
edges in nanoribbons. We study this at the atomic level using an ultrastable
in situ heating holder in an aberration-corrected transmission electron
microscope and discriminating Mo from S at the edge, revealing unique
Mo terminations for all zigzag orientations that remain stable and
atomically flat when cooling back to room temperature. Highly faceted
MoS<sub>2</sub> nanoribbon constrictions are produced with Mo rich
edge structures that have theoretically predicted spin separated transport
channels, which are promising for spin logic applications
Atomically Flat Zigzag Edges in Monolayer MoS<sub>2</sub> by Thermal Annealing
The
edges of 2D materials show novel electronic, magnetic, and
optical properties, especially when reduced to nanoribbon widths.
Therefore, methods to create atomically flat edges in 2D materials
are essential for future exploitation. Atomically flat edges in 2D
materials are found after brittle fracture or when electrically biasing,
but a simple scalable approach for creating atomically flat periodic
edges in monolayer 2D transition metal dichalcogenides has yet to
be realized. Here, we show how heating monolayer MoS<sub>2</sub> to
800 °C in vacuum produces atomically flat Mo terminated zigzag
edges in nanoribbons. We study this at the atomic level using an ultrastable
in situ heating holder in an aberration-corrected transmission electron
microscope and discriminating Mo from S at the edge, revealing unique
Mo terminations for all zigzag orientations that remain stable and
atomically flat when cooling back to room temperature. Highly faceted
MoS<sub>2</sub> nanoribbon constrictions are produced with Mo rich
edge structures that have theoretically predicted spin separated transport
channels, which are promising for spin logic applications