6 research outputs found
Hydrogen Evolution Catalyzed by a Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes
Molybdenum
disulfide has been demonstrated as a promising catalyst
for hydrogen evolution reaction (HER). However, its performance is
limited by fractional active edge sites and the strong dependence
on hydrogen coverage. In this study, we find an enhanced HER performance
in a two-dimensional substoichiometric molybdenum sulfide. Both first-principles
calculations and experimental results demonstrate that the basal plane
is catalytically active toward HER, as evidenced by an optimum Gibbs
free energy and a low reaction overpotential. More interestingly,
the HER performance is insensitive to hydrogen coverage and can be
improved under compressive in-plane biaxial strains. Our results suggest
not only an improved HER performance of substoichiometric molybdenum
sulfide due to its chemical reactive basal plane but also a way to
tune the performance
Orientation and Electronic Structures of Multilayered Graphene Nanoribbons Produced by Two-Zone Chemical Vapor Deposition
The
orientation and electronic structure of multilayered graphene
nanoribbons with an armchair-edge (AGNRs) were determined by low-temperature
scanning tunneling microscopy in this study. The orientation of AGNRs
was found to be an edge-on structure when positioned as a top layer,
while previous reports showed a face-on structure for monolayered
AGNRs on Au(111). According to density functional theory calculations,
AGNRs in a top layer preferentially form as edge-on structures rather
than face-on structures due to the balance of CH−π and
π–π interactions between AGNRs. Scanning tunneling
spectroscopy and density functional theory calculations revealed that
the electronic structures of multilayered AGNRs are similar to those
in a gas-phase due to the lack of interaction between AGNRs and the
Au(111) substrate. The observation of AGNRs in mutilayers might suggest
the conformation-assisted mechanism of dehydrogenation when there
is no contact with the Au(111) substrate
Substoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes
Molybdenum sulfide
(MoS<sub>2</sub>) is widely recognized for its
catalytic activities where the edges of the crystals turn over reactions.
Generating sulfur defects on the basal plane of MoS<sub>2</sub> can
improve its catalytic activity, but generally, there is a lack of
model systems for understanding metal-centered catalysis on the basal
planes. Here, we synthesized a new phase of substoichiometric molybdenum
sulfide (s-MoS<sub><i>x</i></sub>) on a sulfur-enriched
copper substrate. The basal plane of s-MoS<sub><i>x</i></sub> contains chemically reactive Mo-rich sites that can undergo dynamic
dissociative adsorption/desorption processes with molecular hydrogen,
thus demonstrating its usefulness for hydrogen-transfer catalysis.
In addition, scanning tunneling microscopy was used to monitor surface-directed
Ullmann coupling of 2,8-dibromo-dibenzothiophene molecules on s-MoS<sub><i>x</i></sub> nanosheets, where the 4-fold symmetric surface
sites on s-MoS<sub><i>x</i></sub> direct C–C coupling
to form cyclic tetramers with high selectivity
Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus
Two-dimensional black
phosphorus
(BP) has sparked enormous research interest due to its high carrier
mobility, layer-dependent direct bandgap and outstanding in-plane
anisotropic properties. BP is one of the few two-dimensional materials
where it is possible to tune the bandgap over a wide energy range
from the visible up to the infrared. In this article, we report the
observation of a giant Stark effect in electrostatically gated few-layer
BP. Using low-temperature scanning tunnelling microscopy, we observed
that in few-layer BP, when electrons are injected, a monotonic reduction
of the bandgap occurs. The injected electrons compensate the existing
defect-induced holes and achieve up to 35.5% bandgap modulation in
the light-doping regime. When probed by tunnelling spectroscopy, the
local density of states in few-layer BP shows characteristic resonance
features arising from layer-dependent sub-band structures due to quantum
confinement effects. The demonstration of an electrical gate-controlled
giant Stark effect in BP paves the way to designing electro-optic
modulators and photodetector devices that can be operated in a wide
electromagnetic spectral range
Two-Dimensional Polymer Synthesized <i>via</i> Solid-State Polymerization for High-Performance Supercapacitors
Two-dimensional
(2-D) polymer has properties that are attractive
for energy storage applications because of its combination of heteroatoms,
porosities and layered structure, which provides redox chemistry and
ion diffusion routes through the 2-D planes and 1-D channels. Here,
conjugated aromatic polymers (CAPs) were synthesized in quantitative
yield <i>via</i> solid-state polymerization of phenazine-based
precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP)
with different positions of bromine substituents on a phenazine-derived
scaffold, C–C cross coupling was induced following thermal
debromination. CAP-2 is polymerized from monomers that have been prepacked
into layered structure (3-TBQP). It can be mechanically exfoliated
into micrometer-sized ultrathin sheets that show sharp Raman peaks
which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied
as an asymmetric supercapacitor, it delivers a specific capacitance
of 233 F g<sup>–1</sup> at a current density of 1.0 A g<sup>–1</sup>, and shows outstanding cycle performance
Surface Functionalization of Black Phosphorus via Potassium toward High-Performance Complementary Devices
Two-dimensional
black phosphorus configured field-effect transistor
devices generally show a hole-dominated ambipolar transport characteristic,
thereby limiting its applications in complementary electronics. Herein,
we demonstrate an effective surface functionalization scheme on few-layer
black phosphorus, through in situ surface modification with potassium,
with a view toward high performance complementary device applications.
Potassium induces a giant electron doping effect on black phosphorus
along with a clear bandgap reduction, which is further corroborated
by in situ photoelectron spectroscopy characterizations. The electron
mobility of black phosphorus is significantly enhanced to 262 (377)
cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> by over
1 order of magnitude after potassium modification for two-terminal
(four-terminal) measurements. Using lithography technique, a spatially
controlled potassium doping technique is developed to establish high-performance
complementary devices on a single black phosphorus nanosheet, for
example, the p–n homojunction-based diode achieves a near-unity
ideality factor of 1.007 with an on/off ratio of ∼10<sup>4</sup>. Our findings coupled with the tunable nature of in situ modification
scheme enable black phosphorus as a promising candidate for further
complementary electronics