272 research outputs found
Evolution of Interlayer Coupling in Twisted MoS2 Bilayers
Van der Waals (vdW) coupling is emerging as a powerful method to engineer and
tailor physical properties of atomically thin two-dimensional (2D) materials.
In graphene/graphene and graphene/boron-nitride structures it leads to
interesting physical phenomena ranging from new van Hove singularities1-4 and
Fermi velocity renormalization5, 6 to unconventional quantum Hall effects7 and
Hofstadter's butterfly pattern8-12. 2D transition metal dichalcogenides
(TMDCs), another system of predominantly vdW-coupled atomically thin layers13,
14, can also exhibit interesting but different coupling phenomena because TMDCs
can be direct or indirect bandgap semiconductors15, 16. Here, we present the
first study on the evolution of interlayer coupling with twist angles in
as-grown MoS2 bilayers. We find that an indirect bandgap emerges in bilayers
with any stacking configuration, but the bandgap size varies appreciably with
the twist angle: it shows the largest redshift for AA- and AB-stacked bilayers,
and a significantly smaller but constant redshift for all other twist angles.
The vibration frequency of the out-of-plane phonon in MoS2 shows similar twist
angle dependence. Our observations, together with ab initio calculations,
reveal that this evolution of interlayer coupling originates from the repulsive
steric effects, which leads to different interlayer separations between the two
MoS2 layers in different stacking configurations
Systematic Determination of Absolute Absorption Cross-section of Individual Carbon Nanotubes
Determination of optical absorption cross-section is always among the central
importance of understanding a material. However its realization on individual
nanostructures, such as carbon nanotubes, is experimentally challenging due to
the small extinction signal using conventional transmission measurements. Here
we develop a technique based on polarization manipulation to enhance the
sensitivity of single-nanotube absorption spectroscopy by two-orders of
magnitude. We systematically determine absorption cross-section over broad
spectral range at single-tube level for more than 50 chirality-defined
single-walled nanotubes. Our data reveals chirality-dependent one-dimensional
photo-physics through the behaviours of exciton oscillator strength and
lifetime. We also establish an empirical formula to predict absorption spectrum
of any nanotube, which provides the foundation to determine quantum
efficiencies in important photoluminescence and photovoltaic processes
Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals
To explore new constituents in two-dimensional materials and to combine their
best in van der Waals heterostructures, are in great demand as being unique
platform to discover new physical phenomena and to design novel functionalities
in interface-based devices. Herein, PbI2 crystals as thin as few-layers are
first synthesized, particularly through a facile low-temperature solution
approach with the crystals of large size, regular shape, different thicknesses
and high-yields. As a prototypical demonstration of flexible band engineering
of PbI2-based interfacial semiconductors, these PbI2 crystals are subsequently
assembled with several transition metal dichalcogenide monolayers. The
photoluminescence of MoS2 is strongly enhanced in MoS2/PbI2 stacks, while a
dramatic photoluminescence quenching of WS2 and WSe2 is revealed in WS2/PbI2
and WSe2/PbI2 stacks. This is attributed to the effective heterojunction
formation between PbI2 and these monolayers, but type I band alignment in
MoS2/PbI2 stacks where fast-transferred charge carriers accumulate in MoS2 with
high emission efficiency, and type II in WS2/PbI2 and WSe2/PbI2 stacks with
separated electrons and holes suitable for light harvesting. Our results
demonstrate that MoS2, WS2, WSe2 monolayers with very similar electronic
structures themselves, show completely distinct light-matter interactions when
interfacing with PbI2, providing unprecedent capabilities to engineer the
device performance of two-dimensional heterostructures.Comment: 36 pages, 5 figure
- …