16 research outputs found
Strain Anisotropy Driven Spontaneous Formation of Nanoscrolls from Two-Dimensional Janus Layers
Two-dimensional Janus transition metal dichalcogenides (TMDs) have attracted
attention due to their emergent properties arising from broken mirror symmetry
and self-driven polarisation fields. While it has been proposed that their vdW
superlattices hold the key to achieving superior properties in piezoelectricity
and photovoltiacs, available synthesis has ultimately limited their
realisation. Here, we report the first packed vdW nanoscrolls made from Janus
TMDs through a simple one-drop solution technique. Our results, including
ab-initio simulations, show that the Bohr radius difference between the top
sulphur and the bottom selenium atoms within Janus M_Se^S (M=Mo, W) results in
a permanent compressive surface strain that acts as a nanoscroll formation
catalyst after small liquid interaction. Unlike classical 2D layers, the
surface strain in Janus TMDs can be engineered from compressive to tensile by
placing larger Bohr radius atoms on top (M_S^Se) to yield inverted C scrolls.
Detailed microscopy studies offer the first insights into their morphology and
readily formed Moir\'e lattices. In contrast, spectroscopy and FETs studies
establish their excitonic and device properties and highlight significant
differences compared to 2D flat Janus TMDs. These results introduce the first
polar Janus TMD nanoscrolls and introduce inherent strain-driven scrolling
dynamics as a catalyst to create superlattices
Spatial coherence of room-temperature monolayer WSe exciton-polaritons in a trap
The emergence of spatial and temporal coherence of light emitted from
solid-state systems is a fundamental phenomenon, rooting in a plethora of
microscopic processes. It is intrinsically aligned with the control of
light-matter coupling, and canonical for laser oscillation. However, it also
emerges in the superradiance of multiple, phase-locked emitters, and more
recently, coherence and long-range order have been investigated in bosonic
condensates of thermalized light, as well as in exciton-polaritons driven to a
ground state via stimulated scattering. Here, we experimentally show that the
interaction between photons in a Fabry-Perot microcavity and excitons in an
atomically thin WSe layer is sufficient such that the system enters the
hybridized regime of strong light-matter coupling at ambient conditions. Via
Michelson interferometry, we capture clear evidence of increased spatial and
temporal coherence of the emitted light from the spatially confined system
ground-state. The coherence build-up is accompanied by a threshold-like
behaviour of the emitted light intensity, which is a fingerprint of a polariton
laser effect. Valley-physics is manifested in the presence of an external
magnetic field, which allows us to manipulate K and K' polaritons via the
Valley-Zeeman-effect. Our findings are of high application relevance, as they
confirm the possibility to use atomically thin crystals as simple and versatile
components of coherent light-sources, and in valleytronic applications at room
temperature.Comment: 13 pages, 4 figure
The synthesis of competing phase GeSe and GeSe2 2D layered materials.
We demonstrate the synthesis of layered anisotropic semiconductor GeSe and GeSe2 nanomaterials through low temperature (∼400 °C) and atmospheric pressure chemical vapor deposition using halide based precursors. Results show that GeI2 and H2Se precursors successfully react in the gas-phase and nucleate on a variety of target substrates including sapphire, Ge, GaAs, or HOPG. Layer-by-layer growth takes place after nucleation to form layered anisotropic materials. Detailed SEM, EDS, XRD, and Raman spectroscopy measurements together with systematic CVD studies reveal that the substrate temperature, selenium partial pressure, and the substrate type ultimately dictate the resulting stoichiometry and phase of these materials. Results from this work introduce the phase control of Ge and Se based nanomaterials (GeSe and GeSe2) using halide based CVD precursors at ATM pressures and low temperatures. Overall findings also extend our fundamental understanding of their growth by making the first attempt to correlate growth parameters to resulting competing phases of Ge-Se based materials