57 research outputs found
Visualization of defect-induced excitonic properties of the edges and grain boundaries in synthesized monolayer molybdenum disulfide
Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs)
are attractive materials for next generation nanoscale optoelectronic
applications. Understanding nanoscale optical behavior of the edges and grain
boundaries of synthetically grown TMDCs is vital for optimizing their
optoelectronic properties. Elucidating the nanoscale optical properties of 2D
materials through far-field optical microscopy requires a diffraction-limited
optical beam diameter sub-micron in size. Here we present our experimental work
on spatial photoluminescence (PL) scanning of large size ( microns)
monolayer MoS grown by chemical vapor deposition (CVD) using a diffraction
limited blue laser beam spot (wavelength 405 nm) with a beam diameter as small
as 200 nm allowing us to probe nanoscale excitonic phenomena which was not
observed before. We have found several important features: (i) there exists a
sub-micron width strip ( nm) along the edges that fluoresces brighter than the region far inside; (ii) there is another brighter
wide region consisting of parallel fluorescing lines ending at the corners of
the zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonic
peak, as large as meV, in the PL spectra from the edges. Using
density functional theory calculations, we attribute this giant blue shift to
the adsorption of oxygen dimers at the edges, which reduces the excitonic
binding energy. Our results not only shed light on defect-induced excitonic
properties, but also offer an attractive route to tailor optical properties at
the TMDC edges through defect engineering.Comment: 10 pages, 4 figures in Journal of Physical Chemistry C, 201
Photoresponse of Natural van der Waals Heterostructures
Van der Waals (vdW) heterostructures consisting of two dimensional materials
offer a platform to obtain material by design and are very attractive owing to
novel electronic states. Research on 2D van der Waals heterostructures (vdWH)
has so far been focused on fabricating individually stacked atomically thin
unary or binary crystals. Such systems include graphene (Gr), hexagonal boron
nitride (h-BN) and member of the transition metal dichalcogenides family. Here
we present our experimental study of the opto-electronic properties of a
naturally occurring vdWH, known as Franckeite, which is a complex layered
crystal composed of lead, tin, antimony, iron and sulfur. We present here that
thin film franckeite (60 nm < d < 100 nm) behave as narrow band gap
semiconductor demonstrating a wide band photoresponse. We have observed the
band-edge transition at ~ 1500 nm (~830 meV) and high external quantum
efficiency (EQE~3%) at room temperature. Laser power resolved and temperature
resolved photocurrent measurements reveal that the photo-carrier generation and
recombination are dominated by continuously distributed trap states within the
band gap. To understand wavelength resolved photocurrent, we also calculated
the optical absorption properties via density functional theory. Finally, we
have shown that the device has fast photoresponse with rise time as fast as ~ 1
ms. Our study provides a fundamental understanding of the optoelectronic
behavior in a complex naturally occurring vdWH and can open up the
possibilities of producing new type of nanoscale optoelectronic devices with
tailored properties.Comment: 10 pages, 5 figures (to be appeared in ACS NANO
Integrative taxonomy of the Plain-backed Thrush (Zoothera mollissima) complex (Aves, Turdidae) reveals cryptic species, including a new species
Ultra-scaled MoS2 transistors and circuits fabricated without nanolithography
The future scaling of semiconductor devices can be continued only by the development of novel nanofabrication techniques and atomically thin transistor channels. Here we demonstrate ultra-scaled MoS2 field-effect transistors (FETs) realized by a shadow evaporation method which does not require nanofabrication. The method enables large-scale fabrication of MoS2 FETs with fully gated ∼10 nm long channels. The realized ultra-scaled MoS2 FETs exhibit very small hysteresis of current–voltage characteristics, high drain currents up to ∼560 A m−1, very good drain current saturation for such ultra-short devices, subthreshold swing of ∼120 mV dec−1, and drain current on/ off ratio of ∼106 in air ambient. The fabricated ultra-scaled MoS2 FETs are also used to realize logic gates in n-type depletion-load technology. The inverters exhibit a voltage gain of ∼50 at a power supply voltage of only 1.5 V and are capable of in/out signal matching
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