9 research outputs found
Auger Recombination in Chemical Vapor Deposition-Grown Monolayer WS<sub>2</sub>
Reduced
dimensionality and strong Coulombic interactions in monolayer
semiconductors lead to enhanced many-body interactions. Here, we report
Auger recombination, i.e., exciton–exciton annihilation, in
large-area chemical vapor deposition-grown monolayer WS<sub>2</sub>. Using ultrafast spectroscopy, we experimentally determine the Auger
rate to be 0.089 ± 0.001 cm<sup>2</sup>/s at room temperature,
which is an order of magnitude greater than the bulk value. This nonradiative
recombination pathway dominates, regardless of excitation energy,
for exciton densities greater than 8.0 ± 0.6 × 10<sup>10</sup> cm<sup>–2</sup> and below the Mott density. Higher-energy
excitation above the A exciton resonance may initially produce a hot
electron–hole gas that precedes exciton formation. Therefore,
we use resonant excitation of the A exciton to ensure accuracy and
avoid artifacts associated with other photogenerated species
Photoinduced Bandgap Renormalization and Exciton Binding Energy Reduction in WS<sub>2</sub>
Strong
Coulomb attraction in monolayer transition metal dichalcogenides
gives rise to tightly bound excitons and many-body interactions that
dominate their optoelectronic properties. However, this Coulomb interaction
can be screened through control of the surrounding dielectric environment
as well as through applied voltage, which provides a potential means
of tuning the bandgap, exciton binding energy, and emission wavelength.
Here, we directly show that the bandgap and exciton binding energy
can be optically tuned by means of the intensity of the incident light.
Using transient absorption spectroscopy, we identify a sub-picosecond
decay component in the excited-state dynamics of WS<sub>2</sub> that
emerges for incident photon energies above the A-exciton resonance,
which originates from a nonequilibrium population of charge carriers
that form excitons as they cool. The generation of this charge-carrier
population exhibits two distinct energy thresholds. The higher threshold
is coincident with the onset of continuum states and therefore provides
a direct optical means of determining both the bandgap and exciton
binding energy. Using this technique, we observe a reduction in the
exciton binding energy from 310 ± 30 to 220 ± 20 meV as
the excitation density is increased from 3 Ă— 10<sup>11</sup> to
1.2 Ă— 10<sup>12</sup> photons/cm<sup>2</sup>. This reduction
is due to dynamic dipolar screening of Coulomb interactions by excitons,
which is the underlying physical process that initiates bandgap renormalization
and leads to the insulator–metal transition in monolayer transition
metal dichalcogenides
Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS<sub>2</sub>
Transition-metal
dichalcogenides (TMDs) are an exciting class of
2D materials that exhibit many promising electronic and optoelectronic
properties with potential for future device applications. The properties
of TMDs are expected to be strongly influenced by a variety of defects
which result from growth procedures and/or fabrication. Despite the
importance of understanding defect-related phenomena, there remains
a need for quantitative nanometer-scale characterization of defects
over large areas in order to understand the relationship between defects
and observed properties, such as photoluminescence (PL) and electrical
conductivity. In this work, we present conductive atomic force microscopy
measurements which reveal nanometer-scale electronically active defects
in chemical vapor deposition-grown WS<sub>2</sub> monolayers with
defect density varying from 2.3 × 10<sup>10</sup> cm<sup>–2</sup> to 4.5 × 10<sup>11</sup> cm<sup>–2</sup>. Comparing
these defect density measurements with PL measurements across large
areas (>20 ÎĽm distances) reveals a strong inverse relationship
between WS<sub>2</sub> PL intensity and defect density. We propose
a model in which the observed electronically active defects serve
as nonradiative recombination centers and obtain good agreement between
the experiments and model
Graphene As a Tunnel Barrier: Graphene-Based Magnetic Tunnel Junctions
Graphene has been widely studied for its high in-plane
charge carrier mobility and long spin diffusion lengths. In contrast,
the out-of-plane charge and spin transport behavior of this atomically
thin material have not been well addressed. We show here that while
graphene exhibits metallic conductivity in-plane, it serves effectively
as an insulator for transport perpendicular to the plane. We report
fabrication of tunnel junctions using single-layer graphene between
two ferromagnetic metal layers in a fully scalable photolithographic
process. The transport occurs by quantum tunneling perpendicular to
the graphene plane and preserves a net spin polarization of the current
from the contact so that the structures exhibit tunneling magnetoresistance
to 425 K. These results demonstrate that graphene can function as
an effective tunnel barrier for both charge and spin-based devices
and enable realization of more complex graphene-based devices for
highly functional nanoscale circuits, such as tunnel transistors,
nonvolatile
magnetic memory, and reprogrammable spin logic
Nano-“Squeegee” for the Creation of Clean 2D Material Interfaces
Two-dimensional (2D)
materials exhibit many exciting phenomena that make them promising
as materials for future electronic, optoelectronic, and mechanical
devices. Because of their atomic thinness, interfaces play a dominant
role in determining material behavior. In order to observe and exploit
the unique properties of these materials, it is therefore vital to
obtain clean and repeatable interfaces. However, the conventional
mechanical stacking of atomically thin layers typically leads to trapped
contaminants and spatially inhomogeneous interfaces, which obscure
the true intrinsic behavior. This work presents a simple and generic
approach to create clean 2D material interfaces in mechanically stacked
structures. The operating principle is to use an AFM tip to controllably
squeeze contaminants out from between 2D layers and their substrates,
similar to a “squeegee”. This approach leads to drastically
improved homogeneity and consistency of 2D material interfaces, as
demonstrated by AFM topography and significant reduction of photoluminescence
line widths. Also, this approach enables emission from interlayer
excitons, demonstrating that the technique enhances interlayer coupling
in van der Waals heterostructures. The technique enables repeatable
observation of intrinsic 2D material properties, which is crucial
for the continued development of these promising materials
Charge Trapping and Exciton Dynamics in Large-Area CVD Grown MoS<sub>2</sub>
There is keen interest in monolayer
transition metal dichalcogenide
films for a variety of optoelectronic applications due to their direct
band gap and fast carrier dynamics. However, the mechanisms dominating
their carrier dynamics are poorly understood. By combining time-resolved
terahertz (THz) spectroscopy and transient absorption, we are able
to shed light on the optoelectronic properties of large area CVD grown
mono- and multilayer MoS<sub>2</sub> films and determine the origins
of the characteristic two-component excited state dynamics. The photoinduced
conductivity shows that charge carriers, and not excitons, are responsible
for the subpicosecond dynamics. Identical dynamics resulting from
sub-optical gap excitation suggest that charge carriers are rapidly
trapped by midgap states within 600 fs. This process complicates the
excited state spectrum with rapid changes in line-width broadening
in addition to a red-shift due to band gap renormalization and simple
state-filling effects. These dynamics are insensitive to film thickness,
temperature, or choice of substrate, which suggests that carrier trapping
occurs at surface defects or grain boundaries. The slow dynamics are
associated with exciton recombination and lengthen from 50 ps for
monolayer films to 150 ps for multilayer films indicating that surface
recombination dominates their lifetime. We see no signatures of trions
in these MoS<sub>2</sub> films. Our results imply that CVD grown films
of MoS<sub>2</sub> hold potential for high-speed optoelectronics and
provide an explanation for the absence of trions in some CVD grown
MoS<sub>2</sub> films
Enhancing the Purity of Deterministically Placed Quantum Emitters in Monolayer WSe<sub>2</sub>
We
present a method utilizing an applied electrostatic
potential
for suppressing the broad defect bound excitonic emission in two-dimensional
materials (2DMs) which otherwise inhibits the purity of strain induced
single photon emitters (SPEs). Our heterostructure consists of a WSe2 monolayer on a polymer in which strain has been deterministically
introduced via an atomic force microscope (AFM) tip. We show that
by applying an electrostatic potential, the broad defect bound background
is suppressed at cryogenic temperatures, resulting in a substantial
improvement in single photon purity demonstrated by a 10-fold reduction
of the correlation function g(2)(0) value
from 0.73 to 0.07. In addition, we see a 2-fold increase in the intensity
of the SPEs as well as the ability to activate/deactivate the emitters
at certain wavelengths. Finally, we present an increase in the operating
temperature of the SPE up to 110 K, a 50 K increase when compared
with the results when no electrostatic potential is present
Room-Temperature Spin Filtering in Metallic Ferromagnet–Multilayer Graphene–Ferromagnet Junctions
We
report room-temperature negative magnetoresistance in ferromagnet–graphene–ferromagnet
(FM|Gr|FM) junctions with minority spin polarization exceeding 80%,
consistent with predictions of strong minority spin filtering. We
fabricated arrays of such junctions <i>via</i> chemical
vapor deposition of multilayer graphene on lattice-matched single-crystal
NiFe(111) films and standard photolithographic patterning and etching
techniques. The junctions exhibit metallic transport behavior, low
resistance, and the negative magnetoresistance characteristic of a
minority spin filter interface throughout the temperature range 10
to 300 K. We develop a device model to incorporate the predicted spin
filtering by explicitly treating a metallic minority spin channel
with spin current conversion and a tunnel barrier majority spin channel
and extract spin polarization of at least 80% in the graphene layer
in our structures. The junctions also show antiferromagnetic coupling,
consistent with several recent predictions. The methods and findings
are relevant to fast-readout low-power magnetic random access memory
technology, spin logic devices, and low-power magnetic field sensors
Double Indirect Interlayer Exciton in a MoSe<sub>2</sub>/WSe<sub>2</sub> van der Waals Heterostructure
An emerging class
of semiconductor heterostructures involves stacking
discrete monolayers such as transition metal dichalcogenides (TMDs)
to form van der Waals heterostructures. In these structures, it is
possible to create interlayer excitons (ILEs), spatially indirect,
bound electron–hole pairs with the electron in one TMD layer
and the hole in an adjacent layer. We are able to clearly resolve
two distinct emission peaks separated by 24 meV from an ILE in a MoSe<sub>2</sub>/WSe<sub>2</sub> heterostructure fabricated using state-of-the-art
preparation techniques. These peaks have nearly equal intensity, indicating
they are of common character, and have <i>opposite</i> circular
polarizations when excited with circularly polarized light. <i>Ab initio</i> calculations successfully account for these observations:
they show that both emission features originate from excitonic transitions
that are indirect in momentum space and are split by spin–orbit
coupling. Also, the electron is strongly hybridized between both the
MoSe<sub>2</sub> and WSe<sub>2</sub> layers, with significant weight
in both layers, contrary to the commonly assumed model. Thus, the
transitions are not purely interlayer in character. This work represents
a significant advance in our understanding of the static and dynamic
properties of TMD heterostructures