20 research outputs found
Tip-Enhanced Raman Spectroscopy of 2D Semiconductors
Two-dimensional (2D) semiconductors are one of the most extensively studied modern materials showing potentials in large spectrum of applications from electronics/optoelectronics to photocatalysis and CO2 reduction. These materials possess astonishing optical, electronic, and mechanical properties, which are different from their bulk counterparts. Due to strong dielectric screening, local heterogeneities such as edges, grain boundaries, defects, strain, doping, chemical bonding, and molecular orientation dictate their physical properties to a great extent. Therefore, there is a growing demand of probing such heterogeneities and their effects on the physical properties of 2D semiconductors on site in a label-free and non-destructive way. Tip-enhanced Raman spectroscopy (TERS), which combines the merits of both scanning probe microscopy and Raman spectroscopy, has experienced tremendous progress since its introduction in the early 2000s and is capable of local spectroscopic investigation with (sub-) nanometer spatial resolution. Introducing this technique to 2D semiconductors not only enables us to understand the effects of local heterogeneities, it can also provide new insights opening the door for novel quantum mechanical applications. This book chapter sheds light on the recent progress of local spectroscopic investigation and chemical imaging of 2D semiconductors using TERS. It also provides a basic discussion of Raman selection rules of 2D semiconductors important to understand TERS results. Finally, a brief outlook regarding the potential of TERS in the field of 2D semiconductors is provided
Signature of lattice dynamics in twisted 2D homo/hetero-bilayers
Twisted 2D bilayer materials are created by artificial stacking of two
monolayer crystal networks of 2D materials with a desired twisting angle
. The material forms a moir\'e superlattice due to the periodicity of
both top and bottom layer crystal structure. The optical properties are
modified by lattice reconstruction and phonon renormalization, which makes
optical spectroscopy an ideal characterization tool to study novel physics
phenomena. Here, we report a Raman investigation on a full period of the
twisted bilayer (tB) WSe moir\'e superlattice (\textit i.e. 0{\deg} 60{\deg}). We observe that the intensity ratio of two Raman peaks,
and correlates with the evolution of moir\'e period.
The Raman intensity ratio as a function of twisting angle follows an
exponential profile matching the moir\'e period with two local maxima at
0{\deg} and 60{\deg} and a minimum at 30{\deg}. Using a series of
temperature-dependent Raman and photoluminescence (PL) measurements as well as
\textit{ab initio} calculations, the intensity ratio
is explained as a signature of lattice
dynamics in tB WSe moir\'e superlattices. By further exploring different
material combinations of twisted hetero-bilayers, the results are extended for
all kinds of Mo- and W-based TMDCs.Comment: 22 pages, 12 fugure
Giant Optical Anisotropy in 2D Metal-Organic Chalcogenates
Optical anisotropy is a fundamental attribute of some crystalline materials
and is quantified via birefringence. A birefringent crystal not only gives rise
to asymmetrical light propagation but also attenuation along two distinct
polarizations, a phenomenon called linear dichroism (LD). Two-dimensional (2D)
layered materials with high in- and out-of-plane anisotropy have garnered
interest in this regard. Mithrene, a 2D metal-organic chalcogenate (MOCHA)
compound, exhibits strong excitonic resonances due to its naturally occurring
multi-quantum well (MQW) structure and in-plane anisotropic response in the
blue wavelength (~400-500 nm) regime. The MQW structure and the large
refractive indices of mithrene allow the hybridization of the excitons with
photons to form self-hybridized exciton-polaritons in mithrene crystals with
appropriate thicknesses. Here, we report the giant birefringence (~1.01) and
tunable in-plane anisotropic response of mithrene, which stem from its low
symmetry crystal structure and unique excitonic properties. We show that the LD
in mithrene can be tuned by leveraging the anisotropic exciton-polariton
formation via the cavity coupling effect exhibiting giant in-plane LD (~77.1%)
at room temperature. Our results indicate that mithrene is an ideal polaritonic
birefringent material for polarization-sensitive nanophotonic applications in
the short wavelength regime
Exciton tuning in monolayer WSe via substrate induced electron doping
We report on large exciton tuning in WSe monolayers via substrate induced
non-degenerate doping. We observe a redshift of 62 meV for the
exciton together with a 1-2 orders of magnitude photoluminescence (PL)
quenching when the monolayer WSe is brought in contact with highly oriented
pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and
SiO. As the evidence of doping from HOPG to WSe, a drastic increase of
the trion emission intensity was observed. Using a systematic PL and Kelvin
probe force microscopy (KPFM) investigation on WSe/HOPG, WSe/hBN, and
WSe/graphene, we conclude that this unique excitonic behavior is induced by
electron doping from the substrate. Our results propose a simple yet efficient
way for exciton tuning in monolayer WSe, which plays a central role in the
fundamental understanding and further device development.Comment: 14 pages, 10 figure
Optical Response of CVD-Grown ML-WS2 Flakes on an Ultra-Dense Au NP Plasmonic Array
The combination of metallic nanostructures with two-dimensional transition metal dichalcogenides is an efficient way to make the optical properties of the latter more appealing for opto-electronic applications. In this work, we investigate the optical properties of monolayer WS2 flakes grown by chemical vapour deposition and transferred onto a densely-packed array of plasmonic Au nanoparticles (NPs). The optical response was measured as a function of the thickness of a dielectric spacer intercalated between the two materials and of the system temperature, in the 75–350 K range. We show that a weak interaction is established between WS2 and Au NPs, leading to temperature- and spacer-thickness-dependent coupling between the localized surface plasmon resonance of Au NPs and the WS2 exciton. We suggest that the closely-packed morphology of the plasmonic array promotes a high confinement of the electromagnetic field in regions inaccessible by the WS2 deposited on top. This allows the achievement of direct contact between WS2 and Au while preserving a strong connotation of the properties of the two materials also in the hybrid system
High Density, Localized Quantum Emitters in Strained 2D Semiconductors
Two-dimensional chalcogenide semiconductors have recently emerged as a host
material for quantum emitters of single photons. While several reports on
defect and strain-induced single photon emission from 2D chalcogenides exist, a
bottom-up, lithography-free approach to producing a high density of emitters
remains elusive. Further, the physical properties of quantum emission in the
case of strained 2D semiconductors are far from being understood. Here, we
demonstrate a bottom-up, scalable, and lithography-free approach to creating
large areas of localized emitters with high density (~150 emitters/um2) in a
WSe2 monolayer. We induce strain inside the WSe2 monolayer with high spatial
density by conformally placing the WSe2 monolayer over a uniform array of Pt
nanoparticles with a size of 10 nm. Cryogenic, time-resolved, and gate-tunable
luminescence measurements combined with near-field luminescence spectroscopy
suggest the formation of localized states in strained regions that emit single
photons with a high spatial density. Our approach of using a metal nanoparticle
array to generate a high density of strained quantum emitters opens a new path
towards scalable, tunable, and versatile quantum light sources.Comment: 45 pages, 20 figures (5 main figures, 15 supporting figures
Advances in ultrafast plasmonics
In the past twenty years, we have reached a broad understanding of many
light-driven phenomena in nanoscale systems. The temporal dynamics of the
excited states are instead quite challenging to explore, and, at the same time,
crucial to study for understanding the origin of fundamental physical and
chemical processes. In this review we examine the current state and prospects
of ultrafast phenomena driven by plasmons both from a fundamental and applied
point of view. This research area is referred to as ultrafast plasmonics and
represents an outstanding playground to tailor and control fast optical and
electronic processes at the nanoscale, such as ultrafast optical switching,
single photon emission and strong coupling interactions to tailor photochemical
reactions. Here, we provide an overview of the field, and describe the
methodologies to monitor and control nanoscale phenomena with plasmons at
ultrafast timescales in terms of both modeling and experimental
characterization. Various directions are showcased, among others recent
advances in ultrafast plasmon-driven chemistry and multi-functional plasmonics,
in which charge, spin, and lattice degrees of freedom are exploited to provide
active control of the optical and electronic properties of nanoscale materials.
As the focus shifts to the development of practical devices, such as
all-optical transistors, we also emphasize new materials and applications in
ultrafast plasmonics and highlight recent development in the relativistic
realm. The latter is a promising research field with potential applications in
fusion research or particle and light sources providing properties such as
attosecond duration
Micro and Nano Raman Investigation of Two-Dimensional Semiconductors towards Device Application
Recent advances in nanoscale characterization and device fabrications have opened up opportunities for layered semiconductors in nanoelectronics and optoelectronics. Due to strong confinement in monolayer thickness, physical properties of this materials are greatly influenced by parameters such as strain, defects, and doping at the nanoscale. Therefore, understanding the effect of this parameters on layered semiconductors is the prerequisite for any device application. In this doctoral thesis, impact of such parameters on the optical properties of layered semiconductors are studied in nanoscale. MoS2, the most famous transition metal dechalcogenide (TMDC) (n-type semiconductor), and p-type GaSe, a member of metal monochalcogenide (MMC) are investigated in this work. Finally, in outlook, a device made of p-type few layer GaSe and n-type 1L-MoS2 is discussed
Micro and Nano Raman Investigation of Two-Dimensional Semiconductors towards Device Application
Recent advances in nanoscale characterization and device fabrications have opened up opportunities for layered semiconductors in nanoelectronics and optoelectronics. Due to strong confinement in monolayer thickness, physical properties of this materials are greatly influenced by parameters such as strain, defects, and doping at the nanoscale. Therefore, understanding the effect of this parameters on layered semiconductors is the prerequisite for any device application. In this doctoral thesis, impact of such parameters on the optical properties of layered semiconductors are studied in nanoscale. MoS2, the most famous transition metal dechalcogenide (TMDC) (n-type semiconductor), and p-type GaSe, a member of metal monochalcogenide (MMC) are investigated in this work. Finally, in outlook, a device made of p-type few layer GaSe and n-type 1L-MoS2 is discussed