256 research outputs found
Unified description of the optical phonon modes in -layer MoTe
-layer transition metal dichalcogenides provide a unique platform to
investigate the evolution of the physical properties between the bulk (three
dimensional) and monolayer (quasi two-dimensional) limits. Here, using
high-resolution micro-Raman spectroscopy, we report a unified experimental
description of the -point optical phonons in -layer -molybdenum
ditelluride (MoTe). We observe a series of -dependent low-frequency
interlayer shear and breathing modes (below , denoted LSM and
LBM) and well-defined Davydov splittings of the mid-frequency modes (in the
range , denoted iX and oX), which solely involve
displacements of the chalcogen atoms. In contrast, the high-frequency modes (in
the range , denoted iMX and oMX), arising from
displacements of both the metal and chalcogen atoms, exhibit considerably
reduced splittings. The manifold of phonon modes associated with the in-plane
and out-of-plane displacements are quantitatively described by a force constant
model, including interactions up to the second nearest neighbor and surface
effects as fitting parameters. The splittings for the iX and oX modes observed
in -layer crystals are directly correlated to the corresponding bulk Davydov
splittings between the and modes, respectively,
and provide a measurement of the frequencies of the bulk silent and
optical phonon modes. Our analysis could readily be generalized to
other layered crystals.Comment: Main Text (5 Figures, 2 Tables) + Supporting Information (12 Figures
Photothermal heterodyne imaging of individual nonfluorescent nanoclusters and nanocrystals
We introduce a new, highly sensitive, and simple heterodyne optical method
for imaging individual nonfluorescent nanoclusters and nanocrystals. A 2 order
of magnitude improvement of the signal is achieved compared to previous
methods. This allows for the unprecedented detection of individual small
absorptive objects such as metallic clusters (of 67 atoms) or nonluminescent
semiconductor nanocrystals. The measured signals are in agreement with a
calculation based on the scattering field theory from a photothermal-induced
modulated index of refraction profile around the nanoparticle
Photothermal Absorption Spectroscopy of Individual Semiconductor Nanocrystals
Photothermal heterodyne detection is used to record the first
room-temperature absorption spectra of single CdSe/ZnS semiconductor
nanocrystals. These spectra are recorded in the high cw excitation regime, and
the observed bands are assigned to transitions involving biexciton and trion
states. Comparison with the single nanocrystals photoluminescence spectra leads
to the measurement of spectral Stokes shifts free from ensemble averaging
Large-Scale Statistical Analysis of Defect Emission in hBN: Revealing Spectral Families and Influence of Flakes Morphology
Quantum emitters in two-dimensional layered hexagonal boron nitride are
quickly emerging as a highly promising platform for next-generation quantum
technologies. However, precise identification and control of defects are key
parameters to achieve the next step in their development. We conducted a
comprehensive study by analyzing over 10,000 photoluminescence emission lines,
revealing 11 distinct defect families within the 1.6 to 2.2 eV energy range.
This challenges hypotheses of a random energy distribution. We also reported
averaged defect parameters, including emission linewidths, spatial density,
phonon side bands, and the Debye-Waller factors. These findings provide
valuable insights to decipher the microscopic origin of emitters in hBN hosts.
We also explored the influence of hBN host morphology on defect family
formation, demonstrating its crucial impact. By tuning flake size and
arrangement we achieve selective control of defect types while maintaining high
spatial density. This offers a scalable approach to defect emission control,
diverging from costly engineering methods. It highlights the importance of
investigating flake morphological control to gain deeper insights into the
origins of defects and to expand the spectral tailoring capabilities of defects
in hBN
Photothermal Heterodyne Imaging of Individual Metallic Nanoparticles: Theory versus Experiments
We present the theoretical and detailed experimental characterizations of
Photothermal Heterodyne Imaging. An analytical expression of the photothermal
heterodyne signal is derived using the theory of light scattering from a
fluctuating medium. The amplitudes of the signals detected in the backward and
forward configurations are compared and their frequency dependences are
studied. The application of the Photothermal Heterodyne detection technique to
the absorption spectroscopy of individual gold nanoparticles is discussed and
the detection of small individual silver nanoparticles is demonstrated
Gold nanoparticles delivery in mammalian live cells: a critical review
Functional nanomaterials have recently attracted strong interest from the biology community, not only as potential drug delivery vehicles or diagnostic tools, but also as optical nanomaterials. This is illustrated by the explosion of publications in the field with more than 2,000 publications in the last 2 years (4,000 papers since 2000; from ISI Web of Knowledge, ‘nanoparticle and cell’ hit). Such a publication boom in this novel interdisciplinary field has resulted in papers of unequal standard, partly because it is challenging to assemble the required expertise in chemistry, physics, and biology in a single team. As an extreme example, several papers published in physical chemistry journals claim intracellular delivery of nanoparticles, but show pictures of cells that are, to the expert biologist, evidently dead (and therefore permeable). To attain proper cellular applications using nanomaterials, it is critical not only to achieve efficient delivery in healthy cells, but also to control the intracellular availability and the fate of the nanomaterial. This is still an open challenge that will only be met by innovative delivery methods combined with rigorous and quantitative characterization of the uptake and the fate of the nanoparticles. This review mainly focuses on gold nanoparticles and discusses the various approaches to nanoparticle delivery, including surface chemical modifications and several methods used to facilitate cellular uptake and endosomal escape. We will also review the main detection methods and how their optimum use can inform about intracellular localization, efficiency of delivery, and integrity of the surface capping
High-resolution spatial mapping of the temperature distribution of a Joule self-heated graphene nanoribbon
We investigate the temperature distributions of Joule self-heated graphene
nanoribbons (GNRs) with a spatial resolution finer than 100 nm by scanning
thermal microscopy (SThM). The SThM probe is calibrated using the Raman G mode
Stokes/anti-Stokes intensity ratio as a function of electric power applied to
the GNR devices. From a spatial map of the temperature distribution, heat
dissipation and transport pathways are investigated. By combining SThM and
scanning gate microscopy data from a defected GNR, we observe hot spot
formation at well-defined, localized sites.Comment: 4 pages, 3 figures, accepted on App. Phys. Let
Optical detection of single non-absorbing molecules using the surface plasmon of a gold nanorod
Current optical detection schemes for single molecules require light
absorption, either to produce fluorescence or direct absorption signals. This
severely limits the range of molecules that can be detected, because most
molecules are purely refractive. Metal nanoparticles or dielectric resonators
detect non-absorbing molecules by a resonance shift in response to a local
perturbation of the refractive index, but neither has reached single-protein
sensitivity. The most sensitive plasmon sensors to date detect single molecules
only when the plasmon shift is amplified by a highly polarizable label or by a
localized precipitation reaction on the particle's surface. Without
amplification, the sensitivity only allows for the statistical detection of
single molecules. Here we demonstrate plasmonic detection of single molecules
in realtime, without the need for labeling or amplification. We monitor the
plasmon resonance of a single gold nanorod with a sensitive photothermal assay
and achieve a ~ 700-fold increase in sensitivity compared to state-of-the-art
plasmon sensors. We find that the sensitivity of the sensor is intrinsically
limited due to spectral diffusion of the SPR. We believe this is the first
optical technique that detects single molecules purely by their refractive
index, without any need for photon absorption by the molecule. The small size,
bio-compatibility and straightforward surface chemistry of gold nanorods may
open the way to the selective and local detection of purely refractive proteins
in live cells
Thermoelectric spin voltage in graphene
In recent years, new spin-dependent thermal effects have been discovered in
ferromagnets, stimulating a growing interest in spin caloritronics, a field
that exploits the interaction between spin and heat currents. Amongst the most
intriguing phenomena is the spin Seebeck effect, in which a thermal gradient
gives rise to spin currents that are detected through the inverse spin Hall
effect. Non-magnetic materials such as graphene are also relevant for spin
caloritronics, thanks to efficient spin transport, energy-dependent carrier
mobility and unique density of states. Here, we propose and demonstrate that a
carrier thermal gradient in a graphene lateral spin valve can lead to a large
increase of the spin voltage near to the graphene charge neutrality point. Such
an increase results from a thermoelectric spin voltage, which is analogous to
the voltage in a thermocouple and that can be enhanced by the presence of hot
carriers generated by an applied current. These results could prove crucial to
drive graphene spintronic devices and, in particular, to sustain pure spin
signals with thermal gradients and to tune the remote spin accumulation by
varying the spin-injection bias
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