46 research outputs found
Phonon-phonon coupling in bismuth vanadate over a large temperature range across the monoclinic phase
In this work we study phonon-phonon coupling in bismuth vanadate (BiVO4),
known for its second-order transition involving a variety of coupling
mechanisms. Using Raman spectroscopy as a probe, we identify two optical
coupled phonon modes of the VO4 tetrahedron and study them by varying light
polarization and temperature. The coupling manifests in non-Lorentzian
line-shapes of Raman peaks and frequency shifts. We use theoretical framework
of coupled damped harmonic oscillators to model the coupling and capture the
phenomena in the temperature evolution of the coupling parameters. The coupling
is negligible at temperatures below 100 K and later increases in magnitude with
temperature until 400 K. The sign of the coupling parameter depends on the
light polarization direction, causing either phonon attraction or repulsion.
After 400 K the phonon-phonon coupling diminishes when approaching phase
transition at which the phonon modes change their symmetry and the coupling is
no longer allowed
Asymmetry of resonance Raman profiles in semiconducting single-walled carbon nanotubes at the first excitonic transition
Carbon nanotubes are one-dimensional nanoscale systems with strongly pronounced chirality-dependent optical properties with multiple excitonic transitions. We investigate the high-energy G mode of semiconducting single-walled nanotubes of different chiralities at first excitonic transition by applying resonant Raman spectroscopy. The G mode intensity dependence on excitation energy yielded asymmetric resonance Raman profiles similar to ones we reported for the second excitonic transition. We find the scattering efficiency to be strongest at the incoming Raman resonance. Still, the degree of asymmetry is different for the first and second transitions and the first transition profiles provide a narrower line shape due to longer exciton lifetimes. The overall scattering efficiency is up to a factor of 25 times more intense at first excitonic transition, compared to the second transition. The fifth-order perturbation theory, with implemented phonon scattering pathways between excitonic states, excellently reproduced experimental data
Excitonic Resonances in Coherent Anti-Stokes Raman Scattering from Single-Walled Carbon Nanotubes
In this work we investigate the role of exciton resonances in coherent anti-Stokes Raman scattering (er-CARS) in single walled carbon nanotubes (SWCNTs). We drive the nanotube system in simultaneous phonon and excitonic resonances, where we observe a superior enhancement by orders of magnitude exceeding non-resonant cases. We investigated the resonant effects in five chiralities and find that the er-CARS intensity varies drastically between different nanotube species. The experimental results are compared with a perturbation theory model. Finally, we show that such giant resonant non-linear signals enable rapid mapping and local heating of individualized CNTs, suggesting easy tracking of CNTs for future nanotoxology studies and therapeutic application in biological tissues
Excitonic Resonances in Coherent Anti-Stokes Raman Scattering from Single Wall Carbon Nanotubes
In this work we investigate the role of exciton resonances in coherent
anti-Stokes Raman scattering (er-CARS) in single walled carbon nanotubes
(SWCNTs). We drive the nanotube system in simultaneous phonon and excitonic
resonances, where we observe a superior enhancement by orders of magnitude
exceeding non-resonant cases. We investigated the resonant effects in five
chiralities and find that the er-CARS intensity varies drastically
between different nanotube species. The experimental results are compared with
a perturbation theory model. Finally, we show that such giant resonant
non-linear signals enable rapid mapping and local heating of individualized
CNTs, suggesting easy tracking of CNTs for future nanotoxology studies and
therapeutic application in biological tissues.Comment: 17 pages, 6 figure
Dielectric Screening inside Carbon Nanotubes
Dielectric screening plays a vital role in determining physical properties at the nanoscale and affects our ability to detect and characterize nanomaterials using optical techniques. We study how dielectric screening changes electromagnetic fields and many-body effects in nanostructures encapsulated inside carbon nanotubes. First, we show that metallic outer walls reduce the scattering intensity of the inner tube by 2 orders of magnitude compared to that of air-suspended inner tubes, in line with our local field calculations. Second, we find that the dielectric shift of the optical transition energies in the inner walls is greater when the outer tube is metallic than when it is semiconducting. The magnitude of the shift suggests that the excitons in small-diameter inner metallic tubes are thermally dissociated at room temperature if the outer tube is also metallic, and in essence, we observe band-to-band transitions in thin metallic double-walled nanotubes
Separation of specific single-enantiomer single-wall carbon nanotubes in the large-diameter regime
The enantiomer-level isolation of single-walled carbon nanotubes (SWCNTs) in high concentration and with high purity for nanotubes greater than 1.1 nm in diameter is demonstrated using a two-stage aqueous two-phase extraction (ATPE) technique. In total, five different nanotube species of âŒ1.41 nm diameter are isolated, including both metallics and semiconductors. We characterize these populations by absorbance spectroscopy, circular dichroism spectroscopy, resonance Raman spectroscopy, and photoluminescence mapping, revealing and substantiating mod-dependent optical dependencies. Using knowledge of the competitive adsorption of surfactants to the SWCNTs that controls partitioning within the ATPE separation, we describe an advanced acid addition methodology that enables the fine control of the separation of these select nanotubes. Furthermore, we show that endohedral filling is a previously unrecognized but important factor to ensure a homogeneous starting material and further enhance the separation yield, with the best results for alkane-filled SWCNTs, followed by empty SWCNTs, with the intrinsic inhomogeneity of water-filled SWCNTs causing them to be worse for separations. Lastly, we demonstrate the potential use of these nanotubes in field-effect transistors
Global Alignment of Carbon Nanotubes via High Precision Microfluidic Dead-End Filtration
Single wall carbon nanotubes (SWCNTs) dispersed by negatively charged sodium deoxycholate (DOC) or positively charged cetrimonium bromide (CTAB) are shown to assemble into aligned films (3.8 cm2) on polycarbonate membranes by slow flow dead-end filtration. Global alignment (S2D max â 0.85) is obtained on both pristine polyvinylpyrrolidone (PVP) coated membranes and those with an intentional 150â600 nm groove pattern from hot embossing. In all cases, a custom microfluidic setup capable of precise control and measurement of the volume rate, transmembrane pressure, and the filtration resistance is used to follow SWCNT film formation. Conditions associated with the formation of SWCNT crystallites or their global alignment are identified and these are discussed in terms of membrane fouling and the interaction potential between the surface of the membrane and nanotubes. SWCNT alignment is characterized by cross-polarized microscopy, atomic force microscopy, scanning electron microscopy (SEM), and Raman spectroscopy
Resonant anti-Stokes Raman scattering in single-walled carbon nanotubes
The dependence of the anti-Stokes Raman intensity on the excitation laser
energy in carbon nanotubes is studied by resonant Raman spectroscopy. The
complete resonant anti-Stokes and Stokes Raman profiles of the high-energy
longitudinal phonon (G+) are obtained for (8,3), (7,5), (6,4), and (6,5)
single chirality enriched samples. A high asymmetry between the intensity of
the incoming and outgoing resonance is observed in the resonant Raman
profiles. In contrast to Stokes scattering, anti-Stokes scattering is more
intense at the outgoing resonance then at the incoming resonance. The
resonance profiles are explained by a Raman process that includes the phonon-
mediated interactions with the dark excitonic state. The chirality dependence
of the Raman profiles is due to the variation in the exciton-phonon matrix
elements, in agreement with tight-binding calculations. Based on the
asymmetric Raman profiles we present the resonance factors for the Stokes
/anti-Stokes ratios in carbon nanotubes
Fluorescent PolymerâSingleâWalled Carbon Nanotube Complexes with Charged and Noncharged Dendronized Perylene Bisimides for Bioimaging Studies
Fluorescent nanomaterials are expected to revolutionize medical diagnostic, imaging, and therapeutic tools due to their superior optical and structural properties. Their inefficient water solubility, cell permeability, biodistribution, and high toxicity, however, limit the full potential of their application. To overcome these obstacles, a waterâsoluble, fluorescent, cytocompatible polymerâsingleâwalled carbon nanotube (SWNT) complex is introduced for bioimaging applications. The supramolecular complex consists of an alkylated polymer conjugated with neutral hydroxylated or charged sulfated dendronized perylene bisimides (PBIs) and SWNTs as a general immobilization platform. The polymer backbone solubilizes the SWNTs, decorates them with fluorescent PBIs, and strongly improves their cytocompatibility by wrapping around the SWNT scaffold. In photophysical measurements and biological in vitro studies, sulfated complexes exhibit superior optical properties, cellular uptake, and intracellular staining over their hydroxylated analogs. A toxicity assay confirms the highly improved cytocompatibility of the polymerâwrapped SWNTs toward surfactantâsolubilized SWNTs. In microscopy studies the complexes allow for the direct imaging of the SWNTs' cellular uptake via the PBI and SWNT emission using the 1st and 2nd optical window for bioimaging. These findings render the polymerâSWNT complexes with nanometer size, dual fluorescence, multiple charges, and high cytocompatibility as valuable systems for a broad range of fluorescence bioimaging studies
Nanomechanical spectroscopy of 2D materials
We introduce a nanomechanical platform for fast and sensitive measurements of the spectrally resolved optical dielectric function of 2D materials. At the heart of our approach is a suspended 2D material integrated into a high Q silicon nitride nanomechanical resonator illuminated by a wavelength-tunable laser source. From the heating-related frequency shift of the resonator as well as its optical reflection measured as a function of photon energy, we obtain the real and imaginary parts of the dielectric function. Our measurements are unaffected by substrate-related screening and do not require any assumptions on the underling optical constants. This fast (Ïrise ⌠135 ns), sensitive (noise-equivalent power = 90âŁpWâHz), and broadband (1.2â3.1 eV, extendable to UVâTHz) method provides an attractive alternative to spectroscopic or ellipsometric characterization techniques