Influence of C=O groups on the optical extinction coefficient of graphene exfoliated in liquid phase

LiLiquid phase exfoliation of graphite is currently one of the most promising graphene production methods at large scale. For this reason, an accurate calculation of the concentration in graphene dispersions is important for standardization and commercialization. Here, graphene dispersions, at high concentrations, were produced by electrochemical exfoliation. Furthermore, a cleaner methodology to obtain graphene oxide by electrochemical exfoliation at high acid concentrations was implemented. The absorption coefficient for graphene and graphene oxide was determined in the optical range ($\alpha_{660nm}=$ 1414 ($\pm$3\%) mL mg$^{-1}$ m$^{-1}$ and $\alpha_{660nm}=$ 648 ($\pm$ 7\%) mL mg$^{-1}$ m$^{-1}$, respectively) with an exponential dependence with the wavelength. The difference in $\alpha$ for both materials is attributed to an increased presence of C=O groups as evidenced by FTIR, UV-vis and Raman spectroscopy, as well as, in the calculation of the opical extinction coefficient and optical band-gap via Tauc-plots.

Above-aircraft cirrus cloud and aerosol optical depth from hyperspectral irradiances measured by a total-diffuse radiometer

This study develops the use of spectral total and diffuse irradiance measurements, made from a prototype hyperspectral total-diffuse Sunshine Pyranometer (SPN-S), to retrieve layer fine-mode aerosol (τaer) and total optical depths from airborne platforms. Additionally, we use spectral analysis in an attempt to partition the total optical depth it into its τaer and cirrus cloud optical depth (τcld) components in the absence of coarse-mode aerosols. Two retrieval methods are developed: one leveraging information in the diffuse irradiance, and the other using spectral characteristics of the transmitted direct beam, with each approach best suited for specific cloud and aerosol conditions. SPN-S has advantages over traditional sun-photometer systems including no moving parts and a low cost. However, a significant drawback of the instrument is that it is unable to measure the direct beam irradiance as accurately as sun-photometers. To compensate for the greater measurement uncertainty of the radiometric irradiances these retrieval techniques employ ratioed inputs or spectral information to reduce output uncertainty. This analysis uses irradiance measurements from SPN-S and the Solar Spectral Flux Radiometer (SSFR) aboard the National Aeronautics and Space Administration’s (NASA) P-3 aircraft during the 2018 deployment of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) campaign and the 2019 Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) mission to quantify above-aircraft cirrus τcld and derive vertical profiles of layer τaer. Validation of the τaer retrieval is accomplished by comparison with collocated measurements of direct solar irradiance made by the Sky-Scanning Sun-Tracking Atmospheric Research (4STAR) and in situ measurements of aerosol optical depth. For the aggregated 2018 ORACLES results, regression between the SPN-S based method and sun-photometer τaer values yield a slope of 0.96 with an R2 of 0.96, while the root-mean-square error (RMSE) is 3.0 × 10−2. When comparing the retrieved τaer to profiles of integrated in situ measurements of optical extinction, the slope, R2, and RMSE values for ORACLES are 0.90, 0.96, 3.4 × 10−2, and for CAMP2Ex are 0.94, 0.97, 3.4 × 10−2 respectively. This paper is a demonstration of methods for deriving cloud and aerosol optical properties in environments where both atmospheric constituents may be present. With improvements to the low-cost SPN-S radiometer instrument, it may be possible to extend these methods to a broader set of sampling applications, such as ground-based settings.

Estimating the optical extinction of liquid water clouds in the cloud base region

Accurate lidar-based measurements of cloud optical extinction, even though perhaps limited to the cloud base region, are useful. Arguably, more advanced lidar techniques (e.g. Raman) should be applied for this purpose. However, simpler polarisation and backscatter lidars offer a number of practical advantages (e.g. better resolution and more continuous and numerous time series). In this paper, we present a backscatter lidar signal inversion method for the retrieval of the cloud optical extinction in the cloud base region. Though a numerically stable method for inverting lidar signals using a far-end boundary value solution has been demonstrated earlier and may be considered as being well established (i.e. the Klett inversion), the application to high-extinction clouds remains problematic. This is due to the inhomogeneous nature of real clouds, the finite range resolution of many practical lidar systems, and multiple scattering effects. We use an inversion scheme, where a backscatter lidar signal is inverted based on the estimated value of cloud extinction at the far end of the cloud, and apply a correction for multiple scattering within the cloud and a range resolution correction. By applying our technique to the inversion of synthetic lidar data, we show that, for a retrieval of up to 90 m from the cloud base, it is possible to obtain the cloud optical extinction within the cloud with an error better than 5 %. In relative terms, the accuracy of the method is smaller at the cloud base but improves with the range within the cloud until 45 m and deteriorates slightly until reaching 90 m from the cloud base.

Au@Ag Dendritic Nanoforests for Surface-Enhanced Raman Scattering Sensing

The effects of Au cores in Ag shells in enhancing surface-enhanced Raman scattering (SERS) were evaluated with samples of various Au/Ag ratios. High-density Ag shell/Au core dendritic nanoforests (Au@Ag-DNFs) on silicon (Au@Ag-DNFs/Si) were synthesized using the fluoride-assisted Galvanic replacement reaction method. The synthesized Au@Ag-DNFs/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction, and Raman spectroscopy. The ultraviolet-visible extinction spectrum exhibited increased extinction induced by the addition of Ag when creating the metal DNFs layer. The pure Ag DNFs exhibited high optical extinction of visible light, but low SERS response compared with Au@Ag DNFs. The Au core (with high refractive index real part) in Au@Ag DNFs maintained a long-leaf structure that focused the illumination light, resulting in the apparent SERS enhancement of the Ag coverage.

A computational analysis of nanoparticle-mediated optical breakdown

A theoretical model of the optical breakdown phenomena during picosecond and femtosecond laser pulse exposure with gold nanoparticles in water was developed. The model provides new and valuable insight into the dependence of the optical breakdown on the wavelength, morphology and environment in the vicinity of the nanoparticles. The developed model was successfully validated against experimental data, which also revealed some insights to the criterion for optical breakdown. Three studies were performed using the model. In the first study, the effects of the dielectric environment on the optical extinction spectra of individual bare and silica-coated gold nanorods were examined. The experimental extinction spectra of an individual gold nanorod was compared to a calculation from a numerical model that included environmental features present in the measurements and the morphology of the corresponding nanorod measured by transmission electron microscopy. The combination of these experimental and theoretical tools permitted a detailed interpretation of the optical properties of an individual gold nanorod. In the second study, a strongly coupled finite element model of nanoparticle-mediated optical breakdown phenomena was developed. This model was used to theoretically study a 6 ps laser pulse interaction with uncoupled and plasmon coupled gold nanoparticles. The study showed how the one-dimensional assembly of nanoparticles affects the optical breakdown threshold of its surroundings. The optical breakdown threshold had a stronger dependence on the optical near-field enhancement than on the volume of the nanostructure or its absorption cross-section. Finally, a model was developed to study the wavelength dependence of the threshold of gold nanorod-mediated optical breakdown during picosecond and femtosecond near infrared optical pulses. This study showed that the wavelength dependence in the picosecond regime is governed solely by the changes of the nanorod’s optical properties. On the other hand, the optical breakdown during femtosecond pulse exposures was found to depend on the multiphoton ionization and its wavelength dependence when, Eratio, the ratio of the maximum electric field from the outside to the inside of the nanorod was greater than 7. The developed model and conducted research deepens the understanding of the nanoparticlemediated optical breakdown in water and updates the theoretical formulation of the process with the latest findings, which leads to advancing this technology further.

A computational analysis of nanoparticle-mediated optical breakdown

A theoretical model of the optical breakdown phenomena during picosecond and femtosecond laser pulse exposure with gold nanoparticles in water was developed. The model provides new and valuable insight into the dependence of the optical breakdown on the wavelength, morphology and environment in the vicinity of the nanoparticles. The developed model was successfully validated against experimental data, which also revealed some insights to the criterion for optical breakdown. Three studies were performed using the model. In the first study, the effects of the dielectric environment on the optical extinction spectra of individual bare and silica-coated gold nanorods were examined. The experimental extinction spectra of an individual gold nanorod was compared to a calculation from a numerical model that included environmental features present in the measurements and the morphology of the corresponding nanorod measured by transmission electron microscopy. The combination of these experimental and theoretical tools permitted a detailed interpretation of the optical properties of an individual gold nanorod. In the second study, a strongly coupled finite element model of nanoparticle-mediated optical breakdown phenomena was developed. This model was used to theoretically study a 6 ps laser pulse interaction with uncoupled and plasmon coupled gold nanoparticles. The study showed how the one-dimensional assembly of nanoparticles affects the optical breakdown threshold of its surroundings. The optical breakdown threshold had a stronger dependence on the optical near-field enhancement than on the volume of the nanostructure or its absorption cross-section. Finally, a model was developed to study the wavelength dependence of the threshold of gold nanorod-mediated optical breakdown during picosecond and femtosecond near infrared optical pulses. This study showed that the wavelength dependence in the picosecond regime is governed solely by the changes of the nanorod’s optical properties. On the other hand, the optical breakdown during femtosecond pulse exposures was found to depend on the multiphoton ionization and its wavelength dependence when, Eratio, the ratio of the maximum electric field from the outside to the inside of the nanorod was greater than 7. The developed model and conducted research deepens the understanding of the nanoparticlemediated optical breakdown in water and updates the theoretical formulation of the process with the latest findings, which leads to advancing this technology further.

Halloysite Nanotubes with Immobilized Plasmonic Nanoparticles for Biophotonic Applications

Halloysite nanotubes (HNTs) with immobilized gold (Au) and silver (Ag) nanoparticles (NPs) belong to a class of nanocomposite materials whose physical properties and applications depend on the geometry of arrangements of the plasmonic nanoparticles on HNT’ surfaces. We explore HNTs:(Au, Ag)-NPs as potential nano-templates for surface-enhanced Raman scattering (SERS). The structure and plasmonic properties of nanocomposites based on HNTs and Au- and Ag-NPs are studied by means of the transmission electron microscopy and optical spectroscopy. The optical extinction spectra of aqueous suspensions of HNTs:(Au, Ag)-NPs and spatial distributions of the electric fields are simulated, and the simulation results demonstrate the corresponding localized plasmonic resonances and numerous “hot spots” of the electric field nearby those NPs. In vitro experiments reveal an enhancement of the protein SERS in fibroblast cells with added HNTs:Ag-NPs. The observed optical properties and SERS activity of the nanocomposites based on HNTs and plasmonic NPs are promising for their applications in biosensorics and biophotonics.

Color Changes in Ag Nanoparticle Aggregates Placed in Various Environments: Their Application to Air Monitoring

Fresh Ag nanoparticles (NPs) dispersed on a transparent SiO2 exhibit an intense optical extinction band originating in localized surface plasmon resonance (LSPR) in the visible range. The intensity of the LSPR band weakened when the Ag NPs was stored in ambient air for two weeks. The rate of the weakening and the LSPR wavelength shift, corresponding to visual chromatic changes, strongly depended on the environment in which Ag NPs were set. The origin of a chromatic change was discussed along with both compositional and morphological changes. In one case, bluish coloring followed by a prompt discoloring was observed for Ag NPs placed near the ventilation fan in our laboratory, resulted from adsorption of large amounts of S and Cl on Ag NP surfaces as well as particle coarsening. Such color changes deduce the presence of significant amounts of S and Cl in the environment. In another case, a remarkable blue-shift of the LSPR band was observed for the Ag NPs stored in the desiccator made of stainless steel, originated in the formation of CN and/or HCN compounds and surface roughening. Their color changed from maroon to reddish, suggesting that such molecules were present inside the desiccator.

CloudCT 3D volumetric tomography - mission advances

<p> Significant climate uncertainties are associated with insufficient understanding of small warm clouds, due to the nature of their 3D structure and radiative transfer. It is desirable to improve understanding of such clouds and their sensitivity to environmental changes. This requires sensing platforms that are suitable for 3D sensing, and signal analysis tuned to 3D radiative transfer. We approach these challenges in the CloudCT project, funded by the ERC. It is a mission that develops and aims to demonstrate 3D volumetric scattering tomography of clouds. This will be facilitated by an unprecedented large formation of ten cooperating nanosatellites. The formation will simultaneously image cloud fields from multiple directions, at approximately 20m nadir ground resolution. Based on this data, scattering tomography will seek the 3D volumetric distribution of droplet effective radius, liquid water content and optical extinction. In addition to advancement of the technology, CloudCT will yield a global database of 3D macro and microphysical properties of warm cloud fields.</p><p>In this talk, we present advances made on several fronts of the project: modeling, payload, algorithm, and operation. Regarding cloud modeling, we performed LES simulations (using the SAM model with bin microphysics) of warm convective cloud fields (at different environments), at high spatial resolution. Using the simulated clouds properties, several imager and waveband possibilities have been quantitatively considered for the mission. Major consideration criteria are tomographic quality in the face of sensor and photon noise, calibration errors and stray light. Additional criteria are technological availability, platform constraints, calibration requirements and cost.</p><p>We investigated specifically possibilities of visible light (VIS, 463nm, 545nm, 645nm, and 705nm) short wave infra-red (SWIR, 1641 nm), and polarized imagers (POL, 463nm, 545nm, 645nm, and 705nm).  These examinations relied on physical modeling of 3D radiative transfer and the sensing processes. Due to platform constraints in CloudCT, each platform will carry a single camera exclusively (either VIS/NIR or SWIR). Hence, we describe the tradeoff of introducing SWIR cameras and various POL architectures.  </p><p>While CloudCT is mainly designed for simultaneous imaging of each cloud field, it is possible to tolerate a lag of several seconds, as small warm clouds hardly evolve in this time scale (at the 20 meter spatial scale). We exploit this, to add more view-points, using the same number of platforms (10). The added viewpoints correspond to single-scattering angles, where polarization yields enhanced sensitivity to the droplet microphysics. These angles require sampling of <1° in the fogbow region. This dictates requirements for the platform attitude control.  </p><p>On the algorithmic front, we advanced the retrieval to yield results that (compared to the simulated ground truth) have smaller errors than the prior art. Elements of our advancement include initialization by a parametric horizontally-uniform microphysical model. The parameters of this initialization are determined by a fast optimization process.  The optimized initialization is particularly strong, when relying on the detected degree of linear polarization, instead of radiance.</p>