Defects Vibrations Engineering for Enhancing Interfacial Thermal Transport

To push upper boundaries of effective thermal conductivity in polymer composites, a fundamental understanding of thermal transport mechanisms is crucial. Although there is intensive simulation research, systematic experimental investigation on thermal transport in polymer composites is limited. To better understand thermal transport processes, we design polymer composites with perfect fillers (graphite) and defective fillers (graphite oxide); we choose polar polyvinyl alcohol (PVA) as a matrix model; and we identify how thermal transport occurs across heterogeneous interfaces. Measured thermal conductivities of in PVA/defective filler composites is higher than those of PVA/perfect filler composites, while measured thermal conductivities in defective fillers are lower than those of perfect fillers. An effective quantum mechanical model is developed, showing that the vibrational state of the defective level plays a critical role in enhancing the thermal conductivity with increased defect concentration. Our experimental and model results have suggested that defects in polymer composites may enhance thermal transport in polymer composites by promoting vibrational resonant couplings.Comment: Enclosed: (i) Main Manuscript, including 5 main figures. (ii) Supplementary Information, including 16 Supplementary Figures and one self-contained theoretical sectio

Ultrafast and Real-Time Dynamics of Nanomaterials Studied by Advanced Spectroscopic Techniques

Ultrafast and nonlinear spectroscopies are used to study excited-state dynamics and monitor real-time growth dynamics of different types of nanomaterials. In the first study, the growth dynamics of colloidal gold-silver core-shell nanoparticles are studied using in situ second harmonic generation and extinction spectroscopy. The growth lifetimes are studied under different reaction conditions, resulting in different silver shell thicknesses, with spectral comparisons to finite-difference time-domain calculations. The results are consistent with a three-step growth process. During the first step of the nanoparticle growth reaction, rough and uneven surfaces are formed rapidly giving rise to plasmonic hot spots with corresponding broad, red-shifted plasmon spectra. In the second step, the nanoparticle surface becomes smoother, reaching a thermodynamic equilibrium. The Au@Ag nanoparticle growth process has a third, slower step where the nanoparticle surface charge density changes due to chemical reactions resulting in a decreasing SHG signal while the extinction spectrum remains constant. In another study, transient absorption spectroscopy is used to study the ultrafast dynamics of porphyrin- and zinc porphyrin-based colloidal nanomaterials for potential applications in light-harvesting devices. Porphyrin and zinc porphyrin dyes in water exhibit long-lived excited states on the order of several nanoseconds. However, these porphyrin excited-state lifetimes are significantly faster when in the nanoparticle environment due to energy transfer, enhanced intersystem crossing, and electronic delocalization. Finally, the excited-state heating and melting dynamics of aluminum thin film samples at different thicknesses are investigated using ultrafast pump-probe reflectivity with 800 nm light under varying laser pump pulse powers. The data reveal a dramatic change in the carrier relaxation mechanism below and above 150 nm in thickness under varying laser powers due to the role of the characteristic heat penetration depth. Samples with thickness below this length scale show faster heating dynamics and a lower power threshold for melting. Overall, the synthesis, characterization, nonlinear spectroscopy, and ultrafast spectroscopy of a wide variety of nanomaterials are studied for the development of potential nanomedicine, optoelectronic, molecular sensing, and laser-based additive manufacturing applications

Raman Spectroscopy on Brain Disorders: Transition from Fundamental Research to Clinical Applications

Brain disorders such as brain tumors and neurodegenerative diseases (NDs) are accompanied by chemical alterations in the tissues. Early diagnosis of these diseases will provide key benefits for patients and opportunities for preventive treatments. To detect these sophisticated diseases, various imaging modalities have been developed such as computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET). However, they provide inadequate molecule-specific information. In comparison, Raman spectroscopy (RS) is an analytical tool that provides rich information about molecular fingerprints. It is also inexpensive and rapid compared to CT, MRI, and PET. While intrinsic RS suffers from low yield, in recent years, through the adoption of Raman enhancement technologies and advanced data analysis approaches, RS has undergone significant advancements in its ability to probe biological tissues, including the brain. This review discusses recent clinical and biomedical applications of RS and related techniques applicable to brain tumors and NDs

Nonlinear and ultrafast spectroscopy of molecular dye interactions with colloidal plasmonic nanoparticles

Mol. dye interactions with colloidal gold and gold- silver- gold core- shell- shell nanoparticles are studied using second harmonic generation (SHG) , extinction spectroscopy, and transient absorption spectroscopy. The adsorption isotherms of several dyes such as malachite green, brilliant green, and rhodamine 110 to the colloidal nanoparticle surface in water are measured with SHG and the results are fit using the modified Langmuir model to det. the free energies of adsorption and the adsorbate site densities. Complementary measurements of the extinction spectra of the combined dye and nanoparticle solns. with subtractions from the spectra of the dye and nanoparticles alone at corresponding concns. reveal strong polaritonic states from resonant coupling that depend on the dye- plasmonic nanoparticle interactions. The resonant coupling spectroscopy agrees with computational simulations using a multiscale hybrid /classical approach, showing polariton peaks that overlap with a Fano- type profile. The plasmonic spectra of core- shell- shell nanoparticles are controlled by changing the shell thicknesses for improved spectral overlap with the adsorbed dyes, resulting in significantly enhanced resonant coupling peaks. Addnl., transient absorption spectroscopy on excited- state dynamics of the dye- nanoparticle solns. show the convolution of plasmonic and mol. dynamics to study effects from energy transfer, mol. hindrance, and optical field enhancements. The results are compared to fluorescent quenching and enhancement measurements to understand the overall, time- dependent optical and energetic interactions between dye mols. and colloidal plasmonic nanoparticle surfaces. These linear, nonlinear, and ultrafast spectroscopic investigations provide important information that can be utilized for improved plasmon- enhanced mol. sensing applications in aq. soln

Nonlinear and ultrafast spectroscopy of molecular dye interactions with colloidal plasmonic nanoparticles

Mol. dye interactions with colloidal gold and gold- silver- gold core- shell- shell nanoparticles are studied using second harmonic generation (SHG) , extinction spectroscopy, and transient absorption spectroscopy. The adsorption isotherms of several dyes such as malachite green, brilliant green, and rhodamine 110 to the colloidal nanoparticle surface in water are measured with SHG and the results are fit using the modified Langmuir model to det. the free energies of adsorption and the adsorbate site densities. Complementary measurements of the extinction spectra of the combined dye and nanoparticle solns. with subtractions from the spectra of the dye and nanoparticles alone at corresponding concns. reveal strong polaritonic states from resonant coupling that depend on the dye- plasmonic nanoparticle interactions. The resonant coupling spectroscopy agrees with computational simulations using a multiscale hybrid /classical approach, showing polariton peaks that overlap with a Fano- type profile. The plasmonic spectra of core- shell- shell nanoparticles are controlled by changing the shell thicknesses for improved spectral overlap with the adsorbed dyes, resulting in significantly enhanced resonant coupling peaks. Addnl., transient absorption spectroscopy on excited- state dynamics of the dye- nanoparticle solns. show the convolution of plasmonic and mol. dynamics to study effects from energy transfer, mol. hindrance, and optical field enhancements. The results are compared to fluorescent quenching and enhancement measurements to understand the overall, time- dependent optical and energetic interactions between dye mols. and colloidal plasmonic nanoparticle surfaces. These linear, nonlinear, and ultrafast spectroscopic investigations provide important information that can be utilized for improved plasmon- enhanced mol. sensing applications in aq. soln

Growth Dynamics of Colloidal Silver-Gold Core-Shell Nanoparticles Studied by Second Harmonic Generation and Extinction Spectroscopy

The growth dynamics of colloidal silver-gold core-shell (Ag@Au CS) nanoparticles (NPs) in water are monitored in a stepwise synthesis approach using time-dependent second harmonic generation (SHG) and extinction spectroscopy. Three sequential additions of chloroauric acid, sodium citrate, and hydroquinone are added to the silver nanoparticle solution to grow a gold shell around a silver core. The first addition produces a stable urchin-like surface morphology, while the second and third additions continue to grow the gold shell thickness as the surface becomes more smooth and uniform, as determined using transmission electron microscopy. The extinction spectra after each addition are compared to finite-difference time-domain (FDTD) calculations, showing large deviations for the first and second additions due to the bumpy surface morphology and plasmonic hotspots while showing general agreement after the third addition reaches equilibrium. The SHG signal is dominated by the NP surface, providing complementary information on the growth time scales due to changes to the surface morphology. This combined approach of synthesis and characterization of Ag@Au CS nanoparticles with SHG spectroscopy, extinction spectroscopy, and FDTD calculations provides a detailed foundation for investigating complex colloidal nanoparticle growth mechanisms and dynamics in developing enhanced plasmonic nanomaterial technologies

Efficient photoinduced energy transfer in porphyrin-based nanomaterials

© 2020 American Chemical Society. Synthesis, characterization, and ultrafast dynamics of porphyrin- and zinc porphyrin-based nanomaterials are reported. Spherical nanoparticles composed of a group of uniform materials based on organic salts (nanoGUMBOS) are prepared from either porphyrin or zinc porphyrin with trihexyl(tetradecyl)phosphonium in aqueous colloidal suspension with sizes of approximately 50 nm in diameter. Ultrafast excited-state dynamics of porphyrin and zinc porphyrin nanoGUMBOS in water are measured using transient absorption spectroscopy with 400 nm excitation. Results are compared to corresponding measurements of the porphyrin molecular dye parent compounds in water. Porphyrin and zinc porphyrin have long-lived excited states arising from intersystem crossing of the first-excited singlet S1 state to the triplet T1 state. These excited-state lifetimes are significantly faster in porphyrin-based nanoGUMBOS as compared to the corresponding porphyrin molecules due to intermolecular energy transfer, electronic delocalization, and altered chemical environments of the nanomaterials. Additionally, these results demonstrate that porphyrin-based nanoGUMBOS are promising nanomaterials for light harvesting in solar cells and optoelectronics

Monitoring the growth dynamics of colloidal gold-silver core-shell nanoparticles using in situ

© 2019 Author(s). The growth dynamics of gold-silver core-shell (Au@Ag) nanoparticles are studied using in situ time-dependent second harmonic generation (SHG) and extinction spectroscopy to investigate the nanoparticle shell formation. The silver shell is grown by reduction of silver cations onto a 14 nm gold core using ascorbic acid in colloidal aqueous solution under varying reaction concentrations producing Au@Ag nanoparticles of final sizes ranging from 51 to 78 nm in diameter. The in situ extinction spectra show a rapid increase in intensity on the timescale of 5-6 s with blue shifting and narrowing of the plasmonic peak during the silver shell formation. The in situ SHG signals show an abrupt rise at early times of the reaction, followed by a time-dependent biexponential decrease, where the faster SHG lifetime corresponds to the timescale of the shell growth, and where the slower SHG lifetime is attributed to changes in the nanoparticle surface charge density. A large enhancement in the SHG signal at early stages of the reaction is caused by plasmonic hot spots due to the nanoparticle surface morphology, which becomes smoother as the reaction proceeds. The final extinction spectra are compared to finite-difference time-domain (FDTD) calculations, showing general agreement with experiment, where the plasmon peak red shifts and increases in spectral width as the silver shell thickness increases. These in situ SHG and extinction spectroscopy results, combined with FDTD calculations, help characterize the complicated processes involved in colloidal nanoparticle shell formation in real time for developing potential plasmon-enhanced nanomaterial applications

Monitoring the Seed-Mediated Growth of Gold Nanoparticles Using in Situ Second Harmonic Generation and Extinction Spectroscopy

© 2018 American Chemical Society. In situ second harmonic generation (SHG) coupled with extinction spectroscopy is used for real-time monitoring of seed-mediated growth dynamics of colloidal citrate-stabilized gold nanoparticles in water. The time-dependent in situ SHG results capture an early stage of the growth process where a large enhancement in the SHG signal is observed, which is attributed to the formation of plasmonic hot spots from a rough and uneven nanoparticle surface. The temporal peak in the SHG signal is followed by a decay that is fit to an exponential function to characterize the size-dependent nanoparticle growth lifetime, which varies from 0.45 to 1.7 min for final nanoparticle sizes of 66 and 94 nm, respectively. This early growth stage also corresponds to a broadening of the plasmon spectra, as monitored using time-dependent in situ extinction spectroscopy. Over the course of the seed-mediated growth reaction, the nanoparticle becomes more thermodynamically stable through surface reconstruction resulting in a smoother, more uniform surface, corresponding to lower, stable SHG signals and narrower plasmon spectra. With real-time monitoring of nanoparticle formation, in situ SHG spectroscopy combined with in situ extinction spectroscopy provides an important insight for controlling nanoparticle synthesis and surface morphology for potential nanoscale engineering of different colloidal nanomaterials