Narayanan Kuthirummal

Nonlinear optical scattering and absorption in bismuth nanorod suspensions

The authors present the results of optical limiting measurements of ∼10nm wide bismuth nanorods suspended in chloroform. Their Z-scan measurements reveal that optical limiting under 532nm excitation stems from a strong nonlinear scattering (NLS) subsequent to nonlinear absorption (NLA) by suspension. On the other hand, the optical limiting is entirely due to NLA when excited with 1064nm excitation in the nanosecond regime. The occurrence of NLS at one wavelength and absence at another is unusual, especially when compared to the behavior of carbon nanotubes under similar conditions, in which NLS is dominant at both wavelengths.

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene.

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 1B2 state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 1B2 state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 1A1 state that has been implicated as an intermediate step between the initially excited 1 1B2 state and the ground electronic state. The rise and decay times for the 2 1A1 state were found to be 55 and 84 fs, respectively.

Chitosan wound dressing with hexagonal silver nanoparticles for hyperthermia and enhanced delivery of small molecules

Chitosan films were synthesized with hexagonal silver nanoparticles (Ag NP). The unique shape and size of the Ag NP shift the optical absorption into the infrared. Stimulation of the nanoparticles with infrared light was used to generate heat and facilitate intracellular delivery of fluorescently-labeled dextran molecules. Chitosan films prepared with hexagonal or spherical Ag NP were characterized by optical and thermal analyses, and X-ray diffraction. There were found to be slight differences between how the chitosan molecular chains interface with the Ag NP depending upon shape of the nanoparticle. Viability of cells associated with dermal wound healing was evaluated on chitosan films prepared with hexagonal or spherical Ag NP, with both keratinocytes and fibroblasts having normal or moderately enhanced growth on films containing hexagonally-shaped nanoparticles.

A 9 eV superexcited state of 1,3-cyclohexadiene revealed by double resonance ionization photoelectron spectroscopy

Abstract Photoelectron spectra of 1,3-cyclohexadiene (CHD) have been obtained by three-photon, double resonance ionization via the ultrashort lived 1 B 2 ( S 2 ) state, and a previously unknown superexcited valence state at 9.0–9.5 eV. Upon two-photon excitation to the superexcited state there is an ultrafast internal conversion leading to a set of vibrationally excited Rydberg states. Ionization out of these latter states by a third photon results in photoelectron spectra that show two prominent Rydberg series with quantum defects of δ=0.58±0.04 and δ=0.98±0.02, respectively.

Synthesis and optical spectroscopic studies of semiconducting cadmium sulfide nanowires

Optical properties of cadmium sulfide nanowires of 50-100 nm diameter prepared by the pulsed-laser vaporization method have been studied using photoacoustic, UV-Vis, Raman, and photoluminescence spectroscopy. The photoacoustic (PA) technique yielded clean spectra with a steeper absorption edge for as-prepared opaque semiconducting CdS nanowires when compared to the corresponding conventional optical absorption spectra. The PA signal intensity was also significantly higher for nanowires. The Raman spectrum revealed increased exciton-longitudinal-optical-phonon coupling. The appearance of a narrow photoluminescence peak at 491 nm (FWHM approximately 9 nm) and the absence of emission above 500 nm demonstrate the high quality of nanowires. High-resolution transmission electron microscopy showed excellent ordering of the atoms in the [001] planes perpendicular to the growth direction.

Development and characterization of elastic nanocomposites for craniofacial contraction osteogenesis.

Development of resorbable elastic composites as an alternative means to apply contractive forces for manipulating craniofacial bones is described herein. Composites made from the biodegradable elastomer, poly (1,8-octanediol co-citric acid) (POC), and hydroxyapatite (nHA) with a 200 nm diameter (0-20% loadings) were created to develop a material capable of applying continuous contractive forces. The composites were evaluated for variation in their mechanical properties, rate of degradation, and interaction of the hydroxyapatite nanoparticles with the polymer chains. First, an ex vivo porcine model of cleft palate was used to determine the rate of cleft closure with applied force. The closure rate was found to be 0.505 mm N(-1) . From this approximation, the ideal maximum load was calculated to be 19.82 N, and the elastic modulus calculated to be 1.98 MPa. The addition of nHA strengthens POC, but also reduces the degradation time by 45%, for 3% nHA loading, compared to POC without nHA. X-ray diffraction data indicates that the addition of nHA to amorphous POC results in the formation of a semicrystalline phase of the POC adjacent to the nHA crystals. Based on the data, we conclude that amongst the 0-20% nHA loadings, a 3% loading of nHA in POC may be an ideal material (1.21 MPa elastic modulus and 13.17 N maximum load) to induce contraction forces capable of facilitating osteogenesis and craniofacial bone repair.

Hybrid Donor–Acceptor Polymer Particles with Amplified Energy Transfer for Detection and On-Demand Treatment of Breast Cancer

Judicious combination of semiconducting polymers with alternating electron donor (D) and acceptor (A) segments created hybrid nanoparticles with amplified energy transfer and red-shifted emission, while simultaneously providing photothermal capabilities. Hybrid D-A polymer particles (H-DAPPs) passively localized within orthotopic breast tumors, serving as bright fluorescent beacons. Laser stimulation induced heat generation on par with gold nanorods, resulting in selective destruction of the tumor. H-DAPPs can also undergo multiple thermal treatments, with no loss of fluorescence intensity or photothermal potential. These results indicate that H-DAPPs provide new avenues for the synthesis of hybrid nanoparticles useful in localized detection and treatment of disease.

Structural and mechanical characterization of bioresorbable, elastomeric nanocomposites from poly(glycerol sebacate)/nanohydroxyapatite for tissue transport applications

Poly(glycerol sebacate) (PGS)/nanohydroxyapatite (nHA) composites were assessed to develop new materials for closure via tissue transport for nonhealing defects (e.g., cleft palate and large skin wounds). The elastic shape memory polymer, PGS, was reinforced with nHA at 3 and 5% loading to increase the mechanical properties compared with the undoped PGS. Differential scanning calorimetry (DSC) was utilized to identify a glass transition temperature (Tg ) of -25°C. X-ray diffraction demonstrated a reduction in the amorphous nature of the material. The Fourier transform infrared photoacoustic spectral (FTIR-PAS) data showed decreased CO bonding and increased hydrogen bonding with increased nHA incorporation. Composites exhibited Youngs moduli in the range of 0.25-0.5 MPa and tensile strength of 1.5-3 N. No significant difference in extension to break (∼50 mm) with addition of nHA was observed. The elastic modulus significantly increased for 5% PGS/nHA compared to 0 and 3% PGS/nHA and tensile strength significantly increased for 3% PGS/nHA compared to 0 and 5% PGS/nHA. Degradation of 5% nHA/PGS significantly increased during the second week compared to PGS 0 and 3% PGS/nHA. The accelerated degradation for 5% PGS/nHA coupled with decreased flexibility and tensile strength implies an interruption in crosslinking. By maintaining flexibility and extension while increasing tensile strength, the 3% PGS/nHA doped satisfied the force range desired for closure of soft tissue defects. Based on this work, PGS with 3% nHA shape memory polymers should serve as a good candidate for closure of nonhealing soft tissues.

Synthesis and characterization of Ar-annealed zinc oxide nanostructures

Nanostructured zinc oxide samples were synthesized through CVD and annealed in argon. The samples were investigated using SEM, TEM, XRD, and UV/VIS/FTIR photoacoustic spectroscopy. The SEM/TEM images show relatively spherical particles that form elongated, connected domains post-anneal. XRD measurements indicate a typical wurtzite structure and reveal an increase in average grain size from 16.3 nm to 21.2 nm in Ar-annealed samples over pristine samples. Visible photoacoustic spectra reveal the contribution of defect levels on the absorption edge of the fundamental gap of zinc oxide. The steepness parameter of the absorption edge, which is inversely proportional to the width of the absorption edge, decreased from 0.1582 (pristine) to 0.1539 (annealed for 90 minutes) revealing increased density of defect states upon annealing. The FTIR photoacoustic spectra show an intense peak at 412 cm-1 and a shoulder at 504 cm-1 corresponding to the two transverse optical stretching modes of ZnO. These results may indic...

REEXAMINATION OF INFRARED SPECTRA OF Bi NANORODS: L–T TRANSITION OR EXTRINSIC PHASES

Bismuth is a fascinating material system owing to its unusual Fermi surface topology, which depends on size and temperature. Theoretical calculations predict that Bi should undergo a semimetal-to-semiconductor transition as at least one of its dimensions becomes < 50 nm. This prediction was experimentally confirmed by infrared (IR) absorption spectra, which is largely underlain by transitions between the L (electron) and T (hole) pockets of the Fermi surface. In this work, however, we report that in our nanosize samples, the observed IR peak positions are practically independent of temperature, which is hard to reconcile with the predicted behavior of the L–T transition. To help elucidate the origin of these IR peaks, we performed a careful analysis of the IR spectra of Bi nanorods, as well as those of bulk Bi, Bi samples prepared under different conditions and Bi2(CO3)O2 using Fourier transform infrared and photoacoustic spectroscopy measurements. We propose that the observed IR peaks in Bi nanorods arise from the oxygen–carbon containing secondary phases formed on the surface of Bi rather than from the Bi itself. We believe that secondary phases must be taken into account on a general basis in modeling the IR spectra of Bi and that the scenario that ascribes these IR peaks solely to the L–T transitions may not be correct. The results reported herein may also impact the research of Bi-based thermoelectric nanostructures and bulk materials.