Browsing by Author "Sanchez, Stephen R."

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    Assessing Inhomogeneity in Sorted Samples of Single-Walled Carbon Nanotubes through Fluorescence and Variance Spectroscopy
    (The Electrochemical Society, 2017) Kadria-Vili, Yara; Sanchez, Stephen R.; Bachilo, Sergei M.; Weisman, R. Bruce
    Detailed spectroscopic analysis has been used to study the homogeneity of single-walled carbon nanotube fractions carefully prepared by nonlinear density gradient ultracentrifugation sorting. Two distinct colored bands containing (6,5) enantiomers were subdivided into several extracted fractions that were separately diluted with sodium cholate surfactant and characterized by fluorescence, absorption, and variance spectroscopy. Values were measured for emission and absorption peak positions, Stokes shifts, emission peak widths, and emissive quantum yields. In addition, variance data were used to find relative emission per nanotube and to plot covariance slices representing homogeneous emission spectra. It was found that emission from SWCNTs within the upper enantiomer band shifts to shorter wavelengths with increasing depth in the centrifuge tube. In the lower enantiomer band such spectral shifts were not observed, but the emissive quantum yields decreased with depth. Variance analysis revealed spectral differences among SWCNTs within the same fraction of the same band. It is concluded that current methods for density gradient ultracentrifugation sorting produce samples that retain measurable structural and spectral inhomogeneities.
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    (n,m)-Specific Absorption Cross Sections of Single-Walled Carbon Nanotubes Measured by Variance Spectroscopy
    (American Chemical Society, 2016) Sanchez, Stephen R.; Bachilo, Sergei M.; Kadria-Vili, Yara; Lin, Ching-Wei; Weisman, R. Bruce; Smalley-Curl Institute
    A new method based on variance spectroscopy has enabled the determination of absolute absorption cross sections for the first electronic transition of 12 (n,m) structural species of semiconducting single-walled carbon nanotubes (SWCNTs). Spectrally resolved measurements of fluorescence variance in dilute bulk samples provided particle number concentrations of specific SWCNT species. These values were converted to carbon concentrations and correlated with resonant components in the absorbance spectrum to deduce (n,m)-specific absorption cross sections (absorptivities) for nanotubes ranging in diameter from 0.69 to 1.03 nm. The measured cross sections per atom tend to vary inversely with nanotube diameter and are slightly greater for structures ofﾠmod 1ﾠtype than forﾠmod 2. Directly measured and extrapolated values are now available to support quantitative analysis of SWCNT samples through absorption spectroscopy.
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    Variance Spectroscopy
    (American Chemical Society, 2015) Streit, Jason K.; Bachilo, Sergei M.; Sanchez, Stephen R.; Lin, Ching-Wei; Weisman, R. Bruce; Smalley-Curl Institute
    Spectroscopic analysis and study of nanoparticle samples is often hampered by structural diversity that presents a complex superposition of spectral signatures. By probing the spectra of small volumes within dilute samples, we can expose statistical variations in composition to obtain information unavailable from bulk spectroscopy. This new approach is demonstrated using fluorescence spectra of unsorted single-walled carbon nanotube samples to deduce structure-specific abundances and emissive efficiencies. Furthermore, correlations between intensity variations at different wavelengths provide two-dimensional covariance maps that isolate the spectra of homogeneous subpopulations. Covariance analysis is also a sensitive probe of particle aggregation. It shows that well-dispersed nanotube samples can spontaneously form loose aggregates of a type not previously recognized. Variance spectroscopy is a simple and practical technique that should find application in many nanoparticle studies.