Browsing by Author "Weisman, R. Bruce"
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Item Advanced Characterization and Optical Properties of Single-Walled Carbon Nanotubes and Graphene Oxide(2011) Naumov, Anton Viatcheslavovich; Weisman, R. BrucePhotophysical, electronic, and compositional properties of single-walled carbon nanotubes (SWCNTs) and bulk nanotube samples were investigated together with graphene oxide photoluminescence. First, we studied the effect of external electric fields on SWCNT photoluminescence. Fields of up to 10 7 V/m caused dramatic, reversible decreases in emission intensity. Quenching efficiency was proportional to the projection of the field on the SWCNT axis, and showed inverse correlation with optical band gap. The magnitude of the effect was experimentally related to exciton binding energy, as consistent with a proposed field-induced exciton dissociation model. Further, the electronic composition of various SWCNT samples was studied. A new method was developed to measure the fraction of semiconducting nanotubes in as- grown or processed samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results provide important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques. The nature of absorption backgrounds in SWCNT samples was also studied. A number of extrinsic perturbations such as extensive ultrasonication, sidewall functionalization, amorphous carbon impurities, and SWCNT aggregation were applied and their background contributions quantified. Spectral congestion backgrounds from overlapping absorption bands were assessed with spectral modeling. Unlike semiconducting nanotubes, metallic SWCNTs gave broad intrinsic absorption backgrounds. The shape of the metallic background component and its absorptivity coefficient were determined. These results can be used to minimize and evaluate SWCNT absorption backgrounds. Length dependence of SWCNT optical properties was investigated. Samples were dispersed by ultrasonication or shear processing, and then length-fractionated by gel electrophoresis or controlled ultrasonication shortening. Fractions from both methods showed no significant absorbance variations with SWCNT length. The photoluminescence intensity increased linearly with length, and the relative quantum yield gradually increased, approaching a limiting value. Finally, a strong pH dependence of graphene oxide photoluminescence was observed. Sharp and structured excitation/emission features resembling the spectra of molecular fluorophores were obtained in basic conditions. Based on the observed pH-dependence and quantum calculations, these spectral features were assigned to quasi-molecular fluorophores formed by the electronic coupling of oxygen-containing addends with nearby graphene carbon atoms.Item Advanced Optical Detection of Single-Walled Carbon Nanotubes for Biomedical Applications and Photophysical Studies(2017-08-09) Lin, Ching-Wei; Weisman, R. BruceFluorescence of single-walled carbon nanotubes (SWCNTs) is of great interest for biomedical applications and optoelectronic devices because of their unusual emission wavelengths in the short-wave infrared (SWIR; 900-1600 nm). Many applications require developing novel experimental techniques with special capabilities. In this thesis, I demonstrate the use of a SWIR avalanche photodiode (APD) detector to expand biomedical applications and photophysical studies. Taking advantage of the detector’s high sensitivity, we developed a new approach for the noninvasive imaging of nanotubes in biological specimens and observed new photophysical phenomena of SWCNTs. A custom-built system was designed and constructed to detect and locate small amounts of SWCNTs inside living animals. This instrument uses diffuse LED illumination to excite nanotubes, a scanning optical probe to capture emission at specific locations, and a spectrally filtered APD to sensitively detect the nanotube fluorescence. This system also implements a new method called spectral triangulation, which determines the 3D locations of SWCNT emission sources in vivo. The unique feature of spectral triangulation is taking advantage of the differential water absorption in the SWIR region to gauge the path length in tissue between emission source and the probe. The SWCNT fluorescence signals attenuate differently in two selected spectral channels, so that the distance between source and probe can be deduced from the ratio of channel intensities. By probing at more than three positions on the specimen surface, we gauge the depth of the source and triangulate its 3D position with high precision. The SWIR-scanner system and spectral triangulation method were demonstrated first in tissue phantoms and then in living mice. Results show that the depth of a SWCNT source and the attenuation coefficients of nearby tissues can be obtained simultaneously. Future prospects for advancing the detection of SWCNTs in vivo are also quantitatively discussed. The high time resolution provided by APD detection allows exploration of novel SWCNT photophysics. I have developed a novel kinetic apparatus to monitor SWCNT luminescence changes on the sub-microsecond to millisecond time scale, with particular value for detecting weak delayed emission in the SWIR. For the first time, the “pile-up” distortions that commonly hamper traditional time-correlated single photon counting measurements have been overcome by mathematical analysis. This allows data acquisition to be conducted using a low repetition-rate, high power excitation source. This novel system is a powerful tool for studying SWCNT delayed fluorescence or photophysics involving singlet oxygen. Electronic excitation of SWCNTs has almost always been achieved through light absorption, electron injection, or molecular energy transfer, creating singlet excitons in the nanotubes. Very little is therefore known about SWCNT triplet excitons. I describe here the first production of SWCNT triplet excitons through singlet oxygen sensitization. A specialized apparatus was built to perform SWIR delayed luminescence spectrometry (SWIR-DLS) and selectively measure delayed emission spectra at intensities more than 20 times lower than normal SWCNT fluorescence. In these experiments, an optically excited organic sensitizer is quenched by dissolved oxygen to generate excited 1O2. This species then quenched in energy transfer encounters with nanotubes that produce triplet SWCNT excitons. Thermal activation of the SWCNT triplet state to its emissive singlet rate results in the detected delayed fluorescence emission. The delayed spectrum shows strong (n,m) selectivity that reflects the relative energy levels of SWCNT triplet excitons and singlet oxygen. Evidence is also seen for an alternate process that excites (n,m) species with triplet energies higher than 1O2 through triplet exciton ̶ singlet oxygen annihilation (TESOA). These studies demonstrate the use of advanced experimental probes for expanding basic scientific knowledge about carbon nanotubes and developing applications that make use of their remarkable properties.Item Advanced Optical Studies of Single-Wall Carbon Nanotubes Wrapped with Single-Stranded DNA(2019-05-23) Zheng, Yu; Weisman, R. BruceSingle-wall carbon nanotubes (SWCNTs) exist in a variety of discrete structures, giving distinct optical and electronic properties. Purification of as-grown SWCNTs and sorting of SWCNT mixtures into pure (n,m) fractions are required for enabling basic research progress and developing advanced applications. A remarkably effective method for SWCNT sorting is based on structure-selective wrapping interactions of single-stranded DNA (ssDNA) with nanotubes. In this thesis, optical spectroscopy has been used to investigate the ssDNA-SWCNT system for aspects of photophysics and photochemistry. A simple way to reveal the wrapping ability of ssDNA sequences with SWCNT structural forms is studying fluorescence spectral kinetics during displacement of ssDNA coating by sodium deoxycholate (SDC). It is found that the displacement rate dramatically decreases with increasing nanotube diameter for (GT)20-SWCNT samples. Moreover, the structure-specific interactions of (ATT)4-(7,5) hybrids show strong enough enantiomeric differences to cause resolvable shifts in fluorescence spectra and dramatic kinetic differences in coating displacement. The quenching of ssDNA-SWCNT fluorescence by dissolved oxygen was observed and the effect was quickly and fully reversed at room temperature by displacing the O2. The extent of quenching depends on structure-selective interactions of ssDNA with SWCNTs, allowing their specific affinities to be uncovered and studied through quick and nondestructive optical measurements. In addition, methylene blue (MB), a common dye, has been found to significantly quench ssDNA-SWCNT fluorescence through a charge-transfer process. The distinct MB quenching for different (n,m) species also shows the specific affinities of ssDNA-SWCNT hybrids. Structure-selective photochemistry of SWCNTs has been achieved using near-infrared (NIR) monochromatic irradiation in the presence of dissolved oxygen. The covalent functionalization of (8,3), (6,5), and (7,6) species in unsorted SWCNT dispersions was induced by irradiation with wavelengths of 955, 985, and 1130 nm, respectively. After the photoinduced reaction, fluorescence spectrum was significantly and selectively quenched and the absorption spectrum was heavily and selectively distorted. So the optical properties of SWCNT samples can be easily tailored by such nanotube structure-selective photochemistry. Furthermore, the spatial pattern of covalent functionalization sites on SWCNT sidewalls and the tunable and smooth modification of SWCNT energy levels have been readily obtained using ssDNA as a template to covalently functionalize SWCNTs. A room temperature chemical reaction between the nanotube surface and guanine nucleotides in the ssDNA coating proceeds quickly on exposure to singlet oxygen (1O2). This reaction preserves the SWCNT characteristic fluorescence but causes spectral red-shifts that depend on the spatial guanine density of the ssDNA oligo. Customized spatial patterns and depths of band gap modulation can thus be inscribed on the nanotube by selecting the templating ssDNA sequence. In addition, previously studied sparse chemical doping and DNA-templated covalent functionalization show distinct treated emission features, allowing flexible strategies for modifying SWCNTs.Item An integrated interpretation of azomethane photodissociation dynamics(1992) Andrews, Burton Kim; Weisman, R. BruceAlthough it has been known for more than 60 years that azoalkanes (R-N=N-R) dissociate under the influence of light or heat, few definitive conclusions have been reached on the mechanism of their photodissociation. Recently, the issue of whether the photodissociation is concerted or stepwise has been settled through kinetic resolution of the process into two steps, implying a methyldiazenyl radical intermediate. Our ab initio CASSCF quantum chemical calculations confirm the stability of this methyldiazenyl radical, and the dissociative transition state on its ground state ($\sp2$A$\sp\prime$) surface has been located. A barrier height of 410 cm$\sp{-1}$ has been found for this transition state, which leads to the dissociation into methyl radical plus N$\sb2$. The lowest energy path for this decomposition has been calculated using the CASSCF method with a 6-31G* basis set including nine electrons in nine active orbitals. Methyldiazenyl's $\nu\sb5$ mode (651 cm$\sp{-1}$) was found to correspond closely to the dissociation coordinate. In addition, UHF calculations have revealed that the lowest-lying triplet state of azomethane has a perp configuration at its equilibrium geometry, suggesting that first-step photodissociation may occur from the T$\sb1$ surface. Energies and methyldiazenyl vibrational frequencies obtained from the quantum calculations have been combined with existing thermochemical data as input to an energy disposal analysis that includes both statistical and impulsive modelling of product state energy distributions measured with time-resolved coherent anti-Stokes Raman spectroscopy (CARS). It appears that the first dissociative step may have significant impulsive character. In the second step, although some findings fall outside the range spanned by the impulsive and statistical models, the experimental nitrogen vibrational distribution is in excellent agreement with the prediction of the separate statistical ensemble (SSE) model. Further experimental and theoretical research, particularly on the first step, will be required before an adequate understanding of azomethane photodissociation can be achieved.Item 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. BruceDetailed 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.Item Automated Enrichment of Single-Walled Carbon Nanotubes with Optical Studies of Enriched Samples(2013-05-13) Canning, Griffin; Weisman, R. Bruce; Hafner, Jason H.; Kono, JunichiroThe design and performance of an instrument is presented whose purpose is the extraction of samples highly enriched in one species of single-walled carbon nanotubes from density gradient ultracentrifugation. This instrument extracts high purity samples which are characterized by various optical studies. The samples are found to be enriched in just a few species of nanotubes, with the major limitation to enrichment being the separation, rather than extraction. The samples are then used in optical and microscopic studies which attempt to determine the first absorption coefficient (S1) of the (6,5) species of nanotube. Initial experiments give a value of 9.2 ± 2.6 cm2 C atom-1. Future work is proposed to improve upon the experiment in an attempt to reduce possible errorsItem Biomedical studies of single -walled carbon nanotubes using near-infrared fluorescence(2007) Cherukuri, Paul; Weisman, R. BruceExperimental studies will be described aimed at providing a scientific foundation for the use of single-walled carbon nanotubes (SWNTs) in biomedical applications. SWNTs have been found to be a unique class of nanoscale near-infrared (NIR) fluorescence contrast agents that also exhibit novel therapeutic capabilities. In our first study, we found that cultured macrophage cells readily engulf individual nanotubes. The rate of cellular uptake of SWNTs was measured by monitoring their characteristic NIR emissions. Furthermore, we also found that the NIR emissions of individual SWNTs are persistent in both the extracellular and intracellular environment of macrophage cells. Next, we extended our study from simple in vitro systems to the more complex in vivo mammalian animal model. By quantitatively tracking individual SWNTs, we have determined the rabbit's pharmacokinetic SWNT profile without the aid of additional fluorophores or radiolabels. As a final therapeutic application, we have developed SWNTs as novel pharmaceutical agents that efficiently carry siRNA molecules into cancer cells in order to induce targeted apoptosis of specific tumors.Item CARS studies of the quenching of excited sulfur atoms by rare gases: Fine structure selectivity in electronic-to-translational energy transfer(1990) Stout, Joe Edward; Weisman, R. BruceIn the first use of the Raman effect to detect sulfur atoms, transient CARS spectroscopy has revealed novel relaxation effects in sulfur formed through multiple photon excitation of carbon disulfide vapor. Ground term $\sp3P$ sulfur was monitored through its 396 cm$\sp{-1}$ and 573 cm$\sp{-1}$ fine structure transitions ($\sp3P\sb1\leftrightarrow\ \sp3P\sb2$ and $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$) at various delays after the photolysis laser pulse. It was found that quenching of $\sp1D\sb2$ sulfur to the $\sp3P$ term by collision with the rare gases argon, krypton, and xenon gives clear kinetic and spectral evidence of a population inversion between the $\sp3P\sb0$ and $\sp3P\sb2$ fine-structure levels. Kinetic data indicate no such inversion between the $\sp3P\sb1$ and $\sp3P\sb2$ levels. Extensive modeling of the kinetic data taken at the $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$ transition was performed to obtain the branching ratio into the $\sp3P\sb0$ level for the three rate gases studied. Although kinetic models including all possible processes in this system contain too many unknown parameters to be useful, a simple three parameter model gives reasonable fits to the data. This model yields branching ratios of 0.83, 0.75, and 0.73 for the fractional formation of sulfur's $\sp3P\sb0$ level through quenching from $\sp1D\sb2$ by Ar, Kr, and Xe, respectively. The results of MCSCF-CI calculations of the relevant low-lying Ar-S potential curves suggest that quenching proceeds through a single energetically accessible intersection between molecular terms, which, when correlation and coupling rules are considered, leads adiabatically to the $\sp3P\sb0$ product level that is experimentally observed to dominate. Although calculated Xe-S and Kr-S potential energy curves are not available, comparison with the similar rare gas oxide system suggests that a general explanation for the selective quenching mechanism may involve differences in the spin-orbit coupling strengths between molecular terms at the two crossings that adiabatically correlate with the $\sp3P\sb0$ and $\sp3P\sb2$ levels.Item Creating fluorescent quantum defects in carbon nanotubes using hypochlorite and light(Springer Nature, 2019) Lin, Ching-Wei; Bachilo, Sergei M.; Zheng, Yu; Tsedev, Uyanga; Huang, Shengnan; Weisman, R. Bruce; Belcher, Angela M.; Smalley-Curl InstituteCovalent doping of single-walled carbon nanotubes (SWCNTs) can modify their optical properties, enabling applications as single-photon emitters and bio-imaging agents. We report here a simple, quick, and controllable method for preparing oxygen-doped SWCNTs with desirable emission spectra. Aqueous nanotube dispersions are treated at room temperature with NaClO (bleach) and then UV-irradiated for less than one minute to achieve optimized O-doping. The doping efficiency is controlled by varying surfactant concentration and type, NaClO concentration, and irradiation dose. Photochemical action spectra indicate that doping involves reaction of SWCNT sidewalls with oxygen atoms formed by photolysis of ClO- ions. Variance spectroscopy of products reveals that most individual nanotubes in optimally treated samples show both pristine and doped emission. A continuous flow reactor is described that allows efficient preparation of milligram quantities of O-doped SWCNTs. Finally, we demonstrate a bio-imaging application that gives high contrast short-wavelength infrared fluorescence images of vasculature and lymphatic structures in mice injected with only ~100 ng of the doped nanotubes.Item Dye Quenching of Carbon Nanotube Fluorescence Reveals Structure-Selective Coating Coverage(American Chemical Society, 2020) Zheng, Yu; Alizadehmojarad, Ali A.; Bachilo, Sergei M.; Kolomeisky, Anatoly B.; Weisman, R. Bruce; Smalley-Curl InstituteMany properties and applications of single-wall carbon nanotubes (SWCNTs) depend strongly on the coatings that allow their suspension in aqueous media. We report that SWCNT fluorescence is quenched by reversible physisorption of dye molecules such as methylene blue, and that measurements of that quenching can be used to infer structure-specific exposures of the nanotube surface to the surrounding solution. SWCNTs suspended in single-stranded DNA oligomers show quenching dependent on the combination of nanotube structure and ssDNA base sequence. Several sequences are found to give notably high or low surface coverages for specific SWCNT species. These effects seem correlated with the selective recognitions used for DNA-based structural sorting of nanotubes. One notable example is that dye quenching of fluorescence from SWCNTs coated with the (ATT)4 base sequence is far stronger for one (7,5) enantiomer than for the other, showing that coating coverage is associated with the coating affinity difference reported previously for this system. Equilibrium modeling of quenching data has been used to extract parameters for comparative complexation constants and accessible surface areas. Further insights are obtained from molecular dynamics simulations, which give estimated contact areas between ssDNA and SWCNTs that correlate with experimentally inferred surface exposures and account for the enantiomeric discrimination of (ATT)4.Item Enabling in vivo measurements of nanoparticle concentrations with three-dimensional optoacoustic tomography(Wiley, 2014) Tsyboulski, Dmitri A.; Liopo, Anton V.; Su, Richard; Ermilov, Sergey A.; Bachilo, Sergei M.; Weisman, R. Bruce; Oraevsky, Alexander A.In this report, we demonstrate the feasibility of using optoacoustic tomography (OAT) to evaluate biodistributions of nanoparticles in animal models. The redistribution of single-walled carbon nanotubes (SWCNTs) was visualized in living mice. Nanoparticle concentrations in harvested organs were measured spectroscopically using the intrinsic optical absorption and fluorescence of SWCNTs. Observed increases in optoacoustic signal brightness in tissues were compared with increases in optical absorption coefficients caused by SWCNT accumulation. The methodology presented in this report can further be extended to calibrate the sensitivity of an optoacoustic imaging system for a range of changes in optical absorption coefficient values at specific locations or organs in a mouse body to enable noninvasive measurements of nanoparticle concentrations in vivo. Additionally, qualitative information provided by OAT and quantitative information obtained ex vivo may provide valuable feedback for advancing methods of quantitative analysis with OAT.Item Enantiomers of Single-Wall Carbon Nanotubes Show Distinct Coating Displacement Kinetics(American Chemical Society, 2018) Zheng, Yu; Bachilo, Sergei M.; Weisman, R. Bruce; Smalley-Curl InstituteIt is known that specific oligomers of single-stranded DNA (ssDNA) can show remarkable selectivity when coating different structural species of single-wall carbon nanotubes (SWCNTs). We report that (ATT)4ᅠssDNA coatings strongly distinguish between the two optical isomers of (7,5) SWCNTs. This causes resolvable shifts in their fluorescence spectra and differences of 2 orders of magnitude in the room temperature rates of coating displacement, as monitored through changes in nanotube fluorescence wavelength and intensity on exposure to sodium deoxycholate. During coating displacement, the enantiomer with high affinity for the ssDNA oligomer is deduced to form an intermediate hybrid that is not observed for the low affinity enantiomer. These results reveal that enantiomeric differences in SWCNTs complexed with ssDNA are more diverse and dramatic than previously recognized.Item Energy dependence of collisional energy removal from vibrationally excited T(1) pyrazine(1992) Bevilacqua, Thomas Joseph; Weisman, R. BruceIn the first application of a new method for determining this information, rates of collisional vibrational energy removal have been measured for the T$\sb1$ state of pyrazine with vibrational energy contents ranging from 2000 to 5500 cm$\sp{-1}$. This method is the first to enable the measurement of energy removal from electronically excited states in the energy regime above that of well-separated vibrational states. It is also the first method applicable to excited triplet states in any energy regime. Energized T$\sb1$ pyrazine is formed through intersystem crossing from various laser-excited vibronic levels of the S$\sb1$ electronic state. Triplet-triplet transient absorption spectrometry is then used to monitor the T$\sb1$ decay kinetics under various collisional conditions. The new method of analyzing these kinetic data, based on the known variation of T$\sb1$ pyrazine's nonradiative decay rate with vibrational energy, allows the extraction of the rate of vibrational energy removal by the collision partners as a function of pyrazine vibrational energy. The key feature of the results is an approximately tenfold increase in the energy removed per collision within the $\sim$3500 cm$\sp{-1}$ range of energy contents studied. At $\langle$ ET$\sb1\rangle$ $\approx$ 2000 cm$\sp{-1}$, all buffers remove 5 to 10 cm$\sp{-1}$ per collision; at $\sim$5500 cm$\sp{-1}$, the deduced energy removal rates per collision are $\sim$55 cm$\sp{-1}$ for helium, 175 cm$\sp{-1}$ for argon, 160 cm$\sp{-1}$ for H$\sb2$, 530 cm$\sp{-1}$ for SF$\sb6$, and 360 cm$\sp{-1}$ for pyrazine. This increase, which is significantly steeper than found for vibrationally hot ground state molecules at higher energy contents, may be related to differences in the electronic characters of the molecules studied or in the regime of vibrational state densities investigated. The development of this new method for measuring energy removal rates and the intriguing results found with it should serve to stimulate further theoretical and experimental research into collisional relaxation of electronically excited states.Item Fluorescent security ink using carbon nanotubes(2010-03-23) Weisman, R. Bruce; Bachilo, Sergei M.; Booth, Eric Christopher; Rice University; United States Patent and Trademark OfficeThe present invention is directed toward fluorescent inks and markers comprising carbon nanotubes. The present invention is also directed toward methods of making such inks and markers and to methods of using such inks and markers, especially for security applications (e.g., anti-counterfeiting). Such inks and markers rely on the unique fluorescent properties of semiconducting carbon nanotubes.Item Fullerene triplet states in solution(1998) Ausman, Kevin Douglas; Weisman, R. BruceTriplet state pre-equilibration by reversible energy transfer has been observed by transient-absorption spectroscopy in mixed toluene solutions of C70 and C60 and of C70 and C60(CH 3)2. The equilibrium constants governing the asymptotic partitioning of triplet energy in these mixtures were determined as a function of temperature. The enthalpies of these excited states were found from van't Hoff plots of the equilibrium constant data to be -0.1 +/- 0.2 and -3.4 +/- 0.3 kJ mol-1 for C60 and C60 (CH3)2 respectively relative to a C70 triplet energy exchange partner. The corresponding relative entropies are 5.8 +/- 0.5 and -4.0 +/- 1.0 kJ mol-1 K-1 respectively. Transient spectra from high temperature C70/C60(CH3)2 mixed samples revealed evidence of a third, unidentified transient absorber that exhibited different kinetics from the pre-equilibrated triplet pool. Triplet state transient difference spectra and intrinsic decay kinetics were measured and compared for C60 and several derivatives of C 60. These derivatives were C60H2, C60(CH 3)2, ortho-xylyl-C60, N,N'-dimethyl-1,2-ethylenediamine-C 60, C60C(COOCH2CH3)2, and C60O. The spectral locations of the main triplet-triplet absorption peak for these compounds correlates linearly with the observed intrinsic intersystem crossing rate constant. The triplet state persistence of C60 was measured in toluene solution as a function of both ground state concentration and solution temperature. The unimolecular intersystem crossing deactivation channel shows very little thermal activation, whereas the observed bimolecular self-quenching decay channel is found to be highly activated. At room temperature, the deduced exponential lifetime of the solvent-caged encounter complex between triplet and ground state molecules is three orders of magnitude shorter than that of the isolated monomer triplet state. This suggests that the self-quenching process is not a simple perturbation of an isolated molecule's intersystem crossing, but rather occurs through a qualitatively different mechanism. The kinetic data are examined in light of the proposed excimer mechanism for triplet self-quenching. Finally, the self-quenching process is interpreted as a low-probability "mis-step" during energy transfer.Item Graphite Oxide: Structure, Reduction and Applications(2012-09-05) Gao, Wei; Ajayan, Pulickel M.; Weisman, R. Bruce; Barrera, Enrique V.This thesis proposes a modified structure model for graphite oxide (GO), an important precursor in graphene chemistry, develops a new strategy to convert GO back to graphene-like structure, and demonstrates its possible applications in both water purification and supercapacitor technologies. GO, a nontraditional compound first obtained from graphite oxidation over 150 years ago, is now becoming an important player in the production of graphene-based materials, which has high technological relevance. GO structure and reduction have been vigorously investigated, but its precise chemical structure still remains obscure, and the complete restoration of the sp2 carbon lattice has not yet been achieved. In our work, solid state 13C NMR (MAS) analysis offered a piece of evidence for five or six-membered ring lactol structure existing in GO that had never been assigned before, leading to a modified Lerf-Klinowski model for GO. A three-step reduction strategy, involving sodium borohydride (NaBH4), sulfuric acid, and high temperature thermal annealing, described in the thesis, successfully reduced GO back to chemically converted graphene (CCG) with the lowest heteroatom abundance among all those previously reported. In addition to the chemical significance of graphene/CCG production, GO and its derivatives were used as novel adsorbents in water purification. GO-coated sand showed higher retention than ordinary sand for both Rhodamine B and mercuric ion (Hg2+) contaminants in water. Further functionalization of GO with thiophenol resulted in better adsorption capacity toward Hg2+ than that of activated carbon. In addition, free-standing films of GO were treated and reduced with a CO2 laser beam into different conductive reduced GO (RGO) patterns, and directly used as supercapacitor devices which showed good cyclic stability and energy storage capacities comparable to that of existing thin film ultracapacitors. GO turned out to be a solid electrolyte with anisotropic proton conductivity similar to Nafion, while the large amount of trapped water in GO played an important role.Item High Precision Fractionator for use with Density Gradient Ultracentrifugation(American Chemical Society, 2014) Kadria-Vili, Yara; Canning, Griffin; Bachilo, Sergei M.; Weisman, R. Bruce; Richard E. Smalley Institute for Nanoscale Science and TechnologyThe recent application of density gradient ultracentrifugation (DGU) for structural sorting of single-walled carbon nanotube samples has created a need for highly selective extraction of closely spaced layers formed in the centrifuged tube. We describe a novel computer-controlled device designed for this purpose. Through the use of fine needles, systematic needle motions, and slow flow rates, multiple sample layers can be aspirated under program control with minimal cross contamination between layers. The fractionator’s performance is illustrated with DGU-sorted samples of single-walled carbon nanotubes.Item Impact of Sunlight and Natural Organic Matter on the Fate, Transport, and Toxicity of Carbon Based Nanomaterials(2013-09-16) Qu, Xiaolei; Li, Qilin; Alvarez, Pedro J.; Zhong, Weiwei; Weisman, R. BruceThe fast growing production of carbon based nanomaterials (CNMs) and their potential widespread use in consumer products raise concerns regarding their potential risks to human health and ecosystems. The present study investigated the role of photochemical transformation and natural organic matter (NOM) in the fate, transport, and toxicity of fullerenes and carbon nanotubes (CNTs) in natural aquatic systems, providing fundamental information for risk assessment and management. Photochemical transformation of aqueous fullerene nanoparticles (nC60) and CNTs occurs at significant rates under UVA irradiation at intensity similar to that in sunlight. The transformation processes are mediated by self-generated ROS, resulting in changes of surface structure depending on the initial surface oxidation state of CNMs. UVA irradiation leads to oxygenation of nC60 surface and decarboxylation of carboxylated multi-walled carbon nanotubes (COOH-MWNTs). The environmental transport of CNMs is significantly affected by their surface chemistry, concentration and species of electrolytes, and concentration and properties of co-existing NOM. In electrolyte solutions without NOM, the mobility of CNMs is largely decided by their surface chemistry, primarily the oxygen-containing functional groups. In NaCl solutions, UVA irradiation remarkably enhanced the mobility of nC60; conversely, it reduced nC60 stability in CaCl2 solutions. The mobility of COOH-MWNTs in NaCl solutions correlated well with the abundance of surface carboxyl groups. Humic acid, once adsorbed on the nC60 surface, can significantly enhance its stability through steric hindrance. The extent of stabilization depends on the amount and properties of humic acid adsorbed. Humic acid has limited adsorption on UVA-irradiated nC60. Soil humic acid is more efficient in stabilizing nC60 than aquatic humic acid due to its higher molecular weight. Humic acid immobilized onto the silica surface can potential enhance or hinder nC60 deposition, depending on the complex interplay of attractive and repulsive forces. MWNTs are more toxicity to bacteria, Escherichia coli, than COOH-MWNTs due to their higher bioavailability and oxidative capacity. Surface oxidation induced by •OH reduced the toxicity of MWNT while reactions with •OH have little effect on the COOH-MWNT toxicity. Antioxidants such as glutathione can effectively inhibit the antibacterial activity of MWNTs.Item Kinetics and dynamics of azoalkane photofragmentation: Direct studies using transient CARS spectroscopy(1990) Burton, Katherine Ann; Weisman, R. BruceA fundamental and long-standing question surrounds the mechanism of primary bond cleavage in azoalkanes: do the two C-N bonds break in a synchronous or a stepwise manner? When a vapor phase azoalkane absorbs near ultraviolet light, it dissociates into two alkyl radicals and nitrogen. Transient CARS spectroscopy was used here as a time-resolved probe of the photoproducts formed from azoalkanes excited at 355 nm. In a detailed reinvestigation, azomethane was found to dissociate in a stepwise process involving a methyldiazenyl radical intermediate. The diazenyl intermediate was formed in less than 1 ns and lived for 5.3 $\pm$ 1 ns before fragmenting into a methyl radical and nitrogen. Kinetic studies on azoisopropane (AIP) also gave evidence for stepwise photodissociation with a similar diazenyl lifetime. The first methyl radical formed in azomethane photodissociation was found to have 0 to 4 quanta of $\nu\sb2$ excitation, whereas the second methyl radical was predominantly vibrationally unexcited. The nascent rotational temperature of N$\sb2$ from azomethane was found to be 2500 K, and its vibrational distribution was confirmed to be 84% in v = 0 and 16% in v = 1. These results seem consistent with predictions based on the transition state structure computed for methyldiazenyl dissociation. Internal energy distributions were also measured for the nitrogen formed from 3-(methylazo)-3-methyl-butene (MAMB), which was previously shown to undergo stepwise dissociation through a methyldiazenyl intermediate. The rotational and vibrational energy distribution from MAMB were almost identical to those from azomethane, consistent with a common dissociation mechanism. AIP also gave similar nitrogen rotational and vibrational distributions, suggesting that the dissociative transition state of isopropyldiazenyl is similar to that of methyldiazenyl. In summary, direct kinetic measurements have demonstrated stepwise gas phase photodissociation in acyclic azoalkanes. Related measurements of product internal energy distributions should form the basis for a detailed dynamical understanding.Item Method for separating single-wall carbon nanotubes and compositions thereof(2006-07-11) Smalley, Richard E.; Hauge, Robert H.; Kittrell, Carter W.; Sivarajan, Ramesh; Strano, Michael S.; Bachilo, Sergei M.; Weisman, R. Bruce; Rice University; United States Patent and Trademark OfficeThe invention relates to a process for sorting and separating a mixture of (n, m) type single-wall carbon nanotubes according to (n, m) type. A mixture of (n, m) type single-wall carbon nanotubes is suspended such that the single-wall carbon nanotubes are individually dispersed. The nanotube suspension can be done in a surfactant-water solution and the surfactant surrounding the nanotubes keeps the nanotube isolated and from aggregating with other nanotubes. The nanotube suspension is acidified to protonate a fraction of the nanotubes. An electric field is applied and the protonated nanotubes migrate in the electric fields at different rates dependent on their (n, m) type. Fractions of nanotubes are collected at different fractionation times. The process of protonation, applying an electric field, and fractionation is repeated at increasingly higher pH to separated the (n, m) nanotube mixture into individual (n, m) nanotube fractions. The separation enables new electronic devices requiring selected (n, m) nanotube types.
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