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Register a new userElectroabsorption spectroscopy of single walled nanotubes
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J. W. Kennedy
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We have measured the electric field modulated absorption of a sample of single-walled nanotubes (SWNT) suspended in a solid polyvinyl alcohol matrix. The electroabsorption (EA) spectrum roughly follows the first derivative of the absorption with respect to photon energy, scales quadratically with the electric field strength, and shows a pronounced anisotropy of light polarization with respect to the applied electric field direction. These findings indicate a quadratic Stark effect caused by a change in the polarizability of the excited states, which is common to quasi-one dimensional (1D) excitons in organic semiconductors. The EA spectrum is well described by calculations involving electron-electron interaction in the model Hamiltonian of both zigzag and chiral nanotubes. We have calculated the EA spectra for both zigzag and chiral nanotubes within a model Hamiltonian that includes electron-electron interactions. The calculations reproduce the observed quadratic Stark shift of the lowest optical exciton, as well as the more complicated behavior of the EA spectrum in the energy region that corresponds to the next higher exciton. Our findings show that the low-lying absorption bands in semiconducting SWNT are excitonic in origin, in agreement with transient optical measurements that identify the primary photoexcitations in SWNT as quasi-1D excitons with a substantial binding energy.
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We have calculated the binding energy of various nucleobases (guanine (G), adenine (A), thymine (T) and cytosine (C)) with (5,5) single-walled carbon nanotubes (SWNTs) using ab-initio Hartre-Fock method (HF) together with force field calculations. The gas phase binding energies follow the sequence G $>$ A $>$ T $>$ C. We show that main contribution to binding energy comes from van-der Wall (vdW) interaction between nanotube and nucleobases. We compare these results with the interaction of nucleobases with graphene. We show that the binding energy of bases with SWNTs is much lower than the graphene but the sequence remains same. When we include the effect of solvation energy (Poisson-Boltzman (PB) solver at HF level), the binding energy follow the sequence G $>$ T $>$ A $>$ C $>$, which explains the experimentcite{zheng} that oligonucleotides made of thymine bases are more effective in dispersing the SWNT in aqueous solution as compared to poly (A) and poly (C). We also demonstrate experimentally that there is differential binding affinity of nucleobases with the single-walled carbon nanotubes (SWNTs) by directly measuring the binding strength using isothermal titration (micro) calorimetry. The binding sequence of the nucleobases varies as thymine (T) $>$ adenine (A) $>$ cytosine (C), in agreement with our calculation.
We compare Raman spectra from aqueous suspensions of length-separated single-walled carbon nanotubes (SWCNTs) dispersed using either polymer adsorption of single-stranded DNA or miscelle encapsulation with sodium deoxycholate surfactant. The Raman spectral features, other than the D-band, increase monotonically with nanotube length in both dispersion schemes. The intensity ratio of the disorder-induced D to G Raman bands decays as a function of SWCNT length, proportional to 1/L, as expected for endcap defects. While the UV-vis absorption and fluorescence also increase with length for both dispersants, the fluorescence intensity is dramatically lower for DNA-wrapped SWCNTs of equal length. The similarities in the length-dependent D/G ratios exclude defects as an explanation for the fluorescence decrease in DNA versus deoxycholate dispersions.
Diffusion Monte Carlo calculations on the adsorption of $^4$He in open-ended single walled (10,10) nanotubes are presented. We have found a first order phase transition separating a low density liquid phase in which all $^4$He atoms are adsorbed close to the tube wall and a high density arrangement characterized by two helium concentric layers. The energy correction due to the presence of neighboring tubes in a bundle has also been calculated, finding it negligible in the density range considered.
Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically-resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighbouring local lattice distortion such as Stone-Thrower-Wales defects. The stability under the electron beam of these nanotubes has been studied in two extreme cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures.
A boost in the development of flexible and wearable electronics facilitates the design of new materials to be applied as transparent conducting films (TCFs). Although single-walled carbon nanotube (SWCNT) films are the most promising candidates for flexible TCFs, they still do not meet optoelectronic requirements demanded their successful industrial integration. In this study, we proposed and thoroughly investigated a new approach that comprises simultaneous bilateral (outer and inner surfaces) SWCNT doping after their opening by thermal treatment at 400 C under an ambient air atmosphere. Doping by a chloroauric acid (HAuCl$_{4}$) ethanol solution allowed us to achieve the record value of sheet resistance of 31 $pm$ 4 $Omega$/sq at a transmittance of 90% in the middle of visible spectra (550 nm). The strong p-doping was examined by open-circuit potential (OCP) measurements and confirmed by ab initio calculations demonstrating a downshift of Fermi level around 1 eV for the case of bilateral doping.
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