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Register a new userExciton binding energies in carbon nanotubes from two-photon photoluminescence
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One- and two-photon luminescence excitation spectroscopy showed a series of distinct excitonic states in single-walled carbon nanotubes. The energy splitting between one- and two-photon-active exciton states of different wavefunction symmetry is the fingerprint of excitonic interactions in carbon nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are supported by ab-initio calculations of the linear and non-linear optical spectra, prove that the elementary optical excitations of carbon nanotubes are strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole pairs, stable even at room temperature. This alters our microscopic understanding of both the electronic structure and the Coulomb interactions in carbon nanotubes, and has direct impact on the optical and transport properties of novel nanotube devices.
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We show that the photoluminescence intensity of single-walled carbon nanotubes is much stronger in tubes with large chiral angles - armchair tubes - because exciton resonances make the luminescence of zigzag tubes intrinsically weak. This exciton-exciton resonance depends on the electronic structure of the tubes and is found more often in nanotubes of the +1 family. Armchair tubes do not necessarily grow preferentially with present growth techniques; they just have stronger luminescence. Our analysis allows to normalize photoluminescence intensities and find the abundance of nanotube chiralities in macroscopic samples.
ultiwalled carbon nanotubes, prepared by both electric arc discharge and chemical vapor deposition methods, show a strong visible light emission in photoluminescence experiments. All the samples employed in the experiments exhibit nearly same super-linear intensity dependence of the emission bands on the excitation intensity, and negligible temperature dependence of the central position and the line shapes of the emission bands. Based upon theoretical analysis of the electronic band structures and optical transition, it is suggested that besides the electronic transitions across the fundamental gap, the transitions between pi and sigma conduction bands are the major source of the light emissions. A two-step transition mechanism is proposed.
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 present a chemical route to covalently couple the photosystem I (PS I) to carbon nanotubes (CNTs). Small linker molecules are used to connect the PS I to the CNTs. Hybrid systems, consisting of CNTs and the PS I, promise new photo-induced transport phenomena due to the outstanding optoelectronic properties of the robust cyanobacteria membrane protein PS I.
Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (SWCNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100~meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting SWCNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1~ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all our experimental data semi-quantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in SWCNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.
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