Magneto-optical studies of single-wall carbon nanotubes (original) (raw)
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Magneto-optical spectroscopy of carbon nanotubes
Physica E-low-dimensional Systems & Nanostructures, 2005
We review our recent optical experiments on single-walled carbon nanotubes in high magnetic fields. The data revealed magnetic-field-induced optical anisotropy as well as broadening, splittings, and shifts of interband absorption and photoluminescence peaks. Quantitative comparison with theoretical predictions based on the Aharonov-Bohm effect is presented. r
Magneto-optical spectroscopy of excitons in carbon nanotubes
Physica Status Solidi B-basic Solid State Physics, 2006
We describe recent results of optical experiments on single-walled carbon nanotubes in high magnetic fields, probing the influence of a tube-threading magnetic flux on their band structure and excitonic states. The magnetic flux breaks the time-reversal symmetry and thus lifts the K –K ′ valley degeneracy, and the amount of state splitting is determined by the Aharonov–Bohm phase. We show experimental evidence that this field-induced symmetry breaking overcomes the Coulomb-induced exciton splitting which is predicted to make the lowest singlet exciton state optically inactive (or “dark”). Thus, a magnetic field applied parallel to the tube axis “brightens” the dark exciton, resulting in a drastic increase in photoluminescence intensity with magnetic field. We also find that the amount of brightening increases with decreasing temperature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical Processes in Single-Walled Carbon Nanotubes Threaded by a Magnetic Flux
2006
Single-walled carbon nanotubes threaded by a magnetic flux φ are predicted to posses novel magnetic and optical properties, critically depending on the value of φ/φ0 where φ0 is the magnetic flux quantum. This is a consequence of the Aharonov-Bohm phase 2πφ/φ0 influencing the boundary conditions on the Bloch wavefunctions. Here we report results of a series of magneto-optical studies of micelle-suspended single-walled carbon nanotubes in aqueous solutions in high magnetic fields. Their exotic magnetic properties manifest themselves in near-infrared magneto-absorption and magneto-photoluminescence spectra, including static and dynamic magnetic linear dichroism, splittings of exciton peaks, and field-induced band gap shrinkage. We show that these observations are quantitatively consistent with existing theories based on the Aharonov-Bohm effect.
Physical Review B, 2008
We have investigated excitons in highly-aligned single-walled carbon nanotubes (SWCNTs) through optical spectroscopy at low temperature (1.5 K) and high magnetic fields (B ) up to 55 T. SWCNT/polyacrylic acid films were stretched, giving SWCNTs that are highly aligned along the direction of stretch (n). Utilizing two well-defined measurement geometries,n B andn ⊥ B , we provide unambiguous evidence that the photoluminescence energy and intensity are only sensitive to the B-component parallel to the tube axis. A theoretical model of one-dimensional magnetoexcitons, based on exchange-split 'bright' and 'dark' exciton bands with Aharonov-Bohm-phasedependent energies, masses, and oscillator strengths, successfully reproduces our observations and allows determination of the splitting between the two bands as ∼ 4.8 meV for (6,5) SWCNTs. PACS numbers: 78.67.Ch,71.35.Ji,78.55.-m
Magnetic Brightening of Carbon Nanotube Photoluminescence through Symmetry Breaking
Nano Letters, 2007
We report that symmetry breaking by a magnetic field can drastically increase the photoluminescence quantum yield of single-walled carbon nanotubes, by as much as a factor of six, at low temperatures. To explain this we have developed a theoretical model based on fielddependent exciton band structure and the interplay of Coulomb interactions and the Aharonov-Bohm effect. This conclusively explains our data as the first experimental observation of dark excitons 5-10 meV below the bright excitons.
ACS Nano
Organic color-center quantum defects in semiconducting carbon nanotube hosts are rapidly emerging as promising candidates for solid-state quantum information technologies. However, it is unclear whether these defect color-centers could support the spin or pseudospin-dependent excitonic fine structure required for spin manipulation and readout. Here we conducted magneto-photoluminescence spectroscopy on individual organic color-centers and observed the emergence of fine structure states under an 8.5 T magnetic field applied parallel to the nanotube axis. One to five fine structure states emerge depending on the chirality of the nanotube host, nature of chemical functional group, and chemical binding configuration, presenting an exciting opportunity toward developing chemical control of magnetic brightening. We attribute these hidden excitonic fine structure states to field-induced mixing of singlet excitons trapped at sp 3 defects and delocalized band-edge triplet excitons. These findings provide opportunities for using organic color-centers for spintronics, spin-based quantum computing, and quantum sensing.
Polarized excitons and optical activity in single-wall carbon nanotubes
2018
The polarized excitons and optical activity of single-wall carbon nanotubes (SWNTs) are studied theoretically by π-electron Hamiltonian and helical-rotational symmetry. By taking advantage of the symmetrization, the single-particle energy and properties of a SWNT are characterized with the corresponding helical band structure. The dipole-moment matrix elements, magnetic-moment matrix elements, and the selection rules can also be derived. Based on different selection rules, the optical transitions can be assigned as the parallel-polarized, left-handed circularly-polarized, and right-handed circularly-polarized transitions, where the combination of the last two gives the cross-polarized transition. The absorption and circular dichroism (CD) spectra are simulated by exciton calculation. The calculated results are well comparable with the reported measurements. Built on the foundation, magnetic-field effects on the polarized excitons and optical activity of SWNTs are studied. Dark-bright exciton splitting and interband Faraday effect in the CD spectrum of SWNTs under an axial magnetic field are predicted. The Faraday rotation dispersion can be analyzed according to the selection rules of circular polarizations and the helical band structure.
Physical Review B, 2019
We investigate the intrinsic magneto-optical properties of suspended single-walled carbon nanotubes of various chiralities, using a combination of spectrally and time-resolved spectroscopy of individual nanotubes at low temperature. This study elucidates the population and relaxation dynamics of the lowest-energy bright and dark singlet excitons that are mixed with the Aharonov-Bohm effect. The high-quality emission spectra reveal the residual oscillator strength of the dark exciton in zero field, as an optical signature of spin-orbit coupling. They also unveil an asymmetry in the bright exciton recombination line, attributed to the free nature of excitons in these unperturbed nanotubes.
2008
We report the direct observation of spin-singlet dark excitons in individual single-walled carbon nanotubes through low-temperature micro-magneto-photoluminescence spectroscopy. A magnetic field (B) applied along the tube axis brightened the dark state, leading to the emergence of a new emission peak. The peak rapidly grew in intensity with increasing B at the expense of the originally dominated bright exciton peak and became dominant at B > 3 T. This behavior, universally observed for more than 50 tubes of different chiralities, can be quantitatively modeled by incorporating the Aharonov-Bohm effect and intervalley Coulomb mixing. The directly measured dark-bright splitting values were 1-4 meV for tube diameters 1.0-1.3 nm. Scatter in the splitting value emphasizes the role of the local environment surrounding a nanotube in determining its excitonic fine structure.