Excitons In Single-walled Carbon Nanotubes Research Articles

Temperature dependence of the near infrared absorption spectrum of single-wall carbon nanotubes dispersed by sodium dodecyl sulfate in aqueous solution: experiments and molecular dynamics study.

Single-wall carbon nanotubes (SWCNT) dispersed in water with the help of sodium dodecyl sulfate (SDS) surfactants exhibit a temperature dependent near infrared (NIR) exciton spectrum. Due to their biocompatibility and NIR spectrum falling within the transparent window for biological tissue, SWCNTs hold potential for sensing temperature inside cells. Here, we seek to elucidate the mechanism responsible for this temperature dependence, focusing on changes in the water coverage of the SWCNT as the surfactant structure changes with temperature. We compare optical absorption spectra in the UV-Vis-IR range and fully atomistic molecular dynamics (MD) simulations. The observed temperature dependence of the spectra for various SWCNTs may be attributed to changes in the dielectric environment and its impact on excitons. MD simulations reveal that the adsorbed SDS molecules effectively shield the SWCNT, with ~ 70% of water molecules removed from the first two adlayers; this coverage shows a modest temperature dependence. Although we are not able to directly demonstrate how this influences the NIR spectrum, this represents a potential pathway given the strong influence of the water environment on the excitons in SWCNTs. Optical absorption measurements were carried out in the UV-Vis-NIR range using a Varian Cary 5000 spectrophotometer in a temperature-controlled environment. PeakFit™ v. 4.06 was used as peak-fitting program in the spectral range 900-1400nm (890-1380meV) as a function of the temperature. Fully atomistic molecular dynamics simulations were conducted using the NAMD2 package. The CHARMM force field comprising two-body bond stretching, three-body angle deformation, four-body dihedral angle deformation, and nonbonded interactions (electrostatic and Lennard-Jones 6-16 potentials) was employed.

(Digital Presentation) Excitonic Photoluminescence Properties of Locally Functionalized Single-Walled Carbon Nanotubes Using Molecular Modifiers

Excitons in single-walled carbon nanotubes (SWCNTs) show unique properties due to strong quantum confinement and weak dielectric screening effects which give rise to high binding energies based on strong Coulombic interactions. By this feature, stable and mobile excitons are generated in SWCNTs and its radiative recombination produces photoluminescence (PL) in the near infrared (NIR) region. Recently, local chemical functionalization of SWCNTs has attracted great attention because of enhancement of their NIR PL properties.[1-5] This functionalization dopes local defects such as sp3 carbon to the semiconducting sp2 carbon networks of SWCNTs based on the covalent bond formation between the tube walls and modifier molecules. As a result, emissive doped sites that have narrower band gaps and exciton trapping features are created in the locally functionalized SWCNTs (lf-SWCNTs). For lf-SWCNTs, E 11* PL newly appears with red-shifted wavelengths and increased PL quantum yields compared with original E 11 PL of pristine SWCNTs. Currently, chemical functionalization methods and E 11* PL wavelength modulation techniques are being developed for further functionalization of lf-SWCNTs. [1-5] In this presentation, E 11* PL properties of lf-SWCNTs synthesized using newly designed chemical modifiers are discussed. For example, an azide compound is found to produce the emissive doped sites in lf-SWCNTs via a [2+1] cycloaddition reaction. The E 11* PL peak was observed at 1118 nm and N1s peak in X-ray photoelectron spectroscopy was detected around 400 eV regions based on the chemical bond formation. Moreover, solvatochromic PL energy shifts of the E 11* PL showed a different trend from those observed for typical lf-SWCNTs synthesized using diazonium chemistry. This result indicates that the chemical structures of the doped sites could be a key factor to modulate excitonic properties of localized excitons in lf-SWCNTs. Therefore, the molecular chemistry approach would provide unique exciton engineering tools for lf-SWCNTs and develop advanced excitonic NIR materials applicable to quantum cryptography technologies and high-performance bio/medical imaging and sensing.[1] T. Shiraki, Chem. Lett., 50, 397 (2021); [2] T. Shiraki et al., Acc. Chem. Res., 53, 1846 (2020); [3] S. Tretiak et al., Acc. Chem. Res., 53, 1791 (2020); [4] Y. Wang et al., Nat. Rev. Chem., 3, 375 (2019); [5] J. Zaumseil, Adv. Optical Mater. 2101576 (2021).

Brightening of dark excitons in single-walled carbon nanotubes: Investigation by many-body Green’s function theory

Optical excitations of the hybrids, which are assembled by coupling single-walled carbon nanotubes (SWCNTs) with organic molecules through van der Waals interactions, are studied using ab initio manybody Green’s function theory. We take the semiconducting (7,0) SWCNT, the squarylium and oligothiophene molecules as the example. The E11 and E22 absorption peaks of the (7,0) tube can be redshifted by tens of meV. Most importantly, the lowest dark exciton of the (7,0) tube at the lower-energy side of E11 can be brightened by the interaction between the nanotube and molecules. Position of this new satellite absorption peak is influenced by the type of adsorbed molecule. These findings may be useful for tuning the emission energy and emission efficiency of CNTs.

(Invited) Aryl-Functionalized Single-Walled Carbon Nanotubes Embedded into Metallo-Dielectric Antennas

Creating exciton localization via covalent sp3 functionalization of single-walled carbon nanotubes (SWCNTs) is a promising route to enhance the optical quantum yield and can furthermore lead to single-photon emission at room temperatures [1]. Of recent interest are aryl-functionalized SWCNTs that are protected by polyfluorene, for which we have recently shown that a variety of chemically and energetically distinct defect state exist that have their origin in topological variation in the chemical binding configuration of the monovalent aryl groups [2]. Here we extend our study by embedding aryl-functionalized SWCNTs into metallo-dielectric antennas that provide broadband coupling with near-unity light collection efficiencies by trapping air gaps on chip that form cavity modes. Scalable implementation is realized by employing polymer layer dry-transfer techniques that avoid solvent incompatibility issues, as well as a planar design that avoids solid-immersion lenses, resulting in a narrow output cone of ±15° that enables a priori fiber-to-chip butt coupling. In these devices the exciton emission from the aryl-functionalized sides (E11*) remains spectrally sharp with linewidth values near the resolution limit of about 35 meV. Interestingly, we find that the pump-induced dephasing for E11* exciton (0D exciton) is significantly reduced by more than 300% as compared to the E11 exciton (1D exciton), thereby preserving exciton coherence even into the high pump power regime, as needed for practical devices in quantum photonics. We will discuss how these findings provide a foundation to build unified descriptions on emergence of novel optical behavior from the interplay of covalent dopants, dispersants, and excitons in SWCNTs. [1] He, X. et al. Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes. Nature Photonics 11, 577–582 (2017). [2] He, X. et al. Low-Temperature Single Carbon Nanotube Spectroscopy of sp3 Quantum Defects, ACS Nano, Articles ASAP (2017); DOI:10.1021/acsnano.7b03022

Dynamics of the Lowest-Energy Excitons in Single-Walled Carbon Nanotubes under Resonant and Nonresonant Optical Excitation

The dynamics of lowest-energy E11 excitons in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) are investigated using transient absorption spectroscopy and theoretical calculations. In undoped SWCNT samples, E11 excitons under resonant E11 photoexcitation show faster decay than those under high-energy nonresonant photoexcitation. Doping SWCNTs with holes accelerates the E11 exciton decay through exciton-hole interactions; moreover, both resonant and nonresonant photoexcitation in hole-doped SWCNTs lead to nearly identical decay profiles. Theoretical calculations, which are based on a simple model that accounts for bright, dark, and charged exciton states, agree well with the experimental results. We ascribe the fast E11 exciton decay under the resonant E11 photoexcitation to the redistribution of exciton populations in the E11 bright exciton band, which includes optically accessible and inaccessible states; the rapid exciton redistribution is caused by exciton-hole scattering. Our study prov...

Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes.

Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization.

Relative ordering between bright and dark excitons in single-walled carbon nanotubes.

The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1st sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2nd sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

Invited Presentation: Strong Nonlinear Photoluminescence Properties of Trions and Localized Excitons in Single-Walled Carbon Nanotubes

Recently, the optical properties of carrier-doped single-walled carbon nanotubes (SWNTs) have been extensively studied [1-3]. We describe nonlinear photoluminescence (PL) properties of excitons and trions (charged excitons) in hole-doped SWNTs. We found that the PL intensity of the trions in hole-doped SWNTs exhibits a strong nonlinear saturation behavior as the excitation density increases, whereas that of the excitons shows a weak saturation behavior. The strong PL saturation of the trions is attributed to the state filling of trion states due to the limited number of doped holes. Moreover, the radiative lifetime of the trion was evaluated to be about 180 ns, which indicates that the experimentally observed PL from the trions originates from partially dipole-allowed transition in a SWNT. We will also discuss the nonlinear PL properties of localized excitons in oxygen-doped SWNTs [4,5].[1] R. Matsunaga, K. Matsuda, and Y. Kanemitsu , Phys. Rev. Lett. 106, (2011) 037404. [2] J. S. Park, Y. Hirana, S. Mouri, Y. Miyauchi, N. Nakashima, and K. Matsuda, J. Am. Chem. Soc. 134, (2012) 14461.[3] S. Mouri, Y. Miyauchi, M. Iwamura, and K. Matsuda, Phys. Rev. B 87, (2013) 045408.[4] S. Ghosh, S. M. Bachilo, R. A. Simonette, K. M. Beckingham, and R. B. Weisman, Science, 330 (2010) 1656.[5] Y. Miyauchi, M. Iwamura, S. Mouri, T. Kawazoe, M. Ohtsu, and K. Matsuda, Nat. Photonics 6, (2013) 547.

Coherent Phonon Coupled with Exciton in Semiconducting Single-walled Carbon Nanotubes with Several Chiralities

Dynamics of coherent phonon coupled with exciton in single-walled carbon nanotubes (SWNTs) are investigated by using few cycle pulse laser. Modulations corresponding to the radial breathing mode (RBM) observed in the time traces for the four chiral systems have been analyzed. The vibrational modes of the coherent phonon spectra are identified from the 2D distribution of probe photon energy versus Fourier frequency. Resonance conditions and mode frequencies lead to definite chirality assignments. The present study clarifies that the observed probe photon energy dependence of RBM amplitude is due to both the imaginary and real parts of the third-order susceptibility.

Photophysics in Single-Walled Carbon Nanotubes with (6,4) Chirality at High Excitation Densities: Bimolecular Auger Recombination and Phase-Space Filling of Excitons

The photophysics in single-walled carbon nanotubes (SWNTs) at high excitation densities has been attracting significant attention for exciton stability. Exciton–exciton interactions and an exciton Mott transition of quasi-one-dimensional excitons in SWNTs are of fundamental interest because the screening effect of the Coulomb interaction is suppressed in one-dimensional systems. In this study, we investigate exciton dynamics and a bimolecular Auger recombination process in SWNTs with (6,4) chirality using femtosecond transient absorption measurements at high excitation densities in the range of 2.4 × 1013–4.8 × 1015 photons cm–2 per pulse. Temporal evolutions of the change in the absorption coefficient are analyzed with an analytical expression for the bimolecular Auger recombination dynamics. We determine the Auger recombination coefficient and initial exciton number per tube, and the absorption cross section is also obtained. By using the phase-space filling model, the exciton size is estimated to be 3.1 ± 1.3 or 2.2 ± 1.0 nm, which depends on the corresponding wave function. Based on the estimated exciton size, the possibility of the Mott transition of quasi-one-dimensional excitons is discussed.

Photoluminescence studies on exciton photophysics in carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are extremely thin cylinders of hexagonal carbon networks with diameters on the order of 1 nm. Over the past decade, optical properties of SWNTs, dominated by correlated electron–hole bound states known as excitons, have been intensively studied. The exciton photophysics of SWNTs is relevant to nearly all optical phenomena observed in SWNTs, and their detailed understanding is critical for the development of future optoelectronic devices using SWNTs. Here we review some interesting photophysics related to excitons in SWNTs, including fundamental features in optical absorption and photoluminescence spectra, effects of exciton–phonon interactions, anisotropic optical absorption and emission, effects of surrounding materials, radiative and nonradiative lifetimes, and diffusional exciton migration along SWNTs, which have been studied by continuous-wave and time-resolved optical spectroscopy.

Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using 13C-labeled, Highly Enriched (6,5) Single-walled Carbon Nanotubes

A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton-phonon coupling opens up additional absorption and emission channels, some of which may "brighten" the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare (12)C and (13)C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright (1)E(11) singlet exciton, one absorptive sideband ~200 meV above, and one emissive sideband ~140 meV below, the bright singlet exciton. Both sidebands demonstrate a ~50 cm(-1) isotope-induced shift, which is commensurate with exciton-phonon coupling involving phonons of A[Formula: see text] symmetry (D band, ω ~ 1330 cm(-1)). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ~25 meV (for the (6,5) SWCNT) above the bright Γ-momentum singlet. This study represents the first use of (13)C-labeled SWCNTs highly enriched in a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton.

Evidence for Long-lived, Optically Generated Quenchers of Excitons in Single-Walled Carbon Nanotubes

The nonlinear dependence of near-infrared photoluminescence (PL) emission on excitation intensity has been measured for individual nanotubes representing six different (n,m) species. Significant deviations from linearity are observed for intensities as low as ~100 W/cm(2), and an approximate inverse correlation is found between nonlinearity and PL action cross section (brightness). A model in which all PL nonlinearity arises from exciton-exciton annihilation is insufficient to account for the experimental data using realistic parameters. It is proposed that additional nonlinear quenching arises from photoinduced quenching states or species with longer lifetimes than emissive excitons. Evidence is also found for metastable photogenerated PL quenchers with lifetimes up to 20 s.

Effects of localized spins on excitons in single-walled carbon nanotubes with imperfections

Excitons comprising electron–hole pairs are one of the representative many-body effects in carbon nanotubes (CNTs) and are well known to affect the optical properties of CNTs. In addition to the excitons, another fascinating many-body effect in CNTs is magnetism that is inherent in CNTs with certain imperfections, such as adsorbents, defects and interfaces, in their hexagonal atomic network. In this paper, we report on the interesting interplay between these two many-body effects in CNTs: the localized spins originating from imperfections in CNTs can couple to the excitons and change the spin state of excitons. We show that the interaction can be written as the spin–spin interaction expressed by the Kondo-like Hamiltonian. Our calculation reveals that the interaction induces mixing between the singlet and triplet excitons and that this mixing produces optically activated triplet excitons. The results solve a recent mystery regarding the microscopic origin of unexpected behavior in the photoluminescence spectra of CNTs.

Ultrahigh Magnetic Field Optical Study of Single-walled Carbon Nanotubes Film

Excitons in Single-Walled Carbon Nanotubes (SWNTs) have emerged as an ideal candidate for exploring one-dimensional (1-D) exciton physics. Exciton states which dominate optical properties of SWNTs even at room temperature, are not clarify yet. The optical absorption spectra of aligned SWNTs films under ultra high magnetic fields up to 190 T are examined to investigate this issue. Shifting and splitting of the absorption peaks due to Aharonov-Bohm effect was observed clearly above 80 T in the configuration where the magnetic fields were applied in parallel to the alignment of SWNTs. The lowest singlet exciton state has been determined through the analysis of energy splitting of excitons by the application of magnetic fields. Keywords: blue shift, optically active, optically inactive, red shift, single-turn coil system

Radiative lifetimes and coherence lengths of one-dimensional excitons in single-walled carbon nanotubes

Radiative lifetimes and coherence lengths of one-dimensional excitons in single-walled carbon nanotubes

PHOTOLUMINESCENCE AND PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY OF SEMICONDUCTING SINGLE WALL CARBON NANOTUBES

Since the discovery of single wall carbon nanotubes (SWNT) in 1991, this nanomaterial has received an enormous attention from the nanoscience and nanotechnology community, not only due to the clear prospects for applications such as novel nanoelectronic and nano-optoelectronic devices, but also because this unique one dimensional (1D) system offered a new possibility for the investigation of novel physical phenomena in low dimensions. In 2002 it was demonstrated that photoluminescence (PL) could be observed in suspensions of isolated SWNTs and, later on, that PL could also be observed from individual, suspended SWNTs. Since then, there has been an increasing amount of work directed towards the investigation of the optical properties of semiconducting SWNTs by PL and photoluminescence excitation spectroscopy (PLE), and the use of PL and PLE for the qualitative and quantitative identification of SWNTs species within and ensemble of carbon nanotubes. In 2005 it was shown that the observed optical transitions are associated to 1D excitons and, from the point of view of optical properties, the rich physics of excitons in SWNTs has received much attention. For instance, it is now clear that excitons and exciton-phonon interactions play a major role in the mechanisms responsible for the emission and the absorption of light in SWNTs. Also, the interaction of SWNTs with their vicinity, which includes the interaction with organic and inorganic molecules, and the modifications in the excitonic system caused by changes in the dielectric constant, can be readily investigated by PL and PLE. In this talk we presented an overview of our recent work in the optical spectroscopy of SWNTs.1–3 In particular, we showed some of our results on the investigation of the exciton-phonon interaction in semiconducting SWNTs and on the modifications in the PL and PLE spectra associated to the interactions with its surrounding environment. Note from Publisher: This article contains the abstract only.

Stochastic Analysis of Stepwise Fluorescence Quenching Reactions on Single-Walled Carbon Nanotubes: Single Molecule Sensors

The 1D quantum confinement of photogenerated excitons in single-walled carbon nanotubes (SWNT) can amplify the detection of molecular adsorption to where single-molecule discrimination is realizable, even from within living cells and tissues. Toward this aim, we present a type 1 collagen film, similar to those used as 3D cell scaffolds for tissue engineering, containing embedded SWNT capable of reporting single-molecule adsorption of quenching molecules. We utilize hidden Markov modeling to link single-molecule adsorption events to rate constants for H2O2, H+, and Fe(CN)6(3-). Among the three kinds of reactant molecules studied, H2O2 has the highest quenching equilibrium constant of 1.59 at 20 microM, whereas H+ is so insensitive that a similar equilibrium constant is achieved only with a concentration of 0.1 M (pH 1). The results were self-consistent because reverse (unquenching) rate constants (600 micros(-1) for H2O2, 1130 micros(-1) for H+ and 4000 micros(-1) for Fe(CN)6(3-)) were observed to be concentration-independent and the forward (quenching) rate constants varied monotonically with concentration. The quenching rate constants also increased with an increase in the redox potential of the quencher, indicating that electron transfer increases the adsorption equilibrium constant on the nanotube surface and, hence, the dwell time of the quencher. These developments provide the material, analytical, and mechanistic groundwork for SWNT to function as single-molecule stochastic biosensors.

Length-Dependent Optical Effects in Single Walled Carbon Nanotubes

Recently, Heller et al. reported length-dependent effects on the relative photoluminescence (PL) quantum yield of single walled carbon nanotubes (SWNTs) [Heller et al J. Am. Chem. Soc. 2004, 126, 14567-14573]. We propose a simple model involving thermal diffusion of excitons along the nanotube axis and quenching at the ends, to explain the observed trend in their data. By fitting to our model, we extract a diffusion coefficient of 6 cm(2)/s for excitons in SWNTs. Assuming a mono exponential decay of exciton PL, we also predict that effective length-dependent PL lifetimes for these excitons lie in the range of 1-27 ps. Experimental observations are shown to be consistent with stochastic rather than wavepacket-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs in imaging and optical sensing applications.

Structural dependence of exciton in carbon nanotubes

The binding energies and sizes of excitons, and energy splitting of the bright-dark excitons in single-walled carbon nanotubes have been calculated using the nonorthogonal tight-binding model, supplemented by the long-range Coulomb interaction. It is found that the binding energies and the sizes of excitons not only depend on tube's diameter d, but also its chirality. However, the splitting of the bright-dark excitons mostly depends on 1/d2. Our obtained results show that the curvature effect is very important for the exciton excitations in the SWNTs, especially in the smaller diameter ones.