States Of Single-walled Carbon Nanotubes Research Articles

Modulation for Redox States of Single-Walled Carbon Nanotubes: Effect of Wrapping Conjugated Polymers.

Controlling the redox states of single-walled carbon nanotubes (SWNTs) is important for the optimization of their real performances in various fields. By means of in situ photoluminescence (PL) spectroelectrochemical measurements, we report a successful modulation for the redox parameters (redox potentials and electrochemical band gap) of (6,5) and (7,5)SWNTs with a simple change in conjugated polymers (CPs) non-covalently wrapped on the nanotubes. The large shift in the band gap (187 meV for (6,5)SWNTs and 101 meV for (7,5)SWNTs) was connected to the prominent difference in the interactions between the CPs and SWNTs as suggested by molecular dynamics (MD) simulations, while a striking difference in the 𝜋-electrons states of CP/SWNTs enabled the tuning of SWNTs' electronic states. Asymmetrical modulation for the reduction potential (LUMO) and oxidation potential (HOMO) of the SWNTs was observed as well. Our results can be promising for a simple but precise control of the electric states of SWNTs.

Photoemission Study of One-Dimensional Electronic States of (6,5), (6,6), and (7,4) Single-Walled Carbon Nanotubes

We have investigated the one-dimensional electronic states of single-walled carbon nanotubes (SWCNTs) with diameters ranging from 0.8–1.4 nm by optical absorption, photoemission, and inverse photoemission measurements. In the photoemission spectra of (6,5) SWCNTs and (6,6)- and (7,4)-enriched SWCNTs, spike structures caused by Van Hove singularities (VHSs) are clearly observed. The observed VHS positions are compared with the positions calculated using the tight-binding model and density functional theory. A large curvature affects the electronic states of metallic SWCNTs with small diameters.

Entangled two-plasmon generation in carbon nanotubes and graphene-coated wires

We investigate the two-plasmon spontaneous decay of a quantum emitter near single-walled carbon nanotubes (SWCNT) and graphene-coated wires (GCWs). We demonstrate efficient, enhanced generation of two-plasmon entangled states in SWCNTs due to the strong coupling between tunable guided plasmons and the quantum emitter. We predict two-plasmon emission rates more than twelve orders of magnitude higher than in free-space, with average lifetimes of a few dozens of nanoseconds. Given their low dimensionality, these systems could be more efficient for generating and detecting entangled plasmons in comparison to extended graphene. Indeed, we achieve tunable spectrum of emission in GCWs, where sharp resonances occur precisely at the plasmons' minimum excitation frequencies. We show that, by changing the material properties of the GCW's dielectric core, one could tailor the dominant modes and frequencies of the emitted entangled plasmons while keeping the decay rate ten orders of magnitude higher than in free-space. By unveiling the unique properties of two-plasmon spontaneous emission processes in the presence of low dimensional carbon-based nanomaterials, our findings set the basis for a novel material platform with applications to on-chip quantum information technologies.

Effect of thermochemical treatment on the state of SWNT and on the electrical conductivity of epoxy-SWNT composites

An experimental study of single-walled carbon nanotubes (SWNTs) subjected to liquid-phase oxidation with various acid solutions was carried out. The changes in the structure of SWNTs were estimated by the methods of Raman spectroscopy and X-ray structural analysis. Visible decrease of the oriented phase was revealed in the process of liquid-phase oxidation of the samples (the I G/I D-ratio decreases). It was shown that acid treatment leaded to the reduction of the alpha-iron and iron-carbide impurities content in the samples. Composites based on epoxy resin doped by oxidized SWNTs were prepared. Electric conductivity of the composites within SWNT-concentration range from 0 to 1% wt was measured for all used oxidants. It was found that the composites with SWNTs oxidized by mixtures with a predominance of hydrochloric acid have a twice the electric conductivity as compared to composites with untreated SWNTs.

Broadening of van Hove Singularities Measured by Photoemission Spectroscopy of Single- and Mixed-Chirality Single-Walled Carbon Nanotubes

The occupied valence electronic states of single-walled carbon nanotubes (SWCNTs) are responsible for their optoelectronic properties and are unique for each SWCNT chirality. Photoemission spectroscopy (PES) is one of the few methods capable of directly measuring the electron density in the valence states of materials, but there are only a few reports which have observed the valence states of SWCNTs and no examples for single-chirality SWCNTs. Here, we prepare single- and mixed-chirality SWCNT films and characterize their valence states using PES. Chirality-pure SWCNTs were isolated using both gel permeation chromatography and single-stranded DNA-facilitated aqueous two-phase extraction from starting materials consisting of mixed-chirality species. Chirality separation and purity was confirmed with UV-vis-NIR absorption spectroscopy. SWCNT films were prepared for the single-chirality species (10,3), (7,6), (7,3), (6,5), (8,3), (9,1) along with SWCNT chirality mixture of metallic and semiconducting SWCNTs, and as-synthesized mixtures possessing a range of SWCNT diameter. PES using synchrotron radiation was completed for all samples with survey and C 1s core-level spectra obtained to confirm SWCNT coverage, defect level, and purity. Valence band PES was obtained to characterize the valence electronic states and showed significant broadening of the signal, in comparison to calculated density of states, which could not be accounted for by instrument resolution. An inverse diameter dependence of the broadening was observed with greater broadening for smaller-diameter SWCNTs. The broadening is hypothesized to be related to the photohole lifetime, which was found to be significantly longer for wide-diameter SWCNTs. The diameter dependence of the broadening and photohole lifetimes is discussed in terms of both Tomonaga-Luttinger and Landau theories of Fermi liquids.

Tailoring the electrocatalytic oxygen reduction reaction pathway by tuning the electronic states of single-walled carbon nanotubes

Carbon nanomaterials are emerging as a kind of promising electrocatalyst for oxygen reduction reaction (ORR) whether to produce hydrogen peroxide (H2O2) or to be applied in energy conversion devices. However, it is unexplored up to now whether the electronic states of carbon nanomaterials, such as single-walled carbon nanotube (SWNT), will affect its reaction pathway of ORR. Herein, we demonstrate that the selectivity towards H2O2 during ORR of SWNT can be controllably tailored by filling of a series of foreign substances, such as potassium (K), iron (Fe), fullerene (C60) and iodine (I2). This tunability is ascribed to the modification of electronic states of SWNT by electron transfer between SWNT and the filled species, as evidenced by Ultraviolet–visible–near infrared absorption spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The results indicate that the more negatively the carbon atoms are charged, the more preferentially the four electrons route is to undergo. Contrarily, the more positively the carbon atoms are charged, the more tendentiously the two electrons pathway is to conduct.

Enhancing the production of hydrogen peroxide from electrocatalytic oxygen reduction reaction by tailoring the electronic states of single-walled carbon nanotubes: a synergistic effect from interior filling and exterior oxidation

Single-walled carbon nanotubes co-modified with interior filling of phosphomolybdic acid and successive exterior oxidation (O-HPMO@SWCNTs) possess better two-electron oxygen reduction reaction performance than the HPMO-filled SWCNTs, oxidized SWCNTs or pristine SWCNTs.

Modulation of the Local Density of States of Carbon Nanotubes by Encapsulation of Europium Nanowires As Observed by Scanning Tunneling Microscopy and Spectroscopy

Modulation of the local density of states of single-wall carbon nanotubes (SWCNTs) is induced by the encapsulation of europium nanowires (EuNWs). The observation of these modulated density of states using scanning tunneling microscopy/spectroscopy combined with density functional theory calculations is reported. The electronic modulation of SWCNTs by encapsulation of EuNWs is revealed as a Fermi level shift, band gap reduction, and the emergence of localized states in the gap. The present results show that the electronic interaction between EuNWs and SWCNTs is much stronger than that previously reported for nanomaterials encapsulated in SWCNTs.

Multi-exciton emission from solitary dopant states of carbon nanotubes.

By separating the photons from slow and fast decays of single and multi-exciton states in a time gated 2nd order photon correlation experiment, we show that solitary oxygen dopant states of single-walled carbon nanotubes (SWCNTs) allow emission of photon pairs with efficiencies as high as 44% of single exciton emission. Our pump dependent time resolved photoluminescence (PL) studies further reveal diffusion-limited exciton-exciton annihilation as the key process that limits the emission of multi-excitons at high pump fluences. We further postulate that creation of additional permanent exciton quenching sites occurring under intense laser irradiation leads to permanent PL quenching. With this work, we bring out multi-excitonic processes of solitary dopant states as a new area to be explored for potential applications in lasing and entangled photon generation.

Geometry and Self-stress of Single-Wall Carbon Nanotubes and Graphene via a Discrete Model Based on a 2nd-Generation REBO Potential

The main purpose of this paper is to evaluate the self-stress state of single-walled Carbon NanoTubes (CNTs) and Flat Graphene Strips (FGSs) in their natural equilibrium state, that is, the state prior to the application of external loads. We model CNTs as discrete elastic structures, whose shape and volume changes are governed by a Reactive Empirical Bond-Order (REBO) interatomic potential of second generation. The kinematical variables we consider are bond lengths, bond angles, and dihedral angles; to changes of each of these variables we associate a work-conjugate nanostress. To determine the self-stress state in a given CNT, we formulate the load-free equilibrium problem as a minimum problem for the interatomic potential, whose solution yields the equilibrium nanostresses; next, by exploiting the nonlinear constitutive dependence we derive for nanostresses in terms of a list of kinematical variables, we determine the equilibrium values of the latter; finally, from the equilibrium values of the kinematical variables we deduce the natural geometry and, in particular, the natural radius. Our theoretical framework accommodates CNTs of whatever chirality. In the achiral case, the stationarity conditions implied by energy minimization are relatively easy to derive and solve numerically, because we can count on maximal intrinsic symmetries and hence the number of independent unknowns is reduced to a minimum; for chiral CNTs, we prefer to solve the minimum problem directly. The natural-radius predictions we achieve within our discrete-mechanics framework are in good agreement with the results of calculations based on Density Functional Theory (DFT) and Tight Binding (TB) theory; the same is true for our predictions of the self-energy, that is, the energy associated with self-stress (called cohesive energy in the literature); we surmise that our discrete mechanical model may serve as a source of benchmarks for Molecular-Dynamics (MD) simulation algorithms. We find that self-stress depends on changes in both bond and dihedral angles in achiral CNTs and, in addition, on changes in bond length in chiral CNTs. Our analysis applies also to FGSs, whose self-stress and self-energy we evaluate; we find that in FGSs self-stress is associated exclusively with changes in bond angle.

<i>In Situ </i>Photoluminescence Spectroelectrochemistry for Determination of Electronic States of Single-Walled Carbon Nanotubes

The determination of electronic states of single-walled carbon nanotubes (SWNTs) has been a central issue in nanoscience of carbon nanotubes. We have established an in situ photoluminescence (PL) spectroelectrochemistry to determine the electronic properties of the SWNTs. This review article focuses on the fundamentals of the in situ PL spectroelectrochmical method to determine electrochemical potentials of isolated SWNTs and microenviromental effects on the electronic states of the SWNTs, electronic states of SWNTs with a single chirality, and empirical prediction for the electronic potentials of the SWNTs. Such electronic state determination is important for detailed understanding about fundamental properties of the SWNTs and their applications, especially in electronic device applications of the nanotubes. [DOI: 10.1380/ejssnt.2015.179]

Laser-induced pinpoint hydrogen evolution from benzene and water using metal free single-walled carbon nanotubes with high quantum yields.

Metal-free photocatalytic hydrogen evolution occurred efficiently in benzene containing single-walled carbon nanotubes under laser irradiation at 532 nm with an extremely high turnover number of 2 000 000 and a high quantum yield of 130%. The rate of hydrogen evolution increased with increasing laser intensity to exhibit a fourth power dependence, suggesting that hydrogen was evolved via four-photon processes in which the coupling of two radical anions derived from benzene is the rate-determining step and the benzene radical anion is produced by electron transfer from benzene to the doubly excited state of single-walled carbon nanotubes, which requires two photons. Polymerisation of benzene was induced by the photogenerated C6H6˙-, accompanied by hydrogen evolution, resulting in a leverage effect to increase the quantum yield of hydrogen evolution to well over the 25% expected for the four-photon process. Laser-induced hydrogen evolution also occurred in water containing single-walled carbon nanotubes. In contrast to the case of benzene, water was not oxidized but hydrogen evolution from water was accompanied by the multi-oxidation of single-walled carbon nanotubes. The yield of hydrogen based on one mole of single-walled carbon nanotubes with 1.4 nm diameter and 1-5 mm length was determined to be 2 700 000%, when oxidations of single-walled carbon nanotubes occurred to produce the polyhydroxylated product.

Vibrational Excitation in Electron Transport through Carbon Nanotube Quantum Dots.

Electron transport in single-walled carbon nanotubes (SWCNTs) is extremely sensitive to environmental effects. SWCNTs experiencing an inhomogeneous environment are effectively subjected to a disorder potential, which can lead to localized electronic states. An important element of the physical picture of such states localized on the nanometer-scale is the existence of a local vibronic mainfold resulting from the localization-enhanced electron-vibrational coupling. In this Letter, scanning tunneling spectroscopy (STS) is used to study the quantum-confined electronic states in SWCNTs deposited on the Au(111) surface. STS spectra show the vibrational overtones identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. The presence of these vibrational modes in the STS spectra suggests rippling distortion and dimerization of carbon atoms on the SWCNT surface. The present study thus, for the first time, experimentally connects the properties of well-defined localized electronic states to the properties of their associated vibronic states.

Redox properties of a single (7,5)single-walled carbon nanotube determined by an in situ photoluminescence spectroelectrochemical method.

The determination of electronic states of single-walled carbon nanotubes (SWNTs) has been a central issue in science and nanotechnology of carbon nanotubes. We here describe the oxidation and reduction potentials of a single SWNT determined by in situ photoluminescence (PL) spectroelectrochemical measurements. By PL imaging and single SWNT PL spectroscopy, the stepwise quenching behavior of the PL from a single (7,5)SWNT was detected as the outer-applied potentials increased. Based on the analysis of the obtained potential-dependent PL plots using the Nernst equation, the oxidation and reduction potentials of the (7,5) tube are successfully determined as 0.41 V and -0.38 V vs. Ag/AgCl, respectively, which shift from those of the bulk (7,5)SWNTs. We further observed a PL blueshift and narrowing of the line width as the external-applied potential to the single SWNT increases. The present results are important for understanding the electronic properties of a single (n,m)SWNT and its applications.

Microenvironment effect on the electronic potentials of individual (6,5)single-walled carbon nanotubes

As one of the most fundamental properties of carbon nanotubes, the electronic states of single-walled carbon nanotubes (SWNTs) have been investigated using different experimental approaches. In this study, we report a large shift in the redox potentials and band gaps of (6,5)SWNTs with a change in the solubilizer as determined by in situ photoluminescence (PL) spectroelectrochemical measurements in both aqueous and organic solvents. This shift is attributed to the change in the microenvironment around the SWNT surfaces due to interactions between the solubilizers, including a fluorene-based copolymer (PFO-Bpy) and carboxymethyl cellulose (CMC-Na), and the SWNTs, and this behaviour was proved by molecular mechanics simulations. A striking difference was also revealed in the states of the π-electrons in the sidewalls of SWNTs solubilized in different polymers, which indicates that altering the solubilizer enables tuning of the electronic states of SWNTs. The results presented are important for understanding the effects of the microenvironment on the redox states of individual (n,m)SWNTs.

Empirical Prediction of Electronic Potentials of Single-Walled Carbon Nanotubes With a Specific Chirality (n,m)

The determination of the electronic states of single-walled carbon nanotubes (SWNTs) with a specific chirality has been a central issue in the science of SWNTs. Here we present the empirical equations with fitting parameters for the determination of the reduction and oxidation potentials of SWNTs for a wide range of diameters and chiral angles. In these equations, a distinct chirality family dependence of the reduction potentials is observed, while the oxidation potentials show a simple diameter dependence nearly proportional to the inversed nanotube diameter. Based on observations of the asymmetric chirality dependence between the reduction and oxidation potentials, the Fermi levels of the SWNTs were revealed to have a definite chirality family dependence, which indicates that the work functions of the SWNTs with small diameters deviate from the values for the large diameter SWNTs and graphene. We also performed quantum chemical calculations to compare the experiment to the calculations.

Modifying the Density of States of Single-Walled Carbon Nanotubes by Reversible Wrapping with Organometallic Nanofoils: A Scanning Tunneling Spectroscopy Study

Tuning the charge carrier mobility in carbon nanotubes is a key issue for applications in novel electronic or spintronic devices. This property is directly related to the electronic density of states, which is usually modified by introduction of defects or doping with metallic atoms. Both methods can induce strong localization of electronic states, disturbing the overall nanotube conductivity and enhancing electron and spin scattering. Wrapping with organometallic nanofoils is an alternative method which preserves its charge carrier density. In this work, we have prepared single-walled carbon nanotubes (SWCNTs) wrapped with nanofoils of a cobalt-coordinated polymer (NCoDPS). Transmission electron microscopy and Raman scattering reveal that the nanotubes remain intact and are homogeneously covered by the polymer. The electronic structure of SWCNT-NCoDPS was investigated by scanning tunneling spectroscopy. This technique shows that the electronic density of states of these wrapped nanotubes increases, as co...

Effect of Charge of a Matrix Polymer on the Electronic States of Single-Walled Carbon Nanotubes

Abstract The determination of electronic states of single-walled carbon nanotubes (SWNTs) has been a central issue in science and nanotechnology of carbon nanotubes. Previously, we reported the electronic states of (n,m)SWNTs that were embedded in an anionic polymer, carboxymethylcellulose sodium salt (CMC). It is important to reveal how the electronic properties of individual (n,m)SWNTs are affected by the surrounding microenvironment. In this study, we focused on the effect of charge of the SWNT solubilizers. We used a cationic polymer, hydroxyethylcellulose ethoxylate (quaternized) (HEQ) as the SWNT solubilizer in place of CMC, and the isolated (n,m)SWNTs were embedded in a film of HEQ (matrix polymer) on an electrode. We found that the charge of HEQ virtually does not affect the electronic parameters, i.e., oxidation/reduction potentials, Fermi levels, and band gaps of the (n,m)SWNTs, indicating that the presented electronic parameters are the parameters of the intrinsic (n,m)SWNTs.

Electron Transfer Properties of Iodine-Doped Single-Walled Carbon Nanotubes Using Field Effect Transistor

Single-walled carbon nanotubes (SWNTs) are known to have a p-type charge transfer character in the atmosphere. The energy state of SWNTs can be modulated by doping with either an electron donor or an acceptor. In this study, iodine molecules are chosen for intercalation to SWNTs to predict the charge transfer tendency between them. Field-effect transistors (FETs) using iodine intercalated SWNTs (I-SWNTs) are fabricated and their electronic properties are investigated to better understand the charge transfer between iodine and SWNTs by changing gate voltages. Under vacuum, I-SWNT FETs exhibit weak n-type character, indicating that electrons are transferred slightly from the iodine to the SWNTs. After exposure to O2 gas, n-type characters are reduced; however, they still retain their original type.

Effect of Charge of Solubilizers on the Electronic States of Single-Walled Carbon Nanotubes

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