Electronic Structure Of Nanotubes Research Articles

Band gap opening of metallic single-walled carbon nanotubes via noncovalent symmetry breaking

Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.

Modulation of Carbon Nanotube Electronic Structure by Grain Boundary Defects in RbI on Au(111).

The electronic properties of single-walled carbon nanotubes (SWCNTs) are known to be highly sensitive to environmental effects. Here, we use scanning tunneling microscopy and spectroscopy to investigate the electronic properties of SWCNTs deposited on RbI monolayer films grown on Au(111). We find that grain boundary defects in RbI monolayers cause the appearance of spatially confined localized states in the SWCNTs. Our density functional theory calculations show that grain boundary defects in RbI/Au(111) produce a stabilizing electrostatic potential caused by reduced coordination of iodine atoms at the RbI grain boundary. The presented results may offer insights into the performance of devices involving transport through SWCNTs subjected to external electrostatic disorder.

Impact of External Electronic Perturbations on Single-Walled Carbon Nanotube Electronic Structure: Scanning Tunneling Spectroscopy and Density Functional Theory

Understanding the local impact of environmental electronic perturbations on the local density of states (LDOS) of single-walled carbon nanotubes (CNTs) is critical for developing CNT-based devices. We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of CNTs adsorbed on both a metal, Au(111), and a dielectric, monolayer RbI on Au(111), serving as models for stronger and weaker electrostatic interactions, respectively. In both cases, STS revealed modulations in the CNT LDOS corresponding to features in the underlying material. We corroborate our STM/STS results with density functional theory calculations of the electronic structure of semiconducting CNTs in the presence and absence of an external dipole. DFT-calculated CNT LDOS qualitatively matched STM/STS results, providing key insight into the local impact external charges have on CNT electronic structure.

Kinetics, Electronic Properties of Filled Carbon Nanotubes Investigated with Spectroscopy for Applications.

The paper is dedicated to the discussion of kinetics of growth, and electronic properties of filled carbon nanotubes investigated by spectroscopy for applications. The paper starts with discussion of growth of carbon nanotubes inside metallocene-filled carbon nanotubes. Nickelocene, cobaltocene are considered for growth of carbon nanotubes. Then, the investigations of filled carbon nanotubes by four spectroscopic techniques are discussed. Among them are Raman spectroscopy, near edge X-ray absorption fine-structure spectroscopy, photoemission spectroscopy, optical absorption spectroscopy. It is discussed that metal halogenides, metal chalcogenides, metals lead to changes in electronic structure of nanotubes with n- or p-doping. The filling of carbon nanotubes with different organic and inorganic substances results in many promising applications. This review adds significant contribution to understanding of the kinetics and electronic properties of filled SWCNTs with considering new results of recent investigations. Challenges in various fields are analyzed and summarized, which shows the author's viewpoint of progress in the spectroscopy of filled SWCNTs. This is a valuable step toward applications of filled SWCNTs and transfer of existing ideas from lab to industrial scale.

Photoluminescence Dynamics Defined by Exciton Trapping Potential of Coupled Defect States in DNA-Functionalized Carbon Nanotubes.

Chemical reactions between semiconducting single-wall carbon nanotubes (SWCNTs) and single-stranded DNA (ssDNA) achieve spatially patterned covalent functionalization sites and create coupled fluorescent quantum defects on the nanotube surface, tailoring SWCNT photophysics for applications such as single-photon emitters in quantum information technologies. The evaluation of relaxation dynamics of photoluminescence (PL) from those coupled quantum defects is essential for understanding the nanotube electronic structure and beneficial to the design of quantum light emitters. Here, we measured the PL decay for ssDNA-functionalized SWCNTs as a function of the guanine content of the ssDNA oligo that dictates the red-shifting of their PL emission peaks relative to the band-edge exciton. We then correlate the observed dependence of PL decay dynamics on energy red-shifts to the exciton potential energy landscape, which is modeled using first-principles approaches based upon the morphology of ssDNA-altered SWCNTs obtained by atomic force microscopy (AFM) imaging. Our simulations illustrate that the multiple guanine defects introduced within a single ssDNA strand strongly interact to create a deep exciton trapping well, acting as a single hybrid trap. The emission decay from the distinctive trapping potential landscape is found to be biexponential for ssDNA-modified SWCNTs. We attributed the fast time component of the biexponential PL decay to the redistribution of exciton population among the lowest energy bright states and a manifold of dark states emerging from the coupling of multiple guanine defects. The long lifetime component in the biexponential decay, on the other hand, is attributed to the redistribution of exciton population among different exciton trapping sites that arise from the binding of multiple ssDNA strands along the nanotube axis. AFM measurements indicate that those trapping sites are separated on average by ∼8 nm along the nanotube axis.

Theoretical evidence of a significant modification of the electronic structure of double-walled carbon nanotubes due to the interlayer interaction

The recently reported experimental optical spectra of double-walled carbon nanotubes exhibit more peaks than it could be expected based on the layers alone. The appearance of excess peaks has been attributed to the interlayer interaction. In order to elucidate the origin of the excess peaks, we perform calculations of the optical absorption of a particular nanotube using the recursion method with non-orthogonal tight-binding basis functions. Our study shows that the interlayer interaction can give rise to major changes in the electronic structure of this nanotube, manifesting themselves with shifts of the optical transitions and appearance of new optical transitions. The derived absorption spectrum is found to be in excellent agreement with the available experimental data, which justifies the use of the proposed approach for double-walled carbon nanotubes.

Tuning the rate of an outer-sphere electron transfer by changing the electronic structure of carbon nanotubes

Non-adiabatic electron transfer from a single wall carbon nanotube to a model H-like reactant is explored in the framework of quantum mechanical theory. As examples, we consider a pristine semiconducting (8, 0) nanotube, tubes intercalated with Li+ or Cl− ions, and tubes doped with nitrogen. The electronic structure of the nanotubes has been calculated by DFT with periodic boundary conditions. The electronic coupling between the tube and the reactant, which is proportional to the reactant–electrode orbital overlap, is calculated as a function of distance, and used to estimate the effective Landau-Zener factor. Both doping and intercalation shift the nanotube Fermi level either to the valence band or to conduction band, which results in a rise of the rate constant of non-adiabatic electron transfer by up to four orders of magnitude.

Fe-incorporated TiO2 nanotube arrays: Electronic structure and magnetic response

Incorporating Fe atoms into the lattice is shown to significantly alter electronic and magnetic properties of $\mathrm{Ti}{\mathrm{O}}_{2}$ nanotubes synthesized by electrochemical anodization of Ti-Fe alloy sheets. The effects of Fe incorporation on the nanotube morphology, crystallinity, crystal structure, magnetic behavior and electronic structure were investigated with crystallographic and magnetic probes, including synchrotron-based spectroscopy. Results indicate that the iron cations predominately adopt the $\mathrm{F}{\mathrm{e}}^{3+}$ configuration, leading to a large increase of the electronic density of states at the Fermi energy. This increase is anticipated to provide enhanced catalytic action, for instance, in the degradation of water and of air pollutants. These results provide insight for tailoring the functionality of these nanostructures for energy-related applications.

Atomic-Scale Deformations at the Interface of a Mixed-Dimensional van der Waals Heterostructure.

Molecular self-assembly due to chemical interactions is the basis of bottom-up nanofabrication, whereas weaker intermolecular forces dominate on the scale of macromolecules. Recent advances in synthesis and characterization have brought increasing attention to two- and mixed-dimensional heterostructures, and it has been recognized that van der Waals (vdW) forces within the structure may have a significant impact on their morphology. Here, we suspend single-walled carbon nanotubes (SWCNTs) on graphene to create a model system for the study of a 1D–2D molecular interface through atomic-resolution scanning transmission electron microscopy observations. When brought into contact, the radial deformation of SWCNTs and the emergence of long-range linear grooves in graphene revealed by the three-dimensional reconstruction of the heterostructure are observed. These topographic features are strain-correlated but show no sensitivity to carbon nanotube helicity, electronic structure, or stacking order. Finally, despite the random deposition of the nanotubes, we show that the competition between strain and vdW forces results in aligned carbon–carbon interfaces spanning hundreds of nanometers.

Conductivity of AuCl4-Functionalized Carbon Nanotube Networks

We have studied the effect of AuCl4 functionalization on the conductivity of carbon nanotube networks by first-principles electronic structure calculations. The functionalization results from treating carbon nanotube networks by dissolved AuCl3. Band structures and electronic transmission functions for single-walled semiconducting carbon nanotubes with physisorbed AuCl4 anions are computed using the density functional theory. The resulting p-type doping of nanotubes accompanied by a downshift of the Fermi level and balanced by negatively charged AuCl4 anions make the nanotubes metallic. Moreover, the influence of AuCl4 functionalization on the conductance of the junction between two semiconducting carbon nanotubes is considered. Increasing the AuCl4 coverage lowers the Fermi level rapidly until it is pinned by a van Hove singularity of the nanotube electronic structure. For these doping levels our calculations based on the density-functional nonequilibrium Green’s function method show a significant increa...

Cavity-enhanced Raman microscopy of individual carbon nanotubes.

Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics.

Electrochemical polymerization of pyrene derivatives on functionalized carbon nanotubes for pseudocapacitive electrodes.

Electrochemical energy-storage devices have the potential to be clean and efficient, but their current cost and performance limit their use in numerous transportation and stationary applications. Many organic molecules are abundant, economical and electrochemically active; if selected correctly and rationally designed, these organic molecules offer a promising route to expand the applications of these energy-storage devices. In this study, polycyclic aromatic hydrocarbons are introduced within a functionalized few-walled carbon nanotube matrix to develop high-energy, high-power positive electrodes for pseudocapacitor applications. The reduction potential and capacity of various polycyclic aromatic hydrocarbons are correlated with their interaction with the functionalized few-walled carbon nanotube matrix, chemical configuration and electronic structure. These findings provide rational design criteria for nanostructured organic electrodes. When combined with lithium negative electrodes, these nanostructured organic electrodes exhibit energy densities of ∼350 Wh kg−1electrode at power densities of ∼10 kW kg−1electrode for over 10,000 cycles.

Functionalization and defunctionalization of single walled carbon nanotubes: Electrochemical and morphologic consequences

Electrochemical oxidation and subsequent electrochemical reduction of single walled carbon nanotubes was carried out by cyclic voltammetry. Using methods of Raman and absorption spectra it was revealed that oxidation of SWCNT electrodes leads to significant disruption of electronic structure of nanotubes; the structure can be substantially restored in the course of subsequent cathodic polarization. The quantitative relationship between the concentration of various oxygen-containing functional groups present on the pristine surface, electrochemically oxidized and subsequently reduced single walled carbon nanotubes was demonstrated with the use of XPS. It was shown that upon application of anodic potentials, functionalization of metallic SWCNTs proceeds deeper than in the case of semiconductor ones. The correlation between the degree of functionalization of SWCNT electrodes and kinetic parameters of electron transfer were studied using redox couples: [Ru(NH3)6]2+/3+, [Fe(CN)6]4−/3−, and Fe2+/3+. The essential growth of the electron transfer rate constant observed for couple Fe+2/+3 was suggested to results from the coordination of iron ion to oxygen from functional groups, formed at water electrolysis. It was demonstrated that the maximum rate constant for the system is reached at the medium functionalization degree.

Sulfur Dioxide Internal and External Adsorption on the Single-Walled Carbon Nanotubes: DFT Study

Density functional theory is used to investigate sulfur dioxide physisorption inside and outside of single-walled carbon nanotube (SWCNT) of (5,0) and (5,5). This study is conducted at B3LYP/6-31G* level of theory. Sulfur dioxide molecule is studied with axis oriented parallel or perpendicular to the nanotube wall. Both internal and external adsorption on nanotubes are increased with the angle of interaction being at maximum of 90° to the tube axis. The harmonic frequencies are computed from analytical derivatives for all species in order to define the minimum energy structures. The lowest values of HOMO and LUMO energies are obtained in the process of the adsorption on the external wall of (5,5) nanotube. The dipole moment of the SWCNTs-SO2 system is highly increased and is more than that of SWCNTs- H2S and SWCNTs- CO2. The lowest ΔGtot for SO2 on the outside wall of (5,0) is obtained while this quantity is positive and is not a favorable adsorption process for H2S on the nanotubes. The NBO analysis shows the change in the electronic structure of nanotubes could be suitable for fabricating sensors.

High-throughput optical imaging and spectroscopy of individual carbon nanotubes in devices

Single-walled carbon nanotubes are uniquely identified by a pair of chirality indices (n,m), which dictate the physical structures and electronic properties of each species. Carbon nanotube research is currently facing two outstanding challenges: achieving chirality-controlled growth and understanding chirality-dependent device physics. Addressing these challenges requires, respectively, high-throughput determination of the nanotube chirality distribution on growth substrates and in situ characterization of the nanotube electronic structure in operating devices. Direct optical imaging and spectroscopy techniques are well suited for both goals, but their implementation at the single nanotube level has remained a challenge due to the small nanotube signal and unavoidable environment background. Here, we report high-throughput real-time optical imaging and broadband in situ spectroscopy of individual carbon nanotubes on various substrates and in field-effect transistor devices using polarization-based microscopy combined with supercontinuum laser illumination. Our technique enables the complete chirality profiling of hundreds of individual carbon nanotubes, both semiconducting and metallic, on a growth substrate. In devices, we observe that high-order nanotube optical resonances are dramatically broadened by electrostatic doping, an unexpected behaviour that points to strong interband electron-electron scattering processes that could dominate ultrafast dynamics of excited states in carbon nanotubes.

Probing the electronic structure of multi-walled carbon nanotubes by transient optical transmittivity

High-resolution time resolved transmittivity measurements on horizontally aligned free-standing multi-walled carbon nanotubes reveal a different electronic transient behavior from that of graphite. This difference is ascribed to the presence of discrete energy states in the multishell carbon nanotube electronic structure. Probe polarization dependence suggests that the optical transitions involve definite selection rules. The origin of these states is discussed and a rate equation model is proposed to rationalize our findings.

A convenient route to disperse pristine carbon nanotubes in polystyrene with no damage of nanotube electronic structure

A convenient route to prepare polystyrene (PS)/pristine carbon nanotube (CNT) nanocomposite was developed; its electronic structure and electrical conductivity were also studied. Using their well-dispersed system in N-methyl-2-pyrrolodone, pristine multi-walled CNTs (MWCNTs), without any surface organo-modification, were greatly debundled and uniformly dispersed in PS matrix via a simple solution blending procedure. The dispersion structure of the prepared PS/pristine MWCNT nanocomposite was confirmed by scanning electron microscope and laser scanning confocal microscopic images. Moreover, Raman spectroscopy indicated that this method could effectively preserve the electronic structure of the pristine CNTs in the prepared nanocomposite, and the PS/pristine MWCNT nanocomposite showed greatly enhanced electrical conductivity.

Structural, electronic and optical properties of titania nanotubes

This review paper describes primarily recent theoretical calculations with some supporting experimental findings on titania nanotubes. Nanotubes with different types and sizes are discussed in detail in terms of existing theoretical and experimental achievements. Both classical and quantum mechanical simulations are focused on. The properties of these nanotubes have been treated within first principle density functional electronic structure simulation methods. In this paper, we pay particular attention to computational aspects, but when appropriate, relationships with experimental results on titania nanostructures will be mentioned. First, the structural properties of titania nanotubes are reviewed, focusing from experimental growth mechanism to possible theoretical stable structure and orientation. Second, the electronic structure of nanotubes is discussed in terms of band gap modifications of titania and photocatalytic efficiencies in photoelectrochemical devices. Finally, current computational limitations and future directions are described with respect to the performances of nanotube titania based photosensitive devices.

DFT study of CH4 adsorption on the (5,0), (4,4), (5,5) and (6,6) single-walled carbon nanotubes. Calculated binding energies, NMR and NQR parameters

Behavior of a single CH4 molecule adsorbed on external surface of H-capped (4,4) (5,5), (6,6) and (5,0) single-walled carbon nanotubes (SWCNTs) is studied via B97D hybrid density functional and 6-31G* basis set. Binding energies clearly exhibit adsorption dependence on tube diameter. 13C and 1H chemical shielding tensors are calculated at the B971 level using GIAO method. The 1H and 13C(CH4) NMR results reveal that chemical shielding due to CH4 molecule adsorption are also dependent upon the nanotube electronic structure, and radius. 2H nuclear quadrupole coupling constants, C Q, and asymmetry parameter, η, reveal the remarkable effect of CH4 adsorption on electronic structure of the SWCNTs.

Scanning tunnelling spectroscopy on the local electronic structure of Gd@C82 peapods

AbstractThe electronic structure of Gd@C82 metallofullerene peapods is studied experimentally by using scanning tunnelling microscopy and spectroscopy (STM and STS). The spatial modulation of the nanotube electronic structure induced by Gd@C82 encapsulation has been observed depending on the position along the tube axis. Both STM and STS reveal a 1.2 nm periodicity which is consistent with the intermolecular distance of Gd@C82 inside single‐wall carbon nanotubes.