Semiconducting Zigzag Carbon Nanotubes Research Articles

Possibility of the magnitude of the chiral vector having an effect on the terahertz radiation generation by hot electrons in zigzag carbon nanotubes driven by DC-AC fields

The possibility that the magnitude of a chiral or circumferential vector (Chv) influences the generation of terahertz radiations (T-rays) by hot electrons in zigzag carbon nanotubes (z-CNTs) driven by direct current (DC) and alternating current (AC) fields is taken into consideration. The study conducted a semiclassical theoretical analysis by using the Boltzmann kinetic equation to determine current density in relation to high AC field frequency (ω) and z-CNT circumference (|Chv|) when hot electrons were present. A numerical analysis was performed by varying the magnitude of the circumference of the tube at a fixed temperature and the strong hot electron injection rate at a predetermined value. When the circumference of the semiconducting zigzag carbon nanotubes (sz-CNTs) is increased, the current density's lowest and highest peaks are also enhanced (i.e., large amplitudes). This increases positive differential conductivity (PDC), necessitating enhanced domain suppression for higher potential generation of T-rays. In contrast, it was observed that reducing the tube's circumference in metallic zigzag carbon nanotubes (mz-CNTs) resulted in an increase in both the lowest and highest peaks of current density, which enhanced the PDC. These findings imply that larger circumferential sz-CNTs and smaller circumferential mz-CNTs may be more suitable candidates for a higher potential generation of T-rays with applications pertinent to modern science and technology.

Effect of encapsulation of fullerene in semiconducting zigzagcarbon nanotubes on optical and thermoelectric properties

We study the optical and thermoelectric properties of carbon nanotubes (CNTs) with encapsulated C60 fullerene molecules. Using ab-initio methods, we calculate optical and thermoelectric paramters for CNT with fullerenes periodically located inside the nanotube at different distances from each other. Dependencies of these parameters on fullerene concentration and diameter of CNT are analyzed.

Radius dependence of the electrical conductivity of zigzgag carbon nanotubes

The radius dependence of the electrical conductivity of metallic and semiconducting zigzag carbon nanotubes (CNTs) is theoretically studied. The investigation was done semiclassically by solving the Boltzmann transport equation to derive current density as a function of a homogenous axial dc field and radius of the tube. The analysis was numerically carried out by varying the radius of the materials at a constant temperature. Plots of the normalized current density versus dc field applied along the axis of both materials are presented. We observed that in the case of the metallic zigzag CNTs as the radius increases, the electrical conductivity decreases. On the other hand, in the semiconducting zigzag CNT there was an increase as radius increases. This research shows that thinner metallic zigzag CNTs and thicker semiconducting zigzag CNTs are better conductors of electricity. This investigation therefore offers way of obtaining higher electrical conductivity in both materials without doping. This study therefore shows applications in the development of current conducting nano-devices for scientific systems.

Optical properties of semiconducting zigzag carbon nanotubes with and without defects.

The excited states of a series of semiconducting zigzag (n, 0) tubes are studied using the GW method and the Bethe-Salpeter equation within the ab initio many-body perturbation theory. The optical variation rule of the excitation energy with the tube diameter exhibits a family pattern, which arises from the electronic structure of the pristine tube and depends on the value of n mod 3. The introduction of single vacancy and Stone-Wales defects with different orientations affords an effective route for modulating the band structures and optical spectra, resulting in the variation of the selection rules of the excitons and turning dipole-forbidden excitons into dipole-allowed ones. The new localized impurity states in defected tubes will provide additional optically allowed transitions and give rise to pronounced satellite red-shifted peaks. These findings provide inspiration for the tune of optical properties of carbon nanotubes in the future for applications in optoelectronics.

Majorana fermion wavefunctions in carbon nanotubes and carbynes

Electron properties of semiconducting zigzag carbon nanotubes (CNTs) can be described by two uncoupled Dirac equations of dimension (1+1) for the particle with nonzero mass. The solutions of these equations are two charge-neutral Majorana fields. An analogous equation is obtained for the carbon chains. We use the approach, wherein wavefunction of charged particle is represented as the production of a rapidly oscillating exponent and the slowly varying function amplitude depending on the longitudinal coordinate.

Detection of CO2 using CNT-based sensors: Role of Fe catalyst on sensitivity and selectivity

The adsorption of CO2 on surfaces of graphene and carbon nanotubes (CNTs), decorated with Fe atoms, are investigated using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method, neglecting the heat effects. Fe ad-atoms are more stable when they are dispersed on hollow sites. They introduce a large density of states at the Fermi level (NF); where keeping such density low would help in gas sensing. Furthermore, the Fe ad-atom can weaken the CO double bonds of the chemisorbed CO2 molecule, paving the way for oxygen atoms to drain more charges from Fe. Consequently, chemisorption of CO2 molecules reduces both NF and the conductance while it enhances the sensitivity with the increasing gas dose. Conducting armchair CNTs (ac-CNTs) have higher sensitivity than graphene and semiconducting zigzag CNTs (zz-CNTs). Comparative study of sensitivity of ac-CNT-Fe composite towards various gases (e.g., O2, N2, H2, H2O, CO and CO2) has shown high sensitivity and selectivity towards CO, CO2 and H2O gases.

Effect of substitutionally boron‐doped single‐walled semiconducting zigzag carbon nanotubes on ammonia adsorption

We investigate the binding of ammonia on intrinsic and substitutionally doped semiconducting single-walled carbon nanotubes (SWCNTs) on the side walls using density functional calculations. Ammonia is found to be weakly physisorbed on intrinsic semiconducting nanotubes while on substitutional doping with boron its affinity is enhanced considerably reflected with increase in binding energies and charge transfer. This is attributed to the strong chemical interaction between electron rich nitrogen of ammonia and electron deficient boron of the doped SWCNT. On doping, the density of states are changed compared to the intrinsic case and additional levels are formed near the Fermi level leading to overlap of levels with that of ammonia indicating charge transfer. The doped SWCNTs thus are expected to be a potential candidate for detecting ammonia.

Many-body effects on the electronic and optical properties of strained semiconducting carbon nanotubes

We present many-body \textit{ab initio} calculations of the electronic and optical properties of semiconducting zigzag carbon nanotubes under uniaxial strain. The GW approach is utilized to obtain the quasiparticle bandgaps and is combined with the Bethe-Salpeter equation to obtain the optical absorption spectrum. We find that the dependence of the electronic bandgaps on strain is more complex than previously predicted based on tight-binding models or density-functional theory. In addition, we show that the exciton energy and exciton binding energy depend significantly on strain, with variations of tens of meVs per percent strain, but that despite these strong changes the absorbance is found to be nearly independent of strain. Our results provide new guidance for the understanding and design of optomechanical systems based on carbon nanotubes.

Effects of Vacancies on the Electron Transport in Semiconducting Zigzag Carbon Nanotubes

Since the advent of nanoscale material based electronic devices, there has been a considerable interest in exploring carbon nanotubes from fundamental science and technological perspectives. In carbon nanotubes, the atoms form a cylindrical structure with a diameter of the order 1nm. The length of the nanotubes can extend up to several hundred micrometers. Carbon nanotubes exhibit a variety of intriguing electronic properties such as semiconducting and metallic behaviour, due to the quantum confinement of the electrons in the circumferential direction. Much of the study dedicated to describe the behaviour of carbon nanotube-based devices assumes for simplicity the nanotube to be a ballistic material. However, in reality the phonon scattering mechanism exists also in nanotubes, of course, and can generally not be neglected, except in very short nanotubes. In this work, we focus attention on exploring the steady-state electron transport properties of semiconducting single-walled carbon nanotubes, including both phonon scattering and defect (vacancy) scattering, using the semi-classical bulk single electron Monte Carlo method. The electron energy dispersion relations are obtained by applying the zone folding technique to the dispersion relations of graphene, which are calculated using the tight-binding description. The vibrational modes in the carbon nanotubes are studied using a fourth nearest-neighbour force constant model. Both the electron-phonon and the electron-defect interactions are formulated within the tight-binding framework, and their corresponding scattering rates are computed and analyzed. In particular, the dependence of the phonon scattering rate and the defect scattering rate on the diameter of the nanotube, on temperature and on electron energy is studied. It is shown that the differences observed in the scattering rate between different nanotubes mainly stem from the differences in their band structure. A bulk single electron Monte Carlo simulator was developed to study the electron transport in semiconducting zigzag carbon nanotubes. As a first step, we included only electron-phonon scattering, neglecting all other possible scattering mechanisms. With this scattering mechanism, the steady-state drift velocity and the mobility for the nanotubes (8,0), (10,0), (11,0), (13,0) and (25,0) were calculated as functions of the electric-field strength and lattice temperature, and the results are presented and analysed here. The dependence of the mobility on the lattice temperature can be clearly seen at low electric-field strengths. At such electric-field strengths, the scattering is almost entirely due to acoustic phonons, whereas at high electric-field strengths optical phonon emission processes dominate. It is shown that the saturation of the steady-state drift velocity at high electric-field strengths is due to the emission of high-energy optical phonons. The results indicate the presence of Negative differential resistance for some of the nanotubes considered in this work. The discrepancy found in the literature concerning the physical reason for the appearance of negative differential resistance is clarified, and a new explanation is proposed. It is also observed that the backward scattering is dominant over the forward scattering at high electric-field strengths. We then included also defect scattering, actually electron-vacancy scattering, for the nanotubes (10,0) and (13,0). The steady-state drift velocities for these nanotubes are calculated as functions of the density of vacancies, electric-field strength and the lattice temperature, using three different vacancy concentrations. The results indicate the presence of Negative differential resistance at very low concentration of defects, and how this feature may depend on the concentration of defects. The dependence of the steady-state drift velocity on the concentration of defect and the lattice temperature is discussed. The electron distribution functions for different temperatures and electric field strengths are also calculated and investigated for all the semiconducting nanotubes considered here. In particular, a steep barrier found in the electron distribution function is attributed to the emission of high energy optical phonons.

Heat Capacity of Defective Semiconducting Carbon Nanotubes

Using a random tight-binding model within the coherent potential approximation, the effects of boron and nitrogen doping on the temperature dependence of the specific heat of semiconducting zigzag carbon nanotubes are studied. It is shown that the electronic specific heat capacity behaves anomalously when the temperature is lowered. The presence of this anomaly is clarified on the basis of the idea of the so-called Schottky anomaly. More importantly, in the presence of dopants, the position of this anomaly moves towards higher temperatures and its height shifts down as the dopant concentration is increased. Such behavior is attributed to the substantial modifications in the density of states.

High-field electron transport in semiconducting zigzag carbon nanotubes

Electron transport in semiconducting zigzag carbon nanotubes is studied by solving the Boltzmann transport equation using the single-particle Monte Carlo technique. The electronic band structure is based on a standard nearest-neighbour tight-binding parameterization, and the phonon spectrum is calculated using a fourth nearest-neighbour force constant model. The electron–phonon scattering probabilities are calculated within a tight-binding formalism. The steady-state drift velocities for the semiconducting zigzag nanotubes (8, 0), (10, 0), (11, 0), (13, 0), and (25, 0) are computed as functions of electric field strength and temperature, and the results are analysed here. The results show the presence of negative differential resistance at high electric fields for some of the nanotubes. The drift velocity and the low-field mobility reach a maximum value of ≅ 4.67 × 107 cm s−1 and≅ 4 × 104 cm2 V −1 s−1, respectively, for a (25, 0) nanotube.

Structural and electronic properties of conducting Cu nanowire encapsulated in semiconducting zigzag carbon nanotubes: A first‐principles study

AbstractUnder the generalized gradient approximation (GGA), the structural and electronic properties of conducting Cu8 nanowires encapsulated in semiconducting zigzag carbon nanotubes (CNT) have been investigated by using the first‐principles projector‐augmented wave (PAW) potential within the density function theory (DFT) framework. Among these three Cu8 wire‐encapsulated C cables, (11,0) and (13,0) nanocables are formed endothermically, (14,0) nanocable is formed exothermically. Consequently, it is expected that the thicker Cu wire would be pulled spontaneously into wider nanotubes by forces amounting to a fraction of a nanonewton. The high electrical transport behaviors of the Cu8‐encapsulated C cables coming only from the Cu8 nanowire indicate the Cu8‐encapsulated C cables can be applicable for the ultralarge‐scale integration (ULSI) circuits.

Electron‐phonon and electron‐defect scattering rates in semiconducting zigzag carbon nanotubes

AbstractElectron‐phonon and electron‐defect scattering rates have been calculated within a tight‐binding approach for semiconducting zigzag carbon nanotubes. The scattering rates for (5,0), (7,0), (10,0), (13,0) and (25,0) nanotubes have been investigated. The electron‐phonon scattering rate shows both diameter and temperature dependence, and the dependence of the electron‐defect scattering rate on nanotube diameter is similar to that of the electron‐phonon scattering rate. Backscattering and forward scattering for electrons interacting with defects occur with equal probability at all energies. The importance of electron‐defect scattering relative to electron‐phonon scattering depends very much on the energy of the electron (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the Charge Carrier Localization in Zigzag Carbon Nanotube Junctions

In this work, we propose a novel photovoltaic device constructed from the junction of two carbon nanotubes, where the top of the valence band is mainly localized on one of the nanotubes' sides, while the bottom of the conduction one is mainly localized on the other side. This allows the separation of electrons on the side displaying the lower conduction band, from the holes, located at the other side. On the basis of self-consistent charge-density functional tight-binding calculations, we propose that this kind of device can be formed by joining two consecutive semiconducting zigzag carbon nanotubes. We have shown that, for these systems, the existence of the required band energy differences is related to the kind of variations in energy gap with the tube diameters, observed for the zigzag nanotubes. To illustrate the formation of the proposed junction, we have chosen to study a (10,0) tube joined linearly to a (11,0) one through a single pentagon–heptagon defect.

Optical transitions in semiconducting zigzag carbon nanotubes with small diameters: A first-principles broad-range study

We study all possible band-to-band transitions between 12 valence and 16 conduction bands of (8,0), (10,0), and (7,0) nanotubes and calculate the corresponding dipole moments using first-principles methods in a wide ultraviolet-visible-infrared range of photon energies. The goal is to investigate the optical transitions and the selection rules for nanotubes with small diameters in a broad range of energies, taking into account the curvature and rehybridization effects on the dipole moment. Our calculations show the conservation of a modified quantum number, $\stackrel{\fontencoding{LECO}\selectfont\char177{}}{m}$, for all the transitions and reversal of the horizontal parity. Besides the $\ensuremath{\pi}\text{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{\ast}}$ transitions, we observe the possibility of $\ensuremath{\pi}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{\ast}}$, $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{\ast}}$, and $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{\ast}}$ transitions. In fact, the maximum dipole moment for the three nanotubes corresponds to the transition between valence and conduction bands that are both of $\ensuremath{\sigma}$ nature. The maximum transition rate for an (8,0) nanotube happens at approximately 1.42 eV, corresponding to the transition between the first valence and fourth conduction bands and mapping to the infrared region. The maximum of transition rate for (10,0) and (7,0) nanotubes happens at 0.79 eV and 3.03 eV, respectively. There exist high absorption probabilities in the infrared and visible region in all three nanotubes. Also, high absorption probability in the low ultraviolet region is demonstrated, mostly as a result of $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{\ast}}$ transitions.

Electron–Phonon Scattering Rates in Semiconducting Zig-Zag Carbon Nanotubes

Since the advent of nanoscale material based electronic devices, there has been a considerable interest in exploring carbon nanotubes from fundamental science and technological perspectives. In carbon nanotubes, the atoms form a cylindrical structure with a diameter of the order 1nm. The length of the nanotubes can extend up to several hundred micrometers. Carbon nanotubes exhibit a variety of intriguing electronic properties such as semiconducting and metallic behaviour, due to the quantum confinement of the electrons in the circumferential direction. Much of the study dedicated to describe the behaviour of carbon nanotube-based devices assumes for simplicity the nanotube to be a ballistic material. However, in reality the phonon scattering mechanism exists also in nanotubes, of course, and can generally not be neglected, except in very short nanotubes. In this work, we focus attention on exploring the steady-state electron transport properties of semiconducting single-walled carbon nanotubes, including both phonon scattering and defect (vacancy) scattering, using the semi-classical bulk single electron Monte Carlo method. The electron energy dispersion relations are obtained by applying the zone folding technique to the dispersion relations of graphene, which are calculated using the tight-binding description. The vibrational modes in the carbon nanotubes are studied using a fourth nearest-neighbour force constant model. Both the electron-phonon and the electron-defect interactions are formulated within the tight-binding framework, and their corresponding scattering rates are computed and analyzed. In particular, the dependence of the phonon scattering rate and the defect scattering rate on the diameter of the nanotube, on temperature and on electron energy is studied. It is shown that the differences observed in the scattering rate between different nanotubes mainly stem from the differences in their band structure. A bulk single electron Monte Carlo simulator was developed to study the electron transport in semiconducting zigzag carbon nanotubes. As a first step, we included only electron-phonon scattering, neglecting all other possible scattering mechanisms. With this scattering mechanism, the steady-state drift velocity and the mobility for the nanotubes (8,0), (10,0), (11,0), (13,0) and (25,0) were calculated as functions of the electric-field strength and lattice temperature, and the results are presented and analysed here. The dependence of the mobility on the lattice temperature can be clearly seen at low electric-field strengths. At such electric-field strengths, the scattering is almost entirely due to acoustic phonons, whereas at high electric-field strengths optical phonon emission processes dominate. It is shown that the saturation of the steady-state drift velocity at high electric-field strengths is due to the emission of high-energy optical phonons. The results indicate the presence of Negative differential resistance for some of the nanotubes considered in this work. The discrepancy found in the literature concerning the physical reason for the appearance of negative differential resistance is clarified, and a new explanation is proposed. It is also observed that the backward scattering is dominant over the forward scattering at high electric-field strengths. We then included also defect scattering, actually electron-vacancy scattering, for the nanotubes (10,0) and (13,0). The steady-state drift velocities for these nanotubes are calculated as functions of the density of vacancies, electric-field strength and the lattice temperature, using three different vacancy concentrations. The results indicate the presence of Negative differential resistance at very low concentration of defects, and how this feature may depend on the concentration of defects. The dependence of the steady-state drift velocity on the concentration of defect and the lattice temperature is discussed. The electron distribution functions for different temperatures and electric field strengths are also calculated and investigated for all the semiconducting nanotubes considered here. In particular, a steep barrier found in the electron distribution function is attributed to the emission of high energy optical phonons.

Energy splitting between bright and dark excitons in carbon nanotubes

We have calculated the bright-dark energy splitting in semiconducting zigzag carbon nanotubes in a simple tight-binding model for π -electrons . The short-range cut-off of the Coulomb interaction is used as a fitting parameter. We have found that the splitting depends not only on the type of transition, E 11 or E 22 , but also on the structure, or the index, of carbon nanotube, and that family pattern appears in the cut-off parameter dependence of the splitting.

Davydov's solitons in zigzag carbon nanotubes

AbstractNonlinear mechanism of charge transport in low‐dimensional nanosystems is considered, based on the account of the electron–phonon interaction that arises from the deformation dependencies of the on‐site and the hopping interaction energies at intermediatevalues of the corresponding coupling constants. In particular, carbon‐type hexagonal zigzag nanotubes are studied. It is shown that in the adiabatic approximation the electron–phonon coupling results in the self‐trapping of carriers and formation of polaron (soliton) states. We show that the ground state of an electron in a nanotube is a low‐dimensional polaron whose symmetry depends on the strength of the coupling. Namely, at relatively weak coupling, the polaron possesses quasi‐one‐dimensional properties and has an azimuthal symmetry. When the coupling constant exceeds some critical value, the azimuthal symmetry breaks down and two‐dimensional polaron on the nanotube surface is formed. There are also polarons formed by the electrons in the conducting band (or by holes in the valence band) in semiconducting zigzag carbon nanotubes. Such polarons are described by the system of coupled nonlinear Schroedinger equations which admits single‐band polarons, and entangled (hybridized) polarons. These two types of polarons possess different energies and symmetry properties. Single‐band solutions are one‐dimensional polarons: they are azimuthally symmetric and localized along the nanotube axis. The entangled polarons are also self‐trapped along the nanotube axis, but possess an inner structure and are modulated around the nanotube. The entangled polarons break the azimuthal symmetry and their energy is lower than the energy of single‐band polarons. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Theoretical study of quadratic electro-optic effect in semiconducting zigzag carbon nanotubes

Using the perturbation treatment developed by Aspnes and Rowe [Phys. Rev. B 5, 4022 (1972)], an analytic expression for the third-order nonlinear optical susceptibility ${\ensuremath{\chi}}^{(3)}(\ensuremath{\omega};0,0,\ensuremath{\omega})$ is computed and analyzed for single walled zigzag carbon nanotubes. By improving their method, our calculations based on a tight-binding model take into account the transitions between all pairs of valence and conduction bands and thereby the contributions to the third-order susceptibility associated with different energy bands are investigated. With increasing radius of the nanotube, a nonmonotonous increase of the quadratic electro-optic effect has been demonstrated except for the fundamental peak. The nonuniformity is a result of the overlap between two energy bands as well as the reduced effective masses associated with each pair of conduction and valence bands. A nonperturbative numerical calculation is applied to obtain the high-field response as well as to assess the applicability of the low-field perturbation expression.

Ab initiostudy of TCNQ-doped carbon nanotubes

7,7,8,8-tetracyanoquinodimethane (TCNQ: ${\mathrm{C}}_{12}{\mathrm{H}}_{4}{\mathrm{N}}_{4}$), a well known electron acceptor organic molecule, has recently been quite considered for practical applications with carbon nanotubes, such as chiral selectivity and nanoelectronics. In this paper, we present the results of an extensive ab initio theoretical investigation of TCNQ-doped single-walled carbon nanotubes (SWCNTs). We have obtained the energetically most stable geometries (namely, ``bridge-I,'' for zigzag nanotubes, and ``bridge-II,'' for armchair ones), and the respective equilibrium distances (between 0.30 and $0.32\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) for external functionalization of SWCNTs by TCNQ, as well as the minimum diameter (around $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) a SWCNT should have so that it is able to exothermically encapsulate a TCNQ molecule. In addition, the electronic properties of TCNQ-doped small semiconducting zigzag carbon nanotubes were examined in details, clarifying the doping effects near the bandgap and revealing a sizable charge transfer from the nanotube toward the molecule.