Zigzag Carbon Nanotubes Research Articles

Improving the ON/OFF Current Ratio and Ambipolarity of Doping-Less Tunneling Carbon-Nanotube FET Using Drain Engineering Technique

This paper presents a novel structure utilizing a doping-less (DL) tunneling carbon nanotube field-effect transistor (T-CNTFET) with dual drain work functionality aimed at enhancing the ON/OFF current ratio. The proposed design features a zigzag carbon nanotube (CNT) of type (13, 0) with a diameter of 1 nm. It employs HfO2 as the gate oxide, which is 2 nm thick and has a dielectric constant of 16. The CNT serves as an intrinsic semiconductor for the source and drain regions, which are composed of metals with appropriate work functions. By selecting appropriate metals for the source and drain regions, the necessity for doping as a fabrication step is avoided, simplifying construction and reducing costs for the nanoscale device. This design includes two drain electrodes, each measuring 15 nm in length and having different work functions (DWFs). The work function of the drain positioned closest to the channel (DWFAC) is set at 3.9 eV, which is 0.5 eV lower than that of the CNT, while the other drain section has a work function of 3.4 eV, 1 eV lower than the CNT. The electrical properties of the device were examined through quantum numerical simulations using the non-equilibrium Green's function (NEGF) method. The results indicate that the proposed structure significantly decreases the OFF-state current and improves the ON/OFF current ratio. Additionally, the leakage current is substantially lowered, resulting in favorable changes to the device's ambipolarity, along with a reduction in hot carrier effects and subthreshold swing (SS).

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.

A first-principles adsorption study of functionalized carbon, boron nitride, silicon carbon nanotubes with ifosfamide as vehicles for drug delivery: Thermal analysis

We investigated the adsorption of ifosfamide (IFS) on the outer surface of zigzag (10, 0) carbon nanotubes (CNT), boron nitride nanotubes (BNNT), and silicon carbon nanotubes (SiCNT), using density functional theory (DFT) calculations at the PBE-D3 level in a water solvent phase. Based on zero-point corrected binding energies (Ebin), IFS exhibits chemisorption through its O-head and Cl-head on CNT (−1.05 eV) compared to BNNT (−0.93 eV), characterized by covalent interaction. In contrast, IFS undergoes physisorption via its O-head on SiCNT with binding energy of −0.68 eV as the most stable model this interaction is driven by electrostatic forces. The formation of complexes between the drug and nanotubes is influenced by charge transfer dynamics. Our thermodynamic analysis demonstrates the Gibbs free energy (ΔG) and enthalpy energy (ΔH) for all models are exothermic and spontaneous. The observed decrease in binding energy for BNNT and CNT correlates with changes in their energy gap, dipole moment, and charge transfer upon IFS adsorption. Notably, SiCNT exhibits a different response with a significant energy gap change leading to an increase in dipole moment and charge transfer. These findings suggest that these nanotubes demonstrate promising sensitivity to the presence of IFS and could be explored as potential drug delivery systems for this drug.

Theoretical investigation of enhanced nonlinear optical properties of silicene and carbon nanotubes: Potential applications in infrared and ultraviolet optoelectronics

This theoretical study investigates the linear and nonlinear optical properties of zigzag carbon nanotubes (CNTs) and silicene nanotubes (SiNTs) with varying radii, focusing on their behavior in the infrared and ultraviolet energy ranges. In the infrared region, absorption spectra exhibit several peaks resulting from allowed transitions between valence and conduction bands. The number of absorption peaks increases with radius for both nanotube types, with SiNTs showing peaks at lower energy ranges and higher intensities compared to CNTs. Conversely, CNTs display markedly higher absorption intensities in the ultraviolet region. The quadratic electronic optic (DC Kerr) effect reveals sharp peaks near the band gap with multiple sign changes, attributed to allowed optical transitions at band edges. The third-order optical susceptibility for both CNT and SiNT structures show several peaks below the band gap energy due to multiphoton resonance absorption. In the infrared region, two highest and lowest subbands near to the Fermi level play a dominant role in the χTHG(3)(3ω) peaks formation. The position and intensity of χTHG(3)(3ω) peaks demonstrate a strong dependence on nanotube radius and type with higher intensity for the SiNTs. The tunable nature of the optical properties of CNTs and SiNTs by their radius and the enhanced nonlinear optical response of SiNTs, characterized by lower energy peaks and higher intensities, show their significant potential for advanced applications in nonlinear optics, optical detection, and high-energy optical systems.

Electronic properties of InSe/CNT heterojunctions with the modulation of electric field and vacancy defects

We investigate van der Waals (vdW) heterojunctions by combining InSe and zigzag carbon nanotubes (CNT(n,0)) by first-principle calculations. When n ranges from 5 to 7, The heterojunctions show n-type Schottky contact. However, for n of 8, 9, and 11, the heterojunctions still retain the characteristic of semiconductors with bandgaps. The metallized InSe/CNT(10,0) heterojunction has the most amount of charge transfer and the highest tunneling probability. Ohmic contact can be formed in InSe/CNT(n,0) (n = 5–7) heterojunctions under the external electric field. The charge transfer is enhanced and Schottky barrier heights are significantly reduced in heterojunctions with Se vacancy defect. In vacancy defect causes the disappearance of Schottky barrier because of metallization of InSe and more charge transfer than Se vacancy defect in InSe/CNT. Our findings provide a direction for the application of InSe/CNT in tunable nanoelectronic devices.

Impact of diameter and temperature on domain suppression by hot electrons in zigzag carbon nanotubes under constant electric field

The impact of the diameter of zigzag carbon nanotubes (CNTs) and temperature on domain suppression by injection of hot electrons at a strong rate along the axis of the tubes are investigated. The semiclassical Boltzmann kinetic equation was utilized in the theoretical work to derive the current density when hot electrons are available. First, the study was carried on by changing the value of the zigzag carbon nanotube's diameter at a fixed temperature. Second, we vary the temperature while keeping the diameter constant. For the semiconducting zigzag CNTs, increasing both the diameter and temperature enhances the suppression of the electric field domain required for a higher potential of terahertz radiation generation. However, in the metallic zigzag CNTs, we observed an enhancement of domain suppression when the diameter and temperature were decreased. These observations suggest that larger-diameter semiconducting zigzag CNTs at higher temperatures and smaller-diameter metallic zigzag CNTs at lower temperatures are better candidates for the generation of terahertz radiation.

Oxygen Electroreduction Reaction on Iron, Nitrogen-doped Nanocarbons: Structure – Reactivity Relationship

Theoretical calculations at the revPBE0-D3(PCM)/def2-TZVP level were performed to investigate the mechanism of the oxygen reduction reaction (ORR) on the FeN4-doped NCMs (graphene, single-walled (6,6)-armchair and (12,0)-zigzag carbon nanotubes) as catalysts. The effect of the support and relative orientation of FeN4 fragment in the catalyst structure on the ORR activity and stability of the differently adsorbed successive intermediates (O2*, HO2*, O*, HO*, H2O2*, H2O*) is discussed. Special attention is given to the possible poisoning effect of CO. The relative catalytic activity of the metal center and the adjacent C2 site of the carbon support are analyzed depending on the structure of the support. The metal catalytic center on the armchair nanotube and graphene supports is prone to deactivation by poisoning with CO, whereas in the zigzag nanotube supported catalyst the metal center is tolerant to CO. The four-electron mechanism of ORR is shown to be preferable over the two-electron mechanism in both acidic and alkaline media, both on the metal and C2 centers.

Performance analysis of carbon nanotube and graphene nanoribbon based biochemical sensors at atomic scale

Advancements in fabrication technologies have led to the possibility of synthesizing atomic-scale graphene nanoribbon (GNR) and carbon nanotube (CNT) based nanodevices. The purpose of this study was to model the electronic properties and electrical characteristics of these devices by atomistic modeling using density functional theory and the non-equilibrium Green’s function and compare the effects of molecular functionalization and sensing. The potential profile of the device was computed using the three-dimensional Poisson equation for smaller applied bias within one voltage range. Simulations showed a bandgap of 1 eV for armchair GNRs (AGNRs), which were insensitive to functionalized amine molecules, resulting in fewer alterations in the density of states (DoS), transmission spectra and the device current (ΔI). The bandgap further increased to 2 eV upon rolling the GNR into a armchair CNT (ACNT), which further decreased sensitivity. However, changing the configuration of the AGNR to a zigzag GNR (ZGNR) led to remarkable changes in the DoS and transmission spectra and a significant improvement in sensitivity. This improvement increased by 1.5–2 times upon rolling the ZGNR into a zigzag CNT (ZCNT). Thus, at lower dimensions in atomic scale, we found an alteration in device current of the carbon structures that was directly proportional to sensitivity in the following order: ΔI ACNT < ΔI AGNR < ΔI ZGNR < ΔI ZCNT. However, the same was found to fall for ZGNR and ZCNT with an increase in width to length (W/L) ratio. This highlights the importance of smaller atomic structures and this work provides a guideline for effective utilization of these structures for biochemical sensing.

Nonlinear dynamic behavior of carbon nanotubes incorporating size effects

This research endeavors to investigate the geometrically nonlinear dynamic characteristics of Carbon Nanotubes (CNTs) while incorporating size effects based on the Second Strain Gradient (SSG) elasticity theory. To this end, the nonlinear governing equations and its corresponding boundary conditions are deduced in alignment with the Green–Lagrange strain tensor and Hamilton’s principle. Concurrently, the weak form is expounded through the utilization of the C3 continuum Hermite interpolation functions, which ensure the continuity and smoothness of higher-order strain and displacement fields. Subsequently, a perturbation methodology is introduced, incorporating nonlinear phenomena into the context of linear wave propagation within the framework of periodic structures theory. The wave propagation characteristics manifest a pronounced disparity between the models based on linear SSG theory and those through nonlinear SSG theory with stiffness hardening phenomenon. In contrast to the zigzag CNTs, armchair CNTs evince an elevated aptitude for wave control. The application of nonlinear SSG theory in combination with the wave finite element method is significant for comprehending the wave propagation characteristics of complex CNTs.

Clar Covers and Zhang-Zhang Polynomials of Zigzag and Armchair Carbon Nanotubes

We derive here compact formulas for the Zhang-Zhang (ZZ) polynomials of two classes of finite open-ended carbon nanotubes: zigzag nanotubes (n, 0) of length d and armchair nanotubes (n, n) of length d. For zigzag nanotubes, the underlying Clar cover theory is trivial; in contrast, for armchair nanotubes, the Clar theory is complex and abundant in results. The ZZ polynomial formulas have been obtained using the interface theory of benzenoids and the transfer matrix methodology.

Modeling and Simulation of Current Voltage Characteristics for Cylindrical CNTFET Transistor

Abstract: In this study, we present a physics-based analytical model for the current-voltage characteristics of a near-ballistic CNTFET with a single-wall zig-zag carbon nanotube (16, 0). This model is based on the calculation of the potential and threshold voltage in the channel of a cylindrical gate CNTFET. Consequently, a new expression for the mobility law as a function of the CNT geometric parameters is proposed. Subsequently, a new model of I-V characteristics is presented, accounting for the impact of edge resistances. The paper also discusses the DIBL, subthreshold slope (SS), and the impact of some geometric and physical parameters on the drain current. A comparison of the I-V characteristics shows a good agreement between our model and the FETToy model. Keywords: Cylindrical gate CNTFET, I-V characteristics, Threshold voltage, Analytical model, Mobility law.

Band Gap Mechanism for Armchair Single Walled Carbon Nanotubes and Metal Semiconductor Transition for Symmetry Breaking

We have studied structures of single walled carbon nanotubes and metal semiconductor transition for the symmetric breaking. It was found that band gap existed for metallic armchair single walled carbon nanotubes. Calculations were made to analyse the meta-stability of the corrugated structures of armchair single walled carbon nanotubes. The corrugated single walled carbon nanotube structures are always lower in energy than the non corrugated nanotubes. The curvature effect was that the corrugated structure breaks the local symmetry between different carbon atoms. A true gap is created which does not vanish even when an external magnetic field is swept. The corrugation length and band gap gaps are decaying functions of the nanotubes radius and approached zero for carbon nanotube such as graphene. This was also true for zigzag and chiral single wall carbon nanotubes. The obtained results were found in good agreement with previously obtained results.

Molecular dynamics investigation for mechanical and failure behaviors of carbon nanotube-reinforced functionally graded aluminum–copper nanocomposites

In this study, molecular dynamics simulation is carried out to investigate the mechanical properties and failure behavior of a novel carbon nanotube-reinforced aluminum-copper alloy nanocomposite (Al–Cu/CNT). The study explores the influence of several key parameters, including the volume fraction of the carbon nanotubes (CNT), the diameter of CNT, the structure (zigzag, armchair, and chiral) of CNT, and the applied strain rate on the mechanical behavior of Al–Cu /CNT nanocomposite structure. The MD simulation results reveal that increasing the volume fraction of the CNT evidently increases the modulus of elasticity whereas it has no detrimental effect on the failure strain levels of the nanocomposites. The size of the CNT exhibits an inverse relationship with the elastic modulus. It is noted that increasing the size of the armchair CNTs results in lower elastic modulus levels and higher failure strain levels. The failure behavior of the Al–Cu /CNT nanocomposite is observed to vary according to the structure of the CNT. The nanostructure with zigzag CNTs experiences a gradual failure mode whilst armchair CNTs lead to a sudden failure in the nanostructures. The applied strain rate plays a minor role on the elastic modulus, but a slight increase in failure strain levels is observed at higher strain rates. The mechanical behavior of functionally graded Al–Cu alloy reinforced with CNT is also investigated (Al–Cu FGM/CNT). Minor changes are noticed in the elastic modulus and the failure strain levels of Al–Cu FGM/CNT with different material variations.

Pt-and Pd-doped carbon nanotubes with different diameters as CO sensors: A comparative DFT study

The density functional theory (DFT) is employed to assess the electronic characteristics and vibrational frequencies of CO adsorbed by several Zigzag carbon nanotubes with different diameters that are substitutionally doped using Pt and Pd. Computations were based on the Quantum Espresso (PBE) and Gaussian 16/6-31G(d,p) or LanL2DZ basis sets. The formation energy, adsorption energy, and interaction distances between CO and carbon nanotube as a function of the tube diameter were evaluated for Pt- and Pd- doped carbon nanotubes. Such doped nanotubes have relatively higher adsorption energy at smaller diameters, amounting to −2.784 eV and −2.635 eV for Pd-doped and Pt-doped carbon nanotubes, respectively. The adsorption energies of CO gas on Larger tubes Pt and Pd doped carbon nanotube are −2.207 eV, and −2.583 eV eV, respectively. Following CO adsorption, the smaller diameter Pt-doped carbon nanotube has a relatively greater change in electrical conductivity and chemical reactivity than other Pd- or Pt-doped nanotubes with relatively higher diameters. These changes are attributed to energy gaps concerning the alpha and beta HOMO and LUMO molecular orbitals. The computed Raman active mode frequencies extensively depend on diameter pre- and post-adsorption on the Pd- and Pt -nanotubes. This work offers several scientific suggestions concerning the use of Pt and Pd doped carbon nanotubes with different diameters for CO molecule sensor use cases. Such sensor materials having charge movement-based sensing might be employed as acceptors or donors.

High efficiency carbon nanotubes-based single-atom catalysts for nitrogen reduction

Carbon-based single-atom catalysts (SACs) for electrochemical nitrogen reduction reaction (NRR) have received increasing attention due to their sustainable, efficient, and green advantages. However, at present, the research on carbon nanotubes (CNTs)-based NRR catalysts is very limited. In this paper, using FeN3@(n, 0) CNTs (n = 3 ~ 10) as the representative catalysts, we demonstrate that the CNT curvatures will affect the spin polarization of the catalytic active centers, the activation of the adsorbed N2 molecules and the Gibbs free energy barriers for the formation of the critical intermediates in the NRR processes, thus changing the catalytic performance of CNT-based catalysts. Zigzag (8, 0) CNT was taken as the optimal substrate, and twenty transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Tc, Ru, Rh, Pd, W, Re, Ir, and Pt) were embedded into (8, 0) CNT via N3 group to construct the NRR catalysts. Their electrocatalytic performance for NRR were examined using DFT calculations, and TcN3@(8, 0) CNT was screened out as the best candidate with a low onset potential of − 0.53 V via the distal mechanism, which is superior to the molecules- or graphene-support Tc catalysts. Further electronic properties analysis shows that the high NRR performance of TcN3@(8, 0) CNT originates from the strong d-2π* interaction between the N2 molecule and Tc atom. TcN3@(8, 0) CNT also exhibits higher selectivity for NRR than the competing hydrogen evolution reaction (HER) process. The present work not only provides a promising catalyst for NRR, but also open up opportunities for further exploring of low-dimensional carbon-based high efficiency electrochemical NRR catalysts.

Metal Contact Induced Unconventional Field Effect in Metallic Carbon Nanotubes.

One-dimensional carbon nanotubes (CNTs) are promising for future nanoelectronics and optoelectronics, and an understanding of electrical contacts is essential for developing these technologies. Although significant efforts have been made in this direction, the quantitative behavior of electrical contacts remains poorly understood. Here, we investigate the effect of metal deformations on the gate voltage dependence of the conductance of metallic armchair and zigzag CNT field effect transistors (FETs). We employ density functional theory calculations of deformed CNTs under metal contacts to demonstrate that the current-voltage characteristics of the FET devices are qualitatively different from those expected for metallic CNT. We predict that, in the case of armchair CNT, the gate-voltage dependence of the conductance shows an ON/OFF ratio of about a factor of two, nearly independent of temperature. We attribute the simulated behavior to modification of the band structure under the metals caused by deformation. Our comprehensive model predicts a distinct feature of conductance modulation in armchair CNTFETs induced by the deformation of the CNT band structure. At the same time, the deformation in zigzag metallic CNTs leads to a band crossing but not to a bandgap opening.

Water Diffusion in Carbon Nanotubes for Rigid and Flexible Models

We compared the diffusion of water confined in armchair and zigzag carbon nanotubes for rigid and flexible water models. Using one rigid model, TIP4P/2005, and two flexible models, SPC/Fw and SPC/FH, we found that the number of the hydrogen bonds that water forms depends on the structure of the nanotube, directly affecting the diffusion of water. The simulation results reveal that, due to the hydrophobic nature of carbon nanotubes and the degrees of freedom imposed by the water force fields, water molecules tend to avoid the surface of the carbon nanotube. This junction of variables plays a central role in the diffusion of water, mainly in narrow and/or deformed nanotubes, governing the mobility of confined water in a nontrivial way, where the greater the degree of freedom of the water force field, the smaller mobility it will have in confinement as we limit the competition between area and volume and it no longer plays the unique role in changing water diffusivity.

The carrier transport properties of zigzag carbon nanotubes with intrinsic defects: An investigation from first principles

Through first-principles calculations, the effects of intrinsic defects on the carrier transport properties of zigzag single-walled carbon nanotubes are investigated. The calculation includes three intrinsic defects: Stone-Wales (SW) defects, single vacancy (SV) defects, and double vacancy (DV) defects. According to the results of the calculations, SW defects improve the carrier mobility of CNT(7, 0), whereas SV and DV defects hinder these properties of CNT(7, 0). The band structure and current-voltage characteristics provide additional support for this conclusion. In addition, the effect of size on the carrier transport properties of intrinsically defective CNT(n, 0) n = 7–15 is considered. When SW defects are introduced into pure CNT(n, 0), the obtained results indicate that the electron mobilities of 3N + 1 and 3N + 2 group structures will increase significantly, and the carrier transport properties will be enhanced. However, the presence of SV or DV defects will increase the effective carrier mass of pure CNT(n, 0). In these instances, the electron and hole mobility in the majority of structures will decrease.

Review on Molecular Dynamics Simulations of Effects of Carbon Nanotubes (CNTs) on Electrical and Thermal Conductivities of CNT-Modified Polymeric Composites

Due to the unique properties of carbon nanotubes (CNTs), the electrical and thermal conductivity of CNT-modified polymeric composites (CNTMPCs) can be manipulated and depend on several factors. There are many factors that affect the thermal and electrical conductivity of CNTs and CNTMPCs, such as chirality, length, type of CNTs, fabrication, surface treatment, matrix and interfacial interaction between the matrix and reinforcement (CNTs). This paper reviews the research on molecular dynamics (MD) simulations of the effects of some factors affecting the thermal and electrical conductivity of CNTs and CNTMPCs. First, the chirality dependence of the thermal and electrical conductivity of single-walled carbon nanotubes (SWNTs) was analyzed. The effect of chirality on the conductivity of short-length CNTs is greater than that of long-length CNTs, and the larger the chiral angle, the greater the conductivity of the CNTs. Furthermore, the thermal and electrical conductivity of the zigzag CNTs is smaller than that of the armchair one. Therefore, as the tube aspect ratio becomes longer and conductivity increases, while the effect of chirality on the conductivity decreases. In addition, hydrogen bonding affects the electrical and thermal conductivity of the CNTMPCs. The modeling of SWNTs shows that the thermal and electrical conductivity increases significantly with increasing overlap length. MD simulations can be effectively used to design highly conductive CNTMPCs with appropriated thermal and electrical properties. Since there are too many factors affecting the thermal and electrical conductivity of CNTMPCs, this paper only reviews the effects of limited factors on the thermal and electrical conductivity of CNTs and CNTMPCs based on MD simulations, and further detailed studies are required.

A density functional theory study of van der Waals interaction in carbon nanotubes

AbstractIn this article, we investigate the effect of van der Waals force in zigzag carbon nanotubes (CNTs) including single‐wall CNT (SWCNT) and double‐walled CNT (DWCNT) structures with several interaction configurations. The solid‐state density functional theory is employed to calculate the geometric optimization, normal mode frequencies, and IR and Raman spectra with the periodic boundary condition. For SWCNTs, we find that the Raman intensity is not affected by the tube diameter or the electronic structure. The IR absorption, however, increases with the tube diameter. We find that the close metallicity of the electronic structure has a significant impact on the IR simulations. When the van der Waals force is applied outside the CNTs at a distance longer than 3.0, the effect on Raman spectra is minimal but some effects can still be confirmed by IR absorption. When the van der Waals force acts inside the CNTs, the effect on the spectrum can be observed, especially at a distance of 2.8 Å, both IR and Raman can be significantly enhanced in many modes.