Two-Gap Superconductivity in Niobium Carbide-Coated Single-Walled Carbon Nanotubes: A First-Principles Study

Superconductivity in carbon nanotubes is attracting worldwide attention because of the reported high superconducting transition temperature in small-diameter single-walled carbon nanotubes (SWCNTs). However, it is well known that superconductivity in low-dimensional (quasi-1D) systems is not so common due to low density of states (DOS), strong quantum fluctuations and other phenomena in such systems. In this paper, we present theoretical investigations of the proximity effect of superconducting niobium carbide on single-walled carbon nanotube using density functional theory (DFT). The relaxed structure shows that Nb atoms are held around the SWCNT, forming a layer through weak van der Waals’ forces. The stability of the structure has been confirmed by Hirshfeld analysis and Mullikan population analysis as well. The study of the electronic band structure of the pristine and modified SWCNT shows a fascinating condensation of electronic states and a striking shift in the Fermi level. Further, two additional band gaps have appeared below the valence band suggesting some kind of pairing mechanism being operational. This indicates the possibility of superconducting behaviour in SWCNT in proximity of niobium carbide. The relaxed structure thus envisions the feasibility and stability of NbC-coated SWCNTs which will have superconducting properties as well as the remarkable mechanical and optical properties of SWCNTs. This prediction seeks interest of the researchers to try and develop such a novel nanomaterial which, if realized, will prove to be highly significant for many technological applications.     

Electronic structures and three-dimensional effects of boron-doped carbon nanotubes

Abstract

We study boron-doped carbon nanotubes by first-principles methods based on the density functional theory. To discuss the possibility of superconductivity, we calculate the electronic band structure and the density of states (DOS) of boron-doped (10,0) nanotubes by changing the boron density. It is found that the Fermi level density of states D(?_F) increases upon lowering the boron density. This can be understood in terms of the rigid band picture where the one-dimensional van Hove singularity lies at the edge of the valence band in the DOS of the pristine nanotube. The effect of three-dimensionality is also considered by performing the calculations for bundled (10,0) nanotubes and boron-doped double-walled carbon nanotubes (10,0)@(19,0). From the calculation of the bundled nanotubes, it is found that interwall dispersion is sufficiently large to broaden the peaks of the van Hove singularity in the DOS. Thus, to achieve the high D(?_F) using the bundle of nanotubes with single chirality, we should take into account the distance from each nanotube. In the case of double-walled carbon nanotubes, we find that the holes introduced to the inner tube by boron doping spread also on the outer tube, while the band structure of each tube remains almost unchanged.     

Labeling the defects of carbon nanotubes with thiol groups

Herein, we investigate the reactivity of perfect and defective single-wall carbon nanotubes (SWCNTs) with the SH group using first principle periodic calculations. The presence of Stone–Wales (SW) defect sites significantly increases the reactivity of SWCNTs against the thiol group. The most reactive site for the addition of the SH radical is the single vacancy defect; the sulfur atom reconstructs the SWCNT framework and the hydrogen atom becomes attached to a carbon atom. The cluster model calculations performed for perfect SWCNTs confirmed a very low reactivity with the thiol group, even for the small diameter and metallic SWCNTs. The reaction between the perfect SWCNT and SH results thermodynamically unfavorable. The different reactivities observed for perfect and defective SWCNTs suggest that the SH group can be employed to perform a chemical labeling of the defect sites present in carbon nanotubes. The SH radical group is quite unique because, even though it has an unpaired electron, it does not react with sp ² carbon frameworks, unless they have defects or curvature similar to C60. The results are discussed in terms of the recent experimental investigations about thiolated SWCNTs. We were able to explain the Transmission Electron Microscopy images of thiolated nanotubes and the lack of reactivity at the tips. Finally, we discuss a possible route to synthesize sulfur-doped SWCNTs using thiol groups and their electronic properties.     

Preparation of Ag–Fe-decorated single-walled carbon nanotubes by arc discharge and their antibacterial effect

A simple one-step approach for the preparation of Ag–Fe-decorated single-walled carbon nanotubes (Ag–Fe/SWCNTs) by DC hydrogen arc discharge is presented in this article. The growth of SWCNTs and the attachment of Ag and Fe nanoparticles to the SWCNTs occur simultaneously during the arc discharge evaporation process. It has been confirmed that the Ag and Fe nanoparticles in the diameter range of 1–10 nm are well dispersed and tightly attached to the outer surfaces of SWCNTs. The as-grown Ag–Fe/SWCNTs have been purified by high-temperature hydrogen treatment to remove amorphous carbon and carbon shells. Antibacterial tests show that the antibacterial activity of the purified Ag–Fe/SWCNT hybrid nanoparticles is excellent against Escherichia coli. The percentage of the E. coli killed by 100 μg/ml Ag–Fe/SWCNTs can reach up to 85.1 % at a short residence time of 2 h, suggesting that the purified Ag–Fe/SWCNTs may have potential antibacterial applications.     

Selective Growth of Metal‐Free Metallic and Semiconducting Single‐Wall Carbon Nanotubes

A major obstacle for the use of single‐wall carbon nanotubes (SWCNTs) in electronic devices is their mixture of different types of electrical conductivity that strongly depends on their helical structure. The existence of metal impurities as a residue of a metallic growth catalyst may also lower the performance of SWCNT‐based devices. Here, it is shown that by using silicon oxide (SiO_x) nanoparticles as a catalyst, metal‐free semiconducting and metallic SWCNTs can be selectively synthesized by the chemical vapor deposition of ethanol. It is found that control over the nanoparticle size and the content of oxygen in the SiO_x catalyst plays a key role in the selective growth of SWCNTs. Furthermore, by using the as‐grown semiconducting and metallic SWCNTs as the channel material and source/drain electrodes, respectively, all‐SWCNT thin‐film transistors are fabricated to demonstrate the remarkable potential of these SWCNTs for electronic devices.     

Soft X-ray Magnetic Circular Dichroism and Photoemission Studies of II–VI Diluted Ferromagnetic Semiconductor Zn1−x Cr x Te

The electronic structure of the Cr ions in Zn_1−x Cr _x Te (x=0.03 and 0.15) has been investigated using X-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy. Magnetic-field (H) and temperature (T) dependences of the Cr L _2,3 XMCD spectra well corresponded to the behavior of the magnetization measured by a SQUID magnetometer. The line shape of the Cr L _2,3 XMCD spectra was independent of H,T, and x, indicating that the ferromagnetism was originated from the same Cr electronic states independent of Cr concentration. The Cr 3d partial density of states (DOS) showed a peak at the top of the valence band but there was no DOS at the Fermi level, corresponding to their insulating behaviors.     

The nature of contact between Pd leads and semiconducting carbon nanotubes

The contact between semiconducting single-wall carbon nanotubes (SWCNTs) and metallic leads is of central importance to potential electronic device applications. We investigate the nature of the contact of SWCNTs with Pd leads in a fully covered geometry that closely resembles experimental setups. We employ first-principles calculations within density functional theory to obtain the equilibrium structure for representative semiconducting SWCNTs embedded in Pd and analyze their electronic structure features, charge-transfer effects, electrostatic potentials, and Fermi level alignment at the interfaces with the metal contact. We find that there is no electrostatic or Schottky-type barrier to electron transfer between the metal and the nanotube.     

First-principles study of carbon nanotubes with bamboo-shape and pentagon-pentagon fusion defects

A first-principles study of single-walled carbon nanotubes with bamboo-shape (BS) and pentagon-pentagon fusion defects was conducted. Sharp resonances occur on the BS-nanotubes as strong density of electronic states (DOS) localized at carbon atoms adjacent to the partitions, while at the partition the localized DOS was greatly depleted. A strong defect state at -0.1 eV below the Fermi level was generated and the band gap was narrowed for BS-(10, 0) nanotube. Sharp resonant states are observed in the valence and conduction bands of BS-(12, 0) nanotube. The resonant states are attributed to the pentagon defects as exemplified by the study of a (5, 5) nanotube with pentagon-pentagon fusion ring. The high chemical reactivity of the topological defects of the BS-nanotubes is correlated to the presence of localized resonant states.     

Proximity Effect of Magnesium Diboride on Single-Walled Carbon Nanotube: an Ab Initio Study

Magnesium diboride (MgB₂) superconductor with excellent physical properties continues to attract the attention of researchers since its discovery. It derives its versatility from the absence of weak links, large coherence length, and small anisotropy. On the other hand, reports of superconductivity in small-diameter single-walled carbon nanotubes (SWCNTs) suspended between superconducting contacts and proximity induced supercurrents in Ta/SWNTs/Au junctions have also aroused great interest in the scientific community. Proximity induced superconductivity in SWCNTs has opened up new frontiers of research which will lead to many novel discoveries. This paper reports ab initio investigations on the proximity effect of MgB₂ on the electronic structure of a SWCNT. Condensation of electronic states is observed in the electronic band structure of the pristine SWCNT when MgB₂ is held in proximity. An additional band gap is generated below the lowest energy state of the valence band of the pristine CNT which we suggest, is due to Cooper pair formation. This leads to the prediction that SWCNTs will show superconducting properties in proximity of MgB₂. We envision MgB₂-coated SWCNTs as a novel nanomaterial that has a combination of proximity induced superconductivity and inherently unique mechanical and optical properties of SWCNTs.     

Influence of substitutional doping on the electronic properties of carbon nanotubes with Stone Wales defects: density functional calculations

Abstract

In this work, we implemented density function theory to investigate the structural and the electronic properties of nitrogen doped single walled carbon nanotube under different orientations of Stone Wales defect. We have found that, the doped defected structures are more stable than the non-doped defected structures. Furthermore, doping defected carbon nanotubes with a nitrogen atom has significantly narrowed the band gap and slightly shifted the Fermi level toward the conduction band. Moreover, nitrogen substitution creates new band levels just above the Fermi level which exemplifies an n-type doping. However, the induced band gap is indirect band gap compared to direct band gap as in pristine carbon nanotubes. Furthermore, the electronic and structural properties of nitrogen doped carbon nanotube with Stone Wales defects is crucially affected by the dopant site as well as the orientations of Stone Wales defects.     

Electronic Property Modification of Single‐Walled Carbon Nanotubes by Encapsulation of Sulfur‐Terminated Graphene Nanoribbons

The use of carbon nanotubes (CNTs) as cylindrical reactor vessels has become a viable means for synthesizing graphene nanoribbons (GNRs). While previous studies demonstrated that the size and edge structure of the as‐produced GNRs are strongly dependent on the diameter of the tubes and the nature of the precursor, the atomic interactions between GNRs and surrounding CNTs and their effect on the electronic properties of the overall system are not well understood. Here, it is shown that the functional terminations of the GNR edges can have a strong influence on the electronic structure of the system. Analysis of SWCNTs before and after the insertion of sulfur‐terminated GNRs suggests a metallization of the majority of semiconducting SWCNTs. This is indicated by changes in the radial breathing modes and the D and G band Raman features, as well as UV–vis–NIR absorption spectra. The variation in resonance conditions of the nanotubes following GNR insertion make direct (n,m) assignment by Raman spectroscopy difficult. Thus, density functional theory calculations of representative GNR/SWCNT systems are performed. The results confirm significant changes in the band structure, including the development of a metallic state in the semiconducting SWCNTs due to sulfur/tube interactions. The GNR‐induced metallization of semiconducting SWCNTs may offer a means of controlling the electronic properties of bulk CNT samples and eliminate the need for a physical separation of semiconducting and metallic tubes.     

Infrared Organic Light‐Emitting Diodes with Carbon Nanotube Emitters

While organic light‐emitting diodes (OLEDs) covering all colors of the visible spectrum are widespread, suitable organic emitter materials in the near‐infrared (nIR) beyond 800 nm are still lacking. Here, the first OLED based on single‐walled carbon nanotubes (SWCNTs) as the emitter is demonstrated. By using a multilayer stacked architecture with matching charge blocking and charge‐transport layers, narrow‐band electroluminescence at wavelengths between 1000 and 1200 nm is achieved, with spectral features characteristic of excitonic and trionic emission of the employed (6,5) SWCNTs. Here, the OLED performance is investigated in detail and it is found that local conduction hot‐spots lead to pronounced trion emission. Analysis of the emissive dipole orientation shows a strong horizontal alignment of the SWCNTs with an average inclination angle of 12.9° with respect to the plane, leading to an exceptionally high outcoupling efficiency of 49%. The SWCNT‐based OLEDs represent a highly attractive platform for emission across the entire nIR.     

Electrical transport properties of amorphous Se78−x Te22Bi x films

D.C. Conductivity measurements on the thin films of a-Se_78–x Te₂₂Bi _x system (where x = 0, 0.5, 2 and 4) are reported in the temperature range 213–390 K and the density of states (DOS) near the Fermi level is calculated using dc conductivity data. It is found that the conduction in all the samples takes place in the tails of localized states. The conduction in the high temperature region 296–390 K is due to thermally assisted tunneling of electrons in the localized states at the conduction band edge. In the low temperature region 213–296 K conduction takes place through variable range hopping in the localized states near the Fermi level.     

Tailoring the diameter of single-walled carbon nanotubes for optical applications

Single-walled carbon nanotubes (SWCNTs) with specific diameters are required for various applications particularly in electronics and photonics, since the diameter is an essential characteristic determining their electronic and optical properties. In this work, the selective growth of SWCNTs with a certain mean diameter is achieved by the addition of appropriate amounts of CO₂ mixed with the carbon source (CO) into the aerosol (floating catalyst) chemical vapor deposition reactor. The noticeable shift of the peaks in the absorption spectra reveals that the mean diameters of the as-deposited SWCNTs are efficiently altered from 1.2 to 1.9 nm with increasing CO₂ concentration. It is believed that CO₂ acts as an etching agent and can selectively etch small diameter tubes due to their highly curved carbon surfaces. Polymer-free as-deposited SWCNT films with the desired diameters are used as saturable absorbers after stamping onto a highly reflecting Ag-mirror using a simple dry-transfer technique. Sub-picosecond mode-locked fiber laser operations at ∼1.56 μm and ∼2 μm are demonstrated, showing improvements in the performance after the optimization of the SWCNT properties.      

Study of Vacancies and Pd Atom Decoration on the Electronic Properties of Bilayer Graphene

We have investigated the scenario of graphene when irradiated with high energetic protons and subsequently decorated with Pd atoms on one of the layers. Theoretical analyses were performed on graphene 2L (two layers) with vacancies (carbon 3 and 13) (sample A), graphene 2L with vacancies and the two carbon atoms intercalated in between the two carbon layers (sample B), graphene 2L with the vacancies intercalated and subsequently with two Pd atoms on one of the layers, the top layer (called surface) (sample C), and, last but not least, graphene 2L with vacancies intercalated and decorated with six Pd atoms on the surface (sample D). For the four cases enunciated, energy bands were performed which provided information about the semi-metallic behavior, showing more semi-metallic character for the first case, while less metallic behavior occurs for the second and third one. Moreover, sample D showed a mini gap (between the conduction and valence bands) of the order of 0.02 eV and manifest semiconductor behavior. Total and projected density of states were performed in order to provide information about the contributions from each selected atom to the total DOS in the vicinity of the Fermi level in order to analyze the effect on the electronic behavior. Pd d orbitals contribute with ∼6% to the total DOS, while graphene (carbon atoms) p orbitals contribute with ∼5%. Furthermore, a strong hybridization is manifest between these two multiple degenerate orbitals.     

Photochemical modification of single-walled carbon nanotubes using HPHMP photoinitiator for enhanced organic solvent dispersion

Photochemical modification of single-walled carbon nanotubes (SWCNTs) was carried out by covalent attachment of 2-propanol-2-yl radicals on the surface of SWCNTs, which were engendered by the photolysis of 1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (HPHMP) under ultraviolet (UV) light. Pristine single-walled carbon nanotubes (p-SWCNTs) were dispersed in acetone along with HPHMP photoinitiator. After that, the mixture was irradiated by UV light to generate the free radicals which were introduced onto the surface of SWCNTs. The modification of SWCNTs was supported by UV/visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, thermal gravimetric analysis–mass spectrometry (TGA–MS), and transmission electron microscopy (TEM). UV/visible results revealed the loss of van Hove singularities due to covalent modification. The modification was further verified by FT-IR showing the signals at 3421 and 1100 cm⁻¹ due to stretching and bending of O–H group, respectively. Moreover, other peaks at 2927 and 2858 cm⁻¹ indicated the asymmetric and symmetric stretching modes of aliphatic C–H bond, respectively. Raman spectra illustrated that the intensity ratio of the tangential mode to the disorder mode (I _G/I _D), for modified SWCNTs (F-SWCNTs), decreased nearly four times than p-SWCNTs. TGA–MS also evidenced the signal corresponding to m/z 59 at 400 °C indicating the presence of 2-propanol-2-yl groups. TEM and dispersibility data demonstrated that the sidewall modification detached the bundled structure, enhanced the dispersion in common organic solvents and retained the original size of SWCNTs without hefty modification, which could cut or damage the nanotubes.     

Combined XPS and first principles study of double-perovskite Ca2GdTaO6

The electronic structure and the vibrational property of the double perovskite oxide, Ca₂GdTaO₆ (CGT), synthesized by solid-state reaction technique are investigated. Density functional theory calculations performed by the VASP show a direct band gap energy of 3.2 eV. The calculated density of states (DOS) has been compared to the valence band spectrum measured by X-ray photoelectron spectroscopy (XPS). The calculated electronic structures of CGT are qualitatively similar to those of the XPS spectra in terms of spectral features and relative intensities. The DOS of CGT shows that the Gd f, Ta d and O 2p states are hybridized. The band-structure calculation is used to obtain the optical dielectric constant of the sample. The inter-band contributions to the optical properties of CGT have been analysed. Raman spectrum of the sample taken at 488 nm excitation wavelength shows five primary strong peaks at 112, 215, 320, 458 and 767 cm^?1. Lorentzian lines with 17 bands have been used to fit the Raman spectrum. The eigen frequencies of different phonon modes have been theoretically calculated. The calculated results are compared with the experimental data.     

Electronic structural analysis of transparent In2O3-ZnO films by hard X-ray photoelectron spectroscopy

The electronic structural analysis of the conductive transparent films was carried out using bulk sensitive hard X-ray photoelectron spectroscopy (HAXPES). The In₂O₃-ZnO film has amorphous structure before and after annealed, and the conduction band spectrum around Fermi level showed the similar spectra with that of as-deposited amorphous In₂O₃ film. In these amorphous films, the conduction band minimum locates at the deeper level than the crystalline In₂O₃ film. The electronic state which comes from randomness of amorphous structure possibly exists around this level or below. These electrons are expected to act as scattering center. We concluded that the electron mobility depends on the density of this electronic state.