High-field electrical transport in single-wall carbon nanotubes

Using low-resistance electrical contacts, we have measured the intrinsic high-field transport properties of metallic single-wall carbon nanotubes. Individual nanotubes appear to be able to carry currents with a density exceeding 10(9) A/cm(2). As the bias voltage is increased, the conductance drops dramatically due to scattering of electrons. We show that the current-voltage characteristics can be explained by considering optical or zone-boundary phonon emission as the dominant scattering mechanism at high field.     

Band structures of carbon nanotube with spin-orbit coupling interaction

We explore the band structures of single-walled carbon nanotubes (SWCNTs) with two types of spin-orbit couplings. The obtained results indicate that weak Rashba spin-orbit coupling interaction can lead to the breaking of four-fold degeneracy in all tubes even though without the intrinsic SO coupling. The asymmetric splitting between conduction bands and valence bands is caused by both SO couplings at the same time. When the ratio of Rashba spin-orbit coupling to the intrinsic spin-orbit coupling is larger than 3, metallic zigzag nanotube is always metallic conductor, on the contrary it becomes semiconducting properties. However, only when this ratio is equal to about 3 or the intrinsic spin-orbit coupling is much weak, the metallic armchair nanotube still holds the metallic behavior in transport.     

Electronics and Structural Properties of Single-Walled Carbon Nanotubes Interacting with a Glucose Molecule：ab initio Calculations

The adsorption of glucose molecule on single-walled carbon nanotubes (SWCNTs) is investigated by density functional theory calculations. Adsorption energies and equilibrium distances are evaluated, and glucose binding to the typical semiconducting and metallic nanotubes with various diameters and chirality are compared. We also investigated the role of the structural defects on the adsorption capability of the SWCNTs. We could observe larger adsorption energies for the larger diameters semiconducting CNTs, while the story is paradoxical for the metallic CNTs. The obtained results reveal that the adsorption energy is significantly higher for nanotubes with higher chiral angles. Finally, the adsorption energies are calculated for defected nanotubes for various configurations such as glucose molecule approaching to the pentagon, hexagon, and heptagon sites in the tube surface. We find that the respected defects have a minor contribution to the adsorption mechanism of the glucose on SWNTs. The calculation of electron transfers and the density of states supports that the electronic properties of SWCNTs do not change significantly after the gluycose molecular adsorption. Consequently, one can predict that presence of glucose would neither modify the electronic structure of the SWCNTs nor direct to a change in the conductivity of the intrinsic nanotubes.     

Electronics and Structural Properties of Single-Walled Carbon Nanotubes Interacting with a Glucose Molecule: ab initio Calculations

The adsorption of glucose molecule on single-walled carbon nanotubes(SWCNTs)is investigated by density functional theory calculations.Adsorption energies and equilibrium distances are evaluated,and glucose binding to the typical semiconducting and metallic nanotubes with various diameters and chirality are compared.We also investigated the role of the structural defects on the adsorption capability of the SWCNTs.We could observe larger adsorption energies for the larger diameters semiconducting CNTs,while the story is paradoxical for the metallic CNTs.The obtained results reveal that the adsorption energy is significantly higher for nanotubes with higher chiral angles.Finally,the adsorption energies are calculated for defected nanotubes for various configurations such as glucose molecule approaching to the pentagon,hexagon,and heptagon sites in the tube surface.We find that the respected defects have a minor contribution to the adsorption mechanism of the glucose on SWNTs.The calculation of electron transfers and the density of states supports that the electronic properties of SWCNTs do not change significantly after the gluycose molecular adsorption.Consequently,one can predict that presence of glucose would neither modify the electronic structure of the SWCNTs nor direct to a change in the conductivity of the intrinsic nanotubes.     

Ion irradiation effects on conduction in single-wall carbon nanotube networks

We have measured how irradiation by Ar⁺ and N⁺ ions modifies electronic conduction in single-wall carbon nanotube (SWNT) networks, finding dramatically different effects for different thicknesses. For very thin transparent networks, ion irradiation increases localization of charge carriers and reduces the variable-range hopping conductivity, especially at low temperatures. However, for thick networks (SWNT paper) showing metallic conductivity, we find a relatively sharp peak in conductivity as a function of irradiation dose. Our investigation of this peak reveals the important role of thermal annealing extending beyond the range of the irradiating ions, and shows the dependence on the morphology of the samples. We propose a simple model that accounts for the temperature-dependent conductivity. PACS 73.63.Fg; 61.80.-x     

Physics of transparent conductors

Transparent conductors (TCs) are materials, which are characterized by high transmission of light and simultaneously very high electrical DC conductivity. These materials play a crucial role, and made possible numerous applications in the fields of electro-optics, plasmonics, biosensing, medicine, and “green energy”. Modern applications, for example in the field of touchscreen and flexible displays, require that TCs are also mechanically strong and flexible. TC can be broadly classified into two categories: uniform and non-uniform TC. The uniform TC can be viewed as conventional metals (or electron plasmas) with plasma frequency located in the infrared frequency range (e.g. transparent conducting oxides), or ultra-thin metals with large plasma frequency (e.g. graphen). The physics of the nonuniform TC is much more complex, and could involve transmission enhancement due to refraction (including plasmonic), and exotic effects of electron transport, including percolation and fractal effects. This review ties the TC performance to the underlying physical phenomena. We begin with the theoretical basis for studying the various phenomena encountered in TC. Next, we consider the uniform TC, and discuss first the conventional conducting oxides (such as indium tin oxide), reviewing advantages and limitations of these classic uniform electron plasmas. Next, we discuss the potential of single- and multiple-layer graphene as uniform TC. In the part of the paper dealing with non-uniform metallic films, we begin with the review of random metallic networks. The transparency of these networks could be enhanced beyond the classical shading limit by the plasmonic refractive effects. The electrical conduction strongly depends on the network type, and we review first networks made of individual metallic nanowires, where conductivity depends on the inter-wire contact, and the percolation effects. Next, we review the uniform metallic film networks, which are free of the percolation effects and contact problems. In applications that require high-quality electric contact of a TC to an active substrate (such as LED or solar cells), the network performance can be optimized by employing a quasi-fractal structure of the network. We also consider the periodic metallic networks, where active plasmonic refraction leads to the phenomenon of the extraordinary optical transmission. We review the relevant literature on this topic, and demonstrate networks, which take advantage of this strategy (the bio-inspired leaf venation (LV) network, hybrid networks, etc.). Finally, we review “smart” TCs, with an added functionality, such as light interference, metamaterial effects, built-in semiconductors, and their junctions.     

Electronic structures and transport properties of sulfurized carbon nanotubes

The electronic and transport properties of side-walled sulfurized (8, 0) zigzag carbon nanotube were investigated by using density functional theory coupled with a non-equilibrium Green function approach. It is found that the adsorption of the sulfur chains largely reduces the bandgap of the semiconducting (8, 0) carbon nanotube, even changing it into a metallic one. More importantly, the transmission eigenstates around the Fermi level are contributed by not only the sulfur chains but also the complex system made of the sulfur chains and the single-walled carbon nanotube. Our results provide a method to improve the conductivity and utilization rate of the surface in the electrodes of supercapacitor which are made of the carbon nanotubes.     

Time-dependent wave-packet approach to the quantum transport of carbon nanotubes with vacancies

We investigate the vacancy effect on the electronic transport properties of the (5,5)-metallic and (5,0)-semiconducting carbon nanotubes using the time-dependent wave-packet approach based on the Kubo-Greenwood formula within the tight-binding approximation. We found that the metallic and semiconducting carbon nanotubes show different electronic transport properties for the states created by vacancies.     

Electron localization induced by grain boundary disorder in the normal state of high-Tc superconductors

We have studied the effects of superconducting grain boundary disorder on the normal state transport properties of cuprate films. Dip-coated granular YBa₂Cu₃O_7−y (YBaCuO) thick films on polycrystalline MgO substrates were synthesized and networked grains were systematically made less disordered in order to probe the crossover from strong to weak inter-grain disorder. Grain boundary passivation was achieved by metallic inclusions of different forms. We have shown that the normal state of samples exhibit a semiconducting behavior and changes to ‘metallic’ with sharper transitions to the superconducting state as we reduce grain-interfaces disorder, i.e. increase metallic inclusion content. On the basis of electron localization mechanisms, the normal state conductivity is thus shown to undergo a dimensional crossover from 3D to 2D in the frame of the variable-range hopping (VRH) regime. The transition threshold was found to depend on the form of metallic inclusions.     

Nanocomposite oxide thin films grown by pulsed energy beam deposition

Highly non-stoichiometric indium tin oxide (ITO) thin films were grown by pulsed energy beam deposition (pulsed laser deposition-PLD and pulsed electron beam deposition-PED) under low oxygen pressure. The analysis of the structure and electrical transport properties showed that ITO films with a large oxygen deficiency (more than 20%) are nanocomposite films with metallic (In, Sn) clusters embedded in a stoichiometric and crystalline oxide matrix. The presence of the metallic clusters induces specific transport properties, i.e. a metallic conductivity via percolation with a superconducting transition at low temperature (about 6 K) and the melting and freezing of the In-Sn clusters in the room temperature to 450 K range evidenced by large changes in resistivity and a hysteresis cycle. By controlling the oxygen deficiency and temperature during the growth, the transport and optical properties of the nanocomposite oxide films could be tuned from metallic-like to insulating and from transparent to absorbing films.     

Origins of coexistence of conductivity and transparency in SnO(2)

SnO2 is a prototype "transparent conductor," exhibiting the contradictory properties of high metallic conductivity due to massive structural nonstoichiometry with nearly complete, insulator-like transparency in the visible range. We found, via first-principles calculations, that the tin interstitial and oxygen vacancy have surprisingly low formation energies and strong mutual attraction, explaining the natural nonstoichiometry of this system. The stability of these intrinsic defects is traced back to the multivalence of tin. These defects donate electrons to the conduction band without increasing optical interband absorption, explaining coexistence of conductivity with transparency.     

Thermal transport enhancement of multi-walled carbon nanotubes/ high-density polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) enhanced high-density polyethylene (HDPE) composites were prepared and their thermophysical properties were measured. The thermal diffusivity of the composites increases with the increase in the amount of MWCNTs. A thermal diffusivity of more than three times that of pure HDPE was obtained for 38 vol. % MWCNTs/HDPE composites. An equation based on an effective medium approach model was used to discuss the thermal diffusivity enhancement of MWCNTs/HDPE composites as a function of the volume fraction of MWCNTs. The results from this analysis can be a predictive guideline for further improvements in the thermal transport properties of MWCNTs/HDPE composites. Moreover, the intrinsic longitudinal thermal conductivity k_z of an individual MWCNT was deduced from the measured results on the MWCNTs/HDPE composites. PACS 67.55.Hc; 61.46.Fg; 66.30.Xj     

Transparent carbon nanotube coatings

Thin networks of carbon nanotubes (CNTs) are sprayed onto glass or plastic substrates in order to obtain conductive transparent coatings. Transparency and conductivity at room temperature of different CNT material are evaluated. CNT coatings maintain their properties under mechanical stress, even after folding the substrate. At a transparency of 90% for visible light we observe a surface resistivity of 1 kΩ/sq which is already a promising value for various applications.     

The metal-insulator transition in ‘random’ Al−Ge films

Electronic transport properties have been measured on 3500 Å ‘random’ Al?Ge films. The room temperature resistivity exhibits a sharp discontinuous jump at the metal-insulator transition at the critical metallic fraction, φ_c=8.8 vol% Al. A new procedure is described for extracting values for the zero temperature conductivity σ(0) from the metallic low temperature conductivity data. When σ(0) is extrapolated to zero as a function of Al content, the value obtained for the critical aluminum fraction φ_c is in good agreement with the room temperature value.     

Optical properties of nonstoichiometric ZrO<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> according to spectroellipsometry data

Amorphous nonstoichiometric ZrO_x films of different composition have been synthesized by the method of ion-beam sputtering deposition of metallic zirconium in the presence of oxygen at different partial oxygen pressures in the growth zone, and their optical properties have been studied in the spectral range of 1.12–4.96 eV. It is found that light-absorbing films with metallic conductivity are formed at the partial oxygen pressure below 1.04 × 10^–3 Pa and transparent films with dielectric conductivity are formed at the pressure above 1.50 × 10^–3 Pa. It is shown that the spectral dependences of optical constants of ZrO_x films are described well by the corresponding dispersion models: the Cauchy polynomial model for films with dielectric conductivity and the Lorentz–Drude oscillator model for films with metallic conductivity.     

Effect of optical phonons scattering on electronic current in metallic carbon nanotubes

The effect of optical phonons scattering on electronic current has been studied in metallic carbon nanotubes. The current has been calculated self-consistently by total voltage equation and the heat transport equation. The total voltage equation consists of three terms, optical phonons collision term, acoustic phonon scattering term, and contact resistance one. Including LO, A₁, and E₁(2) phonons in collision term, we can reproduce the experimental I-V curves displaying negative differential conductance. Furthermore, one conclusion is made that the more optical phonons are scattered by electron, the lower current is in metallic carbon nanotubes. By comparing the current under different conditions, we can make another conclusion that there should be nonequilibrium optical phonons under high bias in spite of whether the metallic nanotube is suspended or not. This result agrees well with the others [M. Lazzeri, F. Mauri, Phys. Rev. B 73 (2006) 165419]. Based on these results, we do not only explain the experiment, but also propose to design a heat-controlling electronic transistor with metallic carbon nanotubes as its channel, in which the electronic current can be controlled by optical phonons.     

Electronic properties of multi-defected zigzag carbon nanotubes

Electronic properties of multi-defected zigzag single-walled carbon nanotubes are investigated by use of the tight-binding Green's function method. The Stone-Wales defects and the vacancies are considered. We find that the conductance sensitively depends on the realistic defect configurations for the metallic zigzag carbon nanotubes. Interestingly, the electronic transport properties of the nanotubes with three vacancies can be considered as the sum effect of two double-vacancies, while those with Stone-Wal...     

Unique structural and transport properties of molybdenum chalcohalide nanowires

We combine ab initio density functional and quantum transport calculations based on the nonequilibrium Green's function formalism to compare structural, electronic, and transport properties of Mo6S6-xIx nanowires with carbon nanotubes. We find systems with x=2 to be particularly stable and rigid, with their electronic structure and conductance close to that of metallic (13,13) single-wall carbon nanotubes. Mo6S6-xIx nanowires are conductive irrespective of their structure, more easily separable than carbon nanotubes, and capable of forming ideal contacts to Au leads through thio groups.     

Transport between multiple users in complex networks

We study the transport properties of model networks such as scale-free and Erd?s-Rényi networks as well as a real network. We consider few possibilities for the trnasport problem. We start by studying the conductance G between two arbitrarily chosen nodes where each link has the same unit resistance. Our theoretical analysis for scale-free networks predicts a broad range of values of G, with a power-law tail distribution $\Phi_{\rm SF}(G)\sim G^{-g_G}$ , where g_G=2λ-1, and λ is the decay exponent for the scale-free network degree distribution. The power-law tail in Φ_SF(G) leads to large values of G, thereby significantly improving the transport in scale-free networks, compared to Erd?s-Rényi networks where the tail of the conductivity distribution decays exponentially. We develop a simple physical picture of the transport to account for the results. The other model for transport is the max-flow model, where conductance is defined as the number of link-independent paths between the two nodes, and find that a similar picture holds. The effects of distance on the value of conductance are considered for both models, and some differences emerge. We then extend our study to the case of multiple sources ans sinks, where the transport is defined between two groups of nodes. We find a fundamental difference between the two forms of flow when considering the quality of the transport with respect to the number of sources, and find an optimal number of sources, or users, for the max-flow case. A qualitative (and partially quantitative) explanation is also given.