Room Temperature Ballistic Conduction in Carbon Nanotubes

Creating covalent bonds between Cu and C at the interface of metal/open-ended carbon nanotubes

The unique electrical properties of carbon nanotubes (CNTs) are highly desired in many technological applications. Unfortunately, in practice, the electrical conductivity of most CNTs and their assemblies has fallen short of expectations. One reason for this poor performance is that electrical resistance develops at the interface between carbon nanomaterials and metal surfaces when traditional metal-metal type contacts are employed. Here, a method for overcoming this resistance using covalent bond formation between open-ended CNTs and Cu surfaces is investigated experimentally and supported by theoretical calculations. The open-ended CNTs are vertically oriented compared to the substrate and have carboxylic functional groups that react with aminophenyl groups (linkers) grafted on metal surfaces. The covalent bond formation, crosslinking carboxylic and amine, via amide bond formation occurs at 120 °C. The covalent bonding nature of the aminophenyl linker is demonstrated theoretically using (100), (110), and (111) Cu surfaces, and bridge-like bond formation between carbon and two adjacent Cu atoms is revealed. The electrical conductivity calculated for a single intramolecular-type junction supports covalent bond formation between Cu and CNTs. Experimentally, the robustness of the covalent bonding between vertically oriented CNTs is tested by exposing CNTs on Cu to sonication, which reveals that CNTs remain fixed to the Cu supports. Since bonding CNTs to metals was performed at low temperatures, the reported method of covalent bond formation is expected to facilitate the application of CNTs in multiple fields, including electronics.

Superfluidity Meets the Solid State: Frictionless Mass Transport through a (5,5) Carbon Nanotube

Superfluidity is a well-characterized quantum phenomenon which entails frictionless motion of mesoscopic particles through a superfluid, such as ^{4}He or dilute atomic gases at very low temperatures. As shown by Landau, the incompatibility between energy and momentum conservation, which ultimately stems from the spectrum of the elementary excitations of the superfluid, forbids quantum scattering between the superfluid and the moving mesoscopic particle, below a critical speed threshold. Here, we predict that frictionless motion can also occur in the absence of a standard superfluid, i.e., when a He atom travels through a narrow (5,5) carbon nanotube (CNT). Because of the quasilinear dispersion of the plasmon and phonon modes that could interact with He, the (5,5) CNT embodies a solid-state analog of the superfluid, thereby enabling straightforward transfer of Landau's criterion of superfluidity. As a result, Landau's equations acquire broader generality and may be applicable to other nanoscale friction phenomena, whose description has been so far purely classical.

The Effect of Copper-Graphene Composite Architecture on Thermal Transport Efficiency

This paper presents the results of molecular dynamic modeling, revealing that inserting confined graphene layers into copper crystal reduces the thermal conductivity of the whole composite, and the coefficient of thermal conductivity κ decreases upon an increase in the number of graphene layers. The injection of one, two, and three layers of 15 nm graphene leads to a change in the coefficient of thermal conductivity from 380 W/(m·K) down to 205.9, 179.1, and 163.6 W/(m·K), respectively. Decreasing the length of graphene layers leads to a decrease in the density of defects on which heat is dissipated. With one, two, and three layers of 8 nm graphene, the coefficient of thermal conductivity of the composite is equal to 272.6, 246.8, and 240.8 W/(m·K), appropriately. Meanwhile the introduction of an infinite graphene layer results in the growth of κ to 414.2-803.3 W/(m·K).

Dielectrophoretic alignment of carbon nanotubes: theory, applications, and future

Carbon nanotubes (CNTs) are nominated to be the successor of several semiconductors and metals due to their unique physical and chemical properties. It has been concerning that the anisotropic and low controllability of CNTs impedes their adoption in commercial applications. Dielectrophoresis (DEP) is known as the electrokinetics motion of polarizable nanoparticles under the influence of nonuniform electric fields. The uniqueness of this phenomenon allows DEP to be employed as a novel method to align, assemble, separate, and manipulate CNTs suspended in liquid mediums. This article begins with a brief overview of CNT structure and production, with the emphasize on their electrical properties and response to electric fields. The DEP phenomenon as a CNT alignment method is demonstrated and graphically discussed, along with its theory, procedure, and parameters. We also discussed the side forces that arise in DEP systems and how they negatively or positively affect the CNT alignment. The article concludes with a brief review of CNT-based devices fabricated using DEP, as well as the method's limitations and future prospects.

Uncertainties in Electric Circuit Analysis of Anisotropic Electrical Conductivity and Piezoresistivity of Carbon Nanotube Nanocomposites

Electrical conductivity and piezoresistivity of carbon nanotube (CNT) nanocomposites are analyzed by nodal analysis for aligned and random CNT networks dependent on the intrinsic CNT conductivity and tunneling barrier values. In the literature, these parameters are assigned with significant uncertainty; often, the intrinsic resistivity is neglected. We analyze the variability of homogenized conductivity, its sensitivity to deformation, and the validity of the assumption of zero intrinsic resistivity. A fast algorithm for simulation of a gauge factor is proposed. The modelling shows: (1) the uncertainty of homogenization caused by the uncertainty in CNT electrical properties is higher than the uncertainty, caused by the nanocomposite randomness; (2) for defect-prone nanotubes (intrinsic conductivity ~10⁴ S/m), the influence of tunneling barrier energy on both the homogenized conductivity and gauge factor is weak, but it becomes stronger for CNTs with higher intrinsic conductivity; (3) the assumption of infinite intrinsic conductivity (defect-free nanotubes) has strong influence on the homogenized conductivity.

A Meta-Analysis of Conductive and Strong Carbon Nanotube Materials

A study of 1304 data points collated over 266 papers statistically evaluates the relationships between carbon nanotube (CNT) material characteristics, including: electrical, mechanical, and thermal properties; ampacity; density; purity; microstructure alignment; molecular dimensions and graphitic perfection; and doping. Compared to conductive polymers and graphitic intercalation compounds, which have exceeded the electrical conductivity of copper, CNT materials are currently one-sixth of copper's conductivity, mechanically on-par with synthetic or carbon fibers, and exceed all the other materials in terms of a multifunctional metric. Doped, aligned few-wall CNTs (FWCNTs) are the most superior CNT category; from this, the acid-spun fiber subset are the most conductive, and the subset of fibers directly spun from floating catalyst chemical vapor deposition are strongest on a weight basis. The thermal conductivity of multiwall CNT material rivals that of FWCNT materials. Ampacity follows a diameter-dependent power-law from nanometer to millimeter scales. Undoped, aligned FWCNT material reaches the intrinsic conductivity of CNT bundles and single-crystal graphite, illustrating an intrinsic limit requiring doping for copper-level conductivities. Comparing an assembly of CNTs (forming mesoscopic bundles, then macroscopic material) to an assembly of graphene (forming single-crystal graphite crystallites, then carbon fiber), the ≈1 µm room-temperature, phonon-limited mean-free-path shared between graphene, metallic CNTs, and activated semiconducting CNTs is highlighted, deemphasizing all metallic helicities for CNT power transmission applications.

Morphology dependent electrical conduction and breakdown in single TiO2 nanotubes

Understanding the electrical conduction properties of a single nanostructure is essential for gaining insight into the fundamental charge transport through 1D materials and also for exploring the collective behavior of an array of such nanostructures. TiO₂ nanostructures, such as electrochemically grown nanotubes, have been widely studied in recent times for several applications. The electrolyte plays a vital role in deciding the morphology, which, in turn, governs the charge transport behavior. Here we present a comparative study of the charge transport through a single TiO₂ nanotube grown by electrochemical anodization using ethylene glycol and dimethyl sulphoxide electrolytes. The individual nanotubes are assembled into nanodevices using photolithography without relying on complex and sophisticated process like electron beam lithography or focused ion beam deposition. The electric field dependent charge transport properties show Schottky emission at a lower field regime and Poole-Frenkel emission in the higher region. The temperature-dependent electrical conduction (110 K-410 K) is mediated by two thermal activation processes, attributed to shallow impurities in the low-temperature range (T < 230 K) and to the donors at deep intermediate levels at higher temperatures (T > 230 K). The activation energies for EG based nanotubes are found to be higher than those for DMSO nanotubes owing to the double wall morphology of the formed tubes. Also, the study of the electrical breakdown phenomena of these nanotubes reveals three distinct categories of collapse. 'Model A' type breakdown is characterized by a stepwise rise of the current up to the breakdown point and a fall to zero following a non-uniform step by step decrease, which is driven by crack formation near the electrode interface and its propagation. 'Model B' shows a transient rise and fall in current, leading to breakdown due to electromigration, whereas 'Model C' type breakdown observed in a bundle of nanotubes shows a mixed trend of 'Model A' and 'Model B'. The data and analysis provide insight into the current limit through an individual nanotube or bundle of nanotubes and will be useful for designing prototype nanodevices from titania nanostructures.

Assessing Changes in Dielectric Properties Due to Nanomaterials Using a Two-Port Microwave System

Detecting changes in the dielectric properties of tissues at microwave frequencies can offer simple and cost effective tools for cancer detection. These changes can be enhanced by the use of nanoparticles (NPs) that are characterised by both increased tumour uptake and high dielectric constant. This paper presents a two-port experimental setup to assess the impact of contrast enhancement on microwave signals. The study focuses on carbon nanotubes, as they have been previously shown to induce high microwave dielectric contrast. We investigate multiwall carbon nanotubes (MWNT) and their -OH functionalised version (MWNT-OH) dispersed in tissue phantoms as contrast enhancing NPs, as well as salt (NaCl) solutions as reference mixtures which can be easily dissolved inside water mixtures and thus induce dielectric contrast changes reliably. MWNT and MWNT-OH are characterised by atomic force microscopy, and their dielectric properties are measured when dispersed in 60% glycerol–water mixtures. Salt concentrations between 10 and 50 mg/mL in 60% glycerol mixtures are also studied as homogeneous samples known to affect the dielectric constant. Contrast enhancement is then evaluated using a simplified two-port microwave system to identify the impact on microwave signals with respect to dielectric contrast. Numerical simulations are also conducted to compare results with the experimental findings. Our results suggest that this approach can be used as a reliable method to screen and assess contrast enhancing materials with regards to a microwave system’s ability to detect their impact on a target.

Spin current generation and control in carbon nanotubes by combining rotation and magnetic field

We study the quantum dynamics of ballistic electrons in rotating carbon nanotubes in the presence of a uniform magnetic field. When the field is parallel to the nanotube axis, the rotation-induced electric field brings about the spin-orbit interaction which, together with the kinetic, inertial, and Zeeman terms, compose the Schrödinger-Pauli Hamiltonian of the system. Full diagonalization of this Hamiltonian yields the eigenstates and eigenenergies leading to the calculation of the charge and spin currents. Our main result is the demonstration that, by suitably combining the applied magnetic field intensity and rotation speed, one can tune one of the currents to zero while keeping the other one finite, giving rise to a spin current generator.

High Ampacity Carbon Nanotube Materials

Constant evolution of technology is leading to the improvement of electronical devices. Smaller, lighter, faster, are but a few of the properties that have been constantly improved, but these developments come hand in hand with negative downsides. In the case of miniaturization, this shortcoming is found in the inherent property of conducting materials-the limit of current density they can withstand before failure. This property, known as ampacity, is close to reaching its limits at the current scales of use, and the performances of some conductors such as gold or copper suffer severely from it. The need to find alternative conductors with higher ampacity is, therefore, an urgent need, but at the same time, one which requires simultaneous search for decreased density if it is to succeed in an ever-growing electronical world. The uses of these carbon nanotube-based materials, from airplane lightning strike protection systems to the microchip industry, will be evaluated, failure mechanisms at maximum current densities explained, limitations and difficulties in ampacity measurements with different size ranges evaluated, and future lines of research suggested. This review will therefore provide an in-depth view of the rare properties that make carbon nanotubes and their hybrids unique.

Characterization of contact resistances in ceramic-coated vertically aligned carbon nanotube arrays

Despite the technological significance of carbon nanotube (CNT) arrays and metal-oxide coated CNTs for electronic and electrochemical devices such as supercapacitors, lithium-ion batteries, and solar-chemical cells, sub-optimal device performance often results due to large contact resistance between the CNTs and the metallic current collectors or between the CNTs and their ceramic coatings. While contact resistance measurements are regularly carried out on individually contacted CNTs, contact resistance measurements on vertically aligned (VA) CNT arrays are not routine. Here, we demonstrate that two-probe electrical current-voltage measurements and electrochemical impedance spectroscopy can be used to probe the end contact resistance and side contact resistances of coated and uncoated VACNT arrays in order to optimize material deposition and selection.

Quantum Point Contact Single-Nucleotide Conductance for DNA and RNA Sequence Identification

Several nanoscale electronic methods have been proposed for high-throughput single-molecule nucleic acid sequence identification. While many studies display a large ensemble of measurements as "electronic fingerprints" with some promise for distinguishing the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine, and uracil), important metrics such as accuracy and confidence of base calling fall well below the current genomic methods. Issues such as unreliable metal-molecule junction formation, variation of nucleotide conformations, insufficient differences between the molecular orbitals responsible for single-nucleotide conduction, and lack of rigorous base calling algorithms lead to overlapping nanoelectronic measurements and poor nucleotide discrimination, especially at low coverage on single molecules. Here, we demonstrate a technique for reproducible conductance measurements on conformation-constrained single nucleotides and an advanced algorithmic approach for distinguishing the nucleobases. Our quantum point contact single-nucleotide conductance sequencing (QPICS) method uses combed and electrostatically bound single DNA and RNA nucleotides on a self-assembled monolayer of cysteamine molecules. We demonstrate that by varying the applied bias and pH conditions, molecular conductance can be switched ON and OFF, leading to reversible nucleotide perturbation for electronic recognition (NPER). We utilize NPER as a method to achieve >99.7% accuracy for DNA and RNA base calling at low molecular coverage (∼12×) using unbiased single measurements on DNA/RNA nucleotides, which represents a significant advance compared to existing sequencing methods. These results demonstrate the potential for utilizing simple surface modifications and existing biochemical moieties in individual nucleobases for a reliable, direct, single-molecule, nanoelectronic DNA and RNA nucleotide identification method for sequencing.

Stress Writing Textured Graphite Conducting Wires/Patterns in Insulating Amorphous Carbon Matrix as Interconnects

This study reports a mechanical stress-based technique that involves scratching or imprinting to write textured graphite conducting wires/patterns in an insulating amorphous carbon matrix for potential use as interconnects in future carbonaceous circuits. With low-energy post-annealing below the temperature that is required for the thermal graphitization of amorphous carbon, the amorphous carbon phase only in the mechanically stressed regions transforms into a well aligned crystalline graphite structure with a low electrical resistivity of 420 μΩ-cm, while the surrounding amorphous carbon matrix remains insulating. Micro-Raman spectra with obvious graphitic peaks and high-resolution transmission electron microscopic observations of clear graphitic lattice verified the localized phase transformation of amorphous carbon into textured graphite exactly in the stressed regions. The stress-induced reconstruction of carbon bonds to generate oriented graphitic nuclei is believed to assist in the pseudo-self-formation of textured graphite during low-temperature post annealing.

Nanofluid Types, Their Synthesis, Properties and Incorporation in Direct Solar Thermal Collectors: A Review

The global demand for energy is increasing and the detrimental consequences of rising greenhouse gas emissions, global warming and environmental degradation present major challenges. Solar energy offers a clean and viable renewable energy source with the potential to alleviate the detrimental consequences normally associated with fossil fuel-based energy generation. However, there are two inherent problems associated with conventional solar thermal energy conversion systems. The first involves low thermal conductivity values of heat transfer fluids, and the second involves the poor optical properties of many absorbers and their coating. Hence, there is an imperative need to improve both thermal and optical properties of current solar conversion systems. Direct solar thermal absorption collectors incorporating a nanofluid offers the opportunity to achieve significant improvements in both optical and thermal performance. Since nanofluids offer much greater heat absorbing and heat transfer properties compared to traditional working fluids. The review summarizes current research in this innovative field. It discusses direct solar absorber collectors and methods for improving their performance. This is followed by a discussion of the various types of nanofluids available and the synthesis techniques used to manufacture them. In closing, a brief discussion of nanofluid property modelling is also presented.

Carbon Nanotube Interconnects Realized through Functionalization and Sintered Silver Attachment

Carbon nanotubes (CNTs) in the form of interconnects have many potential applications, and their ability to perform at high temperatures gives them a unique capability. We show the development of a novel transfer process using CNTs and sintered silver that offers a unique high-temperature, high-conductivity, and potentially flexible interconnect solution. Arrays of vertically aligned multiwalled carbon nanotubes of approximately 200 μm in length were grown on silicon substrates, using low-temperature photothermal chemical vapor deposition. Oxygen plasma treatment was used to introduce defects, in the form of hydroxyl, carbonyl, and carboxyl groups, on the walls of the carbon nanotubes so that they could bond to palladium (Pd). Nanoparticle silver was then used to bind the Pd-coated multiwalled CNTs to a copper substrate. The silver-CNT-silver interconnects were found to be ohmic conductors, with resistivity of 6.2 × 10(-4) Ωm; the interconnects were heated to temperatures exceeding 300 °C (where common solders fail) and were found to maintain their electrical performance.

Polymer Coating of Carbon Nanotube Fibers for Electric Microcables

Carbon nanotubes (CNTs) are considered the most promising candidates to replace Cu and Al in a large number of electrical, mechanical and thermal applications. Although most CNT industrial applications require macro and micro size CNT fiber assemblies, several techniques to make conducting CNT fibers, threads, yarns and ropes have been reported to this day, and improvement of their electrical and mechanical conductivity continues. Some electrical applications of these CNT conducting fibers require an insulating layer for electrical insulation and protection against mechanical tearing. Ideally, a flexible insulator such as hydrogenated nitrile butadiene rubber (HNBR) on the CNT fiber can allow fabrication of CNT coils that can be assembled into lightweight, corrosion resistant electrical motors and transformers. HNBR is a largely used commercial polymer that unlike other cable-coating polymers such as polyvinyl chloride (PVC), it provides unique continuous and uniform coating on the CNT fibers. The polymer coated/insulated CNT fibers have a 26.54 μm average diameter—which is approximately four times the diameter of a red blood cell—is produced by a simple dip-coating process. Our results confirm that HNBR in solution creates a few microns uniform insulation and mechanical protection over a CNT fiber that is used as the electrically conducting core.

Diagnosing nanoelectronic components using coherent electrons

We present the direct observation, using off-axis electron holography (EH), of the electric potential distribution in the vicinity of a single carbon nanotube electrically biased by two closely spaced contacts. When our results are combined with finite element modeling, we demonstrate the ability to separately observe the electrostatic potential drops across the metal contacts at the interface with the nanotube and along the length of the nanotube itself. We demonstrate that the uneven resistivity of different contacts can cause an asymmetric EH phase shift, which can readily be identified and quantified. EH thus offers a unique and precise approach for in-depth understanding and quick diagnosis of many similar nanoscale electronic devices.

'Inorganics-in-organics': recent developments and outlook for 4G polymer solar cells

Recent developments in solution processable single junction polymer solar cells have led to a significant improvement in power conversion efficiencies from ∼5% to beyond 9%. While much of the initial efficiency improvements were driven through judicious design of donor polymers, it is the engineering of device architectures through the incorporation of inorganic nanostructures and better processing that has continued the efficiency gains. Inorganic nano-components such as carbon nanotubes, graphene and its derivatives, metal nanoparticles and metal oxides have played a central role in improving device performance and longevity beyond those achieved by conventional 3G polymer solar cells. The present work aims to summarise the diverse roles played by the nanosystems and features in state of the art next generation (4G) polymer solar cells. The challenges associated with the engineering of such devices for future deployment are also discussed.

Electrical transport and electromigration studies on nickel encapsulated carbon nanotubes: possible future interconnects

We nominate the nickel filled multiwalled carbon nanotubes (MWNTs) as potential candidates to cope with challenges in persistent scaling for future interconnect technology. The insights into electrical transport through nickel filled carbon nanotubes provide an effective solution for major performance and reliability issues such as the increasing resistivity of metals at reduced scales, electromigration at high current densities and the problem of diffusion and corrosion faced by the existing copper interconnect technology. Furthermore, the nickel filled MWNTs outperform their hollow counterparts, the unfilled MWNTs, carrying at least one order higher current density, with increased time to failure. The results suggest that metal filled carbon nanotubes can provide a twofold benefit: (1) the metal filling provides an increased density of states for the system leading to a higher current density compared to hollow MWNTs, (2) metal out-diffusion and corrosion is prevented by the surrounding graphitic walls.

Conductive AFM for CNT characterization

We report on and emphasize the versatility of conductive atomic force microscopy in characterizing vertically aligned carbon nanotubes (CNTs) aimed to be used in via interconnect technology. The study is conducted on multi-walled CNT arrays vertically grown on a copper-based metal line. Voltage-dependent current mapping and current-voltage characteristics recorded down to single CNT allow for a comprehensive insight into the electric behaviour of the hybrid structure.

Carbon nanotube growth from Langmuir-Blodgett deposited Fe3O4 nanocrystals

We investigate colloidal Fe(3)O(4) nanocrystals as a catalyst system for carbon nanotube (CNT) growth that allows for decoupling the CNT growth step from the catalyst shaping and activation step. The system consists of 6.4 nm Fe(3)O(4) nanocrystals synthesized using a solution-based thermal decomposition reaction and, subsequently, transferred as hexagonally ordered Langmuir-Blodgett (LB) monolayers on TiN substrates. We demonstrate for the first time aligned CNT growth from LB deposited nanocrystals on a metallic underlayer. The hexagonally ordered monolayers of catalyst particles show promising stability up to the CNT growth temperature. In situ TEM heating experiments were performed to find this onset of particle deformation and showed stability of the nanoparticles up to 600 °C. The particle coalescence at high temperatures was also evidenced by the increasing CNT diameter, from 9.5 nm at 580 °C to 16 nm at 630 °C. By choosing to work at temperatures below the onset particle coalescence temperature, equivalent CNT diameters were obtained under different catalyst activation and growth conditions. The high stability of the catalyst on the metallic underlayer enables us to study CNT growth kinetics independently of the catalyst shaping step. This work opens a route towards combining growth studies with an electrical evaluation of the CNT growth as the TiN can be used as the bottom contact.

Re-ordering chaotic carbon: origins and application of textured carbon

Formation of nanocrystals with preferred orientation within the amorphous carbon matrix has attracted lots of theoretical and experimental attentions recently. Interesting properties of this films, easy fabrication methods and practical problems associated with the growth of other carbon nanomaterials such as carbon nanotubes (CNTs) and graphene gives this new class of carbon nanostructure a potential to be considered as a replacement for some applications such as thermal management at nanoscale and interconnects. In this short review paper, the fabrication techniques and associated formation mechanisms of these nanostructured films have been discussed. Besides, electrical and thermal properties of these nanostructured films have been compared with CNTs and graphene.

Electrical tomography using atomic force microscopy and its application towards carbon nanotube-based interconnects

The fabrication and integration of low-resistance carbon nanotubes (CNTs) for interconnects in future integrated circuits requires characterization techniques providing structural and electrical information at the nanometer scale. In this paper we present a slice-and-view approach based on electrical atomic force microscopy. Material removal achieved by successive scanning using doped ultra-sharp full-diamond probes, manufactured in-house, enables us to acquire two-dimensional (2D) resistance maps originating from different depths (equivalently different CNT lengths) on CNT-based interconnects. Stacking and interpolating these 2D resistance maps results in a three-dimensional (3D) representation (tomogram). This allows insight from a structural (e.g. size, density, distribution, straightness) and electrical point of view simultaneously. By extracting the resistance evolution over the length of an individual CNT we derive quantitative information about the resistivity and the contact resistance between the CNT and bottom electrode.

A study of Joule heating-induced breakdown of carbon nanotube interconnects

We investigate breakdown of carbon nanotube (CNT) interconnects induced by Joule heating in air and under high vacuum conditions (10(-5) mbar). A CNT with a diameter of 18 nm, which is grown by chemical vapor deposition to connect opposing titanium nitride (TiN) electrodes, is able to carry an electrical power up to 0.6 mW before breaking down under vacuum, with a corresponding maximum current density up to 8 × 10(7) A cm(-2) (compared to 0.16 mW and 2 × 10(7) A cm(-2) in air). Decoration with electrochemically deposited Ni particles allows protection of the CNT interconnect against oxidation and improvement of the heat release through the surrounding environment. A CNT decorated with Ni particles is able to carry an increased electrical power of about 1.5 mW before breaking down under vacuum, with a corresponding maximum current density as high as 1.2 × 10(8) A cm(-2). The Joule heating produced along the current carrying CNT interconnect is able to melt the Ni particles and promotes the formation of titanium carbon nitride which improves the electrical contact between the CNT and the TiN electrodes.

Nanocontact resistance and structural disorder induced resistivity variation in metallic metal-oxide nanowires

Several systems of metallic metal-oxide nanowires (NWs), including pure RuO2 and as-implanted and annealed Ru(0.98)Cu(0.02)O2 and Ru(0.93)Cu(0.07)O2 NWs, have been employed in two-probe electrical characterizations by using a transmission electron microscope-scanning tunneling microscope technique with a gold tip. Thermal, mechanical, and electron beam exposing treatments are consecutively applied to reduce the electrical contact resistance, generated from the interface between the NW and the gold tip, so as to evaluate the intrinsic NW resistance. It is found that the residual contact resistance cannot be entirely removed. For each system of metallic metal-oxide NWs, several tens of NWs are applied to electrical characterizations and the total resistances unveil a linear dependence on the ratio of the length to the area of the NWs. As a result, the average resistivity and the contact resistance of the metallic metal-oxide NWs could be evaluated at room temperatures. The average resistivities of pure RuO2 NWs agree well with the results obtained from standard two- and four-probe electrical-transport measurements. In addition, the as-implanted Cu-RuO2 NWs reveal disordered crystalline structures in high-resolution TEM images and give higher resistivities in comparison with that of pure RuO2 NWs. The residual contact resistances of all kinds of metallic metal-oxide NWs unveil, more surprisingly, an approximation value of several kilohms, even though the average resistivities of these NWs change by more than one order of magnitude. It is argued that the ductile gold tip makes one or more soft contacts on the stiff metal-oxide NWs with nanometer roughness and the nanocontacts on the NWs contribute to the electrical contact resistance.

On the importance of the electrostatic environment for the transport properties of freestanding multiwall carbon nanotubes

Electrical measurements of freestanding multiwall carbon nanotubes using high resistance tunnelling contacts reveal a power law behaviour, I alpha V alpha + 1, with alpha as high as 5.2, followed by a transition to an offset ohmic behaviour. The freestanding electrode geometry allows for a distinction between the predictions from Luttinger liquid and environmental quantum fluctuation (EQF) theories to be made. The high values of exponents found are explained within the EQF formulism, where reflections resulting from the impedance discontinuity caused by the freestanding geometry are included.

Aharonov-Bohm conductance modulation in ballistic carbon nanotubes

We report on magnetoconductance experiments in ballistic multiwalled carbon nanotubes threaded by magnetic fields as large as 55 T. In the high temperature regime (100 K), giant modulations of the conductance, mediated by the Fermi level location, are unveiled. The experimental data are consistently analyzed in terms of the field-dependent density of states of the external shell that modulates the injection properties at the electrode-nanotube interface, and the resulting linear conductance. This is the first unambiguous experimental evidence of Aharonov-Bohm effect in clean multiwalled carbon nanotubes.

Characterization of Pt Nanoparticles Deposited onto Carbon Nanotubes Grown on Carbon Paper and Evaluation of This Electrode for the Reduction of Oxygen

Multiwalled carbon nanotubes (MWCNTs) were grown on the fibers of a commercial porous carbon paper used as carbon-collecting electrodes in fuel cells. The tubes were then covered with Pt nanoparticles in order to test these gas diffusion electrodes (GDEs) for oxygen reduction in H2SO4 solution and in H2/O2 fuel cells. The Pt nanoparticles were characterized by cyclic voltammetry, transmission electron microscopy, and X-ray photoelectron spectroscopy. The majority of the Pt particles are 3 nm in size with a mean size of 4.1 nm. They have an electrochemically active surface area of 60 m2/g Pt for Pt loadings of 0.1-0.45 mg Pt/cm2. Although the electroactive Pt surface area is larger for commercial electrodes of similar loadings, Pt/MWCNT electrodes largely outperform the commercial electrode for the oxygen reduction reaction in GDE experiments using H2SO4 at pH 1. On the other hand, when the same electrodes are used as the cathode in a H2/O2 fuel cell, they perform only slightly better than the commercial electrodes in the potential range going from approximately 0.9 to approximately 0.7 V and have a lower performance at lower voltages.

Tomonaga-Luttinger liquid and Coulomb blockade in multiwall carbon nanotubes under pressure

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger liquids (LLs). Contrary to individual MWNTs, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, approximately 2.4, can be accounted for only by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.

Transmission electron microscopy study of individual carbon nanotube breakdown caused by Joule heating in air

We present repeated structural and electrical measurements on individual multiwalled carbon nanotubes, alternating between electrical measurements under ambient conditions and transmission electron microscopy (TEM). The multiwalled carbon nanotubes made by chemical vapor deposition were manipulated onto cantilever electrodes extending from a specially designed microfabricated chip. Repeated TEM investigations were then made of the progressive destruction of the nanotube structure induced by Joule heating in air. The electrical measurements indicate that the studied nanotubes behave as diffusive conductors with remarkably predictable electrical properties despite extensive structural damage.

Real-time observation of tubule formation from amorphous carbon nanowires under high-bias Joule heating

The tubule formation process from amorphous carbon nanowires under high-bias-caused Joule heating was observed in real time in a high-resolution transmission electron microscope. The crystallization of the amorphous carbon nanowires occurred in two distinct ways: the formation of tubular graphitic basal planes parallel to the nanowire axis on the surface and the formation of nano-onions in the interior of the nanowire. The tubule formation mechanism is a process of solid-state atom diffusion at high temperatures. Energetically, the tubule formation is caused by the exceptionally low surface energy of the (0002) plane of graphite. Higher input power to the amorphous nanowires generally leads to improved graphitization and, in turn, to increased conductance. The results suggest that nanotube formation in the arc-discharge growth process may involve the formation and crystallization of amorphous carbon.

Electronic Confinement and Coherence in Patterned Epitaxial Graphene

Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.

Establishing Ohmic contacts for in situ current-voltage characteristic measurements on a carbon nanotube inside the scanning electron microscope

Multi-walled carbon nanotubes (CNTs), either on an SiO(2) substrate or suspended above the substrate, were contacted to W, Au and Pt tips using a nanoprobe system, and current-voltage (I-V) characteristics were measured inside a scanning electron microscope. Linear I-V curves were obtained when Ohmic contacts were established to metallic CNTs. Methods for establishing Ohmic contacts on a CNT have been developed using the Joule heating effect when the tips are clean and e-beam exposing the contacting area of the tip when the tips are covered by a very thin contamination layer. When the contact is not good, non-linear I-V curves are obtained even though the CNTs that have been contacted are metallic. The resistance measured from the metal tip-CNT-metal tip system ranges from 14 to 200 k Ω. When the CNT was contacted via with Ohmic contacts the total resistance of the CNT was found to change roughly linearly with the length of the CNTs between the two tips. Field effect measurements were also carried out using a third probe as the gate, and field effects were found on certain CNTs with non-linear I-V characteristics.

Electrowetting in Carbon Nanotubes

We demonstrate reversible wetting and filling of open single-wall carbon nanotubes with mercury by means of electrocapillary pressure originating from the application of a potential across an individual nanotube in contact with a mercury drop. Wetting improves the conductance in both metallic and semiconducting nanotube probes by decreasing contact resistance and forming a mercury nanowire inside the nanotube. Molecular dynamics simulations corroborate the electrocapillarity-driven filling process and provide estimates for the imbibition speed and electrocapillary pressure.

Multichannel ballistic transport in multiwall carbon nanotubes

The electric transport properties of an individual vertical multiwall carbon nanotube (MWCNT) were studied in situ at room temperature in a scanning electron microscope chamber. It was found that the single MWCNT has a large current-carrying capacity, and the maximum current can reach 7.27 mA. At the same time, a very low resistance of about 34.4 ohms and a high conductance of about (460-490)G0 were obtained. The experimental observations imply a multichannel quasiballistic conducting behavior occurring in the MWCNTs with large diameter, which can be attributed to the participation of multiple walls in electrical transport and the large diameter of the MWCNTs.

Nanopipettes for metal transport

Here we demonstrate, for the first time experimentally, a nanopipette action for metals using multiwalled carbon nanotubes. The process relies on electromigration forces, created at high electron current densities, enabling the transport of material inside the hollow core of carbon nanotubes. In this way nanoparticles of iron were transported to and from electrically conducting substrates.