Session X24: Nanotubes: Transport, Synthesis and Growth

Quantum electron transport in toroidal carbon nanotubes

Electron transport under bias is treated in tight-binding approximation using a non-equilibrium Green's function approach. Density-of-states D(E), transmissivity T(E), and current I$_{SD}$ are calculated through a (3,3) armchair nanotorus with laterally attached metallic leads and a magnetic field penetrating the toroidal plane. Plateaus in T(E) through the torus are observed as a function of both the relative angle between leads and magnetic flux. Initial computational studies performed with 1800 atoms and attached leads show substantial computational slowdown when increasing the system size by a factor of two. Results are generated by inverting the device Hamiltonian with a standard recursion method extended to account for unit cell toroidal closure. Significant computational speed-up is expected for a parallelized code on a multiprocessor computer cluster. The dependence of electronic features on torus size and torus curvature is tested for three tori with 900, 1800 and 3600 carbon atoms, respectively. References: 1. M. Jack and M. Encinosa, Quantum electron transport in toroidal carbon nanotubes with metallic leads. ArXiv: quant-ph/0709.0760. 2. M. Encinosa and M. Jack, Dipole and solenoidal magnetic moments of electronic surface currents on toroidal nanostructures. J. Comp.-Aided Mat. Design (Springer), 14 (1) (2007) 65 -- 71.   

\textit{Ab} initio study of solvent effects on electrical transport of molecular bridge between electrodes

The electrical conductance of benzene dithiolate (BDT) between gold electrodes has been actively investigated to realize single molecular devices. However, almost all of previous theoretical studies for the electrical conductance of BDT were done assuming 0K and vacuum in spite that many measurements have been performed at room temperature in solution [1,2]. In this study, we have investigated the electrical transport of BDT molecule between gold electrodes in water solution using \textit{ab initio} nonequilibrium Green's function method and Car-Parrinello molecular dynamics at room temperature. The calculated time-averaged conductance of the BDT in water solution, 0.190 G$_{0}$, is clearly different from the value calculated without water, 0.201 G$_{0}$. Detailed analysis shows that this difference can be attributed to the effect of dipole moments of water molecules on the potential profile of the BDT molecule. [1] X. Xiao \textit{et al}., Nano Lett. \underline {\textbf{4}}, 267 (2004).\textbf{ [}2] M. Kiguchi \textit{et al.}, Appl. Phys. Lett. \underline {\textbf{89}}, 213104 (2006).   

Low-temperature electronic transport in Pt-nanocluster decorated alumina template grown carbon nanotubes

Alumina template grown nanotubes are known to be highly disordered tube when compared to arc-discharge grown tubes. This is due to the particular type of growth process involved. Temperature dependence study reveals a slow power law dependence of the conductance as a function of the temperature. Large value of power law exponents found in pristine tubes, suggest that the transport mechanism takes place through tunneling between adjacent graphene flakes. When platinum-decorated, those devices show a L\"{u}ttinger liquid behavior in the high-T regime and a large suppression of the conductance at low-T due to the interplay of disorder and $e-e$ interactions. Transport properties are studied in light of a recently proposed model for disordered multi-channel quantum wires. Magneto-transport measurements ($\vert $B$\vert <$5T) show the presence of weak localization and a small but distinct Rashba spin-orbit scattering effect in the low-field regime ($\vert $B$\vert <$.5T), the latter attributed to the surface decoration. Coherent transport is found to be recovered with increasing applied electric field.   

Electronic properties of doped semiconductor nanowires

Semiconductor nanowires have shown very promising properties, which opens new opportunities for the design of nanoscale devices. The physics of these nanowires is not yet fully understood. In this context, theory and numerical simulation give valuable insights. We present self-consistent tight binding calculations of donor and acceptor impurities in semiconductor nanowires, either in a free standing configuration or surrounded by a metallic gate and an oxide. We show that the dielectric mismatch between the nanowires and their surroundings increases the binding energies of dopant impurities up to a few hundreds of meV [1]. This decreases the doping efficiency and affect the behavior of nanowire devices. The effect of the environment will be discussed. When the nanowires are surrounded by an oxide, polaronic effects largely contribute to the binding energy of the dopants [2]. \newline [1] M. Diarra, Y. M. Niquet, C. Delerue and G. Allan, Phys. Rev. B 75, 045301 (2007). \newline [2] M. Diarra, C. Delerue, Y.M. Niquet, and G. Allan, J. Appl. Phys., accepted.   

The effect of the sodium and iodine doping on the electronic band structure of the polyicosahedral Si nanowire: A first principles study.

In a previous molecular dynamics study, we predicted a polyicosahedral Si nanowire which has a Si20 fullerene cage per icosahedral Si100 nanodot [1]. The unique cage structure is distinct from the crystalline diamond Si nanowire. Encapsulating a guest atom into the Si20 cage allows us to tune the physical properties of the nanowire. Here, we report on a first-principles study of the effect of the sodium and iodine doping on the electronic band structure of the hydrogen-terminated polyicosahedral Si nanowire [2]. Our calculations reveal that the guest-free polyicosahedral Si nanowire is a semiconductor with a 1.20 eV band gap. We also find that the semiconducting nanowire becomes metallic by the sodium and iodine doping, suggesting that the electronic band structure of polyicosahedral Si nanowires can be tuned by doping appropriate guest atoms. [1] J. Chem. Phys. 125, 074712 (2006). [2] submitted to PRB   

Electronic structures and electron-phonon interactions of boron-doped carbon nanotube

We study the boron-doped single-walled carbon nanotubes using first-principles method based on the density functional theory. The total energy, band structure and density of states are calculated. From the formation energy of boron-doped nanotubes with different diameter, it is found that the narrower tube needs lower energy to substitute a carbon atom with a boron atom. Using the result of different doping rate in the (10,0) tube, we extrapolate the result to low boron density limit and find that the ionization energy of the acceptor impurity level is approximately 0.2 eV. Furthermore, we discuss the doping rate dependence of the density of states at the Fermi level and the electron-phonon interactions which are important for superconductivity.   

Edge states and magnetism in carbon nanotubes with line defects

Under certain conditions, magnetic ordering has been predicted to occur in carbon nanostructures even in the absence of transition metal impurities. These conditions involve situations in which electronic localization takes place, such as at zig-zag edges or defects in graphene sheets and ribbons or in topological defects in carbon nanostructures. In the present work, we apply first-principles calculations to investigate the interplay between electronic and magnetic properties of carbon nanotubes with line defects. We consider three types of defects: lines of C-O-C epoxy groups, and defects resulting from the substitution of the oxygen atoms by CH$_2$ or C$_2$H$_4$ divalent radicals. We find that the line defects behave as pairs of coupled graphene edge states, and a variety of electronic and magnetic ground states is predicted as a function of defect type, nanotube diameter, and a possibly applied transverse electric field.   

Effects of intrinsic spin-orbit coupling in carbon nanotubes

The electronic structure and transport properties of carbon nanotubes (NTs) are the subject of intense theoretical and experimental studies. Tight-binding model predict these quasi-1D systems to be metallic or insulating depending on their chiral wrapping. Furthermore, external magnetic fields applied along the NT's axis, are expected to change these behaviorsby opening (metallic) or closing (insulating) gaps. \\ In this work we present preliminary numerical results obtained solving a tight-binding Hamiltonian [1] for band structures of armchair and zigzag NTs in the presence of the intrinsic spin-orbit (I-SO) interaction and magnetic fields. The I-SO interaction has dramatic effects, opening gaps for metallic NTs that are proportional the I-SO coupling constant. When considering an external field, the I-SO generated gaps show a quadratic dependence on the field strength in contrast with the linear dependence predicted in the absence of this interaction. Insulating tubes show increased gaps that do not close even in the presence of external fields. \\ \noindent [1] F.D. Haldane, Phys. Rev Lett. {\bf 61}, 2015 (1988).   

Transport properties of single vacancies in nanotubes

We present transport, density of states and electronic transport calculations of single vacancies in carbon nanotubes. We confirm that the defect reconstructs into a pentagon and a nonagon following the removal of a single carbon atom. This leads to the formation of a dangling bond. Finally we demonstrate that care must be taken when calculating the density of states of impurities in one dimensional systems in general. We show that obtaining information about the transport properties of such systems with defects solely from the density of states of a periodic DFT calculation can be misleading. The appearance of mini-gaps and oscillations, even in the limit of large unit cells, can be erroneously associated with changes induced by the defect itself instead of a figment of the procedure. In fact, we demonstrate that those mini-gaps vanish if the appropriate approach is taken, namely a Green's function method where the effects of semi-infinite electrodes are considered and a true open system is calculated.   

The structural and electronic properties of vacancy clusters in carbon nanotubes

Carbon vacancies, which can be generated by electron or ion irradiation, significantly modify the structural and electronic properties of carbon nanotubes. Recent experiments showed that vacancy-type defects induce structural changes such as junction, shrinkage, and bending. In this work we study the atomic and electronic structure of vacancy clusters up to six missing atoms in carbon nanotubes through both first-principles and tight-binding calculations. We find that vacancy defects are generally stable when they are aligned along the tube axis, forming a vacancy-chain. Due to the curvature effect, this feature is different from that found for graphene, where vacancies tend to aggregate into a lump. For the even-numbered vacancies in the (5,5) and (9,0) nanotubes, we find that clustering of vacancies leads to the local shrinkage, with a smaller diameter tube sandwiched between two semi-infinite tubes. In this case, the defect levels near the Fermi level are mostly associated with 7- or 8-membered rings, whereas those for odd-numbered vacancies result from the remnant dangling bonds.   

Synthesis of Triangle Tungsten Oxide Nanosheets with Potassium Doped

We report that large quantity of tungsten oxide nanosheets are synthesized by oxidizing tungsten plate directly. Using potassium hydrate as the catalyst and tungsten plate as the substrate, those triangle tungsten oxide nanosheets with thickness of 50-300 nm and wide up to tens of micrometers are obtained on a large scale. The angles of the triangle nanosheets are around 35$^{\circ}$ and 50$^{\circ}$ statistically. We discussed the potential reason of this peculiar phenomenon, besides the several characteriztions results of the grown nanosheets. The probable growth mechanism is also investigated.   

CVD synthesis of graphene

We will report on the use of Chemical Vapor Deposition (CVD) to grow sheets of graphene on substrates suitable for the semiconductor industry. Growth starts with the deposition of seed crystals of graphene on the substrate. CVD growth is found to result in growth at the edges of the seeds, rather than on the surfaces. The result is increases in the size of the seed crystal without additional layers of graphene forming on top of the crystal. This technique holds promise for forming large areas of high quality single layer graphene on inexpensive substrates.   

New opportunities from controlled growth of carbon nanotubes.

Aligned growth of carbon nanotubes (CNT) has been an important issue to researchers involved in CNT, and recent work has proved the possibility of alignment by using special substrates (i.e. quartz, sapphire, etc.) where CNT can grow along crystallographic axes. We present here two applications of aligned tubes produced from quartz substrates. The first is the realization of an integrated platform for using CNTs as electrodes for single molecules toward sensing applications; numerous sensing units can be generated by photolithography without long, tedious e-beam writing steps due to the precise control of location and direction of CNTs over a large area ($\sim $1X1 cm). The second is measurement of the high frequency properties of CNTs, which is difficult due to the high impedance of single-tube devices. By a slight modification of growth parameters, we have achieved growth of 'serpentine' CNTs on on quartz substrates that permit the fabrication of low-impedance devices using multiple identical CNT sections.   

Diffusion-limited and pressure-driven electrodeposition in a microfluidic channel

Self-terminating electrochemical fabrication has previously been devised to create quantum point contacts with single-atom contacts, but the structure of the growth has been poorly controlled. We have introduced a microfluidic channel with which to apply pressure-driven flow during the formation of the junction between two Au electrodes. Without applied flow, dendritic growth and dense branching morphologies were typically observed at the cathode. The addition of applied pressure-driven flow resulted in a densely packed gold structure that filled the channel. Our computer simulation yielded insight into the regimes where the diffusion, flow and electric field between the electrodes individually dominated growth. Proposals for further sophistication in both experiment and simulation will also be presented.   

Continuous and Scalable Fabrication of Transparent Conducting Thin Films of Single Walled Carbon Nanotubes

We report the fabrication of optically transparent and electrically conducting thin films of single-walled carbon nanotubes (SWNT) using the industrially scalable, fast and simple process of Rod-Coating. Rheology was used to study four different surfactants, their capacity to disperse SWNT in water and the viscoelastic properties of the resulting dispersion. A combination of two different surfactants was found ideal to make a uniform dispersion with high concentration of SWNT and the specific viscoelastic properties desired for coating. Rod coating with this coating fluid produced highly uniform, transparent and conducting SWNT thin films. The films were also treated with various strong acids which lead to further significant improvement in properties. Our results show that the choice of surfactant for making the coating dispersion has a strong effect on the electro-optical properties of the SWNT films. Films with sheet resistance of 100 Ohm/sq and 300 Ohm/sq for respective transparency of 70{\%} and 90{\%}, in the visible region, were obtained with this process. The development of this continuous and scalable fabrication process will thus bring the SWNT films closer to commercial application.