refs

Text Box: Welcome to Keivan Esfarjani’s HomePage

The nanotube page

Since their discovery in 1991 by Iijima, carbon nanotubes have attracted a lot of attention. They have indeed very strong mechanical properties, and are very good electrical and thermal conductors. They have applications in many fields from biology to chemical-electrical-mechanical engineering, and materials science. Finding a cheap way to grow a lot of them would therefore be very beneficial.

Growth mechanism of carbon nanotubes by Molecular Dynamics:

We have used the molecular dynamics (MD) technique to investigate their growth on the surface of a catalyst particle such as Ni. First, we used ab-initio density functional calculations to construct an interaction potential between the nickel particle and carbon atoms, then used MD to understand how the tube standing on the metal particle grows after addition of carbon species to its end. We discovered that the growth mechanism takes place via dimer addition to the tube end. Although thermodynamic considerations (phase separation of C-Ni) are important for the formation of carbon layers on Ni, these considerations do not show how the tube can GROW and do not explain why the nanoparticle does not simply get covered with a carbon layer!

It was found that the dimer addition to the tube end closes a hexagon and thus pushes the tube away slightly from the surface due to weakening of existing C-Ni bonds, and the stress caused by hexagon closure.

To see the animation click on the picture below

Electronic structure and non-linear transport in n-p doped CNT junctions:

Carbon nanotubes can be metallic or semiconducting depending on the way the graphene sheet is folded to form them. Here I have plotted two examples: a (4,4) armchair tube which is metallic, and a (7,0) zigzag tube, which is semiconducting. The armchair and zigzag feature refers to the atomic structure of the edge of the tube. There is also a third kind which is neither armchair, nor zigzag. For an obvious reason, they are called chiral nanotubes. The band structure of a CNT can be deduced from that of a graphene sheet by using folding arguments. Neglecting the small change in the hybridizations due to the folding of the planar graphene sheet, one can obtain the band structure of a CNT using the periodic boundary condition on the wavefunction along the tube circumference. This implies that the transverse wavenumber k is quantized along this direction as: kR=integer. This selects only a few parallel lines in the first Brillouin zone (FBZ) of graphene. If these lines pass through one of the 6 corners of the FBZ where the conduction and valence bands meet, then the corresponding nanotube will be metallic; otherwise, it will be semiconducting. The interesting thing about metallic CNTs is that at the Fermi level, the dispersion E(q) is linear, and this would lead to strong interaction effects causing the electrons to form a so-called Luttinger liquid (LL). In LLs, correlations have algebraic decay, and therefore long-ranged.

We proposed a nano diode formed of a n-p doped semiconducting CNT junction. Donor dopants can be Cs, Rb, K or Na, and acceptors could be Br, I or Cl [2].

By calculating self-consistently charge transport in carbon nanotube junctions in the high-field limit, we have found that semiconducting CNTs behave as a diode and display rectifying properties, whereas metallic armchair tubes show negative differential resistance[3]. The latter is caused by a symmetry selection rule: current can be reduced if the symmetries of the incoming and outgoing electron wavefunctions are different. Furthermore, it was shown that transport times in CNT junctions can be very fast and switching speeds were estimated to be of the order of femtoseconds.[4]

Mechanical properties of CNTs with dislocations as defects:

Our total energy calculations of defected CNTS have shown that depending on the applied load, the interaction between the heptagon-pentagon (5-7) defects, which are dislocations, can be repulsive (for large load) or attractive (for small load). This plays an important role in their dynamics under stress.[5]

Dynamics of C60 insertion inside CNTs:

The animation on the top left of the page shows a molecular dynamics simulation of C60 insertion inside a CNT. There is approximately a 3 eV kinetic energy gain as the fullerene enters through the nanotube end. Further collisions with the walls cause friction and heat up the nanotube. [6]

Thermoelectric properties of a point contact made of capped CNTs:

The still figure on the top left of the page shows two capped nanotubes forming a nanocontact. We found that, due to quantum confinement and interference effects, such system can have a large thermopower of the order of 10 k_B/e. Furthermore the Wiedemann-Franz law is violated in such a system leading to potentially large values for the figure of merit. Due to the absence of contact, the thermal conductivity is also expected to be very small, although we have not calculated it yet. [7]

Electronic properties of CNTs doped by magnetic transition metal elements:

We considered doping by Fe, No and Ni atoms inside the tube. First-principles density functional calculations revealed that Ni does not magnetize the tube, whereas Co, and to a larger extent Fe do magnetize it if it is metallic. The exchange coupling between TM and CNT was found to be antiferromagnetic except for chromium. A large up to down ratio of density of states at the Fermi level was found for Fe doped CNTs. This has important consequences on the functionality of the device as a spin valve. The following figure shows the charge isosurfaces for Cr and Ni.[8]