Interlinking, Band Gap Engineering, Tunable Adsorption and Functionalization of Carbon Nanotubes

A First-Principles and Neutron Scattering Study

``First-principles simulation of nanotube-metal contact"
( Appl. Phys. Lett. Vol. 83 , 3180 (2003). ) [Download PDF]

Our recent theoretical study published in Applied Physics Letters (Vol. 83, P. 3180, 2003) suggests that nanotube-metal interface with a controlled contact-resistance may be possible.

Carbon nanotubes are ideal candidates for building blocks of future electronic nanodevices. Although formidable obstacles remain, nanotube based field-effect transistors and other nano-electronic devices have already been demonstrated. However, the main technical problem in such nanotube-based devices is the high energy barrier (i.e. the Schottky barrier) at the metal-nanotube contact that hinders electrons entering the nanotube from a metal connecting wire. Therefore we'll overcome this key problem quicker if we have a good understanding of the nature of nanotube-metal contact and the origin of the Schottky barrier, V_s .
Our calculations indicate that the electronic structure and the potential of a semiconducting nanotube side-contacted on metal electrodes (see figure below) depend strongly on the type of metal, and exhibits marked differences from those of metal-Si heterostructures, which are known to be insensitive to the type of metal. The calculations are performed using first-principles pseudo-potential plane wave method within the density functional theory and the generalized gradient approximation. The metal-nanotube contact is simulated in a supercell approach, consisting of five layers of metals and the (8,0) carbon nanotube as shown in the figure. The atomic positions and the lattice parameters are relaxed during the self-consistent calculations. Mo and Au metals are studied as an example of two ends of the metal-electrode spectrum.

The calculations indicate that the nanotube-Au slab forms a very weakly bounded system with a sizable potential barrier between the tube and metal electrode, V ~3.9 eV, that is comparable with the calculated work function (V ~ 5 eV) of the Au slab. The effect of the nanotube-metal contact on the electronic properties and charge density are minimal as shown in the top inset to figure. These findings, explains why the devices made from Au electrodes have high contact resistance. Because of weak coupling and hence large V, the nanotube-Au contact is reminiscent of the metal-oxide-semiconductor junctions. We also studied the effect of the radial deformation of the nanotube and found that upon radial deformation, the nanotube-metal contact distance decreases and eventually the potential barrier, V_s, collapses.

For the case of nanotube side-bonded to the Mo (110) surface, the situation is totally different. For this case, the interaction is found very strong, as evident by the C-Mo hybridization shown in the charge contour plot (see figure). Moreover, a partial density of state analysis indicates C-Mo bond states near the Fermi level. This suggests that the site of the nanotube at the interface is conducting, while the opposite site farthest from the contact remains semiconducting. Finally the contact barrier is estimated to be around 0.4 eV, much smaller than that of Au-contact.

In conclusion, our work indicates that the type of metals used in an electrode is very important in determining the contact resistance. Other metals and alloys, and in particular, spin-polarized metal electrodes in connection with spintronics, are under current study. The theoretical studies such as our work will be important in pointing out the paths for other researchers to follow in experiments that pursue opportunities to make new nanodevices using carbon nanotubes as the building blocks.

The work was supported by grants from the National Science Foundation and the Scientific and Technical Research Council of Turkey.

``Effects of hydrogen adsorption on single-wall carbon nanotubes: Metallic hydrogen decoration"
( Phys. Rev. B (Rapid Com.) 66 , 121401 (2002). ) [Download PDF]

In this paper, we show that the electronic and atomic structure of carbon nanotubes undergo dramatic changes with hydrogen chemisorption from first principle calculations. Upon uniform exohydrogenation at half coverage, the cross sections of zigzag nanotubes become literally square or rectangular (see Figure below), and they are metallic with very high density of states at the Fermi level, while other isomers can be insulating. For both zigzag and armchair nanotubes, hydrogenation of each carbon atom from inside and outside alternatively yield the most stable isomer with a very weak curvature dependence and a large band gap. The details can be found HERE.

``TOP: A side and top view of a hydrogenated (8,0) SWNT at half coverage. The black and gray represent carbon and hydrogen atoms, respectively. Note that the fully optimized structure has a rectangular cross section. BOTTOM: The electronic density of states (DOS), indicating a very large number of states at the Fermi level. Hence, hydrogentation of an insulating (8,0) SWNT at half coverage induce metallization. The dotted line shows the contribution to the DOS from hydrogen atoms.''

``Matal nanoring and tube formation on carbon nanotubes"
(Phys. Rev. B 66 , 045409 (2002). ) [Download PDF]

In this paper, the structural and electronic properties of aluminum-covered single-wall carbon nanotubes (SWNT) are studied from first principles for a large number of coverages. Aluminum-aluminum interaction, that is stronger than aluminum-tube interaction, prevents uniform metal coverage, and hence gives rise to the clustering. However, a stable aluminum nanoring (see figure below) and aluminum nanotube with well defined patterns can also form around the semiconducting SWNT and lead to metallization. The persistent current in the Al nanoring is discussed to show that a high magnetic field can be induced at the center of SWNT. The details can be found HERE.

"A schematic view of the M₈-nanoring coated on a (8,0) nanotube (where M=metal such as Al, forming a magnetic tip due to a large persistent current in the nanoring''

``Formation of quantum structures on a single nanotube by modulating hydrogen adsorption"
( Applied Phys. Lett. (submitted 2002) ) [Download PDF]

In this work, using first-principles density functional calculations we showed that quantum structures can be generated on a single carbon nanotube by modulating adsorption of hydrogen atom (see figure below). While the hydrogen free part of the tube remains semiconducting or metallic, a wide band gap opens in the part covered by hydrogen. The type of the nanotube, the extent and sequence of hdyrogen-free and hydrogen-covered regions can provide several options to design a desired optoelectronic device. The details can be found HERE.

`` A quantum-well structure formed from a (8,0) nanotube by periodic hydrogenation. Controlling length and H-concentration, one can tune the parameters l₀ and V₀ which determine the properties of the nanodevice.

Nanotube Research Team: