Interlinking, Band Gap Engineering, Tunable Adsorption and Functionalization of Carbon
A First-Principles and Neutron Scattering Study
``First-principles simulation of nanotube-metal contact"
( Appl. Phys. Lett.
Vol. 83 , 3180 (2003).
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, Vs .
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, Vs, 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).
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.
can be found
``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).
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.
can be found
"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)
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.
can be found
`` 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:
Dr. Taner Yildirim (NIST)
Dr. Oguz Gulseren (University of Pennsylvania and NIST)
Prof. Salim Ciraci (UIC, Chicago and Bilkent University, Turkey)
Dr. C. M. Brown and Dr. D. A. Neumann (NCNR, NIST)