O(N) real-space method for ab initio quantum transport calculations: Application to carbon nanotube - metal contacts Page: 4
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BUONGIORNO NARDELLI, FATTEBERT, AND BERNHOLC
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E (eV)FIG. 2. The conductance spectrum of a periodic array of finite
metallic nanocontacts, as shown in the inset. The Fermi level is
taken as a reference.
the idealized side-contact geometry considered here is very
inefficient, which can explain the pathologically high contact
resistance observed in nanotube-metal contacts.13'15',16 In par-
ticular, this example clearly demonstrates that the weak
nanotube-metal coupling is mostly responsible for the weak
electron transport in the combined system, while wave vector
conservation is not a significant factor.17'35 The weak distrib-
uted coupling is also the reason for the measured contact
resistance being inversely proportional to contact
length.14,36'37 A conductance of one has also been observed in
an experiment that measured electron transfer between a liq-
uid metal and a multiwalled nanotube,36 but the conditions of
this experiment allow for several alternative explanations.38
The real-space, principal-layer formalism and the rapid
screening of charge disturbances allow us to carry out related
calculations with modest additional effort. Using the results
of the first calculation, two other contact geometries were
considered. The first is a periodic (infinite) array of narrow
metallic wires crossing an infinite nanotube, with both con-
tacts and tube bridges being 1.5 nm wide. This configuration
and the resulting conductance spectrum are shown in Fig. 2.
The main characteristic is an opening of a semiconducting
gap in the otherwise almost ideal nanotube spectrum. It is
induced by the breaking of the mirror symmetry of the nano-
tube wave functions induced by the localized perturbation of
the nanocontacts. The gap in the electronic band structure at
the Fermi energy is clearly reflected in the local density of
states computed from the Green's function Gc. A similar
result was previously obtained for a copper chain in contact
with a nanotube.39
Finally, we discuss the contact geometry shown in Fig. 3
(inset). It more closely resembles an experimental two-
terminal device, with two semi-infinite contacts connected
by a nanotube bridge, 1.5 nm long. In this geometry, the
system recovers the ideal conductance of an isolated tube
with two conductance channels at the Fermi energy, as
shown in Fig. 3. This behavior is induced by the alignment
of the valence band edge of the nanotube with the Fermi
energy of the metal contacts, triggered by the charge transfer
in the lead regions. In this particular geometry, these condi-
tions restore the two original eigenchannels of the nanotube
and thus conserve the number of conducting channels)
Ca8
6
4
2
-3 -2 -1 0 1 2 3
E (eV)FIG. 3. The conductance spectrum of an ideal two-terminal de-
vice shown in the inset. The Fermi level is taken as a reference.
throughout the system. It is important to note that the weak
nanotube-metal interaction, responsible for the pathologi-
cally high resistance of the nanotube-metal assembly, is not
strengthened. Both eigenchannels are highly localized on the
nanotube, with a negligible fraction on the metal contacts,
and closely resemble the channel40 shown in the right panel
of Fig. 1. Although the nanotube behaves as an ideal ballistic
conductor, the bonding characteristics of the nanotube-metal
system prevent an efficient electron transfer mechanism from
the nanotube to the Al contact. Indeed, inducing defects in
the contact region, e.g., by localized electron
bombardment,16 would drastically increase the bonding
strength of the nanotube-metal assembly and greatly improve
the performance of the device. Alternatively, we have found
that mechanically pushing the nanotube closer to the Al sur-
face by a small amount ( 1 A, with an energy expense of
10 meV/atom) more than doubles the transmission effi-
ciency between the metal and the nanotube. The mechanical
deformation induces a small inward relaxation of the Al sur-
face in the contact region, facilitating stronger hybridization
between the nanotube and the metal contact in the conduct-
ing channels and thus contributing to a higher electron trans-
mission rate between the two systems.
IV. SUMMARY
In summary, we have developed an efficient ab initio
method to compute quantum conductances in nanostructures.
As a first application, the transport properties of carbon
nanotube-metal contacts were investigated. The calculations
provide a clear interpretation of current experimental results
for a variety of contact geometries and suggest avenues for
improving the properties of nanotube-metal assemblies in
potential nanoscale electronic devices, such as rectifiers, ac-
tuators, and nanoswitches.
ACKNOWLEDGMENTS
It is a pleasure to thank Dr. Vincent Meunier for many
fruitful discussions. This work was supported by ONR,
DOE, and NASA. A portion of the work was performed un-
der the auspices of the U.S. Department of Energy by Uni-
versity of California Lawrence Livermore National Labora-
tory under Contract No. W-7405-Eng-48.245423-4
PHYSICAL REVIEW B 64 245423
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Buongiorno Nardelli, Marco; Fattebert, Jean-luc & Bernholc, Jerry. O(N) real-space method for ab initio quantum transport calculations: Application to carbon nanotube - metal contacts, article, December 10, 2001; [College Park, Maryland]. (https://digital.library.unt.edu/ark:/67531/metadc234920/m1/4/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.