|
|
Atomic Wires and Molecular Devices
|
The ultimate in miniaturization of computer circuitry would be to make
circuit elements out of small assemblages of atoms or molecules ("molecular
electronics"). A first step on this road is to study the properties of
atomic wires, short chains of atoms that conduct electricity between two
contacts. We have performed first-principles calculations
on both metallic and covalently bonded atomic wires connecting two
metal electrodes.(1,2) Figure 1 shows the conductance of a linear
carbon-atom chain (a "cumulene") versus number of atoms in the chain.
The conductance is seen to oscillate between about one and two quantum units
of conductance (one unit corresponds to a resistance of 12,900 ohms). The
low-bias conductance can be related to the density of states at the Fermi
level of the electrodes.
|
|
Figure 1
|
At zero bias, there is a large transfer of electronic charge from the
electrodes to the carbon wires, effectively providing doping without
introducing scattering centers. Figure 2 gives a plot of the electrostatic
potential (minus the superposition of the potentials of the free chain and
the pair of bare electrodes) of a metal--7-carbon-atom-chain--metal
system as a
function of position (in atomic units) along and transverse to the wire.
The two barriers at the wire ends reflect the fact that most of the charge
transferred to the wire accumulates at the ends.
Figure 3 shows the difference of electron densities
of the system consisting of two electrodes connected by a 7-carbon-atom
chain with an applied bias of 3 volts, and that of the same system with
no applied bias. The green color indicates an unchanged, red an increased,
and blue a decreased electron density. The primary involvement of the
wire's  states is clear. The figure shows complex changes in
the metal-to-wire charge transfer, polarization by the electric field, and
screening by the electrodes. The electrostatic potential associated with
this charge distribution shows the way in which the voltage drop between
the electrodes is distributed spatially along the wire. It is found in
particular that in the vicinity of the wire, the potential variation extends
deep into the electrodes, and that about half of the potential drop occurs
over the wire itself. (Some studies in the literature conclude that
the drop occurs only at the wire ends.)
|
|
Figure 2 Detail
Figure 3
|
|
In collaboration with scientists from Vanderbilt University, these same
calculational tools have been used to study the conductance of a simple
carbon-ring-based molecule, 1,4-Benzenedithiolate.(3)
A contour plot of
the electron density distribution for the molecule bonded to a pair of metal
electrodes is shown in Fig. 4. (The dots represent atom positions.)
The I-V characteristic for this system was calculated and its shape found
to be similar to that found experimentally; the absolute value of the
current is strongly dependent on the presence of single atoms at
the molecule-electrode interface.(4)
This calculation has also been done in the presence of a third capacitive
terminal (gate); it is found that an appropriate gate field can switch the
molecule into a much more conductive state.(5)
|
|
Figure 4 Detail
|
(1) N. D. Lang, Phys. Rev. B 52, 5335 (1995); Phys. Rev. Lett. 79, 1357 (1997).
(2) N. D. Lang and Ph. Avouris, Phys. Rev. Lett. 84, 358 (2000).
(3) M. Di Ventra, S. T. Pantelides, and N. D. Lang, Phys. Rev. Lett. 84, 979 (2000).
(4) M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, and J. M. Tour, Science 278, 252 (1997).
(5) M. Di Ventra, S. T. Pantelides, and N. D. Lang, Applied Phys. Lett. 76, 3448 (2000).
|
|
|
