period oscillations in both devices, although
is approximately doubled in
the 250- versus 500-nm QD. Because
inversely proportional to the gate capacitance,
, and QD size, this comparison
shows that the true size of the confined QD
can be controlled in a predictable manner in
these modulation-doped nanowires (
The potential of our approach for encoding
coupled quantum structures has been explored
in modulation-doped silicon nanowires that have
structures of the form
are fixed-width tunnel bar-
riers that weakly couple the structure to source
and drain electrodes, and
is a variable-width
barrier that couples the two QDs (Fig. 4D, left
data recorded from rep-
resentative nanowire devices with three dif-
barrier widths coupling the QDs
(Fig. 4D, right panel) demonstrate several
key points. First, the device with the largest
barrier exhibits a single Coulomb oscillation
period that yields a capacitance consistent
with the size of each individual QD deter-
mined from SGM measurements. This result
shows qualitatively that the two QDs are
weakly coupled, and moreover, have sizes
that are similar. Second, the data from the
device with an intermediate-width
exhibits a splitting of each of the Coulomb
oscillation peaks into doublets, which is the
signature of enhanced tunneling conductance
between the QDs (
). This observation
agrees with previous studies (
coupled dots were defined by lithographically
patterned gate electrodes. Last, as the barrier
width is reduced further, a single Coulomb
oscillation period is again observed, although
the capacitance shows that the effective QD
size is twice that of the individual
that is, the structures are fully delocalized.
These studies demonstrate the ability to
synthesize coupled QDs within nanowires,
where the interaction between quantum struc-
tures is defined by synthesis not lithography.
More generally, this work demonstrates the
potential of encoding functional information
into nanostructures during synthesis, which
we believe will open up opportunities for con-
ventional and quantum electronic devices and
circuits in the future.
Reference and Notes
1. P. L. McEuen, M. S. Fuhrer, H. Park,
, 78 (2002).
2. H. Dai,
Acc. Chem. Res.
, 1035 (2002).
3. C. M. Lieber,
Mater. Res. Soc. Bull.
, 486 (2003).
4. L. Samuelson
, 313 (2004).
5. T. Mokari, E. Rothenberg, I. Popov, R. Costi, U. Banin,
, 1787 (2004).