Fabrication of nanogapped singleelectron transistors for the study of individual singlemolecule magn

1

• Fabrication of nano-gapped
  single-electron transistors for the study of
  individual
  single-molecule magnets
• J.J. Henderson1, C.M. Ramsey1, E. del Barco1, A.
  Mishra2, and G. Christou2
• Department of Physics, University of Central
  Florida, Orlando, FL-USA
• Department of Chemistry, University of Florida,
  Gainesville, FL-USA

Preliminary Results
Design and Imaging
Motivation Electron transport properties of
individual molecules have received considerable
attention over the last several years due to the
introduction of single-electron transistor (SET)
devices, which allow the experimenter to probe
electronic, vibrational, or magnetic excitations
in an individual molecule. The properties are
difficult to measure and require correlation and
analysis across hundreds of data files in order
to draw conclusions about their source.
Using several layers of optical lithography (1 µm
resolution) and UHV metal deposition we build the
devices up on a Silicon substrate, then create
the nanowires using electron beam lithography
(spot size down to 20 nm).
The molecules for these measurements were
self-assembled on the surface of a chip for 90
minutes then cooled down to 4 Kelvin. The
results agree well with predictions and the
excitation energies are consistent with values
given in the literature (40meV).
Single Electron Transistors (SETs)
Figure 5. a) Optical micrograph of the completed
device, b) AFM image of the gold nanowire c) SEM
micrograph of wire.
In a three-terminal SET the molecule is situated
between the source and drain leads with an
insulated gate electrode underneath. Current can
flow between the source and drain via a
sequential tunneling process through the
molecular charge levels, which the gate electrode
is used to tune. In particular we have studied
the properties of a Mn12 with sulfurous ligands
tailored to stick to gold.
Figure 4. Autocad Rendering of the design.
Electro-migration to form the nano-gap
Figure 11. Differential conductance versus bias
for the same gates as figure 10.
Figure 10. I-V curves for gate voltages ranging
from -1.5 V to 0 volts that show steps and move
with the changing gate.
The nano gap between the source and drain
electrodes is formed by electro-migration induced
breaking of the nanowire at low temperature (lt 4
K). Vbreak and thus the gap size are tunable by
adding series resistors in the circuit of the
nanowire. The zero bias resistance (ZBR)
provides us with a direct measure of the gap size
without imaging.
?1-3 nm
Figure 2. Energy diagram of our system. Note
that the charge state can be tuned by biasing the
gate.
Figure 1. Schematic of the system of a molecule
coupled to the three terminals of the device.
a)
b)
d)
Figure 6. Breaking voltage as a function of
resistance in series with the nanowire.
Figure 7. Zero bias resistance after breaking as
a function of the Vbreak..
Figure 12. Contour plot of the differential
conductance as a function of Vbias versus Vgate,
for a SMM based SET with a single molecule of
Mn12.
c)
Summary
e)

• We have successfully measured SMM based SETs
  using a Mn12 derivative. See J. Appl. Phys. 101,
  09E102 (2007)
• Measurements at lower temperature (down to 15
  mK) and using fields up to 8 T are currently
  underway yielding huge numbers of data files
  which we will need I2Lab support to correlate and
  analyze.
• We would like to thank the I2Lab at UCF for
  supporting this research.

Figure 3. a) the charge state of the molecule is
not available to accept electrons (no current
flows), b) charge state is available (current
flows), c) I-V curves showing Coulomb Blockade
effect, d) plot of current as a function of Vbias
and Vgate, e) The Coulomb Diamond showing the
differential conductance peaks and the two
available charge states N and N1.
Figure 8. Nanowires broken at varying breaking
currents vary consistently with the dispersion
between the evaporation source and the target
substrate.
Figure 9. Typical tunneling curves after the
electro-migration. Steps indicate peaks in
differential conductance.