FY2010 - Oak Ridge National Laboratory

Director’s R&D Fund—
Science for Extreme Environment: Advanced Materials and Interfacial Processes for Energy
Electrical measurements. Impedance spectroscopy was used to study the brush-based metal–insulator–
metal (MIM) structure: heavily doped Si – 2-nm-thick native silicon oxide – ~30-nm-thick PCHD or PPP
brushes. By using a Randles model, our equivalent circuit modeling with undoped PPP brushes suggested
that the contact resistance in the system was 50 times lower than the measured bulk resistance.
The MIM structure without PCHD and PPP brushes had a measured impedance of 1000 ohm and phase of
90 degree over the entire frequency range that we monitored (20–2 million Hz). For the data sets labeled
“air” (i.e., no connection), the low-frequency values are vibrating dramatically, because we are reaching
the higher measuring limit of impedance amplitude at this frequency. The MIM structure with polymer
brushes showed brush-type-dependent impedance behaviors. At 100 Hz, PCHD brushes had AC
impedances that were 20 times higher than those of PPP brushes, while doped PPP brushes showed
10 times lower impedance after 24 h doping in FeCl 3 solution. The doping process was expected to be
slower in brushes than in powder because the dopant could only diffuse from the brush top surface.
Distinctively different AC phase response was also observed upon the PCHD aromatization and PPP
brush formation. The PCHD brush has a phas angle close to 90 degrees, which suggests a nearly pure
capacitor behavior. The corresponding undoped PPP brush has two maxima in the phase-frequency plot:
one at 20 Hz and another at 100,000 Hz, indicating significant resistor-type contributions from interfacial
and orientational polarization mechanisms. The doped PPP brush has an even stronger resistive behavior,
corresponding to a significantly increased R2 and smaller C value in the Randles Circuit as compared to
PCHD and undoped PPP brushes. In addition, we can extrapolate the frequency-dependent impedance to
infinite low frequency in order to estimate DC resistance. The extracted DC conductivity of undoped PPP
brushes were in the range of 10 -10 to 10 -12 S/cm, which is slightly higher than the reported values for
undoped polyacetylene (10 -9 to 10 -10 S/cm).
A microscopic four-point probe (M4PP) was purchased and installed in our Hitachi SEM. It is used either
inside a scanning electron microscopy (SEM) or under an optical microscope to measure DC conductivity
of ultra-thin films. Because we are moving the M4PP into a newly acquired Zeiss SEM, currently we only
managed to use it to measure the conductivity of Nafion/Pt-C composites films on flexible substrates
(Kapton or Teflon). M4PP measurements on a hard substrate such as a silicon wafer or inside the new
Zeiss SEM are still under way.
Patterning. Micropatterning of PPP brushes was successfully achieved with nickel masking and
photolithography. The rough edge shown in the AFM cross-section scan was attributed to the thick nickel
pattern we used (100 nm), which should be easily optimized later on. Currently we are combining e-beam
lithography and nickel masking to make nanopatterns of PPP brushes. A photo-resist pattern generated by
e-beam lithography will be used to pattern PPP brushes.
05195
Controlled Hierarchical Self-Assembly of Robust Organic
Architectures
Benjamin P. Hay, Radu Custelcean, and Nathan C. Duncan
Project Description
The power to predict and control the assembly of matter at the molecular level is the key to unprecedented
control over many intriguing and useful material properties. Although design concepts for the structure-
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