Download Photo-induced Conductance Modulation of

Photo-induced Conductance Modulation of Nafion-gated
Single-Walled Carbon Nanotubes
H. Munedzimwe3 , S. V. Rotkin1,2 , L.G. Rotkina1
1Center for Advanced Materials and Nanotechnology. 2Physics Department. 3Integrated Business and Engineering Department.
Lehigh University, Bethlehem, PA.
The Problem
• Ionomers (polyelectrolytes) such as Nafion provide
efficient gating of SWCNT Field Effect Transistors
(FETs) by establishing a polarizing charge layer under
bias at much closer proximity to the nanotube channel
than with traditional gate materials e.g. SiO2.
• The greater efficiency stems from the mobility of ions
in the ionomeric material (even in “solid” state).
• Thus, even low gate voltages in such conditions can
provide comparatively large shifts in the Fermi level.
• However, ionomer substrates typically have many
trap sites which randomize IV-curves.
• Predictability and consistency are crucial for
practical device applications but the phenomena
resulting in the randomization are largely unknown.
Special Apparatus
• Keithley Semiconductor
Characterization System
for I-V sourcing and
measurement.
• Spectra-Physics 800 nm
semiconductor Laser.
• Custom-made screening
box for photo-electric studies
Device Topology-SWCNT FET
Cross-section
Plan/ Top view
Color Key
Source
Gold Contact(70 µm)
SWCNTs
Channel
Nafion®
Gold layer
SWCNT layer
Back Gate
Drain
Nafion substrate
Objective
•To study electronic hysteresis in CNT/ Nafion
ionomer FETs by electronic transport and optical
means
Experimental Approach
• Nafion® 117 ionomer:
- A Fluorinated, hydrated
organic polyelectrolyte.
- Molecular structure is illustrated
to the right:
C, F, M, X
**
**“ where X- is a sulfonic or carboxylic
functional group and M+ is a Metal cation”
Quip courtesy of John Payne. “Nafion- perflorosulfonate ionomer.”
[http://www.usrc.usm.edu/mauritz/nafion.html]
• Singe-Walled Carbon Nanotube “Bundles”
-Solution of Bucky-paper in Dichloromethane is sonicated at 50˚C for
1-1.5hrs.
-Small droplets of solution are then
deposited onto the Nafion base
and samples are desiccated.
- Sonication is somewhat defeated
by diffusion during desiccation
hence tubes are in tangles.
- Hence this particular report is
a bulk characterization.
Above, SEM image of SWCNT layer of the FETs.
The nanotubes are in intertwined bundles, as
shown, in near-perfect random orientation.
80,000X Magnification.
Results
• Strong light response was realized in
our samples in white light as well as
under laser illumination.
• Typical photo-modulation was about
14%, subject to improvement with
further optimization procedures, the form
of which are indicative of the nature of
transport events and trapping
mechanisms in the FET arrangement.
• Figs. (A) and (B). Typical light response
vs. time. Red curves depict gate leakage
current (at zero gate bias),
corresponding to trap charging. Blue
curves are the CNT channel current.
• Fig.(C). Channel (drain) and leakage
(gate) currents vs. drain voltage. Note
the strong hysteresis in the nafion-base
under a drain current sweep.
• Both electrostatic and photomodulation signals are clear and
reproducible and sensitive to device
geometry.
A
B
C
Summary
• We developed a photo-electric set-up to study hysteresis in CNT
FET devices.
• The modulation tendencies observed verify that the photo-gating
is a useful technique for both qualitative and quantitative
studies of charge trapping in ionomer substrate CNT FETs.
Acknowledgements
This work was partially supported by DoD-ARL (grant W911NF-06-2-0020) under the LehighArmy Research Laboratory Cooperative Agreement, by ACS PRF (grant 46870-G10) and by
the PA Infrastructure Technology Fund (grant PIT-735-07).