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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).