Photo-Electronic Modulation of Nafion-gated Single-Walled Carbon Nanotube FETs Happiness Munedzimwe 3, S. V. Rotkin 1,2 1 Physics Department 2 Center for Advanced Materials and Nanotechnology 3 Integrated Business and Engineering Department Lehigh University, Bethlehem, PA. Photo-Electronic Modulation of Nafion-gated Single-Walled Carbon Nanotube FETs Happiness Munedzimwe 3, S. V. Rotkin 1,2 1 Physics Department 2 Center for Advanced Materials and Nanotechnology 3 Integrated Business and Engineering Department Lehigh University, Bethlehem, PA. INTRODUCTION MOTIVATION FOR SWNT FILM ELECTRONICS METHODBACKGROUND Ionomers have recently begun to be seriously considered as plausible replacements for silicon back gate in SWNT FETs. Optoelectronic characterization of SWNT on Si FETs is a powerful tool to investigate processes at the Si/SiO 2 interface. Marcus et al (1) have demonstrated that for such systems with a back gate geometry photoelectric modulation of the SWNT drain current (I d ) is mostly due to charge trapping at the Si/SiO 2 interface and the gating events from that charge alteration. Our work investigates photo gating of a SWNT Film FET (a TFT topology) arrangement with Nafion-117 ionomer as a substrate and the gate spacer and shows the predominance of direct nanotube responses in such systems as opposed to ambient events in the former. Hence we show that ionomer gating can be used for accurate characterization of nanotube electronics and with optimization of the TFT design in practical devices Rule: n - m = 3i ⇒ Metallic if i is integer ⇒ Semiconductor if i is non- integer Chiral vector: [2] Ch = na 1 + ma 2 ≡ (n, m) (n, m are integers, 0 ≤ |m| ≤ n) Nanotube Geometry and Electronic Properties [10,10] arm chair [17,0] zig-zag 1 Conducting: 2 Semiconducting Any batch of SWNTs contains metallic, semi- metallic and semi- conducting tubes because of chiral statistics Band profiles of nanotubes show a direct band gap in 66% of nanotubes in a batch.  Single SWNT electronics research is promising with commercial and near commercial applications  However, for both in-situ grown and deposited SWNT selection, separation and reorientation are crucial for single SWNT electronics but are quite difficult.  Major Obstacle  SWNT films with random tubes show p-type conductivity due to ambient O 2 and n-type conductivity with functionalization  We use a back-gated film SWNT TFT geometry with Nafion-117 ionomer substrate for characterizing gating performance and the trapping processes at the Nafion/ CNT layer interface.  Both electrostatic gating and photo gating are employed, with similar signal profiles. Sonication:of Bucky Paper in DichloroMethane at > 60°C Dropper Deposition onto Nafion Desiccation then Metallization Systematic of Final Bulk CNT FET Topology SEM Channel Close-up: Nanotube Forest NAFION 117 SUBSTRATE [4,5]  Nafion is a per fluorinated ionomer where X - is a sulfonic or carboxylic functional group and M + is a metal cation (See Diagram)  Transport via mobile cation-water clusters  Major cation is H +  µ of Na+ in Nafion ~ 0.3e -07 m 2 V -1 s -1 vs. 0.5e -07 m 2 V -1 s -1 (2) in plain aqueous solution => low mobility  Permittivity of 4<ε r < 20 RESULTS V th = 1-3 V V bi = 40-70mV P-channel behavior ELECTROSTATIC GATING PHOTO GATING [3] CITATIONS 1. M. S. Marcus, J. M. Simmons, O. M. Castellini, R. J. Hamers, M. A. Eriksson. Photo gating Carbon Nanotube Transistors J. Appl. Phys. 100 (2006) 2. M.S. & G. Dresselhaus, P. Avouris,Carbon Nanotubes, Springer (2001) 3. Image From Royal Chemistry Society 4.Nafion 5 T. Okada et al. Electrochimica Acta 43, Iss. 24 (Aug. 1998) p Funding Acknowledgements. This work was partially supported by DoD-ARL (Grant W911NF ) under the Lehigh- Army Research Lab. Cooperative Agreement and by the American Chemical Society Petroleum Research Fund (Grant G10). 500nm CONCLUSION: Photo modulating SWNT FETs based on Nafion differentiates increased channel conductivity due to excitonic absorption and photo-generation of charge carriers from real photo-gating which features charge redistribution in the substrate/gate interface which then indirectly changes the channel conductivity. PREPARATION ZERO BIAS ILLUMINATION ´ ´ ´ ´ ´ Ig Vg Id Vg IR illumination under Zero Bias conditions -Peak current generated n SWNT channel = 20uA Nafion is also photoactive -Peak photocurrent in Nafion substrate = 18 nA -Gating effects present from generated Igs current Gating effect reduced by; electrostatic shielding of NT layers away from the back gate mixed chirality of nanotubes IR Illumination under a drain bias of 10 mV Rise time ~ 200s Fall time ~ 100s Photo gating and photocurrent generation usually distinguished by time scale of response (s to min vs. fs to ns respectively) Charge trapping dynamics in Nafion-117 slows down both effects Zero bias conditions give the magnitude of the photocurrent Peak Ig= 18 nA, Id=44 uA