Photo-Electronic Modulation of Nafion-gated Single-Walled Carbon Nanotube FETs

Happiness Munedzimwe3 , S. V. Rotkin1,2 1Physics Department 2Center for Advanced Materials and Nanotechnology 3Integrated Business and Engineering Department Lehigh University, Bethlehem, PA.

Photo-Electronic Modulation of Nafion-gated Single-Walled Carbon Nanotube FETs

Happiness Munedzimwe3 , S. V. Rotkin1,2 1Physics Department 2Center for Advanced Materials and Nanotechnology 3Integrated 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/SiO2 interface. Marcus et al(1) have demonstrated that for such systems with a back gate geometry photoelectric modulation of the SWNT drain current (Id) is mostly due to charge trapping at the Si/SiO2 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 = na1 + ma2 ≡ (n, m)(n, m are integers, 0 ≤ |m| ≤ n)

Nanotube Geometry and Electronic Properties

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 O2 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

Desiccationthen 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 m2V-1s-1 vs. 0.5e-07 m2V-1s-1 (2) in plain aqueous solution => low mobilityPermittivity of 4<εr< 20

RESULTS

Vth = 1-3 VVbi = 40-70mVP-channel behavior

ELECTROSTATIC GATING

PHOTO GATING

[3]

CITATIONS1. 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 www.rsc.org/ej/CC/2001/b102348a/b102348a-f2.gif 4.Nafion < http//www.psrc.usm.edu/mauritz/nafion.html >5 T. Okada et al. Electrochimica Acta 43, Iss. 24 (Aug. 1998) p. 3741-3747

Funding Acknowledgements . This work was partially supported by DoD-ARL (Grant W911NF-07-2-0064) under the Lehigh-Army Research Lab. Cooperative Agreement and by the American Chemical Society Petroleum Research Fund (Grant 46870-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

4 2 0 2 4

3. 10

6

2. 106

106

0

1.

10

6

2. 106

-Ig

Vg

Id

4 2 0 2 4

3.106

2.106

1.106

0

1.106

2.106

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 mVRise time ~ 200sFall 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

-2

0.000032

0.000030

0.000028

0.000024

0.000026

--4 0 2