Erik Sczygelski 1|| , Vinod K. Sangwan 1|| , Chung-Chiang Wu 1 , Heather N. Arnold 1 , Ken
Everaerts 1,2 , Tobin J. Marks 1,2 , Mark C. Hersam 1,2,3 , and Lincoln J. Lauhon 1*
1 Department of Materials Science and Engineering, Northwestern University, Evanston, IL
60208, USA
2 Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
3 Department of Medicine, Northwestern University, Evanston, IL 60208, USA
|| These authors contributed equally.
* Corresponding author.
Section S1: Characterization of Single-walled Carbon Nanotube Thin-Films
Bottom-contact single-walled carbon nanotube (SWCNT) thin film transistors (TFTs) were fabricated on 4-layers of zirconia-based self-assembled nanodielectrics (Z-SAND) using photolithography and reactive-ion etching (RIE). The fabrication details are provided elsewhere
1
.
The SWCNT thin film morphology was characterized with scanning electron microscopy (SEM), see Fig. S1. Preparation of SWCNTs by vacuum filtration resulted in uniform thickness due to self-limiting aggregation of SWCNTs. The density of SWCNTs was characterized by counting
1

the number of nanotubes from scanning electron microscopy (SEM) images using methodology
employed previously 1,2 . The SEM was imported into AutoCAD 12 and individual nanotubes
within squares of 1 µm 2
area were traced using the drawing tools. A nanotube fully enclosed within the square was counted as 1 and a nanotube crossing the edge of the square was counted as ½. Thus, the present devices had SWCNT density of approximately 17.3 CNTs/µm 2
. The average length of SWCNTs is 1.36 µm 1
.
Figure S1: a) Scanning electron micrograph of a patterned SWCNT thin film on 4 layers of Z-
SAND. b) Traced nanotubes in AutoCAD 12 within a square of 1 µm x 1 µm to characterize the density of SWCNTs.
2

Section S2: Electrical Characterization of Single-walled Carbon Nanotube Thin-Film
Transistors
SWCNT TFTs with varying channel length and width (5 – 50 µm) were fabricated on 4 layers of Z-SAND using photolithography and reactive ion-etching. Fig. S2 shows optical images of SWCNT TFTs. Wires were bonded on long electrodes (~3 mm) to enable illumination of wired devices in a scanning confocal microscope. Devices were first screened via electrical characterization in an ambient probe station (Cascade Microtech) before the photocurrent measurements. Fig. S3 shows output and transfer characteristics of the SWCNT TFT discussed in the manuscript (Fig. 2b). Channel current density decreased by 40% during photocurrent measurements done a few days after the initial electrical characterization due to degradation.
The field-effect mobility of the devices was observed to be between 2 – 8 cm
2
/Vs and the on/off ratio ~10 3 . Fig. S4 shows transfer characteristics of the TFTs with varying channel length for a channel width W = 10 µm. The channel current density decreases monotonically with increasing channel length. Scanning photocurrent analysis focused on longer channel length devices because they provided better signal to noise ratios.
3

Figure S2: a) Optical micrograph of SWCNT TFTs with varying channel length and width.
500 nm SiO
2
pads were deposited beneath the metal bond pads to prevent punch through of the ultra-thin Z-SAND gate dielectric. b) An optical micrograph of a device with channel length L of
30 µm and width W of 15 µm.
4

Figure S3: a) Output characteristics of the SWCNT TFT with L = 5 µm and W = 10 µm discussed in the manuscript (Fig. 2b). b) Transfer characteristics of the same device at different drain biases.
Figure S4: Transfer characteristics of SWCNT TFTs with varying channel lengths at V d
= -0.1
V. The channel width of the devices is 10 µm.
5

Section S3: Drain Bias and Light Power dependent Photocurrent
The persistent positive photocurrent in SWCNT TFTs in depletion (Fig. 3 and 4) is ascribed to interband excitation resulting in excess free carriers within SWCNTs. Fig. S5 shows the linear power dependence of the persistent photocurrent amplitude. Fig. S6 shows that the persistent photocurrent increases monotonically with the drain bias. A linear drain bias dependence is not expected for the photothermoelectric effect
3
.
Figure S5: Persistent photocurrent versus laser power (wavelength 1250 nm) at V g
= 2 V
6

3
Figure S6: Persistent photocurrent is plotted as a function gate bias for different drain biases.
7