Broadband Photodetector Based on Carbon Nanotube Fibers

This material is based upon work supported by the National Science Foundation under Grant No. EEC-0540832.

Simon Lee1, Xuan Wang2, Sébastien Nanot2, Xiaowei He2, Colin C. Young3, Dmitri E. Tsentalovich3, Matteo Pasquali3, and Junichiro Kono2

1Department of Biochemistry & Biophysics, Texas A&M University, College Station, Texas, USA 2Department of Electrical & Computer Engineering, Rice University, Houston, Texas, USA

3Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas, USA

www.mirthecenter.org

Motivation

Why carbon nanotubes? • Mechanical strength: strong covalent bonds

yet flexible • Optical properties: sensitive to broadband

absorptions across a wide electromagnetic spectra

• Electrical properties: can be metallic or a semiconductor; high current-carrying capacity; great electron mobility

Background • Our fibers consist of well aligned and densely

packed carbon nanotubes [1]

• Fibers carry over their microscopic characteristics: 1. mechanically strong and flexible 2. electrically conductive 3. thermally conductive 4. optically absorptive within a broad band of the electromagnetic spectra

• The fibers optically absorb energy from a light source, in this case a laser, generating a thermal distribution across the length of the fiber.

Photodetector Fabrication Two ways photodetecting devices were made:

1. Double-fiber photodetector: interconnection between two fibers creates a node

2. Single-fiber photodetector: use current to anneal only half of the fiber. (junction is continuous)

x 0

Current annealed fiberIodine doped fiber

Annealed fiberIodine doped fiber

x 0

Figure (a) corresponds to the double-fiber photodetector with its interconnection, magnified in Figure (b).

Series or parallel circuits can be created by several identical devices to enhance signal.

Doping Dependence Wavelength Dependence

Position Dependence

Conclusion & Future Plans • The CNT fiber’s photodetecting ability is a result to its

photothermoelectric properties that are inherent to the fibers.

• The wavelength dependent graphs require a normalization by calculating the beam size.

• Polarization dependence should be able to be seen in the single fiber devices due to the aligned nature of the fiber.

• Seebeck coefficient depends on doping. Therefore, tests are not limited to iodine doped samples. Other samples such as sulfur-doped and potassium-doped samples were created and tested.

(a)

(b)

References [1] N. Behabtu,et al., “Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity”, Science 339 (2013) 182–186. [2] X. He et al., “Photothermoelectric p-n Junction Photodetector with Intrinsic Broadband Polarimetry Based on Macroscopic Carbon Nanotube Films”, ACS Nano, ASAP,Web (2013) DOI: 10.1021/nn402679u\

Science 339.6116 (2013): 182-186

Phot

ovol

tage

(mV)

0

0.45

0.9

1.35

1.8

Laser Power (mW)0 2.75 5.5 8.25 11

I2 doped 20micron, 660nmAs Spun 20micron, 660nmS Doped 20um, 660nm

Results

Polarization Dependence

-60 -40 -20 0 20 40 60 80 100

-4

-2

0

2

4

ΔI

ΔV

Under illumination

Volta

ge (m

V)

Current (µA)

Without illumination

Laser excitation Heat exchange by gas

• Seebeck effect is generated from the heat produced by laser excitation

dTdVS −= ∫ ⋅−=Δ

2

1

x

x

dTSV

ACS Nano, X. He et al.,

Because of the well aligned nature of the CNT fibers, a polarization dependence should be observed. Polarization dependence has been observed in CNT films [2]

V/Vm

ax

0

0.25

0.5

0.75

1

Position (mm)-3 -2.3 -1.5 -0.8 0 0.8 1.5 2.3

As-Spun Current Annealed

V/Vm

ax

0

0.25

0.5

0.75

1

Position (mm)-3 -2.3 -1.5 -0.8 0 0.8 1.5 2.3 3

As-Spun Annealed