Terahertz Spectroscopy of Large-Scale Graphene on Various Substrates

James Allred,1 Kazunori Serita,2 Makoto Ohshiro,2 Iwao Kawayama,2 Masayoshi Tonouchi,2 Minjie Wang,1 Robert Vajtai,3 Junichiro Kono,1 and Pulickel M. Ajayan3

1Department of Electrical and Computer Engineering and the NanoJapan Program, Rice University, Houston, Texas, U.S.A.

2Institute of Laser Engineering, Osaka University, Suita, Osaka, Japan 3Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas,

U.S.A.

Graphene is a promising material in a variety of scientific and technological fields because of its many astounding properties, such as its high electron mobility at room temperature, incredible mechanical strength, zero-gap band structure, and zero effective mass of electrons. In particular, we are interested in its electron mobility for potential incorporation in high frequency terahertz (THz) electronic devices. The electron mobility of graphene is affected by gas molecules between it and the substrate as well as by charge impurities at the substrate-graphene interface. Therefore, in this study, we have investigated the dependence on various substrates of THz transmission and optical conductivity of large-scale graphene using THz time-domain spectroscopy (THz-TDS). The substrates we examined are indium phosphide (InP), magnesium oxide (MgO), indium arsenide (InAs), gallium arsenide (GaAs), and black polypropylene sheets (BPPS). We found an unexpectedly large dependence of transmittance and conductivity of graphene on the substrate used. We also investigated the effect of cw laser illumination on graphene with THz-TDS.

• Graphene’s electron mobility should be affected by: • Investigate this change via transmission and optical conductivity analysis of graphene on various substrates for potential use in high frequency THz electronic devices.

TERAHERTZ SPECTROSCOPY OF LARGE-SCALE GRAPHENE ON VARIOUS SUBSTRATES

James Allred1, Kazunori Serita2, Makoto Ohshiro2, Iwao Kawayama2, Masayoshi Tonouchi2, Minjie Wang1, Robert Vajtai3, Junichiro Kono1, Pulickel M. Ajayan3

1Department of Electrical and Computer Engineering and the NanoJapan Program, Rice

University, Houston, Texas, U.S.A. 2Institute of Laser Engineering, Osaka University, Suita, Osaka, Japan

3Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas, U.S.A.

2. Graphene on Substrates

7. Conclusion

6. Discussion and Future Work

This research project was conducted as part of the 2013 NanoJapan: International Research Experience for Undergraduates Program with support from a National Science Foundation Partnerships for International Research & Education grant (NSF-PIRE OISE-0968405). For more information on NanoJapan see http://nanojapan.rice.edu. Special thanks to Packard-sensei, Kono-sensei, Tonouchi-sensei, and Sarah Phillips for planning and organizing NanoJapan 2013!

o Charge impurities on substrate- graphene interface o Gas molecules trapped between graphene and substrate

Terahertz: • Frequency range 300 GHz to 30 THz • Terahertz Time Domain Spectroscopy (TDS) measures THz field • From transmitted and reference (no sample) fields, calculations of transmittance and conductivity are possible

Graphene: • Astoundingly high electron mobility at room temperature, zero gap band structure, and zero effective mass of electrons • High potential for high frequency electronics

1. Terahertz and Graphene

3. Terahertz Time-Domain Spectroscopy

9. Acknowledgements

8. References

CVD-grown Graphene on

InP

InP

Beam Splitter

Graphene Sample

Emitter Detector

Time delay

Probe beam

Pump beam

Lock-in Amp.

Ti:Sapphire Laser

Pre. Amp.

CW Laser (365 nm)

Chopper

Grap

hen

e

Sub

strate

Wave Direction

THz Pulse

CW Laser Absorption due

to graphene and substrate

• Graphene grown using chemical vapor deposition (CVD)

• Using conventional THz-TDS, compared transmittance and optical conductivity of graphene on GaAs, BPPS, InP, InAs, and MgO • Measured same properties when illuminated by 365 nm CW laser

• THz pulses generated and detected by GaAs photodiode antennas

4. Graphene Transmittance Dependence on Substrate

Transmittance of Graphene on Various Substrates

0 1 2 3 4 5

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

Am

plit

ude

[a

.u.]

Frequency [THz]

Reference

InP

InP and Graphene

Spectrum

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.2

0.4

0.6

0.8

1.0

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

Tra

nsm

itta

nce

Frequency [THz]

Tra

nsm

itta

nce

Frequency [THz]

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.2

0.4

0.6

0.8

1.0

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.80

0.85

0.90

0.95

1.00

Tra

nsm

itta

nce

Frequency [THz]

Tra

nsm

itta

nce

Frequency [THz]

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-4

-2

0

2

4

6

8

10

Optical S

heet C

onductivity [(p

i*e^2

)/(2

h)]

Frequency [THz]

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-4

-2

0

2

4

6

8

10

12

14

Op

tica

l S

hee

t C

on

du

ctivity [(p

i*e

^2)/

(2h

)]

Frequency [THz]

• Possible dependence of transmission of graphene on substrate • The 365 nm laser clearly affects the transmission and optical conductivity of graphene

Email: jja6@rice.edu Cell: 918-406-0354

6σ ≈ 11.5 x 103 (Ω·cm)-1 6σ ≈ 11.5 x 103 (Ω·cm)-1

• L. Ren, Q. Zhang, J. Yao, Z. Sun, R. Kaneko, Z. Yan, S. Nanot, Z. Jin, I. Kawayama, M. Tonouchi, J. M. Tour, and J. Kono. NanoLett. 2012, 12, 3711−3715 • Mizuno, H. Murakami, I. Kawayama, Y. Takahashi, M. Yoshimura, Y. Mori, J. Darma, and M. Tonouchi. Optics Express 163639th ser. 20.12 (2012): 1-7 • J. Y. Kim, J. H. Rho, C. Lee, S. Bae, S. J. Kim, K. S. Kim, B. H. Hong, and E.J. Choi. J. Nanosci. Nanotechol. 2012, Vol. 12, No. 7

MgO (Magnesium Oxide) InP (Indium Phosphide)

5. Graphene Transmittance Increase Under 365 nm CW Laser

Substrate Dependence • First time observed in the THz region • Reason largely unknown • Most likely related to the substrate-graphene border

CW Laser Dependence • Several possible explanations • One is air molecules affecting Fermi level

• Several more tests needed to confirm mechanism

o Air molecules bind to graphene o Induced electric field moves Fermi level from Dirac point -> increases carrier density -> lowers transmittance o Laser removes molecules, resetting Fermi level

0.5 1.0 1.5 2.0

0.0

0.2

0.4

0.6

0.8

1.0

BPPS

GaAs

InAs

InP

MgO

0.5 1.0 1.5 2.0

0.7

0.8

0.9

1.0

Tra

nsm

itta

nce

Frequency [THz]

Tra

nsm

itta

nce

Frequency [THz]