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: [email protected] 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]