Transcript Carbon-based devices on flexible substrate

Carbon-based devices on flexible substrate
2011.12.26
Chun-Chieh Lu
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Graphene properties
• Dimensions
- Available in a wide range of platelet lengths (typically 1-20 μm) and thicknesses
(approximately 0.34 nm to 100 nm)
- Single layer graphene as thin as 0.34 nm
• Strength
- Fifty times stronger than steel
- Ultra-high Young’s modulus (approximately 1,000 GPa) and highest intrinsic strength (~ 130
GPa estimated)
• Conductivity
- The highest thermal conductivity known today (up to ~ 5,300 W/(mK), five times that of
copper, at a density that is four times lower
- Exceptional in-plane electrical conductivity (up to ~ 20,000 S/cm)
• Transmittance
- Absorptance 2.3% for single layer graphene
Good performance of strength, conductivity and transmittance
 Advantages for transparent and flexible electronics
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Carbon-based transparent conducting film (TCF)
Carbon-based field effect transistor
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• Carbon-based transparent conducting films (TCFs)
Transfer to PET
Transfer to glass
Transmittance vs. Sheet resistance
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• Carbon-based transparent conducting films (TCFs)
Graphene films bending and stretching test
Compared with ITO film:
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• Graphene field-effect transistor on flexible substrate
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Field-effect transistor in flexible regime:
logic gate, portable memory, display driving circuit, electronic tagging, RFID system…
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Field-effect transistor needed in conventional process:
metal contact deposition in high vacuum and gate oxide grown in high temp. process
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For flexible substrate:
(1) To develop low temperature, printing processes for materials that form the channel
region, gate insulator and electrodes on soft substrates with high thermal expansion
coefficients
(2) To overcome intrinsic limitations of mechanical properties associated with
conventional materials and circuits through development of new materials or device
architectures.
•
For example:
High-k inorganic dielectrics (HfO2, Al2O3 and ZrO2) cannot be available for flexible
devices based on plastic substrates due to their high growth temperature.
 Due to low temperature and printing process, ion gel can be used as good gate
dielectric of carbon-based field effect transistor on flexible substrate
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• Graphene field-effect transistor on flexible substrate
Ion gel
凝膠
=
+
+ :[EMIM]
- :[TFSI]
+
solvent
poly(styreneblock-methyl methacrylateblock-styrene) (PS-PMMA-PS)
triblock copolymer
Process:
(1) [EMIM][TFSI] : PS-PMMA-PS : solvent
= 0.7 : 9.3 : 90 (w/w)
(2) Drop-casted onto graphene
(3) After the solvent was removed, an ion gel
film was formed through physical
association .
(4) Deposit gate electrodes (shadow mask)
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• Graphene field-effect transistor on flexible substrate
C-V measurement
Electric double layer (EDL)
• EDL capacitor with a dielectric of sub
nanometer thickness
• Ion gels provided a specific
capacitance of 5.17 μF/cm2 at 10 Hz
• FET devices can operate within a
low voltage region (~3 V) with a high oncurrent (~mA) on Si substrate.
• Due to the ultrahigh capacitance of the ion
gel gate dielectric yielded this low-voltage,
high-current operation
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• Graphene field-effect transistor on flexible substrate
Transfer to PET substrate
• Hole mobility: 203 ± 57 50 cm2/Vs
Electron mobility: 91 ± 50 cm2/Vs (at VD=-1 V)
• Only 20% changes in μ/μo were observed as the • Problem:
Low ON/OFF, gate electrode dimensions…
bending radius was changed from 6 to 0.6 cm
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• Carbon-based logic gate on flexible substrate
Graphene : low ON/OFF but easily for large area fabrication
CNT: high ON/OFF but complicated fabrication
 Using graphene as the electrode and CNT as device channel
 All carbon–based field effect transistor on flexible substrate
5 layer graphene
as electrode (270 Ω/□)
Network SWCNTs
as device channel
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• Carbon-based logic gate on flexible substrate
• Using high source-drain current to break metallic CNTs
• Small hysteresis that is caused by the interface trap states parasitized between
the gate electrode and dielectrics
• Graphene-CNT device showed both a high transmittance of 83.8%, due to highly
transparent monolayer graphene for electrodes
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• Carbon-based logic gate on flexible substrate
• Due to the bundle problem of
CNTs, the random network CNT
channels were optimized
as a function of CNT density and
channel width
Reasonable device performance, such as
an on/off ratio of approximately 103 and a
mobility of approximately 81 cm2/Vs were
achieved at a density of 7.5 SWCNTs μm-1.
• The PMOS inverter consisting of two ptype transistors
The inverter gain was approximately 1.4,
and a supply voltage of 0-5 V was enough
to provide the switching functions
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• Carbon-based logic gate on flexible substrate
Bending and stretching test
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Summary
• Carbon-based materials such as CNTs and graphene have large
potential in flexible electronics
• Due to low temperature and printing process, ion gel can be
used as good gate dielectric of carbon-based field effect
transistor on flexible substrate
• Using multilayer graphene as electrodes and network SWCNTs
as device channel can achieve the all carbon logic gate circuit
for transparent and flexible application
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