Carbon-based devices on flexible substrate

1. Carbon-based devices on flexible substrate 2011.12.26 Chun-Chieh Lu

2. 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

3. Carbon-based transparent conducting film (TCF) Carbon-based field effect transistor

4. Carbon-based transparent conducting films (TCFs) Transfer to PET Transfer to glass Transmittance vs. Sheet resistance

5. Carbon-based transparent conducting films (TCFs) Graphene films bending and stretching test Compared with ITO film:

6. Graphene field-effect transistor on flexible substrate • Field-effect transistor in flexible regime: logic gate, portable memory, display driving circuit, electronic tagging, RFID system… • Field-effect transistor needed in conventional process: metal contact deposition in high vacuum and gate oxide grown in high temp. process • 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

7. Graphene field-effect transistor on flexible substrate = + + Ion gel solvent 凝膠 + :[EMIM] - :[TFSI] poly(styrene- block-methyl methacrylate-block-styrene) (PS-PMMA-PS) triblock copolymer Process: [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)

8. 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/cm2at 10 Hz • FET devices can operate within a • low voltage region (~3 V) with a high on- • current (~mA) on Si substrate. • Due to the ultrahigh capacitance of the ion • gel gate dielectric yielded this low-voltage, • high-current operation

9. 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) • Problem: • Low ON/OFF, gate electrode dimensions… • Only 20% changes in μ/μowereobserved as the • bending radius was changed from 6 to 0.6 cm

10. 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

11. 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

12. Carbon-based logic gate on flexible substrate • The PMOS inverter consisting of two p-type transistors • 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 inverter gain was approximately 1.4, and a supply voltage of 0-5 V was enough to provide the switching functions

13. Carbon-based logic gate on flexible substrate Bending and stretching test

14. 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