Engineering and Technology Degree Level: Ph.D. Abstract ID# 298
Graduate Category: Engineering and Technology Degree Level: Ph.D. Abstract ID# 298 FLEXIBLE SINGLE-‐WALLED CARBON NANOTUBE (SWNT) ELECTRONICS: ELECTROPHORETIC DIRECTED ASSEMBLY AND 2D TRANSFER PRINTING OF THIN FILM SWNT WIRES
Mehmet Cem Apaydin and Ahmed Busnaina
NSF Center for High-‐rate Nanomanufacturing, Northeastern University, Boston, MA •
ABSTRACT
Single-‐walled carbon nanotubes (SWNTs) are one of the promising novel nanomaterials for future nanoscale electronics due to their metallic and semiconducRng properRes, and 1D charge transport capability. The typical requirement of most applicaRons is to form SWNT thin films at a monolayer or few monolayers, but to fully uRlize their unique characterisRcs, SWNTs must be aligned with the direcRon of current flow. Therefore, control over the posiRon of SWNTs and the electronic characterisRcs on the substrate layouts are criRcally important for scalability and reproducibility. In this research, we present the electrophoreRc directed assembly of carbon nanotube wire networks onto the source template and a 2D transfer prinRng method to the final flexible device. An average carbon nanotube density of 40 SWNTs/μm (per unit channel width in micrometers) is obtained for single device. The thickness of the directed assembly is tunable down to a monolayer of SWNTs. Further, a highly efficient 2D transfer prinRng is integrated that enables the boLom-‐up transistor fabricaRon. Transistors are fabricated with commercially available epoxy (SU8) gate dielectric providing enhanced electron mobility. ON/OFF current raRo of up to 103 is obtained from single SWNT wire networks with 10μm transistor node, which is promising for flexible electronics.
.
INTRODUCTION Current PercolaKon in Random SWNT Networks
(a) (b) (c) Figure 1. (a) Nanotube density vs. conductance regime under various percolaRon behavior (b) Random percolaRon network of SWNTs that are striped into 5μm parallel wires (c) The variaRon of ON/OFF raRo (le_) and the transconductance (right) at different widths of SWNT network [1, 2] Current PercolaKon in Aligned SWNT Networks
Figure 2. The simple sRck percolaRon network that displays the current flow pathways of metallic (blue) and semiconducRng (red) SWNTs under different wire widths [3]
GOAL BIOLOGICAL SENSORS
FLEXIBLE ELECTRONICS
ApplicaRons aim to make electronics go faster and more flexible CHEMICAL SENSORS with beLer detecRon CMOS ELECTRONICS capability
•
RESULTS
The assembly at the minimum allowable pulling speed (5.0mm/ min) and below (2.5mm/min). No assembly is observed above the maximum allowable pulling speed (20.0mm/min).
""
(a) (b)
METHOD
1)
Cr/Au/oxide chips are diced and cleaned.
2)
Chips are spin coated with polymethyl methacrylate (PMMA).
3)
PMMA is paLerned by e-‐beam lithography and developed by applying MIBK:IPA (1:3), IPA and DI water sequenRally.
4)
The as-‐received SWNT soluRon (Brewer Science Inc.) is diluted and sonicated.
5)
PaLerned electrode is inserted with a bare counter electrode into an aqueous suspension of SWNTs (with NH4OH added to adjust the ionic concentraRon), and an electric field is applied to drive the negaRvely charged carbon nanotubes (deprotonated – COOH funcRonal groups) towards the paLerned electrode.
6)
A_er specified assembly Rme, paLerned electrode and counter electrode are withdrawn from the SWNT soluRon at uniform pulling speed.
7)
PMMA is removed.
20% of 300nm wide wires display diode behavior at 10μm gap, which means that the current percolaKon is dominated by metal-‐semiconductor SWNT juncKons [4].
Figure 4. ElectrophoreRc assembly with the pulling speed of (a) 2.5mm/min, and (b) 5.0mm/min • The assembled number of monolayers with voltage and pulling speed at fixed soluKon condiKons (pH10.1 and 500ppb nanotube concentraKon), trench depth (150nm)
(a)
(b) Figure 5. The number of assembled monolayers with respect to: (a) the voltage at 20mm/min pulling speed, (b) the pulling speed at 2.0V of applied voltage • The assembled number of SWNT layers by tuning the PMMA thickness
(a)
(b)
(c)
Figure 6. A 300nm-‐wide wire a_er the assembly: (a) AFM micrograph of the wire, (b) SEM micrograph of a similar wire, (c) Cross-‐secRonal view of the red line in inset (a) showing an average approximate thickness of 2nm • Flexible transistor-‐based device arrays
Figure 8. Diode response of some wires at 300nm width • Transistor curves have been obtained with the diodes displayed above. The ON/OFF raKo of 103 has been obtained with an electron mobility of ≈0.17 cm2/V.s per wire.
Figure 9. Transfer curves before (blue) and a_er (red) thermal treatments for 10 hours at 150°C
CONCLUSION • Accomplishments ü IdenRficaRon of the criRcal governing parameters behind the electrophoreRc directed assembly ü Assembly in the scale of metal-‐semiconductor juncRon dominated current percolaRon regime ü Assembly on the order of monolayer thickness ü Transfer to flexible substrates ü FabricaRon and tesRng of the first transistors • Path Forward ü Flexible SWNT transistor fabricaRon at smaller wire widths ü The determinaRon of criRcal device performance metrics ü Flexibility assessment
REFERENCES
[1] N. Pimparkar, Q. Cao, J. A. Rogers, and M. A. Alam, “Theory Figure 3. The experimental flow chart
8)
Photolithography and li_-‐off are done on SWNT wires to generate the Au electrodes.
9)
Polyamic acid (PAA) is spun on the source and drain interconnects.
10) PAA is cured to polyimide. 11) Polyimide is mechanically peeled off with the source and drain SWNT wire interconnects.
(a) (b) Figure 7. The transfer on polyimide chips: (a) 10mm x 10mm chip with an array of devices (b) a close-‐up microscope image of the electrode arrays, which are connected by the SWNT wiring
and PracRce of ‘Striping’ for Improved ON/OFF RaRo in Carbon Thin Film Transistors”, Nano Research, 2009, 2, 167 Nanonet [2] Q. Cao, H. Kim, N. Pimparkar, J. P. Kulkarni, C. Wang, M. Shim, K. Roy, M. A. Alam & J. A. Rogers, “Medium-‐scale Carbon Nanotube Thin-‐film Integrated Circuits on Flexible PlasRc Substrates”, Nature, 2008, 454, 495 [3] S. Somu, H. Wang, Y. Kim, L. Jaberansari, M. G. Hahm, B. Li, T. Kim, X. Xiong, Y. J. Jung, M. Upmanyu, and A. Busnaina, “Topological TransiRons in Carbon Nanotube Networks via Confinement”, ACS Nano, 2010, 4, 4142 Nanoscale [4] M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, Y. Yoon, M. S. C.
Mazzoni, H. J. Choi, J. Ihm,S. G. Louie, A. ZeLl, and P. L. McEuen, “Crossed Nanotube JuncRons”, Science, 2000, 288, 494
Mehmet Cem Apaydin and Ahmed Busnaina
NSF Center for High-‐rate Nanomanufacturing, Northeastern University, Boston, MA •
ABSTRACT
Single-‐walled carbon nanotubes (SWNTs) are one of the promising novel nanomaterials for future nanoscale electronics due to their metallic and semiconducRng properRes, and 1D charge transport capability. The typical requirement of most applicaRons is to form SWNT thin films at a monolayer or few monolayers, but to fully uRlize their unique characterisRcs, SWNTs must be aligned with the direcRon of current flow. Therefore, control over the posiRon of SWNTs and the electronic characterisRcs on the substrate layouts are criRcally important for scalability and reproducibility. In this research, we present the electrophoreRc directed assembly of carbon nanotube wire networks onto the source template and a 2D transfer prinRng method to the final flexible device. An average carbon nanotube density of 40 SWNTs/μm (per unit channel width in micrometers) is obtained for single device. The thickness of the directed assembly is tunable down to a monolayer of SWNTs. Further, a highly efficient 2D transfer prinRng is integrated that enables the boLom-‐up transistor fabricaRon. Transistors are fabricated with commercially available epoxy (SU8) gate dielectric providing enhanced electron mobility. ON/OFF current raRo of up to 103 is obtained from single SWNT wire networks with 10μm transistor node, which is promising for flexible electronics.
.
INTRODUCTION Current PercolaKon in Random SWNT Networks
(a) (b) (c) Figure 1. (a) Nanotube density vs. conductance regime under various percolaRon behavior (b) Random percolaRon network of SWNTs that are striped into 5μm parallel wires (c) The variaRon of ON/OFF raRo (le_) and the transconductance (right) at different widths of SWNT network [1, 2] Current PercolaKon in Aligned SWNT Networks
Figure 2. The simple sRck percolaRon network that displays the current flow pathways of metallic (blue) and semiconducRng (red) SWNTs under different wire widths [3]
GOAL BIOLOGICAL SENSORS
FLEXIBLE ELECTRONICS
ApplicaRons aim to make electronics go faster and more flexible CHEMICAL SENSORS with beLer detecRon CMOS ELECTRONICS capability
•
RESULTS
The assembly at the minimum allowable pulling speed (5.0mm/ min) and below (2.5mm/min). No assembly is observed above the maximum allowable pulling speed (20.0mm/min).
""
(a) (b)
METHOD
1)
Cr/Au/oxide chips are diced and cleaned.
2)
Chips are spin coated with polymethyl methacrylate (PMMA).
3)
PMMA is paLerned by e-‐beam lithography and developed by applying MIBK:IPA (1:3), IPA and DI water sequenRally.
4)
The as-‐received SWNT soluRon (Brewer Science Inc.) is diluted and sonicated.
5)
PaLerned electrode is inserted with a bare counter electrode into an aqueous suspension of SWNTs (with NH4OH added to adjust the ionic concentraRon), and an electric field is applied to drive the negaRvely charged carbon nanotubes (deprotonated – COOH funcRonal groups) towards the paLerned electrode.
6)
A_er specified assembly Rme, paLerned electrode and counter electrode are withdrawn from the SWNT soluRon at uniform pulling speed.
7)
PMMA is removed.
20% of 300nm wide wires display diode behavior at 10μm gap, which means that the current percolaKon is dominated by metal-‐semiconductor SWNT juncKons [4].
Figure 4. ElectrophoreRc assembly with the pulling speed of (a) 2.5mm/min, and (b) 5.0mm/min • The assembled number of monolayers with voltage and pulling speed at fixed soluKon condiKons (pH10.1 and 500ppb nanotube concentraKon), trench depth (150nm)
(a)
(b) Figure 5. The number of assembled monolayers with respect to: (a) the voltage at 20mm/min pulling speed, (b) the pulling speed at 2.0V of applied voltage • The assembled number of SWNT layers by tuning the PMMA thickness
(a)
(b)
(c)
Figure 6. A 300nm-‐wide wire a_er the assembly: (a) AFM micrograph of the wire, (b) SEM micrograph of a similar wire, (c) Cross-‐secRonal view of the red line in inset (a) showing an average approximate thickness of 2nm • Flexible transistor-‐based device arrays
Figure 8. Diode response of some wires at 300nm width • Transistor curves have been obtained with the diodes displayed above. The ON/OFF raKo of 103 has been obtained with an electron mobility of ≈0.17 cm2/V.s per wire.
Figure 9. Transfer curves before (blue) and a_er (red) thermal treatments for 10 hours at 150°C
CONCLUSION • Accomplishments ü IdenRficaRon of the criRcal governing parameters behind the electrophoreRc directed assembly ü Assembly in the scale of metal-‐semiconductor juncRon dominated current percolaRon regime ü Assembly on the order of monolayer thickness ü Transfer to flexible substrates ü FabricaRon and tesRng of the first transistors • Path Forward ü Flexible SWNT transistor fabricaRon at smaller wire widths ü The determinaRon of criRcal device performance metrics ü Flexibility assessment
REFERENCES
[1] N. Pimparkar, Q. Cao, J. A. Rogers, and M. A. Alam, “Theory Figure 3. The experimental flow chart
8)
Photolithography and li_-‐off are done on SWNT wires to generate the Au electrodes.
9)
Polyamic acid (PAA) is spun on the source and drain interconnects.
10) PAA is cured to polyimide. 11) Polyimide is mechanically peeled off with the source and drain SWNT wire interconnects.
(a) (b) Figure 7. The transfer on polyimide chips: (a) 10mm x 10mm chip with an array of devices (b) a close-‐up microscope image of the electrode arrays, which are connected by the SWNT wiring
and PracRce of ‘Striping’ for Improved ON/OFF RaRo in Carbon Thin Film Transistors”, Nano Research, 2009, 2, 167 Nanonet [2] Q. Cao, H. Kim, N. Pimparkar, J. P. Kulkarni, C. Wang, M. Shim, K. Roy, M. A. Alam & J. A. Rogers, “Medium-‐scale Carbon Nanotube Thin-‐film Integrated Circuits on Flexible PlasRc Substrates”, Nature, 2008, 454, 495 [3] S. Somu, H. Wang, Y. Kim, L. Jaberansari, M. G. Hahm, B. Li, T. Kim, X. Xiong, Y. J. Jung, M. Upmanyu, and A. Busnaina, “Topological TransiRons in Carbon Nanotube Networks via Confinement”, ACS Nano, 2010, 4, 4142 Nanoscale [4] M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, Y. Yoon, M. S. C.
Mazzoni, H. J. Choi, J. Ihm,S. G. Louie, A. ZeLl, and P. L. McEuen, “Crossed Nanotube JuncRons”, Science, 2000, 288, 494
