Directed assembly of solution processed single-walled carbon nanotubes via dielectrophoresis: From aligned array to individual nanotube devices
Summary (4 min read)
- University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2010 Published by the American Vacuum Society ARTICLES YOU MAY BE INTERESTED IN Directed assembly of solution processed single-walled carbon nanotubes via dielectrophoresis: From aligned array to individual nanotube devices Paul Stokes and Saiful I. Khondakera Nanoscience Technology Center and Department of Physics, University of Central Florida, 12424 Research Parkway, Orlando, Florida 32826 Received 9 July 2010; accepted 7 September 2010; published 29 November 2010 For practical applications in nanoelectronics, it is important that SWNTs are assembled at selected positions of the circuit with high yield.
- Catalytic particles in the solution tend to make their way into the electrode gap with the SWNTs during assembly process due to their highly conducive nature which can disrupt their device performance.
A. Electrode design and fabrication
- Devices were fabricated on heavily doped silicon substrates capped with a thermally grown 250 nm thick SiO2 layer.
- The electrode patterns were fabricated by a combination of optical and electron beam lithography EBL .
- First, contact pads and electron beam markers were fabricated with optical lithography using double layer resists LOR 3A/ Shipley 1813 developing in CD26, thermal evaporation of 3 nm Cr and 50 nm Au followed by lift-off.
- Smaller electrode patterns were fabricated with EBL using single layer PMMA resists and then developing in 1:3 methyl isobutyl ketone:isopropal alchohol MIBK:IPA .
- The electrode patterns for the alignment of individual tubes use a pair of adjacent taper shaped electrodes with sharp tips separated by 1 m, whereas the electrode patters for aligned arrays of SWNTs were done using 200 m long parallel electrodes with 5 m spacing.
B. Solution preparation
- The authors used three different SWNT solutions for the DEP assembly: i A homemade dimethylformamide DFM solution, ii homemade dichloroethane DCE solution, and iii already suspended, surfactant-free aqueous SWNT solution purchased from Brewer Science Inc.26.
- The DMF-SWNT suspension was made by ultrasonically dispersing HiPCO grown SWNTs Carbon Nanotechnologies Inc. in 5 ml DMF and 1 ml trifluoroacetic acid TFA .
- TFA was used to dissolve any unwanted catalytic particles and amorphous carbon from the bulk material.
- After dissolving the SWNTs in DMF/TFA, the solution was centrifuged, the supernatant was decanted, and the solid is then redispersed for further dispersion/centrifugation/decantation cycles.
- The DCE mixture was made by simply adding a very small pinch of the HiPCO SWNT soot to 4 ml of DCE and then sonicating for 5–10 min before the assembly.
C. Dielectrophoretic assembly
- The directed assembly of SWNTs at predefined electrode positions was done in a probe station under ambient conditions.
- A small drop of SWNT solution was cast onto the chip containing the electrode pairs.
- The induced dipole moment of the nanotube interacting with the strong electric field causes the nanotubes to move in a translational motion along the electric field gradient.
- Hence the authors can set the effective potential of the electrodes to = Vp-p /2 for their simulation.
- B, Vol. 28, No. 6, Nov/Dec 2010 tor was turned off and the sample was blown dry by a stream of nitrogen gas.
A. Controlling the assembly of individual SWNTs
- Figure 2 shows the effect of SWNT concentration on the DEP assembly using the commercial solution diluted in DI water.
- The authors use a simultaneous deposition technique14,22 in this case, applying the ac field between source and gate for 3 min.
- The yield for the 1000 ng/ml and 100 ng/ml concentration is 95%.
- Approximately 10% of the diameters are greater than 3.5 nm, which is an indication of the possible presence of some double-walled nanotubes or large diameter SWNTs.
- It can be seen here that the devices are free of bundles and catalytic particles which stems from the quality of the commercial solution.
B. Comparison of solutions
- This is done because long trapping times are more complex to use as DMF and DCE evaporate quickly in air.
- Figure 4 a shows a representative AFM im- age after the assembly for the DMF solution.
- As can be clearly seen here, the resulting SWNT deposition contains a bundle and catalytic particles shown by the arrows.
- The commercial solution turned out to be stable for months, therefore increasing the reproducibility.
- 26 Therefore, although the DMF and DCE solutions may be optimized for further control, however, due to the reproducible and clean device assembly from the stable commercial solution, the authors only continued further investigation of the devices stemming from the commercial solution.
C. Electrical transport properties of individual nanotube devices
- After the DEP assembly, the room temperature dc electrical transport measurements of the devices were done in a probe station using a DL instruments 1211 current preamplifier combined with a high resolution DAC card interfaced with LABVIEW.
- After two terminal resistance measurements of the as-assembled devices, they were annealed in a tube furnace using ultrahigh purity Ar /H2 1:10 ratio/Ar:H2 at 200 °C for 1 h.
- During cool down, the gas was left flowing until the sample reached room temperature.
- Approximately 70% of their devices show metallic or semimetallic behavior with current on-off ratio Ion / Ioff less than 10, and 30% of the devices show semiconducting behavior with on-off ratios 10.
- The higher percentage of metallic nanotubes during the assembly is expected since, to the first order approximation, DEP tends to attract metallic SWNTs over semiconducting SWNTs because metallic SWNTs have a higher dielectric constant.
1. Metallic nanotube device properties
- Figure 5 b is a histogram of the contact resistance for the metallic SWNT devices before annealing and after annealing.
- The average contact resistances before and after annealing are 100 M and 1 M , respectively.
- The contact resistance is as low as 25 k after annealing for C6B10 Paul Stokes and Saiful I. Khondaker: Directed assembly of solution processed SWNTs C6B10 J. Vac. Sci. Technol.
- B, Vol. 28, No. 6, Nov/Dec 2010 certain devices.
- To characterize the quality of the metallic SWNTs and their contact, the authors measured the ID-VDS characteristics at high bias after annealing.
2. Semiconducting nanotube device properties
- Figure 5 c shows the transfer characteristics, ID-VG, for a representative FET device at VDS=−0.1, 0.5, 1.0, and 2.0 V d 1.7 nm showing p-type transport characteristics.
- The drain current changes by several orders of magnitude with gate voltage and maintains approximately the same off-current for each VDS.
- The maximum mobility is 20 times higher than the highest previous reported values for other solution processed devices and close to what is expected in high quality direct growth CVD devices of similar diameter.
- The authors speculate that the improved device performance stems from the nonexistence of residual surfactant and the cleanliness of the as-assembled devices with the absence of bundles.
D. Large area assembly and device properties
- Large scale parallel arrays of SWNTs are of considerable interests in order to increase device to device homogeneity and their expected higher performance compared to organic electronic FETs.
- The authors used diluted commercial solution 1 g /ml for this assembly along with parallel electrode.
- The array contains a mixture of metallic and semiconducting SWNTs as evident in the I-VG for the asassembled device Fig. 6 b top curve where the device shows semimetallic behavior with an on-off ratio of 3.
- After the second breakdown, the mobility reduces a small amount to 53 cm2 /V s and the on-off ratio increases to 14.
- The devices yield median mobility values of 77, 41, and 15 cm2 /V s after the three breakdowns, respectively.
- The authors demonstrated directed assembly of high quality solution processed carbon nanotube devices via ac dielectrophoresis using commercially available SWNT solutions.
- By optimizing the device design, concentration of the solution, and assembly time, the authors are able to control the assembly of SWNTs from dense arrays to clean individual devices.
- Electronic transport measurements of individual SWNT revealed mobilities more than an order of magnitude improvement over previous solution processed individual SWNT devices and close to the theoretical limit.
- FETs fabricated from aligned 2D arrays of SWNT show high field effect mobility, up to three orders of magnitude higher than solution processed organic FET devices.
- This study shows promise for commercially available SWNT solution for the parallel fabrication of high quality nanoelectronic devices.
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