Field Effect Transistors using Carbon Nanotubes

1. Biosensor Field Effect Transistors using Carbon Nanotubes David Hecht

2. Why Study NTFET’s? Bacteria 1 m 100 nm Virus Proteins 10 nm Biosensor 1 nm DNA 0.1 nm • Size!!! • Diameter ≈ 1nm, length ≈ 1 micron • Quasi 1-D object • High Mobility Semiconductors • Moore’s Law • Can be p-type or n-type • Flexible Transistors/Electronics? • Sensor • Biosensor (walls can be functionalized) • Chemical Sensor • high sensitivity/large surface area

3. How a NTFET Works p-type in air Source Drain Insulating Layer (SiO2) Conducting “Gate” (p type Si) Rprot VRprot Isd Vgate Vsd Vg = 0

4. How a NTFET Works p-type in air S D Insulating Layer (SiO2) Conducting “Gate” (p type Si) Rprot VRprot Isd Vg Vsd Vg = positive, Holes are Depleted

5. How a NTFET Works p-type in air S D Insulating Layer (SiO2) Conducting “Gate” (p type Si) Rprot VRprot Isd Vg Vsd Vg = negative, Holes are enhanced

6. Nanotube FET transistor S D SiO2 AFM image Si back gate Vsd Vg Isd Ideal NTFET Device A A. Max Conductance B. Modulation – Signal to Noise C. Transconductance (slope at zero gate) Mobility of Carriers (electrons or holes) D. Threshold Shift – Changes in Doping p-type C Conductance (S) B D Gate Voltage (V)

7. Effect of charge transfer on the device electronics Detection of gases1 NH3 el. donor NO2 el. acceptor 1Bradley, K.; Gabriel, J.-C. P.; Briman, M.; Star, A.; Grüner, G. “Charge Transfer from Ammonia Physisorbed on Nanotubes,” Phys. Rev. Lett.2003, 91, 218301.

8. Single Nanotube vs. Network Novel active electronic devices • Single tube/fiber channel • greater sensitivity • individual device fabrication (basic research) • Network channel • easier/more consistent device fabrication • Applications

9. Metallic vs. Semiconducting Determines geometry and diameter Armchair: (n, n) Zig-Zag: (n, 0) If n – m is a multiple of 3, the nanotube is metallic. 1/3 of NT’s are metallic,2/3 are semi-conducting!!!!

10. On/Off Ratio Low Lots of Metallic Tubes Metallic Tubes are the Enemy ISD ISD On/Off Ratio High VG VG Vg Few Metallic Tubes D NT Film V Metallic Tubes Act To Screen Potential from outermost tubes of Film!!! x Si gate SiO2 S

11. Deposition Techniques Want: Uniform Film of individually separated NT’s • Direct Deposition • Drop Casting -- Flocculation due to Van der Waals between tubes limits uniformity. • Spin Coating – work in progress • Langmuir-Blodgett/Quasi-Langmuir-Blodgett • Separate Tubes using Solubilization Agents • Starch/Enzymes • PmPV

12. Quasi-Langmuir-Blodgett Film • DEPOSITION METHOD • 1. Dissolve NT’s in Solvent by Sonication. Single tube dissolution is ideal. For a solvent we used a 10:1 mixture of ortho-xylene and 1,2-dichlorobenzene.

13. Alumina Filter (pore size = 20nm)* Quasi-Langmuir-Blodgett Film • DEPOSITION METHOD • 2. Quickly suck fluid through a porous alumina filter (pore size = 20 nm) * From Whatman website

14. Quasi-Langmuir-Blodgett Film • DEPOSITION METHOD • 3. While film is still slightly damp with solvent, wash water over filter. Film will break off as a “raft” and float to top.

15. Substrate Quasi-Langmuir-Blodgett Film DEPOSITION METHOD 4. Put in substrate and suck out water to redeposit the film.

16. Why 10:1 solvent mixture? • Ortho-xylene:Dichlorobenzene solvent mixture used for 3 reasons • 1) High nanotube solubility ≈ 15 mg/L • 2) Specific Gravity < 1 (so rafts can float) • 3) Immiscibile in water

17. Film on Glass Slide Quasi-Langmuir-Blodgett Film ADVANTAGES DISADVANTAGE • Film fairly uniform over large area • Thickness of Film controllable. (20nm-1um) • Room Temperature Technique vs. CVD at 900o C • Film’s too thick. Can’t get monolayer.

18. 200 nm SEM of Nanotube Film1 Does water Immersion affect Films? Temperature Dependence of Resistivity1 • MECHANICAL • Film consists of well separated ropes of ≈10 nm before and after immersion • ELECTRICAL • Immersion in water does not affect DC resistivity. 1N. Peter Armitage

19. Quasi-LB Film: Are They Uniform? 30 nm thick film 80 nm thick film 50 microns Average “roughness” = 36 nm Average “roughness” = 14 nm “Roughness” = Σ|(xi - xave)|/n *Data taken in UCLA Nanolab using Nano-Or 3-D Scope 2000 for 2D profiles, and Dektak 8 profiler to measure film thickness

20. SiO2 Device Fabrication • Steps to making Device • 5000 Ao SiO2 on doped Silicon • commercially bought, HF remove SiO2 from one side • Deposit NT Film • Evaporate Gold source and drain through shadow masking • Use Silver epoxy to attach wires • Clamp onto metal chuck to apply Vg • Dielectric BreakdownBreakdown Electric Field in SiO2 = 1 x 107 V/cm

21. Measurement Setup p-type in air S D • Output (Vg ) ±100 V quasi-AC Sawtooth Waveform. • Output bias voltage (100mV) across SD, and measure Isd • Measure Voltage across Rprot to get Ileakage SiO2 p type Si back gate 200 V VRprot Rprot Isd Vg Vsd

22. Transistor Characteristics Film = 4000 Angstroms thick (≈400 NT layers) => 2% Modulation!!

23. Transistor Characteristics Vg = 0 Vg = 50 Vg = 100 Blew on Sample Notice the slow drift…Need to stabilize temperature for future measurements

24. Transistor Characteristics Film = 800 Angstroms thick (≈80 NT layers) => 20% Modulation!!

25. Vsd vs Isd

26. Transistor Characteristics Film = 300 Angstroms thick (≈30 NT layers) => 40% Modulation!!

27. Exponentially Better with Film thinness

28. Calculation of Mobility • Quadratic Model of MOSFET: • ISD = (μCoxW)[(VGS – VT)VDS – VDS2/2] for VDS << VGS – VT • Slope of IVg curve = μCoxWVDS Plugging in the numbers yields mobility of 0.9 cm2/V*s Mobility's: Single Carbon NT = 105 Silicon = 102-103 NT Network = 101 Organic Semiconductor = 10-4 - 10-1 High Mobility means device can operate at Higher frequency!!! L L

29. Liquid Gating 50 % Modulation!! AluminaFilter on glass Liquid Gating Setup Data for 1000 Angstrom thick sample1 • Larger Modulation than Bottom Gating => liquid penetrates porous film • Liquid Gating useful for protein/biomolecule detection. 1Data taken with aid of Mikhail Briman

30. Nanotube Reflux in Nitric Acid O Original Nanotube H After 20 Hour Reflux in HNO3 O Carboxylic Acid Group OH O After filtering and rinsing in water At PH 7 Becomes Polar O -

31. Did Refluxing Work? 1 Minute after Sonication 10 Minute after Sonication

32. Did the Reflux Work? 24 Hours after Sonication

33. Future Work • Improve Device Characteristics • Thinner Films • More Dispersed Tubes • Separate Semiconducting and Metallic Tubes • Mobility vs. applied pressure • Improve Probe Station • True AC Setup • Gold Pogo Pin probe/Micrometer positioner • Temperature/Humidity Control Chamber • Characteristics vs. Network Density • Photo-lithographic Mask for micro scale geometries • Study 2-D Percolation problem of random array of semiconducting rods • E-Beam Lithography for nano scale geometries • Protein detection

34. Conclusion • Created NT network transistor using room temperature fabrication process • Film too thick to get good characteristics.

35. Thanks • The Gruner Group: Peter Armitage, M. Briman, Erika Artukovic, Liangbing Hu, George Gruner • The Chemists: Erik Richman, Will Molenkamp (Tolbert); Matt Spotnitz (Kaner). • Steve Franz (Nanolab). • MCTP