Nano-Electronics and Nano-technology

1. Nano-Electronics and Nano-technology A course presented by S. Mohajerzadeh,Department of Electrical and Computer Eng,University of Tehran

2. Carbon structures

3. Fullerene • C60, a type of carbon arrangement with 60 carbon atoms placed in 1nm lattice separation. • Discovery: 1985 by Bukminister Fuller. • 12 pentagonal and 20 hexagonal shapes. • Fullerene can be doped (26%) by alkali atoms (sodium) because its empty space is that much.

4. Fullerene Total: 10,000 publications! = 2,000 PhD students?!

5. Multi-wall and single-wall tubes • Transmission electron micrograph of single-wall CNT, (bundles of CNT’s) • Schematic diagram of single-wall tube

6. Multi-wall tubes

8. Physical characteristics • Single wall nanotubes: 1 – 5 nm diameter • Types of nanotube formation: Armchair, Zigzag, Chiral • Multi-wall tubes 2-50 nm concentric tubes, ID : 1.5 – 15 nm, OD : 2.5 – 30 nm • 100 times stronger than steel, r = 1/6 (1.3 – 1.4 g/cm3) • Strong, lightweight materials • kCNT = 2000 (Copper 400) W/m.K • Transmission of heat is better than diamond

9. Chirality vector • Although the fabrication of nanotubes is not by rolling the graphite sheets, they are modeled by this phenomenon; • “Ch” or Chirality vector or circumferential vector is the translation vector of graphite plane onto nanotube. • Axis vector is “T” which is perpendicular to chilarity vector “Ch” and shows the tube axis. • Ch= na1 + m a2 where “a1” and “a2” represent the main constructing vectors of graphite sheet.

10. Chirality vectors

11. Electrical properties • Semiconductor, metallic behavior If n-m=3q then metallic • Armchair structures, metallic, • Chiral and Zigzag structures, semiconductor: • Band gap depends on the diameter • Reducing the diameter leads to higher band gaps.

12. Mechanical properties • Nanotubes are very strong materials. • If a wire of area A is stressed by a weight “W”, the level of stress is S=W/A, • Strain is defined as: ε=ΔL/L and S=E ε • ε is called: Young’s module and it is 0.21TPa for nanotubes!!, 10 times more than steel! • 1 TPa is equivalent to 10millions atmospheric pressure!! • If we bend the tubes, they act like straws, but come back to their original status, self-repairing! • When the tube is severely bent, the “sp2” structure converts onto “sp” orbitals and once the pressure is removed, sp2 orbitals are reconstructed. • Tensile strength is the measure of how much force is needed to take apart a material. • For nanotubes, tensile strength is 45 billion Pascal (GPa) whereas for steel it is only 2GPa!

13. Characterization methods • SEM • TEM • Raman (interaction of incoming light with solid vibrations) • SPM (AFM , STM ,…) • XRD (X-ray diffraction) similar to electron diffraction • TPO, TGA (temperature programmed oxidation) and (thermal gravimetric analysis) • Electrical characterization

14. Applications Electronics Hydrogen storage, Chemical Sensors Fuel Cells Nano-transistors, nano-structures Application in STM Composite materials, Catalysts 4.2, 8, 300 (!)wt% of hydrogen in CNT at 25oC

15. Nano-wires

16. Single electron behavior • FET structure at below 1degree Kelvin! • Electron-by-electron transport through the nanotube, step-wise response

17. Nano-transistors

18. Photonic crystals • Similar to atomic periodicity, a structure with matter periodicity is created to form a band-gap for optical wavelengths. • Only at certain wavelengths, standing waves can be created and at some other wavelengths, transmission is prohibited

19. Field emission devices • Each sharp tip of nanotube acts as a field-emitter device. • The emitted electrons hit the top electro-luminescent material (like ZnS). • Pixels are clusters of nanotubes • Standard micro-meter photo-lithography, • Large area applications • Stable structures are needed for a reliable application

20. Hydrogen storage • Computer simulations of Adsorption of hydrogen ( ) in trigonal arrays of single-walled carbon nanotubes ( )

21. Fabrication (growth) Techniques • Direct current arc-discharge between carbon electrodes in an inert-gas environment • Laser Ablation or Pulsed Laser Vaporization (PLV) • Plasma Enhanced CVD • Catalytic Chemical Vapor Deposition (CVD) CCVD High-pressure CO conversion (HiPCO)

22. Carbon Arc-discharge method • Carbon Atoms are evaporated by a plasma of Helium gas that is ignited by high currents passed through opposing carbon anode and cathode

23. Carbon Arc Discharge

24. CNT by Carbon Arc Discharge • Basic Process • A vacuum chamber is pumped down and back filled with some buffer gas, typically neon or Ar to 500 torr • A graphite cathode and anode are placed in close proximity to each other. The anode may be filled with metal catalyst particles if growth of single wall nanotubes is required. • A voltage is placed across the electrodes, • The anode is evaporated and carbon condenses on the cathode as CNT

25. Pulsed Laser Vaporization /Ablation • Used for the production of SWNTs • Uses laser pulses to ablate (or evaporate) a carbon target • Target contains 0.5 atomic percent nickel and/or cobalt • The target is placed in a tube-furnace • Flow tube is heated to ~1200°C at 500 Torr • 10-200 mg/hr depending on the laser power density

26. Low temperature Low Pressure DC, RF:13.56MHz Microwave:2.47GHz Reactiing gas CH4 ; C2H4 ; C2H6 ; C2H2 ; CO Catalytic metal (Fe, Ni, Co) Plasma CVD Gas inlet Substrate Power suplly Gas outlet

27. High-pressure CO conversion (HiPCO) • New method of growing SWNT • Primary carbon source is carbon monoxide • Catalytic particles are generated by in-situ thermal decomposition of iron penta-carbonyl in a reactor heated to 800 - 1200°C • Process is done at a high pressure to speed up the growth (~10 atm) • Promising method for mass production of SWNTs

28. Chemical Vapor Deposition • Involves heating a catalyst material to high temperatures in a tube furnace and flowing a hydrocarbon gas through the tube reactor. • The materials are grown over the catalyst and are collected when the system is cooled to room temperature. • Key parameters are: • Catalysts support active component • Source of carbon • Operational condition simplicity of apparatus Absolute advantage in Mass Production

29. CVD technique

30. Catalyst: • Support: • Silicon substrates • Quartz substrates • Silica • Zeolites • MgO • Alomina • Active components : • Transition metals i.e.: Co , Fe, Ni / Mo (or oxides of them)

31. Nanometric islands

32. Catalysts effect

33. Sources of carbon: • Carbon monoxide • Hydrocarbons: • Methane • Ethylene • Acetylene • propylene • Acetone • n-pentane • Methanol • Ethanol • Benzene • Toluene , …

34. Operational condition: • Temperature: 600-1100 oC • Pressure: 1-10 atm • Reaction time: 0.5-3 h • Dilutent gas: He, Ar, H2 • Resident time of gases: Volume fraction ( partial pressure) Flow rate

35. Carbon products • Vertical growth, random growth, • Wall thickness in the case of multi-wall growth • Single-wall (shell) nanotube (SWNT) • Multi-wall (shell) nanotube (MWNT) • Graphitic form of carbon • Amorphous form of carbon • selectivity of SWNT & MWNT

36. Carbon Nanotubes, Production by Catalytic Chemical Vapor Deposition (CCVD) • SWNT-reinforced composites needs tons of CNT per year • Laser vaporization and arc discharge: g’s/day SWNT • Carbon source: CO & HC’s: CH4 , C2H2-6 , C6H6 • Conditions: 700-1000 oC, 1-5 atm • Catalyst formulation: Co/Fe/Ni-Mo on SiO2 , zeolite, … • Quantification of SWNT: SEM , TEM, AFM, Raman, TPO • Purification steps: • Caustic to remove silica • Acid to remove metals

37. Carbon Nanotubes CO deposition on Co-Mo/Silica

38. Carbon Nanotubes Characterization-Quantification AFM

39. Carbon Nanotubes Raman characterization Graphite SWNT Disordered C

40. CCVD CNT Cat. & Reaction Eng. Lab. 1mm 20 Kx

41. Storage of Gases • Hydrogen storage • Average storage capacity: at least %8 wt. • 100 km = 1.2 kg H2= 13,500 L(gaseous) For 500 km : 6 kg H2 100 kg CNT CNT 1.2 kg/lit 84 lit. CNT ( 3.1 kg !?) (DOE)