CHAPTER 7 (Chapter 10 in text) Nanotubes, Nanorods and Nanoplates

1. CHAPTER 7 (Chapter 10 in text) Nanotubes, Nanorods and Nanoplates

2. DENOMINATIONS 1 D Nanotubes Nanorods 0 D Nanoparticles Fullerenes 2 D Nanoplates LASER: Light Amplification by Stimulated Emission of Radiation

3. NANOPLATES, e.g . Gold nanoplates

6. CONDITIONS FOR THE FORMATION OF RODS AND PLATES For non-isotropic (non-cubic systems) For minimum surface energy conditions! Tetragonal unit cell This means that since for the cubic system surface energy a and c are equal (since they are symmetric)  a=c (true!)

7. This gives us a great opportunity. If we can modify the surface energies of certain lattice planes for example through preferential attachment of surface active compounds we can influence the surface energy ratio and thereby influence the shape! How about under an agglomerated state? (which configuration has the minimum surface energy?) Considering Figure 10.6.a Considering Figure 10.6.b

8. For configuration a to be stable, then: or Leads to rods! Alternatively, for configuration b to be stable: Leads to the formation of platelets

9. Agglomerates of nanorods reduce energy by increasing aspect ratio. While nanoplates reduce energy by decreasing aspect ratio Hence we have control: using surface active molecules carefully selected to modify the surface energies we can form rods or plates (even for cubic materials (e.g. gold)).

11. What about layered structures Imagine layers held together by van der Waals forces At circumference we have unsatisfied (dangling) bonds. These have negligible effects for large plates For nano it is another story!! The like to satisfy them by curling to make cylinders or tubes! Hence compounds crystallizing in layered structure have tendency to form nanotubes!

12. If no time is allowed to form tubes! Other things happen, they simply join together! As seen in figure 10.8!

13. One-dimensional crystals Nanotubes can also be produced by selecting compounds that crystallize only in one dimension. Not a lot of them!! e.g. silicates called ALLOPHANES (short range ordered aluminosilicates) 2nm 1.3<x<2, 2.5<y<3 1nm Crystallize as tubes (2-5nm dia.) Al2SiO3(OH)4 Si/Al can adjust diameter Here aluminum can be substituted by Fe, Mg, Mn Influence color and diameter

14. Poor strength

15. Functionalization

16. Nanostructures related to compounds with layered structures As we said reducing energy by generating tubes (e.g. graphite, boron nitride, Sulphides) CARBON NANOTUBES Lets first talk about graphite and fullerenes Graphite crystallizes in layered hexagonal structures (C covalently bonded within each layer) This satisfies only three electrons but what about the fourth (delocalized across layer) Hence graphite electrical conductor parallel to layers and insulator perpendicular to it.

17. Within layer very strong (covalent), across very weak van der Waals forces, hence can cleave to form individual layers (called graphenes) 2D structures Fullerenes are combination of hexagons and pentagons When these gaps close you get 3D structures The combination of a large number of these structures leads to spherical shapes (polyeders)

18. 12 pentagons and 20 hexagons Never experimentally found smaller stable ones The soccer ball molecule

19. Can attach molecules to Fullerene surfaces They also appear in many layers as aggregates (nested fullerenes) or onion molecules. Comprising only of pentagons

20. Let us observe the structure of a graphene sheet

21. 0<m<n

22. Based on chirality vector can determine nanotube diameter Single walled nanotubes observed with diameters 1.2-1.4nm Consider the Chiral angle (between e1 vector and c). Zigzag =zero angle Armchair = 30o Metallic electrical conductivity obtained:(2n+m)/3=q=integer

24. Nanotubes closed with fullerene halves/caps

25. Due to stiffness and small diameter = ideal for use as tips scanning force or scanning tunneling microscopes

26. Nanotubes can make great electron emitters (usually needs sharp tips hence needing less operating voltage). Electric field at tip controls electron fields Emission. One would think to use SWCNT but so far practically easier (availability) to use MWCNTs Current density=5.7x1010A/m2 Assuming 1.5nm dia. Made by removing caps through oxidizing environment

27. They can be superior to tungsten tips (more stable, better oxidation resistance)

28. Distance <100microns

29. May replace TV sets and comp monitors

33. Nanotubes and nanorods from non-carbon materials In principle, materials crystallizing in layered structures can form nanotubes and fullerene type structures • Initially MoS2, WS2 then • selenides of Mo and Tungsten • Layered structures with each layer Consisting of 3 sub-layers X-Me-X metal Nonmetallic ion Hence MoS2 and WS2 are used as solid lubricants like graphite (due to similar type bonding within layers and between layers) BN is another option (non conductive though)

34. Zircon and selenium

37. Rolling mechanism

38. Synthesis of Nanotubes Direct current arc discharge method Complex Typical voltages 18-30V currents 50-200A Gas pressure ~50-500 torr Advantage = does nor require the presence of catalyst Since Soot is also produced often followed by oxidation at high Temperatures (1000-1100 K)

39. Laser ablation techniques

40. Chemical Vapor Deposition E.g. Methane + hydrogen+ argon Ni, Fe nanoparticles

44. 1000-1200K Liquid phase expected for Au-Ge system at that temp Poisoning

45. We are talking about a slow process! SWCNT

48. Graphite or Si substrate