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

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

NANOPLATES, e.g. Gold nanoplates

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

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.b Considering Figure 10.6.a

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

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)).

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!

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

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) 1.3<x<2, 2.5<y<3 Crystallize as tubes (2-5nm dia.) Here aluminum can be substituted by Fe, Mg, Mn Influence color and diameter Al 2 SiO 3 (OH) 4 Si/Al can adjust diameter 1nm 2nm

Poor strength

Functionalization

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.

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)

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

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

Let us observe the structure of a graphene sheet

0<  m  <n

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

Nanotubes closed with fullerene halves/caps

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

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 Made by removing caps through oxidizing environment Current density=5.7x10 10 A/m 2 Assuming 1.5nm dia.

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

Distance <100microns

May replace TV sets and comp monitors

Nanotubes and nanorods from non-carbon materials In principle, materials crystallizing in layered structures can form nanotubes and fullerene type structures Initially MoS 2, WS 2 then selenides of Mo and Tungsten  Layered structures with each layer Consisting of 3 sub-layers X-Me-X Nonmetallic ion metal Hence MoS 2 and WS 2 are used as solid lubricants like graphite (due to similar type bonding within layers and between layers) BN is another option (non conductive though)

Zircon and selenium

Rolling mechanism

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

Laser ablation techniques

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

K Liquid phase expected for Au-Ge system at that temp Poisoning

We are talking about a slow process! SWCNT

Graphite or Si substrate