CHAPTER 7(Chapter 10 in text)

Nanotubes, Nanorods and Nanoplates

DENOMINATIONS0 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

224 aacU casurf For non-isotropic (non-cubic systems)

Tetragonal unit cell

22

2 24 aa

VU

a

VccaV casurf

For minimum surface energy conditions!

044442

aca

a

V

a

Ucaca

surf

c

a

c

a

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

246 aacU cab

Considering Figure 10.6.a

228 aacU caa

For configuration a to be stable, then:

22 4628 aacaac caca 1a

c

c

a

c

a

c

a

or

Alternatively, for configuration b to be stable:

c

a

c

a

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 structuresWhat 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 crystalsNanotubes 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)

yx OHSiOOAl )()( 2232

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

Al2SiO3(OH)4

Si/Al can adjust diameter

1nm

2nm

Poor strength

Functionalization

Nanostructures related to compounds with layered structuresNanostructures related to compounds with layered structures

As we said reducing energy by generating tubes (e.g. graphite, boron nitride, Sulphides)

CARBON NANOTUBESCARBON NANOTUBES

Lets first talk about graphite and fullerenes

Graphite crystallizes in layered Graphite crystallizes in layered hexagonal structures hexagonal structures (C covalently bonded within each layer)(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 hexagonsNever 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

5.0225.022 )(0783.0)(3

nmmnnmmnad cc

Single walled nanotubes observed with diameters 1.2-1.4nm

Consider the Chiral angle (between e1 vector and c).

]23arctan[

mn

m

Zigzag =zero angleArmchair = 30o

Metallic electrical conductivity obtained:(2n+m)/3=q=integer

5.022 )(0783.0 nmmnd

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=5.7x1010A/m2

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 MoS2, WS2 then selenides of Mo and Tungsten

Layered structures with each layer Consisting of 3 sub-layers

X-Me-X

Nonmetallic ionmetal

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)

Zircon and selenium

Rolling mechanism

Synthesis of Nanotubes

Direct current arc discharge methodDirect current arc discharge method

Complex

Typical voltages 18-30V currents 50-200AGas 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)

Laser ablation techniques

Chemical Vapor Deposition

E.g. Methane + hydrogen+ argon

Ni, Fe nanoparticles

1000-1200KLiquid phase expected for Au-Ge system at that temp

Poisoning

We are talking about a slow process!

SWCNT

Graphite or Si substrate