Transcript Chapter 7
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)
U surf
4
a ac
2
c a
2
V
a
2
c
c
V a
2
U surf
4
a V a
2
c a
2
For minimum surface energy conditions!
U surf
a
4
a V a
2 4
c a
Tetragonal unit cell
4
a c
4
c a
0
a c
a c
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.a
U a
8
a ac
2
c a
2
Considering Figure 10.6.b
U b
6
a ac
4
c a
2
For configuration a to be stable, then:
8
a ac
2
c a
2 6
a ac
4
c a
2
a c c a
1
or
a c
a c
Leads to rods!
Alternatively, for configuration b to be stable:
a c
a c
Leads to the formation of platelets
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) 2nm
Al
2
O
3 (
SiO
2 )
x
(
H
2
O
)
y
1.3
1nm Crystallize as tubes (2-5nm dia.) Here aluminum can be substituted by Fe, Mg, Mn Al 2 SiO 3 (OH) 4 Si/Al can adjust diameter Influence color and diameter
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
Based on chirality vector can determine nanotube diameter
d
3
a c
c
(
n
2
m
2
nm
) 0 .
5 0 .
0783 (
n
2
m
2
nm
) 0 .
5
Single walled nanotubes observed with diameters 1.2-1.4nm
Consider the Chiral angle (between e1 vector and c).
arctan[
Zigzag =zero angle Armchair = 30 o
3 2
n m
m
]
Metallic electrical conductivity obtained :(2n+m)/3=q=integer
d
0 .
0783 (
n
2
m
2
nm
) 0 .
5
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 Current density=5.7x10
10 A/m 2 Assuming 1.5nm dia.
Made by removing caps through oxidizing environment
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 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)
Laser ablation techniques
Chemical Vapor Deposition E.g. Methane + hydrogen+ argon Ni, Fe nanoparticles
1000-1200K Liquid phase expected for Au-Ge system at that temp Poisoning
We are talking about a slow process!
SWCNT
Graphite or Si substrate