CHAPTER 7
(Chapter 10 in text)
Nanotubes, Nanorods and Nanoplates
DENOMINATIONS
0D
1D
2D
Nanoparticles
Nanotubes
Nanoplates
Fullerenes
Nanorods
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
V
2
V  a c  c  2  U surf  4 a  2 c a
a
a
2
For minimum surface energy conditions!
U surf
a
Tetragonal unit cell
V
 4 a 2  4 c a  4 a c  4 c a  0
a
a a

c 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
Ub  6 a ac  4 c a
2
For configuration a to be stable, then:
8 a ac  2 c a  6 a ac  4 c a
2
2
 ac
1
 ca
Leads to rods!
Alternatively, for configuration b to be stable:
a a

c c
Leads to the formation
of platelets
or
a a

c c
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
Al2O3 (SiO2 ) x ( H 2O) y
1.3<x<2, 2.5<y<3
1nm
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
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
d
3

ac c (n  m  nm)
2
2
0.5
Single walled nanotubes observed with
diameters 1.2-1.4nm
Consider the Chiral angle
(between e1 vector and c).
m
  arctan[ 3
]
2n  m
Zigzag =zero angle
Armchair = 30o
Metallic electrical conductivity
obtained:(2n+m)/3=q=integer
 0.0783(n  m  nm)
2
2
0.5
d  0.0783(n2  m2  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=5.7x1010A/m2
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 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)
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