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Nano materials. The Joys of quantum confinement. Physics 355. Where we’ve been. symmetry bonding in solids crystal structures and diffraction band theory metals semiconductors semiconductor devices phonons magnetism superconductivity. - PowerPoint PPT Presentation
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NANO MAT
ERIALS
THE JO
YS O
F QUANTU
M CONFIN
EMENTPhysics 355
WHERE WE’VE BEEN
symmetry bonding in solids crystal structures and diffraction band theory metals semiconductors semiconductor devices phonons magnetism superconductivity
Nanoscience would be boring if small things were just like big things.Nanoscience would be boring if small things were just like big things.Luckily they are not. Luckily they are not.
The color of gold changes with sizesThe color of gold changes with sizes
The goal of nanoscience is to The goal of nanoscience is to find and understandfind and understandhow physical properties change with size.how physical properties change with size.
graphitegraphite buckyballbuckyball nanotubenanotube
Graphite, for example, takes on interesting shapes if it is Graphite, for example, takes on interesting shapes if it is kept from becoming a big solid.kept from becoming a big solid.
Small things are different.
Why Nano
LET’S LOOK AT THREE ASPECTS OF THE NANOSCALE
1. Number of possible vibrational states, or electronic states, is greatly reduced.
2. Small structures have a large ratio of surface area to volume than macroscopic objects.
3. Ferromagnetism is different on the nanoscale than in the bulk.
QUANTUM CONFINEMENT
In our previous work on phonons, we calculated phonon spectra for essentially a near infinite network of atoms. In reality, these systems are finite. We solve this problem by having periodic boundary conditions.
Numerical (symbols) and analytical (lines) phonon dispersion curves in the first Brillouin zone of a monoatomic atomic chain with N = 16, m = 1, a = 1, and k = 1.
2 0,1,2,3,...
nq n
aN
QUANTUM CONFINEMENT
When the number of atoms is relatively small, the number of allowed states is reduced. Instead of having a quasi-continuum, we have a set of discrete states.
These states are separated by significant energy amounts – this is the essence of quantum confinement.
substrate
thin metal film of
thickness d
vacuum
( ) ( ) ( , )r z x y
For electrons in the thin film:
2 2
2xy
xye
kE
m
QUANTUM CONFINEMENT
( )
1,2,3...
z zik z ik z
z
z Ae Be
k d n n
2
then, 2
zk
d n
2 2 2 2 2
22 2z
ze e
k nE
m d m
Infinite well solution:
Better solution:
2 1,2,3...z I Vk d n n
QUANTUM CONFINEMENTQuantum DotsThe smallest energy for the formation of an electron-hole pair is
Semiconductor quantum dots (QDs) possess size tunable fluorescence and absorption properties.
2 2 2
min 20
1 1.8
42g
eE E
rr
SURFACES AND INTERFACES
Bloch Wave
( ) ( ) ikrk kr u r e
SURFACES AND INTERFACES
( )k( ) ( ) zk zi rk kr u r e e
k( ) ( ) i r i zk kr u r e e
Surface Localized State:
In bulk ferromagnetic materials, the energy required to flip one magnetic moment is on the order of the exchange energy, kBTCurie.
This is true for nano-particles as well.
MAGNETISM ON THE NANOSCALE
In bulk ferromagnetic materials, the energy required to flip one magnetic moment is on the order of the exchange energy, kBTCurie.
This is true for nano-particles as well.
Iron Nanoparticles
BLOCH WALLS
The drawing shows a ferromagnetic material containing a 180o domain wall (center). On the left, the magnetic moments are aligned downward. The hypothetical wall structure is shown if the spins reverse direction over Na atomic distances In real materials, N: 40 to 104. The thickness is typically 0.5 m.
MAGNETISM ON THE NANOSCALE
Assuming no external field, the energy required to rotate the entire magnetic moment of a small particle is
Unfortunately, we do not know Binside. But, it turns out that it is dependent on the shape of the particle and its order of magnitude is µ0M, so we make the approximation that
insideE VMB
20E V M
MAGNETIC PROPERTIES OF IRON NANOPARTICLES
coercivity is the magnetic-field strength necessary to demagnetize a ferromagnetic material that is magnetized to saturation. It is measured in A/m, or traditionally in Oersted. 1 Oe = 79.578 A/m