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Nano materials

VIEWS: 5 PAGES: 14

									                                 A LST
                                I MEN
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                            ERNFINE
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 ics




                          AT CO
     35




                        MANTUM
       5




               N O F QU
           N A JOYS O
               E
             TH
WHERE WE’VE BEEN
symmetry × bonding in solids × crystal structures and diffraction ×
band theory × metals × semiconductors × semiconductor devices ×
phonons × magnetism × superconductivity
                             Why Nano
Small things are different.
Nanoscience would be boring if small things were just like big things.
Luckily they are not.
 The color of gold changes with sizes




Graphite, for example, takes on interesting shapes if it is kept from
becoming a big solid.




              graphite                       buckyball          nanotube



              The goal of nanoscience is to find and understand
                  how physical properties change with size.
LET’S LOOK AT THREE ASPECTS OF THE NANOSCALE

• Number of possible vibrational states, or electronic states,
  is greatly reduced.
• Small structures have a large ratio of surface area to
  volume than macroscopic objects.
• 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.
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.


                     vacuum
thin metal
     film of
 thickness
           d
                    substrate


                                For electrons in the thin film:
QUANTUM CONFINEMENT
Infinite well solution:




                          Better solution:
QUANTUM CONFINEMENT
Quantum Dots
The smallest energy for the formation of an electron-hole pair
is




                  Semiconductor quantum dots (QDs) possess size tunable
                  fluorescence and absorption properties.
SURFACES AND INTERFACES
SURFACES AND INTERFACES

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.




                  Surface Localized State:
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 180 o
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
mm.
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
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

								
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