Transcript Diapositiva 1

Surface reconstructions
Lüth §3.2
Due to coulomb force imbalance, atoms near the surface assume equilibrium positions with different spacing and/or
symmetry from the bulk atoms, creating a different surface structure.
rigid shift of the top layer from the original position
variation of the lattice constant of atoms in the top layer
The general symmetry of a layer might also change, as in the case of the Pt(100) surface, which is reconstructed from a
cubic to a hexagonal structure.
Reconstructions can also be induced or altered by the
adsorption of other atoms onto the surface as the interatomic
forces are changed.
The reconstruction of a surface with adsorption can depend on:
1. The composition of the substrate and of the adsorbate
2. The coverage of the substrate surface layers and of the
adsorbate (namely, how many monolayers are present)
3. The ambient conditions (i.e. temperature, gas pressure,
etc.)
AFM image of Si(111)
reconstructed surface
Lüth Ch. 5
Surface
Phonons
We refer to the ideal surface,
with neither reconstruction
nor relaxation
k ||  g 
Phase velocity (red): angular speed
group velocity (green): speed of the envelope
1 2
g
2
Surface Reflectivity
and surface plasmons
Novotny Ch. 12
Lüth Ch. 5
Metals look grey until intra-band transitions are considered. If not:
Inter-band transitions
Gold absorbs green/blue photons (2.75 eV),
And reflects yellow ones.
These are bulk properties…
Copper absorbs green photons (2.1 eV),
and reflects orange/red ones.
Bulk phonon polaritons
Reflectivity
VACUUM-DIELECTRIC
INTERFACE
VACUUM-DIELECTRIC
INTERFACE
These solutions
Evanescent waves
are partly reflected inside the dielectric
waves
(<0, high reflectivity)
No solutions
w
Ibach Ch. 7
Total internal
reflection
from inside
dielectric
(SEMI)METAL-DIELECTRIC
INTERFACE
Dissipation included
Surface Plasmon Polariton electric field
wp is at 1.0 in ordinate!
(silver)
Free electron
Surface polariton dispersion for free electrons + inter-band transitions
Fuchs-Kliewer SPhP
Red: surface-plasmon-polariton (DE=40 meV)
Blue: surface-phonon-polariton (DE=37 meV)
Loss peak position
wp 
ne 2
m 0
With varying doping
or adsorbate density
we have that:
nphonon is constant
nelectron varies
Moreover,
Phonon-like
Plasmon-like
(Log scale!)
EELS and HREELS
Lüth Panel IX (Ch. 4)
(Ch. 4 Dielectric Theory
EELS: primary energy of the electron beam: 20÷500 eV (but <200 eV for surf. analysis)
HREELS: (High Resolution EELS): energy below 20 eV
Scattering on adsorbed atoms
In order to have momentum conservation K=Dk = kmode
The activated/destroyed mode of the adsorbate must possess a wavevector parallel to Dk.
If e.g. bending oscillations are involved, the scattering is off-specular:
Surface plasmon polariton excitation
Novotny Ch. 12
SPP reflectivity curves
Sensitivity to adsorbates
Localized plasmons
Novotny Ch. 12
Confinement effects in nanogold particles
Electric field distribution
for a
nanogold/dielectric system
Estinction(a)=Absorption-a3+Scattering-a6
“a” being the particle radius
Lycurgus’cup
DIATHRETE CUP