universita` degli studi di padova - Dipartimento di Scienze Chimiche

UNIVERSITA’ DEGLI STUDI DI PADOVA
Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati
Part 6.a – Size effects and applications of metal and
semiconductor nanoparticles
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Technological Interest in Metal Nanocrystals









Novel optical effects
Nanoelectronics
Biolabelling
Single electron devices (capacitors, memory
storage)
Polarizers
Shape control
SERS - molecular detection
Catalysis
Plasmonics and Optical Chips
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HRTEM Ag Nanoparticles
“Monodisperse”
6nm diameter,
crystalline silver
nanoparticles in water
produced by radiolytic
reduction. Solution is
bright yellow!
Inset: Shows metal atoms with lattice spacing
of 2.36Å - same as bulk.
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Plasmons
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Metal nanostructures & surface plasmons
Surface plasmons are waves that propagate along the
surface of a conductor. By altering the structure of a
metal’s surface, the properties of surface plasmons—in
particular their interaction with light—can be tailored,
which offers the potential for developing new types of
photonic device. This could lead to miniaturized photonic
circuits with length scales that are much smaller than
those currently achieved. Surface plasmons are being
explored for their potential in subwavelength optics, data
storage, light generation, microscopy and bio-photonics.
WL Barnes, A Dereux and TW Ebbesen, Nature 424, 824 (2003)
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Plasma Equations
d 2x
me 2  eE . Damping term to model loss can be used.
dt
For time harmonic response, E, x ~ e i t
eE
  2 me x  eE  x 
.
2
me
 ( )  1 
ω 2p

2
Ne 2
, ω 
 0 me
2
p
.
Adding the ionic contributi on to dielectric response,
2
  p2 
Ne
2
 ( )   ()1  2  ,  p 
 0 ()me
  
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Dielectric Constant
Plasma Dielectric Function
Normalized Epsilon
1
Reflection
0
Propagation
-1
-2
0
0.5
1
1.5
2
Normalized Frequency
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Volume EM Waves in Metals
 2E 
2
c
2
  E  0 .
E ~ eiz
2

  ( ) 2



p
2

  2     2  ()1  2   2    p2
c
c
c
  
2
2


At    p ,  is purely imaginary, hence E decays exponentia lly.
 A “bandgap” exists for propagation due to the negative
dielectric constant
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Dispersion of EM in Plasma
2
 2p
c
  
1
Negative Dielectric Constant
E & H out of phase, imaginary 

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Surface Plasmon
 Coherent excitation of plasma near the surface of a metal –
coupled to surface EM wave
 These are essentially light waves that are trapped on the
surface because of their interaction with the free electrons of
the conductor (i.e., guided waves)
 SPs help us to concentrate and channel light using
subwavelength structures.
x
propagation
z
Metal
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Dispersion Curves
 Propagation curve in 1 (dielectric medium) does not
cross the surface plasmon dispersion curve

c


c
1


1 2
c 1   2

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SPR excitation techniques
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SPP at planar metal surfaces
 Plasmon-polariton excitation produces absorbance peak
at specific frequency
 Shift in the absorbance spectrum indicates presence of
analyte molecules (change in dielectric refractive index)
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SPR Applications
Chemical
1. Sensing
2. Reaction kinetics
3. Concentration
measurement
Biological
1. Real time sensing
4. Mass Spectrometry
5. Equilibrium properties
2. Detection of binding
reactions
… and many more
3. Proteomics
4. Plasma membrane studies
5. Drug delivery techniques
…
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and many more
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SPR-based commercial products
SPR biosensor Spreeta from Nomadics-Texas Instruments
Applications






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Beverages
Medical diagnostics
Food safety
Security
Water quality
Research
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Localized Plasmon Resonance @ NPs
Coherent excitation of conduction electrons driven by
the E radiation field.
Theoretically modeled by Mie Theory
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Optical Properties
Mie Theory(1908)
9V0 m3 2
2
 abs  
2
2
c


2



1
m
2
    1   i 2 
Drude free electron model
    1
 2p
 2  i 
Empirically
a
 r    0 
r
 Surface Plasmon Resonance is invariant with
respect to the size on the nanoparticle.
 The FWHM scales with the radius of the
particles.
 Assumes spherical particle
 Particle diameter << l/10
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J.H. Hodak, et al. ; J. Phys Chem. B, 104(43), 9954, 2000.
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Different Plasmon Modes
 Bulk Plasmons:
    0
 Surface Plasmons on Flat
Surface:      m
 Surface Plasmons on a Small
Sphere:     2
m
 Surface Plasmons on a Small
Ellipsoid:      L
m
 Intrinsically sensitive to
surface perturbations.
 “Small” means < 30nm.
 L depends on aspect ratio.
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Dielectric confinement and local field amplification
 The value of the local field near the metal nanocrystal is
El 
3
E
  2 m
 A huge amplification of the local field occurs near the
SP resonance at the pole Re    2 m  0
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Surface Enhanced Raman Scattering
Normal Raman Scattering
I NRS  S   N  I  L   Rfree
Surface Enhanced Raman
Scattering (SERS)
R
I SERS  S   N  I  L   A L   A S    ads
2
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Overview of Novel Effects in Metal
Nanocrystals
 Surface plasmon changes due to electron
density.
 Surface plasmon changes due to shells or
adsorbates e.g. biomolecules.
 Surface plasmon changes with particle size.
 Surface plasmon changes with particle shape.
 Single Electron Effects: Coulomb Blockade.
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Surface Plasmon Spectroscopy
0.04
2
0
1.5

Absorbance
(b)
(c)
(a) Absorption spectrum of 6nm dia.
Silver particles (0.1mM)
(b) Predicted difference spectrum
following injection of 5µM electrons
(c) Experimental difference
spectrum using pulse radiolysis
(N2O, 2-propanol)
-0.04
1
(a)
0.5
-0.08
0
-0.12
300
350
400
450
Blue shift occurs due to
Increased electron
concentration.
500
Wavelength [nm]
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Shape Effects: Metal Nanorods
3.0
0.12
3.5
2.5
2.0
1.5
0.00
200
-2
0.04
)
(nm
C
ext
0.08
300
400
500
600
700
(Left) Nanorods absorb two
colours. The colour depends
on the length of the rod. For
silver, almost all colours of
the rainbow are predicted to
be possible.
Wavelength / nm
700
Gans predicted this effect in
1911.
Peak Position (nm)
b = 10 nm
600
Longitudinal
First proper gold rods made
in 1994 to test this.
a
500
b
400
1
Transverse
2
3
Aspect Ratio
4
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Shape Control - Polarizers, Liquid Crystals,
Nanomechanics
Aims: Use shape control to create new materials
with unusual optical properties
Color of Gold Nanorods depends on aspect ratio
and orientation!
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Primitive Alignment of Ag Rods
Stretch a polymer film
with silver rods (10nm
x 40nm) and hold it
under a polarizer
 Focus a laser onto the silver
rods and they melt back into
yellow nanospheres
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Nanoshells for expanding SPR’s
Optical resonances of gold-silica core nanoshells as
a function of their core/shell ratio.
 More sensitive than
simple nanoparticles.
(gain ~r2/r1)
 Resonance frequency
is a strong function of
geometry
 More resonant
frequencies possible
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Nanoshells in bio-applications
Properties:
 Optical activity in bio-compatible wavelengths.
 Strong tunable absorption in NIR region (700-1300 nm)
(maximum light penetration through tissue in NIR)
 Easy conjugation with specific proteins
 Chemical / Photochemical stability
 Biocompatible, non-toxic to tissue (gold)
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NIR photothermal tumor therapy
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Single Electron Devices from NCs
The capacitance of a small particle is
C = Q/V = e/4πeoa in vacuum. To add
an electron to the particle costs an
energy, U = Q2/2C. If U >>kT, then the
usual I-V curve will show jumps. If
these jumps can be distinguished then
the presence of single electrons can
be confirmed, and a storage or logic
device created. Low temps are usually
needed.
An STM can be used to study the flow of electrons through a single
Gold nanoparticle.
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Single Electron Devices from NCs
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Metal-ceramic nanocomposite materials
 For inductive components in high-frequency electronic devices
 Chemical synthesis of Ni-Fe/SiO2, Co/SiO2, Fe-Co/SiO2, Fe/nickelferrite, Ni-Zn-ferrite/SiO2, Fe-Ni/ polymer, and Co/polymer composite
magnetic materials
 Exchange coupling between nanoparticles  large magnetic
permeability
 Cancellation of magnetic anisotropy
 Small parasite currents
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Inframat Corporation, Farmington, CT
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Motivation for Studying Semiconductor
Nanoparticles
Nanocrystals are of tremendous interest because they
(i)
Have size dependent optical, electronic, catalytic and
magnetic properties.
(ii) Quantum size effects provide direct insight into the
validity of current models of bonding and structure.
(iii) Numerous applications require miniaturisation.
(iv) QSE may provide the fundamental limits to Moore’s law.
(v) Such materials are fun to play with!
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Growing CdSe Quantum Dots
2
1
rapidly inject precursor solution
into hot (350o C) trioctylphosphine
oxide
o C-
reached.
Qui ckTime™ and a
Photo - JPEG decompr essor
are needed to see thi s pi cture.
1
dimethyl cadmium
+
trioctylphosphine selenide
+
trioctylphosphine
Absorbance spectra for a batch of CdSe
quantum dots
0. 8
increas ing
time
0. 6
Abs
QuickTi me™ and a
Photo - JPEG decompressor
are needed to see thi s pi ctur e.
heat the reaction mixture at 260
300 o C until desired particle size is
0. 4
heat
0. 2
0
30 0
35 0
40 0
45 0
50 0
55 0
Wavelen gth (n m)
60 0
65 0
70 0
Similar preps for: InAs, InP, CdS, ZnS, ZnSe, CdTe, PbS,
PbSe, ZnO, alloys and core-shells of these materials.
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Typical Absorption and Fluorescence Kinetic Profiles
100
Emission Intensity (norm.)
2
Absorbance
1.5
1
180"
0.5
0
400
5"
500
600
5"
180"
80
60
40
20
700
Wavelength (nm )
0
400 450 500 550 600 650 700
Wavelength (nm )
CdSe nucleation in octadecene at 275oC; Growth at 250oC; oleic
acid/dodecylamine capping agents (L): Absorption (R):
Luminescence vs time
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Radius vs Time
44
2.4
42
2.2
40
2
38
1.8
FWHM (nm)
Mean Crystallite Radius (nm)
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1.6
34
32
1.4
1.2
0
50mM monomer in
Octadecene at 275oC
30
50
100 150 200 250 300
Time (s)
Particle growth is slow, distribution narrows
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Tweaking…

If you make the nucleation go fast enough…

If you can stop the nanocrystals growing…

If you can stop them sticking together…

If you can make them all the same size…
If you can make them as single crystals with
no defects…

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Quantum Size Effect produces
“Artificial Atoms”
CdSe Nanocrystals ranging from 1nm to 6nm in diameter - to distinguish
so many colours, the size distributions must be very narrow. The growth
kinetics must be carefully controlled.
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600
9
8
7
6
300
5
4
3
2
0
1
400 450 500 550 600 650 700
Wavelength (nm)
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CdSe Diameter (nm)
Emission Intensity (a.u.)
Size Control is Critical!
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Wannier excitons in bulk semiconducors
 Bound electron-hole pairs
that split off the conduction
band due to mutual
attractive (Coulomb)
interaction
 Binding energy and degree
of localization depend on
electrostatic screening
e4
2 K 2
Eexciton  EG  2 2 2 
2  n
2M 
n 2 2
aB  rn 
e2
Bohr radius
with 1   1 m  1 m
e
h



and M  me  mh
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Exciton transitions in cuprous oxide
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Weak Confinement a > aB
 We will not discuss this region. For NCs small, but larger than the
exciton radius, the first evidence of QSE is a slight blue shift of the
exciton due to its feeling trapped.
 The exciton energy increases until it reaches the band edge..at this
point, “normal” excitons no longer exist in the particle..
 The shift is not as dramatic as in the strong regime…
Example: CdSe Nanocrystals
HRTEM CdSe Nanocrystal
(Courtesy M Bawendi, MIT)
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Strong Confinement Regime (a < aB)
 Smaller than exciton radius in bulk crystal.
 Treat e,h as separate entities.
 Spherical box.
2
 2  ml
E  Eg 
2me a 2
e
ml
2
 2  ml
E  Eg 
2mh a 2
h
ml
 Coulomb attraction is less than
confinement energy. Varies as 1/a.
Brus showed that lowest optical
transition obeys:
 2 2
 2 2
1.786
E1s 1s  Eg 


2
2
2mh a
2me a
4 0 a
since 10  
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Summary of QSE
Unoccupied
energy levels
4.0
CuCl
3.5
Energy (eV)
heat
Filled energy
levels
ZnSe
3.0
2.5
CdS
2.0
CdSe
1.5
GaAs
1.0
R < 2 nm
R < 3 nm
R > 5 nm
(bulk)
1
10
NanoCrystal Radius (nm)

1 .8e
E ( R )  E ( ) 

2m * R
R
2
g
2
g
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2
43
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Quantum Size Effects produces
“Artificial Atoms”
7 nm
1.5 nm
Nanocrystal Diameter
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Applications - NCs
Tunable LEDs
Tunable lasers
Smart Glasses
Biolabelling
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Biolabelling
Advantages of QDs as biolabels
 More photostable under laser irradiation. Dyes bleach
very quickly.
 Narrower spectra so more colours in any spectral
region.
 Simple excitation spectra, so readily multiplexed with
simple optics. Dyes require multiple lasers in
microscopes or cytometers.
 Common chemistry for all labels.
Note that gold NCs have long been used as biolabels! Today, interest in all
types of NCs as labels: magnetic,metallic, fluorescent, mixtures thereof.
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Challenges and Conclusions
Challenges for Nanocrystal Engineers
 Shape Control, Core-Shell Synthesis and Passivation.
 Monodispersity and Scale-Up.
 Luminescence and Quantum Yield from UV to NIR.
 QSE theory: matching with computational models.
Challenges for NanoTechnologists
 Ordering of NCs on different lengthscales.
 Integration with Top-Down NT processes.
 Creating Functional nanoscale materials and devices.
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Single Nanocrystal Spectroscopy
Temperature (K)
FWHM
300 K
(15 – 18) nm (50
– 60) meV
(0.06 – 0.3) nm
(0.2 – 1) meV
0.04 nm
120 eV
20 K
10 K
(low exct. Int
185W/cm2)
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Confocal microscope
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