enhanced nonlinear-optical interactions in silicon nanocrystal

Lecture 1
LINEAR OPTICAL EFFECTS IN
NANOSTRUCTURED SOLIDS
Pavel Kashkarov
M.V. Lomonosov Moscow State University,
Physics Department
Russian Research Center “Kurchatov Institute”
Oulu University
August 28, 2008
Outline
I.
Introduction
II. Photonic crystals and anisotropic
layers based on porous silicon
III. Silicon nanocrystals in dielectric matrix
for optoelectronic applications
IV. Silicon nanocrystals for biomedical
applications
V. Conclusions
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August 28, 2008
Photonic crystals and anisotropic layers
based on porous silicon
1.
The idea that nanostructuring of homogenous
and isotropic media results in completely new
optical properties was suggested many years
ago (see excellent textbook “Principles of
Optics” by M. Born and E. Wolf). But at that time
there was not appropriate technology to
fabricate nanostructured materials.
2.
There were suggested two types of structures
with form anisotropy: parallel layers and
periodically arranged nanocylinders. Both
structures show a strong birefringence.
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August 28, 2008
Form Birefringence
Lord Rayleigh
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August 28, 2008
... Photonic crystals and anisotropic layers
based on porous silicon
3.
4.
5.
Only recently an electrochemical technique was
applied to fabricate such structures on
semiconductor substrate (mainly silicon).
Another type of an artificial medium is a
photonic crystal. It turns out that it also can be
easily made by the electrochemical treatment of
semiconductors.
The
obvious
advantage
of
porous
nanostructured media is the possibility to
change the optical properties of the sample by
filling pores by different gaseous, liquid and
solid substances.
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August 28, 2008
Electrochemical nanostucturing of
semiconductors
quantumwires
cross-section of
wires is 1-5 nm
HF
solution
c-Si
thicknessof porous
layer is1-100
 m
metal plate
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August 28, 2008
Different morphologies of porous silicon
Preferential pore growth in <100> crystallographyc direction
microporous
(100) surface, =10 cm
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mesoporous
(110) surface, =3 mcm
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Tailoring of the Refractive Index by
Nanostucturing
80
Porosity (%)
1.6
70
1.8
60
2.0
2.2
50
2.4
40
0
Refractive index
1.4
p-Si (75 mcm)
Effective media approximation
(Bruggeman model )
eff of the disordered heterogeneous mixture:
P
 d 
eff
 d  2
eff
 (1  P)
 Si  
eff
 Si  2
eff
0
2.6
50
100
150
200
250
2
Current Density (mA/cm )
300
where P is porosity
refractive index:
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n


eff
August 28, 2008
Porous Silicon Based Photonic Crystals
c -S i

Ideal Bragg reflector
Bragg condition:
2
Current density (mA/cm )
n1d1+n2d2=/2
100
12 pairs
n1=1.3, n2=2
80
60
40
20
0
0
10 20 30 40 50 60 70 80 90 100
time (sec)
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August 28, 2008
1D Photonic Crystal Based on
Porous Silicon
d2
d1
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n2
n1
August 28, 2008
Linear Optical Properties of Photonic
Crystals Based on Porous Silicon
Wavelength (m)
1,2 1 0,8
0,6
0,4
1,0
(A)
Reflectivity
0,5
0,0
1,0
(B)
0,5
0,0
1,0
(C)
0,5
0,0
8
11
14
17
20
3
23
26
-1
Wavenumber ( 10 cm )
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August 28, 2008
Strong Optical Anisotropy in (110)
Porous Silicon
(1  p )
 eff
1   eff , ii
 2   eff , ii
p
0
 Li (1   eff , ii )
 eff  Li ( 2   eff , ii )
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Generalized
Bruggeman
model
August 28, 2008
Refractive index
Effect of Porosity on Birefringence of
Nanostructured Silicon Films
For layers prepared
at j =100 mA/cm2 :
2.0
1.8
no
1.6
Δn=0.24
<n> = (no+ne)/2 = 1.3
δn=Δn/<n> = 0.18
1.4
ne
1.2
0.4
no-ne
0.3
0.2
80%
0.1
65%
porosity
0.0
medium
Crystalline Si
Iceland spar
Por-Si (110)
Δn
5 10-6
0.15
0.24
60%
25
50
75
Current density, mA/cm
100
2
Nanostructured Si films have large birefringence value δn =18%
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August 28, 2008
Polarization Tunable Photonic Crystals
Reflection spectra
E ^ [001]
E || [001]
Reflectance
1.0
12 pairs
0.8
0.6
0.4
0.2
0.0
1000
1200
1400
1600
Wavelength (nm)
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August 28, 2008
Oxidized Porous Silicon
X-ray diffraction
c-Si
Por-Si
Oxydized
por-Si
• Thermal oxidation of birefringent por-Si film results in
formation of chemically stable, transparent for visible
radiation medium, which possesses optical
anisotropy.
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August 28, 2008
Birefringence of Oxidized Porous Silicon
950 oC 2.5 h
Transparent birefringent
film
0.025
|no - ne|
0.020
0.015
0.010
0.005
10
20
30
40
50
60
70
80
90
2
Current density (mА/сm )
Oxidized por-Si film on a mirror
under a polarizer.
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In oxidized por-Si
birefringence is 2 times
higher than in quartz
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Silicon nanocrystals in dielectric
matrix for optoelectronic applications
1. Silicon was, is and will be the main material for microelectronics,
but application of silicon in optoelectronics is limited.
2. Low probability of radiative electron transition in Si can be
increased by formation of nanoparticles or/and by an introduction of
activators of luminescence.
3. A Er3+ ion possesses rather promising properties as a
luminescence activator in Si. The Er3+ luminescence line
(λ=1.54µm) corresponds to a maximum of transmission in the
quartz fiber waveguides.
4. For formation of light emitting device we combined both
approaches
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August 28, 2008
Light absorption in semiconductors
(interband transitions)
1. Direct gap materials (GaAs, InP, CdTe, CdS )
Е
Е
L
GaAs

f
Eg=1.52 эВ
h  Eg
Eg
0–

i
0
p
p2
p2
Ec ( p)  Eg  * , Ev ( p)   *
2me
2mh
<111>
<000>
<100>

p
h  Ec  Ev

 
ph  pc  pv  0
ph  h /  (  104  103 cm )
pc ,v  h / a0 (a0  107 cm )
pc ,v  ph
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August 28, 2008
Light absorption in semiconductors
(interband transitions)
2. Indirect gap materials (Ge, Si, GaP )
Е
h  Ec  Ev

 

ph  pc  pv  p phon
i′
I
f
Eg1
Eg
h  Eg
i
Eg2
II
f′

p
3. Heisenberg uncertainty principle
p   x   / 2
p   / 2d , where d is a nanoparticle size
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August 28, 2008
Quantum size effect
E
e  h / pe ; ne / 2  d , n  1,2,... 
E3
pe2
h 2n 2
pe  hn / 2d ; Ee 
 2 *
*
2me 4d me
h 2n 2  1
1 
 *  * 
E  Ee  Eh 
2 
4d  me mh 
E g  E g 0  E
E 2
E1
0
d
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z
August 28, 2008
Quantum size effect
2D structure
E
h  E c  E v
m2
Quantum size effect enables
one to modify absorption and
luminescence spectra of
semiconductor nanoparticles
m 1
E e1
Eg
E g0
E h1
The
effect
becomes
appreciable when the size of
a particle is less then 10 nm
m 1
m2
px , p y
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August 28, 2008
Size-controlled Si nanocrystals in nc-Si/SiO2
Superlattices
3nm
Preparation Details:
M. Zacharias et al., APL 80, 661 (2002).
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1.
Alternating evaporation of SiO powder
in vacuum 10-7 mbar or in oxygen
atmosphere under oxygen partial
pressure of 10-4 mbar. This changes the
stoichiometry x of SiOx alternatively
between 1 and 2.
2.
SiO/SiO2 superlatticies are
characterized by the thickness of the
SiO layers varied between 1 and 3 nm
and the thickness of SiO2 layers
between 2 and 3 nm. The number of
periods varied between 30 and 90.
3.
The evaporated samples were annealed
at 1100 oC under N2 atmosphere. Thus
nc-Si/ SiO2 superlattices were
obtained.
4.
Er doped nc-Si/SiO2 superlattice were
produced by implantation with Er ions
(energy 300 keV, doses 1014 – 5·1016
cm-2) followed by TA at 900 oC for 5-60
minutes.
Er
August 28, 2008
Norm alized PL Intensity
Size Dependent PL of nc-Si/SiO2
Structures
d S iO = 6 ... 2 nm
1.0
1.2
1.4
1.6
1.8
2.0
Photon Energy (eV)

Size of Si nanocrystals is controlled by initial thickness of SiO layer in
SiO/SiO2 superlattice

The peak position of PL spectrum is determined by quantum
confinement and excitonic effects in Si nanocrystals in SiO2 matrix.
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August 28, 2008
Effect of Er doping on PL of nc-Si/SiO2
PL Intensity (arb. un.)
4
10
4
4
I13/2 - I15/2
4
I11/2 - I15/2
4
4
I9/2 - I15/2
4
F9/2 - I15/2
hexc= 3.7 eV
T= 300 K
nc-Si/SiO2
0
4
Er3+
dSi = 3 nm
10
-1
10
-2
10
nc-Si/SiO2
nc-Si/SiO2:Er
a-SiO2:Er
1.5 m
(0.8 eV)
-3
0.8
1.0
1.2
1.4
1.6
Photon Energy (eV)
1.8
2.0
P.K.Kashkarov et al., JETP 64,1123 (2003)

Er doping results in both two order of magnitude quenching of the exciton PL
and a strong emission line at 1.5 m

Er-doped a-SiO2 layers does not exhibit efficient Er photoluminescence

Efficient energy transfer from electronic excitation of Si nanocrystals to Er ions
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August 28, 2008
Transient PL Investigation of Er3+ Population
Inversion
1
decay
4
0.5
N1/NEr
Time (ms)
5
3
2
nc-Si/SiO2:Er
d=4 nm
rise
1
0.01
N1/NEr= 1 - rise/decay
0.1
0.1
1
0.01
2
0.1
1
2
Pump power (W/cm )
Pump power (W/cm )
N1
N0
• Lifetime of Er-related PL becomes shorter at Iexc > 0.1 W/cm2 that correlates with the
population inversion of Er3+
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August 28, 2008
Silicon nanocrystals for biomedical
applications
1. Oxygen molecules exist in several forms:
non-active one (ground state) and active
ones (excited states)
2. In the active form oxygen is toxic and
therefore fatal for any live cells
3. This property of active oxygen is used to
clean water containing harmful microbes
and is the base of our immunity
4. In some cases generation of active oxygen
can be applied for cancer therapy
5. But how an oxygen molecule can be
transferred into active form?
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August 28, 2008
Electronic Structure of Molecular Oxygen
=7s
1
S
Excited States:
1.63 eV
 = 50 min
1
• Spin Singlet
• Energy-rich
• High chemical Reactivity:
S(O2) + S(Mol.)
S(Mol.)

0.98 eV
3
Ground State:
S
O
O
• Spin Triplet
• Paramagnetic
• Chemically inert:
T(O2) + S(Mol.)
S(Mol.)
Optical Excitation is inefficient  Photosensitizer is required
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August 28, 2008
Photosensitization: Basic Principle
Energy Transfer
S1
T0
h
S
S0
T
Donor
R
Acceptor
Efficient Energy Transfer requires:
Small Spatial Separation between Donor and Acceptor
• Spectral Overlap of Donor and Acceptor Energy Bands
• High Quantum Efficiency of Donor Luminescence
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August 28, 2008
Si Nanocrystal Assemblies as Photosensitizers?
•Simple Electrochemical Preparation
•Open Nanostructure
•Efficient Photolumenescence
A.G.Cullis and L.T.Canham, Nature 353, 335 (1991)
Exciton Lifetime (µs)
S
10
3
10
2
10
1
T
S
1.2
1.4
1.6
1.8
2.0
2.2
2.4
Energy (eV)
• Broad Tunable Emission Band
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• Long Exciton Lifetime (µs - ms)
August 28, 2008
Effect of Physisorbed Oxygen Molecules on
PL of Si Nanocrystal Assembly
PL Intensity (arb. units)
10
0
10
-1
10
-2
10
-3
10
-4
T=5 K
Vacuum
Physisorbed O 2
1
3
 S
1
0.8
1.2
1.6
• PL Quenching of
Excitonic and Defect
Emission Band
• 1  3S Emission Line
of 1O2
S
2.0
Energy (eV)
 Evidence for Energy Transfer from Excitons to O2
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August 28, 2008
Singlet Oxygen Photosensitization in Water
Exciton PL Intensity (arb. units)
PL transients of porous Si powder dispersed in water
1 mg of nano-Si
(from micropor-Si)
dispersed in 3 ml of
H2O
 I PL (t )dt
vac
hex=3.7 eV
T=300 K
0.1
without O2
PL=90 s
O2 saturated
PL=40 s
0.01
0
50
100
150
200
250
300
Time (s)
 I PL (t )dt  1.7
oxygen
for SO ~ 1-3 s:
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hPL=1.6 eV
1
 E  40%
NSO ~ 1015 – 1016 (1/cm3)
August 28, 2008
Effect of Porous Si on cancer cells
(in vitro experiments with mouse fibroblasts)
Relative Number of Cancer Cells
Cells were counted by using optical density measurements
1.2
in dark
0.8
under illumination
0.4
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Concentration of Porous Si (g/l)
Cancer cell number vs porous Si concentration in the dark (blue symbols)
and after illumination (red symbols)
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August 28, 2008
Number of cells
600
A
G1
400
IG1+ IS+ IG2 (arb. un.)
DNA Analysis
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Concentration of nc-Si (g/l)
200
G2
S
0
0
200
400
600
800
DNA content
Histogram of DNA content for the cancer cells kept in the nutrient solution with dispersed
porous Si (1.5 g/l) in darkness (blue curve) and after illumination (red curve). Symbols G1, S,
and G2 mark different cycles of the cell proliferation. The apoptotic cell region is marked by A.
Inset shows the relative contribution of G1, S and G2 regions vs nc-Si concentration in the dark
(blue symbols) and after illumination (red symbols)
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August 28, 2008
h
Photodynamic cancer therapy
triplet
3О
2
nc-Si
Si
nc-Si
Si
singlet
1О
2
Si
Si
Si
Si
Si
Si
Si
Si Si
Si Si
Si
Si
tumour
nc-Si
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August 28, 2008
General Conclusions
•
Nanostructuring of homogenous and isotropic
Si-crystals enables one to form photonic
media with unique properties
• Ensembles of Si-nanocrystals in a dielectric
matrix are promising base for silicon laser
compatible with microelectronic technology
• Bio-compatible Si-nanoparticles are effective
photosensitizers of singlet oxygen generation
what can be applied for photodynamic cancer
therapy
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August 28, 2008
Thank you for the attention!
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August 28, 2008

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