Light Engineering in
New MaterialsHow material science can help in realizing new
efficient incoherent and coherent light sources
April 15, 2019Lights of Tuscany 2019
Anna Vinattieri, Dept. Physics and Astronomy, UNIFI
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Outline
A little bit of history
Band-gap engineering
Epitaxial growth
Chemical synthesis
Perovskites
A look to the future
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A little bit of history: 150 years from the
Mendeleev Table
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The semiconductor tree
The Mendeleev table is the tree
principal root
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The recognition of material science
relevance
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The nanostructures and
the superconductors
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Heterostructures
and integrated
circuit
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The CCD
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The Graphene: the
perfect lattice
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The
blue/white
laser diodes
The materials for opto-devices
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SemiconductorsSemiconductors
Ordered structures ( bulk crystals and nanostructures)
Ordered structures ( bulk crystals and nanostructures)
InorganicInorganic
OrganicOrganic
AmorphousAmorphous
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The Rosetta Stone of Semiconductors
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The King of Semiconductors for
electronics: Silicon
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32 inch , 80 cm
Unfortunately Si is very bad for
light emitters being an indirect
band-gap semiconductor, so it is
hard to have electrons and holes
recombining in a radiative way !
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The Kings of Semiconductors for light
emitters : III-V and III-Nitrides
In,Ga,Al Arsenide and Nitride Alloys with a band gap value
ranging from 0.8 to 4 eV
We can realize a device emitting light from 1.55 µm ( perfect
for telecomm) and 300 nm ( perfect for UV- lithography and
blue-rays, UV curing, etc.). White LEDs are realized using
nitride alloys.
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colours.
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How materials are realized?
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Czochralski growth (1916): very fast for big
crystals : O and C common contaminants
Epitaxial growth developed in late 1960s in
Bell Labs
AA 2018/19
growth speed
~ 2 - 3
mm/minute
Fisica dei Semiconduttori:Teoria e Applicazioni 13
Molecular Beam Epitaxy
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Epitaxy can be by Molecular Beam,
Chemical Vapour, Liquid Phase
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Epitaxy means in “ growth in an ordered way”: epitaxial growth is
therefore a growth with an high control of the deposition, at the atomic
level
It requires same
crystallographic
lattice and
“same” lattice
constant to avoid
extended defects
which are
detrimental for
the device
operation
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But Nature can help: Self-assembled growth
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Some results
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A superlattice
Quantum dots
What’s the meaning?
We can modify the electronic properties ( energy, band
dispersion, band gap…) at the nanoscale ( few Angstrom up
to tens of nm)
Quantum effects dominate and we can tailor the
material properties the way we prefer
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So very nice results but…..
Highly expensive techniques
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As alternative:
Chemical synthesis
Advantages of chemical synthesis
Low cost synthesis and processing
High tunability of band-gap
Easier integration in photonic structures
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The promising class of materials:
Perovskites
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Same class of material
as CaTiO3
A:Ca2+
B:Ti4+
X:O-
Depending on A, they can be
hybrid with organic cation or
fully inorganic
A: CH3NH3, Cs
B:Pb,Sn
X:Cl,Br,I
Application Fields
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Energy Harvesting
LEDs and Lasers
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https://doi.org/10.1016/j.mattod.2017.03.021
https://doi.org/10.1016/j.mattod.2017.03.021
Different material nanostructuring
means different behavior
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From bulk to plates, wires, dots
Here in Florence
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• Synthesis
• Morphological and structural characterization (XRD, SEM)
• High resolution optical spectroscopy in space and time
Synthesis of nanostructures/thin films
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Then precursors are spin-coated on a
substrate……..solvent evaporation produces
perovskite
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For crystals
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At the optical microscope:
spin-coated samples
High resolution Imaging
SEM AFM
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High resolution SEM
Bulk crystals
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Using an optical microscope Using a SEM
What spectroscopy on such samples?
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➢We need to determine the bandgap: standard cw
spectroscopy ( transmission/ absorption/ photoluminescence)
➢We need to understand the non-radiative recombination
paths: temperature-dependent measurements
➢We need to understand the carrier interactions and the
recombination kinetics: time-resolved experiments, typically
at the picosecond time-scale
Moreover: optical spectroscopy at the
macro or micro scale?
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Macro: we probe macroscopic portions of the sample ( ≥ 100 µm2)
Micro: Spatial resolution at the diffraction limit (≈µm in the vis)
Near Field detection: spatial resolution limited by the fiber tip
( ≈ 100 nm)
Different informations can be extracted!
Here in the labs
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MicroPL
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SNOM
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At the end
Some examples
Superlinear emission: ASE or lasing?
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1.61 1.62 1.63 1.64 1.65
1.61 1.62 1.63 1.64 1.65
Energy (eV)
P = I0
P = 1.6 I0
P = 2.3 I0
PL
Inte
nsi
ty (
a.u.)
P = 3.4 I0
P = 5.4 I0
1.60 1.61 1.62 1.63 1.64
1.60 1.61 1.62 1.63 1.64
Energy (eV)
P= I0
P=5 I0
P=60 I0
PL
In
ten
sity
(a.
u.)
P=85 I0
P=140 I0
Decay kinetics is similar
but polarization
properties are different
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0.0 0.2 0.4 0.6 0.8 1.010
-2
10-1
1
400ps
400ps
Experimental response
SE
SPE
No
rmal
ized
PL
In
ten
sity
Time (ns)
Surface states role
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10-1
1
10-2
10-1
1
Norm
. P
L I
nte
nsi
ty
293 K
11 K
P
DCNorm
. P
L I
nte
nsi
ty
Time 1ns
11K
293K
P
BAA11 K
210 K
P
A
Time 1ns
11K
293KP
2.3 2.4 2.5 2.60.0
0.2
0.4
0.6
0.8
1.0
Norm
aliz
ed P
L I
nte
nsi
ty
C
D
B
Energy (eV)
A
Recombination dynamics slows
down increasing the
temperature!
The smaller the nanocrystals,
the larger the effect
0.00 0.02 0.04 0.06 0.08 0.10
A
Inte
gra
ted
PL
In
ten
sity
1/Temperature (K-1)
T = 10 K
2.2 2.3 2.4 2.5 2.6 2.7
293K
130K
230K
210K
180K
150K
110K
90K
70K
50K
30K
11K
250K
PL
In
ten
sity
(a.
u.)
Energy (eV)
ABC
D
The increase of lifetime with T is counterintuitive ! Usually
non radiative channels are more effective as T increases
But….
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Large crystals or high temperature (smaller barrier, less surface defects)
Small crystals or low temperature (larger barrier, more surface defects)
CB
VB
Surface state
Surface states act as a
reservoir an can release
population as T increases
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Band A (K-1) B (10-3K-2) EB (meV)
A
≈ 10-3
0.18 33
B 0.48 39
C 2.6 67
D 25 130
Thermally activated transfer
0 50 100 150 200 250 300
1
10
P/P
0
Temperature (K)
A
B
C
D
P0= P(T=10K)
c
𝑷 𝑻 = 𝑨𝑻 + 𝑩𝒆−𝑬𝑩𝒌𝑩𝑻
Non-linearities
with two beams
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nanop2018-R
om
e O
cto
ber
1-3
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Non resonant bias CW @ 405
nm + ps pulse @ 370 nm
Ipb- (Ip+Ib) vs time
Negative signal: bleaching related to
localized/bound states
Positive signal: Superlinearity from excitons
formation
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2.26 2.28 2.3 2.32 2.34 2.36
0ps
14.2ps
28.4ps
42.6ps
I pb-(I p
+I b
)
Energy(eV)
0
E0= zero crossing energy
2.26 2.28 2.3 2.32 2.34 2.36
56.8ps85.2ps113.6ps142ps184.6ps227.2ps269.8ps
I pb-(I p
+I b
)
Energy(eV)
0
Conclusions
Optical spectroscopy implemented in several different configurations allows to
provide a dramatic amount of physical information which is fundamental for the
progress of material science.
New classes of artificial-made materials recently realized are of relevance for
opto devices: in these years perovskites are probably the most investigated !
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A look to the future
Controlled deposition of nanocrystals and homogeneous
film
Definition of post-growth treatment for defect annealing
Coupling of perovskites to photonic structures to tailor the
light properties
Optimization of electrical injection for realization of Leds
and Lasers
Eco-friendly materials ( no lead, etc)
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Let’s try to think our lives without
semiconductors: just 3 objects
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Could you survive?
Thanks
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