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Light Engineering in New Materials

How material science can help in realizing new efficient incoherent and coherent light sources

April 15, 2019

Lights 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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Semiconductors Semiconductors Ordered structures ( bulk crystals and nanostructures) Ordered structures ( bulk crystals and nanostructures) Inorganic Inorganic Organic Organic Amorphous Amorphous

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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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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

• peration

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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

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 Intensity (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 Intensity (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 1.0 10

• 2

10

• 1

1

400ps 400ps

Experimental response SE SPE

Normalized PL Intensity Time (ns)

<30ps

30 60 90 120 150 180 210 240 270 300 330 SPE

x y

30 60 90 120 150 180 210 240 270 300 330 SPE A B

1.62 1.63 1.64 1.65 1.66

1.634 1.636 1.638 1.640

Energy (eV) PL Intensity (a.u.)

B A

Energy (eV)

Surface states role

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10

• 1

1 10

• 2

10

• 1

1

• Norm. PL Intensity

293 K 11 K

P

D C

• Norm. PL Intensity

Time

1ns 11K 293K

P

B A

11 K 210 K

P

A Time

1ns 11K 293K

P

2.3 2.4 2.5 2.6 0.0 0.2 0.4 0.6 0.8 1.0

Normalized PL Intensity

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

Integrated PL Intensity 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 Intensity (a.u.) Energy (eV)

A B C 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

50 100 150 200 250 300 1 10

P/P0 Temperature (K)

A B C D

P0= P(T=10K)

c

𝑸 𝑼 = 𝑩𝑼 + 𝑪𝒇

− 𝑭𝑪 𝒍𝑪𝑼

Non-linearities with two beams

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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

Ipb-(Ip+Ib)

Energy(eV) E

0= zero crossing energy

2.26 2.28 2.3 2.32 2.34 2.36 56.8ps 85.2ps 113.6ps 142ps 184.6ps 227.2ps 269.8ps

Ipb-(Ip+Ib)

Energy(eV)

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

• pto 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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