Università degli Studi di Trieste Dipartimento di Ingegneria e Architettura A.A. 2016-2017 Scienza e Tecnologia dei Materiali Ceramici Modulo 2: Materiali Nanostrutturati - Lezione 5 - Vanni Lughi vlughi@units.it 040 558 3769 Dipartimento di Ingegneria e Architettura Università degli Studi di Trieste
Previous lecture: Review Basic nanostructures: overview Nanoparticles Growth kinetics
This lecture: Content Basic nanostructures: overview Nanoparticles Steric stabilization approaches Colloidal synthesis of nanocrystals of binary compounds Colloidal synthesis of metal nanoparticles Kinetics of nucleation and growth in colloidal suspensions Optoelectronic properties Applications
Optoelectronic properties of semiconductor nanocrystals
Artificial atoms nanocrystal atom Quantum dots, just like atoms, have discrete energy levels
Artificial atoms
Energia Absorption and emission in quantum dots
Absorption and emission in quantum dots
Absorption and emission in quantum dots
Core-shell nanocrystals 3.3 ev 2.7 ev 2.2 ev 2.3 ev 1.7 ev 1.5 ev
Core-shell nanocrystals - Fabrication Methods - SILAR Successive Ion Layer Adsorption and Reaction Cation Exchange DOI: 10.1021/acs.chemrev.5b00739
Nanoparticle Engineering Controlling the optical properties
2 x Bohr radius (1) NCs as PHOSPHORS color = f (E G ) (I) COLOR ENGINEERING E G can be easily tuned (i) SIZE (ii) COMPOSITION bulk CdSe Eg (iii) ALLOYING
(1) NCs as PHOSPHORS (I) COLOR ENGINEERING Composition (i) SIZE 2.4 ev 1.7 ev 1.5 ev CdS CdSe CdTe (ii) COMPOSITION (iii) ALLOYING
(1) NCs as PHOSPHORS (I) COLOR ENGINEERING Shell Shell growth increases overall size of the NC different shell materials reduction of E G according (i) SIZE to: - shell thickness - shell composition - original core size (ii) COMPOSITION different core materials (iii) ALLOYING shell thickness (MLs)
(1) NCs as PHOSPHORS (I) COLOR ENGINEERING Shell Shell growth increases overall size of the NC different CdSe core sizes reduction of E G according (i) SIZE to: - shell thickness - shell composition - original core size (ii) COMPOSITION CdSe-ZnSe core-shell NCs (iii) ALLOYING shell thickness (MLs)
(1) NCs as PHOSPHORS (I) COLOR ENGINEERING alloying ZnSe (E G = 2.7 ev) DIFFUSION (alloying) causes E G to change according to composition (in this (i) system, SIZE increase of E G ) CdSe (E G = 1.7 ev) Cd x Zn 1-x Se (ii) COMPOSITION E G (core-shell) E G (alloy) (iii) ALLOYING Vegard s Law E G (Cd x Zn 1-x Se) = x E G (CdSe) + (1-x) E G (ZnSe) + b(1-x)
(1) NCs as PHOSPHORS (I) COLOR ENGINEERING alloying T = 350 C T = 330 C T = 310 C T = 300 C T = 280 C DIFFUSION (alloying) causes E G to change according to composition (in this (i) system, SIZE increase of E G ) (ii) COMPOSITION original CdSe/ZnSe (iii) ALLOYING
λ = 610 nm λ = 570 nm λ = 550 nm λ = 520 nm λ = 500 nm (1) NCs as PHOSPHORS (II) SINGLE EXCITATION WAVELENGTH for different phosphors different excitation wavelenghts organic phosphors optical density at higher energies UV light λ = 350 nm
(1) NCs as PHOSPHORS (III) CONVERSION EFFICIENCY Fluorescence QUANTUM YIELD f ( Probability of BAND-EDGE radiative ) RECOMBINATION high QY (up to 85%) QY tunable; depends on: Single phase NCs: Multi phase NCs: chemistry surface chemistry lattice mismatch VBO/CBO
fluorescence QY % (1) NCs as PHOSPHORS FLUORESCENCE Quantum Yield Influence of SHELL Influence of HEAT TREATMENTS ZnSe CdSe ZnSe (E G = 2.7 ev) CdZnSe CdSe CdSe ZnSe CdSe (E G = 1.7 ev) CdSe NC single phase NC: LOW QY CdSe-ZnSe CdZnSe shell thickness (nm) heat treatment time (min)
CdSe-CdZnSe-ZnSe CdSe-CdSeS-CdS (1) NCs as PHOSPHORS FLUORESCENCE Quantum Yield Shell with alloyed interlayer record QY Shell High QY no shell Low QY Influence of Lattice mismatch 7% 4% Lattice mismatch
(1) NCs as PHOSPHORS SOLID STATE LIGHTING Ideal compromise of: Easy synthesis Color control High Conversion Efficiency pc-led system LED Light Emitting Diode Emission of light (electroluminesce) PHOSPHOR Down conversion of light (photoluminescence) WHITE (POLYCHROMATIC) NCs based phosphors
(1) NCs as PHOSPHORS Available Nanocrystals QY > 70 % green - yellow - red region
(1) NCs as PHOSPHORS 1.3 NCs / polymer composite material LIQUID CdSe x S 1-x - CdS colloidal NCs BLEND in PMMA SOLID λ = 500 nm λ = 530 nm λ = 550 nm λ = 610 nm
(1) NCs as PHOSPHORS 1.3 NCs / polymer composite material pc-led system (still in development) COST-EFFECTIVE white lighting device LED Light Emitting Diode Emission of light (electroluminesce) NCs in polymeric lenses as PHOSPHORS WHITE (POLYCHROMATIC)