PHOTOPHYSICS OF

MOLECULAR MATERIALS AND

SEMICONDUCTOR NANOSTRUCTURES

UNIVERSITÀ DEGLI STUDI MILANO-BICOCCA

OPEN DAY 28 NOVEMBRE 2018 - CORSO DI LAUREA IN SCIENZA DEI MATERIALI

RESEARCH TOPICS: PHYSICS OF LUMINESCENT NANOMATERIALS

Semiconductor Nanostructures

Organic and Hybrid Nanoparticles and Nanotubes

Metal Quantum Clusters

Prof. F. Meinardi Prof. S. Brovelli Dr. A. Monguzzi

DIPARTIMENTO DI SCIENZA DEI MATERIALI U5

Photon managing for Solar Technologies (DOWN and UP photon

conversion)

Luminescent Materials for Photonics and Imaging Applications

(LED, Scintillationg, Bioimaging)

RESEARCH TOPICS: APPLICATIONS

LUMINESCENT SOLAR

CONCENTRATORS

PHOTONS UPCONVERTERS

LED, OLED MATERIALS FOR PHOTONICS

SEMICONDUCTOR NANOCRYSTALS

Cd10Se4(SePh)12(PPr3)4

Bulk CdSe

Quantum Dot

Regime

Cluster Molecule

100,000 atoms

20 nm

100 atoms

2 nm

10 nm

Nanocrystal Quantum Dots Epitaxial Quantum Dots

<10 nm

SEMICONDUCTOR NANOCRYSTALS

SEMICONDUCTOR NANOCRYSTALS

SEMICONDUCTOR NANOCRYSTALS

SEMICONDUCTOR NANOCRYSTALS

TUNABLE OPTICAL, LUMINESCENCE and MAGNETIC PROPERTIES

vs composition, size, shape and doping

SMART WINDOWS

OPEN DAY 28 NOVEMBRE 2018 - CORSO DI LAUREA IN SCIENZA DEI MATERIALI

HYBRID NANOMATERIALS: FLUORESCENT

METAL ORGANIC FRAMEWORKS

Optically inert MOFs = modulable porous systems• Gas storage• Molecular sensors• Chemical catalysis

Luminescent MOFs:• From ions• From Ligands

inorganic node(metal ion or metal

cluster)

organic ligand

Metal-Organic Framework

O.M. Yaghi, H.L. Li, J. Am. Chem. Soc. 1995, 117, 104010

RECOMBINATION MECHANISMS OF

MOLECULAR EXCITONS IN MOFS

Molecular excitons migrate within the

framework reaching energetic traps

metal ion

fluo. ligand

excitation

emission

MOF

Symmetry Monoclinic

a (Å) 16.7

b (Å) 6.9

c (Å) 28.3

β 90.6

Volume (Å3) 3252

Molecules per

unit cell4

Absorption/Photoluminescence Properties

1

𝜏𝑟𝑎𝑑= 2.88 ∙ 10−9 𝑛2

𝑃𝐿 ν 𝑑ν

𝑃𝐿(ν)ν3

𝑑νන𝜀 ν

ν𝑑ν

𝑄𝑌 =𝜏𝑒𝑥𝑝

𝜏𝑟𝑎𝑑

Stickler-Bergrelationship

Einstein Coefficients

RECOMBINATION MECHANISMS OF

MOLECULAR EXCITONS IN MOFS

Axis R (Å) Θ2 R0 (nm) khop (THz)D (cm2s-

1)L (nm)

x 9.2 4 6.0 3.5∙101 0.30 252

y 9.0 1 4.7 9.9 8.1∙10-2 131

z' 6.0 1 4.7 1.1∙102 0.41 296

𝐷𝑖 = 𝑘ℎ𝑜𝑝𝑅2

𝑖

XRD analysis

𝑘ℎ𝑜𝑝 = 𝑘𝐹𝑠 =1

𝜏𝐷

𝑅0𝑅

6

𝑅0 = 0.211 𝜃2 ∙ 𝑛−4 ∙ 𝑄𝑌 ∙ 𝐽 λൗ1 6

𝐽 λ = න0

∞

𝑃𝐿 λ ∙ 𝜀𝐴 λ ∙ λ4 𝑑λ

Förster Rate

Förster Radius

Spectral Overlap

spectroscopy

𝐼

𝐼0= 10−𝐴 = 𝑒−𝜀𝑀𝑥

x (nm) d (nm)

MOF 924 ~100

MOF-bpy 629 ~50

MOF-dabco 564 ~200

Lambert-Beer

1. Excitation light penetration length (x)

Li = (Dit)0.5

2. Singlet diffusion length (L)

L/d >> 1

x/d >> 1

RECOMBINATION MECHANISMS OF

MOLECULAR EXCITONS IN MOFS

ENGINEERED MOFS FOR LOW POWER

PHOTON UPCONVERSION

METAL QUANTUM CLUSTERSBRIDGING THE GAP BETWEEN ATOMS AND COLLOIDAL NANOPARTICLES

Mn clusters combine molecule-like electronic structure with quantum confinement effects

• unmatched flexibility for tailoring their

physical properties

“magic” sizes and electro/magnetic properties are dictated by the s valence electrons of metal constituents

spherical jellium model for s electrons

𝐻 = 𝐻𝑒𝑙 + 𝐻𝑏𝑎𝑐𝑘 + 𝐻𝑒𝑙−𝑏𝑎𝑐𝑘

ENERGY

DELOCALIZATION EXCITED AGGREGATE

FORMATION

R

Rex

Excimers exhibit the absorption spectrum of their molecular constituents and long-lived non-resonant PL

IDEAL large Stokes Shift Emitters

Molecular excimers are typically formed upon collisional interaction between monomers in concentrated solutions

In principle not realizable as stable self-standing particles, unless…

EXCIMER: Excited State Aggregate

METAL QUANTUM CLUSTERSEXCITED STATES INTERACTIONS

• fPL = 12%, t =1.6 ns

trad =13.3 ns

[trad (SB) =12 ns]

• Mirror symmetry PL, distinct

vibronic replica

• No LM-CT emission @ 2 eV

• fPL = 4.5%, ultra-slow decay

• No LM-CT @ 2 eV

Au8 absorption/PLE

Largely Stokes Shifted PL

• Au8 absorption/PLE

D

o

u

b

l

e

encapsulated single clusters

[Molecular photophysics]

Au8-pX

Stokes shift 0.93 eV

Ab

s/P

LE

Ab

s/P

LE

METAL QUANTUM CLUSTERSEXCITED STATES INTERACTIONS

METAL QUANTUM CLUSTERSMANIPULATING SECONDARY INTERACITONS FOR BIOIMAGING

SPECTROSCPY LAB

CW and TR ultrafast photoluminescence spectroscopy

Close cycle cryostat T ~ 1.5 K

Magnetic Field Response ( fino a 5.5 T )

Confocal imaging micro-photoluminescence