semiconductor quantum dots - unicam · quantum dotsare sphericalnanocrystalsof semiconducting...

Quantumdots arespherical nanocrystals ofsemiconductingmaterials constituted fromafew hundreds toafewthousands atoms,characterized bythequantumconfinement ofthecharge carriers.

Owing toquantumconfinement,their optical andelectronicproperties arestrongly dependent ontheparticle size

SizetunablefluorescenceemissionfromCdSe/ZnS coreshellnanocrystals.ThesizeoftheCdSe coredictatestheemissionwavelengththroughquantumconfinement.

d = 2.0 2.5 3.0 4.0 5.0 6.5 nm

Photoactive semiconductornanocrystal quantumdots; A.Credi

(Ed.),Top.Curr.Chem. 2017

Semiconductorquantumdots

ElectronictransitionsinsemiconductormaterialsEnergy

Bandgap

VB

CB

VB

CB_

+

hn' Bandedgeluminescence

hn >Ebg

VB

CB _

+

electron-holepair(exciton)

thermalrelaxation

Insemiconductors,optical excitation causes thepromotionofavalence bandelectroninto theconduction band,leaving behind avalence bandhole.Oncepromotionhas occurred,theCBelectronquickly relaxes into thelowest energy CBstate,while thehole

moves tothetopoftheVB(VBelectrons cascade downinto lower energy valence states).

Photoluminescence canarise fromradiativerecombination oftheCBelectronwiththeVBhole.Thisphenomenon is called bandedge luminescence andthecorresponding emissionbandis characterized bya

very smallStokesshiftrelativetobandedge absorption.

Thecoupled electron-hole pair is electrostatically bound andit is often referred toas asingleentity called exciton,since one cannot exist without theother.

OpticalpropertiesofsemiconductorquantumdotsE

nerg

y

Density of states

↑ ↓

↑ ↓

↑ ↓

band gap

Bulk NP Cluster Atomiclimit

Because ofquantumconfinement,thebandgapofsemiconductornanoparticles increasesbydecreasing thediameter.

Core

Shell

Capping monolayerMethods that enable the preparation of NPs of semiconductor materials

with a precise control of size and shape are available.The majority of these methodologies exploit precipitation processes resulting from the

reaction of appropriate inorganic or organometallic precursors in a solution phase in the presence of organic ligands that eventually form a capping monolayer on the NP surface.

The size and shape accuracy of the product is made possible by a detailed understandingand a precise thermodynamic and kinetic control of the various phases associated with the

formation of the nanocrystals, namely nucleation, growth and termination.

TEM images of CdSe-ZnS core-shell NPs

Size-controlledsynthesisofmonodispersenanoparticles

Engineeringthesurfacethroughligandexchange

Fluorescein

QDCdSe4nm

QDCdSe4nm

Fluo

rescein

§ Broadabsorptionspectrum

§ Sharpgaussian-shapedemission

§ VerysmallStokesshift

§ Opticalpropertiestunablewithsizeandmaterialtype

Photophysicalpropertiesofsemiconductorquantumdots

§ Highluminescencequantumyieldandrelativelylonglifetime

§ Highphotostability

§ Largetwo-photonabsorptioncrosssection

§ Possibilityofsurfacepassivationbyinorganicandorganiclayers

J.Phys.Chem.C 2010,114,7007;PureAppl.Chem. 2011,83,1;ChemPhysChem,2011,12,2280;Chem.Soc.Rev. 2012,41,5728

Core-shell QD:ananocrystal ofagiven semiconductor coated withathin layer ofanother semiconductor.

Core-shellquantumdots

CdSe ZnS

V

R

h+

e-

CdSe

ZnS

Type-I

Both electronandhole areconfined inthecore

CdTe CdSe

V

R

h+

e-

CdTe

CdSe

Type-II CdSe CdTe

V

R

h+

e-

CdSe

CdTe

Type-II

Electronis confined intheshellHole is confined inthecore

Electronis confined inthecoreHole is confined intheshell

CdSe CdS

V

R

h+

e-

CdSe

CdS

Type-I1/2

Electronis delocalized overtheentire QDHole is confined inthecore

Curr.Phys.Chem.2011,1,181;Chem.Soc.Rev.2015,44,4275

QD

Photoinduced electrontransferfromtheconduction bandofthequantumdottotheLUMO

ofabound molecular electronacceptor (A)

Molecule

Photoinduced electrontransferfromtheHOMOofabound molecular electrondonor(D)tothevalence bandofthequantumdot

Photoinduced energy transferbydipole-dipole(FRET)mechanism:theenergy oftheQDexciton level is transferred totheelectroniclevels ofthebound molecule

Theefficiency ofthis process depends onthespectral overlap between theQDemissionandthemolecule absorption,andontheQD-molecule distance.

Therateofthese processes depends ontheelectronic coupling between theQDandthemolecule,andontheir distance

LuminescencemodulationmechanismsinQD-moleculeconjugates

CdTe /[Ru(dpp)3]2+

DaltonTrans.2011,40,12083

CdSe-ZnS /4,4’-bipyridiniumderivatives

J.Mater.Chem.2008,18,2022

!

CdTe /Zn(TPP)

Inorg.Chim.Acta2012,381,247

!

Luminescencequenchinginself-assembledQD-moleculeconjugates

Ratiometric pH sensorBawendi,Noceraetal.,J.Am.Chem.Soc. 2006,128,13320

MaltosesensorinwaterBensonetal.,J.Am.Chem.Soc. 2005,127,12198

QD-moleculeconjugatesasluminescentsensors

RatiometricO2 sensorfororganicsolvents

Chem.Commun. 2011,47,325Bandedgeemission lmax =632nm

Insensitivetooxygen

Fluorescence lmax =375nm

Strongly quenchedbyoxygen

I632/ I398 = (0.061±0.004)+(0.445±0.007)´ pO2

MatteoAmelia

QD-moleculeconjugatesasluminescentsensors

With Nathan McClenaghan, Université de Bordeaux

With Nathan McClenaghan, Université de Bordeaux MarcelloLaRosa

100-µsdomain

CdSe-3

CdSe-3 / 1-PCA

Engineeringtheluminescencelifetimeofsemiconductorquantumdots

Molecularmachinesandmotors

‘Manas IndustrialPalace’FritzKahn,1926

Naturalmolecularmotorsinskeletalmuscles

Myosin Actin

‘Wenn wir wachen’

FritzKahn,1939

‘Wenn wir schlafen‘

sarcomere (~3 µm)

actin filament

myosin filament

expanded form contracted form

(~10

nm

)