Semiconductor quantum dots - Unicam · Quantum dotsare sphericalnanocrystalsof semiconducting...
Transcript of 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
)