TCE : tetrachloroethylene. ⢠dicholorethylene. CH. 3. SH. ⢠Ligand = surfactant. ⢠an organic function stick to the QD surface. ⢠An alkane chain which gives the ...
Physical chemistry of colloidal nanocrystals Emmanuel Lhuillier INSP – september 2016
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Safety Toxicity of nanoparticles remains under discussion, however o Materials include heavy metals o Solvents are terrible and very little of the involved chemicals are safe chemicals
Safety rules need to be respected Wear protective equipement (glove, google, labcoat) Try to keep them confined (fumehood, glovebox, solid matrix…)
Think before doing (read safety datasheet)
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Basic of chemistry Polar solvent
ligand H3C
• H2O : water !!! avoid with QD • DMF-NMF : di/N methyl formamide • Alcohol : MeOH>EtOH>IsoOH
SH
• acetone Ligand = surfactant
Apolar solvent • Alkane : typically between C6 and C18 • Toluene • Chlorinated solvent • CHCl3 : cholorform
• TCE : tetrachloroethylene • dicholorethylene
an organic function stick to the QD surface An alkane chain which gives the QD surface an apolar behavior
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Nanocrystals CH3
CH3
H3C
H3C
=
CH3
SH
HS
SH
HS
SH
SH
H3C
HS SH
CH3
HS SH SH
HS
H3C
CH3
H3C
CH3 CH3
Colloidal quantum dot Grow in solution
A nanoparticle (one in the 1 to 100nm) Not a cluster Presence of quantum confinement
There is a small crystral core and a ligand shell
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Nanocrystals synthesis Vacuum and Ar line
1
Introduce in the flask precursor Generally cation salt, ligand and solvent
Cooling column
2
Degase under vacuum (remove O2 and water)
3
Switch atmosphere to Ar and heat up T depends on material (250°C for Cd; 150°C for Pb; 80°C for Hg)
4
Inject anion precursor and let react (30s to hours)
5
Quench reaction (add ligands and cool down)
6
Clean up (remove greasy solvent and unreacted species)
Thermal controler Septum for solution injection Three-neck flask
Heating mantle From PGS’s lab
Magnetic stirrer
Nanocrystals history Late 1980 : pionnering work by Ekimov, Bruss, Englein, Itoh
o Synthesis in an aqueous phase, poor monodispersity o Quantum confinement was still under discussion 1993 : hot injection method (MIT -Bawendi)
o The method for highly monodisperse nanocrystals synthesis 1996 : Colloidal heterostructure : core shell (U. Chicago –PGS) 2001 Anisotropic growth : rods (U. Berkeley – Alivisatos) – 2006
-2008 platelets 2D (Hyeon et Dubertret) 2004 : Cation exchange (U. Berkeley – Alivisatos)
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Ligands OH H3C
Carboxylic acid : R-COOH • Oleic acid (C18COOH) • Myristic acid (C14COOH)
Thiol: R-SH • Dodecanthiol (C12SH) • Very strong binding agent to gold or mercury • Partly contributes to charge delocalization (formation of chalcogenides)
O
Amine: NH2 • oleylamine (C18NH2) • Highly used but well knwon source of non reproducibility due to low purity (80% or less) • Octadecylamine and hexadecylamine are purer but solid
Phosphine and phosphine oxide • Trioctylphosphine (C8)3P and (C8)3P=0 • Tributylphosphine (C4)3P • Convenient for NMR study
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Cation exchange Cu+
PbSe
1
A procedure to exchange the cation lattice while keeping the anion lattice unchanged
A2+
Pb2+ Cu2Se
Cu+
Mix a solution of QD with Cu+/Ag+ cations In methanol, RT, ≈100% yield
2
Obtain a solution of Cu2X/ Ag2X QD Large second cation excess, heat up
3
ASe
Obtain a solution of AX QD
Cation Exchange Reactions in Ionic Nanocrystals, D. H. Son et al, Science 306, 1009 (2004)
Even if a direct procedure exists, the method generally relies on intermediate cation species used Cu+ and Ag+ based intermediate species Second step is more complex All the challenge is to identify reaction parameters to keep the QD (size, shape) unaffected
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Shell growth Silar method Successive ionic layer adsorption and reaction
High temperature growth of shell (250°C)
C-ALD
Colloidal –Atomic layer deposition
Low temperature (room T) growth of shell
With successive introduction of cations and anions
Well suited for the growth of shell of sensitive material
Request to estimate the size of the QD to determine how much precursor needs to be injected
Self limited process (ie no need accurate determination of the injected amount of precursor)
Risk of secondary nucleation
Cation exchange
PbS
Cook in Cd(OA)2
60-100°C
PbS CdS
Growth of shell at constant radius The shell growth comes with a blue shift The way to make shell on lead chalcogenides
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Band aligment in heterostrcture CdSe Core
CdSSe
PbS CdS
Alloy
Type I
CdSe
CdSe
CdTe Type II
CdZnS
gradient
CdSe CdS ZnS Multi-shell
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Nanocrystals synthesis cleaning Alcohol
transfer QD +greasy solvent +unreacted species
Storage • dark • dry situation • Not to dilute
QD solution
Time (s)
At least turbid, or even presence of aggregate
Centrifuge at few 1000 rpm few min (!!!Ebulk k = -π/R
k = π/R
Only a part of the band k α π/L
VB
Tunable spectrum By tuning the size we tune the color.
The smaller the particle, the bluer the color
diagramm can be accessed
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Optical properties : absorption Absorption (OD)
0.6
Peak band edge o Energy of the peak : relates to the size of the QD
0.4
CdSe: D =1.61x10-9 λ4- 2.66 × 10-6 λ3+ 1.62×10-3 λ2 - 0.43 λ + 41.6
0.2 0.0
350 400 450 500 550 600
Wavelength (nm)
Abs (350nm) relates to the total amount of CdSe more or less independent of size and shape
Sub band gap absorption needs to be as flat as possible otherwise scattering or evidence for second population
W. W. Yu et al, Chem. Mater. 2003, 15, 2854-2860
o Magnitude of the peak : relates to the concentration of the QD solution CdSe: ε= 1600 .∆E .D3
Note : absorption of QD is robust (i.e. poorly sensitive to surface chemistry)
0,6 2 0,4 1
0,2 0,0
0 350 400 450 500 550 600
Wavelength (nm)
Photoluminescence (a.u.)
Absorption (OD)
Optical properties : Photoluminescence
PL linewidth for visible QD value is around 25-35nm. For QD it is mostly limited by inhomogeneous broadening Stokes shift difference inenergy between PL and absorption. Can be tuned from 0 to large value (critical to avoid reabsorption) PL Quantum yield generally measured using a reference of known QY o For core only object PLQY ≈10-20% o For core shell PLQY >50%
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The sub-band gap absorption The sub band gap absorption of a semiconductor is quantified by the Urbach tail α ∼ exp(E/Eu) QD can absorb below the band-edge… and even below the bulk band gap
Eu≈50-60meV for reasonably monodisperse CdSe QD
P. Guyot Sionnest et al, J. Chem Phys 137, 154704 (2012)
Wide dynamic setup for sub band gap absorption
Nanocrystals library
1 9
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Optical properties : range of wavelength 350
530
400
CdS gap
1500
3000
CdSe gap
CdS
E (eV)
CuInSSe
CdSe
PbX
HgX
New material
CsPbCl3 CsPbBr3 CsPbI3
InAs
III-V semiconductor
InP
10000
λ (nm)
II-VI semiconductor
ZnX
700 800
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IR spectroscopy Acid oleic capped QD C=O
C-H
Absorbance (u.a)
IR is a good tool to probe the surface ligands C-H : 3000cm-1 C=O : 1700cm-1 The others are more tricky
4000
3500
3000
2500
2000
1500
1000
-1
Wavelength (cm )
Raman spectroscopy is currently very little used Can be used to confirm the material Is a surface sensitive method
2 2
Infrared QD 1Pe 1Se
Intraband transition
Interband transition
Absorbance (a.u.)
1 intraband signal interband signal
1Sh 0 10
8
6
4
3
-1
2
Wavenumber (x10 cm )
If conduction band is populated by self doping, electrochemistry, optical pumping , one can observe intraband transistion at low energy
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How to switch solvent from a QD solution Motivation : reduce toxicity, better film deposition, change relative density with respect to polar phase
Respect the nature of the solvent : if surface chemistry is unchanged you have to respect the solvent polarity. In other word if you start from a non polar solvent, new solvent has to be apolar too. Alcohol
QD in solvent A
QD solution
Time (s)
At least turbid, or even presence of agregate
Centrifuge at few 1000 rpm few min (!!!
