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Determination of g-factors in II-VI, III-V and VI-VI Semiconductor
Nanocrystals
Efrat LifshitzDept. of Chemistry, Solid State
Institute and Multidisciplinary Center for Nanoscience and Nanotechnology
Technion, Haifa, Israel
The quantum size effectThe quantum size effect
CB
400 500 600 7000.00
0.05
0.10
0.15
0.20
0.25
0.30
OD
λ [nm]
VB
CB
N
S
“artificial atoms”
Common preparation procedures of 0D semiconductors
Common preparation procedures of 0D semiconductors
Bottom up:
Stain growth:Pyramids20X2 nm
Colloidal growth:Spherical 0D, <10 nm
Dots)-or QNCs(nanocrystalssemiconductor PbSeSynthesis of
P
TOP
OA
OO
(C H2)7
C H
C H( C H
2)7
C H3
liquid surfactant
at 120 oC
Pb-Ac/Ph-Et/OA/TOP TOP:Se +
P
P
P
P
OO
OO
O O
OO
Fluorescing materials with Fluorescing materials with
extremely bright and tunable colors in the NIRextremely bright and tunable colors in the NIR
Eye-safe laser
Photonic Crystals
Transparency through the blood
Telecommunication window
II-VI:CdTe IV-VI:PbSe
RB=460 nm, me,h=0.1m0, ε∞=24
III-V: InP
1000 1250 1500 1750 2000
7.0nm6.8nm
6.3nm6.0nm
5.8nm5.4nm
5.1nm4.4nm
3.7nm3.5nm
3.1nm2.3nm
Wavelength [nm]
Abso
rban
ce [a
.u]
PbSe NCs
E. Lifshitz et al. Adv. Fun. Mat., in press
Wire
400nm
10°C
Rod
40°C
Sphere/cube
5nm
400 600 800 1000
Lum
ines
cenc
e (a
.u.)
Wavelength (nm)
400 600 800 1000
Lum
ines
cenc
e (a
.u.)
Wavelength (nm)
p g
Taylor cone
Instability process
⊥
||
Core/shell PbSe/PbS NanocrystalsCore/shell PbSe/PbS Nanocrystals
PbSe core
PbS shell
Chemically rebust
& exhibit high QY!!Type II Type I
“spherical quantum well”
Tunning of the band gap, with size and composition
Shell thickness [ML]
Core diameter [nm]
1S-e
xcito
n en
ergy
[eV]
Shell thickness [ML]
Core diameter [nm]
1S-e
xcito
n en
ergy
[eV]
M. Brumer, A. Kigel, L. Amirav, A. Sashchiuk, E. Lifshitz, Adv. Fun. Mater., (2004) in press
0.8 1.2
3ML
2ML
1ML
Core
Absorbance [a.u.]
PL in
tens
ity [a
.u]
Energy [eV]
Trapped carrier
PbSe
PbSexS1-x
Ksp(PbSe)<Ksp(PbS)
QY=40%
QY=65%
0 10 20 30 40 50
0.1
1
1.91 eV1.98 eV
1.75 eV1.83 eV
1.71 eV1.65 eV
Nor
mal
ized
PL
inte
nsity
time (µsec)1.0 1.2 1.4 1.6 1.8 2.0 2.2
D - h
exciton
PL In
tens
ity
Photon Energy [eV]
Trapped carrier
Trapped carrier
PbSe
PbSexS1-x
Ksp(PbSe)<Ksp(PbS)
QY=40%QY=65%
Device preparation Device preparation
A polymer film embedding PbSe NCs was placed between two glass windows with antireflection-coated surfaces, providing protection of the film and preventing wave-front distortion
Setup for passive Q-switching of laserSetup for passive Q-switching of laser
pumping
Lasing element
M1M2
Q-switch
Laser output
Preliminary results of Q-switching experiments: The free running laser threshold - 6.5 J The threshold with the Q-switch inserted in the cavity - 12 J. Energy of the output pulse of the Q-switched laser - 0.2 µ J.
0 2 4 6 8 10 12 14 16 18 20
0.72
0.76
0.80
0.84
0.88
0.92
Tran
smita
nce
P b S P b S e/P b S P b S eS
E n erg y flu en ce (J /cm 2)
Imax (J/cm2)σgs (cm2)σes (xσgs)Sample0.255.00.39PbSe0.206.30.31PbSe/PbS0.187.00.40PbSe/PbSexS1-x
Stokes Shift versus NC’s diameter
±1/2
± 1/2± 3/2(S+L)
0,±1
±2
1/R (nm)
e-h exchange
1.0 1.5 2.0
A 5.7nm
5.6nm
5.4nm
5.3nm
5.1nm
4.8nm
4.0nm
Absorbance [a.u]
PL in
tens
ity [a
.u]
Energy [eV]
4 5
0 . 8 0
0 . 8 5
0 . 9 0
0 . 9 5
1 . 0 0B 1S-exciton absorption energy [eV]
1S-e
xcito
n PL
ene
rgy
[eV]
N C 's d i a m e t e r [ n m ]
±1/2
± 1/2 lh± 3/2 (hh)
-1/2+1/2
-3/2+3/2
+1/2-1/2
σσ --σσ ++Magnetic field (B)
−+
−+
+−
=σσ
σσ
IIIIDCP
τµ
/11
4 1TkTBg he
+⋅≈ −
Degree of Circular Polarization
-1 0 1 2 3 4 5 6 7
DC
P =
(I σ+- I σ−)
/ (I σ++
I σ−)
Magnetic Field, B [T]
ge-h
Field-induced circular polarized PL
T1/τ=0.025
InP/ZnS NCs
Dependence of DCP on the external magnetic field
τµ
/11
4 1TkTBgDCP he
+⋅≈ −
-1 0 1 2 3 4 5 6 7
0.00
0.05
0.10
0.15
DC
P =
(I σ+- I σ−)
/ (I σ++
I σ−)
1.8eV (Exciton band) 1.45eV (Donor-hole band)
Magnetic Field, B [T]1.0 1.2 1.4 1.6 1.8 2.0 2.2
Donor-hole band (D-h)
exciton band
PL In
tens
ity (a
. u.)
Energy [eV]
L.Langof, L. Fradkin, E. Ehrenfreund, E. Lifshitz, O. I. Micic, and A. J. Nozik. Chem. Phys, 2004, 297, 93
hehe ggg 3−=−
DCP: ge-h= 0.55
ge (electron factor)
+ 1/2
-1/2
∆IPL
MW
eh
±3/2
ODMR
E. Lifshitz, L. Fradkin, A. Glozman, L. Langof, Annu. Rev. Phys. Chem. 2004, 55: 509-57
+
+ σ−σ
21,2
3 −−
21,2
3 +−
21,2
3 −+
21,
23 ++
0,0
3/2J
IODMR
Magnetic Field
σ - σ
(Non-thermalized case)
±
Instrumental Setup
Microwave guide
Laser
mirror mirror
Detector
Magnetic powersupply
Cryostatlens
Pulse GeneratorMicrowave Source
Microwave Amplifier
Lock-In Amplifier
lens
Ref
ere n
c e S
i gn a
l
Signal
Modulation
PL
(a)
OD
MR
inte
nsity
O
DM
R in
tens
ity
0.3 0.4 0.5
(b)
Magnetic field /Tesla
∆H= 3J/βge
FaradayB0 PL
VoightB0 PL
σ - σ +
(x10)
hehehhee SDSSSJBgSBgSH +++= 00 ββ
Spin Hamiltonian
)(31
31
zzyyxxeD ggggTrgg ++===
005.0995.1 ±=xxg
005.0995.1 ±=yyg
005.0845.1 ±=zzg
005.0945.1 ±=Dg
J = 0.225µeV
L. Langof, E. Ehrenfreund, E. Lifshitz, O. I. Micic and A.J. Nozik,, J. Phys. Chem. B, 2002, 106, 1606-1612
3000 4000 5000Magnetic Field [Gauss]
VpIn
In
In
InVpIn
In
nradrad
mw121
212
1
22
mw211
211
1
11
111
hE,)kT/Eexp(11
P)nn(T)nn(nGn
dtdn
P)nn(T)1)(nn(nGn
dtdn
τ+
τ=
τ
ν=∆∆+
=ρ
−−ρ+−
−+τ
−=
−−ρ−+−
−+τ
−=
off
on
microwave
(e)τrad1~τnrad2τrad1 < τnrad1,τrad2
(d)τrad1 < τrad2,τnrad1/2 T1 < τrad
(c)τrad1 < τrad2,τnrad1/2T1 > τrad
(b)τrad1/2~τnrad1/2~T1
(a)τrad1~τrad2 T1 > τ
Lum
ines
cenc
e In
tens
ity
Time [µs]
Time-resolved ODMR
T1/τ~3(Non-thermalized case)
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0
O F FO N
TR-O
DM
R
D e l a y [ µ s ]
g-factors extraction
hDhD ggg 3−=−
DCP: gD-h= 0.72ODMR (associated with trapped e
gD = 1.945
heex ggg 3−=DCP: gex= 0.55
ge= 1.81 ghh= 0.42
L.Langof, L. Fradkin, E. Ehrenfreund, E. Lifshitz, O. I. Micic, and A. J. Nozik. Chem. Phys, 2004, 297, 93
DCP of a various sized NCs
0 1 2 3
0.0
0.1
0.2Linear fitting of the exciton band
42A g(ex)=0.517
48A g(ex)=0.437
80A g(ex)=0.109
DC
P[ (
I σ+-
Iσ-)/
( Iσ+
+ Iσ
- )]
Magnetic field (Tesla)NC size gex gD-h ge
42 Å 0.517 0.488 1.957
48 Å 0.437 0.410 1.937
80 Å 0.109 0.253 1.786))((3
20)( EEEE
Ee gsog
sopgEg +∆++∆−=
Assemblies
ET
1.8 2.0 2.2
Nor
mal
ized
PL
Inte
nsity
Energy [eV]
CdTe(TGA) CdTe(CA) Electrostatic link Covalent link
10 20 30
Nor
mal
ized
PL
Inte
nsity
Delay Time [nsec]
CdTe(TGA) CdTe(CA) Electrostatic link Covalent link
The optical head
XYZ Positioner
Magnet(up to 12 T)
System OverviewOptical head
Nonmagnetic, floating, optical table
Cryostat with the optical head and superconductive
magnet
Cryostat isolation stage
Probe pumping and flushing spot
1.276 1.278 1.280
0
200
400
600
800
9T
8T
7T
6T
5T
4T
3T
2T
1T
0T
InGaAs Single Quantum Dot,Magnetic field dependence, X-1 and X0 lines
Energy (eV)
Cou
nts
Conclusion
Unique synthesis of core-shell nanocrystals, with improved quantum yield, and optical tunability in the IR spectral regime. Utilization of core-shell NCs as passive Q-switch in eye- safe lasers.
Preparation of PbSe wires, rods and tetrapods, using coordinating and templating ligands. Alignment of PbSewires in a polymer fiber by co-electrospinning.
A use of magneto-optical methods (CP-PL and ODMR, single dot spectroscopy) for the determination of carriers’g-factors (spin properties)
Acknowledgments
Students and PostdocsDr. A. Saschiuk L. FradkinL. Langof M. BrumerM. Bashouti A. KrigerDr. M. Sirota Dr. A. GlozmanL. Amirav Dr. J. KolnyR.OcsovskiiS. Fruend
CollaboratorsProf. H. Weller and Dr. A. Eychmueller, Hamburg Univ. Germany,Dr. E. Zussman, Prof. N. Tessler, Dr. S. Berger, Prof. U. Sivan, Prof. D. Gershoni, Technion
Funding DIP, GIF, BSF, ISF, Magneton, MOD, MOS
Thank you for your attention !
Oscilloscope trace of a single laser output pulse as a function of time, using PbSeNCs as a Q-switch
Table: Q-switching performance
Pulse FWHM
Power output
Trans. at1540 nmType of a sample
57 nsec.2.0 mJ87.4 %PbSe core
40 nsec.3.5 mJ86.0 %PbSe/PbSexS1-x
1.276 1.278 1.280
0
2000
4000
6000
8000
10000
12000
14000
12
34
56
7
Power Dependence @ 3T (excitation laser power increases from file 7 to 1)
Energy (eV)
Cou
nts
Confocal microscope ?
MW
MW
PnnT
nnnGndt
dn
PnnTnnnGn
dtdn
)()(
)()1)((
121
212
2
22
211
211
1
11
−−+−
−+−=
−−−+−
−+−=
ρτ
ρτ
;)/exp(1
1kTE∆+
=ρ
n1, n2 – population of the Zeeman split states
G - the generation rate
τ1, τ2 - optical decay times of the spin states (1/τ1,2= 1/τrad1,2+1/τnrad1,2 )
T1 - spin-lattice relaxation time
PMW - the MW power
MWhE ν=∆
Time - resolved ODMR
T1/τ ~ 3
0 500 1000 1500 2000
O FFO N
TR-O
DM
RD e la y [µ s ]
0
10
20
30
40
50
60
PbS NPs
T%
wavelength [nm]
PS microbeads PS microbeads with PbS NPs
A B
C
D
C
C
N
N
Pb 2+
C
C
N
N
22
2 )()(2 ++ →+ EtDAPbEtDAPb
C
C
N
N
C
C
N
N
Pb 2+
Se 2-
z
yx
EtDAPbSeSeEtDAPb 2)( 222 +→+ −+
+−− +→+ HSeHSe 2
C
C
NH
H
NH
H
C
C
N HH
HH
Se
N
Pb 2+
Se 2-
Se 2-
Pb 2+
Pb 2+
Se 2-
Pb 2+
Se 2-
EtDAEtDA
EtDA
EtDA
EtDA
EtDA
EtDA
EtDA
Conductivity mechanism:
1Se
1Sh
1Se
1Sh
• Electric field inside a NC is given by E≈V/(εmL)=7.0×103V/m; εm=ε1+4√2π/(ε2-ε1) [½r/(r+D)]3 is the volume-weighted average of the dielectric constants for the TOPO capping and for the PbSe NCs;
• The resistance, R, over a TOPO coated NC is about 18 kΩ;• The self-capacitance of an isolated NC, C=4πε0εm(r +D)=1.5x10-18 Farad; • The lifetime of an electron sited on a certain NC: τ=RC; The energy associated
with its transfer to a neighboring NC: ET=πħ/τ=76 meV. • Intra-band spacing (~100 meV) > ET; • Conductivity of a typical PbSe wire-like assembly: σ ~7.0 Ω-1cm-1.
Synthesis of PbS Nanoparticles
Pb:S 2:1
Pb: PbO
S: bis-Trimethylsillylsulfide (TMS)
Stabilizer: Oleic Acid
Solvent: Octadecene
Injection of TMS at 150°C into Pb oleate solution
Synthesis of CdTe Nanoparticlesand Tetrapods
Cd:Te 2:1
Cd: CdO
Te: TeTBP (tributylphospine) for dots and TeTOP for tetrapods
Stabilizer: Oleic Acid or tetradecylphosphonic acid (TDPA)
Solvent: Octadecene
Injection of Te precursor at 300°C into Cd-oleate or CdTDPA solution, growth at 250°C
200 nm
A C
B Figure: HR-SEM image of PEO nanofiber, containing a PbSe QW (A); Cross sectional SEM image of a free end of a fiber, with a concentric PbSe QW (B); Cross sectional SEM image of unidirectional aligned QWs-polymer nanofibers, forming a one-dimensional nanorope (C).
M. Bashouti, M. Brumer, A. Zussman, E. Lifshitz, Adv. Mater. (2004) submitted
Transfer of CdTe Q-dots from organic phase to aqueous phase:
1Synthesis of CdTe Q-dots –
The particles are being synthesized in ODE-octadecen H2C=CH-(CH2)15-Me (the solvent).Stabilizers that we use during the reaction –ODPA – n- octadecylphosphonic acid . CH3(CH2)17P(O)(OH)2TDPA- n – tetradecylphosphonic asid CH3(CH2)13P(O)(OH)2Those particles are being stored in their original medium in the glove box.
2Phase transfer –
0.5 ml of CdTe (TDPA/ODE) particles dissolved in 5 ml chloroform . Addition of few drops of TGA solution till we see precipitation .Addition of 1-2 ml of water.After shaking we see that the particles are transferred to the upper phase (the water). TGA – Thioglycolic acid . HO-C(=O)(CH2)-SHPreparation of TGA solution – 1.8 gr of KOH dissolved in 10ml of methanol + 0.5 ml of TGA .
TGA replaces the ODPA/TDPA on the surface of nano-particles. It makes negatively charged organic shell on the surface, which pulls the particles from the organic molecules (not charged ) in to polar solvent ( water).
The water particles are more florescent than those in organic medium.
ODMR scanning microscope based on the gradient index lens (SELFOC). AS-amplitude stabilizer, L1, L2, L3, and L4
– lenses, BS – beamsplitter, P1 and P2 – pinholes.
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
0
10000
20000
30000
40000 ODMR, InP 42A, with Selfoc lens
OD
MR
(a.u
.)
Magnetic field (Tesla)
An ODMR spectrum of a few InP NCslocated on the back side of a SELFOC lens.
Lineshape of the ODMR spectrum inluding hyperfine interaction of P
vacancy with its four nearest Indium neighbors.
3000 4000 5000
Magnetic Field [Gauss]
Phosphor vacancy deep in the nanocrystals
VpIn
In
In
In
Lineshape of the ODMR spectrum including unresolved hyperfine
interaction of P vacancy with its twonearest Indium neighbors.
3000 4000 5000
Magnetic Field [Gauss]VpIn
In Phosphor vacancy at the surface
h e Energy
3/2 1/2 E1=1/2ge H + 3/2gh H +3/4J
3/2 -1/2 E2=-1/2ge H + 3/2gh H -3/4J
-3/2 1/2 E3=1/2ge H - 3/2gh H -3/4J
-3/2 -1/2 E4=-1/2ge H - 3/2gh H +3/4Jσ+σ-
hν
hν
nradrad
mw121
212
1
22
mw211
211
1
11
111
hE,)kT/Eexp(11
P)nn(T)nn(nGn
dtdn
P)nn(T)1)(nn(nGn
dtdn
τ+
τ=
τ
ν=∆∆+
=ρ
−−ρ+−
−+τ
−=
−−ρ−+−
−+τ
−=
IODMR
Magnetic Field
σ - σ+
+ σ
1,1 −
0,1
1,1
0,0
0,0
D
J
−σ π
21,
21 −−
21,
21 +−
21,
21 −+
21,
21 ++
0,0
D J
IODMR
Magnetic Field
Spheres, Rods and Wires Spheres, Rods and Wires
Rod
n=3n=2n=1
Sphere
n=3n=2
n=1
Bulk Wire
n=3n=2n=1
Energy
Wires, rods, cubes and spheres of PbSe synthesized with coordinating ligands
500nm
40°C
400nm
10°C
117°C
E. Lifshitz et al.,
NanoLetters (2003)
Assemblies
ET
1.8 2.0 2.2
Nor
mal
ized
PL
Inte
nsity
Energy [eV]
CdTe(TGA) CdTe(CA) Electrostatic link Covalent link
10 20 30
Nor
mal
ized
PL
Inte
nsity
Delay Time [nsec]
CdTe(TGA) CdTe(CA) Electrostatic link Covalent link
NH2
CdTe NH2
NH2
NH2
NH2
NH2NH2H2N
H2N
H2N
H2N
H2NCO2H
CdTe CO2H
H2OC
CO2H
CO2H
CO2H
CO2H
H2OC
H2OC
H2OC
CO2HH2OC
NH 2 = cysteamine (“CA”) CO2H = thioglycolic acid (“TGA”)
electrostatically linked NCs assemblycovalently bound NCs assembly
CdTe
CdTe
CdTe
CdTeNCdTe
HC
O
CdTe
ET
1.8 2.0 2.2
Nor
mal
ized
PL
Inte
nsity
Energy [eV]
CdTe(TGA) CdTe(CA) Electrostatic link Covalent link
10 20 30
Nor
mal
ized
PL
Inte
nsity
Delay Time [nsec]
CdTe(TGA) CdTe(CA) Electrostatic link Covalent link
τ0 [nsec] – at the apex of the PLSample
5.1CdTe(TGA)
4.2CdTe(CA)
1.9Electrostatic Assembly
1.2Covalent Assembly
1.8 2.0 2.2 2.4 2.6
(A)
ET
Nor
mal
ized
abs
orpt
ion
Inte
nsity
CdTe(CA)
RT
CdTe(TGA)N
orm
aliz
ed P
L In
tens
ity
Energy [eV]1.8 2.0 2.2 2.4
RT
20 meV
80 meV(B)
Nor
mal
ized
PL
Inte
nsity
Energy [eV]
( ) ( )∫∞
−=0
445
26
0 128*10ln*9000
λλλελπ
dPLQNn
kR AnormDD
AD
p
Sample Inter-NCs distance R0
CdTe(TGA) CdTe(TGA) 1.12nm 14.6nm
CdTe(CA) CdTe(CA) 1.14nm 10.6nm
CdTe(TGA) CdTe(CA) 16.2nm
CdTe(CA) CdTe(TGA)
Electrostatically linked
1.13nm 8.6nm
Covalentely linked
0.93nm
0.0
0.4
0.8
12 16 200.0
0.4
0.8
0.0
0.4
0.8
PL [a
.u.]
PL [a
.u.] σ+
σ-
DCP x 10
DC
P
PL
[a.u
.] σ+
σ-
DCP
Ts / τ =0.5 D
CP
Time Delay [µs]
σ+
σ-
DCP
Ts / τ =0.025
D
CP
Circular polarized decay curves
Experimental
Theoretical, simulated with different T1/τ ratios
Trioctylphosphine
Oleic Acid
ResultsResultsA partial surface coverage, leads to a
surface oxidation and to an aggregation
P
P
OO
(CH2)7
CHCH
(CH2)7
CH3
PbSePb
SePbSe
Se
1000 1200 1400 1600 1800
Ab
sorb
ance
[a.u
]
Wavelength [nm]
PbSe Nc's PbSe Nc's after 2 weeks PbSe Nc's after 2 months
PbSe
[weakly-coordinating Ligands]
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