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]