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Reduced and Oxidized Colloid Quantum Dots
I. Introduction on colloidal quantum dots, spectroscopy, dynamics, microscopy
II. Charges, conduction, lasing
Philippe Guyot-Sionnest
The University of Chicago
I. Colloidal Quantum DotsPerceived applications:Screen-printed flat panel displays of large area. Better than OLED.Phosphors for white light LED conversion. Mix at will.Photovoltaic energy conversion. Bandgap optimized.Biolabels. A bit bigger but much better than dyes.Infrared tags for night vision. No organic alternative.Laser “dyes” for infrared (near IR and Atmosphere windows).Nanoelectronic and spintronics self-assembled components. Colloidal molecules.
e-
h+
hν Many start-up companies, Nanosys, Q.dot, Evident tech….
Ø150 M$ of venture capital.
Ø1B$ perceived value
Research: Fabrication, Spectra-size, carrier dynamics, trapping, energy relaxation, carrier transport…
Nanocrystal quantum dots:
The colloid synthesis:1982: Precipitation, ionic precursors, Aqueous solutions. Brus, Henglein, Nozik1986: Micelle “nanoreactor”. Pileni, Brus. Water/micelle in Oil.1993 “organometallic approach”: Purely organic environment, high temperatures and surfactants. Murray and Bawendi2000 “Greener” reagents for II-VI. Peng
The pioneers:
Ekimov and Efros, 1980. Effective Mass Approximation applied to CuCl aggregates and excitonic spectra.
QDs from the Near IR to UV:
1200 2000 2800
Abs
orba
nce
(arb
. uni
ts)
Wavelength (nm)300 400 500
Abs
orba
nce
(arb
. uni
ts)
Photolum
inescence Intensity (arb. units)
Wavelength (nm)
400 500 600 700
Abs
orba
nce
(arb
. uni
ts)
Photolum
inescence Intensity
Wavelength (nm)
CdSe ZnSe ZnOPbSe
300 400 500 600
Abs
orba
nce
(arb
. uni
ts)
Photolum
inescence Intensity (arb. units)
Wavelength (nm)
IR Material Visible Material UV Material
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�� � � Hines JPC 98 Shim JACS 01Wehrenberg JPC 02
Spectroscopy
Continuously size-tunable spectra. Excitonic peaks assigned to transitions between
“particle in the box” quantum states
Murray and Bawendi, CdSe, 1993
1S-1S3/2
1S-2S3/2
1P-1P3/2
Bawendi, Murray, Norris, Efros, 93-96
Some parity rules seen in Linear and nonlinear optical spectra. (and LARGE two-photon cross-section)
Trapping and recombination center
Band-edge fluorescence
Type I Core/shell: CdSe/ZnS, CdSe/CdS, InAs/CdSe, CdSe/ZnSe/ZnS, PbSe/CdSe, etc…
Surface capping molecules or inorganic shell to “passivate” the surface
CdSe: Alkyl amines and alkylphosphine/oxide enhance luminescence. Thiols and pyridine reducePL by orders of magnitude. (different for CdTe)
Phosphors. Lighting, Light-emitting diodes. , Displays, Lasers Fluorescence:
Other materials and shapes e.g. PbSe
Kinetic size and shape control: => sphere, cubes, rods, stars…
Small changes in surfactant compositions lead to large effects on final shape and size monodispersivity.
1200 2000 2800
Abs
orba
nce
(arb
. uni
ts)
Wavelength (nm)
For PbSe nanocrystals, 80 % QY, small shift and long (~0.9 µs) lifetimes at RT.
2
1
12
32
���
����
� +=ε
εεradTT
Role of dielectric confinement in lengthening the lifetime:
ε2 ε1
With εPbSe~ 24, ε1~ 2,
T~ 20Trad ~ 0.4 µs.
JPCB 2002
IntrabandSpectroscopy
Colloid QDs are soluble mid-IR material for linear and nonlinear optic,
light emission, etc….
Carrier dynamics
• Multicarrier effects: Auger.• Intraband relaxation.• Linewidths.
Auger processShort biexciton lifetimes
PRB 60, R2181, 1999, and unpublished
the Auger process is a three-body process,
Its bulk rate is: dn/dt~ γn3, with γ~ 10-29/10-30 cm6s-1
0
0.05
0.1
0.15
0 200 400 600 800∆α
(O.D
.)Delay (ps)
0.14 mJ/cm2
1.15 mJ/cm2
1
10
2 3γ (x
10-3
0 cm
6 s-1)
Radius (nm)
R4
Klimov showed that γ is size dependent ~ R3, Science 287, 1011, 2000
Typical time scales for “biexciton” Auger relaxation:
~ 20 ps for 3 nm diameter, and ~ 500 ps for 9 nm diameter. => much faster than fluorescence. => A significant “colloid” issue for lasing.
1Se-1Pe relaxation rate?• Klimov et al, PRL 1998: 100 fs-
0.5 ps Interband bleach recovery.
• Too fast for the understood phonon-mechanism, ∆E~ 10 ωLO(phonon bottleneck)
• Explanation: electron-hole Auger relaxation, Singh (APL 1994)
150 ps; Efros (Sol. State. Comm. 1995) ~ 2 ps, Zunger (nanolett. 2004)~ 100s of ps.
• An open debate.
Intraband relaxation
Interband linewidths and Acoustic side band by hole-burning
CdSe, Palinginis, Wang et al, PRB 67, 201307 2003
~ 10 µeV observed at low power high rep-rate or cw-hole burning.
Linewidths:
10Kn-ZnO
Shim, PRB 64, 345432, 2001
intraband linewidths and LO-phonon replicas.
Weak Coupling to LO phonon• In polar semiconductors, polar cell motions, ( Cd2+�Se2- Longitudinal
Optical Phonons) can couple to changes in charge distribution.• Moderate magnitude and ~1/R size dependence (larger coupling for
smaller sizes) consistent with the bulk electron-LO coupling.
CdSe-
- -
Moderate Coupling to
acoustic phonons
CdSe
Intraband Photon Echo
InP
T2=8ps;170µ V
)/(sinh20 kTTg LOLOacoustic ωγ �++Γ=Γ
PRB 2001
Coupling to Acoustic Phonons
( ) 221
21
2 /1~)(~
~
RrDg
Tlinewidth
PSe ∆Ψ−Ψ�
γEffective FWHM
Deformation potential: Acoustic phonon shift valence and conduction band energies.
Small particle=> strong overlap of deformation amplitude and electronic wavefunction
Takagahara:
53
22
~~
~~−
−�Rg
Rg ii
ωγ
ωγ �
Brus et al:
Because 1/2k∆(r)2R3~ hν~1/R, so ∆(r)~1/R2
?
Two photon microscopy of single nanocrystal. Blanton et al, Chem. Phys. Lett. 229, 317 (1994), APL 1996.Biological imaging. Webb et al, Science 300, 1434 (2003)
Observation of intensity and spectral wandering.
Single dot microscopyAPL 1996
One-Photon Microscopy
• Visible to the eye.• Narrow emission ~
100 µeV.• Linear Stark effect
Demonstrated.• Spectral and intensity
fluctuations. • Blinking: Nirmal, Brus,
Bawendi. Power law Statistics, Kuno and Nesbitt.
Presumed to be due to charge moving on the surface, ionization, or dynamic surface reorganization.
Empedocles and Bawendi, 1996
Blinking
Time bins
A nanocrystal mystery:
Dots blink on and off with Tonν and Toff
µ.,
ν~ 1.5 ~ µ. Power law is independent of T, Radius, material. Can be seen in ensemble fluorescence (Pelton, APL), like 1/f noise of resistors. Bawendi, PRB 63, 205316, 2001
II. Colloidal QDs and the role of charges.
1. Some Possible Applications.
2. Effect of charges on color.
3. Effect of charges on Transport in close-packed QD films.
4. Effect of charges on Fluorescence
Applications: Solar cells with Colloid Quantum dots.
Alivisatos, Nature 2002 State of the art: 1.5 % efficiency at A.M. 1.5
Light Emitting Diodes
Bawendi, Nature 2002.
State of the art efficiency: 0.52%
Still in question: is it genuine e-h recombination or is it simply energy transfer?
Reduced or Oxidized QDs?
• Nozik, Henglein, Kamat, mid-80’s- mid-90’
• Brus, “A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites” , J. Chem. Phys. 79, 5566-5571 (1983)
µ
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3
Abs
orba
nce
Energy (eV)
Charge and color
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3
Abs
orba
nce
Energy (eV)
1Se
1Pe
visible absorption
��������
n-type!
Before charge transfer After charge transfer
dramatic changes in optical properties.
Shim, Nature 407, 981 (2000).
IR absorption
visible bleach
1Se
1Pe
Electrochromic response:
0
0.2
0.4
0.6
0.8
1
0.2 0.3 0.4 0.5
Abs
orb
an
ce
Energy (eV)1.8 1.9 2 2.1 2.2 2.3 2.4
0
0.2
0.4
0.6
0.8
1
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 0.2 0.4 0.6
No
rma
lized
Ab
sorb
an
ce (
arb
. un
its)
Energy (eV)
1Se
1Pe
1S3/2
2S3/2
2(3)S1 /2
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
−∆
α/α
Wang, Appl. Phys. Lett. 80, 4 (2002). Science 291, 2390 (2001).
0 V
−1.170 V
Thin films of nanocrystals change color with an applied electrochemical potential.
0
0.1
0.2
0.3
0.4
0
0.1
0.2
0.3
0.4
0.5
0.6
0 400 800 1200 1600
No
rmal
ize
d A
bso
rba
nce
(a
rb.
un
its)
−∆α
/α
Time (sec)
OffOff
OffOff
On On On On
Spectral changes resulting form 1Se occupation:
?
P-state charging at more negative potentials
Charges and Conductione− e−
Organic surfactant layer insulates the nanocrystal
σσσσ~10−14 S/cm below 200 K. 1
µ~ 10−4−10−6 cm2/Vs only at very high fields of 107
V/m. 2
1) M. Drndic et al., J. Appl. Phys. 92, 7498 (2002); C. A. Leatherdale et al., Phys. Rev. B 62, 2669 (2000);
2) D. S. Ginger et al., J. Appl. Phys. 87, 1361 (2000).
Nanocrystal solids have been reported to be “excellent” insulators!
Electrochemical tuning of carrier density in nanocrystals:
~5 �m
CdSe NanocrystalSolution
~40 mV
UV/Vis Source
UV/Vis Detector
1
2
3H2N NH2
H2N NH2
H2NNH2
Shell to shell conduction:
10-3
10-2
10-1
100
-1 -0.5 0
10-7
10-6
10-5
10-4
Op
tica
l Ble
ach
(O.D
.)
Potential (V)
a)
Co
ndu
ctance (S
)
10-7
10-6
10-5
10-4
-1 -0.5 0
10-3
10-2
10-1
100
Co
ndu
ctance (S
)
Potential (V)
b)
1Se
1Pe
Op
tica
l Ble
ach
(O
.D.)
1Se
1Pe
Conductivity peak at half filling ~ x(1−x) where x is the filling factor.
6.4 nm CdSe 5.4 nm CdSe
-0.04
-0.02
0
400 500 600 700Op
tica
l Ble
ach
(O
.D.)
W avelength (nm)
Further improvement of conduction by modifying linker:
10-8
10-7
10-6
10-5
10-4
10-3
10-2
-1 -0.5 0
Con
duct
ivity
(S/c
m)
Potential (V)
TOPO/1,7-heptadiamine
Pyridine/1,7-heptadiamine
Pyridine/1,4-phenylenediamine
6.4 nm CdSe
σ � � µ � � �
10-7
10-6
10-5
10-4
10-3
10-2
0.1 1C
ondu
ctan
ce (S
)
# e- in 1Se state
µ~ 10-2cm2/V/s
µ~ 0.5.10-5cm2/V/s
2
Yu, Science, 300, 1277 (2003)
-5
0
5
10
15
0.05 0.1 0.15 0.2 0.25 0.3
ln (
G/n
S)
T-1/2(K-1/2)
-5
0
5
10
15
0 0.05 0.1ln
(G
/nS
)
1/T(K-1)
))/*(exp( 2/1TTG −∝ , Τ∗∼ 5300Κ
���� �� � �� � �
100K
10K
Variable Range Hopping
Coulomb gap
Efros and Shklovskii model of VR-> LnG ~ (T*/T) 1/2
Energy randomness
Mott’s model of VRH -> LnG ~ T-1/4
B0
2*
48.2
ake
Tπεε
=
2
0B0
4
C)4( πεεkageT =
!
ε
Extremely nonlinear I/V: 9 decades for one decade of V
0.001
0.1
10
1000
105
0 0.001 0.002 0.003
4.3K 10K
15K
22K
36K
53K
G (n
S)
E-1/2(V/m)-1/2
"�# $
0.0001
0.01
1
100
104
106
0.1 1 10 100
Cur
rent
(nA
)
Bias (V)
52.5 K
36K
22K
15K10K
4.3K
I ~ V9 ???
EEAG *exp−=
High-Field dependence
0.001
0.1
10
1000
105
0 0.001 0.002 0.003
4.3K10K
15K
22K
36K53K
Con
duct
an
ce (
nS
) (A)
E -1/2(V/m)-1/2
0
2
4
6
8
0 4 8 12 16
Cu
rre
nt
(nA
)
4.3K10K
15K
22K
36K(B)
E (105V/m)
0
0.5
1
1.5
2
Cu
rre
nt
(nA
)
10K
15K
22K
36K(D)
4.3K0.001
0.1
10
1000
105
Co
nd
uct
anc
e(n
S)
4.3K 10K
15K
22K
36K53K
(C)
0
2
4
6
8
0 0.002 0.004 0.006
1/E1/2(V/m)-1/2
r/d
(E)
4.3K
10K15K22K
36K
53K0
2
4
6
8
0 1 2 3 4
r/d
E (105V/m)
22K15K
10K
4.3K(F)
36K
)8
exp(1
)8
2exp(
B
*B
*
TkeEr
ra
TT
TkeEr
ra
TT
ar
AG+−+
+−−=
Simulation --------
%
&eaTk
E2
*B* =
Dielectric constant effect: T*~1/εPbSe: ε~ 300 CdSe: ε~10,
0 0.001 0.002 0.003
G(n
S)
E-1/2 (V/m)-1.2
4.3 K
40K105
1000
10
0.1
0.001
PbSe, 7 nm
0.001
0.1
10
1000
105
0 0.001 0.002 0.003
4.3K10K
15K
22K
36K
53K
G (n
S)
E-1/2(V/m)-1/2
CdSe, 7 nm
Τ∗∼ 600Κ Τ∗∼ 5300Κ
Charge, Fluorescence and Lasing
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Lower ASE threshold of (QD)2-:
0.0
0.4
0.8
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600 620 640 660
PL
Inte
nsity
Wavelength (nm)
0
100
200
300
400
500
600
0 0.5 1 1.5 2
PL
and
AS
E In
tens
ity (a
rb. u
nits
)
Pump Fluence (mJ/cm2)
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0
50
100
150
200
-1000 0 1000 2000 3000 4000 5000
Ligh
t Em
issi
on In
tens
ity a
t 648
nm
(ar
b. u
nits
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Time (ms)
−1.3 V
−1.4 V
−0.7 V
−1.2 V−1.5 V
−1.6 V
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0.5
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2.5
3
3.5
4
0
0.2
0.4
0.6
0.8
1
1.2
-1.6 -1.2 -0.8 -0.4 0
−∆α/
α
AS
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Potential (V)
A
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0.05
0.10
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0.20
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0.2
0.4
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500 550 600 650 700
Abs
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PL Intensity (arb. units)
Wavelength (nm)
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Wavelength (nm)
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Charges in Colloidal Quantum Dots :
III. Reduced lasing-threshold in the conducting state.
• Sean Blanton (1992-1997)• Mark Schmidt (post-doc,1995-1997)• Margaret (Peggy) Hines(1993-1998)• Moonsub Shim (UIUC)(1998-2001) • Congjun Wang(2000-2004)• Brian Wehrenberg(2000-)• Dong Yu(2001-)
PRL 92, 216802 (2004)JPCB 0489830(2004)JACS, 125, 7806, (2003)Science, 300, 1277 (2003)
•Single dot microscopy
•Two-photon spectroscopy
•Dipole moment•CdSe/ZnS•ZnSe•IntrabandSpectroscopy
JPCB, 107, 7355 (2003).APL, 80, 4 (2002). Science, 201, 2390, (2001)JPCB, 104, 1494, (2001)Nature, 407, 981 (2001)
Charges and colloid quantum dots, the work:
Alamin Dhirani ,92-97, STM-molecular electronics, U.Toronto.
Pao-Hong Lin, 94-00, Vib. Dyn. And Mol. Elec. (ITRI Taiwan)
Uwe Schroeder, post-doc 96-98, UHV Vib. Dyn. SFG. Siemens
Chris Matranga, 98-02, Vib. Dyn., UHV, SFG. DOE lab.
Herdis Adams, 01-, STM, mol. Elec.
Mingzhao Liu, 02-, plasmonics
Jiasen Ma, 03- Mol. Elec.
Matt Pelton, 03- post-doc plasmonics
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