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 Dots
Perceived 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-
hν
Many start-up companies,
Nanosys, Q.dot, Evident tech….
h+
150 M$ of venture capital.
1B$ perceived value
Research: Fabrication, Spectra-size, carrier dynamics, trapping, energy relaxation,
carrier transport…
Nanocrystal quantum dots:
The pioneers:
Ekimov and Efros, 1980. Effective Mass
Approximation applied to CuCl aggregates and
excitonic spectra.
The colloid synthesis:
1982: Precipitation, ionic precursors, Aqueous
solutions. Brus, Henglein, Nozik
1986: Micelle “nanoreactor”. Pileni, Brus.
Water/micelle in Oil.
1993 “organometallic approach”: Purely organic
environment, high temperatures and surfactants.
Murray and Bawendi
2000 “Greener” reagents for II-VI. Peng
Spectroscopy
1200
2000
2800
Wavelength (nm)
IR Material
400 500 600 700
Wavelength (nm)
300
400
500
Wavelength (nm)
Visible Material
Shim JACS 01
ZnO
Absorbance (arb. units)
ZnSe
Absorbance (arb. units)
Absorbance (arb. units)
Absorbance (arb. units)
CdSe
Photoluminescence Intensity (arb. units)
Hines JPC 98
Photoluminescence Intensity
PbSe
Wehrenberg JPC 02
300 400 500 600
Wavelength (nm)
UV Material
Photoluminescence Intensity (arb. units)
QDs from the Near IR to UV:
Continuously size-tunable spectra.
Excitonic peaks assigned to transitions between
“particle in the box” quantum states
1P-1P3/2
Murray and Bawendi, CdSe, 1993
1S-2S3/2
1S-1S3/2
Bawendi, Murray, Norris, Efros, 93-96
Some parity
rules seen in
Linear and
nonlinear optical
spectra. (and LARGE
two-photon cross-section)
Fluorescence:
Phosphors. Lighting, Light-emitting diodes.
, Displays, Lasers
Surface capping molecules or inorganic shell to “passivate” the surface
CdSe: Alkyl amines and
alkylphosphine/oxide enhance
luminescence. Thiols and pyridine reduce
PL by orders of magnitude. (different for
CdTe)
Band-edge
fluorescence
Trapping and
recombination
center
Type I Core/shell: CdSe/ZnS, CdSe/CdS,
InAs/CdSe, CdSe/ZnSe/ZnS,
PbSe/CdSe, etc…
Absorbance (arb. units)
Other materials and shapes e.g.
PbSe
1200
2000
2800
Wavelength (nm)
Kinetic size and shape
control: => sphere,
cubes, rods, stars…
Small changes in surfactant
compositions lead to large
effects on final shape and
size monodispersivity.
For PbSe nanocrystals, 80 % QY, small
shift and long (~0.9 µs) lifetimes at RT.
Role of dielectric
confinement in
lengthening the
lifetime:
T = Trad
ε 2 + 2ε 1
3ε 1
With εPbSe~ 24, ε1~ 2,
T~ 20Trad ~ 0.4 µs.
JPCB 2002
ε2
2
ε1
Intraband
Spectroscopy
Colloid QDs are soluble mid-IR material for linear and nonlinear optic,
light emission, etc….
Carrier dynamics
• Multicarrier effects: Auger.
• Intraband relaxation.
• Linewidths.
Auger process
Short biexciton lifetimes
PRB 60, R2181, 1999, and unpublished
6 -1
cm s )
0.15
Klimov showed that γ is
size dependent ~ R3,
γ (x 10
0.1
∆α (O.D.)
Its bulk rate is: dn/dt~ γn3,
with γ~ 10-29/10-30 cm6s-1
10
-30
the Auger process is a threebody process,
4
R
1
2
3
Radius (nm)
2
1.15 mJ/cm
0.05
Science 287, 1011, 2000
2
0.14 mJ/cm
0
0
Typical time scales for “biexciton” Auger relaxation:
200
400
Delay (ps)
~ 20 ps for 3 nm diameter, and ~ 500 ps for 9 nm diameter. => much
faster than fluorescence. => A significant “colloid” issue for lasing.
600
800
Intraband relaxation
1Se-1Pe relaxation rate?
• Klimov et al, PRL 1998: 100 fs0.5 ps Interband bleach
recovery.
• Too fast for the understood
phonon-mechanism, ∆E~ 10
ωLO(phonon bottleneck)
• Explanation: electron-hole
Auger relaxation, Singh (APL 1994)
Efros (Sol. State. Comm. 1995) ~ 2
ps, Zunger (nanolett. 2004)~ 100s of ps.
• An open debate.
150 ps;
Linewidths:
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.
intraband linewidths and LO-phonon replicas.
n-ZnO
10K
Shim, PRB 64, 345432, 2001
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
Γ = Γ0 + g acousticT + γ LO sinh 2 ( ω LO / kT )
InP
T2=8ps;170µ V
Intraband Photon Echo
PRB 2001
CdSe
Coupling to Acoustic Phonons
Deformation potential: Acoustic phonon shift
valence and conduction band energies.
Small particle=> strong overlap of deformation
amplitude and electronic wavefunction
Effective FWHM
linewidth ~ γT
g ~ De2
(Ψ
1S
2
2
)
− Ψ1P ∆ (r ) ~ 1 / R 2
Because 1/2k∆(r)2R3~ hν~1/R, so ∆(r)~1/R2
Takagahara:
?
Brus et al:
γ~
g i2
ωi ~ R −2
γ ~ gω 3 ~ R −5
Single dot microscopy
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.
APL
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.
Empedocles and Bawendi, 1996
Presumed to be due to charge moving on the
surface, ionization, or dynamic surface
reorganization.
Blinking
A nanocrystal mystery:
Dots blink on and off with Tonν and Toffµ.,
Time
bins
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
State of the art efficiency: 0.52%
Still in question: is it genuine e-h recombination or is it simply energy transfer?
Bawendi, Nature 2002.
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
Charge and color
Before charge transfer
After charge transfer
3
3
1Pe
1Se
2
visible absorption
1.5
1
2
1
0.5
0
0
0.5
1
1.5
2
Energy (eV)
2.5
3
visible bleach
1.5
0.5
0
1Pe
1SeIR absorption
2.5
Absorbance
Absorbance
2.5
n-type!
0
0.5
1
2
Energy (eV)
dramatic changes in optical properties.
Shim, Nature 407, 981 (2000).
1.5
2.5
3
Electrochromic response:
Norm alized Absorbance (arb. units)
Absorbance
O ff
O ff
1P e
0.5
0.4
0.3
0.3
0.8
0.2
0.2
0.1
0.1
On
0
0
On
400
On
800
1200
Tim e (sec)
0.6
On
1600
0
0V
0.4
0.4
0.2
0
0.2 0.3 0.4 0.5
0.5
0.5
−1.170 V
0.2
0
1.8 1.9 2 2.1 2.2 2.3 2.4
Energy (eV)
0.4
1S e
0.3
0.3
0.2
0.1
0
-0.1
0.4
1S 3/2
2S 3/2
2(3)S 1 /2
0.2
0.1
0
-0.1
-0.2
-0.2
0 0.2 0.4 0.6 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2
Energy (eV)
Thin films of nanocrystals change color with an applied electrochemical
potential.
Wang, Appl. Phys. Lett. 80, 4 (2002). Science 291, 2390 (2001).
− ∆ α /α
0.6
O ff
0.4
−∆ α /α
0.8
1
0.6
Off
Normalized Absorbance (arb. units)
1
Spectral changes resulting form
1Se occupation:
?
P-state charging at more negative
potentials
Charges and Conduction
e−
e−
Organic surfactant layer insulates
the nanocrystal
Nanocrystal solids have
been reported to be
“excellent” insulators!
σ~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).
Electrochemical tuning of carrier density in
nanocrystals:
~5 m
1
CdSe Nanocrystal
Solution
2
NH2
H2N
NH2
H2N
NH2
H2N
3
UV/Vis Source
~40 mV
UV/Vis Detector
Shell to shell conduction:
-0.04
400 500 600 700
W avelength (nm)
-1
10
-4
10
-5
1Se
10
-2
10
-6
1Pe
10
-3
10
-1
-0.5
P otential (V)
0
-7
10
Optical Bleach (O.D.)
Optical Bleach (O.D.)
-0.02
6.4 nm CdSe a)
10
0
5.4 nm CdSe b)
-1
1S
10
10
10
-5
10
-6
10
-7
e
-3
-1
-4
e
-2
1P
10
-0.5
Potential (V)
0
Conductance (S)
0
10
0
Conductance (S)
Conductivity peak
at half filling ~
x(1−x) where x is
the filling factor.
Optical Bleach (O.D.)
10
Further improvement of conduction by
modifying linker:
-2
-2
10
10
Pyridine/1,4phenylenediamine
-2
-3
-3
10
-5
10
TOPO/1,7heptadiamine
-7
10
6.4 nm CdSe
σ
0.1
1
# e in 1S state
-
µ
2
e
-0.5
0
Potential (V)
-6
10
2
µ~ 0.5.10 cm /V/s
-7
-1
-5
10
-8
10
-5
10
-6
10
-4
10
-4
10
Conductance (S)
Pyridine/1,7heptadiamine
Conductivity (S/cm)
10
2
µ~ 10 cm /V/s
Yu, Science, 300, 1277 (2003)
G ∝ exp(−(T * / T ) )
1/ 2
, Τ∗∼ 5300Κ
15
100K
10
5
0
-5
0
5
0.05
-1
1/T(K )
0.1
0
10K
(K
-1/2
0.3
)
-1/2
0.25
T
0.2
0.15
0.1
-5
0.05
ln (G/nS)
10
ln (G/nS)
15
Variable Range Hopping
Energy randomness
Mott’s model of VRH -> LnG ~ T-1/4
T = e ag k (4πεε )
4
0
B
2
0
!
C
ε
Coulomb gap
Efros and Shklovskii model of VR-> LnG ~ (T*/T) 1/2
2
2
.
8
e
T* =
4πεε 0 ak B
Extremely nonlinear I/V:
$
# "
9 decades for one decade of V
10
6
10
4
10
5
53K
1000
100
1
0.01
36K
36K
G (nS)
Current (nA)
52.5 K
I ~ V9 ???
22K
22K
15K
0.1
15K
10K
1
Bias (V)
10K
4.3K
4.3K
0.0001
0.1
10
0.001
10
100
0
0.001
E
-1/2
0.002
(V/m)
-1/2
G = A exp− E * E
0.003
5
15K
0.1
Conductance(nS)
0
-1/2
(V/m)
1000
10
22K
0.1
15K
4.3K
r/d
15K
10K 4.3K
22K
(F)
4.3K
6
4.3K
36K
(D)
10K
15K
22K
36K
4
2
53K
0
0
0.002 0.004 0.006
1/E
1/2
(V/m )
r T * a eEr
+
exp(−2 −
)
a T 8r k BT
G=A
T * a eEr
+
1 + exp(−
)
T 8r k B T
0.5
0
8
10K
15K
16
22K
1
(E)
6
4.3K
4
8
12
5
E (10 V/m)
36K
1.5
10K
4
0
2
53K
36K
2
10K
-1/2
(C)
0.001
8
15K
2
0
0.001 0.002 0.003
E
5
4
10K
4.3K
Current (nA)
0.001
22K
-1/2
Simulation --------
0
k BT *
E =
2ea
*
0
1
2
5
3
E (10 V/m )
4
%
22K
6
High-Field
dependence
(B)
36K
&
10
Current (nA)
53K
36K
1000
10
8
(A)
r/d
Conductance (nS)
10
Dielectric constant effect: T*~1/ε
PbSe: ε~ 300
CdSe: ε~10,
40K
5
10
10
PbSe, 7 nm
10
53K
36K
10
22K
15K
0.1
0.1
0.001
CdSe, 7 nm
1000
4.3 K
G (nS)
G(nS)
1000
5
10K
4.3K
0
0.001
-1/2
E
Τ∗∼ 600Κ
0.002
-1.2
(V/m)
0.003
0.001
0
0.001
E
0.002
-1/2
-1/2
(V/m)
Τ∗∼ 5300Κ
0.003
Charge, Fluorescence and Lasing
-1
Pump threshold:
1eh /dot
Pump threshold:
0eh /dot!
Lower ASE threshold of (QD)2-:
500
400
1.2
PL Intensity
PL and ASE Intensity (arb. units)
600
0.8
0.4
0.0
600 620 640 660
Wavelength (nm)
300
200
100
0
0
0.5
1
1.5
2
Pump Fluence (mJ/cm )
2
!
"#$
4
3
2.5
2
1.2
1
0.8
0.6
−∆α/α
1
0.5
0
-1.6 -1.2 -0.8 -0.4
Potential (V)
2
1.5
ASE Threshold (mJ/cm )
3.5
A
0.4
0.2
0
0
Light Emission Intensity at 648 nm (arb. units)
%#$&
200
&
'
−1.3 V
150
−1.4 V
100
−1.5 V
−1.2 V
50
−0.7 V
0
-1000
−1.6 V
0
1000 2000 3000
Time (ms)
4000
5000
!
0.20
1.2
1.2
0.20
1.0
0.10
0.6
0.4
0.05
"#$
Absorbance
Absorbance
(−
0.8
0.15
(
(&−
0.10
550
600
650
Wavelength (nm)
0.0
700
0.6
0.4
0.05
0.2
0.00
500
0.8
0.2
0.00
500
550
600
650
Wavelength (nm)
0.0
700
PL Intensity (arb. units)
(
PL Intensity (arb. units)
0.15
1.0
ν
,
)
# .
0
* %+#
- /
)
0
/
/
1223"43#
1
&/
+#
Charges in Colloidal Quantum Dots :
III. Reduced lasing-threshold in the conducting state.
•Single dot
microscopy
•
•
•
•
•
•
•
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-)
•Two-photon
spectroscopy
•Dipole moment
•CdSe/ZnS
•ZnSe
•Intraband
Spectroscopy
Alamin Dhirani ,92-97, STMmolecular electronics,
U.Toronto.
Pao-Hong Lin, 94-00, Vib. Dyn.
And Mol. Elec. (ITRI Taiwan)
Charges and colloid quantum dots, the work:
PRL 92, 216802 (2004)
JPCB 0489830(2004)
JACS, 125, 7806, (2003)
Science, 300, 1277 (2003)
JPCB, 107, 7355 (2003).
APL, 80, 4 (2002).
Science, 201, 2390, (2001)
JPCB, 104, 1494, (2001)
Nature, 407, 981 (2001)
Uwe Schroeder, post-doc 9698, 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