Debye Lecture 9
Multi-Component Nanocrystal Assemblies
C. B. Murray
Designing Nanoscale Materials
Lecture Series by 2004 Debye Institute
Professor
Christopher B. Murray
IBM Research
Ornstein Laboratory 166
Office phone 253 2227
cbmurray@alum.mit.edu
Semiconductor Quantum Dot Arrays
Glassy QD Solids
Crystalline QD Solids
Most of the work in this tutorial can be found in:
C. B. Murray, C. R. Kagan, M. G. Bawendi, Ann. Rev. Mat. Sci.
C. R. Kagan, Thesis, MIT (1996).
C. B. Murray, Thesis, MIT (1995).
Absorption and Fluorescence of
CdSe Quantum Dot Solids
Absorption
60 Å
49 Å
38 Å
28 Å
400
500
600
Wavelength (nm)
700
800
Intensity (arbitrary units)
Absorbance (arbitrary units)
Fluorescence
Quantum Dot Solids
Small-Angle X-ray Scattering
CdSe QD in PVB
CdSe QD Glass
1
Amplitude
Log(Intensity)
CdSe QD in PVB
Form Factor
of Sphere
Radial Distribution Functions
38.5 Å
38.5 Å
CdSe QD in PVB
Form Factor
of Sphere
CdSe QD in PVB
CdSe QD Glass
38.5 Å CdSe QD Glass
0
-1
a
R
1
Amplitude
NN
Log(Intensity)
-2
62 Å
62 Å CdSe QD Glass
0
-1
62 Å
-2
0
Particle Size and Nearest
Neighbor Distance from
Small Angle X-ray Scattering
2
4
6
2θ
R
8
0
2
4
6
2θ
8
10
50
100
150
Distance (Å)
NN
200
Mixed QD Solids
Didn’t satisy radius
ratio rules to form
ordered
intermetallic phases
Ordered
Glassy
18% 38.5 Å/82% 62Å CdSe QDs
Photoconductivity in Quantum Dot Solids
h•
e-
h•
hν
e-
Charge Neutral
2 Charged QDs
C. A. Leatherdale, C. R. Kagan, N. Y. Morgan, S. A. Empedocles,
M. A. Kastner, M. G. Bawendi, Phys. Rev. B, 62, 2669 (2000).
Spectral Response of Photoconductivity
rb. units)
Absorbance (arb. units)
Photocurrent (a
Photocurrent (arb. units)
Photocurrent at -500V
Photocurrent at +500V
Absorption Spectrum
2.6
Ener
2.4
2.2
gy (e
V)
0
-200
-400
2.0
Vo
lta
ge
3.0 2.8
(V
)
400
200
2.0
2.2
2.4
2.6
Energy (eV)
2.8
3.0
Size and Interparticle Distance Dependence of
Photoconductivity
10 K
Absorbance (arb. units)
Photocurrent (arb. units)
-500V
+500V
Absorbance
60.5 Å
49.5 Å
41.4 Å
34.4 Å
2.0
2.5
3.0
Different
ligands
length ~7Å
Different
ligands
length ~10Å
Energy (eV)
• Spectral response maps the size-dependent,
discrete electronic states of QDs
• Photocarriers thermalize to lowest excited
state before being separated
Increased energy required to
overcome binding energy with
decreasing QD size
Temperature Dependence of the Photoconductivity
Absorbance
0.15
0.10
0.05
0.00
2.2
2.4
2.6
2.8
Energy (eV)
Quantum Yield
Intensity
(b)
0.03
0.02
0.01
0.00
0 50 100150
Temperature (K)
1.7 1.8 1.9 2.0 2.1 2.2 2.3
Energy (eV)
Temperature
dependence of the
Quantum Yield
Scale I-V curves to nearly
universal curve
Decrease in photocurrent
and the decrease in
lifetime and Quantum Yield
have the same
temperature dependence
Fluorescence Quenching
Well passivated QDs
Poorly passivated QDs
Deep trap emission quenched at lower
fields than band edge emission
Quenching not observed in PL of isolated QDs in applied field
Quenching not directly proportional to charge separation efficiency
as free charges in film may quench PL by Auger process
Measureable quenching possible sign of free charges in film
Charge Separation versus Geminate Recombination
k
e h
φ
|e>2
d
e
very
rapid
|e>1
|e>0
G
h
γ
Energy offset of initial and
final states
knr kr
Fluorescence
|g>
Efficiency of Charge Separation
η (E,T ) =
k (E,T )
k (E,T ) + k nr (T ) + k r (T )
For
k (E,T ) << k r (T ) + k nr (T )
η (E,T ) = τ (T )k (E,T )
assume weakly
field-dependent
Core
Overcome:
• Coulomb attraction between e- and h
• Charging Energy
Trap
e h
e
e
h
e
h
h
Dipole-Dipole Interaction
Excited Donor
Unexcited Acceptor
Induce Transition
Field Created by
Interaction Energy Transition Dipole of Acceptor
Couple via Mutual Radiation Fields
Created by Transition Dipoles
Field Created by
Transition Dipole of Donor
Transfer Energy
Coulombic Interaction
Electronic Energy Transfer
Transfer of Charge
Neutral
h•
e-
h
•
Excitation as an Entity
e
-
Excited
Donor
D∗
Ground State
Acceptor
+
Ground State
Donor
A
D
Radiationless Energy Transfer
D∗ + A → D + A ∗
Requires Coupling between the
Excited Donor
and
Ground State Acceptor
Near Field
d < 100 Å
Radiative Transfer
D∗ → D + hν
A + hν → A∗
Far Field
No Direct Donor-Acceptor Interaction
Excited
Acceptor
+
A∗
One Step
Process
Two Step Process
“Real” Photon
Mediates Energy
Transfer
Long Range Resonance Transfer of Excitations
|A>1
|D>
very fast
|A>2
|g>
Couple via
Radiation Fields
Created by
Transition Dipoles
|g>
kDA
e
38.5 Å
CdSe QD
Donor
Quenching of Donor
Luminescence
Quantum Yield and Lifetime
h
62 Å
CdSe QD
Acceptor
Enhancement of Acceptor
Luminescence
Quantum Yield and Lifetime
Spectral Overlap of Donor Emission
and Acceptor Absorption
Transition
Dipole of
Excited Donor
Normalized
Spectrum for
Donor Emission
Transition
Dipole of
Ground State Acceptor
Molar Extinction
Coefficient
for
Acceptor Absorption
10 K
62 Å QD
Acceptor
Absorption
38.5 Å QD
Donor
Emission
Intensity (arbitrary units)
Intensity (arbitrary units)
Room Temperature
62 Å QD
Acceptor
Absorption
38.5 Å QD
Donor
Emission
1.9 2.0 2.1 2.2 2.3 2.4
1.9 2.0 2.1 2.2 2.3 2.4
Energy (eV)
Energy (eV)
Spectral Overlap varies with Temperature as:
Spectral Features Shift Blue as T decreases
Narrow
Mixed CdSe Quantum Dot Solid
10 K
Intensity (arbitrary units)
62 Å
CdSe QD
Intensity (arbitrary units)
Room Temperature
Abs
PL
62 Å
Abs
PL
62 Å
38.5 Å
38.5 Å
CdSe QD
1.5
38.5 Å
2.0
2.5
3.0
1.5
Energy (eV)
62 Å
2
Absorbance
Absorbance
38.5 Å
0.0
3.0
1
0.4
0.3
0.2
2.5
Energy (eV)
1
0.4
2.0
0.3
38.5 Å
0.2
2
0.0
62 Å
0.0
0.0
1.5
2.0
2.5
Energy (eV)
3.0
1.5
2.0
2.5
Energy (eV)
3.0
Efficiency of Long Range Resonance Transfer
Ro → “critical radius” -- distance of donor and acceptor separation at which
1
k D * + A →D + A *
Energy Transfer
Rate
τD
Donor Lifetime =
Sum of Rates of
De-excitation by other
competing processes
Spectral Overlap of Donor Emission and Acceptor Absorption
Quantum Yield
of Donor
∞
⎛ ϕ
ν)
R o ∝ ⎜⎜ D4 ∫ FD ( ~ν ) ε A ( ~
n
⎝
0
Index of
Refraction
⇒ 47 Å at Room Temperature
⇒ 67 Å at 10 K
d~ν ⎞⎟
~ν 4 ⎟
⎠
1
6
Ro increases as
ϕD increases
by a factor of ~10
from RT to 10 K
Spectral Overlap
compared to
Room Temperature kDA = 1 × 10 8 sec-1
⇒ RDA = 61.25 Å
Nearest Neighbor Interaction
Photoluminescence of a Mixed QD Solid and Solution
Containing 82% 38.5 Å and 18% 62 Å CdSe Quantum Dots
10 K
38.5 Å
62 Å
Mixed Film
62 Å
Pure Film
38.5 Å
1.8
2.0
2.2
2.4
Intensity (arbitrary units)
Mixed Solution
Mixed Solution
38.5 Å
62 Å
Mixed Film
62 Å
Intensity (arbitrary units)
Intensity (arbitrary units)
Intensity (arbitrary units)
Room Temperature
Pure Film
Quenching of the Luminescence QY
of the small 38.5 Å QDs
accompanied by
Enhancement of the Luminescence QY
of the large 62 Å QDs
in the Mixed CdSe QD Solid
38.5 Å
1.8
Energy (eV)
2.0
2.2
2.4
Energy (eV)
Mixed QD Solid
Excited to the Red
Of the 38.5 Å QDs
Absorptions
C. R. Kagan, C. B. Murray, M. Nirmal, M. G. Bawendi, Phys. Rev. Lett. 76, 1517 (1996).
Intensity (arbitrary units)
Intensity (arbitrary units)
Intensity (arbitrary units)
Photoluminescence Excitation: The Origin of
Emission
Mixed Film
38.5 Å CdSe QDs
1P3/21Pe/2S1/21Se
1S3/21Se
2S3/21Se
PL
PLE
Mixed Solution
PLE
PL
Solution of 62 Å QD
2S3/21Se2S1/21Se
3S1/21Se
PL
1S 1S1P3/21Pe
3/2
e
PLE
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
Energy (eV)
Time Dependence of Energy Transfer in Mixed QD Solids
Intensity (arbitrary units)
Donors--38.5 Å CdSe QD
Pure Film
Mixed Film
Quenching of Donor Lifetime
Ro = 48 Å
Intensity (arbitrary units)
Acceptors--62 Å CdSe QD
Mixed Film
Excitation Red of 38.5 Å QDs
Mixed Film
Excitation Blue of 38.5 Å QDs
Energy Transferred to Acceptor
Calculated from
Quenching of Donor
0
5
10
Time (nsec)
15
20
25
Energy Transfer within the Inhomogeneous
Distribution of Electronic States
Inhomogeneous
Distribution of
Emission
Energies
in a QD Sample
Dispersed System
Close Packed System
No Interaction between QDs
Energy Transfer
between Proximal QDs
QDs Dispersed in Solution → Close Packed in QD Solid
→ Red Shift
→ Narrowingof the Emission Lineshape
→ Asymmetric
C. R. Kagan, C. B. Murray, M. G. Bawendi, Phys. Rev. B 54, 8633 (1996).
Abs
PL
48 Å
30 Å
Absorbance (AU)
Intensity (AU)
Intensity (AU)
39.5 Å
Absorbance (AU)
-0.2
-0.1
0.0
0.1
0.2
Energy (eV)
Absorbance (AU)
Intensity (AU)
62 Å
Absorbance (AU)
Intensity (AU)
Energy Transfer within the Sample
Inhomogeneous Distribution
0.3
0.4
Experimental
Solution Luminescence
∗
Sample
Distribution
Single Dot
Lineshape
Fit Solution
Luminescence
Sample
Distribution
Efficiency
Energy Transfer
Calculated
within
From
Sample
Spectral Overlap
Distribution
Calculated
Film Luminescence
reproduces
Experimental
Film Luminescence
Probability of Energy Transfer
PDA =
Spectral Overlap from the
Absorption Spectrum for the QD Solid,
Emission Spectrum for the QDs in Solution,
Quantum Yield of the QD Solid
R 6o
R 6o + R 6DA
Nearest Neighbor
Distance
in QD Solid
D
Ro
RDA
PDA
30 Å
37.9 Å
41.3 Å
0.38
39.5 Å
35.4 Å
50.4 Å
0.11
48 Å
47.3 Å
59.1 Å
0.21
62 Å
53.9 Å
73.1 Å
0.14
Ro increases
Spectral Overlap increases
Stokes shift between
Absorption and Emission Spectra decreases
Ro and PDA
vary with the
Quantum Yield
for the QD
Solid
PDA decreases
RDA increases faster than Ro
Broad versus Narrow Size Distribution
Energy Transfer as a Function of
Sample Inhomogeneous Distribution
Broad vs Narrow
Sample Inhomogeneous Distribution
Absorbance
Fluorescence
2.1
2.2
2.3
2.4
2.5
2.6
Intensity (AU)
Absorbance (arbitrary units)
Intensity (arbitrary units)
Solution
Film
Luminescence Luminescence
Experimental
Experimental
Fit
Calculated
2.15
2.20
2.25
2.30
2.35
Energy (eV)
Energy (eV)
Peak in Emssion Shifts Red
Red Shift Increases
with Increasing
Sample Inhomogeneous Distribution
Narrow Distribution ∆E = 14.6 meV
Broad Distribution ∆E = 29.6 meV
Emission Lineshape Narrows
Narrow Distribution ∆FWHM=11 meV
Broad Distribution ∆FWHM=23 meV
Emission Lineshape Narrows
from Solution → Film
Narrowning Increases
with Increasing
Sample Inhomogeneous Distribution
Concentration Dependence of Luminscence Lineshape
Pure QD Solid
Pure Solution
Intensity (arbitrary units)
Mixed QD Solids
18% 62 Å QDs
6.2% 62 Å QDs
4.4% 62 Å QDs
3.2% 62 Å QDs
2.2% 62 Å QDs
1.95
2.00
2.05
2.10
Energy (eV)
Luminescence
Lineshape
Shift Blue → Solution
Asymmetric → “Gaussian-like”
as
Decreasing Concentration of 62 Å QDs in Matrix of 38.5 Å QDs
Increases Average Separation between 62 Å QDs
Reduces Probability of Energy Transfer between 62 Å QDs
exchange-spring magnets
concept
A super strong magnet will enable
M
Exchange-Spring Nanocomposites via Self-Assembly
H
material requirements:
High Ms, high Hc, high
(BH)max
M
tS
tS ≤ 2δW (10-20 nm)
H
nanoparticle self-assembly-bottom-up approach
nature limits: Ms vs. Hc
solution: exchangespring magnets
Nanoscale Engineering for Optimum Exchange-Coupling
Nature, 420, 395 (2002)
0.2
m (arb.unit)
m (arb. unit)
0.4
0.0
-0.2
-0.4
A
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
C
-60 -40 -20 0 20 40 60
-60 -40 -20 0 20 40 60
H (kOe)
H (kOe)
Hysteresis loops of FePt-Fe3Pt nanocomposite
derived from Fe3O4:FePt binary assembly (A)
4 nm:4 nm; and (C) 12 nm:4 nm
10
8
B (kG)
TEM images of the binary composite
assemblies of
(A) Fe3O4(4 nm):Fe58Pt42(4 nm);
(B) Fe3O4(8 nm):Fe58Pt42(4 nm);
(C) Fe3O4(12 nm):Fe58Pt42(4 nm);
(D) FePt)Fe3O4 core-shell
(A) Fe58Pt42
(B) FePt:Fe3Pt
36% energy product
enhancement
compared to singlephase FePt!
6
4
2
0
-2
-8
-6 -4 -2
H (kOe)
0
Binary nanocomposites
4 nm:4 nm
4 nm:8nm
TEM images of two different binary
assemblies prepared directly from
particle dispersions of 4 nm FePt as
well as 4 nm Fe3O4 and 8 nm Fe3O4.
HRTEM image of an exchange-coupled
nanocomposite (FePt-Fe3Pt) made from
4nm FePt and 4nm Fe3O4 nanoparticles
under reductive annealing. Shown here is
a modulated structure with FePt and Fe3Pt
in intimate contact, resulting in exchangecoupling.
H. Zeng et al, Nature, 2002, 420, 395.
Nanocomposite magnetics
10
(A) Fe58Pt42
(B) FePt:Fe3Pt
8
(BH)max ~
20.1 MGOe
(160 kJ/m3)
6
B (kG)
m (arb. unit)
0.4 4 nm:4 nm
0.2
4
0.0
2
-0.2
0
-0.4
A
-2
-8
-60 -40 -20 0 20 40 60
12000
H (kOe)
4 nm:8 nm
10000
0.1
8000
0.0
-0.1
-0.2
B
-60 -40 -20 0 20 40 60
H (kOe)
Hysteresis loops at room
temperature with the
composites from 4nm:4nm and
4nm:8nm nanoparticles
respectively.
B (Gauss)
m (arb.unit)
0.2
(BH)max~
14.7 MGOe
(117 kJ/m3)
6000
-6
-4
H (kOe)
-2
0
Ms =1050 emu/cc
Ms
= 1050 emu/cc
Hc= 2.4 T
Hc
= 2.4
TMGOe
(BH)max
~ 24
(BH)max ~24 MGOe
(191 kJ/m3)
4000
2000
0
-10000
-8000
-6000
-4000
-2000
0
H (Oe)
(BH)max, energy product, reflects the
ability for a composite to store the
magnetic energy, the larger the better.
Binary Nanocrystal Array’s a New Class of Nanostructured Materials
Franz Redl, Kyung-Sang Cho and C. B. Murray
Binary Assembly
Composites of:Ferromagnets, Noble Metals,
Semiconductor QDs, Ferroelectrics,
Superconductors, may all be possible.
AB13
New Near IR Magneto-Optic Composite ~13nm Fe2O3 and 5nm PbSe QDots
Binary nanocomposites
via self-assembly of two kinds of NPs
Mix
Self-Assembly
A nanoparticle dispersion in an
organic solvent. The particles are
stabilized by a layer of organic
surfactant to prevent them from
aggregation.
Binary nanocomposite: Magnetic-magnetic
composite or magnetic-semiconductor composite.
PbSe – Au binary mixture
20 nm
Au
PbSe
10 nm
Fe2O3 – Au binary mixture
20 nm
20 nm
PbSe – Ag binary nanoparticle mixture
PbSe (large) – Ag (small) binary nanoparticle mixture
PbSe (large) – Ag (small) binary nanoparticle mixture
PbSe – Ag binary nanoparticle mixture
18
80
60
60
c)
40
d)
40
12
b)
6
d)
0
-20
M / (A m2 kg-1)
arbitrary units
20
a)
c)
20
-40
-60
0
-1000 -500
0
500
1000
-20
-40
-60
0
0
100
200
T/ K
300
-80
-5000-4000-3000-2000-1000 0 1000 2000 3000
H/ Oersted
140
4
a)
-1
εparticle / (l molparticle )
120
b)
100
c)
2
80
d)
60
0
40
800
1200
1600
2000
2400
20
0
800
1600
wavelength / nm
2400
Simultaneous Reaction
A & B Compounds & Alloys
Anneal to remove Organic
Ferromagnets,
Noble Metals,
Semiconductor QDots,
Ferroelectrics,
Superconductors
Binary Assembly
AB2 & AB13
Dicyanobenzene linked Cobalt Nanocrystals
Customize organic linkers (molecular wires)
N C
C N
N C
C N
N C
C N
N C
N C
C N
N C
N C
C N
C N
C N
N C
C N
N C
N C
N C
C N
C N
C N
N C
C N
150 nm
50 nm