Designing Multifunctional Binary
Nanocomposites for Direct
Energy Conversion Applications
Particle-Based Device Technologies
Toronto, Ontario
Jeff Urban
University of Pennsylvania
8/20/2007
Nanocomposites for DEC
• New Frontiers in Material Properties Design
• Opportunities in Thermoelectrics
• Synthesizing Nanocrystal Superlattices
• Electronic Properties of Nanocrystal Films
• Studying Nanocrystal Thermal Conductance
• Future directions and Summary
Building Designer Solids via
Modular Assembly
Kalsin et al. Science, 312 420 (2006)
Shevchenko, E.V., Talapin, D.V. et al. Nature, 439 55 (2006)
It is now possible to construct binary superlattices
composed of a variety of nanocrystals
Combining nanocrystals with desired electronic,
optical, magnetic, and dielectric properties
enables construction of new “metamaterials”
Leunissen et al. Nature, 437 235 (2006)
Electronic Properties of Nanocrystal Solids
Single Crystals
20 nm
Thermoelectrics, Photovoltaics, etc.
Transistors
Models for Transport
Nanocrystalline Solids
Display Cells Battery
Printable Thin-Film Electronic Devices
Nanocomposites for DEC
• New Frontiers in Material Properties Design
• Opportunities in Thermoelectrics
• Synthesizing Nanocrystal Superlattices
• Electronic Properties of Nanocrystal Films
• Studying Nanocrystal Thermal Conductance
• Future directions and Summary
Energy Security: Waste Heat Recovery
100
Ene
rgy
Uni
ts
40 Energy Units
60 Energy Units
~60% of raw energy sources are lost as waste heat
Goal: 10% capture and recovery rate would be valuable
Thermoelectric Applications:
Converting Heat into Electricity
Waste Heat Recovery
•cars, trucks, ships
•chemical plants
•utilities, power plants
Geothermal Power
Generation
Solar Energy
Direct Nuclear to
Electrical Conversion
Remote Power
Generation
Wireless Sensor
Power Supply
Waste Heat is Abundant and Free
image courtesy M. Kanatzidis
The Case For Nanostructured
Thermoelectric Materials
Harman et al. (2002)
Venkatasubramanian et al. (2001)
Nanopatterned
Materials
(ZT > 1)
Discovery of
Bulk Solids
(ZT ≤ 1)
adapted from Majumdar, A. Science (2004)
Evaluating Thermoelectrics: Figures
of Merit
⎛
⎜
Th − Tc ⎜ 1 + .....
ZT − 1
η=
Th ⎜ 1 + .... + Tc
ZT
⎜
Th
⎝
TE Efficiency
⎞
⎟
⎟
⎟
⎟
⎠
Electrical
Conductivity
Seebeck
Coefficient
σS
ZT =
T
κ
2
Thermal Conductivity
Maximum efficiency of TE devices is bound by the Carnot efficiency
As ZT is increased, TE efficiency approaches the Carnot limit
Design Advantages in
Nanocomposite Thermoelectrics
Electrical Conductivity ↑ σ
Nanocrystals can be used
to introduce dopants:
Ag2Te (p-type)
Bi (n-type)
Seebeck
Coefficient
σS2
ZT = κ T
Thermal Conductivity
κ = κe + κl
set by Wiedemann-Franz
reducible by
interface scattering
1 ⎛ ∂n ⎞
S~
⎜ ⎟
n(ε ) ⎝ ∂ε ⎠ ε = Ε f
DOS enhancement in
nanomaterials boosts
Seebeck coefficient
ZT and Electronic Structure
band anisotropy
band degeneracy
band anisotropy is
extreme in PbTe:
ab║~12nm, ab┴~150nm
γ = 4 for PbTe
T
Z max ∝, γ
τ
3/ 2
mxm y
mz
κ lattice
e
1
(r+ )
2
semiconductors with high band anisotropy and degeneracy are optimal
PbTe has a very appealing electronic structure for TE applications
Dreaming of an Ideal Thermoelectric
PbTe
Ag2Te
{
Narrow Band-Gap Semiconductors (PbTe)
Highly Anisotropic Band Structure (PbTe)
High Band Degeneracy (PbTe)
Heavy Elements (PbTe, Ag2Te)
Complex Compositions (BNSL)
High Interface Density (BNSL)
Complex Lattice Structure (BNSL)
Nanocrystal Seebeck Boost (BNSL)
Nanocrystal composites have multiple design advantages
over conventional bulk thermoelectric materials
Doping of PbTe with Ag2Te
Introduction of Ag2Te increases
the conductivity of bulk PbTe by
~one order of magnitude
Ag2Te hole-dopes PbTe and can
diffuse very quickly into the lattice
Noda, Y. et al. J. Jpn. Inst. Metals, 61 180 (1997)
Challenges in Traditional
Approaches to Nanocrystal Doping
+
Bulk semiconductors: solid solubility limit (SSL) accurately predicts
achievable doping levels
Semiconductor nanocrystals: SSL limit is never achieved; realized
doping levels are substantially lower or zero
Possible Explanations: “self-cleansing” effect in nanocrystals;
kinetic constraints preclude realization of thermodynamic SSL limit
Synthesis and Assembly of
Nanostructured Materials
R3PTe
Chemical
Precursors
T=160° C
PbX2 + RCOOH
PbTe nanocrystals
Nanocrystal
Superlattice
Films
Self-Assembly:
Drop Casting
Nanoparticle
Superlattice
Quantum Confined PbTe Nanocrystals
Size-tunable PbTe nanocrystals provide tunable absorbers in the IR
J.J. Urban et al. J. Am. Chem. Soc. (2006)
n2 2
n = n0 +
E + ....
2
Z-scan method enables rapid
characterization of n2
Normalized Transmittance
Nonlinear Optical (NLO)
Properties of PbX Nanocrystals
λ = 1μm to 1.5μm
Z position (mm)
PbX NLO properties interesting due to strong confinement
n2 is negative and modest in size (about -1x10-13 cm2/W for PbX)
Size-Control in PbTe: Ligand Control
of Decomposition Kinetics
p
r
e
c
u
r
s
o
r
s
nucleation
growth
fast
until all
monomer is
consumed
slow
Less oleic acid creates unstable
lead acetateÆmany, small nuclei
Less oleic acid yields smaller NC’s
More oleic acid yields larger NC’s
More oleic acid creates stable
lead oleateÆfewer, larger nuclei
Ag2Te Nanocrystals: p+ Dopants
λmax = 1152nm
10nm
Ag2Te absorbs at an ideal for
biological imaging applications
Good for fundamental characterization—bandgap? mobility?
We have developed the first known synthesis of Ag2Te nanocrystals
Shape and Size Evolution in
Ag2Te Nanocrystals
50nm
50nm
50nm
Nanocomposites for DEC
• New Frontiers in Material Properties Design
• Opportunities in Thermoelectrics
• Synthesizing Nanocrystal Superlattices
• Electronic Properties of Nanocrystal Films
• Studying Nanocrystal Thermal Conductance
• Future directions and Summary
How Nature Builds Multicomponent
Structures: The Brazilian Opal
J.V. Sanders Phil. Mag. A, Vol. 42, No. 6, 705-720 (1980)
Opal consists of large (0.36μm) and small (0.21μm) SiO2 spherical crystals
These crystals co-crystallize into two binary crystalline phases: AB2 and AB13
How Chemists Build
Multicomponent Structures
A
1. High Monodispersity
B
Mix
2. Proper A:B Size Ratio
Self-Assembly
3. Oppositely Charged
20nm
Diversity in Nanocrystal Binary
Superlattices
a
Great diversity in both
stoichiometry and symmetry of
multicomponent nanocrystal
superlattices are accessible
c
b
d
e
f
g
h
i
Binary Superlattices of PbTe and Ag2Te
PbTe
Ag2Te
PbTe and Ag2Te form an AB superlattice whose [100] projection is shown
AB Superlattices: A Closer Look
50nm
Analyzing Nanocrystal Superlattices
NaCl
CuAu
Binary superlattices of NaCl and CuAu symmetry have been prepared
Both superlattice isomers available (i.e. Ag2Te (L)/PbTe (S) and vice versa)
TEM measurements supported by molecular modeling and wide-angle XRD
Nanocomposites for DEC
• New Frontiers in Material Properties Design
• Opportunities in Thermoelectrics
• Synthesizing Nanocrystal Superlattices
• Electronic Properties of Nanocrystal Films
• Studying Nanocrystal Thermal Conductance
• Future directions and Summary
Charge Transport in Nanocrystal Films
α
matching site energies
semiconductor core
capping ligands
Ec
Ev
S
β
D
exchange coupling energy
Gate
Ec
Coulomb charging energy
2Ec
donor
+
ionized donor
e-
EC ~
e2
4πεε 0 d
Advantages of Lead Chalcogenide
Nanocrystals
α
matching site energies
PbSe
fwhm ~36 meV
• small Δα
• high DOS
exchange coupling energy
• large Bohr radius for e- and h+
• rB ~23nm (PbSe); ~46nm (PbTe)
Ec
Absorbance
β
1Sh-1Se
1Ph-1Pe
Coulomb charging energy
• very high static dielectric constant
• ε0 ~250 (PbSe); ~1,000 (PbTe)
0.6
0.8
1.0
photon energy, eV
• High carrier mobility in bulk lead chalcogenides
-PbTe is amongst the highest known with ~106 cm2/Vs
1.2
Poor Conduction in Nanocrystal Solids
PbSe
CdSe
ID [pA]
5 L=4 μm
W=7.5 mm
0
11
ρ=~10 Ω cm
no gate effect
-5
-4
-2
0
2
UDS [V]
4
β low exchange coupling energy
ƒcarrier traps
ƒinefficient carrier injection from electrodes
ƒlarge dot-dot interparticle separation
Activating Electronic Transport in
Semiconductor Nanocrystal Films
Ligand shells create large interparticle
spacings, thus suppressing charge
transport in nanocrystal thin films
semiconductor core
insulating shell
Hydrazine treatment removes these
capping groups, thereby enhancing charge
transport by ~10 orders of magnitude
Building IV-VI Nanocrystal
Devices
S
D
•Drop-casting from hexane:octane (8:1)
•Spin-coating from chloroform
Gate
50 nm
PbSe NCs
GISAXS indicates
excellent film order over
>200 mm length scale
Films are smooth over >mm length scale
SiO2
Si
GISAXS: “Seeing” Nanoscale
Density Correlations
GISAXS patterns for limiting cases
GISAXS provides information about density correlations in x, y, and z
In QD films, data on interparticle spacings and film structure is obtained
GISAXS on Nanocrystal Thin Films
initial
qz(Å-1)
0.3
qz(Å-1)
0
0.3
N2H4 treated
0
-0.3
0
0.3
qy(Å-1)
Hydrazine treated QD’s retain their
size, shape, and individuality
interparticle spacings:
native film: ~1.7nm
after treatment: ~0.3nm
Lead Chalcogenide Nanocrystal TFTs
VG
VDS= 40V
0.08
-3
1.5
20 V
]
1/2
[A
ID [A]
30 V
2.0
ID [mA]
10
-4
10
0.04
-5
1.0
10 V
0.5
0V
0.0
-10 V
-20 V
-30 V
0
10
20
30
VDS [V]
WCi μ sat
(VG − VT ) 2
ID =
2L
10
-6
10
-20
0 20
VG [V]
40
1/2
40 V
L=10 µm
W=2000 µm
2.5
-2
10
IDS
3.0
0.00
Mobility: PbSe 0.95 cm2V-1s-1
PbS 0.1 cm2V-1s-1
PbTe 0.9 cm2V-1s-1
D. V. Talapin & C. B. Murray. Science 310, 86 (2005)
n-type PbTe Nanocrystal Transistors
Hydrazine n-dopes
the PbTe NC films
Treated films act as
gate-modulated NC
transistors
Although linear
regime is shown,
transistor currents
saturate enabling
mobility calculations
μsat ~ 0.9 cm2/Vs
Ion/Ioff ~ 103
μmax = 11.5 cm2/Vs
+80
+60
+40
Vg = +20
PbTe Nanocrystal Solids Annealed at 200°C:
mobility >11 cm2/Vs
VG
15
60V
L=20μm
W=900μm
2
μlin=11.5 cm /Vs
40V
ID [mA]
15
10
ID [mA]
20
10
5
20V
5
L= 20 μm
W= 900 μm
0V
-20V
0
0
5
10
VDS [V]
15
20
0
-60
Vds= 20V
-30
0
30
VG [V]
record mobility for solution-processed semiconductors
compatible with common plastic substrates
60
p-type PbTe Nanocrystal Transistors
Vsd (V)
PbTe also shows
p-type FET
behavior upon
hydrazine removal
-80
Vg (V)
Isd (A)
μsat~ 0.15 cm2/Vs
Ion/Ioff ~ 102
-60
Isd (A)
This n- to pconversion is
fully reversible
Vg = -40
p-Type Electronic Transport in Ag2Te
Vsd (V)
Ag2Te demonstrates no substantial gate modulation of current
Isd (A)
Although chemical
treatment enhances
transport by ~6
orders of magnitude,
these films remain
quite resistive
compared to PbTe
QD solids
PbTe
?
Ag2Te
20nm
Binary Superlattice
Nanocrystal Building Blocks
Multicomponent Nanocrystal Superlattices
Will modified chemical properties translate
into modified physical properties?
J.J. Urban et al. Nature Materials 6, 115 (2007)
p-Type Electronic Transport in 1:1
Mixed PbTe/Ag2Te Composites
Vsd (V)
p-type tansport in
mixed composites
exceeds sum of
individual behavior
Positive gate voltages
reduce, but cannot
shut off transport
1:1 Film
Isd (A)
Ag2Te seems to add
many carriers to the
PbTe system
p-Type Electronic Transport in 5:1
Mixed PbTe/Ag2Te Composites
Vsd (V)
5:1 composites remain
p-type, however are
less conductive than
the 1:1 mixtures
as in 1:1 composites,
positive gate voltages
reduce, but cannot
shut off transport
5:1 Film
Isd (A)
Ag2Te still adds
carriers to PbTe,
evinced by enhanced
conductivity relative
to pure PbTe
Synergistic Effects in p-type Transport
G1:1 Film >100x greater than GPbTe + GAg2Te
1:1 Film
p-type transport
enhanced in binary
nanocrystal solids
5:1 Film
PbTe
Ag2Te
J.J. Urban et al. Nature Materials 6, 115 (2007)
Superlattices After Heating
Even 200°C thermal treatments do not abrogate superlattice order
High-temperature treatments do further reduce organic shells
Nanocomposites for DEC
• New Frontiers in Material Properties Design
• Opportunities in Thermoelectrics
• Synthesizing Nanocrystal Superlattices
• Electronic Properties of Nanocrystal Films
• Studying Nanocrystal Thermal Conductance
• Future directions and Summary
Characterizing Nanocrystal Thin Film
Thermal Properties
Δ R ∼ ΔT
Thin film thermal conductivity measurements are challenging—
surface and substrate effects complicate steady-state techniques
TDTR allows quick, accurate measurement of thin-film thermal
properties by measuring temperature-dependent changes in R
Time-Domain Thermoreflectance
Sample
Probe
Delay Stage
Laser Parameters
Pump
Wavelength:
Pulse Width:
Pulse Energy:
Repetition Rate:
800 nm
200 fs
2.2 nJ
9.8 MHz
TDTR is a pump-probe optical technique used to measure
the thermal properties of materials and film interfaces
Time-Domain Thermoreflectance
ΔR ∼ ΔT ∼ 1/Effusivity
Effusivity = κC
Time- and Temperature-dependent reflection of acoustic
pulses enables characterization of thermal properties
Nanocrystal Thin Film Thermal Properties
Sample
Aluminum
Nanocrystal Film
SiO2
Silicon
Ktherm (W/m·K)
PbTe QD Film
Ag2Te QD Film
Binary QD Film
0.6 W/m·K
0.9 W/m·K
0.9 W/m·K
Ion Beam QD Film
1.4 W/m·K
Bulk PbTe
5.0 W/m·K
TDTR measurements demonstrate that nanostructuring
decreases thermal conductivity for all cases studied
Single-phase nanocrystalline samples exhibit reductions in
thermal conductivity from 5-10x in comparison to bulk solids
Thin-Film Seebeck Meaurements
ΔV
S=
ΔT
ΔT from hot plate
ΔV from voltmeter
Seebeck provides information on entropy
transported per charge carrier
Thin-Film Seebeck Meaurements
ΔV from voltmeter
ΔT from thermocouples
ΔV
S=
ΔT
First Seebeck measurements of chemically
derived quantum dot solids
Thin-Film Seebeck Meaurements
Material
ΔV (V) ΔT (K) Sexp (V/K) Slit (V/K)
p-type Si (5x1018 cm-3)
48.5 mV
24.8
1.95 mV/K
1-2 mV/K
iron bar
0.4 mV
10.5
38.1 μV/K
20 μV/K
PbTe QD Solid
1.2 mV
31.4
>200 μV/K
????????
± 0.5K on Temperature Measurements
± 10 μV on Voltage Measurements
Data on thin film p-Si and Iron are
consistent with literature values
Just collecting initial data on PbTe
Quantum Dot Solids
Acknowledgements
Chris Murray (UPenn)
Mercouri Kanatzidis (MSU/NW)
Cherie Kagan (UPenn)
Chuck Black (BNL)
Bob Sandstrom (IBM)
Dmitri Talapin (MF)
Elena Shevchenko (MF)
Ricardo Ruiz (Hitachi)
Will Green (IBM)
Sarah Cowan (Cornell)
Future Directions
• Nanocrystal Seebeck Measurements
• Environmentally Stable QD Transistors
• Quantum Dot/Polymer Hybrids for PV
• Large-scale Processing of Nanocrystal TFTs
Air-Exposed Nanocrystal Transistors
Vsd (V)
Isd (A)
Air exposure enhances mobility, but decreases gate-effect
Challenge: Retain gate-effect and switching behavior
Mechanism: Further ligand oxidation? Passivating oxide?
Large-scale, Low-cost Printing
Conventional Methods
(Photolithography)
Proposed Method
(Inkjet Printing)
Thermoelectric (TE) Devices:
Refrigerators and Heaters
Heat Source
Active Cooling
p
n
Heat Rejection
Refrigeration Mode
p
n
Heat Sink
Power-Generation Mode
TE devices have no moving parts, produce no waste, and are reliable
Saturation and Mobility Measurements
in PbTe Transistors
Mobilities calculated in saturation regime
⎛ W ⎞⎛ 1 ⎞
2
I ds = Cμ ⎜
⎟⎜ ⎟(Vg − Vt )
⎝ 2 L ⎠⎝ d ⎠
Origins of Nonlinear Electric Transport
where
Ben-Chorin, M. et al. Phys. Rev. B, (1994)
Nonlinear transport has been observed in granular semiconductors
Due to electric-field enhanced thermal activation of carriers from traps
Surface Transfer Doping
Ristein, J. Science (2006)
Surface transfer doping is distinct from classical doping processes
Already demonstrated in Si/SiO, diamond, and nanotube systems
Possible means to enhance conductivity in non-sintered binary systems
Bismuth: n-type dopant for PbTe
50nm
10nm
Mixed Bi/PbTe superlattices may provide a route to n-type devices
images courtesy D. Talapin
GeTe Nanocrystals: Building Block
for Nanocomposite TAGS
GeTe is a central
component of TAGS,
an excellent high-T
TE material
40nm
TAGS is an alloy of
AgSbTe2 and GeTe
GeTe nanocrystals
are rhombohedral
and can link together
to form chains
100nm
GeTe also displays attractive phase change and ferroelectric properties
Why Pursue Photovoltaics?
Power Provided by 1 Hour
of Global Solar Irradiance
=
Power Consumed by
Humanity in 1 Year
=
Convenient methods to harness solar radiation are a huge opportunity
Cheap, efficient PV devices improve energy security and be C-neutral
Barriers are cost, efficiency, ease of patterning, compatability
Realistic Cost/Power Considerations
Lewis, N. Science (2007)
$1/Wp amortizes to target of 5¢/kWh over 30 yr. PV lifetime
Next Generation Inorganic Solar Cells
Common Sources of PV Inefficiency Are:
1. Thermalization of Supra-bandgap photons (λ<<Eg)
2. Transparency to Sub-bandgap photons (λ>>Eg)
Multiple Exciton Generation in Lead
Chalcogenide Quantum Dots
Multiple Exciton Generation (MEG): one photon in, multiple carriers out
Unique lead chalcogenide band
structure makes MEG process efficient
Theory predicts 300% Quantum Yield
for 4nm PbSe QD hit with hν = 4Eg
...this implies 4 excitons per photon!
Can MEG be harnessed to
design high-efficiency solar cells?
Carrier Multiplication in Lead
Chalcogenide Nanocrystals
Theory predicts most efficient MEG processes occur in lead
chalcogenide nanocrystals excited with 2.25 Eg≤ hν ≤3.0 Eg
Initial optical studies of MEG in PbSe, PbS, and PbTe nanocrystals
has demonstrated highest QY carrier multiplication in PbTe
Electrical Measurement of Carrier
Multiplication Effects
hω = 3Eg
e- h+
e- h+
e- h+
Although optically characterized in PbX nanocrystals,
MEG has never been measured electronically
Measuring photocurrent as a function of pump energy in the
nanocrystal film geometry would be the first confirmation
Harnessing MEG in High-Efficiency PV
Common Loss Processes:
1.) thermalization
2,3.) junction/contact V losses
4.) recombination losses
5.) sub-bandgap transparency
MEG captures multi-junction spectral range in one material system
Direct band gap enables efficient photon absorption/carrier conversion
mention/change to meg/polymer composites
Ability to apply high E fields minimizes recombination losses
What’s Wrong With Silicon PV?
Common Loss Processes:
1.) thermalization
2,3.) junction/contact V losses
4.) recombination losses
Indirect band gap decreases efficiency, limits device performance
Limited spectral coverage: bandgap captures small portion of sunlight
Expensive fabrication, limited substrate compatability
GISAXS on Nanocrystal Thin Films
qz (Å-1)
qy (Å-1)
GISAXS interrogates film order in all
three spatial dimensions
Interparticle spacings are obtained
from GISAXS pattern integration
Synergistic Effects in Transport
Conductance enhanced >100-fold in 1:1 binary composites
Real Thermoelectric Modules
Thermoelectric elements are connected in series electrically
Thermoelectric elements are connected thermally in parallel
Measuring Seebeck Voltages in
Nanoscale Samples
MMR device enables measurement of Seebeck voltages from 70-700K
samples as small as 1mm x 2mm may be measured (50nV resolution)
Multi-shell Structures for
Introducing Dopants
Ag
Sb
PbTe
PbTe
PbTe
PbTe
A multi-shell structure might also facilitate nanoparticle doping
Superlattices After Heating
Even 200°C thermal treatments do not abrogate superlattice order
High-temperature treatments do further reduce organic shells
Hydrazine Treated Nanocrystals
Hydrazine treatment decreases interparticle spacing
while preserving nanocrystal individuality
Thermally Treated Superlattices
Both single-component and binary superlattices heated under
vacuum at 150°C retain nanocrystal individuality
Ag2Te: Self-Assembly into Islands
50nm
50nm
10 nm
Ag2Te nanocrystals are monodisperse and self-assemble into islands
p-Type Electronic Transport in 1:1
Mixed PbTe/Ag2Te Composites
Vsd (V)
Ag2Te seems to add
many carriers to the
PbTe system
Isd (A)
Positive gate voltages
reduce, but cannot
shut off transport
Vg (V)
Isd (A)
p-type tansport in
mixed composites
exceeds sum of
individual behavior
p-Type Electronic Transport in 5:1
Mixed PbTe/Ag2Te Composites
Vsd (V)
5:1 composites remain
p-type, however are
less conductive than
the 1:1 mixtures
as in 1:1 composites,
positive gate voltages
reduce, but cannot
shut off transport
Isd (A)
Ag2Te still adds
carriers to PbTe,
evinced by enhanced
conductivity relative
to pure PbTe
Superlattice Assembly Not Simply
Entropy-Driven Crystallization
Nanocrystal Electrophoretic Mobilities
0
1
OA
2
-0.4
0.0
PbSe
PbSe
3x OA
0.0
0.4
0.8
TOPO
-0.4
0.0
0.4
0.8
4
2 -1 -1
Mobility*104 [cm2V-1s-1] Mobility*10 [cm V s ]
Shevchenko, E. et al. Nature, (2006)
Nanoparticle charge states:
1.) critical for BNSL formation
2.) adjustable via chemicals
0.8
Au
PbSe
TOPO
-0.4
0.4
Intensity [a.u.]
Intensity [a.u.]
PbSe
OA
-0.8
Intensity [a.u.]
-1
PbSe
Au
PbSe
One charge per particle
can tip the balance!
Material Tradeoffs in Bulk, SinglePhaseThermoelectrics
Intuitive Picture of (S2σ) Factor and
Cooling/Power Generation
Comparison of Bulk Transport
Characteristics to Quantum Dot Solids
PbTe QD TFT
Bulk PbTe
Binary QD TFT
Heremans et al. Phys. Rev. B (2004)
Binary QD Solids ~102 enhanced conductance vs. bulk PbTe
Comparison of Bulk Seebeck
Characteristics to Quantum Dot Solids
Binary QD TFT?
TBD…
Bulk PbTe
PbTe QD TFT
Heremans et al. Phys. Rev. B (2004)
Have to consider all of the junctional thermovoltages…