2009 NSF NSE Grantees Conference, Dec 7-9, 2009, Arlington, VA
TDS
Nanoscale design for scalable
sustainable energy technologies
Tim Sands
Director, Birck Nanotechnology Center
Purdue University
Birck Nanotechnology Center
TDS
Themes
 The magnitude of the challenge
 The nanoscale opportunity
 Focus on solid state approaches to energy efficiency
 Solid state lighting
 Waste heat conversion
 Solid state refrigeration
 Constraining the problem – filling the gap between basic and
applied research





What is the technological discontinuity to be bridged?
What fundamental problem is to be solved?
Are the source materials abundant and of low toxicity?
Is there potential for scalable manufacturing?
Are there near-term niche application for early solutions?
Birck Nanotechnology Center
TDS
The Terawatt Challenge
Power Consumed [TW]
15
Global Rate of Primary Energy
Consumption
2003 14 TW total
10
2050 30 TW total scenario
Primary source for
2003 data:
International Energy
Agency
5
0.5%
0
Oil
Coal
Gas
Nuclear Biomass Hydro
Other
{
Solar
Wind
GeothermalЙ
Adapted from Richard Smalley’s presentation on “Our Energy Challenge” at the 2004 International Electron
Devices Meeting (IEDM), San Francisco, CA 12/14/04
Birck Nanotechnology Center
Spectrum of opportunities for nano
TDS
Workshop on Nanotechnologies for Thermal and Solar Energy Conversion and
Storage, August 10,11, 2008, Jacksonville, FL
“Nanoscale design to enable the
revolution in renewable energy” J.
Baxter, Z. Bian, G. Chen, D. Danielson,
M.S. Dresselhaus, A.G. Fedorov, T.S.
Fisher, C.W. Jones, E. Maginn, U.
Kortshagen, A. Manthiram, A. Nozik,
D.R. Rolison, T. Sands, L. Shi, D. Sholl and
Y. Wuo, Energy & Environmental Science
2 (2009) 559-588.
Portfolio of solar/thermal/electrochemical
energy conversion, storage, and
conservation technologies, and their
interactions
Birck Nanotechnology Center
TDS
The solid-state part of the solution…
 More efficient devices for…




LED-based lighting
Thermoelectric refrigeration
Thermoelectric and thermophotovoltaic conversion of waste heat
Photovoltaic conversion of solar energy and production of hydrogen
 Added benefits
 Compact
 Robust
 Low environmental impact
 Challenges
 Efficiency breakthroughs needed!
Nanostructured semiconductors?
 Availability and price of raw materials Thin films instead of bulk?
 Manufacturing costs “Bottom-up” nanofabrication?
Birck Nanotechnology Center
TDS
New degrees of freedom in nanocomposites
 Length scales on the order of electron,
phonon and photon wavelengths, and
scattering lengths of electrons,
phonons and electron spin
 Dimensions below critical values for
relaxation of lattice misfit strain and
epitaxial stabilization of high pressure
phases
 High surface (interface)-to-volume
ratios for sensing, catalysis, and
chemical storage
5 nm diameter
CdSe nanocrystal (Manna
et al., J. Cluster Sci. 2002)
Strain relaxation
in nanowire
heterostructures
(Ertekin et al.,
JAP,2005)
Nanoparticles
suspended in
solution (Frankel,
MIT)
Fundamentally new materials with new properties
Birck Nanotechnology Center
TDS
Solid State Lighting
Birck Nanotechnology Center
Solid state lighting – the opportunity
TDS
 Electricity generation accounts for about 37% of primary
energy consumption in the U.S.*
 Lighting accounts for 22% of the nation’s electric power
usage.
 The DoE SSL Goal: a solid-state lamp that is more
efficient, longer lasting and cost competitive compared
to conventional technologies, targeting a system
efficiency of 50% and the color quality of sunlight.
 Implications of Success: 33% reduction in energy
consumed for lighting by 2025, eliminating need for 41
1000MW power plants, and saving consumers $128 B+.
+Navigant Consulting (11/03)
*Annual Energy Outlook (02)
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TDS
LEDs – the technological discontinuity
III-V LEDs cover the visible spectrum,
but not with one materials system
Low cost solution:
Blue (In,Ga)N LED with
partially absorbing yellow
phosphor
Limitations: poor color
rendering, low efficiency due
to Stokes shift
Warm light solution:
Board-level integration of
(In,Ga)N/yellow phosphor
and (Al,Ga,In)P red LEDs
Limitations: “green gap”,
high cost of assembly
Compound Semiconductor, June 2008, pg. 17
Birck Nanotechnology Center
TDS
The nano aspect of a solution?
 Elastic relaxation of lattice misfit strain
 Reduced electric field in recombination volume
 Higher local injection current densities without
Auger recombination
 Greater InN incorporation, longer emission
wavelengths
EL
 Filtering of threading dislocations
 Lower-cost, larger diameter, higher thermal
conductivity substrates (metallized silicon)
 Improved geometry for light extraction
EDS
TEM
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Thermoelectric Waste
Heat Generators
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TDS
Waste heat factoids…
 >60% of primary energy consumed is dissipated as waste heat
 Energy costs account for approximately 8~12% of the total cost of glass
production, and about 15% for steel*
 The world’s data centers are projected to surpass the airline industry as a
greenhouse gas polluter by 2020**
 About 40 percent of the energy content of gasoline burned in automobile IC
engines is lost as exhaust heat and another 30 percent is lost through
engine cooling***
www.greencarcongress.co
m/thermoelectrics
*http://www1.eere.energy.gov/industry/program_areas/industries.html
**Study by McKinsey & Co., Green Enterprise Computing Symposium in Orlando, Fla., May 1, 2008
***http://www.washingtontimes.com/news/2008/aug/12/researchers-eye-exhaust-for-fuel/?page=2
www.eere.doe.gov
Birck Nanotechnology Center
Overview: thermoelectrics
TDS
 Thermoelectric primer
 Recent progress in ZT enhancement by
nanostructuring (MBE and bulk)
 A scalable approach with nanoscale control:
electrodeposition into porous templates
 Mitigating parasitics – removing/replacing the
template, and scaling to thicknesses > 100 mm
 TE element fabrication from nanoscale materials
Birck Nanotechnology Center
TDS
Physical basis of thermoelectricity
 Thermoelectricity arises from the difference between the
average energy of the electrons (or holes) responsible for
conduction and the Fermi energy
In an n-type semiconductor…
Energy
Fermi-Dirac
Occupation
Function
Average energy of
conduction electrons
=
*
Density
of
States
(DOS)
0
1
EF
Density
of
Occupied
States
Birck Nanotechnology Center
TDS
The Seebeck effect (1821)
 Generation of voltage along a conductor subjected to a temperature
difference
 Initially, carriers (electrons or holes) move from hot to cold
 Resulting potential difference opposes further current flow
 Open circuit voltage is proportional to DT
lim {DVoc}= SDT
DT0
where S is the Seebeck coefficient [V/K]
 Completing the circuit through a load  generator
Hot Side
Cold Side
Initial
Electron
energy
Steady State
e
] qDVoc
Birck Nanotechnology Center
TDS
Thermoelectric generators
e-
current
n-type semiconductor
heat
sink
heat
source
p-type semiconductor
hot side
h+
cold side
zT = S2sT/k
Dimensionless figureof-merit
Altenkirch (1909,1911)
•Large Seebeck coefficient (S)  large open circuit voltage for
generators; large Peltier coefficient for refrigerators
•Low thermal conductivity (k)  easier to maintain DT for generators;
reduced conduction of heat back to cold side for refrigerators
•High electrical conductivity (s)  reduced Joule heating
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TDS
Applications of TE today (zT ~ 1)
Materials zT~1; System ZT~0.7
Thermo
electric
modules
Heat
 Radioisotope TE generators powering deep
source
space probes
(PuO2)
 Automotive seat coolers/heaters (500,000/yr)
 Picnic coolers
 DNA PCR precision temperature cyclers
 Temperature stabilizers for laser diodes used in
fiber optic communications systems
http://www.its.caltech.edu/~jsnyder/thermoelectrics/
 Electronics cooling at the cabinet level
 Remote power generators in harsh environments
Can we expect more from thermoelectrics?
Bell, L 2008, “Cooling, heating, generating power, and recovering waste heat with thermoelectric systems,”
Science 321, p.1457
Birck Nanotechnology Center
Applications of TE tomorrow (zT =2-3)
TDS
Materials zT=2-3; System ZT~1.5-2
 Exhaust waste heat recovery for gasoline and diesel engines to
improve mileage by 10%
 Cogenerators for 5-20 kW diesel generators, improving system
efficiency by 5-10%
 Split-spectrum solar generators (PV + TE)
 Industrial waste heat recovery in metal, glass and cement processing
 Flex fuel powered small engines
 Microprocessor cooling
 Greenhouse-gas-free HVAC for vehicles and residences
Bell, L 2008, “Cooling, heating, generating power, and recovering waste heat with thermoelectric systems,”
Science 321, p.1457
Birck Nanotechnology Center
TDS
Materials design challenges
Ss
k
2
zT =
2
T
zT =
S
Lo + (kph/sT)
k = ke + kph = (LosT) + kph
For most bulk materials…
• Increasing s increases ke (Weideman-Franz-Lorenz Law)
• Decreasing kph by adding defects decreases mobility (and s)
• Reducing m* increases mobility (and s), but decreases S
• Increasing carrier concentration decreases S
Birck Nanotechnology Center
TDS
Device design challenges
 CTE and CTE mismatch
 Assembly of 10s to 100s of p-n couples – electrically in
series to match load resistance
 Thermal and electrical parasitic resistances
 Contact resistance – leg length should be greater than ~200
mm (total contact resistance <10% of total resistance for rc = 1
x 10-6 W-cm2 and rTE = 1 mW-cm)
 Maintaining DT – leg length should be between ~100 mm and
1 mm to maximize power density
Birck Nanotechnology Center
TDS
20% efficient TE generators?
Target: >20% efficiency; >104 W/m2 power density
0.4
Tcold =50°C
0.3

ZT= 5
ZT= 4
ZT= 3
Carnot
ZT= 2
0.2
ZT= 1
0.1
0
0
50 100 150 200 250 300 350
400 450 500 550 600
Thot [°C]
 ZT > 2; DT ~ 500K; 10% areal coverage; forced
convection
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Optimum leg length
Mayer and Ram, Nano-Microscale Thermophys Eng, 10 (2006)
103
Power density (W/cm2)
102
Interface
limited
Heat flux
limited
101
100
10-1
10-2
10
100
L (µm)
1000
Leg length for maximum power density decreases with increasing heat
transfer coefficient, minimizing use of less abundant materials
Birck Nanotechnology Center
TDS
Design of bulk thermoelectrics
Ss T
ZT = k
2
• Power factor, S2s, optimized for degenerate
semiconductors
• Heavy masses, low “spring constants”, and large unit cells
to reduce kph
• Solid-solution alloying to reduce kph
“Phonon glass, electron crystal”
 Bi2Te3 and its alloys for cooling near room temperature
Birck Nanotechnology Center
TDS
Materials zT
Sb1.5Bi0.5Te3
State-of-the-art commercial and NASA TE materials (from G.J. Snyder and E.S. Toberer, Nat. Mater. 7
(2008) 105.)
Commercial bulk materials (not intentionally
nanostructured) are limited to materials zT of ~ 1
Birck Nanotechnology Center
TDS
Quantum confinement…the solution?
Hicks and Dresselhaus, PRB 47 (1993) 12727
Ideal DOS: delta function several kT from
EF will maximize power factor if mobility
can be enhanced by eliminating ionized
impurity scattering
S kph
15
Bi2Te3
Electron Energy
well
Figure-of-Merit ZT
After Esaki, 1990
10
1D - quantum wire
5
2D - quantum well
wire
bulk
Density of States
3D - bulk
0
0
1
2
3
4
5
6
Well or Wire Width [nm]
Does not include deleterious
interface or barrier effects!
Birck Nanotechnology Center
TDS
zT enhancement: p-type leg, 300-400K
 1950’s to 2000: Bulk (Bi,Sb)2Te3 with zT~1
 2001: RTI group reports zT = 2.4 for Bi2Te3/Sb2Te3 superlattice grown by
MBE1
• 2008: Boston College/GMZ/Nanjing/MIT group
reports ball milled (Bi,Sb)2Te3 bulk alloy with
nanoscale grain size yielding zT ~ 1.4 at 373K2
• 2009: Wuhan/Clemson group reports zT ~ 1.56 at
300K for melt spun and spark plasma sintered bulk
(Bi,Sb)2Te3 with nanocrystalline grains embedded
in amorphous matrix3
[1] Venkatasubramanian, Nature 413(2001)597
[2] Poudel, Science 320(2008)634
[3] Xie et al. APL 94 (2009)102111
Birck Nanotechnology Center
Is zT > 2 possible for a scalable material?
TDS
 Common thread: zT > 1 achieved by enhancing
scattering of longer wavelength (> 1 nm) phonons,
suppressing lattice thermal conductivity without
reducing power factor
 zT > 2 only reached in MBE material; difficult to scale to
optimal leg lengths of 200 mm to ~1 mm
 Is there a synthesis approach that combines control of
vapor deposition with scaling to practical leg lengths to
achieve zT > 2?
 Can we enhance the power factor (S2s) as well?
Birck Nanotechnology Center
Electrodeposited nanowire arrays?
TDS
 Nanoscale control of grain size and composition modulation
(lateral and radial) with a scalable synthesis technique
 First electrodeposited Bi2Te3 nanowire (~280 nm dia.) arrays into
porous anodic alumina (Sapp et al., 1999)
40 nm diameter Bi2Te3 nanowires
electrodeposited into porous anodic
alumina (Prieto et al., 2001)
Texture control
Sapp, Lakshmi, and Martin, Advan. Mater. 11 (1999) 402; Prieto et al., JACS 123 (2001) 7160.
Birck Nanotechnology Center
(Potential) advantages of nanowires
TDS
 Phonon scattering - Phonon scattering from free surfaces and grain
boundaries decreases the lattice contribution to the thermal conductivity,
thereby increasing zT.
 Lateral elastic relaxation - Lateral elastic relaxation of strain in
nanowires enables coherent interfaces in large lattice misfit
heterostructures.
 Elastic compliance - Improved elastic compliance resulting from free
surfaces and/or interfaces with compliant matrix materials.
 Chemical modification of free surfaces - Access to free surfaces
permits chemical modification that may enhance charge mobility while
suppressing phonon transport; short diffusion lengths  less grain growth
 Crystallographic texture control – interface energy driven
 Quantum confinement – Potential for power factor enhancement, but
only for nanowires with diameters < 5 nm (for Bi2Te3)
Birck Nanotechnology Center
TDS
Challenges of nanowire topology
 Minimizing parasitics
 Leg lengths > 100 mm – thicker than typical PAA
 Minimizing matrix thermal shunt – kmatrix <<kTE and/or matrix
volume fraction, fmatrix, should be as small as possible
 Enhancing mechanical strength and toughness
 Protecting internal (nanowire) surfaces
 Making ohmic contacts
…
Birck Nanotechnology Center
TDS
Bi2Te3 nanowires – property measurements
 Wang et al. (JAP 96 2004) – arrays, PAA removed
 50 nm diameter
 S = 270 mV/K at 300K; Semiconducting
 Zhou et al. (APL 87 2005) – single nanowires
 43.5-120 nm diameter
 Highly variable S and s; k lower than bulk by 28-57%
 Li et al. (Nanotech. 17 2006) – arrays, PAA removed
 40-60 nm diameter
 Semiconducting
 Mavrokefalos et al. (JAP 105 2009) – single
nanowires
 52 and 55 nm diameter
 zT estimated to be 0.1 at 400K; Doping level too high;
minimal reduction (~20%) in k due to surfaces and
grain boundaries; better control of stoichiometry
needed
Mavrokefalos et al.(JAP 2009)
No power factor or ZT measurements of alloy nanowires yet
Birck Nanotechnology Center
TDS
Minimizing matrix effects

nw


comp


k
m 1
1
  
1

k
f
nw
 nw 
5 mm
 Approach 1 – replace PAA matrix with low k polymer
 Approach 2 – eliminate matrix by synthesizing selfsupporting nanowire arrays
Birck Nanotechnology Center
TDS
PAA replacement with SU-8
 PAA selectively etched in 3 wt% KOH for 24h; rinsed
with water and then IPA
 SU-8 2005 spin coated at 2000 rpm to thickness of 40
mm; uv exposed, then hard baked on hot plate at
150°C
NW array
in PAA
Etch PAA
Free-standing
NW array
SU-8 backfilled
NW array
SU-8 infiltration
K.G. Biswas, T.D. Sands, B.A. Cola and X. Xu, APL 94 (2009) 223116
Birck Nanotechnology Center
TDS
Bi2Te3/SU-8 composite
FESEM images of fractured
composite in cross-section
Bi2Te3 NWs
5 mm
cleavage plane
5 mm
500
nm
K.G. Biswas, T.D. Sands, B.A. Cola and X. Xu, APL 94 (2009) 223116
Birck Nanotechnology Center
Photoacoustic measurement of k
TDS
Composite
Bi2Te3/PAA
Bi2Te3/SU-8
kcomp (W/m-K)
1.40 ± 0.07 1.10 ± 0.06
kBi2Te3(W/m-K)
1.44 ± 0.10 1.45 ± 0.09
kmatrix (W/m-K)
1.31 ± 0.10
0.2*
Replacement of PAA with SU-8 reduces the matrix penalty in ZT
from 27% to ~5%. The effect will be larger with lower kTE
K.G. Biswas, T.D. Sands, B.A. Cola and X. Xu, APL 94 (2009) 223116
*data from product literature
Birck Nanotechnology Center
TDS
Composition modulation in Bi2(Te,Se)3
By varying the electrodeposition
potential from +40 mV to -60 mV, the
composition was varied from 0.18 to
0.07 mole fraction Bi2Se3.
Thermal Conductivity Measurements
 (W/m-K)
Bi2Te3
Bi2(Te,Se)3
(pure)
(composition-modulated)
1.48
~0.23
• Substantial suppression of apparent
0.2
thermal conductivity
• Compare to minimum of 0.31 for SU-8
Bi2Te3 and ~0.20 for
(Bi,Sb)2Te3[Chiritescu et al., JAP 106
(2009)]
Cross-section FESEM image of
composition-modulated Bi2(Te,Se)3
nanowires.
Thermal conductivity measurements
obtained by the photoacoustic
technique (Baratunda Cola, Xianfan Xu;
Purdue University).
Birck Nanotechnology Center
TDS
Next step…
 Can we eliminate the matrix altogether?
 Suggests interconnected, self-supporting nanowire array
• Template must be branched as well
• Solution: Branched Porous Anodic Alumina (B-PAA)
Birck Nanotechnology Center
Interconnecting pores in B-PAA
TDS
K.G. Biswas, et al., Appl. Phys. Lett. 95 (2009) 073108
Birck Nanotechnology Center
B-PAA synthesis
TDS
 165-190°C in 0.4 M phosphoric acid with a current density limit
of 1.1 A/cm2 (275X current density for mild anodization)
 Thermal runaway  branched pores and a rate of anodization >
300 mm/hr, 60X faster than mild anodization
Time variation of
voltage, current,
and temperature
during B-PAA
synthesis
K.G. Biswas, et al., Appl. Phys. Lett. 95 (2009) 073108
Birck Nanotechnology Center
TDS
Self-supporting nanowire array
Cross-section FESEM images of self-supporting interconnected Bi2Te3
nanowires after B-PAA template removal by selective etching.
Pt electrode, electrolyte: 0.035M Bi(NO3)3 · 5H2O and 0.05M HTeO2+ in
1M nitric acid at pH = 1; 3 s pulses of current density 5 mA / cm2 followed
by a 3 s standby.
Birck Nanotechnology Center
TDS
Fabrication of elements
B-PAA template before electrodeposition of
(Bi,Sb)2Te3
340 mm
Top view of Au/Ni metallized 370 mm x 370 mm
(Bi,Sb)2Te3 self-supporting nanowire array
element
Side view of Au/Ni metallized 370 mm x 370 mm (Bi,Sb)2Te3
self-supporting nanowire array element of 100 mm thickness
Birck Nanotechnology Center
Where are we?
TDS
 Thermal conductivity measurements (photoacoustic method)
validate PAA replacement approach
 B-PAA process understood and scaled to ~300 mm in thickness
(~1h)
 Element fabrication (electrodeposition, annealing planarization,
metallization, template removal, and dicing) demonstrated
 Present: Optimization of element fabrication process (uniformity
and reproducibility)
 Next: zT measurement and optimization through composition
modulation and post-growth modification (e.g., annealing in
Te/Se vapor)….a platform for systematic analysis of the
nanowire approach
Birck Nanotechnology Center
Another example: nano to bulk
TDS
Sputtered metal/semiconductor superlattices for “thermoelectric metamaterials”
HAADF/STEM image of (Zr,W)N/ScN superlattice Courtesy:
Joel Cagnon and Susanne Stemmer, UCSB
Birck Nanotechnology Center
TDS
Conclusions
 Parasitic thermal and electrical effects must be
overcome before “artificially nanostructured”
materials can be accurately assessed
 Leg (segment) lengths greater than 100 mm are
required
 Electrodeposition into porous templates is a
viable approach to scalable artificially
nanostructured TE materials
 Not there yet!
Birck Nanotechnology Center
TDS
Closing thoughts…
 A portfolio solution to the global energy challenge will
include solid-state devices that conserve energy
 Nanoengineered materials and devices can offer more
efficient use of materials with limited availability
 Size-dependent properties may allow substitution for
abundant non-toxic materials
 Considering scaling and parasitics at the outset alters
materials and device design choices
Birck Nanotechnology Center