2010-Segalman

Rachel A. Segalman UC Berkeley

Organic and Nanocomposite Thermoelectrics

Shannon Yee, Jonathan Malen, Kevin See,

Jeffrey Urban, Arun Majumdar, and Rachel A. Segalman

1


Energy landscape

Global Power Capacity ~ 13 Terawatts

From Fossil fuels

Nuclear

Hydroelectric

Geothermal + Wind + Photovoltaic

http://www.doe.gov


Power genera8on

Tout >Tambient+100 oC

Power = 10 TrillionWaBs Efficiency = Power/Heatin~ 40% Heatin = 25 TW Heatlost = 15 TW

Hot (Thot)

Engine

Heatin

Power

Cold (Tambient)

Heatlost

Global Power Capacity ~ 13 Terawatts

From Fossil fuels

Nuclear

Hydroelectric

Geothermal + Wind + Photovoltaic


Power co-‐genera8on Power = 10 TrillionWaBs Efficiency = Power/Heatin~ 40% Heatin = 25 TW Heatlost = 15 TW

Hot (Thot)

Engine

Heatin

Power

Cold (Tambient)

Heatlost Tout = Tambient+100 oC

Extra Power

Heat cold Efficiency ~ 3 % Extra Power = 0.45 TW US Electrical Capacity = 1 TW (2005)


Trends in bulk materials

Figure of Merit, ZT:

S σ

S2σ Semiconductors Metals

Carrier concentration

kTSZT σ2

=

Bismuth Telluride (low efficiency, expensive)

p n p n

R

Qin

Qout

ZT

Frac

tion

of C

arno

t

Bi 2 Te 3

0

0.2

0.4

0 1 2 3 4 5 σ = Electrical Conductivity

κ = Thermal Conductivity

S = Thermopower=V/ΔT


Electrical conduc8vity in metals

σ(E)

E

Ef

Fermi Distribution

0 1

E

Ef

f(E)

E

Ef

−∂f∂E

⎛

⎝⎜⎞

⎠⎟

D(E)

σ = σ (E) dE

0

∞

∫ σ (E)dE =

−e2

3τ E( )v E( )2

D E( ) ∂f0

∂EdE


Thermopower in metals

S = − 1

eT

σ E( ) E − E f( )dE0

∞

∫σ

σ(E)

E

Ef

E-Ef

E

Ef

E-Ef E

Ef

σ(E)(E-Ef)

Inequality in the size of these humps results from asymmetry in σ(E) at Ef, which results in S


Assymmetry about Ef Insulator

Eave

N-type Semiconductor

Eave

Degenerate Semiconductor

Eave

Metal

Ef

E

Eave

Increasing S

Increasing σ Applies as a function of doping in bulk conjugated polymers, too

(Review: Ali Shakouri and Suquan Li, 1999)


Trends in Bulk Materials

Figure of Merit, ZT:

Semiconductors Metals


σ = Electrical Conductivity

κ = Thermal Conductivity

S = Thermopower=V/ΔT

kTSZT σ2

=

Bismuth Telluride (complex processing, expensive)

p n p n

R

Qin

Qout S σ

S2σ Semiconductors Metals



kTSZT σ2

=

Strategies to achieve high ZT

Heterostructures

S M

M S Incident Energy

Reflected Energy

Transmitted Energy

Interfaces decrease k Large asymmetry in σ(E) increases S

σ(E) of Metal

Effective σ(E)

Met

al

Ef

E

Sem

icon

duct

or

M S

Mahan, Woods, Phys. Rev. Lett (1998)


Majumdar, Science 303, 777 (2004)

Venkatasubramanian et al. Nature 413, 597 (2001)

Bi2Te3/Sb2Te3 Superlattices 2.5-25nm

Recent history

Hsu et al., Science 303, 818 (2004)

AgPb18SbTe20

AgSb rich

Harman et al., Science 297, 2229 (2002)

PbSeTe/PbTe QD Superlattices


Organic-‐inorganic hybrid junc8ons

•  Scalable Manufacturing •  Poten8ally Inexpensive Components

•  Durable/Flexible Devices •  Tunable electronic proper8es

•  Bulk polymers follow the same trends as other bulk materials (S and σ opposing)

Review: Shakouri and Li (1999) Images from Forrest et al.


Molecular thermoelectrics?

Heterostructures

S M M S M

Heterostructures

M M M

Physics Unexplored

Inexpensive thermoelectric materials!


Metal-‐molecule junc8ons have unique poten8al

•  Metal-‐molecule junc8ons have unique proper8es coming from mismatched DOS.

•  Predic8on of junc8on proper8es is the subject of intense research


I = e

πτ E( )

−∞

∞

∫ fHot E( )− fCold E( )⎡⎣ ⎤⎦dE

fCold E( )

0 1

E

Ef

fHot E( )

f E( )

E

fHot E( )− fCold E( )

Ef

SJunction = −

π 2kB2T

3e

∂ ln τ Ef( )( )∂ Ef( )

E

Ef

I

Landauer Formula: M. Buttiker, Y. Imry, R. Landauer, and S. Pinhas, Phys. Rev. B, 31, 6207 (1985)

Sommerfield Expansion: P.N. Butcher, J. Phys. Chem. 2, 4869 (1990)

τ(E)

E

Ef

HOMO

LUMO

V

Au Au Hot Cold

HS SH


Transport in Metal-‐Molecule-‐Metal hybrids

Energy

LUMO

HOMO

EF

EF

G Slope~-‐S

V=-‐SΔT

Hot Cold

Paulsson and Data, Phys. Rev. B 67, 241403 (2003)


Tip approach Tip withdraw

Tip speed: 2 – 40 nm/s

Mica Gold (200 nm)

Gold STM tip

Octanedithiol (ODT) HS SH

Single and multi-molecule Structure-property relationships

B.Xu et.al, Science, 301, pp 1221-1223, 2003 Collaboration: Prof. Arun Majumdar (ARPA-E)


0

4

3

2

1

0.5 nm

Con

duct

ance

(Go)

Distance 3Go2Go1Go

Cou

nt (a

u)Conductance (Go)

Go = 2e2

h = 77µS

12.9 kΩ 1

=

> 1000 times

Conductance of Au-‐Au point contact

Conductance Traces Histogram

Maximum Conductance through an energy level


Sta8s8cal analysis of molecules

0

1 nm

Distance

Conductance Traces

Ω==

KheGo 9.12

12 2

HS SH (CH2)6

Jang, Reddy, Majumdar, Segalman, Nano Letters (2006)

Histogram from Last Steps of >2000 traces

R~ 45 MΩ

0 2 X 10-31.5 X 10-31 X 10-35 X 10-4

Cou

nts

(au)

Conductance (Go)Conductance (10-4 Go)

5 10 15 20

Con

duct

ance

(10-

4 Go)

4

8

12

16


NH2H2N

H2N NH2

H2N NH2

H2N NH2

OCH3

H3CO

H2N NH2

Cl

Cl

BDA

DBDA

TBDA

DMDBDA

DCDBDA

H2NNH2

H2NNH2

H2N NH2

HAD (C6)

ODA (C8)

DDA (C10)

Molecular structure/property rela8onship

Jang, S. Y.; Reddy, P.; Majumdar, A.; Segalman, R. A. Nano Letters 2006. Similar to: Venkataraman, L. et al. Nature (2006)

βaromatic =0.46 per A

βalkane =0.98 per A

0 5 10 15 20 10

15

20

25

DDA ODA

HDA TBDA

DBDA BDA

ln (R

esis

tanc

e)

Length (A o

)

Aromatics Alkanes

o

o

o

o


Length dependence of resistance Tunnel junc8ons

l

Eb

R~ exp(ßN)

) 2 ~ exp( ) ( l mE

E b - τ


STM thermopower measurements

22

STM Tip withdraw STM Tip approach

Tip Movement

Tambient+ΔT

Ambient T Metal STM Tip

Hot Au Substrate

Current Amplifier

Voltage Amplifier

Tambient


Experimental measurement of thermopower

STM Tip approach STM Tip withdraw

Tip speed: 2 – 40 nm/s Reddy, Jang, Segalman, Majumdar, Science (2007)


2 2 1 ( ( ))3 ( )

f

Bjunction

E E

k T ESe E E

π ττ =

∂= −

∂

Paulsson & Datta, Phys. Rev. B (2003)

€

G ~ 2e2

hτ (E) E=E f

Electronic Structure of BDT

HOMO LUMO

HOMO LUMO

10 -2

10 -1

10 0

Tran

smis

sion

, τ (E)

-12 -11 -10 -9 -8 -100 -80 -60 -40 -20

0 20 40 60

Ther

mop

ower

, S ( µ

V/K

)

Energy, E (eV)

SBDT= 7.2 µV/K

~1.2eV

Fermi Level

Fermi Level

G

Reddy, Jang, Segalman, Majumdar, Science (2007)


Temperature dependent fluctua8ons

•  DistribuNons broaden at higher temperatures

•  Amount of change is surprising


Tip

App

roac

hes

to C

ondu

ctan

ce S

etpo

int

Mor

e M

olec

ules

in th

e Ju

nctio

n

For N molecules each with transmission τ1(E)

GN =G0τN E( )

E=Ef

= NG0τ1 E( )E=Ef

= NG1

SN = −π 2kB

2T3e

1Nτ1 E( )

∂ Nτ1 E( )( )∂E

E=Ef

= S1


STM approach STM withdraw STM withdraw

µ1 µ2

Devia8on between µ’s results from Junc8on-‐to-‐Junc8on Varia8ons

Devia8on about a single µ results from Junc8on Evolu8ons (hard to measure)

Two observed fluctua8ons in measurements


Insight from Fluctua8ons The FWHM of Increases with ΔT

ΔSΔT =VFWHM ΔSS

=2.3 ± .37.7 ± .5

= 0.30 ± .04


ΔEEvo

Ef − EHOMO( )~ 0.09 ± .04

ΔEJ −to−J

Ef − EHOMO( )~ 0.21 ± .06

• Junc8on Evolu8ons are unobservable.

• Junc8on-‐to-‐Junc8on Varia8ons are the primary source of devia8on

Rela8ve importance of evolu8ons versus varia8ons


τ E( ) ≈ Γ2

E − EHOMO( )2 + Γ2

ΔSS≈

ΔEE

f− E

HOMO

≈ 0.30 ± .04

Assume τ(E) is Lorentzian:

ΔS ≈ ΔE∂S∂E

E =Ef

+ ΔΓ∂S∂Γ

E =Ef

Assume ∆S is a Perturba8on to S:

ΔΓ ΔE ΔE


Effect of binding geometry has even stronger effect on conductance

•  Experimental conducNvity full width half max ~47% •  DFT of 15 possible binding geometries suggests that

fluctuaNons are due to binding not molecular conformaNon

Venkataraman, Nano Letters 2006 Quek et al, Nano Letters, 2008


Transport devia8ons increase with length of molecule


Tuning electrical conductance and thermopower

HOMO LUMO

HOMO LUMO

10 -2

10 -1

10 0

Tran

smis

sion

, τ (

E)

-12 -11 -10 -9 -8 -100 -80 -60 -40 -20

0 20 40 60

Ther

mop

ower

, S ( µ V/

K)

Energy, E (eV)

~1.2 eV

Fermi Level

Fermi Level

S

σ κ

σ = Electrical Conduc8vity

k = Thermal Conduc8vity

S = Thermopower

kTSZT σ2

=G

S

Bahe8, Malen, Doak, Majumdar, Segalman, Nano LeBers (2008)


SBDT = 7.7±0.5 µV/K

Electron withdrawing subs8tuents

ClCl

ClClHS SH


SBDT = 7.7±0.5 µV/K

Electron dona8ng subs8tuents

HS SH


Effects of electrode-‐molecule bond

SAu-BDCN-Au ~ -1.3 ± 0.3( )µV / K

N-‐type Molecular Junc8ons

CNNC


Tuning Electrical Conductance and Thermopower

HOMO LUMO

HOMO LUMO

10 -2

10 -1

10 0

Tran

smis

sion

, τ (

E)

-12 -11 -10 -9 -8 -100 -80 -60 -40 -20

0 20 40 60

Ther

mop

ower

, S ( µ V/

K)

Energy, E (eV)

~1.2 eV

Fermi Level

Fermi Level

S

σ κ

σ = Electrical Conduc8vity

k = Thermal Conduc8vity

S = Thermopower

kTSZT σ2

=G

S

Bahe8, Malen, Doak, Majumdar, Segalman, Nano LeBers (2008)


2 2

Paulsson and Datta, PRB (2003)

1 ( )3 ( )

f

B

E E

TV EST e E E

π κ ττ

=

⎛ ⎞∂= − = − ⎜ ⎟Δ ∂⎝ ⎠

€

G ~ 2e2

hτ (E) E=E f

Scaling to materials systems

Fermi Levelinorg

G


Design of Molecular Thermoelectrics

SH HS Gold Gold

Ene

rgy (Ef)

Frequency, ω Den

sity

of S

tate

s, D

(ω) Solid

Molecules

Large Mismatch in the Phonon Density of States

Met

al

Mol

ecul

e Met

al

Δ

Den

sity of States

Large S

kTSZT σ2

=LUMO HOMO


Nanocomposite Design

•  High σ /low k Polymer

•  High S inorganic nanoparticles

•  Processing: –  Water Dispersed –  Amenable to

solution processing

σ∼100 S/cm

S~500 µV/K

Te Te

Te

Te

Nano Letters ASAP 2010


System Design

41

Fermi Levelinorg G


Te rods, CTAB coated

Synthesis of Composites

Na2TeO3 + PEDOT:PSS + Na2TeO3 + (CTAB) +

H2O

Xi, G Crystal Growth & Design 2006, 6, 2567-2570."

In-Situ Synthesis of Polymer Coated Te Rods

Pedot:PSS Passivation

Te rods, polymer coated!

Polymer

Te



Porous Nanocomposites


Morphology of Composite

Films easily deposited or spin-coated Films are porous, but fully connected

Voc

TH TC Nano Letters ASAP 2010



Room Temp Proper8es of Nanocomposite Films


Room Temp Proper8es of Nanocomposite Films

Highest reported ZT for aqueous processed nanocomposite

• Remarkable combination of properties • Higher σ than conducting polymer • High S of nanocrystals


Rachel A. Segalman UC Berkeley 48

Summary

•  Thermopower from molecular juncNons

•  Measured thermopowers provide insight towards molecular juncNon physics

•  ZT ~ 0.2 for a nanocomposite processed from H2O at room temperature

•  Novel pla\orm for soluNon processable thermoelectrics

•  Large space to explore transport and for opNmizaNon of ZT towards 1

Rachel A. Segalman UC Berkeley 49

Acknowledgements •  DOE-‐BES LBNL Thermoelectrics Program

•  FaciliNes at the Molecular Foundry •  Dr. Shaul Aloni (TEM)

•  Dr. Arun Majumdar


Segalman Group

Documents

2010-Segalman