Single Molecule Electronics

Single Molecule ElectronicsSingle Molecule Electronics

[email protected]

Arizona Biodesign Institute

Single molecules as a tool for understanding

More Single molecules for better understanding

Photosynthesis and single molecule electronicsPhotosynthesis and single molecule electronicsThe ASU-Physics-Chemistry-Engineering-Motorola group:

Make fixed-gap (useful? T-dep?) devicesMake fixed-gap (useful? T-dep?) devices

The ProgramThe ProgramMeasure Single Molecule in well-defined Measure Single Molecule in well-defined conditionsconditions

Compare with theory (repeat as needed!)Compare with theory (repeat as needed!)

‘‘Calibrated’ molecules and contacts for devicesCalibrated’ molecules and contacts for devices

Many Few-Molecule-Devices have been made but measurements/theories generally do not

agree: For example, DNA is:

AN INSULATOR (D. Dunlap et al. PNAS 90, 7652, 1993)

A SEMICONDUCTOR (D. Porath et al, Nature 403, 635, 2000)

A CONDUCTOR (Fink and Schoenberger, Nature 398, 407,1999)

A SUPERCONDUCTOR (A.Y. Kasumov et al. Science 291, 280, 2001)

The molecule-metal contact The molecule-metal contact problemproblem

1.1. Self-assembly: well defined geometry, Self-assembly: well defined geometry, contactscontacts

2.2. Repeated contact (simpler after answer is Repeated contact (simpler after answer is known)known)

3.3. Fixed nano-gaps: problem of Fixed nano-gaps: problem of manufacturabilitymanufacturability

Making Single Molecule Making Single Molecule JunctionsJunctions

i

Self-assembled Self-assembled nanojunctionnanojunction(Science 294, 571, 2001)

Alkanedithiols – STM Images

-40

-30

-20

-10

0

10

20

30

40

-1.0 -0.5 0.0 0.5 1.0

1I(V)

2I(V)

3I(V)

4I(V)

5I(V)

Cur

rent

(nA

)

Tip bias (V)

IV-Curves are integral multiples

-8-6-4-202468

-1.0 -0.5 0.0 0.5 1.0

1I(V)/12I(V)/23I(V)/34I(V)/45I(V)/5

Tip bias (V)

NI(

V )/N

(nA)

Two Models for ‘quantized ‘ data

Histogram of curve multipliers

0100200300400500600700

0 1 2 3 4 5Current divisor X

• Find X such that variance from curve to curve is minimized

• Over 1000 curves for n=1

IV-Curves of bonded molecules not very stress dependent!

-2

0

2

4

6

8

10

10-5

0.0001

0.001

0.01

0.1

1

10

100

-24 -20 -16 -12 -8 -4 0 4F

orce

(n

N)

Current (n

A)

Distance moved (nm)

Current not stress-dependent – through bond?

Mechanical

Bonded

F

IF(h)

(Nanotechnology 13 5-14, 2002)

0.0001

0.001

0.01

0.1

1

10

100

0 0.2 0.4 0.6 0.8 1

Cur

rent

(n

A)

Tip Bias (V)

Theory Bonded

Mechanical

I is closer to theory for bonded molecules

Comparison with electrochemistry:

)0(2

A

B

EKe

TkR

Tk

E

KEK

B

A

A

exp

)0(

K=105s-1, EA=21kJ/m, R(C8)=300M

R(C8)=950M

More chain lengths give (V) (J. Phys. Chem. B 106 8609-8614, 2002 )

-0.04

0

0.04

-0.05 0 0.05

-0.02

0

0.02

-0.05 0 0.05

0

50

1 2 3 4

-15

-10

-5

0

5

10

15

-1 -0.5 0 0.5 1

Cur

rent

(nA

)

Tip Bias (V)

-4-3-2-101234

-1 -0.5 0 0.5 1

Cur

rent

(nA

)

Tip Bias (V)

(CH2)10

(CH2)12

0

30

1 2 3 4

• Also measure Ohmic region carefully to get (0)

• Data do NOT agree with theory!

I=I0exp[-(v)z]

-8

-6

-4

-2

0

2

4

6

8

-1 -0.5 0 0.5 1

Cur

rent

(nA

)

Tip Bias (V)

Clue: I-V doesn’t fit Clue: I-V doesn’t fit tunneling model welltunneling model well

What if the top contact is not so good? Coulomb Blockade?

Coulomb Blockade: Coulomb Blockade: Quantized charge transferQuantized charge transfer

R1>>h/2e2

Coulomb Blockade

n

n

-12

-8

-4

0

4

8

12

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Cur

ren

t (n

A)

Tip Bias (V)

C8

-6

-4

-2

0

2

4

6

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Cur

ren

t (n

A)

Tip Bias (V)

C10

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Cur

ren

t (n

A)

Tip Bias (V)

C12

 r1 = 1M

c1 = 0.318aF

(from fitting C8)

r8 = 128.36M

r10 = 252M

r12 = 875M

(c1, r1 fixed)

C8=c10 =c12= 0.085aF

(Theory=0.08aF)

Coulomb Blockade fitsCoulomb Blockade fits

(Removes Beta anomaly)(Removes Beta anomaly)

Good agreement for I,

- Carotene- Carotene

- Phenylene-ethylenine oligomers- Phenylene-ethylenine oligomers

--Technique reveals Technique reveals mobilitymobility of Au of Au contactscontacts

Other MeasurementsOther Measurements

CaroteniodCaroteniod

SH

HS

32 Å

t

TRANS

CIS

T/C=4:1

(J. Phys. Chem. B, 107, 6162-6169, 2003)

-10

-5

0

5

10

-1.5 -1 -0.5 0 0.5 1 1.5

Cur

ren

t (n

A)

Tip Bias (V)

-0.2

0

0.2

-0.2 0 0.2-3

-2

-1

0

1

2

3

-1.5 -1 -0.5 0 0.5 1 1.5

Cur

ren

t (n

A)

Tip Bias (V)

a b

R1 R2

C1 C2

Trans

Cis

Measured

No ‘free’ parameters

Simple Tunneling with Simple Tunneling with Coulomb Blockade fits wellCoulomb Blockade fits well

Carotene easily oxidized but vibronic contribution not dominant

Phenylene-ethylynene OligomersPhenylene-ethylynene Oligomers

and Negative Differential and Negative Differential ResistanceResistance

SAc

NO2

AcS

SAcAcS

1 = 1-nitro-2,5-di(phenylethynyl-4´-thioacetyl)benzene2 = 2,5-di(phenylethynyl-4´-thioacetyl)benzene

1

2

Applied Physics Letters 81 3043-3045 (2002)

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0.5 0 0.5 1 1.5

Cur

rent

(nA

)

Tip Bias (V)

Single Molecule NDRSingle Molecule NDR

1.8G

50G

1

202468

1012

0 0.5 1 1.5 2

Co

un

t

Peak Volts

Confirms NDR but see non-reversible behavior

Asymmetry, but molecules NOT oriented?

c.f. Reichert et al. PRL 88 2002

Single Molecule Bonding Fluctuations (Science 300 1413, 2002)

• “Stochastic switching” reported for

• We see the same effect in alkane dithiols

• Significant switching with gold sphere attached

NO2

• Cannot internal electronic changes

• Cannot be top ‘dipping’ into film

• Cannot be bond to sphere breaking

• Rate increases at annealing temperature

• Fluctuations of lower bond

Single Molecule Bonding Single Molecule Bonding Fluctuations – Property of Au-SFluctuations – Property of Au-S

25 60

Transport in gold-Transport in gold-n-n-alkane-gold fits ab-initio alkane-gold fits ab-initio tunneling calculations well: One molecule, tunneling calculations well: One molecule, well defined environment.well defined environment.

Gold nanocluster introduces Coulomb BlockadeGold nanocluster introduces Coulomb Blockade

Carotene – good agreement with Carotene – good agreement with tunnelingtunneling calculationscalculations

Phenylene-ethylenine oligomers, confirm NDR. Phenylene-ethylenine oligomers, confirm NDR.

Technique reveals Technique reveals mobilitymobility of Au contacts of Au contacts

ConclusionsConclusions

Break JunctionsBreak Junctions

(Xu and Tao, Science 301, 1221-1223 2003)

- Geometry unknown BUT

- Calibration available AND

- Most common peak appears to be correct

- Much easier than self-assembled junctions

Single Molecule Conductance from Single Molecule Conductance from Break JunctionsBreak Junctions

1 2 3

PZT

• Are histogram peaks good data points?

• What is the effect of strain?

Gold filaments stretch – Gold filaments stretch – maximum force ca 1nNmaximum force ca 1nN

Xu, Xiao, Tao, JACS 2003

Alkanedithiols:Alkanedithiols:SHHS

RC6=10.5 M

HS SH

N=6:

N=8:

RC8=51 M

SHHSN=10:

RC10=630 M

Major peaks are right peaks

200nm200nm

SiO2

AuAu

1um

500nm

a b

c

d

2um

e

Fixed Gaps: Fixed Gaps: EBL, Electrochemistry, ElectromigrationEBL, Electrochemistry, Electromigration

Kubatkin et al. (2003) – (but 3 devices/paper)

Our Fixed Gaps: 10% “Success Our Fixed Gaps: 10% “Success Rate”? Rate”?

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-20-10

0102030405060708090

77K

Cu

rre

nt(u

A)

SDbias(V)

Vg=-0.5V Vg=-0.25V Vg=0V Vg=0.25V Vg=0.5V

Molecule free molecular transistorMolecule free molecular transistor

Atomic-scale control needed!

1.1. Measurements and theory in good agreement Measurements and theory in good agreement for some (single) moleculesfor some (single) molecules

(n-alkanes, carotenes, BDT (n-alkanes, carotenes, BDT [Tao, nanolets [Tao, nanolets 44 267] 267]))

2.2. Break junctions can give good data and are Break junctions can give good data and are much simplermuch simpler

Summary 1:Summary 1:

What we think we knowWhat we think we know

1.1. Single MoleculeSingle Molecule devices need to be devices need to be assembled with assembled with Atomic Precision!Atomic Precision!

2.2. Fluctuations at room temperature?Fluctuations at room temperature?

3.3. Couplings and molecular vs. contact Couplings and molecular vs. contact properties?properties?

4.4. Redox activity molecules and environment?Redox activity molecules and environment?

Summary 2:Summary 2:

What we don’t know/Can’t doWhat we don’t know/Can’t do

ACKNOWLEGEMENTSACKNOWLEGEMENTSASU PHYSICSXiadong CuiOtto SankeyJohn TomfohrJun LiJin HeASU EngineeringNongjian TaoASU ChemistryAna MooreTom MooreDevens GustXristo ZarateAlex PrimakYuichi TerazanoMotorolaGary HarrisLarry Nagahara