Single Molecule Electronics
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Single Molecule Electronics. [email protected] Arizona Biodesign Institute. Single molecules as a tool for understanding. More Single molecules for better understanding. Photosynthesis and single molecule electronics The ASU-Physics-Chemistry-Engineering-Motorola group:. The Program.

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Single Molecule Electronics

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Single Molecule Electronics

[email protected]

Arizona Biodesign Institute

Single molecules as a tool for understanding

More Single molecules for better understanding


Photosynthesis and single molecule electronics

The ASU-Physics-Chemistry-Engineering-Motorola group:


The Program

Measure Single Molecule in well-defined conditions

Compare with theory (repeat as needed!)

‘Calibrated’ molecules and contacts for devices

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


The molecule-metal contact problem

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)


Making Single Molecule Junctions

  • Self-assembly: well defined geometry, contacts

  • Repeated contact (simpler after answer is known)

  • Fixed nano-gaps: problem of manufacturability


Self-assembled nanojunction

(Science 294, 571, 2001)

i


Alkanedithiols – STM Images


NI(V )/N (nA)

IV-Curves are integral multiples


Two Models for ‘quantized ‘ data


Histogram of curve multipliers

  • 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!

Current not stress-dependent – through bond?

Bonded

F

IF(h)

Mechanical

(Nanotechnology 13 5-14, 2002)


I is closer to theory for bonded molecules

Theory

Comparison with electrochemistry:

Bonded

R(C8)=950M

K=105s-1, EA=21kJ/m, R(C8)=300M

Mechanical


(CH2)10

(CH2)12

More chain lengths give (V)

(J. Phys. Chem. B 106 8609-8614, 2002 )

  • Also measure Ohmic region carefully to get (0)

  • Data do NOT agree with theory!

I=I0exp[-(v)z]


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

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


Coulomb Blockade: Quantized charge transfer

R1>>h/2e2

 Coulomb Blockade

n

n


Coulomb Blockade fits

(Removes Beta anomaly)

C8

C10

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)

Good agreement for I, 


Other Measurements

- Carotene

- Phenylene-ethylenine oligomers

-Technique reveals mobility of Au contacts


32 Å

Caroteniod

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

TRANS

T/C=4:1

CIS


Simple Tunneling with Coulomb Blockade fits well

No ‘free’ parameters

a

b

Measured

R1

R2

C1

C2

Trans

Cis

Carotene easily oxidized but vibronic contribution not dominant


Phenylene-ethylynene Oligomers

and Negative Differential Resistance

Applied Physics Letters 81 3043-3045 (2002)


2

Single Molecule NDR

1

1.8G

50G

Asymmetry, but molecules NOT oriented?

c.f. Reichert et al. PRL 88 2002

Confirms NDR but see non-reversible behavior


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


Single Molecule Bonding Fluctuations – Property of Au-S

  • 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

25

60


Conclusions

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

Gold nanocluster introduces Coulomb Blockade

Carotene – good agreement with tunneling calculations

Phenylene-ethylenine oligomers, confirm NDR.

Technique reveals mobility of Au contacts


Break Junctions

  • - Geometry unknown BUT

  • Calibration available AND

  • Most common peak appears to be correct

  • Much easier than self-assembled junctions

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


Single Molecule Conductance from Break Junctions

PZT

1

2

3

  • Are histogram peaks good data points?

  • What is the effect of strain?


Gold filaments stretch – maximum force ca 1nN

Xu, Xiao, Tao, JACS 2003


Alkanedithiols:

N=6:

RC6=10.5 MW

N=8:

RC8=51 MW

N=10:

RC10=630 MW

Major peaks are right peaks


SiO2

Au

Au

1um

Kubatkin et al. (2003) – (but3 devices/paper)

200nm

200nm

2um

500nm

a

b

Fixed Gaps:

EBL, Electrochemistry, Electromigration

c

d

e


77K

90

Vg=-0.5V

Vg=-0.25V

80

Vg=0V

70

Vg=0.25V

60

Vg=0.5V

50

Current(uA)

40

30

20

10

Molecule free molecular transistor

0

-10

-20

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

SDbias(V)

Atomic-scale control needed!

Our Fixed Gaps: 10% “Success Rate”?


Summary 1:

What we think we know

  • Measurements and theory in good agreement for some (single) molecules

  • (n-alkanes, carotenes, BDT [Tao, nanolets 4 267])

  • Break junctions can give good data and are much simpler


Summary 2:

What we don’t know/Can’t do

  • Single Molecule devices need to be assembled with Atomic Precision!

  • Fluctuations at room temperature?

  • Couplings and molecular vs. contact properties?

  • Redox activity molecules and environment?


ASU PHYSICS

Xiadong Cui

Otto Sankey

John Tomfohr

Jun Li

Jin He

ASU Engineering

Nongjian Tao

ASU Chemistry

Ana Moore

Tom Moore

Devens Gust

Xristo Zarate

Alex Primak

Yuichi Terazano

Motorola

Gary Harris

Larry Nagahara

ACKNOWLEGEMENTS


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