Stuart.Lindsay@ASU.EDU

Interfacing Molecules to Interfacing Molecules to Electronic MaterialsElectronic Materials

Artificial EnzymesArtificial EnzymesHydrogen generation, photovoltaicsHydrogen generation, photovoltaics

+

+

+ +

++

++

+

+

+

++

+

+

Steinberg-Yfrach et al. Nature 392, 479 (1998)

1. Make functional molecules

2. Wire molecules to electrodes

Steps to bio/molecular electronics:Steps to bio/molecular electronics:

3. Make them function on electrodes like they do in solution

4. Make economically-viable devices

Test Case: Single Molecule SwitchTest Case: Single Molecule Switchmade from Oligo Anilinemade from Oligo Aniline

HS

HN

NH

HN

NH

HN

NH

HN

SH

Insulatori

VConductor

-2e-

Insulator

-2e-

Single Molecule Switch?

Molecular electronics vs. solution Molecular electronics vs. solution charge transfer chemistrycharge transfer chemistry

• Charge transfer in nature in solution + ions.

• Charge transfer in molecular electronics electrode to electrode – No water/ions

12

LUMO

HOMO

CHARGED

Why solvent + ions matterWhy solvent + ions matter

e-

EN

ER

GY

e-

LANDAUER

MARCUS

SS

Charge Transfer in DNACharge Transfer in DNA

Barnett et al., Science 294 567 (2001)

The ChallengeThe Challenge

Need to measure single molecule conductance in a conducting solution with independent control of charge state.

How to do this?

0.000 0.005 0.010 0.015 0.020

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.4 0.8 1.2 1.60

2

4

6

Current (nA

)

G (

10-5X

2e2 /h

)

Time (Second)

Distance (nm)

Repeated break junctionRepeated break junction

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

Wiring Single Molecules ReliablyWiring Single Molecules Reliably

Cui et al. Science 274 571 (2001)

Xu and Tao, Science 301 1221 (2003)

GOOD NEWS: 7000:1 G range – worst Gtheor/Gmeas is 3.3

BAD NEWS: All Landauer theory

Operating probes in electrolyteOperating probes in electrolyte

Rev. Sci. Instrum. 60, 3128 - 3130 (1989)

(DNA - Xu et al. Nanoletts 4 1105 2004)

Insulating layer

Controlling ion gradients/electric Controlling ion gradients/electric fields at an electrode surfacefields at an electrode surface

Bigger ion gradient = Bigger electric field at molecule

Measuring transport as a function Measuring transport as a function of oxidation stateof oxidation state

H NH N

O

H NHN

OH N

H NO

HNHN

O

H NH N

O

H NH N

OH N

H NO

HNHN

O

O

O +-

Es

Vts

LOCAL FIELD

SURFACE FIELD

-0.2 0 0.2 0.4 0.6 0.8

Electrochem

ical Current

Surface Potential, ES , V vs. Ag

-0.2 0 0.2 0.4 0.6 0.80

1

2

3

4

5

6

7M

olec

ular

Con

duct

ance

(nS

)

-0.2 0 0.2 0.4 0.6 0.8

G=GMAX- a(ES-b)2

Insulator Conductor Insulator

TIP-SUBSTRATE V FIXED AT 50mV

-0.2 0 0.2 0.4 0.6 0.8

Ofer et al. JACS 112 7869, 1990

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

Neutral molecule

- - - - - -

Cur

rent

(nA

)

Tip-substrate bias (V)

(NO IONS)

FIX ES, VARY TIP FIELD

Oxidized molecule ES=0.4V(H2SO4)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6v

G=Gmax-a(ES-b)2

ES(V) =ES-V

v

=1.4

v

ES=0.3VES=0.25V

Tip-substrate bias (V)

Cur

rent

(nA

)

- - - - - -

MOLECULE FIELD = SURFACE FIELD ± TIP FIELD

We have a two terminal switch!

Vi

i

Vi

i

Vo

Vo

i

Vo

iA B

Vi

0 0.1 0.2 0.3

0

0.2

0.4

0.6

0.2 0.3 0.4 0.5 0.6

0.5

1.0

0.2 0.3 0.4 0.5 0.6

0.4

0.8

Cu

rre

nt (

nA

)

Vts (Volts) ES (V vs. Ag Wire)

- - -

But it will need more than one But it will need more than one molecule:molecule:

Bias sweeps Potential sweeps

SummarySummary

• Made a low-voltage switch based on chemical knowledge, get NDR

• Probe role of fluctuations

• Roadmap for going from chemistry to molecular electronics