Molecular Electronics - McGill Universitycsacs.mcgill.ca/custom/docs/activities/classes/2009-class-blum.pdf · • Making molecular measurements ... Attach molecules to surface and

Molecular Electronics

Amy Szuchmacher BlumMcGill University

Department of Chemistry

Outline

• Why use molecules?

• Making molecular measurements

• Conduction in molecules

• Molecular switches

Moore’s Law

www.intel.com

Production Costs

Technical Challenges

target

Molecular Measurements

Basic 2 Terminal Device

Elucidate the roles that “alligator clips”, metals and the molecular core play in defining I-V

characteristics

molecular core

“alligator clips”

metals

“Alligator Clips”

Attach molecules to surface and drive the self-assembly process

“Connect” molecular electronic structure to the metallic band structure

Reed and Tour Scientific American June 2000

Electronic Characterization Methods

Scanning Probes

Nanopore Hg Drop

S S

Au{111}

S

STM Tip trajectory

STM Tip

S SS SSS S S S S

Break Junction Crosswire Junction

Break Junction

QuickTime™ and a decompressor

are needed to see this picture.

Advantages: Can measure single molecules

Electrical contact to device easy

Stable to T change (low T)

Y. Selzer et al. Nano Lett 5, 61 (2005).

Break Junction Measurements


are needed to see this picture.QuickTime™ and a

decompressorare needed to see this picture.

H.B. Weber et al. Physica E, 18, 231 (2003).

SAM in Nanopore





M.A. Reed Proc IEEE, 87, 652 (1999).

Advantages: SAM formation well understood

Stable once formed (T, V, molecular decompostion)

High yield of functional devices

Problems With the Top Electrode



A. V. Walker et al. Appl. Phys. Lett., 84, 4008 (2005).

Hot metal reacts with SAM (ex: Ti)

Hot metal doesn’t stick to SAM (ex: Au)

Crossed Wire Tunnel Junction

S SAu

Au

• Simple experimental apparatus • Wide variety of wires available (Au, Pd, Pt, …)• Junction contains ~103 molecules• No metallization after SAM deposition

Thiolate-Metal Interaction

Higher conductivity corresponds to hole injection at the Pd-S interface.

Lower barrier for Pd-S compared to Au-S contact consistent with theoretical predictions.

SSAu Pd

SPM techniques

• Molecular junctions made using metal coated AFM tips to contact self-assembled monolayers

• Junction resistance measured as a function of molecular length and contact type

C. Daniel Frisbie University of Minnesota

Conducting Probe AFM

Current vs. Voltage Measurements on Single Molecules

crystalline undecanethiol

(C11) SAM

insert dithiol molecule at defect

sites

bind 2 nm gold nanoparticles to inserted dithiols

25 nm x 25 nm 75 nm x 75 nm 75 nm x 75 nm

A. S. Blum et al. Appl. Phys. Lett., 82, 3322 (2003).

Molecular Film vs. Isolated Molecule

•Intermolecular hopping does not contribute to conductance

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

-0.1 -0.06 -0.02 0.02 0.06 0.1

bias (V)

curr

ent (

nA)

monocomponent OPE SAM OPE inserted in C11 SAM

monocomponent OPE SAM

OPE inserted into C11 SAM


Two Layer Tunnel Junction Model

L. A. Bumm et al. J. Phys. Chem. B., 103, 8122 (1999).

Layer Transconductance:

G = G0 exp(-bihi)

Constant Current Imaging:

GC11 Ggap = GOPV Ggap

b2 = [bC11hc11-a(DSTM-Dh)]/h2

Determining b via STM Imaging

A. S. Blum et al. J. Am. Chem. Soc., 127, 10010 (2005).

Apparent height represents convolution of topography and electronic information.

Molecular Decay Constants

molecule length(Å)

STM apparent height

(Å)

b(Å-1)

C11 12.5 0 1.2

OPE 20.2 4.3 1.03±0.13

OPV 19.3 10.0 0.40±0.12

Ru-OPE (S to final C) 22.4 7.8 0.88±0.12

Ru-OPE(S to final Ru) 18.6 7.8 0.59±0.08

D. S. Seferos et al. J. Am. Chem. Soc., ASAP (2006).

A. S. Blum et al. J. Am. Chem. Soc., 127, 10010 (2005).

Multiple Break Junction Formation





Z. Huang et al. J. Am. Chem. Soc., 129, 13225 (2007).

Advantages: Good statistics

Probes local environment

Don’t need monolayer

Idealized Junction

We think about and model single-molecule junctions.

Measuring an Ensemble of Molecules

Are the properties of molecular wires in a metal-molecule-metal

junction additive? Does G = nG0

Theoretical Predictions

M. Magoga and C. Joachim, Phys. Rev. B 59, 16011 (1999).

S. N. Yaliraki and M. A. Ratner, J. Chem. Phys. 109, 5036

(1998).

N. D. Lang and P. Avouris, Phys. Rev. B 62, 73257329 (2000).

Measurements on Alkanes

Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571-574.

Scaling of Parallel Molecular Wires

sample molecular structure length(nm)

crossed wire

resistance

STM resistance

OPE 2.02 1.7 ± 0.6 MW 1.7 ± 0.4 GW

OPV 2.07 0.5 ± 0.2 MW 0.6 ± 0.1 GW

A.S. Blum et al., J. Phys. Chem. B, 108, 18124 (2004).

Scaling of Molecular Wires in Parallel

Molecular wires act as discrete non-interacting conductance channels (i.e. G = nG0)

Possible Conduction Mechanisms

resonanttunneling

non-coherent charge carrier hopping

non-resonanttunneling

Dominant Mechanisms for Short Molecules

Conduction via Tunneling



Y. Selzer et al. Nano Lett 5, 61 (2005).

Arrhenius plot of current vs. 1/T at bias voltage 0.1 V to 1 V

Basic 2 Terminal Device

molecular coreOBu

BuO

Structure Dependence of Charge Transport

S SAu Au

SSAu

Au

S

SAu

Au

OBu

BuO

Inelastic Electron Tunneling Spectroscopy

Elastic

Inelastic

E

EF

h

EF - eV

A molecular vibration can be

stimulated when

V h/e

I

dI/dV

d2I/dV2

V

V

Vh/e

h/e

h/e

IETS of Alkane Thiol

Observation of both IR and Raman vibrational modes

C-H str.

CH2 rock

v(C-C)T

CH2 wag

Jason Lazorcik, James G. Kushmerick

IETS of Molecular Junctions

HS

AcS SAc

AcSSAc

C11

OPV

OPE

C11

OPE OPV

Nano Letters 4 (2004) 639-642.

In Junction Optical Spectroscopy



D.R. Ward et al. Nano Lett 8, 918 (2008).

Correlated Optical and Conduction Measurements



D.R. Ward et al. Nano Lett 8, 918 (2008).

Outline

• Motivation

• Switching in Molecules

• Virus as a scaffold

• Making molecular circuits

• Molecular memory devices

Molecular Electronics

• How can we make molecular switches?

• Once we have switches, how can we make electrical contact?

Switching in Molecules

Previous Examples of Molecular Switching

Reed/Tour switch in nanopore

voltage based switching

Weiss group (2001), Lindsay group (2003), others switch in STM

stochastic switching

Z. J. Donhauser, et. al, Science 292, 2303-2307 (2001).

Reed et al. Applied Physics Letters

78 (2001) 3735-3737.

Current vs. Voltage Measurements on Single Molecules

crystalline undecanethiol

(C11) SAM

insert dithiol molecule at defect

sites

bind 2 nm gold nanoparticles to inserted dithiols

25 nm x 25 nm 75 nm x 75 nm 75 nm x 75 nm


Stochastic Switching

molecule matrix reference

dodecanethiol(C12)

Z.J. Donhauser et al. Science, 292, 2303 (2001).

decanethiol(C10)

G. K. Ramachandran et al. Science, 300, 1413 (2003).

dodecanethiol(C12)

decanethiol(C10)

R. A. Wassel et al. Nano Lett., 3, 1617 (2003).

undecanethiol(C11)

A. S. Blum et al. Nat. Mater.,

5,167 (2005).

Stochastic Switching

Measurements made at constant bias voltage (1V)

Stochastic switching is dynamic—molecules switch on and off

Molecular Film vs. Isolated Molecule

•Intermolecular hopping does not contribute to conductance

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

-0.1 -0.06 -0.02 0.02 0.06 0.1

bias (V)

curr

ent (

nA)

monocomponent OPE SAM OPE inserted in C11 SAM

monocomponent OPE SAM

OPE inserted into C11 SAM


I/V Measurements on BPDN SAMs

A. S. Blum et al. Nat. Mater., 5, 167 (2005)

Two-State Switching in BPDN

• Is it the same phenomenon as stochastic switching?

• Is it dependent on the experimental setup?

• Is it a molecular property?





0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2

bias (V)

curr

ent (

nA)

Voltage Controlled Switching in STM

500 mV21 500 mV

500 mV (molecules off)

2

1

500 mV (molecules on)

Bias history changes imaging properties of inserted molecules.

Change in molecule appearance is due to

state change, NOT bias dependent imaging

STM of a Non-switching Molecule

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

bias (V)

curr

ent (

nA)

2

1

OPE shows no bias history dependent behavior

Stochastic, but no field based switching observed

1 500 mV 500 mV2





SEM images of micromagnetic trap (MMT)(a) Central region of MMT device.(b) Image of magnetic proximal probe tips.

D. Long et al., Adv. Mater., 16, 814 (2004).

I

V

SEM image of microsphere junction

N S

Magnetic Entrapment of Metallized Silica Microspheres

Crossed-Wire Test Bed

PRL 89, 086802 (2002)JACS 124, 10654-10655 (2002)

JACS 125, 3202-3203 (2003)Nano Letters 3, 897-900 (2003)

Conductance Scaling

1

1000

100-300

D. Long et al., Adv. Mater., 16, 814 (2004).A.S. Blum et al., J. Phys. Chem. B, 108, 18124 (2004).

Molecular Memory Studied by Three Techniques

Consistent switch signature

observed for same molecule in three independent test-

beds.

BPDN

A. S. Blum et al. Nat. Mater., 5, 167 (2005)





Single Molecule Measurements

I

Over 2nm particle

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-2 -1.5 -1 -0.5 0

bias (V)

curr

ent (

nA)

Over C11 film

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

-2 -1.5 -1 -0.5 0

bias (V)

curr

ent (

nA)

In I/V curves, 2 states observed for BPDN molecule, but not for C11 alkane

Controllable Switching

0.0

2.0

4.0

6.0

8.0

10.0

12.0

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1

Bias (V)

Cur

rent

(Offs

et) n

A

h

Controlled switching between states induced by running bias through 0 V

Conductance state can be controlled by the applied voltage

Bias offset demonstrates charging in molecule or film

Reproducibility

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

-3 -2.5 -2 -1.5 -1 -0.5 0

bias (V)

curr

ent (

nA) [

offs

et fo

r cla

rity]

1

10

20

45

Switching between high conductivity state and low conductivity state is reproducible as long as tip stays over moleculeSwitching between high conductivity state and low conductivity state is

reproducible as long as tip stays over molecule

ReproducibilityE. Lörtscher et al. Small 2, 973 (2006).

J. He et al. JACS 128, 14828 (2006).

Z.K. Keane et al., Nano Lett. 6, 1518 (2006).





• What is necessary for two-state behavior?

Switching Mechanisms

Bias offset demonstrates charging in molecule or film

Au

SH

SH

SH

SH

SH

SHSH SH SHSH SH SH SH SH SHSH SH SHSH SH SH SH SHSHSHSH

Au

2 nm Au

Au

SH

SH

SH

SH

SH


Au

SH

SH

SH

SH

SH


Au

2 nm Au


• Is it the same phenomenon as stochastic switching?• No, independent phenomenon.

• Is it dependent on the experimental setup?• No, 3 different test beds show same qualitative

behavior• Is it a molecular property?

• Yes, isolated BPDN molecules show two state switching

• What is necessary for two-state behavior?• Low lying redox states?• NO2 functionality?• Pyridinyl functionality?• Biphenyl ring structure?

Redox Active Molecules as Switches

Ru1Ru2

NN

SSiMe3

4

11.62

15.4

Ru1Ru2

NN 4

SSiMe3

18.62

22.4

Compund 1, distances are crystallographically determined

Compund 2, distances are estimated based on that of 1

Designing a molecular switch:

Low-lying redox states

Conjugated system for conductance

Tong Ren, Purdue A. S. Blum et al. J. Am. Chem. Soc., 127, 10010 (2005).

Redox Active, No Switching!

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

bias (V)

curr

ent (

nA)

Single redox wave with a formal potential (vs. Ag/AgCl/KCl) of E = 0.40 V

Stochastic switching, but no two state behavior in I/V.

Scott Trammel, NRL


• Is it the same phenomenon as stochastic switching?• No, independent phenomenon.

• Is it dependent on the experimental setup?• No, 3 different test beds show same qualitative behavior

• Is it a molecular property?• Yes, isolated BPDN molecules show two state switching

• What is necessary for two-state behavior?• Low lying redox states? NO!• NO2 functionality?• Pyridinyl functionality?• Biphenyl ring structure?

What is needed for switching?

Molecules courtesy Jim Tour

N

SAc

N

SAc

O2N

NO2

N

SAc

N

SAc

SAc

SAc

SAc

SAc

NO2

NO2

SAc

SAc

O2N

NO2

SAc

SAc

BPDN trans-DN cis-DN BP Biphen OPE

Biphenyl-Dinitro

N

AcS

N

SAc

NO2

O2N95% of all molecules probed show 2-state behavior

For any given molecule, 90% of the I/V traces show 2-state behavior

Large change in conductance: high to low ratio is 30 ± 10

Biphenyl

AcS SAc

No two-state behavior observed

Cis-Dinitro

AcS SAc

O2NO2N 85% of all molecules probed show 2-state behavior


Large change in conductance: high to low ratio is 15 ± 8

Bias offset seen for high conductive state, as in BPDN

Bipyridyl

N

AcS

N

SAc 83% of all molecules probed show 2-state behavior


Small change in conductance: high to low ratio is 3.5 ± 2

BP Bias Offset

BPDN BP

Bias offset opposite in direction?

Polaron Model

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

-2.5 -1.5 -0.5 0.5 1.5 2.5

bias (V)cu

rren

t (nA

)

M. Galperin, M. A. Ratner, and A. Nitzan Nano Lett. 5, 125 (2005).

Conductance and Switching

molecule length(Å)

STM apparent height (Å)

b(Å-1)

C11 12.5 0 1.2

OPE 20.2 4.3 1.03±0.13

Biphenyl 24.1 5.1 1.16±0.31

Bipyridyl 24.1 5.3 1.14±0.25

Bipyridyl-Dinitro 24.1 9.3 0.73±0.18

Biphenyl and substituted biphenyls in the low state show the same measured current at 1 V.

What is needed for switching?

N

AcS

N

SAc

NO2

O2N

AcS SAc

O2NO2N

N

AcS

N

SAc

• Molecules with NO2groups have “strong” switching signatures.

• Smaller switching signature still present for pyridyl molecule without NO2 groups!

• No switching for molecule lacking both NO2 and pyridyl groups.

AcS SAc

What is Needed for Switching?

N

AcS

N

SAc

NO2

O2N

AcS SAc

NO2

O2N

AcS SAc

O2NO2N

N

AcS

N

SAc

AcS SAc

AcS SAc

Molecular Structure Switching

Yes

Yes

Yes

Yes (Minimal)

No

No

Conclusions

• BPDN exhibits bias controlled switching between two conductance states independent of measurement technique

• Redox activity is not enough to produce switching

• NO2 groups produce a large difference between high and low states, but not required for conductance switching

• b for high conductance state significantly enhanced vs. low conductance state (and unsubstituted biphenyl)

• Switching mechanism possibly related to charging, but still unknown—IETS is a good probe

Acknowledgements

NRL Rice UniversityJohn C. Yang James M. TourCharles M. Patterson Yuxing Yao Banahalli R. Ratna Jay C. HendersonMartin H. MooreScott Trammell

SAIC Purdue UniversityRanganathan Shashidhar Tong RenDavid P. Long

NIST FundingJames G. Kushmerick DARPA, ONR

Documents

Molecular Electronics - McGill Universitycsacs.mcgill.ca/custom/docs/activities/classes/2009-class-blum.pdf · • Making molecular measurements ... Attach molecules to surface and