Scanned Probe Imaging of Switching Centers in Molecular Devices

HP Labs

Quantum Science Research

Chun Ning (Jeanie) Lau

Dr. Duncan Stewart

Dr. R. Stanley Williams

Prof. Marc Bockrath (Caltech)

Molecular Electronics

Challenges

•Fabrication

•Architecture

•New devices

•Ultimate limit of miniaturization

•Self-assembly low fabrication cost

Fault-tolerant Architecture

HPL TeraMAC1 THz multi-architecture computer• Largest defect-tolerant

computer• 220,000 (3%) defective

components• 106 gates operating

at 106 cycle/sec• addresses problem of

<100% yield

Heath et al, Science (1997).

Nano-imprint Lithography

• fast fabrication of nm scale features over cm area

Substrate

MoldMold

SubstrateSubstrate

Y. Chen, G.Y. Jung et al. (2003).6 Gbits/cm2

Unsolved Question

What are the switching mechanism(s)?

Proposed:• conformational change of molecules• eletrical charge transfer, electron localization• molecule-metal contacts

Molecular Switches

• Previously studied systems: Nanopore, STM, cross-bar

Molecules studied: rotaxane, catanane, OPE, etc

• Potential applications as memory or logic devices

Switching of Metal/Alkane/Metal Junctions

Langmuir-Blodgett films of molecular monolayer sandwiched between m-sized metallic electrodes

V

A

TiPt

Our Experiment

COH

OH3C

C18H36O2

Stearic acid:

• electrical insulator HOMO-LUMO gap ~ 8eV

• no redox centers, mobile subgroups, or charge reception sites

2.6 nm

Switching of Metal/Alkane/Metal Junctions

• Asymmetric electrodes (Ti & Pt)

• Reversible switching dependent on bias direction

-1000

-500

0

500

1000

Cu

rren

t A

)

1.00.50.0-0.5Voltage (V)

1 2

3

4

V

A

Ti

Pt

Stewart et al, Nano Lett., in press.

Novel Scanned-Probe Technique

• Apply ~N force with AFM tip while measuring device conductance

• Simultaneously explores electrical and local mechanical properties

• AFM tip not electrically connected to the device

Si Substrate

Pt

Ti

AFM

• Plot conductance through junction as a function of tip position

• AFM tip applies ~N force pressure ~ 103 – 104 atm

AFM Imaging of Mechanically-induced Conductance Response

Conductance Map (“off” state)

Topography

Conductance (Off)

15 m

A

V

Molecular junction in the “off” state exhibited no observable electrical response to local mechanical perturbation by the AFM tip.

1.0

0.5

0.0

I(

0.50.0V(V)

off

1.0

0.5

0.0

I(

0.50.0V(V)

off

on

Conductance Map (“On” state)

Topography

Conductance (Off)

15 m

Conductance (On)

• A nanoscale conductance peak (“switching center”) emerges when the junction turn “on”.

1.0

0.5

0.0

I(

0.50.0V(V)

off

on

-20

-10

0

10

I (n

A)

-0.5 0.5Vbias

Off

400

200

0

-200

-400

I

-0.4 -0.2 0.0 0.2 0.4

Vbias

on

Off

-1.5

-1.0

-0.5

0.0

0.5

1.0

I

-0.4 -0.2 0.0 0.2 0.4

Vbias

Off

“Switching Centers”

on

Switching “on” of a device is always accompanied by the emergence of a new nanoscale pressure-induced conductance peak.

50 nm

1.6

1.5

1.4

1.3

1.2I

1086420

Tip Position (m)

0.0500.045

Switching “off”

300

200

100

0

I

43210Vbias

The switching center faded and completely vanished with successive switchings to lower conductance states.

1

2

3

Our Experimental Finding

Under mechanical pressure, a single nanoscale conductance peak (switching center) appears when the junction is switched “on”, and disappeared when “off”.

Formation and dissolution of nanoscale structural inhomogenities on the junction give rise to switching.

Lau et al, in preparation.

What are these inhomogenities?

A Simple Model

• Transport across molecular monolayer via tunneling• When switched “on”, electrodes move closer together

within a nanoscale region dominate transport• Conductance only increase when the AFM tip is

compressing the nano-asperity.

Nano-asperity

(top or bottom electrodes)

Applying Pressure with AFM tip

• Monolayer compressed by z (~ 0.2 Å for F ~ 1 N)• Spatial resolution ~ 40 nm : Limited by tip radius and thickness of top electrode.

Elasticity theory (Landau&Lifshitz)

point force applied to semi-infinite plane strain at (x,y,d)

F

z=0

d F

dyx

d

Euzz 2/5222

3

2

3~

E = Young’s modulus ~ 80 GPa for metals and alkane moleculesd = thickness of top electrode ~ 30 nm

A Simple Model• Nano-asperity dominate transport

Model

No free parameters : Goff~ 0.1S, Gon~1.3S, ~1Å-1, z (0,0) ~ 0.2 Å

• switching “on” ↔ growth of asperity

Data

0 500

0.0

127.5

col

row

-0.1625 0.2000

ana15_05fa_short3D_sm

1.2 S 1.4

G = Goff + (Gon-Goff )exp (- z(x, y))

nano-asperity located at (0,0)“off” state conductance

• Good agreement between model and data

Nanoscale Filaments

-20

-10

0

10

I (n

A)

-0.5 0.5Vbias

Off

400

200

0

-200

-400

I

-0.4 -0.2 0.0 0.2 0.4Vbias

on

Off

• G>>conductance quantum GQ Continuous filamentary pathway

• switching ↔ formation and dissolution of nano-filaments

• Nature and growth mechanism of filaments?(thermal migration, electrochemical reaction, electro-migration…)

0 500

0.0

127.5

col

row

-0.1625 0.2000

ana15_05fa_short3D_sm

850 860S

On/off ratio ~ 105

Effect of Force on Conductance

Conductance within a “switching center” increases with increasing applied force.

nm

Current

Vbias = 0.1 V

Force

0.1 N

0.6 N

1.5 N

3 N

tunnel barrier

Conclusion

• Novel experimental technique that probes nanocale conductance pathways through molecular junctions

• New switching mechanism with high on/off ratio due to formation and break-down of conductive nano-filaments

Under Investigation

• filament growth mechanism

• pressure dependence

• other systems (other molecules, different electrodes)

• role of electrodes in metal/molecule/metal structures better engineering of molecule-based devices.

We’re only at the base camp….

•molecular devices

•superconducting nanowires & nanorings

•ferromagnetic nanowires

•single molecules

•carbon nanotubes

•nanosensors

……

mastery of nanoscale systems

Silicon Electronics

Wires & Switches

ArchitectureLithography & Deposition

Complex physical structurePerfect fabrication

Memory & Logic

Nanoelectronics

Wires & Switches

Chemical synthesis & assembly

Simple physical structureImperfect fabrication

Architecture?

Memory & Logic

Nanoscale filaments

Nanoscale filaments grows or shrinks switching

•Electromigration?

•Elecrochemical migration?

•Thermal migration?

•Single dominant pathway Runaway process?

-20

-10

0

10

I (n

A)

-0.5 0.5Vbias

Off

400

200

0

-200

-400

I

-0.4 -0.2 0.0 0.2 0.4

Vbias

on

Off

-1.5

-1.0

-0.5

0.0

0.5

1.0

I

-0.4 -0.2 0.0 0.2 0.4

Vbias

Offon

“Switching Centers”

Switching “on” of a device is always accompanied by the emergence of a new nanoscale pressure-induced conductance peak.

Device A

Nanoscale Filaments?

860

850G

S)

2500Tip Position (nm)

• Unperturbed conductance ~ 800 S ~ 10 GQ

• Small increase in conductance under pressure ~1%

Device B Device A

1.3

1.2

G

S)

2500Tip Position (nm)

• Unperturbed conductance << GQ

• Relatively large increase in conductance under pressure ~16%

Nano-imprint Lithography

Substrate

MoldMold

SubstrateSubstrate

Y. Chen, G.Y. Jung et al.

• nm scale features over cm area

6Gbits/cm2

Fault-tolerant Architecture

HPL TeraMAC1 THz multi-architecture computer• Largest defect-tolerant

computer• 220,000 (3%) defective

components• 106 gates operating

at 106 cycle/sec• Built from programmable gate

arrays• Computes with look-up tables

Collier et al, Science 1998.

Philip Kuekes

Nano-imprint Lithography

Substrate

MoldMold

SubstrateSubstrate

Y. Chen, G.Y. Jung et al.

• fast fabrication of nnm scale features over cm area

6Gbits/cm2

Topography

Conductance image

Nano-conducting Channels

Bottom Electrode

Top Electrode

0.0 0.5 1.0 1.5 2.0 2.5

18.5

19.0

0 2

Tip position (m)

18.0

R (

k

19.0

A local dominant nanoscale conducting channel

Switching On of Molecular Junctions

0.14

0.13

I

1.00.50.0Tip position (m)

A conductance hot-spot appeared under mechanical modulation when the junction was switched on.

• Diameter ~ 50 nm ~ AFM tip radius

• 10% increase in conductance under ~2 N

400

200

0

-200

-400

I

-0.4 -0.2 0.0 0.2 0.4Vbias

Another Device

A new hot spot always appeared after switching “on” the junction

1.025 1.112 1.200Current (nA)

-20

-10

0

10

I (nA

)

-0.5 0.5Vbias

86.0

85.0

I

5002500

Tip Position (nm)

Topography Conductance

85.0 86.0current (A)

Off

On

Off

On

Transport Through Molecular devices

100 nm

Single Molecule Measurements Nanoscale junctions

2m

Pt

Ti

-1.2

-0.9

-0.6

-0.3

0

-2 -1 0 1 2

Cu

rren

t (m

A)

Voltage (V)

0

5

10

15

20

0 0.5 1 1.5 2 2.5

Cu

rre

nt

(nA

)

Voltage (V)

Ti

S

O

CH3

Pt

13

3

V

Chang et al, APL (2003)

T(K)

C18H36OH

Pt

Pt V

Stewart et al, in preparation.diode r~5 x 105

Molecular Electronics

•Ultimate limit of miniaturization

•Self-assembly low fabrication cost

•“Designer molecules”