Scanned Probe Imaging of Switching Centers in Molecular Devices

1
Scanned Probe Imaging of Switching Centers in
Molecular Devices
Chun Ning (Jeanie) Lau Dr. Duncan Stewart Dr. R.
Stanley Williams Prof. Marc Bockrath (Caltech)

• HP Labs
• Quantum Science Research

2
Molecular Electronics

• Challenges
• Fabrication
• Architecture
• New devices

• Ultimate limit of miniaturization
• Self-assembly ? low fabrication cost

3
Fault-tolerant Architecture
Nano-imprint Lithography

• fast fabrication of nm scale features over cm area

• HPL TeraMAC
• 1 THz multi-architecture computer
• Largest defect-tolerant computer
• 220,000 (3) defective components
• 106 gates operating at 106 cycle/sec
• addresses problem of lt100 yield

Heath et al, Science (1997).
6 Gbits/cm2
Y. Chen, G.Y. Jung et al. (2003).
4
Molecular Switches

• Potential applications as memory or logic devices

• Previously studied systems Nanopore, STM,
  cross-bar
• Molecules studied rotaxane, catanane, OPE,
  etc

Unsolved Question

• What are the switching mechanism(s)?
• Proposed
• conformational change of molecules
• eletrical charge transfer, electron localization
• molecule-metal contacts

5
Switching of Metal/Alkane/Metal Junctions
Our Experiment
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Switching of Metal/Alkane/Metal Junctions
1
2
Ti
3
Pt
4

• Asymmetric electrodes (Ti Pt)
• Reversible switching dependent on bias direction

Stewart et al, Nano Lett., in press.
7
Novel Scanned-Probe Technique

• Apply mN force with AFM tip while measuring
  device conductance
• Simultaneously explores electrical and local
  mechanical properties
• AFM tip not electrically connected to the device

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AFM Imaging of Mechanically-induced Conductance
Response

• Plot conductance through junction as a function
  of tip position
• AFM tip applies mN force ? pressure 103 104
  atm

9
Conductance Map (off state)
Topography
off
Conductance (Off)
15 mm
Molecular junction in the off state exhibited
no observable electrical response to local
mechanical perturbation by the AFM tip.
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Conductance Map (On state)
Topography
Conductance (On)
15 mm

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

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Switching Centers
Switching on of a device is always accompanied
by the emergence of a new nanoscale
pressure-induced conductance peak.
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Switching off
1
2
3
1
The switching center faded and completely
vanished with successive switchings to lower
conductance states.
2
3
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Our Experimental Finding
Lau et al, in preparation.

• 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.

What are these inhomogenities?
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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)
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Applying Pressure with AFM tip
Elasticity theory (LandauLifshitz) point force
applied to semi-infinite plane strain
at (x,y,d)
E Youngs modulus 80 GPa for metals and
alkane molecules d thickness of top electrode
30 nm

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

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A Simple Model

• Nano-asperity dominate transport

G Goff (Gon-Goff )exp (- b ? dz(x, y))
nano-asperity located at (0,0)
off state conductance
1.2 mS
1.4
No free parameters Goff 0.1mS, Gon1.3mS,
b1Å-1, dz (0,0) 0.2 Å

• Good agreement between model and data

• switching on ? growth of asperity

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Nanoscale Filaments
On/off ratio 105

• Ggtgtconductance quantum GQ ? Continuous
  filamentary pathway
• switching ? formation and dissolution of
  nano-filaments
• Nature and growth mechanism of filaments?
• (thermal migration, electrochemical reaction,
  electro-migration)

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Effect of Force on Conductance
600 nm
Force
Current
0.1 mN
Conductance within a switching center increases
with increasing applied force.
0.6 mN
1.5 mN
3 mN
Vbias 0.1 V
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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.

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mastery of nanoscale systems

• molecular devices
• superconducting nanowires nanorings
• ferromagnetic nanowires
• single molecules
• carbon nanotubes
• nanosensors

Were only at the base camp.
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Silicon ElectronicsWires SwitchesArchitectu
reLithography DepositionComplex physical
structurePerfect fabricationMemory Logic
NanoelectronicsWires SwitchesChemical
synthesis assemblySimple physical
structureImperfect fabricationArchitecture?Me
mory Logic
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Nanoscale filaments
Nanoscale filaments grows or shrinks ? switching

• Electromigration?
• Elecrochemical migration?
• Thermal migration?
• Single dominant pathway ? Runaway process?

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Switching Centers
Device A
Switching on of a device is always accompanied
by the emergence of a new nanoscale
pressure-induced conductance peak.
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Nanoscale Filaments?
Device A
Device B

• Unperturbed conductance 800 mS 10 GQ
• Small increase in conductance under pressure 1

• Unperturbed conductance ltlt GQ
• Relatively large increase in conductance under
  pressure 16

25
Nano-imprint Lithography

• nm scale features over cm area

6Gbits/cm2
Y. Chen, G.Y. Jung et al.
26
Fault-tolerant Architecture

• HPL TeraMAC
• 1 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

Philip Kuekes
Collier et al, Science 1998.
27
Nano-imprint Lithography

• fast fabrication of nnm scale features over cm
  area

6Gbits/cm2
Y. Chen, G.Y. Jung et al.
28
Nano-conducting Channels
Topography
Top Electrode
A local dominant nanoscale conducting channel
Bottom Electrode
Conductance image
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Switching On of Molecular Junctions
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 mN

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Another Device
Topography
Conductance
Off
1.025 1.112
1.200 Current (nA)
On
Off
A new hot spot always appeared after switching
on the junction
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Transport Through Molecular devices
Stewart et al, in preparation.
diode r5 x 105
Chang et al, APL (2003)
Single Molecule Measurements
Nanoscale junctions
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Molecular Electronics

• Ultimate limit of miniaturization
• Self-assembly ? low fabrication cost
• Designer molecules