Single Molecule Electronics And Nano-Fabrication of Molecular Electronic Systems

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Single Molecule Electronics And
Nano-Fabrication of Molecular Electronic Systems

• S.Rajagopal, J.M.Yarrison-Rice
• Physics Department,
• Miami University Center For Nanotechnology,
  Oxford, OH.

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Highlights

• Organometallic paddlewheel complex
• Fabrication of two electrode and gated
• devices using EBL
• Closing of gap using electrodeposition
• Breaking a nanowire by electromigration
• Characterization of the fabricated nanogap

3
Process Steps
Fabricate nano-gap electrodes with EBL
Close gap to nano-gap using electrodeposition
Characterize the nano-gap
Deposit molecule and study the gap
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The Molecule

• Re-Re Quadruple bond

• Paddlewheel bridging ligands

• Anchoring thiol
• group

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Fabrication of Nanogap Electrodes
C
A
B
300nm
300nm
D
E

• Raith 150 EBL system
• Different gold thickness (100/150/250 nm) on top
  of 30nm Cr

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Fabrication Results

• Two electrode devices

1

• After EBL development

2

• GDS2 design
• Design gap 75nm

• Gap74nm

3

• After metal evaporation
• of Cr/Au
• Gap53nm

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Fabrication Results

• Gated electrode devices

1

• GDS2 gated design
• Design gap 60nm

2

• After metal evaporation of Cr/Au
• Gap10nm

3

• Gated device with 3
• contact pads

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Closing the Gap Using Electrodeposition

• Packaging Wire bonding Epoxy cavity

2
1

• Package Kovar material
• Wire bonding of contact pads to external leads
  Substrate
• temp 150 C
• Epoxy cavity for forming the electrochemical cell

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Factors To Consider

• Method ? Setup ( 2 methods tried )
• Electrolyte composition ( 2 compositions )
• Deposition current
• Electrolyte concentration ( 4 concentrations)

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Closing the Gap Using Electrodeposition

• Electrodeposition Setup 1 (Non Cyanide)

• Method Constant current Monitor the voltage
  across WE
• and RE
• Electrolyte composition 0.42 M Na2SO3 0.42 M
  Na2S2O3
• 0.05 M NaAuCl4
• Non-toxic and without strongly adsorbed ions
• At room temperature

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Results of Electrodeposition (Method 1)

• Time evolution curve of Vgap at a constant
  current of 25 µA on a chart recorder

Stop

• SEM image of fused electrodes after
  electrodeposition

• I-V curve showing hysteresis

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Difficulties with Method 1

• Method requires precise switching on desired gap
  voltage ? Manual ( less precise)
• Open loop system (no feedback)
• Lacks control on deposition rate
• Solution stability problem
• No two fabricated pairs showed the same growth
  pattern with similar initial/final gap voltages

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Modified Setup Self-terminating

• Method Constant current More directional
  growth
• Preset current for desired gap 5/10/20/50nA
• Mix C D 0.4 M Na2SO3 0.4 M Na2S2O3
• 0.01 M Na2Au(S2O3)2 0.3 M Sodium citrate
• Solution more stable (for more than 2 weeks)

J. Xiang, B.Liu, B.Liu, B. Ren, Z.Q. Tian,
Electrochemical Communications vol. 8, pp.
577-580, 2006
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Electrodeposition Results
Mag2.2 Kx I-10nA
Mag36 Kx I-10nA
Left electrode
Right electrode
Mag 15 Kx I-10nA
Left electrode
Right electrode
Abnormal growth
But, fine grain size
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Results Difficulties
I (A)
V (V)

• Growth moderately fine, but not predictable in
  all pairs
• Abnormal growth due to surface contamination
• Small structural shapes of electrode not
  retained
• Initial/Final V of nanogap showed no trend
• All final I/V curves showed huge gaps

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Design and Setup Changed

• New design tried to retain shape and avoid
  folding patterns
• New electrolyte delivery to localize to single
  pair
• Solution modification to minimize deposits on
  other electrode
• Minimize surface contamination

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Results SEM Micrographs

• Out of 8 pairs, 6 pairs showed similarly growth
• A small gap (10nm) could be realized using SEM
  images
• Abnormal growth seems controlled
• Electrode shape retained

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I-V Results of Nanogap
Pair 1
Pair 2
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Steps Ahead

• Design change 2 (Should make growth pattern more
  clear)
• Investigate why no similarity in the I-V curve
• Investigate affect of thickness of insulation
  layer on electrodeposition results (Use thicker
  insulation layer above substrate)
• Effective way of depositing a long (1nm)
  organic molecule across nanogap
• Measure electrical characteristics after
  depositing the molecule

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Conclusion

• Molecule Land
• Paddlewheel complex synthesized.
• Anchoring ligands are attached.
• Final analysis of the complex
• Device Fabrication Land
• Two electrode and Gated electrode device with
  larger nano-gap separation fabricated.
• Electrodeposition parameters determined for
  achieving 10nm gap.
• Fine-tuning of electrodeposition parameters for
  lt10nm gap

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Thank you !