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.

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

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

The Molecule

¤ Paddlewheel bridging ligands

¤ Re-Re Quadruple bond

¤ Anchoring thiol group

¤ Ir-Ir Double bond

¤ Os-Os Triple bond

Fabrication of Nanogap Electrodes

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

A B C

D

300nm

300nm

E

Fabrication Results¤ Two electrode devices

¤ GDS2 design¤ Design gap 75nm

¤ Gap=74nm

¤ After metal evaporation of Cr/Au¤ Gap=53nm

1

2

3

¤ After EBL & development

Fabrication Results¤ Gated electrode devices ¤ GDS2 gated

design¤ Design gap 60nm

¤ After metal evaporation of Cr/Au¤ Gap=10nm

¤ Gated device with 3 contact pads

1

2

3

Closing the Gap Using Electrodeposition

¤ Packaging = Wire bonding + Epoxy cavity

¤ Package: Kovar material¤ Wire bonding of contact pads to external leads ; Substrate temp ~150° C¤ Epoxy cavity for forming the electrochemical cell

21

Factors To Consider

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

Closing the Gap Using Electrodeposition

¤ 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

¤ Electrodeposition Setup 1 (Non Cyanide)

Results of Electrodeposition (Method 1)

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

¤ SEM image of fused electrodes after electrodeposition

Stop

¤ I-V curve showing hysteresis

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

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)

Galvanostat

DMM

Faraday Cage

WE RE/CE

200μ

J. Xiang, B.Liu, B.Liu, B. Ren, Z.Q. Tian, Electrochemical Communications vol. 8, pp. 577-580, 2006

I total

I total = I dep + I tunnel

I dep I tunnel

Electrodeposition ResultsMag=2.2 Kx I=-10nA Mag=36 Kx I=-

10nA

Left electrode

Right electrode

Abnormal growthBut, fine grain

size

Mag= 15 Kx I=-10nA

Left electrode

Right electrode

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

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

700nm

Revamped

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