1. 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.

2. 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

4. The Molecule ¤ Paddlewheel bridging ligands ¤ Re-Re Quadruple bond ¤ Anchoring thiol group ¤ Ir-Ir Double bond ¤ Os-Os Triple bond

5. 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 E

6. 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

7. 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

8. 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

9. Factors To Consider ¤ Method  Setup ( 2 methods tried ) ¤ Electrolyte composition ( 2 compositions ) ¤ Deposition current ¤ Electrolyte concentration ( 4 concentrations)

10. Closing the Gap Using Electrodeposition ¤ Method: Constant current ; Monitor the voltage across WE and RE ¤ Electrolyte composition: 0.42 M Na 2 SO 3 + 0.42 M Na 2 S 2 O 3 + 0.05 M NaAuCl 4 ¤ Non-toxic and without strongly adsorbed ions ¤ At room temperature ¤ Electrodeposition Setup 1 (Non Cyanide)

11. Results of Electrodeposition (Method 1) ¤ Time evolution curve of V gap at a constant current of 25 µA on a chart recorder ¤ SEM image of fused electrodes after electrodeposition Stop ¤ I-V curve showing hysteresis

12. 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

13. 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 Na 2 SO 3 + 0.4 M Na 2 S 2 O 3 + 0.01 M Na 2 Au(S 2 O 3 ) 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

14. Electrodeposition Results Mag=2.2 Kx I=-10nA Mag=36 Kx I=-10nA Left electrode Right electrode Abnormal growth But, fine grain size Mag= 15 Kx I=-10nA Left electrode Right electrode

15. 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

16. 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 700 nm Revamped Previous

17. 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

18. I-V Results of Nanogap Pair 1 Pair 2

19. 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

20. 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 <10nm gap… Conclusion

21. Thank you !