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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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 ¤ Re-Re Quadruple bond ¤ Paddlewheel bridging ligands ¤ Anchoring thiol group ¤ Os-Os Triple bond ¤ Ir-Ir Double bond Fabrication of Nanogap Electrodes A C B 300nm 300nm D E ¤ Raith 150 EBL system ¤ Different gold thickness (100/150/250 nm) on top of 30nm Cr Fabrication Results ¤ Two electrode devices 1 ¤ After EBL & development 2 ¤ GDS2 design ¤ Design gap 75nm ¤ Gap=74nm 3 ¤ After metal evaporation of Cr/Au ¤ Gap=53nm Fabrication Results ¤ Gated electrode devices 1 ¤ GDS2 gated design ¤ Design gap 60nm 2 ¤ After metal evaporation of Cr/Au ¤ Gap=10nm 3 ¤ Gated device with 3 contact pads Closing the Gap Using Electrodeposition ¤ Packaging = Wire bonding + Epoxy cavity 1 2 ¤ Package: Kovar material ¤ Wire bonding of contact pads to external leads ; Substrate temp ~150° C ¤ Epoxy cavity for forming the electrochemical cell Factors To Consider ¤ Method Setup ( 2 methods tried ) ¤ Electrolyte composition ( 2 compositions ) ¤ Deposition current ¤ Electrolyte concentration ( 4 concentrations) 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 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 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 I dep I tunnel I total I total = I dep + I tunnel Faraday Cage 200μ WE RE/CE Galvanostat DMM ¤ 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 Electrodeposition Results Mag=2.2 Kx I=-10nA Mag=36 Kx Left electrode Right electrode Mag= 15 Kx Left electrode I=-10nA Right electrode Abnormal growth But, fine grain size I=-10nA 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 Previous Revamped 700 nm 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 I-V Results of Nanogap Pair 1 1.E-09 8.E-10 2.E-04 I (A) I(A) 4.E-04 6.E-10 0.E+00 -0.06 -0.04 -0.02 0.00 -2.E-04 -4.E-04 0.02 0.04 4.E-10 0.06 2.E-10 V(V) 0.E+00 -0.10 -0.05 0.00 0.05 0.10 V (V) 4.E-04 I(A) 3.E-04 Pair 2 2.E-04 1.E-04 0.E+00 0.E+00 -0.04 -1.E-040.00 -0.02 -2.E-04 -3.E-04 0.02 0.04 0.06 -0.10 -0.05 -1.E-10 V(V) I(A) -0.06 -2.E-10 -3.E-10 -4.E-10 0.00 0.05 V(V) 0.10 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 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 <10nm gap… Thank you !