Electron Transport and Inelastic Electron Tunneling Spectroscopy of

Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction Teresa Esposito 1, Alexandra Krawciz 2, Peter H. Dinolfo 2, Kim Lewis 1 1 Department of Physics, Applied Physics, and Astronomy 2 Department of Chemistry and Chemical Biology Rensselaer Polytechnic Institute, Troy NY 12180

Porphyrin Motivation: – Circuit element for organic electronics Characteristics: – Highly conjugated aromatic molecule – Can be functionalized with a metal ion in the center – Functionalized with a protected thiol group (–SH) to form a covalent bond to gold Objective: – Zn- Porphyrin: use IETS – electrical or conductance switching 2 (Zn-Porphyrin)

Inelastic Electron Tunneling Spectroscopy (IETS) • Measure junction characteristics (I/V, d. I/d. V, and d 2 I/d. V 2) in order to investigate electron transport • Will give information on the vibrational modes of the molecule in the junction 3 http: //en. wikipedia. org/wiki/Inelastic_electron_tunneling_spectroscopy#mediaviewer/File: Second_derivative. gif

Elastic Electron Tunneling • Electrons tunnel from one electrode to the other without losing kinetic energy. Electrons do not interact with the molecule. e- e-e*Vbias eee- e- Energy Au Electrode 4 Molecule’s energy levels Position

Inelastic Electron Tunneling • Electrons donate energy (EV) to the molecule, exciting a vibrational mode and creating a new tunneling pathway. e- e-e*Vbias ee- e- Energy EV Position 5

Molecular Conductance • Modeled by the Landauer Formula • Where T(E) is the transmission function • Nanogaps without porphyrin can be modeled using Simmon’s equation 6

Nanowires • Fabricated using electron beam lithography at the Lurie Nanofabrication Facility at the University of Michigan in Ann Arbor • Au nanowires and contact pads on oxide layer grown on Si substrate • ~80 samples with two 30 nm x 100 nm wires 7

Electromigration • Electrons transfer momentum to nearby metal ions, causing displacement of the ions • Occurs in most metals when there is a high current density (~1012 A/m 2) at a defect • High reproducibility, consistently sized nanogap ~3 -8 nm in width e. Au+ Cathode Current 8 ee- nanowire Anode

Electromigration Point where electromigration occurs 9

SEM Images • Images from the Zeiss Supra 55 SEM Anode Cathode 10 ~6 nm gap

IV for an Empty Nanogap 11

Electronics to measure IETS SRS DS 360 Low distortion function generator NI USB 6259 DAQ board AC/DC Mixer Keithley 2100 Digital Multimeter (DC Voltage) Sample via breakout box to 4. 2 K cryostat SR 570 Low noise current preamplifier SR 830 Lock-in amplifier (d. I/d. V) 12 SR 830 Lock-in amplifier (d 2 I/d. V 2) Keithley 2100 Digital Multimeter (DC current)

Diode Test • In order to test the functionality of the IETS setup, testing was completed with a tunneling diode at 300 K • One peak due to Diodes having two “states” – No current for negative voltage – Increasing current for positive voltage 13

IR Spectroscopy of Porphyrin • Vibrational modes: • 750 – 1750 cm-1: porphyrin core & phenyl-ethynylphenyl (PEP) side groups • 2800 – 3000 cm-1: C-H modes IETS can identify vibration modes intrinsic to porphyrin structure beyond the metal-molecule vibration mode. 14 Calculations completed by Dr. Peter Dinolfo, Department of Chemistry, RPI.

Conclusion and Future Testing • IETS of empty nanogaps at 5 K – No peaks due to tunneling current • IETS of Zn. P-A 1 at 5 K – Look for evidence of switching • Comparison to theoretical calculations of vibrational modes- DFT calculation • Improve electromigration technique in order to thin wires enough such that fewer porphyrins bridge the nanogap • Compare IETS of different analogs of porphyrin 15

Acknowledgements • Dr. Lewis’ Hybrid Electronics & Characterization Lab – Dr. Kim Lewis, Dr. Guougang Qian, Qi Zhou, Andrew Horning, Samuel Ellman, Maria Del Pili Pujol Closa. • Dr. Dinolfo’s Chemistry Group – Dr. Peter H. Dinolfo, Dr. Alexandra Krawicz, Marissa Civic • Dr. Meunier’s Computational Physics group – Dr. Vincent Meunier, Dr. Jonathan Owens • Cleanroom support staff 16

References Qian, G. , Saha, S. , Lewis, K. M. Ap. Phys. Lett. 96, 24307 (2010). Qiu, X. H. , Nanzin, G. V. , Ho, W. Phys. Rev. Lett. 93(19), 196806 (2004). Saha, S. , Owens, J. R. , Meunier, V. , Lewis, K. M. Ap. Phys. Lett. 103, 173101 (2013). Saha, S. , Qian, G. , Lewis, K. M. J. Vac. Sci. Technol B 29(6), 061802 (2011). Simmons J. G. J. Ap. Phys. 34(6), 1793 (1963). Wang, W. , Lee, T. , Kretzschmar, I. , Reed, M. A. Nano. Lett. 4, 643 (2004). 17