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Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction Teresa Esposito 1, Alexandra Krawciz 2, Peter H.

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Presentation on theme: "Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction Teresa Esposito 1, Alexandra Krawciz 2, Peter H."— Presentation transcript:

1 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

2 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 (Zn-Porphyrin) 2

3 Inelastic Electron Tunneling Spectroscopy (IETS) Measure junction characteristics (I/V, dI/dV, and d 2 I/dV 2 ) in order to investigate electron transport Will give information on the vibrational modes of the molecule in the junction 3

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

5 Inelastic Electron Tunneling Electrons donate energy (E V ) to the molecule, exciting a vibrational mode and creating a new tunneling pathway. -e*V bias e-e- e-e- e-e- e-e- e-e- e-e- e-e- 5 Energy Position EVEV

6 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

7 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 x100 nm wires 7

8 Electromigration Electrons transfer momentum to nearby metal ions, causing displacement of the ions Occurs in most metals when there is a high current density (~10 12 A/m 2 ) at a defect High reproducibility, consistently sized nanogap ~3-8 nm in width e-e- e-e- e-e- Current Au + 8 CathodeAnode nanowire

9 Electromigration 9 Point where electromigration occurs

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

11 IV for an Empty Nanogap 11

12 SRS DS360 Low distortion function generator NI USB 6259 DAQ board AC/DC Mixer Keithley 2100 Digital Multimeter (DC Voltage) Sample via breakout box to 4.2K cryostat SR570 Low noise current preamplifier Keithley 2100 Digital Multimeter (DC current) SR830 Lock-in amplifier (dI/dV) SR830 Lock-in amplifier (d 2 I/dV 2 ) Electronics to measure IETS 12

13 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

14 IR Spectroscopy of Porphyrin 14 Calculations completed by Dr. Peter Dinolfo, Department of Chemistry, RPI. Vibrational modes: 750 – 1750 cm -1 : porphyrin core & phenyl-ethynyl- phenyl (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.

15 Conclusion and Future Testing IETS of empty nanogaps at 5K –No peaks due to tunneling current IETS of ZnP-A1 at 5K –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

16 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

17 References Qian, G., Saha, S., Lewis, K. M. Ap. Phys. Lett. 96, (2010). Qiu, X. H., Nanzin, G. V., Ho, W. Phys. Rev. Lett. 93(19), (2004). Saha, S., Owens, J. R., Meunier, V., Lewis, K. M. Ap. Phys. Lett. 103, (2013). Saha, S., Qian, G., Lewis, K. M. J. Vac. Sci. Technol B 29(6), (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

18 Introduction and Experimental Overview Investigating the use of porphyrin as a possible circuit element for organic electronics Porphyrin is assembled into electromigrated gold nanogaps, forming a molecular junction Measure junction characteristics Peaks in the second derivative indicate vibrational modes excited by inelastic electron tunneling 18

19 Peak Width 19 Minimizing peak width will give a more accurate reading of vibrational modes Choice of AC voltage due to thermal broadening of peaks. Minimize W exp Want W modulation ~ W thermal At 5 K V AC_rms ~ 1mV


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