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Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction Teresa Esposito1, Alexandra Krawciz2, Peter H. Dinolfo2,

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Presentation on theme: "Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction Teresa Esposito1, Alexandra Krawciz2, Peter H. Dinolfo2,"— Presentation transcript:

1 Electron Transport and Inelastic Electron Tunneling Spectroscopy of Porphyrin in a Molecular Junction Teresa Esposito1, Alexandra Krawciz2, Peter H. Dinolfo2, Kim Lewis1 1Department of Physics, Applied Physics, and Astronomy 2Department of Chemistry and Chemical Biology Rensselaer Polytechnic Institute, Troy NY 12180

2 Porphyrin Motivation: Characteristics: Objective:
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 In order to increase the computing power of our ever-shrinking electronic devices, circuit elements must be made smaller and smaller. However, we are reaching the limits of cost effective fabrication techniques. As an alternative, we are looking to molecules, which are already small, as possible circuit elements. In particular my group is investigating a class of molecules called porphyrins as a possible circuit element for molecular electronics. Take the carboxyl group off the end to get the thiol (de-protection: add an acid or base) Switch: two different conductance values at the same (Zn-Porphyrin)

3 Inelastic Electron Tunneling Spectroscopy (IETS)
Measure junction characteristics (I/V, dI/dV, and d2I/dV2) in order to investigate electron transport Will give information on the vibrational modes of the molecule in the junction There are many other experimental methods to measure vibrational modes of molecules, such as raman spectroscopy and FTIR. However, these methods are performed with avagadro numbers worth of molecules in a solution or adsorbed onto a surface. IETS is unique because it allows us to learn about the behaviour of the molecule in a junction, which is how it will eventually be used if it is a plausible candidate for organic electronics.

4 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- -e*Vbias e- e- e- The square represents the energy levels in the molecule Energy Molecule’s energy levels Position Au Electrode

5 Inelastic Electron Tunneling
Electrons donate energy (EV) to the molecule, exciting a vibrational mode and creating a new tunneling pathway. e- e- -e*Vbias e- e- e- e- e- This energy lost also corresponds to the energy gained by the molecule, exciting a vibrational mode. Vbias= -0.5 to 0.5 Energy EV Position

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 !!! For high voltage range V=bias voltage Phi=barrier height Beta=correction factor~1=23/24 F=field strength in the insulator- due to the bias voltage of the electrodes=Voltage/gap size FIX – explain simmon’s model and our choice of barrierV/barrier height Approximates a square well

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 Figure 1. A schematic of a broken nanowire shown with 5,15-di-4(S-acetylphenyl)-10,20-diphenyl porphyrin in the nanogap. Thickness of oxide layer????? (guess 100nm) Gold contact pads 1.5mmx1.5mm Ti=2nm Au=18nm

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 (~1012 A/m2) at a defect High reproducibility, consistently sized nanogap ~3-8 nm in width Flip the picture backwards e- Au+ e- e- nanowire Cathode Anode Current

9 Electromigration Point where electromigration occurs
=.3A/(20nm*30nm)=5E14 A/m^2. 2mV/s ramp (PRSN45)

10 SEM Images Images from the Zeiss Supra 55 SEM ~6nm gap Anode Cathode
the gap is approximately 6 nm in width. MANY porphyrins will orient themselves horizontally in the gap Multi-molecule junction Nanowires are nm in size Gap forms near the cathode (bottom image is from PRSN45) Resolution 2.5 nm at 1keV 1.5 nm at 5keV Anode Cathode ~6nm gap

11 IV for an Empty Nanogap Done at room temp, no vaccum,
Matches the overall shape expected from landauer theory Simmon’s modeling has not been done to match the gap size or barrier height (PRSN45)

12 Electronics to measure IETS
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 (d2I/dV2) Electronics are all controlled using labview

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 The easiest way to reduce noise is to increase the signal.

14 IR Spectroscopy of Porphyrin
Vibrational modes: 750 – 1750 cm-1: porphyrin core & phenyl-ethynyl-phenyl (PEP) side groups 2800 – 3000 cm-1: C-H modes PEP: phenyl-ethynyl- phenyl Intrinsic to the molecule not external- coupling to au or bias voltage IETS can identify vibration modes intrinsic to porphyrin structure beyond the metal-molecule vibration mode. Calculations completed by Dr. Peter Dinolfo, Department of Chemistry, RPI.

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 Determine which molecule is best suited for molecular electronics

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

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). AVS style formatting

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 In order to increase the computing power of our ever-shrinking electronic devices, circuit elements must be made smaller and smaller. However, we are reaching the limits of cost effective fabrication techniques. As an alternative, we are looking to molecules, which are already small, as possible circuit elements. In particular my group is investigating a class of molecules called porphyrins as a possible circuit element for organic electronics.

19 Peak Width Minimizing peak width will give a more accurate reading of vibrational modes Choice of AC voltage due to thermal broadening of peaks. Minimize Wexp Want Wmodulation~ Wthermal At 5 K VAC_rms ~ 1mV


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