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 JunctionTeresa Esposito1, Alexandra Krawciz2,Peter H. Dinolfo2, Kim Lewis11Department of Physics, Applied Physics, and Astronomy2Department of Chemistry and Chemical BiologyRensselaer Polytechnic Institute, Troy NY 12180
2 Porphyrin Motivation: Characteristics: Objective: Circuit element for organic electronicsCharacteristics:Highly conjugated aromatic moleculeCan be functionalized with a metal ion in the centerFunctionalized with a protected thiol group (–SH) to form a covalent bond to goldObjective:Zn- Porphyrin: use IETSelectrical or conductance switchingIn 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 transportWill give information on the vibrational modes of the molecule in the junctionThere 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*Vbiase-e-e-The square represents the energy levels in the moleculeEnergyMolecule’s energy levelsPositionAu 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*Vbiase-e-e-e-e-This energy lost also corresponds to the energy gained by the molecule, exciting a vibrational mode.Vbias= -0.5 to 0.5EnergyEVPosition
6 Molecular Conductance Modeled by the Landauer FormulaWhere T(E) is the transmission functionNanogaps without porphyrin can be modeled using Simmon’s equation!!! For high voltage rangeV=bias voltagePhi=barrier heightBeta=correction factor~1=23/24F=field strength in the insulator- due to the bias voltage of the electrodes=Voltage/gap sizeFIX – explain simmon’s model and our choice of barrierV/barrier heightApproximates a square well
7 NanowiresFabricated using electron beam lithography at the Lurie Nanofabrication Facility at the University of Michigan in Ann ArborAu nanowires and contact pads on oxide layer grown on Si substrate~80 samples with two 30 nm x100 nm wiresFigure 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 1.5mmx1.5mmTi=2nmAu=18nm
8 ElectromigrationElectrons transfer momentum to nearby metal ions, causing displacement of the ionsOccurs in most metals when there is a high current density (~1012 A/m2) at a defectHigh reproducibility, consistently sized nanogap ~3-8 nm in widthFlip the picture backwardse-Au+e-e-nanowireCathodeAnodeCurrent
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 gapMulti-molecule junctionNanowires are nm in sizeGap forms near the cathode(bottom image is from PRSN45)Resolution 2.5 nm at 1keV1.5 nm at 5keVAnodeCathode~6nm gap
11 IV for an Empty Nanogap Done at room temp, no vaccum, Matches the overall shape expected from landauer theorySimmon’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 generatorNI USB 6259 DAQ boardAC/DC MixerKeithley 2100 Digital Multimeter (DC Voltage)Samplevia breakout box to 4.2K cryostatSR570 Low noise current preamplifierKeithley 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 TestIn order to test the functionality of the IETS setup, testing was completed with a tunneling diode at 300 KOne peak due to Diodes having two “states”No current for negative voltageIncreasing current for positive voltageThe 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 groups2800 – 3000 cm-1: C-H modesPEP: phenyl-ethynyl- phenylIntrinsic to the molecule not external- coupling to au or bias voltageIETS can identify vibration modes intrinsic to porphyrin structurebeyond 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 5KNo peaks due to tunneling currentIETS of ZnP-A1 at 5KLook for evidence of switchingComparison to theoretical calculations of vibrational modes- DFT calculationImprove electromigration technique in order to thin wires enough such that fewer porphyrins bridge the nanogapCompare IETS of different analogs of porphyrinDetermine 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 GroupDr. Peter H. Dinolfo, Dr. Alexandra Krawicz, Marissa CivicDr. Meunier’s Computational Physics groupDr. Vincent Meunier, Dr. Jonathan OwensCleanroom support staff
17 ReferencesQian, 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 electronicsPorphyrin is assembled into electromigrated gold nanogaps, forming a molecular junctionMeasure junction characteristicsPeaks in the second derivative indicate vibrational modes excited by inelastic electron tunnelingIn 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 WidthMinimizing peak width will give a more accurate reading of vibrational modesChoice of AC voltage due to thermal broadening of peaks. Minimize WexpWant Wmodulation~ WthermalAt 5 K VAC_rms ~ 1mV