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 Astronomy2Department of Chemistry and Chemical Biology

Rensselaer Polytechnic Institute, Troy NY 12180

PorphyrinMotivation:

– 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

NN

N NZn

N

N

N

N

N

N

S

O

S

O

NN

NN

Zn

N

N

N

N

N

N

S

O

S

O

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

http://en.wikipedia.org/wiki/Inelastic_electron_tunneling_spectroscopy#mediaviewer/File:Second_derivative.gif3

Elastic Electron Tunneling• Electrons tunnel from one electrode to the

other without losing kinetic energy. Electrons do not interact with the molecule.

-e*Vbias

e- e-

e-

e-

e-e-

4

Energy

PositionMolecule’s energy levelsAu Electrode

Inelastic Electron Tunneling

• Electrons donate energy (EV) to the molecule, exciting a vibrational mode and creating a new tunneling pathway.

-e*Vbias

e-

e-

e-

e-

e-e-

e-

5

Energy

Position

EV

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

)E(2 2

Th

e

V

IG

bias

0

30

0

30

0

23 21

4exp

21

4exp

8

)/(2

eV

meV

seVm

eV

s

h

sVeJ

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

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

e-

e-

e-

Current

Au+

8

Cathode Anodenanowire

Electromigration

9

214

m

A10

A

IJ ff

Point where electromigration occurs

SEM Images

• Images from the Zeiss Supra 55 SEM

10 ~6nm gap

CathodeAnode

IV for an Empty Nanogap

11

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 to measure IETS

12

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

14

0 500 1000 1500 2000 2500 3000 3500

0.00 0.06 0.12 0.19 0.25 0.31 0.37 0.43Voltage (V)

Inte

nsi

ty (

AU

)

Wavenumber (cm-1)

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.

NNN

N

Zn

NN

N

NN

N

S

O S O

NN N

NZ

n

N

NN

N

N N

SO

SO

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

NN

N NZn

N

N

N

N

N

N

S

O

S

O

NN

NN

Zn

N

N

N

N

N

N

S

O

S

O

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

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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).

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