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Lorentz Force Tunneling Spectrometer for Studying Molecules

on Single Crystal Surfaces

Darin T. ZimmermanBrad A. Petrilla, John R. Rea, Darrell L. Sharp

Penn State Altoona

Glenn AgnoletTexas A&M

Work Supported by NSF (DMR-0072148) & Penn State University

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The condensed matter lab at Penn State Altoona

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Motivation

Identification of surface adsorbates by inelastic electron tunneling spectroscopy (IETS) using an adjustable tunnel junction

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eVB e-

Electrode A

Electrode B

TunnelBarrier

x

Energy

Inelastic tunneling

eV

A

B

A

e-

Inelastic tunneling

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Ne Barrier

C2H2 molecules

Molecules to be identified are adsorbed on the surface or incorporated into the tunnel barrier

Ideal Tunneling Configuration

Ne Barrier

C2H2 molecules

VBias

Pt Tip

Pt Surface

ITe-

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Crossed-Wire Technique1

• Straight wire fixed parallel to external magnetic field• Curved wire deflected in plane by Lorentz force

B

Id

1S. Gregory, Phys. Rev. Lett. 64, 689 (1990)

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Data from neon-acetylene platinum crossed wire junction. At 5% acetylene chemisorbed peaks are observed (top). At 25%, both infrared and Raman active gas-phase peaks appear (bottom). Data taken at 4K with a 20mV modulation.

[Appl. Phys. Lett. 75, 2500 (1999)]

[Rev. Sci. Instrum. 72, 1781 (2001)]

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Advantages• Stable enough to measure vibrational spectra

of adsorbed molecules• Wires easily cleaned and dosed in-situ

Disadvantages• Wire surfaces not well-characterized• Not possible to control straight wire• Force required to make and adjust junction not

reproducible

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Microcoax

Capillary mount

RuO2 sensor

Capillary

Deflection wire tip

Pickup Wire

Platinum crystal

AlN Holder

Close up view of new tip-surface geometry

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Cryomech Pulse-Tube Closed Cycle Refrigerator

He4 Compressor Heated Capillary

Vibration Dampening

Gas Manifold

Turbo pump

Pulse-tube Cryostat

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Close up of magnet bore / junction mount

Cryo-pumping line

Heated Capillary and electrical cables

Adjustable copper support rod

Compression fitting to secure mount

Junction mount

Capillary / bridge to Pt crystal

Magnet bore radiation shield

Superconducting solenoid

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Deflection Wire Gas-dosing capillary

Tungsten filamentAluminum Nitride holder

Brass mount Platinum crystal

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1350K

1200K

>650K

Deflection wire and platinum surface are heated while surroundings are kept <10K

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3.9K

Neon barrier film is grown on cooled surface

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Neon Barrier Film

Neon gas repeatedly sprayed until desired thickness is obtained

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B

Id

Tip deflected downward by DC current (Id ~ 0.5mA) flowing perpendicular to external magnetic field (B = 4T)

IdIdId

IdIdId

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Adjusting Junction

12 16 20 24 28 32 3610-5

10-4

10-3

10-2

Lo

g [

GJ

/ (

2e

2/

h)

]

Force (N)

Semilogarithmic plot showing the orders of magnitude adjustability in the DC conductance with applied force. Data is for two separate Pt-Pt junctions and a neon barrier film.

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Hydrogen/Ne on Platinum

0 20 40 60 80 1000

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40

60

80

100

120

140

(d2 I/d

V2 )

/ (d

I/dV

) (

V-1

)

Bias (mV)

Data from neon-hydrogen adsorbed on platinum. Observed peaks appear to be consistent with rotational / vibrational modes of adsorbed hydrogen.

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Conclusions

• Can form reproducible junctions whose resistance is adjustable over several orders of magnitude

• Experiments performed without any significant vibration isolation

• Junctions are sufficiently stable to perform IETS of molecular adsorbates on metal surfaces

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• Piezoelectric elements provide for scanning capability• e-beam or ion source for surface cleaning

Future Work

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Undergraduate Assistants

Back: Yoonsoo Kang & Brandon Kline; Front: Jamie McCulloch, Justin Huffman,

& Darrell Sharp

Not pictured: Nat Anderson

Darrell and Justin toying with the Lock-In Amplifier

Undergraduate Researchers 2000 - 2003

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Undergraduate Researchers 2003 - 2004

Brad Petrilla (left) and John Rea were involved in taking inelastic electron tunneling spectroscopy data, maintaining the apparatus, and making some much needed improvements to the experiment. Not pictured: Darin Merrill

More pics…

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John working on adjusting the 12m, platinum deflection wire.

Brad putting the finishing touches on new drawings of the apparatus.

Undergraduate Researchers at Work