Lorentz Force Tunneling Spectrometer for Studying Molecules on Single Crystal Surfaces
Darin T. Zimmerman
Brad A. Petrilla, John R. Rea, Darrell L. Sharp
Penn State Altoona
Glenn Agnolet
Texas 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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Identification of surface adsorbates by inelastic electron tunneling spectroscopy
(IETS) using an adjustable tunnel junction
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
A
Energy

A

B
 eV
In elastic tunneling e
e
eV
 


B
Electrode
A
Tunnel
Barrier
Electrode
B x
4

Pt Tip
V
Bias e
-
I
T
Pt Surface
Molecules to be identified are adsorbed on the surface or incorporated into the tunnel barrier
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1
I d
B
• Straight wire fixed parallel to external magnetic field
• Curved wire deflected in plane by Lorentz force
1 S. 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
RuO
2 sensor
Pickup Wire
Platinum crystal
AlN Holder
Deflection wire tip
Capillary
Close up view of new tip-surface geometry
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Cryomech Pulse-Tube Closed Cycle Refrigerator
He 4 Compressor Heated Capillary
Gas Manifold
Vibration
Dampening
Pulse-tube Cryostat
Turbo pump 10

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
Superconducting solenoid
Magnet bore radiation shield
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Deflection Wire
Brass mount
Platinum crystal
Tungsten filament
Gas-dosing capillary
Aluminum Nitride holder
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1200K
>650K
1350K
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
I
I
I
I
I
I
I d d
Tip deflected downward by DC current (I d
~ 0.5mA) flowing perpendicular to external magnetic field (B = 4T)
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10
-2
10
-3
10
-4
10
-5
12 16 20 24
Force (

N)
28 32 36
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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140
120
100
80
60
40
20
0
0 20 40 60
Bias (mV)
80 100
Data from neon-hydrogen adsorbed on platinum.
Observed peaks appear to be consistent with rotational / vibrational modes of adsorbed hydrogen.
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• 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
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Undergraduate Researchers 2000 - 2003
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
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Undergraduate Researchers 2003 - 2004
More pics…
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
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Undergraduate Researchers at Work
John working on adjusting the
12
 m, platinum deflection wire.
Brad putting the finishing touches on new drawings of the apparatus.
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