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

2. The condensed matter lab at Penn State Altoona

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

4. e- e- Inelastic tunneling Inelastic tunneling Electrode A Electrode B
Barrier x
Energy
Inelastic tunneling e-
Inelastic tunneling

5. Ideal Tunneling Configuration
Pt Tip V
Bias
Ne Barrier
Ne Barrier e- IT
Pt Surface C C H H
molecules
molecules 2 2 2 2
Molecules to be identified are adsorbed on the surface or incorporated into the tunnel barrier

6. Crossed-Wire Technique1B I d
Straight wire fixed parallel to external magnetic field
Curved wire deflected in plane by Lorentz force
1S. Gregory, Phys. Rev. Lett. 64, 689 (1990)

7. Data from neon-acetylene platinum crossed wire junction
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)]

8. 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

9. Close up view of new tip-surface geometry
Microcoax
Capillary mount
RuO2 sensor
Capillary
Deflection wire tip
Pickup Wire
Platinum crystal
AlN Holder
Close up view of new tip-surface geometry

10. Cryomech Pulse-Tube Closed Cycle Refrigerator
He4 Compressor
Heated Capillary
Vibration
Dampening
Gas Manifold
Turbo pump
Pulse-tube Cryostat

11. 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

12. Aluminum Nitride holder
Gas-dosing capillary
Deflection Wire
Platinum crystal
Brass mount
Tungsten filament
Aluminum Nitride holder

13. 1200K
>650K
1350K
Deflection wire and platinum surface are heated while surroundings are kept <10K

14. Neon barrier film is grown on cooled surface
3.9K
Neon barrier film is grown on cooled surface

15. Neon Barrier Film
Neon gas repeatedly sprayed until desired thickness is obtained

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

17. Adjusting Junction
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.

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

19. 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

20. Future Work Piezoelectric elements provide for scanning capability
e-beam or ion source for surface cleaning

21. 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

22. 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…

23. Undergraduate Researchers at Work
John working on adjusting the 12 mm, platinum deflection wire.
Brad putting the finishing touches on new drawings of the apparatus.
Undergraduate Researchers at Work