1 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- Electrode A Electrode B Tunnel Barrier x Energy Inelastic tunneling e-e-

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

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

7 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 Microcoax Capillary mount RuO 2 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 He 4 CompressorHeated 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 Deflection Wire Gas-dosing capillary Tungsten filament Aluminum Nitride holder Brass mount Platinum crystal

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

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

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

21 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

22 Undergraduate Researchers Brad Petrilla 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 MerrillJohn Rea More pics…

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