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-0072148) & Penn State University

The condensed matter lab at Penn State Altoona

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

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

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

Crossed-Wire Technique1 B 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)

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)]

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

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

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

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

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

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

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

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

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)

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.

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

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

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

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

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…

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