Post on 18-Dec-2015
1
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
3
Motivation
Identification of surface adsorbates by inelastic electron tunneling spectroscopy (IETS) using an adjustable tunnel junction
4
eVB e-
Electrode A
Electrode B
TunnelBarrier
x
Energy
Inelastic tunneling
eV
A
B
A
e-
Inelastic tunneling
5
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-
6
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)
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
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
Deflection Wire Gas-dosing capillary
Tungsten filamentAluminum Nitride holder
Brass mount Platinum crystal
13
1350K
1200K
>650K
Deflection wire and platinum surface are heated while surroundings are kept <10K
16
B
Id
Tip deflected downward by DC current (Id ~ 0.5mA) flowing perpendicular to external magnetic field (B = 4T)
IdIdId
IdIdId
17
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.
18
Hydrogen/Ne on Platinum
0 20 40 60 80 1000
20
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.
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 2000 - 2003
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…