very BASIC PRINCIPLES OF SCANNING TUNNELING MICROSCOPY

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(very) BASIC PRINCIPLES OF SCANNING TUNNELING
MICROSCOPY
Iván Brihuega
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Simplified STM
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The birth of STM
Si(111)7x7
G. Binnig and H. Rohrer (1982)
(Nobel Prize in 1986)
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Tunnel effect
Tunnel effect in one dimension
Transmission coefficient
V (x)
For E lt V0
V0
E
x
0
d
In the limit ? d gtgt 1
where,
d
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Vacuum tunneling between two planar electrodes
V
STM basis
For low bias voltage (eV ltlt ? )
I ? f(V) exp (-2? d)
V
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Main components and layout of an STM
I ? f(V) exp (-2? d)
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STM tips
W tip mechanically sharpened
W tip electrochemically etched
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STM head design evolution
Vibration damping
Mechanical stiffness
20 cm
1 cm
1985
1997
Lab. Nuevas Microscopías, UAM, Madrid
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Topographic imaging
Constant current

• Constant current imaging
• Measurement of Z (x,y) at I,V constant
• Constant height imaging
• Meausrement of I (x,y) at Z,V constant

V
Constant height
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Image acquisition and display
Z 12 Å
Z 0
Topview with pseudocolor code
3D view with pseudocolor code
3D view with simulated light
Topview numerical derivative
ErSi1.7 (J.A. Martín-Gago, J.M. Gómez-Rodríguez
and J.-Y. Veuillen (1996))
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Scanning tunneling spectroscopy (STS)
Sample density of states
Transmitivity
Tip density of states
Independent tip and sample
Sample bias 0
Sample bias gt 0
Sample bias lt 0
Tip
Sample
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Spectroscopic modes (STS)
Unoccupied states Vsample gt 0
Occupied states Vsample lt 0
e-
eV
e-
eV
Sample
Tip
Sample
Tip
d
d
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dI/dV (V)
I
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Scanning tunneling spectroscopy (STS)

• Method
• Tip is held at fixed x,y position.
• Feedback is off, while holding z position.
• Bias voltage is ramped while measuring the tunnel
  current.
• An I(V) plot is obtained.

Timing sequence
X-Position
Y-Position
Bias Voltage
Off
Feedback
On
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Si(111)-(2x1)
I/V (V)
Bulk gap 1.1 eV Surface gap ? 0.5eV
Feenstra et al. (1986)
First example of I-V measurement at x, y, z
constant with STS
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Pb(111)
I/V (V)
dI/dV (V)
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STS vs. other spectroscopical techniques

• Drawbacks of STS

• Advantages of STS

• Lack of chemical sensitivity (in contrast to XPS
  or AES).

• STS is sensitive to both occupied and unoccupied
  electronic states.

• Unprecedented spatial resolution (atomic
  resolution can be achieved).

• Tip artifacts can have an influence on
  spectroscopic data

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Understanding STM information
STM
Si(111)-(7x7)
Theory
STM
Theory
O.Paz, I. Brihuega, J.M. Gómez-Rodríguez JM.
Soler Phys. Rev. Lett. 94. 056103 (2005)
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INSIDE THE SUBK-STM
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3He Magnet Cryostat
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Influence of vibrations in STM experiments
Au(111)
Someone walking in the lab
Lab quiet
40x40nm2
T 4.2 K
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Vibrations measured with an accelerometer
VIBRATION ANALYSER VA-2
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Vibrations measured with an accelerometer
Vibrations measured on the floor and on the
experimental system (after one damping stage)
Acceleration Data (0.01ms-2/Volt)
Experimental System Vertical scale 50 mV/div
Floor Vertical scale 500 mV/div
Horizontal scale 500 ms/division
FFT of the data to extract the vibration
frequencies Vertical scale 20
dBm/div Horizontal scale 10 Hz/div
Displacement Data (1mm/Volt)
Experimental System Vertical scale 1 V/div
Floor Vertical scale 1 V/div
Horizontal scale 500 ms/division
FFT of the data to extract the vibration
frequencies Vertical scale 20
dBm/div Horizontal scale 10 Hz/div
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Comparison between vibrations on the experimental
system and STM performance
Bad conditions
Good conditions
Vibrations of the experimental system measured
with the accelerometer
STM tip displacement with respect to the surface
Au(111) T 4.2 K
Noise level 5 pm
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Vibrations measured with an accelerometer
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Final improvement new active dampers
STM profile measured with new active dampers
(STACIS). Noise level lt 1pm

Au(111) STM image measured with new active
dampers (STACIS) at 4.2 K
Vibration sensor
STM line profile measured with old active dampers
(Avi400). Noise level approx. 4 pm
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The Broad Superconducting Gap Mystery
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and the most important