Transcript Metode şi tehnici de studiu a suprafeţelor

Methods and Tehniques in
Surface Science
Prof. Dumitru LUCA
“Alexandru Ion Cuza” University, Iasi, Romania
Scanning Tunneling Microscopy (STM)
Z

X
Allows for surface mapping by evidencing
atomic-scale surface features

Characterization of surface topology.

Evidencing surface growth and interatomic
bridging sites.
Resolution:
lateral < 1.0 Å
vertical < 0.1Å
Remark:
What is evidenced is the electron density
at surface, not atom positions!
Images are frequently interpreted as
“atoms”, but this is not necessarily true
under any circumstance.
The tunneling effect
Useful reference: http://www.ntmdt.ru/Documents/153_STM%20Phys_Back.pdf
STM schematics
.
Dedicated only
.
to investigate
conductive
surfaces…
The Inventors of the STM, Gerd Binnig and Heinrich Rohrer have been awarded Nobel
prize in physics in 1986
STM basics
Sample
STM Tip
EV
EV
1
2
EF
EF
Bulk
Bulk
Potential
Barrier
Potential
Barrier
STM Tip
EV
Sample
EV
d
EF
EF
Bulk
Bulk
Potential
Barrier
Electrons flow in both directions
with equal probability!
Potential
Barrier
Useful reference:
http://www.ntmdt.ru/Documents/153_STM%20Phys_Back.pdf
The physical background of STM
STM tip
Sample
EV
EV
d
By applying a biasing voltage V:
EF
EF
Bulk
Potential
barrier
j  1
Bulk
Potential
barrier
av
s
j –current density;
a1, a2 – constants;
V – bias voltage;
s – tip-to-sample distance
Ve
 2 av s
av – the average value of tip and surface work function;
http://www.ntmdt.ru/Documents/169_1.3%20Obser_Phys_Quantities_in_STM.pdf
STM operation
Two modes: constant current and constant inaltime height
The most frequently mode is the constant
curent.
A constant value of approx. Itunnel = (0.1 - 1 nA) is
currently used.
To keep Itunnel = constant, z should be
automatically adjusted by a feedback circuit.
Useful reference: http://www.ntmdt.ru/SPM-Techniques/Principles/
Constant current mode
Tunneling currents up to 0.03 nA are recorded, which are small enough to investigate even
low-conductivity surfaces, including biological samples.
.
Constant height mode
Here, a certain value is set for z, and
Itunnel is directly measured.
No feedback!
 A periodic variation of the tip-tosample distance occurs.
j  1
av
s
Ve
 2 av s
Utilized only for very flat surfaces!
 There, where tip will be positioned
exactly on top of a sample atom, the
tunneling current will be maxim
 When the tip is above a valley, the
tunneling current will be much smaller.
Constant Height Mode
STM Drawbacks:
Complexity in data processing, for certain
surfaces:
The surface image is not straightforwardly
determined by its topography, but also by:
- density of states,
- the sign and value of the bias,
- current value etc.
What information can we get from STM imagining
1. Atom seggregation at surface…
…including seggregtion of impurities at grain boundaries.
This image depicts the (110) surface of the Fe-3.5at%Si
alloy. Aproximately 1/3 of surface atoms are Si (dark
color in the picture) which substitute the Fe atoms!
Carbon atoms cannot be detected, but they mask the Fe
atoms in the central rows of the ladder-type
structure.
Images taken from the paper by Biedermann, M. Schmid , P. Varga, Surf. Sci. 331-333 (1995) 787793.
What information can we get from STM images?
2. Surface growth processes
1.
Pb and Cu are non-miscibile metals:
rPb= 1.37 rCu.


Es Pb = 0.50 J/m2, while Es Cu (1.96 J/m2).
According to the classical theory of growth
phenomena, Cu should grow as small islands on Pb
surface.
2. Pb atomii are very mobile.
 A Cu island grown on Pb has, in its turn, a surface at
its lateral edges, therefore its surface energy
increases.

The most favorable configuration (minimal energy)
occurs when these side wings are covered with Pb.
A compromise between the seggregation
tendency of Cu and the large mobility of Pb
atoms.
Other STM applications
Nanolithography
 The most direct way for surface manufacturing.
 The specimen’s surface under the STM tip can be melted and evaporated.
An example of STM lithography: an STM image of 3 ML of a film during 3 electric pulses
exposure.
Nano-anodization

A voltage is applied between the conductive cantilever tip and the metal surface to be
anodized. Electrochemical processes occur that lead to the formation of oxide
nanostructures.

By using the electro-lithography, the local geometric and composition properties of the
sample surface can be altered.
.
Nano-manipulation
Fe on Cu (111)