Transcript Atomic Force Microscope (AFM)

Scanning Tunneling Microscope (STM)
and
Atomic Force Microscope (AFM)
Topographic scan of a glass surface
Types of Microscope
Smaller
Scale?
Optical
Microscope
Electric
Microscope
Scanning Probe
Microscope
Year of invention
1644
1931
1981
Operation
environment
In air, liquid and
vacuum
in vacuum
In air, liquid and
vacuum
Resolution
Horizontal (x,y)
Vertical (z)
Magnification
0.5μm
NA
1-2x10³
2nm
NA
10-10⁶
0.1-1.0nm
0.01nm
5x10²-10⁸
Sample preparation
Not much
Sample should be
frozen and dried
none
Sample conditions
Cannot be
completely
transparent
Can store in vacuum Surface cannot be
Surface should be
too rough
conductive
Scanning Probe Microscope
PTMS, photothermal microspectroscopy/microscopy
SECM, scanning electrochemical microscopy
BEEM, ballistic electron emission microscopy
SCM, scanning capacitance microscopy
CFM, chemical force microscopy
SGM, scanning gate microscopy
C-AFM, conductive atomic force microscopy
SICM, scanning ion-conductance microscopy
EFM, electrostatic force microscopy
ESTM electrochemical scanning tunneling microscope SPSM spin polarized scanning tunneling microscopy
SSRM, scanning spreading resistance microscopy
FMM, force modulation microscopy
SThM, scanning thermal microscopy
KPFM, kelvin probe force microscopy
STM, scanning tunneling microscopy
MFM, magnetic force microscopy
SVM, scanning voltage microscopy
MRFM, magnetic resonance force microscopy
STP, scanning tunneling potentiometry
NSOM, near-field scanning optical microscopy
SHPM, scanning Hall probe microscopy
(or SNOM, scanning near-field optical microscopy)
SXSTM synchrotron x-ray scanning tunneling microscopy
PFM, Piezoresponse Force Microscopy
PSTM, photon scanning tunneling microscopy
AFM, atomic force microscopy
Of these techniques AFM and STM are the most commonly used
for roughness measurements
STM
• a powerful instrument for imaging surfaces at the atomic level
• developed in 1981
• By Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize
winners in Physics in 1986
• 0.1 nm lateral resolution and 0.01 nm depth resolution
Quantum Tunneling
Quantum Tunneling
V
electron e-
V2= Vo
V1 = 0
A
B
x = -a
C
x=a
V3 = 0
x
Assume the electron has positive energy (E > 0) and E < V2 = Vo.
How can the electron tunnel through the energy barrier
under the above assumption?
Find the wave functions in Region A, B and C respectively to get the answer!
Quantum Tunneling
1. Write down the Schrodinger’s equations in the 3 regions:
 2 d 2 B

 (V0  E ) B
2
2me dx

2

2me
d 2 A,C
dx
2
 E A,C
2. Get the general solutions:
 A ( x)  Ieikx  Re ikx
k
2 me E

 B ( x)  AeKx  Be Kx
k
2me (V0  E )

 C ( x)  Te ikx
k
2me E

I, R and T are related to the incident, reflection and transmission coefficients.
Quantum Tunneling
Consider only the transmission coefficient:
J trans

J inc
q
 *
* 
(

) , we have
From J 
2m i
x
x
J trans  2ikT 2
J inc  2ikI 2

T
I
2
2
Tunneling current    T
2
Quantum Tunneling
From boundary conditions,
2kK 2
2
2
 4kK  2 Ka
T  2
 2
e
2 2
2
2
2 
(k  K ) sinh 2Ka  (2kK )  k  K 
, for large Ka
 T unnelingcurrent  e
2 Ka
The thickness a of the energy barrier can be found
by measuring the tunneling current!
Components of STM
Operation of STM
measure the tunneling current between a conducting tip and conducting sample
when a DC bias voltage is applied.
Resolution in z (perpendicular to surface) is ~0.01 nm and in x/y is ~0.1 nm,
resulting in atomic-resolution images.
Operation of STM
Consider two metals, one is a tip and the other is a surface, separated by a
finite distance  uniform potential barrier.
Electron wave function can tunnel through the barrier and leads to finite
current.
2
4kK  2 KL
The probability is T   2
e
2 
k K 
2
Tunneling current
As the two metals move
apart, the probability of
electrons tunneling
decreases exponentially
and as they move closer
the tunneling increases.
metal 1
L
metal 2
13
Operation of STM
tip
Scan direction
Constant height
The air
gap
(barrier) is
several
nm
I (nA)
Lower current
Higher current
Atomic surface
The current
profile
duplicates the
atomic surface
x (nm)
14
Operation of STM
The resolution of
bending can be
as low as 0.2 nm
15
• The attention paid to the first problem and the
engineering solution to it is the difference between a
good microscope and a not so good microscope - it
need not worry us here, sufficient to say that it is
possible to accurately control the relative positions of
tip and surface by ensuring good vibrational isolation
of the microscope and using sensitive piezoelectric
positioning devices.
• Tip preparation is a science in itself - having said that, it
is largely serendipity which ensures that one atom on
the tip is closer to the surface than all others.
Let us look at the region where the tip approaches the surface in greater detail ....
... the end of the tip will almost invariably show a certain amount of structure,
with a variety of crystal facets exposed ...
… and if we now go down to the atomic scale ....
... there is a reasonable probability of ending up with a truly atomic tip.
• If the tip is biased with respect to the surface by the
application of a voltage between them then electrons
can tunnel between the two, provided the separation of
the tip and surface is sufficiently small - this gives rise to
a tunnelling current.
• The direction of current flow is determined by the
polarity of the bias.
If the sample is biased -ve with respect to the tip, then electrons will flow
from the surface to the tip as shown above, whilst if the sample is biased
+ve with respect to the tip, then electrons will flow from the tip to the surface
as shown below.
The name of the technique arises from the quantum mechanical tunnelling-type
mechanism by which the electrons can move between the tip and substrate.
Quantum mechanical tunnelling permits particles to tunnel through a potential
barrier which they could not surmount according to the classical laws of physics in this case electrons are able to traverse the classically-forbidden region between
the two solids as illustrated schematically on the energy diagram below.
This is an over-simplistic model of the tunnelling that occurs in STM but it is a
useful starting point for understanding how the technique works.
In this model, the probability of tunnelling is exponentially-dependent upon
the distance of separation between the tip and surface : the tunnelling
current is therefore a very sensitive probe of this separation.
• Imaging of the surface topology may then be
carried out in one of two ways:
– in constant height mode (in which the tunnelling current is monitored as the
tip is scanned parallel to the surface)
– in constant current mode (in which the tunnelling current is maintained
constant as the tip is scanned across the surface)
If the tip is scanned at what is nominally
a constant height above the surface,
then there is actually a periodic
variation in the separation distance
between the tip and surface atoms. At
one point the tip will be directly above a
surface atom and the tunnelling current
will be large whilst at other points the tip
will be above hollow sites on the
surface and the tunnelling current will
be much smaller.
A plot of the tunnelling current v's tip position therefore shows a periodic variation
which matches that of the surface structure - hence it provides a direct "image" of
the surface (and by the time the data has been processed it may even look like a
real picture of the surface ! ).
In practice, however, the normal way of
imaging the surface is to maintain the
tunnelling current constant whilst the tip is
scanned across the surface. This is achieved
by adjusting the tip's height above the surface
so that the tunnelling current does not vary
with the lateral tip position. In this mode the tip
will move slightly upwards as it passes over a
surface atom, and conversely, slightly in
towards the surface as it passes over a hollow.
The image is then formed by plotting the tip height (strictly, the voltage
applied to the z-piezo) v's the lateral tip position.
Piezoelectric Tube
Piezoelectric Material
• piezoelectric effect was
discovered by Pierre Curie
and Jacques Curie
• (1880)
•
An applied mechanical
stress will generate a voltage
•
An applied voltage will
change the shape of the
solid by a small amount
•
most well-known
piezoelectric material is
quartz (SiO2)
Bimorph
Two long, thin plates of piezoelectric
material are glued together, with a
metal film sandwiched in between .
Two more metal films cover
the outer surfaces . Both
piezoelectric plates are poled along
the same direction, perpendicular
to the large surface
By applying a voltage, stress
of opposite
sign is developed in both
plates, which generates a
torque
Tube
Making a sharp tip
• By electrochemical etching method
Consider a gold tip prepared in 0.8 M KCN
solution from a gold wire (Au),
the following electrochemical redox reaction
takes place:
4Au(s)+8KCN(aq)+O2(g)+2H2O→
4Au(CN)2−(aq)+4OH−(aq)+8K+(aq)
By applying different voltages, some
part of tip were washed with the
distilled water
Manipulation of atoms using STM
Lithography and micromanipulation
• The interactions between the
STM tip and substrate can be
used to modify the surface in
a controlled way.
• This can be done in a number
of ways.
• Eg Eigler and Schweizer
manipulated xenon atoms on
a Nickel(110) surface under
UHV conditions, with
everything at 4K.
• Obtained by
manipulating CO
on a Pt(111)
surface.
Manipulation of atoms using STM
atom