Transcript of " Scanning tunneling microscope (STM)"
An Najah National University
Faculty of Science
Scanning Tunneling Microscope
Prof: Gassan Saffarini
Prepared by: Balsam Ata
List of contents:
2- The basic of STM……………………………………4
5-Modes of operation……………………………….…..13
5-1-Constant current mode………………………….….13
5-2- Constant Height Mode…………………………….14
6- density of state imaging… ……………………….…15
9- STM related studies…………………………………21
List of figures:
Fig.1: Rectangular potential barrier and particle wave function
Fig.2: Scanning tip……………………………………….…7
Fig.3: Electrochemical Etching……………………………..8
Fig.4 : Scanning Tunneling Microscope schematic………..10
Fig.5: The scanning tunneling microscope…………………11
Fig.6: Voltage biase vs tunneling current………………….12
Fig.7: Constant current mode …. ………………………….13
Fig.8: Constant Height Mode ………………………..……14
Fig.9: STM images show the steps of "quantum corral"
A scanning tunneling microscope (STM)is an instrument for
imaging surfaces at the atomic level . It was invented in 1981
by Gred Binnig and Heinrich Rohrer at IBM Zurich. Five years
later they were awarded the Nobel prize in physics for its
invention . The STM was the first instrument to generate
real-space images of surface with atomic resolution . STM
has good resolution considered to be 0.1 nm lateral resolution
and 0.01 nm depth resolution .
STM gives true atomic resolution on some samples even at
ambient conditions. Scanning tunneling microscopy can be
applied to study conductive surfaces or thin nonconductive films
and small objects deposited on conductive substrates .
The STM is a non-optical microscope which employs principles
of quantum mechanics. A very fine tip is moved over the surface
of the material under study, and a voltage is applied between
probe and the surface. Depending on the voltage and its
characteristics electrons will "tunnel" or jump from the tip to
the surface (or vice-versa depending on the polarity), resulting
in a weak electric current. The size of this current is
exponentially dependent on the distance between tip and the
surface . By scanning the tip over the surface and measuring the
current, one can thus reconstruct the surface structure of the
material under study .
2- The basic of STM.
The STM based on the concept of quantum tunneling ,
quantum tunneling is a microscopic phenomenon where a
particle can penetrate or pass through a potential barrier. This
barrier is assumed to be higher than the kinetic energy of the
particle ,therefore such a motion is not allowed by the laws of
classical mechanics .
To understand the phenomenon, particles attempting to travel
between potential barriers can be compared to a ball trying to
roll over a hill; quantum mechanics and classical mechanics
differ in their treatment of this scenario.
Classical mechanics predicts that particles that do not have
enough energy to classically surmount a barrier will not be able
to reach the other side. Thus, a ball without sufficient energy to
surmount the hill would roll back down. Or, lacking the energy
to penetrate a wall, it would bounce back (reflection) or in the
extreme case, bury itself inside the wall (absorption).
In quantum mechanics, these particles can, with a very small
probability, tunnel to the other side, thus crossing the barrier
.The reason for this difference comes from the treatment of
matter in quantum mechanics as having properties of waves and
particles(wave –particle duality ) .
Problems in quantum mechanics center around the analysis of
the wave function for a system. Using mathematical
formulations of quantum mechanics, such as the Schrödinger
equation, the wave function can be solved. This is directly
related to the probability density of the particle's position, which
describes the probability that the particle is at any given place.
The simplest problems in quantum tunneling are onedimensional such as the rectangular barrier .
Fig. 1. Rectangular potential barrier and particle wave function Ψ .
The wave function
can be found by solving
time_independent Schrödinger equation for the system in one
Where m is the mass of the particles Planck constant/2
,V(x) the height of the barrier ,E the energy of the incident
=0( means no forces act on the particle)
2) When 0<X<L
3) When X>L
The constant F=0 because there is no barrier to reflect the
By applying the boundary conditions
We can find the amplitudes(A,B,C,D,E) of each wave function.
The wave function
represents the incoming particles
moving to the right ,and
represents the reflected particles
moving to the left ;
represents the transmitted particles
moving to the right .The wave function
particles inside the barrier ,some of which end up in region 3
while the others return to region 1 
The transmission probability T for a particle to pass through the
barrier is equal to
T= J transmitted /J incident
where J(x ,t) the probability current which is equal to :
So the approximate transmission probability :
Where L is the width of the barrier, and
3- STM design
The basic components of the STM :
1-The sample:aclean conducting or semiconducting surface.
2-Scanning tip : The tip is the trickiest part in the STM ,it needs
a small curvature to resolve coarse structures .
For atomic resolution a mini tip with a one atomic end is
Fig.2 scanning tip .
Tips typically are made out of tungsten, platinum or a Pt-Ir wire.
A sharp tip can be produced by:
Cutting and grinding
The tungsten wire is put into a solution of NaOH and kept on a
positive potential towards a counter electrode. The etching
process takes place predominately on the surface of the solution.
When the neck is thin enough the wire fractures due to its
weight. Thus actually two tips are produced. The tip has to be
cleaned with de ionized water and pure ethanol or methanol.
Fig.3 Electrochemical Etching .
Most often the tip is covered with an oxide layer and
contaminations from the etchant and is also not sharp enough.
Thus other treatments to the tip, like annealing or field
evaporation are necessary.
3- Piezoelectric tube : piezoelectric controlled height and x, y
scanner. The word piezoelectricity means electricity resulting
from pressure. A piezoelectric substance is one that
produces an electric charge when a mechanical stress is applied
(the substance is squeezed or stretched). Conversely, a
mechanical deformation (the substance shrinks or expands) is
produced when an electric field is applied.
The piezoelectric effect occurs only in non conductive
materials. Piezoelectric materials can be divided in 2 main
groups: crystals and ceramics. The most well-known
piezoelectric material is quartz (SiO2).
4-Vibration isolation system: due to the extreme sensitivity of
tunnel current to height, proper vibration isolation or an
extremely rigid STM body is imperative for obtaining usable
results. In the first STM by Binnig and Rohrer, magnetic
levitation was used to keep the STM free from vibrations; now
mechanical spring or gas spring systems are often used.
5-Computer: the computer may also be used for enhancing the
image with the help of image processing as well as performing
4- STM Operation:
A voltage bias is applied and the tip is brought close to the
sample by some coarse sample-to-tip control, which is turned
off when the tip and sample are sufficiently close. At close
range, fine control of the tip in all three dimensions when near
the sample is typically piezoelectric, maintaining tip-sample
separation W typically in the 4-7 Å (0.4-0.7 nm) range, which is
the equilibrium position between attractive (3<W<10Å) and
repulsive (W<3Å) interactions.
Fig.5 The scanning tunneling microscope  .
In this situation, the voltage bias will cause electrons to tunnel
between the tip and sample, creating a current that can be
measured .The resulting tunneling current is a function of tip
position, applied voltage, and the local density of states (LDOS)
of the sample Information is acquired by monitoring the current
as the tip's position scans across the surface, and is usually
displayed in image form .
It: tunneling current
Vt: tunneling voltage or (voltage bias)
Z= Tip-sample separation, typically 4-10 Å
Θ: work function, typically 3-5 ev .
Fig. 6 Voltage biase vs tunneling current . 
5- Modes of operation.
5-1 Constant Current Mode:
In STM bias voltage is applied between a sharp conductive tip
and a conductive sample, so when the sample is approached to a
few angstroms from the tip, tunneling current occurs, that
indicates proximity of the tip to the sample with very high
accuracy. In Constant Current mode (CCM) of operation when
scanning sample surface the scanner keeps the current constant
by feedback circuit.
Fig.7 Constant current mode .
feedback electronics adjust the height by a voltage to the
piezoelectric height control mechanism . This leads to a
height variation and thus the image comes from the tip
topography across the sample and gives a constant charge
density surface; this means contrast on the image is due to
variations in charge density .
5-2 Constant Height Mode
In constant height mode, the voltage and height are both held
constant while the current changes to keep the voltage from
changing; this leads to an image made of current changes over
the surface, which can be related to charge density.
Fig.8 Constant Height Mode .
In Constant Height mode (CHM) of operation the scanner of
STM moves the tip only in plane, so that current between the tip
and the sample surface visualizes the sample relief. Because in
this mode the adjusting of the surface height is not needed a
higher scan speed can be obtained. CHM can only be applied if
the sample surface is very flat, because surface corrugations
higher than 5-10 A will cause the tip to crash. The weak
feedback is still present to maintain a constant average tipsample distance. As the information on the surface structure is
obtained via the current, a direct gauging of height differences is
no longer possible .
The benefit to using a constant height mode is that it is faster, as
the piezoelectric movements require more time to register the
height change in constant current mode, than the current change
in constant height mode .
6-Density of States imaging
As long as measured in STM current is determined by the
tunneling processes through tip-sample surface gap its value
depends not only on the barrier height but on the electron
density of states also. Accordingly obtained in STM images are
not simply images of sample surface relief (topography), these
images can be hardly affected by the density of electronic states
distribution over the sample surface. Good example of Local
Density of States (LDOS) influence on the STM image is wellknown image of highly oriented pyrolitic graphite (HOPG)
atomic lattice. Only half atoms are visible in STM. Similar case
is image of GaAs atomic lattice.
LDOS determining can also help to distinguish chemical nature
of the surface atoms. LDOS acquisition is provided
simultaneously with the STM images obtaining .
Most of STM applications are in nanotechnology and biology.
Determination of surface structures is one of the most important
applications of the STM.
7-1 Manipulation of Atoms:
One innovative applications of STM recently found is
manipulation of atoms. Example, Iron atoms are placed on
Cu(111) surface at very low temperature (4K), Iron atoms are
first physisorbed on the Cu surface, then the tip is placed
directly over a physisorbed atom and lowered to increase the
attractive force by increasing the tunneling current, the atom
was dragged by the tip and moves across the surface to a desired
position. Then, the tip was withdrawn by lowering the tunneling
Fig.9 STM images show the steps of "quantum corral"
7-2 Scanning tunneling microscope study of iron(II)
growth on metals and insulating
Due to the broad field of applications metal phthalocyanine
(MPc) molecules and their derivatives have attracted intense
interest of researchers within previous decades .They are
important compounds for optical and organic electronic devices
such as organic light-emitting diodes, thin film transistors, and
solar cells . The physical properties of MPc molecular films are
strongly affected not only by the molecular structure but also by
the molecular orientation in thin films as well as by the interface
to the hosting carrier. Therefore, molecule–molecule and
molecule–substrate interactions are important issues for the
formation of highly ordered MPc molecular films and attracted
considerable interest in these planar molecular model systems,
in experiments and in numerical modeling .
The scanning tunneling microscopy (STM) and spectroscopy
(STS) are intensively used to determine both, the geometric and
the electronic structures of MPc molecules deposited on metallic
The adsorption configurations of several MPc molecules on
various metal substrates have been studied with STM under
ultra-high vacuum (UHV) conditions.
7-3 Modification of thin gold films with the scanning
tunneling microscope: Thin gold films, which were deposited
by sputter deposition onto highly oriented graphite surfaces,
were investigated and modified by means of a scanning
tunneling microscope. By applying short voltage pulses to the
vertical piezoelectric element or to the tunneling tip, hole
patterns were generated.
8- STM images:
All images that are obtained STM device is the image of gray
tones and to obtain a color image, computer programs are used
to highlight important features to show it in the picture.
2-D network of 4 nm Au cluster array on Ga As .
Atoms of n-type MoS2, a common dry lubricant. The bright
spots indicate S atoms, which account for its excellent
lubrication properties .
STM image, 35 nm x 35 nm, of single substitutional Cr
impurities (small bumps) in the Fe(001) surface .
STM image, 7 nm x 7 nm, of a single zig-zag chain of Cs atoms
(red) on the GaAs(110) surface (blue) .
STM image of individual silicon (Si) atoms .
Image of DNA .
9-STM related studies:
Many other microscopy techniques have been developed based
9-1 Photon scanning microscopy (PSTM):
which uses photons instead of tunneling electrons to image
9-2 Scanning tunneling potentiometry (STP)
used to study the spatial variation of the electric potential on
thin film surfaces. Topography and potential distribution of the
film surface are measured simultaneously.
9-3 Atomic force microscopy (AFM):
or scanning force microscopy (SFM) is a very high-resolution
type of scanning probe microscopy, with demonstrated
resolution on the order of fractions of a nanometer, more than
1000 times better than the optical diffraction limit.
The Atomic Force Microscope was developed to overcome a
basic drawback with STM - that it can only image conducting or
semiconducting surfaces. The AFM, however, has the advantage
of imaging almost any type of surface, including polymers,
ceramics, composites, glass, and biological samples .
9-4 Spin polarized scanning tunneling microscopy
is a specialized application of scanning tunneling microscopy
(STM) that can provide detailed information of magnetic
phenomena on the single-atom scale additional to the atomic
topology gained with STM. SP-STM opened a novel approach
to static and dynamic magnetic processes as precise
investigations of domain walls in ferromagnetic and ant
ferromagnetic systems, as well as thermal and current-induced
switching of nonmagnetic particles.
9-5 Scanning Tunneling Spectroscopy (STS):
is an extension of Scanning Tunneling Microscopy (STM)
which is used to provide information about the density of
electrons in a sample as a function of their energy .
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