Download Scanning Tunneling Microscopy

Scanning Tunneling Microscopy
Introduction

Invented by Binnig and Rohrer at IBM in 1981 (Nobel Prize in Physics in 1986).
Binnig also invented the Atomic Force Microscope(AFM) at Stanford University in
1986.
Introduction
Topographic (real space) images
Spectroscopic (electronic structure, density of states) images
Introduction
 Atomic resolution, several orders of magnitude better
than the best electron microscope
 Quantum mechanical tunnel-effect of electron
 In-situ: capable of localized, non-destructive
measurements or modifications
 material science, physics, semiconductor science,
metallurgy, electrochemistry, and molecular biology
 Scanning Probe Microscopes (SPM): designed based on
the scanning technology of STM
Theory and Principle
Tunneling Current
 A sharp conductive tip is brought to within a few Angstroms of
the surface of a conductor (sample).
 The surface is applied a bias voltage, Fermi levels shift
 The wave functions of the electrons in the tip overlap those of
the sample surface
 Electrons tunnel from one surface to the other of lower potential.
Theory and Principle
 The tunneling system can be described as the model of
quantum mechanical electron tunneling between two
infinite, parallel, plane metal surfaces
EF is the Fermi level
ψ is the wave function of the electron
Ф is the work function of the metal.
Electrons tunnel through a rectangular
barrier.
Experimental methods
Basic Set-up
 the sample you want to study
 a sharp tip mounted on a
piezoelectric crystal tube to be
placed in very close proximity to
the sample
 a mechanism to control the
location of the tip in the x-y plane
parallel to the sample surface
 a feedback loop to control the
height of the tip above the sample
(the z-axis)
Scanning Tunneling Microscopy concept
The basic principle of scanning tunneling microscopy (STM) is based on the tunneling
current between a metallic tip, which is sharpened to a single atom point and a
conducting material.
A small bias voltage (10mV to 3 V) is applied between an atomically sharp tip and the
sample. If the distance between the tip and the sample is large no current flow. However,
when the tip is brought very close (10 Å) a current (pA to nA) flows across the gap
between the tip and the sample.
Such current is called tunneling current which is the result of the overlapping
wavefunctions between the tip atom and surface atom, electrons can tunnel across the
vacuum barrier separating the tip and sample in the presence of small bias voltage.
Scanning tunneling microscopy probes
•Cutting and grinding
•Electrochemical etching
Scanning electron microscopy image of a
tungsten tip
Scanning Tunneling Microscopy
Theory of the electron tunneling
At low voltage and temperature
I  exp(-2Kd)
d is the distance between tip and sample. If the distance
increased by 1 Angstrom, the current flow decreased by
an order of magnitude, so the sensitivity to vertical
distance is terribly high.
K=
2m / 
m is mass of electron,  is the local
tunneling barrier height or the average
work function of the tip and sample.
How to operate?
Raster the tip across the surface, and
using the current as a feedback
signal.
The tip-surface separation is controlled
to be constant by keeping the
tunneling current at a constant
value.
The voltage necessary to keep the tip at a
constant separation is used to
produce a computer image of the
surface.
What an STM measures?------local density of states
Each plane represents a different value of the tip-sample bias V, and the lateral
position on the plane gives the x,y position of the tip. Filled states are given in
red. The plane at the Fermi energy (V=0) is shown in blue.
Scanning Tunneling Microscopy
Theory of the electron tunneling
Remember !
STM does NOT probe the nuclear position directly, but
rather it is a probe of the electron density, so STM images
do not always show the position of the atoms, and it
depends on the nature of the surface and the magnitude
and sign of the tunneling current.
Scanning Tunneling Microscopy
In case of a negative potential on the sample the occupied states generate the
current, whereas in case of a positive bias the unoccupied states of the sample
are of importance.
Imaging the occupied states of
SiC(000 )3x3
Imaging the unoccupied states of
SiC(000 )3x3
By altering the voltage, a complete different image can be
detected as other states contribute to the tunneling current.
This is used in tunneling spectroscopy.
Scanning Tunneling Microscopy
STM image of graphite
5 nm