Stm 07.08.13

Scanning Tunneling Microscopy

09TT20 CHARACTERIZATION OF TEXTILE POLYMERS

Presentation I

S.Dhandapani – 11MT62

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

Scanning tunneling microscopy is a microscopical technique that allows the investigation of electrically conducting surfaces down to the atomic scale.

Atomic resolution, several orders of magnitude better than the best electron microscope.

Quantum mechanical tunnel-effect of electron.

Material science, physics, semiconductor science, metallurgy, electrochemistry, and molecular biology.

Working Principle of STM

In the scanning tunneling microscope the sample is scanned by a very fine metallic tip.

The tip is mechanically connected to the scanner, an XYZ positioning device realized by means of piezoelectric materials.

 The sample is positively or negatively biased so that a small current, the "tunneling current" flows if the tip is in contact to the sample.

Working Principle of STM 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.

The tunneling system can be described as the model of quantum mechanical electron tunneling between two infinite, parallel, plane metal surfaces

Working Principle of STM This feeble tunneling current is amplified and measured.

With the help of the tunneling current the feedback electronic keeps the distance between tip and sample constant.

If the tunneling current exceeds its preset value, the distance between tip and sample is decreased, if it falls below this value, the feedback increases the distance.

 The tip is scanned line by line above the sample surface following the topography of the sample.

Experimental methods

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)

Basic Set-up

Tunneling Tips

Cut platinum – iridium wires

Tungsten wire electrochemically etched

Tungsten sharpened with ion milling

Best tips have a point a few hundred nm wide

In reality is relatively easy to obtain such tips by etching or tearing a thin metal wire.

Very small changes in the tip-sample separation induce large changes in the tunneling current.

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.

Two Modes of Scanning

Constant Height Mode

Constant Current Mode

Usually, constant current mode is superior.

Tunneling Current The reason for the extreme magnification capabilities of

the STM down to the atomic scale can be found in the physical behavior of the tunneling current.

The tunneling current flows across the small gap that separates the tip from the sample,in better approach of quantum mechanics the electrons are "tunneling" across the gap.

The tunneling current I has a very important characteristic: it exhibits an exponentially decay with an increase of the gap d.

I= K*U*e -(k*d)       k and K are constants, U is the tunneling bias

Tip-sample tunneling contact

Exponential behavior of the tunneling current I with distance d

STM Tips

Tunneling current depends on the distance between the STM probe and the sample

SurfaceTunneling current depends on distance between tip and surface

Tunneling Current• It shows a cross section of a

sample surface with two surface atoms being replaced by foreign atoms, for instance adsorbates (black).

• While at low bias (red) the tip may follow the "actual" topography, there may also be a bias where no contrast is obtained (green) or a bump is seen above the adsorbates (blue).

• This bias dependent imaging is used to create the color images: three individual STM images of the same sample area are obtained at different tunneling bias.

Advantages No damage to the sample

Vertical resolution superior to SEM

Spectroscopy of individual atoms

Relatively Low Cost

Disadvantages Samples limited to conductors and semiconductors

Limited Biological Applications: AFM

Generally a difficult technique to perform

Figures of Merit

Maximum Field of View: 100 μm

Maximum Lateral Resolution: 1 Å

Maximum Vertical Resolution: .1 Å

Interesting Images with STM

Copper Surface

Xenon on NickelSingle atom lithography

Iron on Copper

Quantum Corrals Imaging the standing wave created by interaction of species

Carbon Monoxide Man: CO on Platinum

Graphite is a good example! STM images of graphite

Structure of graphite

Overlay of structure shows only every other atom is imaged

Thank you

Basic Principles of STM

Electrons tunnel between the tip and sample, a small current I is generated (10 pA to 1 nA).

I proportional to e-2κd, I decreases by a factor of 10 when d is increased by 1 Å.

d ~ 6 Å

Bias voltage:

Instrumental Design: Controlling the Tip

Raster scanning

Precise tip control is achieved with Piezoelectrics

Displacement accurate to ± .05 Å