Download Microscopy as a means for Nano

Scanning Electron Microscope
(SEM)




The SEM functions much like an
optical microscope but uses
electrons instead of visible light
waves.
The SEM uses a series a series of
EM coils as lenses to focus and
manipulate the electron beam.
Samples must be dehydrated and
made conductive.
Images are black and white.
http://www.mos.org/sln/SEM/works/slideshow/semmov.html
SEM Images
http://www.mos.org/sln/SEM/works.html
Scanning Tunneling
Electron Microscope (STM)
 Basic principle is tunneling.
 Tunneling current flows
between tip and sample when
separated by less than 100nm.
 The tunneling current gives
us atomic information about
the surface as the tip scans.
http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/index.htmlx
What is the role of piezoelectric part in STM?
 In 1880 Pierre Curie discovered that by applying a
pressure to certain crystals he could induce a
potential across the crystal.
 The STM reverses this process. Thus, by applying
a voltage across a piezoelectric crystal, it will
elongate or compress.
 A typical piezoelectric material used in an STM is
Lead Zirconium Titanate.
http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/index.htmlx
STM Images
http://www.almaden.ibm.com/vis/stm/gallery.html
Atomic Force Microscopy
(AFM)



AFM is performed by scanning a sharp tip on the end of a flexible cantilever across the
sample while maintaining a small force.
Typical tip radii are on the order of 1nm to 10nm.
AFM has two modes, tapping mode and contact mode.

In scanning mode, constant cantilever deflection is maintained.

In tapping mode, the cantilever is oscillated at its resonance frequency.
http://www.nanoscience.com/education/AFM.html
http://www.azom.com/details.asp?ArticleID=3278
AFM Images
http://www.azom.com/details.asp?ArticleID=3278
http://www.nanoscience.com/index.html
Transmission Electron
Microscope (TEM)




Same principle as optical
microscope but with electrons.
Condenser aperture stops high
angle electrons, first step in
improving contrast.
The objective aperture and
selected area aperture are
optional but can enhance
contrast by blocking high angle
diffracted electrons
Advantages: we can look at non
conducting samples, i.e.
polymers, ceramics, and
biological samples.
http://www.unl.edu/CMRAcfem/temoptic.htm
TEM Images
http://www.abdn.ac.uk/emunit/emunit/temcells/index.htm
Antiferromagnetic ordering
Paramagnetic system w/o magnetic field
SUPERPARAMAGNETISM
 Superparamagnetism is a phenomenon in which
magnetic materials may exhibit a behavior
similar to paramagnetism at temperatures below
the Curie or the Néel temperature. This is a
small length-scale phenomenon, where the
energy required to change the direction of the
magnetic moment of a particle is comparable to
the ambient thermal energy. At this point, the
rate at which the particles will randomly reverse
direction becomes significant.
SUPERPARAMAGNETISM-II
 Superparamagnetism occurs when the material is composed of
very small crystallites (1–10 nm). In this case even when the
temperature is below the Curie or Neel temperature (and hence
the thermal energy is not sufficient to overcome the coupling
forces between neighboring atoms), the thermal energy is
sufficient to change the direction of magnetization of the entire
crystallite. The resulting fluctuations in the direction of
magnetization cause the magnetic field to average to zero. Thus
the material behaves in a manner similar to paramagnetism,
except that instead of each individual atom being independently
influenced by an external magnetic field, the magnetic moment
of the entire crystallite tends to align with the magnetic field.
SPINTRONICS
 The research field of Spintronics emerged from experiments on
spin-dependent electron transport phenomena in solid-state
devices done in the 1980s, including the observation of spinpolarized electron injection from a ferromagnetic metal to a
normal metal by Johnson and Silsbee (1985),[3] and the discovery
of giant magnetoresistance independently by Albert Fert et al.[4]
and Peter Grünberg et al. (1988).[5] The origins can be traced
back further to the ferromagnet/superconductor tunneling
experiments pioneered by Meservey and Tedrow,[6] and initial
experiments on magnetic tunnel junctions by Julliere in the
1970s.[7] The use of semiconductors for spintronics can be traced
back at least as far as the theoretical proposal of a spin fieldeffect-transistor by Datta and Das in 1990.[8]
SPINTRONICS-II
 Electrons are spin-1/2 fermions and therefore constitute a twostate system with spin "up" and spin "down". To make a
spintronic device, the primary requirements are to have a system
that can generate a current of spin polarized electrons comprising
more of one spin species -- up or down -- than the other (called a
spin injector), and a separate system that is sensitive to the spin
polarization of the electrons (spin detector). Manipulation of the
electron spin during transport between injector and detector
(especially in semiconductors) via spin precession can be
accomplished using real external magnetic fields or effective
fields caused by spin-orbit interaction.
GIANT MAGNETORESISTANCE
 The simplest method of generating a spin-polarised current in a
metal is to pass the current through a ferromagnetic material. The
most common application of this effect is a giant magnetoresistance
(GMR) device. A typical GMR device consists of at least two layers
of ferromagnetic materials separated by a spacer layer. When the
two magnetization vectors of the ferromagnetic layers are aligned,
the electrical resistance will be lower (so a higher current flows at
constant voltage) than if the ferromagnetic layers are anti-aligned.
This constitutes a magnetic field sensor.
 Two variants of GMR have been applied in devices: (1) current-inplane (CIP), where the electric current flows parallel to the layers
and (2) current-perpendicular-to-plane (CPP), where the electric
current flows in a direction perpendicular to the layers.