Transmission Electron Aberration-corrected Microscope
Encyclopedia
Transmission Electron Aberration-Corrected Microscope (TEAM) is a collaborative research project between four US laboratories and two companies. It is based at the Lawrence Berkeley National Laboratory
in Berkeley, California
and involves Argonne National Laboratory
, Oak Ridge National Laboratory
and Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign
, as well as FEI
and CEOS companies, and is supported by the U.S. Department of Energy. The project's main activity is design and application of a transmission electron microscope (TEM) with a spatial resolution below 0.05 nanometers, which is roughly half the size of an atom of hydrogen
. The project was started in 2004; the operational microscope was built in 2008 and achieved the 0.05 nm resolution target in 2009. The microscope is a shared facility available to external users.
and chromatic aberration
s because of their similarity to aberrations in optical lenses. Those aberrations are reduced by installing in a microscope a set of specially designed auxiliary "lenses" which are called aberration correctors.
(that is scanning TEM) modes. To minimize the mechanical vibrations, the microscope is located in a separate room within a sound-proof enclosure and is operated remotely. The electron source is a Schottky type field emission gun
with a relatively low energy spread of 0.8 eV at 300 keV. In order to reduce chromatic aberrations, this spread is further lowered to 0.13 eV at 300 keV and 0.08 eV at 80 kV using a Wien-filter
type monochromator
. Both the illumination lens, which is located above the sample and is conventionally called the condenser lens, and the collection lens (called the objective lens
) are equipped with fifth-order spherical aberration correctors. The electrons are further energy filtered by a GIF filter and detected by a CCD
camera. The filter makes it possible to select electrons scattered by specific chemical elements and so identify individual atoms in the sample being studied.
), germanium
([114]), gold
([111]) and others, and reaching the spatial resolution below 0.05 nm (about 0.045 nm). In the images of graphene
—a single sheet of graphite
—not only the atoms, but also the chemical bonds could be observed (see top picture). A movie has been recorded inside the microscope showing hopping of individual carbon atoms around a hole punched in a graphene sheet. Similar pictures, resolving carbon atoms and bonds between them, have been independently produced for pentacene
—a planar organic molecule consisting of five carbon rings—using a very different microscopy technique, atomic force microscopy (AFM). In AFM, the atoms are probed not by electrons, but by a sharp vibrating tip.
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
in Berkeley, California
Berkeley, California
Berkeley is a city on the east shore of the San Francisco Bay in Northern California, United States. Its neighbors to the south are the cities of Oakland and Emeryville. To the north is the city of Albany and the unincorporated community of Kensington...
and involves Argonne National Laboratory
Argonne National Laboratory
Argonne National Laboratory is the first science and engineering research national laboratory in the United States, receiving this designation on July 1, 1946. It is the largest national laboratory by size and scope in the Midwest...
, Oak Ridge National Laboratory
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
and Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign
University of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...
, as well as FEI
FEI Company
FEI Company , founded in 1971, is an American supplier of electron microscopy tools to researchers, developers and manufacturers working on the nanoscale. Headquartered in Hillsboro, Oregon, the company employs 1,770 people worldwide....
and CEOS companies, and is supported by the U.S. Department of Energy. The project's main activity is design and application of a transmission electron microscope (TEM) with a spatial resolution below 0.05 nanometers, which is roughly half the size of an atom of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. The project was started in 2004; the operational microscope was built in 2008 and achieved the 0.05 nm resolution target in 2009. The microscope is a shared facility available to external users.
Scientific background
It has long been known that the best achievable spatial resolution of an optical microscope, that is the smallest feature it can observe, is of the order of the wavelength of the light λ, which is about 550 nm for green light. One route to improve this resolution is to use particles with smaller λ, such as high-energy electrons. Practical limitations set a convenient electron energy to 100–300 keV that corresponds to λ = 3.7–2.0 pm. Unfortunately, the resolution of electron microscopes is limited not by the electron wavelength, but by intrinsic imperfections of electron lenses. These are referred to as sphericalSpherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
and chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
s because of their similarity to aberrations in optical lenses. Those aberrations are reduced by installing in a microscope a set of specially designed auxiliary "lenses" which are called aberration correctors.
Hardware
The TEAM is based on a commercial FEI Titan 80–300 electron microscope, which can be operated at voltages between 80 and 300 keV, both in TEM and STEMScanning transmission electron microscopy
A scanning transmission electron microscope is a type of transmission electron microscope . As with any transmission illumination scheme, the electrons pass through a sufficiently thin specimen...
(that is scanning TEM) modes. To minimize the mechanical vibrations, the microscope is located in a separate room within a sound-proof enclosure and is operated remotely. The electron source is a Schottky type field emission gun
Field emission gun
A field emission gun is a type of electron gun in which a sharply-pointed Müller-type emitter is held at several kilovolts negative potential relative to a nearby electrode, so that there is sufficient potential gradient at the emitter surface to cause field electron emission...
with a relatively low energy spread of 0.8 eV at 300 keV. In order to reduce chromatic aberrations, this spread is further lowered to 0.13 eV at 300 keV and 0.08 eV at 80 kV using a Wien-filter
Wien filter
A Wien filter is a device consisting of perpendicular electric and magnetic fields that can be used as a velocity filter for charged particles, for example in electron microscopes and spectrometers. It is named for Wilhelm Wien who developed it in 1898 for the study of anode rays. It can be...
type monochromator
Monochromator
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input...
. Both the illumination lens, which is located above the sample and is conventionally called the condenser lens, and the collection lens (called the objective lens
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
) are equipped with fifth-order spherical aberration correctors. The electrons are further energy filtered by a GIF filter and detected by a CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
camera. The filter makes it possible to select electrons scattered by specific chemical elements and so identify individual atoms in the sample being studied.
Applications
The TEAM has been tested on various crystalline solids, resolving individual atoms in GaN ([211] orientationMiller index
Miller indices form a notation system in crystallography for planes and directions in crystal lattices.In particular, a family of lattice planes is determined by three integers h, k, and ℓ, the Miller indices. They are written , and each index denotes a plane orthogonal to a direction in the...
), germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
([114]), gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
([111]) and others, and reaching the spatial resolution below 0.05 nm (about 0.045 nm). In the images of graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
—a single sheet of graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
—not only the atoms, but also the chemical bonds could be observed (see top picture). A movie has been recorded inside the microscope showing hopping of individual carbon atoms around a hole punched in a graphene sheet. Similar pictures, resolving carbon atoms and bonds between them, have been independently produced for pentacene
Pentacene
Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings. This highly conjugated compound is an organic semiconductor. The compound generates excitons upon absorption of ultra-violet or visible light; this makes it very sensitive to oxidation...
—a planar organic molecule consisting of five carbon rings—using a very different microscopy technique, atomic force microscopy (AFM). In AFM, the atoms are probed not by electrons, but by a sharp vibrating tip.