Transmission Electron Microscope

Introduction

With countless applications across the fields of Materials Science, Geology and Biology¹, the TEM machine has come a long way to become what it has today. Originally used as a means of imaging solid samples, advances in its technology has played a major role in shedding light on the ‘nano’ world. This technology has found many other applications in fields such as forensic science, cancer research and nanotechnology.

In essence, the electron microscope is a microscope that uses electron beams as the illuminating source instead of light alongside the accompanying modified components to manipulate the unique properties of electrons. Electrons have a much shorter wavelength than light and this allows the microscope to resolve smaller objects. Modern electron microscopes are capable of 0.2nm resolution², meaning they are able to discern individual atoms. To give a comparison, the human eye can only distinguish two points that are 0.2 mm apart – if the points are too close, they will simply appear as a single point.

Electron microscopes use a beam of highly energetic electrons to examine objects on a very fine scale. There are two main types of electron microscopes: scanning electron microscope and transmission electron microscope. When a beam of electron shines onto a specimen (which is very thin), electrons many either reflect off the specimen or be transmitted through it. The SEM analyses the reflected electrons whereas the TEM (as its name suggests) analyses the transmitted ones.³ Thus, the TEM is most useful when examining the minute structure of objects whereas the SEM is useful for observing their 3D surface.

History

As with most inventions, the TEM machine was only able to come into being due to several revolutionary theories that surfaced at the time. The most important theory was perhaps the discovery of the electron in 1897 by John Thomson. Using just a cathode ray tube, Thomson was able to show that there were particles of mass which carry a negative charge, are deflected by an electrostatic force and are acted on by a magnetic force. This opened whole new areas of science and research.

In 1924, it was proposed that matter as a whole (including electrons) acted like waves and had their own wavelengths by Louis De Broglie. At the time, many young scientists were already researching into improved microscopes after Ernst Abbe’s discovery of the resolution limit of a microscope. There were several attempts at the use of ultraviolet microscopes, most notably by Kohler and Rohn, although these were never that effective. The discovery that, in a vacuum, accelerated electrons behave like light (in addition to their wave-like properties) stimulated a new generation of scientists to produce a stronger microscope. Equipped with all the necessary knowledge, several teams of scientists engaged in building the first electron microscope until 1931 when Ernst Ruska and Max Knoll finally produced a working model. Ruska subsequently received a Physics Nobel Prize for his work in 1986. ⁴

Components

Many of the electron microscope components are analogous to those of a light microscope. These components have been modified for manipulating electrons however many of their purposes are the same. Rather than a light source, the electron microscope uses an electron gun and the glass lenses are replaced by electromagnetic lenses. Unlike glass lenses, the resolution and magnification of an electromagnetic lens is affected by changing the current through the lens whereas in a light microscope it is done by mechanically changing the lenses. Electron microscopes are normally built underground in order to reduce interference (in the form of vibrations) from environmental sources. As a precaution, they are also built in a room with thick walls as well as a vibration dampening table – you could drop a pencil on the table and it won’t bounce!

The Transmission Electron Microscope can be compared to a slide projector⁵; it has an electron source which is made into a parallel beam by condenser lenses, which passes through an object and is then focussed onto a fluorescent screen.

Most TEM machines sport a columnar design with the electron source located at the top. ⁶ Typical components found inside the column are:

Electron gun – An electron gun consists of a filament (usually made of tungsten) surrounded by a Wehnelt cylinder that directs the beam in a tight, narrow beam. Most TEM machines work by heating the filament so that thermionic electrons are emitted by the tungsten. Alternatively, field emission guns may be used; these are used to pull electrons from atoms by producing a strong electrical field. An anode exists below the Wehnelt cylinder to accelerate the electrons generated. The specimen to be examined will be located below the accelerated beam.

Specimen chamber – Due to the nature of the TEMs, the sample used has to be immobilised and be made very thin. The specimen holder has one or two wells located at its end where the sample will be loaded onto. The specimen has to be kept very still so as to be able to produce clear images free of artefacts. As of such, the sample chamber is very sturdy.

Lenses – Similar to the optical microscope, the TEM has a lens system. However, this is a magnetic lens system rather than a glass one. Electron beams are not affected by glass lenses. However, magnetic fields can affect them as discovered by Han Busch in 1927. The TEM uses an electromagnet so that the strength of the magnetic field can be varied to control the direction and size of the electron beam.

Viewing chamber – Located at the base of the electron microscope, the viewing chamber is where the image of the magnified specimen is projected onto a screen. The surface onto which the image is projected onto is made of a fluorescent material which lights up depending on the density of electrons that arrives on the screen.

Vacuum chamber – Particles can interfere with the electron beams generated by the electron gun in two ways. Firstly, they can easily absorb some of the electrons and thus block the path of the electron beam. Alternatively, electrons may be scattered of knocked out by the particles and onto the specimen; this could distort the surface of the specimen.

How it works

The electron beam generated by the electron gun is concentrated into a more powerful beam by the anode underneath it. It is directed towards the specimen, which sits on a copper grid in the specimen chamber. There, the beam passes through the specimen and based on the varying thickness and density of the material, some of the beam may be scattered or reflected off of the specimen. This means that the electron beam at the other end of the specimen will have regions of varying electron densities. This is then magnified by the magnetic lenses and the image becomes visible when the electron beam hits a fluorescent screen. The brightness of an area is determined by the electron density of that area of the beam. This image can then be viewed directly in the viewing chamber via a pair of binoculars or a camera.

This would normally produce a 2D image. Electron tomography can be used to produce 3D images. A beam of electrons is transmitted through the sample at incremental degrees of rotation around the centre of the target sample. This information can be collected and used to form an image by using computer software⁷ .

Applications and limitations

Despite being considered a rather old technique nowadays, the transmission electron microscope is still found at the forefront of many areas of science such as materials science and biology. The TEM can be used to construct an image of the microstructure of a sample as well as view its morphology and even analyse the composition of these samples. It produced the very first image of a virus!⁸ There has been a lot of research into cancer cells and especially ways of killing these cells safely in the body. The TEM is a core instrument towards the analysis of such cells as it allows scientists to view the cancerous cells in detail. However, a disadvantage of the technique is the fact that the cancerous cells have to be immobilised (thus killed) in an inert matrix as well as put into a vacuum.

Technology companies have used TEMs to view micro-sized objects in order to find and fix faults in their microstructures. It has also found use in semiconductor analysis in the production of computer silicon chips. Electron tomography can be used to view the nanometre-scale structures which semiconductors produce from manufacturing processes in order to measure and control their dimensions.

Transmission electron microscopes have several limitations which many scientists are trying to eliminate. First and foremost, it is impossible to view living specimens since the system is kept under high vacuum⁹ . Environmental TEM uses a specially designed vacuum system to allow researchers to observe objects in a range of conditions close to ‘natural’ conditions. Further developments may allow atmospheric pressure to be achieved, enabling some types of molecular structures to be analysed live. Secondly, the images produced by the TEM are grayscale, although this is not really a problem to most scientists. In addition, the images are usually flat or two-dimensional due to the fact that the specimen must be kept extremely thin for electrons to penetrate them.

These limitations will be more or less be ironed out with continued maturation of the TEM machine, bringing forth new advances in technology.