Evolution of Microscopes

Evolution of Microscopes

The human mind is the most unpredictable intellectual thing that knows no bounds. It makes us look deeper into the matter and further into outer space. When the evolving intellect associated with human curiosity results in the evolution of advanced technologies. On one hand, humans made the Hubble space telescope to view the most remote and imperceptible reaches of the cosmos, while on the other, they made the transmission electron microscope (TEM) to view the atomic arrangements in crystalline and quasicrystalline materials.

Materials of the modern world owe much to strides in microscopy. The development of microscopy commenced in the sixteenth century with the invention of glass lenses that used visible light as the source of illumination. Initially, these lenses were used by biologists as simple magnifying glasses. These single-lens magnifying glasses would provide 10 times magnification, at best. It would, thus, be a misnomer to refer to them as a scientific instruments. But it was the start.

Physicists started experimenting with the phenomenon of refraction of light combined with lens geometry that resulted in this magnification via simple ray diagrams. Among them, the most notable was Dutch scientist Antony van Leeuwenhoek (1632-1723), who found that by increasing the convexity of the glass lens an object could be magnified by more than 250x in comparison to the naked eye. His experiments eventually led to the discovery of bacteria and earned him the title ‘Father of Microscopy’. The findings were, thereafter, used as a foundation to study the effect of lenses put together in series and the instruments came to be known as compound microscopes, which are very much in use till date.

Optical microscopes were used to study the internal structure of metallic materials (microstructure) after different processing conditions like casting, rolling, and heat treatment to understand the correlation between microstructure and its mechanical properties. However, optical microscopy was associated with multiple limitations, the primary limitation being the large wavelength of light (that limited its theoretical resolution to 2000 angstroms) with accompanying constraints being the quality of glass used and the perfection of the lens shape. The large wavelength of visible light signified that its application at atomic-scale resolution was impossible. So, while the resolution power of the optical microscope retains its relevance in studies pertaining to biology, advancement in microscopy was required for the study of solid-state materials at the atomic scale.

In the mid-twentieth century, the use of electrons as a source of illumination created a paradigm shift in the world of microscopy. Studies by physicists in the nineteenth century brought to light electrons (originally known as “cathode rays” by its discoverer J J Thomson), which were negatively charged particles that could be scattered by atoms and subatomic particles, and bent when passed through both electric and magnetic fields. Correspondingly, experiments such as the famous Rutherford scattering experiment led to a better understanding of the internal structure of an atom as being inhomogeneous. It came to be known that since most of the volume of an atom is empty space, electrons with sufficient kinetic energy could pass through thin foils (a condition known as electron transparency). This meant that electrons accelerated at high voltages (more than a few hundred kilovolts) and could pass through solids to enable viewing of its submicroscopic features.

The electron microscope was invented by Max Knoll and Ernst Ruska in the year 1931 at the Berlin Technische Hochschule in Germany. Their invention yielded a resolution as fine as 10 nm, which upon a further increase in acceleration voltage and advancements in lens and illumination technology could be further reduced to 2 nm by the mid-1940s.

Like optical microscopy, electron microscopy, too, was affected by both chromatic and spherical aberrations. The achievement of atomic-scale resolution took several decades of research and involved an upgrade in lens technology, electron guns, electron detectors/cameras, and better vacuum systems. The achievement of atomic-scale resolution took several decades of research and involved an upgrade in lens technology, electron guns, electron detectors/cameras, and better vacuum systems.

Further advancement in lens technology also led to the development of the double-aberration-corrected (both spherical and chromatic aberration corrected) STEMs. The development of atomic resolution imaging also made atomic-scale elemental analysis possible by spectroscopic techniques such as energy dispersive x-ray spectroscopy, wavelength dispersive x-ray spectroscopy, and electron energy loss spectroscopy. The collective advancement in the disciplines of materials, electrical, electronics, and computer technology has brought electron microscopy to its current-day avatar. The advent of 4D-STEM has opened a new dimension of structure-property correlation in functional materials such as those in battery technology, semiconductor industry, ferroelectric and ferromagnetic industry, and optoelectronics industry.

The heavy dependence of such an advanced characterization technique on the increased computational power can be realized from the fact that a single dataset of 4D-STEM imaging acquired in 164 seconds has a size of 420 GB.

 

• Dan Shechtman, an Israeli scientist, won the 2011 Nobel Prize in Chemistry for the discovery of quasicrystals (materials having a five-fold symmetry,
• The entire aircraft industry is heavily dependent on aluminum alloys, the strengthening which could not be ascertained until the strengthening particles in the microstructure, called GP zones, were observed with the help of electron microscopes.
• TEM studies on the shape memory effect led to the development of a new class of materials called shape memory alloys that find widespread biomedical applications such as unclogging of blood vessels using Nitinol stents.
• A recent achievement in the history of electron microscopy was the development of magnetic-field-free atomic-resolution STEM. The instrument was used to demonstrate that each iron atom itself acts as small magnet and established a method for the observation of atomic magnetic fields. This would further propel research and development in magnetic semiconductors and spintronic devices 

In the years 1946-48, Asia’s first horizontal microscope was built by a team of scientists led by N N Dasgupta, associated with Saha Institute of Nuclear Physics and the University of Calcutta, India

Keep learning, Keep Observing.