[Guest Post] Avatars of Microscopes: A Brief History

 

Avatars of Microscopes: A Brief History

 

The human mind knows no bounds; it makes humans look deeper into matter and further into outer space. The evolving intellect associated with the human curiosity has concurrently led to the evolution of advanced technologies. On one hand, humans made the Hubble Space Telescope to view the remotest and imperceptible reaches of the cosmos, on the other hand, they made the Transmission Electron Microscope (TEM) to view the atomic arrangements in crystalline and quasicrystalline materials. Both instruments are an assembly of multiple components made from various materials, which are chosen based on the required properties. Therefore, finding a structure-property correlation is important for its application. This structure-property correlation necessitates advancements in both property testing equipments (like mechanical, electrical, magnetic property, to name a few) and correspondingly, the microscopes for observing the underlying microstructure. One may spare a minute to introspect how materialistic humans are today! Only a caveman like Tarzan today could live without smartphones or even a mundane object like the razor blade! Everything is a product of innovation. Materials of the modern world owe much to strides in microscopy. Philosophers since ancient times have emphasized how a deeper look at the microcosm has aided the understanding of the macrocosm. The strides made in microscopy could be attributed to this philosophy as humans are able to resolve features that are 0.1 mm fine at best!

The development of microscopy commenced in the 16^th 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 could provide 10 times magnification at best. It would thus be a misnomer to call it a scientific instrument, but nonetheless, it was a start. Physicists started experimenting the phenomenon of refraction of light combined with the lens geometry that resulted in this magnification via simple ray diagrams. Among them, the most notable was Antony van Leeuwenhoek, a Dutch scientist, 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 of ‘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 even today. One could see how these microscopes were developed to achieve a magnification of almost 1000x. Though these microscopes were used for studying the details of the building blocks of life- cells, gradually it started finding applications in the world of materials as well. Optical microscopes were used to study the internal structure of metallic materials (microstructure) after different processing conditions like casting, rolling, heat treatment, etc. 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) and accompanying constraints being the quality of glass used and the perfection of lens’ shape. The large wavelength of visible light signified that its application in atomic scale resolution was impossible. So, while the resolution power of the optical microscope retains its relevance in studies pertaining to biology, more advancement in microscopy was requisite for studying solid state materials in the atomic scale.

In the mid-20^th century, the use of electrons as a source of illumination created a paradigm shift in the world of microscopy. Studies by physicists in the 19^th century brought to light the electrons (originally known as the 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 was realized 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 few hundred kilovolts) could pass through solids to enable viewing of its submicroscopic features. One must not be astonished how fundamental discoveries in physics had set the stage for the invention of electron microscopes. Rather one must appreciate the interrelation of various disciplines of science and technology in everyday life.

            The electron microscope was invented by Max Knoll and Ernst Ruska in the year 1931 at the Berlin Technische Hochschsule in Germany. Their invention finally yielded a resolution as fine as 10 nm, which upon further increase in acceleration voltage and advancements in lens and illumination technology could be further reduced to 2 nm by mid-1940s. However, like any other scientific instrument, there were other factors that required refinement for minimization of aberrations and greater resolution. 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 the upgradation in lens technology, electron guns, electron detectors/camera and better vacuum systems. The electron guns evolved from using tungsten filament (that yielded the coarsest resolution), to lanthanum hexaboride filament, to field emission guns by the 1960s. Likewise, the electromagnetic lenses which initially functioned as an electron beam focusing gadgetry were gradually engineered to perform in-plane rastering (movement in x-y directions on a horizontal plane) of the beam to scan the specimen (these TEMs are known as Scanning Transmission Electron Microscopes or STEMs). Further advancement in the 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, to name a few.

The collective advancements in 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, optoelectronics industry, to name a few. (The term 4D-STEM was coined to refer to the recording of 2D images of a convergent electron probe over a 2D grid of probe positions.) 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!

The relationship of materials and microscopy is akin to that of the immortalized lovers Romeo and Juliet. Deeper insights into the micromechanisms of hardening and softening of materials under different processing and alloying conditions was understood by studying its microstructures. It is because of microscopy that Dan Shechtman, an Israeli scientist, won the Nobel Prize in Chemistry in 2011 for the discovery of quasicrystals (materials having a 5-fold symmetry, which was otherwise thought to be physically impossible). The entire aircraft industry is heavily dependent on aluminum alloys, the strengthening in 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 the 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 a small magnet and established a method for observation of atomic magnetic fields. This would further propel the research and development in magnetic semiconductors and spintronic devices.

While the electron microscopes used today have a vertical optic axis, microscopes having a horizontal axis were also being developed initially. In fact, 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 University of Calcutta, India. It is rather unfortunate that India was at par with the world in development of electron microscopes only in the initial stages. However, it provides an opportunity to budding Indian scientists from various disciplines to come together to compete with the world in manufacturing Made in India electron microscopes.

About the author: Soumita is currently pursuing her doctoral research at the Department of Materials Engineering at IISc Bangalore. Her study focuses on characterization of bulk-nanostructured materials by electron microscopy and X-ray diffraction techniques.