‘There’s plenty of room at the bottom’- this was envisioned by Prof. Richard Feynman in his famous lecture at Caltech in 1960. He explained that all matter is built of tiny building blocks or Legos called atoms. An interlock between suitable combinations of the atoms creates different structures with distinct properties and functions. Alterations in the arrangement of atoms, their combinations, or absence of an atom from its allocated position may lead to drastic changes in properties exhibited by the material. For example, an inert oxide with no Oxygen vacancies (absence of Oxygen atoms where it is supposed to be in the crystal) can become an active, reducible oxide due to the Oxygen vacancies. The presence of Oxygen vacancies makes it reducible so that it can regain its Oxygen and become inert at the cost of reducing another material in its environment. Sometimes, a small change in Si crystal, like replacing a few atoms of Si with Ga or As changes the conductivity of the parent Si remarkably. This forms the basis for semiconductor physics.
Prof. Feynman suggested in his quote that it is important to look at how atoms build themselves within the material to understand behavior of the material. Now, let us visualize the atomic dimension he was aspiring to look at.
How small is an atom? More than a million atoms can line up to form the width of a human hair which is the smallest object visible to naked human eyes! A single strand of hair is about 100 microns wide, which means the size of an atom is about 1/one-million times the width of a hair, that is 0.0001 microns or 0.1 nanometer (nm) or 1 Angstrom.
Now, how do we see objects that are so tiny? Magnifying glasses can be used to magnify the finer details; however, it does not improve resolution. At this juncture, it is good to point out the difference between magnification and resolution. Magnification is the enlargement of a smaller object whereas, Resolution is the ability to distinguish between two points in the object. For example, the ability to view the headlights of a car as two distinct sources instead of a blurred bilobed source is called resolution. For any optical/lens system, the resolution is limited by the wavelength of the illumination source used. The resolution is roughly half the wavelength of the source used (Ernst Abbe in 1870). So, in a visible light microscope using light of wavelength 500 nm, one cannot distinguish two lines drawn about 250 nanometers apart, which is 1/400 times the thickness of human hair. But we aim to resolve spaces between 1/one-million times the thickness of human hair! Thus, to have better resolution, sources having a smaller wavelength is necessary. Can electrons act as such a source of ‘illumination’?
In 1925, electrons were discovered to also behave like light waves having very low wavelengths (Louis de Broglie) and electron movement could be guided by an electromagnetic field and hence, electromagnetic lenses could be used to converge electron beam to a focus (Hans Busch, 1926). These two discoveries laid the foundation for the concept of electron microscopes. In 1933, Knoll and Ruska built the first working transmission electron microscope (TEM) which immediately surpassed the resolution of visible light microscopes. Transmission electron microscopes are called so because the electrons (light source) transmit through the specimen and creates a “shadow” of the object (specimen) in the projection plane. Thus, the specimen needs to be thin enough to be “electron transparent” such that the building blocks of the specimen and other finer details are visible.
Even though the theoretical resolution of an electron microscope (using high energy electrons of wavelength 0.0027 nm) is about 0.01 Angstrom, practically a resolution of only about 1 Angstrom is achieved. Angstrom is usually the unit of length used to measure the distance between two atoms in a molecule or a crystal. Resolution in a TEM depends on several other factors, including the build of the electron source, apertures, and lenses in the path of the electrons. Any lens system has aberrations (spherical aberration) which basically makes a point object appear like a blob at the image plane. This hampers the resolution of the microscope. After many years of research and development, microscopes were developed with corrected lens aberrations (1997) which, after further improvement, now gives a resolution of about 50-70 picometer (pm) which is roughly the size of a hydrogen atom. With recent advances in TEM, atomic columns can be viewed clearly leading to precise observations on the position of different atoms, presence of atomic impurity, or a lack of an atom (vacancy). The materials scientists finally have one of the most powerful equipment to look within a material at the level of its building blocks or atoms!
Why is it important to look at the building blocks of materials?
Let us look at an example of a study on Molybdenum disulphide (MoS2), where the materials scientists and industries reaped benefit from observing the presence of defects like vacancies in the material. Vacancy in a material means the absence of one or more atoms at the lattice site (a site where the atom was allocated to sit). Molybdenum disulphide (MoS2) is a 2-dimensional layered material which shows semiconductor properties and is a potential candidate in building thin and flexible electronics. Earlier, it was impossible to comment accurately on such defects in a material. However, with the advent of TEM, correction of aberration in its lens system, and advancements in image processing it is possible to spot sulphur atoms and their absences (holes/ vacancies) in the MoS2 sheet. The development of a semiconductor and modification of bandgap in a semiconductor lies in the generation of its charge carriers (holes or electrons) and prevention of the recombination of the charge carriers. The charge carrier density affects the bandgap of the material. Earlier, the creation of defects like vacancies was only hypothesized while attributing the reason for a modified bandgap. Now the scientists have direct proof by literally seeing the vacancy in the building block of the material, thus making Feynman’s vision a reality.
The development of an electron microscope and its contribution as an indispensable tool to materials scientists and now biological scientists have come a long way. 2017 Nobel prize in Chemistry was awarded for the development of cryo-electron microscopy for high-resolution structure determination of biomolecules in solution. As I write this article today, the Kavli Prize 2020 for the field of nanoscience has been awarded to four eminent scientists with contribution to the development of aberration-corrected TEM. Electron microscopy is still in the process of evolution. Along with the improvement of the microscope, a lot of work is being done on post-processing of data, methods for large scale data analysis, and image analysis. Thus, the field of electron microscopy is evolving rapidly to develop new interfaces with biological science, computation, and data science along with materials science creating immense opportunities for new discoveries to happen.
Author:
Dipanwita Chatterjee is currently a post-doc at Norwegian University of Science and Technology, Trondheim and she works on method development in Transmission Electron Microscopy to study broadly materials for transport. She has a knack for simplifying science for a better understanding of people from a broader field. She loves to travel to places around the world, experience new food and culture, meet new people. She has an interest in learning about people from different walks of life and their work. She is trained in Odissi which is one of the 8 Indian classical dance forms. She is at a juncture in life where she wants to find her professional niche from the versatile interests that she has. Connect with her on Instagram or LinkedIn or drop an email at dipanwita.chatterjee06@gmail.com to share creative ideas or talk about any interesting project that you might want to collaborate on.
Editors:
Rajamani Selvam received her Ph.D. in Neuroscience at the University of Connecticut Health, Farmington, CT. She is currently pursuing a fellowship where she studies the blood-brain barrier. She is interested in a career in science policy/regulatory affairs. She spends her free time by performing demonstrations and hands-on exercises for STEM activities to middle and high schoolers. She is also a mentor for 1000 girls 1000 futures program and Freedom English Academy. Away from science, she is an artist and enjoys leisure travel.
Amrita Anand is in her 4th year of Ph.D. in Genetics and Genomics at the Baylor College of Medicine, Houston. She studies the reprogramming potential of certain key factors in the regeneration of mouse inner ear hair cells. She has been actively pursuing Science communication over the last three years as she enjoys bridging the gap between scientists and non-experts. As an editor, she wants to make science more accessible to the public and also hopes the hard work behind the science gets due credit. Twitter handle- @_amritanand
Cover image- CSIRO Science image and Pexels
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