From: veritasium
For a long time, it was considered impossible to directly see atoms [00:00:24]. However, modern technology allows zooming in on a piece of metal to reveal individual atoms [00:00:00]. This capability, achieved through electron microscopy, represents a significant leap in atom visualization technology [00:00:14].
Why Visible Light Cannot See Atoms
Atoms cannot be seen with visible light because of their minuscule size [00:00:45]. Visible light has wavelengths ranging between 380 and 750 nanometers [00:00:48], whereas an atom is significantly smaller, approximately 0.1 nanometers [00:00:52]. When the wavelength of light is much larger than the object it attempts to image, the light simply diffracts, or bends, around the object, making it impossible to resolve [00:00:57].
Wave-Particle Duality and Electron Wavelengths
To visualize atoms, a medium with a much smaller wavelength is required [00:01:06]. The best candidate for this is not light, but electrons [00:01:11]. This concept stems from the wave-particle duality of matter.
In 1924, French physicist Louis de Broglie proposed that everything exhibits wavelike properties, not just light, but also matter such as atoms, molecules, and even humans [00:01:17]. The wavelength of any object is defined by Planck’s constant divided by its momentum (mass times velocity) [00:01:30].
In electron microscopes, electrons are accelerated to very high speeds, such as 300 kilovolts (kV) [00:01:39]. These “relativistic particles” can move at about 80% of the speed of light [00:01:46]. At such speeds, their wavelength is calculated to be between 2 to 3 picometers [00:01:56]. This is over 100,000 times smaller than visible light, theoretically offering a resolution 100,000 times greater [00:02:05].
History and Development of Electron Microscopes
Shortly after de Broglie’s discovery, scientists in Germany began developing microscopes utilizing these high-speed electrons [00:02:13].
The Electromagnetic Lens
A challenge arose because electrons cannot be bent using traditional glass lenses [00:02:21]. German physicist Hans Busch suggested using an electromagnetic lens [00:02:28]. Although Busch published his findings in 1926, he never built one [00:02:33].
Ernst Ruska, a young PhD student, used Busch’s paper as inspiration [00:02:38]. Ruska built his first prototype by coiling wire around iron, leaving a gap in the middle [00:02:46]. Passing current through the coil induced a donut-shaped magnetic field, which served as his lens [00:02:53].
To test it, Ruska boiled electrons off a tungsten filament (similar to an incandescent light bulb) [00:03:02]. These free electrons were then accelerated through a positively charged anode down to the electromagnetic lens [00:03:09]. As an electron approaches the lens, the magnetic field exerts a force, known as the Lorentz force [00:03:15]. This force constantly steers the electron into a circular motion, pushing it inwards and spiraling it into the center of the lens, thereby focusing the electron beam [00:03:28].
The First Transmission Electron Microscope (TEM)
By 1931, Ruska and his colleague Max Knoll built the first working electron microscope based on this design [00:03:54]. This early Transmission Electron Microscope (TEM) was basic but functional [00:04:02].
The image was formed when the focused electron beam struck an incredibly thin sample (around 100 nanometers thick) at the focal point [00:04:10]. Thinner parts of the sample allowed more electrons to pass through, creating an electron imprint [00:04:20]. A second electromagnetic lens magnified this imprint onto a fluorescent detector, producing the final image [00:04:27].
Initially, the TEM offered minimal magnification, no better than optical microscopes [00:04:39]. Ruska’s persistence in adding more lenses led to magnifications exceeding 10,000 times by the mid-1930s, far surpassing optical microscopes for imaging insects, bacteria, and viruses [00:04:50].
The Spherical Aberration Problem
Just as the TEM gained prominence, German physicist Otto Scherzer published a paper in 1936 claiming that the microscope faced an unavoidable flaw: spherical aberration [00:05:11].
Spherical aberration occurs because the magnetic field in an electromagnetic lens does not scale linearly [00:06:01]; it is stronger near the edges [00:06:05]. This causes electrons further from the optical axis to be over-deflected, focusing before the rays in the middle [00:06:08]. The result is a blurred focus spread across the optical axis instead of a single point [00:06:21]. This distortion worsens with higher magnification [00:06:28].
While optical lenses can correct spherical aberration by combining converging and diverging lenses [00:06:50], this is physically impossible with magnetic lenses [00:07:29]. All magnetic field lines must form closed loops, starting at one pole and ending at the other [00:07:45]. As electrons pass through the magnetic field, they are always pushed towards the axis, meaning all electromagnetic lenses inherently converge the beam and can never diverge it [00:07:50]. Scherzer’s proof stated that a radially symmetric magnetic lens could not diverge [00:08:30]. This presented a significant roadblock for electron microscopy, as spherical aberration seemed unavoidable regardless of electron acceleration [00:08:41].
Alternative Atom Imaging and the Scanning TEM
Due to the spherical aberration roadblock, resolution advancements in electron microscopes slowed significantly [00:08:55].
In 1955, the field ion microscope, a different technology, captured the first generally accepted images of atoms [00:09:00]. This method involved shooting helium or neon atoms at a sharp, positively charged needle tip, which ionized the gas atoms and ejected them perpendicularly to the surface, forming an impression of the tip’s atomic structure [00:09:09]. However, this method was limited to the very tip of the needle and produced unimpressive images [00:09:29].
During the 1980s and 1990s, probe microscopes emerged, which could also image atoms [00:13:25]. These worked by gliding a tiny stylus across a sample, detecting variations in quantum effects or nanoscale forces to map the surface [00:13:29]. Lacking lenses, they weren’t limited by spherical aberration and could even produce 3D images [00:13:41]. Yet, this was described more as “feeling atoms” rather than “seeing atoms” [00:13:51].
Meanwhile, work continued on the transmission electron microscope [00:11:06]. British-American physicist Albert Crewe made significant advancements, replacing the random electron source with a more directed one that pulled electrons off a sharpened tungsten tip using a stronger electric field [00:11:17]. This produced a narrow beam over a thousand times brighter [00:11:34].
Crewe paired this new beam with technology inspired by cathode ray tube (CRT) televisions [00:11:43]. Just as CRT TVs scan an electron beam across a phosphor-coated screen to create an image, Crewe designed a scanning electron beam for the TEM [00:11:51]. Instead of creating a full imprint at once, Crewe’s scanning TEM (STEM) mapped the sample bit by bit [00:12:10]. Though Manfred von Ardenne had built an early prototype in the 1930s, Crewe revived and drastically improved the design, producing the first image of single atoms with an electron microscope in 1970 [00:12:22].
Despite these advancements, Scherzer’s spherical aberration problem persisted, setting a hard limit on resolution [00:13:05]. Crewe himself eventually gave up on correcting it [00:13:12].
Aberration Correction: Breaking the Symmetry Limit
A crucial insight emerged: Scherzer’s theorem proved a diverging, radially symmetric lens was impossible, but if radial symmetry was abandoned, the theorem might not apply [00:14:04]. While breaking symmetry typically distorts the image, three scientists — Knut Urban, Max Haider, and Harold Rose — pursued a “crazy idea” [00:14:26]. They hoped to find a small, diverging part within a purposely distorted image that could correct the spherical aberration [00:14:46].
Their solution involved using a complex system of electromagnets with six, eight, or even ten separate coils, known as hexapole, octopole, and decapole magnets, to create bumpy magnetic fields [00:15:08]. As an electron beam passed through a hexapole, it would twist and squeeze the 2D image into a triangular saddle shape, stretching the interior and creating a slight concave bow in the middle—an effect of small divergence [00:15:22]. They then passed the beam through a second hexapole that worked in reverse, unbending the image back into a circular shape [00:15:45]. Theoretically, the remnants of that tiny divergence in the center would then have spherical aberration pointing in the opposite direction [00:15:55].
If their calculations and engineering were precise, this system could almost completely counteract spherical aberration [00:16:05]. Many in the field doubted its feasibility [00:16:15].
In May 1997, with only two months of funding left, their new lens iteration was still on the drawing board [00:16:29]. Miraculously, by July 23rd, just a week before funding ran out, the lens was ready [00:16:41]. After initial instability, they switched off the equipment for 24 hours to allow the magnets to settle [00:16:50]. At 2 AM on July 24th, the picture began to stabilize, revealing clear images of atoms with no aberration [00:17:00].
After over 60 years of failed attempts, Urban, Rose, and Haider achieved the seemingly impossible, reducing the TEM’s resolution to 0.13 nanometers [00:17:19]. This breakthrough transformed average TEM images from blurry to remarkably clear [00:17:31].
Knut Urban initially presented these results at a small, unnoticed conference session due to the group’s reputation [00:17:38]. However, as word spread, a crowd of hundreds formed to witness the stunningly sharp images [00:17:50].
Shortly after, Ondrej Krivanek independently achieved the same aberration correction for Crewe’s scanning TEM (STEM) [00:20:07]. In 2020, all four were awarded the prestigious Kavli Prize in Nanoscience for their accomplishments [00:20:18].
Current Importance
Aberration correction is now essential for seeing atoms and measuring atomic distances [00:20:36]. It is a game-changer for research in materials science, materials engineering, and chemical engineering, enabling scientists to relate material properties directly to their atomic structure [00:20:50]. Consequently, almost every university today requires a microscope with aberration correction [00:21:07].