WASHINGTON — For the first time, physicists have photographed the structure of an atom down to its electrons.
The pictures, soon to be published in the journal Physical Review B, show the detailed images of a single carbon atom's electron cloud, taken by Ukrainian researchers at the Kharkov Institute for Physics and Technology in Kharkov, Ukraine.
This is the first time scientists have been able to see an atom's internal structure directly. Since the early 1980s, researchers have been able to map out a material's atomic structure in a mathematical sense, using imaging techniques.
Quantum mechanics states that an electron doesn't exist as a single point, but spreads around the nucleus in a cloud known as an orbital. The soft blue spheres and split clouds seen in the images show two arrangements of the electrons in their orbitals in a carbon atom. The structures verify illustrations seen in thousands of chemistry books because they match established quantum mechanical predictions.
David Goldhaber-Gordon, a physics professor at Stanford University in California, called the research remarkable.
"One of the advantages [of this technique] is that it's visceral," he said. "As humans we're used to looking at images in real space, like photographs, and we can internalize things in real space more easily and quickly, especially people who are less deep in the physics."
To create these images, the researchers used a field-emission electron microscope, or FEEM. They placed a rigid chain of carbon atoms, just tens of atoms long, in a vacuum chamber and streamed 425 volts through the sample. The atom at the tip of the chain emitted electrons onto a surrounding phosphor screen, rendering an image of the electron cloud around the nucleus.
Field emitting electron microscopes have been a staple of scientists’ probing the very small since the 1930s. Up to this point, the microscopes were only able to reveal the arrangement of atoms in the sample.
The sharper a sample’s pointed tip inside the vacuum chamber, the greater the resolution of the final image on the screen said Igor Mikhailovskij, one of the paper's authors. In the last year, physicists learned to manipulate carbon atoms into chains. With the tip of the sample now just a single atom wide, the microscope was able to resolve the electron's orbitals. The Kharkov researchers are the first to produce real images of the electrons of a single atom, making the predictions of quantum mechanics visible.
While tools like the scanning tunneling microscope already map the structure of electrons in a sample of many atoms, "it's always good to have complimentary approaches," Goldhaber-Gordon said. "Sometimes something puzzling in one view becomes crystal clear in the other view. Each one gets you a step closer to a full understanding."
Goldhaber-Gordon also pointed out that the technique may not be widely applicable because the high resolution was due to the sample's specific structure.
"At the moment it's more important for displaying quantum mechanics very directly than for learning new things about materials," he said. "But that could change if [the Ukrainian team] develop new capabilities."
—Mike Lucibella & Lauren Schenkman
Inside Science News Service
Wow, so our chemistry teachers weren't lying to us! I remember peering at the little diagrams of fat blobs impaled on the x, y, and z axes, and thinking, "Yeah, right." Sure, you can solve Schrodinger's equation for a hydrogen atom and "see" for yourself, but quantum mechanics is maddeningly hard to wrap your mind around. And for those of us who can't see the answer in a math function, it's so wonderful to be able to look at a photo.
In the paper, the researchers identify the sphere and sort of dumbell shape are s and p orbitals. Carbon has six electrons total and four valence electrons, which occupy the 2s orbital and two of the 2p orbitals. That's exactly what we see in these photos. For comparison, here are the chemistry textbook versions of 2s (left) and 2p (right), courtesy of Wikipedia. The 2s orbital has been chopped in half so you can see the location of the nucleus:
Interestingly, this new observation was possible thanks to the marriage of nanotechnology and a pretty hoary piece of technology. The nanotech is pretty cool; to fabricate an atomic carbon chain, researchers peel one strip of atoms off of graphene, a single-atom-thick sheet of graphite, the 100-percent-carbon stuff in the lead of your number 2 pencil, the way you'd pull a thread out of a piece of fabric. (Earlier methods involved using an electron beam to burn a hole through a sheet of graphene, leaving just a thin line of carbon atoms joining two regions.)
Meanwhile, Erwin Muller invented the field-emission electron microscope in 1936. It works on a simple principle: get a strong, localized electric field between a sharpened sample and a screen coated with a fluorescent material, and the electric field will rip electrons off the sample (emission) and send them flying into your screen. Its successor, the field-ion microscope, pulled off whole ions, allowing Muller to see the (albeit blurrily) the atoms in the tip of a tungsten needle.
This retro tech is the key, in my opinion, to why this research is so exciting for the general public. Physicists and materials sciences have developed dozens of imaging techniques over the years. They can probe oxidation states and atomic structure with x-rays, and can make gorgeous maps of materials using scanning tunneling microscopy. The STM especially is a really incredible invention, and allows us to explore the nanoscale world and witness phenomena that are the direct consequence of quantum mechanics.
But, as Goldhaber-Gordon says, there's something visceral about these FEEM images. Let's go back to the STM for a second. The STM scans a needle-like tip across a sample surface and measures how many electrons tunnel (a quantum-mechanical phenomenon) between the surface and the scope's needle-like tip in order. Because the tunneling current decreases exponentially as a function of distance between material and scanning tip, the current can be used to adjust the microscope continuously to keep it at the same distance away from the sample it's scanning. These values are what form the resulting, often breathtaking, image.
Meanwhile, the FEEM image works on the same basic principle as a film camera, except one is a case of photons impinging a piece of film and the other has electrons splatting on a fluorescent screen. What comes out, a physical mark of the quantum world, is somehow more believable to a member of the general public (myself included) than any other explanation. There's nothing like a Hubble photograph to drive home how rich and vast the universe is; similarly, these images make the baffling laws governing the incomprehensibly small simply more believable.