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The field ion microscope (FIM), developed by Erwin Müller in the 1950s, enabled the first direct observation of individual atoms on a surface, notably capturing images of platinum atoms in 1955. This was a significant milestone in the field of microscopy and surface science. Here's how it was done and why individual atoms could be observed:
How Field Ion Microscopy Works:
1. Sharp Tip and High Voltage: The FIM operates by placing a sharp metal tip, often made of platinum, in a strong electric field created by applying a high voltage between the sample tip and a phosphor screen. The tip is cooled to very low temperatures using liquid helium. The sharpness of the tip is crucial, as it enhances the electric field strength at the tip due to the geometric effect (field enhancement at sharp points).
2. Ionization of Gas Atoms: A gas, typically helium or neon, is introduced into the vacuum chamber where the FIM is housed. The strong electric field at the surface of the sharp tip ionizes the gas atoms when they are close to the tip.
3. Projection of Ions: The ionized gas atoms (ions) are accelerated away from the tip towards a phosphor screen due to the high voltage. The trajectory of these ions depends on their point of ionization near the tip surface, effectively projecting the surface atomic structure onto the screen.
4. Magnification: The arrangement of the ions striking the phosphor screen reproduces the arrangement of the atoms on the tip, magnified by a factor of about 10 million times. This magnification allows individual atoms to be visualized as discrete points of light on the screen.
Why Individual Atoms Can Be Seen:
Despite the naturally smooth surface and electron cloud of platinum, individual atoms can be seen under a field ion microscope due to several factors:
1. Quantum Mechanical Tunneling: The phenomenon of field ionization, integral to FIM, relies on quantum mechanical tunneling. The strong electric field near the tip causes the tunneling of electrons from the gas atoms to the tip, leading to ionization just above the atomic sites.
2. Atomic Resolution: The ability of FIM to provide atomic resolution lies in the field ionization process being highly localized to the immediate vicinity of individual atoms on the tip's surface. This localization is enhanced by the sharpness of the metallic tip, which can be prepared to expose individual atoms.
3. Discrete Detection: Each ionized atom corresponds to a discrete point on the detector (phosphor screen), enabling the individual atomic positions to be distinguished as separate light spots. Thus, despite platinum's dense electron cloud in its bulk form, the manipulation of the electric field in FIM allows the discrete atomic structure to be resolved.
The development of the field ion microscope was a profound step forward, providing the first-ever images of individual atoms and offering a powerful tool for studying the atomic structure of materials at the highest possible resolution. This technology paved the way for further advancements in nanotechnology and materials science.
