An Excursion to Nano-Land on Penn State's Ultra Stable Scanning Tunneling Microscope
by Tom Imerito
Several years ago, while working on a story about nanotechnology, I had the good fortune to come upon a friendly and generous Penn State professor of physics and chemistry, named Paul Weiss. (Paul now heads UCLA’s California Nanosystems Institute) Little did I know when we first met, that Penn State’s Weiss Group Laboratory was home to the mother of all scanning probe microscopes (SPMs), the renowned ultra-stable, extreme-high-vacuum, low temperature scanning tunneling microscope (STM), of which there are fewer than 10 in the world.
STMs resolve objects that defy capture by conventional optics. The objects are invisible because they are smaller than one-half the wavelength of visible light. STMs work by subjecting a sample to a one-atom-wide stream of electrons from a very sharp, metal, needle-like tip, which, when extremely close to the sample, makes a mind-boggling leap into the land of quanta where, under certain conditions, electrons can “tunnel,” or relocate from the tip to the sample, in apparent disregard for the laws of classical physics. As the very sharp STM tip and a sample material are brought together, the first glimpses of tunneling current are used to activate a set of piezoelectric pillars that deform in response to the current, keeping the tip a uniform distance from the sample, thereby scanning a topographic map of the surface at extremely small scales.
When I learned that Weiss’s STM was capable of resolving objects as small as 100 femtometers, (roughly one-thousandth the diameter of an atom), I became captivated by the idea of “seeing” things with which light has great difficulty coping.
My proverbial Virgil on my descent into this invisible world was a brilliant and affable fifth-year doctoral student named Thomas J. (T.J.) Mullen, who, the day before, had won the prestigious Rustum and Della Roy Award for his work in Self and Directed Assembly of Precise Chemical Patterns Exploiting Intermolecular Interactions.
I spent the morning under Mullen’s tutelage, getting to know his personal STM. After imaging some organic molecules on a gold substrate, we went next door to go for a ride on the ultra-stable, extreme-high-vacuum, low-temperature STM, the reason for my visit. For those of us who associate the word microscope with objectives, barrels, stages, glass slides and focusing knobs, calling this apparatus a microscope is a little like calling an aircraft carrier a boat. Leaving just enough space for a researcher to get around, the ultra-stable STM fills a 12-foot by 18-foot basement room whose foundation is situated beneath the subfloor in an excavation that is isolated from the rest of the building by dead space.
Inside the double-doored room, an abundance of stainless steel tubes, flanges, chambers, peepholes, pressure locks, pneumatic lifts and dosing rams extends about 10 feet above and five feet below the floor. While snooping around, I spied a barrel-sized vessel suspended in the hole beneath the floor. This was the low-temperature vacuum chamber…the place where it all happens… or rather, almost nothing happens. This is where, once a sample is in place, the air is evacuated, so there are no vagrant molecules floating in the atmosphere, and the heat is removed until molecular vibrations slow to the point at which, for purposes of observation, they are negligible. By the time scanning starts, the environment inside the sample chamber is sufficiently stable to permit weeks-long, time-lapse movies of molecular and atomic activity to be made.
Just outside the chamber, on the STM’s computer, a researcher named Rong Zhang imaged a single layer island of oxygen atoms on a palladium substrate at a temperature of 4 Kelvin (in Fahrenheit, that’s 452 degrees below zero). I asked if she would record an image for me, which she happily did, thereby making me the proud owner of an image of 83 oxygen atoms laid out in a remarkably precise hexagonal grid. With that, my mission was complete. I have seen the invisible. And I have the evidence to prove it.
This article first appeared in Tom Imerito’s TEQ column, Innovation Chronicles.
© Copyright 2007, Thomas P. Imerito / dba Science Communications