How do you make an atomically sharp needle?
September 3, 2014
Scanning Tunneling Microscope (STM), the Nanoscope I. As I was a largely unemployed writing student in those days, I often drove my 1979 diesel Rabbit up to Santa Barbara to hang out at the lab.Back in the late 1980s, my friend Dan was a student at University of California, Santa Barbara and an employee of Digital Instruments, one of the very first companies to sell a practical commercial
STMs were the first microscopes to be able to resolve individual atoms (they have since been largely supplanted by atomic force microscopes). Atoms are smaller than wavelengths of light so they can't be seen, but they can be detected. A tungsten needle, called a tip, was held above the sample not too differently from the needle in a record player. It was moved by piezoelectric crystals, which are sensitive enough to move the tips in sub-nanometer increments. The Nanoscope would oscillate the tip back and forth across the sample, scanning its whole surface; and every time it got close enough to an atom on the surface, an effect called quantum tunneling would occur in which electrons from an applied charge would "tunnel" through the space between the tip and the sample (that space is a vacuum, because there's not enough room for air molecules). The topography could then be displayed in a 3D image on a computer screen.
For this process to work, the tungsten tips would have to be atomically sharp. Sometimes the tips would "crash" (strike the sample) and would no longer be usable, so Nanoscope owners often had to buy new supplies of tips. (I believe they were priced at $5 each in those early days.) My friend Dan, before he became an engineer there professionally, worked part time making these atomically sharp tips.
How? Well, it seems a difficult problem, which Digital Instruments had solved by trial and error. You take a spool of tungsten wire, and cut it with scissors. They had tried many different brands and models of scissors. The winning pair with the best success rate was a cheap model from the local discount megastore. Cut tips would be observed under an electron microscope, and (believe it or not) if they looked sharp under the microscope, they were almost certain to actually be atomically sharp. Somewhere in the world, someone resolved atoms using tips that I cut with a pair of K-Mart scissors.
If only all seemingly insurmountable problems in life were so easily solved.
As a footnote, I feel compelled to mention that Dr. Virgil Elings, the founder of Digital Instruments and an avid motorcycle racer, once invited all employees to a day at the Jim Hall Kart Racing School, with $20 offered to anyone who could beat him. I tagged along and kicked everyone's ass, beating Virgil by a tenth of a second. He refused to pay up, claiming the offer was for "employees only." I remain bitter to this day. ;-)
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