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Twists and turns keep TEM on top

Fri, 01/14/2011 - 7:02am

Paul Headshot with Name and TitleOn Wednesday, the National Institute of Technology and Standards--more simply, NIST--demonstrated that in microscopy, there’s always a new way "slice a sample".

In 1931, when the first transmission electron microscope was built in a laboratory in Germany, researchers tested their handiwork by firing the electron gun at mesh gratings, gauging the ability of their instrument to deflect electrons. In recent work at NIST, the grate has become a star player. Built using nanoscale openings, the new grating uses the shape of its apertures to finesse the orbital momentum of electrons generating by the gun, creating a corkscrew shape in free space.

ElectronsNIST-175

NIST researchers twisted the flat electron wavefronts into a fan of helices using a very thin film with a 5-micron-diameter pattern of nanoscale slits, which combines the wavefronts to create spiral forms similar to a pasta maker extruding rotini. This method produces several electron beams fanning out in different directions, with each beam made of electrons that orbit around the direction of the beam.

By "twisting" the angular momentum of the electrons by 100 times, NIST can fan the beam 10 times more widely than a conventional electron microscope. The wider beam deflects within the sample more readily, offering the possibility of allowing TEM to be used on a greater variety of specimens, such as biological samples, which tend to be invisible the powerful beams.

The nanograting is the latest in a stream of inventions that has positioned TEM firmly near the top of the commercial lab technology food chain, with a price tag to match. The TEM has a deserved reputation among researchers. It’s big, It’s capable. It’s complex. It’s finicky. It’s pricey. And it requires a healthy dose of training and experience to extract the true value of its prodigious resolving power. In short, it’s like the nuclear reactor of the lab; nothing else can perform the way the TEM can, but few inventions cost so much to build and demand so much expertise to create and operate.

And yet, this hasn’t stopped demand for TEMs, which command a hundred million dollar market despite the limited array of customers. Because we can’t do without it’s ability to so handily outperform the best optical microscopes, the technology base has received almost continual attention since Max Knoll and Ernst Ruska at Siemens in Germany improved on earlier cathode ray oscilloscope designs to create that very first TEM. The idea that a magnetic “lens” could be used to deflect a beam of electrons (a cathode) dated back to the mid-19th century, but, amazingly, Knoll and Ruska had, at the time, no concept of the wave nature of electrons. They were more concerned with the function of the devices to ever think about putting an actual specimen in the beamline. In 1932, de Broglie’s hypothesis, by then a 5-year-old paper, came to their attention. Mathematically, the theory suggested that electrons, whose wavelength is a tiny fraction of the wavefront of light, could be used to show much finer detail than any conventional microscope, even down to the atomic level.

TEM1-125

The first practical TEM, Originally installed at I. G Farben-Werke and now on display at the Deutsches Museum in Munich, Germany

They quickly abandoned test images of mesh grids and gratings and began inserting samples of objects in the path of the electron beam. The first image to show better resolution than an optical microscope? Cotton fibers, which were promptly destroyed by the electron beam.

In 1939, the first commercial TEM was built. After World War II, a steady stream of improvements emerged: sample preparation techniques, more advanced electromagnetic lens and ultrahigh vacuum equipment, and field-emission guns to allow scanning-type TEMs.

Interestingly, the give-and-take dynamic between universities and electron microscope vendors that emerged after World War II continues to this day. Publicly-funded institutions push the limits of electron beam technology to solve theoretical problems, while industry partners provide technical and manufacturing resources, as well as experience, in return for the regular addition of features to meet new industrial applications. Perhaps the most advanced recent example has been the collaboration between FEI Company, Lawrence Livermore National Laboratory, and others on the TEAM microscope, an R&D 100 Award winning STEM (scanning transmission electron microscope) that is capable of resolving to one-half Angstrom.

While the TEAM microscope may live up to capable but cantankerous reputation of previous TEMs (which often require their own sound-proof enclosures and gigantic vibration-free stages) that’s the price to pay for the world’s best imaging resolution. Not many can pay for it, so hopefully NIST’s innovation, and its potentially small price tag as an additional feature in existing TEM designs, is a sign of things to come.

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