Vortex Beams opens new possibilities for electron microscopy

Vortex beams render completely new possibilities for electron microscopy. A method of producing extremely intense vortex beams has been discovered at the Vienna University of Technology (TU Vienna).

Nowadays, electron microscopes are an essential tool, especially in the field of materials science. At TU Vienna, electron beams are being created that possess an inner rotation; these vortex beams cannot only be used to display objects, but to investigate material-specific properties with minute precision. A new breakthrough in research now allows scientists to produce much more intense vortex beams than ever before.

In a tornado, the individual air particles do not necessarily rotate on their own axis, but the air suction overall creates a powerful rotation. The rotating electron beams that have been generated at TU Vienna behave in a very similar manner. Vortex beams can only be explained in terms of quantum physics: the electrons behave like a wave, and this quantum wave can rotate like a tornado or a water current behind a ship's propeller.

After the vortex beam gains angular momentum, it can also transfer this angular momentum to the object that it collides. The angular momentum of the electrons in a solid object is closely linked to its magnetic properties. For materials science it is therefore a huge advantage to be able to make statements regarding angular momentum conditions based on these new electron beams.

Peter Schattschneider and Michael Stöger-Pollach (USTEM, TU Vienna) have been working together with a research group from Antwerp on creating the most intense, clean and controllable vortex beams possible in a transmission electron microscope. The first successes were achieved two years ago: at the time, the electron beam was shot through a minuscule grid mask, whereby it split into three partial beams: one turning right, one turning left and one beam that did not rotate.

Now, a new, much more powerful method has been developed: researchers use a screen, half of which is covered by a layer of silicon nitride. This layer is so thin that the electrons can penetrate it with hardly any absorption, however they can be suitably phase-shifted. After focusing using a specially adapted astigmatic lens, an individual vortex beam is obtained.

 

More exotic applications of vortex beams are also conceivable: in principle, it is possible to set all kinds of objects in rotation, even individual molecules using these beams, which possess angular momentum. Vortex beams could therefore also open new doors in nanotechnology.

Cold atoms could replace hot gallium in focused ion beams

Scientists at the National Institute of Standards and Technology (NIST) have developed a radical new method of focusing a stream of ions into a point as small as one nanometer (one billionth of a meter). Because of the versatility of their approach—it can be used with a wide range of ions tailored to the task at hand—it is expected to have broad application in nanotechnology both for carving smaller features on semiconductors than now are possible and for nondestructive imaging of nanoscale structures with finer resolution than currently possible with electron microscopes. Researchers and manufacturers routinely use intense, focused beams of ions to carve nanometer-sized features into a wide variety of targets. In principle, ion beams also could produce better images of nanoscale surface features than conventional electron microscopy. But the current technology for both applications is problematic. In the most widely used method, a metal-coated needle generates a narrowly focused beam of gallium ions. The high energies needed to focus gallium for milling tasks end up burying small amounts in the sample, contaminating the material. And because gallium ions are so heavy, if used to collect images they inadvertently damage the sample, blasting away some of its surface while it is being observed. Researchers have tried using other types of ions but were unable to produce the brightness or intensity necessary for the ion beam to cut into most materials.

The NIST team took a completely different approach to generating a focused ion beam that opens up the possibility for use of non-contaminating elements. Instead of starting with a sharp metal point, they generate a small "cloud" of atoms and then combine magnetic fields with laser light to trap and cool these atoms to extremely low temperatures. Another laser is used to ionize the atoms, and the charged particles are accelerated through a small hole to create a small but energetic beam of ions. Researchers have named the groundbreaking device "MOTIS," for Magneto-Optical Trap Ion Source.

Because the lasers cool the atoms to a very low temperature, they're not moving around in random directions very much. As a result, when ions are accelerated, they travel in a highly parallel beam, which is necessary for focusing them down to a very small spot, explains Jabez McClelland of the NIST Center for Nanoscale Science and Technology. The team was able to measure the tiny spread of the beam and show that it was indeed small enough to allow the beam to be focused to a spot size less than 1 nanometer. The initial demonstration used chromium atoms, establishing that other elements besides gallium can achieve the brightness and intensity to work as a focused ion beam .

MONOLITHIC COMB DRIVE: A NANOSCALE MANIPULATOR

Jason Vaughn Clark, an assistant professor of electrical and computer engineering and mechanical engineering created a tiny motorized positioning device that has twice the dexterity of similar devices being developed for applications that include biological sensors and more compact, powerful computer hard drives. The device, called a monolithic comb drive, might be used as a "nanoscale manipulator" that precisely moves or senses movement and forces. The devices also can be used in watery environments for probing biological molecules.




The advantage of this device is that it can shrink the size of the overall sensor instrument. The sensors generally detect objects using two different componenets. A probe is moved while at the same time the platform holding the specimen is positioned. The new technology would replace both components with a single one - THE MONOLITHIC COMB DRIVE.



The researchers expected the sensors to work faster and at higher resolution. Also due to the single component they are small enough to fit on a microchip.The higher resolution might be used to design future computer hard drives capable of high-density data storage and retrieval. It could possibly be used to fabricate or assemble miniature micro and nanoscale machines.



Structure wise, the new monolithic device has a single structure with two perpendicular comb drives. It is so called because it contains comb drive components that are not mechanically and electrically separate. Conventional comb drives are structurally decoupled to keep opposite charges separated. Along with that there are certain advantages of comb drive overother technologies. In contrast to piezoelectric actuators that typically deflect, or move, a fraction of a micrometer, comb drives can deflect tens to hundreds of micrometers. And unlike conventional comb drives, which only move in one direction, new device can move in two directions - left to right, forward and backward - an advance that could reallyopen up the door for many applications.

SECRETS OF NANO WORLD: REVEAL BY SUPER X RAY MICROSCOPE

Researchers have been working on such super-resolution microscopy concepts for electrons and x-rays for many years. A novel super-resolution X-ray microscope developed by a team of researchers from Paul Scherrer Institute (PSI) and EPFL (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE). They combine the high penetration power of x-rays with high spatial resolution, thereby creating possibility to shed light on the detailed interior composition of semiconductor devices and cellular structures.

The new instrument uses a Megapixel Pilatus detector which has excited the synchrotron community for its ability to count millions of single x-ray photons over a large area. This key feature makes it possible to record detailed diffraction patterns while the sample is raster-scanned through the focal spot of the beam. In contrast, conventional x-ray (or electron) scanning microscopes measure only the total transmitted intensity.

These diffraction data are then treated with an algorithm. An image reconstruction algorithm was developed that deals with the several tens of thousands of diffraction images and combines them into one super-resolution x-ray micrograph explains PSI researcher Pierre Thibault, first author on the publication. Even in order to achieve images of the highest precision, the algorithm not only reconstructs the sample but also the exact shape of the light probe resulting from the x-ray beam.

Conventional electron scanning microscopes can provide high-resolution images, but usually only for the surface of the specimen, and the samples must be kept in vacuum. The Swiss team's new super-resolution microscope bypasses these requirements, meaning that scientists will now be able to look deeply into semiconductors or biological samples without altering them. It can be used to non-destructively characterize nanometer defects in buried semiconductor devices and to help improve the production and performance of future semiconductor devices with sub-hundred-nanometer features. A further very promising application of the technique is in high-resolution life science microscopy, where the penetration power of X-rays can be used to investigate embedded cells or sub-cellular structures. Finally, the approach can also be transferred to electron or visible laser light, and help in the design of new and better light and electron microscopes.