SUMMARY / RELATED TOPICS

Transmission electron microscopy

Transmission electron microscopy is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device. Transmission electron microscopes are capable of imaging at a higher resolution than light microscopes, owing to the smaller de Broglie wavelength of electrons; this enables the instrument to capture fine detail—even as small as a single column of atoms, thousands of times smaller than a resolvable object seen in a light microscope. Transmission electron microscopy is a major analytical method in the physical and biological sciences. TEMs find application in cancer research and materials science as well as pollution and semiconductor research, but in other fields such as paleontology and palynology.

TEM instruments boast an enormous array of operating modes including conventional imaging, scanning TEM imaging, diffraction and combinations of these. Within conventional imaging, there are many fundamentally different ways that contrast is produced, called "image contrast mechanisms." Contrast can arise from position-to-position differences in the thickness or density, atomic number, crystal structure or orientation, the slight quantum-mechanical phase shifts that individual atoms produce in electrons that pass through them, the energy lost by electrons on passing through the sample and more. Each mechanism tells the user a different kind of information, depending not only on the contrast mechanism but on how the microscope is used—the settings of lenses and detectors. What this means is that a TEM is capable of returning an extraordinary variety of nanometer- and atomic-resolution information, in ideal cases revealing not only where all the atoms are but what kinds of atoms they are and how they are bonded to each other.

For this reason TEM is regarded as an essential tool for nanoscience in both biological and materials fields. The first TEM was demonstrated by Max Knoll and Ernst Ruska in 1931, with this group developing the first TEM with resolution greater than that of light in 1933 and the first commercial TEM in 1939. In 1986, Ruska was awarded the Nobel Prize in physics for the development of transmission electron microscopy. In 1873, Ernst Abbe proposed that the ability to resolve detail in an object was limited by the wavelength of the light used in imaging or a few hundred nanometers for visible light microscopes. Developments in ultraviolet microscopes, led by Köhler and Rohr, increased resolving power by a factor of two; however this required expensive quartz optics, due to the absorption of UV by glass. It was believed that obtaining an image with sub-micrometer information was not possible due to this wavelength constraint. In 1858, Plücker observed the deflection of "cathode rays" by magnetic fields.

This effect was used by Ferdinand Braun in 1897 to build simple cathode-ray oscilloscope measuring devices. In 1891, Riecke noticed that the cathode rays could be focused by magnetic fields, allowing for simple electromagnetic lens designs. In 1926, Hans Busch published work extending this theory and showed that the lens maker's equation could, with appropriate assumptions, be applied to electrons. In 1928, at the Technical University of Berlin, Adolf Matthias, Professor of High voltage Technology and Electrical Installations, appointed Max Knoll to lead a team of researchers to advance the CRO design; the team consisted of several PhD students including Bodo von Borries. The research team worked on lens design and CRO column placement, to optimize parameters to construct better CROs, make electron optical components to generate low magnification images. In 1931, the group generated magnified images of mesh grids placed over the anode aperture; the device used two magnetic lenses to achieve higher magnifications, arguably creating the first electron microscope.

In that same year, Reinhold Rudenberg, the scientific director of the Siemens company, patented an electrostatic lens electron microscope. At the time, electrons were understood to be charged particles of matter. Knoll's research group was unaware of this publication until 1932, when they realized that the De Broglie wavelength of electrons was many orders of magnitude smaller than that for light, theoretically allowing for imaging at atomic scales. In April 1932, Ruska suggested the construction of a new electron microscope for direct imaging of specimens inserted into the microscope, rather than simple mesh grids or images of apertures. With this device successful diffraction and normal imaging of an aluminium sheet was achieved; however the magnification achievable was lower than with light microscopy. Magnifications higher than those available with a light microscope were achieved in September 1933 with images of cotton fibers acquired before being damaged by the electron

Johan du Toit

Johannes Willem du Toit is a South African rugby union player for the Stormers in Super Rugby and Western Province in the Currie Cup and in the Rugby Challenge. He can play as a lock. Du Toit grew up in the nearby Swartland area of the Western Cape, he was selected to represent his local Boland Cavaliers union at both the Under-13 Craven Week in 2008 and the Under-18 Craven Week in 2013. At the start of 2017, Du Toit moved to Cape Town, where he joined the Western Province Currie Cup team. Du Toit is the younger brother of Pieter-Steph a professional rugby player that represented the South African national team since 2013; the two brothers were contracted to the Sharks at the same time before reuniting at the Stormers from 2017 onwards

Dominic Daley

Dominic Daly was an Irishman who immigrated to America some time around 1800, was executed for murder, in what has been believed to be a miscarriage of justice. The date of Daly's birth and arrival in the United States has been lost, it is known that he worked in Boston, Massachusetts. In November 1805, the body of a young farmer, Marcus Lyon, was found on the open road near town of Wilbraham, Massachusetts. Daly and a fellow Irishman, James Halligan, were traveling in the area at the time, heading for New Haven, when they were arrested for the murder on November 12, 1805, in Northampton, for which their captor was paid $500; the pair protested their innocence, but were held in prison for nearly five months, being charged with highway robbery from the assault of Lyon. Despite their long confinement, they were granted defense attorneys only 48 hours before their trial. Once the trial began, they were convicted within minutes, under such flimsy evidence that one of the defense attorneys was led to declare that it was based on outright bigotry.

At their request, the Rev. Jean-Louis Lefebvre de Cheverus, the Catholic bishop of Boston, went at great personal risk to assist them in their last moments, he celebrated a Catholic Mass for them in their prison cell. This is believed to have been the first time; the next day, an estimated 15,000 people viewed the execution on June 5, 1806. The two Irishmen publicly forgave the prosecutors of the case, they were hanged. On St. Patrick's Day 1984, Governor Michael Dukakis of Massachusetts issued a proclamation exonerating Daley and Halligan. List of wrongful convictions in the United States The National Registry of Exonerations Exoneration profiles at The Innocence Project