Questions about samples consisting entirely of a collection of microparticles come from people involved in a wide variety of industries dealing with powders, from Pharmaceuticals to industrial minerals, powdered coatings to industrial hygiene concerns. The questions often asked are morphological. How big? How long? How wide?, and often pose a special problem for the microscopist. The time and tedium associated with answering these questions for a significant subset of the sample particles without an automated system causes the cost to skyrocket and sends the requester off to the light scatterers who at least give cheap answers, never mind that nobody knows what they really mean (What is the average size of a particle with an aspect ratio of 5? And did you calibrate your scattering system for my particles in my dispersant or are you still using latex spheres in water?). A common additional question is “How many are quartz, or titania, or organic, or….?” The inherent advantages of microscopy—the ability to determine detailed morphology in two dimensions (and maybe three), and to discriminate different particle types in a mixture—were usually overwhelmed by the cost and time involved.

The preparation of electron micrographs for 2″ x 2″ slide presentations is usually done by photographing EM prints with a 35 mm camera. However, for some of our better electron micrographs, many of us have enjoyed printing the negatives directly on 2″ x 2″ Kodak Projector Slide Plates (#s 140 6875 & 140 6867), which can yield projected images of particularly pleasing tonal quality.

A previous issue of Microscopy Today (issue 94-2, March 1994} contained a brief article my me describing a method for making 2″ x 2″ EM slides with Projector Slide Plates. Unfortunately, it now appears that Kodak has raised the price of these plates to a level that is well beyond what most of us can afford. My first inkling of this came after the article was in the hands of the editor. I telephoned Kodak and confirmed that the price was now about $560 per box of 36 plates, roughly 10 times the former price. However, it still seemed possible that this had been an oversight on the part of Kodak, and that it could be rectified.

It was suggsted by a few that since the film is outgassing it is OK to backfill a TEM with lab air which has a high moisture content because the film in the camera chamber has a significant outgassing rate. My general comment is that you want to keep as much water and oxygen out of a vacuum system as possible. There is no need to add more to what may already be there.

In a vacuum system without any leaks or severe outgassing problems and with properly cleaned surfaces, the major factor which limits the ultimate pressure and pumpdown time is the water adsorbed on the exposed surfaces of the system. Depending on the type of surface, water molecules car either adsorb or chemisorb onto the surface. A chemisorbed species is characterized by having an exchange of electrons with the atoms on the surface and will have a higher energy associated with the desorption of the molecule from the surface.

A quiet report from an international work shop, this fall in London on World Ecology, points out how primitive our knowledge of basic soil science really is. It also records the determination of the participants to bring out the need to study soil in relation to maintaining biodiversity because of man's rapid destruction of global habitat and the inability for soil to recover without it's natural canopy of plant and animal life. It is further pointed out that we have to quickly learn how to reclaim soil, perhaps by artificial means, after insult. The programs they envision will create exciting professional opportunities both for career change and for students over a broad spectrum from basic research to applied engineering. The land fill crises has further underscored the need to find ways to return sewage sludge and other organic waste to useful crop production.

A variety of energy emissions occur as a result of primary beam interaction with the specimen surface. Secondary electrons, x-rays, visible photons, near IR photons, and Auger electrons are emitted during inelastic scattering of electrons. Backscattered electrons (BSE) are emitted during elastic scattering of primary electrons. Backscattered electrons are those electrons which pass through the electron cloud of an atom and change direction without much energy loss. BSEs may diffuse into the sample or may escape from the sample surface. In practice, the primary electron beam penetrates deeply into low Z (atomic number) materials and produces few BSEs while high Z materials retard primary beam penetration and emit large numbers of BSEs. According to Murata et al., the higher the atomic number, the smaller the mean free path between electron scattering events (i.e. 528 Å for Al vs. 50 Å for Au at 30 KeV) and the higher the probability of scattering.

Part 1 of this series, in the August '94 issue of this newsletter, contained an overview of digital imaging for TEM. The key point was that digital imaging systems are getting better and costing less. This article contains a brief summary of the pro's and con's of digital imaging for TEM.

Although different laboratories have different situations, there are common issues that all can consider when evaluating the value of a digital imaging system for TEM.

Information Density: As things stand today, atypical 1024 pixel by 1024 pixel digital imaging system cannot compare with film! Typical film used for EM can display feature sizes as small as 10 μm. This is like having a film negative made up of 10,400 pixels by 7,800 pixels. Put this way it's tough to see how a factor of between 7 and 10 can be ignored.

When most of us use a microscope, we are looking through a specimen, or more and more frequently, at the “top” surface. It usually doesn't occur to us to look sideways. But that is what was done recently in the laboratory of Charles Lieber at Harvard University. As reported by Daniel Frisbie, Lawrence Rozsnyai, Aleksandr Nay, Mark Wrighton, and Prof, Lieber, an atomic force microscope (AFM) was modified to perform as a “chemical force microscope” (CFM). With the CFM they were able to precisely measure adhesive and frictional forces. These forces can be thought of as acting parallel to the surface of the specimen, whereas most observations are made perpendicular to the surface.

It is very important to be able to deliver tiny droplets of liquid onto any surface. There are two general methods for this. One, a glass fiber brush seaied into a capillary; and, the other, a polyethylene micrapipette. Both are capable of delivering droplets as small as 100 μm in diameter. Description of the glass fiber brush follows. Part II, later in this publication, will describe the polyethylene micropipette.

We have borrowed this clever idea from Fred Schneider of IBM (private communication). A bundle of 20 to 30 glass fibers, 5 to 8 mm long and about 20 μm in diameter, is sealed with a microflame into one end of a glass capillary. A melting point capillary 1 to 2 mm in diameter and open at both ends is convenient (Fig. 1), To use for small drop deposition, the capillary is partly filled (2 to 3 cm. long column of the desired liquid} so that the sealed-in end of the glass fiber bundle is fully immersed.

One of the major difficulties with the older DEC PDP-11 based EDS systems is that they were designed as standalone units. Their manufacturers believed that the systems were capable of performing all the spectral processing that the users would ever need, and therefore did not allow for transfer of data to other computer systems. While some of the vendors have attempted to offer file transfer capabilities as upgrades, typically these upgrades are based on a serial connection using a file transfer program like Kermit. Although such solutions are acceptable for the transfer of small numbers of spectra, they cannot seriously handle hundreds of spectra or digital images data without tying up the EDS computer for extended periods of time. In multi-user environments, it is often desirable to move the data taken in one users microscope session immediately, to avoid filling to capacity the disk space on the acquisition system and rendering the system unusable for the next user.

Scanning Probe Microscopy (SPM) was introduced to the scientific community twelve years ago, and its inventors (Binning and Rohrer) were awarded the Nobel Prize in Physics in 1986. In the last decade it expanded from a single technique limited to a single medium of imaging, namely ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), to an array of techniques operating in essentially any medium (vacuum, air or solution with and without electrochemical control). In all of these techniques, a probe, positioned in close proximity to the sample, scans its surface and monitors some property that is related to the surface topography or to any other surface property. Not only can surface structures and properties be investigated at ultrahigh resolution with SPM, but surfaces can also be modified by design. The latter capability is ushering in a new era of nanotechnology.

The presence of internal boundaries can significantly influence many important properties of materials, such as fracture toughness, creep, electrical conductivity and magnetic behavior. Interfacial structure, chemical composition and bonding, on a nanometer length scale, are often controlling and sought after factors influencing these properties. An electron spectroscopic technique, known as energy-loss near edge structure (ELNES) analysis, can be utilized to probe compositional and bonding variations with a spatial resolution less than 1 nm and is therefore well suited to this endeavor.

When a fast electron passes through a material in an electron microscope, it collides with the electrons bound to the atoms in that sample. As a result, the fast electron often gives up a small fraction of its kinetic energy to the bound electrons. The laws of quantum mechanics dictate that these so-called inelastic scattering events will only take place if the bound electron can gain enough energy to enter an empty energy level.

Photographic film has played a central role in the recording of microscope images since the invention of photography in the 1830s. Photography rapidly creates an accurate record of the microscopic specimen and avoids the subjectivity that was inherent in written and artistic descriptions of visual observations. Both instant and conventional films offer high resolution, the ability to record a large tonal gradation, and a stable media for long-term data storage.

Newer techniques, such as thermal, ink jet, and laser prints, have grown in popularity because they offer ease-of-use or lower per-image cost. Despite recent improvements, none of these techniques offer the resolution or stability of a photograph. In general, people have been forced to sacrifice quality and permanence in order to achieve the lower per-image cost and ease-of-use promised by alternative recording systems.

Having been brought up on monocular microscopes I find the omnipresent binocular systems a luxury. To support this viewpoint I'd like to suggest some benefits you may not have considered.

Because I'm used to monocular viewing I sometimes use two different oculars, say 10X and 25X, in order to scan quickly to find an area of interest and then to examine the detail with higher magnification. Occasionally I use both oculars simultaneously and “concentrate” on either image to the exclusion of the other. A better way is to set the interocular distance at the extreme setting most different from your own interocular distance. By moving your head about a centimeter either way you can use either ocular.