Formvar films

The thickness of a film Formvar 100 mesh layer on a grid is more robust than for one with a smaller apertures, such as a 400 mesh, for example (SPl Supplies). Typically, the so-called holey films are the most suitable for the analysis of powder catalysts. Figure 12.14a, b show images of a 400 mesh grid coated with a thin carbon layer sample without and one with a catalyst sample of AU/AI2O3, respectively, by the deposition method of alcohol droplets. 

Fig. 114. Transmission electron micrograph (TEM) at limiting aperture coverage of PbS crystals formed by the slow (30 min) infusion of H2S to an AA monolayer in the Lauda film balance (kept at it = 26 mNm 1 surface pressure) floating on an aqueous 5.0xl0 4M Pb(N03)2 solution. The PbS particulate film was deposited on a formvar-coated, 200-mesh copper grid [647]...

Saran (Dow polyvinylidene dichloride) is a tough, chemically resistant plastic available in a variety of forms that are useful in the laboratory. Saran pipe or tubing can easily be welded to itself or sealed to glass and is useful for handling corrosive solutions. Thin Saran film, available commercially as a packaging material, is useful for windows, support films, etc. Mylar (du Pont polyethylene terephthalate) film and other polyester films are also useful for these purposes. Mylar is chemically inert and has excellent electrical properties for electrical insulation and for use as a dielectric medium in capacitors. Much thinner than these are films that can be made in the laboratory by allowing a dilute ethylene dichloride solution of Formvar (polyvinyl acetal) to spread on a water surface and dry. 

Since most materials are opaque to the electron beam, even when only a few hundred nanometers thick, special problems arise in the production of suitable mounted specimens. Specimen support films are usually made of plastic or carbon, though other materials have also been used. Suitable film solutions may be made up of 2% w/v formvar (polyvinyl formal) in ethylene dichloride or chloroform. 

An alternative procedure is to clean a microscope slide with detergent and polish with a clean cloth without rinsing away the detergent, so as to form a hydrophilic layer at the surface, which facilitates stripping. The slide is dipped in a solution of formvar in ethylene dichloride (0.3% to 0.7% depending on the film thickness required) and allowed to drain dry. The film may be floated on to a water surface and mounted on grids as before. If individual grids are required, the film may be cut into small... 

The procedure for preparing holey carbon films (carbon film containing small holes of 100 nm diam.) is somewhat more complex. In this case the microscope slide is initially dipped into a 0.25% solution of formvar in ethylene dichloride and then held over a source of water vapor for sufficient time for an array of droplets to collect on the formvar surface. Excess formvar is drained off the slide and allowed to dry. During this stage the water droplets evaporate leaving behind a wide size range of bubbles in the formvar film. Carbon is evaporated onto this surface following the procedure described above so that the eventual result is the formation of a holey carbon film. 

Figure 6. Anomalous reduction of scattering intensity from H of Formvar (Ch/L/iLL), as a function of momentum transfer q [Chatzidimitriou-Dreismann 2003 (a)]. The results are normalized by dividing the measured cross section ratios of H to C + O by the "conventional" ratio 21.6. VESUVIO results for full squares 0 1 mm formvar foils and open circles 0 2 mm foils. Large open triangles high-energy electron-proton scattering measurement from Formvar films of 50 — 100 A thickness, as described in Ref. [Chatzidimitriou-Dreismann 2003 (a)].

Using an electron spectrometer with an improved energy analyzer, Vos observed ECS from protons of formvar [Vos 2002 (a) Vos 2002 (b)]. This is an amorphous polymer widely used in electron microscopy since it makes extremely thin and flexible films and contains no double bonds (producing additional peaks in the energy loss spectra thus obscuring the ECS-peak). 

Figure 11. ECS energy-loss spectrum of a formvar film taken using 25 keV electrons, resulting to a mean momentum transfer hq to protons with q = 61.8 A-1. Inset the fit of the //-recoil peak (about 8 eV) with a Gaussian, before and after subtraction of the background taken from [Chatzidimitriou-Dreismann 2003 (a)].

Various thin films of formvar (about 50 — 100 A thick) were prepared by standard procedures. An ECS spectrum (which is a function of energy transfer AE from the electron to the impinging nucleus) is shown in Fig. 11. As is done in NCS investigations [Sears 1984 Watson 1996], the electron spectra were fitted with Gaussians. The "background" of the peaks is mainly due to interactions of the incident electrons with electrons in the sample. 

Figure 14. Anomalous reduction of NCS and ECS intensity from H of formvar, as a function of momentum transfer hq. The q-range corresponds to scattering times tsc 200 — 1000 x 10-18 s. Small squares and circles values of ratios R, XJ> of NCS peak-areas measured in the detector angular range 32° — 68°, relative to the conventionally expected value RCOnv Full squares (open circles) results for foils of thickness 0.1 mm (0.2 mm). Large open triangles ratios Rexp/Rcon-u measured by ECS from films of 50 -100 A thickness, using electrons with kinetic energies 15 - 30 keV [Chatzidimitriou-Dreismann 2003 (a)].

Scanning transmission electron microscopy (STEM), coupled with EDX, has been used to determine metal particle sizes. The specimens for STEM are prepared by dispersing the sample ultrasonicaUy in methanol and placing one drop of the suspension onto a Formvar film supported on a copper grid. 

There are many difficulties in transferring monolayer samples for electron microscope studies—the evaporation or removal of the interposed water film between the monolayer and the Formvar, vibration, mechanical problems, and various other strains. Therefore, we do not claim a one-to-one correspondence between the state of a film on the water surface and the structures observed in the micrographs. Nevertheless, it is of interest to compare the sequence of changes that occur during... 

The blank sample of Figure 3A was obtained by raising the Formvar-covered screen through a clean water—air interface before the film was spread. Such micrographs establish both the flat smooth surface of the Formvar as well as the fine texture of the vapor-deposited germanium. 

Samples for transmission electron microscopy were prepared in the following manner. Films were grown with different current rates up to various thicknesses on platinum working electrodes. The film thickness was controlled by the period of current flow. The films were transferred onto carbon coated electron microscope grids by stripping with formvar and subsequently removing the formvar with methylene chloride. As-synthesized films were directly used for scanning electron microscopy. 

Plastic films can be formed from a number of polymers two of the most common are collodion (nitrocellulose) and polyvinyl formal (Formvar). A number of other plastic materials can be used for thin films, some with specific advantages (101). Plastic films are usually prepared either by spreading a drop of solution of the plastic (in an appropriate solvent) onto a clean water surface, or by stripping off a film that has been coated onto a clean, flat surface (such as a glass slide) onto the surface of clean water. 

A micro-adaptation of energy dispersive X-ray fluorescence (EDXRF) has been used to compare the Ni content of normal and carcinomatous human breast tissue (Rizk and Sky-Peck, 1984). Surgical specimens were homogenized (200 g/L in water) in a Teflon homogenizer. Ten 5 aliquots of each specimen were layered onto Formvar films in a slide holder, air-dried under laminar flow, and placed in the vacuum chamber of the instrument at 200 /im of mercury. By X-irradiation for 500 s (18 mA, 35 kV) 16 elements were determined simultaneously. The Ni scattered peak was observed at 7 keV and Ni concentrations of about 1 mg/kg dry wt. were caiculated after computer correction of the background continuum, as described (Sky-Peck and Joseph, 1981). 

Extensive documentation on the preparation of holey film substrates for cryo-TEM sample preparation is available in the literature. Holey films or nets have been produced [10,25-27] from a glycerol-water mixture in a solution of a mixture of formvar and triafol in dichloromethane spread on mica. After floating the thin film off the mica support, the films were treated with ethyl acetate to transform pseudoholes into holes or nets. 

Procedure. The material to be studied is obtained by the microspreading method for SCs (see Section V). The support used is copper grids (100-200 mesh) covered with Formvar film (0.8% in 1,2-dichloroethane (Pelco International, Redding, CA). The plastic-covered grids are stabilized by evaporation of a thin carbon film (light brown color in the porcelain or filter paper used as control) in a shadow-cast unit (vacuum evaporator). 

Slides must be coated with a thin plastic film [e.g., Formvar, 1% (w/v) in chloroform]. It can also be easily prepared from polystyrene labware (e.g.. Falcon petri dishes) by dissolving 0.75% (w/v) crumbled material in chloroform. Slides are dipped in the solution and air dried. The interesting area is scaled with a scalpel or a glass cutter. Small drops of 1% (v/v) hydrofluoric acid are put on the edges (hydrofluoric acid is toxic and corrosive wear gloves and full-face protection). A thin layer of the glass is etched underneath the plastic film by the acid and the film is lifted. Drops of water are added until the film comes off... 

The most commonly used support films are either ultrathin carbon films or Formvar films stabilized with carbon. Essentially the carbon is vaporized in a vacuum chamber by passing a high current through two carbon rods. The carbon is deposited on to a... 

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