Carbon footprint
The carbon footprint is a proven variant of the surface footprint for the investigation of the surface morphology of massive objects with a transmission electron microscope (TEM).
The carbon footprint consists of a thin (approx. 10 to 20 nm) layer of carbon , which is produced in a vapor deposition apparatus in a high vacuum . For this purpose, two electrodes made of spectral carbon are brought into contact with each other either by spring pressure or by movement with a vacuum-tight rotary feedthrough and heated with 20 to 50 A alternating current at the point of contact so that the carbon evaporates. It has proven to be advantageous if the electrodes are separated from one another at a small distance while the current is flowing, so that the evaporation takes place from an arc. With longer interruptions after short bursts of evaporation, heat-sensitive samples can be protected. The sample to be vaporized is arranged in such a way that the carbon atoms reach it at a distance of about 10 cm, predominantly directly, without colliding with the remaining molecules in the air , and condense there to form an amorphous, mechanically and chemically very resistant layer . In order to map the relief of the sample to be examined, the vapor deposition takes place at a certain angle of inclination (e.g. 45 °). A carbon layer is created with a thickness distribution that reproduces the relief, analogous to the shading when exposed to light. In the case of surfaces that rise in the steaming direction, the thickness is greater than that of surfaces that slope down. On the screen of the electron microscope and in the positive photographic image, thicker object areas appear darker than thinner ones. Steeply rising steps appear, depending on whether they rise or fall in the steaming direction, dark or light, individual elevations result in a light cast shadow on the side that was turned away from the steaming source . The vapor deposition process can be easily controlled if a piece of paper with a right-angled edge is arranged next to the object to be vaporized in such a way that part of the paper is shaded from the edge up so that the formation of a vapor shadow can be observed.
In order to bring the carbon layer into an electron microscope as a surface impression, it has to be detached from the original object. The detachment of the carbon layer from the original object is the real problem and requires a different approach depending on the material properties of the original object. In the simplest case, the carbon layer can be removed by carefully pushing the object coated with carbon into water when the water penetrates between the object surface and the vapor layer due to its surface tension. In other cases, the original object is dissolved in a suitable solvent or, in the case of metals, in dilute nitric acid or hydrochloric acid , and the detached layer is fished up with a glass rod and transferred to water. The carbon footprint, which has been cleaned of intermediate products of the detachment process, is fished from the surface of the water with a mesh screen and, after drying, placed on filter paper in the object space of the electron microscope. The circular mesh panels with a diameter of 2 mm either consist of a fabric made of bronze wire with a diameter of 20 micrometers or are galvanically produced from copper with bars of approximately the same width. The mesh size of the mesh panels is 0.1 mm.
For this work you need small Petri dishes , a binocular dissecting microscope , sharp watchmaker tweezers and a steady hand.
When the original object is made of a polymer such as polyethylene , the attempt to dissolve directly fails because the original object swells when it is dissolved, thereby destroying the carbon layer. The solution to the problem was found in the form of the self-shadowed carbon footprint with a metallic support layer for very rough fracture surfaces of polyethylene. In this process, after vapor deposition of the carbon, gold , about 20 nm thick, is vapor deposited while rotating the object in such a way that all protrusions of the original surface are enveloped with gold on all sides. The object was then galvanically coated with a copper layer approximately 0.1 mm thick, so that the carbon layer was reliably protected when the polyethylene was subsequently dissolved in boiling xylene . Then the copper was dissolved with nitric acid and the gold after adding hydrochloric acid. As a result, extremely rough surfaces could also be imaged with the transmission electron microscope, as can actually only be achieved with the scanning electron microscope (see photo).
It often proves to be useful not to make the carbon imprint directly from the original object, but from a varnish matrix. This is made by covering the surface of the original object with a solution of nitrocellulose in amyl acetate, so that a thin layer of varnish is created after the solvent has evaporated. This layer of lacquer is removed from the original object. The carbon print is then obtained from the contact side of the paint print as described above. It is then a two-step carbon print or die print.
Individual evidence
- ↑ Heinz Müller: Preparation of technical-physical objects for the electron microscopic examination. Academic publishing company Geest & Portig, Leipzig 1962.
- ↑ Heinz HW Preuß: fracture surface morphology and character of the fracture of polyethylene bodies, doctoral dissertation, University of Leipzig . Leipzig 1963.
- ↑ HHW Preuss: A morphological contribution to the brittle breakage of polyethylene . In: Plastics and rubber . Volume 10, issue 6. VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig 1963, p. 330 -335 u. 338 .