Silver Coating for SEM
The following abstract, method results (introduction only) is
reproduced by kind permission of A.A. Mills.
Scanning Microscopy, Vol. 2, No.3, 1988 (Pages 1265-1271)
Silver as a removable coating for Scanning Electron Microscopy
A.A. Mills
Department of Geology,
The University of Leicester.
Leicester.
LE1 7RH.
U.K.
Abstract
A thin film of silver, applied by sputtering or vacuum evaporation, provides an excellent conformal conductive coating for scanning electron microscopy of insulating specimens. When no longer required it is easily removed with Farmer's reducer - a dilute aqueous solution of potassium ferricyanide and sodium thiosulphate. No damage was apparent to fine structure in the calcite matrix of ostracode shells, or to other biological tissues. No problems have been encountered with grain in the silver film at magnifications up to 15,000 X, or in the storage of coated specimens in a desiccator for periods exceeding six months.
Introduction
Many specimens for which scanning electron microscopy (SEM) is invaluable
are electrical insulators, for example microfossils and dried biological
preparations. To promote the emission of secondary electrons, and to
prevent charging of the surface (with consequent repulsion of bath incoming
and secondary electrons) it is usual to coat such specimens with a very
thin layer of metal. Nowadays gold (sometimes over a thin undercoat of
carbon) is commonly employed for the majority of work, although refractory
metals have been recommended for the very highest magnifications. These
coatings are normally applied by sputtering in a glow discharge, for
this technique is omni-directional and tends to give a fine-grained deposit,
whilst the apparatus required is comparatively simple and inexpensive
since a high vacuum is not required. An alternative, older, technique
(which also allows aluminium to be deposited) is evaporation of a molten
bead of the chosen metal in a high vacuum. The inherent directionality
of this method means that samples must generally be moved continuously
by a rotating/nodding table.
Problems arise when it is desired to return a specimen to its original uncoated
condition, for example to allow successive treatments or because too thick
a coating has been accidentally applied. Even samples which have been correctly
coated may be rendered unsuitable for subsequent optical and analytical examination,
due to the highly reflective nature of the gold film and its interference with
X-ray emission. For these reasons there is frequently a reluctance to allow
SEM examination of certain material, e.g.,type specimens and archaeological
artefacts.
Removal of Gold and Aluminium Coatings
Attempts have therefore been made to remove the metal film by suitable
reagents, which must obviously not attack the substrate. It is well-known
that gold is recovered from siliceous ores by complexing with aqueous
cyanide under oxidising (aerobic) conditions, and two groups, have independently
utilised this reaction. A major obstacle is the highly toxic nature of
cyanides, necessitating efficient fume hoods and a high degree of supervision
and control unwanted in most laboratories. A less objectionable reagent
is ferric chloride in alcohol, but it requires some six hours a gold
/ palladium film from a smooth PTFE surface , and appears likely to attach
many specimens.. Mercury amalgamates gold, but does not remove it completely
and adds its own background.
Aluminium dissolves in week acids and alkalies with the evolution of hydrogen.
Sylvester and Bradley therefore hoped that soaking in a dilute solution of
sodium hydroxide would enable this metal to be removed from calcite microfossils
without damage to the matrix. Unfortunately, he was later obliged to acknowledge
that insufficiently careful exposure to alkali could result in dissolution
of fine structure.
Advantages of a Silver Film
Silver would appear to have -much to commend it as an alternative to gold. It is the most conductive metal known. possesses a high secondary electron coefficient. and is readily applied by sputtering or evaporation to follow irregular17 contours better than any other material. Unlike gold. its X-ray emission lines are well-separated from those of the biologically important sulphur and phosphorus. Its cost is only a fraction of gold and the platinum metals. The unique applicability of silver to photography has resulted in extensive research upon its complex ions and their solubility. Quite early in the history of photography it was found that ~dark. over-exposed negative could be rendered less opaque ('reduced') by aqueous oxidising agIBts in the presence of sodium th~osulphate. The metallic silver forms the Ag ion. which is promptly complexed by the thiosulphate so that still more silver dissolves. No gas is evolved. The negative would be removed from the reagent and thoroughly washed when a sufficient amount of silver had been abstracted from the image. One of the mildest of these 'reducers' is that formulated by Farmer in 1884, employing very dilute potassium ferricyanide as the oxidising agent. As paper, albumen and gelatine were apparently unaffected, it was thought that this reagent might well prove suitable for dissolving silver from a variety of coated specimens without damage to the matrix. Ferricyanides do not possess the extreme toxicity of the simple cyanides, and may be purchased and used in the same way as ordinary laboratory and photographic chemicals.
Materials and Methods
Farmer's Reducer
The formulation used is based on that given by Jacobson:
Solution A
Sodium thiosulphate (crystals) 25
g
Water to
250 ml
Kodak 'Photoflo' 2
drops
Solution B
Potassium ferricyanide 10
g
Water to
100 ml
These solutions appear to be stable indefinitely at room temperature if kept in securely stoppered amber glass bottles. Immediately before use, the following mixture is to be prepared:
Water 50
ml
Solution A 50
ml
Solution B 3
ml
It was found that the resulting pale yellow solution had a pH of about
5, the same as the CO2-equilibrated tap water used for its
preparation. It was unstable, losing activity
and colour after about 2 hours at room temperature. A neutral mixture may be
prepared by substituting pH 7 phosphate buffer (conveniently prepared from
a BDH tablet) for water in the above dilution. However, all the tests to be
described in the paper were conducted with the ordinary solution prepared with
tap water.
It should be noted that calcium carbonate has a significant solubility in water.
In nature, calcite microfossils are protected against percolating groundwater
by the sacrificial dissolution of fossils above and around them. Once removed
from this environment to the laboratory, such fossils should presumably be
washed only with distilled water that has been allowed to stand in contact
with CaCO (e.g. marble chips) and filtered. Otherwise needles and similar fine
structures will be particularly at risk. This equilibrated 'hard' water could
be used to prepare and dilute the Farmer's reducer. A very brief final rinse
in distilled water is probably permissible: the common practice of 'soaking
overnight' is not.
Results
Silver mirror on glass
A silver mirror was made by evaporating the metal on to a microscope
slide cleaned with Chromic acid. Sufficient was deposited to give a semi-transparent
film: silvery when placed on a dark background and viewed by reflected
light, but behaving as a blue filter when examined by transmitted light.
The coated glass slide was immersed in freshly-prepared Farmer's reducer. The
silver was gently dissolved in a controlled manner, as shown by the gradual
and uniform loss of colour in transmitted light, until none remained after
3 minutes. No gas was evolved. It was decided that a 10 minute immersion should
allow an ample margin to deal with specimens with convoluted surfaces.
The reagent had no effect upon gold films. Alloys of silver and gold have not
been investigated.
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