Thin Bismuth Film Preparation

Thin bismuth films are the material at the center of the Craig dossier. The source describes preparation methods from the 1920s and, in its appended bibliography, from later mainstream research.

Methods compared by Craig (1926)

Craig compared casting, electroplating, evaporating, sputtering, and metallic spraying, finding the last three particularly successful.

• Casting. Used for thicker plates.
• Dipping. Mica dipped into molten bismuth.
• Spraying. The Schoop and Gravitas metal-spraying processes, with nitrogen used to prevent oxidation; the Metals Coating Company of Philadelphia cooperated on this route.
• Electroplating. Reported as giving poor results.
• Evaporation.
• Cathodic sputtering. Reported to give the thinnest films. Craig's sputtering setup used 20,000 V secondary, 30 microns vacuum, 5 mA for about 20 minutes, with a Crookes dark space of about 2 cm and a bismuth cathode disc 3.5 in diameter. Separately, the sprayed film used for the Table I measurements was 3.5 × 8.0 cm and 0.012 cm thick.

Craig also used sulfur encapsulation as a protective coating for brittle bismuth plates and to exclude surface conduction.

Heaps's molten-drop casting (1927)

C. W. Heaps prepared his own plates by a different route: a vertically clamped glass tube holding a bismuth rod, with a clean horizontal glass plate about 10 cm below, heated until a single large drop falls and spreads into a thin uniform circular film.

Later methods in the appended bibliography

• Sono-electroplating. Chiba and Kojima (2009) report deposition from a Bi(III)-EDTA bath (BiOCH₃COO + EDTA-4Na in 2 mol/dm³ CH₃COOH–2 mol/dm³ CH₃COONa buffer, pH adjusted to 4.1), with films obtained at 10–100 mA/cm². Ultrasonic agitation accelerates mass transfer via micro-jet and shock-wave pressure; best conditions reported as 0.10 mol/dm³ BiY⁻, pH 4.0–5.5, 298 K, 10 mA/cm², with sonicated films smoother and denser than stationary-state films.
• Electrodeposition. O'Brien et al. (2008) deposited polycrystalline Bi films on gold from bismuth nitrate; texture depends strongly on deposition potential and Bi(III) concentration but weakly on thickness, while morphology depends strongly on deposition potential and thickness.
• Epitaxial growth. Partin et al. (1988) grew 0.1–2 µm epitaxial films by MBE on BaF₂ ⟨111⟩.
• Electroless plating. Murata Manufacturing Co., Ltd. holds a 1994 patent on an electroless bismuth plating bath, a process previously regarded as impossible.
• Ex situ pre-plating. Hutton, Hocevar and Ogorevc (2005) pre-plated bismuth films potentiostatically onto a single carbon fibre for microelectrode work.

See also

Hall Effect, Bismuth-Film Electrodes (BFEs), concepts/quantum-size-effect-bismuth-films.