For single letters, simply use Unicode (copy-pasting the symbol directly into the title in Zotero). I have set "Export unicode as plain-text latex commands" in the BBT settings, and also in the BBT the Automatic export tab.
For longer or more complicated subscripts and superscripts, use HTML tags. E.g., "Procedure for solution growth of ZnS, CdS and Zn1-xCdxS thin films by successive ionic-layer adsorptions and reactions".
This automatic title-casing which Zotero and BBT applies works fine in most cases. This presupposes that titles in Zotero are entered using sentence-case.
But I have observed an issue for single-letter non-capitalised words, e.g., Laflere1974, "On the differential capacitance of the n- and p-type..." which becomes rendered with capital "N-" in the thesis bibliography, which is not correct.
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Putting single (or double) braces around the letter "{n}" in Zotero only causes the braces to be printed in the bibliography.
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The BBT FAQ describes the definitive solution: enclose the letter in
<span class="nocase">...</span>.
Add them to the end of the given name field in Zotero, after a comma.
- https://forums.zotero.org/discussion/comment/130244/ (see the last comment)
- How to not mess up your bibliographies with BibTeX, Claus O. Wilke (2015)