Make a one column appendix in a two column document: revtex, aps, prb

1. In general latex docs, switching between onecolumn and twocolumn styles  in a twocolumn document it is enough to type
\onecolumn

It starts a new page with the onecolumn doc. But, if you are writing a paper and if the previous does not help... try next advice.

2. In case you are writing a paper (for prb for instance) and want to make an appendix that is one column you should add  the following line before you start your appendix:

\onecolumngrid
\appendix
\section*{Supplementary Material}