Y label with bold superscript

Hi,

I`m trying to put my superscript type in bold style. Could somebody help me?

My text is inside a barplot.

#!/bin/R
barplot(x, ylab=expression(paste("org.", cm^-2)))

# so I tried this way,

barplot(x, ylab=expression(bold(paste("org.", cm^-2)))) # but my
superscript maintain its shape in regular font type.

Thank you all.