pen vs write of

pen is a noun but write of is not a noun.

Word	Noun	Adjective	Verb	Adverb
pen	Yes	No	Yes	No
write of	No	No	Yes	No

As verbs, write of is a hyponym of pen; that is, write of is a word with a more specific, narrower meaning than pen:

pen (noun)	write of (noun)
female swan
a writing implement with a point from which ink flows
an enclosure for confining livestock
a correctional institution for those convicted of major crimes
a portable enclosure in which babies may be left to play

pen (verb)	write of (verb)
produce a literary work	write about a particular topic