Hi, Jack —
I assume the question concerns Permanent Deacons as Transitional Deacons almost always wear them.
It depends on the Bishop they serve. Yes, they can and they usually also wear a separate symbol of the diaconate (usually a deacon stole draping a cross). In the Archdiocese of Boston, for example, they are asked not to wear them.
Canonically, Canon 284 says, clerics are to wear . . . and Deacons are clerics; however, Canon 288 exempts permanent deacons from Canon 284 and leaves it up to particular law.