1. A memristor array comprising: a lower layer of crossbars; an upper layer of crossbars intersecting the lower layer of crossbars; memristor cells interposed between intersecting crossbars, each memristor cell comprising a mobile dopant movable by a switching voltage to change an electrical resistance of that memristor cell; pores separating adjacent memristor cells; and a capping layer covering said pores that varies in thickness, being thicker on one side of a memristor cell than an opposite side of that memristor.