A substrate assembly including a photosensitive etching glass substrate; and a first substrate and a second substrate for interposing both main surfaces of the photosensitive etching glass substrate between them. The concave gripping surface terminates in a circumferential lip that encircles a longitudinally extending concave tack receiving area at one end of the shaft opposite the flat top surface adapted for striking by a tack hammer. 0 m, at least 90% of all the aluminum oxide particles have a particle diameter of at most 1. The electrodes are connected to electric current or voltage. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire and a coaxial heterostructure nanowire.