route [23]. After heating at 120 C for 2 days and then sintering at 750 C for 4 h, these plates were cut into small sections, which were then sintered at 900 C for 12 h to decrease the sizes of the voids. The 1.5 mm thick plates were infiltrated with nickel (ca. 100 lm) using the previously described electrochemical deposition methodÐan electrode was fabricated from the opal sections, immersed into a commercially available plating solution (Technic Inc.) and nickel was deposited into the opal by a constant current method at low current density (0.1 mA cm ±2 ) over a 72 h period [19,20].
Plates of the thereby fabricated silica/nickel-mesh composite (typically 10 7 1.5 mm) were attached to a polishing wand and protected on four sides by pieces of single crystal silicon plates (typically two pieces of 4 10 2 mm and two of 4 7 2 mm). The composite pieces were first polished with a piece of silicon carbide paper (1200/4000) to remove the silica layers that had not been infiltrated with metal. After reaching the nickel mesh layer, the plate appearance changes from opalescent white to black. The polishing was then carefully continued with a piece of silicon carbide paper (1000/3000) for about 30 min to obtain a thin sheet (ca. 50 lm thick). If the sample is inverted and polished further for about 20 min or more, very thin sheets can be obtained that range in thickness from about 15 lm to less than 1 lm. Alternatively, the 50 lm sheet can be attached to a Mo disk (5 mm) with neoprene glue, inserted into a precision ion-polishing system (Gatan Model 691), and further polished on both sides. Thin composite pieces, produced either mechanically or by ion milling, can be manipulated using tweezers. Thin plates of nickel wire-mesh were obtained after soaking the Ni-infiltrated SiO 2 opal plates in a 2 % HF solution for 2 h to remove the silica spheres. Because the thin wire mesh pieces readily fragment if handled, dissolution of the silica was carried out directly on samples already mounted for SEM imaging. SEM was carried out on a JEOL JSM 5410 SEM.