{"id":1372,"date":"2013-07-25T18:27:56","date_gmt":"2013-07-25T18:27:56","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/?p=1372"},"modified":"2013-07-25T18:29:59","modified_gmt":"2013-07-25T18:29:59","slug":"lithographically-defined-zno-nanowire-growth","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/lithographically-defined-zno-nanowire-growth\/","title":{"rendered":"Lithographically-defined ZnO Nanowire Growth"},"content":{"rendered":"

Photovoltaics (PVs) with inorganic zinc oxide nanowire (ZnO NW) active layers are possible for future generations of technology due to low-temperature processing, abundant materials, and application on many surfaces. For maximum performance, vertical alignment, density, and electrical properties of the NW, growth must be controlled. NWs with varying sizes, orientations, and pitch can be grown via a low-temperature, hydrothermal, seeded growth process with a zinc nitrate hexahydrate and hexamethylenetetramine solution[1<\/a>]<\/sup>. To achieve an ideal NW spacing for PVs[2<\/a>]<\/sup>, growth spacing can be controlled with poly(methyl methacrylate) (PMMA) or block copolymers (BCPs) pre-patterned masks. We grew arrays of ZnO NWs that were masked by both poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) BCP and PMMA hole arrays.<\/p>\n

For a BCP nanopattern mask, a thin film of PS-b-PDMS was self-assembled on a ZnO seed-layer to form a single layer of oxidized, parallel PDMS cylinders (pitch = 35 nm) (Figure 1a). The BCP mask controlled the density of NWs during the subsequent hydrothermal growth process to about 4.5×1010<\/sup> unmasked vs. 2.4\u00d71010<\/sup> NWs\/cm2 <\/sup>masked (Figure 1b and 1c). The BCP mask can be further directed by electron-beam lithography-based template (EBL)[3<\/a>]<\/sup>[4<\/a>]<\/sup>. For a PMMA resist mask, arrays of EBL-defined holes were patterned in 50-200 nm thick PMMA on a ZnO seed-layer (Figure 2a inset). The ZnO NWs grew only through the templating holes (Figure 2a and b). A hydrothermal-solution concentration of 2 mM and 0.2 mM poly(ethyleneimine) (PEI) and hole pitch of 275 nm resulted in primarily orthogonal, roughly 125-200 nm diameter NWs. This work expands on previous experimental work by others, though we report masks of smaller (275-nm) pitch, on a spin-coated seed layer[5<\/a>]<\/sup>.<\/p>\n\n\t\t