Anisotropic wet etched silicon substrates for reoriented and selective growth of ZnO nanowires and enhanced hydrophobicity

Control of zinc oxide nanowire array properties with electron-beam lithography templating for photovoltaic applications

Hydrothermally synthesized zinc oxide nanowire arrays have been used as nanostructured acceptors in emerging photovoltaic (PV) devices. The nanoscale dimensions of such arrays allow for enhanced charge extraction from PV active layers, but the device performance critically depends on the nanowire array pitch and alignment. In this study, we templated hydrothermally-grown ZnO nanowire arrays via high-resolution electron-beam-lithography defined masks, achieving the dual requirements of high-resolution patterning at a pitch of several hundred nanometers, while maintaining hole sizes small enough to control nanowire array morphology. We investigated several process conditions, including the effect of annealing sputtered and spincoated ZnO seed layers on nanowire growth, to optimize array property metrics-branching from individual template holes and off-normal alignment. We found that decreasing template hole size decreased branching prevalence but also reduced alignment. Annealing seed layers typically improved alignment, and sputtered seed layers yielded nanowire arrays superior to spincoated seed layers. We show that these effects arose from variation in the size of the template holes relative to the ZnO grain size in the seed layer. The quantitative control of branching and alignment of the nanowire array that is achieved in this study will open new paths toward engineering more efficient electrodes to increase photocurrent in nanostructured PVs. This control is also applicable to inorganic nanowire growth in general, nanomechanical generators, nanowire transistors, and surface-energy engineering.

Biologically inspired tunable hydrophilic/hydrophobic surfaces: a copper oxide self-assembly multitier approach

In this study, a fabrication method for biologically inspired superhydrophobic micro- and nano-structured tier surfaces, each made of a self-assembled copper oxide, is presented. The method is controllable and applicable to bulk production when compared to existing high-end fabrication techniques. By modulating wet chemistry, different shapes and scales of tier structures were created. We demonstrated that their wetting behaviors are closely related to morphological information such as pitch, height and shape. To characterize their wetting behaviors, several experiments were designed and executed. In static water contact angle (WCA) measurements, morphological modulation led to wide WCA range (17°-95°). After hydrophobic self-assembly monolayer of 1-dodecanethiol, their WCA was escalated into superhydrophobic regime. In dynamic WCA, the contact angle hysteresis is greatly reduced by hybridizing the micro- and nano-tier (multiple tiers) when compared to utilizing a single tier. Also, the modification of the surface structure influences the rate of evaporation. In an analytical approach, the multiple tiers show a lower surface free energy compared to that of the single tier. By hybridizing different scales and shapes of tiers-such as hemispheric and conic shapes-the multiple tiers can efficiently reduce the surface energy barrier. Eventually, these manipulations lead to a subtle WCA hysteresis during the liquid motion testing. The analytical results are consistent with the dynamic WCA measurements. The multiple tiers also stabilize the Cassie regime and result in an increased hydrophobicity, which is more than when a single tier is employed.