The surface-guided growth of horizontal nanowires (NWs) allows assembly and alignment of the NWs on the substrate during the synthesis, thus eliminating the need for additional processes after growth. One of the major advantages of guided growth over postgrowth assembly is the control on the NWs direction, crystallographic orientation, and position. In this study, we use the guided growth approach to synthesize high-quality, single-crystal, aligned horizontal ZnS NWs on flat and faceted sapphire surfaces, and show how the crystal planes of the different substrates affects the crystal structure and orientation of the NWs. We also show initial results of the effect of Cu doping on their photoluminescence. Such high-quality aligned ZnS NWs can potentially be assembled as key components in phosphorescent displays and markers due to their unique optical properties. The ZnS NWs have either wurtzite or zinc-blende structure depending on the substrate orientations and contain intrinsic point defects such as sulfur vacancies, which are common in this material. The crystallographic orientations are consistent with those of guided NWs from other semiconductor materials, demonstrating the generality of the guided growth phenomenon. The successfully grown ZnS NWs and the Cu doping are the first step toward the fabrication of optoelectronic devices based on ZnS nanostructures.
Large-scale on-chip integration of organic nanowire-based devices requires the deterministic assembly of organic small molecules into highly-aligned nanowires. In this work, phthalocyanine molecules are self-assembled into horizontally-aligned nanowires after generating parallel hydrophobic nanogrooves on a sapphire surface. In contrast to previous self-oriented inorganic nanowires, these molecular nanowires are separated from their supporting sapphire by an ultrathin amorphous layer, indicating a complete elimination of lattice matching between nanowires and substrates. Therefore, small molecules beyond phthalocyanines hold promise to form aligned nanowires using this graphoepitaxial self-assembly strategy. The excellent alignment and high crystallinity of these nanowires enable the desired in-situ integration of nanowire-based devices without additional postgrowth processing steps. As a proof of concept, self-oriented CuPc nanowires are integrated into photodetector arrays directly on their growth substrate after electrode arrays are transferred onto the nanowires. Compared to previous CuPc photodetectors constructed using other approaches, these detectors exhibit a faster response to the spectrum in the 488-780 nm range (rise and fall times are 0.05-0.43 s and 0.38-2.34 s, respectively) while offering comparable detectivities (2.49 × 10 10 Jones on average). This graphoepitaxial self-assembly offers new opportunities for the aligned growth of organic crystalline nanowires and their large-scale in-situ integration into functional devices.
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