Abstract
One-dimensional (1-D) ultrathin (15 nm) and thin (100 nm) aligned 1-D (0001) and () oriented zinc oxide (ZnO) nanowire (NW) arrays were fabricated on copper substrates by one-step electrochemical deposition inside the pores of polycarbonate membranes. The aspect ratio dependence of the compressive stress because of the lattice mismatch between NW array/substrate interface and crystallite size variations is investigated. X-ray diffraction results show that the polycrystalline ZnO NWs have a wurtzite structure with a = 3.24 Å, c = 5.20 Å, and [002] elongation. HRTEM and SAED pattern confirmed the polycrystalline nature of ultrathin ZnO NWs and lattice spacing of 0.58 nm. The crystallite size and compressive stress in as-grown 15- and 100-nm wires are 12.8 nm and 0.2248 GPa and 22.8 nm and 0.1359 GPa, which changed to 16.1 nm and 1.0307 GPa and 47.5 nm and 1.1677 GPa after annealing at 873 K in ultrahigh vacuum (UHV), respectively. Micro-Raman spectroscopy showed that the increase in E2 (high) phonon frequency corresponds to much higher compressive stresses in ultrathin NW arrays. The minimum-maximum magnetization magnitude for the as-grown ultrathin and thin NW arrays are approximately 8.45 × 10−3 to 8.10 × 10−3 emu/g and approximately 2.22 × 10−7 to 2.190 × 10−7 emu/g, respectively. The magnetization in 15-nm NW arrays is about 4 orders of magnitude higher than that in the 100 nm arrays but can be reduced greatly by the UHV annealing. The origin of ultrathin and thin NW array ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of ZnO NWs. The n-type conductivity of 15-nm NW array is higher by about a factor of 2 compared to that of the 100-nm ZnO NWs, and both can be greatly enhanced by UHV annealing. The ability to tune the stresses and the structural and relative occupancies of ZnO NWs in a wide range by annealing has important implications for the design of advanced photonic, electronic, and magneto-optic nano devices.
Highlights
One-dimensional (1-D) inorganic nanostructures have stimulated great interest because of their unique physical and chemical properties [1,2,3,4] such as flexibility of nanostructures [5,6,7], metal-insulator transition [4,8], superior mechanic toughness [6], higher luminescence efficiency, and lower lasing threshold [8,9]
During the growth of the zinc oxide (ZnO) NWs in the pores, the current remains nearly constant in region (II)
The decrease in full-width half maximum (FWHM) and the increase in peak intensity both indicate the increase in crystallite size and an improvement in the crystalline quality of wellaligned ZnO NW arrays after ultrahigh vacuum (UHV) annealing. These results demonstrate that 15-nm ZnO NWs have lower lasing power threshold than 100-nm NWs due to the higher crystallinity of the ultrathin ZnO NW arrays
Summary
One-dimensional (1-D) inorganic nanostructures have stimulated great interest because of their unique physical and chemical properties [1,2,3,4] such as flexibility of nanostructures [5,6,7], metal-insulator transition [4,8], superior mechanic toughness [6], higher luminescence efficiency, and lower lasing threshold [8,9]. Many techniques have been employed to fabricate 1-D nano architectures, such as EBL [24], NIL [25], VLS [26], CVD [27], sol-gel [28], hydrothermal process [29], and thermal evaporation [30] Electrochemical deposition demonstrates another important approach to the synthesis of 1-D nanostructures [31]. Sheng et al [33] demonstrated the conversion of mechanical energy into a 1.2-V electrical energy due to the 0.19% of strain induced in the aligned ZnO NWs. The electrical, optical, and magnetic properties of 1-D nanostructure are affected by the residual stress [34]. One possibility that has been often cited for the compressive intrinsic stress is the development of free surfaces or variation in crystal size of nanostructures before other competing stress is generated during process
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