For the integration of ZnO nanowires in future devices, the controlled growth on a Si substrate is of utmost interest. We report the spontaneous and position-controlled epitaxial growth of highly oriented ZnO nanowires on Si using a catalyst-free CVD approach with an AlN buffer and a ZnO seed layer. The length, diameter, and density of the nanowires were analyzed for a wide range of growth parameters, i.e., growth time, substrate temperature, oxygen concentration, and carrier flow rate. Subtle changes lead to variations in nanowire dimensions and density but maintain the nontapered and uniform hexagonal shape. The use of the AlN buffer layer allowed for epitaxial growth of the ZnO seed layer and nanowires on Si, as confirmed by high-resolution X-ray diffraction. The high alignment of nanowires with low crystal tilt and twist was confirmed by ω-scans with a full width at half-maximum of 0.33 and 0.64° of the (0002) and (101̅0) reflection, respectively. Finally, after optimizing growth parameters, catalyst-free, position-controlled growth of ZnO NWs was demonstrated by lithographic patterning and selective etching of the ZnO seed layer.
Comparative studies of photoluminescence (PL) of undoped and Er-doped size-controlled nanocrystalline Si/SiO2 superlattice structures show that the optical excitation of Si nanocrystals can be completely transferred to the Er3+ ions in surrounding SiO2, resulting in a strong PL line at 1.5 μm. The PL yield of the Er-doped structure increases for higher photon energy of excitation and for smaller nanocrystal sizes. This highly efficient sensitizing of the Er-related PL is explained by a strong coupling between excitons confined in Si nanocrystals and neighboring Er3+ ions in their upper excited states.
Quantum-size effects are essential for understanding the terahertz conductivity of semiconductor nanocrystals, particularly at low temperatures. We derived a quantum mechanical expression for the linear terahertz response of nanocrystals; its introduction into an appropriate effective medium model provides a comprehensive microscopic approach for the analysis of terahertz conductivity spectra as a function of frequency, temperature, and excitation fluence. We performed optical pump--terahertz probe experiments in multilayer Si quantum dot networks with various degrees of percolation at 300 and 20 K and with variable pump fluence (initial carrier density) over nearly three orders of magnitude. Our theoretical approach was successfully applied to quantitatively interpret all the measured data within a single model. A careful data analysis made it possible to assess the distribution of sizes of nanocrystals participating to the photoconduction. We show and justify that such conductivity-weighted distribution may differ from the size distribution obtained by standard analysis of transmission electron microscopy images.
Nanowires with twinned morphology have been observed in many cubic-phase materials including spinel. We study systematically the formation of multitwinned Zn2TiO4 nanowires based on a solid−solid reaction of ZnO nanowires with a conformal shell of TiO2, which is deposited by atomic layer deposition (ALD). By varying the solid-state reaction temperature, reaction time, and TiO2 shell thickness, the formation process is carefully analyzed with the help of transmission electron microscopy. It is found that the multitwins develop through an oriented attachment of initially separated spinel nanobricks and a simultaneous Ostwald ripening process. The oriented assembly of the individual bricks is strongly dependent on annealing conditions, which is required to favor the motion and interaction of the bricks. This mechanism differs dramatically from those proposed for twinned nanowires grown with the presence of metal catalysts. Our result provides new insights on controlling the morphology and crystallinity of designed 1-D nanostructures based on a solid-state reaction route.
Ionic liquid assisted growth of ultra-long ZnO nanowires from thermal chemical vapor deposition and the incorporation of dopants into the ZnO lattice have been investigated. We find that decomposed components of the ionic liquid at higher temperatures facilitate ultra-long vapor-liquid-solid ZnO nanowires that exhibit an unusual a-axis orientation. In particular, the ionic liquid BMImBF4 has been studied and the mechanism of the nanowire growth model in response to the use of the ionic liquid has been explained. We show that boron which is part of the investigated ionic liquid incorporates into the ZnO lattice and serves as a donor source. Electrical measurements were conducted and have shown an enhanced electrical conductivity (ρ = 0.09 Ω cm) when using the ionic liquid assisted growth approach. This work represents a step towards the controlled doping for designing future nanowire devices.
