We report metalorganic vapor-phase epitaxial growth and structural and photoluminescent characteristics of ZnO nanorods. The nanorods were grown on Al 2 O 3 (00•1) substrates at 400°C without employing any metal catalysts usually needed in other methods. Electron microscopy revealed that nanorods with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates. The mean diameter of the nanorods is as narrow as 25 nm. In addition, x-ray diffraction measurements clearly show that ZnO nanorods were grown epitaxially with homogeneous in-plane alignment as well as a c-axis orientation. More importantly, from photoluminescence spectra of the nanorods strong and narrow excitonic emission and extremely weak deep level emission were observed, indicating that the nanorods are of high optical quality.

2 Control of the interlayer twist angle in two-dimensional (2D) van der Waals (vdW)heterostructures enables one to engineer a quasiperiodic moiré superlattice of tunable length scale 1-7 . In twisted bilayer graphene (TBG), the simple moiré superlattice band description suggests that the electronic band width can be tuned to be comparable to the vdW interlayer interaction at a 'magic angle' 8 , exhibiting strongly correlated behavior. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favoring interlayer commensurability, which competes with the intralayer lattice distortion 9-15 . Here we report the atomic scale reconstruction in TBG and its effect on the electronic structure. We find a gradual transition from incommensurate moiré structure to an array of commensurate domain structures as we decrease the twist angle across the characteristic crossover angle, θc ~1°. In the twist regime smaller than θc where the atomic and electronic reconstruction become significant, a simple moiré band description breaks down. Upon applying a transverse electric field, we observe electronic transport along the network of onedimensional (1D) topological channels that surround the alternating triangular gapped domains, providing a new pathway to engineer the system with continuous tunability.In the absence of atomic scale reconstruction, a small rigid rotation of the vdW layers relative to each other results in a moiré pattern, whose long wavelength periodicity is determined by the twist angle. For unreconstructed TBG, atomic registry varies continuously across the moiré period between three distinct types of symmetric stacking configurations: energetically favorable AB and BA Bernal stacking and unfavorable AA stacking (Fig. 1a). This quasiperiodic moiré superlattice, associated with the incommensurability of the twisted layers, modifies the band structure significantly. In the small twist regime, low-energy flat bands appear at a series of magic angles ( ≤ 1.1°) where the diverging density of states (DOS) and vanishing Fermi velocity, associated with strong electronic correlation, are predicted 8 . The recent experiment demonstrated the presence of the first magic angle near ~1.1° where Mott insulator and unconventional superconductivity were observed 6,7 . The TBG moiré band calculation, however, assumes a rigid rotation of layers ignoring atomic scale reconstruction. Despite the weak nature of vdW interaction and the absence of dangling bonds, recent experimental works on similar material systems suggestthere is substantial lattice reconstruction at vdW interfaces, especially at small twist angle close to global commensuration between two adjacent layers 9,10 . Atomic scale reconstruction at vdW B 92, 155438 (2015).

This paper presents a review of current research activities on ZnO nanorods (or nanowires). We begin this paper with a variety of physical and chemical methods that have been used to synthesize ZnO nanorods (or nanowires). There follows a discussion of techniques for fabricating aligned arrays, heterostructures and doping of ZnO nanorods. At the end of this paper, we discuss a wide range of interesting properties such as luminescence, field emission, gas sensing and electron transport, associated with ZnO nanorods, as well as various intriguing applications. We conclude with personal remarks on the outlook for research on ZnO nanorods.

There has been considerable interest in the growth of one-dimensional (1D) semiconductor nanostructures including nanowires. [1±3] In addition to nanowires, semiconductor nanoneedles are of particular interest because their tips show a sharp curvature, offering potential applications as probing tips with high spatial resolution in both vertical and horizontal dimensions or field-emission tips due to the increased field-enhancement factor. [4,5] Nevertheless, semiconductor nanoneedles have rarely been studied, while numerous semiconductor nanowires including Si, InP, GaN, GaAs, GaP, and ZnO have been synthesized. [1±3,6] To prepare the nanowires, metal-catalysis-assisted vapor±liquid±solid (VLS) deposition has been widely employed because this method was used for Si microwhisker growth in the 1960s. [7] Since as-grown nanowires prepared by the VLS method yield a metal nanoparticle on their tips, nanoneedles with sharp tips are difficult to prepare using the VLS method. However, we recently developed a metal-catalyst-free growth method for preparing ID nanostructures, [8] which enables the growth of sharp-tipped ZnO nanoneedles.One-dimensional ZnO nanowires and nanorods have been studied recently for optoelectronic nanodevice applications as the material has a wide and direct fundamental bandgap energy of 3.4 eV, a large excitonic binding energy of~60 meV, and high mechanical and thermal stabilities. [9] Both ZnO films and nanostructures have been prepared, typically on sapphire substrates as the substrate shows a good lattice match with ZnO. [10±12] Nevertheless, the use of Si substrates enables the deposition of nanomaterials on large and cheap substrates, offering possible mass production of the nanomaterials. More importantly, the preparation of nanomaterials on Si represents a breakthrough for nanomaterial integration in Si-based electronic devices. Despite the importance of the nanomaterials growth on Si substrates, vertically well-aligned one-dimensional ZnO nanostructure growth on Si has rarely been reported. [6,13] Here, we report metal±organic chemical vapor deposition (MOCVD) of vertically well-aligned ZnO nanoneedles on Si substrates and their structural and optical characteristics.The surface morphology of as-grown ZnO nanoneedles on Si substrates was investigated using scanning electron microscopy (SEM). As shown in Figures 1a±1c, a high density of ZnO nanoneedles is vertically aligned over the entire substrate and they exhibit sharp tips. The diameter and aspect ratio of ZnO nanoneedles depend on growth conditions including growth time. Typically, nanoneedles grown for 1 h exhibit mean lengths of 740 ± 50 nm and mean diameters of 40 ± 5 nm (Figs. 1d and 1e). Normalized standard deviation values (a standard deviation divided by a mean) in nanoneedle diameter and length distributions are as small as 0.16 and 0.06, respectively. These values are comparable to those of ZnO nanorods grown on Al 2 O 3 (0001) substrates and are one or two orders of magnitude smaller than those prepared by the catalyst-as...

We report on ferromagnetic characteristics of Zn 1Ϫx Mn x O ͑xϭ0.1 and 0.3͒ thin films grown on Al 2 O 3 (00•1) substrates using laser molecular-beam epitaxy. By increasing the Mn content, the films exhibited increases in both the c-axis lattice constant and fundamental band gap energy. The Curie temperature obtained from temperature-dependent magnetization curves was 45 K for the film with xϭ0.3, depending on the Mn composition in the films. The remanent magnetization and coercive field of Zn 0.9 Mn 0.1 O at 5 K were 0.9 emu/g and 300 Oe, respectively. For Zn 0.7 Mn 0.3 O, the remanent magnetization at 5 K increased to 3.4 emu/g.