The nanonail flower

The excellent physical and optical properties of ZnO make its nanostructures suitable for various device applications, including in optoelectronics, nanophotonics, piezotronics, sensing, and solar cells, among others. ZnO is a wide bandgap (3.4 eV) semiconductor which is transparent in the visible range. Wurtzite ZnO has the fastest growth rate along its (0 0 0 1) face which makes it easy to grow into one-dimensional (1D) nanocrystals. The 1D geometry, coupled with the dislocation-free single-crystalline nature, high index of refraction and atomically smooth surfaces, allow for sufficient end-facet reflectivity and photon confinement in a volume of just a few cubic wavelengths of the material. As a result of the strong field localization inside sub-cubic wavelength volumes, they enable large emitter-field interaction strengths. Manipulating light with a ZnO 1D nanocrystal is interesting because the envisioned photons interaction within a small cavity can be experimentally verified. To study the optical cavity effect of nanocrystals, novel and regularly shaped nanostructures, beyond nanorods, are in demand.

The evolution of a ZnO nucleus seed normally leads to the formation of a hexagonally shaped nanorod/nanowire. The growth mechanism can be easily understood as the fastest growth rate is along the axis direction. However, ZnO nanobelts, nanoneedles, nanohelix, and nanonails have also been reported, indicating complex growth mechanisms of ZnO crystal. Diverse morphologies of ZnO nanostructures provide opportunities to investigate the crystal growth behaviors. The nanonail structure, consisting of a nanorod shank and a lotus-shaped head, is exceptional because it indicates that the growth along the [0 0 0 1] direction is not always dominant. The growth process of such a structure can be described as two steps. First, a nanorod grows along the axis by alternatively staking O and Zn atomic layers over the top facet; Second, the growth along the axial direction slows down and the radial growth starts to become dominant, leading to the formation of a hexagonally shaped nanonail head. Hence, there was a change in growth conditions that suddenly hastened the radial growth to gradually thicken the nanorod while the epitaxy over top facet continued.

Previously, we grew vertically aligned ZnO nanonails on Si substrates using a chemical vapor deposition (CVD) method and studied the optical whispering-gallery mode (WGM) resonances within individual nanonails by using spatially resolved cathodoluminescence (CL). The formation mechanism of ZnO nanonails remains unclear. Nevertheless, our CL study on a single nanonail revealed that the tapered neck section has more oxygen vacancy defects. Therefore, the change of Zn/O ratio in growth species could be a trigger for the formation of nanonails.

In synthesis, a powder mixture of ZnO and graphite was used as precursor; the furnace temperature was set to be 900 °C, but Zn started to vaporize from the source at 750 °C. With the temperature exceeding 750 °C and rising, oxygen in the gas flow was consumed both at the zinc source and the nanorod growth region. The reaction between ZnO and carbon supplied zinc vapor which combined with oxygen and condensed into ZnO crystals. The concentration of Zn in the vapor phase was not constant during the growth process of ZnO nanorods: it would reach a peak and then decrease until the zinc source was exhausted. Hence, there was a point that the ratio of Zn/O was sufficiently unbalanced to start the radial growth of a ZnO nanorod, forming the nanonail neck section with insufficient oxygen. However, the growth temperature is also an important factor. We found that nanonails were grown at 550–650 °C: areas beyond this temperature range only formed nanorods/nanowires.

The hexagonally shaped nanonail head can act as WGM resonator. This means the nanonail head may be regarded as a 2D hexagonal cavity in which photons are confined by the sides and circulate around the cavity to give rise to WGM resonances. The small size of the nanonail cavity enables us to observe low-order resonances which are interesting for fundamental studies. Our study reveals that the WGM resonances from individual ZnO nanonails have different features from those of ZnO micro/nanorod cavities. This means the nanonail cannot be treated as a 2D cavity, as its 3D confinement of photons enhances the WGM resonances.

This issue's cover image shows a flower of ZnO nanonails grown via the CVD method. The image was taken using a Hitachi S-4800 field-emission scanning electron microscopy (FE-SEM), with 5 kV accelerating voltage. In our previous work, arrays of ZnO nanonails were vertically grown onto seed-layer-coated Si substrates. In this work, we increased the amount of precursors, leading to the formation of the nanonail flower. As can be seen from the SEM image, these nanonails were grown in a radial pattern with a nanoparticle at the center. The average diameter of the nanonail head is ∼1.5 μm, and the shank length is about 3 μm. These nanonails are quite uniform in length, indicating the simultaneous growth of the heads. We believe that with detailed investigation of the relationships between growth conditions, such as Zn to O ratio in vapor phase and temperature, and the growth rates over different crystal facets, the growth mechanism of ZnO nanonails can be clearly understood and a rational design of the CVD facility can be achieved to grow ZnO nanostructures with desired shapes for applications in nanophotonics, optoelectronics, and lab-on-a-chip applications.

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DOI: 10.1016/j.mattod.2014.06.005