We report that the Zn-terminated ZnO (0001) polar surface is chemically active and the oxygen-terminated $(000\overline{1})$ polar surface is inert in the growth of nanocantilever arrays. Longer and wider “comblike” nanocantilever arrays are grown from the (0001)-Zn surface, which is suggested to be a self-catalyzed process due to the enrichment of Zn at the growth front. The chemically inactive $(000\overline{1})\mathrm{\text{−}}\mathrm{O}$ surface typically does not initiate any growth, but controlling experimental conditions could lead to the growth of shorter and narrower nanocantilevers from the intersections between $(000\overline{1})\mathrm{\text{−}}\mathrm{O}$ with $\pm (01\overline{1}0)$ surfaces.

Atomic imaging of three-dimensional structures has required a crystal in diffraction or a lens in electron imaging. Whereas diffraction achieves very high resolution by averaging over many cells, imaging gives localized structural information, such as the position of a single dopant atom. However, lens aberrations limit electron imaging resolution to about 1 angstrom. Resolution is reduced further by low contrast from weakscattering or from the limitations on electron dose for radiation-sensitive molecules. We show that both high resolution and high contrast can be achieved by imaging from diffraction with a nanometer-sized coherent electron beam. The phase problem is solved by oversampling and iterative phase retrieval. We apply this technique to image a double-wall carbon nanotube at 1-angstrom resolution, revealing the structure of two tubes of different helicities. Because the only requirement for imaging is a diffraction pattern sampled below the Nyquist frequency, our technique has the potential to image nonperiodic nanostructures, including biological macromolecules, at diffraction intensity–limited resolutions.

A superconducting interface: Interfaces between materials can harbor quantum states that belong to neither of the materials. A classic example is the superconducting interface between two insulating oxides, LaAlO ₃ and SrTiO ₃ , with a critical temperature of around 200 millikelvin. Liu et al. observed superconductivity at a different interface—formed between KTaO ₃ as a substrate and an overlayer of either EuO or LaAlO ₃ —at a considerably higher temperature of about 2 kelvin. Transport measurements displayed anisotropy, which may indicate an unusual superconducting state. Science , this issue p. 716