(Non-carbon) anisotropic nanomaterials

Abstract

Nanomaterials research has evolved tremendously during the last 20 years. Since the initial systematic discovery of (nano)size-related properties and, in general, the search for a correlation between particle size and the physical/chemical properties of materials (even though this is still, for some materials, a matter of intense study and debate), a plethora of new research directions have been followed by an increasing number of researchers, with the objective of finding and understanding new effects that are directly determined by the nanoscale dimensions of the materials but are strongly affected by other parameters. Among these parameters, shape (or the degree of anisotropy) has been identified as a major tool for engineering the properties (mechanical, optical, electronic, magnetic, etc.) of nanomaterials, with the additional attractive feature of displaying different properties in different directions. The development and characterisation of one-dimensional nanostructures has been strongly encouraged since the discovery of carbon nanotubes, back in 1991, which are recognised nowadays as the main example of materials for nanotechnology. It was a few years thereafter when other anisotropic nanomaterials started to get attention, among which nanowires and nanorods were most often studied, but whole ranges of new shapes were being constantly reported. This explosion of anisotropic nanomaterials research is reflected in the graph displayed in Fig. 1, which summarizes a literature search on the number of papers including selected keywords (nanotubes, nanowires, nanorods, nanobelts, nanodisks/ nanoplates) in their titles, over the past ten years. For all the selected keywords, the plot reveals an exponential increase, even though the first published papers are in some cases only a few years old. This themed issue was conceived as a collection of selected contributions by some of the researchers who are most active and recognised as leaders in the field of anisotropic nanomaterials, but deliberately leaving carbon nanomaterials (carbon nanotubes in particular) aside, since they constitute a field on their own. While, obviously, not all of the leading groups could be represented here, the final collection of papers definitely provides an excellent overview of current research directions, and I would like to express my gratitude to all contributors for their effort and support toward this initiative. In what follows, a brief description of the papers included in this issue is provided, which intends to serve as an introduction to encourage further reading. Three excellent feature articles cover extremely hot topics. Moore and Wang describe the formation of nanowires, nanorods, nanobelts and other fancy nanostructures made of ZnS, nicely showing that the final morphology is directly determined by the growth direction from the original seeds (Fig. 2). Yacaman and co-workers concentrate on the shape evolution of noble metals, describing the various morphologies that typically originate during the growth of small (twinned) metal nanoparticle seeds, including amazing examples from their high resolution electron microscopy (HRTEM) studies (Fig. 3). While these two feature articles are related to the growth processes leading to anisotropic nanomaterials, the one by Okamoto and Kimura deals with anisotropic metal nanomaterials characterization. In particular, they demonstrate the capability of directly imaging optical surface plasmon resonance modes within metal nanoparticles of various morphologies using a novel microscopy technique, namely scanning near field optical microscopy (SNOM). The