2D Nanonets_______________________________________________________ by Céline TERNON

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One dimensional (1D) nanostructures such as nanotubes (NTs) or nanowires (NWs) have been attracting a large interest in the last decades. These 1D nanomaterials exhibit interesting electrical, optical, mechanical and chemical properties thanks to their high aspect ratio and large surface area. Because of their surface properties, NWs have been developed in many applications such as solar cells, biological and chemical sensors, catalysts or batteries. However, due to time consuming, complex and expensive technological techniques, the industrialization of devices based on a unique NWs is limited. Moreover, due to a large variability between NW properties, there is an obvious lack of reproducibility from one device to another. As a consequence, it is difficult to exploit unique properties of NWs into actual devices.

On the contrary, materials composed of randomly oriented NWs offer a good reproducibility and have been attracting recent interest in many applications. Also called “nanonets”, for NANOstructured NETworks as proposed by G. Grüner, such materials show several interesting properties arising either from the individual components, the NWs or NTs, or from the structural properties of the network itself. These properties include:

•  High surface area/high porosity. Due to the component geometry, the surface area drastically increases, in comparison with thin films. As a consequence, addition of functional materials is favored due to the high porosity.
• Electrical conductance. The so-defined nanonets are above the percolation threshold and the larger the NW conductivity, the better the network conductance. Factors like NW-NW contacts are playing a major role and have to be studied and optimized.
• Optical transparency. A network of high aspect ratio nanostructures has high transparency approaching 100% when aspect ratio tends to infinity. However, when dealing with absorbing materials, the transparency decreases when aspect ratio decreases.
• Mechanical robustness and flexibility. A random network of interwoven NWs has significantly higher flexibility than that of a thin film.
• Reproducibility and fault tolerance. At the macroscopic scale, the nanonet involves several millions of NWs so that the observed properties result from a large scale statistical averaging on individual NW properties. Moreover, breaking a conducting path in the network leaves many others still present and the overall properties of the nanonet are stable. As a consequence, in contrast to individual components, nanonets offer high reproducibility of the physical properties coupled with easy preparation.
• High quality components. It is easy to grow defect free NWs over large area so that the electrical and optical properties are improved
• Functionalization ability. By attaching chemical species or nanoscale materials such as nanoparticles (NPs) with well-defined functionality, it is possible to add properties induced by the synergy between NWs and NPs.

When the nanonet thickness is significantly thinner than the length scale of its individual components, the nanonet is considered as two-dimensional (2D) because the percolation properties of such network show typical 2D characteristics. Carbon nanonets (composed of carbon NTs, functionalized or not) and metallic nanonets (composed of silver or copper NWs) have been studied for a decade and are widely described in the literature particularly for the fabrication of transparent and conductive thin films for photovoltaic applications. In contrast, there are currently very few studies on 2D nanonets based on nanomaterials other than carbon or silver, such as, for example, Si, ZnO, and other semiconducting NWs, despite the high potential of such materials. 
From Serre et al, Nanotechnology 26 (2015) 015201

• G. Grüner group
• M. Alam group