Emergent motifs of macroscopic nanowire assemblies

Abstract

In the past two decades, the field of nanowire assemblies has flourished, with activity focusedonboth fundamental and applied research. Nanowire assemblies of a diverse range of compositions with tailored physical properties have been constructed using nanowires as building blocks. Many physical properties are significantly altered when the nanowire diameter is close to or below certain length scales, such as the exciton Bohr radius, the wavelength of light, the phonon mean free path, the critical size of magnetic domains, or the exciton diffusion length. However, individual nanowirebased nanodevices and their disordered structuresmaybe impractical formeeting the growing demands of microelectronic and photonic device fabrication, because of the poor reproducibility and complex fabrication processes. Macroscopic-scale integration of nanowires into ordered or controlled assembled structures provides insights into the phenomena of aggregation and anewway to tailor theproperties of nanowires [1]. Despite the impressive possibility of manipulating individual nanowires, engineering of nanowires by surfactantassisted assembly appears to be impractical to meet the growing demand for energy conversion coatings and new high-performance structural materials [2]. Assembly strategies have been employed to prepare nanowire thin films on the wafer scale to demonstrate novel collective properties and practical device fabrication (Fig. 1). Two wellknown and contrasting design strategies for nanowire assembly are employed: top-down and bottom-up. Top-down approaches seek to create nanowire-based devices using larger, externally controlled methods to direct their assembly, such as lithography, thin-film deposition, and etching, while in the bottom-up approach individual nanowires are built up into more complex nanowire assemblies via self-assembly methods. Compared with top-down strategies, bottom-up approaches have lower processing costs and higher production efficiency, offering more flexibility in selecting nanoscale building blocks and fabrication processes. Current progress on nanowire assembly methods offers considerable opportunities by introducing tiny but long-lasting forces ranging from molecular interactions, shear forces, and external field forces to electrostatic interactions [3]. Molecular interaction-driven assembly of nanowires usually employs interfaces as the assembly platform; these include methods such as the Langmuir Blodgett strategy involving water–air interfaces and the evaporation approach, using liquid–air or more complex interfaces, such as oil–water–air interfaces [4,5]. Shear forces, which are unaligned forces pushing two parts of the body