Significance One-dimensional (1D) nanomaterials, such as nanowires (NWs), nanorods (NRs) and nanotubes (NTs), are the smallest units for achieving the efficient transportation of electrons and excitons, which are considered to be the ideal building blocks for constructing micro/nano functional devices. 1D nanomaterials have potential application prospects in nano-optoelectronics, nanosensing, energy storage, biomedicine, and other such fields because of their unique optical, electrical, magnetic, thermal, and mechanical characteristics as well as other excellent characteristics. Currently, the techniques used to synthesize the ordered 1D nanomaterials are quite mature. However, the efficient assembly of 1D nanomaterials remains a challenge that must be urgently solved. The gaps between 1D nanomaterials and integrated devices in various fields can be bridged by assembling 1D nanomaterials into two-dimensional (2D) or three-dimensional (3D) micro/nanoarchitectures. In addition, the properties of 1D nanomaterials must be completely utilized. Thus, to realize the high-precision and highly directional assembly of 1D nanomaterials in 2D/3D spaces is the key to explore their potential applications.
Various methods, such as the lithography and etching technologies, the methods in which external force or field approaches, the template-assisted methods, the biorecognition methods involving near-field manipulation, and the electro-hydrodynamic (EHD) printing method, are used for assembling 1D nanomaterials into 2D and 3D ordered mesoscale structures. Unfortunately, the inherent disadvantages associated with these methods considerably limit their wider applications. In case of the usage of the external force approach, it is difficult to precisely control the density and placement of NWs using the shear force-based approaches. The application of the magnetic field-based method is only restricted to the ferromagnetic and super-paramagnetic material-based NWs. In addition, the electric field-based method requires the highly unified process conditions and the preparation of electrodes in advance. Furthermore, the assembly of 1D nanomaterials in 3D space is still in the initial research stage. The traditional assembly methods such as the Langmuir-Blodgett (LB), contact printing, and EHD printing methods, can used to realize the deposition of 2D and 2.5D structures, such as arrays and mesh grids, by stacking 1D nanomaterials. However, it is still difficult to accurately control the vertical assembly of 1D nanomaterials using these traditional assembly methods. Therefore, the high precision, highly directional, and controllable assembly of 1D nanomaterials in 3D space requires a further investigation.
Recently, two-photon polymerization (TPP) laser direct writing has emerged as a promising technique for assembling nanomaterials owing to its real 3D nanofabrication capability and sub-diffraction-limited resolution. TPP fabrication can achieve designable, highly directional, and high-precision assembly of 1D nanomaterials in 3D space because of the laser-induced trapping force and micro/nanoscale laser writing resolution. Currently, some research groups have assembled 1D nanomaterials, including Au NRs, Ag NWs, CNTs, and ZnO NWs, via laser direct writing. However, some challenges remain with respect to the highly directional assembly, integration and application of the assembled nanomaterials and the LSPRs of metal nanomaterials. Hence, the existing research must be summarized for guiding the future development of this field in a rational manner.
Progress In this study, first, the background of 1D nanomaterial assembly techniques is introduced. In addition, the mechanism and state of the art of non-laser assembly techniques are summarized. Furthermore, the existing challenges associated with this field are discussed. Second, the recent progress of the laser assembly techniques of 1D nanomaterials is reviewed. Both 1D metallic and semiconducting nanomaterials, including Au NRs, Ag NWs, CNTs, and ZnO NWs, are reviewed and discussed. For assembling 1D metal nanomaterials, Do et al. have deposited an individual Au NW from an optical trap using two different laser wavelengths to avoid the influence of LSPRs (
Conclusion and Prospect Compared with the traditional non-laser assembly techniques, the laser assembly methods, especially the femtosecond laser direct writing technology, exhibit advantages on the assembly of 1D nanomaterials because of their high spatial resolution and true 3D micro-nano manufacturing capability. A femtosecond laser exhibits high peak power and short pulse duration, and thus the nanomaterials can be accurately controlled with respect to its energy and momentum. Although the femtosecond laser direct writing technology has made some progresses on the assembly of 1D nanomaterials, several problems remain to be resolved, including some irregularities observed in the assembled 1D nanomaterials, the LSPRs of metal nanomaterials, and the low efficiency of the laser assembly methods. Thus, the regularity, flexibility, and efficiency of the laser direct writing technology may be further improved by modifying the components of the 1D nanomaterial composite resin, introducing vectorial electromagnetic fields, or employing parallel laser direct writing manufacturing.
