Controlled synthesis and property of horizontally aligned carbon nanotubes

Horizontally aligned carbon nanotubes (HACNTs) refers to the horizontally aligned CNT arrays grown and parallel on flat substrates, with relatively large intertube distances. Due to their perfect structures, excellent properties and lengths up to millimeters, even centimeters, HACNTs show great potential as building blocks for transparent displays, nano electronics, quantum lines, field emission transistors, superstrong tethers, and aeronautics and aerospace materials, etc. Great development was obtained on the study of ultralong CNTs during the past decade. In this review, we summarize the recent progress on the growth mechanism, structure-controlled synthesis, optical visualization, controllable manipulation, mechanical, electrical and thermal properties, as well as the superlubricity between the shells of HACNTs. First we review the growth mechanism of ultralong CNTs, pointing out the key for obtaining CNTs with perfect structures, extraordinary properties and macroscale lengths. We then discuss the controlled synthesis of ultralong CNTs with various chirality, diameters, lengths, and morphologies, etc. After that, we discuss the optical visualization, controllable manipulation, structure and property characterization, and superlubricity of HACNTs. Finally, an outlook is presented to shed light on the future study of ultralong CNTs.