Fabrication of AFM Single-Walled Carbon Nanotube Tips

Abstract

Probe is a key part of atomic force microscopy (AFM) and the curvature radius of its tip limits the lateral resolution. Furthermore, as an expendable, its life has a direct effect on operation cost. Single-walled carbon nanotubes (SWNTs) are ideal materials for the probe tips used in atomic force microscopy (AFM), since they have intrinsically small diameters, high aspect ratios and unique wearability. Herein, the fabrication of AFM SWNT tips is systemically investigated, including mechanical method and direct growth method. In the former, substrates with isolated SWNTs are first prepared by chemical vapor deposition and then scanned with conventional silicon probes under AFM. During scanning, SWNTs can be picked up from substrates and attached to the pyramid of silicon probes via attractive Van del Waals forces. Preparation of substrates is decisive to this method, since only the SWNTs disengaged from surface are possible to pick up. Different substrates have been compared and the results show that the substrates full of gaps are prior to the planar ones. SWNTs are suspended across gaps and suitable to the following assembly. The mechanical method is easily implemented and can produce AFM SWNT tips one by one, while the direct growth method is promising to manufacture them in a large scale, in spite of more difficulty. In the latter, catalyst for the growth of SWNTs is directly placed on individual commercial probes or pyramid arrays. Through chemical vapor deposition, grown SWNTs protrude from the original tips and create new ones. Due to the random orientation of SWNT growth, the yield of successful SWNT tips is generally low for the direct growth method. In this article, Electric filed is used to direct the growth orientation of SWNTs during chemical vapor deposition. Both electrodes are located in the furnace to apply a static electric field along the pyramid axis during growth. The intensity can be varied from 0.1V/um to IV/um, and the direction is towards the surface of wafers. The growth of SWNTs is oriented by electric field and the yield of SWNT tips is raised. The AFM SWNT tips derived from the above two methods have been utilized to image Au layers in the tapping mode. Compared with conventional silicon probes, SWNT tips are longer lasting to keep a high resolution.