Electron beam‐induced etching of carbon nanotubes by environmental transmission electron microscope

Abstract

It is crucially needed to develop a novel process to cut and connect nanomaterials artificially for further progress in nanotechnology. Gas‐mediated electron beam‐induced etching (EBIE) is promising to carve nanomaterials via electron beam‐induced chemical reactions between the nanomaterials and precursor gases. In this study, we have investigated the EBIE process of multi‐walled carbon nanotubes (CNTs) in oxygen gas using environmental transmission electron microscopy (ETEM). Commercially available multi‐walled CNTs (NanoIntegris Inc.) were supported on a Cu micro grid with a carbon supporting film of 3 mm in diameter. A grid with multi‐walled CNTs was fixed on a specimen holder and transferred to an ETEM (FEI Tecnai F20 equipped with an environmental‐cell [1]) operated at 200 kV. As a precursor gas for EBIE, oxygen gas (99.9999%) was introduced into the ETEM. To carry out the EBIE of multi‐walled CNTs, a focused electron beam (about 0.2 nm in diameter) was repeatedly scanned across multi‐walled CNTs in oxygen gas of 100 Pa at room temperature in scanning TEM (STEM) mode. Figure 1 shows the EBIE process of a multi‐walled CNT in oxygen gas. A focused electron beam was scanned repeatedly along a white dotted line in Fig. 1(a) with the rate of 103 s −1 . STEM images and TEM images of the multi‐walled CNT were recorded after every 62 scans. The electron dose was estimated and shown in each image in Fig. 1. As the electron dose increases, the multi‐walled CNT is etched gradually. Figures 1(b), 1(c), 1(f), and 1(g) clearly show that the etching proceeds from the outer walls to inner walls of the multi‐walled CNT. Finally, the multi‐walled CNT is cut completely along the scanning direction (Fig. 1(d), (h)). The cutting surface remains cylindrical. We confirmed that the areas that do not receive electron dose are not etched. On the other hand, multi‐walled CNTs cannot be cut in a vacuum even after receiving the electron dose much higher to cut in oxygen gas. The scanning areas in multi‐walled CNTs are amorphized. This means that both electron irradiation and oxygen gas are needed for EBIE of multi‐walled CNTs. Oxygen atoms from dissociation of oxygen molecules by electron irradiation [2] react with carbon atoms in multi‐walled CNTs to produce CO and/or CO 2 which desorb from the surface of multi‐walled CNTs. We have succeeded in cutting of multi‐walled CNTs at desired positions at room temperature by EBIE in ETEM. In the presentation, we will show the dependence of EBIE of multi‐walled CNTs on the pressure of oxygen gas, accelerating voltage of electrons, and gas species. The mechanism of EBIE of multi‐walled CNTs will be discussed.