A practical method for performing full-tilt-angle electron tomography of submicron-sized particles

Abstract

In this study, a practical method for performing full-tilt-angle electron tomography for particles with sizes of several 100 nanometers was developed. To achieve this, we designed a specimen holder for a 200-kV transmission electron microscope (TEM) and a protocol that can pick up the particle of interest in a focused ion beam (FIB) instrument equipped with a scanning electron microscope (SEM). In the protocol, carbon deposition through electron beams was used to attach the particle to the tip of a tungsten (W) needle. This technique prevents surface etching and radiation-induced damage by Ga ion beams and ensures clear TEM observations. Subsequently, the W needle was detached from the FIB–SEM system and fixed to the developed specimen holder. The holder can be tilted by ±90° in a TEM through a goniometer, even with an objective lens having a pole piece with a narrow gap for atomic resolution. By employing this procedure, full-tilt-angle tomography was demonstrated in this study using TEM images of a polystyrene latex particle and annular dark-field scanning TEM images of a ZnO tetrapod particle. Clear three-dimensional (3D) reconstructions of the external shapes of these submicron-sized particles were obtained without the missing wedge effect. By carefully placing these particles on the eucentric axis, a tilt series for tomography is obtained between −90° and + 90° without position compensation against the shifts induced by tilting. Using this advantage, full-tilt-angle fast tomography of submicron-sized ZnO particles was achieved. Based on a 25-s movie recorded during continuous tilting, we realized 3D reconstruction having a quality similar to that of conventional tomography, where the measurement is performed for over more than an hour. The mechanical stability of a particle attached to the W needle was assessed through atomic-resolution side-view observation of a plate-like flake of TiSe₂ using an additional tilting mechanism incorporated into the developed holder.