Structural changes of Au nanocones during in situ cold‐field emission observed by high‐resolution TEM

Abstract

In situ transmission electron microscopy (TEM) allows imaging on the atomic scale of complex physical phenomena, which are induced by an externally applied stimuli. This provides a directly observed correlation between material structure and properties, which promotes the understanding of the material and the triggered phenomena. Here, a Nanofactory in situ TEM biasing holder with a nanomanipulator has been used to both manipulate Au nanostructures and also to enable the studies of electron cold‐field emission (CFE). The conical shaped Au nanostructures are produced by hole‐mask colloidal lithography on an electron‐transparent carbon film in macroscopic short‐range‐ordered arrays [1]. Individual nanocones typically feature a tip radius of around 5 nm and a height of around 180 nm. The entire macro‐array is then transferred to a mechanically cut Au‐wire that subsequently was inserted into the in situ TEM holder (Fig. 1). The nanomanipulator of the TEM holder can move in 3D, with both coarse and fine motion. The coarse control utilizes a slip‐stick mechanism for a mm‐ranged motion. The piezo‐driven fine control has a range of 10 μm and a resolution on the sub‐Å level. The Au nanocones in Fig. 1 were transferred to the nanomanipulator, making the configuration seen in Fig. 2. The nanomanipulator was thereafter positioned opposite a nanocone that was in direct contact with the Au‐wire (Fig. 2). During the experiment, the electrical potential between the two cones was increased until the electric field around the cathode nanocone was sufficiently high (several volts per nm) to initiate CFE. Earlier work using a similar TEM holder reports about in situ CFE experiments using carbon‐based nanotips over a distance of a few hundreds of nanometer [2–4]. Here, the distance between the two Au nanocones is around 20 nm, allowing for simultaneous imaging at high resolution of both cones during CFE. This allows a better understanding of the CFE process and the effects of electron bombardment. At 115 V applied voltage with a CFE current, i e , of 4 μA, structural changes of the anode Au nanocone were observed. The change in structure started with a faceting at the apex of the anode nanocone. At the same time, the anode nanocone material was redistributed forming an elongated structure, making the anode nanocone thinner over a region that stretched over more than 30 nm from the tip towards the base. The elongation was a multi‐stage process, taking about 5 s to complete. See images in Figs. 3a and 3b, which are separated by 0.4 s. The electron bombardment current was kept in the μA‐range and resulted in an amorphization of the outmost atomic layers of the anode apex around 7 s after the structural changes of the nanocone had occurred. During these events, no structural changes were observed on the cathode. This indicates that the structural changes to the anode Au nanocone is an effect of electron bombardment by the emitted and accelerated electrons.