Melting gold without heat

Materials World magazine
19 Dec 2018

Gold’s unusual behaviour when under an electric field has been observed, as Ellis Davies reports.

The surface of a gold object has, for the first time, been melted at room temperature by researchers at Chalmers University of Technology, Sweden. This occurred following periodic increases in the electric field observed via an electron microscope, and researchers say that the results give a new fundamental knowledge of gold. Although the study is in the interest of pure science, researchers envision that with further work it could have a number of applications, including use in catalysts, sensors or field-effect transistor technology and new concepts for contactless components.

It has been previously known that the application of an intense electric field to a gold surface can cause atoms to evaporate. However, it had never before been observed in real time at atomic resolution. During an experiment to observe this, researchers saw that, at an electric field lower than needed to evaporate atoms, the outmost atomic gold layers melted.

Ludvig de Knoop, Master of Science in Engineering Physics at Chalmers’ Department of Physics, told Materials World about his interest in the phenomena in gold. ‘We were interested in working with metals because they are conductive, and we chose gold because it does not oxidise. If we had been working with a metal that oxidises the surface would have been covered in an oxide layer, hindering any observation of the gold surface.’

The discovery showed a novel behavior for gold under a strong electric field. The surface-melted outmost layer can also be reverted back to a solid when the electric field is decreased. ‘In other words, the discovery shows us that we can controllably switch a few atomic layers of gold between surface melted and solid using an electric field,’ said Knoop.

Crank up the field

Researchers used a transmission electron microscope to zoom in on a gold cone two million times in order to see individual gold atoms. The gold cone was mounted in a specialised sample holder used to apply a voltage of 100V. ‘Since the size of the cone and the distance to the grounded electrode are extremely small, the [resulting] electric field is large – around 25,000,000,000V/m,’ Knoops explained. ‘Being able to apply this large electric field while simultaneously observing the movement of the atoms with the microscope allowed us to see live how the outmost atomic layers of gold melted. The technique is called in situ transmission electron microscopy.’

To understand the mechanism behind the observed melting, ab initio molecular dynamics simulations, led by shared first author Mikael Juhani Kuisma, were made. These concluded that the reason for the surface melting was that the energy cost of form surface defects vanishes when the intense electric field is applied. ‘In other words, we have a high density of surface defects in our surface melted layer,’ said Knoop.

When the electric field is applied, it excites the gold’s atoms causing them to break all structure and connections to each other. Knoop explained that although a few atomic surface layers of gold have been seen to melt at close to room temperature, the team learnt that the surface melted not from an increase
in temperature, but from an increased density of surface defects.

As it stands, the study shows only a way to melt very small amounts of gold surface. But, researchers have plans to apply this technique to other materials. ‘At the moment we are writing an article where we observe the effects of electron bombardment on gold cones,’ said Knoop. ‘Following this, we want to see if we can surface-melt other metals and materials by applying an intense electric field.

‘To reach such a high electric field, we have to be working on the nanoscale. This could be done using existing techniques from the semiconductor industry. We also want to stress that it is only a few atomic layers that are melted. Therefore, the technique will surely not be used to melt any larger sized objects.’