Samuel Jarman, SciencePOD

Lab-manufactured diamond is an extremely versatile material, widely used in modern technology for its extreme hardness and high thermal conductivity. A diverse range of properties determined by the chemical makeup of its outermost atomic layers can also be found at its surface.

Today, researchers are increasingly interested in how these surface properties can be exploited for use in practical applications. So far, however, our understanding of how these surfaces respond to artificial treatments has remained limited.

In new research published in Applied Surface Science, a team led by Rich Mildren at Macquarie University, Australia, show how diamond surface properties can be significantly altered by removing just a small fraction of the top layer of carbon atoms using pulsed ultraviolet light.

The team’s discoveries could help researchers to overcome challenges in a wide array of emerging technologies including quantum computing, biosensing, and, especially, in diamond-based electronics.

“If practical diamond transistors can be made, this could lead to major advances in high power and high frequency electronics and in electronics used in extreme environments such as in space or at high temperatures,” Mildren says. “These properties are needed for boosting the efficiency of power-hungry applications such as electric trains and cars, and powering data centres.”

So far, efforts to alter the surface properties of diamond have largely involved an approach named ablation: a broad family of techniques for removing tiny amounts of material in highly controlled ways. However, since diamond is so hard and chemically inert, many of these techniques come with significant challenges.

“Surface processing is usually performed using plasma-based treatments, or with ion or electron beams,” Mildren explains. “Although a high level of precision is possible for these techniques, it is difficult to achieve as their control systems either don’t have atomic level accuracy or are extremely difficult to implement.”

In their study, Mildren’s team investigated a gentler approach. It involved illuminating diamond surfaces with short pulses of ultraviolet (UV) laser light, with an energy per unit area below the threshold required for ablation to occur. Instead, the laser forced some carbon atoms on the surface to absorb two photons simultaneously, exciting them to higher energy levels.

“Due to this two-photon excitation, carbon atoms are ejected at the surface at a rate set by the laser pulse energy,” Mildren explains. “In that way, we can control the amount of ejection according to the laser pulse energy and the number of pulses.”

With such a high level of control, the team could manipulate diamond surfaces with unprecedented precision, removing less than 5% of their top layer of carbon atoms. To test the impact of this alteration they applied a chemical treatment to bond hydrogen atoms to the diamond’s surface to make it electrically conductive – a process named ‘hydrogen termination’.

Using advanced x-ray spectroscopy, the researchers could then determine how the diamond’s surface chemistry evolved as they varied the laser’s energy. As their measurements revealed, the electrical conductivity of their hydrogen-terminated surfaces was boosted by a factor of 10 in some cases, a far more drastic transformation than they expected.

Based on their results, the team are now confident that their approach will pave the way for a far broader exploration of the possibilities presented by manipulating diamond surfaces.

“There was a great sense of excitement when we learnt what we had achieved. At the same time, these discoveries raise new scientific questions,” Mildren says. “We don’t know how the laser is breaking the surface chemical bonds to induce the modifications, or how the changes in surface structure enable it to be more conductive. So there is much to learn, and many more discoveries to be made.”