QUT physics student, David Sommers has discovered diamond can generate strong electrical current when exposed to visible light, a finding that could lead to a new species of optoelectronics.
Sommers made the discovery while working with Associate Professor Dongchen Qi from the QUT School of Chemistry and Physics, who recently proved diamonds could become conductive by modifying the surface with metal oxide.
As part of an undergraduate physics capstone research project, Qi gave Sommers a diamond chip with an electronic circuit already formed on the surface so he could investigate if diamond had photocurrent properties.
“I was trying to determine if the electronic circuit could be powered by photocurrent — the transfer of energy from light to an electron within the diamond structure,” Sommers said.
“Diamond is insulating, not conducting, so you can’t actually get any charge carriers like electrons to move through it in great numbers.
“Modifying the diamond surface with a thin metal oxide layer creates an energy offset to extract electrons from carbon bonds, which basically glue the diamond atomic structure together, and trap them in the oxide layer.
“Electrons that remain in the carbon bonds can then wander around by skipping in and out of ‘holes’ left behind by electrons that transferred to the oxide.”
In semiconductors, holes refer to the absence of electrons but are essentially the same in conducting electricity.
“This process is very akin to a bubble moving through water,” Sommers explained.
“A bubble in water is simply a void. Inside the bubble is nothing but the absence of water, but when it appears to move through water what we are actually seeing is the movement of molecules around the void.”
More current for less photo energy
Sommers thinks the modified surface layer is the reason his diamond needed less photo energy to free electrons.
“We think the oxide layer introduced energy states to the diamond bandgap, offsetting the amount of light energy needed to free electrons,” Sommers said.
Semiconductors including diamond comprise a valence band with lots of electrons and a conduction band with no electrons.
The energy difference between them is called the bandgap and represents the amount of energy needed to free an electron to the conduction band and create holes in the valence band.
“Our proposed mechanism is that we didn’t actually need to excite electrons to the conduction band — we could get hole current in the valence band using less photoenergy by jumping electrons to the surface instead,” Sommers said.
The diamond bandgap is large compared to other materials — about 5.5 electron volts (eV) — which corresponds to a wavelength of 225 nm for electromagnetic radiation — also known as ‘light’.
Shorter wavelengths with less distance between each wave have more energy than longer, loose waves — so the waves with smaller measurements have more energy.
Sommers and Qi expected only ultraviolet light, a short electromagnetic wavelength below 225 nm, would have enough energy to excite the electrons to diamond’s conduction band.
“An incoming photon would need an energy equal to or greater than the energy gap to free an electron, but I achieved photocurrent with a green laser about 532 nm, less than half the energy needed,” Sommers said.
“I also tested white light, which comprises all visible wavelengths — red, orange, yellow, green, blue, indigo, violet — and measures between 400 nm and 1000 nm.
“Its power was lower than the laser, but it had a larger beam diameter, so more electrons could be photo-excited.
“That created more hole carriers in the valence band and, therefore, more current.”
Sommers recently completed QUT’s Nanotechnology Minor, which focuses on skills and knowledge needed to synthesise and characterise nanomaterials and helped him understand the principles governing his nano-scale discovery.
Supervisor Qi, who is also a Chief Investigator with the QUT Centre for Materials Science said Sommers’ findings could be useful in the creation of switches and transistors in new electronic devices.
“In electronics, you need transistors, which form the binary logic of electronic devices and allow them to do certain computations. If a transistor as a switch lets current through, for example, it could represent a one or zero — on or off,” Qi said.
“Researchers are always looking for new semiconductor and optoelectronic materials, which is what this diamond platform could offer.”
The large bandgap of diamond also means it can sustain very high voltages without breaking down, according to Qi.
“Having such a large bandgap makes diamond potentially suited for electronics in high power operations like electricity grids, and technologies in extreme conditions like satellites and deep space applications,” Qi said.
“The generation of photocurrent with visible light adds a whole new dimension to diamond electronics,” he said.