"What we are demonstrating now is a tool kit that will help us to tune and control the charge state and help to stabilize the systems for applications," says José Antonio Garrido, a researcher at the Technische Universitat Munchen. Although the optical properties of so-called "NV centres" in diamond had already proved their credibility for a range of applications, researchers had struggled to control their optical response.

"The future of diamond electronics seemed difficult, but NV centres generated a new wave of interest," says Garrido. For most applications NV centres in a negatively charged state offer the best optical properties. "However, these states were observed to change to a neutral state – although it wasn’t clear why – and when that happens these properties that are so important are lost."

Garrido, together with colleagues at the Technische Universitat Munchen, Universitat Stuttgart and Universitat Leipzig, has demonstrated an easily scalable transistor-type system that can reversibly control the charge state of these NV centres – defects that comprise a nitrogen atom substituted in the place of a carbon atom together with a vacancy at a neighbouring lattice site. Both the neutral and negative charge states of the NV centres fluoresce, but a non-fluorescent "dark state", thought to be positive, has also been observed.

Solid state of the art

Garrido and his team exploited the effects of exposure to hydrogen, which makes diamond conducting, and exposure to oxygen, which makes it insulating, to fabricate an in-plane gated field-effect transistor. The source and drain are connected by a conducting strip of diamond with a narrowed waist in the middle defined by oxygen-exposed diamond. The system is gated at this 100 nm wide waist.

An electric field applied across the gates alters the electronic properties of the system by bending the electron energy bands. This band-bending changes the charge state of the NV centre from dark, to neutral, to negative at high enough voltages.

Combining expertise

Garrido and his team had previously demonstrated that an electrolyte-gated structure could be used to manipulate the charge states of diamond NV centres. "Members of our team were already using these electrolyte-gated field-effect transistor systems in biosensors," explains Garrido. "We knew how the presence of charges or biomolecules alters the properties of the transistor and decided to study the system with NV centres."

However, a water-based electrolyte gated system presents significant limitations for electronic applications. In addition, only changes between negative and neutral states could be achieved because larger voltages caused chemical reactions at the diamond surface. The present solid-state system overcomes these shortcomings.

Next steps

The researchers now plan to exploit the control offered by the solid-state system to to study the dark state, which is little understood because no fluorescence has been detected from it. They are also looking to progress from manipulating single centres, and ensembles of centres, to independent control of centres positioned close together. "This would be particularly useful for applications in quantum computing," adds Garrido.

Full details are reported in Nano Letters dx.doi.org/10.1021/nl4047619.