The researchers blasted carbon ions through holes to create vacancies and heated the diamond to make the vacancies mobile within the crystal | Credit: F.J. Heremans and D. Awschalom/U. Chicago and K. Ohno/UCSBAtomic-scale defects can be made in diamond more accurately than ever before, thanks to a team of US-based researchers. This could advance a range of applications such as quantum computing and atomic-scale sensing.
David Awschalom at the University of Chicago and colleagues developed a route for precisely introducing diamond defects called nitrogen vacancy centers into diamond film. In these defects two neighboring carbon atoms in the diamond lattice are replaced with a nitrogen and a vacant spot. Each nitrogen vacancy center therefore contains an unpaired electron, and it is the spin of this electron that is so useful.
It has previously been difficult to control the positioning of nitrogen vacancy center defects within a 3D diamond structure while also preserving their desirable long spin-lifetimes, explains Awschalom. ‘Our work demonstrates a crystal growth technique in combination with nano-lithography that meets both these requirements,’ he says.
‘The key concept of our approach is to separate the incorporation of nitrogen atoms from the creation of vacancies,’ he explains. The first step of the work published in Applied Physics Letters [Ohno K. et al., Appl. Phys. Lett. (2014) DOI: 10.1063/1.4890613) was to introduce a very thin layer of nitrogen atoms during the growth of the diamond film. The nitrogen layer was kept extremely thin by significantly slowly down the speed of growth. ‘This enables control over the depth position of the nitrogen vacancy centers at the nanometer scale,’ says Awschalom.
The second step involved shooting carbon atoms into the tiny pin-prick holes of a mask placed over the film. These collide with the carbon atoms in the lattice creating vacancies beneath each hole. The team then heated the material, causing the vacancies to become mobile and travel downwards to form nitrogen vacancy centers in the nitrogen layer.
‘This ultimately allows the full 3D control of the nitrogen vacancy center position,’ says Awschalom. This level of control opens up the fabrication technique to a multitude of potential uses. The ability to add a nitrogen vacancy center to a functional “sweet spot” of a photonic crystal, for example, can greatly enhance its photoluminescence emission.
The team also showed that these nitrogen vacancy centers can hold a specific spin for longer than 300 microseconds, an order of magnitude longer than has been achieved using other fabrication methods. This long spin lifetime means the material can hold quantum information for longer – useful when using the electron’s spin as a quantum-analogue of a computing bit.
This material could also be used to enhance the tips of a diamond-based scanning probe microscope being used to image magnetic fields. The longer spin lifetimes should enable smaller magnetic signals to be detected than previously possible. The team plan to test this theory by measuring nuclear spins of hydrogen atoms within a biological molecule. This could improve our understanding of how photosynthesis works.
‘We anticipate that these devices not only provide improved performances, but will also trigger new exciting scientific opportunities, as advancements in material quality open up new doors for the science community,’ says Awschalom.