Single GeV center in diamond. (left) Fluorescence mapping and (right) atomic structure model. Credit: Tokyo Institute of Technology
Germanium defects in a diamond crystal lattice act as a reliable source for single photons. Pure diamonds are naturally colorless, but gaps in the crystal structure or impurities of other elements can create colors and even emit fluorescence. Recently, researchers have shown that the fluorescent lattice defects could be useful as single photon sources for quantum cryptography and as bright luminescent makers in living cells.
Now a new type of diamond crystal defect that fluoresces to produce single photons in a narrow, high energy wavelength band has been demonstrated. The defects, which have been named germanium-vacancy (GeV) centres, are easy to fabricate in a reliable, reproducible way.
Iwasaki and co-workers were inspired by recent work that demonstrated fluorescence from nitrogen-vacancy (NV) and silicon-vacancy (SiV) defects in diamond. They used an ion implantation method to insert germanium atoms into diamond films, before heating the films at 800 ⁰C. The resulting samples showed fluorescence only after heating, which induces diffusion of vacancies in the diamond lattice. They concluded the fluorescence was produced by combined defects, each comprising a germanium atom side-by-side with a vacancy.
The biggest step forward was when they managed to create the same types of defects through a different method, microwave plasma chemical vapor deposition (MPCVD). MPCVD involves reactions of volatile chemicals on a substrate, and is often used to make synthetic diamonds. The defects in the sample prepared using MPCVD gave off more consistent fluorescence with a narrower and more stable peak. MPCVD provides closer control over the fabrication process, and is less likely to produce unwanted damage to the samples than ion implantation.
Luminescence characteristics of GeV color center in diamond formed by ion implantation.
The GeV centres produced single-photon bursts of fluorescence centred at a wavelength of around 602 nm, representing a higher energy fluorescence than SiV centres. They also created the films through the less destructive method of chemical vapor deposition, producing films with narrower and more stable emission peaks of ensemble GeV centres which are useful for biomarkers.
The work opens up a promising new avenue for developing sources of single photons, which are essential for quantum cryptography. Iwasaki and co-workers are also hopeful that they could incorporate GeV centres in nanodiamonds for use as biological markers. http://www.nature.com/articles/srep12882 http://www.chemeurope.com/en/news/154629/new-diamond-structures-produce-bright-luminescence.html
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