Atomic impurities, or defects, in natural diamond lead to the colour seen in pink, blue and yellow diamonds. One such defect, the NV, occurs when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site. NVs in nanodiamonds could be ideal as biological probes because they are non-toxic, photostable and can easily be inserted into living cells. They are also capable of detecting the very weak magnetic fields that come from electronic or nuclear spins, and so can be used as highly sensitive magnetic resonance probes capable of monitoring local spin changes in a material over distances of a few tens of nanometres. And, in contrast to conventional magnetic resonance imaging techniques in biology in which millions of spins are required to produce a measurable signal, the NV defects can detect individual target spins with spatial precision measured in nanometres.

The main problem until now, however, was that the spin coherence time in these NVs was annoyingly short because of the high concentration of paramagnetic impurities (namely nitrogen) in diamond nanocrystals grown by conventional high-pressure high-temperature (HPHT) processes. A team led by Dirk Englund of the Massachusetts Institute of Technology (MIT) has now developed a top-down fabrication technique using a self-assembling porous metal mask and reactive ion etching process that produces extremely pure nanocrystals devoid of paramagnetic impurities. These nanocrystals contain NVs that are able to preserve their spin states for as long as 210 µs.

The defects also have record magnetic field sensitivities of 290 nT Hz–1/2, which means that they can be used as magnetic field probes that have a diameter of just 50 nm. “And that is not all: the NVs can be produced in their billions or more per shot, without much effort on our part, thanks to the simple ‘self-guiding’ or porous metal mask we employed,” MIT team member Matthew Trusheim told nanotechweb.org.

Single photon sources and solid-state qubits

As well as being ideal as magnetic field sensors, the NVs might be placed in photonic structures and used as single photon sources or as solid-state quantum bits (qubits) entangled with photons, he adds. Quantum computers exploit the counterintuitive idea that tiny objects can exist in more than one state at the same time. Rather than processing classic bits – which are either 0 or 1 – such devices instead manipulate qubits that can be 0 and 1 at the same time. Vast numbers of logic operations could then be possible in parallel, making these computers theoretically far faster than ordinary machines.

Until now, qubits made from nanodiamond NVs were incredibly fragile and the quantum information they held was rapidly destroyed by interactions with noise in the surrounding environment. The new result, published in Nano Lett. DOI: 10.1021/nl402799u, could go a long way in changing all this.

Gold and palladium mask

The researchers made their highly pure nanocrystals of diamond by first depositing a metal mask (made of gold and palladium) onto a high-purity bulk diamond substrate. The mask self-assembles into droplets that are tens of nanometres in size. “Next, we used an oxygen plasma etch, where reactive ions are accelerated onto the substrate surface to remove the diamond,” explained Trusheim. “The metal mask blocks the incoming ions, forming small regions where the diamond is preserved. These regions are then removed mechanically and become our nanodiamond.”

The team, which includes scientists from Columbia University, the City College of New York and the University at Albany-State of New York, says that it would now like to use these NV-fluorescent nanodiamonds in real sensing applications. “For example, they could be used to detect the electric fields coming from neural action potentials in the body, or to detect magnetic proteins in living cells,” said Trusheim. “We are also integrating these diamond structures into photonic networks to efficiently interface photonic qubits with spin qubits,” he revealed.

Further reading

Silicon carbide shows promise for quantum computing (Nov 2011)
Diamond downsizes classical MRI and NMR (Feb 2013)
Nanodiamond thermometer takes temperature of biological cells (Aug 2013)
Nanocavity helps graphene absorb more light (Oct 2012)
Gated graphene makes high-contrast modulator (Feb 2013)