This 3D image of a nanodiamond, produced by Bragg coherent diffraction imaging, shows surface coloration indicative of local strain. Image: Stephan Hruszkewycz.
This 3D image of a nanodiamond, produced by Bragg coherent diffraction imaging, shows surface coloration indicative of local strain. Image: Stephan Hruszkewycz.

Quantum mechanics, the physics that governs nature at the atomic and subatomic scale, contains a host of new physical phenomena for exploring quantum states at the nanoscale. Though tricky, there are ways to exploit these inherently fragile and sensitive systems for quantum sensing.

One nascent technology in particular makes use of point defects, or single-atom misplacements, in nanoscale materials, such as diamond nanoparticles, for measuring electromagnetic fields, temperature, pressure, frequency and other variables with unprecedented precision and accuracy. Quantum sensing could revolutionize medical diagnostics, lead to the development of new drugs, improve the design of electronic devices and more.

For use in quantum sensing, the bulk nanodiamond crystal surrounding the point defect must be highly perfect. Any deviation from perfection, such as additional missing atoms, strain in the crystalline lattice of the diamond or the presence of other impurities, will adversely affect the quantum behavior of the material. Unfortunately, highly perfect nanodiamonds are quite expensive and difficult to make.

A cheaper alternative, say researchers at Argonne National Laboratory and the University of Chicago, is to take defect-ridden, low-quality, commercially-manufactured diamonds, and then ‘heal’ them. In a paper published in APL Materials, the researchers describe a method for healing diamond nanocrystals under high-temperature conditions, while visualizing the crystals in three dimensions using an X-ray imaging technique.

“Quantum sensing is based on the unique properties of certain optically-active point defects in semiconductor nanostructures,” said F. Joseph Heremans, an Argonne National Laboratory staff scientist and co-author of the paper.

These defects include nitrogen-vacancy (NV) centers in diamond, which are created when a nitrogen atom replaces a carbon atom adjacent to a vacancy in the diamond lattice structure. These defects are extremely sensitive to their environment, making them useful probes of local temperatures, as well as electric and magnetic fields, with a spatial resolution more than 100 times smaller than the thickness of a human hair.

Because diamonds are biologically inert, quantum sensors based on diamond nanoparticles, which can operate at room temperature and detect several factors simultaneously, could even be placed within living cells. Here, according to Heremans, they could “image systems from the inside out”.

Heremans and his colleagues, which included Argonne’s Wonsuk Cha and Paul Fuoss, as well as David Awschalom from the University of Chicago, set out to map the distribution of the crystal strain in nanodiamonds and to track the healing of these imperfections. They did this by subjecting the nanodiamonds to high temperatures: up to 800°C in an inert helium environment.

“Our idea of the ‘healing’ process is that gaps in the lattice are filled as the atoms move around when the crystal is heated to high temperatures, thereby improving the homogeneity of the crystal lattice,” explained Stephan Hruszkewycz, also a staff scientist at Argonne and lead author on the paper.

The researchers monitored this nanodiamond healing with a three-dimensional (3D) microscopy method called Bragg coherent diffraction imaging, which involved illuminating the nanodiamonds with a coherent X-ray beam at the Advanced Photon Source at Argonne. The X-rays scatter off the nanodiamonds to form a coherent diffraction pattern; a series of these patterns can be used to reconstruct the 3D shape of the nanocrystal, “and, more importantly, the strain state of the crystal,” Hruszkewycz said.

The researchers found that nanodiamonds ‘shrink’ during the high-temperature annealing process, and surmise that this occurs because of a phenomenon called graphitization. This occurs when the surface of the material is converted from the normal diamond lattice arrangement into graphite, where the carbon atoms are arranged in a chicken-wire-like formation.

This study marks the first time that Bragg coherent diffraction imaging has been shown to be useful at such high temperatures. According to Hruszkewycz, this capability “enables the exploration of structural changes in important nanocrystalline materials at high temperatures that are difficult to access with other microscopy techniques”.

He added that the research represents “a significant step towards developing scalable methods of processing inexpensive, commercial nanodiamonds for quantum sensing and information processing.”

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.