Nanodiamonds, or diamond nanoparticles, are diamonds with a size below 100 nanometers. They can be produced by impact events such as an explosion or meteoritic impacts. Because of their inexpensive, large-scale synthesis, potential for surface functionalization, and high biocompatibility, nanodiamonds are widely investigated as a potential material in biological and electronic applications and quantum engineering.
History
In 1963, Soviet scientists at the All-Union Research Institute of Technical Physics noticed that nanodiamonds were created by nuclear explosions that used carbon-based trigger explosives.
Structure and composition
There are three main aspects in the structure of diamond nanoparticles to be considered: the overall shape, the core, and the surface. Through multiple diffraction experiments, it has been determined that the overall shape of diamond nanoparticles is either spherical or elliptical. At the core of diamond nanoparticles lies a diamond cage, which is composed mainly of carbons. While the core closely resemble the structure of a diamond, the surface of diamond nanoparticles actually resemble the structure of graphite. A recent study shows that the surface consists mainly of carbons, with high amounts of phenols, pyrones, and sulfonic acid, as well as carboxylic acid groups, hydroxyl groups, and epoxide groups, though in lesser amounts. Occasionally, defects such as nitrogen-vacancy centers can be found in the structure of diamond nanoparticles. 15N NMR research confirms presence of such defects. A recent study shows that the frequency of nitrogen-vacancy centers decreases with the size of diamond nanoparticles.
Production methods
Other than explosions, methods of synthesis include hydrothermal synthesis, ion bombardment, laser bombardment, microwave plasma chemical vapor deposition techniques, ultrasound synthesis, and electrochemical synthesis. In addition, the decomposition of graphitic C3N4 under high pressure and high temperature yields large quantities of high purity diamond nanoparticles. However, detonation synthesis of nanodiamonds has become the industry standard in the commercial production of nanodiamonds: the most commonly utilized explosives being mixtures of trinitrotoluene and hexogen or octogen. Detonation is often performed in a sealed, oxygen-free, stainless steel chamber and yields a mixture of nanodiamonds averaging 5 nm and other graphitic compounds. In detonation synthesis, nanodiamonds form under pressures greater than 15 GPa and temperatures greater than 3000K in the absence of oxygen to prevent the oxidation of diamond nanoparticles. The rapid cooling of the system increases nanodiamond yields as diamond remains the most stable phase under such conditions. Detonation synthesis utilizes gas-based and liquid-based coolants such as argon and water, water-based foams, and ice. Because detonation synthesis results in a mix of nanodiamond particles and other graphitic carbon forms, extensive cleaning methods must be employed to rid the mixture of impurities. In general, gaseous ozone treatment or solution-phase nitric acid oxidation is utilized to remove sp2 carbons and metal impurities. Nanodiamonds are also formed by dissociation of ethanol vapour. and via ultrafast laser filamentation in ethanol.
Potential applications
The N-V center defect consists of a nitrogen atom in place of a carbon atom next to a vacancy (empty space instead of an atom) within the diamond’s lattice structure. Recent advances (up to 2019) in the field of nanodiamonds in quantum sensing applications using NVs have been summarized in the following review.
Applying a microwave pulse to such a defect switches the direction of its electron spin. Applying a series of such pulses (Walsh decoupling sequences) causes them to act as filters. Varying the number of pulses in a series switched the spin direction a different number of times. They efficiently extract spectral coefficients while suppressing decoherence, thus improving sensitivity. Signal-processing techniques were used to reconstruct the entire magnetic field.
The prototype used a 3 mm-diameter square diamond, but the technique can scale down to tens of nanometers.
Micro-abrasive
Nanodiamonds share the hardness and chemical stability of visible-scale diamonds, making them candidates for applications such as polishes and engine oil additives for improved lubrication.