How creating defective nanodiamonds could revolutionize nanotechnology and quantum computing

June 8, 2016

This electron microscope image shows a hybrid nanoparticle consisting of a nanodiamond (roughly 50 nanometers wide) covered in smaller silver nanoparticles that enhance the diamond’s optical properties. (credit: Min Ouyang)

University of Maryland researchers have developed a method to quickly and inexpensively assemble diamond-based hybrid nanoparticles from the ground up in large quantities while avoiding many of the problems with current methods.

These hybrid nanoparticles could speed the design of room-temperature qubits for quantum computers and create brighter dyes for biomedical imaging or highly sensitive magnetic and temperature sensors, for example.

When impurities are better

Synthetic diamonds of various colors (from defects) grown by the high-pressure high-temperature technique (credit: Wikipedia/
public domain)

The basic trick in creating a interesting or useful diamond is, ironically: Add a defect in the diamond’s crystal lattice. It’s similar to doping silicon to give it special electronic properties (such as making it work as a transistor).

Pure diamonds consist of an orderly lattice of carbon atoms and are completely transparent. However, pure diamonds are quite rare in natural diamond deposits; most have defects resulting from non-carbon impurities such as nitrogen, boron and phosphorus. Such defects create the subtle and desirable color variations seen in gemstone diamonds.

This altered bond is also the source of the optical, electromagnetic, and quantum physical properties that will make a nanodiamond useful when paired with other nanomaterials.

Nitrogen vacancy impurity

Model of nitrogen-vacancy center in diamond (credit: Wikipedia/public domain)

The most useful impurity — and used in the Maryland study — is the famous “nitrogen vacancy” defect: Sticking in a single nitrogen atom where a carbon atom should be, with an empty space right next to it.

As KurzweilAI has shown in several articles, a nitrogen vacancy in a diamond (or other crystalline materials) can lead to a variety of interesting new properties, such as a highly sensitive way to detect neural signals, an ultrasensitive real-time magnetic field detector, and importantly, making a nanodiamond behave as a quantum bit (qubit) for use in quantum computing and other applications.

Nearly all qubits studied to date require ultra-cold temperatures to function properly. A qubit that works at room temperature would represent a significant step forward, helping use quantum circuits in industrial, commercial and consumer-level electronics. That’s of special interest to Ougang’s team.

Volume production of hybrid nanoparticles

A synthetic route for hybrid nanodiamond nanoparticles. (a) Different growth stages, ending in (S6) growth of metal nanoparticles on the nanodiamond surface. (b) Transmission electron microscope image showing hybrid nanodiamond-silver nanostructures made by following the synthetic scheme in (a). Scale bar, 200 nm. (credit: J. Gong et al./Nature Communications)

Ougang’s and colleagues’ main breakthrough, though, is their method for constructing the hybrid nanoparticles. Other researchers have paired nanodiamonds with complementary nanoparticles using relatively imprecise methods, such as manually installing the diamonds and particles next to each other onto a larger surface one by one.

These top-down methods are costly, time consuming, and introduce a host of complications. “Our key innovation is that we can now reliably and efficiently produce these freestanding hybrid particles in large numbers,” explained Ouyang, who also has appointments in the UMD Center for Nanophysics and Advanced Materials and the Maryland NanoCenter, with an affiliate professorship in the UMD Department of Materials Science and Engineering.

His team’s method also enables precise control of the hybrid particles’ properties, such as the composition and total number of non-diamond particles.

“A major strength of our technique is that it is broadly useful and can be applied to a variety of diamond types and paired with a variety of other nanomaterials,” Ouyang said. “It can also be scaled up fairly easily. We are interested in studying the basic physics further, but also moving toward specific applications.”

Abstract of Nanodiamond-based nanostructures for coupling nitrogen-vacancy centres to metal nanoparticles and semiconductor quantum dots

The ability to control the interaction between nitrogen-vacancy centres in diamond and photonic and/or broadband plasmonic nanostructures is crucial for the development of solid-state quantum devices with optimum performance. However, existing methods typically employ top-down fabrication, which restrict scalable and feasible manipulation of nitrogen-vacancy centres. Here, we develop a general bottom-up approach to fabricate an emerging class of freestanding nanodiamond-based hybrid nanostructures with external functional units of either plasmonic nanoparticles or excitonic quantum dots. Precise control of the structural parameters (including size, composition, coverage and spacing of the external functional units) is achieved, representing a pre-requisite for exploring the underlying physics. Fine tuning of the emission characteristics through structural regulation is demonstrated by performing single-particle optical studies. This study opens a rich toolbox to tailor properties of quantum emitters, which can facilitate design guidelines for devices based on nitrogen-vacancy centres that use these freestanding hybrid nanostructures as building blocks.