Nanomaterials have the potential to improve many next-generation technologies. They promise to speed up computer chips, increase the resolution of medical imaging devices and make electronics more energy efficient. But imbuing nanomaterials with the right properties can be time consuming and costly. A new, quick and inexpensive method for constructing diamond-based hybrid nanomaterials could soon launch the field forward.

University of Maryland researchers developed a method to build diamond-based hybrid nanoparticles in large quantities from the ground up, thereby circumventing many of the problems with current methods. The technique is described in the June 8, 2016 issue of the journal Nature Communications.

The process begins with tiny, nanoscale diamonds that contain a specific type of impurity: a single nitrogen atom where a carbon atom should be, with an empty space right next to it, resulting from a second missing carbon atom. This “nitrogen vacancy” impurity gives each diamond special optical and electromagnetic properties.

By attaching other materials to the diamond grains, such as metal particles or semiconducting materials known as “quantum dots,” the researchers can create a variety of customizable hybrid nanoparticles, including nanoscale semiconductors and magnets with precisely tailored properties.

“If you pair one of these diamonds with silver or gold nanoparticles, the metal can enhance the nanodiamond’s optical properties. If you couple the nanodiamond to a semiconducting quantum dot, the hybrid particle can transfer energy more efficiently,” said Min Ouyang, an associate professor of physics at UMD and senior author on the study.

Evidence also suggests that a single nitrogen vacancy exhibits quantum physical properties and could behave as a quantum bit, or qubit, at room temperature, according to Ouyang. Qubits are the functional units of as-yet-elusive quantum computing technology, which may one day revolutionize the way humans store and process information. 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, facilitating the integration of quantum circuits into industrial, commercial and consumer-level electronics. The new diamond-hybrid nanomaterials described in Nature Communications hold significant promise for enhancing the performance of nitrogen vacancies when used as qubits, Ouyang noted.

While such applications hold promise for the future, Ouyang and colleagues’ main breakthrough is their method for constructing the hybrid nanoparticles. Although other researchers have paired nanodiamonds with complementary nanoparticles, such efforts relied on relatively imprecise methods, such as manually installing the diamonds and particles next to each other onto a larger surface one by one. These methods are costly, time consuming and introduce a host of complications, the researchers say.

“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.

The method developed by Ouyang and his colleagues, UMD physics research associate Jianxiao Gong and physics graduate student Nathaniel Steinsultz, also enables precise control of the particles’ properties, such as the composition and total number of non-diamond particles. The hybrid nanoparticles could speed the design of room-temperature qubits for quantum computers, brighter dyes for biomedical imaging, and highly sensitive magnetic and temperature sensors, to name a few examples.

“Hybrid materials often have unique properties that arise from interactions between the different components of the hybrid. This is particularly true in nanostructured materials where strong quantum mechanical interactions can occur,” said Matthew Doty, an associate professor of materials science and engineering at the University of Delaware who was not involved with the study. “The UMD team’s new method creates a unique opportunity for bulk production of tailored hybrid materials. I expect that this advance will enable a number of new approaches for sensing and diagnostic technologies.”

The special properties of the nanodiamonds are determined by their nitrogen-vacancies, which cause defects in the diamond’s crystal structure. 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.

The nanoscale diamonds used in the study were created artificially, and have at least one nitrogen vacancy. This impurity results in an altered bond structure in the otherwise orderly carbon lattice. The altered bond is the source of the optical, electromagnetic and quantum physical properties that make the diamonds useful when paired with other nanomaterials.

Although the current study describes diamonds with nitrogen substitutions, Ouyang points out that the technique can be extended to other diamond impurities as well, each of which could open up new possibilities.

“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 explained. “It can also be scaled up fairly easily. We are interested in studying the basic physics further, but also moving toward specific applications. The potential for room-temperature quantum entanglement is particularly exciting and important.”

Marianna Kharlamova (the Lomonosov Moscow State University Department of Materials Science) examined different types of carbon nanotubes’ “stuffing” and classified them according to the influence on the properties of the nanotubes. The researcher’s work was published in the high-impact journal Progress in Materials Science (impact factor — 26.417).

An 87 pages long overview summarized the achievements of scientists in the field of the investigation of the electronic properties of single-walled carbon nanotubes (SWNTs). ‘A detailed systematic study of 430 works was conducted, including 20 author’s works, most of which had been published during the last 5 years, as the area under study is actively developing,’ says Marianna Kharlamova. Apart from analytical systematization of the existing data, the author considers the theoretical basis of such studies — the band theory of solids, which describes the interaction of the electrons in a solid.

The Many Faces of carbon: diamonds, balls, tubes

Carbon has several forms of existence (allotropic modifications) and can be found in different structures. It forms coal and carbon black, diamond, graphite, from which slate pencils are made, graphene, fullerenes and others. The whole organic chemistry is based on carbon which forms the molecular backbone. In diamonds the carbon atoms are kept on a strictly specified positions of the crystal lattice (which leads to its hardness). In graphite, the carbon atoms are arranged in hexagonal layers resembling honeycombs. Each layer is rather weakly interacting with the one above and the one below, so the material is easily separated into flakes which look to us like a pencil mark on the paper. If you take one such layer of hexagons and roll it into a tube, you get what is called a carbon nanotube.

A single-walled nanotube is a single rolled layer, and a multi-walled looks like the Russian ‘matryoshka’ doll, consisting of several concentric tubes. The diameter of each tube is a few nanometers, and the length is up to several centimeters. The ends of the tube are closed by hemispheric “caps” — halves of fullerene molecules (fullerenes are another form of elemental carbon resembling a soccer ball stitched together from hexagons and pentagons). To make and fill the carbon nanotube is much more challenging than to stuff a wafer curl : to tailor these structures scientists use laser ablation techniques, thermal dispersion in an arc discharge or vapor deposition of hydrocarbons from the gas phase.

SWNT is no cookie

What is so special about them then? The properties of the graphite (electrical conductivity, ductility, metallic shine) remind metals, yet carbon nanotubes have quite different properties, which can be used in electronics (as components of prospective nanoelectronic devices — gates, memory and data transmission devices etc.) and biomedicine (as containers for targeted drug delivery). The conductivity of carbon nanotubes can be changed depending on the orientation of the carbon hexagons relative to the tube axis, on what is included in its wall besides carbon, on which atoms and molecules are attached to the outer surface of the tube, and what it is filled with. Besides, single-walled carbon nanotubes (or SWNTs) are surprisingly tear-proof and refract light in a particular way.

Marianna Kharlamova was the first to classify types of nanotubes’ “stuffing” according to their impact on the electronic properties of SWNTs. The author of the review considers the method of filling SWNTs as the most promising for tailoring their electronic properties.

‘This is due to four main reasons,’ Marianna Kharlamova says. ‘Firstly, the range of substances that can be encapsulated in the SWNT channels is wide. Second, to introduce the substances of different chemical nature into the SWNT channels several methods have been developed: from the liquid phase (solution, melt), the gas phase, using plasma, or by chemical reactions. Third, as a result of the encapsulation process, high degree of the filling of SWNT channels can be achieved, which leads to the significant change in the electronic structure of nanotubes. Finally, the chemical transformation of the encapsulated substances allows controlling the process of tailoring the electronic properties of the SWNTs by selecting an appropriate starting material and conditions of the nanochemical reaction.’

The author herself conducted experimental studies of the filling of nanotubes with 20 simple substances and chemical compounds, revealed the influence of “stuffing” on the electronic properties of nanotubes, found the correlation between the temperature of the formation of inner tubes and the diameter of the outer tubes, and explained which factors influence the degree of the nanotubes’ filling.

Electron movements form the basis of electronics as they enable storage, processing and transfer of information. State-of-the-art electronic circuits have reached maximum clock rates of several billion switching cycles per second, limited by the heat accumulated in the process of switching power on and off.

The electric field of light changes its direction a trillion times per second and is able to move electrons in solids at this speed. Thus light waves could form the basis for future electronic switching once the induced electron motion and its influence on heat accumulation is precisely understood.

Physicists from the Laboratory for Attosecond Physics have already found out that it is possible to manipulate the electronic properties of matter at optical frequencies. In a follow-up experiment the researchers, in a manner similar to their previous approach, shot extremely strong, femtosecond-laser pulses (one femtosecond is a millionth of a billionth of a second) onto silicon dioxide glass.

A single oscillation

The light pulse comprises only a single strong oscillation cycle of the field, hence the electrons are moved left and right only once. The full temporal characterization of the light field after transmission through the thin glass plate now, for the first time, provides direct insight into the attosecond electron dynamics, induced by the light pulse in the solid.

This measurement technique reveals that electrons react with a delay of only some ten attoseconds (one attosecond is a billionth of a billionth of a second) to the incoming light. This time delay in the reaction determines the energy transferred between light and matter.

Since it is now possible to measure this energy exchange within one light cycle, the parameters of the light-matter interaction can be understood and optimized to reach out for the ultimate speed in signal processing. The more reversible the exchange and the smaller the residual energy left in the medium after the light pulse has passed, the more suitable the interaction for future light field-driven electronics.

Cool relationship

To understand the observed phenomena and identify the best set of experimental parameters to that end, the experiments were backed up by a novel simulation method based on first principles developed at the Center for Computational Sciences at University of Tsukuba. The theorists there used the K computer, currently the fourth fastest supercomputer in the world, to compute electron movement inside solids with unprecedented accuracy.

Finally, the researchers succeeded in optimizing the energy consumption by adapting the amplitude of the light field. At certain field strengths energy is transferred from the field to the solid during the first half of the pulse cycle and is almost completely emitted back in the second half of the light.

These findings verify that a potential switching medium for future light-driven electronics need not overheat. Thus the ‘cool relationship’ between glass and light might provide an opportunity for dramatically accelerating electronic signal and data processing, up to the ultimate limit.