Researchers produce nanostructured diamonds by irradiating graphite with laser

A paper in Nature's Scientific Reports describes experiment involving light source that emits 25-femtosecond pulses

By José Tadeu Arantes  |  Agência FAPESP – The formation of diamonds in nature depends essentially on the presence of carbon under high pressure (in the range of 15 gigapascals, or just over 148,000 atmospheres) and high temperature (around 2,500 degrees Celsius).

These conditions are present deep inside the earth’s crust but can also be reproduced in a laboratory. A well-known process for synthesizing diamonds involves compressing graphite while heating it to a high temperature using an electric current. The carbon atoms in the graphite are rearranged into a different crystal structure to make conventional diamonds. 

Another form of diamond made up of nanocrystals has been produced in the lab, also under high pressure and temperature. It is even harder and stronger than natural diamonds but costs a great deal to produce because of the equipment required.

Brazilian researchers have developed a viable alternative. With this method, the same pressure and temperature levels were reached using a shockwave generated by ultrashort laser pulses. The experiment is described in the article “Synthesis of diamond-like phase from graphite by ultrafast laser driven dynamical compression”, published in Scientific Reports, an online journal published by Nature Publishing Group. 

“The laser we used not only generates high-energy pulses but emits them at very short intervals lasting only 25 femtoseconds [25×10-15 seconds] and concentrates them in a very small area with a radius of 65 micrometres [65×10-6 metres]. All these factors converged to enable us to reach the pressure and temperature levels required for the shockwave,” physicist Narcizo Marques de Souza Neto told Agência FAPESP.

Souza Neto, a researcher with the National Synchrotron Light Laboratory (LNLS) in Brazil, designed the experiment in the context of projects supported by FAPESP. 

“We succeeded in producing a highly desirable final nanomaterial for several applications  with relatively modest resources,” said physicist Francisco Carlos Barbosa Maia, a postdoctoral fellow at LNLS and lead author of the paper. Among the applications are use in electronic components and as coatings for artificial joints, cell markers, and drug vectors.

D-Scan technique

The experiment also stands out for its simplicity. It used polycrystalline graphite, the most common form of the material, instead of the far more expensive highly oriented pyrolytic graphite (HOPG) used in other studies. Although the laser used in the experiment produces high-powered ultrashort pulses, it is affordable for midsize labs in Brazil and elsewhere.

“The procedure involved moving a graphite block in front of a focused laser beam [see photo] so that streams of laser pulses overlapped at each position on the block, in a form quantified by a technique called D-Scan, which we developed,” said Ricardo Elgul Samad, a researcher with the Energy & Nuclear Research Institute (IPEN) in Brazil. Samad is an expert in high-intensity ultrashort pulse lasers and also participates in research projects supported by FAPESP.

As a result of the irradiation, several micrometre-scale crystals were formed in the range of 10-50 μm. They included nanometric crystallites of a carbon allotrope similar to diamonds. 

The micrometre-scale crystals were studied using Raman microspectroscopy (RM), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). “Besides diamond-like nanometric crystallites, we found another notable carbon formation in which the atoms are arranged in an onion-like structure,” said microscopy specialist Jefferson Bettini, a researcher with the National Nanotechnology Laboratory (LNNano), also in Brazil.

Based on this discovery, the researchers proposed a mechanism for transforming graphite into the diamond-like allotrope. The mechanism follows an indirect pathway that depends on the morphology of the starting material, specific thermodynamic events produced by ultrashort laser pulses, and the formation of natural catalysts such as onion-like structures and nanometre-scale graphite granules.

New synchrotron light source

These results are highly interesting, yet for the researchers they are only a first step toward more groundbreaking achievements. “When we begin operating Sirius, the new synchrotron light source, in 2018, we’ll be able to reach pressures of more than 1 terapascal, equivalent to 10 million atmospheres, and temperatures as high as 50,000 degrees Celsius in shockwave experiments,” Souza Neto said.

The existing synchrotron light source at LNLS is second generation. Sirius, which is scheduled to go live in 2018, will be one of the world’s only fourth-generation synchrotron light sources, alongside the Max 4 facility now under construction in Sweden. Many experiments that cannot be performed in Brazil today will be feasible with Sirius. 

According to Souza Neto, the experiment was a proof-of-concept test of the use of high-intensity ultrashort laser pulses to generate a shockwave, with a view to future development using Sirius. “The synthesis and study of new phases of matter at very high pressure and temperature could lead to the discovery of materials with extraordinary properties for everyday applications,” he said.

“In this sense lasers are key to processes that require extreme conditions. They can be used to produce electromagnetic fields, pressures and temperatures that have never been accessible before,” said Nilson Dias Vieira Junior, a researcher with IPEN.

The article published by Scientific Reports was written by Francisco Carlos Barbosa Maia (LNLS), Ricardo Elgul Samad (IPEN), Jefferson Bettini (LNNano), Raul de Oliveira Freitas (LNLS), Nilson Dias Vieira Junior (IPEN) and Narcizo Marques de Souza Neto (LNLS). Another highlight of the experiment is the synergy achieved among the three institutions involved.

LNLS and LNNano are both national laboratories open to researchers from anywhere in Brazil and from abroad. They are located on the same campus, at the National Energy & Materials Research Center (CNPEM) in Campinas, São Paulo State. IPEN is located on the main campus of the University of São Paulo (USP) in São Paulo City.