Tag Archives: NaCl

Deriving graphene-like films from salt

This research comes from Russia (mostly). A July 29, 2016 news item on ScienceDaily describes a graphene-like structure derived from salt,

Researchers from Moscow Institute of Physics and Technology (MIPT), Skolkovo Institute of Science and Technology (Skoltech), the Technological Institute for Superhard and Novel Carbon Materials (TISNCM), the National University of Science and Technology MISiS (Russia), and Rice University (USA) used computer simulations to find how thin a slab of salt has to be in order for it to break up into graphene-like layers. Based on the computer simulation, they derived the equation for the number of layers in a crystal that will produce ultrathin films with applications in nanoelectronics. …

Caption: Transition from a cubic arrangement into several hexagonal layers. Credit: authors of the study

Caption: Transition from a cubic arrangement into several hexagonal layers. Credit: authors of the study

A July 29, 2016 Moscow Institute of Physics and Technology press release on EurekAlert, which originated the news item,  provides more technical detail,

From 3D to 2D

Unique monoatomic thickness of graphene makes it an attractive and useful material. Its crystal lattice resembles a honeycombs, as the bonds between the constituent atoms form regular hexagons. Graphene is a single layer of a three-dimensional graphite crystal and its properties (as well as properties of any 2D crystal) are radically different from its 3D counterpart. Since the discovery of graphene, a large amount of research has been directed at new two-dimensional materials with intriguing properties. Ultrathin films have unusual properties that might be useful for applications such as nano- and microelectronics.

Previous theoretical studies suggested that films with a cubic structure and ionic bonding could spontaneously convert to a layered hexagonal graphitic structure in what is known as graphitisation. For some substances, this conversion has been experimentally observed. It was predicted that rock salt NaCl can be one of the compounds with graphitisation tendencies. Graphitisation of cubic compounds could produce new and promising structures for applications in nanoelectronics. However, no theory has been developed that would account for this process in the case of an arbitrary cubic compound and make predictions about its conversion into graphene-like salt layers.

For graphitisation to occur, the crystal layers need to be reduced along the main diagonal of the cubic structure. This will result in one crystal surface being made of sodium ions Na? and the other of chloride ions Cl?. It is important to note that positive and negative ions (i.e. Na? and Cl?)–and not neutral atoms–occupy the lattice points of the structure. This generates charges of opposite signs on the two surfaces. As long as the surfaces are remote from each other, all charges cancel out, and the salt slab shows a preference for a cubic structure. However, if the film is made sufficiently thin, this gives rise to a large dipole moment due to the opposite charges of the two crystal surfaces. The structure seeks to get rid of the dipole moment, which increases the energy of the system. To make the surfaces charge-neutral, the crystal undergoes a rearrangement of atoms.

Experiment vs model

To study how graphitisation tendencies vary depending on the compound, the researchers examined 16 binary compounds with the general formula AB, where A stands for one of the four alkali metals lithium Li, sodium Na, potassium K, and rubidium Rb. These are highly reactive elements found in Group 1 of the periodic table. The B in the formula stands for any of the four halogens fluorine F, chlorine Cl, bromine Br, and iodine I. These elements are in Group 17 of the periodic table and readily react with alkali metals.

All compounds in this study come in a number of different structures, also known as crystal lattices or phases. If atmospheric pressure is increased to 300,000 times its normal value, an another phase (B2) of NaCl (represented by the yellow portion of the diagram) becomes more stable, effecting a change in the crystal lattice. To test their choice of methods and parameters, the researchers simulated two crystal lattices and calculated the pressure that corresponds to the phase transition between them. Their predictions agree with experimental data.

Just how thin should it be?

The compounds within the scope of this study can all have a hexagonal, “graphitic”, G phase (the red in the diagram) that is unstable in 3D bulk but becomes the most stable structure for ultrathin (2D or quasi-2D) films. The researchers identified the relationship between the surface energy of a film and the number of layers in it for both cubic and hexagonal structures. They graphed this relationship by plotting two lines with different slopes for each of the compounds studied. Each pair of lines associated with one compound has a common point that corresponds to the critical slab thickness that makes conversion from a cubic to a hexagonal structure energetically favourable. For example, the critical number of layers was found to be close to 11 for all sodium salts and between 19 and 27 for lithium salts.

Based on this data, the researchers established a relationship between the critical number of layers and two parameters that determine the strength of the ionic bonds in various compounds. The first parameter indicates the size of an ion of a given metal–its ionic radius. The second parameter is called electronegativity and is a measure of the ? atom’s ability to attract the electrons of element B. Higher electronegativity means more powerful attraction of electrons by the atom, a more pronounced ionic nature of the bond, a larger surface dipole, and a lower critical slab thickness.

And there’s more

Pavel Sorokin, Dr. habil., [sic] is head of the Laboratory of New Materials Simulation at TISNCM. He explains the importance of the study, ‘This work has already attracted our colleagues from Israel and Japan. If they confirm our findings experimentally, this phenomenon [of graphitisation] will provide a viable route to the synthesis of ultrathin films with potential applications in nanoelectronics.’

The scientists intend to broaden the scope of their studies by examining other compounds. They believe that ultrathin films of different composition might also undergo spontaneous graphitisation, yielding new layered structures with properties that are even more intriguing.

Here’s a link to and a citation for the paper,

Ionic Graphitization of Ultrathin Films of Ionic Compounds by A. G. Kvashnin, E. Y. Pashkin, B. I. Yakobson, and P. B. Sorokin. J. Phys. Chem. Lett., 2016, 7 (14), pp 2659–2663 DOI: 10.1021/acs.jpclett.6b01214 Publication Date (Web): June 23, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall.

A selection of science songs for summer

Canada’s Perimeter Institute for Theoretical Physics (PI) has compiled a list of science songs and it includes a few Canadian surprises. Here’s more from the July 21, 2016 PI notice received via email.

Ah, summer.

School’s out, the outdoors beckon, and with every passing second a 4.5-billion-year-old nuclear fireball fuses 620 million tons of hydrogen so brightly you’ve gotta wear shades.

Who says you have to stop learning science over the summer?

All you need is the right soundtrack to your next road trip, backyard barbeque, or day at the beach.

Did we miss your favourite science song? Tweet us @Perimeter with the hashtag #SciencePlaylist.

You can find the list and accompanying videos on The Ultimate Science Playlist webpage on the PI website. Here are a few samples,

“History of Everything” – Barenaked Ladies (The Big Bang Theory theme)

You probably know this one as the theme song of The Big Bang Theory. But here’s something you might not know. The tune began as an improvised ditty Barenaked Ladies’ singer Ed Robertson performed one night in Los Angeles after reading Simon Singh’s book Big Bang: The Most Important Scientific Discovery of All Time and Why You Need to Know About It. Lo and behold, in the audience that night were Chuck Lorre and Bill Prady, creators of The Big Bang Theory. The rest is history (of everything).

“Bohemian Gravity” – A Capella Science (Tim Blais)

Tim Blais, the one-man choir behind A Capella Science, is a master at conveying complex science in fun musical parodies. “Bohemian Gravity” is his most famous, but be sure to also check out our collaboration with him about gravitational waves, “LIGO: Feel That Space.”

“NaCl” – Kate and Anna McGarrigle

“NaCl” is a romantic tale of the courtship of a chlorine atom and a sodium atom, who marry and become sodium chloride. “Think of the love you eat,” sings Kate McGarrigle, “when you salt your meat.”

This is just a sampling. At this point, there are 15 science songs on the webpage. Surprisingly, rap is not represented. One other note, you’ll notice all of my samples are Canadian. (Sadly, I had other videos as well but every time I saved a draft I lost at least half or more. It seems the maximum allowed to me is three.).

Here are the others I wanted to include:

“Mandelbrot Set” – Jonathan Coulton

Singer-songwriter Jonathan Coulton (JoCo, to fans) is arguably the patron saint of geek-pop, having penned the uber-catchy credits songs of the Portal games, as well as this loving tribute to a particular set of complex numbers that has a highly convoluted fractal boundary when plotted.

“Higgs Boson Sonification” – Traq 

CERN physicist Piotr Traczyk (a.k.a. Traq) “sonified” data from the experiment that uncovered the Higgs boson, turning the discovery into a high-energy metal riff.

“Why Does the Sun Shine?” – They Might Be Giants

Choosing just one song for this playlist by They Might Be Giants is a tricky task, since They Definitely Are Nerdy. But this one celebrates physics, chemistry, and astronomy while also being absurdly catchy, so it made the list. Honourable mention goes to their entire album for kids, Here Comes Science.

In any event, the PI list is a great introduction to science songs and The Ultimate Science Playlist includes embedded videos for all 15 of the songs selected so far. Happy Summer!

Table salt, acting as a heat scavenger, enables cheaper silicon nanostructures

Recyclable, easily obtained, and inexpensive, there’s a magic ingredient which could make producing silicon nanostructures easier and cheaper according an Aug. 8, 2013 news item on Nanowerk,

Chemists at Oregon State University have identified a compound that could significantly reduce the cost and potentially enable the mass commercial production of silicon nanostructures – materials that have huge potential in everything from electronics to biomedicine and energy storage.

This extraordinary compound is called table salt.

The Aug. 8, 2013 Oregon State University news release, which originated the news item, describes the principle at work and possible applications for the technique,

By melting and absorbing heat at a critical moment during a “magnesiothermic reaction,” the salt prevents the collapse of the valuable nanostructures that researchers are trying to create. The molten salt can then be washed away by dissolving it in water, and it can be recycled and used again.

The concept, surprising in its simplicity, should open the door to wider use of these remarkable materials that have stimulated scientific research all over the world.

“This could be what it takes to open up an important new industry,” said David Xiulei Ji, an assistant professor of chemistry in the OSU College of Science. “There are methods now to create silicon nanostructures, but they are very costly and can only produce tiny amounts.

“The use of salt as a heat scavenger in this process should allow the production of high-quality silicon nanostructures in large quantities at low cost,” he said. “If we can get the cost low enough many new applications may emerge.”

Silicon, the second most abundant element in the Earth’s crust, has already created a revolution in electronics. But silicon nanostructures, which are complex structures much smaller than a speck of dust, have potential that goes far beyond the element itself.

Uses are envisioned in photonics, biological imaging, sensors, drug delivery, thermoelectric materials that can convert heat into electricity, and energy storage.

Batteries are one of the most obvious and possibly first applications that may emerge from this field, Ji said. It should be possible with silicon nanostructures to create batteries – for anything from a cell phone to an electric car – that last nearly twice as long before they need recharging.

Existing technologies to make silicon nanostructures are costly, and simpler technologies in the past would not work because they required such high temperatures. Ji developed a methodology that mixed sodium chloride and magnesium with diatomaceous earth, a cheap and abundant form of silicon.

When the temperature reached 801 degrees centigrade, the salt melted and absorbed heat in the process. This basic chemical concept – a solid melting into a liquid absorbs heat – kept the nanostructure from collapsing.

The sodium chloride did not contaminate or otherwise affect the reaction, researchers said. Scaling reactions such as this up to larger commercial levels should be feasible, they said.

The study also created, for the first time with this process, nanoporous composite materials of silicon and germanium. These could have wide applications in semiconductors, thermoelectric materials and electrochemical energy devices.

Here’s a link to and a citation for the published paper,

Efficient Fabrication of Nanoporous Si and Si/Ge Enabled by a Heat Scavenger in Magnesiothermic Reactions by Wei Luo, Xingfeng Wang, Colin Meyers, Nick Wannenmacher, Weekit Sirisaksoontorn, Michael M. Lerner & Xiulei Ji. Scientific Reports 3, Article number: 2222 doi:10.1038/srep02222 Published 17 July 2013

This article is open access.