Tikalon Blog by Dev Gualtieri

SnIP - An Inorganic Double Helix

October 27, 2016

Sulfur is an ubiquitous element on Earth, since it forms compounds with many other elements. One example is FeS₂, iron pyrite, also called fool's gold because of its golden luster. Another example is PbS, lead(II) sulphide, that's found in mineral form as galena. Lead sulfide was used as a "cat's whisker" detector in crystal radio sets to form a point-contact diode, and it's an excellent infrared photoconductor.

A "cat's whisker" galena radio detector. Certain crystal facets are better for this application, so the whisker is moved across the mineral to find a "sweet spot."

(Photo by J.A. Davidson, via Wikimedia Commons.)

Sulfur is interesting among inorganic chemicals in its tendency to form rings and polymeric chains. Rings of eight sulfur atoms exist in liquid sulfur; and, by Heating to high temperature and quenching to room temperature, a sulfur polymer can be formed (see figure).

S8 ring and a polymer chain of eight sulphur atoms.

It's an easy process to transition from an S₈ ring to a polymer chain.

Heating breaks the S₈ rings, and these combine into chains.

(Created using Inkscape.)

There's a lot of interesting science behind this polymeric transition. While sulfur melts to a low-viscosity light yellow liquid at about 120°C, its viscosity increases by four orders of magnitude in the temperature range of 159°C to 184°C.[1] At these high temperatures, there is an average of a million sulfur atoms in polymer chains.[1] The simplicity of this process is demonstrated in a YouTube video.[2]

Sulfur is interesting for another reason. Although known for its rotten egg odor, it has a connection to the rose in Juliet's famous speech in Shakespeare's Romeo and Juliet,

What's in a name? that which we call a rose
By any other name would smell as sweet;

When I was a young student of chemistry, sulfur was known also as sulphur, the first being the more common American usage, and the later being the common British English usage. I still type sulphur, which is how I learned it from the chemistry textbook my father had in his G.I. Bill college days, and my spell checker corrects me. The International Union of Pure and Applied Chemistry adopted the sulfur spelling in 1990.[3] The Royal Society of Chemistry switched from sulphur to sulfur in its publications in 1992, although there's still considerable controversy about the change.[4] Still, sulfur by any other name...

Inorganic polymers are rare (aluminum phosphate, AlPO₄, will also form polymer chains in solution), but another interesting member has been added to the list. A team of German scientists led by members at the Technical University of Munich has discovered a double helix inorganic material, SnIP, that's a semiconductor with interesting optical and electronic properties.[5-7] Since the synthesized form is a fiber, it also has mechanical flexibility.[5-6]

The double helix structure of SnIP.

Although it's not clear from the references, my conjecture is that the red atoms are phosphorus, the green atoms are tin, and the violet atoms are iodine.

(Technical University of Munich image by Prof. Tom Nilges.)

This is an exciting discovery, since SnIP is comprised of the abundant and inexpensive elements, tin, iodine, and phosphorus. The research team has been able to synthesize gram quantities of SnIP, which functions as a semiconductor much like gallium arsenide, but without the toxicity.[6-7] Centimeter-length fibers of SnIP have been produced, and they're extremely flexible. These fibers can be separated into nano-sized fibers of about 20 nanometer size comprised of just a few double helix strands.[6-7]

A racemic mixture of right- and left handed double helices is produced in just minutes by the demonstrated synthesis process.[5] The SnIP double helices can be suspended in solvents such as toluene to produce thin layers by solvent evaporation.[6] SnIP is stable up to about 500°C (930°F), so it could be used in concentrator solar cells.[6-7] SnIP has a band gap of 1.86 eV,[5] so it's slightly more responsive to infrared light than gallium arsenide, which has a band gap of 1.43 eV.

A scanning electron microscope image of SnIP fibers.

(Max Planck Institute for Solid State Research, Stuttgart, image by Viola Duppel.)

Since SnIP could be doped with other elements, it might be used as a photocatalyst and a thermoelectric material.[6] Theoretical calculations indicate that quite a few doping elements are possible. Since SnIP helices are formed in both right- and left-handed forms, this might lead to some interesting optoelectronic applications.[6] This research was funded in part by the German Research Foundation (Deutsche Forschungsgemeinschaft).[5]

SnIP - An Inorganic Double Helix

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