Fibers of the first known atomic-scale inorganic double helix discovered recently are flexible, photoluminescent, and semiconducting, potentially making them useful for solar cells or in flexible optical devices. Tom Nilges, of the Technical University of Munich, and his colleagues were experimenting with using tin and tin iodide (SnI₄) to convert red phosphorous to black phosphorus under vacuum and high temperature. After they slowly lowered the temperature of the furnace, the researchers retrieved dark fibers, produced on a gram scale, from the oven. X-ray diffraction and transmission electron microscopy of the fibers revealed they had created a double helix structure similar to that of DNA.

Computer models confirmed a helix of alternating tin and iodide atoms surrounding an inner helix of phosphorous. Because phosphorous can adopt a variety of structures, its helix may guide the formation of the outer tin-iodide helix, Nilges says. In this inorganic system, coulombic interactions between phosphorous and tin atoms hold the two helices together, which differs from the hydrogen bonds that connect strands in a DNA double helix.

Before this, a nontemplated, carbon-free double helical inorganic structure on an atomic scale did not exist. Previous inorganic double helices have more complicated structures than this material including a double helix made using vanadium-oxo subunits, held together with inorganic ligands, as well as phosphorous and potassium cations. Nanoparticles of cadmium telluride alone or in combination with cadmium sulfide also produce non-atomistic nanoscale or mesoscale helices.

The composition of this double helix, SnIP, makes it a quantum confined semiconductor similar to perovskites and solid-state dye-sensitized materials used in solar cells. The 1.86 eV bandgap of bulk SnIP is similar to that of gallium arsenide, a common semiconductor in solar cells. Nilges says they are developing ways to make thin films using SnIP fibers, and they hope to test their long-term stability for applications like solar cells.

Based on computer modeling, the researchers think they could use other elements to make inorganic double helices, provided the ingredients are mixed in the proper proportions during the synthesis. They are also investigating the possibility of introducing other elements into the helix to generate materials with different electronic properties.

Nicholas Kotov, of the University of Michigan, is excited about the potential for combining this material’s semiconducting properties with the chirality of its double helix. Separating the left-handed and right-handed helices, though challenging, could generate materials useful for polarized optics, he says.

Read the abstract in Advanced Materials.