The dark tarnish that creeps across old silverware is an unsightly nuisance for antique collectors. But the chemistry of tarnishing has now inspired a simple method to make a silver-based semiconductor, which mimics some of the optical and electronic properties of atom-thin materials (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b08878).
Two-dimensional materials like graphene and molybdenum disulfide have been tested in applications from photovoltaics to sensors, but these gossamer sheets are fragile and can be difficult to integrate into devices. So J. Nathan Hohman of the Molecular Foundry at Lawrence Berkeley National Laboratory has been developing materials that pack 2-D properties into more robust 3-D structures, using organic ligands to sandwich flat atomic layers and also fine-tune their optoelectronic qualities. “It’s a really nice way to package a 2-D material, and by introducing organic ligands we have an infinitely configurable system,” Hohman says.
Earlier this year, Hohman’s team showed that one such material, silver benzeneselenolate, was a semiconductor that also luminesced deep blue under UV light. The crystals are stable to air and water and could be useful in applications such as fluorescence resonance energy transfer (FRET) microscopy, Hohman says. In an homage to J. R. R. Tolkien, they dubbed it mithrene, a nod to the silvery metal called mithril from “The Lord of the Rings” (ACS Appl. Nano Mater. 2018, DOI: 10.1021/acsanm.8b00662).
But their solution-based synthesis of mithrene was finicky and gave unwanted by-products. They needed a process that worked with no solvent at moderate temperatures—just like the reaction between silver and sulfur compounds that tarnishes the metal with an insoluble black sulfide.
So the researchers put a glass slide coated with silver in a jar, along with an open vial of diphenyl diselenide. After sealing the jar, they simply left it in an 80 ⁰C oven for eight days. The silver slowly grew a coating of pure, golden mithrene crystals up to a few hundred nanometers thick. “I was surprised that it worked so well,” Hohman says. Indeed, the method seems remarkably robust—Hohman even raided his grandmother’s silverware to prove that they could plate one of her teaspoons with mithrene.
Rather like the corrosion of metal, water plays a crucial role. Silver attracts a thin film of water from the atmosphere, and Hohman’s team found that this veneer of solvent hosts the redox reaction between silver metal and diphenyl diselenide that ultimately forms mithrene. Since reaction intermediates can move around in the water, it may also explain why the process doesn’t merely coat the silver in a self-assembled monolayer of phenyl selenide.
Silver benzeneselenolate was first made more than 15 years ago by John F. Corrigan at Western University, but his team did not study the material’s semiconductor or luminescence properties (Z. Anorg. Allg. Chem. 2002, DOI: 10.1002/1521-3749(200211)628:11<2483::AID-ZAAC2483>3.0.CO;2-U). Corrigan says he’s delighted it’s been rediscovered and surprised that the tarnishing synthesis was so effective. “It’s very interesting—at first blush you’d just expect the formation of a monolayer,” Corrigan says.
Hohman hopes that synthesis by corrosion could become a more general tactic for making crystalline films. His team has already combined silver with a tellurium-based ligand in the same way, and it is trying the method on other metals including tin, indium, and copper.
The big question, of course, is whether mithrene glows in the presence of Tolkien’s malevolent orcs. “That,” Hohman says with a laugh, “is currently an untested hypothesis.”