As we place ever-greater demands on the processing ability of electronic components, researchers have been seeking a semiconducting material that would improve on silicon’s performance. While a great deal of attention has been devoted to the prospects of carbon in the form of nanotubes or the atom-thin film graphene, this semi-metal does not offer a band gap, the energy range between electron states that can be manipulated as an on-off “switch” for enabling transistor logics or data storage.
Meanwhile the unlikely choice of phosphorus has emerged as a much more promising contender. Depending on their thickness, thin films of black phosphorus — an allotrope of this element’s familiar white or red forms — can be “tuned” to different semiconducting bandgaps that would make this material far more adaptable to various types of optical electronic switching. The electronic properties of these films have been outlined by recent articles in Nature Materials and Nature Communications that detail the work of investigators at McGill University, Polytechnique Montréal, Université de Montréal and France’s Laboratoire d’Étude des Microstructures.
Université de Montréal chemist Richard Martel says these findings reveal the potential of black phosphorus as well as the challenges of working with this unusual semiconductor. “It undergoes an oxidation process, which is fairly slow but it’s there,” Martel says, noting that light, oxygen and water contribute to this degradation. He addressed this problem with a glove box built by his students, which allowed him and his colleagues to study samples of black phosphorus in an atmosphere without water or oxygen. This modest facility even includes an atomic force microscope so that samples could be manipulated and measured at extremely small scales. “We could study the monolayer, the bilayer, the tri-layer and so forth using optical and Raman spectroscopy,” Martel says. “Then we could enclose the sample in a clean environment and go to the electrical and optical characterization tools.”
Martel adds that before black phosphorus can topple silicon from its semiconducting throne, it will require a better manufacturing method. Currently only small amounts can be produced by subjecting white phosphorus to pressures on the order of up to 12,000 atmospheres for an extended period. “We need some kind of catalyst that can scale up the growth to the volume that would be needed for use on a wafer scale,” he says.