Since graphene (a sheet of carbon just one atom thick) was discovered in 2004, the list of 2D crystals has grown considerably. Now, researchers at Leipzig University and Jacobs University Bremen, both in Germany, have added a new member to the 2D family: germanium phosphide (GeP3). This metallic layered material, which was first synthesized around 1970 but forgotten about since, becomes a semiconductor with a small bandgap when thinned down to a mono- or bi-layer. As such, it could find use in a variety of applications in photovoltaics and electronics.
The family of 2D crystals has been steadily growing over the last decade or so and now includes the 2D Dirac semimetals (graphene, silicon, germanene and stanene), insulators (such as hexagonal boron nitride) and direct-bandgap semiconductors (such as the transition-metal dichalcogenides, or transition-metal dichalcogenides (TMDCs) and phosphorene).
Many of these materials, and especially the TMDCs, are already being used in real-world applications. However, for the case of TMDCs, they are not ideal for making photovoltaic devices because their relatively large bandgap (of nearly 2 eV) does not allow them to efficiently collect all solar-emission wavelengths. For such applications, the most commonly used compounds are based on silicon and the group III-V semiconductors (such a GaAs, InAs and InP), all of which have bandgaps below 1.5 eV. Ideally, semiconductors with a bandgap in the 0.3–1.5 eV range would be better.
Now, a team of researchers led by Thomas Heine says that it has rediscovered GeP3 The researchers have studied the material’s stability, mechanical, electronic and optoelectronic properties in detail using first-principles computer simulations. “These methods accurately describe structural and electronic properties of materials,” says Heine, “and we already applied them in 2014 to predict the viability of noble-metal dichalcogenides (such as (PtSe2 and PtS2), which were then synthesized in the laboratory just one year later.”
Interesting contender for solar cells
GeP3 mono- and double layers have similar electronic properties to phosphorene (often called the alternative 2D “wonder material”), but they are much more stable. Phosphorene’s instability is, in fact, its main drawback. GeP3 is also a semiconductor with a small band gap (the monolayer material has a bandgap of 0.55 eV and the bilayer material a 0.43 eV one). It has a carrier mobility as high as that of phosphorene’s and, happily, absorbs light in the visible range with a large absorption coefficient, thus making it an interesting contender for solar cells.
The researchers also calculated that GeP3 has a cleavage energy of just 1.14 J/m2, which means that it could easily and simply be produced by exfoliating (or shaving off) the bulk material. And that is not all: its indirect bandgap could be turned into a direct one by applying strain to it. A direct bandgap is better for applications such as transistors that need to be turned on and off easily.
Applications for GeP3
Applications for GeP3 include 2D electronics (both flexible and rigid), solar cells, components for tandem solar cells and single-material transistors, Heine tells nanotechweb.org.
The Germany team says that it is now busy looking for other 2D materials with interesting physical properties and studying how these properties can be changed using external effects (such as strain) and quantum confinement (that is, reducing layer thickness).
The present work is detailed in Nano Letters DOI: 10.1021/acs.nanolett.6b05143.
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