2D materials have dramatically different electronic and mechanical properties from their 3D counterparts and so may find use in a host of novel device applications. Until now, however, most research in this field has focused on the most famous of all 2D materials, graphene, but the fact that this material lacks a direct electronic bandgap between its valence and conduction bands means that scientists are now starting to look at other 2D candidates too. A bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off.

Among the promising newcomers are the transition metal dichalcogenides (TMDCs). These van der Waals materials have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te), and go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. The monolayers also efficiently absorb and emit light and so could be ideal for making a variety of optoelectronics devices such as light-emitting diodes and solar cells.

However, there is a problem in that TMDCs respond to light relatively slowly. They also have a large bandgap of between roughly 1.5 to 2 eV and so are only suitable for device applications that work in the visible part of the electromagnetic spectrum. A material with a direct and small bandgap, as well as a fast photoresponse, could thus bridge the gap between graphene (a zero-gap semiconductor) and TMDCs with their large bandgaps.

Few-layer black phosphorus, which can be obtained by mechanically cleaving black phosphorus crystals (in the same way that graphene layers are mechanically exfoliated from bulk graphite), is one such material.

Direct and small bandgap

Although researchers have known about bulk black phosphorus (one of the allotropes of the element phosphorus) since the 1960s, it is only very recently that they have tried to isolate single layers of the material. Just as in graphene, phosphorene atoms are arranged in a hexagonal lattice but with its direct and small bandgap (of 0.3 eV for the bulk material and between 0.33 and 0.81 eV for the device made in this work), phosphorene can quickly switch between insulating and conducting states. The material is still thin enough to confine electrons though so that charge flows quickly through the structure, something that leads to high charge mobilities – crucial for making ultrafast photodetectors and other electronics devices.

When exposed to visible and near-infrared light, FETs made from phosphorene show a photoresponse that reaches 4.8 mA/W. This is faster than both MoS2 and WS2-based photodetectors. Importantly (and unlike MoS2 and WS2), the material is also “ambipolar”, which allows it to conduct with both electrons and holes. This property means that it can be used to construct p-n junctions (the building blocks of modern electronics and photovoltaics).

Finally, phosphorene’s hole mobility can reach nearly 300 cm2/Vs, which is about three to five times that of MoS2. To compare, silicon’s hole mobility is just 100 cm2/Vs.

Good optical sensors and solar cells

The researchers obtained their phosphorene (between 3 and 8 nm thick) by exfoliating bulk black phosphorus. “Once we exfoliated the black phosphorus flakes, we fabricated FETs from them in our cleanroom facilities at the Kavli Institute of Nanoscience at Delft,” explained Michele Buscema, who is in Herre van der Zant's Lab. “In contrast to conventional FETs fabricated in the microelectronics industry, our transistors have their conduction channel exposed to allow light to reach the channel. By shining light of different wavelengths onto the channel, we can determine how the transistors respond to this light.”

According to the team, the FETs could make good optical sensors and solar cells. “Few-layer black phosphorus is particularly suited for detection applications in the near infrared and so in night-vision imaging, for example, where the TMDCs do not work because of their large bandgap,” he told nanotechweb.org.

The Delft team says that it is now looking at exploiting the ambipolar behaviour of black phosphorus to build p-n junctions and solar cells. “We are also busy trying to combine the material with other 2D structures – a topic that is already creating a lot of excitement in our community right now,” added Buscema.

The current work is detailed in Nano Letters DOI: 10.1021/nl5008085.