Black phosphorus (BP) has a direct and small bandgap, as well as a fast photoresponse. It could thus bridge the gap between graphene (a sheet of carbon just one atom thick that is a zero-bandgap semiconductor) and the transition metal dichalcogenides. TMDCs are van der Waals materials with 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). They are only suitable for device applications that work in the visible part of the electromagnetic spectrum because of their large bandgaps

BP can be obtained by mechanically cleaving black phosphorus crystals (in the same way that graphene layers are mechanically exfoliated from bulk graphite). However, this technique cannot produce large quantities of the material. A team led by Werner Blau at Trinity College Dublin has now succeeded in producing the 2D semiconductor using liquid-phase exfoliation, a process that is industrially scalable. The researchers then measured the nonlinear optical saturation in the phosphorene using a tuneable femtosecond laser and the well established “z-scan technique”.

'Pump-and-probe' test

“Here, the sample is moved through the focus of the lens,” explains Blau. As the cross-section of the laser beam decreases and then increases along the propagation direction of the laser, that is, in the z-direction (hence the name of the technique), the incident light intensity increases and then decreases depending upon the position of the sample, often by as much as a factor of 10,000.

Measuring the transmitted light as a function of the z-position gives the researchers a quantitative measure of how the material responds to light. However, since this technique does not provide any information about how quickly the material responds to light, they need to perform additional experiments – in this case a “pump-and-probe” test (which is a well-known method in ultrafast laser spectroscopy). “Here, a strong pump laser pulse excites the sample, and we measure how the excited carriers ‘recover’ by sending in a weaker probe pulse that has been delayed in space by sending it on a longer, controlled, path,” says Blau

BP's photoresponse is faster

Fundamentally, low-dimensional semiconductors have an enhanced optical response thanks to the density of electron states being modified in these materials, he adds. “In the case of BP, its photoresponse is faster than the TMDCs MoS2 and WS2, and graphene.

"As for its bandgap, it goes from being 0.3 eV in the bulk to 1.88 eV in the monolayer material, which is an extremely wide energy range. The advantage of BP over graphene is that we have access to the same wavelength regions in both materials, but BP has an intrinsic bandgap, which means that we can make semiconductor-type electronics and optoelectronics devices from it.”

Fengnian Xia of Yale University, who was not involved in this work, agrees. "This work is very novel," he says, "and covers an important infrared wavelength range using an emerging two-dimensional material."

The Dublin team, reporting its work in ACS Nano DOI: 10.1021/acsnano.6b02770 says that it is now busy trying to make devices like a broadband saturable absorber mirror from BP and use it in different optoelectronic applications, such as photovoltaics, too. “We are also looking into other interesting 2D semiconductors, such antimonene,” Blau tells