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New 2D Approach For Mid IR Light Emitters

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Black phosphorus and TMDC-based van der Waals heterostructures show promise mid-infrared light-emission applications

Mid infrared spectra have been widely used for thermal imaging, molecule characterisations, and communications. A suitable light source is the key component of such technologies.

2D semiconductors offer a promising direction for next-generation MIR devices owing to their exotic optical properties, as well as the ultimate thickness limit. More importantly, van der Waals (vdW) heterostructures — combining the best of various 2D materials at an artificial atomic level—provide many new possibilities for constructing MIR light emitters of large tuneability and high integration.

In a recent report in the journal Light: Science & Applications, Chinese researchers have proposed a novel vdW heterostructure for MIR light-emission applications, built from black phosphorous and Transition Metal Dichalcogenides (TMDCs) such as WSe2 and MoS2.

Since the rediscovery of thin-film black phosphorous in 2014, it has received much attention. Black phosphorus consists of multiple layers with 2D structures, weakly bonded to one another by van der Waals forces. It has features such as in-plane anisotropy, high carrier mobility, and tuneable band gap, etc., making black phosphorous a promising material for applications in electronics and optoelectronics.

Black phosphorous has a thickness-dependent (0.3-2 eV) bandgap, and the bandgap size can be further tuned through introducing external electric field or chemical doping. Because of these reasons, thin-film black phosphorous has been regarded as a star MIR material. Previous research mainly focused on the luminescence properties of monolayer and few-layer black phosphorous flakes (with layer number < 5 layers). However, the latest reports indicate that thin-film black phosphorous (> 7 layers) shows remarkable photoluminescence properties in MIR region.

According to density functional theory (DFT) calculation, the black phosphorous-WSe2 heterostructure forms a type-I band alignment. Hence, the electron and hole pairs in the monolayer WSe2 can be efficiently transported into the narrow-bandgap black phosphorous, thereby enhancing the MIR photoluminescence of thin-film black phosphorous. An enhancement factor ~200 percent was achieved in the 5nm-thick black phosphorous-WSe2 heterostructure.

On the other hand, the black phosphorous-MoS2 heterostructure forms a type-II band alignment. A natural PN junction is formed at the interface between p-type black phosphorous and n-type MoS2. When a positive voltage bias is applied between black phosphorous and MoS2 (Vds > 0), electrons in the conduction band of MoS2 can cross the barrier and enter into the conduction band of black phosphorous. At the same time, the majority of holes are blocked at the interface inside black phosphorous due to the large Schottky barrier of the valence band. As a result, an efficient MIR electroluminescence is achieved in the black phosphorous-MoS2 heterostructure

The picture above shows: a, Schematic diagram of the black phosphorous-WSe2 heterostructure. Under the excitation of light, the electron and hole pairs in WSe2 can be efficiently transmitted to black phosphorous, thereby enhancing its MIR photoluminescence. b, Schematic diagram of the black phosphorous-MoS2 heterojunction diode. Under a positive bias voltage between black phosphorous and MoS2, the electrons on the conduction band of MoS2 can overcome the barrier, enter into the conduction band of black phosphorous, and recombine with abundant holes in black phosphorous. Thereby electroluminescence is achieved

According to the research team, the black phosphorous-TMDC vdW heterostructures show many merits, such as simple fabrication process, high efficiency, and good compatibility with silicon technology. Hence, it provides a promising platform for investigating silicon-2D hybrid optoelectronic systems.

'Black phosphorus-based van der Waals heterostructures for mid-infrared light-emission applications' by Xinrong Zong et al; Light: Science & Applications, volume 9, Article number: 114 (2020)

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