Probing interlayer excitons in a vertical van der Waals p-n junction using a scanning probe microscopy technique

Significance 

Electronic and optoelectronic properties of different heterogeneous structures rely mainly on the existing semiconductor p-n junctions as a result of the doping processes. Two-dimensional semiconductors exhibit excellent properties that enable the building of ultra-thin van der Waals heterostructures. As such, several two-dimensional semiconductors have been investigated. In particular, transition metals dichalcogenides, especially MoS2 has attracted significant attention of researchers. This can be attributed to the facts that such compounds can be thinned down to monolayer thickness, are flexible and exhibit nonlinear response.

To exploit these properties, the current trend is mainly focused on creating vertically stacked van der Waals heterostructures from a combination of several two-dimensional layers. Unfortunately, effective fabrication methods that guarantee the required precision and control of the optoelectronic properties have not yet been developed.

Alternatively, research has shown that interlayer excitons in van der Waals heterostructures involve only two types of band alignment. Type I involves separation of the electron and holes using a barrier layer while type II involves both electrons and holes from adjacent layer to create a p-n junction interface. Recently, diode rectification was observed in two-dimensional type II interfaces. However, such a phenomenon is yet to be reported in optoelectronics. Therefore, to further advance the optoelectronic properties of two-dimensional heterostructures, development of exciton devices for optoelectronics is highly desirable.

To this note, a group of researchers at the Chemnitz University of Technology: Mahfujur Rahaman, Christian Wagner, Ashutosh Mukherjee, Adan Lopez-Rivera, Sibylle Gemming, and Dietrich Zahn studied the diode rectification behavior of the two-dimensional p-n interface. Fundamentally, the p-n junction was made of a n-type bilayer of MoS2 and few layers of p-type of GaSe. The resulting interlayer excitons were examined by measuring the photocurrent using current sensing atomic force microscopy. A strong rectification behavior was shown at the interface. For instance, a rectification ratio of 104 at ± 1V was recorded. Additionally, the current-voltage relationship showed pronounced photovoltaic effects especially at excitations below the bandgap due to the dissociation of the interlayer excitons at the interface.

For the first time, Mahfujur Rahaman and colleagues reported the interlayer excitons in photovoltaic applications of van der Waals heterojunctions. To support their study, calculations with the density functional perturbation theory were performed to validate the experimental results by confirming the interlayer exciton formation at the p-n interface. Interestingly this was proved by the provided photoluminescence measurements at the p-n junction. Altogether, the study provides essential instincts into the new van der Waals heterostructures and will therefore advance the development of two-dimensional material based optoelectronic devices. The work is published in the journal, Journal of Physics: Condensed Matter.

Probing interlayer excitons in a vertical van der Waals p-n junction using a scanning probe microscopy technique - Advances in Engineering
Fig. Schematic of the vdW device for photovoltaic measurements (a). Photoluminescence spectra of the vdWH showing interlayer excitons formation (b). Inset is the optical image of the vdWH. Demonstration of interlayer excitons assisted photocurrent generation (c).

Reference

Rahaman, M., Wagner, C., Mukherjee, A., Lopez-Rivera, A., Gemming, S., & Zahn, D. (2019). Probing interlayer excitons in a vertical van der Waals p-n junction using a scanning probe microscopy technique. Journal of Physics: Condensed Matter, 31(11), 114001.

Go To Journal of Physics: Condensed Matter

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