Abstract
Using a conventional Raman experimental apparatus, we demonstrate that the photoluminescent (PL) yield from ultrasonication-exfoliated transition metal dichalcogenides (TMDs) (MoS2 and WS2) can be increased by up to 8-fold by means of a laser etching procedure. This laser etching process allows us to controllably pattern and reduce the number of layers of the solution-exfoliated material, overcoming the key drawback to solvent-based exfoliation of two-dimensional (2D) semiconducting materials for applications in optoelectronics. The successful laser thinning of the exfoliated 2D crystals was investigated systematically by changes in both Raman and PL spectra. A simple proof-of-principle of the scalability of this laser etching technique for solution-exfoliated TMD crystals was also demonstrated. As well as being applicable for individual materials, it is also possible to use this simple laser etching technique to investigate the structure of solution-generated van der Waals heterostructures, consisting of layers of both MoS2 and WS2.
Highlights
Research into two-dimensional (2D) materials has continued to gain interest, first with graphene and with other layered materials, such as the semiconducting transition metal tduicnhgasltceongednisiduelfisd(eT(MWDSs2)),.1m−4olTybMdDensumaredoisfulpfiadretic(uMlaor Si2n)t,earensdt due to their unique electronic structure, and they exhibit a significant change in the band structure when reduced in thickness from bulk to a monolayer.[5−7] The transition to a direct gap semiconductor means that photons with an energy equal to the band gap can be absorbed or emitted, whereas the indirect band gap material requires an additional phonon absorption or emission to compensate for the difference in the momentum, making absorption much less efficient.[8]
Exfoliated monolayer transition metal dichalcogenides (TMDs) can be produced by several methods including micromechanical exfoliation,[1] chemical vapor deposition (CVD),[12−14] chemical exfoliation,[15] electrochemical lithium intercalation,[16−18] and solvent-assisted ultrasonication.[19,20]
A similar effect has been observed previously for graphene films, which showed that, contrary to expectations, spectrum of the aggregated flakes before etching to that of the after laser etching the defect-related D band decreased in same area after the laser etching, as seen in Figure 1d, we see that there is a significant improvement in the signal intensity intensity due to selective removal of the highly reactive edges and defect sites.[24]
Summary
Research into two-dimensional (2D) materials has continued to gain interest, first with graphene and with other layered materials, such as the semiconducting transition metal tduicnhgasltceongednisiduelfisd(eT(MWDSs2)),.1m−4olTybMdDensumaredoisfulpfiadretic(uMlaor Si2n)t,earensdt due to their unique electronic structure, and they exhibit a significant change in the band structure when reduced in thickness from bulk (indirect gap) to a monolayer (direct gap).[5−7] The transition to a direct gap semiconductor means that photons with an energy equal to the band gap can be absorbed or emitted, whereas the indirect band gap material requires an additional phonon absorption or emission to compensate for the difference in the momentum, making absorption much less efficient.[8]. Heterostructures of different semiconducting 2D materials are of great interest because of the ability to modify the electronic structure, introducing novel properties, and because of the fundamental interactions between each of the layers stacked together by van der Waals forces.[30−32] Owing to the close interaction between each of the exfoliated layers, it is possible for excited electrons and holes to transfer between the respective layers These heterostructures have been demonstrated experimentally using mechanically exfoliated flakes manually stacked on top of one another;[33−35] this process is time consuming and lacks scalability. The resultant stacks can be modified through the use of laser etching, and the resultant 3D layered structure can be characterized
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