Light–matter interaction in transition metal dichalcogenides and their heterostructures

Abstract

Abstract:The investigation of two-dimensional van der Waals materials is a vibrant, fast moving and still growing interdisciplinary area of research. Two-dimensional van der Waals materials are truly two-dimensional crystals with strong covalent in-plane bonds and weak van der Waals interaction between the layers with a variety of different electronic, optical and mechanical properties. A very prominent class of twodimensional materials are transition metal dichalcogenides and amongst them particularly the semiconducting subclass. Their properties include bandgaps in the near-infrared to the visible range, decent charge carrier mobility together with high (photo-)catalytic and mechanical stability and exotic many body phenomena. These characteristics make the materials highly attractive for both fundamental research as well as innovative device applications. Furthermore, the materials exhibit a strong light matter interaction providing a high sun light absorbance of up to 15% in the monolayer limit, strong scattering cross section in Raman experiments and access to excitonic phenomena in van der Waals heterostructures. This review focuses on the light matter interaction in MoS2, WS2, MoSe2, and WSe2 that is dictated by the materials complex dielectric functions and on the multiplicity of studying the first order phonon modes by Raman spectroscopy to gain access to several material properties such as doping, strain, defects and temperature. Two-dimensional materials provide an interesting platform to stack them into van der Waals heterostructures without the limitation of lattice mismatch resulting in novel devices for application but also to study exotic many body interaction phenomena such as interlayer excitons. Future perspectives of semiconducting transition metal dichalcogenides and their heterostructures for applications in optoelectronic devices will be examined and routes to study emergent fundamental problems and manybody quantum phenomena under excitations with photons will be discussed.

Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

“…GaAs and InAs. Semiconducting TMDs such as MoSe2 and WSe2 that have a direct band gap in the monolayer limit [29][30][31] excel on further properties due to a strong light-matter interaction [20] with an absorbance of up to 15% of visible light in just one monolayer [32][33][34]. They possess also fascinating spin-and valley-properties with both degrees of freedom locked by the lack of an inversion center in the monolayer crystals enabling optical valley polarization [35][36][37][38].…”

Hydrogen evolution activity of individual mono-, bi-, and few-layer MoS 2 towards photocatalysis

“…Hence, the illumination dependent increase in the currents is close to the noise level of the experimental setting. Reasons for the enlarged photocurrent for the monolayer compared to the bulk might be caused by a higher absorbance for monolayers [11,19]. Overall, the increase in current density is rather low compared to the high illumination intensities.…”

“…The modes are also sensitive to structural changes, defects, strain, temperature as well as the mentioned doping level of the investigated MoS 2 flakes [19]. Therefore, an in-situ -Raman measurement as in Fig.…”

“…In contrast, atomically thin, two-dimensional semiconducting transition metal dichalcogenides (TMDs), such as MoS 2 , offer the unique combination of a band gap in the visible range of about 1.9 eV for monolayers [6][7][8], high sunlight absorption of up to 15% [9][10][11], catalytically active sites for HER [4,[12][13][14][15][16] or CO 2 -reduction [17] and photocatalytic stability under harsh reaction conditions [18]. Moreover, semiconducting TMDs are earth abundant, inexpensive, and sustainable (photo-)catalysts which can be combined in lateral or vertical heterostructures [19][20][21][22]. All of such structures can be further integrated in hybrid devices based on carbon, gallium-nitride or silicon platforms.…”

“…Parameters (1)(2)(3) are reasonably well fulfilled in TMDs and their heterostructures. Moreover, it was demonstrated that 2H-MoS 2 features catalytic active edge sites for HER along the (10-10)-direction [32], while the (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)-direction undergoes an anodic corrosion limiting the functionalization as a photo-anode for the oxygen evolution reaction (OER) [32]. The two-dimensional character of TMDs further offers opportunities for structural and defect engineering to locally increase the catalytic activity and to create anchoring sites for co-catalysis.…”