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The exciton properties play a crucial role in controlling the optical properties of molybdenum disulfide (MoS2). In this work, horizontally oriented MoS2 (H-MoS2), horizontally and vertically oriented MoS2 (HV-MoS2), and vertically oriented MoS2 (V-MoS2) on the same SiO2/Si substrate have been synthesized and investigated using temperature-dependent photoluminescence spectroscopy from 7 to 300 K. Except for direct excitons called as A and B peaks, indirect exciton named as I peak is discovered with the splitting of three peaks. Compared with direct excitons, indirect excitons appear to be more susceptible to the orientation of MoS2. The exciton activation energies are larger, and the exciton–phonon coupling is stronger in V-MoS2 than in H-MoS2. The exciton properties of HV-MoS2 are more similar to those of V-MoS2, but there are some unusual phenomena. Our work provides a reference for optoelectronic applications based on transition-metal dichalcogenides represented by MoS2 grown with horizontal and vertical orientations. [ABSTRACT FROM AUTHOR]

The linear molybdenum disulfide (MoS2) with different nanostructures due to atomic arrangements is expected to show promising prospects in the next generation electronics and optoelectronic devices. In this work, three typical linear MoS2 nanostructures were grown on the SiO2/Si (285 nm, 500 μm) substrate by a modified chemical vapor deposition (CVD) method and were systematically analyzed by using angle-resolved polarized (ARP) Raman spectra. The results are as follows. The first and the second linear MoS2 are identified as nanoribbon-like structures with a tiled layered arrangement similar with the sheet MoS2, where the first linear MoS2 is grown directly on the substrate and the second linear MoS2 is coated on the upper surface of the cylindrical-shaped molybdenum oxide (MoO3). The third linear MoS2 is a nanotube-like structure, and its atomic structure includes vertical layered arrangement in addition. All three linear MoS2 produce a small-size effect, and the second linear MoS2 is more like an intermediate material between the first linear MoS2 and the third linear MoS2 during the growth of CVD. This study provides an effective approach to detect the optical anisotropy of linear nanostructures of two-dimensional materials which has potential applications in ARP optoelectronics devices. [ABSTRACT FROM AUTHOR]

Study of exciton recombination process is of great significance for the optoelectronic device applications of two-dimensional transition metal chalcogenides (TMDCs). This research investigated the decoupling MoS 2 structures by photoluminescence (PL) measurements. First, PL intensity of the bilayer MoS 2 (BLM) is about twice of that of the single layer MoS 2 (SLM) at low temperature, indicating no transition from direct bandgap to indirect bandgap for BLM due to the decrease of interlayer coupling which can be shown by Raman spectra. Then, the localized exciton emission appears for SLM at 7 K but none for BLM, showing different exciton localization characteristics. The PL evolution with respect to the excitation intensity and the temperature further reveal the filling, interaction, and the redistribution among free exciton states and localized exciton states. These results provide very useful information for understanding the localized states and carrier dynamics in BLM and SLM.

Recently, two-dimensional (2D) van der Waals heterostructures (vdWHs) have exhibited emergent electronic and optical properties due to their peculiar phonons and excitons, which lay the foundation for the development of photoelectronic devices. The dielectric environment plays an important role in the interlayer coupling of vdWHs. Here, we studied the interlayer and extra-layer dielectric effects on phonon and exciton properties in WS 2 /MoS 2 and MoS 2 /WS 2 vdWHs by Raman and photoluminescence (PL) spectroscopy. The ultralow frequency (ULF) Raman modes are insensitive to atomic arrangement at the interface between 1LW and 1LM and dielectric environments of neighboring materials, and the layer breathing mode (LBM) frequency follows that of WS 2 . The shift of high-frequency (HF) Raman modes is attributable to interlayer dielectric screening and charge transfer effects. Furthermore, the energy of interlayer coupling exciton peak I is insensitive to atomic arrangement at the interface between 1LW and 1LM and its energy follows that of MoS 2 , but the slight intensity difference in inversion vdWHs means that the substrate's dielectric properties may induce doping on the bottom layer. This paper provides fundamental understanding of phonon and exciton properties of such artificially formed vdWHs structures, which is important for new insights into manipulating the performances of potential devices.