Hybridization with oxide semiconductors provides a versatile strategy for tailoring physicochemical properties of two-dimensional materials (2DMs). However, the direct impacts of specific interface interactions have not yet been very well categorized, in particular at an atomic level. In the present work, through a chemical vapor deposition (CVD) method, we successfully grew monolayer MoS2 flakes on an atomically smooth rutile TiO2 single crystal with (100), (110), and (001) terminations. We found that the fabrication of comparable high-quality MoS2 on all of the TiO2 substrates can only be achieved via finely varying the growth parameters. Moreover, the photoluminescence of MoS2 also changes against the substrate terminations, showing a gradually reduced A 0/A – exciton ratio following the sequence of (100) > (110) > (001). Detailed X-ray photoelectron spectroscopy measurements showed the same tendency for the binding energy shifts of both Ti and O in the MoS2/TiO2 samples, which were attributed to the varied dipole fields established at the MoS2/TiO2 interfaces. Our work not only reinforces the important role of interface charge redistribution in tailoring the properties of hybridized systems but also emphasizes that the facet effect may be applied as an efficient strategy for optimizing the photocatalytic activities of compositional systems.
innovative devices such as synapse transistor, [15][16][17] nonvolatile memory, [18,19] nanopower generators, [20] and so on. For most application circumstances, a proper support is usually necessary which either holds the transferred MoS 2 thin film or directly serves as the synthetic substrate. In fact, recent studies have witnessed the effective modulation of the properties of the MoS 2 films rendered from the different supports. [21][22][23][24][25][26][27][28][29][30][31][32] On the one hand, distinct substrate materials can induce different charge separation at the interfaces and establish varied dipole fields at the corresponding heterojunctions, which strictly depends on the actual components as well as the atomic structures of the interfaces. [33,34] On the other hand, additional tuning effects can be obtained via nanostructuring the substrate surface, which has provided a more-open strategy for tuning the transportation and luminescence properties of MoS 2 . [35][36][37][38][39][40][41][42][43][44] However, in the latter case, the substrate surfaces were usually prepared through complicated and expensive processes such as nanosphere or electron beam lithographies. Meanwhile, onsite synthesis of the MoS 2 films has not been realized on these sophisticatedly structured substrates. The transfer process of the MoS 2 films inevitably arouses the formation of unwanted film wrinkles and interfacial contaminants, which hinders the optimization of the obtained heterostructures. [24,45] Nevertheless, direct synthesis of high quality 2DMs on the largely corrugated substrate remains an urgent yet challenging task. The structure and morphology of the substrate surface play critical roles in tuning the properties of the supported two-dimension materials (2DM). In this work, a simple strategy to engineer the SrTiO 3 single crystal into a trenched structure which is composed of atomically flat terraces and high steps of several nanometers is developed. Through the conventional chemical vapor deposition method, high quality single-layered MoS 2 nanosheets are successfully fabricated directly on the trenched SrTiO 3 (Tr-STO) substrate, which thus result in a heterostructure with well-defined interface and controllable corrugated morphology. The corrugated MoS 2 /Tr-STO sample displays a drastically suppressed photoluminescence as compared to those grown on atomically flat substrates. Detailed scanning probe microscopy in combination with optical spectroscopy measurements demonstrates that the photoluminescence quenching occurs exclusively in the MoS 2 area carpeting the high SrTiO 3 steps, which can be attributed to the significantly reduced bandgaps hence massively enriched free charges in these regions. This work not only provides a new strategy to tailor the 2DM properties by simply engineering the substrate surface corrugations, but also brings deep insights into the dependence of properties of the hybridized system on the interface morphologies.
Monolayer transition-metal dichalcogenides (TMDs) have registered attractive potential applications in optoelectronics, spintronics, and valleytronics. Here, we develop a simple chemical vapor deposition method to directly grow large-size monolayer WS 2 on an atomically flat rutile TiO 2 (110) substrate. The variety of microscopic and spectroscopic characterizations reveals the highly single crystallinity of the as-grown WS 2 and the atomically clean WS 2 / TiO 2 interface. Such structural perfection renders a neutral excitonic photoluminescence of WS 2 with extremely small spectral broadening of around 20 meV at cryogenic temperature. Moreover, the low concentration of defect leads to the suppressed intervalley scattering, while the efficient interfacial electron transfer reduces the Coulomb interaction between the valleys of the WS 2 , which thus enhance the valley polarization and coherence significantly so that it can reserve up to room temperature under ambient conditions. These results demonstrate that the substrate tuning corroborated by interface control can be a promising strategy to utilize the TMDs in practical valleytronics.
Self-assembly films have demonstrated an efficient method to functionalize the surfaces of variously different materials. In this work, we preliminarily explored the modification effect of 10,12-pentacosadiynoic acid (PCDA) on the optical properties of monolayer molybdenum disulfide (MoS<sub>2</sub>) grown on a rutile titanium dioxide (r-TiO<sub>2</sub>) (110) single crystal surface. Atomic force microscopy (AFM) characterizations directly revealed that the PCDA molecules self-assemble into the same lamella structure as on pure MoS<sub>2</sub>, which can be further polymerized into conductive polydiacetylene (PDA) chains under ultraviolet light (UV) irradiation. Detailed photoluminescence (PL) measurements observed clearly increased luminescence of negative trions (A<sup>−</sup>) yet decreased total intensities for MoS<sub>2</sub> upon adding the PCDA assembly, which is further enhanced after stimulating its polymerization. These results indicate that the PCDA assembly and its polymerization have different electron donability to MoS<sub>2</sub>, which hence provides a deepened understanding of the interfacial interactions within a multicomponent system. Our work also demonstrates the self-assembly of films as a versatile strategy to tune the electronic/optical properties of hybridized two-dimensional materials.
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