Hybrid organic-inorganic perovskite materials have received substantial research attention due to their impressively high performance in photovoltaic devices. As one of the oldest functional materials, it is intriguing to explore the optoelectronic properties in perovskite after reducing it into a few atomic layers in which two-dimensional (2D) confinement may get involved. In this work, we report a combined solution process and vapor-phase conversion method to synthesize 2D hybrid organic-inorganic perovskite (i.e., CH3NH3PbI3) nanocrystals as thin as a single unit cell (∼1.3 nm). High-quality 2D perovskite crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence while the thickness or composition is changed. Due to the high quantum efficiency and excellent photoelectric properties in 2D perovskites, a high-performance photodetector was demonstrated, in which the current can be enhanced significantly by shining 405 and 532 nm lasers, showing photoresponsivities of 22 and 12 AW(-1) with a voltage bias of 1 V, respectively. The excellent optoelectronic properties make 2D perovskites building blocks to construct 2D heterostructures for wider optoelectronic applications.

The two-fold valley degeneracy in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) (Mo,W)(S,Se) 2 is suitable for "valleytronics", the storage and manipulation of information utilizing the valley degree of freedom. The conservation of luminescent photon helicity in these 2D crystal monolayers has been widely regarded as a benchmark indicator for charge carrier valley polarization. Here we perform helicityresolved Raman scattering of the TMDC atomic layers. In drastic contrast to luminescence, the dominant first-order zone-center Raman bands, including the low energy breathing and shear modes as well as the higher energy optical phonons, are found to either maintain or completely switch the helicity of incident photons. These experimental observations, in addition to providing a useful tool for characterization of TMDC atomic layers, shed new light on the connection between photon helicity and valley polarization. KeywordsValleytronics, transition metal dichalcogenide, pseudospin, Raman scattering, shear mode, breathing mode 3 Manuscript textSince the discovery of graphene, 1 the mechanical, electronic, chemical and optical properties of various two-dimensional (2D) materials as well as their heterostructures have been widely investigated. [2][3][4][5] A prominent example is the semiconducting transition metal dichalcogenides (TMDCs) that exhibit rich physical phenomena, including indirect to direct bandgap transition, 6, 7 large exciton and trion binding energy, [8][9][10][11] strong photoluminescence and electroluminescence, 7, 12-14 superior transistor performance with large on-off ratio [15][16][17] and reasonably high mobility, 5, 18, 19 and perhaps most strikingly, the capability to address the valley degree of freedom. [20][21][22][23][24] Manipulation of valley polarized carriers excited by circularly polarized light has led to recent observation of the valley Hall effect 25 that opens up potential for applications in 'valleytronics' envisioned before in graphene. 26, 27 Here we apply circularly polarized light to excite electrons in TMDC atomic layers and measure the helicity of the photons emitted after they are inelastically scattered by phonons. We discovered that while some phonons maintain helicity from incident to emitted photon, others can switch it completely. Our results can be explained by the symmetry of participating lattice vibrations in the Raman scattering process. The helicity selection rules provide clean Raman spectra and prove to be a powerful tool for resolving phonon mode assignment and characterization. Importantly, the helicity of Raman scattered photons is independent of layer number and excitation laser wavelength, drastically different from observations in valley pumping of TMDC with helicity resolved luminescence. Our experiments consequently provide new insights into the relation between the photon helicity and valleytronics in semiconducting TMDCs. 4Layered TMDC materials have a graphite-like structure with each graphene sheet replaced wi...

Two-dimensional layered transition metal dichalcogenides (TMDs) show intriguing potential for optoelectronic devices due to their exotic electronic and optical properties. Only a few efforts have been dedicated to large-area growth of TMDs. Practical applications will require improving the efficiency and reducing the cost of production, through (1) new growth methods to produce large size TMD monolayer with less-stringent conditions, and (2) nondestructive transfer techniques that enable multiple reuse of growth substrate. In this work, we report to employ atmospheric pressure chemical vapor deposition (APCVD) for the synthesis of large size (>100 μm) single crystals of atomically thin tungsten disulfide (WS2), a member of TMD family, on sapphire substrate. More importantly, we demonstrate a polystyrene (PS) mediated delamination process via capillary force in water which reduces the etching time in base solution and imposes only minor damage to the sapphire substrate. The transferred WS2 flakes are of excellent continuity and exhibit comparable electron mobility after several growth cycles on the reused sapphire substrate. Interestingly, the photoluminescence emission from WS2 grown on the recycled sapphire is much higher than that on fresh sapphire, possibly due to p-type doping of monolayer WS2 flakes by a thin layer of water intercalated at the atomic steps of the recycled sapphire substrate. The growth and transfer techniques described here are expected to be applicable to other atomically thin TMD materials.

We report the discovery of room temperature in-plane ferroelectricity in van der Waals In2Se3 with β′ phase.