A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waals Heterojunctions

Chalcogenide Materials and Derivatives for Photovoltaic Applications

Two-dimensional (2D) heterostructures with the Z-scheme can significantly improve the recombination of the photogenerated charge pairs and increase the overpotential compared with the corresponding monolayers. Based on first-principles calculations, we demonstrate the photocatalytic performance with the Z-scheme of three 2D AuSe/SnS heterostructures identified from nine stacking types of AuSe and SnS monolayers using their formation energies. The thermal stabilities of the chosen heterostructures are assured by molecular dynamics simulations. The band alignment and built-in electric field support the heterostructures to drive the photocatalytic hydrogen and oxidation evolution reactions with the direct Z-scheme. Moreover, the optical absorption of both AuSe and SnS monolayers is obviously observed in the visible light and UV ranges, indicating their excellent response to solar light. The highest solar-to-hydrogen energy conversion efficiency of the heterostructures can reach 23.96%. The change in Gibbs free energy in the hydrogen evolution reaction with the heterostructures is in the range of 0.63–0.97 eV, indicating that these reactions are not difficult to carry out. Therefore, the present AuSe/SnS heterostructures could be promising candidates for producing hydrogen from photocatalytic water splitting with the Z-scheme driven by solar light.

Two-dimensional heterostructures of AuSe/SnS for the photocatalytic hydrogen evolution reaction with a Z-scheme

Two-dimensional MoSSe/g-GeC van der waals heterostructure as promising multifunctional system for solar energy conversion

Constructing two-dimensional (2D) van der Waals (vdW) heterostructures is becoming a promising way for photocatalytic water splitting to produce hydrogen. In this Letter, we perform a 2D vdW blue phosphorous/β-AsP (BP/β-AsP) heterostructure based on density functional theory calculations. The type II band alignment in the BP/β-AsP vdW heterostructure is beneficial for separating the photogenerated electrons and holes and suppressing their recombination. The BP/β-AsP heterostructure not only keeps a suitable band edge position for the water splitting reaction but also significantly improves the optical absorption in the visible and ultraviolet light region. Appropriate uniaxial strain can change the indirect bandgap of the BP/β-AsP heterostructure into a direct one. The present findings indicate that the BP/β-AsP heterostructure is a promising candidate for applications in photovoltaic devices and photocatalysis.

Two-dimensional BP/β-AsP van der Waals heterostructures as promising photocatalyst for water splitting

The lead-free perovskites derivatives of Cs3Sb2X9 (X = Cl, Br, I) have been synthesized, but their photocatalytic properties are not explored. To evaluate the feasibility for the visible light catalytic performance, we calculate the structural, electronic, optical and charge transfer properties of Cs3Sb2X9, based on the hybrid density functional theory of HSE06 with the projector augmented wave potential. The results show the decrease of band energy gaps and the redshift of absorption edges from X = Cl to I. The absolute potential of the valence band maximum and conduction band minimum is determined to justify the feasibility of the photocatalytic water splitting or CO2 reduction. The calculated carrier mobilities reveal that the high electron mobilities of Cs3Sb2I9 are beneficial to the reducing powers for hydrogen generation and CO2 reduction. The present results indicate that Cs3Sb2I9 is appropriate for the photocatalytic water splitting to produce hydrogen or the CO2 reduction driven by the visible light.

Theoretical insight into the optoelectronic properties of lead-free perovskite derivatives of Cs3Sb2X9 (X = Cl, Br, I)

Te-doped perovskite NaTaO3 as a promising photocatalytic material for hydrogen production from water splitting driven by visible light

Exploring stable photocatalysts with superior optical absorption and high energy conversion efficiency is the key to water splitting. By means of the first-principles calculations, we report a ternary Sn2S2P4 monolayer with excellent stabilities. Remarkably, the material presents an indirect bandgap of 1.77 eV with the band edge perfectly crossing the redox potential of water. Monolayer Sn2S2P4 exhibits noticeable optical absorption and photocurrent density in the visible range and has adequate driving forces to trigger overall water splitting. Anisotropic and high carrier mobility facilitate the fast transport of photogenerated carriers. Moreover, a solar-to-hydrogen efficiency that reaches as high as 17.51% is theoretically predicted, thereby indicating that the Sn2S2P4 monolayer is a promising candidate for overall photocatalytic water splitting.

Yu-Liang Liu

Papers

Two-dimensional MoSSe/g-GeC van der waals heterostructure as promising multifunctional system for solar energy conversion

Two-dimensional BP/β-AsP van der Waals heterostructures as promising photocatalyst for water splitting

Theoretical insight into the optoelectronic properties of lead-free perovskite derivatives of Cs3Sb2X9 (X = Cl, Br, I)

Te-doped perovskite NaTaO3 as a promising photocatalytic material for hydrogen production from water splitting driven by visible light

First principles study of photoelectrochemical water splitting in monolayer Sn2S2P4 with high solar-to-hydrogen efficiency