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

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2

We present first-principles calculations of silicene/graphene and germanene/graphene bilayers. Various supercell models are constructed in the calculations in order to reduce the strain of the lattice-mismatched bilayer systems. Our energetics analysis and electronic structure results suggest that graphene can be used as a substrate to synthesize monolayer silicene and germanene. Multiple phases of single crystalline silicene and germanene with different orientations relative to the substrate could coexist at room temperature. The weak interaction between the overlayer and the substrate preserves the low-buckled structure of silicene and germanene, as well as their linear energy bands. The gap induced by breaking the sublattice symmetry in silicene on graphene can be up to 57 meV.

/pdf/stability-and-electronic-properties-of-two-dimensional-3vzj9yspmn.pdf

Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene

Despite widespread discussion, the role of van der Waals dispersion forces in wetting remains unclear. Here we show that nonlocal correlations contribute substantially to the water-metal bond and that this is an important factor in governing the relative stabilities of wetting layers and 3D bulk ice. Because of the greater polarizability of the substrate metal atoms, nonlocal correlations between water and the metal exceed those between water molecules within ice. This sheds light on a long-standing problem, wherein common density functional theory exchange-correlation functionals incorrectly predict that none of the low temperature experimentally characterized icelike wetting layers are thermodynamically stable.

http://absimage.aps.org/image/MAR12/MWS_MAR12-2011-000153.pdf

To wet or not to wet? Dispersion forces tip the balance for water ice on metals

In this paper, we present a detailed investigation of the structural, electronic, and optical properties of pristine, nitrogenated, and fluorinated MgO monolayers using ab initio calculations. The two dimensional (2D) material stability is confirmed by the phonon dispersion curves and binding energies. Full functionalization causes notable changes in the monolayer structure and slightly reduces the chemical stability. The simulations predict that the MgO single layer is an indirect semiconductor with an energy gap of 3.481 (4.693) eV as determined by the GGA-PBE (HSE06) functional. The electronic structure of the MgO monolayer exhibits high sensitivity to chemical functionalization. Specifically, nitrogenation induces metallization of the MgO monolayer, while an indirect–direct band gap transition and band gap reduction of 81.34 (59.96)% are achieved by means of fluorination. Consequently, the functionalized single layers display strong optical absorption in the infrared and visible regimes. The results suggest that full nitrogenation and fluorination may be a quite effective approach to enhance the optoelectronic properties of the MgO monolayer for application in nano-devices.

/pdf/structural-electronic-and-optical-properties-of-pristine-and-5dp3d2teic.pdf

Structural, electronic and optical properties of pristine and functionalized MgO monolayers: a first principles study

Defective and doped MgO monolayer as promising 2D materials for optoelectronic and spintronic applications

In practice, modifying the fundamental properties of low-dimensional materials should be realized before incorporating them into nanoscale devices. In this paper, we systematically investigate the nitrogen (N) doping and oxygen vacancy (OV) effects on the electronic and magnetic properties of the beryllium oxide (BeO) monolayer using first-principles calculations. Pristine BeO single layer is a non-magnetic insulator with an indirectK-Γ gap of 5.300 eV. N doping induces a magnetic semiconductor nature, where the spin-up and spin-down band gaps depend on the dopant concentration and N-N separation. Creating one OV leads to the energy gap reduction of 31.06% with no spin-polarization, which is due to the abundant 2p electrons of the Be atoms nearest the OV site. The further increase to two OVs and varying the OV-OV distance affect the band gap values, however the spin independence is retained. The magnetic semiconducting behavior is also obtained by the simultaneous N doping and OV presence. Calculations reveal significant magnetization of the BeO@1N, BeO@2N-n, BeO@NOV-nsystems, which is produced mainly by the spin-up N-2p state. Except for the BeO@NOV-1 and BeO@NOV-2, whose magnetic properties are created by the spin-up 2p state of the Be atoms closest to the OV site. The variation of the N-N and N-OV distances keeps the ferromagnetic ordering in the BeO@2N and BeO@NOV layers. Results presented herein may propose efficient methods to artificially modify the physical properties of BeO monolayer, leading to the formation of novel two-dimensional (2D) materials for optoelectronic and spintronic applications.

Nitrogen doping and oxygen vacancy effects on the fundamental properties of BeO monolayer: a DFT study.

Fluorinating the graphene-like BeO monolayer: A spin-polarized first principles study of the electronic, magnetic and optical properties

Designing new two-dimensional (2D) materials based on the Janus structure has attracted great attention due to their novel properties induced by out-of-plane symmetry breaking. In this work, we have systematically investigated the structural and electronic properties of TMSeO (TM = Mo and W) monolayers, which can be obtained by oxygenation of the TMSe2 counterpart. Phonon dispersion curves confirm good dynamical stability. MoSeO and WSeO pristine monolayers are Γ–K indirect gap semiconductors with energy gaps of 0.815(1.324) and 1.323(1.894) eV as determined using the PBE(HSE06) functional, respectively. Our calculations show that a single vacancy of each constituent atoms leads to magnetism-free 2D materials, although it induces a considerable energy-gap reduction. A small lattice constant mismatch between the MoSeO and WSeO monolayers allows the formation of heterostructures. Depending on the stacking, vertical heterostructures (VHSs) exhibit different electronic features, including being a semiconductor with a variable band gap, a semimetal, and metallic in nature. Meanwhile, in lateral heterostructures (LHSs) the semiconductor behavior remains, for which the band gap increases according to an increase in the WSeO block composition. This work provides a deeper insight into the TMSeO monolayers as well as their effective gap engineering based on the formation of defects and heterostructures. The obtained results may introduce new 2D materials, which have the potential for applications in high-performance optoelectronic nano devices.

Exploring the electronic band gap of Janus MoSeO and WSeO monolayers and their heterostructures

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