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Band engineering of two-dimensional transition metal dichalcogenides (2D TMDCs) is of great significance with regard to both fundamental exploration and practical application. Here we report on a study of the band evolution of monolayer and bilayer TMDCs (WS2, WSe2, and MoS2) under vertical electric fields. Our results show that the electric field has a negligible influence on the bandgaps of monolayer TMDCs. For bilayer TMDCs, our results show that their intralayer direct bandgaps are also immune to the electric field. However, the indirect bandgaps of bilayer TMDCs can be effectively tuned by a vertical electric field. Interestingly, we find that the field tunability of the bandgap in bilayer WSe2 is much larger than those in bilayer WS2 and MoS2.

Among transition-metal nitrides, the mononitride γ-MoN with a rock-salt structure has drawn particular attention because it has been predicted to possess excellent mechanical and electronic properties, especially the high superconducting temperature around 30 K. However, synthesis of bulk γ-MoNx with the nitrogen concentration, x, more than 0.5 is still challenging, leading to contradictions on its phase stability and properties. In this work, we formulated a high-pressure synthesis reaction for the formation of single-crystal γ-MoNx with a remarkably high nitrogen concentration value of x ≈ 0.67. This nitride possesses a high asymptotic hardness of ∼24 GPa, which is so far the second hardest among metal nitrides. Impressively, the expected superconductivity is absent in the as-synthesized product. We further performed density functional theory calculations to clarify the structural stability and the absence of superconductivity in stoichiometric γ-MoN. We find that the ground state of γ-MoN is theoretically explored to be a Mott insulator with an antiferromagnetic phase, while a paramagnetic configuration is adopted at the ambient conditions. Such magnetic properties would explain the structural stability and the absence of superconductivity in the as-synthesized γ-MoNx with a high nitrogen concentration.

Low-dimensional organometal halide perovskites have attracted more and more attention because of their good optoelectronic properties and improved stability compared to three-dimensional analogues. In this work, we investigated the charge separation mechanism in multiple quantum well (MQW) perovskite films, which are composed of a mixture of layered perovskites (or quantum wells) with different bandgaps. Despite inefficient dissociation of photo-generated excitons in large-bandgap quantum wells due to the large exciton binding energy, efficient charge separation can occur at the MQW perovskite/electron-extracted-layer interface via energy and/or charge transfer from large-bandgap quantum wells to small-bandgap quantum wells. The MQW perovskite solar cell exhibits a 25-fold improvement in device efficiency, as compared to a pure 2D analogue.

Stacking two-dimensional materials into van der Waals heterostructures with distinct interlayer twisting angles opens up new strategies for electronic structure and physical property engineering. Here, we investigate how the interlayer twisting angles affect the photoluminescence (PL) and Raman spectra of the MoS2/graphene heterostructures. Based on a series of heterostructure samples with different interlayer twisting angles, we found that the PL and Raman spectra of the monolayer MoS2 in these heterostructures are strongly twisting angle dependent. When the interlayer twisting angle evolves from 0° to 30°, both the PL intensity and emission energy increase, while the splitting of the E2g Raman mode decreases gradually. The observed phenomena are attributed to the twisting angle dependent interlayer interaction and misorientation-induced lattice strain between MoS2 and graphene.

Graphene nanostructures are potential building blocks for nanoelectronic and spintronic devices. However, the production of monolayer graphene nanostructures with well-defined zigzag edges remains a challenge. In this paper, we report the patterning of monolayer graphene nanostructures with zigzag edges on hexagonal boron nitride (h-BN) substrates by an anisotropic etching technique. We found that hydrogen plasma etching of monolayer graphene on h-BN is highly anisotropic due to the inert and ultra-flat nature of the h-BN surface, resulting in zigzag edge formation. The as-fabricated zigzag-edged monolayer graphene nanoribbons (Z-GNRs) with widths below 30 nm show high carrier mobility and width-dependent energy gaps at liquid helium temperature. These high quality Z-GNRs are thus ideal structures for exploring their valleytronic or spintronic properties.

In this letter, we report a pilot study on epitaxy of monolayer graphene nanoribbons (GNRs) on hexagonal boron nitride (h-BN). We found that GNRs grow preferentially from the atomic steps of h-BN, forming in-plane heterostructures. GNRs with well-defined widths ranging from ∼15 nm to ∼150 nm can be obtained reliably. As-grown GNRs on h-BN have high quality with a carrier mobility of ∼20 000 cm2 V−1 s−1 for ∼100-nm-wide GNRs at a temperature of 1.7 K. Besides, a moire pattern induced quasi-one-dimensional superlattice with a periodicity of ∼15 nm for GNR/h-BN was also observed, indicating zero crystallographic twisting angle between GNRs and h-BN substrate. The superlattice induced band structure modification is confirmed by our transport results. These epitaxial GNRs/h-BN with clean surfaces/interfaces and tailored widths provide an ideal platform for high-performance GNR devices.

High-quality continuous uniform monolayer graphene was grown on polycrystalline PtRh20 alloy foils by low pressure chemical vapor deposition. The morphology of graphene was investigated by Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. Analysis results confirm that high quality single-layer graphene was fabricated on PtRh20 foil at 1050 °C using a lower flux of methane under low pressure. Graphene films were transferred onto the SiO2/Si substrate by the bubbling transfer method. The mobility of a test field effect transistor made of the graphene grown on PtRh20 was measured and reckoned at room temperature, showing that the carrier mobility was about 4000 cm2 V−1 s−1. The results indicate that desired quality of single-layer graphene grown on PtRh20 foils can be obtained by tuning reaction conditions.

We propose a concept of transparent electrode for solar cells surpassing conventional transparent conductive oxide. Transparent electrode requires low electrical resistivity, high optical transparency, and high optical haze. Although transparent conductive oxide by chemical vapor deposition is widely used as a transparent electrode for solar cells, a breakthrough of the trade-off between electrical and optical properties is required for further improvement of solar cell efficiency. We demonstrate solution-processed electrode fabrication by using nanoimprint technology and metal nanoparticle ink. Silver mesh electrode is self-aligned on nanoimprinted texture with concave pattern as a template for mesh grid. Our electrode concept can realize desired high optical haze by nanoimprinted texture, as well as low electrical resistivity and high optical transparency by metal mesh electrode simultaneously, which boosts solar cell efficiency.

Identifying the role of defects that limits graphene's quality is important for various graphene devices on SiO2. In this paper, monolayer graphene samples with defect densities varying from ∼0.04 μm−2 to ∼10 μm−2 on SiO2 are characterized by both microscopic imaging and electrical transport measurements. We found that the height of graphene on SiO2 is directly related to its defect densities with a reverse correlation, which in turn degrade graphene's quality through a complicated mechanism rather than defects scattering only. We suggest that, at relative high defect density regime, graphene-SiO2 coupling is greatly enhanced causing an increasing charged impurity scattering significantly.

A scheme of identification of scattering mechanisms in epitaxial graphene (EG) on SiC substrate is developed and applied to three EG samples grown on SiC (0001), (11 (2) over bar0), and (10 (1) over bar0) substrates. Hall measurements combined with defect detection technique enable us to evaluate the individual contributions to the carrier scatterings by defects and by substrates. It is found that the dominant scatterings can be due to either substrate or defects, dependent on the substrate orientations. The EG on SiC (11 (2) over bar0) exhibits a better control over the two major scattering mechanisms and achieves the highest mobility even with a high carrier concentration, promising for high performance graphene-based electronic devices. The method developed here will shed light on major aspects in governing carrier transport in EG to harness it effectively. (C) 2014 AIP Publishing LLC.

Vibrational spectra of gas-source molecular beam epitaxy grown dilute InNxAs1−x/InP (001) alloys are obtained using a Fourier-transform infrared (IR) spectroscopy. A triply degenerate NAs local vibrational mode of Td-symmetry is observed near 438 cm−1 corresponding to the In-N bond energy. The analysis of composition dependent infrared reflectivity spectra in InNAs has predicted a two-phonon-mode behavior. In In(Ga)-rich GaInNAs alloys the observed splitting of the NAs local mode into a doublet for the NAs–Ga1(In1)In3(Ga3) pair-defect of C3v-symmetry is consistent with our simulated results based on a sophisticated Green's function theory.

Graphene shows promise on strain sensor applications, but the piezoresistive sensitivity of perfect graphene is low due to its weak electrical conductivity response upon structural deformation. In this paper, we used nanographene films for ultra-sensitive strain sensors. The piezoresistive sensitivity of nanographene films with different thicknesses and conductivities was systematically investigated and a nearly inverse proportional correlation was found. A gauge factor over 300, the highest so far for graphene-based strain sensors, was achieved. A charge tunneling model was used to explain the piezoresistive characteristics of nanographene films, which indicates our results provide a different rout toward ultra-sensitive strain sensors.

Charge trap and loss characteristics of high-k based trapping layer structures are investigated by electrostatic force microscopy, which proves that the interfaces provide dominate trap sites. The effects of post-deposition anneal of HfO2 and Al2O3 single layer are determined. Based on aforementioned findings, we demonstrate the HfO2/Al2O3 bi-layers trapping structure with improved performance. The lateral charge spreading properties are also evaluated by extracted diffusion coefficients to further understand the interface effect. The study may provide insights into fundamental assessment and optimization for charge trapping structures, especially for high-density NAND flash applications.

We report on tuning interfacial properties of epitaxially-grown graphenes with different kinds of metal substrates based on scanning tunneling microscopy experiments and density functional theory calculations. Three kinds of metal substrates, Ni(111), Pt(111), and Ru(0001), show different interactions with the epitaxially grown graphene at the interfaces. The different interfacial interaction making graphene n-type and p-type doped, leads to the polarity change of the thermoelectric property of the graphene/metal systems. These findings may give further insights to the interfacial interactions in the graphene/metal systems and promote the use of graphene-based heterostructures in devices.

Mg3N2 nanocubes were prepared by vaporized bulk magnesium in ammonia atmosphere associated with plasma metal reaction. Then the product transformed to Mg(NH2)2 hollow nanospheres after it was reacted with NH3 based on the Kirkendall effect. The electron microscopy results suggested that the obtained hollow nanospheres were around 100nm and the shell thickness was about 10nm. Because of its short distance for Mg2+ diffusion and large specific surface area for interaction between Mg(NH2)2 and MgH2, the structure dramatically enhanced the hydrogen desorption kinetics of Mg(NH2)2–2MgH2.

The relationship between hardness (H), reduced modulus (E-r), unloading work (W-u), and total work (W-t) of indentation is examined in detail experimentally and theoretically. Experimental study verifies the approximate linear relationship. Theoretical analysis confirms it. Furthermore, the solutions to the conical indentation in elastic-perfectly plastic solid, including elastic work (W-e), H, W-t, and W-u are obtained using Johnson's expanding cavity model and Lame solution. Consequently, it is found that the W-e should be distinguished from W-u, rather than their equivalence as suggested in ISO14577, and (H/E-r)/(W-u/W-t) depends mainly on the conical angle, which are also verified with numerical simulations. (C) 2008 American Institute of Physics.

A manganese nitride surface containing a well-ordered array of MnN-bonded manganese clusters is investigated. The clusters are composed of a quadrant array of Mn atoms forming a tetramer. Scanning tunneling microscopy is used to image and resolve the clusters into their constituent atoms and their structure and arrangement is presented. The Mn–Mn and Mn–N bond lengths are estimated from the experimental data and compared with theoretical predictions by Rao and Jena [Phys. Rev. Lett. 89, 185504 (2002)] for free, N-doped Mn clusters. The possible effect of the bond lengths on the magnetic properties of the clusters is discussed.

Terahertz radiation from longitudinal optical (LO) phonon–plasmon coupling modes in InSb films is observed using an ultrafast photoconductive antenna detector. We demonstrate a response frequency of up to 7 THz for a low-temperature-grown GaAs-based photoconductive antenna gated with 25 fs laser pulses. It is found that the emission frequencies of the coupling modes are dependent only on the residual carrier density, not on the excitation carrier density. It is also found that the LO phonon–plasmon oscillations in semiconductors can serve as an efficient THz source.