Significance A new class of heterostructures consisting of layered transition metal dichalcogenide components can be designed and built by van der Waals (vdW) stacking of individual monolayers into functional multilayer structures. Nonetheless, the optoelectronic properties of this new type of vdW heterostructure are unknown. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe 2 and MoS 2 . We observe spatially direct absorption but spatially indirect emission in this heterostructure, with strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN dielectric layers into the vdW gap. The generic nature of this interlayer coupling is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties through customized composite layers.

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows to resolve submonolayer growth, including individual, localized C=C dimers in a diamond-like carbon matrix for AES C/Si ratio of ∼3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.PACS numbers: 81.05. Uw, 65.40.De, Graphene has been shown recently to possess many of the remarkable electronic properties of carbon nanotubes [1], while lending itself more readily to the planar paradigm of integrated-circuit fabrication processes. Epitaxial graphene (epigraphene) on silicon carbide (SiC) surfaces is emerging as an attractive process alternative to the painstaking layer-by-layer exfoliation of graphite crystals [1,2,3,4,5]. Epigraphene shares the key electron transport properties of free-standing exfoliated films [1,2,3]. However, in contrast to the exfoliated films, new features in the electronic structures appear in epitaxial graphene grown on the (0001) surface of 6H-SiC, whose origins remain controversial [6,7,8,9]. The novel electronic properties and the choice of the type of substrate are significant for the design of nonlinear devices and underscores the importance of substrate interactions in epitaxial films. This Letter is focussed on Si-terminated 6H-SiC(0001).The interaction of epigraphene with SiC substrate is mediated by a monolayer of C atoms, so called "buffer layer", arranged in a honeycomb lattice, like graphene, but bonded in sp 3 configuration, with each atom forming a covalent bond to a Si atom beneath [5]. This buffer layer evolves from C-rich, high temperature surface reconstructions of 6H-SiC (0001) upon thermal desorption of Si atoms around T=1100• C. Annealing to higher temperature (1250• C) results in further desorption of Si, which promotes the formation of a second carbon layer, and deprives the original (topmost) carbon atoms of their covalent bonds to Si atoms, inducing sp 2 bonding configuration, i.e., into that of a graphene layer [2,10]. This paper presents a detailed investigation of the processes by which Si loss occurs, and its effect on the bonding, electronic and mechanical properties of the resulting carbon film. Our observations are made possible by the polarization analysis of the Raman signal from the sample surface. Although unpolarized Raman spectroscopy has been widely used in characterizing graphene layers [11,12,13,14,15,16,17,18,19], this is, to the best our k...

A MoS 2 /graphene hybrid aerogel synthesized with two-dimensional MoS 2 sheets coating a high surface area graphene aerogel scaffold is characterized and used for ultrasensitive NO 2 detection. The combination of graphene and MoS 2 leads to improved sensing properties with the graphene scaffold providing high specifi c surface area and high electrical and thermal conductivity and the single to few-layer MoS 2 sheets providing high sensitivity and selectivity to NO 2 . The hybrid aerogel is integrated onto a low-power microheater platform to probe the gas sensing performance. At room temperature, the sensor exhibits an ultralow detection limit of 50 ppb NO 2 . By heating the material to 200 °C, the response and recovery times to reach 90% of the fi nal signal decrease to <1 min, while retaining the low detection limit. The MoS 2 / graphene hybrid also shows good selectivity for NO 2 against H 2 and CO, especially when compared to bare graphene aerogel. The unique structure of the hybrid aerogel is responsible for the ultrasensitive, selective, and fast NO 2 sensing. The improved sensing performance of this hybrid aerogel also suggests the possibility of other 2D material combinations for further sensing applications.

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