Toward Ferroelectric Control of Monolayer MoS₂

The chemical vapor deposition (CVD) of molybdenum disulfide (MoS₂) single-layer films onto periodically poled lithium niobate is possible while maintaining the substrate polarization pattern. The MoS₂ growth exhibits a preference for the ferroelectric domains polarized “up” with respect to the surface so that the MoS₂ film may be templated by the substrate ferroelectric polarization pattern without the need for further lithography. MoS₂ monolayers preserve the surface polarization of the “up” domains, while slightly quenching the surface polarization on the “down” domains as revealed by piezoresponse force microscopy. Electrical transport measurements suggest changes in the dominant carrier for CVD MoS₂ under application of an external voltage, depending on the domain orientation of the ferroelectric substrate. Such sensitivity to ferroelectric substrate polarization opens the possibility for ferroelectric nonvolatile gating of transition metal dichalcogenides in scalable devices fabricated free of exfoliation and transfer

Superlinear Composition-Dependent Photocurrent in CVD-Grown Monolayer MoS_{2(1–<i>x</i>)}Se_2<i>x</i> Alloy Devices

Transition metal dichalcogenides (TMDs) have emerged as a new class of two-dimensional materials that are promising for electronics and photonics. To date, optoelectronic measurements in these materials have shown the conventional behavior expected from photoconductors such as a linear or sublinear dependence of the photocurrent on light intensity. Here, we report the observation of a new regime of operation where the photocurrent depends superlinearly on light intensity. We use spatially resolved photocurrent measurements on devices consisting of CVD-grown monolayers of TMD alloys spanning MoS₂ to MoSe₂ to show the photoconductive nature of the photoresponse, with the photocurrent dominated by recombination and field-induced carrier separation in the channel. Time-dependent photoconductivity measurements show the presence of persistent photoconductivity for the S-rich alloys, while photocurrent measurements at fixed wavelength for devices of different alloy compositions show a systematic decrease of the responsivity with increasing Se content associated with increased linearity of the current–voltage characteristics. A model based on the presence of different types of recombination centers is presented to explain the origin of the superlinear dependence on light intensity, which emerges when the nonequilibrium occupancy of initially empty fast recombination centers becomes comparable to that of slow recombination centers

Chemical Vapor Deposition Growth of Few-Layer MoTe₂ in the 2H, 1T′, and 1T Phases: Tunable Properties of MoTe₂ Films

Chemical vapor deposition allows the preparation of few-layer films of MoTe₂ in three distinct structural phases depending on the growth quench temperature: 2H, 1T′, and 1T. We present experimental and computed Raman spectra for each of the phases and utilize transport measurements to explore the properties of the 1T MoTe₂ phase. Density functional theory modeling predicts a (semi-)­metallic character. Our experimental 1T films affirm the former, show facile μA-scale source-drain currents, and increase in conductivity with temperature, different from the 1T′ phase. Variation of the growth method allows the formation of hybrid films of mixed phases that exhibit susceptibility to gating and significantly increased conductivity

Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange

We demonstrate bandgap tuning of a single-layer MoS₂ film on SiO₂/Si <i>via</i> substitution of its sulfur atoms by selenium through a process of gentle sputtering, exposure to a selenium precursor, and annealing. We characterize the substitution process both for S/S and S/Se replacement. Photoluminescence and, in the latter case, X-ray photoelectron spectroscopy provide direct evidence of optical band gap shift and selenium incorporation, respectively. We discuss our experimental observations, including the limit of the achievable bandgap shift, in terms of the role of stress in the film as elucidated by computational studies, based on density functional theory. The resultant films are stable in vacuum, but deteriorate under optical excitation in air

Gold Dispersion and Activation on the Basal Plane of Single-Layer MoS₂

Gold islands are typically associated with high binding affinity to adsorbates and catalytic activity. Here we present the growth of dispersed nanoscale gold islands on single layer MoS₂, prepared on an inert SiO₂/Si support by chemical vapor deposition. This study offers a combination of growth process development, optical characterization, photoelectron spectroscopy at submicron spatial resolution, and advanced density functional theory modeling for detailed insight into the electronic interaction between gold and single-layer MoS₂. In particular, we find the gold density of states in Au/MoS₂/SiO₂/Si to be far less well-defined than Au islands on other 2-dimensional materials such as graphene, for which we also provide data. We attribute this effect to the presence of heterogeneous Au adatom/MoS₂-support interactions within the nanometer-scale gold cluster. Theory predicts that CO will exhibit adsorption energies in excess of 1 eV at the Au cluster edges, where the local density of states is dominated by Au 5d_<i>z</i>2 symmetry