Scalable and Transfer-Free Fabrication of MoS2/SiO2 Hybrid Nanophotonic Cavity Arrays with Quality Factors Exceeding 4000.

We report the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS2) - silicon dioxide (SiO2) one-dimensional, free-standing photonic-crystal cavities capable of enhancement of the MoS2 photoluminescence at the narrow cavity resonance. We demonstrate continuous tunability of the cavity resonance wavelength across the entire emission band of MoS2 simply by variation of the photonic crystal periodicity. Device fabrication started by substrate-scale growth of MoS2 using chemical vapor deposition (CVD) on non-birefringent thermal oxide on a silicon wafer; it was followed by lithographic fabrication of a photonic crystal nanocavity array on the same substrate at more than 50% yield of functional devices. Our cavities exhibit three dominant modes with measured linewidths less than 0.2 nm, corresponding to quality factors exceeding 4000. All experimental findings are found to be in excellent agreement with finite difference time domain (FDTD) simulations. CVD MoS2 provides scalable access to a direct band gap, inorganic, stable and efficient emitter material for on-chip photonics without the need for epitaxy and is at CMOS compatible processing parameters even for back-end-of-line integration; our findings suggest feasibility of cavity based line-narrowing in MoS2-based on-chip devices as it is required for instance for frequency-multiplexed operation in on-chip optical communication and sensing.

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.

Large-scale arrays of single- and few-layer MoS2 nanomechanical resonators.

We report the fabrication of large-scale arrays of suspended molybdenum disulfide (MoS2) atomic layers, as two-dimensional (2D) MoS2 nanomechanical resonators. We employ a water-assisted lift-off process to release chemical vapor deposited (CVD) MoS2 atomic layers from a donor substrate, followed by an all-dry transfer onto microtrench arrays. The resultant large arrays of suspended single- and few-layer MoS2 drumhead resonators (0.5-2 µm in diameter) offer fundamental resonances (f0) in the very high frequency (VHF) band (up to ∼120 MHz) and excellent figures-of-merit up to f0 × Q ≈ 3 × 10(10) Hz. A stretched circular diaphragm model allows us to estimate low pre-tension levels of typically ∼15 mN m(-1) in these devices. Compared to previous approaches, our transfer process features high yield and uniformity with minimal liquid and chemical exposure (only involving DI water), resulting in high-quality MoS2 crystals and exceptional device performance and homogeneity; and our process is readily applicable to other 2D materials.

Nanoscale plasmonic phenomena in CVD-grown MoS(2) monolayer revealed by ultra-broadband synchrotron radiation based nano-FTIR spectroscopy and near-field microscopy.

Nanoscale plasmonic phenomena observed in single and bi-layers of molybdenum disulfide (MoS(2)) on silicon dioxide (SiO(2)) are reported. A scattering type scanning near-field optical microscope (s-SNOM) with a broadband synchrotron radiation (SR) infrared source was used. We also present complementary optical mapping using tunable CO(2)-laser radiation. Specifically, there is a correlation of the topography of well-defined MoS(2) islands grown by chemical vapor deposition, as determined by atomic force microscopy, with the infrared (IR) signature of MoS(2). The influence of MoS(2) islands on the SiO(2) phonon resonance is discussed. The results reveal the plasmonic character of the MoS(2) structures and their interaction with the SiO(2) phonons leading to an enhancement of the hybridized surface plasmon-phonon mode. A theoretical analysis shows that, in the case of monolayer islands, the coupling of the MoS(2) optical plasmon mode to the SiO(2) surface phonons does not affect the infrared spectrum significantly. For two-layer MoS(2), the coupling of the extra inter-plane acoustic plasmon mode with the SiO(2) surface transverse phonon leads to a remarkable increase of the surface phonon peak at 794 cm(-1). This is in agreement with the experimental data. These results show the capability of the s-SNOM technique to study local multiple excitations in complex non-homogeneous structures.

Nanoscale plasmonic phenomena in CVD-grown MoS2 monolayer revealed by ultra-broadband synchrotron radiation based nano-FTIR spectroscopy and near-field microscopy: publisher's note.

This publisher's note amends the Acknowledgments of a recent publication [Opt. Express24, 1154 (2016)10.1364/OE.24.001154].

Scalable fabrication of a hybrid field-effect and acousto-electric device by direct growth of monolayer MoS2/LiNbO3.

Lithium niobate is the archetypical ferroelectric material and the substrate of choice for numerous applications including surface acoustic wave radio frequencies devices and integrated optics. It offers a unique combination of substantial piezoelectric and birefringent properties, yet its lack of optical activity and semiconducting transport hamper application in optoelectronics. Here we fabricate and characterize a hybrid MoS2/LiNbO3 acousto-electric device via a scalable route that uses millimetre-scale direct chemical vapour deposition of MoS2 followed by lithographic definition of a field-effect transistor structure on top. The prototypical device exhibits electrical characteristics competitive with MoS2 devices on silicon. Surface acoustic waves excited on the substrate can manipulate and probe the electrical transport in the monolayer device in a contact-free manner. We realize both a sound-driven battery and an acoustic photodetector. Our findings open directions to non-invasive investigation of electrical properties of monolayer films.

Superlinear composition-dependent photocurrent in CVD-grown monolayer MoS2(1-x)Se2x 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.