Ariana E. Nguyen

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.

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.

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.

Band structure characterization of WS2 grown by chemical vapor deposition

Growth by chemical vapor deposition (CVD) leads to multilayer WS2 of very high quality, based on high-resolution angle-resolved photoemission spectroscopy. The experimental valence band electronic structure is considered to be in good agreement with that obtained from density functional theory calculations. We find the spin-orbit splitting at the K¯ point to be 420  ± 20 meV with a hole effective mass of −0.35  ± 0.02 me for the upper spin-orbit component (the branch closer to the Fermi level) and −0.43  ± 0.07 me for the lower spin-orbit component. As predicted by theory, a thickness-dependent increase of bandwidth is observed at the top of the valence band, in the region of the Brillouin zone center. The top of the valence band of the CVD-prepared films exhibits a substantial binding energy, consistent with n-type behavior, and in agreement with transistor characteristics acquired using devices incorporating the same WS2 material.

Chemical vapor deposition growth of a periodic array of single-layer MoS2 islands via lithographic patterning of an SiO2/Si substrate

The growth of micron-scale single-layer MoS2 islands is seeded and distributed at high fidelity by means of a regular array of micron-scale holes that extend through the oxide layer of a 300 nm SiO2/Si substrate. Low coverages exhibit individual sub-micron MoS2 islands directly adjacent to the seed positions. At moderate coverage the seed holes are encircled by merged MoS2 islands, whose overall shape and internal grain boundaries reveal coalescence out of several initial crystallites. Seeded islands are strictly monolayer in height, non-overlapping and they offer high photoluminescence as well as conventional Raman signatures.

Recent progress on the scalable fabrication of hybrid polymer/SiO2 nanophotonic cavity arrays with an encapsulated MoS2 film

We review1 the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS2) - silicon dioxide (SiO2) one-dimensional (1D), freestanding photonic-crystal cavities (PCCs) capable of enhancement of the MoS2 photoluminescence (PL) at the narrow cavity resonance. As demonstrated in our prior work [S. Hammer et al., Sci. Rep. 7, 7251 (2017)]1, geometric mode tuning over the wide spectral range of MoS2 PL can be achieved by changing the PC period. In this contribution, we provide a step-by-step description of the fabrication process and give additional detailed information on the degradation of MoS2 by XeF2 vapor. We avoid potential damage of the MoS2 monolayer during the crucial XeF2 etch by refraining from stripping the electron beam (e-beam) resist after dry etching of the photonic crystal pattern. The remaining resist on top of the samples encapsulates and protects the MoS2 film during the entire fabrication process. Albeit the thickness of the remaining resists strongly depends on the fabrication process, the resulting encapsulation of the MoS2 layer improves the confinement to the optical modes and gives rise to a potential enhancement of the light-matter interaction.

Large-area grown MoS 2 and its integration in geometrically tunable photonic crystal cavities

Monolayer Transition Metal Dichalcogenides (TMDs) have recently attracted great interest in the field of photonics because of their distinctive optical and spin properties. In contrast to bulk TMD materials, which are indirect bandgap semiconductors, monolayer TMDs are highly optically active due to a direct bandgap ranging between 1 and 2 eV [1]. The coupling of TMD excitons to optical fields can be dramatically enhanced by integration with nanophotonic resonators. For instance, enhanced light extraction, signatures of Purcell enhanced emission and even lasing in coupled systems of TMD monolayers transferred onto two-dimensional GaP-based photonic crystal cavities (PCCs) [2,3] as well as microdisk cavities [4] was reported.

Combined electrical transport and capacitance spectroscopy of a MoS2-LiNbO3 field effect transistor

We have measured both the current-voltage ( ISD- VGS) and capacitance-voltage (C- VGS) characteristics of a MoS2-LiNbO3 field effect transistor. From the measured capacitance, we calculate the electron surface density and show that its gate voltage dependence follows the theoretical prediction resulting from the two-dimensional free electron model. This model allows us to fit the measured ISD- VGS characteristics over the entire range of VGS. Combining this experimental result with the measured current-voltage characteristics, we determine the field effect mobility as a function of gate voltage. We show that for our device, this improved combined approach yields significantly smaller values (more than a factor of 4) of the electron mobility than the conventional analysis of the current-voltage characteristics only.

Single- and few-layer transfer-printed CVD MoS 2 nanomechanical resonators with enhancement by thermal annealing

We report on experimental demonstration of single- and few-layer chemical vapor deposited (CVD) molybdenum disulfide (MoS2) nanomechanical resonators by employing a scalable transfer-print method. CVD MoS2 crystals are released from their growth substrates (with well-preserved, uniform crystal quality) by a water-assisted lift-off technique, and suspended over pre-patterned arrays of microtrenches through an all-dry, aligned transfer process. We measure device performance from the resulting single- and few-layer drumhead resonators (1-2 μm in diameter), showing fundamental resonance frequencies (f0) up to ~80 MHz, and figures-of-merit (f0×Q) up to ~2.4×1010 Hz. We quantify the uniformity of resonance performance in arrays of devices, and show that 250 °C thermal annealing can enhance the performance and uniformity.