Large-area MoS2 Films Research Articles

Transferred large area single crystal MoS2 field effect transistors

Transfer of epitaxial, two-dimensional (2D) MoS2 on sapphire grown via synthetic approaches is a prerequisite for practical device applications. We report centimeter-scale, single crystal, synthesized MoS2 field effect transistors (FETs) transferred onto SiO2/Si substrates, with a field-effect mobility of 4.5 cm2 V−1 s−1, which is among the highest mobility values reported for the transferred large-area MoS2 transistors. We demonstrate simple and clean transfer of large-area MoS2 films using deionized water, which can effectively avoid chemical contamination. The transfer method reported here allows standard i-line stepper lithography process to realize multiple devices over the entire film area.

(Invited) Integration of 2-D Materials for Electronic Device Application

In this work, we present our recent progresses on the integration of 2-d materials (TMD, BP, TMC) for the electronic device application. In spite of the recent heightened interest in molybdenum disulfide (MoS2) as a two-dimensional material with substantial bandgaps and reasonably high carrier mobility, a method for the layer-controlled and large-scale synthesis of high quality MoS2 films has not previously been established. Here, we demonstrate that layer-controlled and large-area CVD MoS2 films can be achieved by treating the surfaces of their bottom SiO2 substrates with oxygen plasma process. In addition, by using various polymers, we present that the doping level of MoS2 layers can be controlled for realization of electronic devices.Black phosphorous (BP) is known as a most stable allotrope of phosphorous. Recent studies have shown that BP is an elemental layered material, making it possible to obtain layered structures through mechanical cleavage. In this study, we report the preparation of thickness-controlled few-layer black phosphorus (BP) films through the modulated plasma treatment of BP flakes. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface, which results in enhanced field-effect transistor (FET) performance.

Cu/MoS2/ITO based hybrid structure for catalysis of hydrazine oxidation

We have successfully demonstrated large-area and continuous MoS2 films grown on indium tin oxide (ITO) substrates by RF sputtering followed by a post-annealing process.

Layer-controlled CVD growth of large-area two-dimensional MoS2 films.

In spite of the recent heightened interest in molybdenum disulfide (MoS2) as a two-dimensional material with substantial bandgaps and reasonably high carrier mobility, a method for the layer-controlled and large-scale synthesis of high quality MoS2 films has not previously been established. Here, we demonstrate that layer-controlled and large-area CVD MoS2 films can be achieved by treating the surfaces of their bottom SiO2 substrates with the oxygen plasma process. Raman mapping, UV-Vis, and PL mapping are performed to show that mono, bi, and trilayer MoS2 films grown on the plasma treated substrates fully cover the centimeter scale substrates with a uniform thickness. Our TEM images also present the single crystalline nature of the monolayer MoS2 film and the formation of the layer-controlled bi- and tri-layer MoS2 films. Back-gated transistors fabricated on these MoS2 films are found to exhibit the high current on/off ratio of ∼10(6) and high mobility values of 3.6 cm(2) V(-1) s(-1) (monolayer), 8.2 cm(2) V(-1) s(-1) (bilayer), and 15.6 cm(2) V(-1) s(-1) (trilayer). Our results are expected to have a significant impact on further studies of the MoS2 growth mechanism as well as on the scaled layer-controlled production of high quality MoS2 films for a wide range of applications.

Optical properties of large-area ultrathin MoS2 films: Evolution from a single layer to multilayers

We investigated the optical properties of ultrathin MoS2 films (number of layers: N = 1, 2, 4, and 12) using Raman spectroscopy, photoluminescence (PL) spectroscopy, and spectroscopic ellipsometry. We estimated the layer thicknesses based on Raman spectra. We characterized the microstructural properties of a single-layer MoS2 film using atomic force microscopy. We measured the lowest-energy A and B excitons using PL spectroscopy. We measured the ellipsometric angles (Ψ and Δ) of MoS2 thin films using spectroscopic ellipsometry, and obtained the dielectric functions as the films' thickness changed from a single layer to multi-layers. We determined the films' optical gap energies from the absorption coefficients. Applying the standard critical point model to the second derivative of the dielectric function (d2ε(E)/dE2), we determined several critical point energies. The d2ε(E)/dE2 spectra showed doublet peaks around 3 eV corresponding to the C and D transitions, as well as doublet peaks around 2 eV corresponding to the A and B transitions. These doublet structures at 3 eV are attributed to the transitions in the Brillouin zone between the Γ and K points.

A direct comparison of CVD-grown and exfoliated MoS2 using optical spectroscopy

MoS2 is a highly interesting material, which exhibits a crossover from an indirect band gap in the bulk crystal to a direct gap for single layers. Here, we perform a direct comparison between large-area MoS2 films grown by chemical vapor deposition (CVD) and MoS2 flakes prepared by mechanical exfoliation from mineral bulk crystal. Raman spectroscopy measurements show differences between the in-plane and out-of-plane phonon mode positions in CVD-grown and exfoliated MoS2. Photoluminescence (PL) mapping reveals large regions in the CVD-grown films that emit strong PL at room-temperature, and low-temperature PL scans demonstrate a large spectral shift of the A exciton emission as a function of position. Polarization-resolved PL measurements under near-resonant excitation conditions show a strong circular polarization of the PL, corresponding to a valley polarization.