Sina Najmaei

Thermal effects on the characteristic Raman spectrum of molybdenum disulfide (MoS2) of varying thicknesses

In this letter, thermal effects on the Raman spectra of molybdenum disulfide with thicknesses ranging from bulk to monolayer were evaluated. We quantitatively determined the laser-induced heating effects on the peak position and the line-width of the Raman spectrum. We found considerable thickness-dependent red-shifts as well as line-width changes for both E2g1 and A1g vibrating modes as laser power was increased. Our results enrich the knowledge of phononic behaviors of this material and demonstrate the important effects of the anharmonic terms in the lattice potential energy.

Optoelectronic devices based on two-dimensional transition metal dichalcogenides

In the past few years, two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ranging from the valley–spin coupling to the indirect-to-direct bandgap transition when scaling the materials from multi-layer to monolayer. These properties are appealing for the development of novel electronic and optoelectronic devices with important applications in the broad fields of communication, computation, and healthcare. One of the key features of the TMDC family is the indirect-to-direct bandgap transition that occurs when the material thickness decreases from multilayer to monolayer, which is favorable for many photonic applications. TMDCs have also demonstrated unprecedented flexibility and versatility for constructing a wide range of heterostructures with atomic-level control over their layer thickness that is also free of lattice mismatch issues. As a result, layered TMDCs in combination with other 2D materials have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. In this article, we review the recent progress in the synthesis of 2D TMDCs and optoelectronic devices research. We also discuss the challenges facing the scalable applications of the family of 2D materials and provide our perspective on the opportunities offered by these materials for future generations of nanophotonics technology.

Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light–Matter Interactions

High-quality organic and inorganic van der Waals (vdW) solids are realized using methylammonium lead halide (CH3 NH3 PbI3 ) as the organic part (organic perovskite) and 2D inorganic monolayers as counterparts. By stacking on various 2D monolayers, the vdW solids exhibit dramatically different light emissions. Futhermore, organic/h-BN vdW solid arrays are patterned for red-light emission.

Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation.

We fabricate transistors from chemical vapor deposition-grown monolayer MoS2 crystals and demonstrate excellent current saturation at large drain voltages (Vd). The low-field characteristics of these devices indicate that the electron mobility is likely limited by scattering from charged impurities. The current-voltage characteristics exhibit variable range hopping at low Vd and evidence of velocity saturation at higher Vd. This work confirms the excellent potential of MoS2 as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.

Modifying the Ni-MoS 2 Contact Interface Using a Broad-Beam Ion Source

Charge transport at the contacts is a dominant factor in determining the performance of devices using 2D MoS2. Using a low-energy beam of Ar ions, the interface between Ni and MoS2 was modified to improve the performance in 2D field-effect transistors (FETs). This broad-beam ion source is integrated into an ultrahigh vacuum, physical vapor deposition system that allowed for in situ modification of the MoS2 immediately prior to Ni contact deposition. The contact resistance decreased leading to a corresponding and highly reproducible boost in the on-current by up to four times. Spectroscopic analysis of the ion beam-modified MoS2 suggests that there are generated defects, which supply dangling bonds that improve carrier injection between the Ni metal contact and MoS2. This approach for modifying the Ni-MoS2 interface opens a promising new path for reducing the impact of contacts on MoS2 FET performance.

Cross-Plane Carrier Transport in Van der Waals Layered Materials

The mechanisms of carrier transport in the cross-plane crystal orientation of transition metal dichalcogenides are examined. The study of in-plane electronic properties of these van der Waals compounds has been the main research focus in recent years. However, the distinctive physical anisotropies, short-channel physics, and tunability of cross layer interactions can make the study of their electronic properties along the out-of-plane crystal orientation valuable. Here, the out-of-plane carrier transport mechanisms in niobium diselenide and hafnium disulfide are explored as two broadly different representative materials. Temperature-dependent current-voltage measurements are preformed to examine the mechanisms involved. First principles simulations and a tunneling model are used to understand these results and quantify the barrier height and hopping distance properties. Using Raman spectroscopy, the thermal response of the chemical bonds is directly explored and the insight into the van der Waals gap properties is acquired. These results indicate that the distinct cross-plane carrier transport characteristics of the two materials are a result of material thermal properties and thermally mediated transport of carriers through the van der Waals gaps. Exploring the cross-plane electron transport, the exciting physics involved is unraveled and potential new avenues for the electronic applications of van der Waals layers are inspired.

Photoluminescence quenching in hybrid gold/MoSe 2 nanosheets

Transition Metal Dichalcogenide (TMD) materials have increasingly gained attention, due to their unique optical, spintronic, and electronic properties [1]. These properties at the monolayer limit are by part a result of the ultimate confinement imposed on their excitonic transitions that originate a direct band-gap and a lack of inversion symmetry in their crystallographic structure [2]. Many promising recent efforts in control of excitonic transitions in these materials have been devoted to study of their coupling with plasmonic nanoresonators. Plasmonic nanoresonators are known for their ability to control and modify the optical response of materials in their proximity [3]. These findings have motivated the study of emergent phenomena associated with the plasmon exciton interaction in these hybrid systems [4-5], which include enhancement [6] and quenching of the TMDs photoluminescence [7].