Edge effects in second-harmonic generation in nanoscale layers of transition-metal dichalcogenides

Abstract

791Transition metal dichalcogenides (MoS 2 , WS 2 , and others) are considered as materials of two dimen sional electronics, which can complement graphene in developing nanoelectronic devices. In contrast to graphene which has no band gap, two dimensional layered dichalcogenides have a band gap E g ~ 1.2-1.4 eV [1], transforming from indirect gap bulk semi conductors to direct gap ones [2,3]. Thus, varying the layer thickness, the most important material parame ters, i.e., optical transitions, conductivity, electron mobility, electron relaxation parameters, and others can be controllably varied [4]. Size effects appear in the band structure of transition metal dichalcogenides (TMDs) at thicknesses from one monolayer to 5-7 layers; therefore, thicker nanoscale TMD films are considered as bulk materials.Abstract-The results of studying the optical properties of nanoscale single crystals of MoS 2 :Cl 2 and WS 2 :Br 2 semiconductor compounds are presented. In microscopic images obtained at the wavelength of the second (400 nm), edge effects are detected, which consist in enhancement or reduction in the second harmonic sig nal intensity. Unlike previously proposed interference mechanisms of edge effects, non interference mecha nisms are considered. The occurrence of edge effects is associated with either an increased Cl 2 and Br 2 halo gen molecule concentration or with an electrically induced second harmonic caused by band bending at the edges of individual crystal layers.

Nonlinear optical susceptibility of atomically thin WX2 crystals

“…This nonlinearity value exceeded the value at photon energy closest to the direct bandgap (~1.8-1.9 eV) by at least an order of magnitude. A peak value of about 500 pm/V has been reported by Mishina et al [13] at a fixed photon energy of ~3.1 eV. The value was obtained by comparing SHG signals from different samples and is an order of magnitude smaller than the result obtained in Refs.…”

Facile Fabrication of P(Electrodeposition)/N(Solvothermal) 2D‐WS2‐Homojunction Based High Performance Photo Responsive, Strain Modulated Piezo‐Phototronic Diode

“…The n type solvothermally grown and spin coated WS 2 has the Fermi level located slightly above the Fermi level of p type WS 2 . Under strain free conditions, the positive and negative charge center corresponds in p‐n junction.i.e the electrons and holes are immobilized in the space charge region generating a built‐in potential, but the charge centres gets altered under strain due to the non‐centro symmetric property of WS 2 . It is noteworthy, that the built ‐in‐potential separates the charge carriers when visible light is illuminated as shown in Figure b(i).…”

“…Under strain free conditions, the positive and negative charge center corresponds in p-n junction.i.e the electrons and holes are immobilized in the space charge region generating a built-in potential, but the charge centres gets altered under strain due to the non-centro symmetric property of WS 2 . [23][24][25] It is noteworthy, that the built -in-potential separates the charge carriers when visible light is illuminated as shown in Figure 7b(i). When subjected to external strain, the WS 2 layers are stretched and as already mentioned due to the non-centro symmetry, the charge centers get dispositioned leading to polarization of charges.…”

Ab Initio Investigation of Atomistic Insights into the Nanoflake Formation of Transition-Metal Dichalcogenides: The Examples of MoS2, MoSe2, and MoTe2

“…Two-dimensional TMDs have found potential applications in biomedicine, , optoelectronics, − catalysis, , and energy storage. , The great interest in this class of materials is in part due to the chemical formula MQ 2 , where M is a transition metal (TM) and Q = S, Se, and Te, allowing a large variety of atomic combinations resulting in a broad range of possibilities for their properties. , Furthermore, the edges of 2D TMDs can exhibit unique properties, such as topological states , and the coexistence of metallic and semiconducting states . Besides edge effects, − other factors have been investigated to change the properties of 2D TMDs such as the variation of the number of layers , and the spacial group symmetry …”