Thickness Identification of Thin InSe by Optical Microscopy Methods

Abstract

Indium selenide (InSe), as a novel van der Waals layered semiconductor, has attracted a large research interest thanks to its excellent optical and electrical properties in the ultra-thin limit. Here, we discuss four different optical methods to quantitatively identify the thickness of thin InSe flakes on various substrates, such as SiO2/Si or transparent polymeric substrates. In the case of thin InSe deposited on a transparent substrate, the transmittance of the flake in the blue region of the visible spectrum can be used to estimate the thickness. For InSe supported by SiO2/Si, the thickness of the flakes can be estimated either by assessing their apparent colors or accurately analyzed using a Fresnel-law based fitting model of the optical contrast spectra. Finally, we also studied the thickness dependency of the InSe photoluminescence emission energy, which provides an additional tool to estimate the InSe thickness and it works both for InSe deposited on SiO2/Si and on a transparent polymeric substrate.

Apparent Colors of 2D Materials

“…The origin of the contrast can be understood, thanks to Fresnel's equations, with a physical model like the one depicted in Figure 1, and it is used in several works. [3,4,45] These figures can be used to build up color charts that correlate the thickness of the graphene flakes with their apparent color. These color charts will allow other users a fast thickness estimation, but they present the limitation that will be only valid substrates with 90 nm or 300 nm SiO 2 .…”

“…Optical microscopy (OM)-based methods have been proven to be the most suited ones to rapidly identify atomically thin 2D materials flakes. [2][3][4][5][6] Nowadays there are methods based on Raman spectroscopy, photoluminescence, microtransmittance/ reflectance, Rayleigh scattering, or optical path interferometry that allow the accurate thickness determination of a wide range of 2D materials. Nonetheless, these more sophisticated OMbased methods are always preceded by a coarse thickness estimation based on the apparent color of the flakes.…”

The Study of Optical Properties of III2–VI3 Defect Semiconductor Group Compounds Ga2S3, Ga2Se3, In2S3, and In2Se3

“…The group III–VI compounds belong to a native defect crystal style that includes III and VI elements and structural vacancies comprised in a unit cell. The crystals can form multiple crystalline phases and stacking formula, e.g., ε‐GaSe, β‐GaSe, ε‐InSe, β‐InSe, α‐In 2 Se 3 , γ‐In 2 Se 3 , and β‐In 2 S 3 , [ 15,30,32,35–38 ] and multidisciplinary stoichiometry as GaS, GaSe, Ga 2 S 3 , Ga 2 Se 3 , InS, In 2 S 3 , and In 6 S 7 [ 11,34,39,40,41 ] owing to the ionic misvalency between the III and VI elements as compared with the other renowned III–V compounds such as GaAs, GaN, and InP with strong and matched valence. For III–VI compounds, a highly symmetric s ‐like orbital of In (or Ga) is constructed at the conduction‐band edge to form a direct bandgap in their bulk or multilayered form for supporting strong photoluminescence (PL) at room temperature.…”

Optical microscopy–based thickness estimation in thin GaSe flakes

“…[13][14][15][16] Since then, the isolation of a new 2D material is always followed by experimental efforts to develop optical microscopy-based recipes to identify and determine the number of layers of that novel 2D material. [17][18][19][20][21][22][23][24][25] In this work, we provide a set of simple optical microscopy methods to assess the thickness of GaSe flakes. GaSe is a material of the layered metal-monochalcogenide III-VI semiconductor family and it holds the great possibilities in optoelectronics and nonlinear optics because of the fast photoresponsivity, high carrier mobility and nonlinear optical properties.…”