Quantitative Raman Spectrum and Reliable Thickness Identification for Atomic Layers on Insulating Substrates

Abstract

We demonstrate the possibility in quantifying the Raman intensities for both specimen and substrate layers in a common stacked experimental configuration and, consequently, propose a general and rapid thickness identification technique for atomic-scale layers on dielectric substrates. Unprecedentedly wide-range Raman data for atomically flat MoS(2) flakes are collected to compare with theoretical models. We reveal that all intensity features can be accurately captured when including optical interference effect. Surprisingly, we find that even freely suspended chalcogenide few-layer flakes have a stronger Raman response than that from the bulk phase. Importantly, despite the oscillating intensity of specimen spectrum versus thickness, the substrate weighted spectral intensity becomes monotonic. Combined with its sensitivity to specimen thickness, we suggest this quantity can be used to rapidly determine the accurate thickness for atomic layers.

Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS2

“…The interference between light scattered of the MoS 2 membrane and the silicon at the bottom of the well may affect the relative intensities of the A 1g and E 1 2g modes (Fig S6a). To rule this out as the cause of our observed changes to Raman mode intensities, we closely follow a model developed in other works 10,11 , originally used to determine the effect of the substrate thickness on the Raman mode intensities. In our case, instead of a layer of SiO 2 , we have a vacuum cavity of distance d 2 which changes as the device bulges down under high pressures.…”

“…The effect of strain on the band gap of 2D TMD's has been reported in a number of studies [9][10][11][12]20,23,24 , including uniaxial strains of up to ~4 % 25 and biaxial strains of up to ~3 % produced in highly localized sub-micron areas 26 . Band gap shifts in MoS 2 of ~300 meV have been induced by using very large hydrostatic pressures 27 , and tensile strain has induced shifts of as much as ~100 meV 11 .…”

“…Monolayer MoS 2, a 2D atomic crystal, has been shown in both theory 6,7 and experiment [8][9][10][11][12] to be an ideal candidate for strain engineering. It belongs to the class of 2D transition metal dichalcogonides (TMD's), and as a direct-gap semiconductor 13 has received significant interest as a channel material in transistors 14 , photovoltaics 15 and photodetection 16 devices.…”

“…Parameters such as the band gap energy or carrier mobility of a semiconductor, which are often crucial to the electronic or photonic device performance, can be highly sensitive to the application of only small strains. The combination of this sensitivity with the ultra-high strains possible at the nanoscale could lead to an unprecedented ability to modify the electrical or photonic properties of materials in a continuous and reversible manner.Monolayer MoS 2, a 2D atomic crystal, has been shown in both theory 6,7 and experiment [8][9][10][11][12] to be an ideal candidate for strain engineering. It belongs to the class of 2D transition metal dichalcogonides (TMD's), and as a direct-gap semiconductor 13 has received significant interest as a channel material in transistors 14 , photovoltaics 15 and photodetection 16 devices.…”

“…This reversible modulation of the band gap could be used to make wavelength tunable phototransistors 16 , or MoS 2 strain sensors that have a sensitivity comparable to their state of the art silicon counterparts 20 . Moreover it has been suggested that strain could also improve the performance of MoS 2 transistors 21 , or could be used to create broadband light absorbers for energy harvesting 22 .The effect of strain on the band gap of 2D TMD's has been reported in a number of studies [9][10][11][12]20,23,24 , including uniaxial strains of up to ~4 % 25 and biaxial strains of up to ~3 % produced in highly localized sub-micron areas 26 . Band gap shifts in MoS 2 of ~300 meV have been induced by using very large hydrostatic pressures 27 , and tensile strain has induced shifts of as much as ~100 meV 11 .…”