Effective Optical Properties of Laterally Coalescing Monolayer MoS₂

Abstract

transition metal dichalcogenides (TMDCs) exhibit compelling dimension-dependent exciton-dominated optical behavior influenced by thickness and lateral quantum confinement effects. Thickness quantum confinement effects have been observed; however, experimental optical property assessment of nanoscale lateral dimension monolayer TMDCs is lacking. Here, we employ ex situ spectroscopic ellipsometry to evaluate laterally coalescing monolayer metalorganic chemical vapor deposited MoS 2 . A multisample analysis is used to constrain Bruggeman and Maxwell−Garnett effective medium approximations and the effective dielectric functions are derived for laterally coalesced and uncoalesced MoS 2 films (∼10−94% surface coverage for ∼10−140 nm lateral grain sizes). This analysis demonstrates the ability to probe MoS 2 optical exciton behavior at growth-relevant grain sizes in relation to chemical vapor nucleation density, ripening, and lateral growth conditions. Our analysis is pertinent toward expected in situ epitaxial 2D TMDC film growth metrology to enable the facile development of monolayer films with targeted process-dependent optical properties.

Exfoliation procedure-dependent optical properties of solution deposited MoS2 films

“…Wendumu et al calculated changes in optical absorption characteristics as a function of lateral flake size (i.e., 3.6–6.5 nm) showing spectral shifts in peak exciton behavior 64 . Complementary experimental work shows similar changes in peak exciton wavelength shifts for lateral nanoscale grain sizes from metalorganic chemical vapor deposited monolayer MoS 2 56 . The reported lateral dimensions in prior work are substantially less than the average lateral MoS 2 flake sizes from the exfoliation methods used in this work.…”

“…For each film, the potential influence of effective medium mixing conditions were assessed in the optical dispersion data analysis. This assessment likewise incorporated a multi-sample analysis to constrain effective medium fit parameters involving MoS 2 flakes and air, void, or dielectric fraction filling in the film (e.g., linear, Bruggeman, and Maxwell-Garnett mixing methods) 54 , 55 , as shown in prior work for coalescing monolayer metalorganic chemical vapor deposited MoS 2 films 56 . However, such assessment involving our MoS 2 films produced unphysical effective optical dispersions with high mean squared errors (MSEs > 100) and was not included in the optical dispersion data analysis.…”

Exfoliation Procedure-Dependent Optical Properties of Solution Deposited MoS2 Films

“…For each film, the potential influence of effective medium mixing conditions were assessed in the optical dispersion data analysis. This assessment likewise incorporated a multi-sample analysis to constrain effective medium fit parameters involving MoS 2 flakes and air/void/dielectric fraction filling in the film (e.g., linear, Bruggeman, and Maxwell-Garnett mixing methods) 53,54 , as shown in prior work for coalescing monolayer metalorganic chemical vapor deposited MoS 2 films 55 . However, such assessment for our MoS 2 films produced unphysical effective optical dispersions with high mean squared errors (MSEs >100) and was not included in the optical dispersion data analysis.…”