Energy Scaling of Compositional Disorder in Ternary Transition‐Metal Dichalcogenide Monolayers

Abstract

to play a crucial role as essential building blocks in future high-tech devices. [1][2][3][4][5][6][7][8] Already in the late 1960s, early studies on the structural and electronic properties of bulk TMDs had started and are detailed in a report by Wilson and Yoffe. [9] TMDs are layered materials which combine a transition metal (M: Mo, W, Ti, Zr, etc.) and a chalcogen (X: S, Se, or Te) in the general formula MX 2 with one layer of M atoms sandwiched between two layers of X atoms. [10] Group VIB TMDs with a 2H structural phase (e.g., MoSe 2 ) are the most explored representatives of such systems. They are characterized by an intrinsic bandgap within the visible and near-infrared regions. [11] Furthermore, the material system exhibits a transition from an indirect to a direct bandgap located at the K or K′ points of the hexagonal Brillouin zone, linked to a decrease in the number of layers from the bulk crystal to a monolayer. [12,13] Moreover, such materials also possess a strong spin-orbit interaction due to the presence of a relatively heavy transition metal along with huge exciton binding energies [14][15][16] resulting from a strong Coulomb interaction and a lack of dielectric screening. These properties lead to a valence-band splitting that strongly affects Alloying semiconductors is often used to tune the material properties desired for device applications. The price for this tunability is the extra disorder caused by alloying. In order to reveal the features of the disorder potential in alloys of atomically thin transition-metal dichalcogenides (TMDs) such as Mo x W 1−x Se 2 , the exciton photoluminescence is measured in a broad temperature range between 10 and 200 K. In contrast to the binary materials MoSe 2 and WSe 2 , the ternary system demonstrates non-monotonous temperature dependences of the luminescence Stokes shift and of the luminescence linewidth. Such behavior is a strong indication of a disorder potential that creates localized states for excitons and affects the exciton dynamics responsible for the observed non-monotonous temperature dependences. A comparison between the experimental data and the results obtained by Monte Carlo computer simulations provides information on the energy scale of the disorder potential and also on the shape of the density of localized states created by disorder. Statistical spatial fluctuations in the distribution of the chemically different material constituents are revealed to cause the disorder potential responsible for the observed effects. A deeper understanding of the disorderinduced effects is vital for prospective TMD alloy-based devices.

Comment on “Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin‐Lead Triiodide Perovskites”

“…In two dimensions, a slightly different combination of material parameters determines the scale of energy disorder. [ 6,7 ]…”

“…In two dimensions, a slightly different combination of material parameters determines the scale of energy disorder. [6,7] Equation (1) permits to elucidate the dependence of the disorder energy scale ε 0 on the alloy composition x in the lead-tin perovskites. If the parameters α, m, and N did not depend on x, ε 0 (x) would solely be proportional to x 2 (1 − x) 2 and would display a maximum at x = 0.5.…”

Strain Switching in van der Waals Heterostructures Triggered by a Spin‐Crossover Metal–Organic Framework

“…The PL peaks can be fitted well by considering a Pearson IV distribution (see Experimental Section), as already reported for other transition metal dichalcogenide monolayers. [ 32,33 ] The obtained PL maximum and broadening, Γ, are shown in Figure 5e and Figure 5f, respectively. For the reference WSe 2 monolayer, the thermal dependence of the PL position and broadening is well described by the Varshni and Rudin–Reinecke–Segall laws [ 34,35 ] (see Experimental Section), respectively, in all the temperature range.…”

“…Mater. 2022, 34, 2110027 monolayers [32,33] (see Section 5 in the Supporting Information for further details). The thermal dependence of the PL maximum, E(T), was modelled by the Varshni law for a semiconductor [34] and the line broadening, Γ(T), to the Rudin-Reinecke-Segall law, [35] respectively.…”

“…The measurement protocol was as follows: once the temperature setpoint was reached, we waited 10 min for a better thermal stabilization and, then, we measured the PL for the vdWH and for the WSe 2 reference monolayer. The spectra for vdWH type G were fitted following a Pearson IV distribution, as already reported for MoS 2 , MoSe 2 , and WSe 2 monolayers [32,33] (see Section 5 in the Supporting Information for further details). The thermal dependence of the PL maximum, E(T), was modelled by the Varshni law for a semiconductor [34] and the line broadening, Γ(T), to the Rudin-Reinecke-Segall law, [35] respectively.…”