Composition-dependent Raman modes of Mo_1−xW_xS₂ monolayer alloys

Abstract

Two-dimensional (2D) transition-metal dichalcogenide alloys with tunable band gaps have promising applications in nanoelectronics and optoelectronics. Characterization of structures of 2D alloys, such as composition and atom mixing, is of fundamental importance to their applications. Here, we have conducted systematic Raman spectroscopic studies on Mo1-xWxS2 monolayers (0 ≤x≤ 1). First-order Raman modes and second-order Raman modes have been observed in the range of 100-480 cm(-1) in the 2D alloys. The out-of-plane A1' modes and in-plane E' modes showed one-mode and two-mode behaviors, respectively. The broadening of A1' and E' modes in the alloys has been observed. The disorder-related Raman peaks at ∼360 cm(-1) were only observed in the 2D alloys but not in the two end materials. Modified random-element-isodisplacement (MREI) model has been adopted to successfully predict mode behaviors of A1' and E' modes in the monolayer alloys. Further, composition-dependent A1' and E' frequencies can be well fitted by the MREI model, giving composition-dependent force constants.

Substrate-free layer-number identification of two-dimensional materials: A case of Mo0.5W0.5S2 alloy

“…The peaks at 347 and 365 cm −1 may be the disorder-related Raman peaks. 25 The A 1g and E 1 2g peaks exhibit broadened profiles due to the disorder effect in the alloy compared with that (gray curves) in bulk MoS 2 , as shown in Fig. 1(a).…”

“…In common alloys, random distribution of atoms will result in disorder effect on their vibration modes in terms of peak weakening and broadening. 25 For multilayer 2D alloys exfoliated from the bulk layered alloys, how the alloy layer modifies their C and LB modes is an open issue. Whether the monatomic chain model (MCM) based on 2D crystals 20 can be applied to the C and LB modes in multilayer alloys for layer-number identification is not clear.…”

“…Since, the point group of the bulk alloy is D 6h , the mode at 416cm −1 is assigned as A 1g , 25 , as it lies between A 1g of MoS 2 (409cm −1 ) and WS 2 (420cm −1 ). There are two modes at 380 cm −1 and 352 cm −1 , assigned as MoS 2 -like and WS 2 -like E 1 2g , 25 respectively. The peaks at 347 and 365 cm −1 may be the disorder-related Raman peaks.…”

Preparation, Structure and Properties of Two-dimensional Semiconductor Alloys

“…在组份依赖 的发射能量图中可以看出, 带隙值与组份不是线性关 系, 而是呈现弯曲效应(bowing effect, 能带弯曲). 合金 的带隙值随组份变化规律一般可用二次方程拟合, 即 ( ) ( ) [49] , ≈ 417 和 ≈355 cm -1 [16] , ≈ 241 和≈287 cm -1 [15] , ≈250 和≈262 Figure 11 Raman spectra of 2D semiconductor alloys with different compositions (a) Mo x W 1-x S 2 monolayer [50] , (b) MoS 2x Se 2(1-x) monolayer [51] and (c) Mo x W 1-xSe 2 monolayer [35a] 单层合金中 A' 1 和 E'的单模或双模行为可以用改进 的随机元素等位移模型(Modified random element isodisplacement, MREI)解释 [50] . MREI 模型基于两个假设 [52] : …”

“…The schematics of force constants used in MREI model and (b) composition-dependent Raman frequencies of E' and A' 1 modes in Mo x W 1-x S 2 alloys [50] 通过解上述运动方程, 根据解的个数可以判断是单 模行为还是双模行为. 将解的形式和实验数据拟合, 可 以给出各个力的常数信息, 以及 A' 1 和 E'频率随组份变 化的规律.…”