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We present an experimental investigation of vibrational strong coupling of C=O bonds in poly(methyl methacrylate) to surface lattice resonances (SLRs) on arrays of gold particles in infrared and Raman spectra. SLRs are generated from the enhanced radiative coupling of localized resonances in single particles by diffraction in the array. Compared to previous studies in Fabry-Perot cavities, particle arrays provide a fully open system that easily couples with external radiation while having large field confinement close to the array. We control the coupling by tuning the period of the array, as evidenced by the splitting of the C=O vibration resonance in the lower and upper vibro-polaritons of the IR extinction spectra. Despite clear evidence of vibrational strong coupling in IR transmission spectra, both Raman spectroscopy and micro-Raman mapping do not show any polariton signatures. Our results suggest that the search for vibrational strong coupling in Raman spectra may need alternative cavity designs or a different experimental approach.

The unique optical and electronic properties of two-dimensional transition metal dichalcogenides (2D TMDs) make them promising materials for applications in (opto-)electronics, catalysis and more. Specifically, alloys of 2D TMDs have broad potential applications owing to their composition-controlled properties. Several important challenges remain regarding controllable and scalable fabrication of these alloys, such as achieving control over their atomic ordering (i.e. clustering or random mixing of the transition metal atoms within the 2D layers). In this work, atomic layer deposition is used to synthesize the TMD alloy Mo1−x W x S2 with excellent composition control along the complete composition range 0 ⩽ x ⩽ 1. Importantly, this composition control allows us to control the atomic ordering of the alloy from well-mixed to clustered while keeping the alloy composition fixed, as is confirmed directly through atomic-resolution high-angle annular dark-field scanning transmission electron micrography imaging. The control over atomic ordering leads to tuning of the bandgap, as is demonstrated using optical transmission spectroscopy. The relation between this tuning of the electronic structure and the atomic ordering of the alloy was further confirmed through ab-initio calculations. Furthermore, as the atomic ordering modulates from clustered to well-mixed, the typical MoS2 and WS2 A1g vibrational modes converge. Our results demonstrate that atomic ordering is an important parameter that can be tuned experimentally to finely tune the fundamental properties of 2D TMD alloys for specific applications.

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4S2 compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. The effects of temperature on A and B exciton peak energies and line widths in optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 to 293 K, the energy of the A and B exciton peaks decreases, while their line width increases due to exciton-phonon interactions. The exciton-phonon interactions in the alloy are closer to those for MoS2 than those for WS2. This suggests that exciton wave functions in the alloy have a larger amplitude on Mo atoms than that on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy, the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also contain W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results, we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo