The overall emission is also shifted to higher energies while being in resonance with the light scattered by those two processes. When the excitation energy increases towards non-resonant conditions, the intensity saturates at substantially lower level. The difference between the resonant and non-resonant conditions may correspond to a fundamental change in the processes involved in the optical response.
Out of the resonance only the PL emission is observed. In the resonance, the optical response is also due to cascade Raman scattering processes, which involve optical and acoustic phonons. In the latter case, the energy of the photoexcited carriers is lost in a series of scattering processes, while the intensity of the emission in the former process basically does not change with increasing the excitation energy. The difference between the two mechanisms responsible for the optical emission can be further appreciated by inspecting the spectra excited with laser light of particularly selected energies as shown in Fig.
It is clearly seen in Fig. The resonance of the light scattered by vibrational modes of the crystal with the neutral exciton results in the enhancement of the Raman scattering features with no strong effect on the background PL emission see Fig. Moreover, it can be seen in Fig.
Additional structures emerging on higher-energy slopes of the principal phonon modes in the investigated monolayer are related to multiphonon scattering by optical LO and acoustic phonons. The process involves 1 an optical excitation of an exciton, 2 its relaxation with the emission of an optical phonon down to the vicinity of the band, 3 the subsequent emission of acoustic phonon s , and 4 finally its radiative recombination The peak due to an additional acoustic process, which follows the emission of the optical phonon is broader than the LO phonon peak with the energy occurring at the largest value of the crystal momentum allowed by the exciton dispersion.
The cascade scattering can be strongly enhanced by an outgoing resonance with excitonic complexes, as observed in several semiconductor systems A resonance with the recombination of a free electron and a hole localized on a carbon acceptor in GaAs e, A 0 also leads to a similar effect 40 , Raman peaks related to the combined processes are dispersive, which reflects the exciton dispersion No clear dispersion of the relevant peaks observed in our data most likely results from limited spectral resolution of the experimental set-up.
The resonance of the scattered light with the neutral exciton results in quite different spectra, as shown in Fig. The resonance induces a strong enhancement of several Raman peaks, clearly visible in Fig. This may point out to a different exciton-phonon interaction as compared with the charged exciton. In the case of the charged exciton, the strong optical response may be related to cascade Raman scattering involving both optical and acoustic phonons.
The resonance with the neutral exciton gives rise mainly to the enhancement of Raman scattering by discrete modes. The enhanced modes, whose attribution has already been discussed, can clearly be seen in Fig. Our results underline a complicated character of exciton-phonon interactions in thin TMD layers. This statement is even more valid in view of recent results reported by C.
Chow 43 and co-workers, who showed virtually no effect of the Raman scattering on the emission due to the negatively charged exciton in monolayer MoSe 2 and a crucial effect of multiple LA M phonon emission on the neutral exciton. This might look surprising as both WS 2 and MoSe 2 share the same crystallographic structure. It is moreover known that critical differences between the influence of resonant excitation on the Raman scattering in different materials can exist 19 , which can be explained by theoretical calculations.
The explanation presented in ref. We can, however, stress two points which may be important for the possible analysis. First is a crucial difference in the electronic structure of MoSe 2 and WS 2. Monolayer WS 2 is a darkish material, in which the energetically lowest transition is optically inactive Monolayers of MoSe 2 are bright, which means that in their case the energetically lowest transition is optically active Next, a closer inspection of our results shows that the resonantly enhanced emission due to the charged exciton is blue-shifted as compared to the emission excited out-of-resonance see Fig.
Recently, it has been reported that the PL spectrum due to the negatively charged exciton in monolayer WS 2 is composed of two lines associated with two possible states of that exciton: intravalley singlet and intervalley triplet for details see ref. In consequence, the observed blue shift may suggest that the resonance involves the intervalley triplet state of the charged exciton.
It contrasts with monolayer MoSe 2 , where the charged exciton is ascribed to the intervalley singlet state. This could explain the difference between the resonant-excitation effect on the charged exciton emission seen in our results and reported in ref.
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These facts may be of importance for the explanation of data and we do believe that they will trigger some interest in establishing their theoretical framework. We have presented a study of low-temperature optical emission from monolayer WS 2 excited resonantly in the energy range corresponding to the neutral and charged excitons.
A clear difference between the Raman scattering excitation spectra detected at the energy of negatively charged and neutral excitons has been observed reflecting the differences in the electron-phonon interactions involved. It has also been shown that the RSE spectroscopy employed in our experiment represents a sensitive tool to study electron-phonon interactions in thin films of TMD materials.
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The WS 2 monolayer under investigation was prepared by mechanical exfoliation of a bulk crystal purchased from HQ Graphene. The monolayers were then identified based on their optical contrast and cross-checked with the use of room-temperature Raman scattering and PL measurements. The investigated sample was placed on a cold finger in a continuous flow cryostat mounted on x-y motorized positioners.
Laser Diffraction Technology
The non-resonant PL measurements were carried out using To study the optical response of the system as a function of excitation energy, a dye laser based on Rhodamine 6 G was used providing a tunable wavelength range extending from about nm to almost nm. The signal was collected via the same microscope objective, sent through a 0.
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Published online Jul PMID: Pasteura 5, Warszawa, Poland. Corresponding author. Received Mar 29; Accepted May This article has been cited by other articles in PMC. Abstract Resonant Raman scattering is investigated in monolayer WS 2 at low temperature with the aid of an unconventional technique, i. Introduction Raman scattering spectroscopy is an acknowledged characterization tool of layered materials, such as, for example graphene 1 — 5 , boron nitride 6 — 8 , or semiconducting transition metal dichalcogenides TMDs 9 — Open in a separate window.
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Conclusions We have presented a study of low-temperature optical emission from monolayer WS 2 excited resonantly in the energy range corresponding to the neutral and charged excitons. Methods The WS 2 monolayer under investigation was prepared by mechanical exfoliation of a bulk crystal purchased from HQ Graphene.
Author Contributions M. Notes Competing Interests The authors declare that they have no competing interests. Footnotes Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Contributor Information Maciej R. References 1. Zhao J, et al. Recent progress in the applications of graphene in surface-enhanced Raman scattering and plasmon-induced catalytic reactions. Ding Q, et al.
Dai Z-G, et al. Plasmon-driven reaction controlled by the number of graphene layers and localized surface plasmon distribution during optical excitation. Light Science Application. Zhang X, et al. Reich S, et al. Resonant Raman scattering in cubic and hexagonal boron nitride. Cai Q, et al. Angewandte Chemie International Edition. The unique Raman fingerprint of boron nitride substitution patterns in graphene. Fan J-H, et al. Journal of Applied Physics. Multiphonon resonant Raman scattering in MoS 2. Applied Physics Letters.
Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material. ACS Nano. Grzeszczyk M, et al. Raman scattering of few-layers MoTe 2. Yang X, et al. Plasmon-exciton coupling of monolayer MoS 2 -Ag nanoparticles hybrids for surface catalytic reaction. Materials Today Energy. Raman spectroscopy in graphene. Physics Reports. Gorbachev RV, et al. Lee C, et al.
Analysis of the light-scattering cross section for surface ripples on solids
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