Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS<sub>2</sub>

Lu, Xin; Utama, M. Iqbal Bakti; Wang, Xingzhi; Xu, Weigao; Zhao, Wenguang; Owen, Man Hon Samuel; Xiong, Qihua

doi:10.1002/smll.201701039

Cited by 41 publications

(41 citation statements)

References 36 publications

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“…3(c). It is clear from this systematic experimental analysis-which goes well beyond what had been done before now [50,51]-that the same behavior is common to all atomically thin group-VI TMDs.…”

Section: Resultssupporting

confidence: 58%

Enhanced Electron-Phonon Interaction in Multivalley Materials

et al. 2019

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We report a combined experimental and theoretical investigation that reveals a new mechanism responsible for the enhancement of electron-phonon coupling in doped semiconductors in which multiple inequivalent valleys are simultaneously populated. Using Raman spectroscopy on ionic-liquid-gated monolayer and bilayer MoS 2 , WS 2 , and WSe 2 over a wide range of electron and hole densities, we find that phonons with a dominant out-of-plane character exhibit strong softening upon electron accumulation while remaining unaffected upon hole doping. This unexpected-but very pronounced-electron-hole asymmetry is systematically observed in all monolayers and bilayers. By performing first-principles simulations, we show that the phonon softening occurs when multiple inequivalent valleys are populated simultaneously. Accordingly, the observed electron-hole asymmetry originates from the much larger energy separation between valleys in the valence bands-as compared to the conduction band-that prevents the population of multiple valleys upon hole accumulation. We infer that the enhancement of the electron-phonon coupling occurs because the population of multiple valleys acts to strongly reduce the efficiency of electrostatic screening for those phonon modes that cause the energy of the inequivalent valleys to oscillate out of phase. This robust mechanism is likely to play an important role in several physical phenomena, possibly including the occurrence of superconductivity in different transition metal dichalcogenides.

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Section: Resultssupporting

confidence: 58%

Enhanced Electron-Phonon Interaction in Multivalley Materials

et al. 2019

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“…3(c). It is clear from this systematic experimental analysis -which goes well beyond what had been done until now [49,50]-that the same behavior is common to all atomically-thin TMDs.…”

Section: Resultssupporting

confidence: 54%

Enhanced electron-phonon interaction in multi-valley materials

Ponomarev,

Sohier,

Gibertini

et al. 2019

Preprint

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“…In close agreement with the experiments [11,37], the linewidth of the A 1g mode shows a steeper increase as a function of doping, and is generally larger than the E 2g linewidth. This proves that the major part of the experimentally observed doping-induced phonon linewidth modifications in TMDs [11,37] is underlain by the EHP scattering processes due to the EPC (other contribution might be anharmonic coupling), as it is, for example, the case in MgB 2 where nonadiabatic effects as well play a significant role [9]. The linewidth, similarly to the frequency shift, shows a great sensitivity on the topology of the valleys, and therefore can also be used in Raman spectroscopy as an indicator of the Lifshitz transition.All in all, the phonon dynamics in doped single-layer k x a / 2 π k y a / 2 π energ y [eV] K Σ carrier density frequency shift A NA FIG.…”

supporting

confidence: 88%

“…2(i) and 2(l)]. In close agreement with the experiments [11,37], the linewidth of the A 1g mode shows a steeper increase as a function of doping, and is generally larger than the E 2g linewidth. This proves that the major part of the experimentally observed doping-induced phonon linewidth modifications in TMDs [11,37] is underlain by the EHP scattering processes due to the EPC (other contribution might be anharmonic coupling), as it is, for example, the case in MgB 2 where nonadiabatic effects as well play a significant role [9].…”

supporting

confidence: 84%

Broken adiabaticity induced by Lifshitz transition in MoS2 and WS2 single layers

Novko

2020

Commun Phys

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The breakdown of the adiabatic Born-Oppenheimer approximation is striking dynamical phenomenon, however, it occurs only in a handful of layered materials. Here I show that adiabaticity breaks down in doped single-layer transition metal dichalcogenides in a quite intriguing manner. Namely, significant nonadiabatic coupling, which acts on frequencies of the Raman-active modes, is prompted by a Lifshitz transition due to depopulation and population of multiple valence and conduction valleys, respectively. The outset of the latter event is shown to be dictated by the interplay of highly non-local electron-electron interaction and spin-orbit coupling. In addition, intense electron-hole pair scatterings due to electron-phonon coupling are inducing phonon linewidth modifications as a function of doping. Comprehending these intricate dynamical effects turns out to be a key for mastering characterization of electron doping in two-dimensional nano-devices by means of Raman spectroscopy.The Born-Oppenheimer approximation, which assumes that electrons adiabatically follow the motion of lattice degrees of freedom, is a fundamental starting point in quantum mechanics. Recent studies have, however, shown that striking nonadiabatic (NA) effects visible in vibrational Raman spectra can be found in metallic layered materials, such as graphene [1][2][3], graphite intercalation compounds [4], and magnesium diboride [4,5], as well as in doped semiconductors, such as boron-doped diamond [6]. The strength of this dynamical electronphonon coupling (EPC) can, in fact, be modified as a function of carrier concentration, which makes the Raman spectroscopy a quite powerful tool for characterization of doped two-dimensional and layered materials [7]. However, to make this characterization complete, the precise understanding of the microscopic processes underlying the NA modifications of phonons is needed [1,4,8,9]. Such renormalization of phonons is actually considered quite rare, and apart from these few cases it is not clear in what other two-dimensional materials should adiabaticity break down.Contrary to the aforesaid examples, the vibrational spectra of the single-layer transition metal dichalcogenides (TMDs) under bias voltage is believed to be adiabatic [10,11]. In other words, the gate-induced phonon renormalization can be qualitatively described by means of adiabatic density functional theory (DFT) calculations [11]. Despite this common belief, there are few worthwhile indications that point to the importance of the NA effects in doped TMDs [12][13][14][15]. For instance, recent study on carrier-induced phonon modifications in atomically-thin TMDs had shown that DFT results are largely overestimating the corresponding frequency shifts of the Raman-active modes [12]. Moreover, the theoretical considerations have predicted the importance of the NA coupling in charge transfer dynamics of TMDs heterostructures [13,14] as well as in exciton-mediated Raman scattering of MoS 2 [15]. Additionally, the effect beyond adiabatic approximation, ...

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Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS₂

Cited by 41 publications

References 36 publications

Enhanced Electron-Phonon Interaction in Multivalley Materials

Enhanced Electron-Phonon Interaction in Multivalley Materials

Enhanced electron-phonon interaction in multi-valley materials

Broken adiabaticity induced by Lifshitz transition in MoS2 and WS2 single layers

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Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS2

Cited by 41 publications

References 36 publications

Enhanced Electron-Phonon Interaction in Multivalley Materials

Enhanced Electron-Phonon Interaction in Multivalley Materials

Enhanced electron-phonon interaction in multi-valley materials

Broken adiabaticity induced by Lifshitz transition in MoS2 and WS2 single layers

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Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS₂