Robust room temperature valley polarization in monolayer and bilayer WS<sub>2</sub>

Nayak, Pramoda K.; Lin, F. J.; Yeh, Chao‐Hui; Huang, Jer‐Shing; Chiu, Po‐Wen

doi:10.1039/c5nr08395h

Cited by 72 publications

(75 citation statements)

References 40 publications

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“…It is worth noting that the possibility of P c enhancement being determined by the change of detuning energy is ruled out, because as the electrical/optical doping increases, the A − emission energy goes through red shift, which enlarges the detuning energy and may slightly reduce P c rather than increases it . In regards of the P 0 value, while some studies treated P 0 as 1 for simplicity, previous time resolved PL has measured P 0 = 0.7 ± 0.1 in monolayer WSe 2 system with around 150 meV detuning energy . As has been theoretically pointed out, the change of τ v is around six times at 80 K when the carrier concentration is tuned from 8 × 10 11 cm −2 to 8 × 10 12 cm −2 , which relatively corresponds to our previously estimated doping state (Δ n = 7.69 × 10 12 cm −2 ).…”

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confidence: 96%

Engineering Valley Polarization of Monolayer WS₂: A Physical Doping Approach

Feng

Cong

Konabe

et al. 2019

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The emerging field of valleytronics has boosted intensive interests in investigating and controlling valley polarized light emission of monolayer transition metal dichalcogenides (1L TMDs). However, so far, the effective control of valley polarization degree in monolayer TMDs semiconductors is mostly achieved at liquid helium cryogenic temperature (4.2 K), with the requirements of high magnetic field and on‐resonance laser, which are of high cost and unwelcome for applications. To overcome this obstacle, it is depicted that by electrostatic and optical doping, even at temperatures far above liquid helium cryogenic temperature (80 K) and under off‐resonance laser excitation, a competitive valley polarization degree of monolayer WS2 can be achieved (more than threefold enhancement). The enhanced polarization is understood by a general doping dependent valley relaxation mechanism, which agrees well with the unified theory of carrier screening effects on intervalley scattering process. These results demonstrate that the tunability corresponds to an effective magnet field of ≈10 T at 4.2 K. This work not only serves as a reference to future valleytronic studies based on monolayer TMDs with various external or native carrier densities, but also provides an alternative approach toward enhanced polarization degree, which denotes an essential step toward practical valleytronic applications.

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confidence: 96%

Engineering Valley Polarization of Monolayer WS₂: A Physical Doping Approach

Feng

Cong

Konabe

et al. 2019

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“…To date, most of the remarkable properties of WS 2 have been observed by using mechanically exfoliated flakes. In particular, valley polarization has been measured for micrometer-sized WS 2 grains on SiO 2 [7,8]. The scalable synthesis of continuous WS 2 films displaying high polarization coherence is instrumental in the implementation of a WS 2 -based valleytronic technology.…”

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confidence: 99%

Scalable synthesis of WS ₂ on graphene and h-BN: an all-2D platform for light-matter transduction

et al. 2016

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By exhibiting a measurable bandgap and exotic valley physics, atomically-thick tungsten disulfide (WS2) offers exciting prospects for optoelectronic applications. The synthesis of continuous WS2 films on other two-dimensional (2D) materials would greatly facilitate the implementation of novel all-2D photoactive devices. In this work we demonstrate the scalable growth of WS2 on graphene and hexagonal boron nitride (h-BN) via a chemical vapor deposition (CVD) approach. Spectroscopic and microscopic analysis reveal that the film is bilayer-thick, with local monolayer inclusions. Photoluminescence measurements show a remarkable conservation of polarization at room temperature peaking 74% for the entire WS2 film. Furthermore, we present a scalable bottomup approach for the design of photoconductive and photoemitting patterns.

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“…In contrast, the σ − dipoles in σ − MCM decay faster, resulting in negative values in the calculated DCP. [30] We have demonstrated such flexibility through controllable peak shift of valley emission DCP by further tuning the chiroptical modes in MCMs. Figure 2b shows the simulated local optical chirality (C) at the wavelength of 748 nm in σ + (θ = 15°), achiral (θ = 30°), and σ − (θ = 45°) MCMs upon σ + and σ − excitation.…”

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confidence: 99%

Room‐Temperature Active Modulation of Valley Dynamics in a Monolayer Semiconductor through Chiral Purcell Effects

Zhang

et al. 2019

Advanced Materials

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of free-electron spins. Such valley pseudospins feature robust spin-valley locking and direct coupling to photon spins. [3,4] As a result, excitons in monolayer semiconductors possess additional valley degrees of freedom that are directly addressable through external means, making them promising carriers for information storage and processing in valleytronic devices. [1,2,5,6] Application of valley degree of freedom in optoelectronic devices requires the capability to access and manipulate the valley behaviors. The optical selection rule (i.e., selective coupling to photons with σ + and σ − circular polarization at K and K′ valleys, respectively) in monolayer semiconductors enables the direct addressing of a specific valley by excitation using circularly polarized lasers at cryogenic temperature. The valley information can then be characterized by substantial handedness in far-field photoluminescence (PL) due to the spin conservation during recombination of valley excitons. [7,8] The incorporation of monolayer semiconductors in optical cavities with polarization-dependent responses to an incident laser can further enhance the asymmetric excitation of valley excitons and the resulting PL handedness. [9][10][11] However, the phonon-assisted intervalley scattering accelerates dramatically when temperature is increased, resulting in volatile valley states and significantly reduced handedness of far-field PL at room temperature. [8,12,13] Recent efforts have shown that near-field couplings between optical cavities and valley spins are promising in solving the cryogenic-temperature limitation and enabling room-temperature control of valley dynamics. Specifically, spin-orbit couplings between valley-polarized exciton recombination and circularly polarized surface plasmons can spatially separate the valley spins into opposite inplane directions. [14][15][16] The formation of exciton-polaritons via strong coupling between valley excitons and optical cavities can also prevent the valley polarization from complete vanishing upon enhanced intervalley scattering. [17][18][19][20][21][22] Such valley-optical cavity hybrid systems exhibited room-temperature far-field PL of maintained handedness, which would benefit the application of valley degree of freedom. [19,22] However, modulation of valley behaviors at room temperature is still limited by the requirement of precise spatial and spectral overlap between excitons and optical cavities in current approaches. In addition, it remains unclear how to distinguish the effects of spindependent excitation and near-field-controlled relaxation on Spin-dependent contrasting phenomena at K and K′ valleys in monolayer semiconductors have led to addressable valley degree of freedom, which is the cornerstone for emerging valleytronic applications in information storage and processing. Tunable and active modulation of valley dynamics in a monolayer WSe 2 is demonstrated at room temperature through controllable chiral Purcell effects in plasmonic chiral metamaterials. The strong spindependent...

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Robust room temperature valley polarization in monolayer and bilayer WS₂

Cited by 72 publications

References 40 publications

Engineering Valley Polarization of Monolayer WS₂: A Physical Doping Approach

Engineering Valley Polarization of Monolayer WS₂: A Physical Doping Approach

Scalable synthesis of WS ₂ on graphene and h-BN: an all-2D platform for light-matter transduction

Room‐Temperature Active Modulation of Valley Dynamics in a Monolayer Semiconductor through Chiral Purcell Effects

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Robust room temperature valley polarization in monolayer and bilayer WS2

Cited by 72 publications

References 40 publications

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

Scalable synthesis of WS 2 on graphene and h-BN: an all-2D platform for light-matter transduction

Room‐Temperature Active Modulation of Valley Dynamics in a Monolayer Semiconductor through Chiral Purcell Effects

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Robust room temperature valley polarization in monolayer and bilayer WS₂

Engineering Valley Polarization of Monolayer WS₂: A Physical Doping Approach

Engineering Valley Polarization of Monolayer WS₂: A Physical Doping Approach

Scalable synthesis of WS ₂ on graphene and h-BN: an all-2D platform for light-matter transduction