Valleytronics in 2D materials

Schaibley, John; Yu, Hongyi; Clark, Genevieve; Rivera, Pasqual; Ross, Joel; Seyler, Kyle L.; Yao, Wang; Xu, Xiaodong

doi:10.1038/natrevmats.2016.55

Cited by 2,084 publications

(1,735 citation statements)

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“…Meanwhile, the carrier mobilities are also far below theoretical predictions [3,5]. Furthermore, the photoluminescence spectra not only show a strong excitonic effect, but also ensue a broad defect-activated emission peak within the optical band gap [1,2]. In addition, the photoluminescence quantum yield is unexpectedly low for a direct-gap semiconductor [6].…”

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confidence: 87%

“…According to previous studies, the SQEs are subject to the circularly polarized optical selection rule, especially under high magnetic field. Such circular dichroism on SQEs indicates the trigonal symmetry for their atomic structures, much as the pristine semiconducting TMD monolayers for valley selectivity [1,2]. To this end, we measured the circularly polarized photoluminescence spectra from the WSe 2 monolayer on sapphire, which was grown with the same method as that on graphite.…”

Section: Prl 119 046101 (2017) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

“…Direct-gap semiconducting transition metal dichalcogenide (TMD) monolayers in the form of MX 2 (M ¼ Mo, W; X ¼ S, Se) have spurred great interests for electronics, optoelectronics, and valleytronics [1,2]. However, typical TMD-based field effect transistor devices show n-or p-type behavior without intentional doping [3,4], contradicting what one would expect from a perfect crystal structure.…”

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confidence: 99%

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Defect Structure of Localized Excitons in a WSe2 Monolayer

et al. 2017

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Section: Prl 119 046101 (2017) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

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Defect Structure of Localized Excitons in a WSe2 Monolayer

et al. 2017

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“…functional and quantum materials--the key properties of functional materials are often born out of strong structural and electronic interactions that are quantum mechanical in nature. Notable examples include colossal magnetoresistance manganites (1), ferroelectrics (2), and valleytronic materials (3). The targeted functions are usually accompanied by symmetry breaking, induced through changes in temperature or under other external perturbations.…”

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Reversible structure manipulation by tuning carrier concentration in metastable Cu ₂ S

Tao

Chen

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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The optimal functionalities of materials often appear at phase transitions involving simultaneous changes in the electronic structure and the symmetry of the underlying lattice. It is experimentally challenging to disentangle which of the two effects--electronic or structural--is the driving force for the phase transition and to use the mechanism to control material properties. Here we report the concurrent pumping and probing of Cu 2 S nanoplates using an electron beam to directly manipulate the transition between two phases with distinctly different crystal symmetries and charge-carrier concentrations, and show that the transition is the result of charge generation for one phase and charge depletion for the other. We demonstrate that this manipulation is fully reversible and nonthermal in nature. Our observations reveal a phase-transition pathway in materials, where electron-induced changes in the electronic structure can lead to a macroscopic reconstruction of the crystal structure. functional and quantum materials--the key properties of functional materials are often born out of strong structural and electronic interactions that are quantum mechanical in nature. Notable examples include colossal magnetoresistance manganites (1), ferroelectrics (2), and valleytronic materials (3). The targeted functions are usually accompanied by symmetry breaking, induced through changes in temperature or under other external perturbations. A prominent question is whether the symmetry reduction has an origin in the lattice (e.g., in the form of displacement of atoms, and could be described reasonably well using first-principles calculations) or the electronic degrees of freedom (charge, spin, and orbital) (4-6). It is of great interest to distinguish the roles of these factors in phase transitions.Cu 2 S provides an intriguing example for addressing the above "chicken-and-egg" question (6), which is critical to the understanding of a wide range of functional and quantum materials. It is a fast ionic conductor (7) with highly mobile Cu ions. A phase transition in the bulk material occurs near 100°C from a semiconducting (8) monoclinic symmetry low-chalcocite phase (9) [hereafter called the "L-s phase" (i.e., low, semiconducting); space group P2 1 /c] to an electrically insulating (10) hexagonal symmetry high-chalcocite phase (7, 11) [hereafter called the "H-i phase" (i.e., high, insulating); space group P6 3 /mmc]. The coinciding changes in the bulk electrical conductivity and crystal structure present a possibility for exploring the relationship between electronic and structural phase transitions. Difficulties in the synthesis of stoichiometric Cu 2 S material and the lack of detailed theoretical treatments of both the crystal and the electronic structures of Cu 2 S have hindered the understanding of the phase transition (7, 11-13). Results and DiscussionWe have recently synthesized high-quality Cu 2 S nanoplates (Materials and Methods), which are on the order of 10-nm thick and 100 nm in lateral dimension ( Fig. 1 A and B)....

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“…TMDs offer something that graphene doesn't have: a bandgap, which makes them immediately suitable candidates for semiconductor-based electronics and optoelectronics applications -even at room temperature. Additionally, the excitonic transitions in the ±K valleys (the local minimum and maximum in the conduction and valence band, respectively) can be selectively addressed with circularly polarized light, which has subsequently opened the door to the pursuit of valleytronic devices 4 , where the valley degree of freedom is used to carry information.…”

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confidence: 99%

As thin as it gets

2017

Nature Mater

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editorialGraphene 1 is arguably the most famous material of the last decade; the fascination with its properties has spread beyond the scientific community, especially since the Nobel Prize was awarded in 2010 for its isolation. While research on graphene itself is still extremely active, one cannot overlook the trigger it constituted for the pursuit of atomically thin forms of other materials, such as semiconductors, boron nitride and, more recently, Xenes. All these 2D materials offer endless possibilities for fundamental research, as well as the demonstration of improved or even entirely novel technologies.Transition metal dichalcogenides (TMDs) are atomically thin semiconductors of the type MX 2 , where a transition metal atom (M = Mo, W, We and so on) is sandwiched between two chalcogen atoms (X = S, Se, Te and so on) 2,3 . TMDs offer something that graphene doesn't have: a bandgap, which makes them immediately suitable candidates for semiconductor-based electronics and optoelectronics applications -even at room temperature. Additionally, the excitonic transitions in the ±K valleys (the local minimum and maximum in the conduction and valence band, respectively) can be selectively addressed with circularly polarized light, which has subsequently opened the door to the pursuit of valleytronic devices 4 , where the valley degree of freedom is used to carry information.2D materials have a lot to offer in terms of optoelectronics applications, and in a wide range of wavelengths -from the microwave to the visible. Tony Low, Frank Koppens and colleagues review the field of polaritonics in layered 2D materials on page 182 of this issue. Graphene provides a solid alternative to metal plasmonics, due to the combination of high intrinsic mobilities (manifested when encapsulated in hexagonal boron nitride) with tunable carrier density (and therefore wavelength range). However, TMDs (in particular MoS 2 ) can exhibit much larger light confinement -about an order of magnitude higher than graphene. Hexagonal boron nitride (h-BN) is also a lot more than an ideal dielectric; its in-plane anisotropy renders it naturally hyperbolic (the principal components of the dielectric tensor have opposite signs). Hyperbolic phononpolaritons do not suffer from losses and, although their lifetimes in multi-layered h-BN are comparable to those of optical phonons, the slow group velocity limits the overall propagation length. Theoretical investigations indicate that other 2D materials should also exhibit similar hyperbolic properties.Excitonic effects are also well pronounced in layered materials. Exciton binding energies are about an order of magnitude higher than those of bulk semiconductors, such as Si, Ge and III-V or II-VI alloys, which allows for the direct observation of trions and biexcitons that are hard to see in bulk semiconductors. Exciton-polaritons have been recently observed in 2D materials inserted in microcavities 5,6 , although polariton condensation is still elusive.The latest group of 2D materials to be taking the stage...

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Valleytronics in 2D materials

Cited by 2,084 publications

References 173 publications

Defect Structure of Localized Excitons in a WSe2 Monolayer

Defect Structure of Localized Excitons in a WSe2 Monolayer

Reversible structure manipulation by tuning carrier concentration in metastable Cu ₂ S

As thin as it gets

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Valleytronics in 2D materials

Cited by 2,084 publications

References 173 publications

Defect Structure of Localized Excitons in a WSe2 Monolayer

Defect Structure of Localized Excitons in a WSe2 Monolayer

Reversible structure manipulation by tuning carrier concentration in metastable Cu 2 S

As thin as it gets

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Reversible structure manipulation by tuning carrier concentration in metastable Cu ₂ S