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1. Giant valley splitting in monolayer WS2 by magnetic proximity effect

   https://doi.org/10.1038/s41467-019-11966-4  

   Norden, Tenzin ; Zhao, Chuan ; Zhang, Peiyao ; Sabirianov, Renat ; Petrou, Athos ; Zeng, Hao ( September 2019 , Nature Communications)

   Lifting the valley degeneracy of monolayer transition metal dichalcogenides (TMDs) would allow versatile control of the valley degree of freedom. We report a giant valley exciton splitting of 16 meV/T for monolayer WS₂, using the proximity effect from an EuS substrate, which is enhanced by nearly two orders of magnitude from that obtained by an external magnetic field. More interestingly, a sign reversal of the valley splitting is observed as compared to that of WSe₂on EuS. Using first principles calculations, we investigate the complex behavior of exchange interactions between TMDs and EuS. The sign reversal is attributed to competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions for Eu- and S- terminated EuS surface sites. They act differently on the conduction and valence bands of WS₂compared to WSe₂. Tuning the sign and magnitude of the valley exciton splitting offers opportunities for control of valley pseudospin for quantum information processing.

    

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2. Metal‐Guided Selective Growth of 2D Materials: Demonstration of a Bottom‐Up CMOS Inverter

   https://doi.org/10.1002/adma.201900861  

   Chiu, Ming‐Hui ; Tang, Hao‐Ling ; Tseng, Chien‐Chih ; Han, Yimo ; Aljarb, Areej ; Huang, Jing‐Kai ; Wan, Yi ; Fu, Jui‐Han ; Zhang, Xixiang ; Chang, Wen‐Hao ; et al ( March 2019 , Advanced Materials)

   2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large‐area electronics and circuits strongly relies on wafer‐scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal‐guided selective growth (MGSG), is reported. The success of control over the transition‐metal‐precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p‐ and n‐type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom‐up complementary metal‐oxide‐semiconductor inverter based on p‐type WSe₂and n‐type MoSe₂is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

    

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3. Conversion of Classical Light Emission from a Nanoparticle‐Strained WSe 2 Monolayer into Quantum Light Emission via Electron Beam Irradiation

   https://doi.org/10.1002/adma.202208066  

   Xu, David D. ; Vong, Albert F. ; Lebedev, Dmitry ; Ananth, Riddhi ; Wong, Alexa M. ; Brown, Paul T. ; Hersam, Mark C. ; Mirkin, Chad A. ; Weiss, Emily A. ( December 2022 , Advanced Materials)

   Solid‐state single photon emitters (SPEs) within atomically thin transition metal dichalcogenides (TMDs) have recently attracted interest as scalable quantum light sources for quantum photonic technologies. Among TMDs, WSe₂monolayers (MLs) are promising for the deterministic fabrication and engineering of SPEs using local strain fields. The ability to reliably produce isolatable SPEs in WSe₂is currently impeded by the presence of numerous spectrally overlapping states that occur at strained locations. Here nanoparticle (NP) arrays with precisely defined positions and sizes are employed to deterministically create strain fields in WSe₂MLs, thus enabling the systematic investigation and control of SPE formation. Using this platform, electron beam irradiation at NP‐strained locations transforms spectrally overlapped sub‐bandgap emission states into isolatable, anti‐bunched quantum emitters. The dependence of the emission spectra of WSe₂MLs as a function of strain magnitude and exposure time to electron beam irradiation is quantified and provides insight into the mechanism for SPE production. Excitons selectively funnel through strongly coupled sub‐bandgap states introduced by electron beam irradiation, which suppresses spectrally overlapping emission pathways and leads to measurable anti‐bunched behavior. The findings provide a strategy to generate isolatable SPEs in 2D materials with a well‐defined energy range.

    

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4. Vapor–liquid assisted chemical vapor deposition of Cu 2 X materials

   https://doi.org/10.1088/2053-1583/ac8435  

   Shehzad, M Arslan ; Lee, Yea-Shine ; Cheng, Matthew ; Lebedev, Dmitry ; Tyner, Alexander C ; Das, Paul Masih ; Gao, Zhangyuan ; Goswami, Pallab ; dos Reis, Roberto ; Hersam, Mark C ; et al ( August 2022 , 2D Materials)

   Abstract Transition metal dichalcogenides (TMDs) are known for their layered structure and tunable functional properties. However, a unified understanding on other transition metal chalcogenides (i.e. M 2 X) is still lacking. Here, the relatively new class of copper-based chalcogenides Cu 2 X (X = Te, Se, S) is thoroughly reported. Cu 2 X are synthesized by an unusual vapor–liquid assisted growth on a Al 2 O 3 /Cu/W stack. Liquid copper plays a significant role in synthesizing these layered systems, and sapphire assists with lateral growth and exfoliation. Similar to traditional TMDs, thickness dependent phonon signatures are observed, and high-resolution atomic images reveal the single phase Cu 2 Te that prefers to grow in lattice-matched layers. Charge transport measurements indicate a metallic nature at room temperature with a transition to a semiconducting nature at low temperatures accompanied by a phase transition, in agreement with band structure calculations. These findings establish a fundamental understanding and thrust Cu 2 Te as a flexible candidate for wide applications from photovoltaics and sensors to nanoelectronics. 

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5. Experimental observation of topological Z2 exciton-polaritons in transition metal dichalcogenide monolayers

   https://doi.org/10.1038/s41467-021-24728-y  

   Li, Mengyao ; Sinev, Ivan ; Benimetskiy, Fedor ; Ivanova, Tatyana ; Khestanova, Ekaterina ; Kiriushechkina, Svetlana ; Vakulenko, Anton ; Guddala, Sriram ; Skolnick, Maurice ; Menon, Vinod M. ; et al ( July 2021 , Nature Communications)

   The rise of quantum science and technologies motivates photonics research to seek new platforms with strong light-matter interactions to facilitate quantum behaviors at moderate light intensities. Topological polaritons (TPs) offer an ideal platform in this context, with unique properties stemming from resilient topological states of light strongly coupled with matter. Here we explore polaritonic metasurfaces based on 2D transition metal dichalcogenides (TMDs) as a promising platform for topological polaritonics. We show that the strong coupling between topological photonic modes of the metasurface and excitons in TMDs yields a topological polaritonic Z₂phase. We experimentally confirm the emergence of one-way spin-polarized edge TPs in metasurfaces integrating MoSe₂and WSe₂. Combined with the valley polarization in TMD monolayers, the proposed system enables an approach to engage the photonic angular momentum and valley and spin of excitons, offering a promising platform for photonic/solid-state interfaces for valleytronics and spintronics.

    

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