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1. Low-Temperature 2D/2D Ohmic Contacts in WSe ₂ Field-Effect Transistors as a Platform for the 2D Metal–Insulator Transition

   https://doi.org/10.1021/acsami.0c21440  

   Stanley, Lily J.; Chuang, Hsun-Jen; Zhou, Zhixian; Koehler, Michael R.; Yan, Jiaqiang; Mandrus, David G.; Popović, Dragana (February 2021, ACS Applied Materials & Interfaces)

   null (Ed.)

   We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multi-terminal WSe_2 Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe_2 devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe_2 devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature (T) and density (n_s) dependence of the conductivity \sigma(T,n_s) demonstrate scaling behavior consistent with a metal-insulator quantum phase transition driven by electron-electron interactions, but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering. 

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2. Effects of rhenium dopants on photocarrier dynamics and optical properties of monolayer, few-layer, and bulk MoS ₂

   https://doi.org/10.1039/C7NR07227A  

   Li, Yuanyuan; Liu, Qingfeng; Cui, Qiannan; Qi, Zeming; Wu, Judy Z.; Zhao, Hui (January 2017, Nanoscale)

   We report a comprehensive study on the effects of rhenium doping on optical properties and photocarrier dynamics of MoS 2 monolayer, few-layer, and bulk samples. Monolayer and few-layer samples of Re-doped (0.6%) and undoped MoS 2 were fabricated by mechanical exfoliation, and were studied by Raman spectroscopy, optical absorption, photoluminescence, and time-resolved differential reflection measurements. Similar Raman, absorption, and photoluminescence spectra were obtained from doped and undoped samples, indicating that the Re doping at this level does not significantly alter the lattice and electronic structures. Red-shift and broadening of the two phonon Raman modes were observed, showing the lattice strain and carrier doping induced by Re. The photoluminescence yield of the doped monolayer is about 15 times lower than that of the undoped sample, while the photocarrier lifetime is about 20 times shorter in the doped monolayer. Both observations can be attributed to diffusion-limited Auger nonradiative recombination of photocarriers at Re dopants. These results provide useful information for developing a doping strategy of MoS 2 for optoelectronic applications. 

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3. Layer and material-type dependent photoresponse in WSe ₂ /WS ₂ vertical heterostructures

   https://doi.org/10.1088/2053-1583/ac3c9c  

   Fu, ZhuangEn; Hill, Josh W; Parkinson, Bruce; Hill, Caleb M; Tian, Jifa (December 2021, 2D Materials)

   Abstract Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe 2 /WS 2 vertical heterostructures as well as in pristine WS 2 and WSe 2 layers. For WS 2 , we find that photocurrents increase with increasing layer thickness, whereas for WSe 2 the layer dependence is more complex and depends on both the layer number and applied bias ( V b ). We further find that photocurrents in the WSe 2 /WS 2 heterostructures exhibit anomalous layer and material-type dependent behaviors. Our results advance the understanding of photoresponse in atomically thin WSe 2 /WS 2 heterostructures and pave the way to novel nanoelectronic and optoelectronic devices. 

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4. Stimulated Emission from Below the Bandgap in Giant Quantum Shells

   https://doi.org/10.1021/jacs.5c09795  

   Waters, Amelia D; Bondarchuk, Mykhailo V; Hicks, Christopher M; Smith, Sean; Nazar, Divesh; Kannen, Maxwell Marshal; Harankahage, Dulanjan; Thennakoon, Siddhartha; Huang, Jiamin; Anzenbacher, Edison; et al (August 2025, Journal of the American Chemical Society)

   Colloidal semiconductor nanocrystals (NCs) have emerged as promising candidates for developing solutionprocessable optical gain media with potential applications in integrated photonic circuits and lasers. However, the deployment of NCs in these technologies has been hindered by the nonradiative Auger recombination of multiexciton states, which shortens the optical gain lifetime and reduces its spectral range. Here, we demonstrate that these limitations can be overcome by using giant colloidal quantum shells (g-QSs), comprising a quantum-confined CdSe shell grown over a large (∼14 nm) CdS bulk core. Such bulk-nanoscale architecture minimizes exciton− exciton interactions, leading to suppressed Auger recombination and one of the broadest gain bandwidths reported for colloidal nanomaterials, spanning energies both above and, remarkably, below the bandgap. Ultrafast transient absorption and photoluminescence measurements demonstrate that the high-energy portion of optical gain arises from states containing more than 15 excitons per particle, while the unusual sub-bandgap gain behavior results from an Auger-assisted radiative recombination, a mechanism that has traditionally been viewed as a loss pathway. Collectively, these results reveal a unique gain regime associated with bulk-nanocrystal hybrid systems, which offers a promising path toward solution-processable light sources. 

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5. Macroscopic transition metal dichalcogenides monolayers with uniformly high optical quality

   https://doi.org/10.1038/s41467-023-37500-1  

   Li, Qiuyang; Alfrey, Adam; Hu, Jiaqi; Lydick, Nathanial; Paik, Eunice; Liu, Bin; Sun, Haiping; Lu, Yang; Wang, Ruoyu; Forrest, Stephen; et al (April 2023, Nature Communications)

   Abstract The unique optical properties of transition metal dichalcogenide (TMD) monolayers have attracted significant attention for both photonics applications and fundamental studies of low-dimensional systems. TMD monolayers of high optical quality, however, have been limited to micron-sized flakes produced by low-throughput and labour-intensive processes, whereas large-area films are often affected by surface defects and large inhomogeneity. Here we report a rapid and reliable method to synthesize macroscopic-scale TMD monolayers of uniform, high optical quality. Using 1-dodecanol encapsulation combined with gold-tape-assisted exfoliation, we obtain monolayers with lateral size > 1 mm, exhibiting exciton energy, linewidth, and quantum yield uniform over the whole area and close to those of high-quality micron-sized flakes. We tentatively associate the role of the two molecular encapsulating layers as isolating the TMD from the substrate and passivating the chalcogen vacancies, respectively. We demonstrate the utility of our encapsulated monolayers by scalable integration with an array of photonic crystal cavities, creating polariton arrays with enhanced light-matter coupling strength. This work provides a pathway to achieving high-quality two-dimensional materials over large areas, enabling research and technology development beyond individual micron-sized devices. 

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