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1. Mechanically rollable photodetectors enabled by centimetre-scale 2D MoS ₂ layer/TOCN composites

   https://doi.org/10.1039/D0NA01053G  

   Yoo, Changhyeon; Ko, Tae-Jun; Han, Sang Sub; Shawkat, Mashiyat Sumaiya; Oh, Kyu Hwan; Kim, Bo Kyoung; Chung, Hee-Suk; Jung, Yeonwoong (June 2021, Nanoscale Advances)

   null (Ed.)

   Two-dimensional (2D) molybdenum disulfide (MoS 2 ) layers are suitable for visible-to-near infrared photodetection owing to their tunable optical bandgaps. Also, their superior mechanical deformability enabled by an extremely small thickness and van der Waals (vdW) assembly allows them to be structured into unconventional physical forms, unattainable with any other materials. Herein, we demonstrate a new type of 2D MoS 2 layer-based rollable photodetector that can be mechanically reconfigured while maintaining excellent geometry-invariant photo-responsiveness. Large-area (>a few cm 2 ) 2D MoS 2 layers grown by chemical vapor deposition (CVD) were integrated on transparent and flexible substrates composed of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs) by a direct solution casting method. These composite materials in three-dimensionally rollable forms exhibited a large set of intriguing photo-responsiveness, well preserving intrinsic opto-electrical characteristics of the integrated 2D MoS 2 layers; i.e. , light intensity-dependent photocurrents insensitive to illumination angles as well as highly tunable photocurrents varying with the rolling number of 2D MoS 2 layers, which were impossible to achieve with conventional photodetectors. This study provides a new design principle for converting 2D materials to three-dimensional (3D) objects of tailored functionalities and structures, significantly broadening their potential and versatility in futuristic devices. 

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2. Soft Biomorph Actuators Enabled by Wafer‐Scale Ultrathin 2D PtTe ₂ Layers

   https://doi.org/10.1002/admt.202100639  

   Okogbue, Emmanuel; Mofid, Sohrab_Alex; Yoo, Changhyeon; Han, Sang_Sub; Jung, Yeonwoong (October 2021, Advanced Materials Technologies)

   Abstract Biomorph actuators composed of two layers with asymmetric thermal expansion properties are widely explored owing to their high mechanical adaptability. Electrothermal nanomaterials are employed as the Joule heating components in them for controlled thermal expansion, while their large integration thickness often limits resulting actuation performances. This study reports high‐performance ultrathin soft biomorph actuators enabled by near atom‐thickness 2D platinum ditelluride (PtTe₂) layers—a new class of emergent metallic 2D transition metal dichalcogenides. The actuators employ wafer‐scale 2D PtTe₂layers sandwiched in between two polymer films of largely mismatched thermal expansion coefficients, which are electrically biased to generate Joule heating. This electrical‐to‐thermal conversion causes the asymmetric expansion of the polymers achieving outstanding actuation motions; i.e., large bending curvature, fast responsiveness, as well as high reversibility and endurance, which surpass the performances of previously explored graphene‐based actuators with much smaller dimensions. Furthermore, the 2D PtTe₂layers‐enabled actuators are demonstrated to function as soft grippers in lifting and relocating heavier objects, implying the great potential of near atom‐thickness materials in biomimetic devices. 

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3. Large-area 2D PtTe ₂ /silicon vertical-junction devices with ultrafast and high-sensitivity photodetection and photovoltaic enhancement by integrating water droplets

   https://doi.org/10.1039/D0NR05670G  

   Shawkat, Mashiyat Sumaiya; Chowdhury, Tanvir Ahmed; Chung, Hee-Suk; Sattar, Shahid; Ko, Tae-Jun; Larsson, J. Andreas; Jung, Yeonwoong (November 2020, Nanoscale)

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   2D PtTe 2 layers, a relatively new class of 2D crystals, have unique band structure and remarkably high electrical conductivity promising for emergent opto-electronics. This intrinsic superiority can be further leveraged toward practical device applications by merging them with mature 3D semiconductors, which has remained largely unexplored. Herein, we explored 2D/3D heterojunction devices by directly growing large-area (>cm 2 ) 2D PtTe 2 layers on Si wafers using a low-temperature CVD method and unveiled their superior opto-electrical characteristics. The devices exhibited excellent Schottky transport characteristics essential for high-performance photovoltaics and photodetection, i.e. , well-balanced combination of high photodetectivity (>10 13 Jones), small photo-responsiveness time (∼1 μs), high current rectification ratio (>10 5 ), and water super-hydrophobicity driven photovoltaic improvement (>300%). These performances were identified to be superior to those of previously explored 2D/3D or 2D layer-based devices with much smaller junction areas, and their underlying principles were confirmed by DFT calculations. 

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4. Humidity‐Driven High‐Performance Electrothermal Actuation of Vertically Stacked 2D PtTe ₂ Layers/Cellulose Nanofibers

   https://doi.org/10.1002/aisy.202200269  

   Yoo, Changhyeon; Han, Sang Sub; Okogbue, Emmanuel; Bae, Tae-Sung; Jang, Jae Hyuck; Cao, Justin; Chung, Hee-Suk; Jung, Yeonwoong (January 2023, Advanced Intelligent Systems)

   A new type of soft actuators based on a vertical stack of nanoporous 2,2,6,6‐tetramethylpiperidine‐1‐oxyl‐oxidized cellulose nanofibers (TOCNs) and atomically thin 2D platinum ditelluride (PtTe₂) layers is reported. The actuation of TOCNs is driven by the interfacing 2D PtTe₂layers whose electrothermal proficiency precisely controls their hydration/dehydration states sensitive to mechanical deformation. These vertically stacked TOCN/2D PtTe₂actuators present excellent actuation characteristics such as high linearity of bending curvature versus applied voltage and well‐preserved reversibility during cyclic operations. Most notably, they exhibit an extremely large weight‐lifting ratio, i.e., ≈1000 times the mass of the TOCN layers, confirming superior mechanical robustness. Furthermore, complicated actuations such as twisting in a 3D manner are demonstrated by judiciously controlling the surface wettability of TOCN layers. This study unveils opportunities for CNFs and 2D materials for actuator applications, as well as suggests new design strategies broadly applicable to soft robotics and biomimetic devices. 

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5. Manufacturing strategies for wafer-scale two-dimensional transition metal dichalcogenide heterolayers

   https://doi.org/10.1557/jmr.2020.27  

   Wang, Mengjing; Li, Hao; Ko, Tae-Jun; Sumaiya Shawkat, Mashiyat; Okogbue, Emmanuel; Yoo, Changhyeon; Han, Sang Sub; Islam, Md Ashraful; Oh, Kyu Hwan; Jung, Yeonwoong (June 2020, Journal of Materials Research)

   Modern electronics have been geared toward exploring novel electronic materials that can encompass a broad set of unusual functionalities absent in conventional platforms. In this regard, two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors are highly promising, owing to their large mechanical resilience coupled with superior transport properties and van der Waals (vdW) attraction-enabled relaxed assembly. Moreover, 2D TMD heterolayers based on chemically distinct constituent layers exhibit even more intriguing properties beyond their mono-component counterparts, which can materialize only when they are manufactured on a technologically practical wafer-scale. This mini-review provides a comprehensive overview of recent progress in exploring wafer-scale 2D TMD heterolayers of various kinds. It extensively surveys a variety of manufacturing strategies and discusses their scientific working principles, resulting 2D TMD heterolayers, their material properties, and device applications. Moreover, it offers extended discussion on remaining challenges and future outlooks toward further improving the material quality of 2D TMD heterolayers in both material and manufacturing aspects. 

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