Search for: All records

Self-supervised skeleton-based action recognition has attracted more attention in recent years. By utilizing the unlabeled data, more generalizable features can be learned to alleviate the overfitting problem and reduce the demand for massive labeled training data. Inspired by the MAE [1], we propose a spatial-temporal masked autoencoder framework for self-supervised 3D skeleton-based action recognition (SkeletonMAE). Following MAE's masking and reconstruction pipeline, we utilize a skeleton-based encoder-decoder transformer architecture to reconstruct the masked skeleton sequences. A novel masking strategy, named Spatial-Temporal Masking, is introduced in terms of both joint-level and frame-level for the skeleton sequence. This pre-training strategy makes the encoder output generalizable skeleton features with spatial and temporal dependencies. Given the unmasked skeleton sequence, the encoder is fine-tuned for the action recognition task. Extensive ex- periments show that our SkeletonMAE achieves remarkable performance and outperforms the state-of-the-art methods on both NTU RGB+D 60 and NTU RGB+D 120 datasets. 

Polymer semiconductors (PSCs) are essential active materials in mechanically stretchable electronic devices. However, many exhibit low fracture strain due to their rigid chain conformation and the presence of large crystalline domains. Here, a PSC/elastomer blend, poly[((2,6‐bis(thiophen‐2‐yl)‐3,7‐bis(9‐octylnonadecyl)thieno[3,2‐b]thieno[2′,3′:4,5]thieno[2,3‐d]thiophene)‐5,5′‐diyl)(2,5‐bis(8‐octyloctadecyl)‐3,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione)‐5,5′‐diyl]] (P2TDPP2TFT4) and polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) are systematically investigated. Specifically, the effects of molecular weight of both SEBS and P2TDPP2TFT4 on the resulting blend morphology, mechanical, and electrical properties are explored. In addition to commonly used techniques, atomic force microscopy‐based nanomechanical images are used to provide additional insights into the blend film morphology. Opposing trends in SEBS‐induced aggregation are observed for the different P2TDPP2TFT4 molecular weights upon increasing the SEBS molecular weight from 87 to 276 kDa. Furthermore, these trends are seen in device performance trends for both molecular weights of P2TDPP2TFT4. SEBS molecular weight also has a substantial influence on the mesoscale phase separation. Strain at fracture increases dramatically upon blending, reaching a maximum value of 640% ± 20% in the blended films measured with film‐on‐water method. These results highlight the importance of molecular weight for electronic devices. In addition, this study provides valuable insights into appropriate polymer selections for stretchable semiconducting thin films that simultaneously possess excellent mechanical and electrical properties.

 

Constructing functional molecular systems for solar energy conversion and quantum information science requires a fundamental understanding of electron transfer in donor–bridge–acceptor (D–B–A) systems as well as competitive reaction pathways in acceptor–donor–acceptor (A–D–A) and acceptor–donor–acceptor′ (A–D–A′) systems. Herein we present a supramolecular complex comprising a tetracationic cyclophane having both phenyl-extended viologen (ExV 2+ ) and dipyridylthiazolothiazole (TTz 2+ ) electron acceptors doubly-linked by means of two p -xylylene linkers (TTzExVBox 4+ ), which readily incorporates a perylene (Per) guest in its cavity (Per ⊂ TTzExVBox 4+ ) to establish an A–D–A′ system, in which the ExV 2+ and TTz 2+ units serve as competing electron acceptors with different reduction potentials. Photoexcitation of the Per guest yields both TTz + ˙–Per + ˙–ExV 2+ and TTz 2+ –Per + ˙–ExV + ˙ in <1 ps, while back electron transfer in TTz 2+ –Per + ˙–ExV + ˙ proceeds via the unusual sequence TTz 2+ –Per + ˙–ExV + ˙ → TTz + ˙–Per + ˙–ExV 2+ → TTz 2+ –Per–ExV 2+ . In addition, selective chemical reduction of TTz 2+ gives Per ⊂ TTzExVBox 3+ ˙, turning the complex into a D–B–A system in which photoexcitation of TTz + ˙ results in the reaction sequence 2 *TTz + ˙–Per–ExV 2+ → TTz 2+ –Per–ExV + ˙ → TTz + ˙–Per–ExV 2+ . Both reactions TTz 2+ –Per + ˙–ExV + ˙ → TTz + ˙–Per + ˙–ExV 2+ and TTz 2+ –Per–ExV + ˙ → TTz + ˙–Per–ExV 2+ occur with a (16 ± 1 ps) −1 rate constant irrespective of whether the bridge molecule is Per + ˙ or Per. These results are explained using the superexchange mechanism in which the ionic states of the perylene guest serve as virtual states in each case and demonstrate a novel supramolecular platform for studying the effects of bridge energetics within D–B–A systems. 

Understanding molecular design rules for stretchable polymer semiconductors is important for enabling next generation stretchable electronic circuits. To simultaneously improve both electrical properties and mechanical stretchability, a design strategy is reported in introducing conjugated rigid fused‐rings with bulky side groups in semiconducting polymers. In this work, the understanding of this design concept is improved by systematically investigating the effect of different types of bulky side groups asymmetrically substituted on conjugated polymer semiconductor backbones. Specifically, four types of side groups are investigated, including naphthalene (NaPh), biphenyl (PhPh), thienylphenyl (ThPh), and alkylphenyl (C4Ph), asymmetrically substituted on benzodithiophene units, namely asy‐BDT. With the four types of side groups installed on BDT‐containing conjugated polymers in an asymmetrical fashion, it is observed that they reduced the polymer chain aggregation and film crystallinity, hence improving the film stretchability. Furthermore, the fully conjugated polymer backbone allows maintenance of good charge carrier mobilities. Specifically, polymer PDPP‐C4Ph (with C4Ph side groups) shows the highest mobility in the fully stretchable transistor and maintained its mobility even after being subjected to hundreds of stretching‐releasing cycles at 25% strain. Overall, the results provide anunderstanding of the use of asymmetrically substituted fused‐ring conjugated polymer structures to tune mechanical and charge transport properties.

 

Medical devices are commonly implanted underneath the skin, but how to real‐time noninvasively monitor their migration, integrity, and biodegradation in human body is still a formidable challenge. Here, the study demonstrates that benzyl violet 4B (BV‐4B), a main component in the FDA‐approved surgical suture, is found to produce fluorescence signal in the first near‐infrared window (NIR‐I, 700–900 nm) in polar solutions, whereas BV‐4B self‐assembles into highly crystalline aggregates upon a formation of ultrasmall nanodots and can emit strong fluorescence in the second near‐infrared window (NIR‐II, 1000–1700 nm) with a dramatic bathochromic shift in the absorption spectrum of ≈200 nm. Intriguingly, BV‐4B‐involved suture knots underneath the skin can be facilely monitored during the whole degradation process in vivo, and the rupture of the customized BV‐4B‐coated silicone catheter is noninvasively diagnosed by NIR‐II imaging. Furthermore, BV‐4B suspended in embolization glue achieves hybrid fluorescence‐guided surgery (hybrid FGS) for arteriovenous malformation. As a proof‐of‐concept study, the solid‐state BV‐4B is successfully used for NIR‐II imaging of surgical sutures in operations of patients. Overall, as a clinically translatable solid‐state dye, BV‐4B can be applied for in vivo monitoring the fate of medical devices by NIR‐II imaging.

 

Molecular aggregation and crystallization during film coating play a crucial role in the realization of high‐performing organic photovoltaics. Strong intermolecular interactions and high solid‐state crystallinity are beneficial for charge transport. However, fast crystallization during thin‐film drying often limits the formation of the finely phase‐separated morphology required for efficient charge generation. Herein, the authors show that twisted acceptor‐donor‐acceptor (A‐D‐A) type compounds, containing an indacenodithiophene (IDT) electron‐rich core and two naphthalenediimide (NDI) electron‐poor units, leads to formation of mostly amorphous phases in the as‐cast film, which can be readily converted into more crystalline domains by means of thermal annealing. This design strategy solves the aforementioned conundrum, leading to an optimal morphology in terms of reduced donor/acceptor domain‐separation sizes (ca. 13 nm) and increased packing order. Solar cells based on these acceptors with a PBDB‐T polymer donor show a power conversion efficiency over 10% and stable morphology, which results from the combined properties of desirable excited‐state dynamics, high charge mobility, and optimal aggregation/crystallization characteristics. These results demonstrate that the twisted A‐D‐A motif featuring thermally‐induced crystallization behavior is indeed a promising alternative design approach toward more morphologically robust materials for efficient organic photovoltaics.

 

Numerous strategies are developed to impart stretchability to polymer semiconductors. Although these methods improve the ductility, mobility, and stability of such stretchable semiconductors, they nonetheless still need further improvement. Here, it is shown that 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) is an effective molecular additive to tune the properties of a diketopyrrolopyrrole‐based (DPP‐based) semiconductor. Specifically, the addition of F4‐TCNQ is observed to improve the ductility of the semiconductor by altering the polymer’s microstructures and dynamic motions. As a p‐type dopant additive, F4‐TCNQ can also effectively enhance the mobility and stability of the semiconductor through changing the host polymer’s packing structures and charge trap passivation. Upon fabricating fully stretchable transistors with F4‐TCNQ‐DPP blended semiconductor films, it is observed that the resulting stretchable transistors possess one of the highest initial mobility of 1.03 cm²V⁻¹s⁻¹. The fabricated transistors also exhibit higher stability (both bias and environmental) and mobility retention under repeated strain, compared to those without F4‐TCNQ additive. These findings offer a new direction of research on stretchable semiconductors to facilitate future practical applications.