More Like this

1. The Critical Role of Electron‐Donating Thiophene Groups on the Mechanical and Thermal Properties of Donor–Acceptor Semiconducting Polymers

   https://doi.org/10.1002/aelm.201800899  

   Zhang, Song; Ocheje, Michael U.; Huang, Lifeng; Galuska, Luke; Cao, Zhiqiang; Luo, Shaochuan; Cheng, Yu‐Hsuan; Ehlenberg, Dakota; Goodman, Renée B.; Zhou, Dongshan; et al (March 2019, Advanced Electronic Materials)

   Abstract Organic semiconducting donor–acceptor polymers are promising candidates for stretchable electronics owing to their mechanical compliance. However, the effect of the electron‐donating thiophene group on the thermomechanical properties of conjugated polymers has not been carefully studied. Here, thin‐film mechanical properties are investigated for diketopyrrolopyrrole (DPP)‐based conjugated polymers with varying numbers of isolated thiophene moieties and sizes of fused thiophene rings in the polymer backbone. Interestingly, it is found that these thiophene units act as an antiplasticizer, where more isolated thiophene rings or bigger fused rings result in an increased glass transition temperature (T_g) of the polymer backbone, and consequently elastic modulus of the respective DPP polymers. Detailed morphological studies suggests that all samples show similar semicrystalline morphology. This antiplasticization effect also exists inpara‐azaquinodimethane‐based conjugated polymers, indicating that this can be a general trend for various conjugated polymer systems. Using the knowledge gained above, a new DPP‐based polymer with increased alkyl side chain density through attaching alky chains to the thiophene unit is engineered. The new DPP polymer demonstrates a record lowT_g, and 50% lower elastic modulus than a reference polymer without side‐chain decorated on the thiophene unit. This work provides a general design rule for making low‐T_gconjugated polymers for stretchable electronics. 

   more » « less
2. Toward the Prediction and Control of Glass Transition Temperature for Donor–Acceptor Polymers

   https://doi.org/10.1002/adfm.202002221  

   Zhang, Song; Alesadi, Amirhadi; Selivanova, Mariia; Cao, Zhiqiang; Qian, Zhiyuan; Luo, Shaochuan; Galuska, Luke; Teh, Catherine; Ocheje, Michael U.; Mason, Gage T.; et al (May 2020, Advanced Functional Materials)

   Abstract Semiconducting donor–acceptor (D–A) polymers have attracted considerable attention toward the application of organic electronic and optoelectronic devices. However, a rational design rule for making semiconducting polymers with desired thermal and mechanical properties is currently lacking, which greatly limits the development of new polymers for advanced applications. Here, polydiketopyrrolopyrrole (PDPP)‐based D–A polymers with varied alkyl side‐chain lengths and backbone moieties are systematically designed, followed by investigating their thermal and thin film mechanical responses. The experimental results show a reduction in both elastic modulus and glass transition temperature (T_g) with increasing side‐chain length, which is further verified through coarse‐grained molecular dynamics simulations. Informed from experimental results, a mass‐per‐flexible bond model is developed to capture such observation through a linear correlation betweenT_gand polymer chain flexibility. Using this model, a wide range of backboneT_gover 80 °C and elastic modulus over 400 MPa can be predicted for PDPP‐based polymers. This study highlights the important role of side‐chain structure in influencing the thermomechanical performance of conjugated polymers, and provides an effective strategy to design and predictT_gand elastic modulus of future new D–A polymers. 

   more » « less
3. Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo ₂ O ₄ films

   https://doi.org/10.1063/5.0097869  

   Chen, Xuegang; Wu, Qiuchen; Zhang, Le; Hao, Yifei; Han, Myung-Geun; Zhu, Yimei; Hong, Xia (June 2022, Applied Physics Letters)

   The inverse spinel ferrimagnetic NiCo₂O₄possesses high magnetic Curie temperature T_C, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo₂O₄thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo₂O₄films on MgAl₂O₄substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while T_Cfor 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity [Formula: see text] and its scaling relation with the longitudinal conductivity ([Formula: see text]) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo₂O₄. 

   more » « less
4. Characterization of nanoscale morphology and mechanical properties of conjugated polymer thin films dynamically exposed to a secondary solvent

   https://doi.org/10.1016/j.polymer.2023.126625  

   House, Krystal L; Christian, Kent H; Emge, Thomas J; Pacheco, Haydee; Haber, Richard A; O'Carroll, Deirdre M (February 2024, Polymer)

   Solution processing techniques are often used to enhance intra and interchain order in semiconducting conjugated polymer thin films used in the active layer of organic optoelectronic devices. We investigate the nanomechanical properties of conjugated polymer thin films arising from solution processing techniques. We find that Young’s Modulus data measured by AM-FM AFM can detect additional changes in film properties not discernible by other commonly used bulk thin-film characterization techniques. For PBDB-T-SF, we detect an increase in molecular order that is not noticable by UV–visible absorption spectroscopy, X-ray diffraction or nanoindentation. PCDTBT, the most amorphous of the polymers studied, shows no changes in absorption or X-ray diffraction data, yet clear changes in Young’s Modulus were detected by AFM. Our study demonstrates the applicability of nanomechanical measurements for characterizing local structural variations in conjugated polymer films. This work is relevant to ongoing efforts to control and understand the complex structure-property-processing relationships of conjugated polymer thin films. 

   more » « less
5. Digital Tuning of the Transition Temperature of Epitaxial VO ₂ Thin Films on MgF ₂ Substrates by Strain Engineering

   https://doi.org/10.1002/admi.202001790  

   Howard, Sebastian A.; Evlyukhin, Egor; Páez Fajardo, Galo; Paik, Hanjong; Schlom, Darrell G.; Piper, Louis F. J. (March 2021, Advanced Materials Interfaces)

   Abstract Straining the vanadium dimers along the rutilec‐axis can be used to tune the metal‐to‐insulator transition (MIT) of VO₂but has thus far been limited to TiO₂substrates. In this work VO₂/MgF₂epitaxial films are grown via molecular beam epitaxy (MBE) to strain engineer the transition temperature (T_MIT). First, growth parameters are optimized by varying the synthesis temperature of the MgF₂(001) substrate (T_S) using a combination of X‐ray diffraction techniques, temperature dependent transport, and soft X‐ray photoelectron spectroscopy. It is determined thatT_Svalues greater than 350 °C induce Mg and F interdiffusion and ultimately the relaxation of the VO₂layer. Using the optimized growth temperature, VO₂/MgF₂(101) and (110) films are then synthesized. The three film orientations display MITs with transition temperatures in the range of 15–60 °C through precise strain engineering. 

   more » « less