Ferroelectricity and the phase transition in large area evaporated vinylidene fluoride oligomer thin films

Organic ferroelectric materials, including the well-known poly(vinylidene fluoride) and its copolymers, have been extensively studied and used for a variety of applications. In contrast, the VDF oligomer has not been thoroughly investigated and is not widely used, if used at all. One key advantage the oligomer has over the polymer is that it can be thermally evaporated in vacuum, allowing for the growth of complex heterostructures while maintaining interfacial cleanliness. Here, we report on the ferroelectric properties of high-quality VDF oligomer thin films over relatively large areas on the order of mm2. The operating temperature is identified via differential scanning calorimetry and pyroelectric measurements. Pyroelectric measurements also reveal a stable remanent polarization for these films which persists over very long time scales, an important result for nonvolatile data storage. Temperature dependent pyroelectric and capacitance measurements provide compelling evidence for the phase transition in these films. Capacitance-voltage and currentvoltage measurements are used to confirm ferroelectricity, quantify the dielectric loss, and calculate the spontaneous polarization. Finally, piezoresponse force microscopy is used to demonstrate large area, low-voltage ferroelectric domain reading/writing in VDF oligomer thin films. This work enables new channels for VDF oligomer applications and research

Delayed Severe Drug Induced Cholestasis After Anabolic Steroids Exposure

Anabolic steroids therapy is linked to a distinctive form of acute cholestasis that generally arises within 1 to 4 months of starting therapy but may be delayed to as long as 6 to 24 months in some reported cases. We are presenting an unusual case where acute cholestasis happened 5 weeks after discontinuing short course (7 days) of anabolic steroids. A 20-year-old African American male without significant past medical history presented with jaundice, mild nausea and generalized pruritis. He admitted to self-initiation of oral Stanozolol 40 mg daily for 7-days about 5 weeks before his admission. No prior incidence and no significant alcohol use. Vitals signs unremarkable and physical exam was unremarkable except for jaundice and sclera icterus. No hepatosplenomegaly. Liver function profile with ALT 44 IU/L, AST 72 IU/L, ALP 266 IU/L, & total bilirubin of 21.4 mg/Dl. Extensive hepatitis workup was negative and a CT-guided liver biopsy was performed which showed marked hepatocanalicular cholestasis. This is reflective of drug induced cholestasis. Our case highlights the possibility of delayed liver injury even after a short anabolic steroids course which typically happen while taking the medication for an extended period (more than 1 month)

Persistent opto-ferroelectric responses in molecular ferroelectrics

Persistent photoresponses require optical excitations to metastable states, which are rare of ionic origin due to the indirect photon-ion interaction. In this work, we explore the photoinduced metastable proton states in the proton-transfer type molecular ferroelectric croconic acid. We observe that, after the photoexcitation, the changes of structural and ferroelectric properties relax in ∼10^3s, indicating persistent photoresponses of ionic origin. In contrast, the photoconductivity relaxes within 1 s. The 10^3s timescale suggests that the ionic metastable states result from proton transfer both along and out of the hydrogen bonds. This discovery unveils an ionic mechanism for the phototunability, which offers persistent opto-ferroelectric control for proton-transfer type molecular ferroelectrics

Ferroelectricity and the phase transition in large area evaporated vinylidene fluoride oligomer thin films

Organic ferroelectric materials, including the well-known poly(vinylidene fluoride) and its copolymers, have been extensively studied and used for a variety of applications. In contrast, the VDF oligomer has not been thoroughly investigated and is not widely used, if used at all. One key advantage the oligomer has over the polymer is that it can be thermally evaporated in vacuum, allowing for the growth of complex heterostructures while maintaining interfacial cleanliness. Here, we report on the ferroelectric properties of high-quality VDF oligomer thin films over relatively large areas on the order of mm2. The operating temperature is identified via differential scanning calorimetry and pyroelectric measurements. Pyroelectric measurements also reveal a stable remanent polarization for these films which persists over very long time scales, an important result for nonvolatile data storage. Temperature dependent pyroelectric and capacitance measurements provide compelling evidence for the phase transition in these films. Capacitance-voltage and currentvoltage measurements are used to confirm ferroelectricity, quantify the dielectric loss, and calculate the spontaneous polarization. Finally, piezoresponse force microscopy is used to demonstrate large area, low-voltage ferroelectric domain reading/writing in VDF oligomer thin films. This work enables new channels for VDF oligomer applications and research

Temperature dependence of flexoelectric response in ferroelectric and relaxor polymer thin films

We report the temperature dependence of the flexoelectric response in thin films of both ferroelectric and relaxor forms of vinylidene fluoride polymers. The ferroelectric samples were depoled to minimize piezoelectric response by heating them beyond their Curie temperature and then cooling in zero applied electric field. In both the relaxor ferroelectric polymer and the paraelectric state of the ferroelectric copolymer, the flexoelectric coefficient was proportional to the dielectric constant over a limited range of temperatures, in agreement with general theoretical principles. The enhancements in flexoelectric response were also observed near the Curie transition temperature for the ferroelectric polymer and near the dielectric relaxation temperature for the relaxors. The broad dielectric anomaly in these systems provides greater temperature stability for these enhancements

Measurement of the flexoelectric response in ferroelectric and relaxor polymer thin films

We report measurements of the flexoelectric response (electric polarization induced by a strain gradient) in thin films of both ferroelectric and relaxor forms of vinylidene fluoride polymers. By using a simple cantilever measurement technique, while monitoring remanent polarization through the pyroelectric response, we are able to measure the flexoelectric response in thin films as well as isolate and correct for piezoelectric contributions, which would otherwise dominate the flexoelectric measurement

Study of flexoelectric effect in VDF-based polymers and design of novel organic ferroelectrics

In the first part of the work the strain gradient induced electromechanical coupling known as Flexoelectricity is investigated in vinylidene fluoride (VDF) based polymers. An experimental procedure is proposed and applied to distinguish the piezoelectric contributions from flexoelectric ones in the ferroelectric materials. The magnitudes of flexoelectric coupling coefficient has been measured in the ferroelectric, paraelectric and relaxor states. A comprehensive study of the temperature dependence of flexoelectric effect has also been carried out which shows scaling of the effect with the dielectric constant for very narrow temperature range. The second part delves into new organic ferroelectric systems of diisopropylammonium (DIPAX) salts. Organic systems have advantages over their inorganic counterparts like being lightweight, ecofriendly, easy processing condition and integration in flexible electronics. The synthesis, thermal, electrical and structural properties of bulk crystals are discussed. To be integrated into future devices two approaches for thin film fabrication has been discussed and performed