Nanoscale Ferroelectric and Piezoelectric Properties of Sb₂S₃ Nanowire Arrays

We report the first observation of piezoelectricity and ferroelectricity in individual Sb₂S₃ nanowires embedded in anodic alumina templates. Switching spectroscopy-piezoresponse force microscopy (SS-PFM) measurements demonstrate that individual, <i>c</i>-axis-oriented Sb₂S₃ nanowires exhibit ferroelectric as well as piezoelectric switching behavior. Sb₂S₃ nanowires with nominal diameters of 200 and 100 nm showed <i>d</i>_33(eff) values around 2 pm V^–1, while the piezo coefficient obtained for 50 nm diameter nanowires was relatively low at around 0.8 pm V^–1. A spontaneous polarization (<i>P</i>_s) of approximately 1.8 μC cm^–2 was observed in the 200 and 100 nm Sb₂S₃ nanowires, which is a 100% enhancement when compared to bulk Sb₂S₃ and is probably due to the defect-free, single-crystalline nature of the nanowires synthesized. The 180° ferroelectric monodomains observed in Sb₂S₃ nanowires were due to uniform polarization alignment along the polar <i>c</i>-axis

Bismuth Self-Limiting Growth of Ultrathin BiFeO₃ Films

Bismuth ferrite (BiFeO₃) is a widely studied material, because of its interesting multiferroic properties. Bismuth self-limiting growth of single-phase BiFeO₃ (BFO) has previously been achieved using molecular beam epitaxy (MBE), but the growth of BFO by chemical vapor deposition (CVD) has proved to be very challenging, because of the volatile nature of bismuth. The growth window regarding temperature, pressure, and precursor flow rates that will give a pure perovskite BFO phase is normally very small. In this work, we have studied the metal–organic CVD (MOCVD) growth of epitaxial BFO thin films on SrTiO₃ substrates and found that by carefully controlling the amount of the iron precursor, Fe­(thd)₃ (where thd = 2,2,6,6 tetra-methyl-3,5-heptanedionate), we were able to achieve bismuth self-liming growth, for the first time. The effect of the volume of the bismuth and iron precursors injected on the growth of BFO thin films is reported, and it has been found that the phase-pure films can be prepared when the Bi/Fe ratios are between 1.33 and 1.81 under temperature and pressure conditions of 650 °C and 10 mbar, respectively, and where the O₂ gas flow was kept constant to 1000 sccm out of a total gas flow of 3000 sccm. Piezoresponse force microscopy (PFM) studies demonstrate the presence of bipolar switching in ultrathin BFO films

Anomalous motion of charged domain walls and associated negative capacitance in copper–chlorine boracite

During switching, the microstructure of a ferroelectric normally adapts to align internal dipoles with external electric fields. Favorably oriented dipolar regions (domains) grow at the expense of those in unfavorable orientations and this is manifested in a predictable field-induced motion of the walls that separate one domain from the next. Here, the discovery that specific charged 90°domain walls in copper–chlorine boracite move in the opposite direction to that expected, increasing the size of the domain in which polarization is anti-aligned with the applied field, is reported. Polarization–field (P–E) hysteresis loops, inferred from optical imaging, show negative gradients and on-transient negative capacitance, throughout the P–E cycle. Switching currents (generated by the relative motion between domain walls and sensing electrodes) confirm this, insofar as their signs are opposite to those expected conventionally. For any given bias, the integrated switching charge due to this nomalous wall motion is directly proportional to time, indicating that the magnitude of the negative capacitance component should be inversely related to frequency. This passes Jonscher’s test for the misinterpretation of positive inductance and gives confidence that field-induced motion of these specific charged domain walls generates a measurable negative capacitance contribu tion to the overall dielectric respons