Mengjun Bai

Determination of the optical dispersion in ferroelectric vinylidene fluoride (70%)/trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report measurements of the optical dispersion in ferroelectric Langmuir–Blodgett films of polyvinylidene fluoride (70%)-trifluoroethylene (30%) copolymer, using variable-angle spectroscopic ellipsometry over a wide spectral range from infrared to ultraviolet. Film thickness averaged 1.78±0.07 nm per deposition layer for films ranging from 5 to 125 deposition layers as determined from multi-sample analysis. This deposition rate was consistent with capacitance measurements, yielding a dielectric constant of 9.9±0.4 normal to the film, by quartz microbalance measurements, and by atomic force microscopy.

Ferroelectric nanomesa formation from polymer Langmuir–Blodgett films

We report the fabrication and characterization of nanoscale ferroelectric structures consisting of disk-shaped nanomesas averaging 8.7±0.4nm in height and 95±22nm in diameter, and nanowells 9.8±3.3nm in depth and 128±37nm in diameter, formed from Langmuir–Blodgett films of vinylidene fluoride copolymers after annealing in the paraelectric phase. The nanomesas retain the ferroelectric properties of the bulk material and so may be suitable for use in high-density nonvolatile random-access memories, acoustic transducer arrays, or infrared imaging arrays. The nanomesa and nanowell patterns may provide useful templates for nanoscale molding or contact-printing.

Effects of annealing conditions on ferroelectric nanomesa self-assembly

We report the results of studies of the effects of annealing conditions on the morphology of ferroelectric nanomesas. The nanomesa patterns were fabricated by self-assembly from continuous ultra-thin Langmuir–Blodgett fi lms of copolymers of vinylidene fl uoride and trifl uoroethylene. Annealing in the paraelectric phase induced surface reorganization into disc-shaped ferroelectric nanomesas approximately 9 nm thick and 100 nm in diameter. Several factors affect the nanomesa dimensions, such as polymer composition, substrate material, deposition conditions, and annealing temperature. The height and diameter of the nanomesas both increase with increasing annealing temperature. Annealing studies in the ferroelectric–paraelectric coexistence region show that only the paraelectric phase is mobile. From this we conclude that the paraelectric phase supports a kind of plastic crystalline fl ow connected with dynamic disorder of the polymer conformation.

Evidence for multiple polytypes of semiconducting boron carbide (C2B10) from electronic structure

Boron carbides fabricated via plasma enhanced chemical vapour deposition from different isomeric source compounds with the same C2B10H12 closo-icosahedral structure result in materials with very different direct (optical) band gaps. This provides compelling evidence for the existence of multiple polytypes of C2B10 boron carbide and is consistent with electron diffraction results.

Heterojunction diode fabrication from polyaniline and a ferroelectric polymer

We have fabricated a p–n heterojunction diode by vapor depositing a thin film of polyaniline on top of the crystalline copolymer: poly(vinylidene fluoride with trifluoroethylene). The formation of a diode is expected from the band offsets of the two polymers near the Fermi level. The interface between the two components was investigated, and an abrupt interface was found that is very different from the inorganic analog.

Nanomesa and nanowell formation in Langmuir–Blodgett polyvinylidene fluoride trifluoroethylene copolymer films

In this letter, we report an energetics-based model to explain the self-organizing nanomesa and nanowell patterns recently observed in Langmuir-Blodgett (LB) films of polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] copolymers. The feature size of nanomesas and nanowells has been estimated using a linear stability analysis, and the morphology of the nanomesas and nanowells has been revealed by the numerical simulation, both in good agreement with experiments. A number of other model predictions regarding the nanomesa and nanowell formation also agree with experimental observations. The model can be used to guide the design and optimization of nanostructures in the P(VDF-TrFE) LB films and possibly in other thin polymer films.

Langmuir-Blodgett films of polyethylene

The possibility to obtain surface layers on water and prepare solid multilayer Langmuir–Blodgett films of medium-density polyethylene is shown. The polymer film on water is stable, demonstrates a reversible surface pressure-area isotherm up to 15 mN/m, and can be deposited onto a substrate using the Langmuir–Blodgett technique in a wide range of surface pressures. The thickness of a single deposited layer is 5.1 nm on average. The dielectric and optical constants of multilayer films are near their bulk values. The films exhibit high dielectric strength of at least 200 MV/m.

Infrared spectroscopic ellipsometry study of vinylidene fluoride (70%)-trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report the studies of the molecular conformation and chain orientations through characterization of the vibrational modes in crystalline Langmuir–Blodgett films of the polyvinylidene fluoride/trifluoroethylene copolymer. The infrared spectra obtained by polarized reflectometry and ellipsometry showed that the ferroelectric phase has predominantly all-trans conformation and the paraelectric phase has predominantly alternating trans-gauche conformation, as in solvent-formed films of the same copolymer. The results showed that the polymer chains are predominantly parallel to the film plane with a random in-plane orientation and most of the ferroelectric phase vibrational mode behaviors are consistent with the published mode assignments. The ferroelectric phase optical dispersion curves in the infrared range were extracted from the data analysis based on a uniaxial model.

Magnetoelectric effects in ferromagnetic cobalt/ferroelectric copolymer multilayer films

Interactions between a ferromagnet, cobalt, and a ferroelectric copolymer, poly(vinylidene fluoride with trifluoroethylene) in thin film heterostructures result in a 5% change in the ferroelectric polarization on application of a perpendicular 6 kG magnetic field corresponding to a magnetoelectric coupling coefficient of α=5.45 V/cm Oe. The effect disappears on magnetic saturation, ruling out conventional strain coupling. A simple model posits that the ferroelectric film develops in-plane strain gradients, a consequence of the coupling to strain gradients present at the domain walls in the multidomain Co layer, resulting in the measured polarization change via the flexoelectric effect.

Mesoscopic structures in two-dimensional ferroelectric polymers

Abstract Unique insight into the nature of ferroelectricity is emerging from the study of the first two-dimensional ferroelectric films, which are made by Langmuir-Blodgett deposition of vinylidene fluoride copolymers. These films are the first truly two-dimensional ferroelectrics, with thickness-independent bulk ferroelectric properties and a separate surface phase transition. The films offer a unique system for the study the relationship between ferroelectric properties and mesoscopic structure. The structure at all length scales was probed using complementary techniques: X-ray and neutron diffraction, scanning tunneling microscopy, atomic force microscopy, electric field microscopy, low-energy electron diffraction, angle-resolved inverse photoemission spectroscopy, infrared ellipsometry, electron energy-loss spectroscopy, scanning electron microscopy, optical polarization microscopy. We also introduce new results from dynamic pyroelectric scanning microscopy.