Nathaniel Ng

Manipulating Ferroelectric Domains in Nanostructures Under Electron Beams

Freestanding BaTiO3 nanodots exhibit domain structures characterized by distinct quadrants of ferroelastic 90° domains in transmission electron microscopy (TEM) observations. These differ significantly from flux-closure domain patterns in the same systems imaged by piezoresponse force microscopy. Based upon a series of phase field simulations of BaTiO3 nanodots, we suggest that the TEM patterns result from a radial electric field arising from electron beam charging of the nanodot. For sufficiently large charging, this converts flux-closure domain patterns to quadrant patterns with radial net polarizations. Not only does this explain the puzzling patterns that have been observed in TEM studies of ferroelectric nanodots, but also suggests how to manipulate ferroelectric domain patterns via electron beams.

Anomalous polarization switching in organic ferroelectric field effect transistors

The authors demonstrate organic ferroelectric field effect transistors using poly(vinylidene fluoride-trifluoroethylene) as dielectric in bottom common gate and patterned gate devices. Drain current hysteresis is resulted from the dipole switching at channel region due to gate-source bias. For common gate device, an additional anomalous polarization switching is observed due to gate-drain bias. This switching has no effect on the hysteresis direction yet incurs a strong peak in the off drain current leading to unstable and uncontrollable off state in memory device. Reduction of gate-drain overlapping using patterned metal gate shows diminishing the anomalous switching hence improves performance of the ferroelectric transistors.

Width of phonon sidebands in the Brownian oscillator model

The line width dependence of zero-phonon lines and phonon sidebands on temperature, bath dissipation, and electron-phonon coupling is studied for an underdamped Brownian oscillator model with an Ohmic dissipative bath. Factors determining the line widths vary from the zero-phonon lines to the phonon sidebands. The control-parameter space of line broadening has been mapped out, revealing that the line widths of the zero-phonon lines and phonon sidebands are linearly dependent on both the temperature and the Huang-Rhys factor. It is also found that the dependence of the line widths on the bath damping factor is linear for the zero-phonon lines and quadratic for the phonon sidebands.

Lateral size and thickness dependence in ferroelectric nanostructures formed by localized domain switching

Abstract Ferroelectric nanostructures can be formed by local switching of domains using techniques such as piezo-force microscopy (PFM). Understanding the dependence of the switching behavior on the lateral size of the electrode is important to determine the minimum feature size for writing ferroelectric nanostructures. To understand these lateral size effects, we use the time-dependent Ginzburg–Landau equations in a two-dimensional square to rectangle ferroelectric transition to simulate localized switching of domains for PFM-type and parallel-plate capacitor configurations. Our investigations indicate that fringing electric fields lead to switching via intermediate 90° domains even in the absence of substrate or clamping effects for films of sufficient thicknesses, and via 180° rotations at smaller thicknesses. The voltage required to switch the domain increases by decreasing the lateral size, and at very small lateral sizes the coercive voltage becomes so large that it becomes virtually impossible to switch the domain.

Electron-phonon interactions in ce3+-doped yttrium aluminum garnet nanophosphors.

Ce 3+-doped yttrium aluminum garnet nanophosphors with sizes near 30 and 250 nm have been synthesized by using chemical gelation and solvothermal methods, respectively. The size-dependent electron-longitudinal-optical-phonon coupling is investigated by fitting measured photoluminescence spectra within the framework of the Brownian oscillator model. Results show that the coupling strength is in a decreasing order from the bulk material to the nanophosphors of much smaller sizes.

Surface morphology effects on polarization switching in nanoscale ferroelectrics

The effects of surface morphology on polarization switching in thin ferroelectric films are investigated using a real-space, time-dependent Ginzburg-Landau model that incorporates electrostatic interactions. We consider a two-dimensional uni-axial ferroelectric film with a thickness that varies sinusoidally. Polarization switching, starting from a single domain remnant state, is simulated for several surface modulation amplitudes and wavelengths. We demonstrate that surface heterogeneities produce inhomogeneities in the electric field within the film. These inhomogeneities become preferential sites for easy nucleation of reverse domains. This has a profound effect on the external field necessary to switch the polarization. Increasing the surface undulation amplitude significantly reduces the coercive field compared to the ideal flat film, even for very small amplitude modulations in the thickness. Although surface roughness decreases the field required to form reverse domains, it also hinders subsequent domain wall migration. In fact, for very high amplitude and small wavelength surface morphologies, complete switching to a single domain state becomes impossible. This is because the domain walls become trapped near the peaks in the modulated surface. The technological implications of the present results for utilization of surface roughness and for surface morphology design are discussed.

Depletion-layer-induced size effects in ferroelectric thin films: A Ginzburg-Landau model study

A Ginzburg-Landau model is used to demonstrate how depletion layers give rise to thickness-dependent ferroelectric properties in thin films. It is shown that free charge layers at the film-electrode interface can result in an internal electric field in the bulk of the film even when no external voltage is applied. At high values of the donor dopant density and small thicknesses, this internal electric field can be strong enough to lead to the formation of a domain pattern. This causes a drop in the remnant polarization; a direct demonstration of the important role free charge plays in thin ferroelectric films.

Surface morphology induced localized electric field and piezoresponse enhancement in nanostructured thin films.

Nanostructured piezoelectric and ferroelectric thin films are being increasingly used in sensing and actuating microdevices. In this work, we report the experimental discovery of localized electric field enhancement in nanocolumnar piezoelectric thin films and its significant impact on piezoresponse. The magnitude of electric field enhancement is associated with nonflat surface morphologies and is in agreement with theoretical and finite element models. The influence of this surface morphology induced enhancement on piezoresponse is demonstrated using phase field simulations, which also illustrates surface morphology induced strain enhancement. The observed enhancement can be effectively harnessed to improve the sensitivity of related piezoelectric thin film applications.

Electron-beam driven relaxation oscillations in ferroelectric nanodisks

Using a combination of computational simulations, atomic-scale resolution imaging and phenomenological modelling, we examine the underlying mechanism for nanodomain restructuring in lead zirconate titanate nanodisks driven by electron beams. The observed subhertz nanodomain dynamics are identified with relaxation oscillations where the charging/discharging cycle time is determined by saturation of charge traps and nanodomain wall creep. These results are unusual in that they indicate very slow athermal dynamics in nanoscale systems, and possible applications of gated versions are discussed.