Zhijun Hu

Regular arrays of highly ordered ferroelectric polymer nanostructures for non-volatile low-voltage memories

Ferroelectric nanostructures are attracting tremendous interest because they offer a promising route to novel integrated electronic devices such as non-volatile memories and probe-based mass data storage. Here, we demonstrate that high-density arrays of nanostructures of a ferroelectric polymer can be easily fabricated by a simple nano-embossing protocol, with integration densities larger than 33 Gbits inch(-2). The orientation of the polarization axis, about which the dipole moment rotates, is simultaneously aligned in plane over the whole patterned region. Internal structural defects are significantly eliminated in the nanostructures. The improved crystal orientation and quality enable well-defined uniform switching behaviour from cell to cell. Each nanocell shows a narrow and almost ideal square-shaped hysteresis curve, with low energy losses and a coercive field of approximately 10 MV m(-1), well below previously reported bulk values. These results pave the way to the fabrication of soft plastic memories compatible with all-organic electronics and low-power information technology.

Effect of Nanoconfinement on the Collapse Transition of Responsive Polymer Brushes.

Nanopatterned brushes of a thermo-responsive polymer, poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO2MA), displaying a collapse temperature in the physiological range were synthesized for grafting diameters from a few micrometers down to 35 nm. The reversible collapse transition of the nanobrushes was studied in water as a function of their lateral confinement, down to ensembles of brushes containing only approximately 300 chains. The confinement results in a considerable broadening of the collapse transition and in an increase of the degree of vertical swelling, which can be explained by the internal structure of the nanodroplets derived from a theoretical model of dry nanobrushes. These results enable the rational design of responsive surfaces having a tunable topography engineered at the nanometer scale, which is of direct interest for the development of soft nanoactuators and new substrates for cell adhesion studies.

Control of crystal orientation in soft nanostructures by nanoimprint lithography

Nanoimprint lithography (NIL) is a low-cost and high resolution technique consisting of replicating the nanofeatures of a hard mold by pressing it into a film of a soft material, in order to create nanostructures of virtually any shape. When NIL is applied to materials capable to self-organize, it may also control their crystallographic orientation and morphology provided imprinting conditions are appropriately selected. The principles governing this ordering involve a decreased nucleation probability in the nanostructures, graphoepitaxial alignment, rheological chain alignment, nanoconfinement in the cavities of the mold, and the effect of pressure on the microstructure of soft materials. The ordering can be rationalized based on the knowledge of the structure of the material and more specifically on the notion of basic structural element of the material. The control over the internal structure of nanomolded materials afforded by NIL translates into improved performance of soft functional nanodevices, as is demonstrated in this highlight.

Nanoscale Design of Multifunctional Organic Layers for Low-Power High-Density Memory Devices

We demonstrate the design of a multifunctional organic layer by the rational combination of nanosized regions of two functional polymers. Instead of relying on a spontaneous and random phase separation process or on the tedious synthesis of block copolymers, the method involves the nanomolding of a first component, followed by the filling of the resulting open spaces by a second component. We apply this methodology to fabricate organic nonvolatile memory diodes of high density. These are built by first creating a regular array of ferroelectric nanodots by nanoimprint lithography, followed by the filling of the trenches separating the ferroelectric nanodots with a semiconducting polymer. The modulation of the current in the semiconductor by the polarization state of the ferroelectric material is demonstrated both at the scale of a single semiconductor channel and in a microscopic device measuring about 80,000 channels in parallel, for voltages below ca. 2 V. The fabrication process, which combines synergetically orthogonal functional properties with a fine control over their spatial distribution, is thus demonstrated to be efficient over large areas.

An organic ferroelectric field effect transistor with poly(vinylidene fluoride-co-trifluoroethylene) nanostripes as gate dielectric

We demonstrate the fabrication of an organic Ferroelectric Field Effect Transistor (FeFET) incorporating a ferroelectric gate dielectric made of nanostripes obtained by nanoimprinting poly(vinylidene fluoride-co-trifluoroethylene) over a layer of semiconducting poly(triarylamine). The imprinting process results in a decreased switching voltage for the ferroelectric, by a factor of ca. 1.5, resulting in a decreased operating voltage compared to a reference FeFET with a continuous ferroelectric layer. The transistor consists of a large number of nanostripe-gated transistors placed in parallel, which also offers interesting possibilities for a strong size reduction of organic FeFETs.

Multiferroic Nanopatterned Hybrid Material with Room-Temperature Magnetic Switching of the Electric Polarization.

A nanopatterned hybrid layer is designed, wherein the electric polarization can be flipped at room temperature by a magnetic field aided by an electrical field. This is achieved by embedding ferromagnetic nanopillars in a continuous organic ferroelectric layer, and amplifying the magnetostriction-generated stress gradients by scaling down the supracrystalline cell of the material.

Two-Step Polarization Switching in Ferroelectric Polymers.

The correlation between hierarchical structures and polarization switching in ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene) has been probed by combining transmission electron microscopy studies with piezoresponse force microscopy observations. Differences are demonstrated between homogeneous and anisotropic thin films with well-defined lamellar orientation, with the later exhibiting quadrangular domain shape and double hysteresis. We propose that the polarization switching within lamella is dominated by domain wall flow motion, while the amorphous components between lamellae impede full polarization switching. The coupling between lamellae is controlled by a creep process. These results and interpretations explain well the seemingly contradicting polarization reversal dynamics reported and offer opportunities to change domain reversal speed by making ferroelectric polymer nanostructures.

Organic ferroelectric/semiconducting nanowire hybrid layer for memory storage

Ferroelectric materials are important components of sensors, actuators and non-volatile memories. However, possible device configurations are limited due to the need to provide screening charges to ferroelectric interfaces to avoid depolarization. Here we show that, by alternating ferroelectric and semiconducting nanowires over an insulating substrate, the ferroelectric dipole moment can be stabilized by injected free charge carriers accumulating laterally in the neighboring semiconducting nanowires. This lateral electrostatic coupling between ferroelectric and semiconducting nanowires offers new opportunities to design new device architectures. As an example, we demonstrate the fabrication of an elementary non-volatile memory device in a transistor-like configuration, of which the source-drain current exhibits a typical hysteretic behavior with respect to the poling voltage. The potential for size reduction intrinsic to the nanostructured hybrid layer offers opportunities for the development of strongly miniaturized ferroelectric and piezoelectric devices.

Controllable Hierarchical Surface Patterns of Supramolecular Hydrogels: Harnessing Buckling Instability by Confinement

Patterned surfaces of responsive polymers find applications in diverse fields. However, it is still a great challenge to fabricate hierarchical patterns with long-range orders. Herein controllable hierarchical surface patterns that can be fabricated by combining nanoembossing techniques with the surface instability of supramolecular hydrogels are presented. Nanoembossed nanostripe arrays of polyethylene glycol (PEG)-based polyurethane-urea supramolecular hydrogels are fabricated and exposed to water, whereby the lateral expansion of nanostripes is confined and leads to the formation of folded in-plane or out-of-plane patterns depending on the aspect ratios. The direction of folds is perpendicular to the nanostripes. Both the amplitude and the wavelength of out-of-plane folds are proportional to the thickness of nanostripes. Therefore, hierarchical structures, in which one periodicity is defined by the nanoembossing processes and the other is determined by surface buckling, can be quickly fabricated in supramolecular hydrogel thin films.

Local polarization switching in stressed ferroelectric polymers

Ferroelectric polymers are used in flexible organic ferroelectric memories, ferroelectric polarization enhanced organic solar cells, and organic multiferroics. Therefore, understanding their polarization switching mechanism under bending is important for the operation of such devices. Here, we study locally by piezoresponse force microscopy (PFM) polarization switching in bent thin films of the ferroelectric polymer poly(vinylidene fluoride-ran-trifluoroethylene). In bent samples, higher probability of domain nucleation, faster domain wall propagation, and lower coercive field are consistently observed by PFM. We ascribe these observations to a decrease of the domain wall pinning energy, resulting from the mechanical energy stored in the sample due to bending in the presence of the compression gradient generated below the PFM tip.