Ngai Yui Chan

Palladium Nanoparticle Enhanced Giant Photoresponse at LaAlO3/SrTiO3 Two-Dimensional Electron Gas Heterostructures

With LaAlO3 surface modification by Pd nanoparticles, LaAlO3/SrTiO3 (LAO/STO) interfacial two-dimensional electron gas presents a giant optical switching effect with a photoconductivity on/off ratio as high as 750% under UV light irradiation. Pd nanoparticles with a size around 2 nm are deposited on top of the LAO surface, and the LAO/STO interface exhibits a giant response to UV light with a wavelength shorter than 400 nm. This giant optical switching behavior has been explained by the Pd nanoparticles catalytic effect and surface/interface charge coupling.

Ferroelectric, piezoelectric, and leakage current properties of (K0.48Na0.48Li0.04)(Nb0.775Ta0.225)O3 thin films grown by pulsed laser deposition

Lead-free (K0.48Na0.48Li0.04)(Nb0.775Ta0.225)O3 (KNLNT) thin films were deposited on Pt(111)/Ti/SiO2/Si(001) substrates using pulsed laser deposition. The film exhibited a well-defined ferroelectric hysteresis loop with a remnant polarization 2Pr of 22.6 μC/cm2 and a coercive field Ec of 10.3 kV/mm. The effective piezoelectric coefficient d33,f of the KNLNT thin films was found to be about 49 pm/V by piezoelectric force microscope. The dominant conduction mechanisms of Au/KNLNT/Pt thin film capacitor were determined to be bulk-limited space-charge-limited-current and Poole–Frenkle emission at low and high electric field strengths, respectively, within a measured temperature range of 130–370 K.

Enhanced Surface-and-Interface Coupling in Pd-Nanoparticle-coated LaAlO3/SrTiO3 Heterostructures: Strong Gas- and Photo-Induced Conductance Modulation

Pd nanoparticle (NP) coated LaAlO3/SrTiO3 (LAO/STO) heterointerface exhibits more notable conductance (G) change while varying the ambient gas (N2, H2/N2, and O2) and illuminating with UV light (wavelength: 365 nm) than a sample without the NPs. Simultaneous Kelvin probe force microscopy and transport measurements reveal close relationships between the surface work function (W) and G of the samples. Quantitative analyses suggest that a surface adsorption/desorption-mediated reaction and redox, resulting in a band-alignment modification and charge-transfer, could explain the gas- and photo-induced conductance modulation at the LAO/STO interface. Such surface-and-interface coupling enhanced by catalytic Pd NPs is a unique feature, quite distinct from conventional semiconductor hetero-junctions, which enables the significant conductance tunability at ultrathin oxide heterointerfaces by external stimuli.

New fabrication of high-frequency (100-MHz) ultrasound PZT film kerfless linear array [Correspondence]

The paper describes the design, fabrication, and measurements of a high-frequency ultrasound kerfless linear array prepared from hydrothermal lead zirconate titanate (PZT) thick film. The 15-μm hydrothermal PZT thick film with an area of 1 × 1 cm, obtained through a self-separation process from Ti substrate, was used to fabricate a 32-element 100-MHz kerfless linear array with photolithography. The bandwidth at -6 dB without matching layer, insertion loss around center frequency, and crosstalk between adjacent elements were measured to be 39%, -30 dB, and -15 dB, respectively.

Room-temperature large magnetic-dielectric coupling in new phase anatase VTiO4

The synthetic new-phase VTiO4, as a new solid solution structure of anatase type, brings a large magnetodielectric ratio (Δε/ε0) of 7.2% at 300 K, representing a new simple-oxide structural catalogue exhibiting a room-temperature large magnetic-dielectric effect.

Facile fabrication of highly ordered poly(vinylidene fluoride-trifluoroethylene) nanodot arrays for organic ferroelectric memory

Nano-patterned ferroelectric materials have attracted significant attention as the presence of two or more thermodynamically equivalent switchable polarization states can be employed in many applications such as non-volatile memory. In this work, a simple and effective approach for fabrication of highly ordered poly(vinylidene fluoride–trifluoroethylene) P(VDF-TrFE) nanodot arrays is demonstrated. By using a soft polydimethylsiloxane mold, we successfully transferred the 2D array pattern from the initial monolayer of colloidal polystyrene nanospheres to the imprinted P(VDF-TrFE) films via nanoimprinting. The existence of a preferred orientation of the copolymer chain after nanoimprinting was confirmed by Fourier transform infrared spectra. Local polarization switching behavior was measured by piezoresponse force microscopy, and each nanodot showed well-formed hysteresis curve and butterfly loop with a coercive field of ∼62.5 MV/m. To illustrate the potential application of these ordered P(VDF-TrFE) nanodot arrays, the writing and reading process as non-volatile memory was demonstrated at a relatively low voltage. As such, our results offer a facile and promising route to produce arrays of ferroelectric polymer nanodots with improved piezoelectric functionality.

Dynamic modulation of the transport properties of the LaAl O3/SrTi O3 interface using uniaxial strain

Among the interfacial transport modulations to the LaAlO3/SrTiO3 (LAO/STO) heterostructure, mechanical strain has been proven to be an effective approach by growing the LAO/STO films on different substrates with varying lattice mismatches to STO. However, this lattice-mismatch-induced strain effect is static and biaxial, hindering the study of the strain effect in a dynamic way. In this work we realize dynamic and uniaxial strain to the LAO/STO oxide heterostructure at low temperature, through mechanical coupling from a magnetostrictive template. This anisotropic strain results in symmetry breaking at the interface and induces further splitting of the electronic band structure and therefore produces different conductivities along the x and y in-plane directions. In particular, we observe that along the strained direction the interface conductivity decreases by up to 70% under a tensile strain, while it increases by 6.8% under a compressive strain at 2 K. Also, it is revealed that the modulation on the interfacial transport property can be anisotropic, i.e., the resistance changes differently when an excitation current is parallel or perpendicular to the strain direction. This approach of strain engineering provides another degree of freedom for control of transport properties of oxide heterostructures and opens an additional way to investigate strain effects in materials science.

Elastic softening near the phase transitions in (1 − x)Bi1/2Na1/2TiO3-xBaTiO3 solid solutions

Phase transitions in sol–gel-derived (1 − x)(Bi1/2Na1/2TiO3)-xBaTiO3 (0 x 0.06) solid solutions were investigated via dynamic mechanical, dielectric, and ferroelectric analyses. Structural phase transition was observed from both noncentrosymmetric ferroelectric to nonpolar (antiferroelectric) forms (FE-AFE) at temperatures ranging from ~120 °C to ~210 °C, and from antiferroelectric to paraelectric forms (AFE-PE) at temperatures ranging from ~250 °C to ~320 °C for 0 x 0.06. The former transition is accompanied by a significant and broad elastic softening or soft-mode process, and the latter is of typical diffused type. The dynamic mechanical scaling exponent and critical scaling exponent are used to characterize these two phase transitions, respectively. The temperature-dependent polarization showed an abnormal trend unlike other reported ferroelectrics, confirming the presence of two different phases: FE and AFE. The d33 piezoelectric constant increases with increasing x up to the morphotropic phase boundary. It is suggested that the dynamic mechanical scaling exponents could be used to characterize the mobility of the polar domains and the elastic softening processes, which are closely related to abnormal pyroelectric properties and piezoelastic hardening behaviors.

Photocatalytic microreactors for water purification: Selective control of oxidation pathways

This paper analyzes the different reaction pathways that play major roles in the process of photocatalytic degradation, and presents a novel microfluidic photoelectrocatalytic reactor for selective control of them. This is accomplished by applying positive or negative bias potential to the photocatalytic reaction chamber and to select either the hole-driven or electron-driven oxidation pathway. The experimental results show that the negative bias exhibits higher performance in degrading the model chemical of methylene blue. Such selective control brings several important benefits. It provides another degree of freedom for photocatalysis, and enables detailed kinetic study on the reaction mechanisms. And, the bias eliminates the electron/hole recombination, which is one of the fundamental limits in conventional photocatalytic systems. In additional, the experiment shows that the bias produces a synergetic effect of electrocatalysis and photocatalysis and significantly enhances the degradation efficiency. The photoelectrocatalytic microreactor shows high stability and may be scaled up for high-performance water purification.