Evolution of microstructure, strain and physical properties in oxide nanocomposite films.

We, using LSMO:ZnO nanocomposite films as a model system, have studied the effect of film thickness on the physical properties of nanocomposites. It shows that strain, microstructure, as well as magnetoresistance strongly rely on film thickness. The magnetotransport properties have been fitted by a modified parallel connection channel model, which is in agreement with the microstructure evolution as a function of film thickness in nanocomposite films on sapphire substrates. The strain analysis indicates that the variation of physical properties in nanocomposite films on LAO is dominated by strain effect. These results confirm the critical role of film thickness on microstructures, strain states, and functionalities. It further shows that one can use film thickness as a key parameter to design nanocomposites with optimum functionalities.

Room Temperature Ferrimagnetism and Ferroelectricity in Strained, Thin Films of BiFe0.5Mn0.5O3.

Highly strained films of BiFe0.5Mn0.5O3 (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (TC ∼ 600K), with a room temperature saturation moment (MS ) of up to 90 emu/cc (∼ 0.58 µB /f.u) on high quality (001) SrTiO3. X-ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe3+ and Mn3+. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.

A new class of room-temperature multiferroic thin films with bismuth-based supercell structure.

Intergrowth of two partially miscible phases of BiFeO(3) and BiMnO(3) gives a new class of room-temperature multiferroic phase, Bi(3) Fe(2) Mn(2) O(10+δ) , which has a unique supercell (SC) structure. The SC heterostructures exhibit simultaneously room-temperature ferrimagnetism and remanent polarization. These results open up a new avenue for exploring room-temperature single-phase multiferroic thin films by controlling the phase mixing of two perovskite BiRO(3) (R = Cr, Mn, Fe, Co, Ni) materials.

Extremely high tunability and low loss in nanoscaffold ferroelectric films.

There are numerous radio frequency and microwave device applications which require materials with high electrical tunability and low dielectric loss. For phased array antenna applications there is also a need for materials which can operate above room temperature and which have a low temperature coefficient of capacitance. We have created a nanoscaffold composite ferroelectric material containing Ba(0.6)Sr(0.4)TiO(3) and Sm(2)O(3) which has a very high tunability which scales inversely with loss. This behavior is opposite to what has been demonstrated in any previous report. Furthermore, the materials operate from room temperature to above 150 °C, while maintaining high tunability and low temperature coefficient of tunability. This new paradigm in dielectric property control comes about because of a vertical strain control mechanism which leads to high tetragonality (c/a ratio of 1.0126) in the BSTO. Tunability values of 75% (200 kV/cm field) were achieved at room temperature in micrometer thick films, the value remaining to >50% at 160 °C. Low dielectric loss values of <0.01 were also achieved, significantly lower than reference pure films.

Growth dynamics of barium titanate thin films on polycrystalline Ni foils using polymer-assisted deposition technique.

Polymer-assisted deposition (PAD) technique was developed to fabricate ferroelectric BaTiO(3) (BTO) thin films directly on polycrystalline nickel foils. The growth dynamics was systematically studied to optimize the single-phase BTO films with good dielectric properties. It is critical to pretreat nickel foils with hydrogen peroxide (H(2)O(2)) solution to form thin nickel oxide layers on the surfaces for the growth of BTO films. Both the concentration of H(2)O(2) solution and the pretreated time were found to strongly affect the dielectric constant of BTO films, which may be associated with the oxygen diffusion from nickel oxide buffer layers to BTO layers during annealing. The BTO thin films with optimized growth conditions have good crystal structure and electrical properties, suggesting that the as-grown BTO films by PAD technique can be utilized for new devices development and energy storage applications.

High quality p-type Ag-doped ZnO thin films achieved under elevated growth temperatures.

By correlating the effects of substrate temperature, oxygen pressure and laser energy on the electrical and microstructural properties of Ag-doped ZnO films grown on a sapphire (0001) substrate, p-type conductivity is achieved under various substrate temperatures in the wide range of 250-750 °C. All of the samples were deposited by pulsed-laser deposition under various designed conditions. Hall measurements indicate that the best conductivity is achieved in Ag-ZnO films under a substrate temperature of 500 °C, a partial oxygen pressure of 250-300 mTorr and laser energy between 330 and 345 mJ. The hole-carrier concentration is 2.29 × 10(18) cm(-3), the resistivity is 0.9 Ω cm and the mobility is 3.03 cm(2) V(-1) s(-1). Transmission-electron microscopy (TEM) studies on the p-type films reveal similar microstructural properties to one another, but different properties to that of the n-type films deposited at the same temperatures with different deposition parameters.

Self-separated PZT thick films with bulk-like piezoelectric and electromechanical properties.

Self-separated Pb(Zr(0.52)Ti(0.48))O(3) (PZT) films were processed by a hydrothermal deposition and a rapid thermal separation method, followed by a sol-gel filling and sintering process. The films possess excellent piezoelectric and electromechanical properties close to those of bulk material. The maximum remnant polarization is over 30 µC/cm(2) and the electromechanical coupling factor (k(t)) reaches as high as 0.52. The unique microstructure characteristics of the PZT films, such as their highly dense structure, columnar grains, well-connected grain boundaries, and well-dispersed nanopores, could all contribute to the enhanced piezoelectric and electromechanical properties.

Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain.

Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO(3), is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO(3) (ref. 4) and SrTiO(3) (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO(3) to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm(2)O(3) throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO(3) matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO(3) transition temperatures so far.

Microstructure, magnetic, and low-field magnetotransport properties of self-assembled (La0.7Sr0.3MnO3)0.5:(CeO2)0.5 vertically aligned nanocomposite thin films.

Two-phase (La(0.7)Sr(0.3)MnO(3))(0.5):(CeO(2))(0.5) (LSMO:CeO(2)) heteroepitaxial nanocomposite films were grown on SrTiO(3) (STO) (001) by pulsed laser deposition (PLD). X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that LSMO:CeO(2) films epitaxially grow on STO as self-assembled vertically aligned nanocomposite (VAN). Magnetic and magnetotransport measurements demonstrate that the LSMO phase in the VAN structure behaves differently from its epitaxial single-phase counterpart, e.g. greatly enhanced coercivity (H(C)) and low-field magnetoresistance (LFMR). The enhanced properties in the VAN system are attributed to the interaction between the perovskite and the secondary phase or phase boundary. The results suggest that the growth of functional oxide in another oxide matrix with vertical heteroepitaxial form is a promising approach to achieve new functionality that may not be easily realized in the single epitaxial phase.

Nanoporous thin films with controllable nanopores processed from vertically aligned nanocomposites.

Porous thin films with ordered nanopores have been processed by thermal treatment on vertically aligned nanocomposites (VAN), e.g., (BiFeO(3))(0.5):(Sm(2)O(3))(0.5) VAN thin films. Uniformly distributed nanopores with an average diameter of 60 nm and 150 nm were formed at the bottom and top of the nanoporous films, respectively. Controllable porosity can be achieved by adjusting the microstructure of VAN (BiFeO(3)):(Sm(2)O(3)) thin films and the annealing parameters. In situ heating experiments within a transmission electron microscope (TEM) column at temperatures from 25 to 850 degrees C, provides significant insights into the phase transformation, evaporation and structure reconstruction during the annealing. The in situ experiments also demonstrate the possibility of processing vertically aligned nanopores (VANP) with one phase stable in a columnar structure. These nanoporous thin films with controllable pore size and density could be promising candidates for thin film membranes and catalysis for fuel cell and gas sensor applications.

Growth-controlled surface roughness in Al-doped ZnO as transparent conducting oxide.

The surface morphology of Al(2)O(3)-doped ZnO (AZO, 2 wt%) thin films varies from a uniform layer to nanorod structure by simply controlling oxygen pressure during growth. All AZO films were deposited on sapphire(0001) substrates using a pulsed laser deposition (PLD) technique. In the low oxygen pressure regime (vacuum approximately 50 mTorr), AZO films grow as a smooth and uniform layer. In the high oxygen pressure regime (100-250 mTorr) AZO thin films with nanorods have formed. Detailed cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) studies reveal that, besides the obvious variation in the film morphology, the in-plane d spacing of AZO film increases and the out-of-plane d spacing decreases, as oxygen pressure increases. A bilayer AZO film with a nanorod structure on top of a uniform layer was demonstrated by controlling the oxygen pressure for the two layers. Electrical resistivity and optical transmittance measurements were carried out to correlate with the microstructures obtained under different oxygen pressures. The bilayer AZO films could find applications as a transparent conducting oxide (TCO) with a unique light trapping function in thin film solar cells.

Epitaxial ternary nitride thin films prepared by a chemical solution method.

It is indispensable to use thin films for many technological applications. This is the first report of epitaxial growth of ternary nitride AMN2 films. Epitaxial tetragonal SrTiN2 films have been successfully prepared by a chemical solution approach, polymer-assisted deposition. The structural, electrical, and optical properties of the films are also investigated.