J.E. ten Elshof

Dielectrophoretic alignment of metal and metal oxide nanowires and nanotubes: A universal set of parameters for bridging prepatterned microelectrodes

Nanowires and nanotubes were synthesized from metals and metal oxides using templated cathodic electrodeposition. With templated electrodeposition, small structures are electrodeposited using a template that is the inverse of the final desired shape. Dielectrophoresis was used for the alignment of the as-formed nanowires and nanotubes between prepatterned electrodes. For reproducible nanowire alignment, a universal set of dielectrophoresis parameters to align any arbitrary nanowire material was determined. The parameters include peak-to-peak potential and frequency, thickness of the silicon oxide layer, grounding of the silicon substrate, and nature of the solvent medium used. It involves applying a field with a frequency >10(5) Hz, an insulating silicon oxide layer with a thickness of 2.5 μm or more, grounding of the underlying silicon substrate, and the use of a solvent medium with a low dielectric constant. In our experiments, we obtained good results by using a peak-to-peak potential of 2.1 V at a frequency of 1.2 × 10(5) Hz. Furthermore, an indirect alignment technique is proposed that prevents short circuiting of nanowires after contacting both electrodes. After alignment, a considerably lower resistivity was found for ZnO nanowires made by templated electrodeposition (2.2-3.4 × 10(-3) Ωm) compared to ZnO nanorods synthesized by electrodeposition (10 Ωm) or molecular beam epitaxy (MBE) (500 Ωm).

Local control over nucleation of epitaxial thin films by seed layers of inorganic nanosheets.

Nanosheets of Ti0.87O2 and Ca2Nb3O10 were synthesized and transferred onto Si substrates by Langmuir-Blodgett deposition. Using pulsed laser deposition, SrRuO3 films were formed on top of these samples. The underlying nanosheets determined both the morphology and crystallographic orientation of the films. SrRuO3 grew preferentially in the [110]pc direction on Ti0.87O2 nanosheets, while growth proceeded in the [001]pc direction on Ca2Nb3O10 nanosheets (pc refers to the pseudocubic unit cell of SrRuO3). Besides macroscopic control over the out-of-plane crystal direction, single crystal orientations were measured by electron backscatter diffraction on the level of individual nanosheets, indicating that epitaxial growth was achieved on the nanosheets as imposed by their well-defined crystal lattices. The nanosheets also had a clear effect on the magnetic properties of the films, which showed anisotropic behavior only when a seed layer was used. A monolayer consisting of a mixture of both types of nanosheets was made to locally control the nucleation of SrRuO3. In this context, SrRuO3 was used as model material, as it was used to illustrate that nanosheets can be a unique tool to control the orientation of films on a (sub-)micrometer length scale. This concept may pave the way to the deposition of various other functional materials and the fabrication of devices where the properties are controlled locally by the different crystallographic orientations.

Tribochemistry of Bismuth and Bismuth Salts for Solid Lubrication.

One of the main trends in the past decades is the reduction of wastage and the replacement of toxic compounds in industrial processes. Some soft metallic particles can be used as nontoxic solid lubricants in high-temperature processes. The behavior of bismuth metal particles, bismuth sulfide (Bi2S3), bismuth sulfate (Bi2(SO4)3), and bismuth oxide (Bi2O3) as powder lubricants was studied in a range of temperatures up to 580 °C. The mechanical behavior was examined using a high-temperature pin-on-disc setup, with which the friction force between two flat-contact surfaces was recorded. The bismuth-lubricated surfaces showed low coefficients of friction (μ ≈ 0.08) below 200 °C. Above the melting temperature of the metal powder at 271 °C, a layer of bismuth oxide developed and the friction coefficient increased. Bismuth oxide showed higher friction coefficients at all temperatures. Bismuth sulfide exhibited partial oxidation upon heating but the friction coefficient decreased to μ ≈ 0.15 above 500 °C, with the formation of bismuth oxide-sulfate, while some bismuth sulfate remained. All surfaces were studied by X-ray diffraction (XRD), confocal microscopy, high-resolution scanning electron microscopy (HR-SEM), and energy-dispersive X-ray spectroscopy (EDS). This study reveals how the partial oxidation of bismuth compounds at high temperatures affects their lubrication properties, depending on the nature of the bismuth compound.

Surface micropatterning with zirconia and calcium phosphate ceramics by micromoulding in capillaries

An increasing demand exists for biomaterials that are able to actively participate in the process of repair and regeneration of damaged or diseased organs and tissues. Patterning of surfaces of biomaterials with distinct chemical or physical cues is an attractive way to obtain spatial control over their interactions with the biological system. In the current study, micromoulding in capillaries method was used to pattern silicon substrates with bioinert yttria-stabilised zirconia or with bioactive calcium phosphate ceramics, both widely used biomaterials in orthopaedics and dentistry. Micrometer-scale patterns consisted of parallel lines with varying width and spacing. Both ceramics were successfully deposited on the substrate in a pattern defined by the mould. While the yttria-stabilised zirconia pattern was highly homogenous and smooth (Rq = 5.5 nm), the calcium phosphate pattern, consisting of dicalcium phosphate anhydrous before, and of β-tricalcium phosphate after annealing, exhibited a less homogenous morphology and higher roughness (Rq = 893 nm). Both materials allowed attachment and proliferation of the MG-63 osteosarcoma cell line, independent of the pattern used. While a preferential orientation of cells in the direction of the pattern lines was observed for all patterns, this effect was more pronounced on the lines with a width of up to 20 μm on both yttria-stabilised zirconia and calcium phosphate ceramics, as compared to wider patterns. Furthermore, the cells retained an elongated morphology for a longer period of time on narrow patterns. Micromoulding in capillaries appeared to be a suitable method to pattern both types of ceramics, however further optimisation is needed to improve homogeneity and obtain better control over the chemical phase and crystalline structure of calcium phosphate patterns.

Control of oxygen sublattice structure in ultra-thin SrCuO2 films studied by X-ray photoelectron diffraction

Epitaxial and atomically smooth ultra-thin SrCuO2 films are grown on SrTiO3 substrates using pulsed laser deposition. The structural and chemical aspects of these single-layer films of various thickness are characterized using in situ X-ray photoelectron diffraction (XPD) and photoelectron spectroscopy. By comparing XPD scans to multiple-scattering electron diffraction simulations, we demonstrate a structural transformation from bulk-planar to chain-type SrCuO2 as the film thickness is reduced from 9 to 3 unit-cells. This observation is in agreement with the recent theoretical prediction [Z. Zhong, G. Koster, and P. J. Kelly, Phys. Rev. B 85, 121411(R) (2012)] and opens new pathways for structural tuning in ultra-thin films of polar cuprates

Film transfer enabled by nanosheet seed layers on arbitrary sacrificial substrates

An approach for film transfer is demonstrated that makes use of seed layers of nanosheets on arbitrary sacrificial substrates. Epitaxial SrTiO3, SrRuO3, and BiFeO3 films were grown on Ca2 Nb 3O10 nanosheet seed layers on phlogopite mica substrates. Cleavage of the mica substrates enabled film transfer to flexible polyethylene terephthalate substrates. Electron backscatter diffraction, X-ray diffraction, and atomic force microscopy confirmed that crystal orientation and film morphology remained intact during transfer. The generic nature of this approach is illustrated by growing films on zinc oxide substrates with a nanosheet seed layer. Film transfer to a flexible substrate was accomplished via acid etching

Mass-fractal growth in niobia/silsesquioxane mixtures: a small-angle X-ray scattering study

The nucleation and growth of niobium pentaethoxide-derived clusters in ethanol was monitored at 298–333u2005K by small-angle X-ray scattering. The data were analyzed with a newly derived model for polydisperse mass-fractal-like structures.

Tuning of large piezoelectric response in nanosheet-buffered lead zirconate titanate films on glass substrates

Renewed interest has been witnessed in utilizing the piezoelectric response of PbZr0.52Ti0.48O3 (PZT) films on glass substrates for applications such as adaptive optics. Accordingly, new methodologies are being explored to grow well-oriented PZT thin films to harvest a large piezoelectric response. However, thin film piezoelectric response is significantly reduced compared to intrinsic response due to substrate induced clamping, even when films are well-oriented. Here, a novel method is presented to grow preferentially (100)-oriented PZT films on glass substrates by utilizing crystalline nanosheets as seed layers. Furthermore, increasing the repetition frequency up to 20u2009Hz during pulsed laser deposition helps to tune the film microstructure to hierarchically ordered columns that leads to reduced clamping and enhanced piezoelectric response evidenced by transmission electron microscopy and analytical calculations. A large piezoelectric coefficient of 250u2009pm/V is observed in optimally tuned structure which is more than two times the highest reported piezoelectric response on glass. To confirm that the clamping compromises the piezoelectric response, denser films are deposited using a lower repetition frequency and a BiFeO3 buffer layer resulting in significantly reduced piezoelectric responses. This paper demonstrates a novel method for PZT integration on glass substrates without compromising the large piezoelectric response.

Effect of Water Addition during Preparation on the Early-Time Photodynamics of CH3NH3PbI3 Perovskite Layers

The effect of water addition during preparation of a CH3 NH3 PbI3 layer on the photodynamics is studied by femtosecond transient absorption. Both the regular perovskite and the aqueous analogue show charge thermalisation on a timescale of about 500u2005fs. This process is, however, less pronounced in the latter layer. The spectral feature associated with hot charges does not fully decay on this timescale, but also shows a long-lived (sub-ns) component. As water molecules may interfere with the hydrogen bonding between the CH3 NH3+ cations and the inorganic cage, this effect is possibly caused by immobilisation of cation motion, suggesting a key role of CH3 NH3+ dipole reorientation in charge thermalisation. This effect shows the possibility of controlling hot charge carrier cooling to overcome the Shockley-Queisser limit.

Polarizaton recovery in lead zirconate titanate thin films deposited on nanosheets-beffered Si (oo1)

Fatigue behavior of Pb(Zr,Ti)O3 (PZT) films is one of the deterrent factors that limitsnthe use of these films in technological applications. Thus, understanding and minimizationnof the fatigue behavior is highly beneficial for fabricating reliable devicesnusing PZT films. We have investigated the fatigue behavior of preferentially orientednPZT films deposited on nanosheets-buffered Si substrates using LaNiO3 bottom andntop electrodes. The films show fatigue of up to 10% at 100 kHz, whereas no fatiguenhas been observed at 1 MHz. This frequency dependence of the fatigue behavior isnfound to be in accordance with Dawber–Scott fatigue model that explains the origin ofnthe fatigue as migration of oxygen vacancies. Interestingly, a partial recovery of remnantnpolarization up to 97% of the maximum value is observed after 4 109 cyclesnwhich can be further extended to full recovery by increasing the applied electric field.nThis full recovery is qualitatively explained using kinetic approach as a manifestationnof depinning of domains walls. The understanding of the fatigue behavior andnpolarization recovery that is explained in this paper can be highly useful in developingnmore reliable PZT devices