Chemical and biological sensors based on ZnO microwires usually rely on the change in the wire conductance with the ambient gas composition. Yet, sensitivity and recovery time of the conductance are important limitations in these applications. We treated ZnO:Sb micro-wires with single droplets of solvents for very short times and found a significant enhancement of the persistent photo-conductance and a reduction of the recovery time of the resistance by more than an order of magnitude when treated with isopropanol droplets. Placing a solvent droplet on a ZnO:Sb micro-wire in situ during measurement allowed us to make a direct comparison between the behavior before, during, and after solvent treatment. Isopropyl alcohol (IPA) has the most significant response among the solvents that we studied. Two possible mechanisms have been proposed as possibly being responsible for this huge enhancement of the wire resistance: surface transformations of isopropanol and the catalyzing effect of Sb atoms. Both mechanisms seem to combine and enhance the oxygen adsorption to the wire surface causing a very large rise in the resistance.

Nanostructured ZnO has been widely investigated as a gas sensing material. Antimony is an important dopant for ZnO that catalyses its surface reactivity and thus strengthens its gas sensing capability. However, there are not enough studies on the gas sensing of antimony-doped ZnO single wires. We fabricated and characterized ZnO/ZnO:Sb core–shell micro-wires and demonstrated that individual wires are sensitive to oxygen gas flow. Temperature and light illumination strongly affect the oxygen gas sensitivity and stability of these individual wires. It was found that these micro- and nano-wire oxygen sensors at 200°C give the highest response to oxygen, yet a vanishingly small effect of light and temperature variations. The underlying physics and the interplay between these effects are discussed in terms of surface-adsorbed oxygen, oxygen vacancies and hydrogen doping.

We developed a simple, inexpensive undergraduate laboratory experiment covering concepts and applications related to thermoelectric effects. Students use commercially available thermoelectric plates for producing electric current or for cooling and heating, then utilize them to perform experimental investigations that involve cooling. These investigations include studying supercooling and flash-freezing of water, as well as the temperature dependence of the resistivity of metals and semiconductors. The experiment allows students to easily add more components to investigate additional phenomena, thus lending itself as a potential open-ended ‘final project’ in the lab. The activities emphasize experiment design and scientific investigation. They also develop some of the main goals of advanced physics laboratories, such as the exposure to new technologies and experimental skills, data collection and automation/control, as well as data analysis and the clear communication of the results. This experiment can be integrated into the physics curriculum of electronics or advanced laboratory courses at the sophomore or higher levels.

The structural, transport, and magnetic properties of permalloy (Ni81Fe19) thin films prepared by oblique angle deposition have been investigated. With the increasing oblique angle, column-like microstructures developed on the films while their porosity augmented. Due to the unique columnar microstructures, a deposition-angledependent uniaxial anisotropy was induced in the films. While the normally deposited film was electrically isotropic, the films grown at non-zero angles showed enhanced anisotropic conduction. The magnetization data showed that for the films deposited at angles 70 degrees the easy magnetization axis was along the films in-plane direction and perpendicular to the incident flux direction. As the oblique angle exceeded 70 degrees the magnetization easy-axis reoriented along the long-columnar axis. The systematic increase of oblique angle triggered a noticeable enhancement in the coercivity , especially for the extreme oblique angles (alpha 70 degrees). The experimental results demonstrated that oblique angle deposition, particularly at angles > 70 degrees, can substantially transform the electro-magnetic properties of thin films.

This paper reports the behavior of water droplets on a topography-based, radial gradient on a metallic surface. The radial gradient was designed to concentrate water in the center region through the spontaneous motion and coalescence of individual droplets on the gradient pattern. In this work, parallel microchannels having a fixed width (similar to 25 mu m) and depth (100 mu m) but variable spacing were used to impart a net surface tension force to the droplet for actuation. The diameter of the radial gradient (i.e. circular test region) was approximately 18.9 mm (or, similar to 0.75 in.). The surfaces were fabricated using a 355 nm YVO4 laser system and then characterized using droplet injection (5-38 mu L) and spray testing techniques. Injected droplets were observed to move spontaneously away from the hydrophobic outer ring region of the circle towards the hydrophilic center as designed by the gradient. Droplet travel distances as large as 4 mm were measured depending on the droplet volume and injection site location. In application, such a design could be useful for promoting droplet coalescence on a surface and subsequently removal by gravity through the formation of sufficiently large droplets from two or more otherwise small droplets.

We have systematically studied the effect of oblique angle deposition (OAD) on the transport properties of Permalloy (Ni81Fe19) thin films. Films deposited at oblique angles larger than 50 degrees exhibit a resistivity minimum that can be further manipulated by changing the oblique angle. A minimum in temperature dependent resistivity is manifested as a deviation from the usual phonon-scattering behavior. Furthermore, a change in the magnetic anisotropy in the films is clearly demonstrated in their magnetoresistance that shifts from negative to positive with increasing glancing angle. OAD results in the formation of columns-like microstructures on the films, while their porosity greatly enhances. The microstructural evolution induced by OAD modifies the fundamental electron scattering process in the films. The modifications in the scattering mechanism outcomes in a resistivity minimum in the films grown at high oblique angles. The possible mechanisms responsible for the observed behavior are discussed.

The spontaneous motion of liquid droplets on solid surfaces is the result of an unbalanced surface tension force, which is sometimes called the "Marangoni effect". This can be triggered by either a difference in surface temperature or a heterogeneity in the topography or chemistry of the surface passively or actively. The imbibition of liquid within capillary tubes, horizontal ice wicking on either hydrophilic or hydrophobic substrates, and inkjet printing for example are just some classic illustrations of where the Lucas-Washburn equation can predict droplet behavior characteristics fairly well. In contrast, this study reveals an example of droplet behavior not previously studied that is not well-predicted by the Lucas-Washburn equation, namely the motion of ferrofluid droplets in the presence of uniform magnetic field. When a ferrofluid droplet is horizontally exposed to an external uniform magnetic field on a biphilic surface tension gradient in the shape of a wedge, it appears to violates the Lucas-Washburn equation which predicts that droplet travel distance should scale with the square root of time (i.e. l~t^(1/2)). Rather, our experimental results suggest that the movement of the ferrofluid droplet is slower following the relationship, (l~t^(1/3)). Furthermore, due to the relatively high viscosity of water-based ferrofluid droplets, we observed that at the beginning of the motion, the visco-capillary effect dominates the effects of the magnetism, and the droplets tend to follow the well-known relationship, (l~t^(1/10)). This initial stage of droplet spreading is known as "Tanner's Law".

The application of external uniform magnetic fields to ferrofluid droplets affects their magnetic order at the nanoscale as well as their shape at the macroscale, thus changing their contact angle with the surface. In this work, the effects of external uniform magnetic fields on the contact angles between different oil-based ferrofluid droplets and a handful of horizontal surfaces of varying wettability were studied. The contact angle is no longer constant around the ferrofluid droplet; rather, it varies in a rich yet predictable way. Droplets dispensed in the presence of the magnetic field on oleophobic surfaces adjust such that the contact angle increases at the front and back ends and decreases at the two perpendicular positions. The opposite behavior is reported for ferrofluid droplets on oleophilic surfaces. These direction-dependent changes in the contact angle can have a significant impact on the behavior of ferrofluid droplets on gradient surfaces where they can either diminish or enhance the surface tension gradient. Our work is fundamentally relevant to potential applications involving the controlled movement of ferrofluid droplets on surfaces like the lab-on-a-chip under the combined effects of a magnetic field and either a surface tension gradient or an electric field (i.e., electrowetting). It is important to understand how the two effects interact for the optimal utilization of these effects in future applications.

We synthesized Sb-doped ZnO (ZnO: Sb) microrods with varying Sb content and carried out a systematic study on their structural, optical and photoluminescent properties. Scanning electron microscopy revealed a hexagonal morphology of the as grown microrods, while the x-ray photoelectron spectroscopy (XPS) and Ultra Violet-Visible spectroscopy results indicated the incorporation of Sb dopants into the ZnO lattice. XPS and x-ray diffraction analysis revealed that all ZnO: Sb microrods with different Sb doping possessed typical wurtzite structure and had no other impurity phases. Furthermore, the XPS results showed that Sb ions are in an oxidation state between 3 + and 5+, indicating the existence of an acceptor complex in the ZnO: Sb microrods. In addition, another deep acceptor originated form 2 + oxygen vacancies was identified. Photoluminescence (PL) measurements confirmed the formation of the (Sb-Zn-2V(Zn)) shallow acceptor states in the ZnO: Sb microrods. PL measurements at low temperature showed strong violet luminescence, which is originated from free-electron to acceptor level (FA) transitions. The FA emission showed a slight blue shift with the increase of the temperature. As a result of Sb incorporation into the ZnO lattice, we observed a red shift in the ZnO: Sb microrods' energy gap with the increase of Sb doping. This red shift is attributed to the formation of acceptor levels inside the ZnO band gap. The identification of this acceptor level in these homogeneous single-phase ZnO: Sb microrods provides strong promise of p-type conductivity of ZnO by Sb doping.

A series of Co2FeAl Heusler alloy films, fabricated on Si/SiO2 substrates by magnetron sputtering-oblique angle deposition technique, have been investigated by magnetization and transport measurements. The morphology and magnetic anisotropy of the films strongly depended on the deposition angle. While the film deposited at zero degree (i.e. normal incidence) did not show any anisotropy, the films deposited at higher angles showed unusually strong in-plane anisotropy that increased with deposition angle. The enhanced anisotropy was well-reflected in the direction-dependent magnetization and the coercivity of the films that increased dramatically from 30 Oe to 490 Oe. In a similar vein, the electrical resistivity of the films also increased drastically, especially for deposition angles larger than 60 degrees. These anisotropic effects and their relation to the morphology of the films are discussed. (C) 2018 Elsevier B.V. All rights reserved.

Most materials of practical interest are neither completely wetting nor completely non-wetting. “Surface wettability” then refers to the degree that a surface is hydrophilic (i.e. water-loving) or hydrophobic (i.e. water-fearing). Through careful design, it is possible to alter the natural wettability of a surface to be more water-loving or water-fearing. This is principally achieved by modifying the surface chemistry and/or surface roughness. In some cases, modifying the surface may bring operational benefit or advantage. For example, aluminum and copper (which are used in the construction of heat exchangers) tend to retain water in application, which can degrade performance. Modifying the surface however to be superhydrophilic can help to spread out the condensate, reduce the air-side pressure drop, and facilitate drainage. Moreover, by creating a wettability pattern or gradient, it is possible to predetermine the initiating sites for condensation on a surface as well as facilitate droplet motion and/or control the water droplet movement path. In the first part of this review, the current state of the art of surface wettability modification and control techniques are presented, which includes topographical manipulation, chemical modification, as well as methods for creating gradient surfaces and patterned wettability. In the second part of this review, possible applications and the potential impact of these methodologies in energy systems are discussed with a special focus on heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems and components.

This paper explores the fluid property commonly called surface tension, its effect on droplet shape and contact angle, and the major influences of contact angle behaviour (i.e. surface roughness and surface chemistry). Images of water droplets placed on treated copper surfaces are used to measure the contact angles between the droplets and the surface. The surface wettability is manipulated either by growing a self-assembled monolayer on the surface to make it hydrophobic or by changing the surface roughness. The main activities in this experiment, then, are (1) preparing and studying surfaces with different surface wettability and roughness; (2) determining the shape and contact angles of water droplets on these surfaces; and (3) demonstrating the spontaneous motion of water droplets using surface tension gradients.

This work demonstrates the controlled motion and stopping of individual ferrofluid droplets due to a surface tension gradient and a uniform magnetic field. The surface tension gradients are created by patterning hydrophilic aluminum regions, shaped as wedges, on a hydrophobic copper surface. This pattern facilitates the spontaneous motion of water-based ferrofluid droplets down the length of the wedge toward the more hydrophilic aluminum end due to a net capillarity force created by the underlying surface wettability gradient. We observed that applying a magnetic field parallel to the surface tension gradient direction has little or no effect on the droplet's motion, while a moderate perpendicular magnetic field can stop the motion altogether effectively "pinning" the droplet. In the absence of the surface tension gradient, droplets elongate in the presence of a parallel field but do not travel. This control of the motion of individual droplets might lend itself to some biomedical and lab-on-a-chip applications. The directional dependence of the magnetoviscosity observed in this work is believed to be the consequence of the formation of nanoparticle chains in the fluid due to the existence of a minority of relatively larger magnetic particles.

The charge carriers type in antimony-doped ZnO (ZnO:Sb) microwires was studied using the hot probe technique. The wires were grown by a simple thermal evaporation method. Contrary to the expected p-type behavior reported for Sb doped ZnO thin films and nanowires, our hot probe measurements of representative single Sb-doped ZnO wires show a stable n-type behavior. The hot probe technique is a simple and efficient way to determine the charge carrier type from thermoelectric measurements on a single semiconductor wire and could offer an alternative to Hall effect measurements. The technique relies on creating a temperature gradient across the wire (i.e., heating one side of the wire relative to the other) and monitoring the resulting open-circuit voltage between the two ends. We also performed Energy Dispersive X-ray Spectroscopy measurements to identify and monitor the elemental composition in these ZnO:Sb wires.

We studied the interfacial contact between GaMnAs and superconducting Nb micro-structures both with and without removing the native GaMnAs surface oxide. Our results show that a strong Schottky barrier forms at the interface when the oxide layer is left between Nb and GaMnAs. This barrier can be confused for Andreev Reflection and erroneously used to extract spin polarization. A simple acid etch is shown to remove the oxide film, thus decreasing the interface resistance, removing the Schottky barrier, and causing a clear Andreev reflection effect. One key recommendation for point contact Andreev reflection studies is to push the tip hard enough into contact and verify that the total resistance is not too high.

We carried out a systematic study of magnetic order and magnetic interlayer coupling in Fe/(Ga,Mn)As bilayers using superconducting quantum interference device magnetometry, polarized neutron reflectometry, element-specific x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and x-ray specular reflectivity. Our results clearly show that Fe/(Ga,Mn)As bilayers are strongly exchange coupled at the interface. However, contrary to recent reports in the literature, we observe a ferromagnetic rather than antiferromagnetic coupling between the magnetic moments of the Mn ions and the Fe layer. It is interesting in this context that the surface region of the (Ga,Mn)As layer that is in direct contact with the Fe film displays a nearly identical coercivity to that of Fe (indicating perfect ferromagnetic coupling of that region), while the bulk of the (Ga,Mn)As layer, which is more weakly ferromagnetically coupled with Fe, shows a significantly smaller coercive field.

Magnetization measurements on a series of Fe films grown by molecular beam epitaxy on GaAs (001) substrates and capped with a thin Au layer reveal interesting exchange bias (EB) properties at low temperatures. The observed exchange bias decreases rapidly with increasing temperature, and completely disappears above 30 K. While the Fe samples were not grown with an intentionally deposited antiferromagnetic (AFM) layer, X-ray reflectometry, X-ray absorption near-edge spectroscopy carried out near the L-edge of Fe, and comparison with similar Fe/GaAs samples capped with Al, which do not show exchange bias, suggest that the exchange bias in the GaAs/Fe/Au multilayers is caused by an AFM Fe oxide at the Fe/Au interface formed by penetration of oxygen through the Au capping layer. The observed exchange bias is accompanied by a strikingly asymmetric magnetization reversal of the Fe films occurring when the magnetic field is applied at angles away from the easy axis of the film. The observed asymmetry can be interpreted in terms of a competition between cubic, uniaxial, and unidirectional magnetic anisotropy characteristic of the exchange-biased Fe film.