In a recently developed multilayer (ML) scheme, two or more 10-nm thick Ca0.3Y0.7Ba2Cu3O7-x spacers were inserted into BaZrO3-doped YBa2Cu3O7-x (BZO/YBCO) films to enable dynamic diffusion of Ca ions from the spacers to BZO/YBCO layers. In these ML BZO/YBCO nanocomposite films, significantly enhanced pinning has been attributed to Ca/Cu substitution on the Cu-O planes of YBCO, leading to reduced lattice mismatch and hence defects at the BZO/YBCO interface. In this work, we further probe the Ca diffusion in the five-layer ML films by varying the thickness of the two Ca0.3Y0.7Ba2Cu3O7-x spacers in the range of 1 nm-10 nm and also the thickness of the three BZO/YBCO layers in the range of 50-330 nm. Ca diffusion has been found highly effective if the spacer layer thickness exceeds 2 nm and can diffuse through large BZO/YBCO thicknesses up to 330 nm (total film thickness similar to 1 mu m) along the BZO/YBCO interface. The critical current density exhibits enhanced and almost thickness-independent trends in the ML BZO/YBCO nanocomposite films. Significantly enhanced pinning is illustrated in up to 5 folds enhancement of J(c) at 65 K and 9.0 T. At lower temperatures, the enhanced pinning extends to a broad range of the orientations of magnetic field (B). At 20 K and 9.0 T, the I-c is up to 654 A/cm-width at B//c, which is close to 753 A/cm-width at B//ab due to the intrinsic pinning, has been achieved. This result suggests that the ML scheme provides an interesting approach to improve pinning in nanocomposite films.

Improved pinning efficiency of c-axis-aligned BaZrO3 nanorods (BZO-NRs) in BZO/YBa2Cu3O7 nanocomposite thin films was obtained recently using a multilayer (ML) approach, in which diffusion of Ca ions from two thin (Ca0.3Y0.7)BCO spacers (10 nm in thickness) sandwiched with three BZO/YBa2Cu3O7 layers. The subsequent Ca (30% larger)/Cu substitution at the Cu-O planes of YBCO was found energetically preferable by inducing c-axis elongation of the YBa2Cu3O7 lattice near the BZO-NRs/YBa2Cu3O7 interface to enable a coherent interface via reducing the BZO/YBCO lattice mismatch from originally 7.7% to 1.4%, leading to significantly enhanced J(c) (B) and F-p in thin ML films of 150 nm in thickness. This work investigates whether improved pinning could be achievable in thicker BZO-NRs/YBa2Cu3O7 ML films with the thickness increased to 1000 nm. Interestingly, similar pinning enhancement has been observed in thick BZO-NRs/YBa2Cu3O7 ML films across a wide temperature range of 20-80 K. In particular, the thicker BZO-NRs/YBa2Cu3O7 ML films outperform their thinner counterparts in both higher value and less anisotropy of J(c) (B). At 1000 nm thickness, I-c (30 K, 9T) reaches up to similar to 680 A/cm-width with a variation of similar to 85% over the entire angular range of B field orientations. This result illustrates the critical role of Ca diffusion at the BZO-NRs/YBCO interface for improving pinning efficiency of BZO-NRs in a wide range of temperatures and B fields.

High critical current (I-c) in high magnetic fields (B) with minimal variations with respect to the orientation of the B field is demanded by many applications such as high-field magnets for fusion systems. Motivated by this, this work studies 6 vol. % BaZrO3/YBa2Cu3O7 (BZO/YBCO) multilayer nanocomposite films by stacking two 10 nm thick Ca0.3Y0.7Ba2Cu3O7 (CaY-123) spacers with three BZO/YBCO layers of thickness varied from 50 to 330 nm to make the total film thickness of 150-1000 nm. The Ca diffusion from the spacers into BZO/YBCO was shown to dramatically enhance pinning efficiency of c-axis aligned BZO nanorods, which yields high and almost thickness independent critical current density (J(c)) in the BZO/YBCO multilayer nanocomposite films. Remarkably, enhanced Jc was observed in these multilayer samples at a wide temperature range of 20-80 K and magnetic fields up to 9.0 T. In particular, the thicker BZO/YBCO multilayer films outperform their thinner counterparts in both higher value and less anisotropy of Jc at lower temperatures and higher fields. At 20 K and 9.0 T, Ic is up to 654 A/cm-width at B//c in the 6% multilayer (1000 nm) sample, which is close to 753 A/cm-width at B//ab due to the intrinsic pinning. This result illustrates the critical role of the Ca cation diffusion into the YBCO lattice in achieving high and isotropic pinning in thick BZO/YBCO multilayer films.

One-dimensional artificial pinning centers (1D-APCs) in YBa2Cu3O7-x nanocomposite films provide strong collective pinning at magnetic field B//c-axis. In this work, we reveal a 1D-APC/YBa2Cu3O7-x interface is preferred for high pinning efficiency of individual 1D-APCs including BaHfO3 and BaZrO3. The coherent 1D-APC/YBa2Cu3O7-x interface may be obtained via either growth of the nanocomposite films at optimal condition or Ca-diffusion to dynamically reduce the interface strain during the nanocomposite film growth. Interestingly, the high pinning efficiency of the 1D-APCs with coherent interfaces with YBCO not only lead to a high critical current density (J(c)) in magnetic fields up to 9.0 T at H//c-axis but also enhanced J(c) over a larger angular range when H is away from H//c-axis up to theta = 60-80 degree than that in the case the interface is defective. This result suggests the importance of understanding and engineering the APC/YBCO interface for optimal pinning in nanocomposite films.

C-axis aligned BaZrO3 (BZO) nanorods formed via strain-mediated self-assembly in BZO-doped YaBa2Cu3O7-x (BZO/YBCO) nanocomposite films can provide strong pinning to the quantized magnetic vortices. While the strain initiated from the BZO/YBCO lattice mismatch plays a critical role in nucleation and evolution of the BZO nanorods, it also leads to a highly defective BZO/YBCO interface and hence reduced pinning efficiency of BZO nanorods. This work reports a recent study in probing the effect of BZO/YBCO interface on the pinning efficiency of the BZO nanorods as the interface is repaired dynamically during the BZO nanorod growth using Ca doping. Within the BZO doping range of 2-8 vol.%, significantly enhanced pinning efficiency of the BZO nanorods have been observed. A peak enhancement up to five-fold of critical current density at 9.0 T and 65-77 K has been obtained in the 6 vol.% BZO/YBCO nanocomposites after the interface repair. This result not only illustrates the critical importance of the BZO/YBCO interface in the pinning efficiency, but also provides a facile scheme to achieve such an interface to restore the pristine pinning efficiency of the BZO nanorods.

An important research goal in the applications of high temperature superconductor YBa 2 Cu 3 O 7-δ (YBCO) thin films is increasing both the critical current density and also the isotropic nature of the film. YBCO is inherently anisotropic due to its layered perovskite structure. The critical current density of YBCO thin films is enhanced by increasing the flux pinning sites in the film by the addition of insulating nano-phase materials, such as BaZrO 3 (BZO) nanorods, which are also anisotropic in nature. Using a multilayer pulsed laser deposition technique has been shown to produce films with inclusions that are more isotropic in nature. However, the defective BZO nanorod interface, resulting from its lattice mismatch with YBCO, prevents obtaining optimum pinning force. This research explores the effect of Ca doped YBCO space layers in the multilayer composite film, on the BZO nanorod/YBCO interface, over a wide range of conditions of 65–5 K and 0–9T that are suitable for various applications. The interplay of combining these three variables: BaZrO 3 addition to YBCO, multilayer film growth resulting from varying pulsed laser deposition conditions, and employing calcium doped YBCO space layers, and the resulting impact on film microstructures and superconducting properties, will be presented.

After theoretical discovery of quantized magnetic vortices in type II superconductors by Abrikosov, which received 2003 Nobel Prize in Physics, vortex pinning has been an important topic of research for high critical current densities in applied magnetic fields desired for a variety of applications in electric and electronic devices and systems. The small vortex core size in high temperature superconductors (HTSs), of a few nanometers, has prompted an intensive research in development of nanoscale artificial pinning centers (APCs) in so-called HTS nanocomposites. Exciting results of much enhanced in-field critical current densities and pinning force densities have been achieved. This talk intends to highlight the progress made recently in HTS nanocomposites towards controllable generation of APCs with desired morphologies, dimension, concentration, and pinning efficiency for targeted applications. The future research in HTS nanocomposites to meet the need of practical applications will also be discussed.

The in-field critical current density of YBa 2 Cu 3 O 7-δ (YBCO) thin films has been shown to be enhanced by addition of the flux pinning sites in the film by the addition of insulating nano-phase materials, such as BaZrO 3 (BZO) and BaHfO 3 (BHO) nanorods. This research explores optimization of the BHO nanorod additions and illustrates its beneficial properties for a wide range of temperatures and magnetic fields suitable for various applications. Pulsed laser deposition produced films on SrTiO 3 substrates. Critical current density measured for fields ranging from H = 0–9T with H//c, and temperatures from 5–65 K, provides a detailed picture of the pinning effects.

AbstractThis work examines the pinning enhancement in BaZrO3(BZO) +Y2O3doubly-doped (DD) YBa2Cu3O7(YBCO) nanocomposite multilayer (DD-ML) films. The film consists of two 10 nm thin Ca0.3Y0.7Ba2Cu3O7-x(CaY-123) spacers stacking alternatively with three BZO + Y2O3/YBCO layers of 50 nm each in thickness that contain 3 vol% of Y2O3and BZO doping in the range of 2–6 vol%. Enhanced magnetic vortex pinning and improved pinning isotropy with respect to the orientation of magnetic field (B) have been achieved in the DD-ML samples at lower BZO doping as compared to that in the single-layer counterparts (DD-SL) without the CaY-123 spacers. For example, the pinning force density (Fp) of ∼58 GNm−3in 2 vol.% of DD-ML film is ∼110% higher than in 2 vol% of DD-SL at 65 K andB//c-axis, which is attributed to the improved pinning efficiency byc-axis aligned BZO nanorods through diffusion of Calcium (Ca) along the tensile-strained channels at BZO nanorods/YBCO interface for improvement of the interface microstructure and hence pinning efficiency of BZO nanorods. An additional benefit is in the considerably improvedJc(θ) and reducedJcanisotropy in the former over the entire range of the B orientations. However, at higher BZO doping, the BZO nanorods become segmented and misoriented, which may change the Ca diffusion pathways and reduce the benefit of Ca in improving the pinning efficiency of BZO nanorods.

This work examines the pinning enhancement in BaZrO _3 (BZO) +Y _2 O _3 doubly-doped (DD) YBa _2 Cu _3 O _7 (YBCO) nanocomposite multilayer (DD-ML) films. The film consists of two 10 nm thin Ca _0.3 Y _0.7 Ba _2 Cu _3 O _7-x (CaY-123) spacers stacking alternatively with three BZO + Y _2 O _3 /YBCO layers of 50 nm each in thickness that contain 3 vol% of Y _2 O _3 and BZO doping in the range of 2–6 vol%. Enhanced magnetic vortex pinning and improved pinning isotropy with respect to the orientation of magnetic field (B) have been achieved in the DD-ML samples at lower BZO doping as compared to that in the single-layer counterparts (DD-SL) without the CaY-123 spacers. For example, the pinning force density ( F _p ) of ∼58 GNm ^−3 in 2 vol.% of DD-ML film is ∼110% higher than in 2 vol% of DD-SL at 65 K and B // c -axis, which is attributed to the improved pinning efficiency by c -axis aligned BZO nanorods through diffusion of Calcium (Ca) along the tensile-strained channels at BZO nanorods/YBCO interface for improvement of the interface microstructure and hence pinning efficiency of BZO nanorods. An additional benefit is in the considerably improved J _c ( θ ) and reduced J _c anisotropy in the former over the entire range of the B orientations. However, at higher BZO doping, the BZO nanorods become segmented and misoriented, which may change the Ca diffusion pathways and reduce the benefit of Ca in improving the pinning efficiency of BZO nanorods.

Recently, we reported that incorporating thin Ca(0.3)Y(0.2)Ba(2)Cu(3)O(7-x )spacer layers in BaZrO3/YBa2Cu3O7-x (BZO/YBCO) films in multilayers (ML) could lead to a coherent BZO/YBCO interface due to reduced lattice mismatch of similar to 1.4% and hence improved pinning by the BZO 1D artificial pinning centers (BZO 1D-APCs). It was hypothesized that the highly strained YBCO columns around the BZO 1D-APCs due to the larger lattice constant of BZO than YBCO's in c-axis by 7.7% provide channels to facilitate Ca diffusion more preferably near the BZO/YBCO interface, leading to an enlarged YBCO's c-axis lattice constant as a consequence of the Ca/Cu substitution on the YBCO's Cu-O planes. Confirmation of this hypothesis is hence important to understand the mechanism of Ca diffusion in BZO/YBCO ML films for further enhancement of pinning efficiency of the BZO 1D-APCs. Motivated by this, this work presents a comparative study of two sets of BZO/YBCO ML films: singly-doped (SD-ML, BZO only) and the doubly-doped (DD-ML, BZO + Y2O3). The modulated strain field in the SD-ML films is in contrast to the non-modulated strain in the DD-ML samples as confirmed by transmission electron microscopy. Interestingly, improved pinning by >four times of the reference SD single-layer (SD-SL) film' was observed in the SD-ML samples even when the constituent BZO/YBCO layer thickness was varied between 50-100 nm, indicating that the Ca diffusion along the strained BZO/YBCO interface and the Ca/Cu substitution on YBCO's Cu-O planes indeed correlate with the modulated strain field. In contrast, pinning degrades by >twice in DD-ML samples with respect to their DD-SL counterpart's. This result therefore suggests that strained BZO/YBCO interfaces serve as channels for Ca diffusion in SD-ML, which leads to the coherent BZO/YBCO interfaces and hence improved pinning of BZO 1D-APCs. This effect diminishes when such a modulated strain field is removed in the DD-ML samples.

We successfully use two-dimensional hexagonal boron nitride (h-BN) as a continuous and ultrathin insulator to facilitate the conversion of excitons into free carriers at organic donor-acceptor (D-A) interfaces. Monolayer- (0.33 nm) thick h-BN with a lateral size on the order of 1 cm2 is inserted between zinc phthalocyanine (ZnPc) and perylenetetracarboxylic diimide (PTCDI) organic films that form a generic D-A interface. We find that h-BN increases the photon-to-free-carrier conversion yield of the D-A heterostructure by 130% compared with identical samples without a h-BN layer, even though h-BN lowers the initial electron-transfer rate from ZnPc to PTCDI. The enhanced photon-to-free-carrier conversion yield is attributed to a larger charge-separation yield of charge-transfer (CT) excitons at the D-A interface. h-BN can prohibit the formation of tightly bound CT excitons and impede electron-hole recombination, which can improve the charge-separation yield.

High-field applications require high concentrations of strong pinning centers. In this article, BaZrO 3 doped YBa 2 Cu 3 O 7 (BZO/YBCO) nanocomposite films with BZO doping up to 8 vol.% were fabricated in a multilayer (ML) format by inserting two 10 nm thick Ca 0.3 Y 0.7 Ba 2 Cu 3 O 7-x spacer layers in the BZO/YBCO nanocomposite films for improved pinning and enhanced critical current density Jc at high fields. Significant Jc enhancement was observed in all the BZO/YBCO ML films of BZO doping in the entire range of 2–8 vol.% as compared to their SL counterpart's. At 65 K, the enhancement of peak pinning force density ( Fp, max ) is 71, 67, 296, and 47% for 2, 4, 6, and 8 vol.% BZO/YBCO ML films, respectively. In addition, the B max (the location of the Fp, max ) is shifted towards higher values for BZO/YBCO ML films by up to 33%. Interestingly, at high BZO doping of 8 vol.%, the enhanced modulated strain field was found to reduce the detrimental effect of Ca ion diffusion on T c , leading to Jc enhancement at a strong field up to 9 T at all orientations of the magnetic field.

A fully flexible strain sensor consisting of vertically aligned ZnO nanowires on graphene transferred on polyethylene terephthalate with prefabricated Au/Ti electrodes (ZnO-VANWs/Gr)/PET) has been obtained. The ZnO-VANWs were grown in solution using a seedless hydrothermal process and are single-crystalline of (0001) orientation that provides optimal piezoelectric gating on graphene when deformed mechanically. The change of the graphene channel conductance under such a piezoelectric gating through transduction of the mechanical deformation on the ZnO-VANWs/Gr was used to detect the strain induced by the deformation. Under applied normal forces of 0.30, 0.50, and 0.70 N in a dynamic manner, the ZnO-VANWs/Gr/PET strain sensors exhibited a high response and response times of ∼0.20 s to both force on and off were achieved. Under mechanical bending curvatures of 0.18, 0.23, 0.37, and 0.45 cm-1, high sensitivity of the gauge factors up to ∼248 and response times of 0.20 s/0.20 s (rise/fall) were achieved on the ZnO-VANWs/Gr/PET strain sensors. Moreover, the response changes polarity when the directions of bending alters between up and down, corresponding to the polarity change of the space charge on the ZnO-VANWs/Gr interface as a consequence of the compressive and tensile strains along the ZnO-VANWs. This result shows that the low-cost and scalable ZnO-VANWs/Gr/PET strain sensors are promising for applications in stress/strain monitoring, wearable electronics, and touch screens.

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.

A nanohybrid piezoelectric strain sensor was fabricated by growing vertically aligned (0001)-oriented crystalline zinc oxide nanowires directly on graphene (ZnO-VANWs/Gr) using a facile seedless hydrothermal process. Under mechanical strains, the induced piezoelectric effect on the ZnO-VANWs transduces to a piezoelectric gating effect at the ZnO-VANWs/Gr interface, resulting in a modulation of the conductivity of the Gr channel through electrostatic doping. The vertical alignment of the (0001)-oriented ZnO-VANWs on Gr is ideal to achieving high strain sensitivity, and a low-defect ZnO-VANWs/Gr interface obtained in the seedless hydrothermal process is key to realizing high sensitivity and fast response. Indeed, a high sensitivity up to 3.15 X 10(-2) kPa(-1) was obtained on the ZnO-VANWs/Gr strain sensors at lower pressures of 1.1 X 10(-6)-11 Torr, together with a fast response time of similar to 0.10 s. In particular, these results represent enhancement factors of similar to 7 and 8, respectively, as compared to strain sensors of a similar structure, except having a polycrystalline ZnO seed layer on Gr for the growth of ZnO-VANWs. Therefore, our result illustrates the critical importance of the low-defect interface of the ZnO-VANWs with Gr formed in the seedless ZnO-VANW growth for realizing an optimal electrostatic gating of Gr. In addition, the ZnO-VANWs/Gr nanohybrids can be readily scaled up using the seedless hydrothermal process for commercial applications in optoelectronics and sensors.

Graphene, a single layer conductor, can be combined with other functional materials for building efficient optoelectronic devices. However, transferring large-area graphene onto another material often involves dipping the material into water and other solvents. This process is incompatible with water-sensitive materials such as organometal halide perovskites. Here, a dry method is used and succeeded, for the first time, in stacking centimeter-sized graphene directly onto methylammonium lead iodide thin films without exposing the perovskite film to any liquid. Photoemission spectroscopy and nanosecond time-resolved photoelectrical measurement show that the graphene/perovskite interface does not contain significant amount of contaminants and sustain efficient interfacial electron transfer. The use of this method in fabricating graphene-on-perovskite photodetectors is further demonstrated. Besides a better photoresponsivity compared to detectors fabricated by the conventional perovskite-on-graphene structure, this dry transfer method provides a scalable pathway to incorporate graphene in multilayer devices based on water-sensitive materials.

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.