Search

The intrinsic origin of the ferromagnetic ordering in Zn1-xMnxO thin films grown by pulsed-laser deposition was investigated. The ferromagnetic behaviors for a Zn1-xMnxO (x=0.26) film grown at 700 °C under oxygen pressures of 10-1 Torr were observed at 4 and 300 K. The anomalous Hall effect (AHE) was found at temperatures of up to 210 K for the Zn0.74Mn0.26O thin film. The anomalous Hall coefficients (RA) were determined to be approximately proportional to the square of resistivity in the low field region, indicating the side-jump process for the AHE. Our results provide direct experimental evidence that a carrier-mediated mechanism is responsible for the ferromagnetic ordering in Zn1-xMnxO thin films grown by pulsed-laser deposition. [ABSTRACT FROM AUTHOR]

Spontaneously regulated in-plane polarity inversion of a-oriented GaN domains has been demonstrated for the first time. Crystallographic analysis revealed that each domain grown on circular-hole-patterned r-plane sapphire substrates has basal faces with oppositely oriented in-plane polarity. The inverted orientation of in-plane polarity on the opposite basal faces is not due to merging between in-plane polarity-inverted domains nucleated on the patterned r-plane sapphire substrate, but it was found to be due to spontaneous formation of an inversion domain boundary on the growth fronts of existing domains. This result provides new insights into controlling the in-plane polarity of a-oriented GaN, because the nucleation of in-plane polarity-inverted domains of a-oriented GaN on r-plane sapphire is symmetrically not allowed. [ABSTRACT FROM AUTHOR]

We report real-time high temperature scanning electron microscopy observations of the growth of bismuth nanowires via the on-film formation of nanowires (OFF-ON) method. These observations provide experimental evidence that thermally induced-stress on a Bi film is the driving force for the growth of Bi nanowires with high aspect ratios, uniform diameter, and high-quality crystallinity. Our results show that immobile grain boundaries in the Bi film are required for the growth of nanowires so that grain broadening resulting in hillock formation can be prevented. This study not only provides an understanding of the underlying mechanism, but also affords a strategy for facilitating nanowire growth by OFF-ON. [ABSTRACT FROM AUTHOR]

N-containing gaseous compounds, such as trimethylamine (TMA), triethylamine (TEA), ammonia (NH3), nitrogen monoxide (NO), and nitrogen dioxide (NO2) exude irritating odors and are harmful to the human respiratory system at high concentrations. In this study, we investigated the sensing responses of five sensor materials—Al-doped ZnO (AZO) nanoparticles (NPs), Pt-loaded AZO NPs, a Pt-loaded WO3 (Pt-WO3) thin film, an Au-loaded WO3 (Au-WO3) thin film, and N-doped graphene—to the five aforementioned gases at a concentration of 10 parts per million (ppm). The ZnO- and WO3-based materials exhibited n-type semiconducting behavior, and their responses to tertiary amines were significantly higher than those of nitric oxides. The N-doped graphene exhibited p-type semiconducting behavior and responded only to nitric oxides. The Au- and Pt-WO3 thin films exhibited extremely high responses of approximately 100,000 for 10 ppm of triethylamine (TEA) and approximately −2700 for 10 ppm of NO2, respectively. These sensing responses are superior to those of previously reported sensors based on semiconducting metal oxides. On the basis of the sensing response results, we drew radar plots, which indicated that selective pattern recognition could be achieved by using the five sensing materials together. Thus, we demonstrated the possibility to distinguish each type of gas by applying the patterns to recognition techniques. [ABSTRACT FROM AUTHOR]

In this work, we have investigated the microstructural and hydrogen storage properties of a 60 layer film of Pd(40 nm)/Ti( x nm)/Mg(360 nm)/Ti( x nm) ( x = 10, 20, 30, 40, and 50) using an ultra-high vacuum DC magnetron sputtering system. The highest hydrogen absorption capacity of the Pd/Ti/Mg/Ti film was 4.46 wt% for x = 50. The hydrogen absorption rate accelerated remarkably with increased thickness of the Ti layer, from 209 min for x = 10 to 39 min for x = 40 – overall, an increase by a factor of ∼5. These results demonstrate that the Ti layer in the Pd/Ti/Mg/Ti films serves as a blocking layer preventing the formation of Mg Pd intermetallic phases and provides additional diffusion paths for hydrogen atoms, which leads to enhanced hydrogen storage properties. [ABSTRACT FROM AUTHOR]

We present the effect of multilayer deposition on the magnetic properties of MnBi thin films. The multilayered MnBi films were fabricated with alternating deposition of Bi and Mn with N ( N = 2–10) layers (L), followed by in situ thermal annealing. As N increases, the even-layered and odd-layered films show a similar layer-number-dependence of the magnetic properties, such as an increasing behavior for H c and a concave-up behavior for M r and ( BH ) max . Higher magnetic properties were obtained at 9L and 10L. According to the electron microscopy analyses, different morphologies and elemental distributions were observed at different N (e.g., smooth-surfaced MnBi film with Bi islands at 2L and uneven-surfaced MnBi film at 10L). The different distributions of microstructure and chemical components are associated with the layer-dependent magnetic properties of the MnBi films. [ABSTRACT FROM AUTHOR]

Al 2 O 3 nanoparticles-embedded single-crystalline Bi nanowires were successfully synthesized without using any templates via the spontaneous growth of Bi thin films covered with Al 2 O 3 nanoparticles. It is experimentally confirmed that Al 2 O 3 nanoparticles were embedded into the single-crystalline Bi nanowires by using the high-resolution transmission electron microscopy and energy dispersive X-ray spectrometry. The temperature-dependent thermal conductivities of individual Al 2 O 3 nanoparticles-embedded single-crystalline Bi nanowires were measured directly using suspended micro-devices. The thermal conductivities of the Al 2 O 3 nanoparticles-embedded Bi nanowires were found to be extremely low compared with those of pure Bi nanowires of similar diameters. Moreover, the thermal conductivity of the Al 2 O 3 nanoparticles-embedded Bi nanowires was not size-dependent, i.e., it did not vary for nanowires with significantly different diameters. This result suggests that the phonon-boundary scattering is not the dominant phonon scattering mechanism in these systems while the phonon-boundary scattering is dominant in the pure single-crystalline nanowires. From the experimental measurements and theoretical calculation, these drastic reduction and unique tendency in the thermal conductivities of Al 2 O 3 nanoparticles-embedded Bi nanowires were explained by the combined effect of the phonon-boundary scattering and impurity scattering occurring between the embedded Al 2 O 3 nanoparticles and Bi matrix. [ABSTRACT FROM AUTHOR]

We report the effects of various tensile velocities on the nanocrack formation in a Pd thin film on an elastomeric polydimethylsiloxane substrate and its H 2 sensing properties. A tunable nanocrack along the x and y axes was created by mechanical stretching/compression cycles with varying tensile velocities. From the microstructural analyses, we found that the tensile velocity has a significant effect on the crack density but little effect on the average crack width. The Pd nanogap sensor prepared under a high tensile velocity showed a high performance with a low detection limit of 500 ppm of H 2 in air. Our results indicate that the higher crack density with the narrow nanocrack width (55–100 nm) propagated over the entire film provides the enhanced H 2 sensing properties in air. [ABSTRACT FROM AUTHOR]

[Display omitted] • We report on a breath-isoprene analyzer based on a gas chromatography column with catalyst-coated In 2 O 3 thin films. • We found that Pd-coated In 2 O 3 thin films were superior to Au- and Pt-coated films. • The analyzer was able to selectively detect isoprene in air within 2 min at 196 °C. • The detection limit was 0.4 ppb, which is the lowest level reported to date. • Our study demonstrates that the analyzer can be utilized to rapidly and accurately detect isoprene in breath. Monitoring breath isoprene concentration plays a role in non-invasive diagnosis of blood-cholesterol levels. In this study, we developed a highly sensitive and selective isoprene analyzer based on a Pd-coated In 2 O 3 thin film integrated in a miniaturized gas chromatography column. To improve the sensing performance, the In 2 O 3 thin films were fabricated with high uniformity and crystallinity using a dual-ion-beam sputtering system, and various metal catalysts (Au, Pt, and Pd) were loaded on the film surface. Among them, the Pd catalyst afforded the highest sensing reaction and the lowest detection limit (approximately 0.4 ppb), which is the best performance ever reported. It also significantly lowered the optimal operating temperature of the sensor from 432 °C to 196 °C. The excellent isoprene sensing performance of the Pd-coated In 2 O 3 film can be attributed to the high density of oxygen vacancies, efficient reduction-reoxidation of PdO, and the Mars-van-Krevelen catalytic reaction. We further optimized the isoprene sensing performance of a Pd-coated In 2 O 3 film by varying the Pd thickness and discovered that 1 nm of Pd deposition showed the optimal status of discontinuous islands for sensing. We expect that our device can be applied to a portable breath isoprene analyzer. [ABSTRACT FROM AUTHOR]

Abstract: Hybrid thin films, in which a Pd thin film was sandwiched between thin poly(methyl methacrylate) PMMA layers, were prepared on a flexible substrate using a combination of sputtering and spin coating. In real-time electrical response tests to H2, Pd-PMMA hybrid nanogap sensors operated in an on–off manner for both air and N2 carriers. Due to the selective penetration of H2 and the density reduction of the cracks that formed in the trilayer of the hybrid thin films, the detection limit of the hybrid nanogap sensors (6000ppm H2 in air and 600ppm H2 in N2) was drastically reduced compared to that of Pd nanogap sensors without PMMA layers. Hybrid thin films have great promise in realizing sensors that can be used in air without additional filter system. [Copyright &y& Elsevier]

Abstract: Extremely narrow nanogaps with a width less than 10nm have been achieved in Pd thin films on a PDMS substrate by the combination of tensile straining and fine control of initial hydrogen concentration. For the Pd/PDMS structures pre-elongated by 25%, the width of nanogaps was found to decrease with decreasing concentration of the initial feed of hydrogen gas. Hydrogen sensors based on the nanogaps with reduced widths resulted in a dramatic improvement in hydrogen detection limit, and the minimum detection limit obtained in this study was 10ppm, which is the lowest ever reported for hydrogen sensors employing only a Pd thin film. These results provide an unusual opportunity to use these sensors in specific fields, where ultra-high precision hydrogen detection is required even with one-time use. [Copyright &y& Elsevier]

Abstract: We have successfully fabricated sub-100 nm nanogaps in Pd–Ni alloy thin films on an elastomeric substrate by simple stretching. The nanogaps-containing Pd–Ni films were utilized as hydrogen-sensing sponges and their performance was demonstrated to dominate over the performance of similar mobile thin films comprised of pure Pd in major aspects such as the response time, sensitivity in high H2 concentrations, and H2 detection limit. Notably, Pd87.5Ni12.5 hydrogen sensing sponges showed ultra-high sensitive and reversible On-Off behaviors and low detection limit of ∼100 ppm, which were attributed to the reduced nanogap width and the enhanced volume expandability of Pd–Ni lattice. The effects of Ni added to Pd and a search for an optimum Ni concentration were also systematically studied. [Copyright &y& Elsevier]

Abstract: We have investigated a lithography-free technique for On-Off type hydrogen sensors using a cracked palladium (Pd) film on an elastomeric substrate. Cracks were induced in a sputtered Pd film simply by undergoing hydrogen absorption and desorption processes. Compared to the same thickness of a Pd film on a Si/SiO2 substrate that relied on the electron scattering mechanism, a cracked Pd film on an elastomeric substrate operated as a reversible On-Off hydrogen sensor based on the crack open-close mechanism when exposed to hydrogen. The thickness of a Pd film on the elastomeric substrate plays a significant role in determining the sensing mode of the cracked Pd film. The cracked Pd film with a thickness of 9–11 nm on the elastomeric substrate showed reversible and perfect On-Off responses under a wide range of hydrogen concentrations with large current variations and a fast response time of less than 1 s. [Copyright &y& Elsevier]

Abstract: We investigated the hydrogen (H2) sensing properties of palladium (Pd)–nickel (Ni) alloy films with varying Ni content and discussed them in light of structural deformations. The Pd–Ni alloys operated reversibly upon H2 absorption and desorption and their sensitivities decreased linearly with Ni content added to Pd. This was attributed to reduction in the lattice constant and interstitial volume caused by the Ni addition, allowing fewer hydrogen atoms to penetrate into the Pd–Ni alloy with higher Ni content. Interestingly, the response time of the Pd–Ni alloys was much shorter than that of pure Pd, presumably due to the fast permeation of hydrogen atoms through microscopic imperfections in the alloys. Unlike pure Pd, the Pd–Ni alloys showed an almost linear relationship between the sensitivity and H2 concentration without hysteretic behaviors, enabling the detection of low concentration of H2 down to 0.01%. These results provide a significant understanding of the role of Ni in the Pd–Ni thin films for improving the H2 sensing properties of the Pd-based alloy film sensors. [Copyright &y& Elsevier]

Abstract: We report the fabrication of a novel hydrogen sensor that utilizes the electrical resistance changes in the palladium thin films with nanometer thicknesses. The sensing mechanism is based on transitory absorption of hydrogen atoms into the palladium layer, which leads to the reversible alteration of the electrical resistance. In concentrated hydrogen ambient, the excess hydrogen absorption process leads to mechanical deformation on the surface of the palladium films, corresponding to the phase transition from α-phase to β-phase. The reversible sensing process results in a hysteresis curve for resistive properties, of which the height (sensitivity) could be controlled by manipulating the thickness of the palladium layers. The peel-off phenomena on the surface of the palladium film were suppressed by decreasing the thickness of the film. At the thickness of 20nm, a hysteresis curve of resistance was obtained without any structural change in the palladium thin film. These results provide a significant insight to the fundamental understanding of the relationship between the electrical sensitivity of pure Pd thin films and related structural deformation, which is essential to develop robust H-sensors with high sensibility. [Copyright &y& Elsevier]