Isotropical conductive adhesives with very-long silver nanowires as conductive fillers

Isotropical conductive adhesives (ICAs) have garnered great attention from the researchers in electronic industry as a potential substitute to lead-bearing solders for novel microsystem. In this paper, silver nanowires with a diameter of approximately 390 nm and a length of over 100 μm were synthesized by a polyol process. The ICAs composed of an epoxy-based binder containing silver nanowire were prepared and the curing behaviors, electrical properties, hygroscopicity, and the tensile shear strength of ICAs were investigated. Silver nanowires affect the curing behavior of epoxy resin and reduce the cross-linking density. The resistivity of the ICAs filled with 10, 35 and 45 wt% silver nanowire cured at 150 °C is 8.9 × 10^?3, 1.69 × 10^?5 and 4.9 × 10^?6 Ω cm respectively. The resistivity of the ICAs filled with silver nanowire cured at 150 °C is little change after aged under 85 °C/85 % for 264 h. With the increase of the loading of silver nanowire the tensile shear strength of the ICAs increase. The reasons for the effects of silver nanowires on the curing behavior and the electrical property and hygroscopicity and the tensile shear strength were discussed in terms of the morphology, distribution, and higher activity of silver nanowires.     

Ion implantation effects of negative oxygen on copper nanowires

Copper nanowires of diameter 80 nm were synthesized in polycarbonate membrane using template technique. Samples were then implanted with 160 keV O^?1 ion beam with varying particle fluence of 1?×?10¹², 5?×?10¹² and 1?×?10¹³ ions/cm². The SRIM (Stopping and range of ions in matter) software was used to study the processes involved. Compositional analysis confirms implantation of oxygen ions and the stoichiometry of Cu:O was found to be 6:1 by weight % when implanted at 1?×?10¹³ ions/cm². Scanning electron microscopy reveals no changes in morphology of nanowires on implantation. X-ray diffraction analysis showed no shifting in the ‘2θ’ position of diffraction peaks however some new diffraction peaks of oxygen were seen. Implantation with oxygen ion led to the increased crystallite size and reduced strain. The conductivity of the nanowires was found to increase linearly with the ion fluence presenting constructive effect of negative ion implantation on copper nanowires.     

Employing Al buffer layer with Al droplets-distributed surface to obtain high-quality and stress-free GaN epitaxial films on Si substrates

Al buffer layers with Al droplets-distributed surface have been employed to grow high-quality and stress-free GaN epitaxial films on Si substrates. The Al droplets are proved to efficiently improve the quality of as-grown GaN. On the one hand, they can act as nucleation seeds to facilitate the epitaxial growth, improving the crystalline quality and surface morphology of as-grown GaN epitaxial films. On the other hand, they also can compensate the huge compressive stress produced by Al buffer layer during the cooling process, achieving stress-free film. The density and volume of Al droplets greatly impact the properties of as-grown GaN epitaxial films. The GaN epitaxial film grown on the Al buffer layer with many small Al droplets uniformly distributed on it shows the best crystalline quality with the full-width at half maximum (FWHM) of GaN(0002) and GaN(10–12) as 0.5° and 0.7°, respectively, and flat surface with the smallest surface root-mean-square roughness of 3.8 nm. In addition, it also exhibits relatively better photoelectric properties with an FWHM of near band gap emission peak of 18 nm, carrier concentration of 2.0 × 10¹⁷ cm^?3, and mobility of 137.1 cm²/Vs. This work has revealed the advantages of Al buffer layer and the important effects of buffer layer surface on achieving high-quality GaN by PLD, which is of significance for various applications of GaN-based devices.     

Irradiation effects on AlGaN HFET devices and GaN layers

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with protons as well as carbon, oxygen, iron and krypton ions of high (68 and 120 MeV) and low (2 MeV) energy with fluences in the range from 1 × 10⁷ to 1 × 10¹³ cm^?2. High energy irradiation with protons, carbon and oxygen produced no degradation in devices while krypton irradiation at the fluence of 1 × 10¹⁰ cm^?2 resulted in a small reduction of 2% in the transconductance. Similarly, for GaN samples irradiated with protons, carbon and oxygen at high energy no changes were seen by XRD, PL and Hall effect, while changes in lattice constant and a reduction in PL intensity were observed after irradiation with high energy krypton. Low energy irradiation with carbon and oxygen at a fluence of 5 × 10¹⁰ cm^?2 results in small change in the device performance while remarkable changes in device characteristics are seen at a fluence of 1 × 10¹² cm^?2 for carbon, oxygen, iron and krypton irradiation. Similarly changes are also observed by XRD, PL and Hall effect for the thick GaN layer irradiated at the fluence of 1 × 10¹² cm^?2. The device results and GaN layer properties are strongly correlated.     

Self‐Supporting GaN Nanowires/Graphite Paper: Novel High‐Performance Flexible Supercapacitor Electrodes

Flexible supercapacitors have attracted great interest as energy storage devices because of their promise in applications such as wearable and smart electronic devices. Herein, a novel flexible supercapacitor electrode based on gallium nitride nanowire (GaN NW)/graphite paper (GP) nanocomposites is reported. The outstanding electrical conductivities of the GaN NW (6.36 × 10² S m^?1) and GP (7.5 × 10⁴ S m^?1) deliver a synergistically enhanced electrochemical performance that cannot be achieved by either of the components alone. The composite electrode exhibits excellent specific capacitance (237 mF cm^?2 at 0.1 mA cm^?2) and outstanding cycling performance (98% capacitance retention after 10 000 cycles). The flexible symmetric supercapacitor also manifests high energy and power densities (0.30 mW h cm^?3 and 1000 mW cm^?3). These findings demonstrate that the GaN/GP composite electrode has significant potential as a candidate for the flexible energy storage devices.     

Facile Synthesis of Bimetallic Ag/Ni Core/Sheath Nanowires and Their Magnetic and Electrical Properties

This paper describes a facile method for coating Ag nanowires with uniform, ferromagnetic sheaths made of polycrystalline Ni. A typical sample of these core/sheath nanowires had a saturation magnetization around 33 emu g^?1. We also demonstrated the use of this magnetic property to align the nanowires by simply placing a suspension of the nanowires on a substrate in a magnetic field and allowing the solvent to evaporate. The electrical conductivity of these core/sheath nanowires (2 × 10³ S cm^?1) was two orders of magnitude lower than that of bulk Ag (6.3 × 10⁵ S cm^?1) and Ni (1.4 × 10⁵ S cm^?1). This is likely caused by the transfer of electrons from the Ag core to the Ni sheath due to the difference in work function between the two metals. The electrons are expected to experience an increased resistance due to spin‐dependent scattering caused by the randomized magnetic domains in the polycrystalline, ferromagnetic Ni sheath. Studies on the structural changes to the Ni coating over time under different storage conditions show that storage of the nanowires on a substrate under ambient conditions leads to very little Ni oxidation after 6 months. These Ag/Ni core/sheath nanowires show promise in areas such as electronics, spintronics, and displays.     

Material and technology developments of the totally sputtering-made p/n GaN diodes for cost-effective power electronics

Mg-doped GaN (Mg–GaN) films have been deposited on Si (100) substrates by radio-frequency reactive sputtering technique with single cermet targets. The targets can be made by hot pressing the mixture of metallic Ga and Mg powders and ceramic GaN powder. X-ray diffraction results showed that Mg–GaN films had a wurtzite structure with a preferential nonpolar $ m - \left( {10\bar{1}0} \right) $ growth plane. Mg–GaN with 10.2 % Mg has transformed into p-type conductivity and has the carrier concentration of 9.37 × 10¹⁶ cm^?3, the highest mobility of 345 cm² V^?1 s^?1, and the highest conductivity of 3.23 S cm^?1. The band gap of Mg–GaN films retrieved from the absorption spectra is 2.93–3.06 eV. Furthermore, we have also fabricated a totally sputtering-made and cost-effective GaN diode with the ideality factors of 5.0 and 4.9 for the as-deposited and the annealed, respectively.     

Modifications induced by silicon and nickel ion beams in the electrical conductivity of zinc nanowires

This paper presents the modification in electrical conductivity of Zn nanowires under swift heavy ions irradiation at different fluences. The polycrystalline Zn nanowires were synthesized within polymeric templates, using electrochemical deposition technique and were irradiated with 80 MeV Si⁷⁺ and 110 MeV Ni⁸⁺ ion beams with fluence varying from 1 × 10¹² to 3 × 10¹³ ions/cm². I–V characteristics of exposed nanowires revealed a decrease in electrical conductivity with increase in ion fluence which was found to be independent of applied potential difference. But in the case of high fluence of Ni ion beam (3 × 10¹³ ions/cm²), electrical conductivity was found to increase with potential difference. The analysis found a significant contribution from grain boundaries scattering of conduction electrons and defects produced by ion beam during irradiation on flow of charge carriers in nanowires.     

Effects of growth temperature on electrical and structural properties of sputtered GaN films with a cermet target

GaN films have been deposited at 100–400 °C substrate temperature on Si (100) and sapphire (0001) substrates by RF reactive sputtering in an (Ar + N₂) atmosphere. A (Ga + GaN) cermet target for sputtering was made by hot pressing the mixed powders of metallic Ga and ceramic GaN. The effects of substrate temperature on the GaN formation and its properties were investigated. The diffraction results showed that GaN films with a preferential (10–10) growth plane had a wurtzite crystalline structure. GaN films became smoother at higher substrate temperature. The Hall effect measurements showed the electron concentration and mobility were 1.04 × 10¹⁸ cm^?3 and 7.1 cm² V^?1 s^?1, respectively, for GaN deposited at 400 °C. GaN films were tested for its thermal stability at 900 °C in the N₂ atmosphere. Electrical properties slightly degraded after annealing. The smaller bandgap of ~3.0 eV is explained in terms of intrinsic defects and lattice distortion.     

Low-temperature growth of well-aligned ZnO nanowire arrays by chemical bath deposition for hybrid solar cell application

Well-aligned ZnO nanowire arrays were grown on indium tin oxide coated glass substrates by a facile chemical bath deposition technique. Morphologies, crystalline structure and optical transmission were investigated by field-emission scanning electron microscope, X-ray diffraction and UV–visible transmission spectrum, respectively. The results showed that ZnO nanowires were aligned in a dense array approximately perpendicular to substrate surface, they were wurtzite-structured (hexagonal) ZnO. In addition, the nanowire arrays exhibited high optical transmission (>85 %) in the visible region. Furthermore, an inverted inorganic/polymer hybrid solar cell was built using as-grown well-aligned ZnO nanowire arrays as inorganic layer, under the AM 1.5 illumination with a light intensity of 80 mW/cm², the device showed an open circuit voltage (V_oc) of 0.44 V, a short circuit current (J_sc) of 3.23 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.68 %.     

Nanoscale size dependence parameters on lattice thermal conductivity of Wurtzite GaN nanowires

A detailed calculation of lattice thermal conductivity of freestanding Wurtzite GaN nanowires with diameter ranging from 97 to 160 nm in the temperature range 2–300 K, was performed using a modified Callaway model. Both longitudinal and transverse modes are taken into account explicitly in the model. A method is used to calculate the Debye and phonon group velocities for different nanowire diameters from their related melting points. Effect of Gruneisen parameter, surface roughness, and dislocations as structure dependent parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the experimentally measured curves. It was observed that Gruneisen parameter will decrease with decreasing nanowire diameters. Scattering of phonons is assumed to be by nanowire boundaries, imperfections, dislocations, electrons, and other phonons via both normal and Umklapp processes. Phonon confinement and size effects as well as the role of dislocation in limiting thermal conductivity are investigated. At high temperatures and for dislocation densities greater than 10¹⁴ m^?2 the lattice thermal conductivity would be limited by dislocation density, but for dislocation densities less than 10¹⁴ m^?2, lattice thermal conductivity would be independent of that.     

The fabrication and pyroelectric properties of single crystalline PZT nanorod synthesized by hydrothermal reaction

In this paper, the effect of surfactant polyvinylalcohol (PVA) and polyacrylate acid (PAA) on shape evolution of Pb(Zr_0.3Ti_0.7)O₃ (PZT) nano materials synthesized by hydrothermal method was studied. PZT nanorod array was grown on the conduction substrate surface of (100) Nb–SrTiO₃ with optimized PVA and PAA concentrations. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to characterize the PZT nanomaterials. The results demonstrated that the optimization concentration of PVA and PAA were 0.8 and 3.2 g L^?1, respectively. The pyroelectric coefficient of the PZT nanorod array was 1.75 × 10^?9 C cm^?2 K^?1 before poling and 2.56 × 10^?9 C cm^?2 K^?1 after poling. This low temperature synthesized PZT nanorod array shows great potential application in pyroelectric nanodevices.     

Enhanced electrochemical reduction of hydrogen peroxide by Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowire electrode

Crystalline Co₃O₄ nanowire arrays with different morphologies grown on Ni foam were investigated by varying the reaction temperature, the concentration of precursors, and reaction time. The Co₃O₄ nanowires synthesized under typical reaction condition had a diameter range of approximately 500–900 nm with a length of 17 µm. Electrochemical reduction of hydrogen peroxide (H₂O₂) of the optimized Co₃O₄ nanowire electrode was studied by cyclic voltammetry. A high current density of 101.8 mA cm^?2 was obtained at ?0.4 V in a solution of 0.4 M H₂O₂ and 3.0 M NaOH at room temperature compared to 85.8 mA cm^?2 at ?0.35 V of the Co₃O₄ nanoparticle electrode. Results clearly indicated that the Ni foam supported Co₃O₄ nanowire electrode exhibited superior catalytic activity and mass transport kinetics for H₂O₂ electrochemical reduction.     

Large third-order nonlinearity of nonpolar A-plane GaN film at 800 nm determined by Z-scan technology

We report an investigation on the optical third-order nonlinear property of the nonpolar A-plane GaN film. The film sample with a thickness of ~2?μm was grown on an r-plane sapphire substrate by metal-organic chemical vapor deposition system. By performing the Z-scan method combined with a mode-locked femtosecond Ti:sapphire laser (800?nm, 50?fs), the optical nonlinearity of the nonpolar A-plane GaN film was measured with the electric vector E of the laser beam being polarized parallel (//) and perpendicular (⊥) to the c axis of the film. The results show that both the third-order nonlinear absorption coefficient β and the nonlinear refractive index n₂ of the sample film possess negative and large values, i.e. β_//?=??135?±?29?cm/GW, n_2//?=??(4.0?±?0.3)?×?10^?3?cm²/GW and β_⊥?=??234?±?29?cm/GW, n_2⊥?=??(4.9?±?0.4)?×?10^?3?cm²/GW, which are much larger than those of conventional C-plane GaN film, GaN bulk, and even the other oxide semiconductors.     

Enhanced leakage current properties of HfO2/GaN gate dielectric stack by introducing an ultrathin buffer layer

The band alignments of HfO₂/GaN, HfO₂/SiO₂/GaN and HfO₂/Al₂O₃/GaN gate dielectric stacks were comparatively investigated by using X-ray photoelectron spectroscopy. It was observed that the introduction of an ultrathin buffer layer film (SiO₂ or Al₂O₃) in HfO₂/GaN stack can make the band alignments more symmetrical with larger barrier height as identified by the valence band offsets and electron energy loss spectrum measurements. At room temperature, the leakage current density as function of temperature is 4.1 × 10^?6, 3 × 10^?7 and 9.8 × 10^?8 A cm^?2 at the bias of 1 V for the HfO₂/GaN, HfO₂/Al₂O₃/GaN and HfO₂/SiO₂/GaN gate dielectric stacks, correspondingly. With temperature increase from room temperature to 300 °C, the HfO₂/SiO₂/GaN gate dielectric stack exhibits lowest lower leakage current density than that of others. The HfO₂/GaN high-k gate dielectric stack with an ultrathin SiO₂ buffer layer appears to be a promising candidate for future GaN based high temperature metal-oxide-semiconductor (MOS) devices applications.     

An effective three-dimensional surface-enhanced Raman scattering substrate based on oblique Si nanowire arrays decorated with Ag nanoparticles

Silicon nanowire arrays (SiNWAs) decorated with metallic nanoparticle heterostructures feature promising applications in surface-enhanced Raman scattering (SERS). However, the densely arranged SiNWAs are usually inconvenient for the following decoration of metallic nanoparticles, and only the top area of silicon nanowires (SiNWs) contributes to the SERS detection. To improve the utilization of the heterostructure, herein, oblique SiNWAs were grown separately, and Ag nanoparticles (AgNPs) were uniformly deposited by magnetron sputtering to get the three-dimensional (3D) SiNWAs decorated with AgNPs (AgNPs-SiNWAs) SERS substrate. The large open surfaces of oblique SiNWs would create more surface area available for the formation of hotspots and improve the adsorption and excitation of analyte molecules on the wire. The optimized AgNPs-SiNWAs substrate exhibits high sensitivity in detecting chemical molecule Rhodamine 6G, and the detection limit can reach 1 × 10^?10 M. More importantly, the substrate also can be used as an effective DNA sensor for label-free DNA detection.     

Bandgap variation in grain size controlled nanostructured CdO thin films deposited by pulsed-laser method

Cadmium oxide (CdO) thin films were prepared by pulsed laser deposition technique. Their structure, surface morphology, optical and electrical properties have been investigated. With a decrease in the laser energy density, the average grain size of the CdO film can be adjusted from 108 to 25 nm. High-resolution TEM observation showed that more crystalline defects like lattice distortion, dislocation and amorphous structure existed in the small grained (25 nm) CdO film, and X-ray photoelectron spectroscopy analysis confirmed that the film had more oxygen vacancies. The electrical and optical properties of the films significantly depended on the grain size. With the grain size decreasing to 25 nm, the optical band gap energy of the CdO film increased obviously from 2.82 to 3.33 eV. This change in the nature of material from semimetal to a wide band gap semiconductor, combining with its higher optical transmission (92 %) in visible light region, higher carrier concentration (1.25 × 10²¹ cm^?3) and lower electrical resistivity (2.8 × 10^?4 cm^?3), makes the nano-grained CdO film very useful in optoelectronic applications.     

Effects of nanoscale size dependent parameters on lattice thermal conductivity in Si nanowire

The effects of nanoscale size dependent parameters on lattice thermal conductivity are calculated using the Debye-Callaway model including transverse and longitudinal modes explicitly for Si nanowire with diameters of 115, 56, 37 and 22 nm. A direct method is used to calculate the group velocity for different size nanowire from their related calculated melting point. For all diameters considered, the effects of surface roughness, defects and transverse and longitudinal Gruneisen parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the reported experimental curve. The obtained fitting value for mean Gruneisen parameter has a systematic dependence on all Si nanowire diameters changing from 0·791 for 115 nm diameter to 1·515 for the 22 nm nanowire diameter. The dependence also gave a suggested surface thickness of about 5–6 nm. The other two parameters were found to have partially systematic dependence for diameters 115, 56, and 37 nm for defects and 56, 37 and 22 nm for the roughness. When the diameters go down from 115 to 22 nm, the concentration of dislocation increased from 1·16 × 10¹⁹cm⁻³ to 5·20 × 10¹⁹cm⁻³ while the surface roughness P found to increase from 0·475 to 0·130 and the rms height deviation from the surface changes by about 1·66 in this range of diameter. The diameter dependence also indicates a strong control of surface effect in surface to bulk ratio for the 22 nm wire diameter.     

Growth and characterizations of dual ion beam sputtered CIGS thin films for photovoltaic applications

The growth of CIGS thin films on soda-lime glass substrates at different substrate temperatures by dual ion beam sputtering system in a single-step route from a single quaternary sputtering target with the composition of Cu (In_0.70 Ga_0.30) Se₂ was reported. The effects of the substrate temperature on structural, optical, morphological and electrical properties of CIGS films were investigated. Stoichiometry of one such film was investigated by X-ray photoelectron spectroscopy. All CIGS films had demonstrated a strong (112) orientation located at 2θ ~26.70^o, which indicated the chalcopyrite structure of films. The value of full-width at half-maximum of (112) peak was reduced from 0.58° to 0.19° and crystallite size was enlarged from 14.98 to 43.05 nm as growth temperature was increased from 100 to 400 °C. However, atomic force microscope results showed a smooth and uniform surface at lower growth temperature and the surface roughness was observed to increase with increasing growth temperature. Hall measurements exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 10¹⁸ cm^?3 and mobility of 28.60 cm² V^?1 s^?1 for CIGS film grown at 100 °C. Film absorption coefficient was found to enhance nominally from 1 × 10⁵ to 2.3 × 10⁵ cm^?1 with increasing growth temperature from 100 to 400 °C.     

Nanocrystalline copper selenide thin films by chemical spray pyrolysis

Thin films of copper selenide were deposited onto amorphous glass substrates at various substrate temperatures by computerized spray pyrolysis technique. The as deposited copper selenide thin films were used to study a wide range of characteristics including structural, surface morphological, optical and electrical, Hall Effect and thermo-electrical properties. X-ray diffraction study reveals that the films are polycrystalline in nature with hexagonal (mineral klockmannite) crystal structure irrespective of the substrate temperature. The crystalline size is found to be in the range of 23–28 nm. The SEM study reveals that the grains are uniform with uneven spherically shaped and spread over the entire surface of the substrates. EDAX analysis confirmed the nearly stoichiometric deposition of the film at 350 °C. The direct band gap values are found to be in the range 2.29–2.36 eV depending on the substrate temperature. The Hall Effect study reveals that the films exhibit p-type conductivity. The values of carrier concentration and mobility for the film are found to be 5.02 × 10¹⁷ cm^?3 and 5.19 × 10^?3 cm² V^?1 s^?1; respectively for film deposited at 350 °C.