Synthesis of Ag2S and Ag2Se nanoparticles in self assembled block copolymer micelles and nano-arrays fabrication

This paper describes a simple approach based on self assembled polystyrene-block-poly(4-vinylpyridine) (PS-P4VP) copolymer micelles for the fabrication of ordered 2-dimesional nano-arrays of Ag₂S and Ag₂Se. The nanoparticles were synthesized directly inside the P4VP micelles core at room temperature and nano-arrays were obtained by spin coating on silicon wafer. High resolution-transmission electron microscopy and powder X-ray diffraction characterization suggests the nanoparticles are crystalline. Higher band gap energy (1.76 eV for Ag₂S and 1.35 eV for Ag₂Se) compared to their bulk materials obtained from the absorption studies reveals that the nanoparticles are within the quantum confinement regime.     

Rapid synthesis of Ag2Se nanocrystals by sonochemical reaction

Ag₂Se nanocrystals were rapidly synthesized by the sonochemical reaction between Ag ions and Se powders in diluted ammonia water. By using different complexing agents (NH₃, citric acid or KSCN) in the reaction system, Ag₂Se nano-spheres with different sizes were obtained. X-ray diffraction patterns showed that the samples obtained was orthorhombic β-Ag₂Se. The morphology was characterized by scanning electron microscopy (SEM). This mild method may be extended to prepare other chalcogenides nanocrystalline at room temperature.     

Structural,thermal and transport studies on Ag1 − x Cuxl (0.05 ⩽ x ⩽ 0.25) solid electrolyte

Preparation and characterization studies on polycrystalline samples of Ag_{1 – x}Cu_xl wherex=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, have been reported. Samples were analysed using powder X-ray diffraction (XRD) and differential scanning calorimetric (DSC) techniques in order to identify the compositions and phase transition temperatures. A.c. electrical conductivity studies were carried out on pelleted specimens of various compositions in the frequency range 65.5 kHz to 1 Hz and over the temperature range 293–412 K. DSC results obtained in the temperature range 373–473 K have shown that the ß- to -phase transition temperature is enhanced from 426 K to 438 K whenx is increased from 0.05 to 0.25. XRD results have indicated that there is a shift ind-spacing when the Cul content is increased, suggesting changes in the crystal structure. Typical XRD patterns recorded for the composition Ag_0.95Cu_0.05l at three different temperatures (room temperature, 373 and 473 K, respectively) have confirmed that both face-centred cubic and hexagonal phases would be present at room temperature and at 373 K as well, whereas at 473 K the structure would be purely body-centred cubic in nature. A.c. impedance analysis of the above samples appears to suggest that their electrical conductivity, predominantly due to the migration of Ag⁺ ions, lies in the order of 10^–4S cm^–1 at room temperature.     

Morphological and optical properties of PdxAg1-x alloy nanoparticles

Alloy nanoparticles (NPs) can offer a wide range of opportunities for various applications due to their composition and structure dependent properties such as multifunctionality, electronic heterogeneity, site-specific response, and multiple plasmon resonance bands. In this work, the fabrication of self-assembled Pd_xAg_1-x NPs alloy nanostructures with distinct size, density, shape, and composition is demonstrated via the solid-state dewetting of sputtered Pd/Ag thin films on c-plane sapphire. The initial stage of bilayer dewetting exhibits the nucleation of voids, followed by the expansion of voids and cluster breakdown and finally shape transformation along with the temperature control. Bilayer composition shows a substantial influence on the dewetting such that the overall dewetting is enhanced along with the increased Ag composition, i.e. Pd_0.25Ag_0.75 > Pd_0.5Ag_0.5 > Pd_0.75Ag_0.25. On the other hand, the size and density of NPs can be efficiently controlled by varying the initial thickness of bilayers. Reflectance peaks in UV and near-infrared (NIR) regions and a wide absorption band in the visible region arisen from the surface plasmon resonance are observed in reflectance spectra. The peak intensity depends on the composition of Pd_xAg_1-x NPs and the NIR peaks gradually blue-shift with the size decrement.     

Polarized memory switching effects in Ag2Se/Se/M thin film sandwiches

The current-voltage characteristics of Ag₂Se/Se/M thin film sandwiches were studied as functions of the shapes of the electrodes, which were either symmetrical or asymmetrical, as well as their composition (M is a metal (gold, chromium or silver) or Ag₂Se). A polarized memory switching effect independent of air exposure was observed.The first commutation occurs at a formation voltage V_F which is dependent on the selenium thickness. After some cycles this voltage stabilizes to a value V_S (where V_S<V_F). V_S is a function of temperature, frequency and the maximum reverse voltage but is independent of the selenium thickness.All these results are explained in terms of Ag⁺ ionic diffusion: the formation is related to the growth of Ag₂Se dendrites through selenium as shown by temperature-dependent experiments. Furthermore the variations in V_S are accounted for by ionic motion localized at the M-Se interface.     

Effect of annealing atmosphere on phase formation and electrical characteristics of bismuth ferrite thin films

Bismuth ferrite thin films were deposited on Pt/Ti/SiO₂/Si substrates by a soft chemical method and spin-coating technique. The effect of annealing atmosphere (air, N₂ and O₂) on the structure and electrical properties of the films are reported. X-ray diffraction analysis reveals that the film annealed in air atmosphere is a single-phase perovskite structure. The films annealed in air showed better crystallinity and the presence of a single BFO phase leading to lower leakage current density and superior ferroelectric hysteresis loops at room temperature. In this way, we reveal that BFO film crystallized in air atmosphere by the soft chemical method can be useful for practical applications, including nonvolatile digital memories, spintronics and data-storage media.     

High‐Performance Flexible Organic Nano‐Floating Gate Memory Devices Functionalized with Cobalt Ferrite Nanoparticles

Nano‐floating gate memory (NFGM) devices are transistor‐type memory devices that use nanostructured materials as charge trap sites. They have recently attracted a great deal of attention due to their excellent performance, capability for multilevel programming, and suitability as platforms for integrated circuits. Herein, novel NFGM devices have been fabricated using semiconducting cobalt ferrite (CoFe₂O₄) nanoparticles (NPs) as charge trap sites and pentacene as a p‐type semiconductor. Monodisperse CoFe₂O₄ NPs with different diameters have been synthesized by thermal decomposition and embedded in NFGM devices. The particle size effects on the memory performance have been investigated in terms of energy levels and particle–particle interactions. CoFe₂O₄ NP‐based memory devices exhibit a large memory window (≈73.84 V), a high read current on/off ratio (read I_on/I_off) of ≈2.98 × 10³_, and excellent data retention. Fast switching behaviors are observed due to the exceptional charge trapping/release capability of CoFe₂O₄ NPs surrounded by the oleate layer, which acts as an alternative tunneling dielectric layer and simplifies the device fabrication process. Furthermore, the NFGM devices show excellent thermal stability, and flexible memory devices fabricated on plastic substrates exhibit remarkable mechanical and electrical stability. This study demonstrates a viable means of fabricating highly flexible, high‐performance organic memory devices.     

Metal nanocrystal memory with sol-gel derived HfO2 high-κ tunnel oxide

An all-solution processed metal-oxide-semiconductor (MOS) capacitor structure containing gold (Au) nanoparticles (NPs) within HfO₂ high-κ oxide was fabricated. The ultra-thin (~ 10 nm) HfO₂ high-κ tunnel oxide layer was prepared by sol-gel process and showed good electrical properties, which were critical to superior memory property of the MOS structure. Au NPs with particle size of about 3.3 nm were synthesized by chemical reduction method and then self-assembled onto HfO₂ tunnel oxide. Finally, a Si/HfO₂/Au NPs/HfO₂ memory structure was constructed after the substrate had been covered with a sol-gel-derived HfO₂ control oxide layer (~ 13 nm). By utilizing high-quality HfO₂ as tunnel oxide, the MOS structure containing Au NPs showed memory effect even at a low voltage of ± 3 V. Although its memory window was only 0.8 V by a swapping voltage between ± 5 V, the MOS showed desirable retention characteristics. Therefore, we have fabricated nanocrystal memory device with sol-gel derived HfO₂ high-k tunnel oxide which are attractive for low operation voltage non-volatile memory applications.     

Phase diagram and high ionic conductivity of the system Na2WO4Ag2WO4

The phase diagram of the system Na₂WO₄Ag₂WO₄ was determined with DTA and x-ray diffractometry. The intermediate β-phase of Na₂WO₄ disappears above 0.03 mole fraction of Ag₂WO₄. The intersolubility of both compounds is good in the low temperature γ-phase of Na₂WO₄ and in the high temperature α-phase. The electrical ac conductivity was measured as a function of temperature through the phase diagram.     

Cation Exchange Synthesis and Unusual Resistive Switching Behaviors of Ag2Se Nanobelts

Ag₂Se nanobelts are prepared through employing ZnSe nanobelts as templates via a facile cation exchange approach. The templates are derived from precursor ZnSe·0.5N₂H₄ nanobelts, which are synthesized by a simple hydrothermal method. As‐synthesized precursor nanobelts are with 200 nm in width and several hundreds of micrometers in length. Annealed in N₂, they are transformed into ZnSe nanobelts with preserving their initial morphology. Following with a complete replacement of Zn²⁺ by Ag⁺, Ag₂Se nanobelts with single crystalline are obtained via a cation‐exchange reaction. Combined with the Langmuir–Blodgett assembly technique, regular films of ZnSe nanobelts can be achieved on transparent glass substrates and Si wafers with interdigital Au electrode arrays. Further, the optical and electrical evolutions are investigated from ZnSe nanobelts to Ag₂Se nanobelts. Finally, the resistive switching characteristic are carefully explored for Ag₂Se nanobelts regularly arranged on interdigital Au microelectrodes. The results indicate that it is analogous to complementary resistive switching behaviors, which is different from that of traditional two terminal devices about previously reported Ag₂Se. In order to clarify this phenomenon, a possible mechanism has been proposed and indirectly demonstrated through in situ SEM (scanning electron microscropy) observation.     

Evaluation of the toxicities of silver and silver sulfide nanoparticles against Gram‐positive and Gram‐negative bacteria

In this study, the endogenous lipid signalling molecules, N ‐myristoylethanolamine, were explored as a capping agent to synthesise stable silver nanoparticles (AgNPs) and Ag sulphide NPs (Ag₂ S NPs). Sulphidation of the AgNPs abolishes the surface plasmon resonance (SPR) maximum of AgNPs at 415 nm with concomitant changes in the SPR, indicating the formation of Ag₂ S NPs. Transmission electron microscopy revealed that the AgNPs and Ag₂ S NPs are spherical in shape with a size of 5–30 and 8–30 nm, respectively. AgNPs and Ag₂ S NPs exhibit antimicrobial activity against Gram‐positive and Gram‐negative bacteria. The minimum inhibitory concentrations (MIC) of 25 and 50 μM for AgNPs and Ag₂ S NPs, respectively, were determined from resazurin microtitre plate assay. At or above MIC, both AgNPs and Ag₂ S NPs decrease the cell viability through the mechanism of membrane damage and generation of excess reactive oxygen species.Inspec keywords: cellular biophysics, biomembranes, transmission electron microscopy, nanomedicine, microorganisms, molecular biophysics, antibacterial activity, nanofabrication, silver, biomedical materials, surface plasmon resonance, nanoparticles, materials preparation, silver compounds, lipid bilayersOther keywords: Gram‐negative bacteria, Gram‐positive bacteria, endogenous lipid signalling molecules, N‐myristoylethanolamine, capping agent, silver nanoparticles, Ag sulphide NPs, sulphidation, surface plasmon resonance, concomitant changes, transmission electron microscopy, minimum inhibitory concentrations, resazurin microtitre plate assay, cell viability, membrane damage, reactive oxygen species, Ag toxicities, Ag, Ag₂ S     

Ag<Subscript>2</Subscript>Se complex nanostructures with photocatalytic activity and superhydrophobicity

Single-crystalline Ag₂Se complex nanostructures have been synthesized via a solvothermal route in which selenophene (C₄H₄Se) as a selenylation source reacts with AgNO₃ at a temperature of 240 °C. An orthorhombic phase β-Ag₂Se nanostructure was identified by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectroscopy. The wettability of the as-synthesized β-Ag₂Se nanostructure was studied by measurement of the water contact angle (CA). Static water CA values of over 150° were obtained, which can be attributed to the β-Ag₂Se complex nanostructure having a combination of micro- and nanostructures. The superhydrophobic Ag₂Se nanostructure may find applications in self-cleaning. Additionally, the photocatalytic activity of the as-synthesized β-Ag₂Se nanostructure was evaluated by photodegradation of rhodamine B (RhB) dye under ultraviolet (UV) light irradiation.     

Multiferroic Bi0.7Dy0.3FeO3 thin films directly integrated on Si for integrated circuit compatible devices

Magnetoelectric multiferroic Bi_0.7Dy_0.3FeO₃ (BDFO) thin films deposited on p-type Si (100) substrate using pulsed laser deposition technique demonstrated a saturated ferroelectric and ferromagnetic hysteresis loop at room temperature. More interestingly, the observed change in electric polarization with applied magnetic field in these films indicated the presence of room temperature magnetoelectric coupling behavior. Using high-frequency capacitance-voltage measurements, the fixed oxide charge density, interface trap density and dielectric constant were estimated on Au/BDFO/Si capacitors. These results suggest the integrated circuit compatible application potential of BDFO films in the field of micro-electro-mechanical systems and non-volatile memories.     

Thermoanalytical study of the polymorphism and melting behavior of Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

We have developed a procedure for the synthesis of phase-pure α- and β-Cu₂V₂O₇. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu₂V₂O₇ (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu₂V₂O₇, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu₂V₂O₇, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu₂V₂O₇, CuVO₃, and other compounds.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Photocatalytic Reduction of Graphene Oxide–TiO2 Nanocomposites for Improving Resistive‐Switching Memory Behaviors

Graphene oxide (GO)‐based resistive‐switching (RS) memories offer the promise of low‐temperature solution‐processability and high mechanical flexibility, making them ideally suited for future flexible electronic devices. The RS of GO can be recognized as electric‐field‐induced connection/disconnection of nanoscale reduced graphene oxide (RGO) conducting filaments (CFs). Instead of operating an electrical FORMING process, which generally results in high randomness of RGO CFs due to current overshoot, a TiO₂‐assisted photocatalytic reduction method is used to generate RGO‐domains locally through controlling the UV irradiation time and TiO₂ concentration. The elimination of the FORMING process successfully suppresses the RGO overgrowth and improved RS memory characteristics are achieved in graphene oxide–TiO₂ (Go‐TiO₂) nanocomposites, including reduced SET voltage, improved switching variability, and increased switching speed. Furthermore, the room‐temperature process of this method is compatible with flexible plastic substrates and the memory cells exhibit excellent flexibility. Experimental results evidence that the combined advantages of reducing the oxygen‐migration barrier and enhancing the local‐electric‐field with RGO‐manipulation are responsible for the improved RS behaviors. These results offer valuable insight into the role of RGO‐domains in GO memory devices, and also, this mild photoreduction method can be extended to the development of carbon‐based flexible electronics.     

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

Abstract

High-quality thermoelectric La_0.2Sr_0.8TiO₃ (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO₃(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10^?4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO₃ single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.     

Concentration dependences at the critical temperatures in vacuum topotaxial Ag2Se thin layers

Ag₂Se thin epitaxial layers were grown under vacuum conditions. The starting components were evaporated sequentially on various substrates and followed by annealing to obtain topotaxial layers, which were either poly- or monocrystalline, depending on the substrate. Ag₂Se undergoes a reversible first order phase transition at about 133 °C. The temperatures of the phase transitions are different on heating and cooling and these transitions and associated hysteresis can be demonstrated also by Hall measurements of electron concentration and electron mobility. The mobility shows a sudden decrease at the critical temperature on heating and a sudden increase on cooling. In the present experiments concentration dependences of the mobilities and of the resistivity at the critical temperatures defining the onset of hysteresis (four points), were measured for both types of structure. The μ(n) and ρ(n) dependences show a c₀n^-c power type character with exponents −0.33 and −0.43 for mobilities and −0.58 and −0.72 for the resistivity. The slopes are only slightly different for the poly- and monocrystalline samples. Also the ratios of c₀-s are about 1.4 and 1.6, respectively.     

Les composes Ag7AsS6 et Ag7AsSe6 etude des proprietes thermiques,cristallographiques et electriques

The preparation and the partial phase diagrams Ag₂X“As₂X₅” (X = S or Se) are described. Peritectic decompositions occur at 560°C for Ag₇AsS₆ and 360°C for Ag₇AsSe₆. Phase changes are observed at 250°C for Ag₇AsS₆ and 150°C for Ag₇AsSe₆. For each compound, the low temperature form is cubic P2₁3, and the high temperature form has the Ag₈GeTe₆ structural type, F$\text{4}$3m. The high temperature forms are not quenchable. Ionic and electronic conductivity have been measured in Ag₇AsS₆ and Ag₇AsSe₆. Ionic conductivity was measured using RbAg₄I₅ as a blocking electrode for electronic conduction. Electromotive force measurements confirm transference numbers. At room temperature, ionic conductivities were 1,5.10^?6 (Ω.cm)^?1 and 0,08 (Ω.cm)^?1 for Ag₇AsS₆ and Ag₇AsSe₆ respectively.     

Studies on nonvolatile resistance memory switching in ZnO thin films

Six decades of research on ZnO has recently sprouted a new branch in the domain of resistive random access memories. Highly resistive and c-axis oriented ZnO thin films were grown by us using d.c. discharge assisted pulsed laser deposition on Pt/Ti/SiO₂/Si substrates at room temperature. The resistive switching characteristics of these films were studied in the top-bottom configuration using current-voltage measurements at room temperature. Reliable and repeated switching of the resistance of ZnO thin films was obtained between two well defined states of high and low resistance with a narrow dispersion and small switching voltages. Resistance ratios of the high resistance state to low resistance state were found to be in the range of 2–5 orders of magnitude up to 20 test cycles. The conduction mechanism was found to be dominated by the Ohmic behaviour in low resistance states, while Poole-Frenkel emission was found to dominate in high resistance state. The achieved characteristics of the resistive switching in ZnO thin films seem to be promising for nonvolatile memory applications.