Nanogranular Thin Films Research Articles

Homogeneous and Nanogranular Prussian Blue to Enable Long-Term-Stable Electrochromic Devices.

The increasing demand for the state-of-the-art electrochromic devices has received great interest in synthesizing Prussian blue (PB) nanoparticles with a uniform diameter that exhibit excellent electrochromism, electrochemistry, and cyclability. Herein, we report the controllable synthesis of sub-100 nm PB nanoparticles via the coprecipitation method. The diameter of PB nanoparticles can be modulated by adjusting the reactant concentration, the selection of a chelator, and their purification. The self-assembled nanogranular thin films, homogeneously fabricated by using optimized PB nanoparticles with an average diameter of 50 nm as building blocks via the blade coating technique enable excellent performance with a large optical modulation of 80% and a high coloration efficiency of 417.79 cm2 C-1. It is also demonstrated by in situ and ex situ observations that the nanogranular PB thin films possess outstanding structural and electrochemical reversibility. Furthermore, such nanogranular PB thin films can enjoy the enhanced long-term cycling stability of the PB-WO3 complementary electrochromic devices having a 91.4% optical contrast retention after 16,000 consecutive cycles. This work provides a newly and industrially compatible approach to producing a complementary electrochromic device with extraordinary durability for various practical applications.

Improved electrochromic performance of potentiostatically electrodeposited nanogranular WO3 thin films

Improved electrochromic performance of potentiostatically electrodeposited nanogranular WO3 thin films

Facile synthesis of nanogranular PPy thin films for sensitive and selective detection of toxic NO gas

The pyrrole monomers have been synthesized by in situ oxidative polymerization and followed by the drop casting of polypyrrole thin films onto glass. The optimized PPy thin film for gas sensor showed the highest NO 2 sensitivity of 40.08 % at room temperature with a stable response even after 45 days with a retention of stability of around 86 %. The gas sensing results suggested that the nanogranular PPy sensor exhibited an excellent response toward NO 2 and also showed high sensitivity even at low concentrations up to 10 ppm. • Synthesis of pyrrole monomers by in situ oxidative polymerization for NO 2 gas sensor. • Achieve sensitivity of 40.08 % at room temperature with a stable response even after 45 days. • Selective sensing towards NO 2 within a wide range of concentrations from 10 to 100 ppm. • The sensor exhibits reliable long-term stability, reproducibility and reversibility. Nanogranular polypyrrole (PPy) have been synthesized by in-situ oxidative polymerization of pyrrole monomer followed by deposition of PPy thin films onto a glass substrate by drop casting method. The deposited thin films were characterized for their structural, morphological, compositional studies using x-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and x-ray photoelectron spectroscopic (XPS) studies. The XRD patterns revealed that the synthesized films are amorphous in nature. FE-SEM microscopic images exhibit agglomerated granular-like morphology of PPy thin films with high degree of roughness. The XRD patterns confirm the amorphous nature of PPy thin films. XPS analysis reveals the formation of PPy. The gas sensing properties of PPy thin film based gas sensor were studied in terms of sensitivity, response-recovery time, selectivity and stability by varying the concentration of nitrogen dioxide (NO 2 ) gas. The optimized PPy thin film gas sensor showed highest sensitivity of 40.08 % at room temperature with stable response even after 45 days with retention of stability around 86%. The gas sensing results suggested that nanogranular PPy sensor exhibited an excellent response toward NO 2 and also showed high sensitivity even at low concentrations upto 10 ppm level.

Design of various Ni–Cr nanostructures and deducing their magnetic anisotropy

Understanding the effects of interparticle interactions is a vital problem because magnetic nanoparticles showcase a variety of magnetic configurations due to different contributions to their total energy. To derive reliable and robust properties from magnetic nanoparticles, it is, thus, necessary to understand the competition between particle anisotropy and interparticle interactions that define the magnetic state of nanoparticles, where size control plays an important role. Here, we apply the random anisotropy model (RAM) that considers various magnetic interactions to selectively prepared NiCr nanostructures (NiCr dense nanoclusters, nanogranular NiCr thin films, and Ag(NiCr) nanocomposites) with different interparticle interactions. The estimated single-particle magnetic anisotropy K values (2.82 − 12.3 × 104 J/m3) and careful analysis of magnetization behavior for these nanostructures reveal that orbital hybridization, surface segregation, and interface character govern the magnetic interactions among nanoparticles. Our study demonstrates how magnetic behaviors vary in these different magnetic systems consisting of superparamagnetic (SPM) and ferromagnetic (FM) contributions specific to magnetic interactions.

Structure and tunneling magneto-dielectric properties of Co–SrF2 nano-granular thin films

Nano-granular thin films with a new composition designed to enhance the tunneling magneto-dielectric (TMD) effect of Co–SrF2 were successfully deposited using the co-sputtering method. This study explored the influence of the crystallinity of the matrix material on nano-granular structures. Cobalt contents were controlled from 0 to 19 at. % by changing the power of the cobalt cathode from 0 to 205 W. The structures in which the Co granules (Φ 3–4 nm) were homogeneously dispersed in the crystallized SrF2 matrix were clearly observed by transmission electron microscopy and its diffraction patterns. The dielectric constant of the Co19–(SrF2)81 thin film peaked at the giant value of 648 and the saturation magnetization peaked at 3.1 kG, but this material still exhibited a TMD response Δε′/ε′0 of 1.0%. For the Co16–(SrF2)84 thin film, the TMD response peaked at 3.5%, which is higher than previously reported. This study confirmed that increased matrix crystallinity can enhance the TMD response. It also provides motivation for other microstructure improvements.

Anomalous Nernst effect in the ceramic and thin film samples of La0.7Sr0.3CoO3 perovskite

We report the anomalous Nernst effect (ANE) in large-grain ceramics, nanogranular ceramics, and nanogranular thin films of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$, measured over the temperature range 5--300 K. The study is complemented with thermopower, resistivity, and magnetic measurements. The temperature-dependent ANE below the Curie temperature ${T}_{\mathrm{C}}$ (240--250 K) is analyzed with the help of longitudinal resistivity and Seebeck using a previously proposed formula derived by a combination of Onsager reciprocity, the Mott formula, and the relation between transverse and longitudinal resistivity, ${\ensuremath{\rho}}_{xy}\ensuremath{\propto}{\ensuremath{\rho}}_{xx}^{n}$. We observe a characteristic exponent n \ensuremath{\sim} 0.4 in agreement with the universal scaling for the bad-metal-type conduction regime. The nanogranular samples are characterized by higher resistivity, lower saturated magnetization, and a higher coercive field compared to large-grain ceramics. On the other hand, the magnitude of the ANE is independent of grain size. This observation likely insinuates that the characteristic length scale characterizing the ANE in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3}$ is below the grain size of nanogranular samples \ensuremath{\sim}40 nm. Therefore, the ANE associated with the bad-metal regime is independent of barriers associated with grain surface, which are responsible for activated resistivity and lowered magnetization due to a magnetically ``dead'' layer. The observation that the advantage of the higher coercive field of nanogranular samples is not deteriorated by a lower ANE is important for possible applications in zero magnetic field.

Optical and Magnetic Properties of Ag-Ni Bimetallic Nanoparticles Assembled via Pulsed Laser-Induced Dewetting.

Pulsed laser-induced dewetting (PLiD) of Ag0.5Ni0.5 thin films results in phase-separated bimetallic nanoparticles with size distributions that depend on the initial thin film thickness. Co-sputtering of Ag and Ni is used to generate the as-deposited (AD) nanogranular supersaturated thin films. The magnetic and optical properties of the AD thin films and PLiD nanoparticles are characterized using a vibrating sample magnetometer, optical absorption spectroscopy, and electron energy loss spectroscopy (EELS). Magnetic measurements demonstrate that Ag0.5Ni0.5 nanoparticles are ferromagnetic at room temperature when the nanoparticle diameters are >20 nm and superparamagnetic <20 nm. Optical measurements show that all nanoparticle size distributions possess a local surface plasmon resonance (LSPR) peak that red-shifts with increasing diameter. Following PLiD, a Janus nanoparticle morphology is observed in scanning transmission electron microscopy, and low-loss EELS reveals size-dependent Ag and Ni LSPR dipole modes, while higher order modes appear only in the Ag hemisphere. PLiD of Ag–Ni thin films is shown to be a viable technique to generate bimetallic nanoparticles with both magnetic and plasmonic functionality.

Finite-size effects in ultrafast remagnetization dynamics of FePt

We investigate the ultrafast magnetization dynamics of FePt in the L10 phase after an optical heating pulse, as used in heat assisted magnetic recording. We compare continuous and nanogranular thin films and emphasize the impact of the finite size on the remagnetization dynamics. The remagnetization speeds up significantly with increasing external magnetic field only for the continuous film, where domain wall motion governs the dynamics. The ultrafast remagnetization dynamics in the continuous film are only dominated by heat transport in the regime of high magnetic fields, whereas the timescale required for cooling is prevalent in the granular film for all magnetic field strengths. These findings highlight the necessary conditions for studying the intrinsic heat transport properties in magnetic materials.

Fabrication of nanogranular TiO2 thin films by SILAR technique: Application for NO2 gas sensor

The present investigation reports on the Nitrogen dioxide (NO2) gas sensing properties of the nanogrannular Titanium dioxide (TiO2) thin films synthesized on a glass substrate using simplistic successive ionic layer adsorption and reaction (SILAR) technique at room temperature. The deposition of TiO2 thin films was carried out by varying the SILAR reaction cycles. The synthesized TiO2 thin films showed a granular morphology of size 50–100 nm with an orthorhombic structure. The photoluminescence spectra of nanogranular TiO2 thin films exhibits absorption band edge in the visible region (∼442 nm). The gas sensing performance are evaluated as a function of nanogranular size of TiO2, the operating temperature, response and recovery time and sensitivity of NO2 gas from 50 to 200 ppm. A high response towards NO2 has been obtained at an operating temperature of 250 0C. Moreover, the nanogranular TiO2 thin films based gas sensor shows better gas sensitivity towards NO2 gas at 50 ppm with good selectivity.

Long-lived photoinduced absorption in granular molybdenum disulfide thin films

We present results of the differential photoinduced absorption spectra investigation of the molybdenum disulfide nanogranular thin films. The films were produced by means of a sulfurization of pre-deposited by magnetron sputtering thin molybdenum films. It has been shown that photoexcitation with the light quanta with the energy higher than the bandgap leads to a modification of the absorption spectra in the visible range. Possible nature of the photoinduced absorption spectra will be discussed.

An ellipsometric analysis to model the order-disorder transition in Au-SiO2 nano-granular thin films induced by thermal annealing

We present an ellipsometric analysis to model the order-disorder transition in nano-granular gold embedded in a silica matrix (Au-SiO2). The optical measurements were performed by means of Reflection Spectroscopic Ellipsometry (RSE) over the spectral wavelength range 300–2500 nm. Upon thermal annealing of Au-SiO2 in the temperature range of 200 to 800 °C, a clear order-disorder textural transition in the film is exhibited. Analyzing the surface roughness by atomic force microscopy (AFM), the as-prepared film shows a narrow height distribution peak around ~3 nm, which reveals a good uniformity of the particles size. When the annealing temperature increases, the particles increasingly tend to coalesce into bigger sizes leading to non-homogeneous films showing increasingly larger surface roughness. We systematically determined the evolution of the film surface roughness parameters by using the AFM cumulative height distribution, which was introduced in RSE in order to evaluate the effective optical constants of the Au–SiO2 films, and to numerically fit the recorded Ψ, Δ and T spectra using a multiple Gaussian oscillators model. As a result, a reliable determination of the Au-SiO2 effective thicknesses and optical constants was achieved. Additionally, we fitted the experimental data to the Maxwell-Garnett (MG) effective medium approximation (EMA). It is shown that MG-EMA does not fit for the lower annealing temperature due to the particle self-organization. However, when the particles disorder takes place, as a consequence of the annealing temperature increase, the MG-EMA fit is increasingly improved. All in all, a clear evidence of the order-disorder transition in the Au-SiO2 nano-granular system induced by thermal annealing is provided.

Compositional Dependence of Optical and Structural Properties of Nanogranular Mixed ZrO2/ZnO/SnO2 Thin Film

A study on the optical properties and photoluminescence (PL) spectra of ternary oxide nanogranular thin films comprising Zr, Zn, and Sn revealed that the change in component ratio could direct the roadmap to improve characteristics of the films. Grazing angle X-ray diffraction analysis showed that incorporation of Sn atoms into the tetragonal structure of Zn/Zr thin film resulted in an amorphous structure. The band gap of film was tunable by precisely controlling the concentration of components. The widening of band gap could correlate to the quantum confinement effect. PL spectra of the composite thin films under excitation at 365 nm showed a sharp red emission with relatively Gaussian line shape, which was intensified in the optimum percentage ratio of 50/30/20. This nearly red emission is attributed to the radiative emission of electrons captured at low-energy traps located near the valence band. An optimum red emission is strongly desirable for use in white LEDs. The comparative study on FTIR spectra of unary, binary, and ternary thin films confirmed successful composition of three different metal oxides in ternary thin films. Detailed investigation on FTIR spectra of ternary compounds revealed that the quenching in PL emission at higher percentage of Sn was originally due to the hydroxyl group.

Magnetic Nanoparticles in “Amorphous Ferromagnetic Metal-Insulator” Nanogranular thin Films

A lot of factors limit possible applications of magnetic nanoparticles in medicine for drug delivery and magnetic hyperthermia therefore it is of primary importance to understand influence of classical and quantum-size effects, surface layers, interparticle distance, shape of nanoparticles on their magnetic properties. Magnetic nanogranular thin films, known also as nanocomposites, can be considered as a convenient model for such investigations as it is possible to tune easily many of mentioned above parameters by varying of the fabrication conditions. The ion-beam sputtering technique has been developed to prepare “amorphous ferromagnetic metal-insulator” nanocomposites with different concentration and parameters simultaneously in one technological cycle. This feature is achieved by using of a composite target (consisting of a metal and dielectric parts) with an asymmetric arrangement of the dielectric parts on the metal base. Influence of sputtering conditions and post-fabrication treatment on structural, magnetic, electrical and magnetotransport properties of magnetic nanocomposites in a wide range of metal volume fraction and distance between magnetic nanoparticles is being discussed.

Structural Characteristics and Magnetic Properties of Al2O3 Matrix-Based Co-Cermet Nanogranular Films

Magnetic micro- and nanogranular materials prepared by different methods have been used widely in studies of magnetooptical response. However, among them there seems to be nothing about magnetic nanogranular thin films prepared by a rf cosputtering technique for both metals and insulators till now. This paper presented and discussed preparation, structural characteristics, and magnetic properties of alumina (Al2O3) matrix-based granular Co-cermet thin films deposited by means of the cosputtering technique for both Co and Al2O3. By varying the ferromagnetic (Co) atomic fraction, x, from 0.04 to 0.63, several dominant features of deposition for these thin films were shown. Structural characteristics by X-ray diffraction confirmed a cermet-type structure for these films. Furthermore, magnetic behaviours presented a transition from paramagnetic- to superparamagnetic- and then to ferromagnetic-like properties, indicating agglomeration and growth following Co components of Co clusters or nanoparticles. These results show a typical granular Co-cermet feature for the Co-Al2O3 thin films prepared, in which Co magnetic nanogranules are dispersed in a ceramic matrix. Such nanomaterials can be applied suitably for our investigations in future on the magnetooptical responses of spinplasmonics.

Electrochromic performance of sol–gel derived amorphous MnO2–ZnO nanogranular composite thin films

In this work, the MnO2–ZnO thin films with nominal molar ratios of 8%, 16% and 25% of Zn to Mn were deposited on glass and Indium Tin Oxide (ITO) substrates by dip-coating sol–gel method and annealed in air at 300°C. The films were characterized by Cyclic Voltammetry measurement (CV), X-ray Diffraction spectroscopy (XRD), Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray spectroscopy (EDX) and UV–Visible spectrophotometry. The XRD patterns indicated the amorphous structure for all samples. The CV measurements were performed in the potential window between −2V and 2V at a potential scan rate of 20mV/s and 50mV/s. The results of CVs at a scan rate of 20mV/s in the first cycle showed that the sample with 25% of Zn content with the maximum anodic and cathodic charge density has a better performance compared to other samples and the capacitance decreases with increasing cycle number of all samples at 50mV/s. Optical constants and optical band gaps of thin films were investigated by transmission and reflection. The AFM images revealed that roughness increases with increasing Zn concentration. The FE-SEM images showed nearly similar morphology for all thin films.

Interplay effects of humidity and UV light sensitivities of Zn0.9Mg0.1O nanogranular thin films

Zn0.9Mg0.1O nanogranular thin films with a thickness of 250nm were deposited by sol–gel spin coating process on ITO and glass substrates. Their morphology and microstructure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and atomic force microscopy (AFM). The surface was dominated by zinc oxide and magnesium carbonates. The electrical properties were governed by capillary condensation. This study demonstrates the sensitivity of these films to humidity and UV light and the interplay between both effects, already observed for individual ZnO nanowires. This highlights the ability of the Zn0.9Mg0.1O thin films to be integrated in humidity and UV sensing devices but with some cautious to allows a separations of the two environmental parameters.

High frequency characteristics of (Ni75Fe2)x(ZnO)1−x granular thin films with tunable damping coefficient**Project supported by the National Natural Science Foundation of China (Grant Nos. 50901050 and 51101079).

The effect of the volume fraction of ferromagnetic metal (x) in (Ni75Fe25)x(ZnO)1−x nanogranular thin films on microstructural, soft-magnetic, and high-frequency properties was investigated. Good soft-magnetic properties were obtained in a broad x range, with 0.55 < x < 0.82. High resolution transmission electron microscopy (HRTEM) observations reveal that the grain size of the samples is lower than 14 nm, and that it decreases with decreasing x. Of special interest, our investigation of the permeability spectra indicates that these films exhibit an adjustable frequency linewidth of resonance peak, dependant upon changing x. Correspondingly, large and adjustable damping coefficients (αeff) from 0.023 to 0.043 were achieved by decreasing x from 0.82 to 0.55. Combined with the HRTEM results, the variation of αeff with x was analyzed in detail.

Study of thickness-dependent magnetic and transport properties of Fe/Al nanostructures

The paper presents magnetic and transport properties of compositionally modulated Fe/Al multilayer structures (MLS), with an overall atomic concentration ratio of Fe:Al = 3:1, 2:1 and 1:1. All MLS show soft ferromagnetic behaviour at room temperature (RT) with an in-plane easy axis of magnetization. In each case, coercivity increases continuously and magnetization decreases with an increase in temperature due to enhancement in the anisotropy as a result of non-uniform and disordered formation of thin intermixed (dead) FeAl layer at the interfaces. The Curie temperature obtained for the MLS is much less than that of bcc Fe but is well above RT. The observed magnetic behaviour is mainly attributed to the formation of different FeAl phases and increase in anti-ferromagnetic interlayer coupling with addition of Al. The formation of these phases is also supported by resistivity results. The results of this research enabled us to understand that by controlling of layers thickness and temperature in multilayer systems, the nanogranular thin films with good resistive and soft magnetic properties can be obtained.

Temperature dependent rotation of the uniaxial easy axis of magnetization in granular SiO2–(Co/Fe/CoFe2)–Si(1 1 1) multilayers

We have observed temperature dependent rotation of the uniaxial easy axis of magnetization and introduced the phenomenon of the superspin reorientation transition (SSRT) for the nanogranular thin films and multilayers. The effect is demonstrated on the nanogranular SiO2–M(x)–Si(111) multilayers, where M is the metal (Co, Fe or CoFe2) of the nominal thickness x=3, 5 and 10nm, respectively. The morphology and layer thickness have been examined using the transmission electron microscopy, X-ray reflectivity measurement and atomic force microscopy. Magnetic property measurements demonstrated that the orientation of the uniaxial in-plane easy axis of magnetization given by the film shape anisotropy was thickness independent at temperatures above 200K. Below 50K, the films shape anisotropy has been overcome and the uniaxial easy axis was rotated into different directions for each samples series, forming an angles from 40 up to 150° with respect to the film plane. Such effect of the low temperature magnetization rotation has been attributed to the specific arrangement of the nanoparticles in the individual layers and has been assigned to the SSRT phenomena.

Air-stable nanogranular Fe thin films formed by Chemical Vapor Deposition of triiron dodecacarbonyl as catalysts for carbon nanotube growth

Thin, air-stable, nanogranular iron-based films of 10±1.2, 100±14 and 400±46nm thicknesses were prepared by Chemical Vapor Deposition (CVD) of triiron dodecacarbonyl, Fe3(CO)12, onto silicon wafers at 150°C in an inert environment at a standard pressure of 100kPa. These thin nanogranular structured films, composed of sintered elemental Fe nanoparticles of 4.1±0.7nm diameter, are protected from oxidation by a very thin layer of carbon. The thickness of the Fe based films was controlled by changing the coating parameters such as coating time, concentration of the iron precursor, or by repeating the coating process. The saturation magnetization of these thin Fe coatings was similar to that of bulk iron. The electric resistivity of these films was thickness dependent. The morphology of these coatings was altered by annealing the films under argon, at a standard pressure of 100kPa, at different temperatures (e.g., 350, 550 and 750°C). In the present manuscript, by CVD using ethylene as a carbon precursor, these Fe based films were used as a platform for synthesizing carpets of crystalline and vertically aligned carbon nanotubes.