Layer engineering in optoelectronic and photonic properties of single and few layer phosphorene using first-principles calculations.

Developing devices for optoelectronic and photonic applications-based nanomaterials has been one of the most critical challenges in the last decade. In this work, we use first-principles density functional theory combined with non-equilibrium Green's function to highlight for the first time the sensitivity of optoelectronic and photonic properties toward the exfoliation process. All the studied structures were relaxed and their relevant phonon modes confirm the high structural stability. The obtained phosphorene layers remained semiconducting with a direct band gap like the respective bulk structure with 10 layers. We also examined the effects of the thickness on the electron-hole interaction by calculating absorption energy combined with electron relaxation lifetimes. Additionally, we explore the optoelectronic properties, which can also be influenced by the exfoliation. Finally, we found that the current-voltage (I-V) characteristic shows higher sensitivity toward the bulk structure than the other 2D forms of phosphorene structures, meaning that the Schottky barrier at the interface of the bulk phosphorene is much lower than mono, and few layer phosphorene.

The Influence of TiO2 Nanoparticles Morphologies on the Performance of Lithium-Ion Batteries.

Anode materials based on the TiO2 nanoparticles of different morphologies were prepared using the hydrothermal method and characterized by various techniques, such as X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and N2 absorption. The TiO2 nanoparticles prepared were used as anode materials for lithium-ion batteries (LIBs), and their electrochemical properties were tested using discharging/charging measurements. The results showed that the initial morphology of the nanoparticles plays a minor role in battery performance after the first few cycles and that better capacity was achieved for TiO2 nanobelt morphology. The sharp drop in the specific capacity of LIB during their first cycles is examined by considering changes in the morphology of TiO2 particles and their porosity properties in terms of size and connectivity. The performance of TiO2 anode materials has also been assessed by considering their phase.

Synthesis of TiO2 Nanobelt Bundles Decorated with TiO2 Nanoparticles and Aggregates and Their Use as Anode Materials for Lithium-Ion Batteries.

TiO2 nanobelt bundles decorated with TiO2 aggregates were prepared using an easy and scalable hydrothermal method at various temperatures (170, 190, 210, and 230 °C). It was demonstrated that the synthesis temperature is a key parameter to tune the number of aggregates on the nanobelt surface. Prepared TiO2 aggregates and nanobelt bundles were used to design anode materials in which the aggregates regulated the pore size and connectivity of the interconnected nanobelt bundle structure. A galvanostatic technique was employed for the electrochemical characterization of TiO2 samples. Using TiO2 as a model material due to its small volume change during the cycling of lithium-ion batteries (LIBs), the relationship between the morphology of the anode materials and the capacity retention of the LIBs on cycling is discussed. It was clearly found that the size and connectivity of the pores and the specific surface area had a striking impact on the Li insertion behavior, lithium storage capability, and cycling performance of the batteries. The initial irreversible capacity was shown to increase as the specific surface area increased. As the pore size increased, the ability of the mesoporous anatase to release strain was stronger, resulting in better cycling stability. The TiO2 powder prepared at a temperature of 230 °C displayed the highest discharge and charge capacities (203.3 mAh/g and 140.8 mAh/g) and good cycling stability.

Effects of Adsorbate Diffusion and Edges in a Transition from Particle to Dendritic Morphology during Silver Electrodeposition.

During silver electrodeposition on Au nanoparticle (NP)-covered highly oriented pyrolitic graphite, a transition from an initial growth of microsized particles to the growth of dendrites with pine tree shape (nanotrees) is observed, which is an advancement for material growth with hierarchical surface roughness. Using kinetic Monte Carlo simulations of an electrodeposition model, those results are explained by the interplay of diffusive cation flux in the electrolyte and relaxation of adsorbed atoms by diffusion on quenched crystal surfaces. First, simulations on NP-patterned substrates show the initial growth of faceted silver particles, followed by the growth of nanotrees with shapes similar to the experiments. Next, simulations on electrodes with large prebuilt particles explain the preferential nanotree growth at corners and edges as a tip effect. Simulations on wide flat electrodes relate the nanotree width with two model parameters describing surface diffusion of silver atoms: maximal number of random hops (G) and probability of hop per neighbor (P). Finally, simulations with small electrode seeds confirm the transition from initially compact particles to the nucleation of nanotrees and provide estimates of the transition sizes as a function of those parameters. The simulated compact and dendritic deposits show dominant (111) surface orientation, as observed in experiments. Extrapolations of simulation results to match microparticle and nanotree sizes lead to G = 4 × 1011 and P = 0.03, suggesting to interpret those sizes as diffusion lengths on the growing surfaces and giving diffusion coefficients 2 to 3 × 10-13 m2/s for deposited silver atoms. These results may motivate studies to relate diffusion coefficients with atomic-scale interactions.

A Micropowered Chemoresistive Sensor Based on a Thin Alumina Nanoporous Membrane and SnxBikMoyOz Nanocomposite.

This work presents and discusses the design of an efficient gas sensor, as well as the technological process of its fabrication. The optimal dimensions of the different sensor elements including their deformation were determined considering the geometric modeling and the calculated moduli of the elasticity and thermal conductivity coefficients. Multicomponent SnxBikMoyOz thin films were prepared by ionic layering on an anodic alumina membrane and were used as gas-sensitive layers in the sensor design. The resistance of the SnxBikMoyOz nanostructured film at temperatures up to 150 °C exceeded 106 Ohm but decreased to 104 Ohm at 550 °C in air. The sensitivity of the SnxBikMoyOz composite to concentrations of 5 and 40 ppm H2 at 250 °C (10 mW) was determined to be 0.22 and 0.40, respectively.

Spatially Ordered Matrix of Nanostructured Tin-Tungsten Oxides Nanocomposites Formed by Ionic Layer Deposition for Gas Sensing.

The process of layer-by-layer ionic deposition of tin-tungsten oxide films on smooth silicon substrates and nanoporous anodic alumina matrices has been studied. To achieve the film deposition, solutions containing cationic SnF2 or SnCl2 and anionic Na2WO4 or (NH4)2O·WO3 precursors have been used. The effect of the solution compositions on the films deposition rates, morphology, composition, and properties was investigated. Possible mechanisms of tin-tungsten oxide films deposition into the pores and on the surface of anodic alumina are discussed. The electro-physical and gas-sensitive properties of nanostructured SnxWyOz films have been investigated. The prepared nanocomposites exhibit stable semiconductor properties characterized by high resistance and low temperature coefficient of electrical resistance of about 1.6 × 10-3 K-1. The sensitivity of the SnxWyOz films to 2 and 10 ppm concentrations of ammonia at 523 K was 0.35 and 1.17, respectively. At concentrations of 1 and 2 ppm of nitrogen dioxide, the sensitivity was 0.48 and 1.4, respectively, at a temperature of 473 K. At the temperature of 573 K, the sensitivity of 1.3 was obtained for 100 ppm of ethanol. The prepared nanostructured tin-tungsten oxide films showed promising gas-sensitivity, which makes them a good candidate for the manufacturing of gas sensors with high sensitivity and low power consumption.

Hydrothermal Synthesis of TiO2 Aggregates and Their Application as Negative Electrodes for Lithium-Ion Batteries: The Conflicting Effects of Specific Surface and Pore Size.

TiO2 aggregates of controlled size have been successfully prepared by hydrothermal synthesis using TiO2 nanoparticles of different sizes as a building unit. In this work, different techniques were used to characterize the as-prepared TiO2 aggregates, e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer, Emmett and Teller technique (BET), field emission gun scanning electron microscopy (FEGSEM), electrochemical measurements etc. The size of prepared TiO2 aggregates varied from 10-100 nm, and their pore size from around 5-12 nm; this size has been shown to depend on synthesis temperature. The mechanism of the aggregate formations was discussed in terms of efficiency of collision and coalescence processes. These newly synthetized TiO2 aggregates have been investigated as potential negative insertion electrode materials for lithium-ion batteries. The influence of specific surface areas and pore sizes on the improved capacity was discussed-and conflicting effects pointed out.

Large-Scale Synthesis Route of TiO2 Nanomaterials with Controlled Morphologies Using Hydrothermal Method and TiO2 Aggregates as Precursor.

TiO2 of controlled morphologies have been successfully prepared hydrothermally using TiO2 aggregates of different sizes. Different techniques were used to characterize the prepared TiO2 powder such as XRD, XPS, FEGSEM, EDS, and HRTEM. It was illustrated that the prepared TiO2 powders are of high crystallinity with different morphologies such as nanobelt, nanourchin, and nanotube depending on the synthesis conditions of temperature, time, and additives. The mechanism behind the formation of prepared morphologies is proposed involving nanosheet intermediate formation. Furthermore, it was found that the nanoparticle properties were governed by those of TiO2 nanoparticles aggregate used as a precursor. For example, the size of prepared nanobelts was proven to be influenced by the aggregates size used as a precursor for the synthesis.

Different Electrochemical Sensor Designs Based on Diazonium Salts and Gold Nanoparticles for Pico Molar Detection of Metals.

We report a comparison of sensors' performance of different hybrid nanomaterial architectures modifying an indium tin oxide (ITO) electrode surface. Diazonium salts and gold nanoparticles (AuNPs) were used as building units to design hybrid thin films of successive layers on the ITO electrode surface. Different architectures of hybrid thin films were prepared and characterized with different techniques, such as TEM, FEG-SEM, XPS, and EIS. The prepared electrodes were used to fabricate sensors for heavy metal detection and their performances were investigated using the square wave voltammetry (SWV) method. The comparison of the obtained results shows that the deposition of AuNPs on the ITO surface, and their subsequent functionalization by diazonium salt, is the best performing architecture achieving a high sensitivity in terms of the lower detection limit of pico molar.

Dye-Doped ZnO Microcapsules for High Throughput and Sensitive Optofluidic Micro-Thermometry.

The main objective of this work is to show the proof of concept of a new optofluidic method for high throughput fluorescence-based thermometry, which enables the measure of temperature inside optofluidic microsystems at the millisecond (ms) time scale (high throughput). We used droplet microfluidics to produce highly monodisperse microspheres from dispersed zinc oxide (ZnO) nanocrystals and doped them with rhodamine B (RhB) or/and rhodamine 6G (Rh6G). The fluorescence intensities of these two dyes are known to depend linearly on temperature but in two opposite manner. Their mixture enables for the construction of reference probe whose fluorescence does not depend practically on temperature. The use of zinc oxide microparticles as temperature probes in microfluidic channels has two main advantages: (i) avoid the diffusion and the adsorption of the dyes inside the walls of the microfluidic channels and (ii) enhance dissipation of the heat generated by the focused incident laser beam thanks to the high thermal conductivity of this material. Our results show that the fluorescence intensity of RhB decreases linearly with increasing temperature at a rate of about -2.2%/°C, in a very good agreement with the literature. In contrast, we observed for the first time a nonlinear change of the fluorescence intensity of Rh6G in ZnO microparticles with a minimum intensity at a temperature equal to 40 °C. This behaviour is reproducible and was observed only with ZnO microparticles doped with Rh6G.

Crossover from compact to branched films in electrodeposition with surface diffusion.

We study a model for thin film electrodeposition in which instability development by preferential adsorption and reduction of cations at surface peaks competes with surface relaxation by diffusion of the adsorbates. The model considers cations moving in a supported electrolyte, adsorption and reduction when they reach the film surface, and consequent production of mobile particles that execute activated surface diffusion, which is represented by a sequence of random hops to neighboring lattice sites with a maximum of G hop attempts (G≫1), a detachment probability ε<1 per neighboring particle, and a no-desorption condition. Computer simulations show the formation of a compact wetting layer followed by the growth of branched deposits. The maximal thickness z_{c} of that layer increases with G but is weakly affected by ε. A scaling approach describes the crossover from smooth film growth to unstable growth and predicts z_{c}∼G^{γ}, with γ=1/[2(1-ν)]≈0.43, where ν≈0.30 is the inverse of the dynamical exponent of the Villain-Lai-Das Sarma equation that describes the initial roughening. Using previous results for related deposition models, the thickness z_{c} can be predicted as a function of an activation energy for terrace surface diffusion and the temperature, and the small effects of the parameter ε are justified. These predictions are confirmed by the numerical results with good accuracy. We discuss possible applications, with a particular focus on the growth of multifuncional structures with stacking layers of different porosity.

First-principles investigation of methanethiol adsorption and dissociation mechanisms on the high-Miller-index vicinal surface Cu(4 1 0).

In this work, we present detailed investigations of methanethiol adsorption on a Cu(4 1 0) surface within the framework of the self-consistent first-principles calculations as implemented in the Vienna ab initio simulation package (VASP). In particular, the adsorption sites, the surface coverage rate and electronic properties have been determined and compared to experimental values. The results indicate that the favorable adsorption site in the case of low coverage rate is a bridge on the step followed by the hollow site on the terrace. The adsorption significantly affects the outermost layer of the surface mainly for a higher coverage rate in a (2 × 2) supercell. The nature of the chemisorption process on the surface is analyzed by means of the density of states which, combined with charge density difference and atomic charge calculations, confirms the ionic character of the S-Cu bond. The specific effect of the presence of steps is highlighted by comparing the adsorption on the (1 0 0) terrace to the adsorption on the extended Cu(1 0 0) surface. Compared to the flat Cu(1 0 0), it is found here that while the stability is almost the same at p(2 × 2) coverage, the CH3S/Cu(4 1 0) becomes more stable than CH3S/Cu(1 0 0) at c(2 × 2) coverage with 0.30 eV per molecule. The mechanism of methanethiol dissociation is explored by the nudged elastic band method and demonstrates that the most favorable path is dissociation followed by migration of hydrogen from the step to its most stable position (hollow on the terrace) with energy barriers less than 0.5 eV.

Electrochemical synthesis using a self-assembled Au nanoparticle template of dendritic films with unusual wetting properties.

This report describes the electrochemical preparation of dendritic silver films with unusual wetting properties coming from the use of a self-assembled gold nanoparticle (Au NP) template. It shows that the Au NP self-assembled monolayer on the highly ordered pyrolytic graphite (HOPG) surface is responsible for the formation of the dendritic morphology, which is not observed for the same deposition conditions on a bare HOPG substrate. An interesting evolution of the wetting properties of these films during the electrodeposition process is observed. Field emission scanning electron microscopy (FEGSEM), energy-dispersive spectrometry (EDS), x-ray photoelectron spectroscopy (XPS) and contact angle measurements are used to reveal the dendritic structure of the deposited silver film at a later stage of the electrodeposition process. They also reveal surprising wetting properties in terms of hydrophobic surface.