Morphology and Optical Properties of Gas-Phase-Synthesized Plasmonic Nanoparticles: Cu and Cu/MgO.

In this paper, an investigation of the properties of Cu and Cu/MgO nanoparticles (NPs) is presented. The NPs were obtained with gas-phase synthesis, and the MgO shells or matrices were formed via the co-deposition method on inert substrates. SEM and AFM were used to investigate the NP morphology on Si/SiOx, quartz, and HOPG. The Cu NPs revealed flattening of their shape, and when they were deposited on HOPG, diffusion and formation of small chains were observed. The embedding of Cu NPs in MgO was confirmed by TEM and EDX maps. XPS showed that Cu was in its metallic state, regardless of the presence of the surrounding MgO. UV-Vis revealed the presence of an intense localized surface plasmon resonance (LSPR) for Cu/MgO and for "bare" NPs. These results confirmed the role of MgO as a protective transparent medium for Cu, and the wavelength position of the LSPR in the Cu/MgO system was consistent with calculations. The wavelength position of the LSPR observed for "bare" and post-oxidized Cu NPs was probably affected by the formation of copper oxide shells after exposure to air. This study paves the way for the use of Cu/MgO NPs as plasmonic nanomaterials in applications such as photovoltaics and sensor technology.

Graphene Confers Ultralow Friction on Nanogear Cogs.

Friction-induced energy dissipation impedes the performance of nanomechanical devices. Nevertheless, the application of graphene is known to modulate frictional dissipation by inducing local strain. This work reports on the nanomechanics of graphene conformed on different textured silicon surfaces that mimic the cogs of a nanoscale gear. The variation in the pitch lengths regulates the strain induced in capped graphene revealed by scanning probe techniques, Raman spectroscopy, and molecular dynamics simulation. The atomistic visualization elucidates asymmetric straining of CC bonds over the corrugated architecture resulting in distinct friction dissipation with respect to the groove axis. Experimental results are reported for strain-dependent solid lubrication which can be regulated by the corrugation and leads to ultralow frictional forces. The results are applicable for graphene covered corrugated structures with movable components such as nanoelectromechanical systems, nanoscale gears, and robotics.

Ag/MgO Nanoparticles via Gas Aggregation Nanocluster Source for Perovskite Solar Cell Engineering.

Nanocluster aggregation sources based on magnetron-sputtering represent precise and versatile means to deposit a controlled quantity of metal nanoparticles at selected interfaces. In this work, we exploit this methodology to produce Ag/MgO nanoparticles (NPs) and deposit them on a glass/FTO/TiO2 substrate, which constitutes the mesoscopic front electrode of a monolithic perovskite-based solar cell (PSC). Herein, the Ag NP growth through magnetron sputtering and gas aggregation, subsequently covered with MgO ultrathin layers, is fully characterized in terms of structural and morphological properties while thermal stability and endurance against air-induced oxidation are demonstrated in accordance with PSC manufacturing processes. Finally, once the NP coverage is optimized, the Ag/MgO engineered PSCs demonstrate an overall increase of 5% in terms of device power conversion efficiencies (up to 17.8%).

ZnO Thin Films Growth Optimization for Piezoelectric Application.

The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode.

Friction and Adhesion of Different Structural Defects of Graphene.

Graphene structural defects, namely edges, step-edges, and wrinkles, are susceptible to severe mechanical deformation and stresses under tribo-mechanical operations. Applied forces may cause deformation by folding, buckling, bending, and tearing of these defective sites of graphene, which lead to a remarkable decline in normal and friction load bearing capacity. In this work, we experimentally quantified the maximum sustainable normal and friction forces, corresponding to the damage thresholds of the different investigated defects as well as their pull-out (adhesion) forces. Horizontal wrinkles (with respect to the basal plane, i.e., folded) sustained the highest normal load, up to 317 nN, during sliding, whereas for vertical (i.e., standing) wrinkles, step-edges, and edges, the load bearing capacities are up to 113, 74, and 63 nN, respectively. The related deformation mechanisms were also experimentally investigated by varying the normal load up to the initiation of the damage from the defects and extended with the numerical results from molecular dynamics and finite element method simulations.

Tribological characteristics of few-layer graphene over Ni grain and interface boundaries.

The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10(-5) Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off adhesion forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer(s). Finite element simulations showed good agreement with experiments and led to a rationalization of the observations. Thus, with interfacial grain boundaries the FLG tribology can be effectively tuned.

Frictional transition from superlubric islands to pinned monolayers.

The inertial sliding of physisorbed submonolayer islands on crystal surfaces contains unexpected information on the exceptionally smooth sliding state associated with incommensurate superlubricity and on the mechanisms of its disappearance. Here, in a joint quartz crystal microbalance and molecular dynamics simulation case study of Xe on Cu(111), we show how superlubricity emerges in the large size limit of naturally incommensurate Xe islands. As coverage approaches a full monolayer, theory also predicts an abrupt adhesion-driven two-dimensional density compression on the order of several per cent, implying a hysteretic jump from superlubric free islands to a pressurized commensurate immobile monolayer. This scenario is fully supported by the quartz crystal microbalance data, which show remarkably large slip times with increasing submonolayer coverage, signalling superlubricity, followed by a dramatic drop to zero for the dense commensurate monolayer. Careful analysis of this variety of island sliding phenomena will be essential in future applications of friction at crystal/adsorbate interfaces.

Influence of size, shape and core-shell interface on surface plasmon resonance in Ag and Ag@MgO nanoparticle films deposited on Si/SiO x.

Ag and Ag@MgO core-shell nanoparticles (NPs) with a diameter of d = 3-10 nm were obtained by physical synthesis methods and deposited on Si with its native ultrathin oxide layer SiO x (Si/SiO x ). Scanning electron microscopy and transmission electron microscopy (TEM) images of bare Ag NPs revealed the presence of small NP aggregates caused by diffusion on the surface and agglomeration. Atomic resolution TEM gave evidence of the presence of crystalline multidomains in the NPs, which were due to aggregation and multitwinning occurring during NP growth in the nanocluster source. Co-deposition of Ag NPs and Mg atoms in an oxygen atmosphere gave rise to formation of a MgO shell matrix surrounding the Ag NPs. The behaviour of the surface plasmon resonance (SPR) excitation in surface differential reflectivity (SDR) spectra with p-polarised light was investigated for bare Ag and Ag@MgO NPs. It was shown that the presence of MgO around the Ag NPs caused a red shift of the plasmon excitation, and served to preserve its existence after prolonged (five months) exposure to air, realizing the possibility of technological applications in plasmonic devices. The Ag NP and Ag@MgO NP film features in the SDR spectra could be reproduced by classical electrodynamics simulations by treating the NP-containing layer as an effective Maxwell Garnett medium. The simulations gave results in agreement with the experiments when accounting for the experimentally observed aggregation.

Controlled AFM detachments and movement of nanoparticles: gold clusters on HOPG at different temperatures.

The effect of temperature on the onset of movement of gold nanoclusters (diameter 27 nm) deposited on highly oriented pyrolytic graphite (HOPG) has been studied by atomic force microscopy (AFM) techniques. Using the AFM with amplitude modulation (tapping mode AFM) we have stimulated and controlled the movement of individual clusters. We show how, at room temperature, controlled detachments and smooth movements can be obtained for clusters having dimensions comparable to or smaller than the tip radius. Displacement is practically visible in real time and it can be started and stopped easily by adjusting only one parameter, the tip amplitude oscillation. Analysing the energy dissipation signal at the onset of nanocluster sliding we evaluated a detachment threshold energy as a function of temperature in the range 300-413 K. We also analysed single cluster thermal induced displacement and combining this delicate procedure with AFM forced movement behaviour we conclude that detachment threshold energy is directly related to the activation energy of nanocluster diffusion and it scales linearly with temperature as expected for a single-particle thermally activated process.