Z.H. Cao

Ultrathin diamond-like carbon film coated silver nanoparticles-based substrates for surface-enhanced Raman spectroscopy.

We have demonstrated that by coating with a thin dielectric layer of tetrahedral amorphous carbon (ta-C), a biocompatible and optical transparent material in the visible range, the Ag nanoparticle-based substrate becomes extremely suitable for surface-enhanced Raman spectroscopy (SERS). Our measurements show that a 10 A or thicker ta-C layer becomes efficient to protect the oxygen-free Ag in air and prevent Ag ionizing in aqueous solutions. Furthermore, the Ag nanoparticles substrate coated with a 10 A ta-C film shows a higher enhancement of Raman signals than the uncoated substrate. These observations are further supported by our numerical simulations. We suggest that biomolecule detections in analytic assays could be easily realized using ta-C-coated Ag-based substrate for SERS especially in the visible range. The coated substrate also has higher mechanical stability, chemical inertness, and technological compliance, and may be useful, for example, to enhance TiO(2) photocatalysis and solar-cell efficiency by the surface plasmons.

Fabrication of centimeter-sized single-domain two-dimensional colloidal crystals in a wedge-shaped cell under capillary forces.

Self-assembly of colloidal spheres confined within cells of different shapes formed with two slides under capillary forces are studied. It is found that by controlling the shape of the cell the curvature of the drying front can result in a significant effect on the self-organization process. A curved drying front formed within parallel slides is always associated with growth of colloidal crystal structures with a high density of disorder. We demonstrate that single-domain two-dimensional colloidal crystals with centimeter size can be grown under capillary forces under a straight drying front formed in a wedge-shaped cell. These findings are demonstrated by laser diffraction, microscopy imaging methods and off-normal optical transmission measurements. The present growth method should be of importance in expanding colloidal crystal applications in angle-resolved nanosphere lithography, as well as in preparation of high-quality quasi-three-dimensional plasmonic crystals.

Thickness and grain size dependent mechanical properties of Cu films studied by nanoindentation tests

The hardness and the elastic modulus of Cu films with thickness (t) and grain size (d) have been investigated by nanoindentation tests. The d and the indentation depth increase linearly with the increase in t. The hardness rises with the decrease in t, whereas the elastic modulus is independent of t and it is about 20% less than conventional coarse-grained Cu. The enhanced hardness is attributed to the smaller d and the indentation depth. The analysis of load–displacement curves indicates that the scope of the critical shear stress for different thick Cu films ranges from 3.2 to 4.1 GPa, which is similar to the theoretical shear stress of single crystalline Cu. The present results are explained by the dislocation mediated mechanism even if d reaches about 16.4 nm for the Cu film with t = 180 nm.

Indentation size dependent plastic deformation of nanocrystalline and ultrafine grain Cu films at nanoscale

Nanoindentation creep tests were performed in the depth range from about 28 to 190 nm on nanocrystalline (NC) and ultrafine grain (UFG) Cu films. Pronounced indentation size effects on hardness, creep strain rate (e), and strain rate sensitivity (mc) are observed. Both e and mc are dependent not only on contact depth (hc) but also on grain size. The experiment results and analysis support that the creep deformation of NC and UFG Cu is dominated by grain-boundary-mediated process and diffusion along the interface of tip sample, respectively, under a critical hc and dislocation-mediated process begin to work as hc increases further.

1D partially oxidized porous silicon photonic crystal reflector for mid-infrared application

A functional structure material with good thermal insulation properties, which can also reflect infrared light within a certain range, was obtained by introducing a photonic crystal structure as a reflector into oxidized porous silicon. The porous silicon multilayer was fabricated based on alternative changes in etching current intensity. The oxidation process was performed to ensure a full oxidation of the high porosity layers and a partial oxidization of the low porosity layers to achieve good thermal insulation properties while maintaining enough contrast of the refractory index. The microstructure was characterized by scanning electron microscopy and the optical reflectance spectra by Fourier transform infrared spectroscopy. A band gap centred at 3 µm with a bandwidth of 1.3 µm was successfully obtained.

Achieving high strength and high electrical conductivity in Ag/Cu multilayers

In this work, we investigated the microstructure evolution of Ag/Cu multilayers and its influences on the hardness and electric resistivity with individual layer thickness (h) ranging from 3 to 50 nm. The hardness increases with the decreasing h in the range of 5–20 nm. The barrier to dislocation transmission by stacking faults, twin boundaries, and interfaces leads to hardness enhancement. Simultaneously, in order to get high conductivity, the strong textures in-layers were induced to form for reducing the amount of grain boundaries. The resistivity keeps low even when h decreases to 10 nm. Furthermore, we developed a facile model to evaluate the comprehensive property of Ag/Cu multilayers—the results indicate that the best combination of strength and conductivity occurs when h = 10 nm.

Symmetric and anti-symmetric magnetic resonances in double-triangle nanoparticle arrays fabricated via angle-resolved nanosphere lithography

We report experimentally that for a particular high-symmetry planar periodic arrangement of metal double-triangle nanoparticle arrays fabricated via angle resolved nanosphere lithography, both anti-symmetric and symmetric magnetic resonances can be explicitly excited at off-normal incidence. Further, we demonstrate that the underlying mechanism for the formation of these two modes is a result of direct interactions with the incident electric and magnetic fields, respectively. As a consequence, with increasing the incident angle there is a relatively small blue-shift in the transmission for the electric-field induced anti-symmetric mode, while a remarkable red-shift is observed for the magnetic-field induced symmetric mode.

Rolling deformation induced reduction of rate sensitivity and enhancement of hardness in nanocrystalline NiFe alloys

Both hardness (H) and rate sensitivity (m) of nanocrystalline NiFe alloys were studied by nanoindentation testing. It was found that H increases, and m decreases after rolling in the alloys. It is interesting that the decrease in m by rolling is totally contrary to the conventional coarse grain alloys. The dislocation density is remarkably enhanced by rolling deformation, which leads to the hardening behaviour of the samples. The dislocation absorbed at the grain boundary (GB) and/or sub-GB and grain growth by rolling are responsible for the reduced m of the rolled alloys.

Annealing improved ductility and fracture toughness of nanocrystalline Cu films on flexible substrates

In this paper, the effect of annealing temperature (T) on the ductility of 50 nm thick nanocrystalline (NC) Cu films adhered to flexible substrates was investigated by a uniaxial tension test. It was found that the ductility and the fracture toughness (Gc) can be significantly improved through an annealing treatment. The crack onset strain of the 300 °C annealed Cu film is 18.1%, which is about twice that of the as-deposited NC Cu film. In addition, Gc of the 300 °C annealed Cu film is 1833 J m−2, which is nearly three times that of the as-deposited NC Cu film. Focused ion beam results indicate that the as-deposited film fractures with delamination and strain localization coevolving, while the as-annealed film fractures by adhering well to the substrate. At a higher T, the tensile residual stress is lower, the microstructure is more stable, and a diffusion or compound interface is generated, resulting in a better bonding between the film and the substrate. In this case, the strain localization is suppressed more effectively, causing improved ductility and Gc. Whether the film is as-deposited or as-annealed, the saturated crack spacing is about 1.41 µm, which accords well with the theoretical analysis. Intergranular fracture is suggested to be the main fracture mechanism.

Order-disorder transition and Curie transition in Ni70Fe30 nanoalloy

This letter addresses the issue of the order-disorder and Curie transitions in Ni70Fe30 nanoalloy. The ordered phase is observed at room temperature while the disordered phase appears when the nanoalloy is heated up to 773 K. By means of mechanical spectroscopy, x-ray diffraction, and vibrating sample magnetometer measurements, the order-disorder and Curie transition temperatures of 20 nm Ni70Fe30 nanoalloy are determined to be 636 and 728 K, both lower than the corresponding values in the coarse-grained form. Moreover, the reduction in these two critical temperatures is consistent with the predictions of a thermodynamic analytical model.