Sen-Jiang Yu

Controlled Formation of Surface Patterns in Metal Films Deposited on Elasticity-Gradient PDMS Substrates

Controlled surface patterns are useful in a wide range of applications including flexible electronics, elastomeric optics, fluidic channels, surface engineering, measurement technique, biological templates, stamps, and sensors. In this work, we report on the controlled formation of surface patterns in metal films deposited on elasticity-gradient polydimethylsiloxane (PDMS) substrates. Because of the temperature gradient during the curing process, the PDMS substrate in each sample successively changes from a purely liquid state at one side to a purely elastic state at the opposite side. It is found that surface folds appear in the liquid or viscous PDMS region while wrinkles form in the elastic region. In the transition region from the liquid to elastic PDMS, a nested pattern (i.e., the coexisting of folds and wrinkles) can be observed. The folding wave is triggered by the intrinsic stress during the film deposition and its wavelength is independent of the film thickness. The wrinkling wave is induced by the thermal compression after deposition and its wavelength is proportional to the film thickness. The report in this work could promote better understanding of the effect of substrate elasticity on the surface patterns and fabrication of such patterns (folds and wrinkles) by tuning the substrate property.

Tunable Formation of Ordered Wrinkles in Metal Films with Controlled Thickness Gradients Deposited on Soft Elastic Substrates

Controlled wrinkled surface is useful for a wide range of applications, including flexible electronics, smart adhesion, wettability, stamping, sensoring, coating, and measuring. In this work, thickness-gradient-guided spontaneous formation of ordered wrinkling patterns in metal films deposited on soft elastic substrates is revealed by atomic force microscopy, theoretic analysis, and simulation. It is observed that in the thicker film region, broad cracks form, and the film surface remains flat. In the thinner film region, the cracks attenuate along the direction of the thickness decrease, and various wrinkle patterns including branched stripes, herringbones, and labyrinths can coexist. The interplay between the residual compression and the thickness gradient leading to the formation of such wrinkling patterns is discussed based on a nonlinear wrinkling model. The simulated wrinkling patterns as well as the variation trends of the wrinkle wavelength and amplitude along the gradient direction are in good agreement with the experimental observations. The report in this work could promote better understanding and fabrication of such ordered wrinkling patterns by tunable thickness gradient.

FORMATION MECHANISM AND SURFACE EVOLUTION OF SILVER FILMS SPUTTERING DEPOSITED ON SILICONE OIL SUBSTRATES

The formation mechanism and surface evolution of thin silver films deposited on silicone oil substrates by a DC-magnetron sputtering method are reported. As the film thickness increases, the deposited silver atoms first form compact clusters, then transfer to ramified aggregates and finally form a continuous film on the liquid substrate. After deposition, the surface morphology of the silver film is susceptible to evolve successively in the atmosphere condition, resulting in the formation of broad cracks and straight-sided (or worm-like) wrinkles. The evolution behaviors and underlying physical mechanisms of the cracks and wrinkles are presented and discussed in detail.

GROWTH MECHANISM OF IRON FILMS ON SILICONE OIL SURFACES PREPARED BY SPUTTERING METHOD

The growth mechanism and characteristic ordered patterns of iron (Fe) films deposited on the silicone oil surfaces by a DC-magnetron sputtering method are presented in this paper. It is found that as the film thickness increases, the iron atoms deposited on the oil surface first form compact clusters, then transfer to ramified aggregates and web-shaped structures, and finally form a continuous iron film. The average branch width of the ramified aggregates is about 0.34 μm, which is almost independent of the sputtering power, i.e., the deposition rate. In the continuous iron films, large spatially disk-shaped patterns are observed, which result from spontaneous ordered organization of the iron atoms and atomic clusters driven by the internal stress in this nearly free sustained film system.

Large scale synthesis of FeS coated Fe nanoparticles as reusable magnetic photocatalysts

The FeS coated Fe nanoparticles were prepared by using high temperature reactions between the commercial Fe nanoparticles and the S powders in a sealed quartz tube. The simple method developed in this work is effective for large scale synthesis of FeS/Fe nanoparticles with tunable shell/core structures, which can be obtained by controlling the atomic ratio of Fe to S. The structural, magnetic and photocatalytic properties of the nanoparticles were investigated systematically. The good photocatalytic performance originating from the FeS shell in degradation of methylene blue under visible light and the high saturation magnetization originating from the ferromagnetic Fe core make the FeS/Fe nanoparticles a good photocatalyst that can be collected and recycled easily with a magnet. An exchange bias up to 11 mT induced in Fe by FeS was observed in the Fe/FeS nanoparticles with ferro/antiferromagnetic interfaces. The enhanced coercivity up to 32 mT was ascribed to the size effect of Fe core.

In-situ photoluminescence monitoring of GaN in plasma exposure

We have investigated in-situ photoluminescence properties of GaN surfaces exposed to Ar plasma. With increasing plasma exposure time, both intensities of near-band-edge (NBE) peak and yellow band luminescence (YL) significantly decrease, whereas that of blue band luminescence (BL) is seen to gradually increase. Additionally, the YL/NBE intensity ratio is almost invariant, but the BL/NBE intensity ratio increases dramatically with increasing the plasma exposure time. The results suggest that the increase in the BL intensity is probably due to the plasma-induced damage accumulation and the in-situ monitoring of the BL/NBE intensity ratio can be used to observe the plasma-induced damage.

The shape of telephone cord blisters

Formation of telephone cord blisters as a result of buckling delamination is widely observed in many compressed film-substrate systems. Here we report a universal morphological feature of such blisters characterized by their sequential sectional profiles exhibiting a butterfly shape using atomic force microscopy. Two kinds of buckle morphologies, light and heavy telephone cord blisters, are observed and differentiated by measurable geometrical parameters. Based on the Föppl-von Kármán plate theory, the observed three-dimensional features of the telephone cord blister are predicted by the proposed approximate analytical model and simulation. The latter further replicates growth and coalescence of the telephone cord into complex buckling delamination patterns observed in the experiment.

THICKNESS DEPENDENCE OF BUCKLING PATTERNS OF Ta FILMS SPUTTERED ON GLASS SUBSTRATES

Tantalum (Ta) films deposited on glass substrates have been prepared by a direct current magnetron sputtering method, and buckling patterns induced by residual compressive stress are investigated in detail. When the film thickness increases, the buckling morphologies evolve from straight-sided buckle network to wavy or wormlike wrinkles gradually, and finally change into telephone cord buckles. The geometrical parameters of the buckling patterns are found to increase linearly with the film thickness. Based on the geometrical parameters of the buckling patterns, the mechanical properties of the Ta films are also discussed in the frame of continuum elastic theory.

Mechanical Behaviors Of Quenched Iron Film Sputtering Deposited On Glass Substrate

A characteristic wedge-shaped iron (Fe) film system, deposited on glass substrate by a DC-magnetron sputtering method and quenched by silicone oil during deposition, has been successfully fabricated. The mechanical behaviors of the quenched and non-quenched Fe films have been compared and analyzed. The internal stress is found to transform from tension to compression in nature after quenching, which results in the formation of buckling pattern in the quenched Fe film while crack pattern in the non-quenched film. It is proposed that the origin of the compressive stress is mainly due to freezing of the Fe atoms (or clusters) with high kinetic energy and doping of the oil molecules into the Fe film defects.

Impurity induced wrinkling patterns in metal films deposited on soft elastic substrates

Metal (iron and nickel) films have been deposited on soft elastic polydimethylsiloxane (PDMS) substrates by direct current sputtering technique and the impurity induced wrinkling patterns are investigated by using optical microscopy and atomic force microscopy. It is found that the metal films can spontaneously form disordered wrinkles due to the isotropic compressive stress. In the vicinity of film impurities such as extraneous particles, linear defects, cracks and thickness-gradient film edges, the stress field becomes anisotropic owing to symmetry breaking and thus complex wrinkling patterns including straight stripes, herringbones, crossings, labyrinths and their transitions can be observed. The morphological evolutions, structural characteristics and physical mechanisms of the impurity induced wrinkles have been discussed and analyzed based on the continuum elastic theory.