We demonstrate that crack propagation in uniaxially strained reduced graphene oxide (rGO) films is substantially dependent on the film thickness, for films in the sub-micron regime. rGO film on flexible polydimethylsiloxane (PDMS) substrate develop quasi-periodic cracks upon application of strain. The crack density and crack width follow contrasting trends as film thickness is increased and the results are described in terms of a sequential cracking model. Further, these cracks also have a tendency to relax when the strain is released. These features are also reflected in the strain-dependent electrical dc and ac conductivity studies. For an optimal thickness (3-coat), the films behave as strain-resistant, while for all other values it becomes strain-responsive, attributed to a favorable combination of crack density and width. This study of the film thickness dependent response and the crack propagation mechanism under strain is a significant step for rationalizing the application of layered graphene-like systems for flexible optoelectronic and strain sensing applications. When the thickness is tuned for enhanced extent of crack propagation, strain-sensors with gauge factor up to ∼470 are realized with the same material. When thickness is chosen to suppress the crack propagation, strain-resistive flexible TiO2- rGO UV photoconductor is realized.
The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused Ga+ ion beam with an acceleration voltage of 30 keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.
The confinement of water in quasi two-dimensional layers is intriguing because its physical properties can be significantly different when compared to those of the bulk fluid. This work describes spectroscopic ellipsometry study of confined water layers trapped between sheets of graphene oxide at varied thermal annealing temperatures. The wavelength-dependent refractive index of graphene oxide changes abruptly with annealing temperatures for Tann ≈ 125–160 °C, and we demonstrate that these changes are primarily governed by the expulsion of trapped water. This expulsion is associated with the decrease of interlayer separation of graphene oxide sheets from 7.8 Å to 3.4 Å. Graphene oxide annealed at high temperatures lacks trapped water layers and robust estimates of refractive index can be obtained within a Lorentz oscillator model. The trends in oscillator parameters are extended to lower annealing temperatures, where trapped water is present, in order to estimate the refractive index of confined water, whose value is found to be enhanced as compared to that of bulk. Temperature-dependent ellipsometry data show anomalous changes in ellipsometric parameters over a wide temperature interval (−10 to 10 °C) about the ice-point and these may be attributed to possible phase transition(s) of confined water.
Incorporation of silver ions into a dye-sensitized poly(vinyl alcohol)/acrylamide photopolymer is observed to give better performance compared to other metal-ion-doped photopolymer holographic recording media. Plane-wave transmission gratings were recorded in the photopolymer films using a He-Ne laser, and various holographic parameters were optimized so as to explore maximum potential of the material for various holographic applications. Silver-doped films showed good energy sensitivity, and gratings recorded in optimized film exhibited a diffraction efficiency of more than 75%. The potential of the material for holographic data storage applications is also studied using peristrophic multiplexing.
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