Ultrahigh Pore Density Nanoporous Films Produced by the Phase Separation of Eutectic Al–Si for Template‐Assisted Growth of Nanowire Array

Abstract

turn changes the effective contrast between the alternating polymer and LC rich layers in the structure. This is a simple and flexible system by which photoresponsive photonic crystals can be fabricated in a single-step process. In addition to the one-dimensionally periodic systems such as those presented here, complex two-and three-dimensionally periodic HPDLC photonic materials could be fabricated [13] with the Azo-LC materials to make dynamic photonic crystals of any structure in a one-step process. Future studies will include fabricating novel photo-optic photonic structures, in addition to probing the ultimate response time of the system, as well as the effects of excitation intensity, wavelength, and azobenzene concentration. ExperimentalHolographic-polymer-dispersed liquid crystal structures containing azobenzene-derived liquid crystals were prepared from a standard prepolymer syrup containing 35 % E7, to which was added an additional 3.5 % BMAB. The preparation of a standard syrup can be found in [10] and [11], for example. The BMAB was synthesized according to [14], and can be commercially obtained from BEAMCo Inc, Winter Park, FL. Sample cells were prepared by placing a small amount of syrup to which a small amount of 8 lm glass spherical spacers were added on indium tin oxide (ITO) coated glass. The resulting cell contains approximately an 8±10 lm layer of photo sensitive syrup between two glass plates. Bragg reflection gratings were prepared using two counter propagating beams incident on the sample cell. This produced a one dimensionally periodic holographic structure with a grating vector perpendicular to the surface of the cell. The initial diffraction efficiency of these gratings was probed at normal incidence and ranged from 45 to 55 %. Control gratings prepared from an identical syrup without BMAB showed diffraction efficiencies of 50 to 55 %. Current studies in bulk have shown that 10 % BMAB in E7 is sufficient to induce an isothermal N±I transition at room temperature [15]. In order to minimize the attenuation of the triggering UV light through the thickness of the sample, it is desirable to use as low a concentration of Azo-LC as is sufficient to observe a complete, isothermal N±I transition. Thus approximately 3.5 % of the weight of the syrup in BMAB was added to the syrup, which constitutes roughly 10 % of the liquid crystal content. A convenient wavelength, 540 nm, was selected for holographic recording such that it did not overlap the cis absorption substantially, which provided for an accurate measurement of the diffraction efficiency.Once recorded, the reflection gratings are characterized in an apparatus which allowed for the introduction of both a UV laser beam (up to 80 mW cm ±2 of 365 nm from and Ar+ laser) and a doubled Nd:YVO 4 CW beam of 532 nm (up to 100 mW cm ±2 ) while the visible transmission spectrum and absorbance at 365 nm are simultaneously recorded. The UV beam was near the peak absorption of the trans conformer of the Azo-LC and produced a discernable change in color ...

Template-assisted synthesis of porous molybdenum dioxide nanofibers and nanospheres by redox etching method

“…The integration of chemical etching and template method was used to widen the nanochannels of porous alumina for production of bone-shaped nanomaterials [12]. Ultra-high pore density nanoporous films have been fabricated via nanophase separation of the eutectic Al-Si system followed by the removal of Al cylinders through chemical etching [13].…”

Substrate bias effect on Al–Si and Al–Ge thin film structure

“…Among these materials, phase-separated Al-Si films, composed of Al nanocylinders (nanowires) embedded in an amorphous Si matrix, are very interesting because the fabrication method, co-sputtering deposition from an Al-Si target at growth temperatures less than 100°C, is quite simple and allows the use of various kinds of substrates [9,11]. In addition, the nanoporous material fabricated from the Al-Si film can be used as a template for various kinds of ultrahigh density nanowires [9,12] and nanoporous semiconductor silicon thin films [13]. According to the growth mechanism of phase-separated Al-Si thin films [11], sputtered materials separate into two phases to minimize the total free energy on the growth surface and the evolution of phase separation limited by the film deposition forms an Al nanowire array embedded in an amorphous Si matrix.…”

“…The characteristic feature of such material is determined by an extremely small diameter, which is difficult to achieve with conventional lithographic techniques. Several nanowire fabrication processes have been developed by using an inexpensive bottom-up method, such as the recently developed approach in which the deposited materials separate laterally into two phases during film growth [7][8][9]. This interesting phenomenon is being used to create nanowire arrays because there is no need to use a catalyst with this technique.…”

“…The obtained nanowires, embedded in a matrix, are perpendicular to the substrate and parallel to each other, and their length and density can be easily controlled by deposition conditions. Various kinds of nanowires embedded in a matrix, such as Fe-LaSrFeO 4 films [7], BaTiO 3 -CoFe 2 O 4 films [8], Al-Si films [9], and Al-Ge films [10], have been developed based on the lateral phase separation technique. Among these materials, phase-separated Al-Si films, composed of Al nanocylinders (nanowires) embedded in an amorphous Si matrix, are very interesting because the fabrication method, co-sputtering deposition from an Al-Si target at growth temperatures less than 100°C, is quite simple and allows the use of various kinds of substrates [9,11].…”

“…Various kinds of nanowires embedded in a matrix, such as Fe-LaSrFeO 4 films [7], BaTiO 3 -CoFe 2 O 4 films [8], Al-Si films [9], and Al-Ge films [10], have been developed based on the lateral phase separation technique. Among these materials, phase-separated Al-Si films, composed of Al nanocylinders (nanowires) embedded in an amorphous Si matrix, are very interesting because the fabrication method, co-sputtering deposition from an Al-Si target at growth temperatures less than 100°C, is quite simple and allows the use of various kinds of substrates [9,11]. In addition, the nanoporous material fabricated from the Al-Si film can be used as a template for various kinds of ultrahigh density nanowires [9,12] and nanoporous semiconductor silicon thin films [13].…”