Amirkianoosh Kiani

Micro/nano scale amorphization of silicon by femtosecond laser irradiation

This research aimed to investigate the feasibility of using direct amorphization of silicon induced by femtosecond laser irradiation for maskless lithography. A thin layer of amorphous silicon of predetermined pattern was first generated by irradiation by a femtosecond laser of Mega Hertz pulse frequency. The following KOH etching revealed that the amorphous silicon layer acted as an etch stop. Line width less than 1/67 the focused spot size was demonstrated and hence the proposed maskless lithography process has the potential of producing submicron and nanoscale features by employing a laser beam of shorter wavelength and a high NA focusing lens. Scanning Electron Microscope (SEM), a Micro-Raman and Energy Dispersive X-ray (EDX) spectroscopy analyses were used to evaluate the quality of amorphous layer and the etching process.

Maskless lithography using silicon oxide etch-stop layer induced by megahertz repetition femtosecond laser pulses

In this study we report a new method for maskless lithography fabrication process by a combination of direct silicon oxide etch-stop layer patterning and wet alkaline etching. A thin layer of etch-stop silicon oxide of predetermined pattern was first generated by irradiation with high repetition (MHz) ultrafast (femtosecond) laser pulses in air and at atmospheric pressure. The induced thin layer of silicon oxide is used as an etch stop during etching process in alkaline etchants such as KOH. Our proposed method has the potential to enable low-cost, flexible, high quality patterning for a wide variety of application in the field of micro- and nanotechnology, this technique can be leading to a promising solution for maskless lithography technique. A Scanning Electron Microscope (SEM), optical microscopy, Micro-Raman, Energy Dispersive X-ray (EDX) and X-ray diffraction spectroscopy were used to analyze the silicon oxide layer induced by laser pulses.

Direct patterning of silicon oxide on Si-substrate induced by femtosecond laser

In this study we report for the first time a method for direct patterning of silicon oxide on a silicon substrate by irradiation with a femtosecond laser of Mega Hertz pulse frequency under ambient condition. Embossed lines of silicon oxide with around 3 approximately 4 microm width and less than 100 nm height were formed by controlling the parameters such as laser pulse power and frequency rate. A Scanning Electron Microscope (SEM), an optical microscopy and a Micro-Raman and Energy Dispersive X-ray (EDX) spectroscopy were used to analyze the silicon oxide layer.

Synthesis of Bioactive Three-Dimensional Silicon-Oxide Nanofibrous Structures on the Silicon Substrate for Bionic Devices' Fabrication

Bionic devices are implants that replace biological functions that have been lost due to damaged or lost tissue. The challenge of this area is to find the appropriate materials to match the biocompatible criteria with the same mechanical and electrical performance. In this research, a new method is introduced for the enhancement of silicon biocompatibility by fabrication of a 3D nanofibrous layer on the silicon surface, induced by nanosecond laser pulses at a high repetition rate and power. It was found that the laser treatment with smaller line spacing and a higher overlap number enhanced the biocompatibility of silicon. The results display a promising improvement in the biocompatibility of silicon for the production of biomedical devices such as sensors, bio-MEMS and nano-biomaterial fabrications.

Synthesis of Electrical Conductive Silica Nanofiber/Gold Nanoparticle Composite by Laser Pulses and Sputtering Technique

Biocompatible-sensing materials hold an important role in biomedical applications where there is a need to translate biological responses into electrical signals. Increasing the biocompatibility of these sensing devices generally causes a reduction in the overall conductivity due to the processing techniques. Silicon is becoming a more feasible and available option for use in these applications due to its semiconductor properties and availability. When processed to be porous, it has shown promising biocompatibility; however, a reduction in its conductivity is caused by its oxidization. To overcome this, gold embedding through sputtering techniques are proposed in this research as a means of controlling and further imparting electrical properties to laser induced silicon oxide nanofibers. Single crystalline silicon wafers were laser processed using an Nd:YAG pulsed nanosecond laser system at different laser parameters before undergoing gold sputtering. Controlling the scanning parameters (e.g., smaller line spacings) was found to induce the formation of nanofibrous structures, whose diameters grew with increasing overlaps (number of laser beam scanning through the same path). At larger line spacings, nano and microparticle formation was observed. Overlap (OL) increases led to higher light absorbance’s by the wafers. The gold sputtered samples resulted in greater conductivities at higher gold concentrations, especially in samples with smaller fiber sizes. Overall, these findings show promising results for the future of silicon as a semiconductor and a biocompatible material for its use and development in the improvement of sensing applications.

Optical absorption enhancement in 3D silicon oxide nano-sandwich type solar cell.

Recent research in the field of photovoltaic and solar cell fabrication has shown the potential to significantly enhance light absorption in thin-film solar cells by using surface texturing and nanostructure coating techniques. In this paper, for the first time, we propose a new method for nano sandwich type thin-film solar cell fabrication by combining the laser amorphization (2nd solar cell generation) and laser nanofibers generation (3rd solar cell generation) techniques. In this novel technique, the crystalline silicon is irradiated by megahertz frequency femtosecond laser pulses under ambient conditions and the multi-layer of amorphorized silicon and nano fibrous layer are generated in the single-step on top of the silicon substrate. Light spectroscopy results show significant enhancement of light absorption in the generated multi layers solar cells (Silicon Oxide nanofibers / thin-film amorphorized silicon). This method is single step and no additional materials are added and both layers of the amorphorized thin-film silicon and three-dimensional (3D) silicon oxide nanofibrous structures are grown on top of the silicon substrate after laser irradiation. Finally, we suggest how to maximize the light trapping and optical absorption of the generated nanofibers/thin-film cells by optimizing the laser pulse duration.

Mammalian fibroblast cells show strong preference for laser-generated hybrid amorphous silicon-SiO2 textures.

Background In this study, we investigated a method to produce bioactive hybrid amorphous silicon and silicon oxide patterns using nanosecond laser pulses. Methods Microscale line patterns were made by laser pulses on silicon wafers at different frequencies (25, 70 and 100 kHz), resulting in ablation patterns with frequency-dependent physical and chemical properties. Results Incubating the laser-treated silicon substrates with simulated body fluid demonstrated that the physicochemical properties of the laser-treated samples were stable under these conditions, and favored the deposition of bone-like apatite. More importantly, while NIH 3T3 fibroblasts did colonize the untreated regions of the silicon wafers, they showed a strong preference for the laser-treated regions, and further discriminated between substrates treated with different frequencies. Conclusions Taken together, these data suggest that laser materials processing of silicon-based devices is a promising avenue to pursue in the production of biosensors and other bionic devices.

Leaf-like nanotips synthesized on femtosecond laser-irradiated dielectric material

Nanotips are the key nanostructures for the improvement of field emission, flat panel displays, force microscopy, and biosensor applications. We propose a single-step, rapid synthesis method to generate nanotips using femtosecond laser irradiation at megahertz frequency with a background flow of nitrogen gas at ambient conditions. Two different types of leaf-like nanotips can be grown on the target surface: randomly oriented multiple tips growing from a single large droplet and single tips growing from small droplets. In this report, we explain the mechanism accountable for the formation of such nanotips using known concepts of laser breakdown of dielectric materials, plasma plume generation, plasma interactions with incoming laser pulses and surrounding gas, as well as known thermal properties of target material. Nitrogen gas plays an interesting role for the resultant structural changes on the target surface and thus it is given special attention in our discussion. Our unique fabrication technique has enabled us to produce tips with nanoscale apexes with a stem and length ranging from few hundred nanometers to few micrometers.

Bioactivity enhancement of titanium induced by Nd:Yag laser pulses

Purpose In this research, the effect of laser properties such as laser power and laser dwell time on the surface morphology and oxidizing of titanium have been investigated in order to enhance the bioactivity of laser textured titanium sheets. Methods The Ti samples were irradiated with nanosecond pulses to create the predetermined point patterns on the surface of sample sheets with specific laser parameters. Final bioactivity of the treated samples were evaluated through the use of simulated body fluid (SBF), followed by material characterization techniques such as X-ray diffraction (XRD) and energy dispersive (EDX). Results It was observed that by increasing the roughness of the titanium surface samples using a range of dwelling time, and with different powers, titania with higher levels of surface energy in micro/sub-micro scales are produced. The use of laser results in a one-step heat increase and the oxidation of titanium, which results in creation of titania with higher cell adhesion abilities. Conclusions It was concluded that the variation of the surface roughness, surface morphology, and oxidation level of the material has a direct effect on the cell adhesion rate to the surface of the titanium. Upon completion of the analysis, it is concluded that using a higher power and a lower dwelling time results in better bioactivity improvement than using higher dwelling times and lower powers.

Synthesis of 3D nanostructured metal alloy of immiscible materials induced by megahertz-repetition femtosecond laser pulses

AbstractIn this work, we have proposed a concept for the generation of three-dimensional (3D) nanostructured metal alloys of immiscible materials induced by megahertz-frequency ultrafast laser pulses. A mixture of two microparticle materials (aluminum and nickel oxide) and nickel oxide microparticles coated onto an aluminum foil have been used in this study. After laser irradiation, three different types of nanostructure composites have been observed: aluminum embedded in nickel nuclei, agglomerated chain of aluminum and nickel nanoparticles, and finally, aluminum nanoparticles grown on nickel microparticles. In comparison with current nanofabrication methods which are used only for one-dimensional nanofabrication, this technique enables us to fabricate 3D nanostructured metal alloys of two or more nanoparticle materials with varied composite concentrations under various predetermined conditions. This technique can lead to promising solutions for the fabrication of 3D nanostructured metal alloys in applications such as fuel-cell energy generation and development of custom-designed, functionally graded biomaterials and biocomposites.