Mohamed Y. Elsayed

Silicon-based nanostructures as surface enhanced Raman scattering substrates

We report surface enhanced Raman scattering substrates that are low cost and easy to fabricate using silver nanoparticles on silicon wafer and silicon nanowires decorated with the nanoparticles. Numerical simulations showed enhancement factors around 107 to 108 at the hot spots, enabling single molecule detection. We demonstrated experimentally the substrates using reduced Graphene oxide solution.

Lithography-free wide-angle antireflective self-cleaning silicon nanocones

Low-cost, wideband, and wide-angle antireflective layers are of prime importance to photovoltaic and other optoelectronic applications. We report a novel fabrication methodology of random textured silicon nanocones (SiNCs) array through metal-assisted chemical etching combined with oxidation. The optical properties of the fabricated structure are studied theoretically and experimentally. The random textured SiNCs array showed very promising broadband antireflective properties through the entire visible wavelength range at different incident angles up to ±60°. In addition, the nanostructures inherently could become self-cleaning due to the high contact angle. This random cheap textured SiNCs array increases the absorption efficiency of photodetectors and reduces its cost.

Integrated lab-on-a-chip sensor using shallow silicon waveguide multimode interference (MMI) device

The objective of this work was to develop an integrated general purpose label-free optical sensor using standard photolithography on silicon-on-insulator platform for lab on chip applications. Shallow silicon waveguides have weak confinement in the silicon with lots of field in the cladding. This is advantageous in sensor applications due to the high light matter interaction. Here, we use our shallow strip waveguide platform to design a sensor employing a multimode interference (MMI) section. Utilizing a multi-mode section as short as 4 mm, the sensor exhibits sensitivity ranging from 417 nm / RIU to 427 nm / RIU with a figure of merit from 32 to 133.

Black silicon based on simple fabrication of mesoporous silicon nanowires for solar energy harvesting

We report large-scale fabrication of black silicon based on mesoporous silicon nanowires (SiNWs) with reflectance <;0.1% in the solar irradiance peak (wavelength range 400 to 800 nm). The black silicon reflects <;2% at different incident angles up to ±60°. The black silicon consists of mesporous SiNWs of diameters in the range 130-750 nm, matching the wavelength of the incident light for improved light trapping inside the nanostructure. SiNWs were synthesized through catalytic wet chemical etching. This study shows that black silicon surfaces based on mesoporous SiNWs can be used as an efficient broadband absorber for solar energy harvesting without any need for antireflective coating.

Silver-decorated silicon nanowires array as surface-enhanced Raman scattering (SERS) substrate

Raman scattering is an excellent analysis tool because a wealth of information can be obtained using a single measurement. It can also be configured as a diagnostic tool as a label free sensing method. In that case, enhancing the Raman signal is important to improve the sensitivity and detect low concentrations of analytes. A nanoparticle showing a particular Raman enhancement shows a much higher enhancement when it is on a nanowire. This was also confirmed experimentally. We report on a simple fabrication method of silver nanoparticles and silicon nanowires decorated with these nanoparticles. The nanowires were fabricated using metal assisted chemical etching. The nanoparticles were formed using electrodeposition. Samples were then immersed in Pyridine. An enhancement factor of around 6 to 8×105 was observed for silver nanoparticles alone. By depositing the same nanoparticles on silicon nanowires, the enhancement factor jumped 10-fold to 7×106. Finite Difference Time Domain simulations showed that a range of enhancement factors is possible up to 109.

Semiconductor plasmonic gas sensor

We designed a semiconductor plasmonic slot waveguide operating with mid infrared (MIR) wavelengths. By using Indium Arsenide, we were able to achieve (MIR) plasmonics with dopant concentration around 1020 cm-3. A dispersion analysis studying the guided modes of such waveguide was performed to understand the effects of different design parameters. By tuning the carrier concentration and optimizing the slot waveguide geometry, we were able to operate in the fingerprint region of the IR spectrum. The waveguide worked as an ultracompact gas sensor by exploiting the IR absorption spectra of analytes for good selectivity.

Random textured silicon oxide nanocones for high-performance thin silicon solar cells

High reflection losses combined with low absorption capabilities and high velocity surface recombination are the main problems that deteriorate the efficiency of thin silicon solar cells. Therefore, Low cost and easy scalable fabrication of wide band, angle and self-cleaning antireflection coatings are of great importance for different optical applications especially solar cells. Random textured silicon nanocones are fabricated through electroless metal assisted chemical etching (EMACE) combined with ambient oxidation. Theoretical studies using Finite difference Time Domain (FDTD) simulation guided the experimental procedures in terms of dimensions and tolerance to reach the optimum dimensions and superior optical properties. The Optical numerical and experimental studies are revealed wide antireflection properties and strong trapping effects up to 60° through the entire visible wavelength. The textured structure modified the hydrophobicity of the solar cell into hydrophobic surface with self-cleaning properties.

Advances in Micro- and Nanotechnologies for Stem Cell-Based Translational Applications

The application of stem cell-based therapeutics in improving human health is the ultimate anticipated outcome of stem cell research. Yet there exists a recognized lag between laboratory discoveries confirming the potential of stem cells and their actual application in routine clinical care. This lag can be appreciated if we contrast the number of research articles published in the field of stem cells with the number of products used in clinical practice. For example, a search for the keyword “stem cell” found 279,213 publications on PubMed, whereas only five products, all based on hematopoietic stem cells, were approved for therapeutic use by the US Food and Drug Administration [1]. This chapter will provide an overview of emerging technologies applied to help overcome such challenges with a highlight on their potential for clinical applications.

Graphene plasmonic electro-absorption modulator

We designed a plasmonic absorption electro-optical modulator using graphene-insulator-graphene configuration. By applying an electric field, we are able to tune the graphenes fermi level and thus modulate the optical absorption. Having a width of 80 nm, this modulator easily integrates with electronic components, and reaches a 3dB extinction ratio with a length of only 400 nm.