|Ph.D Student||Ossama Assad|
|Subject||Advances towards Efficient Functionalization and|
Deposition of Silicon Nanowires
|Department||Department of Chemical Engineering||Supervisor||Full Professor Haick Hossam|
|Full Thesis text|
The ability of silicon nanowires (Si NWs) to carry electrical current makes them promising building blocks in various nano-electronic and sensing device concepts. Nevertheless, there are still many challenges in creating such devices. The current thesis is designed to solve two of the most crucial obstacles facing realization of Si NWs in real-world, technological applications.
The first obstacle relates to the formation of highly-stabilized non-oxidized Si NWs that can be functionalized with variety of (bio)molecules. This obstacle was treated by a formation of alkyl monolayers on oxide-free Si NWs using a two-step chlorination/alkylation process. X-ray photoelectron spectroscopic (XPS) analysis demonstrated that functionalization of Si surfaces with -CH=CH-CH3 and -C≡C-CH3 organic molecules gives nearly full coverage of the Si atop sites, like that observed for methyl-terminated Si surfaces. These densely packed monolayers can create a structurally and electronically robust Si/organic interface that provide a better resistance against oxidation than conventional alkene-derived monolayers. Interestingly, the -CH=CH-CH3 monolayers provided greater protection against oxidation attacks of the Si NWs, than the -C≡C-CH3 monolayers. Subsequent functionalization of the -CH=CH-CH3 terminated surfaces were achieved with the mediation of N-Bromosuccinimide (NBS). After bromination, a wide variety of functionalities can be immobilized to the surface, e.g. through the conversion of Br to OH group or converting the Br to NH2 and polyphenylenediamine (PPD), both of which are accomplished without the use of any catalyst. This approach may be useful for the immobilization of inorganic nanomaterials on top of the Si surface.
The second obstacle relates to the difficulty to assemble highly-aligned arrays of NWs on large areas in a cost-effective manner. In this context, we have shown that spray-coating of NWs suspension under controlled conditions of temperature, droplet size, spray-coating angle, and air flow is a successful method for preparing well-aligned and well-controlled NW arrays. The quality and reproducibility of the spray-coated NW arrays were demonstrated by the production of highly uniform Si NW FETs. The transfer of highly aligned and controlled density NWs to both large crystalline substrates and flexible plastic substrates provides advances toward achieving technological implementation of these materials in several areas of electronics and/or (bio)sensors.
The reported approach to form a wide spectrum of organically-functionalized, non-oxidized Si NWs as well as the approach to deposit the NWs using a simple technique is expected to open up a wide range of opportunities for producing stable molecule-based (opto)electronic and (bio)sensing devices.