Silicon nanotubes and selected investigations in energy and biomaterial applications

Abstract

Silicon (Si) remains the key material for electronic devices. The ability to fabricate silicon in nanostructure form is important not only to the next generation of such devices, but also in other applications relying on enhanced surface area, low dimensionality, and structural control, such as photovolatics, battery technology, biosensing/therapeutics and magnetic resonance imaging (MRI) contrast agents. In this presentation, we describe the basic fabrication steps for arrays of Silicon nanotubes (Si NTs) based on (1) deposition of silane (SiH4) on a preformed ZnO nanowire array template on FTO glass or Si wafer substrates, followed by (2) sacrificial etching of the ZnO phase, resulting in the desired nanotube product. Hollow nanotube inner diameter is adjustable by size selection of the initial ZnO nanowire, while shell thickness control is achieved by concentration/duration of silicon deposition. High surface area silicon structures are of great interest in battery applications.^In one study, the synthesis and characterization of porous Si NTs arrays on stainless steel substrates was achieved. We show that this type of self-supported Si NTs electrode exhibits interesting electrochemical properties for high performance Lithium Ion Batteries (LIB). For example, a gravimetric capacity of 800 mAhg-1 up to 180 cycles was measured in a porous Si NT anode with a 10 nm sidewall thickness. Fe3O4 nanoparticles (Fe3O4 NPs) are currently under investigation for possible utlity in magnetic assisted drug delivery and possible MRI contrast agents. Fe3O4 NPs of different sizes (5 and 8 nm) were infiltrated into SiNTs with 40- and 70-nm wall thicknesses. The infiltration process performed at room temperature is supported by a magnetic field to assure optimal filling of the nanotubes. To determine the efficiency of MRI contrast agents, we measured the relaxivities of the above materials, both longitudinal r1 and transverse r2 in water and PBS at 37 °C.^The r2/r1 ratios are higher when present in the nanotubes than for Fe3O4 NPs in suspension. All samples showed a relaxivity ratio r2/r12, placing them in the category of negative contrast agent. Organometal perovskites have implications in photovoltaics and light emitting diodes (LEDs).7 We demonstrate here the formation of organolead perovskite (CH3NH3PbI3) nanostructures of 30 nm, 70 nm and 200 nm thickness whose width is dictated by the inner diameter of a silicon nanotube (Si NT) template. Structural characterization of these structures is achieved via a combination of electron microscopies (SEM, TEM, and high resolution lattice imaging) with associated energy dispersive elemental analysis. After structural characterization, the photophysical properties of these perovskite nanostructures, in terms of optical absorption and photoluminescence (PL) as a function of temperature, were evaluated.^The above studies set the stage for further investigations of the utility of these semiconducting nanotubes for a broad spectrum of possible useful applications.