Silicon nanowire
Silicon nanowires, also referred to as SiNWs, are a type of semiconductor nanowire most often formed from a silicon precursor by etching of a solid or through catalyzed growth from a vapor or liquid phase. Such nanowires have promising applications in lithium-ion batteries, thermoelectrics and sensors. Initial synthesis of SiNWs is often accompanied by thermal oxidation steps to yield structures of accurately tailored size and morphology.
SiNWs have unique properties that are not seen in bulk silicon materials. These properties arise from an unusual quasi one-dimensional electronic structure and are the subject of research across numerous disciplines and applications. The reason that SiNWs are considered one of the most important one-dimensional materials is they could have a function as building blocks for nanoscale electronics assembled without the need for complex and costly fabrication facilities. SiNWs are frequently studied towards applications including photovoltaics, nanowire batteries, thermoelectrics and non-volatile memory.
Applications
Owing to their unique physical and chemical properties, silicon nanowires are a promising candidate for a wide range of applications that draw on their unique physico-chemical characteristics, which differ from those of bulk silicon material.SiNWs exhibit charge trapping behavior which renders such systems of value in applications necessitating electron hole separation such as photovoltaics, and photocatalysts. Recent experiment on nanowire solar cells has led to a remarkable improvement of the power conversion efficiency of SiNW solar cells from <1% to >17% in the last few years.
The ability for lithium ions to intercalate into silicon structures renders various Si nanostructures of interest towards applications as anodes in Li-ion batteries. SiNWs are of particular merit as such anodes as they exhibit the ability to undergo significant lithiation while maintaining structural integrity and electrical connectivity.
Silicon nanowires are efficient thermoelectric generators because they combine a high electrical conductivity, owing to the bulk properties of doped Si, with low thermal conductivity due to the small cross section.
Silicon nanowire field-effect transistor (SiNWFET)
Charge trapping behavior and tunable surface governed transport properties of SiNWs render this category of nanostructures of interest towards use as metal insulator semiconductors and field effect transistors, where the silicon nanowire is the main channel of the FET which connect the source to the drain terminal, facilitating electron transfer between the two terminals with further applications as nano-electronic storage devices, in flash memory, logic devices as well as chemical, gas and biological sensors.Since SiNWFET was first reported in 2001, it has caused wide concern in the sensor area, because of their superior physical properties such as high carrier mobility, high current switch ratio, and close to ideal subthreshold slope. Furthermore, it is cost-efficient and could be manufactured on large scale, since it is combined with CMOS fabricating technology. Specifically, in bioresearch, SiNWFET has high sensitivity and specificity to biological targets and could offer label-free detection after being modified with small biological molecules to match the target object. What's more, SiNWFET could be fabricated in arrays and be selectively functionalized, which enables the simultaneous detection and analysis of multiple targets. Multiplexed detection could greatly improve throughput and efficiency of biodetection.
Synthesis
Several synthesis methods are known for SiNWs and these can be broadly divided into methods which start with bulk silicon and remove material to yield nanowires, also known as top-down synthesis, and methods which use a chemical or vapor precursor to build nanowires in a process generally considered to be bottom-up synthesis.Top down synthesis methods
These methods use material removal techniques to produce nanostructures from a bulk precursor- Laser beam ablation
- Ion-beam etching
- Thermal evaporation oxide-assisted growth
- Metal-assisted chemical etching
Bottom-up synthesis methods
- Vapour–liquid–solid growth – a type of catalysed CVD often using silane as Si precursor and gold nanoparticles as catalyst.
- Molecular beam epitaxy – a form of PVD applied in plasma environment
- Precipitation from a solution – a variation of the VLS method, aptly named supercritical fluid liquid solid, that uses a supercritical fluid as Si precursor instead of vapor. The catalyst would be a colloid in solution, such as colloidal gold nanoparticles, and the SiNWs are grown in this solution
Thermal oxidation