In contrast to the lithographic and etching techniques used in the top-down methodology, the bottom-up approach involves the direct growth of one-dimensional nanostructures onto a substrate. The “bottom-up” approach is thought to be a potential alternative. The idea is to build-up nanosized structures and devices by using nanoscale building blocks to initiate growth directly at desired positions and with designed dimensions and properties.Based on the bottom-up principle, the synthesis of nanowires of common and technologically relevant semiconductor materials such as Si, GaAs, InP, and ZnO has become a focus in current interdisciplinary materials science research.
Growth of semiconductor nanowires

Different techniques, such as a number of self organization methods for arranging the metal particles have been successfully developed. The as-prepared substrates are placed in a reaction tube or chamber, heated until the clusters melt and form liquid droplets; this is frequently achieved by dissolving the semiconductor material to form an alloy with a reduced melting temperature as compared to the pure metal used. A suitable temperature in the range of 300–11008C is chosen according to the binary phase diagram between the metal and the target material. For epitaxial growth of NWs, direct contact of the droplet to the crystalline substrate must be assured. In a second step, a gas containing the growth material flows through the reaction tube. As the surface of the liquid droplet has a much larger sticking coefficient than the solid substrate, the precursor atoms prefer to deposit on the surface of the liquid and forms an alloy. Continued incorporation of precursor atoms into the liquid droplet leads to a super saturation of the semiconductor component. As a consequence, crystal growth occurs at the solid–liquid interface by precipitation and NW growth commences.
Lieber group at Harvard University reported new activities on the growth of Si NWs based on the Si–Au eutectic. The Si NWs were grown by chemical vapor deposition (CVD) and were “harvested” by cutting material from the substrate and then bringing them into suspension. By using Langmuir–Blodgett techniques, the individual NWs were assembled parallel to the surface of a handling substrate, which enabled the Lieber group to study basic physical properties of Si NWs and demonstrate devices such as diodes and biosensors.