Low dimensional nanosystems are known to have properties that can be markedly different fromrntheir bulk counterparts. For semiconductor crystals such as silicon when the carriers (excitons)rnare confined in dimensions less than their de Broglie wavelength (typically a few nanometers)rnquantum mechanical size effects emerge.rnAdvances in the design, synthesis and characterization of nano-materials are expected to providernthe unprecedented ability to manipulate matter at the most fundamental level. When the physicalrndimensions of a device are reduced to the nanometer scale, quantum phenomena becomernprevalent modifying the optical and electronic properties of the material. This may enable therndesign and manufacturing of materials with properties tailored for the particular application.rnThis paper examines the unique properties of silicon-based nanocristals that are fundamentallyrndifferent from those of the bulk crystalline material and make nanoscale silicon attractive for usernin a variety of important technological applications such as light emitting devices for potonicrnapplication,optical communications, photovoltaic solar cells,for memory devices, optical andrnelectrical properties of nanoscale silicon and several techniques towards developing siliconbasedrnnanostructures using nanotechnology tools are explored.rnMoreover, the different synthesis methods of silicon nanostructures and the accompanyingrncharacterizations of silicon nanocrystals and porous silicon are illustrated briefly. Theoreticalrnanalysis of the electronic structures of silicon nanocrystals of small number of atoms usingrnpseudo potential-DFT method is presented.rnIn general, for the last fiften years so many researches and jornals are published on siliconrnnanosystems,in this work we try to give an overview of the applications properties and dimondrncristal structure of elemental silicon