Silicon nanoparticles are synthesized by pyrolysis of 1% silane (SiH4) in argon in a microwave plasma reactor. As-prepared silicon nanoparticles show the luminescence behaviour at room temperature. Covalent attachment of organic molecules to the surface of silicon nanoparticles is achieved by thermally induced hydrosilylation process. The surface oxide layers of silicon nanoparticles are first removed by hydrofluoric acid and the surface is further terminated by alkyl molecules. Blue shift in the photoluminescence (PL) of silicon nanoparticles is obtained after the surface passivation of silicon nanoparticles. The surface chemistry of nanoparticles is analysed by diffuse reflectance infrared fourier transform spectroscopy (DRIFT) technique. The surface coverage of functionalized silicon nanoparticles with alkyl molecules is confirmed by the presence of Si-CH2 bending vibration mode at 1461 cm-1 in the DRIFT spectrum. An almost complete disappearance of Si-O bending vibration and stretching modes as compared to untreated silicon nanoparticles indicate nearly complete removal of surface oxide upon etching with hydrofluoric acid. On further room temperature air oxidation of functionalized sample, the increase in the intensity of oxygen\silicon related modes and decrease in intensity of hydrogen\silicon related modes are observed owing to incomplete coverage of surface with alkyl molecules. This accompanies with a red shift and a decrease in PL with time as compared to freshly functionalized silicon nanoparticles. The change in particle shape and morphology before and after the passivation are investigated by transmission electron microscopy (TEM) technique. The electron diffraction pattern confirms that as-prepared silicon nanoparticles are crystalline in nature and the process does not destroy the crystallinity of silicon nanoparticles.