  A convenient method has been developed to thin electron beam fabricated
Silicon nanopillars under controlled surface manipulation by transforming the
surface of the pillars to an oxide shell layer followed by the growth of
sacrificial ammonium silicon fluoride coating. The results show the formation
of an oxide shell and a Silicon core without significantly changing the
original length and shape of the pillars. The oxide shell layer thickness can
be controlled from few nanometers up to few hundred nanometers. While down
sizing in diameter, smooth Si pillar surfaces of less than 10 nm roughness
within 2 {\mu}m were produced after exposure to vapors of HF and HNO3 mixture
as evidenced by transmission electron microscopy (TEM) analysis. The attempt to
expose for long durations leads to the growth of a thick oxide whose strain
effect on pillars can be assessed by coupled LO-TO vibrational modes of Si-O
bonds. Photoluminescence (PL) on the pillar structures which have been down
sized exhibit visible and infrared emissions, which are attributable to
microscopic pillars and to the confinement of excited carriers in Si core,
respectively. The formation of a smooth core-shell structures while reducing
the diameter of the Si pillars has a potential in fabricating nanoscale
electronic devices and functional components.
