Nanocrystalline Ge films were prepared by isotropic chemical etching on single-crystalline Ge substrates with 100 and 111 orientations. The structural and optical properties have been investigated by transmission electron microscopy (TEM), electron diffraction (ED), Raman photoluminescence (PL), and infrared spectroscopy. The average size of nanocrystals (NCs) was estimated by fitting of the Raman spectra using a phonon-confinement model developed for spherical semiconductor NCs. Considered collectively TEM, ED, and Raman results indicate that all films contain high density of 3-4 nm diameter, diamond-structured Ge NCs with disordered surfaces. There are indications that surface of nanoparticles is mainly hydrogen terminated even for air-stabilized samples. Red PL is observed at room temperature upon excitation by 1.96 eV with peak energy of similar to 1.55 eV and correlates well with recent theoretical calculations of the enlarged optical gap in Ge NCs of similar size. (C) 2008 American Institute of Physics.