High density and ultrasmall size of Ge quantum dots (QDs) have been achieved directly on Si(0 0 1) (2 x 1) reconstruction surface. Their detailed morphology was observed by atomic force microscope (AFM) and shows that small pyramids, small domes, huts, and multi-headed large domes coexist in the film grown at 400 degrees C, while small domes and multi-headed large domes formed at 450 degrees C. Their low temperature photoluminescence (PL) showed that a very strong non-phonon (NP) peak with a large blue shift of 0.19 eV at 14 K, which can be attributed to their very high areal density, 5.2 x 1011 cm(-2), and sub-10-nm mean size, 7.6 +/- 2.3 nm. (C) 2011 Elsevier B. V. All rights reserved.