We show that two-dimensional Si/Ge nanostructures can be imaged with chemical sensitivity on the nanometer scale using a scanning tunneling microscope (STM). Using an atomic layer of Bi terminating the surface, we can distinguish between Si and Ge. The apparent height measured by the STM is similar to 0.09 nm higher at areas consisting of Ge than on areas consisting of Si. This distinction between Si and Ge enabled us to fabricate and characterize two-dimensional Si/Ge nanostructures in a controlled way. Si/Ge nanostructures consisting of alternating Si and Ge rings having a width of 5-10 nm were grown around a 2D Si core island on a Si(111) substrate. The thickness of the Si and Ge rings is only one atomic layer (0.3 nm). Intermixing of Ge and Si is observed if the growth conditions are not chosen properly. The optimized growth conditions were obtained by lowering the temperature and decreasing the growth rate to prevent intermixing and nucleation of secondary islands. The present fabrication method is expected to be applied for nanostructured devices. (C) 2004 Elsevier B.V. All rights reserved.