We show that two-dimensional Si/Ge nanostructures with a thickness of a single atomic layer can be imaged with chemical sensitivity using a scanning tunneling microscope (STM). An atomic layer of Bi terminating the surface is used to distinguish between Si and Ge. This distinction between Si and Ge enabled us to fabricate two-dimensional Si/Ge nanostructures in a controlled way by self-organized growth. Si/Ge nanoring structures consisting of alternating Si and Ge rings having a width of ∼5 nm were grown around a Si core on a Si(111) substrate by molecular beam epitaxy (MBE). The thickness of the Si and Ge rings is only one atomic layer (0.3 nm). Alternating Si/Ge nanowires with a width of ∼3.5 nm and a thickness of 0.3 nm were also fabricated using alternating Si/Ge deposition in the step flow growth mode.