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We have conducted a scanning tunnelling microscopy (STM) investigation of the bare Si(111)-7x7 surface reconstruction and of submonolayer potassium (K) adsorption at room temperature, employing a specially prepared metal tip. On the bare Si surface, different dangling-bond states were distinguished by STM images at different bias voltages. On the K adsorption, two kinds of adsorption geometry, single-atom bright spots on top of the adatoms and dimerlike protrusions at rest atom sites, were mapped in bias-dependent STM images with atomic resolution. A new surface state and a surface-energy gap induced by the alkali adsorbate were identified.