We present a noncontact atomic force microscopy (NC-AFM) image of a mixed monolayer composed of propiolate (HCdropCCOO(-)) and formate (HCOO-) molecules prepared on a TiO2(1 1 0)-(1 x 1) surface. An isolated former molecule can be regarded as a needle with a single-atom diameter perpendicular to the surface. The height difference between the propiolate and formate in the NC-AFM image was smaller than the physical height difference. A simple simulation that employed van der Waals forces between the tip and monolayer-covered surface reproduced the experimental topography. (C) 2002 Elsevier Science B.V. All rights reserved.