We have studied self-assembly of 2-thiocytosine, a substituent of deox
yribonucleic acid (DNA) bases, adsorbed on a Au(111) surface by scanni
ng tunneling microscopy (STM) under ambient conditions in air. STM ima
ges always obtained under ambient conditions are similar to those of a
n ordered array of cytosine on a Au substrate in solution. However, tw
o kinds of STM images have been obtained in this work, one of which wa
s observed for the first time.