COVERAGE-DEPENDENT SUPERSTRUCTURES IN CHEMISORBED NTCDA MONOLAYERS - A COMBINED LEED AND STM STUDY

The long-range order and local geometric arrangement of 1,4,5,8-naphth alene-tetracarboxylic-dianhydride (NTCDA) monolayers on Ag(111) have b een studied with low-energy electron diffraction (LEED) and scanning t unneling microscopy (STM). Two distinct superstructures are detected w hich appear separately or may even coexist, depending on coverage. Mor eover, the unit cells of these two phases are correlated to each other , leading to an arrangement of parallel stripes of alternating superst ructures. The structures can reversibly be transferred into one anothe r by additional dosing or by thermal description. For certain tunnelin g conditions it is possible to image the substrate through the monolay er without disturbing it. Thus, for one structure, the exact adsorptio n site could be determined by the simultaneous observation of contrast from substrate and adsorbate. (C) 1998 Elsevier Science B.V. All righ ts reserved.