Phase-dependent desorption from biphenyl-substituted alkanethiol self-assembled monolayers induced by ion irradiation

Using laser ionization in combination with time-of-flight mass spectrometry, we have studied ion-induced desorption of neutral particles from self-assembled monolayers (SAMs) of omega-(4 '-methylbiphenyl-4-yl) alkane thiols (CH3(C6H4)(2)(CH2)(n)SH, BPn, n = 2, 4 6) formed on Au(111) substrates. Because BPn/Au(111) SAMs with it = even exhibit polymorphism, the effect of purely structural changes on emission yield and fragmentation pattern could be studied without interference from changes in the chemical composition. In spite of the high energy of the primary ion beam (15 keV), the mass spectra reveal a striking sensitivity of the desorption process to rather subtle changes in the structure of the layer. Depending on the SAM structure, substantial differences in the ratio between the cleavage of the molecule-substrate and the C-S bonds are observed. For applications of SAMs as resists in ion beam lithography, the results demonstrate that well-defined removal of molecules requires exact control of the SAM structure.