Thermal stability of self assembled monolayers of n-hexanethiol on Au(001) and Au(111)

FERRON J.; RUANO G.; CRISTINA L.J.

Bexco, Busan

Congreso; International Vacuum Congress. IVC 20; 2016

Thiols selfassembled monolayers (SAMs) on noble metals are currently a hot topic due tothe possibility of tunning the chemical identity of their surfaces to act asvariety of devices; sensors, microelectronics, catalysts as well asapplications in medicine and biochemistry. Due to its large chemical stability,interesting optical properties, and biocompatibility Gold represents a naturalchoice as SAMs support material, and it has been indeed used in manytechnological applications in this field. However, thiol coated goldnanoparticles, polycrystalline and nanostructured gold surfaces challenge thegeneral understanding of the thiol-gold interplay, mainly due to thecoexistence of {001} and {111} faces. Thermal stabilityof thiol SAMs is of crucial importance in scaling these devices out of thelaboratory to real life applications. In this work, using X ray photoemissionspectroscopy (XPS), we studied the thermal stability of SAMs of n-hexanethiol(C6T) on Au(001) and  Au(111) in ultrahigh vacuum (UHV) at different temperatures ranging from 300 to 380K. S 2p3/2core level peak for SAMs of thiols on Au can be usually decomposed into threedifferent components corresponding to; atomically adsorbed sulphur (BE ~161eV), chemisorbed S atoms through thiolate bond in the thiol-Au interface (~162eV), and S atoms in either unbound thiol (free terminal−SH groups) or disulfidespecies (in S−S bonds formed between neighboring HT chains) (at BE ~163-164 eV).We found that SAMson both substrates desorb, even at RT, continuously in the range of days in UHVstorage, being the kinetics of this desorption similar for both faces. Asexpected, the rate of desorption increases steadily with temperature, goingfrom days at RT to minutes at 380K. However, the behaviour is not so smoothwith relation to the final steady state. In fact, the startling result in thisexperiment is that at RT the thiolate: Au ratio is lower than the onecorresponding to 340 K. For both cases the S-S S-H S2p signal ratio tounderlying gold is constant, showing that the final equilibrium has been indeedreached.   For the desorption at 360K and 380K theprocess recovers the expected trend and cleavage of C-S bond is observed. Thisexperimental result suggests the existence of a couple of coexisting mechanismswith so quite close energetic barriers, that changes of only 20K are able ofchanging the desorption conditions.