Structures of Self-Assembled Monolayer Films of Organosulfur Compounds Adsorbed on Gold Single Crystals: Electron Diffraction Studies

Transmiss ion e lect ron microscopy and d i f f rac t ion have been used to s tudy the pack ing ar ranqemencs of docosyi mercaptan (CH3(CHl)rtSH,RSH), didocosyl sulf ide (RSR), and didocosyi disulf ide (RSSR) on gold single,cr]€tal foils. The specimens include gold t-rlms in (100) and (11f ) orientatiors. Docosyl mercaptax and didocosyl disulf ide adsorbed on (111) faces assemble in coincidence latt ice s[ructures rneidine hexaqonai over layers c i ;x ; ) in rvh ich the shor t in terchain d is tance ( t .9? A) ind icates f ree ly ro tar ing merhy iene ch la ins. On a (100) sur face boch a lky l groups exh ib i t an in-p lane s t ructure cons isc ing of a base-centered square array in which the in terchain d is tance (4 .54 A) impi ies a h indered chain movemenr . The spac ing here is a rarional muitiple (10/9) of the sublartice spacing, suggesting that the overlayer is ref,eared in the cencered Iatt ice c(10x10), Although docosyl mercaptan and didocosyl disulf ide presenc the same in-plane structures on a (111) surface, on a ( i00) surface the dia-l l iy ' l disuif ide eshibits addit iona-l phases as well . These phases are incommensurat€ wirh the predominant subsurface (100) lattice; however. the inrerchain spacings indicate that they are repeated in the base-centered square over layers c(7 l2)v '2x1 l2)w'2) and c(5x5) . Thei r translat ional correiat ion iengths are greater than rhe correiat ion iengths of the hexagona-l overiayer and the c(10x10) square overlayer, but the orientational correiations are similar and are iong-range in all phases. Measurements of the diffraction intensitv and spoc displacement with sample tilting indicate that mercapcan and disul, f ide chain incl inacion has a probabil i ty distr ibution that is axial ly symmecric abour the vector normal to either substrate. The measurement of the average chain inciination from the falloff of diffracted beaintensity with beam tiit is complicated by anomalous attenuation associated with dynamical scattering from lhe substrare. Estimates of tilt angle indicare thac the hcp phase has the greatest average tiit (25-35"); this is fol lowed by the c(10x10) square overlayer (6-12') ard f inai ly by the c((712)^/2x(712)v'D anci c(5x5) square overlayers (<6"). The phases of didocosyl sulf ide (RSR) on gold are a"nbiguous. Al l di f fract ion data indicate structures that a-re isomorphous with didocosyi disulf ide (RSSR). The weaker binding of the sulf ide compared to the disu.Lf ide, however, suggests the possibi l i iy that adsorbed disuif ide impurit ies may mask the true suif ide phase.