X-ray interferometryholography was utilized to investigate the profile structures of n-hexadecyltrichlorosilane self-assembled monolayers (SAM's), chemisorbed onto the SiO, surface layer of Ge/Si multilayer substrates fabricated by molecular beam epitaxy, in their initial "as deposited" form and throughout an annealing process. This approach permitted an unambiguous determination of the electron density profiles of the various layers within the inorganic multilayer substrate and the chemisorbed SAM overlayer, including the smoothness/ roughness of the interfaces between adjacent layers, to relatively high spatial resolution withour any prior assumptions. The corresponding in-plane structures of these various forms of the SAM's were investigated by high AQ-resolution X-ray diffraction. The interpretation of the latter results was greatly facilitated by comparison with analogous kinematical structure factors calculated from molecular dynamics computer simulations of an ensemble of alkyl chain molecules "chemisorbed" onto a planar surface as a function of in-plane molecular density and temperature. Our results indicate that the initial "as deposited" and "hightemperature" forms of these SAM's consist of small domains of highly tilted chains (relative to the normal to the monolayer plane) within a positionally-disordered distorted hexagonal in-plane lattice, consistent with the dominance of chain-chain interactions. Upon cooling and drying, substantial structural reorganization takes place within the S A M and the SiO, substrate surface layer resulting in the predominant "annealed" form of these SAM's consisting of larger domains of much less tilted chains within a positionally-disordered hexagonal in-plane lattice, consistent with the dominance of chain headgroup-surface interactions. While such annealing produces only a modest increase in the in-plane interchain correlation length, the so-"annealed" SAM's are then structurally stable over the 293-363 K temperature range investigated, consistent with their intradomain structure.
Previous work has recently employed x-ray interferometry for the unique determination of the profile structures of ultrathin Langmuir-Blodgett multilayer films of Cd-arachidate and of tethered protein monolayers on the surface of Ge/Si multilayer substrates [1, 2]. These studies utilized the inorganic substrate as the reference structure for the interferometrie phasing of the meridional x-ray diffraction I (Qxy=0A-1, QZ) from the inorganic-organic composite structure. The substrates, fabricated by magnetron sputtering, contained only broad profile features (≥20 A), thereby limiting the spatial resolution of the organic profile structures so-determined. Molecular beam epitaxy (MBE) permits the fabrication of the multilayer reference structure with profile features as narrow as a single atomic monolayer, thereby providing delta-function-like features in the reference structure. The reference structure can then be tailored such that the autocorrelation function of the inorganic-organic composite profile structure [obtained by a Fourier transform of its meridional diffraction I (Qxy=0A-1, Qz) data without phase information] contains only the organic profile structure itself over a particular range of the profile coordinate z. This approach for uniquely determining the unknown profile structure of the organic overlayer is x-ray holography by analogy to simple off-axis holography with much longer wavelength radiation. We have initially utilized MBE fabricated Ge/Si multilayer substrates of the type N(Ge2Si30), e.g., for N=2 or 3 superlattice unit cells, each containing two Ge monolayers and thirty Si monolayers, to thereby determine the profile structures of four different organic overlayers, namely a) self-assembled alkylsiloxane monolayers, b) Langmuir-Blodgett cadmium alkylcarboxylate monolayers, c) a Langmuir-Blodgett Cd-arachidate head-to-head bilayer deposited on a)-above and, d) a covalently tethered protein monolayer.
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