X-ray studies of self-assembled organic monolayers grown on hydrogen-terminated Si(111).
The structure of self-assembled monolayers (SAMs) of undecylenic acid methyl ester (SAM-1) and undec-10-enoic acid 2-bromo-ethyl ester (SAM-2) grown on hydrogen-passivated Si(111) were studied by X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The two different SAMs were grown by immersion of H-Si(111) substrates into the two different concentrated esters. UV irradiation during immersion was used to create Si dangling bond sites that act as initiators of the surface free-radical addition process that leads to film growth. The XRR structural analysis reveals that the molecules of SAM-1 and SAM-2 respectively have area densities corresponding to 50% and 57% of the density of Si(111) surface dangling bonds and produce films with less than 4 angstroms root-mean-square roughness that have layer thicknesses of 12.2 and 13.2 angstroms. Considering the molecular lengths, these thicknesses correspond to a 38 degrees and 23 degrees tilt angle for the respective molecules. For SAM-2/Si(111) samples, XRF analysis reveals a 0.58 monolayer (ML) Br total coverage. Single-crystal Bragg diffraction XSW analysis reveals (unexpectedly) that 0.48 ML of these Br atoms are at a Si(111) lattice position height that is identical to the T1 site that was previously found by XSW analysis for Br adsorbed onto Si(111) from a methanol solution and from ultrahigh vacuum. From the combined XPS, XRR, XRF, and XSW evidence, it is concluded that Br abstraction by reactive surface dangling bonds competes with olefin addition to the surface.
Jin
Hua
H
Department of Materials Science and Engineering, and Nanoscale Science, Engineering Center, Northwestern University, Evanston, Illinois 60208, USA.
Kinser
C Reagan
CR
Bertin
Paul A
PA
Kramer
Donald E
DE
Libera
Joseph A
JA
Hersam
Mark C
MC
Nguyen
Sonbinh T
ST
Bedzyk
Michael J
MJ
D006859:Hydrogen / Q000737:chemistry* / Q000528:radiation effects; D008567:Membranes, Artificial; D018625:Microscopy, Atomic Force; D017646:Organosilicon Compounds / Q000138:chemical synthesis* / Q000737:chemistry / Q000528:radiation effects; D012680:Sensitivity and Specificity; D012825:Silicon / Q000737:chemistry* / Q000528:radiation effects; D013052:Spectrometry, X-Ray Emission / Q000379:methods; D013499:Surface Properties; D014466:Ultraviolet Rays; D014961:X-Ray Diffraction / Q000379:methods; D014965:X-Rays
D008567:Membranes, Artificial; D017646:Organosilicon Compounds; D006859:Hydrogen; D012825:Silicon