Structure, Composition, and Phase Behavior of Mixed Biphenylthiol Self-Assembled Monolayers
EMSL Project ID
24103
Abstract
Controlling nanoscale surface and interface properties is of high interest in molecular electronics, sensors, biocompatible materials, and many other fields. One route to such surface functionalization is the self-assembly of two component mixtures from solution. In order for these systems to be used for specific applications, however, an understanding of the structure and phase behavior of mixed self-assembled monolayers (SAMs) is essential. Recently, biphenylthiols adsorbed on gold films have attracted some attention as good candidates for advanced surface design due to the quality of the SAMs formed, their rigidity, and higher conductivity than the much studied alkanethiol SAMs.The objectives of the work proposed here are to determine the relative coverages, molecular orientation, phase behavior, and nanoscale patterning of a range of two component SAMs composed of biphenylthiols and alkanethiols on gold films using a combination of x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD) and scanning probe microscopy (SPM). In our lab at Central Washington University (CWU), mixed SAMs of varying mole fractions of two biphenylthiols (4'-trifluoromethyl-4-mercaptobiphenyl and 4'-hydroxy-4-mercaptobiphenyl) have been studied using reflection absorption infrared spectroscopy (RAIRS) and scanning tunneling microscopy (STM). RAIRS data show that the symmetric C-F stretch is actually more intense for certain mixed SAMs than for a pure 4'-trifluoromethyl-4-mercaptobiphenyl SAM. This may be due to an enhancement of the adsorption of this molecule due to the presence of the 4'-hydroxy-4-mercaptobiphenyl component or it may be due to a change in the tilt angle of the molecule. Mixed systems of alkanethiols of different chain lengths and insertion of 4'-hydroxy-4-mercaptobiphenyl in alkanethiol matrices similarly yield RAIRS results which indicate changes in molecular orientation relative to the corresponding single component SAMs. STM results show a wide range of lateral structure from pure mixing to nanoscale phase separation.
XPS of F and O core-levels will allow for definitive coverage determinations of each component of the mixed biphenylthiol system described above and thus allow for a clearer interpretation of the RAIRS data and thus of the molecular structure of this two component system. Although a powerful method for surface structural investigations, XPD has not yet been applied to SAMs studies. Here, we propose to examine the polar angle dependence of the C1s signal from single and two component SAMs to directly determine the tilt angle of the molecules through forward scattering of the emitted photoelectrons. EMSL's SPM facilities are superior to ours and will result in higher quality imaging of phase separation and mixing of the two component systems.
The measurements proposed here will complement the RAIRS and STM data already accumulated at CWU and provide the data necessary for a comprehensive understanding of some of the mixed SAMs we are studying. The molecular orientation derived from XPD, the coverages determined from XPS, and the phase behavior from SPM measurements are essential for elucidating the three-dimensional molecular scale structure of these interesting systems. Such a comprehensive examination of mixed SAMs is important for understanding the factors governing the self-assembly of two component systems and has potential impact for fine tuning our ability to exploit the natural process of self-assembly for advanced surface design.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-05-31
End Date
2008-05-07
Status
Closed
Released Data Link
Team
Principal Investigator