Direct structural observation of a molecular junction by high-energy x-ray reflectometry

doi:10.1073/pnas.0508070103

Direct structural observation of a molecular junction by high-energy x-ray reflectometry

*Department of Chemistry, Columbia University, New York, NY 10027;
^‡Department of Physics, Brookhaven National Laboratory, Upton, NY 11973;
^§Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel;
^¶Complex Materials Consortium Collaborative Access Team (CMC-CAT), Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439; and
^∥Department of Physics, Harvard University, Cambridge, MA 02138

Edited by Stuart Lindsay, Arizona State University, Tempe, AZ, and accepted by the Editorial Board December 12, 2005
↵ ^†M.L. and J.B. contributed equally to this work. (received for review September 30, 2005)

Abstract

We report a direct angstrom resolution measurement of the structure of a molecular-size electronic junction comprising a single (or a double) layer of alkyl-thiol and alkyl-silane molecules at the buried interface between solid silicon and liquid mercury. The high-energy synchrotron x-ray measurements reveal densely packed layers comprising roughly interface-normal molecules. The monolayer’s thickness is found to be 3–4 Å larger than that of similar layers at the free surfaces of both mercury and silicon. The origins of this and the other unusual features detected are discussed in this article. Measurements of the bilayer junction with an applied potential did not show visible changes in the surface normal structure.

Footnotes

**To whom correspondence should be addressed. E-mail: ocko{at}bnl.gov
Author contributions: M.L., J.B., P.P., M.D., C.N., and B.M.O. designed research; M.L., J.B., E.S., I.K., M.D., and B.M.O. performed research; M.L., J.B., P.P., M.D., and B.M.O. analyzed data; and M.L., J.B., E.S., I.K., P.P., M.D., C.N., and B.M.O. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNS office. S.L. is a guest editor invited by the Editorial Board.
Abbreviations:

SAM,

self-assembled monolayer;

OTS,

octadecyltrichlorosilane;

C18SH,

octadecanethiol;

vdW,

van der Waals;

I–V,

current–voltage.