Infrared spectroscopy of the organic monolayer sandwiched between a Hg electrode and a Si substrate

Abstract

We have successfully observed the vibrational spectra of organic monolayers sandwiched between a liquid Hg electrode and a Si substrate by means of a newly developed reflection absorption (RA) device. The vibrational spectra of organic monolayers between two electrodes can be observed under a certain bias voltage. The monolayers were fabricated by the reaction of hydrogen-terminated Si(111) with 1-octadecene. A metal/insulator/semiconductor structure was prepared using liquid Hg as a metal electrode and the organic monolayer as an insulator. Infrared (IR) light entered from the Si substrate side with an incident angle of 75 degrees. The reflected IR light from the metallic Hg was detected by a mercury-cadmium-telluride detector. We obtained RA spectra using a bare H-Si(111) substrate as a reference. The absorbance of the RA spectrum was comparable with that of the transmission spectrum for the octadecyl-terminated Si(111) without Hg. The C-H stretching modes in the CH(2) group show blueshifts, and the C-H antisymmetric stretching modes in the CH(3) are broadened in comparison with the transmission spectrum. Under a certain bias voltage, we observed changes in band shape. We concluded that the variation was due to the temperature increase by resistive heating of the substrate.

Molecular Orientation of the Decyl Monolayer Chemically Bonded to Si(111) Determined by Angle-Dependent Transmission Infrared Spectroscopy

“…The peak assignments are based on a similar system, i.e., C18-Si. 33,[35][36][37] The peaks at 2851 and 2921 cm À1 are CH 2 symmetric (d þ ) and antisymmetric (d À )…”

“…2) and ( 6), considering the standard deviation of background noise (6 Â 10 À6 ). 33) The estimated orientation angles () and k bulk are summarized in Table I, where the orientation angles mean the angles between the dynamic dipole moments and the surface normal. The calculated absorbance curves by eq.…”

“…Because the preparation of the monolayer and the setup for IR measurement are depicted elsewhere in detail, 32,33) we briefly describe the present experimental method. Pieces of double-sided polished n-type Si(111) wafers (25 Â 10 mm 2 ; 1-3.5 Ácm; thickness = 600 m) were cleaned by hot (100 C) sulfuric acid/hydrogen peroxide mixture (SPM) and 5% HF aqueous solution.…”

Tailoring Electric Field Standing Waves in Reflection–Absorption Infrared Spectroscopy to Enhance Absorbance in Buried Molecular Layers

“…Highly sensitive and selective techniques are required to probe buried layers and interfaces that can be found in the natural environment − as well as in manufactured systems, − where specific reaction mechanisms and transport processes occur that are still only partially understood. For instance, electrochemical and catalytic reactions, − transport within the lipid bilayer of the cell, ,,, and partition of solutes in multiphase fluids ,, can be understood by scrutinizing precisely interfaces and buried molecular layers where these phenomena take place. , Conventional in situ and non-invasive methods such as sum frequency generation (SFG), ,,,− Fourier transform infrared (FTIR) spectroscopy, ,− ,,− grazing incidence X-ray diffraction (GIXD), − and Raman spectroscopy , have been applied to improve our understanding of these processes. , Despite the high selectivity of SFG and GIXD for interfaces, they lack the capacity to investigate the bulk of buried molecular layers. As for the usual methodologies and techniques relying on FTIR and Raman spectroscopy, they are commonly found to have a poor surface sensitivity or to be unselective to the depth profile of multilayer systems.…”