Probing the Molecular Conformation of Self-Assembled Monolayers at Metal/Semiconductor Interfaces by Vibrational Sum Frequency Generation Spectroscopy

Abstract

The conformation of self-assembled monolayers (SAMs) buried at metal-semiconductor interfaces plays a crucial role in the functioning of the molecular junctions thus formed. We have studied by vibrational sum frequency generation (SFG) spectroscopy the influence of deposited gold films on the molecular orientation of a series of n-alkyl monolayers that are covalently bonded to silicon(111) via Si-C linkages (≡Si-C n H 2n+1 , n ) 10, 12, 14, and 18). The SFG spectra indicate that the gold contacts (overlayers) introduce significant gauche defects within the initially highly oriented alkyl chains. The methyl tilt angles of the buried n-alkyl monolayers depend on the gold deposition method (vacuum evaporation vs sputtering) and the alkyl chain length. These observations are supported by solid-state electrical measurements.

Revealing molecular conformation–induced stress at embedded interfaces of organic optoelectronic devices by sum frequency generation spectroscopy

“…3B and Table 1 ) at this stage from the initial value of 5.73, indicating that pentacene deposition induces drastic ODTS disordering (movie S1). The increase in gauche defects in densely packed alkyl molecules upon deposition of a superimposing layer has been witnessed in other systems ( 39 , 47 , 48 ). The trans-gauche conformational change of the CH 2 -CH 2 segments implies the bending down of the alkyl chain of ODTS ( Fig.…”

“…2B ) modes display very weak signals at approximately 2853 and 2916 cm −1 , respectively ( 32 , 41 – 43 ). The dominance of the CH 3 -ss mode in the SFG SSP spectra indicates that the alkyl chains are predominantly in an all-trans configuration with high ordering, i.e., they are oriented normally to the surface with the CH 3 groups pointing outside ( 39 – 41 ), as schematically shown in Fig. 2E (top) and fig.…”

“…2D ): one centered at approximately 2877 cm −1 corresponding to the CH 3 symmetric stretch ( Fig. 2B ) and the other at approximately 2941 cm −1 arising from its Fermi resonance ( 39 ). The CH 2 symmetric and asymmetric stretching (ss and as, respectively, in Fig.…”

Theoretical Investigation of Vibrational Sum-Frequency Generation Signatures of Functionalized H—Si(111)

“…The simulations were carried out with the IR and visible incident beams at 50°and 70°, respectively, with respect to the sample surface normal. 50 The visible wavelength was kept constant at 532 nm, whereas the IR frequency was swept in the CH stretching region (2800−3100 cm −1 ) and in the SiH stretching region (2000−2200 cm −1 ).…”

Influence of Precursors of Alkyl Self Assembled Monolayers on the Alkyl Chain Structure Studied by sum Frequency Generation

“…SFG is a well established technique for analyzing alkyl chain structures of SAMs on solid surfaces. [9,10] We used two different alkylsilanes having C8 chains, Octyldimethylmethoxysilane (ODMS) and Octyltrichlorosilane (OTCS) with one OCH3 precursor and three Cl precursors, respectively. The chain length of the molecules studied in this work is much shorter than that of C12 or C18 chains in previous reports.…”