Projets par an
Résumé
The sum frequency generation (SFG) signatures of octadecyltrichlorosilane (OTS) and dodecyldimethylchlorosilane (DDCS) monolayers on silica were simulated in the C–H stretching region for three polarization combinations (ppp, sps, and ssp), showing the impact of the additional Silinked methyl groups of DDCS on its SFG signatures. These simulations are based on a twostep procedure where (i) the molecular properties (vibrational frequencies, IR and Raman intensities) are evaluated using first principles methods and (ii) the threelayer model is employed to calculate the macroscopic responses using these molecular responses, the geometry of the experimental setup, and the optical properties of the layers. These first principles calculations adopt the own Nlayered integrated orbital molecular mechanics (ONIOM) approach, which divides the system and enables different levels of approximation to be applied to its different parts. Here, the same ωB97XD exchangecorrelation functional is used for all parts, while the underlying silica layers are described with a smaller atomic basis set (STO3G, 321G, or 631G) than the alkylsilane and the top silica layer (6311G*). Calculations show that for describing the lower layer the minimal STO3G basis set already provides reliable spectral profiles. For OTS, the results are compared to the experiment, demonstrating a good agreement for ppp and sps configurations, provided the refractive index of the layer nl is set to 1.1. To highlight the origin of the SFG signatures, two chemical models were used, one that includes explicitly the SiO2 surface in the first principles calculations (adsorbedmodel) and the other that only considers the silane chain (isolatedmodel). Simulations show that OTS and DDCS display similar spectral patterns where, for ppp and sps configurations, the r− CH3 stretching vibrations are dominant in comparison to the r+ stretching ones. Still, in the case of DDCS, the r− peak presents a shoulder, which is assigned to the vibrations of the Silinked methyl groups. This shoulder vanishes when these CH3 groups are frozen. Then, using the isolatedmodel, the rotation angle (ξ) is gradually changed, showing that in the ppp SFG spectrum the r−/r+ intensity ratio decreases from 73.4 at 0° to 1.7 at 180°
langue originale  Anglais 

Numéro d'article  074703 
Pages (de  à)  074703 
journal  The journal of chemical physics 
Volume  150 
Numéro de publication  7 
Les DOIs  
Etat de la publication  Publié  21 févr. 2019 
Projets

CÉCI – Consortium des Équipements de Calcul Intensif
CHAMPAGNE, B., Lazzaroni, R., Geuzaine , C., Chatelain, P. & Knaepen, B.
1/01/18 → 31/12/22
Projet: Recherche

Elaboration of quantum chemistry methods for the simulation and the interpretation of nonlinear vibrational signatures of functionalized surfaces
TETSASSI FEUGMO, C. G. & CHAMPAGNE, B.
1/10/11 → 31/12/18
Projet: Projet de thèse
Équipement

Plateforme Technologique Calcul Intensif
Benoît Champagne (!!Manager)
Plateforme technologique Calcul intensifEquipement/installations: Plateforme technolgique