Evidence of the Grafting Mechanisms of Diazonium Salts on Gold Nanostructures

Abstract

International audienceGold nanostructures (GNS) were chemically functionalized using four different diazonium salts: benzene-diazonium-tetrafluoroborate (DS), 4-decylbenzene-diazonium-tetrafluoroborate (DS-C10H21), 4-carboxybenzene-diazonium-tetrafluoroborate (DS-COOH), and 4-(aminoethyl)-benzene-diazonium-tetrafluoroborate (DS-(CH2)2NH2). Effective chemical grafting on GNS was shown by surface-enhanced Raman spectroscopy (SERS); aromatic ring deformations in the range of 1570–1591 cm–1 are of particular interest. The very strong band observed around 1075 cm–1, related to CH in-plane bending for mono- and para-substituted benzenes (coupled with ring-N stretching mode), provided further irrefutable evidence of the grafting. SERS enhancement of these two bands ascertains the perpendicular orientation of the aromatic rings on the GNS. X-ray photoelectron spectroscopy (XPS) analyses of chemically grafted flat gold surfaces suggest azophenyl radical pathways when using DS, DS-(CH2)2-NH2, or DS-C10H21. It was shown that coating at the interface is the result of a Au–N covalent bond; growth of the layers is via N═N. These XPS results agree with those provided by SERS without excluding the aryl radical pathways. For DS-COOH, the results provided by SERS, XPS, and density functional theory calculations show (i) effective chemical grafting of the GNS via a covalent bond between gold and carboxylate forms and (ii) growth via multilayers in the meta position between aromatic rings through either N═N or C–C bonds

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

“…In this last case an azo-bridge stretching N=N has been also reported [50] and at 66 meV (530 cm −1 ) deformation of the C-N=N bond is also expected [42]. Furthermore, stretching of the N-H can produce the band at 410 meV (3300 cm −1 ) [42] and the stretching of the N ≡ N bond can be recognized in the peak around 297 meV (2400 cm −1 ) [34,42,48]. Note that azo-bonds are remarkably evident in our IETS measurements, while they are typically difficult to identify by infrared spectroscopy since they do not respond to characteristic selection rules, being symmetric in nature and thus inactive in IR.…”

“…2, realized by comparison to existing vibrational spectroscopy studies in literature [21,34,35,[42][43][44][45][46][47][48][49][50][51][52][53]. Typical signatures of expected modes related to bonds of the AQ molecule are present in the whole energy range.…”

“…Note finally that some peaks can be also attributed to residual traces of organic solvents, such as a C-C stretching vibration at 130 meV (1050 cm −1 ) [34,35,42,47,48], a CH 2 stretching at 345-360 meV (2780-2900 cm −1 ). Hydroxyl O-H group bands are attributable to the peaks at 410 meV (3300 cm −1 ) and 437 meV (3525 cm −1 ) and can be also related to vibrations of water molecules [42].…”

“…Most evident vibrational bands are highlighted by black vertical lines. Their assignment is done following the literature [21,34,35,[42][43][44][45][46][47][48][49][50][51][52][53][54] and is summarized in Table II. While IETS is typically difficult to interpret since there are no strict theoretical determined selection rules and propensity rules have only been proposed [63,64], in IR spectroscopy selection rules exist, simplifying the assignment of the vibrational modes. By comparing Tables I and II common modes between IETS and IR spectra can be recognized in the energy range 100 meV < E < 200 meV.…”

Raman Characterization of Phenyl-Derivatives: From Primary Amine to Diazonium Salts

“…This paper reports on a preliminary study designed as a leadin to further examinations for investigating, improving and/ or controlling reaction pathways using synthesized diazonium salt (i.e., to examine and strengthen the nature of the interface between Gold NanoStructures (GNS, electron-beam nanolithographied SERS substrates) and organic coatings) [46].…”