Trapping of molecules in alkanethiol self-assembled monolayer matrices
TL;DR: In this article, a self-assembled monolayer (SAM) surfaces were used as a matrices to preserve species in the presence of NH+4 in water solutions with octadecanethiol.
Abstract: Reactions of NH+ 4 in water solutions with self-assembled monolayer (SAM) surfaces result in the deposition of ammonia in the monolayers. Similarities between this and the soft landing of ions seen in low energy gas phase ion—surface collisions suggest that monolayers can be used as matrices to preserve species. Ammonia can be incorporated only into thicker SAMs such as those made from octadecanethiol. Reactions with larger ions do not lead to trapping. Trapped ammonia desorbs during prolonged exposure of the SAMs to vacuum.
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TL;DR: In this article, the melting properties of octadecanethiol and octanethiol protected silver clusters were studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy.
Abstract: Melting of crystalline solids (superlattices) of octadecanethiol and octanethiol protected silver clusters has been studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. These solids have been compared with the silver thiolate layered compounds in view of their similarity in alkyl chain packing and x-ray diffraction patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silver reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggregation of clusters during heating/cooling cycles. DSC shows two distinct transitions, first corresponding to alkyl chain melting and the second corresponding to superlattice melting. Only alkyl chain melting is observed in variable temperature IR and increased order is manifested upon repeated heating/cooling cycles. Alkyl chain assembly shows strong interchain coupling leading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche defects, whereas significant increase in defect structures is not seen in superlattices. Repeated heating/cooling cycles increase interchain interactions within a cluster and the superlattice order collapses.
46 citations
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TL;DR: In this article, the thermal chemistry of the neutralized sandwich cluster cations on each substrate was studied with temperature programmed desorption (TPDP) and it was determined that the activation energies of the deposited clusters are incorporated into the SAM matrix and firmly trapped inside the alkyl chains of the SAM.
Abstract: V(benzene)2 sandwich cluster cations produced in the gas phase
were size-selectively deposited onto a self-assembled monolayer of
n-hexadecanethiols (HDT-SAM) chemisorbed on a Au(111) surface as well as onto
a bare Au(111) surface. The thermal chemistry of the neutralized clusters on
each substrate was studied with temperature programmed desorption (TPD).
From the analyses of the threshold in the TPD, the desorption
activation energies of the clusters deposited were determined to be 64.4
±12.8 kJ/mol for the Au(111) and 130 ±10 kJ/mol for the HDT-SAM.
The remarkably large desorption activation energy from the SAM suggests that
the deposited clusters are incorporated into the SAM matrix and
firmly trapped inside the alkyl chains of the SAM.
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TL;DR: The results suggest that modified monolayers can be made by simple electrochemical procedures and the processes are similar to the corresponding gas-phase events.
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References
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TL;DR: In this article, it was shown that metal fluorides are the most abundant products in the scattered ion spectra and that multiple fluorine abstractions can occur by a direct reaction mechanism within a single scattering event.
Abstract: Low-energy (<100 eV) ion/surface reactions of metal ions, Ti*+, Cr*+, Fe,+, Mo*+, and W,+, at a fluorinated self-assembled monolayer surface give fluorine-containing scattered ions, MF„+, n < 5. The metal fluorides are the most abundant products in the scattered ion spectra. The single fluorine abstractions are all endothermic and driven by the projectile translational energy. Multiple fluorine abstraction occurs with high efficiency for Mo,+ and W,+ projectiles, where the thermochemistry is most favorable. Polyatomic projectile ions derived by partial fragmentation of the Cr, Mo, and W hexacarbonyls also yield metal fluoride scattered ions, in addition to more complex species formed by fluorine abstraction by ions which retain one or more carbon atoms or carbonyl groups. Feand Ti-containing projectile ions, bearing one or two cyclopentadienyl (Cp) groups, also exhibit fluorine abstraction products, with and without retention of a Cp group. TiCl„+ (n = 1-4) projectiles exhibit fluorine abstraction, as well. Similar fluorine abstraction products are generated upon collisions at a liquid perfluorinated polyether surface. Gas-phase ion/molecule reactions and thermochemical considerations suggest that multiple fluorine abstractions can occur by a direct reaction mechanism within a single scattering event and that multiple fluorine atoms are probably derived from a single fluorocarbon chain. Angle-resolved scattering data support a single-collision, multiple-atom abstraction mechanism. The scattered ions leave the surface with very low translational energy and collision energy variations suggest that in these ion/surface reactions, projectile dissociation is concerted with or occurs prior to fluorine abstraction. There is no evidence that electron transfer to the surface is involved, as it is in alkyl group abstraction reactions by ions at hydrocarbon surfaces.ion products are generated upon collisions at a liquid perfluorinated polyether surface. Gas-phase ion/molecule reactions and thermochemical considerations suggest that multiple fluorine abstractions can occur by a direct reaction mechanism within a single scattering event and that multiple fluorine atoms are probably derived from a single fluorocarbon chain. Angle-resolved scattering data support a single-collision, multiple-atom abstraction mechanism. The scattered ions leave the surface with very low translational energy and collision energy variations suggest that in these ion/surface reactions, projectile dissociation is concerted with or occurs prior to fluorine abstraction. There is no evidence that electron transfer to the surface is involved, as it is in alkyl group abstraction reactions by ions at hydrocarbon surfaces.
54 citations
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TL;DR: The most significant conclusion of the study is that selective chemical modification of monolayer surfaces can be achieved by using reactive ion beams, which lead to new covalent bonds at the surface and in the scattered ions.
41 citations
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TL;DR: In this article, X-ray photoelectron spectroscopy was used to study self-assembled monolayers of varying chain lengths and the binding energies and peak shapes were determined for all the samples.
25 citations
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TL;DR: Evidence is presented which suggests that in some cases molecular projectiles undergo surface-induced dissociation to yield atomic species which subsequently react with the surface.
24 citations
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TL;DR: In this paper, the Hartree-Fock approximation and first Born approximation were used for the calculation of inelastic mean free paths of electrons in solids, and the results for the application of the method to the six solid media amorphous carbon, silicon, germanium, Al2O3, SiO2, and GeO2; are presented.
22 citations