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.

http://www.dstuns.iitm.ac.in/listpdf/80.pdf

Melting of monolayer protected cluster superlattices

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.

Trapping of V(benzene)2 sandwich clusters in a n-alkanethiol self-assembled monolayer matrix

http://www.dstuns.iitm.ac.in/listpdf/68.pdf

Ion/Surface Reactions at Monolayers in Solution: A Combined Surface Enhanced Raman-X-Ray Photoelectron Spectroscopic Investigation of the Chemical Modification of a 2-Mercaptobenzothiazole Monolayer on Polycrystalline Au Films.

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.

http://www.dstuns.iitm.ac.in/listpdf/64.pdf

M. Venkataramanan

Papers

Trapping of molecules in alkanethiol self-assembled monolayer matrices