Monolayer-Protected Cluster Superlattices:  Structural, Spectroscopic, Calorimetric, and Conductivity Studies

Abstract

Alkanethiol-protected silver clusters of average diameter 4.0 ( 0.5 nm form single-phase superlattice solids, and their X-ray powder diffractograms have been fully indexed to single cubic unit cells. Whereas alkanethiols with five or more carbon atoms form superlattices, the corresponding cluster with four carbons yield only separated clusters. The superlattice solids can be recrystallized from nonpolar solvents. No such superlattices are seen for the corresponding gold clusters. The superlattice collapses upon heating, but the solid retains the structure even at 398 K, much above the melting point of crystalline alkanes and the corresponding self-assembled monolayer. In situ variable-temperature X-ray diffraction investigations did not show any solid-state phase transitions in the superlattice. Temperature-dependent infrared spectroscopy reveals the melting of the alkyl chain, and it is seen that the chain as a whole achieves rotational freedom prior to the collapse of the superlattice. Calorimetric investigations show distinct monolayer and superlattice melting transitions. The chemical nature of the cluster-molecule interaction is similar to that of the previously investigated gold and silver systems, as revealed by NMR, mass, infrared, and X-ray photoelectron spectroscopies and thermogravimetry analyses. Conductivity measurements clearly manifest the superlattice melting transition. Diffusion constants in solution measured by NMR show that the relative decrease in the diffusion constant with increasing monolayer chain length is smaller for silver than for gold, suggested to be a signature of intercluster interaction even in solution. Corroborative evidence is provided by the variable-temperature UV/vis investigations of the clusters.

3D Monolayers of 1,4-Benzenedimethanethiol on Au and Ag Clusters: Distinct Difference in Adsorption Geometry with the Corresponding 2D Monolayers

“…In one of our recent investigations of metal cluster liquids by X-ray photoelectron spectroscopy, we found the presence of Ag(I) on the surface of the clusters (20). This investigation was possible on a smooth film of the sample, which was prepared by melting the monolayer-capped metal superlattice solid (21), leading to a free flowing liquid.…”

Dynamics of Alkyl Chains in Monolayer-Protected Au and Ag Clusters and Silver Thiolates: A Comprehensive Quasielastic Neutron Scattering Investigation

“…The preparation and characterization of these nanosystems have been discussed previously. 53 Thiolates have been synthesized as per the method discussed. 22 The metal clusters are characterized by various spectroscopies, diffraction, and microscopy.…”

“…22 The metal clusters are characterized by various spectroscopies, diffraction, and microscopy. 53 Thiolates have been characterized by spectroscopy and diffraction. [18][19][20][21][22] The phase behavior of the systems has been studied by DSC using a Netsch DSC 4000.…”

Structure and dynamics of monolayers on planar and cluster surfaces

“…We have studied the alkanethiols on silver clusters by IR spectroscopy [70]. Presence of d + and d -modes at 2850 and 2920 cm -1 in the case of long-chain alkanethiols (n = 8) shows the crystallinelike environment.…”