Showing papers by "Thalappil Pradeep published in 2000"

Picosecond optical nonlinearity in monolayer-protected gold, silver, and gold-silver alloy nanoclusters

Abstract: Monolayer-protected Au, Ag, and Au:Ag alloy nanoclusters have been synthesized using octanethiol and octadecanethiol as capping agents. The particle-size distribution is narrow with an average core size of 3--4 nm. Optical nonlinearity induced by 35 ps pulses at 532 nm has been investigated in these samples using the Z-scan technique. It is found that in general, they behave either as saturable absorbers or reverse saturable absorbers depending on the intensity of excitation. Au and Ag clusters show nearly the same efficiency for optical limiting, but the alloy clusters are found to be less efficient in limiting and are less photostable. The observed effects are explained in terms of the electron dynamics of the excited-state species.

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 inve...

Melting of monolayer protected cluster superlattices

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.

Crystalline solids of alloy clusters

Abstract: AuAg alloy clusters protected with octadecanethiol (ODT) monolayers form crystalline solids whose powder X-ray diffractograms can be indexed to single cubic unit cells. These solids can also be regarded as superlattices of alloy clusters. The bulk Au:Ag ratio is significantly different from the metal salt feed ratio used for the synthesis of the alloy clusters. The surface plasmon resonance band of the alloy clusters shift monotonically with change in composition and only one distinct band is seen in the composition range, indicating that the single metal core containing alloy clusters are not forming. The shifts observed can be linearly related to the compositions. Atomic emission spectroscopy gives similar compositions as determined from absorption spectroscopy. The monolayer coverage determined from thermogravimetry increases with an increase in silver concentration. Thermal stability of the monolayers is comparable to pure Au and Ag clusters and no change in desorption temperature is observed with composition. Both monolayer melting and superstructure melting transitions are seen in the differential scanning calorimetry of the solids. Infrared spectroscopy suggests that the monolayers are essentially solidlike and the conformations are similar to the monolayers on pure Ag clusters. Some of the bands exhibit factor group splitting in the Au-rich composition, which is observed in other compositions upon annealing.

Distinct liquid phase in metal-cluster superlattice solids

Abstract: Variable temperature x-ray diffraction and differential scanning calorimetry experiments on the superlattice solids of alkanethiol protected silver nanoclusters show that the translational periodicity collapses around 398 K resulting in a liquid phase, which upon cooling, reverts into the parent crystalline phase. This reversibility is seen in a narrow temperature window of 400-448 K. If the heating is done above 473 K, the reversibility is lost, and the liquid in the cooling cycle freezes into a disordered phase. The stability of different phases, encountered in the experiments, is discussed in terms of a phenomenological free energy.

2-Mercaptobenzothiazole protected Au and Ag clusters

Abstract: Nanometer sized gold and silver clusters protected with 2-mercaptobenzothiazole monolayers have been prepared and characterized by various spectroscopic methods. Optical absorption spectra show features assigned to charge-transfer excitation between the monolayer and the cluster, in addition to a red shifting and reduction of plasmon absorption. The monolayers on clusters are compared with the corresponding 2D-monolayers investigated previously. The dominant adsorbate geometries on these clusters are different. Whereas the temperature dependent dynamics are minimal for 2D-SAMs, they are significant for monolayers on 3D surfaces, which is attributed to the decreased packing density on the cluster surfaces. The thermal stability of these monolayers is high and comparable to that of alkanethiolate monolayers. The monolayers undergo irreversible structural changes upon heating, which have been studied by differential scanning calorimetry and IR spectroscopy.

Ionic dissociation of NaCl on frozen water

Abstract: Low-energy reactive ion scattering (RIS) experiments show that NaCl dissociates almost completely on condensed ice surfaces prepared on Ru(001) even at 100 K forming solvated ions. NaCl by itself does not dissociate on a Ru(001) substrate; however, submonolayer coverages of water sets-in dissociation. Na+ ions thus created are immobile such that they do not migrate across one water bilayer (BL) over several minutes. Accumulation of positive charge at the ice surface makes Na+ diffuse into the sublayers.

Metal Ion Reactivity with 1,4-Benzenedimethanethiol Monolayers on Gold

Abstract: Exposure of Cu2+ ions to 1,4-benzenedimethanethiol (BDMT) monolayers on Au in solution results in the formation of copper functionalized monolayers which have been investigated in detail by surface-enhanced Raman spectroscopy (SERS), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Upon exposure to copper ions, the free thiol groups at the surface of the monolayer disappear indicating the replacement of protons with copper ions. The reaction leads to a red shift in the C−S stretching frequency, but most of the other features are unaffected. Relative intensities of the peaks are largely the same; however, some new features are observed suggesting minor changes in the adsorbate structure that makes additional modes observable. Thermal stability of the monolayers has been reduced substantially as a result of reaction with metal ions, indicating that the chemical binding at the Au−monolayer interface is affected. XPS shows that copper is present at the surface. Electrochemical behavior bef...

A Method To Study the Phase Transition and Desorption of Self-Assembled Monolayers on Planar Gold Surfaces

Abstract: In-plane resistance of thin gold films (thickness ∼1000 A) after the formation of self-assembled monolayers has been measured as a function of temperature. We attribute the observed increase in the slope of the resistance vs temperature plot in the case of octadecanethiol and octanethiol monolayers at characteristic temperatures to phase transitions. The structural transition of 1,4-benzenedimethanethiol is also manifested similarly. Propanethiol does not show any phase transition, as expected. The observed changes in the alkanethiol monolayers are related to the decrease in the lifetime of the parallel vibrational motion of the adsorbate on the gold surface. The increase in slope is followed by a decrease which is attributed to the commencement of desorption of monolayers from the gold surface.

Activation of Dioxygen by Halocarbon Ions

Abstract: Tandem mass spectrometry is used to show that low energy collisions of ionized halocarbenes, including CBr 2 •+ and CBrCl •+ , with molecular oxygen lead to (i) decarbonation with formation of the dihalogen molecular cation and (ii) oxygenolysis yielding BrCO + . Both reactions may occur via the same ion-molecule addition product of molecular oxygen and the ionized carbene. Reaction is favored at low collision energies, such as are encountered in an ion trap. Insights into the energetics of the reactions of CBr 2 •+ with O 2 are obtained from ab initio molecular orbital theory. Other aspects of the positive ion chemistry of dioxygen with various halogen-containing ions are also discussed. Dioxygen activation by halocarbon ions suggests additional channels that might affect the fate of halocarbons and the ozone balance in the atmosphere.