27 research outputs found
Metallic Glasses from the Bottom-up
The main challenge in understanding the relation between the structure and properties of metallic glasses is describing their structure at the atomic level. Currently, their structures are considered simply disordered and indeed our understanding of their structure is as undefined as this term. Following the most advanced structural models of metallic glasses that are based on metal clusters, a bottom-up approach to fabrication of metallic glasses using cluster beam technology is introduced. Using metal clusters to fabricate metallic glasses from the bottom-up, that is, formation of cluster-assembled metallic glasses, provides us with the possibility of varying their structure at the atomic level while keeping their composition unchanged. A unique feature working with cluster-assembled metallic glasses is the independent control of their structure and composition. The advantages of this approach are presented, and its potential toward the resolution of structure–property puzzle in metallic glasses is demonstrated along with the main challenges
Circular Dichroism and Isotropy – Polarity Reversal of Ellipticity in Molecular Films of 1,1’-Bi-2-Naphtol
We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1’-Bi-2-naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.</p
Dissociation energies of AgRG (RG = Ar, Kr, Xe) and AgO molecules from velocity map imaging studies
The near ultraviolet photodissociation dynamics of silver atom rare gas
dimers have been studied by velocity map imaging. AgRG (RG = Ar, Kr, Xe)
species generated by laser ablation are excited in the region of the C <- X
continuum leading to direct, near threshold dissociation generating Ag* (2P3/2)
+ RG (1S0) products. Images recorded at excitation wavelengths throughout the C
<- X continuum, coupled with known atomic energy levels, permit determination
of the ground X (2SIGMA+) state dissociation energies of 85.9 +/- 23.4 cm-1
(AgAr), 149.3 +/- 22.4 cm-1 (AgKr) and 256.3 +/- 16.0 cm-1 (AgXe). Three
additional photolysis processes, each yielding Ag atom photoproducts, are
observed in the same spectral region. Two of these are markedly enhanced in
intensity upon seeding the molecular beam with nitrous oxide, and are assigned
to photodissociation of AgO at the two photon level. These features yield an
improved ground state dissociation energy for AgO of 15965 +/- 81 cm-1, which
is in good agreement with high level calculations. The third process results in
Ag atom fragments whose kinetic energy shows anomalously weak photon energy
dependence and is assigned tentatively to dissociative ionization of the silver
dimer Ag2
The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters
Iron oxide nanoclusters are of interest for a broad range of applications, but limited experimental information on their oxidation mechanism is available outside of the gas phase. Here, the oxidation of graphene-supported size-selected Fe-n clusters is studied using high-resolution X-ray Photoelectron Spectroscopy.Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe-n clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p(3/2) binding energy of metallic and oxidized clusters on the cluster size. Binding energies are also linked to chemical reactivity through the asymmetry parameter which is related to electron density of states at the Fermi energy. Upon oxidation, iron atoms in clusters reach the oxidation state Fe(II) and the absence of other oxidation states indicates a Fe-to-O ratio close to 1:1, in agreement with previous theoretical calculations and gas-phase experiments. Such knowledge can provide a basis for a better understanding of the behavior of iron oxide nanoclusters as supported catalysts
Circular Dichroism and Isotropy – Polarity Reversal of Ellipticity in Molecular Films of 1,1’-Bi-2-Naphtol
We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1’-Bi-2-naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.</p
Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study
The possibility of depositing precisely mass-selected Ag clusters (Ag-1, Ag-3, and Ag-7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d(5/2) core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag-7 deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level
Effect of Thiol-Ligands on the Optical Response of Supported Silver Clusters
International audienc
Infrared spectroscopy of gas-phase M(+)(CO2)n (M = Co, Rh, Ir) ion-molecule complexes
The structures of gas-phase M(+)(CO2)n (M = Co, Rh, Ir; n = 2-15) ion-molecule complexes have been investigated using a combination of infrared resonance-enhanced photodissociation (IR-REPD) spectroscopy and density functional theory. The results provide insight into fundamental metal ion-CO2 interactions, highlighting the trends with increasing ligand number and with different group 9 ions. Spectra have been recorded in the region of the CO2 asymmetric stretch around 2350 cm(-1) using the inert messenger technique and their interpretation has been aided by comparison with simulated infrared spectra of calculated low-energy isomeric structures. All vibrational bands in the smaller complexes are blue-shifted relative to the asymmetric stretch in free CO2, consistent with direct binding to the metal center dominated by charge-quadrupole interactions. For all three metal ions, a core [M(+)(CO2)2] structure is identified to which subsequent ligands are less strongly bound. No evidence is observed in this size regime for complete activation or insertion reactions
Reappraising the Luminescence Lifetime Distributions in Silicon Nanocrystals
Abstract The luminescence dynamics in ensembles of nanocrystals are complicated by a variety of processes, including the size-dependence of the radiative and non-radiative rates in inhomogeneous broadened samples and interparticle interactions. This results in a non-exponential decay, which for the specific case of silicon nanocrystals (SiNCs) has been widely modeled with a Kohlrausch or “stretched exponential” (SE) function. We first derive the population decay function for a luminescence decay following exp[− (t/τ) β ]. We then compare the distributions and mean times calculated by assuming that either the luminescence decay or the population decay follows this function and show that the results are significantly different for β much below 1. We then apply these two types of SE functions as well as other models to the luminescence decay data from two thermally grown SiNC samples with different mean sizes. The mean lifetimes are strongly dependent on the experimental setup and the chosen fitting model, none of which appears to adequately describe the ensemble decay dynamics. Frequency-resolved spectroscopy (FRS) techniques are then applied to SiNCs in order to extract the lifetime distribution directly. The rate distribution has a half width of ~ 0.5 decades and mainly resembles a somewhat high-frequency-skewed lognormal function. The combination of TRS and FRS methods appear best suited to uncovering the luminescence dynamics of NC materials having a broad emission spectrum