Fragmentation channels of non-fullerene cationic carbon clusters.

The unimolecular fragmentation channels of highly excited small cationic carbon clusters have been measured with a time-of-flight mass spectrometer after photofragmentation. The dominant channel is loss of the neutral trimer, for all CN+N = 10-27 clusters except for N = 11, 12 which decay by monomer emission, and C25+ which shows competing loss of C2 and C3. The results permit to quantify the role of the rotational entropy in the competition between monomer and trimer decays with the help of energies calculated with density functional theory.

Pentagon, Hexagon, or Bridge? Identifying the Location of a Single Vanadium Cation on Buckminsterfullerene Surface.

Buckminsterfullerene C60 has received extensive research interest since its discovery. In addition to its interesting intrinsic properties of exceptional stability and electron-accepting ability, the broad chemical tunability by decoration or substitution on the C60-fullerene surface makes it a fascinating molecule. However, to date, there is uncertainty about the binding location of such decorations on the C60 surface, even for a single adsorbed metal atom. In this work, we report the gas-phase synthesis of the C60V+ complex and its in situ characterization by mass spectrometry and infrared spectroscopy with the help of quantum chemical calculations and molecular dynamics simulations. We identify the most probable binding position of a vanadium cation on C60 above a pentagon center in an η5-fashion, demonstrate a high thermal stability for this complex, and explore the bonding nature between C60 and the vanadium cation, revealing that large orbital and electrostatic interactions lie at the origin of the stability of the η5-C60V+ complex.

Small chromium-doped silicon clusters CrSin: structures, IR spectra, charge effect, magnetism and chirality.

A series of small chromium-doped silicon clusters CrSin with n = 3-10 in the cationic, neutral and anionic charge states were investigated using quantum chemical methods. The CrSin+ cations with n = 6-10 were produced in the gas phase and characterized by far-IR multiple photon dissociation (IR-MPD) spectroscopy. Good agreement between experimental spectra in the 200-600 cm-1 frequency range and those determined for the lowest-energy isomers by density functional theory calculations (B3P86/6-311+G(d)) provide a strong support for the geometrical assignments. An extensive structural comparison for the three different charge states shows that the structural growth mechanism inherently depends on the charge. While the structures of the cationic clusters are preferentially formed by addition of the Cr dopant to the corresponding pure silicon cluster, it favors substitution in both the neutral and anionic counterparts. The Si-Cr bonds of the studied CrSin+/0/- clusters are polar covalent. Apart from a basket-like Cr@Si9- and an endohedral Cr@Si10- cage, the Cr dopant takes an exohedral position and bears a large positive charge in the clusters. The exohedrally doped clusters also have a high spin density on Cr, manifesting the fact that the intrinsic magnetic moment of the transition metal dopant is well conserved. Three CrSin clusters have a pair of enantiomeric isomers in their ground state, namely the cationic n = 9 and the neutral and anionic n = 7. Those can be distinguished from each other by their electronic circular dichroism spectra, calculated using time-dependent density functional theory. Those enantiomers, being intrinsically chiral inorganic compounds, might be used as building blocks of optical-magnetic nanomaterials because of their high magnetic moments and ability to rotate the plane of polarization.

Gas phase deposition of well-defined bimetallic gold-silver clusters for photocatalytic applications.

Cluster beam deposition is employed for fabricating well-defined bimetallic plasmonic photocatalysts to enhance their activity while facilitating a more fundamental understanding of their properties. AuxAg1-x clusters with compositions (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1) spanning the metals' miscibility range were produced in the gas-phase and soft-landed on TiO2 P25-coated silicon wafers with an optimal coverage of 4 atomic monolayer equivalents. Electron microscopy images show that at this coverage most clusters remain well dispersed whereas EXAFS data are in agreement with the finding that the deposited clusters have an average size of ca. 5 nm and feature the same composition as the ablated alloy targets. A composition-dependant electron transfer from Au to Ag that is likely to impart chemical stability to the bimetallic clusters and protect Ag atoms against oxidation is additionally evidenced by XPS and XANES. Under simulated solar light, AuxAg1-x clusters show a remarkable composition-dependent volcano-type enhancement of their photocatalytic activity towards degradation of stearic acid, a model compound for organic fouling on surfaces. The Formal Quantum Efficiency (FQE) is peaking at the Au0.3Ag0.7 composition with a value that is twice as high as that of the pristine TiO2 P25 under solar simulator. Under UV the FQE of all compositions remains similar to that of pristine TiO2. A classical electromagnetic simulation study confirms that among all compositions Au0.3Ag0.7 features the largest near-field enhancement in the wavelength range of maximal solar light intensity, as well as sufficient individual photon energy resulting in a better photocatalytic self-cleaning activity. This allows ascribing the mechanism for photocatalysis mostly to the plasmonic effect of the bimetallic clusters through direct electron injection and near-field enhancement from the resonant cluster towards the conduction band of TiO2. These results not only demonstrate the added value of using well-defined bimetallic nanocatalysts to enhance their photocatalytic activity but also highlights the potential of the cluster beam deposition to design tailored noble metal modified photocatalytic surfaces with controlled compositions and sizes without involving potentially hazardous chemical agents.

Radiative cooling in silver and palladium doped gold clusters.

The emission of photons from a thermally populated electronic excited state, via the process of recurrent fluorescence, has been recognized as a prominent cooling channel in hot molecules and small metal clusters. For the latter case, however, only monometallic species have been investigated to date. An active radiative cooling channel has a stabilizing effect and can favor the size and composition specific production of selected clusters. In this work, the influence of silver and palladium doping on the radiative cooling of gold cluster cations is studied. The quenching of metastable fragmentation due to radiation of laser-excited Aun+, AgAun-1+ and PdAun-1+ (n = 11-15) clusters is investigated in a single-pass molecular beam setup. The observed high radiation rates, with values in the range from 103 to 105 s-1, are consistent with recurrent fluorescence. The rates present a pronounced odd-even staggering with higher values for the clusters with closed-shell electronic configurations. While substitution of Au with Ag does not alter the odd-even pattern with cluster size, replacing Au with Pd shifts the pattern by one atom. The experimental observations are discussed in terms of the dissociation energy of the clusters, which sets their effective temperature during photon emission, and the low-lying electronic excited states involved in the photon emission process. Linear-response time-dependent density functional theory calculations on selected species are used to illustrate the significant effect of the electronic structure on the radiation rates. For n = 14, substitution of Au with Ag lowers the energy of the lowest-energy transition in the cluster, which in addition has a higher oscillator strength, favoring radiative cooling. The opposite effect is seen in Pd doped clusters. Based on this analysis, conclusions can be drawn about the significance of radiative cooling in laser-excited alloy clusters, with a concomitant fast stabilization at high internal energy conditions.

Influence of oxidation on the magnetism of small Co oxide clusters probed by Stern-Gerlach deflection.

We report on the magnetic properties of small neutral suboxide ConOm (n = 5-18 and m = 0-10, m ≤ n) clusters produced by laser vaporisation and gas aggregation. Their magnetism is probed experimentally by means of Stern-Gerlach magnetic deflection. The results imply that the cobalt atoms couple ferromagnetically not only in pure Con clusters, as known from previous investigations, but also in their oxidized counterparts. It was found that the magnetic moment per cobalt atom is mostly enhanced in the oxide clusters with respect to the pure cobalt clusters and generally increases with the oxygen content in the studied composition range. The spin magnetism of selected clusters is also investigated by density functional theory (DFT) calculations. The computations allow to attribute the effect of oxidation on the magnetic response of the ConOm clusters to electron transfer from the cobalt 3d and 4s valence orbitals to oxygen. The cobalt 3d levels preferentially donate electrons of minority spin, but both spin orientations are involved in the transfer of cobalt 4s electrons.

Spin excitations of individual magnetic dopants in an ionic thin film.

Individual magnetic transition metal dopants in a solid host usually exhibit relatively small spin excitation energies of a few meV. Using scanning tunneling microscopy and inelastic electron tunneling spectroscopy (IETS) techniques, we have observed a high spin excitation energy around 36 meV for an individual Co substitutional dopant in ultrathin NaCl films. In contrast, the Cr dopant in the NaCl film shows much lower spin excitation energy around 2.5 meV. Electronic multiplet calculations combined with first-principles calculations confirm the spin excitation induced IETS, and quantitatively reveal the out-of-plane magnetic anisotropies for both Co and Cr. They also allow reproducing the experimentally observed redshift in the spin excitations of Co dimers and ascribe it to a charge and geometry redistribution.

Evolution of Vibrational Spectra in the Manganese-Silicon Clusters Mn2Sin, n = 10, 12, and 13, and Cationic [Mn2Si13].

A comparison of DFT-computed and measured infrared spectra reveals the ground state structures of a series of gas-phase silicon clusters containing a common Mn2 unit. Mn2Si12 and [Mn2Si13]+ are both axially symmetric, allowing for a clean separation of the vibrational modes into parallel (a1) and perpendicular (e1) components. Information about the Mn-Mn and Mn-Si bonding can be extracted by tracing the evolution of these modes as the cluster increases in size. In [Mn2Si13]+, where the antiprismatic core is capped on both hexagonal faces, a relatively simple spectrum emerges that reflects a pseudo-D6d geometry. In cases where the cluster is more polar, either because there is no capping atom in the lower face (Mn2Si12) or the capping atom is present but displaced off the principal axis (Mn2Si13), the spectra include additional features derived from vibrational modes that are forbidden in the parent antiprism.

The wavelength-dependent non-linear absorption and refraction of Au25 and Au38 monolayer-protected clusters.

In the past decade, the structural and electronic properties of monolayer-protected metal clusters, which can be produced size-selected in macroscopic amounts, have received a lot of attention. Their great potential for optical applications has been identified. In the high intensity regime, monolayer-protected metal clusters show pronounced nonlinear absorption and refraction. Naturally, these phenomena are wavelength-dependent, however, such dependence is largely unexplored. Here, we quantify the wavelength-dependent non-linear optical absorption and refraction cross sections of atomically precise Au25(DDT)18 and Au38(DDT)24 clusters, using the z-scan technique in combination with a tunable nanosecond laser source. Qualitatively different non-linear optical phenomena were found to take place at different excitation wavelengths (two-photon and excited-state absorption, intensity saturation and non-linear refraction). Both clusters have high nonlinear absorption cross sections at 532 nm, and present a (local) maximum at 640 nm, together with a maximum in the absorption saturation. The nonlinear refraction is always negative for Au25(DDT)18, while it changes sign for Au38(DDT)24. Depending on the wavelength, the underlying mechanism of the nonlinear absorption effects is two-photon absorption or excited state absorption. The obtained very high nonlinear cross sections, on the order of 107-109 GM, demonstrate the great potential of those clusters as nonlinear absorption or refraction materials in optical applications.

Antimicrobial and Aging Properties of Ag-, Ag/Cu-, and Ag Cluster-Doped Amorphous Carbon Coatings Produced by Magnetron Sputtering for Space Applications.

Inside a spacecraft, the temperature and humidity, suitable for the human crew onboard, also creates an ideal breeding environment for the proliferation of bacteria and fungi; this can present a hazard to human health and create issues for the safe running of equipment. To address this issue, wear-resistant antimicrobial thin films prepared by magnetron sputtering were developed, with the aim to coat key internal components within spacecrafts. Silver and copper are among the most studied active bactericidal materials, thus this work investigated the antibacterial properties of amorphous carbon coatings, doped with either silver, silver and copper, or with silver clusters. The longevity of these antimicrobial coatings, which is heavily influenced by metal diffusion within the coating, was also investigated. With a conventional approach, amorphous carbon coatings were prepared by cosputtering, to generate coatings that contained a range of silver and copper concentrations. In addition, coatings containing silver clusters were prepared using a separate cluster source to better control the metal particle size distribution in the amorphous carbon matrix. The particle size distributions were characterized by grazing-incidence small-angle X-ray scattering (GISAXS). Antibacterial tests were performed under both terrestrial gravity and microgravity conditions, to simulate the condition in space. Results show that although silver-doped coatings possess extremely high levels of antimicrobial activity, silver cluster-doped coatings are equally effective, while being more long-lived, despite containing a lower absolute silver concentration.

Impact of the gas dynamics on the cluster flux in a magnetron cluster-source: Influence of the chamber shape and gas-inlet position.

Although producing clusters by physical methods offers many benefits, low deposition rates have prevented cluster-beam deposition techniques from being adopted more widely. The influence of the gas aerodynamics inside the condensation chamber of a magnetron cluster-source on the cluster throughput is reported, leading to an improved understanding of the influence of gas aerodynamics on cluster transport. In the first part of this paper, the influence of the carrier gas's inlet position on the cluster flux is studied. In particular, two inlet configurations were investigated, i.e., from the rear of the chamber and from within the magnetron sputtering source. It was found experimentally that the latter configuration can lead to an increased cluster flux, under the same conditions of gas pressure and power applied to the magnetron. This behavior is explained with the help of simulations. In the second part of this paper, the gas dynamics behavior inside four chamber shapes, namely, two cylindrical shapes with different cross-sectional diameters and two conical shapes with different apex angles, was simulated. The modeling showed that the fraction of clusters successfully leaving the aggregation zone can be increased by up to eight times from the worst to the best performing chamber geometries studied. Finally, the cluster throughput was determined experimentally using a quartz microbalance in two of the four chamber designs. It was found that the cluster flux increased up to one order of magnitude, reaching ∼20 mg/h for a condensation chamber with a smaller cross section and a conical exit.

Enhanced Methanol Electro-Oxidation Activity of Nanoclustered Gold.

Size-selected 3 nm gas-phase Au clusters dispersed by cluster beam deposition (CBD) on a conducting fluorine-doped tin oxide template show strong enhancement in mass activity for the methanol electro-oxidation (MEO) reaction compared to previously reported nanostructured gold electrodes. Density functional theory-based modeling on the corresponding Au clusters guided by experiments attributes this high MEO activity to the high density of exposed under-coordinated Au atoms at their faceted surface. In the description of the activity trends, vertices and edges are the most active sites due to their favorable CO and OH adsorption energies. The faceted structures occurring in this size range, partly preserved upon deposition, may also prevent destructive restructuring during the oxidation-reduction cycle. These results highlight the benefits of using CBD in fine-tuning material properties on the nanoscale and designing high-performance fuel cell electrodes with less material usage.

Stability of cationic silver doped gold clusters and the subshell-closed electronic configuration of AgAu14.

Silver doping is a valuable route to modulate the structural, electronic, and optical properties of gold clusters. We combine photofragmentation experiments with density functional theory calculations to investigate the relative stability of cationic Ag doped Au clusters, AgAuN-1 + (N ≤ 40). The mass spectra of the clusters after photofragmentation reveal marked drops in the intensity of AgAu8 +, AgAu14 +, and AgAu34 +, indicating a higher relative stability of these sizes. This is confirmed by the calculated AgAuN-1 + (N ≤ 17) dissociation energies peaking for AgAu6 +, AgAu8 +, and AgAu14 +. While the stability of AgAu6 + and AgAu8 + can be explained by the accepted electronic shell model for metal clusters, density of states analysis shows that the geometry plays an important role in the higher relative stability of AgAu14 +. For this size, there is a degeneracy lifting of the 1D shell, which opens a relatively large HOMO-LUMO gap with a subshell-closed 1S21P41P21D6 electronic configuration.

Photofragmentation Patterns of Cobalt Oxide Cations ConOm+ (n = 5-9, m = 4-13): From Oxygen-Deficient to Oxygen-Rich Species.

Cobalt oxide clusters, ConOm+ (5 ≤ n ≤ 9 and 4 ≤ m ≤ 13), are produced by laser vaporization and studied by time-of-flight mass spectrometry. Specific stoichiometries are mass separated and photofragmented using 355 nm laser light. The preferred fragmentation channels of m = n-1, m = n-2, and m ≥ n species are investigated. Loss of oxygen molecules is the favorable dissociation channel of m ≥ n clusters. While ConOn-2+ clusters decay via the loss of a Co atom, the photofragmentation behavior of ConOn-1+ species interestingly can be divided into two regimes: the n ≤ 6 clusters tend to lose an oxygen atom, but for n > 6 they favorably dissociate via the loss of a cobalt atom. The geometric structures of selected m = n - 2 species are studied using density functional theory calculations. Dissociation energies for different evaporation channels are calculated and thermodynamically favorable channels are found to correspond to the experimental observations.

A platinum-nickel bimetallic nanocluster ensemble-on-polyaniline nanofilm for enhanced electrocatalytic oxidation of dopamine.

We report a new approach to design flexible functional material platforms based on electropolymerized polyaniline (PANI) polymer nanofilms modified with bimetallic nanoclusters (NCs) for efficient electro-oxidation of small organic molecules. Composition defined ligand free Pt0.75Ni0.25 NCs were synthesized in the gas phase using the Cluster Beam Deposition (CBD) technology and characterized using RToF, HAADF-STEM, XAFS and XPS. NCs were then directly deposited on PANI coated templates to construct electrodes. Dopamine (DP) molecules were used as a representative organic analyte and the influence of the NC-PANI hybrid atomistic structure on the electrochemical and electrocatalytic performance was investigated. The as prepared, nearly monodispersed, Pt0.75Ni0.25 NCs of ca. 2 nm diameter featuring a PtOx surface combined with a shallow platelet-like Ni-O(OH) phase formed a densely packed active surface on PANI at ultralow metal coverages. Electrochemical measurements (EIS and CV) show a 2.5 times decrease in charge transfer resistance and a remarkable 6-fold increase at lower potential in the mass activity for Pt0.75Ni0.25 NCs in comparison with their pure Pt counterparts. The enhanced electrochemical performance of the Pt0.75Ni0.25 NC hybrid's interface is ascribed to the formation of mixed Pt metal and Ni-O(OH) phases at the surface of the alloyed PtNi cores of the bimetallic NCs under electrochemical conditions combined with an efficient charge conduction pathway between NCs.

Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au20 clusters.

The free-standing Au20 cluster has a unique tetrahedral shape and a large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of around 1.8 electron volts. The "magic" Au20 has been intensively used as a model system for understanding the catalytic and optical properties of gold nanoclusters. However, direct real-space ground-state characterization at the atomic scale is still lacking, and obtaining fundamental information about the corresponding structural, electronic, and dynamical properties, is challenging. Here, using cluster-beam deposition and low-temperature scanning tunneling microscopy, atom-resolved topographic images and electronic spectra of supported Au20 clusters are obtained. We demonstrate that individual size-selected Au20 on ultrathin NaCl films maintains its pyramidal structure and large HOMO-LUMO gap. At higher cluster coverages, we find sintering of the clusters via Smoluchowski ripening to Au20n agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size.

Altering CO binding on gold cluster cations by Pd-doping.

The introduction of dopant atoms into metal nanoparticles is an effective way to control the interaction with adsorbate molecules and is important in many catalytic processes. In this work, experimental and theoretical evidence of the influence of Pd doping on the bonding between small cationic AuN+ clusters and CO is presented. The CO adsorption is studied by combining low-pressure collision cell reactivity and infrared multiple photon dissociation spectroscopy experiments with density functional theory calculations. Measured dissociation rates of cluster-CO complexes (N ≤ 21) allow the estimation of cluster-CO binding energies, showing that Pd doping increases the CO adsorption energy to an extent that is size-dependent. These trends are reproduced by theoretical calculations up to N = 13. In agreement with theory, measurements of the C-O vibrational frequency suggest that for the doped PdAuN-1+ (N = 3-5, 11) clusters, CO adsorbs on an Au atom, while for N = 6-10 and N = 12-14, CO interacts directly with the Pd dopant. A pronounced red-shifting of the C-O vibrational frequency is observed when CO interacts directly with the Pd dopant, indicating a significant back-donation of electron charge from Pd to CO. In contrast, the blue-shifted frequencies, observed when CO interacts with an Au atom, indicate that σ-donation dominates the Au-CO interaction. Studying such systems at the sub-nanometre scale enables a fundamental comprehension of the interactions between adsorbates, dopants and the host (Au) species at the atomic level.

Stability of small cationic platinum clusters.

The relative stability of small cationic platinum clusters is investigated by photofragmentation experiments. Mass spectra show a smooth intensity distribution except for a local intensity minimum at Pt5+, revealing enhanced stability of the platinum tetramer Pt4+. The possibility that radiative cooling competes with statistical fragmentation after photoexcitation is examined and it is shown that clusters in the N = 3-8 size range do not radiate on the time scale of the experiment. In the absence of radiative cooling, the mass spectra of photofragmented clusters can be well explained by dissociation energies computed using density functional theory. The large calculated HOMO-LUMO gap for Pt4+ (∼1.2 eV) is attributed to its highly symmetric structure and provides an explanation for the surprisingly low reactivity of this cluster in different gas-phase reactions.

Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping.

The reactivity of small metallic clusters, nanoparticles composed of a countable number of atoms (typically up to ∼100 atoms), has attracted much attention due to the fascinating properties these objects possess toward a variety of molecules. Cluster reactivity often is significantly different from the homologous bulk, with gold as prototypical example. Bulk gold is the noblest of all metals, whereas small gold clusters react with carbon monoxide, molecular oxygen, and hydrocarbons, among others. Furthermore, cluster reactivity is strongly size and composition dependent, allowing a wide range of tuning possibilities. The study of cluster reactivity usually follows two routes of investigation. In the first, research aims for fundamental understanding of mechanisms, mainly driven by curiosity. One consequence of the inherent small size of a cluster is that atoms can arrange themselves very differently from the crystallographic structure of the homologous bulk. In addition, quantum confinement effects dominate the electronic structure of a cluster with atom-like electronic shells instead of the electronic bands in bulk. These features result in a very rich and size-dependent interaction of a cluster with small molecules, governed by a fine interplay between the geometry and the electronic structure of the system. An alternative research approach uses the investigation of chemical reactions of isolated small clusters in the gas phase as model systems for the reactions taking place in more complex systems. This offers several advantages compared to more conventional methods and techniques used to study such complex systems. First, clusters can be produced under well-defined conditions, with control over size, composition, and charge state. Second, clusters in the gas phase solely interact with the molecule(s) chosen by the researcher, since contaminations are limited by the high vacuum conditions of the experiments. Third, due to the small number of atoms involved, detailed quantum chemical calculations can be performed on the systems under investigation. Thus, even though gas phase clusters differ significantly in size and in environmental conditions from those encountered, for example, in industrial catalysis, they can be used to unravel the complicated nature of a metal-molecule chemical bonding process. In this Account, both routes of investigation are discussed. The nature of the interaction between small gas phase clusters with diverse molecules is described, stressing the broader relevance of these studies. Particular emphasis is given to the effect of heteroatom doping. By adding a different element to a cluster, its geometric and electronic structure is modified, thereby altering its reactivity. Specifically, the effect of varying size and composition of doped gold, platinum, and aluminum clusters on their reactivity toward diverse molecules, relevant for catalytic applications, is discussed. Most studies presented here combine experiments based on mass spectrometric techniques with density functional theory calculations, allowing a deep understanding of the reaction mechanisms at a molecular level.

Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites.

Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag-Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent-density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag4(H2O) x2+ (x = 2 and x = 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.