Multifacets of Fullerene-Metal Clusters: From Fundamental to Application

ConspectusBuckminsterfullerene, C60, was discovered through a prominent mass peak containing 60 atoms produced from laser vaporization of graphite, driven by Kroto's interest in understanding the formation mechanisms of carbon-containing molecules in space. Inspired by the geodesic dome-shaped architecture designed by Richard Buckminster Fuller, after whom the particle was named, C60 was found to have a football-shaped structure comprising 20 hexagons and 12 pentagons. It sparked worldwide interest in understanding this new carbon allotrope, resulting in the awarding of the Noble Prize in Chemistry to Smalley, Kroto, and Curl in 1996.Intrinsically, C60 is an exceptional species because of its high stability and electron-accepting ability and its structural tunability by decorating or substituting either on its exterior surface or interior hollow cavity. For example, metal-decorated fullerene complexes have found important applications ranging from superconductivity, nanoscale electronic devices, and organic photovoltaic cells to catalysis and biomedicine. Compared to the large body of studies on atoms and molecules encapsulated by C60, studies on the exteriorly modified fullerenes, i.e., exohedral fullerenes, are scarcer. Surprisingly, to date, uncertainty exists about a fundamental question: what is the preferable exterior binding site of different kinds of single atoms on the C60 surface?In recent years, we have developed an experimental protocol to synthesize the desired fullerene-metal clusters and to record their infrared spectra via messenger-tagged infrared multiple photon dissociation spectroscopy. With complementary quantum chemical calculations and molecular dynamics simulations, we determined that the most probable binding site of a metal, specifically a vanadium cation, on C60 is above a pentagonal center in an η5 fashion. We explored the bonding nature between C60 and V+ and revealed that the high thermal stability of this cluster originates from large orbital and electrostatic interactions. Through comparing the measured infrared spectra of [C60-Metal]+ with the observational Spitzer data of several fullerene-rich planetary nebulae, we proposed that the complexes formed by fullerene and cosmically abundant metals, for example, iron, are promising carriers of astronomical unidentified spectroscopic features. This opens the door for a real consideration of Kroto's 30-year-old hypothesis that complexes involving cosmically abundant elements and C60 exhibit strong charge-transfer bands, similar to those of certain unidentified astrophysical spectroscopic features. We compiled a VibFullerene database and extracted a set of vibrational frequencies and intensities for fullerene derivatives to facilitate their potential detection by the James Webb Space Telescope. In addition, we showed that upon infrared irradiation C60V+ can efficiently catalyze water splitting to generate H2. This finding is attributed to the novel geometric-electronic effects of C60, acting as "hydrogen shuttle" and "electron sponge", which illustrates the important role of carbon-based supports in single-atom catalysts. Our work not only unveils the basic structures and bonding nature of fullerene-metal clusters but also elucidates their potential importance in astrophysics, astrochemistry, and catalysis, showing the multifaceted character of this class of clusters. More exciting and interesting aspects of the fullerene-metal clusters, such as ultrafast charge-transfer dynamics between fullerene and metal and their relevance to designing hybrid fullerene-metal junctions for electronic devices, are awaiting exploration.

Recent advances in copper chalcogenides for CO2 electroreduction

Transforming CO2 through electrochemical methods into useful chemicals and energy sources may contribute to solutions for global energy and ecological challenges. Copper chalcogenides exhibit unique properties that make them potential catalysts for CO2 electroreduction. In this review, we provide an overview and comment on the latest advances made in the synthesis, characterization, and performance of copper chalcogenide materials for CO2 electroreduction, focusing on the work of the last five years. Strategies to boost their performance can be classified in three groups: (1) structural and compositional tuning, (2) leveraging on heterostructures and hybrid materials, and (3) optimizing size and morphology. Despite overall progress, concerns about selectivity and stability persist and require further investigation. This review outlines future directions for developing the next-generation of copper chalcogenide materials, emphasizing on rational design and advanced characterization techniques for efficient and selective CO2 electroreduction.

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.

Co3-xFexO4 inverse opals with tunable catalytic activity for high-performance overall water splitting

The development of earth-abundant and high-performance bifunctional catalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline electrolytes is required to efficiently produce hydrogen by electrochemical water splitting, but remains a challenge. We have fabricated mesoporous cobalt iron oxide inverse opals (m-CFO IO) with different mole ratios of cobalt and iron by a wet chemical method using polystyrene beads as a hard template, followed by calcination in air. The performance of the m-CFO IO as OER and HER electrocatalysts was investigated. The as-prepared catalyst with equal concentrations of Fe and Co exhibits remarkable OER and HER performances with low overpotentials of 261 and 157 mV to attain 10 mA cm^-2 and small Tafel slopes of 63 and 56 mV dec^-1, respectively. An alkaline water electrolyzer with a two-electrode configuration achieves 10 mA cm^-2 at 1.55 V with excellent long-term stability, outperforming the combination of noble metal IrO₂ and Pt/C benchmark catalysts. The superior catalytic performance is ascribed to the synergistic effects of particle size, crystallinity, oxygen efficiency, a large number of active sites, and the large specific surface area of the porous inverse opal structure.

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

A series of small chromium-doped silicon clusters CrSi_n with n = 3-10 in the cationic, neutral and anionic charge states were investigated using quantum chemical methods. The CrSi_n⁺ 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 CrSi_n^+/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 CrSi_n 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. Au_xAg_1-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, Au_xAg_1-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 Au_0.3Ag_0.7 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 Au_0.3Ag_0.7 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 Au_n⁺, AgAu_n-1⁺ and PdAu_n-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.

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.

Au3-Decorated graphene as a sensing platform for O2 adsorption and desorption kinetics

The adsorption and desorption kinetics of molecules is of significant fundamental and applied interest. In this paper, we present a new method to quantify the energy barriers for the adsorption and desorption of gas molecules on few-atom clusters, by exploiting reaction induced changes of the doping level of a graphene substrate. The method is illustrated for oxygen adsorption on Au₃ clusters. The gold clusters were deposited on a graphene field effect transistor and exposed to O₂. From the change in graphene's electronic properties during adsorption, the energy barrier for the adsorption of O₂ on Au₃ is estimated to be 0.45 eV. Electric current pulses increase the temperature of the graphene strip in a controlled way and provide the required thermal energy for oxygen desorption. The oxygen binding energy on Au₃/graphene is found to be 1.03 eV and the activation entropy is 1.4 meV K^-1. The experimental values are compared and interpreted on the basis of density functional theory calculations of the adsorption barrier, the binding energy and the activation entropy. The large value of the activation entropy is explained by the hindering effect that the adsorbed O₂ has on the fluxional motion of the Au₃ cluster.

C2 product formation in the CO2 electroreduction on boron-doped graphene anchored copper clusters

A possible remedy for the increasing atmospheric CO₂ concentration is capturing and reducing it into valuable chemicals like methane, methanol, ethylene, and ethanol. However, a suitable catalyst for this process is still under extensive research. Small sized copper clusters have gained attention in recent years due to their catalytic activity in the CO₂ reduction reaction. Although C₂₊ products have a higher economic value, the formation of C₁ products was investigated most thoroughly. Graphene is a promising support for small copper clusters in the electrochemical reduction of CO₂. It exhibits good mechanical and electrical properties, but the weak interaction between copper and graphene is an issue. Our DFT computations reveal that small Cu clusters on the boron-doped graphene (BDG) support are promising catalysts for the electrochemical reduction of CO₂. We found facile reaction pathways towards various C₁ (carbon-monoxide, formic acid, formaldehyde, methanol or methane) and C₂ (ethanol or ethylene) products on Cu₄ and Cu₇ clusters on BDG. The reactivity is cluster-size tunable with Cu₄ being the more reactive agent, while Cu₇ shows a higher selectivity towards C₂ products.

Interaction of graphene with Aunclusters: a first-principles study

The interaction between Au_n(n= 1-6) clusters and graphene is studied using first-principles simulations, based on density functional theory. The computed binding energy between Au_nand graphene depends on the number of atoms in the cluster and lies between -0.6 eV and -1.7 eV, suggesting (weak) chemisorption of the clusters on graphene, rather than physisorption. Overall, the electronic properties, spin-orbit interaction and spin texture, as well as the transport properties of graphene strongly depend on the precise size of the Au_nclusters. Doping of graphene is predicted for clusters with an odd number of Au atoms, due to overlap between Ausand carbonp_zstates close to the Fermi level. On the other hand, there is no charge transfer between even size Au clusters and graphene, but a gap is formed at the Dirac cone, due to the breaking of the pseudo spin inversion symmetry of graphene's lattice. The adsorbed Au_nclusters induce spin-orbit interactions as well as spin and pseudo spin interactions in graphene, as indicated by the splitting of the electronic band structure. A hedgehog spin texture is also predicted for adsorbed clusters with an even number of Au atoms. Ballistic transport simulations are performed to study the influence of the adsorbed clusters on graphene's electronic transport properties. The influence of the cluster on the electron transmission across the structure depends on the mixing of the valence orbitals in the transport energy window. In the specific case of the Au₃/graphene system, the adsorbed clusters reduce the transmission and the conductance of graphene. The Au₃clusters act as 'scattering centers' for charge carriers, in agreement with recent experimental studies.

Bonding Nature between Noble Gases and Small Gold Clusters

Noble gases are usually seen as utterly inert, likewise gold, which is typically conceived as the noblest of all metals. While one may expect that noble gases bind to gold via dispersion interactions only, strong bonds can be formed between noble gas atoms and small gold clusters. We combine mass spectrometry, infrared spectroscopy, and density functional theory calculations to address the bonding nature between Au_n⁺ (n ≤ 4) clusters and Ar, Kr, and Xe. We unambiguously determine the geometries and quantitatively uncover the bonding nature in Au_nNg_m⁺ (Ng = Ar, Kr, Xe) complexes. Each Au cluster can form covalent bonds with atop bound noble gas atoms, with strengths that increase with the noble gas atomic radius. This is demonstrated by calculated adsorption energies, Bader electron charges, and analysis of the electron density. The covalent bonding character, however, is limited to the atop-coordinated Ng atoms.

Evolution of Vibrational Spectra in the Manganese-Silicon Clusters Mn2Sin, n = 10, 12, and 13, and Cationic [Mn2Si13]. J Phys Chem A 2022;126:1617-1626. [PMID: 35238570 PMCID: PMC9084549 DOI: 10.1021/acs.jpca.1c10027]

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-D_6d 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.

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...

Spectroscopic evidence of a Xe-Xe bond in the linear Xe2Au+Xe ion

The closed-shell nature of Xe atoms precludes the formation of Xe-Xe bonds other than based on Van Der Waals interactions. Here, we present experimental evidence showing that the complexation of...

The size-dependent influence of palladium doping on the structures of cationic gold clusters

The physicochemical properties of small metal clusters strongly depend on their precise geometry. Determining such geometries, however, is challenging, particularly for clusters formed by multiple elements. In this work, we combine infrared multiple photon dissociation spectroscopy and density functional theory calculations to investigate the lowest-energy structures of Pd doped gold clusters, PdAu _n-1 ⁺ (n ≤ 10). The high-quality experimental spectra allow for an unambiguous determination of the structures adopted by the clusters. Our results show that the Pd-Au interaction is so large that the structures of PdAu _n-1 ⁺ and Au _n ⁺ are very different. Pd doping induces a 2D to 3D transition at much smaller cluster sizes than for pure Au _n ⁺ clusters. PdAu _n-1 ⁺ clusters are three-dimensional from n = 4, whereas for Au _n ⁺ this transition only takes place at n = 7. Despite the strong Au-Pd interaction, the Au _n-1 ⁺ cluster geometries remain recognizable in PdAu _n-1 ⁺ up to n = 7. This is particularly clear for PdAu₆ ⁺. In PdAu₈ ⁺ and PdAu₉ ⁺, Pd triggers major rearrangements of the Au clusters, which adopt pyramidal shapes. For PdAu₄ ⁺ we find a geometry that was not considered in previous studies, and the geometry found for PdAu₈ ⁺ does not correspond to the lowest-energy structure predicted by DFT, suggesting kinetic trapping during formation. This work demonstrates that even with the continuous improvement of computational methods, unambiguous assignment of cluster geometries still requires a synergistic approach, combining experiment and computational modelling.

Screening of transition metal doped copper clusters for CO2 activation

Activation of CO₂ is the first step towards its reduction to more useful chemicals. Here we systematically investigate the CO₂ activation mechanism on Cu₃X (X is a first-row transition metal atom) using density functional theory computations. The CO₂ adsorption energies and the activation mechanisms depend strongly on the selected dopant. The dopant electronegativity, the HOMO-LUMO gap and the overlap of the frontier molecular orbitals control the CO₂ dissociation efficiency. Our calculations reveal that early transition metal-doped (Sc, Ti, V) clusters exhibit a high CO₂ adsorption energy, a low activation barrier for its dissociation, and a facile regeneration of the clusters. Thus, early transition metal-doped copper clusters, particularly Cu₃Sc, may be efficient catalysts for the carbon capture and utilization process.

Water Splitting by C60 -Supported Vanadium Single Atoms

Water splitting is an important source of hydrogen, a promising future carrier for clean and renewable energy. A detailed understanding of the mechanisms of water splitting, catalyzed by supported metal atoms or nanoparticles, is essential to improve the design of efficient catalysts. Here, we report an infrared spectroscopic study of such a water splitting process, assisted by a C₆₀ supported vanadium atom, C₆₀ V⁺ +H₂ O→C₆₀ VO⁺ +H₂ . We probe both the entrance channel complex C₆₀ V⁺ (H₂ O) and the end product C₆₀ VO⁺ , and observe the formation of H₂ as a result from resonant infrared absorption. Density functional theory calculations exploring the detailed reaction pathway reveal that a quintet-to-triplet spin crossing facilitates the water splitting reaction by C₆₀ -supported V⁺ , whereas this reaction is kinetically hindered on the isolated V⁺ ion by a high energy barrier. The C₆₀ support has an important role in lowering the reaction barrier with more than 70 kJ mol^-1 due to a large orbital overlap of one water hydrogen atom with one carbon atom of the C₆₀ support. This fundamental insight in the water splitting reaction by a C₆₀ -supported single vanadium atom showcases the importance of supports in single atom catalysts by modifying the reaction potential energy surface.

An octacoordinated Nb atom in the NbAl8H8+ cluster

The NbAl₈H₈⁺ cluster was formed in a molecular beam and characterized by mass spectrometry and infrared spectroscopy. Density functional theory calculations show the lowest-energy isomer is a high symmetry singlet with the Nb atom placed at the center of a distorted hexagonal Al ring and coordinated by two AlH moieties, therefore exhibiting octacoordination. The unprecedented high-symmetric geometry is attributed to the 20 valence electrons; the central Nb atom adheres to the 18-electron rule and two additional delocalized electrons stabilize the hexagonal ring.

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.

Doping Platinum with Germanium: An Effective Way to Mitigate the CO Poisoning

The vulnerability towards CO poisoning is a major drawback affecting the efficiency and long-term performance of platinum catalysts in fuel cells. In the present work, by a combination of density functional theory calculations and mass spectrometry experiments, we test and explain the promotional effect of Ge on Pt catalysts with higher resistance to deactivation via CO poisoning. A thorough exploration of the configurational space of gas-phase Pt_n ⁺ and GePt_n-1 ⁺ (n=5-9) clusters using global minima search techniques and the subsequent electronic structure analysis reveals that germanium doping reduces the binding strength between Pt and CO by hindering the 2π-back-donation. Importantly, the clusters remain catalytically active towards H₂ dissociation. The ability of Ge to weaken the Pt-CO interaction was confirmed by mass spectrometry experiments. Ge can be a promising alloying agent to tune the selectivity and improve the durability of Pt particles, thus opening the way to novel catalytic alternatives for fuel cells.

Argon tagging of doubly transition metal doped aluminum clusters: The importance of electronic shielding

The interaction of argon with doubly transition metal doped aluminum clusters, Al_nTM₂ ⁺ (n = 1-18, TM = V, Nb, Co, Rh), is studied experimentally in the gas phase via mass spectrometry. Density functional theory calculations on selected sizes are used to understand the argon affinity of the clusters, which differ depending on the transition metal dopant. The analysis is focused on two pairs of consecutive sizes: Al_6,7V₂ ⁺ and Al_4,5Rh₂ ⁺, the largest of each pair showing a low affinity toward Ar. Another remarkable observation is a pronounced drop in reactivity at n = 14, independent of the dopant element. Analysis of the cluster orbitals shows that this feature is not a consequence of cage formation but is electronic in nature. The mass spectra demonstrate a high similarity between the size-dependent reactivity of the clusters with Ar and H₂. Orbital interactions provide an intuitive link between the two and further establish the importance of precursor states in the reactions of the clusters with hydrogen.

Long-term stability of isolated advancement genioplasty, and influence of associated risk factors: A systematic review

The aim of this review was to investigate the skeletal and soft tissue stability of isolated advancement genioplasty after more than 1 year, and to observe the influence of associated risk factors. A literature search was performed on PubMed, Web of Science, Embase, ScienceDirect, and Cochrane. Only studies with at least 10 patients who underwent an isolated advancement genioplasty, and with a follow-up period of at least 1 year, were included. Of the 2224 records initially identified, eight articles met the eligibility criteria. The mean age of the total study population was 23 years and ranged from 19.1 to 26.5 years in the individual studies. The average surgical advancement at pogonion was 8.2 mm and ranged from 6.2 to 11.7 mm in the individual studies. After 1 year, the horizontal hard tissue relapse at the level of pogonion varied from 0.1 to 2.1 mm. In two studies, this was reported as statistically significant. Regarding the soft tissue, the horizontal relapse varied from 0.3 to 2.9 mm, which was also considered statistically significant in two studies. Isolated advancement genioplasty was found to be a predictable and stable orthognathic procedure in the sagittal plane at both soft and hard tissue levels. The amount of relapse was not associated with the fixation method or with the amount of surgical advancement.

Transient transmission of THz metamaterial antennas by impact ionization in a silicon substrate

The picosecond dynamics of excited charge carriers in the silicon substrate of THz metamaterial antennas was studied at different wavelengths. Time-resolved THz pump-THz probe spectroscopy was performed with light from a tunable free electron laser in the 9.3-16.7 THz frequency range using fluences of 2-12 J/m². Depending on the excitation wavelength with respect to the resonance center, transient transmission increase, decrease, or a combination of both was observed. The transient transmission changes can be explained by local electric field enhancement, which induces impact ionization in the silicon substrate, increasing the local number of charge carriers by several orders of magnitude, and their subsequent diffusion and recombination. The studied metamaterials can be integrated with common semiconductor devices and can potentially be used in sensing applications and THz energy harvesting.

The effect of size, charge state and composition on the binding of propene to yttrium-doped gold clusters

The catalytic activity of metal clusters can be easily tuned by their size, charge state, or the introduction of dopant atoms. Here, the dopant-, charge- and size-dependent propene adsorption on gold (Au_n⁺) and yttrium doped gold (Au_n−1Y⁺) clusters (n = 4–20) was investigated using combined gas-phase reaction studies and density functional theory computations. The increased charge transfer between the cluster and propene in the cationic clusters considerably enhances the propene binding on both pure and yttrium-doped species, compared to their neutral cluster counterparts, while yttrium-doping lowers the propene binding strength in a size-dependent way compared to the pure gold clusters. Chemical bonding and energy decomposition analysis indicate that there is no covalent bond between the cluster and propene. The preferred propene binding site on a cluster is indicated by the large lobes of its LUMO, together with the low coordination number of the adsorption site. In small yttrium-doped gold clusters propene can not only bind to the electron-deficient yttrium atom, but also to the partially positively-charged gold atoms. Therefore, by controlling the charge of the clusters, as well as by introducing yttrium dopants, the propene binding strength can be tuned, opening the route for new catalytic applications.

The catalytic activity of metal clusters can be easily tuned by their size, charge state, or the introduction of dopant atoms.

Titanium Nitride Nanodonuts Synthesized from Natural Ilmenite Ore as a Novel and Efficient Thermoplasmonic Material

Nanostructures of titanium nitride (TiN) have recently been considered as a new class of plasmonic materials that have been utilized in many solar energy applications. This work presents the synthesis of a novel nanostructure of TiN that has a nanodonut shape from natural ilmenite ore using a low-cost and bulk method. The TiN nanodonuts exhibit strong and spectrally broad localized surface plasmon resonance absorption in the visible region centered at 560 nm, which is well suited for thermoplasmonic applications as a nanoscale heat source. The heat generation is investigated by water evaporation experiments under simulated solar light, demonstrating excellent solar light harvesting performance of the nanodonut structure.

Stability of cationic silver doped gold clusters and the subshell-closed electronic configuration of AgAu14. J Chem Phys 2020;153:244304. [PMID: 33380086 DOI: 10.1063/5.0033487]

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, AgAu_N-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 AgAu_N-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.

Au nanoparticle-decorated ZnO nanorods as fluorescent non-enzymatic glucose probe

ZnO nanorods (NRs) synthesized by a hydrothermal method and decorated with Au nanoparticles (NPs) were used for fluorescent non-enzymatic glucose detection. The detection is based on the photoluminescence (PL) quenching of ZnO NRs/Au NPs (at 382 nm under 325 nm excitation) exposed to glucose. The sensor exhibits a high sensitivity of (22 ± 2) % mM^-1 (defined as percentage change of the PL peak intensity per mM) and a limit of detection (LOD) as low as 0.01 mM, along with an excellent selectivity and a short response time (less than 5 s). In comparison with a fluorescent non-enzymatic ZnO nanostructure-based glucose sensor, the addition of Au NPs significantly enhances the sensitivity. This is attributed to the surface plasmon resonance, which increases not only the photoluminescence intensity but also the photo-oxidation property of the ZnO NRs. Thus, ZnO NRs/Au NPs can act as an efficient photocatalyst for glucose detection. Most importantly, the probe is applicable to glucose detection in human blood serum. The outstanding performance of the material and its cost-effectiveness allow for potential application in single-use, noninvasive glucose devices.Graphical abstract A sensitive non-enzymatic fluorescent glucose probe-based ZnO nanorod decorated with Au nanoparticles.

The structures of cationic gold clusters probed by far-infrared spectroscopy

Determining the precise structures of small gold clusters is an essential step towards understanding their chemical and physical properties. Due to the relativistic nature of gold, its clusters remain planar (2D) up to appreciable sizes. Ion mobility experiments have suggested that positively charged gold clusters adopt three-dimensional (3D) structures from n = 8 onward. Computations predict, depending on the level of theory, 2D or 3D structures as putative energy-minimum for n = 8. In this work, far-infrared multiple photon dissociation spectroscopy, using Ar as tagging element, is combined with density-functional theory calculations to determine the structures of Au_n⁺ (n≤ 9) clusters formed by laser ablation. While the Au frameworks in Au₆Ar_m⁺ and Au₇Ar_m⁺ complexes are confirmed to be planar and that in Au₉Ar_m⁺ three-dimensional, we demonstrate the coexistence of 3D and planar Au₈Ar_m⁺ (m = 1-3) isomers. Thus, it is revealed that at finite temperatures, the formal 2D to 3D transition takes place at n = 8 but is not sharp.

Room temperature single electron transistor based on a size-selected aluminium cluster

Single electron transistors (SETs) are powerful devices to study the properties of nanoscale objects. However, the capabilities of placing a nano-object between electrical contacts under pristine conditions are lacking. Here, we developed a versatile two point contacting approach that tackles this challenge, which is demonstrated by constructing in situ a prototypical SET device consisting of a single aluminium cluster of 66 ± 5 atoms, deposited directly in a gold nanogap using an innovative cluster beam deposition technique. The gate driven conductance measurements demonstrate Coulomb blockade oscillations at room temperature correlating with an extracted charging energy of 0.14 eV, which is five times larger than k_BT at 300 K. Our work provides a model SET device platform to probe the quantum features of nano-objects with high precision.

Interaction of carbon monoxide with doped metal clusters

Highlight of experimental and computational studies about the interaction of CO with transition and coinage metal clusters, particularly discussing the influence of dopant atoms.

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 Au_20n agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size.

Non-covalent Interactions and Charge Transfer between Propene and Neutral Yttrium-Doped and Pure Gold Clusters

The dopant and size‐dependent propene adsorption on neutral gold (Au_n) and yttrium‐doped gold (Au_n−1Y) clusters in the n=5–15 size range are investigated, combining mass spectrometry and gas phase reactions in a low‐pressure collision cell and density functional theory calculations. The adsorption energies, extracted from the experimental data using an RRKM analysis, show a similar size dependence as the quantum chemical results and are in the range of ≈0.6–1.2 eV. Yttrium doping significantly alters the propene adsorption energies for n=5, 12 and 13. Chemical bonding and energy decomposition analysis showed that there is no covalent bond between the cluster and propene, and that charge transfer and other non‐covalent interactions are dominant. The natural charges, Wiberg bond indices, and the importance of charge transfer all support an electron donation/back‐donation mechanism for the adsorption. Yttrium plays a significant role not only in the propene binding energy, but also in the chemical bonding in the cluster‐propene adduct. Propene preferentially binds to yttrium in small clusters (n<10), and to a gold atom at larger sizes. Besides charge transfer, relaxation also plays an important role, illustrating the non‐local effect of the yttrium dopant. It is shown that the frontier molecular orbitals of the clusters determine the chemical bonding, in line with the molecular‐like electronic structure of metal clusters.

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.