Highly Stable [C60AuC60]+/- Dumbbells

Assembly-inspired multiferroicity with nontrivial Chern insulating phase from exohedral metallofullerenes

Fullerene-assembled low-dimensional materials have been experimentally realized in polymorphic forms and have attracted significant interest very recently. Here, we predict a two-dimensional (2D) honeycomb lattice material TM2(C60)3 (TM = Cr, Mo, and W) assembled from exohedral metallofullerene clusters TM(C60)3 that could exhibit planar triangular geometries. According to first-principles calculations combined with Monte Carlo simulations, we suggest that these 2D assembled materials exhibit various exotic physical properties, including ferromagnetism, ferroelectricity, and quantum anomalous Hall effect. Interestingly, mechanical strains could effectively tune their magnetic moments and switch the conducting spin channel of the Dirac bands at the Fermi level. Our work provides a new cluster-assembly design strategy toward cluster-assembled 2D materials based on fullerene characters.

Mixed Cluster Ions of Magnesium and C60

Magnesium clusters exhibit a pronounced nonmetal-to-metal transition, and the neutral dimer is exceptionally weakly bound. In the present study, we formed pristine Mgnz+ (n = 1-100, z = 1-3) clusters and mixed (C60)mMgnz+ clusters (m = 1-7, z = 1, 2) upon electron irradiation of neutral helium nanodroplets doped with magnesium or a combination of C60 and magnesium. The mass spectra obtained for pristine magnesium cluster ions exhibit anomalies, consistent with previous reports in the literature. The anomalies observed for C60Mgn+ strongly suggest that Mg atoms tend to wet the surface of the single fullerene positioning itself above the center of a pentagonal or hexagonal face, while, for (C60)mMgnz+, the preference for Mg to position itself within the dimples formed by fullerene cages becomes apparent. Besides doubly charged cluster ions, with the smallest member Mg22+, we also observed the formation of triply charged ions Mgn3+ with n > 24. The ion efficiency curves of singly and multiply charged ions exhibit pronounced differences compared to singly charged ions at higher electron energies. These findings indicate that sequential Penning ionization is essential in the formation of doubly and triply charged ions inside doped helium nanodroplets.

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.

Ab Initio Study of Structural, Electronic and Magnetic Properties of TM&(B@C60) (TM = V, Cr) Sandwich Clusters and Infinite Molecular Wires

The geometrical structure, electronic and magnetic properties of B-endoped C₆₀ (B@C₆₀) ligand sandwich clusters, TM&(B@C₆₀)₂ (TM = V, Cr), and their one-dimensional (1D) infinite molecular wires, [TM&(B@C₆₀)]_∞, have been systematically studied using first-principles calculations. The calculations showed that the TM atoms can bond strongly to the pentagonal (η⁵-coordinated) or hexagonal rings (η⁶-coordinated) of the endoped C₆₀ ligands, with binding energies ranging from 1.90 to 3.81 eV. Compared to the configurations with contrast-bonding characters, the η⁶- and η⁵-coordinated bonding is energetically more favorable for V-(B@C₆₀) and Cr-(B@C₆₀) complexes, respectively. Interestingly, 1D infinite molecular wire [V&(B@C₆₀)-η⁶]_∞ is an antiferromagnetic half-metal, and 1D [Cr&(B@C₆₀)-η⁵]_∞ molecular wire is a ferromagnetic metal. The tunable electronic and magnetic properties of 1D [TM&(B@C₆₀)]_∞ SMWs are found under compressive and tensile stains. These findings provide additional possibilities for the application of C₆₀-based sandwich compounds in electronic and spintronic devices.

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation

Complexes of atomic gold with a variety of ligands have been formed by passing helium nanodroplets (HNDs) through two pickup cells containing gold vapor and the vapor of another dopant, namely a rare gas, a diatomic molecule (H₂, N₂, O₂, I₂, P₂), or various polyatomic molecules (H₂O, CO₂, SF₆, C₆H₆, adamantane, imidazole, dicyclopentadiene, and fullerene). The doped HNDs were irradiated by electrons; ensuing cations were identified in a high-resolution mass spectrometer. Anions were detected for benzene, dicyclopentadiene, and fullerene. For most ligands L, the abundance distribution of AuL_n⁺ versus size n displays a remarkable enhancement at n = 2. The propensity towards bis-ligand formation is attributed to the formation of covalent bonds in Au⁺L₂ which adopt a dumbbell structure, L-Au⁺-L, as previously found for L = Xe and C₆₀. Another interesting observation is the effect of gold on the degree of ionization-induced intramolecular fragmentation. For most systems gold enhances the fragmentation, i.e., intramolecular fragmentation in AuL_n⁺ is larger than in pure L_n⁺. Hydrogen, on the other hand, behaves differently, as intramolecular fragmentation in Au(H₂)_n⁺ is weaker than in pure (H₂)_n⁺ by an order of magnitude.

An intense source for cold cluster ions of a specific composition

The demand for nanoscale materials of ultra-high purity and narrow size distribution is addressed. Clusters of Au, C₆₀, H₂O, and serine are produced inside helium nanodroplets using a combination of ionization, mass filtering, collisions with atomic or molecular vapor, and electrostatic extraction, in a specific and novel sequence. The helium droplets are produced in an expansion of cold helium gas through a nozzle into vacuum. The droplets are ionized by electron bombardment and subjected to a mass filter. The ionic and mass-selected helium droplets are then guided through a vacuum chamber filled with atomic or molecular vapor where they collide and "pick up" the vapor. The dopants then agglomerate inside the helium droplets around charge centers to singly charged clusters. Evaporation of the helium droplets is induced by collisions in a helium-filled radio frequency (RF)-hexapole, which liberates the cluster ions from the host droplets. The clusters are analyzed with a time-of-flight mass spectrometer. It is demonstrated that using this sequence, the size distribution of the dopant cluster ions is distinctly narrower compared to ionization after pickup. Likewise, the ion cluster beam is more intense. The mass spectra show, as well, that ion clusters of the dopants can be produced with only few helium atoms attached, which will be important for messenger spectroscopy. All these findings are important for the scientific research of clusters and nanoscale materials in general.

Hydrogenated Gold Clusters from Helium Nanodroplets: Cluster Ionization and Affinities for Protons and Hydrogen Molecules

We report the mass spectrometric detection of hydrogenated gold clusters ionized by electron transfer and proton transfer. The cations appear after the pickup of hydrogen molecules and gold atoms by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. We focus on the size distributions of the gold cluster cations and their hydrogen content, the electron energy dependence of the ion yield, patterns of hydrogenated gold cluster cation stability, and the presence of "magic" clusters. Ab initio molecular orbital calculations were performed to provide insight into ionization energies and proton affinities of gold clusters as well as into molecular hydrogen affinities of the ionized and protonated gold cluster cations.

Charged Clusters of C60 and Au or Cu: Evidence for Stable Sizes and Specific Dissociation Channels

We have doped helium nanodroplets with C60 and either gold or copper. Positively or negatively charged (C60) mM n± ions (M = Au or Cu) containing up to ≈10 fullerenes and ≈20 metal atoms are formed by electron ionization. The abundance distributions extracted from high-resolution mass spectra reveal several local anomalies. The sizes of the four most stable (C60) mAu n± ions identified in previous calculations for small values of m and n ( m ≤ 2 and n ≤ 2, or m = 1 and n = 3) agree with local maxima in the abundance distributions. Our data suggest the existence of several other relatively stable ions including (C60)2Au3± and (C60)3Au4-. Another feature, namely the absence of bare (C60)2±, confirms the prediction that (C60)2M± dissociates by loss of C60± rather than loss of M. The experimental data also reveal the preference for loss of (charged or neutral) C60 over loss of a metal atom from some larger species such as (C60)3M3+. In contrast to these similarities between Au and Cu, the abundance distributions of (C60)3Au n- and (C60)3Cu n- are markedly different. In this discussion, we emphasize the similarities and differences between anions and cations, and between gold and copper. Also noteworthy is the observation of dianions (C60) mAu n2- for m = 2, 4, and 6.