Nanostructured supramolecular networks from self-assembled diamondoid molecules under ultracold conditions.

Diamondoid molecules and their derivatives have attracted attention as fascinating building blocks for advanced functional materials. Depending on the balance between hydrogen bonds and London dispersion interactions, they can self-organize in different cluster structures with functional groups tailored for various applications. Here, we present a new approach to supramolecular aggregation where self-assembly of diamondoid acids and alcohols in the ultracold environment of superfluid helium nanodroplets (HNDs) was analyzed by a combination of time-of-flight mass spectrometry and computational tools. Experimentally observed magic numbers of the assembled cluster sizes were successfully identified and computed cluster structures gave valuable insights into a different conglomeration mode when compared to previously explored less-polar diamondoid derivatives. We have confirmed that functional groups acting as good hydrogen bond donors completely take over the self-organization process, resulting in fascinating pair-wise or cyclic supramolecular assemblies. Particularly noteworthy is that mono- and bis-substituted diamondoid derivatives of both series engage in completely different modes of action, which is reflected in differing non-covalent cluster geometries. Additionally, formed cyclic clusters with a polar cavity in the center and a non-polar diamondoid outer layer can be of high interest in porous material design and provide insights into the structural requirements needed to produce bulk materials with desired properties.

Diamondoid ether clusters in helium nanodroplets.

Diamondoid ethers were introduced into superfluid helium nanodroplets and the resulting clusters were analyzed by time-of-flight mass spectrometry. Clusters of higher abundances (magic number clusters) were identified and the corresponding potential cluster geometries were obtained from GFN2-xTB and DFT computations. We found that the studied diamondoid ethers readily self-assemble in helium nanodroplets and that London dispersion attraction between hydrocarbon subunits acts as a driving force for cluster formation. On the other hand, hydrogen bonding between ether oxygens and trace water molecules fosters the eventual breakdown of the initial supramolecular aggregate.

London dispersion dominating diamantane packing in helium nanodroplets.

Diamantane clusters formed inside superfluid helium nanodroplets were analyzed by time-of-flight mass spectrometry. Distinct cluster sizes were identified as "magic numbers" and the corresponding feasible structures for clusters consisting of up to 19 diamantane molecules were derived from meta-dynamics simulations and subsequent DFT computations. The obtained interaction energies were attributed to London dispersion attraction. Our findings demonstrate that diamantane units readily form assemblies even at low pressures and near-zero Kelvin temperatures, confirming the importance of the intermolecular dispersion effect for condensation of matter.

Shell-Isolated Au Nanoparticles Functionalized with Rhodamine B Fluorophores in Helium Nanodroplets.

Nanoparticles consisting of three different materials in a layered core@shell@shell structure are synthesized in cold helium droplets by sequential doping. Upon the formation of Au core particles, a first shell layer is formed by adding either Ar, isopropyl alcohol, or hexane. Subsequently, the droplets are doped with rhodamine B (RB) molecules; fluorescence spectra recorded upon laser excitation at 532 nm provide insight into the structure of the formed complexes. For the two-component Au@RB system, the RB fluorescence is quenched in the presence of the Au core. If an intermediate isolating shell layer is introduced (Au@shell@RB), the fluorescence increases again. The results demonstrate that shell-isolated nanoparticles can be formed inside He nanodroplets and functionalized in situ with additional molecules. As the structure of the particles depends on the pickup sequence, the approach can be exploited for the synthesis and investigation of a large variety of different combinations of plasmonic metals, intermediate layers, and molecules.

Synthesis of nanosized vanadium(v) oxide clusters below 10 nm.

Vanadium oxide clusters with a mean diameter below 10 nm are investigated by high resolution Scanning Transmission Electron Microscopy (STEM), Electron Energy Loss Spectroscopy (EELS) and UV-vis absorption spectroscopy. The clusters are synthesised by sublimation from bulk vanadium(v) oxide, in combination with a pick-up by superfluid helium droplets. The latter act as reaction chambers which enable cluster growth under fully inert and solvent-free conditions. High-resolution STEM images of deposited vanadium oxide particles allowing for the determination of lattice constants, clearly indicate a dominating presence of V2O5. This finding is further supported by UV-vis absorption spectra of nanoparticles after deposition on fused silica substrates, which indicates that the oxidation state of the material is preserved over the entire process. From the results of the UV-vis measurement, the band gap of the nanosized V2O5 could be determined to be 3.3 eV. The synthesis approach provides a route to clean V2O5 clusters as it does not involve any surfactant or solvents, which is crucial for an unbiased measurement of intrinsic catalyst properties.

Vanadium(V) oxide clusters synthesized by sublimation from bulk under fully inert conditions.

Oxide nanoparticles in the size range of a few nanometers are typically synthesized in solution or via laser ablation techniques, which open numerous channels for structural change via chemical reactions or fragmentation processes. In this work, neutral vanadium oxide nanoparticles are instead synthesized by sublimation from bulk in combination with a pickup by superfluid helium droplets. Mass spectroscopy measurements clearly demonstrate the preservation of the bulk stoichiometric ratio of vanadium to oxygen in He-grown nanoparticles, indicating a tendency towards tetrahedral coordination of the vanadium centers in finite geometries. This unexpected finding opens up new possibilities for a combined on-the-fly synthesis of nanoparticles consisting of metal and metal-oxide layers. In comparison to mass spectra obtained via direct ionization of vanadium oxide in an effusive beam, where strong fragmentation occurred, we observe a clear preference for (V2O5) n oligomers with even n inside the He nanodroplets, which is further investigated and explained using the electronic structure theory.

Spectroscopy of gold atoms and gold oligomers in helium nanodroplets.

The 6p 2P1/2 ← 6s 2S1/2 and 6p 2P3/2 ← 6s 2S1/2 transitions (D lines) of gold atoms embedded in superfluid helium nanodroplets have been investigated using resonant two-photon ionization spectroscopy. Both transitions are strongly blue-shifted and broadened due to the repulsive interaction between the Au valence electron and the surrounding helium. The in-droplet D lines are superimposed by the spectral signature of Au atoms relaxed into the metastable 2D states. These features are narrower than the in-droplet D lines and exhibit sharp rising edges that coincide with bare atom transitions. It is concluded that they originate from metastable 2D state AuHen exciplexes that have been ejected from the helium droplets during a relaxation process. Interestingly, the mechanism that leads to the formation of these complexes is suppressed for very large helium droplets consisting of about 2 × 106 He atoms, corresponding to a droplet diameter on the order of 50 nm. The assignment of the observed spectral features is supported by ab initio calculations employing a multiconfigurational self-consistent field method and a multi-reference configuration interaction calculation. For large helium droplets doped with Au oligomers, excitation spectra for mass channels corresponding to Aun with n = 2, 3, 4, 5, 7, and 9 are presented. The mass spectrum reveals even-odd oscillations in the number of Au atoms that constitute the oligomer, which is characteristic for coinage metal clusters. Resonances are observed close by the in-droplet D1 and D2 transitions, and the corresponding peak forms are very similar for different oligomer sizes.

Thermally induced alloying processes in a bimetallic system at the nanoscale: AgAu sub-5 nm core-shell particles studied at atomic resolution.

Alloying processes in nanometre-sized Ag@Au and Au@Ag core@shell particles with average radii of 2 nm are studied via high resolution Transmission Electron Microscopy (TEM) imaging on in situ heatable carbon substrates. The bimetallic clusters are synthesized in small droplets of superfluid helium under fully inert conditions. After deposition, they are monitored during a heating cycle to 600 K and subsequent cooling. The core-shell structure, a sharply defined feature of the TEM High-Angle Annular Dark-Field images taken at room temperature, begins to blur with increasing temperature and transforms into a fully mixed alloy around 573 K. This transition is studied at atomic resolution, giving insights into the alloying process with unprecedented precision. A new image-processing method is presented, which allows a measurement of the temperature-dependent diffusion constant at the nanoscale. The first quantification of this property for a bimetallic structure <5 nm sheds light on the thermodynamics of finite systems and provides new input for current theoretical models derived from bulk data.