N. Malinowski

Cage substitution in metal–fullerene clusters

Photofragmentation mass spectra of metal–fullerene clusters C60Mx and C70Mx (M={Fe, Co, Ni, Rh}; x=0,…,30) reveal the existence of a reaction channel which yields clusters having the composition C59−2nM and C69−2nM (n=0,…,10). Enhanced abundances of clusters with 44, 50, and 60 atoms, as well as the presence of clusters containing almost exclusively an even number of atoms, indicate that one carbon atom of the fullerene cage is replaced by a transition metal atom. Additional tandem time-of-flight (TOF) experiments on mass selected C59M and C69M indicate that the initial fragmentation step of this new kind of substitutionally doped fullerenes is the loss of a neutral MC molecule. Measurements on preselected C70Rh3 and C70Ir2 were performed in order to monitor in detail their laser induced transformation into C69Rh and C69Ir.

The Quantum Magnetism of Individual Manganese-12-Acetate Molecular Magnets Anchored at Surfaces

The high intrinsic spin and long spin relaxation time of manganese-12-acetate (Mn(12)) makes it an archetypical single molecular magnet. While these characteristics have been measured on bulk samples, questions remain whether the magnetic properties replicate themselves in surface supported isolated molecules, a prerequisite for any application. Here we demonstrate that electrospray ion beam deposition facilitates grafting of intact Mn(12) molecules on metal as well as ultrathin insulating surfaces enabling submolecular resolution imaging by scanning tunneling microscopy. Using scanning tunneling spectroscopy we detect spin excitations from the magnetic ground state of the molecule at an ultrathin boron nitride decoupling layer. Our results are supported by density functional theory based calculations and establish that individual Mn(12) molecules retain their intrinsic spin on a well chosen solid support.

Metal coated fullerene molecules and clusters

Coevaporation of C60 and an alkali metal in a gas aggregation cell yields a distribution of clusters with composition (C60)nMx with 0≤x<150, n=1,2,3, and M=Li, Na, and K. For singly ionized clusters the mass peaks are strong for odd values of x but only after reaching the composition C60M7+. It is suggested that the onset of even–odd alternation marks the end of electron transfer between metal and C60 and the beginning of metal–metal bonding.

Bonding character of bimetallic clusters AunXm (X=Al, In, Cs)

Bimetallic cluster ions of composition AunXm+ (X=Al,In,Cs) have been studied using time-of-flight mass spectrometry. The mass spectra of gold–aluminum clusters exhibit electronic shell effects for arbitrary composition. Differences in the sequence of shell closings for gold-rich and aluminum-rich clusters can be explained in terms of the differing free electron densities of the two materials. Spectra of gold-indium clusters indicate the formation of electronic shells only for gold-rich species. Among clusters with a higher indium content, the series In+(InAu)n is found to have enhanced stability. This indicates an appreciable charge transfer from gold to indium atoms. Similar spectra are found for the system gold-cesium, where Cs+(CsAu)n are most stable.

A close look at proteins: submolecular resolution of two- and three-dimensionally folded cytochrome c at surfaces

Imaging of individual protein molecules at the single amino acid level has so far not been possible due to the incompatibility of proteins with the vacuum environment necessary for high-resolution scanning probe microscopy. Here we demonstrate electrospray ion beam deposition of selectively folded and unfolded cytochrome c protein ions on atomically defined solid surfaces in ultrahigh vacuum (10(-10) mbar) and achieve unprecedented resolution with scanning tunneling microscopy. On the surface folded proteins are found to retain their three-dimensional structure. Unfolded proteins are observed as extended polymer strands displaying submolecular features with resolution at the amino acid level. On weakly interacting surfaces, unfolded proteins refold into flat, irregular patches composed of individual molecules. This suggests the possibility of two-dimensionally confined folding of peptides of an appropriate sequence into regular two-dimensional structures as a new approach toward functional molecular surface coatings.

Electrospray Ion Beam Deposition: Soft-Landing and Fragmentation of Functional Molecules at Solid Surfaces

The ion beam deposition (IBD) of rhodamine dye molecules on solid surfaces in high vacuum is explored in order to characterize the possibility of fabricating molecular coatings or nanostructures from nonvolatile molecules. Molecular ion beams with a well-defined composition are deposited on silicon oxide surfaces with a controlled kinetic energy. Photoluminescence spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS) are employed in order to characterize the sample with respect to coverage, homogeneity, and the fraction of intact landed ions (soft-landing ratio). We find that homogeneous rhodamine films of defined composition can be produced at energies of 2-100 eV. The coverage is found to be proportional to the ion dose. Soft-landing is observed for energies up to 35 eV.

First principles calculations of Si doped fullerenes: Structural and electronic localization properties in C59Si and C58Si2

Si-doped heterofullerenes C59Si and C58Si2, obtained from C60 by replacing one and two C atoms with Si atoms, are investigated via first principles calculations. Static geometry optimizations show that structural deformations occur in the vicinity of the dopant atoms and give rise to Si–C bonds significantly larger than the ordinary C–C bonds of the fullerene cage. In the case of C58Si2, the lowest energy isomer has two Si atoms located at distances corresponding to third nearest neighbors. The electronic structure of these heterofullerenes, although globally close to that of C60, is characterized by a strong localization of both the HOMO’s and the LUMO’s on the Si sites. Charge transfer occurs from the dopant atoms to the nearest neighbor C atoms, contributing to the formation of polar Si–C bonds. A detailed analysis of the charge localization, based on the electron localization function and maximally localized Wannier function approaches, reveals that the bonding of Si in the fullerene cage consists of ...

Metal-coated fullerenes

Clusters of C60 and C70 coated with alkali or alkaline earth metals are investigated using photoionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra of clusters with composition C60Mx and C70Mx (x = 0… 500; M ϵ| Ca, Sr, Ba) seem to be caused by the completion of distinct metal layers around a central fullerene molecule. The first layer around C60 or C70 contains 32 or 37 atoms, respectively, equal to the number of carbon rings constituting the fullerene cage. Unlike the alkaline earth metal-coated fullerenes, the electronic rather than the geometric configuration seems to be the factor determining the stability of clusters with composition (C60)nMx and (C70)nMx, M ϵ| Li, Na,K,Rb, Cs. The units C60M6 and C70M6 are found to be particularly stable building blocks of the clusters. At higher alkali metal coverage, metal-metal bonding and an electronic shell structure appear. An exception was found for C60Li12, which is very stable independently of charge. Semiempirical quantum chemical calculations support that the geometric arrangement of atoms is responsible for the stability in this case.

Producing and detecting very large clusters

The cluster source we use, a low pressure, rare gas condensation cell, is capable of producing clusters containing more than 45 000 atoms or having masses exceeding 2 500 000 amu. Details of this source and the dependence of the cluster size distribution on adjustable working parameters (oven temperature, inert gas pressure, inert gas type) are discussed in this report. Measurements of the mass-dependent velocity distributions of the clusters emitted by the source are presented and compared to a simple model calculation. The clusters are mass-analyzed with a time-of-flight mass spectrometer and detected by a multi-channel plate. The dependence of the detectability of large clusters on the acceleration voltage is investigated.

Thermally induced structural transition in (C60)n clusters

Abstract Both neutral and charged (C 60 ) n clusters were heated to temperatures at which they evaporated molecules. Particularly stable clusters resisted evaporation and manifested themselves as enhanced peaks in the mass spectra. At 490 K, the set of enhanced mass peaks (e.g., n =13, 19, 39, 46, 49, 55, 116, 131, 147) indicated the presence of icosahedral structures. A different set of intense peaks (e.g., n =38, 48, 58, 68, 71, 75, 77, 84, 98) dominated the mass spectra at temperatures near 585 K. The new set correlates rather well with that expected for close-packed and decahedral structures. The two sets of enhanced peaks were found to be independent of the cluster charge at all temperatures investigated.