Laser ablation of Co/Ge mixtures: a new type of endohedral structure, a semiconductor cage trapping a metal atom

Abstract

In experiments on Co/Ge binary clusters by laser vaporization, a remarkably strong signal in the mass spectrum was presented, which was assigned to the cluster anion consisting of ten germanium atoms and one cobalt atom, [CoGe 10 ]À . For this cluster anion we suggest an endohedral structure ± aGe 10 cage trapping a Co atom in its interior. Reactions between cobalt clusters and germanium clusters using a laser double ablation reactor also confirmed the endohedral structure. The fact that an atom or a group of atoms can be trapped in a fullerene cage 1,2 has been generally recognized since 1991 when techniques to produce metallofullerene were developed 3,4 and the samples were produced in bulk. This provided the conditions to directly study the characteristics of a metallofullerene, 4 and consequently numerous experimental explorations of endohedral fullerenes were conducted. Theoretical studies on the structures and the formation mechanism 5,6 of the new complexes were also developed. But for almost a decade, most of the species with endohedral structures produced and studied mainly had carbon fullerene cages. 7±13 Recently, a W-encapsulating Si 12 cage cluster was reported. 14 Here we give a new experimental result, which might indicate that a transition metal atom (Co) was trapped in a semiconductor cage (Ge 10 ). This recognition comes from the analysis of mass spectra of Co/ Ge binary cluster anions produced by laser ablation and also from the reactions between cobalt clusters and germanium clusters in a laser double ablation reactor (LDAR). EXPERIMENTALThe binary cluster anions of germanium and cobalt were generated and analyzed under the following conditions. The samples were prepared with germanium (purity 99.99%) and cobalt (purity 99.9%) powders, mixed well in different atomic ratios and pressed to form tablets. The experiments to produce and detect Co/Ge binary cluster anions were performed by a vaporization laser and the first stage of a home-made tandem time-of-flight mass spectrometer (T-TOFMS). A detailed description of the T-TOFMS is given elsewhere. 15 In our experiments the first stage of the T-TOFMS is only used to produce the clusters. Briefly, the second harmonic of a Q-switched Nd: YAG laser (532 nm, 10 mJ/pulse, 10 pulse/s) was focused on the surface of a tablet sample held in the vacuum chamber (at 10 À6 Torr) of the spectrometer to produce the cluster anions. The produced cluster anions were extracted and accelerated with a pulse voltage of À1.2 kV, and then drifted in a fieldfree region 3.5 m long. The anions were detected by a dual micro-channel plate detector held in the end of the field-free region, and recorded to give the mass spectrum. Typically, 1000±2000 laser shots were accumulated and stored in an IBM-PC computer.

[Co@Ge10]3−: An Intermetalloid Cluster with Archimedean Pentagonal Prismatic Structure

“…[22] Subsequent calculations were performed to predict the possible structures of endohedral [CoGe 10 ]…”

Titanium Digermanium: Theoretical Assignment of Electronic Transitions Underlying Its Anion Photoelectron Spectrum

“…The germanium element is well-known for its use as semiconductor materials in electronic devices. − Because of the high p-type mobility, germanium has been placed under consideration for potential future high-performance electronic transistors . Similar to silicon-based materials, metal-doped germanium materials are believed to bring in improved properties in comparison to the pure ones. − In the intensive search for such new materials, several transition metal-doped germanium clusters have been experimentally synthesized and characterized. − …”

Structural and Magnetic Properties of CoGen− (n=2–11) Clusters: Photoelectron Spectroscopy and Density Functional Calculations

“…Some theoretical and experimental studies suggested that the doping of transition metals (TMs) could improve the stabilization of the cage structures of germanium-based clusters and tailor their properties at the same time. [8,11,[27][28][29] Moreover, stable TM-doped germanium clusters may be used as building blocks for cluster-assembled materials. [4,5,30] It has been reported that cobalt-doped germanium nanomaterials have low resistivity, high thermal stability, and room temperature ferromagnetism; this results in potential applications in effective on-chip interconnects and nanoelectrodes for highly integrated nanoelectronic devices, spintronic devices, and field-emission displays.…”

“…Zhang et al generated CoÀGe clusters by laser vaporization and observed a relatively strong CoGe 10 À signal in the mass spectrum. [27] Li et al investigated the structure of CoGe 10 À through density functional theory (DFT) calculations and suggested that the most stable structure of CoGe 10 À was a D 4d bicapped tetragonal antiprism and the second most stable structure was a C 3v tetracapped trigonal prism. [3] Subsequently, the CoGe 10 3À cluster anion was synthesized and cocrystallized with the anion [Co(C 8 H 12 ) 2 ] À in the compound [K(2,2,2-crypt)] 4 -[Co@Ge 10 ]À[Co(C 8 H 12 ) 2 ]·toluene (2,2,2-crypt = 4,7,13,16,21,24hexaoxa-1,10-diazabicyclo [8.8.8]hexacosane), and single-crystal X-ray structural characterization suggested that CoGe 10 3À exhibited a D 5d pentagonal prismatic structure.…”