Jürgen Walter

Magnetization studies in quasi two-dimensional palladium nanoparticles encapsulated in a graphite host

Abstract:We show that the numerical method based on the off-equilibrium fluctuation-dissipation relation does work and is very useful and powerful in the study of disordered systems which show a very slow dynamics. We have verified that it gives the right information in the known cases (diluted ferromagnets and random field Ising model far from the critical point) and we used it to obtain more convincing results on the frozen phase of four-dimensional spin glasses. Moreover we used it to study the Griffiths phase of the diluted and the random field Ising models.In this work, magnetization studies on quasi bi-dimensional nanoparticles of Pd encapsulated in a graphite host as a function of magnetic field and temperature are presented. We found that an important magnetic signal can be attributed to the Pd nanoparticles. The obtained magnetic behaviour is interpreted in view of theoretical studies which predict ferromagnetism in slabs of Pd with some monolayers of thickness.

Quasi two-dimensional palladium nanoparticles encapsulated into graphite

Encapsulated palladium nanoparticles (average size in lateral dimension: 53 nm) have been synthesized by reduction of a PdCl2-graphite intercalation compound precursor in a hydrogen atmosphere. The carbon lattice acts as a template (monolayer < particle thickness £ 5 layers). The nanoparticles were observed by TEM and their chemical nature was proved by XRD and SAED. Hexagonal, cubic and intermediate bodies could be observed.

Se atoms and Se6 molecules as guests in Se-carbons - prepared by reduction of a SeCl4-graphite precursor

A SeCl4 -graphite intercalation compound precursor was reduced by a solution of lithium diphenylide in tetrahydrofuran at room temperature. X-ray diffraction measurements gave two distinguishable stages. One stage represented a Se-atom intercalation the other represented an intercalation of Se6 molecules. The in-plane diffraction patterns were estimated by selected-area electron diffraction, the existence of two different guest species (atoms and molecules) could be proved. The Se6 -molecule phase shows an incommensurate lattice with regard to the host lattice, but they are in the same orientation. The lattice parameter of intercalated Se6 is a Se 6-guest = 1158±36 pm, c Se 6-guest = 483±38 pm, which fits with the lattice parameter of non-intercalated Se6 molecules. Se atom domains show a 2 × agraphite superlattice with respect to the host lattice, which is a commensurate superstructure. Raman scattering data showed the occurrence of an acceptor-type graphite intercalation compound. Three different types of spectra could be obtained, two kinds of spectra consists of doublets at 1588 cm-1 and 1608 cm-1 , with different intensity ratios. These two kinds of spectra are certainly attributed to Se-atom domains, with different stages. A third type of spectrum show bands at higher wavenumbers (1646 cm-1 and 1653 cm-1 ). These bands are probably correlated to Se6 -molecule domains. They represent maybe very early stages of nanoparticle formation.

Electron diffraction and scanning tunneling microscope studies of TaCl5-graphite intercalation compounds

Abstract Tantalum(V) chloride (TaCl5) was intercalated into highly oriented pyrolytic graphite and into natural graphite flakes. X-ray diffraction measurements show that second stages were obtained. The fresh intercalation compounds were investigated by scanning tunneling microscopy in air and by electron diffraction. The commensurate ( 7 × 7 ) superlattice of tantalum(V) chloride shows a rotation of ±19.1° against the graphite lattice. Additional superstuctures of longer dimensions were detected by scanning tunneling microscopy. A Moire pattern and some bands were observed. The graphite intercalation compounds were immersed in water for two hours at room temperature and dried at 100°C in air. X-ray diffraction showed that the original stages were preserved after exposure to water. The scanning tunneling micrographs of samples exposed to water show a pattern similar to that of pristine graphite, the superlattice had disappeared. Electron diffraction studies on such samples gave evidence that the intercalate is rearranged and forms amorphous layers. In the case of water-immersed samples, no surface structures of the intercalate were observable with scanning tunneling microscopy.

XPS study on pristine and intercalated tantalum carbosulfide

Abstract Core level and valence band spectra of pristine and Fe, Co, Cu, or Ni intercalated tantalum carbosulfide (Ta 2 S 2 C) have been studied by X-ray photoelectron spectroscopy. The aim was to compare pristine tantalum carbosulfide (a layered compound, stacking sequence: S–Ta–C–Ta–S) to tantalum carbides (TaC and Ta 2 C) and tantalum sulfide (TaS 2 ) and compare intercalated with pristine carbosulfides. The chemical shift of carbon layers in the carbosulfides was comparable to a graphitic carbon monolayer on (111) faces of TaC. Therefore, the carbon in carbosulfides does not have a carbidic character, it is a single carbon layer with graphitic character. From an electronic point of view, the carbosulfides should no longer be called complex carbides. Sulfur and tantalum in the pristine compound showed shifts comparable to tantalum sulfide. X-ray photoelectron valence band spectra can be only interpreted as fingerprints. However, intercalated carbosulfides with guests at the octahedral sites between two adjacent sulfur layers (Fe, Co, and Ni) show valence band spectra distinguishable from Cu intercalated compounds (Cu is located in tetragonal sites between sulfur and tantalum layers). Distinguishable from all valence band spectra of intercalated materials is the valence band spectrum of the pristine compound.

H–T phase diagram and the nature of vortex-glass phase in a quasi-two-dimensional superconductor: Sn-metal layer sandwiched between graphene sheets

Abstract The magnetic properties of a quasi-two-dimensional (2D) superconductor, Sn-metal graphite (MG), are studied using DC and AC magnetic susceptibility. Sn-MG has a unique layered structure where Sn metal layer is sandwiched between adjacent graphene sheets. This compound undergoes a superconducting transition at Tc=3.75 K at H=0. The H–T diagram of Sn-MG is similar to that of a quasi-2D superconductors. The phase boundaries of vortex liquid, vortex glass, and vortex lattice phase merge into a multicritical point located at T ∗ =3.4 K and H ∗ =40 Oe. There are two irreversibility lines denoted by Hgl (de Almeida–Thouless type) and Hgl′ (Gabay–Toulouse type), intersecting at T0′=2.5 K and H0′=160 Oe. The nature of slow dynamic and nonlinearity of the vortex glass phase is studied.

Magnetic-field-induced superconductor-metal-insulator transitions in bismuth metal graphite

Bismuth metal graphite (MG) has a unique layered structure where Bi nanoparticles are encapsulated between adjacent sheets of nanographites. The superconductivity below T c (= 2.48 K) is due to Bi nanoparticles. The Curie-like susceptibility below 30 K is due to conduction electrons localized near zigzag edges of nanographites. A magnetic-field-induced transition from metallic to semiconductorlike phase is observed in the in-plane resistivity ρ a around H c (25 kOe) for both H ⊥ c and H‖‖c (c: c axis). A negative magnetoresistance in ρ a for H ⊥ c (0 40 kOe) suggest the occurrence of a two-dimensional weak-localization effect.

BiCl3-graphite : an environmentally stable low-dimensional compound

Abstract Bismuth(III) chloride has been intercalated from the gas phase into highly oriented pyrolytic graphite. The as-prepared intercalation compound was investigated by X-ray diffraction and by selected area electron diffraction. The product was a mixture of second and third stages, with an in-plane lattice showing the typical rectangular structure of BiCl3-graphite. The sample was exposed to boiling water for 15 weeks and the stages as well as the in-plane lattice were investigated at different periods of time. In the first 4 weeks, no stage conversion could be detected, with the in-plane lattice showing the original diffraction pattern with very well resolved diffraction spots. X-ray diffraction measurements gave evidence of increasing stage disorder by exposure to the boiling water. A stage conversion from a second to a fifth stage was detected between the 4th and the 6th week. A partial deintercalation with appearance of the graphite (002) reflection was also observed. Between 6 and 15 weeks in boiling water, the sample showed different in-plane lattices dependent on the region of the sample examined. Some areas showed the original rectangular pattern, a few other areas a Moire pattern, both types giving very well resolved diffraction spots. The Moire pattern indicates increasing domain twinning. Other regions in the sample were totally emptied of intercalated materials, and a few other regions showed a diffraction pattern with more or less well developed diffraction rings. These results gave evidence for the beginning decomposition of the intercalation compound.

Magnetism and superconductivity in M c Ta 2 S 2 C ( M = Fe , Co, Ni, and Cu)

Magnetic properties of

Successive superconducting transitions in Ta2S2C studied by electrical resistivity and nonlinear ac magnetic susceptibility

{\mathrm{M}}_{c}{\mathrm{Ta}}_{2}{\mathrm{S}}_{2}\mathrm{C}