A. Prodan

STM/AFM investigations of β-MoTe2, α-MoTe2 and WTe2

There is controversy in the literature concerning the correspondence of STM images to the atomic positions on some transition metal layered dichalcogenide surfaces. Although it is difficult to differentiate between metal and chalcogen atoms in these crystals with hexagonal symmetry, like α-MoTe 2 , this can be done in cases of β-MoTe 2 and WTe 2 with changed metal-Te distances. Contrary to published STM images of WTe 2 our STM images of β-MoTe 2 show details which resemble the structure of both corrugated topmost Te and metal layers. The d z2 orbitals of metal atoms protruding vertically upward may provide the tunneling current in this case. The detection of surface or sub-surface atoms depends on the tip electronic condition. The STM results are compared with those from AFM.

The surface superstructures in niobium disulfide and diselenide intercalated by Cu, Co and Fe

Abstract A series of intercalated transition-metal dichalcogenides with nominal compositions AxNbCh2, with A=Cu, Co, Fe, Ch=S, Se and x=0.25, 0.33, were studied by scanning tunneling microscopy (STM). Evidence is given that intercalation influences the STM images. Intercalated Cu atoms occupy tetrahedral interstices in the van der Waals gaps, forming pairs with Nb atoms and adopting the 2Hb–MoS2 stacking of the host structure. Effects associated with non-stoichiometry, such as formation of agglomerates and incomplete ordering of the intercalated Cu are frequently observed. Fe and Co atoms occupy octahedral interstices in the gaps, form chains with alternating Nb and stabilize the 2Ha–NbS2 polytype. In case of (Fe,Co)xNbS2 2a0×2a0 and √3a0×√3a0 hexagonal superstructures are formed for x=1/4 and x=1/3, respectively, while in case of CuxNb(S,Se)2 only the x=1/4 fraction orders into a 2a0×2a0 hexagonal superstructure at room temperature. In all cases the superstructures are visible at the surfaces and are attributed to changes in the local densities of states caused by a charge transfer between the intercalated and the surface atoms. Typically these superstructures were observed to change during prolonged scanning, probably as a result of tip-induced fluctuations in the charge density. The experimental STM images are compared to simulated images calculated with the extended Huckel tight binding approximation.

The surface and domain structure of NbTe2

Scanning tunneling microscopy of freshly cleaved NbTe2 van der Waals surfaces reveals that Te vacancies regularly appear in the topmost chains of the Te surface layer. As a result of reduced symmetry with regard to the parent CdI2 structure microdomains of three possible orientational variants are formed, separated by ortho (601)- or meta (1201)boundaries, while the para (1801)-boundaries are not detected. Regions close to the boundaries are often modulated with periodicities of the adjacent domains, confirming that the boundaries in neighboring Te–Nb–Te triple layers do not necessarily coincide. r 2002 Elsevier Science B.V. All rights reserved.

Charge density waves in some Nb and Ta chalcogenides

Abstract A brief description of the competing mechanisms important for the stability of composition- and temperature-dependent modulation phases of Nb 1− x Ta x Te 4 is given, with some emphasis on the microstructural changes at the two high temperature phase transitions in TaTe 4 . The charge density wave sliding in NbSe 3 is explained on the basis of a structural model which takes into account the easy switching between the two modulation wavevectors along the same trigonal prismatic columns. The dependence of the interpolytypic transitions and various charge density wave transition temperatures on the amount of intercalated Ag in 1T-TaS 2 is discussed.

Intercalation of vacuum deposited silver into 1T-TaS2 and its influence on charge density waves

Abstract It is shown that silver vapour deposited onto (00.1) faces of 1T-TaS 2 crystals at 300–500 K starts to intercalate into van der Waals gaps of the host crystal already during the deposition process, even at room temperature deposition. This is demonstrated by superstructure reflections in electron diffraction patterns. The intercalated silver also lowers the transition temperatures among various 1T-TaS 2 CDW phases. These effects show that the octahedral coordination of Ta atoms changes into the trigonal prismatic one. In crystals with silver deposited above 500 K, where the transformation of 1T-TaS 2 into the stable 2H a form takes place, the observed superstructures are in agreement with the literature results for electrochemically silver intercalated 2H a -TaS 2 crystals.

Effect of V Substitution on the Composite Zr-Ti-Cr-V-Ni Intermetallic Hydrides

Hydrogen storage in reversible metal hydrides is attractive because it can be stored at relatively low pressures with a high volumetric density. In the present research work have been investigated compounds with nominal compositions Zr0.8Ti0.2Cr1.2V0.4Ni0.4 and Zr0.8Ti0.2Cr0.8V0.8Ni0.4 that have been prepared by arc-melting under argon atmosphere. Structural characteristics have been studied by using X-ray powder diffraction while the patterns have been analyzed by using the Rietveld analysis. A main hexagonal Laves phase MgZn2 (C14) and a secondary MgCu2 (C15)-type of, have been found in the powdered compounds. The bulk samples have composite microstructures but the V-rich is characterized by a dendritic microstructure. The high resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray analysis (EDAX) have been used for the morphology and quantitative analysis, respectively. Hydrogenations and dehydrogenations have been obtained after crucial activation procedure. The alloys were found to be more active under hydrogen after activation while the desorbed amount of hydrogen has been measured by using a Sievert-type apparatus.

The Instability of the NbTe2 Surface Structure

Low energy electron diffraction from clean NbTe 2 surfaces shows very diffuse reflections. The effect is attributed to an anisotropic heating of the irradiated Te-Nb-Te surface layer. Diffraction patterns for electron energies below 90 eV correspond to an overlapped contribution from numerous domains, belonging to three orientational variants. Electrons of higher energies stabilize the parent high-temperature CdI 2 structure. A similar effect is observed during scanning tunneling microscopy, where the expected surface corrugation is usually lost for tunneling currents of a few ten nA.

The growth of gold on (001) surfaces of α- and β-MoTe2

Abstract The growth of gold on (001) faces of α- and β-MoTe2 was studied by STM and TEM and it was shown that it is influenced by the different surface topographies of both polytypes. The morphology and orientation of gold islands on α (001) faces are typical for the growth on atomically smooth substrates with low surface free energies. Contrary, gold nucleates preferentially along the troughs parallel to the [010] direction of β-MoTe2 and with its (211) plane parallel to the substrate. At higher coverages however, new, more isotropically shaped islands are formed which tend to agglomerate into larger finger-like aggregates preferentially aligned along the [100] direction of β-MoTe2.

NbS3: A unique quasi-one-dimensional conductor with three charge density wave transitions

Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS

Scanning tunneling microscopy of defects in NbSe2

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