V. Marinković

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.

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.

Charge-density wave modulation in surface layers of TaS2 and NbS2

Abstract NbS 2 , TaS 2 and NbSe 2 layered crystals were studied by means of scanning tunneling microscopy and transmission electron diffraction. NbSe 2 showed a well ordered atomic surface only and TaS 2 a strong nearly commensurate charge-density wave modulation, both in agreement with earlier observations. Contrary, NbS 2 surfaces displayed a corrugation with the periodicity of NbS 2 lattice but with irregular amplitude fluctuations. Two possible structural models for this effect are discussed.

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.

Growth and intercalation of silver deposited onto Nb1+xS2

Abstract Growth of silver thermally deposited at temperatures between 300 and 570 K onto non-stoichiometric single crystals of 3R-Nb 1+ x S 2 ( x ≈ 0.09) was studied by transmission electron microscopy and scanning tunnelling microscopy. The observed reduced amount of silver deposited at elevated temperatures and the appearance of partial dislocations and dislocation loops not present in the as-grown disulphide crystals indicated that silver intercalated into their van der Waals gaps during the very deposition. Morphology of silver islands and intercalation rates were dependent on the deposition temperature. Post-deposition intercalation of silver by gradual disappearance of hexagonally shaped islands could be observed even at room temperature.

Anisotropic growth of Au and Ag on (001) WTe2 and β-MoTe2 surfaces between 350 and 700 K

Abstract The (001) faces of WTe2 and β-MoTe2 crystals are periodically corrugated perpendicularly to their [010] direction. Nucleation and growth of Au and Ag deposited at temperatures between 350 K and 700 K were investigated by TEM, STM and AFM at room temperature. As already observed for room-temperature deposition, both metals grow with their (112) planes parallel to the substrate (001) surface, contrary to the usually observed (111) growth on the flat transition metal dichalcogenide surfaces. As a result of a preferential diffusion along the existing substrate surface troughs, both metals grow in the form of elongated islands. The deposited Ag reacts with Te from the substrate crystals causing destruction of their surfaces which is more pronounced on WTe2 crystals.

Nucleation and growth of noble metals on transition-metal di-tellurides

Abstract Transition-metal di-tellurides (α- and β-MoTe 2 and WTe 2 ) were used as substrates for nucleation and growth studies of noble metals. They represent a group of chemically closely related compounds with different surface topographies. Nucleation and growth of Ag and Au at room temperature were studied by means of UHV-STM, AFM and TEM. The results revealed that the growth and orientation of these metals are influenced by the topography of the substrate surfaces. Contrary to the growth on atomically flat α-MoTe 2 , there is an enhanced diffusion and nucleation along the periodic surface troughs on β-MoTe 2 and WTe 2 . The topography of their (001) surfaces is responsible for the orientation of metal (112) planes being parallel to the substrate surface.)

The growth of WS2 whiskers

WS2 whiskers up to 5 × 0.2 × 0.2 mm were grown by physical vapour transport and were found to correspond to the 2H polytype.