J.C. Bennett

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.

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.

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

_{3}

Crystallographic Aspects of the CDW Instabilities in NbSe3

(phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at

Defect-induced room-temperature modulation inNbSe2

T_{P1}

Alternative model for the structural modulation inNbSe3andm−TaS3

= 360\,K and

Charge density waves in NbSe3: The models and the experimental evidence

T_{P2}

The surface structure and charge distribution of ZrSe3 and ZrTe3

= 150\,K, another CDW transition occurs at a much higher temperature