Charge Density Waves and Alkali Metal Intercalation in Layered Materials

Doctoral thesis, 1999

In this thesis the following issues are discussed: 1. Fundamental properties of two polytypes of TaS2 studied by scanning tunneling microscopy (STM) and spectroscopy (STS). 2. The influence of Na intercalation on charge density waves (CDW's) in 1T-VSe2 and 4Hb-TaS2 studied by STM and STS. 3. Cleaning methods for scanning tunneling microscopy tungsten tips. Variable temperature STS of the known, but not fully explained, metal-insulator transition in 1T-TaS2 shows an abrupt transition at ~190 K, from a CDW depletion to an opening of a deep pseudogap at EF. Two structures in the spectra are interpreted as Hubbard bands, showing that the transition is driven by electron correlation. In the gap, localized states are formed at EF, explaining the low temperature resistivity of 1T-TaS2. Single-layer steps are fabricated by STM etching at 4.2 K in 4Hb-TaS2 to allow studies of the two different layers under the same tunneling conditions. The semiconducting T-layer has a strong .ROOT.13x.ROOT.13 CDW below 600 K, while the metallic H-layer has a weak 3x3 CDW below 22 K. Images of the H-layer at 4.2 K shows the 3x3 CDW at negative bias voltage. At positive sample bias of more than 50 mV, the .ROOT.13x.ROOT.13 CDW from the underlying T-layer is observed also in the H-layer via an interlayer tunneling process. Small amounts of intercalated Na in VSe2 and TaS2 lead to non-uniform distributions below the first layer. In 1T-VSe2 two types of areas, clean and intercalated, are found. In 4Hb-TaS2 clean areas and several intercalated domains, differing in Na concentration or coordination, are observed. The single valence electron of the alkali metal is donated to the host lattice upon intercalation, and influences the CDW formation. Below the CDW transition temperatures, the CDW is modified locally in the intercalated areas, leading to different CDW wavelengths, symmetries and energy gaps. The H-layers of TaS2, only investigated above the transition temperature, show no CDW's. STS shows changes of the CDW gaps and shifts of density of states peaks, also in the clean areas so that the effect on intercalation is not purely located to intercalated domains, although the main influence is found there. In 4Hb-TaS2 metal-semiconductor transitions occur in the intercalated areas of the T-layers, but not in H-layers. Cleaning methods of DC-etched tungsten tips for STM use are investigated using Transmission Electron Microscopy images of the tip, field emission currents from the tip, and STM images/STS spectra of a clean Cu(111) surface. Imaging and spectroscopy can be improved by treating tips with sputtering in a Ne- or Ar- gas environment either a few seconds or until decapitation occurs, and this should be followed by a careful heating via electron bombardment.