Bin Subhan, Mohammed Kashim; (2018) Manipulation of the properties of Molybdenum Disulphide via chemical doping. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Transition metal dichalocogenides (TMDCs) are a group of materials with great potential for next generation applications due to their layered structure and diverse set of ground states. MoS2 is a prototypical member of this family which can be commonly found in nature and has been shown to superconduct when heavily charge-doped. The aim of this work was to investigate the change in the electron and phonon properties of heavily charge-doped bulk MoS2, as well as to search for the emergence of new ground states and to examine the properties and relationships between these electronic states. The layered structure and semiconducting characteristic of MoS2 make it an ideal candidate for manipulating the electronic properties as well as tuning the dimensionality of this material via chemical intercalation. In the first part of this thesis, SQUID magnetometry and four point probe transport measurements are used to identify the presence of a new temperature dependent state around 85 ± 3 K in K0.4MoS2. Scanning Tunnelling Microscopy/Spectroscopy measurements strongly suggest that this new state is a charge density wave (CDW). The CDW has a commensurate 2√3a₀ x 2√3a₀ R30 superlattice (where a₀ is the lattice parameter of MoS2) with an associated gap in the electronic density of states of 2Δ = 6.1 ± 0.1 mV centred on EF. The mechanism of this CDW is consistent with Fermi surface nesting, which has been ruled out for most other CDW TMDC systems. The CDW is found to be present in imaging above and below Tc = 6.9 K, suggesting that CDW and superconductivity may coexist in this system. A further 2a₀ x 2a₀ superlattice is also determined from spectroscopy measurements, which does not show any gap in the density of states, indicating that this electronic superlattice is not CDW related. Raman Spectroscopy is employed to examine the phonon properties of Li, Na, K, Rb and Ca intercalated MoS2. These measurements show the presence of a new mode around 185 - 210 cm⁻¹ at room temperature for the different intercalant systems. This mode is shown to exhibit strong temperature effects with the peak position shifting by around 15 cm⁻¹ between 77 K and 420 K. There is also a remarkably large increase in the linewidth of this Raman mode by around 30 cm⁻¹ in the same temperature range. A further temperature dependent mode is also found in the K and Rb intercalated MoS2 samples, appearing in measurements below 173 K, which is consistent with CDW order. The electronic structure of Li, Na and K intercalated MoS2 is determined using Angle Resolved Photoemission Spectroscopy. These measurements show that there is sufficient doping to move the Fermi level into the conduction band in all the intercalated compounds. For K0.4MoS2, a 2a₀ x 2a₀ superstructure is also observed in the Fermi surface map. Band splitting of the valence band maxima and conduction band minima is found in the Rb0.5MoS2 compound which, supported by Density Functional Theory calculations, is shown to be a feature of the electronic structure of doped, bulk MoS2.

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