TUNING THE STRUCTURAL AND ELECTRONIC PROPERTIES OF TRANSITION-METAL INTERCALATED WS2

Tuning the structural and electronic properties of layered materials is critical for the development of thin, flexible semiconductors that are capable of overcoming Moore’s law. Intercalation of transition metals (TMs) into the interlayer gaps of a two-dimensional host material is one of the most promising methods toward modifying the electronic properties without disrupting the chemical bonds within the layers. Previous studies have shown that the intercalation of TMs into Bi2Se3, SnS2, TaS2, and NbS2 altered the electronic, optical, and magnetic properties of the material due to orbital hybridization between the d-orbitals of the intercalant and the bands of the host material. However, the synthesis of intercalated 2D materials using compositionally-limited because the process is driven by a charge transfer reaction from the intercalant to the conduction band of the host material, which is difficult to achieve on group VI TMDs (MoS2, WS2) with high energy conduction bands. As a result, only metal atoms that are highly reducing, like alkali metals, can be effectively intercalated into WS2. Meanwhile, alkali metal-intercalated WS2 materials are unstable under ambient conditions, which significantly limits further device application. In this dissertation, we developed a solution-phase synthetic method to successfully intercalate a broad range of redox-active TM cations into WS2 and access a variety of intercalation morphologies. With these different intercalated structures, the electronic properties of WS2 can be systematically adjusted.

First, we synthesized vanadium-intercalated WS2, and structural characterization reveals that solvated vanadium cations are uniformly intercalated in WS2, which significantly increases the interlayer spacing from 6.2 Å to 14.2 Å. Raman and X-ray absorption spectroscopy (XAS) experiments indicate a strong interaction between the vanadium intercalants and the WS2 basal plane. Electronic transport measurements show that the vanadium-intercalated WS2 is an n-type semiconductor with room-temperature conductivity of 12 S/cm, 2 orders of magnitude higher than pristine WS2. The electronic properties can be further tuned by varying the concentration of V intercalants.

We further synthesized TM-intercalated WS2 using 17 different metal precursors, varying the identity, reduction potential, charge density, and ionic radius in order to determine the key properties that influence intercalation. With detailed structural characterization, we determined that both charge density and reduction potential of the precursor are critical toward achieving selective intercalation over secondary nucleation. The strength of the host-guest interaction is also dependent on the transition metal identity. With the strongest interaction between the TM intercalants and WS2 basal plane, FeCl3-WS2 has the lowest work function of 4.97 eV and the highest conductivity of 110 S/cm.