Study of TaS2 polymorphic Vander Waals heterostructures by means of low temperature scanning tunnelling microscopy/spectrocopy

Abstract

2D layered materials based on Van der Waals heterostructures are promising candidates to substitute conventional semiconductor based heterostructures. Prior to the production of reliable devices fundamental understanding of their structural and electronic properties is desirable. This thesis presents a study on the structural and electronic properties of two Van der Waals heterostructures constituted by the two polymorphic phases of TaS2. Namely, 2D-2D 1H/1T-TaS2 and 2D-3D 1T/2H-TaS2. Both heterostructures reveal new electronic properties that originate due to the vertical stacking of the 2H and 1T phases. These systems are inspected by scanning tunnelling microscopy, known for its high spatial resolution in topography and spectroscopy, which is perfect to characterize electronic and structural properties in a non disruptive manner. All the experiments are performed in ultra high vacuum and at low temperature. The polymorphic phases of TaS2 have contrasting properties. At low temperatures, 1T- TaS2 exhibits a strong charge density wave modulation that results in a Mott insulating phase while 2H-TaS2 is a metal that develops a superconducting phase on its own. However, the small difference in their formation energies explains why both phases have been found to co-exist in the same 2H crystal studied in this thesis. The individual 2H and 1T areas are carefully characterized, demonstrating that their properties are in complete agreement to those previously reported on individual 2H and 1T crystals. Thus, the properties of the heterostructures studied here are a product of vertically stacking the individual single layers. 2D-2D 1H/1T-TaS2 heterostructures have been already studied in 4Hb-TaS2 crystals in the decade of the 90s and more recently in 1T-TaS2 crystals and HOPG where an increase in the critical superconducting temperature has been reported. One intriguing property of this material is the transparency of the 1H layer to the CDW of the 1T layer underneath, we have demonstrated for the first time that this effect is dominated by the position in energy of the Upper Hubbard sub-band of the 1T layer. We have also validated that the transparency effect is a purely electronic effect as first stated by Han and Ekvall. Contrary to previous publications, no increase in the superconducting critical temperature is measured, instead, what could be a Coulomb gap appears in the local density of states of the 1H/1T heterostructure possibly hindering the expected boost in the critical temperature.