Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Matthias Batzill, Ph.D.

Committee Member

Manh-Huong Phan, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Committee Member

Garrett Matthews, Ph.D.

Committee Member

Michael Cai Wang, Ph.D.

Keywords

2D Materials, Ferromagnetism, Monolayer, TiSe2, Transition Metal Dichalcogenides, VSe2

Abstract

Since the isolation of graphene in 2004, two-dimensional (2D) layered materials, specially the transition metal dichalcogenides (TMDs), have attracted immense interest from theoreticians and experimentalist due to the diversity of properties presented in this family of materials. The main reason for the interest in such materials has been the observation of emergent properties as a consequence of the reduced dimensions, i.e. the monolayer regime. Initially the monolayer regime was obtained via the scotch-tape method. The implementation of exfoliation techniques was successful since layered 2D materials are composed of stacked layers held together by weak van der Walls forces that permits separations of the layers and therefore reducing the material dimensions. Despite this, exfoliation techniques have limitations as they are not as successful in obtaining thin forms of certain TMDs as it was done in graphite to obtain graphene. Some TMDs might react to the ambient in which the exfoliation is done impeding the obtention of pristine thin samples. By now, there are different ways to reach the monolayers regimes that allow synthetizing monolayer forms of TMDs that were not obtained via exfoliation methods. This dissertation implements molecular beam epitaxy (MBE), an ultra-high vacuum deposition method that provides a clean environment for layer-by-layer synthesis of TMD materials.

One of the most coveted properties in TMDs in recent years has been ferromagnetism, especially if present in the monolayer regime. Although ferromagnetism was observed in other thin 2D materials (i.e. Cr2Ge2Te6 and CrI3), it was not observed in any TMD. One of the proposed TMDs to become ferromagnetic, especially as its dimensions are reduced, has been monolayer VSe2. Reports preceding the work presented here, were not successful synthetizing VSe2 in the monolayer regime. That limitation was overcome here by the implementation of MBE synthesis allowing coveted experimental characterization of VSe2 in the monolayer limit. Ferromagnetism was in fact observed in our thin vanadium selenides samples increasing the interest in the field of 2D magnets immensely as the observed ferromagnetism persist up to room temperature, a feature not observed in the previously obtained 2D magnets. Despite this, subsequent reports have questioned if the observed ferromagnetism is an intrinsic property of VSe2. In fact, data presented here indicate that the observed ferromagnetism might be a property of self-intercalation vanadium selenides compounds (i.e. V5Se8 and V3Se4) that are very similar to VSe2 and can therefore be incorrectly labeled as TMDs although the origin of the observed ferromagnetism in our samples as well as samples from other groups remains unexplained and controversial. Presented in this work is the characterization via angle-resolved photoemission spectroscopy (ARPES), vibrating sample magnetometer (VSM) and X-ray magnetic circular dichroism (XMCD) revealing a lack of magnetism in stoichiometric VSe2 and an increasing ferromagnetism as vanadium atoms are intercalated into the TMD layers of VSe2 causing transitions to the intercalated compounds as evidenced by scanning tunneling microscope (STM), X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED) and transmission electron microscopy (TEM). The synthesis of VSe2 and the vanadium intercalated compounds via MBE is detailed here as well.

TMDs have other interesting properties that are more widely observed in many of the materials in this family. One of the more interesting of these properties is the charge density wave (CDW). This lattice distortion, which is usually observed at low temperatures, is accompanied by a metal-insulator transition and has been widely studied in TMDs, including in VSe2. Another material that possesses interesting CDW properties is TiSe2. CDW studies in TiSe2 include studies of the models describing its origin as well as how to tune or control the CDW state. Here, we study the CDW tunability in different monolayer TiSe2 heterostructures obtained by depositing the TiSe2 monolayer on substrates with different charge screening properties. Such choice of substrate, alongside the reduction to the monolayer limit, affect the CDW properties as verified by scanning tunneling spectroscopy (STS) and ARPES, including the CDW transition temperature and the CDW energy gap, by virtue of the excitonic insulator model as the driving mechanism of the TiSe2 CDW state. Further CDW tunability is obtained via electron doping which effectively suppresses the CDW. Such suppression has been associated to competing states i.e. competition between the CDW and superconductivity. Although superconductivity was not directly verified in the experiments presented here due low temperature requirements, the suppression of the CDW phase is verified by potassium deposition via ARPES. Details of the synthesis and characterization of TiSe2 is extended to include intercalation compounds which show different properties than TiSe2 i.e. no CDW state as in un-doped TiSe2.

Such intercalation compounds display very interesting properties that differ from VSe2 (emergent ferromagnetism) and TiSe2 (CDW suppression) which provide fertile routes for the exploration of the self-intercalated structures to complement the on-going studies of the TMD properties.

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