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dc.rights.licenseRestricted to current Rensselaer faculty, staff and students. Access inquiries may be directed to the Rensselaer Libraries.
dc.contributorWang, G.-C. (Gwo-Ching), 1946-
dc.contributorLu, T.-M. (Toh-Ming), 1943-
dc.contributorYamaguchi, Masashi
dc.contributorGall, Daniel
dc.contributor.authorLittlejohn, Aaron James
dc.date.accessioned2021-11-03T09:01:12Z
dc.date.available2021-11-03T09:01:12Z
dc.date.created2018-07-27T15:12:08Z
dc.date.issued2018-05
dc.identifier.urihttps://hdl.handle.net/20.500.13015/2220
dc.descriptionMay 2018
dc.descriptionSchool of Science
dc.description.abstractFor elemental semiconductors such as Ge, successful growth by van der Waals epitaxy has been challenging and therefore it has not been achieved thus far. I will first present the observation of Ge(111) film epitaxially grown on mica at a narrow substrate temperature range around 425 oC despite the large lattice mismatch (23%) and the lack of high in-plane symmetry in the mica(001) surface. X-ray pole figure analysis reveals there exist multiple rotational domains in the epitaxial Ge film with the dominant in-plane orientations qualitatively in agreement with a superlattice area mismatch model. Crystallinity and electrical properties degrade upon deviation from the ideal growth temperature, as shown by Raman spectroscopy, X-ray diffraction and Hall effect measurements. Furthermore, films grown at or below the critical growth temperature, Tc can successfully be exfoliated from the mica substrate, confirming the van der Waals nature of the interface. The establishment of qualitatively predictable superlattices in a van der Waals heterostructure reveals that although not chemically bound to the substrate, the film atoms’ nucleation is governed by the locations of the surface atoms in the substrate, perhaps indicating a stronger interaction than previously believed. By contrast, films grown above Tc cannot be exfoliated, suggesting heat-induced changes to the mica(001) surface. The observation of Ge epitaxy through van der Waals forces is a step toward the growth of single crystal Ge film without the constraints of lattice and symmetry matching with the substrate.
dc.description.abstractAtmospheric pressure chemical vapor deposition was employed for the synthesis of layered transition metal dichalcogenide vanadium disulfide. By tuning several key growth parameters, we achieved VS2 flakes with diameters over 100 μm, larger than those reported in the literature. In addition, ultrathin flakes with thicknesses of several atomic layers are achieved without the use of an exfoliation method as is typically required. X-ray diffraction and high-resolution transmission electron microscopy confirm the flakes’ monocrystalline quality. Raman spectra are collected which, for the first time, agree with the vibrational modes for the trigonal phase of VS2 as determined by density functional theory calculations. Through electron backscatter diffraction pole figure analysis, transmission electron microscopy and optical microscopy a complex epitaxial relationship with nine preferred in-plane orientations is observed in some regions of the VS2/mica samples. Remarkably, this is qualitatively in agreement with a superlattice area mismatch model, providing further evidence of the ability of electrostatic interactions to dictate the nucleation of film atoms in van der Waals heterostructures. Finally, magnetic force microscopy demonstrates room temperature ferromagnetic signal in ultrathin flakes in agreement with several density functional theory calculations. The discovery of an ultrathin ferromagnetic metal such as VS2 may have dramatic impact on emerging fields such as spintronics and quantum computing.
dc.description.abstractTo date, many materials have been successfully grown via van der Waals epitaxy in which no chemical bonds are established through the sharing of electrons between the substrate and epitaxial layer. Due to this weaker interfacial interaction, the constraint of lattice matching as is required for traditional chemical epitaxy is lifted. This thesis is divided into two sections, each reporting the growth and subsequent characterizations of a technologically relevant material on the van der Waals mica(001) surface: 3D germanium and 2D vanadium disulfide.
dc.language.isoENG
dc.publisherRensselaer Polytechnic Institute, Troy, NY
dc.relation.ispartofRensselaer Theses and Dissertations Online Collection
dc.subjectPhysics
dc.titlevan der Waals substrate mediated heteroepitaxy of germanium and vanadium disulfide films
dc.typeElectronic thesis
dc.typeThesis
dc.digitool.pid179064
dc.digitool.pid179065
dc.digitool.pid179066
dc.rights.holderThis electronic version is a licensed copy owned by Rensselaer Polytechnic Institute, Troy, NY. Copyright of original work retained by author.
dc.description.degreePhD
dc.relation.departmentDept. of Physics, Applied Physics, and Astronomy


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