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In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta-to-beta' phase transformation was investigated by temperature dependent transmission electron microscopy and Raman spectroscopy, combined with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. STS measurements show that the indirect bandgap of monolayer beta' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications like non-volatile memory devices. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta' In2Se3 is visualized with scanning tunneling microscopy and spectroscopy (STM/S) combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials. / Doctor of Philosophy / Two-dimensional (2D) materials are materials consisting of a single layer or a few layers of atoms. They exhibit unique and interesting properties distinct from their bulk counterparts. Over the past decade, much effort has been devoted to a large family of 2D materials — 2D metal chalcogenides that exhibit fascinating structural and electronic properties. These 2D metal chalcogenides can also be stacked together to form various heterostructures. The scanning tunneling microscope (STM) is a powerful tool to study these materials with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. It can also be used to manipulate nanometer-scale structures on the material surface. In this dissertation, we use scanning tunneling microscopy and spectroscopy (STM/S) to investigate 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers transform from beta phase to a more stable beta' phase when the sample is cooled down from room temperature to 77 K. This thermally driven beta-to-beta' phase transformation was found to be reversible by temperature dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in 2D ferroelectric beta' In2Se3 is visualized by using STM/S combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: In this thesis the structural dynamics, upon photo-excitation, of the charge-densitywave (CDW) material 4Hb-TaSe2 is investigated on the time-scale of atomic motion and simultaneously on the spatial-scale of atomic dimensions. CDW materials have been of interest since their discovery in the 1970’s because of their remarkable non-linear and anisotropic electrical properties, gigantic dielectric constants, unusual elastic properties and rich dynamical behaviour. Some of these exotic properties were extensively investigated in thermal equilibrium soon after their discovery but only recently have ultrafast techniques like femtosecond spectroscopy become available to study their out-of-equilibrium behaviour on the time-scale of atomic motion. By studying their behaviour on this time-scale a more in-depth understanding of their macroscopic properties can be gained. However, to do investigations on the atomic time-scale and simultaneously directly observe the evolution of the atomic arrangements is another challenge. One approach is through the previously mentioned technique of femtosecond pump-probe spectroscopy but converting the usual ultrashort optical probing source to an ultrashort electron or x-ray source that can diffract off the sample and reveal structural detail on the atomic level. Here, the femto-to-picosecond out-of-equilibrium behaviour upon photo-excitation in 4Hb-TaSe2 is investigated using an ultrashort electron probe source. Two variations of using an electron probe source are used: conventional scanning Femtosecond Electron Diffraction (FED) and a new approach namely Femtosecond Streaked Electron Diffraction (FSED). The more established FED technique, based on femtosecond pumpprobe spectroscopy, is used as the major investigating tool while the FSED technique, based on ultrafast streak camera technology, is an attempt at broadening the scope of available techniques to study structural dynamics in crystalline material on the subpicosecond time-scale. With these two techniques, the structural dynamics during the phase transition from the commensurate- to incommensurate-CDW phase in 4Hb-TaSe2 is observed through diffraction patterns with a temporal resolution of under 500 fs. The study reveals strong coupling between the electronic and lattice systems of the material and several time-constants of under and above a picosecond are extracted from the data. Using these time-constants, the structural evolution during the phase transition is better understood and with the newly gained knowledge, a model of all the processes involved after photo-excitation is proposed. / AFRIKAANSE OPSOMMING: In hierdie tesis word die strukturele dinamika van die lading-digtheid-golf (LDG) materiaal 4Hb-TaSe2 ondersoek op die tydskaal van atomiese bewegings en gelyktydig op die ruimtelikeskaal van atomiese dimensies. LDG materie is al van belang sedert hul ontdekking in die 1970’s as gevolg van hul merkwaardige nie-lineêre en anisotrope elektriese eienskappe, reuse diëlektriese konstantes, ongewone elastiese eienskappe en ryk dinamiese gedrag. Sommige van hierdie eksotiese eienskappe is omvattend ondersoek in termiese ewewig kort na hul ontdekking, maar eers onlangs is dit moontlik deur middle van ultravinnige tegnieke soos femtosekonde spektroskopie om hulle uit-ewewigs gedrag te bestudeer op die tydskaal van atomiese beweging. Deur die gedrag op hierdie tydskaal te bestudeer kan ’n meer insiggewende begrip van hul makroskopiese eienskappe verkry word. Om ondersoeke in te stel op die atomiese tydskaal en gelyktydig direk die evolusie van die atoom posisie te waarneem is egter ’n moeilike taak. Een benadering is deur middle van femtosekonde “pump-probe” spektroskopie maar dan die gewone optiese “probe” puls om te skakel na ’n electron of x-straal puls wat van die materiaal kan diffrak en dus strukturele inligting op die atomiese vlak kan onthul. Hier word die femto-tot-pico sekonde uit-ewewig gedrag in 4Hb-TaSe2 ondersoek met behulp van elektron pulse. Twee variasies van die gebruik van ’n elektron bron word gebruik: konvensionele “Femtosecond Electron Diffraction” (FED) en ’n nuwe benadering, naamlik, “Femtosecond Streaked Electron Diffraction” (FSED). Die meer gevestigde FED tegniek, wat gebaseer is op femtosekonde “pump-probe” spektroskopie, word gebruik as die hoof ondersoek metode terwyl die FSED tegniek, wat gebaseer is op die ultra vinnige “streak camera” tegnologie, ’n poging is om beskikbare tegnieke uit te brei wat gebruik kan word om strukturele dinamika in materie te bestudeer op die sub-picosekonde tydskaal. Met behulp van hierdie twee tegnieke, word die strukturele dinamika tydens die fase oorgang van die ooreenkomstige tot nie-ooreenkomstige LDG fase in 4Hb-TaSe2 deur diffraksie patrone met ’n tydresolusie van minder as 500 fs waargeneem. Die studie toon ’n sterk korrelasie tussen die elektroniese sisteem en kristalrooster. Verskeie tydkonstantes van onder en bo ’n picosekonde kon ook uit die data onttrek word en gebruik word om die strukturele veranderinge beter te verstaan. Hierdie nuwe kennis het ons in staat gestel om ’n model van al die betrokke prosesse voor te stel.

abstract: Early detection and treatment of disease is paramount for improving human health and wellness. Micro-scale devices promote new opportunities for the rapid, cost-effective, and accurate identification of altered biological states indicative of disease early-onset; these devices function at a scale more sensitive to numerous biological processes. The application of Micro-Electro-Mechanical Systems (MEMS) in biomedical settings has recently emerged and flourished over course of the last two decades, requiring a deep understanding of material biocompatibility, biosensing sensitively/selectively, biological constraints for artificial tissue/organ replacement, and the regulations in place to ensure device safety. Capitalizing on the inherent physical differences between cancerous and healthy cells, our ultra-thin silicone membrane enables earlier identification of bladder cancer—with a 70% recurrence rate. Building on this breakthrough, we have devised an array to multiplex this sample-analysis in real-time as well as expanding beyond bladder cancer. The introduction of new materials—with novel properties—to augment current and create innovative medical implants requires the careful analysis of material impact on cellular toxicity, mutagenicity, reactivity, and stability. Finally, the achievement of replacing defective biological systems with implanted artificial equivalents that must function within the same biological constraints, have consistent reliability, and ultimately show the promise of improving human health as demonstrated by our hydrogel check valve. The ongoing proliferation, expanding prevalence, and persistent improvement in MEMS devices through greater sensitivity, specificity, and integration with biological processes will undoubtedly bolster medical science with novel MEMS-based diagnostics and therapeutics. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2018

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<p>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.</p> <p>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.</p> <p>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.</p>