Heat generated by electronic devices must be dissipated in order to ensure reliability and prevent device failure. In order to design devices properly, it is important to have precise knowledge of materials' thermal properties at the nano and micro scales. Here we present a series of experimental studies of heat transport for two different types of material: a two dimensional (2D) material such as MoS2 and micron scale particles. We used frequency domain thermoreflectance (FDTR) to conduct all thermal property measurements. This technique can measure thin film thermal properties as well as characterize the interface between two materials. Molybdenum disulfide (MoS2), a transition metal dichalcogenide, is a 2D material that has potential applications as a transistor in nanoelectronics due to its semiconductor properties. We studied cross plane thermal transport across exfoliated monolayer and few layer MoS2 deposited on two distinct substrates: SiO2 and Muscovite mica. The cross plane direction is critical in layer structure devices since the largest thermal resistances are found along this way. The results show enhanced thermal transport across monolayer MoS2 on both substrates indicating that monolayer MoS2 has superior thermal properties for its use in electronic devices. On the other hand, thermally conductive micro particles are used as fillers in composite materials in order to improve the thermal conductivity of the host or matrix material. They can be embedded in polymers for die attach applications as well as in metals to create more efficient heat sinks. We developed new FDTR based thermal models that apply to isolated particles as well as particles surrounded by another material. We tested the models with isolated diamond and silicon micron size particles and with diamond particles embedded in tin. We were able to obtain the thermal conductivity of individual particles, an effective particle volume and the thermal interface conductance between a particle and its surrounding matrix. This technique could have important applications in industry since it could be used to measure in situ the thermal interface conductance between particles and their matrix, often the highest thermal resistance in composite materials.

abstract: A highly uniform and repeatable method for synthesizing the single-layer transition metal dichalcogenides (TMDs) molybdenum disulfide, MoS2, and tungsten disulfide, WS2, was developed. This method employed chemical vapor deposition (CVD) of precursors in a custom built cold-wall reaction chamber designed to allow independent control over the growth parameters. Iterations of this reaction chamber were employed to overcome limitations to the growth method. First, molybdenum trioxide, MoO3, and S were co-evaporated from alumina coated W baskets to grow MoS2 on SiO2/Si substrates. Using this method, films were found to have repeatable coverage, but unrepeatable morphology. Second, the reaction chamber was modified to include a pair of custom bubbler delivery systems to transport diethyl sulfide (DES) and molybdenum hexacarbonyl (MHC) to the substrate as a S and Mo precursors. Third, tungsten hexacarbonyl (WHC) replaced MHC as a transition metal precursor for the synthesis of WS2 on Al2O3, substrates. This method proved repeatable in both coverage and morphology allowing the investigation of the effect of varying the flow of Ar, varying the substrate temperature and varying the flux of DES to the sample. Increasing each of these parameters was found to decrease the nucleation density on the sample and, with the exception of the Ar flow, induce multi-layer feature growth. This combination of precursors was also used to investigate the reported improvement in feature morphology when NaCl is placed upstream of the substrate. This was found to have no effect on experiments in the configurations used. A final effort was made to adequately increase the feature size by switching from DES to hydrogen sulfide, H2S, as a source of S. Using H2S and WHC to grow WS2 films on Al2O3, it was found that increasing the substrate temperature and increasing the H2S flow both decrease nucleation density. Increasing the H2S flow induced bi-layer growth. Ripening of synthesized WS2 crystals was demonstrated to occur when the sample was annealed, post-growth, in an Ar, H2, and H2S flow. Finally, it was verified that the final H2S and WHC growth method yielded repeatability and uniformity matching, or improving upon, the other methods and precursors investigated. / Dissertation/Thesis / Doctoral Dissertation Physics 2019

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This past decade, 2D materials beyond graphene, and most specifically transition metal dichalcogenides (TMDCs) have gained remarkable attention due to their novel applications in electronics and optoelectronics applications. This work reports large-area growth and structural, optical, and electronic transport properties of few-layer MoS2 thin films fabricated using a hybrid approach based on the magnetron sputtering method. In the first part of this dissertation, properties of optimally annealed MoS2 on different substrates such as amorphous BN, SiO2, Si, Al2O3 are discussed using diffraction, spectroscopic, and transport techniques. Later, we show that the physical properties of large-area sputtered MoS2 thin films can be dramatically improved by an ex-situ high-temperature sulfurization process as it leads to the formation of defect-free MoS2 by removing sulfur vacancies. Sharp film-substrate interface along with high bulk structural order is demonstrated as inferred from diffraction and spectroscopic methods. We show that sulfur vacancies can obscure the MoS2 A-B exciton peaks along with a sharp increase in dc conductivity of MoS2. In the last part of my dissertation, we outline the growth of a novel thermoelectric material (SnSe) and new magnetic inverse-Heuslers (of nominal composition MnxFeSi) using the co-sputtering method. These are some of the first attempts, to our knowledge, to grow such materials in thin-film form. Detailed structure-property relations are thoroughly discussed.

The increased deployment for renewable energy sources to mitigate the climate crisis has accelerated the need to develop efficient energy storage devices. Batteries are at the top of the list of the most in-demand devices in the current decade. Nowadays, research is in full swing to develop a battery that meets the needs of today’s renewable energy systems, which are intermittent by nature. Within the framework of improving the performance of batteries, there are parameters in the composition of the battery that play an important role in its performance: electrode materials, electrolytes, separators, and other factors. The key to battery development is the manufacture of electrode materials with optimal properties. Two-dimensional (2D) materials have led to advances in this field, firstly, using graphite as the anode in lithium-ion batteries (LIBs). However, when using the standard graphite as the anode for sodium-ion batteries (NIBs), the large ionic size and energetic instability of Na+ limit intercalation, resulting in a low storage capacity. Therefore, other 2D materials with large interlayer spacing need to be identified for use as electrodes. In this dissertation, our approach is focus on optimizing anode electrode materials by in situ conversion of 2D materials to obtain hybrid materials. These hybrids materials will synergistically improve the performance of LIBs and NIBs by combining the advantages of individual 2D materials. Starting with converted Ti0.87O2 nanosheets to the TiO2/TiS2 hybrid nanosheets. Then, taking advantage of the properties of MXene, we developed hybrid electrodes based on MXenes by converted V2CTx MXene into V2S3@C@V2S3 heterostructures. Finally, we boosted the redox kinetics and cycling stability of Mo2CTx MXene by using a laser scribing process to construct a multiple-scale Mo2CTx/Mo2C-carbon (LS-Mo2CTx) hybrid material.

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The development of digital technology permits the storage and processing of binary data at high rates, with high precision and density. Therefore, over the past few decades, Moore's law has pushed the development of scaling semiconductor devices for computing hardware. Although the current downward scaling trend has reached its scaling limits, a new "More-than-Moore" (MtM) trend has been emphasized as a diversified function of data collection, storage units, and processing devices. The function diversification defined in MtM can be viewed as an alternative form of "scaling down" for electronic systems, as it incorporates non-computing functions into digital ones, allowing digital devices to interact directly with the environment around them. Two-dimensional (2D) materials display promising potential for combining optical sensing and data storage with broadband photoresponse, outstanding photoresponsivity, rapid switching speed, multi-bit data storage, and high energy efficiency. In this work, in-solution 2D materials flakes (Hafnium Diselenide (HfSe2) and Germanium Selenide (GeSe) have been studied as a charge-trapping layer in non-volatile memory through the seamless fabrication process. Furthermore, the behavior of fabricated non-volatile memories under light illumination has been investigated towards in-memory light sensing. Atomic Force Microscopy, RAMAN spectroscopy, and X-ray Diffraction Spectroscopy characterized the charge-trapping materials. The electrical characterization of Metal Oxide Semiconductor (MOS) Capacitor memory revealed a memory window of 4V for the HfSe2 device under ±10V biasing. Intriguingly, the GeSe device exhibited an extraordinarily wide memory window of 11V under the same electrical biasing. Furthermore, the memory endurance for both materials as charge trapping layer (CTL) exceeds the standard threshold of electrical programming and erasing cycles. The accelerated retention test at different temperatures showed the memory device's stability and reliability for both materials. Under light stimuli with electrical readout voltage, the MOS memory exhibited wavelength and intensity-responsive behavior. The MOS memory of HfSe2 has demonstrated remarkable capabilities in storing the detected light signal, while also exhibiting a noteworthy increase in the memory window of approximately 1.8 V when subjected to a laser wavelength of 405 nm. Meanwhile, the GeSe device's CV measurement revealed a similar trend with the greatest memory window enhancements occurring in relation to 465 nm laser wavelength. Under ±6 V biasing in the absence of light, the memory window was found to be 8.3 V. However, following exposure to a 465 nm laser, this value increased significantly to 9.9 V, representing an increment of 1.6 V. In addition, both devices exhibited distinct sensing of various light intensities and an enhanced memory window as a result of the observable Vt shift caused by altering the levels of illumination. This memory enhancement suggests that photoexcited carriers in the CTL layer were responsible for the optical memory behavior. The 2D materials as CTL pave the way for a reconfigurable optical memory with multilevel optical data storage capacity. This research represents a significant step towards the development of a new generation of memory devices that can store and retrieve data using light signals.

The family of semiconducting 2H-phase group-VI transition metal dichalcogenides (TMDs) have been suggested to be promising candidates for hosting optically accessible spin qubits due to their desirable optical and electrical properties, however, experimental progress towards this goal has been impeded by the difficulties associated with the fabrication of clean structures with quality contacts. In this thesis, we present the complex process for obtaining functional contacts to two particular TMDs, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), from which we use as the foundation for the fabrication of three important gate defined quantum structures: quantum dots, a charge detector, and a long 1D channel. These structures all play an important role in furthering the understanding of these materials and are the building blocks for achieving functional spin qubits. More precisely, we investigate the contact resistances associated with various cleaning procedures and contact architectures and report a recipe that results in an ultra-low contact resistance even at cryogenic temperatures. We then demonstrate electrical control of hole quantum dots, the host of the spin qubit, in gated heterostructure devices based on monolayer WSe2 and study its properties. With a similar structure, we demonstrate that a gate-defined nano-constriction is sensitive to the charge occupation of a nearby quantum dot and is therefore suitable to be used as a charge sensor, a valuable component of elaborate quantum circuits. Finally, we demonstrate the realization of a gate-defined quantum confined 1D channel in a high mobility monolayer WSe2 sample and observe an anomalous conductance quantization in units of e2/h. These results pave the way for the development of quantum devices based on electrostatically confined quantum dots defined in semiconducting TMDs and push forward our understanding of their electronic properties.

This work presents a step forward in the use of 2D layered materials (2DLM), specifically hexagonal boron nitride (h-BN), for the fabrication of memristors. In this study, we fabricate, characterize, and use h-BN based memristors with Ag/few-layer h-BN/Ag structure to implement a fully functioning artificial leaky integrate-and-fire neuron on hardware. The devices showed volatile resistive switching behavior with no electro-forming process required, with relatively low VSET and long endurance of beyond 1.5 million cycles. In addition, we present some of the failure mechanisms in these devices with some statistical analyses to understand the causes, as well as a statistical study of both cycle-to-cycle and device-to-device variabilities in 20 devices. Moreover, we study the use of these devices in implementing a functioning artificial leaky integrate-and-fire neuron similar to a biological neuron in the brain. We provide SPICE simulation as well as hardware implementation of the artificial neuron that are in full agreement, showing that our device could be used for such application. Additionally, we study the use of these devices as an activation function for spiking neural networks (SNNs) by providing a SPICE simulation of a fully trained network, where the artificial spiking neuron is connected to the output terminal of a crossbar array. The SPICE simulations provide a proof of concept for using h-BN based memristor for activation function for SNNs.