abstract: Layer structured two dimensional (2D) semiconductors have gained much interest due to their intriguing optical and electronic properties induced by the unique van der Waals bonding between layers. The extraordinary success for graphene and transition metal dichalcogenides (TMDCs) has triggered a constant search for novel 2D semiconductors beyond them. Gallium chalcogenides, belonging to the group III-VI compounds, are a new class of 2D semiconductors that carry a variety of interesting properties including wide spectrum coverage of their bandgaps and thus are promising candidates for next generation electronic and optoelectronic devices. Pushing these materials toward applications requires more controllable synthesis methods and facile routes for engineering their properties on demand. In this dissertation, vapor phase transport is used to synthesize layer structured gallium chalcogenide nanomaterials with highly controlled structure, morphology and properties, with particular emphasis on GaSe, GaTe and GaSeTe alloys. Multiple routes are used to manipulate the physical properties of these materials including strain engineering, defect engineering and phase engineering. First, 2D GaSe with controlled morphologies is synthesized on Si(111) substrates and the bandgap is significantly reduced from 2 eV to 1.7 eV due to lateral tensile strain. By applying vertical compressive strain using a diamond anvil cell, the band gap can be further reduced to 1.4 eV. Next, pseudo-1D GaTe nanomaterials with a monoclinic structure are synthesized on various substrates. The product exhibits highly anisotropic atomic structure and properties characterized by high-resolution transmission electron microscopy and angle resolved Raman and photoluminescence (PL) spectroscopy. Multiple sharp PL emissions below the bandgap are found due to defects localized at the edges and grain boundaries. Finally, layer structured GaSe1-xTex alloys across the full composition range are synthesized on GaAs(111) substrates. Results show that GaAs(111) substrate plays an essential role in stabilizing the metastable single-phase alloys within the miscibility gaps. A hexagonal to monoclinic phase crossover is observed as the Te content increases. The phase crossover features coexistence of both phases and isotropic to anisotropic structural transition. Overall, this work provides insights into the controlled synthesis of gallium chalcogenides and opens up new opportunities towards optoelectronic applications that require tunable material properties. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

abstract: The structure of glass has been the subject of many studies, however some details remained to be resolved. With the advancement of microscopic imaging techniques and the successful synthesis of two-dimensional materials, images of two-dimensional glasses (bilayers of silica) are now available, confirming that this glass structure closely follows the continuous random network model. These images provide complete in-plane structural information such as ring correlations, and intermediate range order and with computer refinement contain indirect information such as angular distributions, and tilting. This dissertation reports the first work that integrates the actual atomic coordinates obtained from such images with structural refinement to enhance the extracted information from the experimental data. The correlations in the ring structure of silica bilayers are studied and it is shown that short-range and intermediate-range order exist in such networks. Special boundary conditions for finite experimental samples are designed so atoms in the bulk sense they are part of an infinite network. It is shown that bilayers consist of two identical layers separated by a symmetry plane and the tilted tetrahedra, two examples of added value through the structural refinement. Finally, the low-temperature properties of glasses in two dimensions are studied. This dissertation presents a new approach to find possible two-level systems in silica bilayers employing the tools of rigidity theory in isostatic systems. / Dissertation/Thesis / Doctoral Dissertation Physics 2018

There is a movement in the electronic industry toward building electronic devices with dimensions smaller than is currently possible. Atomically thin 2D material, such as graphene, bilayer graphene, hBN and MoS2 are great candidate for this goal and they have a potential set of novel electronic properties compare to their bulk counterparts due to the exhibition of quantum confinement effects. To this goal, we have investigated the electric field screening of multilayer 2D materials due to the presence of impurity charge in the interface and vertical electric fifield from back gate. Our result shows a dramatic difference of screening behavior in high and low charging limit, which depends on the number of layers as well. We also have an extensive study on quantum tunneling effect in graphene and bilayer graphene heterojunctions. The peculiar electronic properties of graphene lead to an unusual scattering effect of electron in graphene n-p junction. We implement the cohesive tunneling effect to explain the nonlinear electron transport in ultrashort channel graphene devices. This nonlinear behavior could make them tremendously useful for ultra-fast electronic applications.

abstract: Transition metal di- and tri-halides (TMH) have recently gathered research attention owing to their intrinsic magnetism all the way down to their two-dimensional limit. 2D magnets, despite being a crucial component for realizing van der Waals heterostructures and devices with various functionalities, were not experimentally proven until very recently in 2017. The findings opened up enormous possibilities for studying new quantum states of matter that can enable potential to design spintronic, magnetic memory, data storage, sensing, and topological devices. However, practical applications in modern technologies demand materials with various physical and chemical properties such as electronic, optical, structural, catalytic, magnetic etc., which cannot be found within single material systems. Considering that compositional modifications in 2D systems lead to significant changes in properties due to the high anisotropy inherent to their crystallographic structure, this work focuses on alloying of TMH compounds to explore the potentials for tuning their properties. In this thesis, the ternary cation alloys of Co(1-x)Ni(x)Cl(2) and Mo(1-x)Cr(x)Cl(3) were synthesized via chemical vapor transport at a various stoichiometry. Their compositional, structural, and magnetic properties were studied using Energy Dispersive Spectroscopy, Raman Spectroscopy, X-Ray Diffraction, and Vibrating Sample Magnetometry. It was found that completely miscible ternary alloys of Co(1-x)Ni(x)Cl(2) show an increasing Néel temperature with nickel concentration. The Mo(1-x)Cr(x)Cl(3) alloy shows potential magnetic phase changes induced by the incorporation of molybdenum species within the host CrCl3 lattice. Magnetic measurements give insight into potential antiferromagnetic to ferromagnetic transition with molybdenum incorporation, accompanied by a shift in the magnetic easy-axis from parallel to perpendicular. Phase separation was found in the Fe(1-x)Cr(x)Cl(3) ternary alloy indicating that crystallographic structure compatibility plays an essential role in determining the miscibility of two parent compounds. Alloying across two similar (TMH) compounds appears to yield predictable results in properties as in the case of Co(1-x)Ni(x)Cl(2), while more exotic transitions, as in the case of Mo(1-x)Cr(x)Cl(3), can emerge by alloying dissimilar compounds. When dissimilarity reaches a certain limit, as with Fe(1-x)Cr(x)Cl(3), phase separation becomes more favorable. Future studies focusing on magnetic and structural phase transitions will reveal more insight into the effect of alloying in these TMH systems. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2020

Organic photovoltaics (OPVs) have received tremendous attention in recent years due to their numerous attractive attributes such as, the potential for high power conversion efficiency (PCE), mechanical flexibility, and the potential for large-scale manufacturing via low-cost techniques. To date, the record PCE values for bulk-heterojunction (BHJ) OPVs exceed 18% for single-junction cells thanks to the rapid development of donors and acceptors materials for active layer. However, the progress of hole-transporting layer (HTL) systems, which is a key device component to reduce the additional performance losses of OPVs, has been limited with only a handful of materials available like PEDOT:PSS and MoOX. In this thesis, I introduce serval materials to unitize as hole-selective contact in high-performance OPVs. Firstly, the application of liquid-exfoliated two-dimensional transition metal disulfides (TMDs) is demonstrated as the HTLs in OPVs. The solution processing of few-layer WS2 suspensions was directly spun onto transparent indium-tin-oxide (ITO) electrodes yield solar cells with superior power conversion efficiency (PCE), improved fill-factor (FF), enhanced short-circuit current (JSC), and lower series resistance than devices based on PEDOT:PSS. Based on PM6:Y6:PC71BM BHJ layer, the cells with WS2 HTL exhibit the highest PCE of 17% thanks to the favorable photonic structure and reduced bimolecular recombination losses in WS2-based cells. Next, the self-assembled monolayer (SAM) namely 2PACz is utilized as hole-selective contact directly onto the ITO anode. The 2PACz modifies the work function of ITO while simultaneously affecting the BHJ layer’s morphology deposited atop. This ITO-2PACz anode is utilized in OPV with PM6:BTP-eC9:PC71BM, showing a remarkable PCE of 18.0%. The enhanced performance is attributed to reduced contact-resistance, lower bimolecular recombination losses, and improved charge transport within the BHJ layer. Lastly, the previously 2PACz SAM was functionalized with bromide functional groups, namely Br-2PACz, which is investigated as hole-extracting interlayers in OPVs. The highest occupied molecular orbital (HOMO) energy of Br-2PACz was measured at -6.01 eV, and significant changes the work function of ITO electrodes upon chemical functionalization. OPV cells based on PM6:BTP-eC9:PC71BM using ITO/Br-2PACz anodes exhibit a maximum PCE of 18.4%, outperforming devices with ITO/PEDOT:PSS (17.5%), resulting from lower interface resistance, improved hole transport, and longer carrier lifetimes.

In this work, general properties of fundamental gas sensors are described. Thesis is mainly focused on chemoresistive and ChemFET types, which are further used in experimental part. Subsequently, properties, preparation and transfer methods of chosen 1D and 2D materials are described. Experimental part of this work describes design and fabrication of chips, which combine the sensing principals mentioned above for utilization of 1D and 2D materials as an active layer. Transfer methods of individual materials on fabricated chips are described and these materials are characterized by Raman spectroscopy and field effect transistor characteristics measurements. Finally, the response of chosen materials to oxidative and reductive gases is measured.

La miniaturisation des MOSFET a permis une forte diminution des transistors et des puces, ainsi qu’une augmentation exponentielle des capacités de calcul. Cette miniaturisation ne peut néanmoins continuer ainsi: de nos jours, un microprocesseur peut contenir des dizaines de milliards de transistors et la chaleur dégagée par ces composants peut fortement détériorer ses performances. De plus, du fait de leur principe même de fonctionnement, la tension d’alimentation des MOSFET ne peut être réduite sans en impacter les performances. De nouvelles architectures telles que le TFET -basé sur l’effet tunnel bande-à-bande et pouvant fonctionner à des tensions d’alimentation très basses- ainsi que de nouveaux matériaux pourraient donc apporter une alternative au MOSFET silicium. Les monocouches de dichalcogènures de métaux de transitions (TMDs) -des semiconducteurs à bande interdite directe d’environ 1 à 2 eV- possèdent un fort potentiel pour l’électronique et la photonique. De plus, dans le cas de contraintes appropriées, ils peuvent conduire un alignement de bandes présentant un broken-gap; cette configuration permet de surpasser les limites habituelles du TFETs, à savoir de faibles courants dus à l’effet tunnel sur lequel ces dispositifs reposent. Dans ce travail de thèse, des hétérojonctions planaires de TMD sont modélisées via une approche atomistique de liaisons fortes, et une configuration broken-gap est observée dans deux d’entre elles (MoTe2/MoS2 et WTe2/MoS2). Leur potentiel dans le cadre de transistors à effet tunnel (TFETs) est évalué au moyen de simulations de transport quantique basées sur un modèle TB atomistique ainsi que la théorie des fonctions de Green hors-équilibre. Des TFETs type-p et type-n basés sur ces hétérojonctions sont simulés et présentent des courants ON élevés (ION > 103 µA/µm) ainsi que des pentes sous-seuil extrêmement raides (SS < 5 mV/dec) à des tensions d’alimentation très faibles (VDD = 0.3 V). Plusieurs architectures novatrices basées sur ces TFETs et découlant de la nature 2D des matériaux utilisés sont également présentées, et permettent d’atteindre des performances encore plus élevées. / Nowadays, microprocessors can contain tens of billions of transistors and as a result, heat dissipation and its impact on device performance has increasingly become a hindrance to further scaling. Due to their working mechanism, the power supply of MOSFETs cannot be reduced without deteriorating overall performance, and Si-MOSFETs scaling therefore seems to be reaching its end. New architectures such as the TFET, which can perform at low supply voltages thanks to its reliance on band-to-band tunneling, and new materials could solve this issue. Transition metal dichalcogenide monolayers (TMDs) are 2D semiconductors with direct band gaps ranging from 1 to 2 eV, and therefore hold potential in electronics and photonics. Moreover, when under appropriate strains, their band alignment can result in broken-gap configurations which can circumvent the traditionally low currents observed in TFETs due to the tunneling mechanism they rely upon. In this work, in-plane TMD heterojunctions are investigated using an atomistic tight-binding approach, two of which lead to a broken-gap configuration (MoTe2/MoS2 and WTe2/MoS2). The potential of these heterojunctions for use in tunnel field-effect transistors (TFETs) is evaluated via quantum transport computations based on an atomistic tight-binding model and the non-equilibrium Green’s function theory. Both p-type and n-type TFETs based on these in-plane TMD heterojunctions are shownto yield high ON currents (ION > 103 µA/µm) and extremely low subthreshold swings (SS < 5 mV/dec) at low supply voltages (VDD = 0.3 V). Innovative device architectures allowed by the 2D nature of these materials are also proposed, and shown to enhance performance even further.

abstract: Two-dimensional quantum materials have garnered increasing interest in a wide variety of applications due to their promising optical and electronic properties. These quantum materials are highly anticipated to make transformative quantum sensors and biosensors. Biosensors are currently considered among one of the most promising solutions to a wide variety of biomedical and environmental problems including highly sensitive and selective detection of difficult pathogens, toxins, and biomolecules. However, scientists face enormous challenges in achieving these goals with current technologies. Quantum biosensors can have detection with extraordinary sensitivity and selectivity through manipulation of their quantum states, offering extraordinary properties that cannot be attained with traditional materials. These quantum materials are anticipated to make significant impact in the detection, diagnosis, and treatment of many diseases. Despite the exciting promise of these cutting-edge technologies, it is largely unknown what the inherent toxicity and biocompatibility of two-dimensional (2D) materials are. Studies are greatly needed to lay the foundation for understanding the interactions between quantum materials and biosystems. This work introduces a new method to continuously monitor the cell proliferation and toxicity behavior of 2D materials. The cell viability and toxicity measurements coupled with Live/Dead fluorescence imaging suggest the biocompatibility of crystalline MoS2 and MoSSe monolayers and the significantly-reduced cellular growth of defected MoTe2 thin films and exfoliated MoS2 nanosheets. Results show the exciting potential of incorporating kinetic cell viability data of 2D materials with other assay tools to further fundamental understanding of 2D material biocompatibility. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019

Low-dimensional semiconductors have been intensely explored as alternative active materials for future generation ultra-scaled smart electronics. However, significant roadblocks (e.g., poor carrier mobilities, instability, and vague potential in scaling-up) exist that prevent the realization of the current state-of-the-art low-dimensional materials’ potential for energy-efficient electronics. We first time developed hydrothermal method to solution-grown two-dimensional Te, which exhibits attractive attributes, e.g., high room-temperature mobility, large on-state current density, air-stability, and tunable material properties through a low-cost, scalable process, to tackle these challenges.

<p>The miniaturization of a MOSFET is the constant driving force in semiconductor technology over the decades. This scaling enables the realization of the ever complex and functional integration on a single chip where over tens of billions of transistors densely packed. Silicon (Si) is always the golden performer until recent years when the shrinking of a transistor becomes more and more difficult, due to phenomena such as short channel effect and mobility degradation, which is a challenge especially for atomic level scaling. The dawning of low dimensional materials, such as graphene, transition metal dichalcogenides (TMDs), black phosphorus (BP), with their natural atomically thin two-dimension (2D) layered structure and other novel properties, might serve as an alternative solution for ultimate scaling. However, the understanding of the electronic transport in these Van der Waals materials is still lacking. </p><p> In this research, the exploration of this material was first initiated on the vertical heterojunctions where two materials’ interfaces meet. Many previous literatures claimed this hetero-interface creates a P/N junction that results in a diode-like rectification. Yet, by careful analysis and comparing with our “real” vertical structures where the lateral components were eliminated, it is proved this rectification is a direct result from the contact region. The Schottky barrier on the drain side together with the gate effect is the true culprit.</p><p> Realizing how the Schottky barrier could be dominating in these 2D FETs, the second study is the Schottky barrier effect on the contact resistances and furthermore the mobility of the device. Because of the existence of the Schottky barrier between the channel and contact, the contact resistance is not negligible, unlike the ohmic contact for conventional Si MOSFETs. By comparing the intrinsic and extrinsic mobilities of TMD materials, It is found that the contact resistance’s response to the back gate, namely, the rate of how it changes with the back gate has a huge factor in determining whether the extrinsic field-effect mobility underestimates or overestimates its intrinsic mobility. This opens a new insight on the understanding of the transport mechanism under contacts for different TMDs.</p> With the understanding of the Schottky barrier FETs, lastly, the flexibility of these 2D materials is utilized to create high performance three-dimensionally stacked multi-channel FETs, from the inspiration of the Si gate-all-around nanosheet structure. A first-ever 3D integrated high performance MoS₂ device with two channels on top of each other was designed and fabricated, where the current is doubled with an extra layer of channel. The potential of these novel material to be implemented on the future generations of high-performance devices is demonstrated, shedding light on the prospect for extending the Moore’s Law with proper assistance from new materials.